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ISSN 1990 7931, Russian Journal of Physical Chemistry B, 2015, Vol. 9, No. 2, pp. 185–192. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © V.P. Tarasov, G.A. Kirakosyan, K.E. German, 2015, published in Khimicheskaya Fizika, 2015, Vol. 34, No. 4, pp. 20–28.
185
INTRODUCTION
In NMR, the secondary isotope effect Δ(А) is
defined as the change in spectral parameter Θ (chem
ical shift of spin A or A–X spin coupling constant)
caused by the replacement of light isotope L
X by heavy
isotope HX in the AXn system [1]:
1
Δ(A) = Θ(AH
Xn) – Θ(AL
XH
Xn – 1). (1)
The values of these effects are extremely small. In
particular, 1
Δ(A) is usually about 0.1–0.2% of the
spin–spin coupling constant (SSCC) value. The magni
tude of the isotope shift depends on the NMR shielding
range and is proportional to the ratio (m'– m)/m', where
m' and m are the masses of the heavy and light isotopes,
respectively. Isotope shifts have been determined and
examined for the substitution of both light isotopes
(1
H/2
H/3
H) and many heavy isotopes (12
C/13
C,
14N/15N, 16O/18O,32S/34S, 35Cl/37Cl, and 79Br/81Br [2]);
at the same time, the isotope effect on SSCC has been
mainly determined for substitution of hydrogen by
deuterium (1
H/2
Н) [3, 4] or by tritium (1
H/3
Н), i.e.,
in systems with the largest possible mass change [5].
Theoretical aspects of isotope effects on magnetic
shielding and spin coupling constants have been devel
oped by Jameson and Osten [2, 3].
Favorable 99
Tc and 17
O NMR parameters of the
anion in aqueous and nonaqueous solutions
(optimal spin–lattice relaxation times Т1 (~ 0.15 s),
small line width (5–15 Hz), and large chemical shift
ranges) have made it possible to measure, at relatively
low magnetic fields (Н0 = 2.1 T), the 99Tc NMR iso
tope shifts induced by the 16
O/18
O substitution and
spin coupling constants 1
J(17
O–99
Tc) in samples
enriched in 17
O and 18
O [6, 7]. Cho et al. [8] have mea
sured, for the first time, the displacement of the 99Tc
chemical shift (Н0 = 7.04 T) as a function of tempera
ture for three isotopomers at natural oxygen isotope
abundance levels and the temperature dependence of the
1J(99Tc–17O) spin coupling constant for the [Tc16O3
17
O]–
isotopomer.
The present study deals with experimental mea
surements of the temperature dependences of 99Tc
NMR isotope shifts and 1
J(17
O–99
Tc) and 1
J(99
Tc–17
O)
TcO4
−
Oxygen Isotope Effect on NMR Parameters
of Pertechnetate Anion
V. P. Tarasova, b, G. A. Kirakosyana, b, and K. E. Germanb
a
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
b Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
e mail: tarasov@igic.ras.ru
Received March 26, 2014
Abstract—The effects of oxygen isotope substitution 16O ↔ 17O ↔ 18O in the coordination sphere of the
pertechnetate anion ( ) on the NMR 99Tc chemical shifts and 99Tc–17O and 17O–99Tc spin coupling
constants have been studied by 17
O and 99
Tc NMR. The isotope shifts 16/17
Δ and 16/18
Δ in 99
Tc NMR and the
spin coupling constants of the Tc 16
O3
18
O–
, Tc 16
O3
17
O–
, Tc 16
O2
17
O 18
O–
, and
isotopomers have been measured. For the Tc 16O3
18O– and Tc 16O3
17O– anions in an ammonium pertechne
tate solution, the temperature dependences of the isotope shift in the temperature range 278–333 K are
described by linear relationships 16/18Δ = –0.616 + 6.45 × 10–4T (ppm) and 16/17 Δ = –0.302 +2.67 × 10–4T
(ppm), respectively. For the Tc16
O3
17O–
anion in a sodium pertechnetate solution, the magnitude of the
1Δ(16/17O) isotope shift nonlinearly decreases with increasing temperature. The nonlinear temperature
dependence of the J(99
Tc–17
O) spin coupling constant and the extreme point on the curve of the 1
Δ(16/18
O)
isotope shift versus temperature for the isotopomers in an NaTcO4 solution are presumably related to equilib
rium between contact and water separated ion pairs.
Keywords: 17
O and 99
Tc NMR, pertechnetate anion, temperature dependence, isotope shift, isotope effects
on spin coupling constants
DOI: 10.1134/S1990793115020281
TcO4
–
−
TcO4
−
Tc O O
16 17
2 2,
−
Tc O O
16 18
2 2.
STRUCTURE OF CHEMICAL COMPOUNDS.
SPECTROSCOPY
186
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015
TARASOV et al.
constants and determination of additivity of changes
in their values induced by the 16
O/17
O and 16
O/18
O iso
tope substitution in the anion in concentrated
solutions.
EXPERIMENTAL
Concentrated solutions of NaTcO4 (I), Tc2O7 (II),
and NH4TcO4 (III) with concentrations ~0.7–
4.07 mol/L in water enriched in the 17
O (1.7%) and
18
O (0.8%) isotopes were studied. The 17
O and 99
Tc
NMR spectra were recorded in fields of 7.04 and 14.1 T
on Bruker Avance 300 and Avance 600 spectrome
ters, respectively. Nuclear spins were excited by 30°
pulses (the 90° pulse width was 10 and 15 μs for 99
Tc
and 17
O, respectively). The sweep width was 1500 and
10000 Hz (32K memory size) and the number of scans
was 200 and 500 for technetium 99 and oxygen 17,
respectively. The pulse repetition time was 1 s. The
solutions were placed in standard 5 mm NMR tubes.
The temperature of the samples was controlled to
within ±0.1° by a BVT 3200 unit. To avoid a temper
ature gradient along the sample tube, the specified
temperature was maintained using air at a high flow
rate of 400–500 L/min. Before each measurement,
samples were kept at a given temperature for at least
15–20 min. At each temperature, spectra were
recorded twice.
RESULTS AND DISCUSSION
Isotope Substitution 16
O ↔ 17
O ↔ 18
O
Intermolecular oxygen exchange between the
anion and isotope enriched water leads to the
formation of technetium isotopomers containing
three different oxygen isotopes in the coordination
sphere. The overall number of possible isotopomers
Z = 15 is determined by the formula
Z = (n + s – 1)!/n!(s – 1)! (2)
where n = 4 is the technetium coordination number in
the pertechnetate anion, and s = 3 is the number of
oxygen isotopes (16
O, 17
O, 18
O). For random distribu
tion, the content C of each isotopomer is determined
as [9]
C = n!(r16)a(r17)b (r18)c/(a ! b ! c), (3)
where r16, r17, and r18 are the mole fractions of the oxy
gen isotopes (r16 + r17 + r18) = 1; coefficients a, b, and
с determine the number of the corresponding isotopes
in the pertechnetate anion so that a + b + c = 4.
The contents of all 15 isotopomers calculated for
natural and enriched oxygen abundance levels (the
latter was used in this study) are summarized in the
table. It should be noted that the temperature depen
dences of isotope effects (99
Tc NMR chemical shift
and 1
J(99
Tc–17
O) constant) have been studied in a
magnetic field of 7.04 T only for the first three isoto
TcO4
−
TcO4
−
pomers [8] listed in the table. As shown below, for the
solutions studied in this work, the 99Tc NMR parame
ters are actually observed only for the first six isoto
pomers listed in the table.
16
O–18
O and 16
O–17
O Substitution Effects
on 99Tc NMR Parameters
In all the samples under consideration, the isotope
effects on the 99
Tc shielding constant are manifested as
the upfield shift of the signals of the Tc 16
O3
18O–
and
Tc 16
O3
17
O–
isotopomers by, respectively, –0.434 and
–0.215 ppm from the signal of the anion. The
99
Tc NMR spectrum of the Tc 16
O3
18O–
isotopomer is
a narrow singlet, while the spectrum of the Tc16O3
17
O–
isotopomer is a sextet caused by technetium spin cou
pling to the oxygen 17 spin (17I = 5/2) with the
1
J(99
Tc–17
O) SSCC.
