Vibrational spectroscopic investigations and computational study of 5-tert-Butyl-N-(4-trifluoromethylphenyl)pyrazine-2-carboxamide

Article abstract of DOI:10.1016/j.saa.2013.04.101

(Figure Presented) Pyrazine and its derivatives form an important class of compounds present in several natural flavors and complex organic molecules. Quantum chemical calculations of the equilibrium geometry, harmonic vibrational frequencies, infrared in

Full text of DOI:10.1016/j.saa.2013.04.101

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Vibrational spectroscopic investigations and computational study
of 5-tert-Butyl-N-(4-trifluoromethylphenyl)pyrazine-2-carboxamide
d
Tomy Joseph a,d, Hema Tresa Varghese b, C. Yohannan Panicker c, , K. Viswanathan ,
⇑
Martin Dolezal e, T.K. Manojkumar f, Christian Van Alsenoy g
a Department of Physics, St. Xavier’s College, Vaikom, Kothavara, Kottayam, Kerala, India
b Department of Physics, Fatima Mata National College, Kollam, Kerala, India
c Department of Physics, TKM College of Arts and Science, Kollam, Kerala, India
d Department of Physics, Karpagam University, Pollachi Main Road, Eachanari, Coimbatore, Tamilnadu, India
e Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Heyrovskeho 1203, Hradec Kralove 500 05, Czech Republic
f Indian Institute of Information Technology and Management – Kerala, Technopark Campus, Trivandrum, Kerala, India
g Department of Chemistry, University of Antwerp, B2610 Antwerp, Belgium
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Quantum chemical calculations of the equilibrium geometry, harmonic vibrational frequencies, infrared
intensities and Raman activities of 5-tert-Butyl-N-(4-trifluoromethylphenyl)pyrazine-2-carboxamide in
the ground state were carried out by using density functional methods. Potential energy distribution
of normal modes of vibrations was done using GAR2PED program. Nonlinear optical behavior of the
examined molecule was investigated by the determination of first hyperpolarizability. The calculated
HOMO and LUMO energies show the chemical activity of the molecule. The stability of the molecule aris-
ing from hyper-conjugative interaction and charge delocalization has been analyzed using NBO analysis.
The calculated geometrical parameters are in agreement with that of similar derivatives. The stability of
the molecule arising from hyper-conjugative interaction and charge delocalization has been analyzed
using NBO analysis.
ꢀ
ꢀ
IR, Raman and NBO were reported.
The wavenumbers are calculated
theoretically using Gaussian09
software.
The wavenumbers are assigned using
PED analysis.
The geometrical parameters are in
agreement with that of similar
derivatives.
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyrazine and its derivatives form an important class of compounds present in several natural flavors and
complex organic molecules. Quantum chemical calculations of the equilibrium geometry, harmonic
vibrational frequencies, infrared intensities and Raman activities of 5-tert-Butyl-N-(4-trifluoromethyl-
phenyl)pyrazine-2-carboxamide in the ground state were carried out by using density functional meth-
ods. Potential energy distribution of normal modes of vibrations was done using GAR2PED program.
Nonlinear optical behavior of the examined molecule was investigated by the determination of first
hyperpolarizability. The calculated HOMO and LUMO energies show the chemical activity of the mole-
cule. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization
Received 6 January 2013
Received in revised form 14 April 2013
Accepted 24 April 2013
Available online 6 May 2013
Keywords:
FT-IR
FT-Raman
⇑
Corresponding author. Tel.: +91 9895370968.
E-mail address: cyphyp@rediffmail.com (C. Yohannan Panicker).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.04.101

