Vibrational Spectral Studies and Electronics Properties of Non-Linear Optical...
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1. Synthesis, structural and spectral analysis of 1-(pyrazin-2-yl)
piperidin-2-ol by density functional theory
M. Suresh a
, M. Syed Ali Padusha a,⇑
, K. Govindarasu b
, E. Kavitha b
a
PG & Research Department of Chemistry, Jamal Mohamed College, Tiruchirappalli 620 020, India
b
Department of Physics (Engg.), Annamalai University, Annamalainagar 608 002, India
h i g h l i g h t s
The FTIR and FT-Raman spectra of
PPOL were reported.
UV–Vis spectra was recorded and
compared with calculated values.
1
H NMR spectra was recorded and
analyzed.
The first order hyperpolarizability
was calculated.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:
Received 18 August 2014
Received in revised form 24 October 2014
Accepted 20 November 2014
Available online 28 November 2014
Keywords:
1-(Pyrazin-2-yl) piperidin-2-ol
FTIR
FT-Raman
NBO
UV–Vis
1
H NMR
a b s t r a c t
The organic compound 1-(pyrazin-2-yl) piperidin-2-ol (abbreviated as PPOL) has been synthesized and
characterized by IR, Raman, 1
H NMR and UV–Vis spectroscopy. The Fourier-transform Raman (3500–
50 cmÀ1
) and infrared spectra (4000–400 cmÀ1
) were recorded in the solid state and interpreted by com-
parison with theoretical spectra derived from density functional theory (DFT) calculations. The optimized
geometry, frequency and intensity of the vibrational bands of the compound was obtained by the density
functional theory using 6-31G(d,p) basis set. In the optimized geometry results shows that geometry
parameters are good agreement with XRD values. Stability of the molecule arising from hyper conjugative
interactions, charge delocalization has been analyzed using natural bond orbital (NBO) analysis. In calcu-
lation of electronic absorption spectra, TD-DFT calculations were carried out in the both gas and solution
phases. 1
H NMR chemical shifts were calculated by using the gauge-invariant atomic orbital (GIAO)
method. 1
H NMR analysis is evident for O–HÁ Á ÁO intermolecular interaction of the title molecule. The
thermodynamic properties of the title compound have been calculated at different temperatures and
the results reveal that the standard heat capacities (Cp,m), standard entropies (Sm) and standard enthalpy
changes (Hm) increase with rise in temperature. In addition, HOMO and LUMO energies and the first-
order hyperpolarizability have been computed.
Published by Elsevier B.V.
Introduction
Piperidine is a widely used building block and chemical reagent
in the synthesis of organic compounds, including pharmaceuticals.
The piperidine structural motif is present in numerous natural
alkaloids. Piperidine is also commonly used in chemical degrada-
http://dx.doi.org/10.1016/j.saa.2014.11.063
1386-1425/Published by Elsevier B.V.
⇑ Corresponding author. Tel.: +91 9865447289, +91 9865042624.
E-mail addresses: mscsuresh2@gmail.com (M. Suresh), padusha_chem@yahoo.
co.in (M. Syed Ali Padusha).
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
2. tion reactions, such as the sequencing of DNA in the cleavage of
particular modified nucleotides. Pyrazine and its derivatives form
an important class of compounds present in many natural flavors
and complex organic molecules. Pyrazines are responsible for fla-
vor of foodstuffs as diverse as cooked meats, cheese, tea and coffee.
Pyrazine is less basic in nature than pyridine, pyridazine and
pyrimidine [1]. Piperazines are currently the most important build-
ing blocks in drug discovery, with a high number of positive hits
encountered in biological screens of this heterocycle and its cong-
eners across a number of different therapeutic areas [2]; anticancer
[3], antifungal [4], antibacterial, antimalarial and antipsychotic
agents [5], as well as HIV protease inhibitors [6]. Our molecule 1-
(pyrazin-2-yl) piperidin-2-ol consist of one piperidin ring and pyr-
azin ring, these rings are bonded through C–N bridge. It has the fol-
lowing properties; appearance: Pale yellowish white solid;
molecular formula: C10H14N2O; molecular weight: 178.24 g/mol.
Recently Prabavathi et al. [7] reported FT-IR, FT-Raman and DFT
quantum chemical study on the molecular conformation, vibra-
tional and electronic transitions of 1-(m-(trifluoromethyl) phenyl)
piperazine. Mahalakshmi et al. [8] analyzed Molecular structure,
Vibrational spectra (FTIR and FT Raman) and Natural bond orbital
analysis of 4-aminomethylpiperidine: DFT study. Revised assign-
ment of the vibrational spectra of methylpyrazines based on scaled
DFT force fields and Vibrational spectroscopic and quantum chemi-
cal study of the chlorine substitution of pyrazine given by Endre´di
et al. [9,10]. FT-Raman, FT-IR spectra and total energy distribution
of 3-pentyl-2,6-diphenylpiperidin-4-one: DFT method [11]. Krish-
nakumar and Prabavathi [12] reported Scaled quantum chemical
calculations and FTIR, FT-Raman spectral analysis of 2-methylpyra-
zine. Theoretical and experimental vibrational spectroscopic study
of 4-(1-pyrrolidinyl) piperidine investigated by Cemal Parlak [13].
Chis et al. [14] studied Experimental and DFT study of pyrazinamide.
Literature survey clearly reveals that to the best of our knowledge,
no experimental and computational vibrational, NMR and UV spec-
troscopic study on free PPOL have published in the literature yet.
In the present work, we report the results of theoretical and
experimental (IR and Raman, UV and NMR) spectra of PPOL mole-
cule along with NBO analysis. Therefore, the aim of this study is to
fully determine the molecular structure, vibrational modes and
wavenumbers are determined and by using quantum chemical cal-
culations. Detailed interpretations of the vibrational spectra of our
compound have been made based on the calculated potential
energy distribution (PED). Therefore, the present work aims to pro-
vide a complete description on the molecular geometry, molecular
vibrations and electronic features of PPOL.
Experimental
Synthesis
To a solution of 2-aminopyrazine (1.52 g, 0.016 mol) in acetoni-
trile (20 mL), 3,4-dihydro-2H-pyran (1.2 mL, 1.4 mmol) was added
followed by Cerium Chloride heptahydrate (Ce(III)
Cl3Á7H2O) (1.8 g,
0.005 mol) at 0 °C. The reaction mixture was kept in an oil bath
maintained at 60 °C and stirred well for 2 h. Progress of the reac-
tion was continuously monitored by LCMS. The reaction mixture
was diluted with ethyl acetate and washed with water. The organic
layer was dried over Na2SO4 and concentrated by vacuum. The
crude mass obtained was purified by column Chromatography
packed with 60/120 silica gel and eluted with 25–35% ethyl acetate
in petroleum ether.
FT-IR, FT-Raman, UV–Vis and 1
H NMR spectral measurements
The FT-IR spectrum of 1-(pyrazin-2-yl) piperidin-2-ol com-
pound was recorded in the range of 4000–400 cmÀ1
on a Perkin
Elmer, RXI model FT-IR spectrometer using KBr pellet technique.