The multiplet in the 99Tc NMR spectrum of the
anion caused by the coupling of the techne
tium spin to two equivalent oxygen 17 spins consists of
11 lines; the multiplicity is determined by the formula
(2nI + 1) = (2 × 2 × 5/2 + 1) = 11, (4)
where n = 2 is the number oxygen 17 spins, and I =
5/2 is the nuclear spin of oxygen 17. According to Pas
cal’s triangle, the ratio of the intensities in this multip
let is 1 : 2 : 3 : 4 : 5 : 6 : 5 : 4 : 3 : 2 : 1. Figure 1 shows
the 99
Tc NMR spectra (H0 = 14.1 T) of the Tc2O7 +
H2
17O system (saturated solution) at room tempera
ture (298 K). For the Tc 16
O3
17
O–
anion, the SSCC
measured as the average of all the five distances
between the sextet components and the SSCC value
obtained by dividing the distance between the outer
most multiplet components by 5 are J(99Tc–17O) =
(131.22 ± 0.20) and (131.39 ± 0.20) Hz, respectively.
For the anion, the SSCC value measured
as the average of the distances between the nine
observed (not overlapped) lines and the SSCC value
obtained by dividing the distance between the outer
most lines of the undecet by 10 are 1J(99Tc–17O) =
(131.97 ± 0.20) and (131.99 ± 0.20) Hz, respectively.
For the Tc2O7 + H2
17O system, comparison of the
SSCCs for the and Tc 16
O3
17O–
isoto
pomers shows that the magnitude of the isotope effect
on SSCC is ΔJ = |1J( ) – 1J(Tc 16O3
17
O–)| =
(0.7 ± 1.0) Hz.
Figure 2 displays the 99Tc NMR spectra (H0 = 7.04 T)
of the concentrated NH4TcO4 solution (0.7 M) at 318 K.
These spectra recorded under relatively “mild” condi
tions show, in addition to the signal of the iso
topomer at 0.0 ppm, the singlet due to the Tc 16
O3
18O–
isotopomer at 1
Δ = –0.432 ppm, the sextet due to the
Tc16
O3
17
O–
isotopomer with 1
J(99
Tc–17
O) = (131.34 ±
Tc O16
4
−
Tc O O16 17
2 2
−
Tc O O16 17
2 2
−
Tc O O16 17
2 2
−
Tc O O16 17
2 2
−
Tc O16
4
−
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015
OXYGEN ISOTOPE EFFECT ON NMR PARAMETERS 187
0.67) Hz at 1
Δ = –0.217 ppm, the singlet of the
isotopomer at 1
Δ = –0.852 ppm, the sextet
of the Tc16
O2
17O 18
O–
isotopomer with 1
J(99
Tc–17
O) =
(131.76 ± 0.93) Hz at 1
Δ = –0.671 ppm, and the
11 membered multiplet due to the isoto
pomer at 1
Δ ≈ –0.44 ppm with the SSCC (131.34 ±
0.67) Hz. Then, the isotope effect on SSCC is ΔJ =
|1
J(Tc 16
O2
17
O 18
O–
)–1
J(Tc 16
O3
17
O–
)|=(0.42± 1.60)Hz
and ΔJ = |1
J( ) – 1
J(Tc 16
O3
17
O–
)| = (0 ±
1.4) Hz. Thus, the error of measurement of the isotope
effect on the J(99
Tc–17
O) SSCC exceeds the value of
this effect. It is likely that there is a weak tendency for
an increase in the |J(99
Tc–17
O)| value with an increase
−
Tc O O16 18
2 2
Tc O O16 17
2 2
−
Tc O O16 17
2 2
−
in the mass of isotopomer. This tendency has been pre
viously reported in [6].
Temperature Dependence of the Chemical Shift
of the Isotopomer
Temperature dependences of the 99
Tc NMR chemi
cal shift of the anion for NaTcO4 and NH4TcO4
solutions are linear with similar slopes ΔT(NaTc 16
O4) =
–40.25 + 0.143T and ΔT (NH4Tc 16O4) = –36.69 +
0.123T. These data point to a weak influence of the
type of cation or concentration on the magnetic
shielding of the technetium nucleus in the iso
topomer. The temperature induced shifts for
Tc O
16 –
4
Tc O16
4
−
Tc O16
4
−
Tc O16
4
−
Content of technetium isotopomers for random distribution of 16O, 17O, and 18O isotopes in pertechnetate anion
No. Isotopomer
Relative isotopomer concentration
16
O : 17
O : 18
O* =
0.99757 : 0.00038 : 0.00205
16
O :17
O : 18
O** =
0.975 : 0.017 : 0.008
1 0.9903153 0.90151
2 Tc16
O3
18O– 8.2129477 × 10–3
0.03164
3 Tc16
O3
17
O– 1.5237026 × 10–3 0.06328
4 2.5092604 × 10–5
4.2 × 10–4
5 Tc16
O2
17O18
O– 9.302623 × 10–6
1.7 × 10–3
6 8.6429484 × 10–7 3.33 × 10–3
7 3.437676 × 10–8
2.4 × 10–6
8 1.946833 × 10–8
1.46 × 10–5
9 Tc16
O17
O2
18O– 3.55224 × 10–9
2.92 × 10–5
10 2.189564 × 10–10 1.95 × 10–5
11 1.7661006 × 10–11 5.3 × 10–9
12 1.309499 × 10–11 2.1 × 10–13
13 3.641046 × 10–12 8.6 × 10–8
14 Tc17O3
18
O– 4.499504 × 10–13
1.71 × 10–7
15 2.085136 × 10–14
8.6 × 10–8
* Natural isotope abundance.
** Solutions of technetium compounds (NaTcO4, NH4TcO4, and Tc2O7) in water enriched in 17
O and 18
O to ~1.7 and ~0.8%, respec
tively.
4TcO
−
−
Tc O
16
4
Tc O O16 18
2 2
−
Tc O O16 17
2 2
−
−
Tc O O16 18
3
−
Tc O O O
16 17 18
2
Tc O O16 17
3
−
Tc O18
4
−
Tc O O17 18
3
−
Tc O O17 18
2 2
−
Tc O17
4
−
188
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015
TARASOV et al.
800 600 400 200 0 –200 –400 –600
×2048
×16
Hz
Fig. 1. 99
Tc NMR spectra (H0 = 14.1 T) of a Tc2O7 solution in H2
17O (4.5 M) at 298 K. The satellite lines due to the 99
Tc–17
O
spin coupling in the anion are asterisked.Tc O O
16 17
2 2
−
600 400 200 –2000 –400 –600
×128
Hz
Fig. 2. 99
Tc NMR spectra (H0 = 7.04 T) of an NH4TcO4 solution in H2
17O (0.7 M) at 318 K. The satellite lines due to the 99
Tc–17
O
spin coupling in the anion are asterisked.Tc O O
16 17
2 2
−
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015
OXYGEN ISOTOPE EFFECT ON NMR PARAMETERS 189
measured on heating from 283 to 333 K are 7.183 and
6.66 ppm for the sodium and ammonium cation,
respectively. These values are nearly twice as large as
the measured temperature induced shifts for diluted
KTcO4 (0.1 M) and NH4TcO4 (0.01 M) solutions [8].
