J. Clarkson, D.R. Armstrong, C.H. Munro & W.E. Smith, "Vibrational Analysis of the Phenylazonaphthol Pigment Ca4B", J. Raman Spectrosc., 29, 421-429, 1998.

1. JOURNAL OF RAMAN SPECTROSCOPY, VOL. 29, 421È429 (1998)
Vibrational Analysis of the Phenylazonaphthol
Pigment Ca4B
J. Clarkson,¤ D. R. Armstrong, C. H. Munro” and W. E. Smith1*
Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK
Resonance Raman scattering and surface-enhanced resonance Raman scattering (SERRS), for the commercial
phenylazonaphthol pigment Ca4B and the structural analogue (CI 15800 in and and SERRS for SolventH
2
O D
2
O
Yellow 14 (SY14) in and are presented. The greater signal-to-noise ratio and the advantage of Ñuores-H
2
O D
2
O
cence quenching mean that SERRS gives more information than resonance scattering. The spectra conÐrm that CI
15800 is closely related in structure to Ca4B, indicating that the calcium-complexing site in Ca4B is mainly
associated with the keto and carboxyl groups, with the sulphonic acid group playing a minor part. A semiempirical
calculation using the PM3 Hamiltonian is used to assign scattering from CI 15800 and Ca4B. The most intense
Raman scattering, due to in-plane modes with the largest displacements on the phenyl and naphthol rings, is
correctly assigned. Further, the calculation predicts changes due to deuterium exchange of the hydrogen associated
with the hydrazo group which are borne out by experiment. Hence Raman scattering provides a good in situ probe
of the hydrogen-bonded network essential to the properties of these compounds. 1998 John Wiley & Sons, Ltd.(
J. Raman Spectrosc. 29, 421È429 (1998)
INTRODUCTION
The Raman spectra of azobenzene and related dyes
have been extensively investigated but the vibrational
spectra of phenylazonaphthol based dyes less so.1 This
is despite the fact that they constitute the largest family
of azo dyes used in the textile and food industry and
they are one of the largest families of photoconductive
materials2,3 used in photocopiers and laser printers.
Ca4B toner is the calcium salt of 4-(4-methyl-2-
sulphophenyl)azo-3-hydroxy-2-naphthalic acid (also
known as Lithol Rubine B, Color Index number
15850 : 1, Pigment Red, P.R.57 : 1) and is considered the
worldwide standard process red for printing.4,5 The
molecular form of Ca4B can exhibit ketohydrazo to
enolazo tautomerism and in solution these tautomers
exist in equilibrium with each other. NMR studies show
that the ketohydrazo form dominates in the solid
state.6,7 Additional features believed to be important for
the pigmentary properties of this material are the
dimeric structure in the solid state and the hydrogen-
bonded network associated with a molecule of water in
the structure. Raman scattering can provide an ideal in
situ probe of the molecular structure which gives rise to
* Correspondence to: W. E. Smith, Department of Pure and
Applied Chemistry, University of Strathclyde, 295 Cathedral Street,
Glasgow G1 1XL, UK.
E-mail: w.e.smith=strath.ac.uk
¤ Present address: Department of Chemistry, Leeds University,
Leeds, UK.
” Present address: Department of Chemistry, University of Pitts-
burgh, Pittsburgh, PA, USA.
the visible adsorption and of the nature of the
hydrogen-bonding network which a†ects packing and
other physical properties. This paper is designed to
provide the assignments required for such studies.
The azoÈhydrazo tautomerism of azonaphthol dyes
has been the subject of studies by NMR, UVÈvisible,
infrared and Raman spectroscopy.8h11 Several 2-
azonaphthol derivatives have been shown to exist in
both the azo and hydrazo forms in the solid state,12
although theoretical calculations on 1-phenyl-2-azon-
aphthol have predicted that the hydrazo form is ther-
modynamically favoured for that molecule.13
Electron-withdrawing substituents and hydrogen
bonding, both intra- and intermolecular, and the forma-
tion of hydrogen-bonded dimers are shown to favour
the hydrazo tautomer.8h18 These Ðndings are consistent
with a Raman spectroscopic study on ionic
phenylazonaphthols15 and with a series of papers on
the tautomerization of azonaphthol dyes and
pigments9,78h81 which included a Raman spectroscopic
study,9 where an assignment of the hydrazo modes was
made. Since then there have been only a few reports on
the resonance Raman spectra of azonaphthol dyes,19h22
including one reporting surface-enhanced Raman scat-
tering.23 These reports detail the assignments of the azo
and the hydrazo Raman bands for the two tautomers. A
recent report has assigned the Raman spectra of two
bisazo pigments derived from benzene-2@-azonaph-
thols.24 However, these pigments were shown to be in
the azo form, in contrast to most monoazo dyes derived
from benzene-2@-azonaphthols.4
The assignment of azo bands for azobenzene dyes has
been investigated extensively1,25h46 and azobenzene has
been the subject of a recent study in this laboratory.47
CCC 0377È0486/98/050421È09 $17.50 Received 21 October 1996
( 1998 John Wiley & Sons, Ltd. Accepted 8 February 1998

2. 422 D. R. ARMSTRONG ET AL .