The spectrum was recorded in the room temperature, with scan-
ning speed of 10 cmÀ1
, and spectral resolution: 4 cmÀ1
. FT-Raman
spectrum of the title compound was recorded using 1064 nm line
of Nd:YAG laser as excitation wavelength in the region 3500–
50 cmÀ1
on a BRUKER RFS 27: FT-Raman Spectrometer equipped
with FT-Raman molecule accessory. The spectral resolution was
set to 2 cmÀ1
in back scattering mode. The laser output was kept
at 100 mW for the solid sample. The ultraviolet absorption spectra
of PPOL were examined in the range 200–800 nm using Perkin
Elmer, lambda-35 model UV spectrometer. The UV pattern is taken
from a 10–5 M solution of PPOL, dissolved in DMSO solvent. 1
H
NMR spectra are recorded on Bruker AVANCE III 500 MHz (AV
500) spectrometer; chemical shifts are expressed in ppm (d units)
relative to TMS signal as internal reference in DMSO solvent.
Computational details
The optimized geometry of and vibrational frequencies were
calculated at the B3LYP/6-31G(d,p) level with the Gaussian 09 pro-
gram [15]. The molecular structure optimization and correspond-
ing vibrational harmonic frequencies were calculated using DFT
calculations [16] with the Becke’s three-parameter hybrid func-
tional (B3) [17] for the exchange part and the Lee–Yang–Parr
(LYP) correlation function [18], for the computation of molecular
structure, vibrational frequencies and energies of optimized struc-
tures by using Gaussian 09 suite of quantum chemical codes.
Firstly, the title molecule was optimized, after then the optimized
structural parameters were used in the vibrational frequency and
calculations of electronic properties. The vibrational wavenumber
assignments were carried out by combining the results of the
Gauss view 5.08 [19] and VEDA4 programs [20]. The calculated
IR spectrum, plotted using the pure Lorentzian band shape with
a band width of FWHM of 10 cmÀ1
has been compared with the
experimental FT-IR spectrum and is found to be well comparable
to that of the spectral data obtained by DFT/B3LYP method. The
natural bonding orbital (NBO) calculations [21] were performed
using Gaussian 09 [15] package at the same level in order to under-
stand various second order interactions between the filled orbitals
of one subsystem and vacant orbitals of another subsystem, which
is a measure of the intermolecular delocalization or hyper conjuga-
tion. UV–Vis spectra, electronic transitions, vertical excitation
energies, absorbance and oscillator strengths were computed with
the time-dependent DFT method. The changes in the thermody-
namic functions (the heat capacity, entropy, and enthalpy) were
investigated for the different temperatures from the vibrational
frequency calculations of molecule. 1
H NMR chemical shifts were
calculated with GIAO approach [22,23] by applying B3LYP method
[24,25]. The theoretically predicted IR and Raman spectra at
B3LYP/6-31G(d,p) level calculation along with experimental FT-IR
and FT- Raman spectra are shown in Figs. 1 and 2.
Prediction of Raman intensities
The Raman activities (Si) calculated by Gaussian 09 program
[15] has been converted to relative Raman intensities (IR
). The the-
oretical Raman intensity (IR
), which simulates the measured
Raman spectrum, is given by the equation [26,27]:
IR
i ¼ Cðm0 À miÞ4
mÀ1
i BÀ1
i Si ð1Þ
where Bi is a temperature factor which accounts for the intensity
contribution of excited vibrational states, and is represented by
the Boltzman distribution:
Bi ¼ 1 À ðexp À hmic=kTÞ ð2Þ
272 M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282
3. In Eq. (1) m0 is the frequency of the laser excitation line (in this
work, we have used the excitation frequency m0 = 9398.5 cmÀ1
,
which corresponds to the wavelength of 1064 nm of a Nd:YAG
laser), mi is the frequency of normal mode (cmÀ1
), while Si is the
Raman scattering activity of the normal mode Qi. Ii
R
is given in arbi-
trary units (C is a constant equal 10À12
). In Eq. (2) h, k, c, and T are
Planck and Boltzman constants, speed of light and temperature in
Kelvin, respectively. Thus, the presented theoretical Raman intensi-
ties have been computed assuming Bi equal 1. The theoretical
Raman spectra have been calculated by the Raint program [28].
The simulated spectra were plotted using a Lorentzian band shape
with a half-width at half-height (HWHH) of 10 cmÀ1
.
Results and discussion
Conformation stability analysis (PES scan analysis)
A detailed potential energy surface (PES) scan on dihedral angle
N10–C8–N1–C6 have been performed at B3LYP level to reveal the
possible conformations of the title compound. The PES scan was
carried out by minimizing the potential energy in all geometrical
parameters by changing the torsion angle at every 20° from
À180° to +180° rotation around the bond. The result obtained in
PES scan study is provided as Supplementary Material (Fig. S1).
The minimum energy was obtained at 0.0° in the potential energy
curve of energy À590.251 Hartree and above and below this tor-
sion angle, the energy rises. Therefore, the most stable conformer
is for 0 torsion angle for N10–C8–N1–C6 rotation. Further results
are based on the most stable conformer of PPOL molecule to clarify
molecular structure and assignments of vibrational spectra.
Structural analysis
The first task for the computational work was to determine the
optimized geometry of PPOL. The atomic numbering scheme of
PPOL was shown in Fig. 3. The calculated bond lengths and bond
angles are given in Table 1. The molecular structure of PPOL has
not been studied and no experimental data have been published
yet. Thus we compared it the as a similar structures [29,30]. The
C–C bond length of the piperidine ring and pyrazin ring varies from
1.535–1.537 Å by DFT method, 1.518–1.537 Å by XRD and 1.389–
1.424 Å by DFT, 1.376–1.402 by XRD. The calculated C–C bond
length shows good agreement with XRD values. The C–H bond
length of the pyrazin ring are C12–H25 = 1.087 Å, C13–
H26 = 1.089 Å and C9–H24 = 1.083 Å calculated by DFT method,
which is slightly greater than that of experimental value at
0.930 Å respectively. The C–H bond length of the piperidine ring
varies from 1.090–1.101 Å by DFT method and 0.95–1.013 Å by
XRD. The steric hinderance on N atom and Coulomb interaction
between the H atoms of CH2 group give rise to the lone pair to
be oriented in axial position while the pyrazin group stays at equa-
torial position. The C–N bond lengths are predicted to be slightly
shorter than the C–C bond lengths. Gundersen and Rankin [31]
reported N–C (1.469 Å) and C–C (1.530 Å) by using electron diffrac-
tion technique to be approximately equal to the calculated bond
length of N–C (1.466 Å and 1.465 Å) and C–C (1.54 Å and
1.539 Å) by B3LYP method. In the present study N1–C8 bond
4000 3500 3000 2500 2000 1500 1000 500
3446
3234
3053
2932
2861
2399
1921
1832
1605
1524
1370
1277
1173
1024
896
823
755
611
514
Wavenumber (cm-1
)
Experimental
515553
615
746
822
915
9761015
1076
1184
1261
1322
1414
146815141560
29292959
3067
3651 B3LYP/6-31G(d,p)
Transmittance%IRIntensity(arb.units)
Fig. 1. Comparison of experimental and theoretical B3LYP/6-31G(d,p) FT-IR spectra for 1-(pyrazin-2-yl) piperidin-2-ol.