The temperature dependence of the technetium
shielding constant σ(T) in the isotopomer (if
the four Tc–O bonds in the anion are equivalent and
chemical exchange is absent) is described in the first
approximation as [10]
σ(T) ≈ σe + (∂σ/∂Δr)e · 4T
〈Δr 〉, (5)
where T
〈Δr〉 is the rovibrational average Tc–16
O bond
extension at a given temperature Т, σe is the shielding
at the equilibrium Tc–16
O bond length re. In the
explicit form, the temperature dependence σ(T) is
deduced from detailed analysis of T
〈Δr〉 represented by
the sum of the vibrational and rotational contribu
tions:
T〈Δr〉 = T〈Δr〉vib + T〈Δr 〉rot. (6)
For molecules of symmetry Td, the rotational contri
bution is [2]
T
〈Δr 〉rot = (4Bere /hc ) kT, (7)
where Be is the rotational constant of the and
ωe is the stretching vibration frequency in cm–1. The
other notations are conventional. For the iso
topomer, the numerical value of the temperature coef
ficient in Eq. (7) is 0.15 × 10–4
Å/K at re = 1.711 Å, and
ωe = 912 cm–1. The rotational contribution T〈Δr〉rot is
one order of magnitude lower than the vibrational
contribution, nevertheless, in a given temperature
range, it is precisely the change in the rotational con
tribution that is responsible for the linear dependence
of the chemical shift. Then, the change in magnetic
shielding with a change in temperature can be esti
mated as
σ(T) – σ(283 K) = (∂σ/∂Δr)e · 4 (T
〈Δr〉 – 283
〈Δr〉); (8)
(∂σ/∂Δr)e istemperatureindependent,being–2030ppm/Å
for [2]. The observed behavior of the chemical
shift as a function of temperature is completely deter
mined by the change in the thermally averaged bond
length 〈Δr(Tc–16
O)〉. Then, this bond extension with
increasing temperature from 283 to 333 K is (T
〈Δr 〉 –
283
〈Δr〉)=8.85×10–4
Å for NaTcO4 and 8.2 × 10–4
Å for
NH4TcO4. For the free pertechnetate anion, the cal
culated Tc–O bond length is 1.717 Å, and for the
hydrated anion, this length is 1.714 Å [8]. These values
are consistent well with the value 1.711 Å obtained from
single crystal diffraction experiments [11].
Tc O16
4
−
2
eω
Tc O16
4
−
Tc O16
4
−
Tc O16
4
−
Isotope Shifts as a Function of Temperature
The chemical shift and isotope shift as a function of
temperature are inherently related to each other. For
highly symmetric ions, such as the 99
Tc NMR
isotope shift caused by the substitution of one 18
O
atom for a 16
O atom in this anion can be written by
analogy with Eq. (8):
(9)
where 18
〈Δr〉 and 16
〈Δr〉 are the rovibrational average
bond extension at a given temperature for the isoto
pomers under consideration. At 300 K, the shift is
1
Δ(99
Tc) = –0.425 ppm; then, the change in the aver
age bond length (18
〈Δr〉 – 16
〈Δr〉) is 2.12 × 10–4
Å. For
the complete substitution of 16
O for 18
O in , the
average bond extension turns out to be 2.12 × 10–4 × 4 =
8.5 × 10–4
Å, which numerically corresponds to the
temperature effect on the bond length change.
The temperature factor of the isotope effect is
described by a rather complex function F(T) [2]:
(10)
where μ and μ* are the reduced masses of the light and
heavy isotopomers, respectively; ω is the Tc–O
stretching frequency in cm–1. The F(T) function con
tains hyperbolic cotangent, depends only slightly on
temperature, and predicts an insignificant decrease in
the isotope shift magnitude with increasing tempera
ture.ThenumericalevaluationoftheF(283K)/F(333K)
ratio by Eq. (10) at ω = 912 cm–1 gives 1.06 and 1.1 for
the Tc 16O3
17O– and Tc 16O3
18O– isotopomers, respec
tively. Indeed, with increasing temperature from 283
to 333 K for ammonium pertechnetate, the 99
Tc NMR
isotope shifts Δ16/17
and Δ16/18
decrease by a factor of
1.05 and 1.09, respectively, as compared with the value
at 283 K.
The temperature dependences of 99
Tc NMR iso
tope shifts 1Δ(16/18O) and 1Δ(16/17O) for the
Tc 16
O3
18O–
and Tc 16
O3
17O–
anions in aqueous solu
tions of ammonium pertechnetate are shown in Figs. 3
and 4. To a first approximation, both dependences are
linear and are described as
(11а)
(11b)
For a sodium pertechnetate solution, the tempera
ture behavior of the isotope shift 1
Δ(16/17
O) for the
Tc 16
O3
17O–
anion exhibits small deviation from lin
Tc O16
4,−
Δ
1
Tc
99
( ) σ Tc O
18
O
16
3( ) σ Tc O
16
4( )–=
= ∂σ/∂Δr( )e Δr〈 〉
18
Δr〈 〉
16
–( ),
TcO4
–
F T( ) hωc/2kT( )coth{=
– μ/μ*( )
1/2
coth hc μ/μ*( )ω/2kT[ ]},
Δ
1
O
16/18
( ) 0.616– 6.45 10
14–
× T (ppm)+=
for the Tc O
16
3 O
18 –
anions,
Δ
1
O
16/17
( ) 0.302– 2.67 10
4–
× T (ppm)+=
for the Tc O
16
3 O
17 –
anions.
190
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015
TARASOV et al.
earity. A quite unexpected fact is the existence of an
extreme point on the curve of 1Δ(16/18O) versus tem
perature for a NaTcO4 solution (Fig. 5). The reasons of
such a behavior of 1
Δ(16/18
O) and 1
Δ(16/17
O) are not
quite clear. We can assume that, at such a high concen
tration (4.07 M), the observed behavior of the isotope
shift is due to equilibrium between the contact and sol
vent separated ion pairs, i.e., [Na]+
[TcO4]–
↔
[Na(H2O)n]+
[TcO4]–
. With increasing temperature,
this equilibrium shifts to the right. In the contact ion
pair [Na]+
[TcO4]–
, the magnitude of the isotope shift
increases with increasing temperature, while, in the
solvent separated ion pair [Na(H2O)n]+[TcO4]–, the
magnitude of the isotope shift |1
Δ(16/18
O)| decreases
with increasing temperature. There are only solvent
separated ion pairs in ammonium pertechnetate solu
tions. Therefore, the isotope shift magnitude
|1Δ(16/18O) | decreases with increasing temperature, as
shown in Fig. 3.
It should be noted that a noticeable effect of the
solution concentration and the type of counterion on
the Δ(1H/2H) isotope shift in the 14, 15N NMR spectra
of the cation has been reported for aqueous solu
tions of ammonium salts [12]. It has been demon
strated that the magnitude of the Δ(1
H/2
H) isotope
shift increases linearly with an increase in concentra
tion.
Temperature Dependence of 1J(17O–99Tc)
and 1J(99Tc–17O) Spin–Spin Coupling Constants
The 17O NMR spectra of an aqueous solution of
NaTcO4 have been studied in the temperature range
283–343 K. The spectra are decets caused by 17
O–99
Tc
spin coupling (technetium 99 spin is I = 9/2) with the
1J(17O–99Tc) constant. The narrowing of the multiplet
lines with increasing temperature (Fig. 6) is evidence
that the 17O NMR line shape is completely dominated
by the quadrupole relaxation of the technetium spin.
The oxygen chemical exchange is so slow that it has no
effect on the 17O NMR line width and position. The
minimalspin–latticerelaxationtimeoftechnetiumspins
in an aqueous solution is T1 min(99Tc) = 0.16 s [7, 13].
Therefore, the effects of the quadrupole relaxation of
technetium nuclei do not change the position of the
outermost multiplet lines since the condition [1]
J(17O–99Tc)T1(99Tc) ~ 20
is met.
Of the three isotopomers corresponding to a given
spectrum (entries 3, 5, and 6 in the table), only the
Tc 16O3
17O– isotopomer (no. 3) is actually observed.
The content of this isotopomer is 20–30 times as large
as the content of the other two isotopomers (table).
The 17
O NMR line width of each of the multiplet com
ponents 17
Δνm corresponds to a definite spin state m of
the 99Tc isotope. The lifetime of each spin state m of
the technetium nucleus is larger than the inverse of the
SSCC magnitude |1
J(17
O–99
Tc)|–1
, and the observed
17O NMR line widths of the multiplet 17Δνobs can be
presented as
17
Δνobs = 17
ΔνQ Cm + 17
Δνex, (12)
where 17
ΔνQ is the intrinsic line width of oxygen 17 in
the pertechnetate anion, 17
Δνex is the line width due to
chemical exchange, Cm is the sum of squared matrix
NH4
+
0.40
340270 330320310300290280
0.41
0.42
0.43
0.44
Т, К
–1
Δ, ppm
Fig. 3. 99Tc NMR isotope shift (–1Δ) vs. temperature for
the Tc16
O3
18O–
isotopomer in an aqueous solution of
NH4TcO4 (0.7 M); –1Δ (16/18O) = –0.616 + 6.45 × 10–4 T
(ppm). H0 = 7.04 T.