In contrast, the assignment of hydrazo bands has been
attempted only in a tentative fashion and for hydroxy-
azobenzene dyes.22,25,30 A few studies on hydrazo
systems have exploited the fact that the hydrazo hydro-
gen on the b-nitrogen can be readily exchanged by deu-
terium by solvating the dye in a deuterated solvent, e.g.
The subsequent changes observed in theD
2
O.19,22
Raman spectra help to assign the bands that involve the
hydrazo group.
This paper reports Raman scattering following deute-
rium exchange for the commercial pigment Ca4B. In
addition, two related systems were studied, the dyes CI
15800 and Solvent Yellow 14 (SY14) (CI 12055) (Fig. 1).
Ca4B has three groups which may bond with the
calcium and/or contribute to the hydrogen-bonding
network and so a†ect the solid-state structure. The sul-
phonic acid group is not present in CI 15800 and
neither the sulphonic acid group nor the carboxylate
group is present in SY14. Comparison of the three
systems enables the relevance to structure of each group
to be assessed.
Resonance Raman scattering was recorded from
Ca4B and from CI 15800 but owing to Ñuorescence a
spectrum from SY14 could not be recorded. However,
surface-enhanced resonance Raman scattering (SERRS)
from colloidal silver in both water and wasD
2
O
recorded for all three compounds. SY14 was the subject
of a recent SERRS investigation and this study makes
some use of the data obtained.48 Fluorescence quen-
ching in SERRS has been known for some time but was
found to be widespread in azo dyes recently.49
To aid in the deÐnition of the assigned modes, molec-
ular orbital calculations were carried out for all three
molecules. Semiempirical methods [modiÐed neglect of
diatomic overlap (MNDO), Austin model 1 (AM1) and
parametric method 3 (PM3)] were found, following suit-
Figure 1. Structures of SY14, CI 15800 and Ca4B.
able correction, to yield results comparable to those
from ab initio methods for the hydrazo structure in
SY14. These semiempirical methods require a fraction
of the computing time of ab initio methods50h52 and are
more suitable for calculations on large molecules such
as dyes and were employed in this study. It should be
noted that these calculations are only suitable for the
hydrazo tautomer of phenylazonaphthols. The PM3
calculations do not predict azo vibrations accurately,47
although a more recent calculation indicates that the
density matrix method is more e†ective.53
EXPERIMENTAL
SY14, sodium citrate (Aldrich) and silver nitrate
(Johnson Matthey) were of analytical grade. Ca4B was
synthesized by the diazotization of 2-amino-5-methyl-
sulphonic acid (4B acid), coupling to 3-hydroxy-2-naph-
thoic acid (BONA) and slaking with calcium chloride,
using standard pigment procedures.4,5 The synthesis of
CI 15800 was very similar to that of Ca4B, except that
aniline was used in place of 4B acid. CI 15800 was not
slaked and the free acid was used in the experiments.
CHN analyses were within 0.4% of the theoretical
value.
Citrate-reduced colloidal silver was prepared accord-
ing to the Lee and Meisel method.52 A 90 mg amount
of silver nitrate in 500 ml of distilled water was rapidly
heated to 100 ¡C, 10 ml of a 1% solution of sodium
citrate were added with vigorous stirring and the solu-
tion was kept at 100 ¡C for 1 h under constant stirring.
Ca4B and CI 15800 are both slightly soluble in water
and dilute suspensions, equivalent if fully dissolved to a
solution of a concentration of 10~4 M, were added to
the colloidal silver to obtain SERRS. For the deuterium
exchange experiments, Ca4B and CI 15800 were dis-
persed in Colloidal silver was centrifuged at 4000D
2
O.
rpm for 1 h and the supernatant replaced with ToD
2
O.
this were added Ca4B and CI 15800 dispersed in D
2
O.