M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282 273
4. length is found to be 1.393 Å by DFT which is comparingly shorter
than other N–C bond lengths. This is due to the conjugation to the
pyrazin group and also by the contribution of its electronegativity
character. The resonance effect between C and N atoms and Cou-
lomb repulsive interaction between CH2 groups makes the other
N–C bond lengths i.e., N1–C6 = 1.471 Å and N1–C2 = 1.487 Å to
be shorter than the C–C bond lengths (1.53 Å) in the piperazine
ring. Generally, the C–N–C bond angles are slightly larger than
the C–C–C or N–C–C bond angles [32]. Gundersen and Rankin
reported the C–N–C (110.70°), C–C–C (109.60°) and N–C–C
(110.50°) bond angles by using electron diffraction technique. In
the present study, the bond angles of C2–N1–C6 (110.7°), C2–C3–
C4 (111.8°) C3–C4–C5 (111.2°) C4–C5–C6 (110.4°) and N1–C6–C5
(112.7°) calculated by B3LYP method. The dihedral angle between
piperidine and pyrazin ring is C2–N1–C8–N10 = À135.46° and C6–
N1–C8–C9 = À166.05°. Similarly dihedral angle between piperi-
dine and hydroxyl group is C3–C2–O7–H23 = À43.48°. From the
theoretical values, it is found that most of the optimized bond
lengths and bond angles are slightly larger than the experimental
values due to fact that the theoretical calculations belong to iso-
lated molecules in gaseous phase and the experimental results
belong to molecules in solid state.
Vibrational assignments
There are 72 fundamental modes of vibrations associated with
PPOL molecule. In agreement with C1 point group symmetry, all
the 72 vibrations are distributed as 25 stretching and 24 in-plane
and 23 torsion vibrations. The fundamental vibrational wavenum-
bers of PPOL was calculated by DFT with 6-31G(d,p) basis set is
given in Table 2. The resulting vibrational wavenumbers for the
optimized geometries, IR intensities as well as Raman intensities
and experimental FT-IR, FT-Raman frequencies are also listed.
The normal modes of vibration were assigned on the basis of
PED. DFT vibrational unsealed wavenumbers are known to be
4000 3500 3000 2500 2000 1500 1000 500 0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
3233
30733032
29402909
2845
27352681
2404
1563
1476
1355
1274
1204
1123
10591005
944
880
809
600
519
448
371
296
180
10970
Rananintensity(arb.units)
Wavenumber (cm-1
)
Experimental
0.0
0.2
0.4
0.6
0.8
1.0
61
131
238
331361
515
600
746
822
976
105310991153
1214
1353
1414
15141560
29292959
3067
Ramanintensity(Arb.units)
B3LYP/6-31G(d,p)
Fig. 2. Comparison of experimental and theoretical B3LYP/6-31G(d,p) FT-Raman spectra for 1-(pyrazin-2-yl) piperidin-2-ol.
Fig. 3. Optimized molecular structure and atomic numbering of 1-(pyrazin-2-yl)
piperidin-2-ol.
274 M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282
7. higher than the experimental wavenumbers due to neglect of
anharmonicity effects. To bring the theoretical values closer to
experimental values, we used the scale factor: 0.9608 [33].
Pyrazin ring vibrations
The hetroaromatic structure shows the presence of C–H
stretching vibrations in the region 3000–3100 cmÀ1
, which is
the characteristic region for ready identification of C–H stretch-
ing vibrations [34]. Accordingly, in the present study, the C–H
stretching vibrations of the pyrazin ring are observed at 3234
and 3053 cmÀ1
in the FTIR and 3023, 3073 and 3032 cmÀ1
in
Raman spectra respectively. The computed wavenumbers at
3101, 3066 and 3037 cmÀ1
are assigned to C–H stretching vibra-
tions of pyrazin ring. Joseph et al. [35] reported the pyrazine CH
stretching modes are assigned at 3130, 3123 cmÀ1
theoretically
and experimentally bands are observed at 3117 cmÀ1
. Three C–
H in-plane bending vibrations of the pyrazin ring were identified
at 1528, 1376 and 1530 cmÀ1
in IR and Raman respectively. The
C–H in-plane bending vibrational modes of pyrazin can be
observed in the 1600–1000 cmÀ1
region of the spectrum [36].
In our case C–H in-plane bending modes observed at
1274 cmÀ1
in FT-Raman spectra. The theoretically predicted
wavenumbers at 1430 and 1285 cmÀ1
by DFT method is assigned
C–H in-plane bending vibrations of pyrazin ring which is in
agreement with literature [35]. The out-of-plane CH modes of
the pyrazine ring observed at 944 cmÀ1
in FT-Raman spectra.
The theoretically computed wavenumber at 934 and 920 cmÀ1
by DFT method is assigned C–H out-of-plane bending vibrations
of pyrazin ring (Mode. Nos: 45, 46) which is good agreement with
literature [35] reported by Joseph et al. The identification of C–N
vibrations is a difficult task, since the mixing of vibrations is pos-
sible in this region. However, with the help of the force field cal-
culations, the C–N vibrations are identified and assigned in this
study. Krishnakumar and Prabavathi [12] reported the bands
appearing at 1021, 1178, and 1249 cmÀ1
in both IR and Raman
spectra of 2MP have been attributed to C–N stretching vibration
modes. In our case, the bands appearing at 1524 cmÀ1
in FTIR and
1204 cmÀ1
in FT-Raman spectrum assigned C–N stretching vibra-
tion of pyrazin ring. This vibration theoretically assigned at 1513,
1216, 1155 and 1075 cmÀ1
by DFT method. The C–N–C inplane
bending vibration observed the weak IR band at 611 cmÀ1
. The
computed wavenumbers at 976 and 612 cmÀ1
by DFT method
are identified as dCNC vibrations. The ring C–C stretching modes
appear in the spectra of the investigated molecules in the
1500–1000 cmÀ1
region [36]. In our present study C–C stretching
modes appears at the strong band at 1605 cmÀ1
in FTIR and med-
ium band at 1563 cmÀ1
and weak band at 1059 cmÀ1
in FT-
Raman spectrum (mode. No’s: 14 and 40). The theoretically pre-
dicted wavenumbers at 1561 and 1051 cmÀ1
for these modes
shows good agreement with experimental findings. The pyrazine
ring stretching modes are observed by Joseph et al. [35] at 1531,
1293, 1213, 1162 cmÀ1
in the IR spectrum, 1523, 1291 cmÀ1
in
the Raman spectrum and at 1527, 1481, 1291, 1207, 1177 cmÀ1
theoretically. The pyrazin ring breathing mode of 2-methylpyra-
zine was found at 1195 cmÀ1
[36]. In our work pyrazin ring
breathing mode is observed at 1123 cmÀ1
in FT-Raman spectrum
and computed wavenumber at 1138 cmÀ1
by DFT method is
assigned to pyrazin ring breathing mode.