0.210
340270 330320310300290280
0.215
0.220
0.225
Т, К
–1Δ, ppm
Fig. 4. 99
Tc NMR isotope shift (–1
Δ) vs. temperature for the
Tc 16
O3
17O–
isotopomer in an aqueous solution of
NH4TcO4 (0.7 M). The shift was referenced to the
signal and measured as the midpoint of the distances between
the corresponding lines of the multiplet due to the 99
Tc–17
O
spin coupling (for six non overlapped lines); –1
Δ (16/17
O) =
–0.302 + 2.67 × 10–4 T (ppm). H0 = 7.04 T.
−
Tc O
16
4
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015
OXYGEN ISOTOPE EFFECT ON NMR PARAMETERS 191
elements corresponding to the probability coefficients
of the transitions Δm = ±1 and Δm = ±2 for the tech
netium spin due to quadrupole interactions. For the
ten spin states of technetium nucleus m = ±9/2, ±7/2,
±5/2, ±3/2, and ±1/2, the Cm values are 3.854, 7.854,
8.125, 7.125, and 6.250, respectively [6]. The narrow
est lines are observed for the two outermost and two
central multiplet components. The 17O–99Tc SSCC
value was measured as the distance between the outer
most multiplet lines divided by 9. The observed tem
perature dependences of the 17O–99Tc and 99Tc–17O
SSCCs for an NH4TcO4 solution show a clear decrease
in their magnitude (Fig. 7). For a NaTcO4 solution, the
temperature dependences (Fig. 8) have an extremum,
like the temperature dependence of the isotope shift
1
Δ(16/18
O) for this solution. However, the error of mea
surement of the 17O–99Tc and 99Tc–17O spin coupling
constants exceeds the accuracy required to reliably
confirm or refute the existence of an extremum. It
should be noted that the theoretical evaluation of the
sign of the reduced SSCC for the pertechnetate anion
gives the positive value K(99Tc–17O) = +350 × 1020 cm–3
[14]. Then, the experimental J(99Tc–17O) value must
be negative since the gyromagnetic ratio is negative for
oxygen 17 and positive for technetium 99.
It has been shown above that the substitution of 16
O
for 18
O leads to the change in the average bond length:
|18
〈Δr〉 – 16
〈Δr〉| = 2.12 × 10–4
Å; at the same time, the
SSCC changes by ΔJ = |1
J(Tc 16
O2
17
O 18
O–
) –
1
J(Tc 16
O3
17
O–
)| = (0.42 ± 1.60) Hz. This change ΔJ
can be presented as
ΔJ = (∂J/∂Δr)e (18〈Δr〉 – 16〈Δr〉)
= (∂J/∂Δr)e × 2.12 × 10–4.
Then, we obtain the numerical estimate (∂J/∂Δr)e ~
2000 Hz/Å.
CONCLUSIONS
The effects of oxygen isotope substitution 16O ↔
17
O ↔ 18
O in the coordination sphere of the pertech
netate anion ( ) on the magnetic shielding of 99
Tc
nuclei and 17
О–99
Тс and 99
Tc–17
O spin–spin coupling
in concentrated solutions of NaTcO4, NH4TcO4, and
Tc2O7 have been studied in the temperature range
−
TcO4
32000 31500 31000 30500 30000 29500 29000
283 K
303 K
333 K
Hz
Fig. 6. Temperature dependence of the 17
O NMR (7.04 T) line shape for an aqueous solution of NaTcO4 in H2
17O (4.07 M).
0.400
340330320310300290280
0.404
0.408
0.412
0.416
0.420
–1
Δ, ppm
Т, К
Fig. 5. 99
Tc NMR isotope shift (–1
Δ) vs. temperature for
the Tc 16
O3
18O–
isotopomer in an aqueous solution of
NaTcO4 (4.07 M). H0 = 7.04 T.
192
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015
TARASOV et al.
278–350 K by 17O and 99Tc NMR in magnetic fields of
7.04 and 14.1 T. The isotope shifts 16/17
Δ and 16/18
Δ and
the spin coupling constants have been measured for
the Tc 16
O3
17
O–
,
Tc16
O2
17
O 18
O–
, and isotopomers of the
fifteen possible Tc16
Ok
17
On
18
O4 – k – n (k, n = 0–4) iso
topomers of the pertechnetate anion. For the
Tc16
O3
18O–
and Tc16
O3
17O–
anions in an ammonium
pertechnetate solution, the temperature dependences
of the isotope shift in the temperature range 278–333 K
are described by linear relationships 16/18
Δ = –0.616 +
6.45 × 10–4Т (ppm) and 16/17Δ = –0.302 + 2.67 × 10–4 Т
(ppm), respectively. In the framework of the Jame
son–Osten theory [3], the observed temperature
dependences of isotope shifts have been considered to
be a result of vibrational and rotational averaging of
Tc–O bond lengths. The change in Tc–O bond length
caused by isotope substitutions is on the order of 10–4
Å.
The fact that, for the Tc 16
O3
17
O–
anion in a sodium
pertechnetate solution, the magnitude of the
1
Δ(16/17
O) isotope shift nonlinearly decreases with
increasing temperature, as well as the presence of an
extreme point on the curve of the temperature depen
dence of the 1
Δ(16/18
O) isotope shift in 99
Tc NMR
spectra of a concentrated NaTcO4 solution, presum
ably reflects the influence of equilibrium between con
tact and water separated ion pairs. The change in
J(99
Tc–17
O) with temperature for the Tc 16
O3
17
O–
anion is characterized by a poorly pronounced extre
mum. However, the error of measurement of the
SSCC values is larger than or comparable with tem
perature induced changes.
−
Tc O16
4, −
Tc O O16 18
3 , −
Tc O O16 17
2 2,
−
Tc O O16 18
2 2,
REFERENCES
1. N. M. Sergeyev, NMR Basic Princ. Progr. 22, 31
(1990).
2. C. J. Jameson and H. J. Osten, Annu. Rep. NMR Spec
trosc. 17, 1 (1986).
3. C. J. Jameson and H. J. Osten, J. Am. Chem. Soc. 108,
2497 (1986).
4. V. P. Tarasov, V. I. Privalov, Yu. A. Buslaev, and U. Eich
hoff, Z. Naturforsch. B 39, 1230 (1984).
5. C. Than, H. Morimoto, H. Andres, and P. G. Williams,
J. Labelled Comp. Radiopharm. 38, 693 (1996).
6. V. P. Tarasov, V. I. Privalov, G. A. Kirakosyan, A. A. Gor
bik, and Yu. A. Buslaev, Dokl. Akad. Nauk SSSR 263,
1416 (1984).
7. V. P. Tarasov, V. I. Privalov, and Yu. A. Buslaev, Mol.
Phys. 50, 1141 (1983).
8. H. Cho, W. A. de Jong, B. K. McNamara, B. M. Rapko,
and I. E. Burgeson, J. Am. Chem. Soc. 126, 1158
(2004).
9. G. Galingaert and H. A. Beaty, J. Am. Chem. Soc. 61,
2748 (1939).
10. C. J. Jameson and H. J. Osten, J. Chem. Phys. 81, 4300
(1984).
11. B. Krebs and K. D. Hasse, Acta Crystallogr., Sect. B 36,
1334 (1976).
12. P. E. Hansen and A. Lycka, Acta Chim. Scand. 43, 222
(1989).
13. V. P. Tarasov, V. I. Privalov, and Yu. A. Buslaev, Dokl.
Akad. Nauk SSSR, 262, 1433 (1982).
14. V. G. Yarzhemskii, V. P. Tarasov, and V. I. Nefedov,
Koord. Khim. 9, 1329 (1983).
Translated by G. Kirakosyan
131.6
340320300280
132.0
132.4
1
J, Hz
Т, К
Fig. 8. Temperature dependence of the 1
J(17
O–99
Tc) (᭿)
and 1
J(99
Tc–17
O) (᭹) spin–spin coupling constants for
the Tc 16
O3
17O–
isotopomer in an aqueous solution of
NaTcO4 (4.07 M); H0 = 7.04 Т.