The Ðnal concentration of the dyes was approximately
10~4 M.
SY14 is less soluble in water and is soluble in ethanol.
To observe SERRS, SY14 was dissolved in a 50 : 50
mixture of water and ethanol to 10~4 M and a drop was
added to 2 cm3 of colloidal silver. For the deuterium
exchange experiment the colloidal silver was centrifuged
and the supernatant replaced with To this wasD
2
O.
added a drop of a SY14 dissolved in a 50 : 50 mixture of
and ethanol at a concentration of 10~4 M. TheD
2
O
Ðnal concentration of all compounds in the colloidal
silver was between 10~5 and 10~6 M, which, given
efficient surface adsorption, would result in above
monolayer coverage of the compounds on the silver
surface.
Raman spectra were obtained on a modiÐed Cary 81
double monochrometer equipped with a thermoelectri-
cally cooled photomultiplier tube and photon counting
detection system. The monochromatic excitation source
was a Spectra-Physics Model 2020 argon ion
continuous-wave laser using 457.9 nm radiation at 100
mW. The slit width was 5 cm~1.
The semiempirical calculations for hydrazone tauto-
meric forms of SY14 and CI 15800 were performed with
( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 421È429 (1998)

3. VIBRATIONAL ANALYSIS OF PHENYLAZONAPHTHOL PIGMENT Ca4B 423
the MOPAC network of programs (version 6) using the
PM3 Hamiltonian. The calculated wavenumbers were
multiplied by 0.89 to correct for anharmonicity.
RESULTS AND DISCUSSION
Resonance Raman scattering of Ca4B and CI 15800 in
andH
2
O D
2
O
Resonance Raman scattering from suspensions of Ca4B
dispersed in and (Fig. 2) is similar to thoseH
2
O D
2
O
obtained from other ketohydrazo dyes. At 10~4 M there
is some solubility apparent in the suspensions. This
“bleedÏ is small and the spectrum is identical with poorer
signal-to-noise ratio spectra from KBr discs so that it is
thought to be due predominantly to the solid state. The
most noticeable di†erences between the spectra in Fig. 2
are that the peak at 1228 cm~1 in is replaced withH
2
O
one at 1212 cm~1 in (Table 1) and the peak at 962D
2
O
cm~1 in is replaced with one at 934 cm~1 inH
2
O D
2
O.
The spectra and the di†erences between andH
2
O D
2
O
suspensions indicate an exchangeable hydrogen on an
NÈH bond, conÐrming that Ca4B in the solid state is
in the hydrazone form. No appreciable di†erence is
observed in the resonance Raman spectra of these com-
pounds at pH/D 13 in either or where theH
2
O D
2
O,
azo form dominates (data not shown).
CI 15800 has a similar solubility to Ca4B in andH
2
O
and Raman scattering was obtained under theD
2
O
same conditions (Fig. 3). Wavenumber shifts upon deu-
terium exchange are similar to those for Ca4B (Table 1).
In particular, both CI 15800 and Ca4B have peaks at
about 1230 and 960 cm~1 in which are replacedH
2
O
by peaks at about 1215 and 935 cm~1 in ThereD
2
O.
are intensity di†erences but the similarities suggest that
Figure 2. Resonance Raman spectra of 10É3 M Ca4B at pH 7.
Excitation at 457.9 nm. (A) Dispersed in (B) dispersed inH
2
O;
D
2
O.
Figure 3. Resonance Raman spectra of 10É2 M Cl 15800 at pH.
Excitation at 457.9 nm. (A) Dispersed in (B) dispersed inH
2
O;
D
2
O.
the NÈH bond and associated hydrogen-bonding
network are very similar in the two compounds.