Piperidin ring vibrations
The infrared bands observed at 2980–2900 cmÀ1
region are
assigned C–H stretching for piperidin [37,38]. In our case C–H
stretching vibration of the piperidin ring assigned at 2926 cmÀ1
by DFT method, which is evident from Table 2 PED column con-
tribute to 88%. For the assignment of CH2 group frequencies, basi-
cally six fundamentals can be associated to each CH2 group
537827520.918.37dCCN(14)pyrazin+sCNCC(33)pyrazin
54755w7737433.259.35dCCN(24)pyrazin+sCNCC(17)pyrazin
55611w6376122.986.50dCNC(55)pyrazin
56600w6256013.348.11tNC(11)piperidine+dCNC(20)pyrazin
5758055710.452.74cCCCN(24)piperidine
58514w519w5405187.736.65dCCN(54)pyrazin
595174974.943.26dCCN(19)piperidine+dCCC(13)piperidine
605014810.522.64dCCC(19)piperidine+cCCCN(16)piperidine
61448w4374203.701.40sCNCC(72)pyrazin
624244086.595.31dCCN(14)piperidine+dCCC(15)piperidine+cCCCN(10)piperidine
63371w38737238.233.54dCNC(25)piperidine+sHOCC(22)piperidine
64379364120.6212.55sHOCC(58)piperidine
653473339.913.82dCCO(49)piperidine
66296w3163042.464.24dCCN(51piperidine)+cOCNC(10)piperidine
672662553.974.54cCCCN(10)piperidine+cCCCC(36)piperidine
682492393.048.43dCNC(18)pyrazin+dCCN(10)pyrazin+sNCCN(15)pyrazin+cOCNC(20)piperidine
69180w1561500.226.39sCCCN(19)piperidine+cOCNC(32)piperidine
701341290.3421.52sNCCN(31)pyrazin+sCNCC(16)pyrazin+sCCNCpyrazin(12)+cCCCN(10)piperidine
7170vs65623.81100.00dCNC(10)pyrazin+cCCCN(11)piperidine+sCNCN(66)pyrazin
7254510.4157.56sCCNC(19)pyrazin+cCCCN(42)piperidine
m–stretching;d–in-planebending;c–out-of-planebending;s–torsion;q–rocking;w–weak;s–strong;vs–verystrong;vw–veryweak.
a
IIR–IRIntensity(KmmolÀ1
).
b
IRa–Ramanintensity(Arbunits)(intensitynormalizedto100%).
M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282 277
8. namely CH2 symmetric stretching, CH2 antisymmetric stretching,
CH2 scissoring and CH2 rocking which belongs to in-plane vibration
and two out-of-plane vibrations viz. The antisymmetric CH2
stretching vibrations are generally observed below 3000 cmÀ1
,
while the symmetric stretch will appear between 3000 and
2700 cmÀ1
[39–41]. The CH2 antisymmetric stretching vibrations
are observed at 3025, 2980, 2965 and 2960 cmÀ1
in DFT method.
They are very pure modes since their PED contributions are above
78%. The CH2 symmetric stretching vibrations are observed at
2932 and 2861 cmÀ1
in FTIR and 2940, 2909 and 2845 cmÀ1
in
FT-Raman spectrum. This vibration theoretically predicted at
2930, 2908 and 2895 cmÀ1
by DFT method, the contribution of
PED for these modes is above 77%. The fundamental CH2 vibrations
due to scissoring, wagging, twisting and rocking appear in the fre-
quency region 1500–800 cmÀ1
. The shift in wavenumber of these
bands is due to the nature of atom and molecule groups linked
to CH2 [42]. Four medium and weak CH2 scissoring vibrations were
calculated at 1454, 1441, 1434 and 1430 cmÀ1
for PPOL. These
vibrational modes are extremely pure with PED contribution max-
imum of about 76%. The CH2 wagging vibrations were calculated at
1349, 1098 and 1051 cmÀ1
by B3LYP method. For these calculated
values, vibrations were observed in Raman spectrum at 1059 cmÀ1
.
The CH2 twisting bending vibrations are observed at 1246 and
1225 cmÀ1
by B3LYP method.
The stretching of CN modes are observed at 1180–1100 cmÀ1
region for piperidine [37,38]. We assigned this band obtained at
1072 and 1016 cmÀ1
by DFT method. The bands obtained at
1005 cmÀ1
FT-Raman spectra have been assigned to C–N stretching
vibrations. The dCCN vibrations identified at 799 and 497 cmÀ1
by
DFT method. The cCN vibrations also identified the calculated
wavenumbers at 557, 481 and 408 cmÀ1
by DFT method. Generally,
the C–C stretching vibrations in aromatic compounds form six
bands in the region 1650–1430 cmÀ1
. These bands show intention
to shift to the lower wavenumber with heavy substituents and
increasing in number of substituents on the compound gives rise
to these vibrations to observe in wide region of FT-IR spectrum.
Piperidin ring C–C stretching modes were shown at 1350–
760 cmÀ1
region [37,38]. In our present study C–C stretching
modes of piperidin ring is observed at 1024 cmÀ1
in FTIR and
809 cmÀ1
in FT-Raman spectrum. The calculated wavenumbers at
1032, 914, 903 and 818 cmÀ1
by DFT method is assigned as C–C
stretching vibrations, these modes are not pure but they contrib-
utes drastically from other vibrations and are substituent-sensi-
tive. The C–C–C inplane bending vibration observed at 957, 497
and 481 cmÀ1
by DFT method. Mode no. 67 also identified as C–
C–C out-off plane bending vibration piperidin ring. The observed
wavenumber at 3446 cmÀ1
and calculated frequency at
3647 cmÀ1
is assigned to O–H stretching vibration. A weak IR band
at 3446 cmÀ1
as against computed band at 3647 cmÀ1
is clearly a
deviation attributable to O–HÁ Á ÁO Inter molecular bonding. The
C–O–H inplane bending vibrations observed at 1370 cmÀ1
in FTIR
spectrum. The computed wavenumbers at 1381 cmÀ1
are identi-
fied as C–O–H inplane bending modes. The C–O stretching vibdra-
tion of the piperidin ring computed at 1098 cmÀ1
by DFT method.
The O–H out of plane bending vibration observed the weak FT-
Raman band at 371 cmÀ1
. The computed wavenumbers at
372 cmÀ1
by DFT method is identified as O–H out-of-plane bending
mode.