130.4
340320300280
130.8
131.2
131.6
132.0
Т, К
1
J, Hz
Fig. 7. Temperature dependence of the 1
J(17
O–99
Tc) (᭿)
and 1J(99Tc–17O) (᭹) spin–spin coupling constants for
the Tc 16
O3
17O–
isotopomer in an aqueous solution of
NH4TcO4 (0.7 M); H0 = 7.04 T.

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2015 phys chemruss-v.9#2p.185-192

  • 1. ISSN 1990 7931, Russian Journal of Physical Chemistry B, 2015, Vol. 9, No. 2, pp. 185–192. © Pleiades Publishing, Ltd., 2015. Original Russian Text © V.P. Tarasov, G.A. Kirakosyan, K.E. German, 2015, published in Khimicheskaya Fizika, 2015, Vol. 34, No. 4, pp. 20–28. 185 INTRODUCTION In NMR, the secondary isotope effect Δ(А) is defined as the change in spectral parameter Θ (chem ical shift of spin A or A–X spin coupling constant) caused by the replacement of light isotope L X by heavy isotope HX in the AXn system [1]: 1 Δ(A) = Θ(AH Xn) – Θ(AL XH Xn – 1). (1) The values of these effects are extremely small. In particular, 1 Δ(A) is usually about 0.1–0.2% of the spin–spin coupling constant (SSCC) value. The magni tude of the isotope shift depends on the NMR shielding range and is proportional to the ratio (m'– m)/m', where m' and m are the masses of the heavy and light isotopes, respectively. Isotope shifts have been determined and examined for the substitution of both light isotopes (1 H/2 H/3 H) and many heavy isotopes (12 C/13 C, 14N/15N, 16O/18O,32S/34S, 35Cl/37Cl, and 79Br/81Br [2]); at the same time, the isotope effect on SSCC has been mainly determined for substitution of hydrogen by deuterium (1 H/2 Н) [3, 4] or by tritium (1 H/3 Н), i.e., in systems with the largest possible mass change [5]. Theoretical aspects of isotope effects on magnetic shielding and spin coupling constants have been devel oped by Jameson and Osten [2, 3]. Favorable 99 Tc and 17 O NMR parameters of the anion in aqueous and nonaqueous solutions (optimal spin–lattice relaxation times Т1 (~ 0.15 s), small line width (5–15 Hz), and large chemical shift ranges) have made it possible to measure, at relatively low magnetic fields (Н0 = 2.1 T), the 99Tc NMR iso tope shifts induced by the 16 O/18 O substitution and spin coupling constants 1 J(17 O–99 Tc) in samples enriched in 17 O and 18 O [6, 7]. Cho et al. [8] have mea sured, for the first time, the displacement of the 99Tc chemical shift (Н0 = 7.04 T) as a function of tempera ture for three isotopomers at natural oxygen isotope abundance levels and the temperature dependence of the 1J(99Tc–17O) spin coupling constant for the [Tc16O3 17 O]– isotopomer. The present study deals with experimental mea surements of the temperature dependences of 99Tc NMR isotope shifts and 1 J(17 O–99 Tc) and 1 J(99 Tc–17 O) TcO4 − Oxygen Isotope Effect on NMR Parameters of Pertechnetate Anion V. P. Tarasova, b, G. A. Kirakosyana, b, and K. E. Germanb a Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia b Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia e mail: tarasov@igic.ras.ru Received March 26, 2014 Abstract—The effects of oxygen isotope substitution 16O ↔ 17O ↔ 18O in the coordination sphere of the pertechnetate anion ( ) on the NMR 99Tc chemical shifts and 99Tc–17O and 17O–99Tc spin coupling constants have been studied by 17 O and 99 Tc NMR. The isotope shifts 16/17 Δ and 16/18 Δ in 99 Tc NMR and the spin coupling constants of the Tc 16 O3 18 O– , Tc 16 O3 17 O– , Tc 16 O2 17 O 18 O– , and isotopomers have been measured. For the Tc 16O3 18O– and Tc 16O3 17O– anions in an ammonium pertechne tate solution, the temperature dependences of the isotope shift in the temperature range 278–333 K are described by linear relationships 16/18Δ = –0.616 + 6.45 × 10–4T (ppm) and 16/17 Δ = –0.302 +2.67 × 10–4T (ppm), respectively. For the Tc16 O3 17O– anion in a sodium pertechnetate solution, the magnitude of the 1Δ(16/17O) isotope shift nonlinearly decreases with increasing temperature. The nonlinear temperature dependence of the J(99 Tc–17 O) spin coupling constant and the extreme point on the curve of the 1 Δ(16/18 O) isotope shift versus temperature for the isotopomers in an NaTcO4 solution are presumably related to equilib rium between contact and water separated ion pairs. Keywords: 17 O and 99 Tc NMR, pertechnetate anion, temperature dependence, isotope shift, isotope effects on spin coupling constants DOI: 10.1134/S1990793115020281 TcO4 – − TcO4 − Tc O O 16 17 2 2, − Tc O O 16 18 2 2. STRUCTURE OF CHEMICAL COMPOUNDS. SPECTROSCOPY
  • 2. 186 RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015 TARASOV et al. constants and determination of additivity of changes in their values induced by the 16 O/17 O and 16 O/18 O iso tope substitution in the anion in concentrated solutions. EXPERIMENTAL Concentrated solutions of NaTcO4 (I), Tc2O7 (II), and NH4TcO4 (III) with concentrations ~0.7– 4.07 mol/L in water enriched in the 17 O (1.7%) and 18 O (0.8%) isotopes were studied. The 17 O and 99 Tc NMR spectra were recorded in fields of 7.04 and 14.1 T on Bruker Avance 300 and Avance 600 spectrome ters, respectively. Nuclear spins were excited by 30° pulses (the 90° pulse width was 10 and 15 μs for 99 Tc and 17 O, respectively). The sweep width was 1500 and 10000 Hz (32K memory size) and the number of scans was 200 and 500 for technetium 99 and oxygen 17, respectively. The pulse repetition time was 1 s. The solutions were placed in standard 5 mm NMR tubes. The temperature of the samples was controlled to within ±0.1° by a BVT 3200 unit. To avoid a temper ature gradient along the sample tube, the specified temperature was maintained using air at a high flow rate of 400–500 L/min. Before each measurement, samples were kept at a given temperature for at least 15–20 min. At each temperature, spectra were recorded twice. RESULTS AND DISCUSSION Isotope Substitution 16 O ↔ 17 O ↔ 18 O Intermolecular oxygen exchange between the anion and isotope enriched water leads to the formation of technetium isotopomers containing three different oxygen isotopes in the coordination sphere. The overall number of possible isotopomers Z = 15 is determined by the formula Z = (n + s – 1)!/n!(s – 1)! (2) where n = 4 is the technetium coordination number in the pertechnetate anion, and s = 3 is the number of oxygen isotopes (16 O, 17 O, 18 O). For random distribu tion, the content C of each isotopomer is determined as [9] C = n!(r16)a(r17)b (r18)c/(a ! b ! c), (3) where r16, r17, and r18 are the mole fractions of the oxy gen isotopes (r16 + r17 + r18) = 1; coefficients a, b, and с determine the number of the corresponding isotopes in the pertechnetate anion so that a + b + c = 4. The contents of all 15 isotopomers calculated for natural and enriched oxygen abundance levels (the latter was used in this study) are summarized in the table. It should be noted that the temperature depen dences of isotope effects (99 Tc NMR chemical shift and 1 J(99 Tc–17 O) constant) have been studied in a magnetic field of 7.04 T only for the first three isoto TcO4 − TcO4 − pomers [8] listed in the table. As shown below, for the solutions studied in this work, the 99Tc NMR parame ters are actually observed only for the first six isoto pomers listed in the table. 