Surface-enhanced resonance Raman scattering of Ca4B,
CI 15800 and SY14
Resonance Raman scattering from SY14 could not be
obtained from aqueous solutions on suspension because
Table 1. Resonance Raman peak positions for CI 15800 and
Ca4B in andH
2
O D
2
Oa
Ca4B CI 15 800
H
2
O D
2
O H
2
O D
2
O
l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1
1604 st 1602 st 1600 st 1600 st
1560 st 1555 st 1560 m 1558 m
1490 st 1489 st 1495 st 1495 st
1455 m 1454 m 1455 w 1458 m
1390 st 1390 m 1388 m 1390 m
1362 st 1358 st 1365 st 1365 st
1328 w 1328 w 1326 w 1326 w
1260 m 1260 m 1265 m 1264 m
1228 m 1212 m 1232 m 1215 m
1180 m 1179 m 1180 m 1180 st
1160 w 1160 w 1160 w 1160 w
1115 m 1120 m
1020 w 1035 w 1045 w 1040 w
1000 w 1018 w 1012 w
962 m 934 w 968 m 940 w
888 w 875 w
820 w 850 w
785 w 800 w 800 w
740 w 740 w 750 w 750 w
708 w 708 w 718 w 718 w
660 w
600 w 600 w
500 w 500 w 475 w 470 w
a st, Strong; m, medium; w, weak.
( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 421È429 (1998)

4. 424 D. R. ARMSTRONG ET AL .
it is virtually insoluble in water and does not suspend
well, presumably because the surfaces of the particles
are non-polar and do not wet. Further, the solid-state
spectra obtained from KBr or silver discs are of very
poor quality owing to intense Ñuorescence. The Ñuores-
cence quenching associated with SERRS makes it pos-
sible to obtain good spectra from SY14 in addition to
Ca4B and CI 15800. Colloidal SERRS requires adher-
ence of the particles or molecules to the silver colloid
surface which is negatively charged. Thus, wetted (polar)
pigment particle surfaces or some solubility is required.
The similarities between the SERRS and resonance data
suggest that particles rather than molecules dominate
the spectrum. They must either adhere to the surface or
be reformed from adsorbed molecules from the bleed on
the surface (Figs 4 and 5). To obtain solubility for SY14,
measurements were carried out in silver colloid sus-
pended in 50 : 50 waterÈethanol as solvent. To obtain
deuterated SERRS, the colloidal silver was centrifuged,
the water removed and the colloid resuspended in D
2
O.
This is a technique that has proved useful in obtaining
SERS from deuterium-exchanged molecules and mol-
ecules which otherwise would not give SERS owing to
insolubility in water.54,55
SERRS (Figs 4È6 and Table 2) gave a very high
signal-to-noise ratio and revealed more information
than resonance Raman scattering. These are di†erences
particularly in the low-energy region compared with
Ca4B and CI 15800. Further, SERRS for SY14 (Fig. 6)
indicates di†erences in intensity and wavenumber on
deuterium exchange compared with Ca4B and CI
15800. All compounds show a deuterium-sensitive
Figure 4. SERRS of 10É4 M Ca4B from colloidal silver. Excitation
at 457.9 nm. (A) Dispersed in (B) dispersed inH
2
O; D
2
O.
Figure 5. SERRS of 10É4 M CI 15800 from colloidal silver. Exci-
tation at 457.9 nm. (A) Dispersed in (B) dispersed inH
2
O; D
2
O.
Figure 6. SERRS of 10É4 M SY14 from colloidal silver. (A) Dis-
persed in excitation at 514.5 nm; (B) dispersed in exci-H
2
O, D
2
O,
tation at 457.9 nm.
( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 421È429 (1998)

5. VIBRATIONAL ANALYSIS OF PHENYLAZONAPHTHOL PIGMENT Ca4B 425
Table 2. SERRS wavenumbers for Ca4B, CI 15800 and SY14 in
andH
2
O D
2
O
Ca4B CI 15800 SY14
H
2
O D
2
O H
2
O D
2
O H
2
O D
2
O
l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1
1605 st 1608 st 1598 st 1600 st 1595 st 1652 w
1552 st 1555 st 1554 m 1558 st 1555 m 1610 m
1490 st 1490 st 1495 st 1492 st 1495 st 1600 st
1449 m 1455 m 1450 m 1455 m 1480 st 1552 m
1390 m 1412 w 1395 m 1395 st 1450 nm 1495 st
1362 st 1359 st 1365 st 1366 st 1484 st
1332 st 1332 m 1330 m 1330 m 1455 m
1290 m 1275 m 1425 m
1260 st 1260 m 1260 m 1260 m 1385 st 1380 st
1234 st 1236 st 1345 st 1342 m
1219 m 1211 st 1315 w 1311 w
1190 m 1265 m 1265 m
1176 st 1180 st 1178 st 1181 st 1235 st 1212 st
1160 m 1160 m 1155 w 1160 m 1215 m