NBO analysis
By the use of the second-order bond–antibond (donor–acceptor)
NBO energetic analysis, insight in the most important delocaliza-
tion schemes was obtained. The change in electron density (ED)
in the (r⁄
, p⁄
) antibonding orbitals and E(2) energies have been cal-
culated by natural bond orbital (NBO) analysis [43] using DFT
method to give clear evidence of stabilization originating from var-
ious molecular interactions. The larger the E(2) value, the more
intensive is the interaction between electron donors and electron
acceptors, i.e., the increasing donating tendency from electron
donors to electron acceptors and the greater the extent of conjuga-
tion of the whole system. Delocalization of electron density
between occupied Lewis-type (bond or lone pair) NBO orbitals
and formally unoccupied (anti bond or Rydberg) non-Lewis NBO
orbitals correspond to a stabilizing donor–acceptor interaction.
The result of interaction is a loss of occupancy from the concentra-
tions of electron NBO of the idealized Lewis structure into an empty
non-Lewis orbital. For each donor (i) and acceptor (j), the stabiliza-
tion energy E(2) associates with the delocalization i ? j is estimated
as.
E2 ¼ DEij ¼ qi
Fði; jÞ
2
ej À ei
qi is the donor orbital occupancy; Ei, Ej is the diagonal elements and
Fij is the off diagonal NBO Fock matrix element. Natural bond orbital
analysis is used for investigating charge transfer or conjugative
interaction in the molecular system.
Table 3
Second order perturbation theory analysis of Fock matrix in NBO basis for 1-(pyrazin-2-yl) piperidin-2-ol.
Donor (i) ED (i)(e) Acceptor (j) ED (j)(e) E(2)
a
(kJ molÀ1
) E(j)–E(i)b
(a.u) F(i,j)c
(a.u)
p(C8–N10) 1.700 p⁄
(C9–N11) 0.335 14.72 0.13 0.061
p⁄
(C12–C13) 0.297 26.03 0.33 0.083
p(C9–N11) 1.753 p⁄
(C8–N10) 0.445 17.77 0.30 0.068
p⁄
(C12–C13) 0.297 19.60 0.33 0.072
p(C12–C13) 1.648 p⁄
(C8–N10) 0.445 16.15 0.26 0.059
p⁄
(C9–N11) 0.335 21.12 0.27 0.068
LP(1)N1 1.753 p⁄
(C8–N10) 0.445 36.01 0.25 0.090
LP(2)O7 1.933 r⁄
(N1–C2) 0.066 14.51 0.63 0.085
LP(1)N10 1.914 r⁄
(C8–C9) 0.045 9.55 0.86 0.082
r⁄
(C12–C13) 0.033 8.90 0.92 0.082
LP(1)N11 1.925 r⁄
(C8–C9) 0.045 10.89 0.86 0.087
r⁄
(C12–C13) 0.033 8.92 0.91 0.081
p⁄
(C8–N10) 0.445 p⁄
(C9–N11) 0.335 348.36 0.01 0.087
p⁄
(C12–C13) 0.297 100.77 0.33 0.078
p⁄
(C9–N11) 0.335 p⁄
(C12–C13) 0.297 139.68 0.02 0.078
ED means electron density.
a
E(2) means energy of hyper conjugative interactions.
b
Energy difference between donor and acceptor i and j NBO orbitals.
c
F(i,j) is the Fock matrix element between i and j NBO orbitals.
278 M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282
9. In our present study, the hyperconjugative interaction and elec-
tron density transfer from p(C8–N10) ? p⁄
(C9–N11) and p⁄
(C12–
C13) which leads stabilization energy of about 14.72 and
26.03 kJ/mol. The interaction from p(C9–N11) ? p⁄
(C8–N10) and
p⁄
(C12–C13) results in a stabilization energy of 17.77 and
19.60 kJ/mol. The interaction from lone pair LP(1)N1 ? p⁄
(C8–
N10) and LP(2)O7 ? r⁄
(N1–C2) results in a stabilization energy
of 34.01/14.51 kJ/mol respectively (Table 3). The important inter-
actions in the title molecule having p⁄
(C9–N11) ? p⁄
(C12–C13)
with that of antibonding results the stabilization of 139.68 kJ/
mol. The maximum energies occurs from p⁄
(C8–N10) to antibond-
ing p⁄
(C9–N11) with delocalization energy 348.36 kJ/mol.
Static polarizability and first order hyperpolarizability
Quantum chemical calculations have been shown to be useful in
the description of the relationship between the electronic structure
of the systems and its NLO response [44]. The computational
approach allows the determination of molecular NLO properties
as an inexpensive way to design molecules by analyzing their
potential before synthesis and to determine high-order hyperpo-
larizability tensors of molecules.
The static polarizability (a) and the hyper polarizability (b) and
the electric dipole moment (l) of the PPOL are calculated by finite
field method using 6-31G (d,p) basis set. To calculate all the elec-
tric dipole moments and the first hyper polarizabilities for the iso-
lated molecule, the origin of the Cartesian coordinate system (x, y,
z) = (0, 0, 0) was chosen at own center of mass of PPOL. The NLO
activity provide the key functions for frequency shifting, optical
modulation, optical switching and optical logic for the developing
technologies in areas such as communication, signal processing
and optical interconnections [45,46].
In discussing nonlinear optical properties, the polarization of
the molecule by an external radiation field is often approximated
as the creation of an induced dipole moment by an external electric
field. Under the weak polarization condition, we can use a Taylor
series expansion in the electric field components to demonstrate
the dipolar interaction with the external radiation electric field.
The first static hyperpolarizability (bo) and its related properties
(b, ao and Da) have been calculated using B3LYP/6-31G(d,p) level
based on finite field approach. In the presence of an applied electric
field, the energy of a system is a function of the electric field and
the first hyperpolarizability is a third rank tensor that can be
described by a 3 Â 3 Â 3 matrix. The 27 components of the 3D
matrix can be reduced to 10 components because of the Kleinman
symmetry [47]. The matrix can be given in the lower tetrahedral
format. It is obvious that the lower part of the 3 Â 3 Â 3 matrices
is a tetrahedral. The components of b are defined as the coefficients
in the Taylor series expansion of the energy in the external electric
field. When the external electric field is weak and homogeneous,
this expansion is given below:
E ¼ Eo
À laFa À 1=2aabFaFb À 1=6babcFaFbFc þ Á Á Á Á Á Á
where Eo
is the energy of the unperturbed molecules, Fa is the field
at the origin, la, aab and babc are the components of dipole moment,
polarizability and first hyperpolarizability, respectively.
The total static dipole moment l, the mean polarizability ao, the
anisotropy of the polarizability Da and the mean first hyperpolar-
izability bo, using the x, y and z components are defined as:
Table 4
The electric dipole moment, polarizability and first order hyperpolarizability of 1-(pyrazin-2-yl) piperidin-2-ol by B3LYP/6-31G(d,p) method.