16 O–18 O and 16 O–17 O Substitution Effects on 99Tc NMR Parameters In all the samples under consideration, the isotope effects on the 99 Tc shielding constant are manifested as the upfield shift of the signals of the Tc 16 O3 18O– and Tc 16 O3 17 O– isotopomers by, respectively, –0.434 and –0.215 ppm from the signal of the anion. The 99 Tc NMR spectrum of the Tc 16 O3 18O– isotopomer is a narrow singlet, while the spectrum of the Tc16O3 17 O– isotopomer is a sextet caused by technetium spin cou pling to the oxygen 17 spin (17I = 5/2) with the 1 J(99 Tc–17 O) SSCC. The multiplet in the 99Tc NMR spectrum of the anion caused by the coupling of the techne tium spin to two equivalent oxygen 17 spins consists of 11 lines; the multiplicity is determined by the formula (2nI + 1) = (2 × 2 × 5/2 + 1) = 11, (4) where n = 2 is the number oxygen 17 spins, and I = 5/2 is the nuclear spin of oxygen 17. According to Pas cal’s triangle, the ratio of the intensities in this multip let is 1 : 2 : 3 : 4 : 5 : 6 : 5 : 4 : 3 : 2 : 1. Figure 1 shows the 99 Tc NMR spectra (H0 = 14.1 T) of the Tc2O7 + H2 17O system (saturated solution) at room tempera ture (298 K). For the Tc 16 O3 17 O– anion, the SSCC measured as the average of all the five distances between the sextet components and the SSCC value obtained by dividing the distance between the outer most multiplet components by 5 are J(99Tc–17O) = (131.22 ± 0.20) and (131.39 ± 0.20) Hz, respectively. For the anion, the SSCC value measured as the average of the distances between the nine observed (not overlapped) lines and the SSCC value obtained by dividing the distance between the outer most lines of the undecet by 10 are 1J(99Tc–17O) = (131.97 ± 0.20) and (131.99 ± 0.20) Hz, respectively. For the Tc2O7 + H2 17O system, comparison of the SSCCs for the and Tc 16 O3 17O– isoto pomers shows that the magnitude of the isotope effect on SSCC is ΔJ = |1J( ) – 1J(Tc 16O3 17 O–)| = (0.7 ± 1.0) Hz. Figure 2 displays the 99Tc NMR spectra (H0 = 7.04 T) of the concentrated NH4TcO4 solution (0.7 M) at 318 K. These spectra recorded under relatively “mild” condi tions show, in addition to the signal of the iso topomer at 0.0 ppm, the singlet due to the Tc 16 O3 18O– isotopomer at 1 Δ = –0.432 ppm, the sextet due to the Tc16 O3 17 O– isotopomer with 1 J(99 Tc–17 O) = (131.34 ± Tc O16 4 − Tc O O16 17 2 2 − Tc O O16 17 2 2 − Tc O O16 17 2 2 − Tc O O16 17 2 2 − Tc O16 4 −
  • 3. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015 OXYGEN ISOTOPE EFFECT ON NMR PARAMETERS 187 0.67) Hz at 1 Δ = –0.217 ppm, the singlet of the isotopomer at 1 Δ = –0.852 ppm, the sextet of the Tc16 O2 17O 18 O– isotopomer with 1 J(99 Tc–17 O) = (131.76 ± 0.93) Hz at 1 Δ = –0.671 ppm, and the 11 membered multiplet due to the isoto pomer at 1 Δ ≈ –0.44 ppm with the SSCC (131.34 ± 0.67) Hz. Then, the isotope effect on SSCC is ΔJ = |1 J(Tc 16 O2 17 O 18 O– )–1 J(Tc 16 O3 17 O– )|=(0.42± 1.60)Hz and ΔJ = |1 J( ) – 1 J(Tc 16 O3 17 O– )| = (0 ± 1.4) Hz. Thus, the error of measurement of the isotope effect on the J(99 Tc–17 O) SSCC exceeds the value of this effect. It is likely that there is a weak tendency for an increase in the |J(99 Tc–17 O)| value with an increase − Tc O O16 18 2 2 Tc O O16 17 2 2 − Tc O O16 17 2 2 − in the mass of isotopomer. This tendency has been pre viously reported in [6]. Temperature Dependence of the Chemical Shift of the Isotopomer Temperature dependences of the 99 Tc NMR chemi cal shift of the anion for NaTcO4 and NH4TcO4 solutions are linear with similar slopes ΔT(NaTc 16 O4) = –40.25 + 0.143T and ΔT (NH4Tc 16O4) = –36.69 + 0.123T. These data point to a weak influence of the type of cation or concentration on the magnetic shielding of the technetium nucleus in the iso topomer. The temperature induced shifts for Tc O 16 – 4 Tc O16 4 − Tc O16 4 − Tc O16 4 − Content of technetium isotopomers for random distribution of 16O, 17O, and 18O isotopes in pertechnetate anion No. Isotopomer Relative isotopomer concentration 16 O : 17 O : 18 O* = 0.99757 : 0.00038 : 0.00205 16 O :17 O : 18 O** = 0.975 : 0.017 : 0.008 1 0.9903153 0.90151 2 Tc16 O3 18O– 8.2129477 × 10–3 0.03164 3 Tc16 O3 17 O– 1.5237026 × 10–3 0.06328 4 2.5092604 × 10–5 4.2 × 10–4 5 Tc16 O2 17O18 O– 9.302623 × 10–6 1.7 × 10–3 6 8.6429484 × 10–7 3.33 × 10–3 7 3.437676 × 10–8 2.4 × 10–6 8 1.946833 × 10–8 1.46 × 10–5 9 Tc16 O17 O2 18O– 3.55224 × 10–9 2.92 × 10–5 10 2.189564 × 10–10 1.95 × 10–5 11 1.7661006 × 10–11 5.3 × 10–9 12 1.309499 × 10–11 2.1 × 10–13 13 3.641046 × 10–12 8.6 × 10–8 14 Tc17O3 18 O– 4.499504 × 10–13 1.71 × 10–7 15 2.085136 × 10–14 8.6 × 10–8 * Natural isotope abundance. ** Solutions of technetium compounds (NaTcO4, NH4TcO4, and Tc2O7) in water enriched in 17 O and 18 O to ~1.7 and ~0.8%, respec tively. 4TcO − − Tc O 16 4 Tc O O16 18 2 2 − Tc O O16 17 2 2 − − Tc O O16 18 3 − Tc O O O 16 17 18 2 Tc O O16 17 3 − Tc O18 4 − Tc O O17 18 3 − Tc O O17 18 2 2 − Tc O17 4 −
  • 4. 188 RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015 TARASOV et al. 800 600 400 200 0 –200 –400 –600 ×2048 ×16 Hz Fig. 1. 99 Tc NMR spectra (H0 = 14.1 T) of a Tc2O7 solution in H2 17O (4.5 M) at 298 K. The satellite lines due to the 99 Tc–17 O spin coupling in the anion are asterisked.Tc O O 16 17 2 2 − 600 400 200 –2000 –400 –600 ×128 Hz Fig. 2. 99 Tc NMR spectra (H0 = 7.04 T) of an NH4TcO4 solution in H2 17O (0.7 M) at 318 K. The satellite lines due to the 99 Tc–17 O spin coupling in the anion are asterisked.Tc O O 16 17 2 2 −
  • 5. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015 OXYGEN ISOTOPE EFFECT ON NMR PARAMETERS 189 measured on heating from 283 to 333 K are 7.183 and 6.66 ppm for the sodium and ammonium cation, respectively. These values are nearly twice as large as the measured temperature induced shifts for diluted KTcO4 (0.1 M) and NH4TcO4 (0.01 M) solutions [8]. The temperature dependence of the technetium shielding constant σ(T) in the isotopomer (if the four Tc–O bonds in the anion are equivalent and chemical exchange is absent) is described in the first approximation as [10] σ(T) ≈ σe + (∂σ/∂Δr)e · 4T 〈Δr 〉, (5) where T 〈Δr〉 is the rovibrational average Tc–16 O bond extension at a given temperature Т, σe is the shielding at the equilibrium Tc–16 O bond length re. In the explicit form, the temperature dependence σ(T) is deduced from detailed analysis of T 〈Δr〉 represented by the sum of the vibrational and rotational contribu tions: T〈Δr〉 = T〈Δr〉vib + T〈Δr 〉rot. (6) For molecules of symmetry Td, the rotational contri bution is [2] T 〈Δr 〉rot = (4Bere /hc ) kT, (7) where Be is the rotational constant of the and ωe is the stretching vibration frequency in cm–1. The other notations are conventional. For the iso topomer, the numerical value of the temperature coef ficient in Eq. (7) is 0.15 × 10–4 Å/K at re = 1.711 Å, and ωe = 912 cm–1. The rotational contribution T〈Δr〉rot is one order of magnitude lower than the vibrational contribution, nevertheless, in a given temperature range, it is precisely the change in the rotational con tribution that is responsible for the linear dependence of the chemical shift. Then, the change in magnetic shielding with a change in temperature can be esti mated as σ(T) – σ(283 K) = (∂σ/∂Δr)e · 4 (T 〈Δr〉 – 283 〈Δr〉); (8) (∂σ/∂Δr)e istemperatureindependent,being–2030ppm/Å for [2]. The observed behavior of the chemical shift as a function of temperature is completely deter mined by the change in the thermally averaged bond length 〈Δr(Tc–16 O)〉. Then, this bond extension with increasing temperature from 283 to 333 K is (T 〈Δr 〉 – 283 〈Δr〉)=8.85×10–4 Å for NaTcO4 and 8.2 × 10–4 Å for NH4TcO4. For the free pertechnetate anion, the cal culated Tc–O bond length is 1.717 Å, and for the hydrated anion, this length is 1.714 Å [8]. These values are consistent well with the value 1.711 Å obtained from single crystal diffraction experiments [11]. Tc O16 4 − 2 eω Tc O16 4 − Tc O16 4 − Tc O16 4 − Isotope Shifts as a Function of Temperature The chemical shift and isotope shift as a function of temperature are inherently related to each other. For highly symmetric ions, such as the 99 Tc NMR isotope shift caused by the substitution of one 18 O atom for a 16 O atom in this anion can be written by analogy with Eq. (8): (9) where 18 〈Δr〉 and 16 〈Δr〉 are the rovibrational average bond extension at a given temperature for the isoto pomers under consideration. At 300 K, the shift is 1 Δ(99 Tc) = –0.425 ppm; then, the change in the aver age bond length (18 〈Δr〉 – 16 〈Δr〉) is 2.12 × 10–4 Å. For the complete substitution of 16 O for 18 O in , the average bond extension turns out to be 2.12 × 10–4 × 4 = 8.5 × 10–4 Å, which numerically corresponds to the temperature effect on the bond length change. The temperature factor of the isotope effect is described by a rather complex function F(T) [2]: (10) where μ and μ* are the reduced masses of the light and heavy isotopomers, respectively; ω is the Tc–O stretching frequency in cm–1. The F(T) function con tains hyperbolic cotangent, depends only slightly on temperature, and predicts an insignificant decrease in the isotope shift magnitude with increasing tempera ture.ThenumericalevaluationoftheF(283K)/F(333K) ratio by Eq. (10) at ω = 912 cm–1 gives 1.06 and 1.1 for the Tc 16O3 17O– and Tc 16O3 18O– isotopomers, respec tively. Indeed, with increasing temperature from 283 to 333 K for ammonium pertechnetate, the 99 Tc NMR isotope shifts Δ16/17 and Δ16/18 decrease by a factor of 1.05 and 1.09, respectively, as compared with the value at 283 K. The temperature dependences of 99 Tc NMR iso tope shifts 1Δ(16/18O) and 1Δ(16/17O) for the Tc 16 O3 18O– and Tc 16 O3 17O– anions in aqueous solu tions of ammonium pertechnetate are shown in Figs. 3 and 4. To a first approximation, both dependences are linear and are described as (11а) (11b) For a sodium pertechnetate solution, the tempera ture behavior of the isotope shift 1 Δ(16/17 O) for the Tc 16 O3 17O– anion exhibits small deviation from lin Tc O16 4,− Δ 1 Tc 99 ( ) σ Tc O 18 O 16 3( ) σ Tc O 16 4( )–= = ∂σ/∂Δr( )e Δr〈 〉 18 Δr〈 〉 16 –( ), TcO4 – F T( ) hωc/2kT( )coth{= – μ/μ*( ) 1/2 coth hc μ/μ*( )ω/2kT[ ]}, Δ 1 O 16/18 ( ) 0.616– 6.45 10 14– × T (ppm)+= for the Tc O 16 3 O 18 – anions, Δ 1 O 16/17 ( ) 0.302– 2.67 10 4– × T (ppm)+= for the Tc O 16 3 O 17 – anions.
  • 6. 190 RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015 TARASOV et al. earity. A quite unexpected fact is the existence of an extreme point on the curve of 1Δ(16/18O) versus tem perature for a NaTcO4 solution (Fig. 5). The reasons of such a behavior of 1 Δ(16/18 O) and 1 Δ(16/17 O) are not quite clear. We can assume that, at such a high concen tration (4.07 M), the observed behavior of the isotope shift is due to equilibrium between the contact and sol vent separated ion pairs, i.e., [Na]+ [TcO4]– ↔ [Na(H2O)n]+ [TcO4]– . With increasing temperature, this equilibrium shifts to the right. In the contact ion pair [Na]+ [TcO4]– , the magnitude of the isotope shift increases with increasing temperature, while, in the solvent separated ion pair [Na(H2O)n]+[TcO4]–, the magnitude of the isotope shift |1 Δ(16/18 O)| decreases with increasing temperature. There are only solvent separated ion pairs in ammonium pertechnetate solu tions. Therefore, the isotope shift magnitude |1Δ(16/18O) | decreases with increasing temperature, as shown in Fig. 3. It should be noted that a noticeable effect of the solution concentration and the type of counterion on the Δ(1H/2H) isotope shift in the 14, 15N NMR spectra of the cation has been reported for aqueous solu tions of ammonium salts [12]. It has been demon strated that the magnitude of the Δ(1 H/2 H) isotope shift increases linearly with an increase in concentra tion. Temperature Dependence of 1J(17O–99Tc) and 1J(99Tc–17O) Spin–Spin Coupling Constants The 17O NMR spectra of an aqueous solution of NaTcO4 have been studied in the temperature range 283–343 K. The spectra are decets caused by 17 O–99 Tc spin coupling (technetium 99 spin is I = 9/2) with the 1J(17O–99Tc) constant. The narrowing of the multiplet lines with increasing temperature (Fig. 6) is evidence that the 17O NMR line shape is completely dominated by the quadrupole relaxation of the technetium spin. The oxygen chemical exchange is so slow that it has no effect on the 17O NMR line width and position. The minimalspin–latticerelaxationtimeoftechnetiumspins in an aqueous solution is T1 min(99Tc) = 0.16 s [7, 13]. Therefore, the effects of the quadrupole relaxation of technetium nuclei do not change the position of the outermost multiplet lines since the condition [1] J(17O–99Tc)T1(99Tc) ~ 20 is met. Of the three isotopomers corresponding to a given spectrum (entries 3, 5, and 6 in the table), only the Tc 16O3 17O– isotopomer (no. 3) is actually observed. The content of this isotopomer is 20–30 times as large as the content of the other two isotopomers (table). The 17 O NMR line width of each of the multiplet com ponents 17 Δνm corresponds to a definite spin state m of the 99Tc isotope. The lifetime of each spin state m of the technetium nucleus is larger than the inverse of the SSCC magnitude |1 J(17 O–99 Tc)|–1 , and the observed 17O NMR line widths of the multiplet 17Δνobs can be presented as 17 Δνobs = 17 ΔνQ Cm + 17 Δνex, (12) where 17 ΔνQ is the intrinsic line width of oxygen 17 in the pertechnetate anion, 17 Δνex is the line width due to chemical exchange, Cm is the sum of squared matrix NH4 + 0.40 340270 330320310300290280 0.41 0.42 0.43 0.44 Т, К –1 Δ, ppm Fig. 3. 99Tc NMR isotope shift (–1Δ) vs. temperature for the Tc16 O3 18O– isotopomer in an aqueous solution of NH4TcO4 (0.7 M); –1Δ (16/18O) = –0.616 + 6.45 × 10–4 T (ppm). H0 = 7.04 T. 0.210 340270 330320310300290280 0.215 0.220 0.225 Т, К –1Δ, ppm Fig. 4. 99 Tc NMR isotope shift (–1 Δ) vs. temperature for the Tc 16 O3 17O– isotopomer in an aqueous solution of NH4TcO4 (0.7 M). The shift was referenced to the signal and measured as the midpoint of the distances between the corresponding lines of the multiplet due to the 99 Tc–17 O spin coupling (for six non overlapped lines); –1 Δ (16/17 O) = –0.302 + 2.67 × 10–4 T (ppm). H0 = 7.04 T. − Tc O 16 4