1138 w 1180 m 1170 m
1114 w 1118 m 1112 m 1120 m 1150 w 1160 w
1092 w 1092 w 1146 w
1042 w 1038 w 1042 w 1040 m 1100 m 1105 m
1020 w 1008 w 1020 w 1016 m 1080 w 1080 w
960 st 960 st 1060 w
936 w 939 m 1040 w 1038 w
890 w 880 w 870 w 882 w 1005 w 1000 w
850 w 862 m 990 m 965 w
820 w 820 w 925 w
780 w 782 w 775 w 872 w 875 w 870 w
745 w 742 m 748 m 748 w 845 w 840 w
710 w 715 w 712 w 718 w 790 w
682 w 660 w 768 w
664 w 664 w 662 w 650 w 735 m 730 m
600 w 602 w 602 w 602 w 620 w 628 w
570 w 570 w 595 w 585 w
554 w 553 w 550 w 552 w 550 w 542 w
532 w 530 w 510 w 500 w
515 w 510 w 500 w 498 w 475 m 464 m
495 w 495 w 430 w 438 w
470 w 465 w 468 w 465 w 420 w 415 w
445 w 445 w 440 w 440 w 370 w 370 w
426 w 425 w 352 w
370 w 365 w 385 w 385 w 312 w
330 w 340 w 368 w 364 w 352 w
310 w 305 w 314 w 304 w 227 w 300 w
265 w 260 w 250 w 250 w 238 w
220 w 220 w 224 m
SERRS peak at about 1235 cm~1 in but SY14H
2
O
shows di†erences in the other deuterium exchange sen-
sitive peaks when compared with Ca4B and CI 15800.
The conclusion reached is that the solid-state structure
of SY14 di†ers more profoundly from the other two and
that the carboxyl group plays a larger part than the
sulphonic acid group in controlling the structure of
Ca4B. The proximity of the carboxyl and keto groups
and the affinity of calcium for them make this reason-
able.
Normal-mode analysis of CI 15800 and SY14
Normal-mode displacements and wavenumbers for CI
15800 obtained from semiempirical calculations with
the PM3 Hamiltonian were used to assign the spectra of
Figure 7. Numbering scheme used for CI 15800 in PM3 semi-
empirical calculations and in describing normal modes.
( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 421È429 (1998)

6. 426 D. R. ARMSTRONG ET AL .
CI 15800 and Ca4B. The numbering of atoms is given
in Fig. 7. The calculation predicts a non-planar molecu-
lar geometry. The bond has a considerableC
8
xO
17
positive z component, where z is the axis perpendicular
to the plane of the paper (the xy plane). The benzene
part of the molecule is not planar with the naphthalene
moiety, but slightly twisted with respect to it. However,
for simplicity the molecule has been drawn as planar for
the normal-mode diagrams.
The predicted wavenumbers scaled to compensate for
anharmonicity are given in the range 1700È700 cm~1
for CI 15800 and SY14 and the corresponding
deuterium-exchanged compounds (Table 3). As
observed in the Raman spectra, many of the modes of
both dyes do not change upon deuterium exchange
whereas a few are diagnostic of the change. The modes
placed together in Table 3 exhibit the same energy con-
tributions from the speciÐc atoms in both the non-
deuterated and deuterated molecules. Those placed with
no mode opposite exhibit unique patterns of energy
contributions. For example, even though the mode pre-
dicted at 1224 cm~1 for SY14 is very close in wavenum-
ber to the 1225 cm~1 mode observed upon deuterium
exchange, the energy contributions are not the same.
This mode involves the hydrazo part of the molecule
including the exchangable H in the H(20)ÈN(19) bond
which makes a dominant contribution and the form of
the mode changes upon deuterium exchange. Overall,
the changes predicted are consistent with those
observed experimentally and give conÐdence in the use
of the calculation in assigning the spectra.
Assignment of CI 15800 and Ca4B vibrational spectra
The two highest energy non CÈH stretching modes, l
82
and can be assigned to the CO stretch and thel
81
,
asymmetric stretch, respectively. The CO stretchCO
2
~
cannot be assigned to any of the observed bands of CI
15800 or Ca4B as no Raman bands are observed above
1605 cm~1. However, Ca4B does have an IR band at
1655 cm~1 and is assigned to the asymmetricl
81
CO
2
vibration. is centred on the naphthol part of thel
80molecule, with a large C(9)ÈC(10) stretch contribution
and has been assigned to the 1620 cm~1 IR active band
of Ca4B and the 1618 cm~1 IR active band of CI 15800.