Dipole moment, l (Debye) Polarizability a First order hyperpolarizability b
Parameter Value (DB) Parameter a.u. esu (Â10À24
) Parameter a.u. esu (Â10À33
)
lx 0.9303 axx 91.9329 13.62446 bxxx 61.8771 534.5563
ly 0.5302 axy 16.6400 2.466048 bxxy 39.8133 343.9471
lz 0.3250 ayy 132.1786 19.58887 bxyy À1.1601 À10.0221
l 1.1190 axz À10.5440 À1.56262 byyy À179.6156 À1551.7
ayz 29.8733 4.427223 bxxz 14.4250 124.6176
azz 122.0604 18.08935 bxyz À30.2341 À261.192
ao 115.3906 17.1008 byyz À222.9429 À1926
Da 40.5944 6.0160 bxzz 18.6586 161.1916
byzz À138.8572 À1199.59
bzzz 32.0096 276.5309
btot 239.4286 2068.4236
b = (2.068 Â 1030
esu)
400 600 800 1000 1200
0.0
0.5
1.0
1.5
2.0
2.5
3.0
261
312
340
Absorbance
wavelength (nm)
Experimental UV spectrum
Fig. 4. The UV–Visible spectrum (DMSO) of 1-(pyrazin-2-yl) piperidin-2-ol.
Table 5
Comparison of experimental and calculated absorption wavelength (k, nm), excitation
energies (E, eV) and oscillator strength (f) of 1-(pyrazin-2-yl) piperidin-2-ol.
TD-DFT/B3LYP/6-31G(d,p) Experimental
k (nm) E (eV) f (a.u) Major contributes k (nm) Abs
DMSO
317.24 3.9083 0.0729 HÀ1 ? L, H ? L 340.13 2.7279
291.76 4.2496 0.0095 HÀ1 ? L, H ? L 311.85 2.3874
250.06 4.9581 0.2345 HÀ1 ? L+1, H ? L+1 261.30 1.5542
Gas phase
309.52 4.0057 0.0480 HÀ1 ? L, H ? L
292.93 4.2325 0.0163 HÀ1 ? L, H ? L
250.88 4.9419 0.0490 HÀ1 ? L+1, H ? L+1
M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282 279
10. Dipole moment is
l ¼ ðl2
x þ l2
y þ l2
z Þ
1=2
Static polarizability is
ao ¼ ðaxx þ ayy þ azzÞ=3
Total polarizability is
Da ¼ 2À1=2
ðaxx À ayyÞ2
þ ðayy À azzÞ2 þ ðazz À axxÞ2
þ 6a2
xz
h i1=2
First order hyperpolarizability is
b ¼ b2
x þ b2
y þ b2
z
1=2
where
bx ¼ ðbxxx þbxyy þbxzzÞ
by ¼ ðbyyy þbyzz þbyxxÞ
bz ¼ ðbzzz þbzxx þbzyyÞ
b ¼ ðbxxx þbxyy þbxzzÞ2
þðbyyy þbyzz þbyxxÞ2
þðbzzz þbzxx þbzyyÞ2
h i1=2
Since the values of the polarizabilities (a) and hyperpolarizabil-
ity (b) of the Gaussian 09 output are reported in atomic units (a.u.),
the calculated values have been converted into electrostatic units
(esu) (For a: 1 a.u. = 0.1482 Â 10À24
esu; For b: 1 a.u. = 8.639
 10À33
esu). The mean polarizability ao and total polarizability
Da of our title molecule are 17.1008 Â 10À24
esu and
6.0160 Â 10À24
esu respectively. The total molecular dipole
moment and first order hyperpolarizability are 1.1190 Debye and
2.068 Â 10À30
esu, respectively and are depicted in Table 4. The
first order hyperpolarizability of PPOL molecule is approximately
six times greater than that of urea (b of urea is 0.3728 Â 10À30
esu
[48]). The calculated first hyperpolarizability of the title compound
PPOL is agree very well comparable with the reported values of
similar derivatives [49]. This result indicates the good nonlinearity
of the title molecule.
Electronic properties
UV–Vis spectral analysis
The chemical structure of PPOL is composed of a conjugated
system of double bonds and aromatic ring. Natural bond orbital
analysis indicates that molecular orbitals are mainly composed of
atomic orbital, so above electronic transitions are mainly derived
from the contribution of bands p ? p⁄
. UV–Vis absorption spec-
trum of the sample in DMSO is shown in Fig. 4. In the present
study, the maximum absorption wavelengths (kmax), excitation
energies (DE) and oscillator strengths (f ) of the title molecules in
the gas and solvent (DMSO) phase are computed using TDDFT/
B3LYP/6-31G(d,p). The observed peaks in the spectrum may cause
one electron excitation from HOMO ? LUMO, HOMOÀ1 ? LUMO
and HOMOÀ1 ? LUMO+1 and HOMO ? LUMO+1 are presented
in Table.5. In the electronic spectrum of PPOL, the strong intensity
peaks at the maximum absorption wavelength of 309.52 (gas),
317.24 (DMSO) are caused by p ? p⁄
transitions and the smaller
intensity bands calculated above 260 nm in all the phases of PPOL
are strongly forbidden and therefore, the value of its oscillator
strength nearly equals to zero. Due to the Frank–Condon principle,
the maximum absorption peak (kmax) in an UV–Visible spectrum
corresponds to vertical excitation. Experimentally, electronic
absorption spectra of title molecule in DMSO solvent showed three
bands at 340.13, 311.85 and 261.30 nm.
HOMO–LUMO analysis
The analysis of frontier molecular orbitals describes one elec-
tron excitation from the highest occupied molecular orbital
(HOMO) to the lowest unoccupied molecular orbital (LUMO). The
energy of HOMO is directly related to the ionization potential
and the energy of LUMO is related to the electron affinity. The
HOMO–LUMO energy gap is an important stability index and also
it reflects the chemical activity of a molecule [50]. The HOMO–
LUMO energy gap for PPOL was computed at the B3LYP/6-
31G(d,p) level of theory. The eigen values of LUMO and HOMO
and their energy gap reflect the chemical activity of the molecule.
Generally, if the energy gap between the HOMO and LUMO
decreases, it is easier for the electrons of the HOMO to be excited.
The energy of HOMO, to easier it is for LUMO to accept electrons
when the energy of LUMO is low. The total energy, energy gap
and dipole moment have an effect on the stability of a molecule.
Surfaces for the frontier orbitals were drawn to understand the
bonding scheme of the present compound. The features of these
MO can be seen in Fig. 5. From the figure the HOMO is localized
on the whole molecule and LUMO is localized on the pyrazine ring
of the molecule.
The calculated energy values of the HOMOÀ1 and HOMO are
À6.5221 eV and À5.6603 eV. Similarly, the LUMO+1 and LUMO
energy values are À0.1388 eV and À0.9878 eV. The energy gap
between HOMO and LUMO indicates the molecular chemical sta-
bility. In this molecule, the value of energy separation between
the HOMO and LUMO is À5.6603 eV. In this molecule, the value
of energy separation between the HOMOÀ1 and LUMO+1 is
6.3833 eV.