  • 7. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015 OXYGEN ISOTOPE EFFECT ON NMR PARAMETERS 191 elements corresponding to the probability coefficients of the transitions Δm = ±1 and Δm = ±2 for the tech netium spin due to quadrupole interactions. For the ten spin states of technetium nucleus m = ±9/2, ±7/2, ±5/2, ±3/2, and ±1/2, the Cm values are 3.854, 7.854, 8.125, 7.125, and 6.250, respectively [6]. The narrow est lines are observed for the two outermost and two central multiplet components. The 17O–99Tc SSCC value was measured as the distance between the outer most multiplet lines divided by 9. The observed tem perature dependences of the 17O–99Tc and 99Tc–17O SSCCs for an NH4TcO4 solution show a clear decrease in their magnitude (Fig. 7). For a NaTcO4 solution, the temperature dependences (Fig. 8) have an extremum, like the temperature dependence of the isotope shift 1 Δ(16/18 O) for this solution. However, the error of mea surement of the 17O–99Tc and 99Tc–17O spin coupling constants exceeds the accuracy required to reliably confirm or refute the existence of an extremum. It should be noted that the theoretical evaluation of the sign of the reduced SSCC for the pertechnetate anion gives the positive value K(99Tc–17O) = +350 × 1020 cm–3 [14]. Then, the experimental J(99Tc–17O) value must be negative since the gyromagnetic ratio is negative for oxygen 17 and positive for technetium 99. It has been shown above that the substitution of 16 O for 18 O leads to the change in the average bond length: |18 〈Δr〉 – 16 〈Δr〉| = 2.12 × 10–4 Å; at the same time, the SSCC changes by ΔJ = |1 J(Tc 16 O2 17 O 18 O– ) – 1 J(Tc 16 O3 17 O– )| = (0.42 ± 1.60) Hz. This change ΔJ can be presented as ΔJ = (∂J/∂Δr)e (18〈Δr〉 – 16〈Δr〉) = (∂J/∂Δr)e × 2.12 × 10–4. Then, we obtain the numerical estimate (∂J/∂Δr)e ~ 2000 Hz/Å. CONCLUSIONS The effects of oxygen isotope substitution 16O ↔ 17 O ↔ 18 O in the coordination sphere of the pertech netate anion ( ) on the magnetic shielding of 99 Tc nuclei and 17 О–99 Тс and 99 Tc–17 O spin–spin coupling in concentrated solutions of NaTcO4, NH4TcO4, and Tc2O7 have been studied in the temperature range − TcO4 32000 31500 31000 30500 30000 29500 29000 283 K 303 K 333 K Hz Fig. 6. Temperature dependence of the 17 O NMR (7.04 T) line shape for an aqueous solution of NaTcO4 in H2 17O (4.07 M). 0.400 340330320310300290280 0.404 0.408 0.412 0.416 0.420 –1 Δ, ppm Т, К Fig. 5. 99 Tc NMR isotope shift (–1 Δ) vs. temperature for the Tc 16 O3 18O– isotopomer in an aqueous solution of NaTcO4 (4.07 M). H0 = 7.04 T.
  • 8. 192 RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B Vol. 9 No. 2 2015 TARASOV et al. 278–350 K by 17O and 99Tc NMR in magnetic fields of 7.04 and 14.1 T. The isotope shifts 16/17 Δ and 16/18 Δ and the spin coupling constants have been measured for the Tc 16 O3 17 O– , Tc16 O2 17 O 18 O– , and isotopomers of the fifteen possible Tc16 Ok 17 On 18 O4 – k – n (k, n = 0–4) iso topomers of the pertechnetate anion. For the Tc16 O3 18O– and Tc16 O3 17O– anions in an ammonium pertechnetate solution, the temperature dependences of the isotope shift in the temperature range 278–333 K are described by linear relationships 16/18 Δ = –0.616 + 6.45 × 10–4Т (ppm) and 16/17Δ = –0.302 + 2.67 × 10–4 Т (ppm), respectively. In the framework of the Jame son–Osten theory [3], the observed temperature dependences of isotope shifts have been considered to be a result of vibrational and rotational averaging of Tc–O bond lengths. The change in Tc–O bond length caused by isotope substitutions is on the order of 10–4 Å. The fact that, for the Tc 16 O3 17 O– anion in a sodium pertechnetate solution, the magnitude of the 1 Δ(16/17 O) isotope shift nonlinearly decreases with increasing temperature, as well as the presence of an extreme point on the curve of the temperature depen dence of the 1 Δ(16/18 O) isotope shift in 99 Tc NMR spectra of a concentrated NaTcO4 solution, presum ably reflects the influence of equilibrium between con tact and water separated ion pairs. The change in J(99 Tc–17 O) with temperature for the Tc 16 O3 17 O– anion is characterized by a poorly pronounced extre mum. However, the error of measurement of the SSCC values is larger than or comparable with tem perature induced changes. − Tc O16 4, − Tc O O16 18 3 , − Tc O O16 17 2 2, − Tc O O16 18 2 2, REFERENCES 1. N. M. Sergeyev, NMR Basic Princ. Progr. 22, 31 (1990). 2. C. J. Jameson and H. J. Osten, Annu. Rep. NMR Spec trosc. 17, 1 (1986). 3. C. J. Jameson and H. J. Osten, J. Am. Chem. Soc. 108, 2497 (1986). 4. V. P. Tarasov, V. I. Privalov, Yu. A. Buslaev, and U. Eich hoff, Z. Naturforsch. B 39, 1230 (1984). 5. C. Than, H. Morimoto, H. Andres, and P. G. Williams, J. Labelled Comp. Radiopharm. 38, 693 (1996). 6. V. P. Tarasov, V. I. Privalov, G. A. Kirakosyan, A. A. Gor bik, and Yu. A. Buslaev, Dokl. Akad. Nauk SSSR 263, 1416 (1984). 7. V. P. Tarasov, V. I. Privalov, and Yu. A. Buslaev, Mol. Phys. 50, 1141 (1983). 8. H. Cho, W. A. de Jong, B. K. McNamara, B. M. Rapko, and I. E. Burgeson, J. Am. Chem. Soc. 126, 1158 (2004). 9. G. Galingaert and H. A. Beaty, J. Am. Chem. Soc. 61, 2748 (1939). 10. C. J. Jameson and H. J. Osten, J. Chem. Phys. 81, 4300 (1984). 11. B. Krebs and K. D. Hasse, Acta Crystallogr., Sect. B 36, 1334 (1976). 12. P. E. Hansen and A. Lycka, Acta Chim. Scand. 43, 222 (1989). 13. V. P. Tarasov, V. I. Privalov, and Yu. A. Buslaev, Dokl. Akad. Nauk SSSR, 262, 1433 (1982). 14. V. G. Yarzhemskii, V. P. Tarasov, and V. I. Nefedov, Koord. Khim. 9, 1329 (1983). Translated by G. Kirakosyan 131.6 340320300280 132.0 132.4 1 J, Hz Т, К Fig. 8. Temperature dependence of the 1 J(17 O–99 Tc) (᭿) and 1 J(99 Tc–17 O) (᭹) spin–spin coupling constants for the Tc 16 O3 17O– isotopomer in an aqueous solution of NaTcO4 (4.07 M); H0 = 7.04 Т. 130.4 340320300280 130.8 131.2 131.6 132.0 Т, К 1 J, Hz Fig. 7. Temperature dependence of the 1 J(17 O–99 Tc) (᭿) and 1J(99Tc–17O) (᭹) spin–spin coupling constants for the Tc 16 O3 17O– isotopomer in an aqueous solution of NH4TcO4 (0.7 M); H0 = 7.04 T.