The CÈC(21È26) phenyl ring can be considered as a
monosubstituted phenyl unit which displays phenyl
mode character in the modes of CI 15800. The modes
all show similarities to the benzene model
79
Èl
75
8a/b.55 Additionally, modes and have a largel
79
l
78
C(8)ÈN(18) (CxN) stretch contribution with havingl
79
a larger contribution than Both of these modes arel
78
.
una†ected by deuterium substitution of the b-nitrogen
of the hydrazo. Previous studies on 1-phenyl-2-azon-
aphthol dyes have incorrectly assigned the CxN stretch
to peaks sensitive to deuterium substitution19,22 These
two modes exhibit a phase pair relationship for the CÈ
C(1È6) and CÈC(21È26) rings which relates to benzene
mode 8a/b. In mode the CÈC(1È6) unit is vibratingl
78
in the opposite phase to mode while other relatedl
79
,
vibrations are in a similar sense for both modes. This
phase pair relationship is a repeated feature of several
other modes of CI 15800. Modes are dominatedl
74
Èl
71
by CÈC(1È6) or CÈC(21È26) displacements and can be
Table 3. PM3 predicted wavenumbers for CI
15800 and SY14
CI 15800 SY 14
NH ND NH ND
l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1
l
82
1747 l
82
1747 l
74
1705 l
74
1703
l
81
1699 l
81
1699 l
73
1661 l
73
1617
l
80
1648 l
80
1648 l
72
1607 l
72
1607
l
79
1608 l
79
1608 l
71
1600 l
71
1599
l
78
1596 l
78
1596 l
70
1592 l
70
1592
l
77
1589 l
77
1589 l
69
1505 l
69
1501
l
76
1578 l
76
1578 l
68
1467 l
68
1466
l
75
1578 l
75
1576 l
67
1430 l
67
1430
l
74
1448 l
74
1448 l
66
1416 l
66
1420
l
73
1406 l
73
1405 l
65
1394 l
65
1403
l
72
1403 l
72
1402 l
64
1347
l
71
1381 l
71
1378 l
63
1306 l
64
1306
l
70
1340 l
70
1340 l
62
1289 l
63
1290
l
69
1264 l
62
1225
l
69
1268 l
61
1224
l
68
1251 l
60
1211 l
61
1210
l
67
1233 l
68
1234 l
59
1191 l
60
1193
l
67
1216 l
58
1176 l
59
1183
l
66
1200 l
57
1152 l
58
1148
l
65
1181 l
66
1181 l
56
1113 l
57
1114
l
65
1175 l
55
1110 l
56
1109
l
64
1170 l
64
1169 l
54
1095 l
55
1096
l
63
1159 l
53
1056 l
54
1056
l
62
1098 l
63
1100 l
52
1046 l
53
1046
l
61
1082 l
62
1081 l
51
1040 l
52
1040
l
60
1069 l
61
1069 l
51
1035
l
59
1044 l
60
1044 l
50
1027 l
50
1026
l
59
1034 l
49
1015
l
58
1030 l
58
1030 l
49
1011 l
48
1011
l
57
1027 l
57
1028 l
48
1002 1002
l
56
1016
l
55
1000 l
56
1000 l
47
1000 l
47
1000
l
54
998 l
55
998 l
46
978
l
53
987 l
54
987 l
46
960
l
52
985 l
53
985 l
45
939
l
52
957 l
45
939
l
51
945 l
44
927
l
50
920 l
51
915 l
44
915 l
43
915
l
49
894 l
50
985 l
43
912 l
42
912
l
48
983 l
49
893 l
42
902 l
41
902
l
48
887 l
41
878 l
40
878
l
47
865 l
40
868 l
39
868
l
46
864 l
47
864
l
45
855 l
46
859
l
45
852 l
39
838 l
38
837
l
44
838 l
44
838 l
38
817
l
43
823 l
43
822 l
37
811 l
37
811
l
42
800 l
42
799 l
36
797 l
36
800
l
41
788 l
35
793
l
40
785 l
41
786 l
35
778 l
34
777
l
40
761 l
34
759 l
33
759
l
39
748 l
39
748
l
38
739 l
38
738 l
33
741 l
32
740
l
37
716 l
37
716 l
32
709 l
31
709
assigned as related to benzene mode 19a and 19b. The
full assignment of 1600È1300 cm~1 modes is given in
Table 4.