204
T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
Pyrazine
Carboxamide
PED
has been analyzed using NBO analysis. The calculated geometrical parameters are in agreement with that
of similar derivatives. The stability of the molecule arising from hyper-conjugative interaction and charge
delocalization has been analyzed using NBO analysis.
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
Experimental
Pyrazine and its derivatives form an important class of com-
pounds present in several natural flavors and complex organic
molecules [1]. 2-Chloropyrazine and 2,6-dichloropyrazine are
mainly found as medical and agricultural drug intermediates
[1–3]. Pyrazine derivatives are important drugs with antibacte-
rial, diuretic, hypolipidemic, antidiabetic, hypnotic, anticancer
and antiviral activity. Pyrazinamide (PZA), another first-line TB
drug, was discovered through an effort to find antitubercular
nicotinamide derivatives [4]. The activity of PZA appears to be
pH dependent, since it is bactericidal at pH 5.5, but inactive
at neutral pH. PZA is used in combinations with INH and rifam-
picin. It is especially effective against semi-dormant mycobacte-
ria. Its mechanism of action appears to involve its hydrolysis to
pyrazinoic acid via the bacterial enzyme pmcA [5]. PZA can also
be metabolized by hepatic microsomal deamidase to pyrazinioic
acid, which is a substrate for xanthine oxidase, affording 5-
hydroxypyrazinoic acid. The acid is believed to act as an anti-
metabolite of nicotinamide and interferes with NAD biosynthe-
sis. A different analog of PZA, 5-chloropyrazine-2-carboxamide,
has previously been shown to inhibit mycobacterial fatty acid
synthase I [6]. Pyrazinamide is a member of the pyrazine family
and it is known as a very effective antimycobacterial agent,
with a well established role in tuberculosis treatment [7]. Pyra-
zinamide is bactericidal to semidormant mycobacteria and re-
duces total treatment time [8]. Although the exact
biochemical basis of pyrazinamide activity in vivo is not known,
under acidic conditions it is though to be a prodrug of pyrazi-
noic acid, a compound with antimycobacterial activity [9]. The
finding that pyrazinamide-resistant strains lose amidase (pyraz-
inamidase or nicotinamidase) activity and the hypothesis that
amidase is required to convert pyrazinamide to pyrazinoic acid
intracellularly led to the recent synthesis and study of various
prodrugs of pyrazinoic acid [10]. The resistance of PZA arises
by the absence of the enzyme, Pmc A. The major side effect
of PZA is dose-related hepatotoxicity. Pyrazinoic acid disrupts
membrane energetics and inhibits membrane transport function
in M.tuberculosis [11]. The dynamical pattern of the 2-amino-
pyrazine-3-carboxylic acid molecule by inelastic and incoherent
neutron scattering, Raman spectroscopy and ab initio calcula-
tions was reported by Pawlukojc et al. [12]. Billes et al. [13]
calculated the vibrational frequencies of the three parent dia-
zines (pyrazine, pyridazine and pyrimidine) applying ab initio
quantum chemical methods, Moller-Pleassett perturbation and
local density function methods. Various compounds possessing
–NHCO- groups, e.g. substituted amides, acyl and thioacyl ani-
lides, benzanilides, phenyl carbamates, etc., were found to inhi-
bit photo synthetic electron transport [14,15]. Therefore, the
vibrational spectroscopic studies of the amides of pyrazine-2-
carboxylic acids are added areas of interest. In the present
work, FT-IR and FT-Raman spectra of 5-tert-Butyl-N-(4-trifluoro-
methylphenyl)pyrazine-2-carboxamide are reported both experi-
mentally and theoretically. The HOMO and LUMO analysis have
been used to elucidate information regarding charge transfer
within the molecule. The stability of the molecule arising from
hyper-conjugative interaction and charge delocalization has
been also analyzed using NBO analysis.
All organic solvents used for the synthesis were of analytical
grade. The solvents were dried and freshly distilled under argon
atmospheres. Melting point was determined using a SMP 3 melting
point apparatus (BIBBYB Stuart Scientific, UK) and are uncorrected.
The reactions were monitored and the purity was checked by TLC
(Merck UV 254 TLC plates, Darmstadt, Germany) using petroleum
ether/EtOAc (9:1) as developing solvent. Purification of compounds
was made using Flash Master Personeal chromatography system
from Argonaut Chromatography (Argonaut Technologies, Redwood
City, CA, USA) with gradient of elution from 0% to 20% ethyl-acetate
in hexane. As sorbent, Merck Silica Gel 60 (0.040–0.063 mm) was
used (Merck). Elemental analysis was performed on an automatic
microanalyser CHNS-O CE instrument (FISONS EA 1110, Milano,
Italy). 1H- and 13C-NMR spectra were recorded (at 300 MHz for
1H and 75 MHz for 13C) in CDCl3 solutions at ambient temperature
on a Varian Mercury-Vx BB 300 instrument (Varian, Palo Alto, CA,
USA). The chemical shifts were recorded as d values in ppm and
were indirectly referenced to tetramethylsilane (TMS) via the sol-
vent signal (7.26 for 1H and 77.0 for 13C in CDCl3).
The 5-tert-butylpyrazine-2-carboxylic acid [16] (50.0 mmol)
and thionyl chloride (5.5 mL, 75.0 mmol) in dry toluene (20 mL)
was refluxed for about 1 h. Excess thionyl chloride was removed
by repeated evaporation with dry toluene in vacuo. The crude acyl
chloride dissolved in dry acetone (50 mL) was added drop wise to a
stirred solution of the 4-triofluoromethylaniline (50.0 mmol) and
pyridine (50.0 mmol) in dry acetone (50 mL) kept at room temper-
ature. After the addition was complete, stirring was continued for
30 min, then the reaction mixture was poured into cold water
(100 mL) and the crude amide was collected and purified by the
column chromatography.
Fig. 1. FT-IR spectrum of 5-tert-Butyl-N-(4-trifluoromethylphenyl)pyrazine-2-
carboxamide.