HOMO energy = À5.6603 eV HOMOÀ1 energy = 6.5221 eV
LUMO energy = À0.9878 eV LUMO+1 energy = À0.1388 eV
HOMO–LUMO energy
gap = 4.6725 eV
HOMOÀ1–LUMO + energy
gap = 6.3833 eV
ELUMO+1 = -0.1388eV
ELUMO = -0.9878eV
EHOMO = -5.6603eV
EHOMO-1= -6.5221eV
∆E=4.6725eV ∆E=6.3833eV
Fig. 5. The atomic orbital compositions of the frontier molecular orbital for 1-
(pyrazin-2-yl) piperidin-2-ol.
280 M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282
11. 1
H NMR analysis
The molecular structure of PPOL compound was optimized by
using B3LYP method in conjunction with 6-31G(d,p) as basis set.
Then, gauge-including atomic orbital (GIAO) 1
H chemical shift cal-
culations of the compound were made. The GIAO [51,52] method is
one of the most common approaches for calculating nuclear mag-
netic shielding tensors. For the same basis set size GIAO method is
often more accurate than those calculated with other approaches
[53]. The chemical shifts with respect to tetramethylsilane (TMS).
The recorded 1
H NMR spectra of PPOL in DMSO solution is shown
in Fig. 6. The H24–H26 protons belonging to pyrazin ring backbone
were observed at 8.02 ppm, 7.81 ppm, 8.01 ppm, respectively.
These chemical shifts also coincide very well with theoretically
computed values at 8.59 ppm, 7.81 ppm, 8.07 ppm for H24, H25
and H26 respectively. The electronegative oxygen atom (O7) pres-
ent in the carbon atom (C2) of the piperidin ring deshields the pro-
ton (H23) O12–H23 which reduces the electron density on H23
resulted in downfield chemical shift. The H16 and H17 protons of
the piperidin ring appeared as doublet, this is observed at
1.57 ppm (H16 and H17) in DMSO solvent and it is also calculated
at 1.60 ppm (H16 and H17) by B3LYP method in DMSO solvent. The
entire experimental chemical shift values are good agreement with
calculated chemical shift values by DFT method shown in Table 6.
Thermodynamic properties
The temperature dependence of the thermodynamic properties
heat capacity at constant pressure (Cp), entropy (S) and enthalpy
change (DH0 ? T) for PPOL was also determined by B3LYP/6-
31G(d,p) level of calculation in the temperature range 100–
1000 K and listed in Table 7. Table 7 depicts that the entropies,
Fig. 6. Experimental 1
H NMR spectrum of 1-(pyrazin-2-yl) piperidin-2-ol.
Table 6
The observed and predicted 1
H NMR isotropic chemical shifts (with respect to TMS, all
values in ppm) for 1-(pyrazin-2-yl) piperidin-2-ol.
Atom position Experimental B3LYP/6-31G(d,p)
H14 4.00 4.75
H15 0.96 0.86
H16 1.57 1.60
H17 1.57 1.60
H18 1.54 1.55
H19 1.74 1.65
H20 – 0.98
H21 2.54 2.64
H22 3.79 4.01
H23 1.41 1.25
H24 8.02 8.59
H25 7.81 7.81
H26 8.01 8.07
Table 7
Thermodynamic properties at different temperatures at the B3LYP/6-31G (d,p) level
for 1-(pyrazin-2-yl) piperidin-2-ol.
T (K) S0
m (cal molÀ1
KÀ1
) C0
p,m (cal molÀ1
KÀ1
) DH0
m (kcal molÀ1
)
100 299.09 76.4 5.26
200 369.19 133.74 15.71
298.15 434.29 197.53 31.92
300 435.52 198.77 32.29
400 501.68 263.24 55.44
500 566.58 318.73 84.62
600 628.83 363.93 118.84
700 687.78 400.61 157.13
800 743.3 430.73 198.74
900 795.53 455.82 243.11
1000 844.68 476.92 289.77
M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282 281
12. heat capacities, and enthalpy changes were increasing with tem-
perature due to the fact that the molecular vibrational intensities
increase with temperature [54]. These observed relations of the
thermodynamic functions vs. temperatures were fitted by qua-
dratic formulas, and the corresponding fitting regression factors
(R2
) are 0.997, 0.969 and 0.970 for heat capacity, entropy and
enthalpy changes respectively. The correlation graphics of temper-
ature dependence of thermodynamic functions for PPOL molecule
are shown in Supplementary Material (Fig. S2). Vibrational zero-
point energy of the molecule PPOL is 557.52 kJ/mol.
Conclusion
The present work for the proper vibrational frequency assign-
ments for the compound of 1-(pyrazin-2-yl) piperidin-2-ol from
the FT-IR and FT-Raman spectra have been recorded for the first
time. The equilibrium geometries, harmonic vibrational frequen-
cies, IR intensities and Raman intensities of the title compound
were determined and analyzed by B3LYP levels of theory utilizing
6-31G (d,p) basis set. The energies of important MOs and the kmax
of the compound were also evaluated from TD-DFT method and in
good agreement with the experimental values. The calculated
HOMO and LUMO energies show that charge transfer and p-elec-
tron delocalization occurs within the molecule. Based on the fre-
quencies scaled and the principle of statistic thermodynamics,
thermodynamic properties ranging from 100 to 1000 K were
obtained and it is obvious that, the gradients of C0
p and S0
m to
the temperature decrease, but that of DH0
m increases, as the tem-
perature increases. The NBO analysis performed in this study
enabled us to know about the conjugative interactions and other
type of interactions taking place within the molecular species.
The calculated first order hyperpolarizability was found to be
2.068 Â 10À30
, which is six times greater than reported in
literature for urea. This study also demonstrates that the title com-
pound can be used as a good non-linear optical material. This
ab initio calculated nonzero b values show that our title molecule
can be good candidates for non linear optical activity. The 1
H NMR
magnetic isotropic chemical shifts were calculated by B3LYP/6-
31G(d,p) level of theory and compared with experimental findings.
Acknowledgements
The authors thank the Principal and Management for providing
necessary facilities to carry out this work at the laboratory of Post
Graduate and Research Department of Chemistry, Jamal Mohamed
College, Trichy-20. Authors M. Suresh and M. Syed Ali Padusha
thank the University Grants Commission, New Delhi for financial
assistance through Major Research Project (Ref. No: 41-261/2012
(SR), Dated: 13-07-2012). We thank the Sophisticated Analytical
Instrumentation Facility, Indian Institute of Technology Madras,
Chennai for providing analytical support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.11.063.
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![tion reactions, such as the sequencing of DNA in the cleavage of
particular modified nucleotides. Pyrazine and its derivatives form
an important class of compounds present in many natural flavors
and complex organic molecules. Pyrazines are responsible for fla-
vor of foodstuffs as diverse as cooked meats, cheese, tea and coffee.