Modes do not show any recognizablel
69
Èl
66benzene mode character (Fig. 8). However, they do
show varying degrees of NÈH stretching and bending
contributions. Indeed, modes and arel
69
, l
68
l
66
( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 421È429 (1998)

7. VIBRATIONAL ANALYSIS OF PHENYLAZONAPHTHOL PIGMENT Ca4B 427
Table 4. Band positions and assignments for Ca4B and CI 15800
Ca4B CI 15800
IR SERRS IR SERRS Mode Assignment
l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l8 /cmÉ1 l l8 /cmÉ1 Descriptiona
1655 m l
81
1699 CO
2
asymmetric
1620 m 1618 st l
80
1648 C–C(1–10)
1601 m 1605 st 1595 st 1598 st l
79
1608 N(18)–C(7), C–C(1–6) 8a/b
1561 st 1569 st l
78
1596 N(18)–C(7), C–C(1–6), 8a/b
1550 st 1552 st 1553 st 1554 m l
77
1589 C–C(1–6), 8a/b
1498 st 1490 st 1496 st 1495 st l
76
1578 C–C(1–6), 8a/b
1482 st 1482 st l
75
1578 C–C(20–26), 8a/b
1450 st 1449 m 1448 st 1450 m l
74
1448 C–C(1–6), 19b
1407 st 1398 st l
73
1406 C–C(20–26), 19b
1389 m 1390 m 1389 st 1395 m l
72
1403 C–C(20–26), 19b
1365 m 1362 st 1356 m 1365 st l
71
1381 C–C(20–26), 19b
1324 m 1332 st 1326 m 1330 m l
70
1340 CO
2
symmetric
1288 st 1290 m 1275 m l
69
1268 C–C, N–H
1265 st 1260 st 1267 st 1260 m l
67
1233 C–C(1–10)
1249 st 1234 st 1233 w 1236 st l
68
1251 C–C, N–H
1211 st 1190 m l
66
1200 C–C(21–26), N–H
1185 st 1176 st 1187 st 1178 st l
65
1181 C–C(1–6), 14
1158 m 1160 m 1146 st 1155 w l
64
1170 C–C(21–26), 14
1138 w l
63
1159 C–C(21–26), 14, N–H
1128 w 1114 w 1112 m l
62
1098 C–C(21–26), 3
1091 m 1092 w 1091 w l
61
1082 C–C(1–6), 3
1075 w 1076 w l
60
1069 C–C(1–10), N–N
1030 m 1042 w 1039 m 1042 w l
59
1044 C–C(21–26), 9a
1020 st 1020 w 1013 st 1020 w l
56
1016 C–C, N–N
954 m 960 st 956 m 960 st l
51
945 C–C(21–26), 18a, N–H
877 m 890 w 870 w l
48
893 C–C(1–6), 5
864 m 850 w l
45
856 C–C(1–6), 11
821 st 820 w 825 st l
43
823 C–C(21–26), 17b
786 m 780 w 785 w 775 w l
41
788 C–C(21–26), 12, N–H
766 st 765 st l
40
785 C–C(21–26), 12
749 st 745 w 748 st 748 m l
39
748 C–C(21–26), 10a
699 m 710 w 712 w l
37
716 C–C(1–10)
685 m 682 w l
35
686 C–C(1–16), 11
664 w 662 w l
34
673 C–C(1–10)
645 w l
35
583 C–C(1–10)
626 m 618 w 615 w l
32
615 C–C(1–10), N–H
612 m 600 w 595 m 602 w l
30
583 C–C(1–6), 6a
570 w l
29
565 C–C(21–26), 16a
554 w 550 w l
28
559 C–C(21–26), 6b
520 m 530 st 532 w l
27
543 C–C(1–10)
506 m 515 w 492 st 500 w l
26
516 C–C, N–N
494 m 495 w l
25
511 C–C(1–10)
470 w 468 w l
24
478 C–C(1–10), N–N
445 w 440 w l
22
437 C–C(1–10)
416 st 426 w 416 st l
21
422 C–C(21–26), 4
370 w 385 w l
19
394 C–C(21–26), N–N
330 w 368 w l
18
333 C–C(1–10)
310 w 314 w l
16
300 C–C(1–6)
265 w 250 w l
14
261 C–C, N–N
220 w l
12
216 C–C(1–10)
a Atoms contributing most to the mode are given in the last column, followed by the parent
benzene mode55 where appropriate.
heavily dependent on the NH group as these vibrations
are signiÐcantly altered upon deuterium exchange
(Table 3). These modes also show a large contribution
from the C(21)ÈN(19) stretch. Therefore, and arel
69
l
68
assigned to the deuterium-sensitive Raman bands of
Ca4B and 1290 and 1234 cm~1, respectively. Although
has a contribution from an NÈH wag, this mode isl
67
not predicted to change upon deuterium substitution
and is therefore assigned to the Ca4B Raman band at
1260 cm~1. The ability of the PM3 calculation to
predict the observed changes in the spectra upon deute-
rium substitution is surprisingly good and gives further
conÐdence in the assignment.
The di†erences in the Raman spectra of the
deuterium-sensitive bands between Ca4B and CI 15800
are due to the di†erences caused by the sulphonate and
methyl groups on the phenyl ring of Ca4B. The sul-
phonate in particular may a†ect these vibrations as this
group will be involved in the hydrogen-bonding
network involving the hydrazo NH. This may explain
( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 421È429 (1998)

8. 428 D. R. ARMSTRONG ET AL .
Figure 8. PM3 calculated CI 15800 normal-mode displacements that display hydrazo character.
the di†erence in the wavenumber of the Raman bands
assigned to which has a large contribution from thel
69
,
NÈH wag.
has a large contribution from NÈH wag and alsol
63
from phenyl mode 14, and is also noticed to transform
upon deuterium substitution. Therefore, this mode is
assigned to the weak Raman band at 1138 cm~1 of
Ca4B. There is one other noticeable Raman band
present at 960 cm~1 in both Ca4B and CI 15800. This
band is assigned as which has a contribution froml
51
,
phenyl mode 18a.
The low-wavenumber vibrations are difficult to assign
owing to the large number and close spacing of the
wavenumbers. However, a tentative assignment has
been attempted for CI 15800 and Ca4B (Table 4). Many
of these vibrations are complex out-of-plane modes with
little or no recognizable phenyl mode character.
CONCLUSIONS
Both the experimental results and theoretical calcu-
lations show that CI 15800 is a better model than SY14
for Ca4B. The similarity of the CI 15800 and Ca4B
spectra and the di†erence between them and the spec-
trum of SY14 indicate the importance of the carbox-
ylate group in determining structure.
The normal-mode calculations reveal that the Raman
activity of the high-wavenumber modes comes from
modes with large contributions from the phenyl ring
and the second naphthalene ring [CÈC(1È6) and CÈ
C(21È26), respectively]. The hydrazo vibrations are also
clear and deÐned in the normal-mode calculations and
Raman spectra by exchanging the NH hydrogen for
( 1998 John Wiley & Sons, Ltd. J. Raman Spectrosc. 29, 421È429 (1998)

9. VIBRATIONAL ANALYSIS OF PHENYLAZONAPHTHOL PIGMENT Ca4B 429
deuterium. The wavenumber of the CxN stretch is not
sensitive to deuterium exchange and is predicted at 1608
cm~1. The deuterium-sensitive modes of the hydrazo
group involve deformation of the CÈNÈH bond, as in
and In mode an NÈN stretch isl
69
, l
68
l
66
. l
56
involved in addition to an NÈH deformation. There is a
very good Ðt between theory and experiment, validating
the use of the PM3 calculation in assigning the spectra
and conÐrming the form of the vibrations observed
experimentally. The hydrogen-bonding network is a key
feature of the solid state structure and properties of
Ca4B. For example, the commercial system has one
molecule of water per dimer and removal of this water
alters the structure. Raman scattering can now be used
to probe this change informatively.
In both the resonance and SERRS spectra of Ca4B
and CI 15800, it is the in-plane high-energy modes that
are the most intense. These modes involve either the
phenyl or the naphthol part of the molecule or both
ring systems. The dominance of the phenyl part in some
of the resonance Raman active modes is unexpected. It
was thought that only the naphthol-dominated modes
would contribute to the resonance scattering since only
the naphthol and hydrazo parts were planar and clearly
conjugated and thus might form the e†ective chromo-
phore. The results indicate that the chromophore
extends over the whole molecule.
The out-of-plane low-energy modes are weak in reso-
nance but are easily detectable by SERRS. SERRS was
clearly related to resonance scattering but enabled
much more vibrational information to be obtained
owing to the higher signal-to-noise ratio. In addition,
the Ñuorescence quenching inherent in the process
extended the range of molecules which could be studied
with excitation at resonant wavenumbers. It could be
used more widely in studies of dyes and pigments.
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