Pyrazine is less basic in nature than pyridine, pyridazine and
pyrimidine [1]. Piperazines are currently the most important build-
ing blocks in drug discovery, with a high number of positive hits
encountered in biological screens of this heterocycle and its cong-
eners across a number of different therapeutic areas [2]; anticancer
[3], antifungal [4], antibacterial, antimalarial and antipsychotic
agents [5], as well as HIV protease inhibitors [6]. Our molecule 1-
(pyrazin-2-yl) piperidin-2-ol consist of one piperidin ring and pyr-
azin ring, these rings are bonded through C–N bridge. It has the fol-
lowing properties; appearance: Pale yellowish white solid;
molecular formula: C10H14N2O; molecular weight: 178.24 g/mol.
Recently Prabavathi et al. [7] reported FT-IR, FT-Raman and DFT
quantum chemical study on the molecular conformation, vibra-
tional and electronic transitions of 1-(m-(trifluoromethyl) phenyl)
piperazine. Mahalakshmi et al. [8] analyzed Molecular structure,
Vibrational spectra (FTIR and FT Raman) and Natural bond orbital
analysis of 4-aminomethylpiperidine: DFT study. Revised assign-
ment of the vibrational spectra of methylpyrazines based on scaled
DFT force fields and Vibrational spectroscopic and quantum chemi-
cal study of the chlorine substitution of pyrazine given by Endre´di
et al. [9,10]. FT-Raman, FT-IR spectra and total energy distribution
of 3-pentyl-2,6-diphenylpiperidin-4-one: DFT method [11]. Krish-
nakumar and Prabavathi [12] reported Scaled quantum chemical
calculations and FTIR, FT-Raman spectral analysis of 2-methylpyra-
zine. Theoretical and experimental vibrational spectroscopic study
of 4-(1-pyrrolidinyl) piperidine investigated by Cemal Parlak [13].
Chis et al. [14] studied Experimental and DFT study of pyrazinamide.
Literature survey clearly reveals that to the best of our knowledge,
no experimental and computational vibrational, NMR and UV spec-
troscopic study on free PPOL have published in the literature yet.
In the present work, we report the results of theoretical and
experimental (IR and Raman, UV and NMR) spectra of PPOL mole-
cule along with NBO analysis. Therefore, the aim of this study is to
fully determine the molecular structure, vibrational modes and
wavenumbers are determined and by using quantum chemical cal-
culations. Detailed interpretations of the vibrational spectra of our
compound have been made based on the calculated potential
energy distribution (PED). Therefore, the present work aims to pro-
vide a complete description on the molecular geometry, molecular
vibrations and electronic features of PPOL.
Experimental
Synthesis
To a solution of 2-aminopyrazine (1.52 g, 0.016 mol) in acetoni-
trile (20 mL), 3,4-dihydro-2H-pyran (1.2 mL, 1.4 mmol) was added
followed by Cerium Chloride heptahydrate (Ce(III)
Cl3Á7H2O) (1.8 g,
0.005 mol) at 0 °C. The reaction mixture was kept in an oil bath
maintained at 60 °C and stirred well for 2 h. Progress of the reac-
tion was continuously monitored by LCMS. The reaction mixture
was diluted with ethyl acetate and washed with water. The organic
layer was dried over Na2SO4 and concentrated by vacuum. The
crude mass obtained was purified by column Chromatography
packed with 60/120 silica gel and eluted with 25–35% ethyl acetate
in petroleum ether.
FT-IR, FT-Raman, UV–Vis and 1
H NMR spectral measurements
The FT-IR spectrum of 1-(pyrazin-2-yl) piperidin-2-ol com-
pound was recorded in the range of 4000–400 cmÀ1
on a Perkin
Elmer, RXI model FT-IR spectrometer using KBr pellet technique.
The spectrum was recorded in the room temperature, with scan-
ning speed of 10 cmÀ1
, and spectral resolution: 4 cmÀ1
. FT-Raman
spectrum of the title compound was recorded using 1064 nm line
of Nd:YAG laser as excitation wavelength in the region 3500–
50 cmÀ1
on a BRUKER RFS 27: FT-Raman Spectrometer equipped
with FT-Raman molecule accessory. The spectral resolution was
set to 2 cmÀ1
in back scattering mode. The laser output was kept
at 100 mW for the solid sample. The ultraviolet absorption spectra
of PPOL were examined in the range 200–800 nm using Perkin
Elmer, lambda-35 model UV spectrometer. The UV pattern is taken
from a 10–5 M solution of PPOL, dissolved in DMSO solvent. 1
H
NMR spectra are recorded on Bruker AVANCE III 500 MHz (AV
500) spectrometer; chemical shifts are expressed in ppm (d units)
relative to TMS signal as internal reference in DMSO solvent.
Computational details
The optimized geometry of and vibrational frequencies were
calculated at the B3LYP/6-31G(d,p) level with the Gaussian 09 pro-
gram [15]. The molecular structure optimization and correspond-
ing vibrational harmonic frequencies were calculated using DFT
calculations [16] with the Becke’s three-parameter hybrid func-
tional (B3) [17] for the exchange part and the Lee–Yang–Parr
(LYP) correlation function [18], for the computation of molecular
structure, vibrational frequencies and energies of optimized struc-
tures by using Gaussian 09 suite of quantum chemical codes.
Firstly, the title molecule was optimized, after then the optimized
structural parameters were used in the vibrational frequency and
calculations of electronic properties. The vibrational wavenumber
assignments were carried out by combining the results of the
Gauss view 5.08 [19] and VEDA4 programs [20]. The calculated
IR spectrum, plotted using the pure Lorentzian band shape with
a band width of FWHM of 10 cmÀ1
has been compared with the
experimental FT-IR spectrum and is found to be well comparable
to that of the spectral data obtained by DFT/B3LYP method. The
natural bonding orbital (NBO) calculations [21] were performed
using Gaussian 09 [15] package at the same level in order to under-
stand various second order interactions between the filled orbitals
of one subsystem and vacant orbitals of another subsystem, which
is a measure of the intermolecular delocalization or hyper conjuga-
tion. UV–Vis spectra, electronic transitions, vertical excitation
energies, absorbance and oscillator strengths were computed with
the time-dependent DFT method. The changes in the thermody-
namic functions (the heat capacity, entropy, and enthalpy) were
investigated for the different temperatures from the vibrational
frequency calculations of molecule. 1
H NMR chemical shifts were
calculated with GIAO approach [22,23] by applying B3LYP method
[24,25]. The theoretically predicted IR and Raman spectra at
B3LYP/6-31G(d,p) level calculation along with experimental FT-IR
and FT- Raman spectra are shown in Figs. 1 and 2.
Prediction of Raman intensities
The Raman activities (Si) calculated by Gaussian 09 program
[15] has been converted to relative Raman intensities (IR
). The the-
oretical Raman intensity (IR
), which simulates the measured
Raman spectrum, is given by the equation [26,27]:
IR
i ¼ Cðm0 À miÞ4
mÀ1
i BÀ1
i Si ð1Þ
where Bi is a temperature factor which accounts for the intensity
contribution of excited vibrational states, and is represented by
the Boltzman distribution:
Bi ¼ 1 À ðexp À hmic=kTÞ ð2Þ
272 M. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 271–282](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)