T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
205
B3PW91/6-31Gꢂ and B3LYP/SDD basis sets to predict the molec-
ular structure and vibrational wavenumbers. Calculations were
carried out with Becke’s three parameter hybrid model using
the Lee–Yang-Parr correlation functional (B3LYP) method. Molec-
ular geometries were fully optimized by Berny’s optimization
algorithm using redundant internal coordinates. Harmonic vibra-
tional wavenumbers were calculated using analytic second deriv-
atives to confirm the convergence to minima on the potential
surface. The Stuttagard/Dresden effective core potential basis
set (SDD) [19] was chosen particularly because of its advantage
of using faster calculations with relatively better accuracy and
structures [20]. Then frequency calculations were employed to
confirm the structure as minimum points in energy. At the opti-
mized structure (Fig. 3) of the examined species, no imaginary
wavenumber modes were obtained, proving that a true minimum
on the potential surface was found. The DFT method tends to
over estimate the fundamental modes; therefore scaling factor
(0.9613) has to be used for obtaining a considerably better agree-
ment with experimental data [21]. The observed disagreement
between theory and experiment could be a consequence of the
anharmonicity and of the general tendency of the quantum
chemical methods to over estimate the force constants at the ex-
act equilibrium geometry. The optimized geometrical parameters
(B3LYP/SDD) are given in table 1. The assignments of the calcu-
lated wavenumbers are aided by the animation option of GAUSS-
Fig. 2. FT-Raman spectrum of 5-tert-Butyl-N-(4-trifluoromethylphenyl)pyrazine-2-
carboxamide.
5-tert-Butyl-N-(4-trifluoromethylphenyl)pyrazine-2-carboxam-
ide. Yield 57%; Anal. Calcd. for C16H16F3N3O (323.3): 59.44% C,
4.99% H, 13.00% N; Found: 59.54% C, 3.97% H, 12.94% N; M.p.
117.2–118.0 °C; Log P: 3.64; Clog P: 4.22870; 1H-NMR (CDCl3) d:
9.81 (1H, bs, 20H), 9.40 (1H, d, J = 1.5 Hz, 7H), 8.64 (1H, d,
J = 1.5 Hz, 26H), 7.93–7.85 (2H, m, AA0, BB0, 16H, 17H), 7.69–7.61
(2H, m, AA0, BB0, 15H, 21H), and 1.45 (9H, s, 31H, 32H, 33H, 34H,
35H, 36H, 37H, 38H, 39H). 13C-NMR (CDCl3) d: 165.1, 160.1,
145.9, 140.6, 140.4, 140.1, 126.4 (q, J = 3.8 Hz), 126.0 (q,
J = 32.7 Hz), 124.0 (q, J = 271.7 Hz), 119.5, 39.1, and 28.2 [17].
The FT-IR spectrum (Fig. 1) was recorded using KBr pellets on a
DR/Jasco FT-IR 6300 spectrometer in KBr pellets. The spectral res-
olution was 2 cmꢁ1. The FT-Raman spectrum (Fig. 2) was obtained
on a Bruker RFS 100/s, Germany. For excitation of the spectrum the
emission of Nd:YAG laser was used, excitation wavelength
1064 nm, maximal power 150 mW, measurement on solid sample.
VIEW program, which gives
a visual presentation of the
vibrational modes [22]. The potential energy distribution (PED)
is calculated with the help of GAR2PED software package [23].
Results and discussion
IR and Raman spectra
The observed IR, Raman bands and calculated wavenumbers
(scaled) and assignments are given in table 2. The C@O stretching
vibrations are expected in the range 1715–1600 cmꢁ1 and the
deformation modes of C@O in the range 460–800 cmꢁ1 [24]. In
the case of 2-aminopyrazine-3-carboxylic acid [12] the C@O modes
are reported as follows: torsional C@O 101 (Raman), 111 (HF);
rocking C@O 537 (Raman), 571 (HF); wagging C@O 723 (Raman),
736 (HF); bending C@O 912 (Raman), 886 (HF) and stretching
mode C@O 1718 (Raman), 1786 cmꢁ1 (HF). For the title compound,
the C@O modes are observed at 1613, 538 cmꢁ1 in the IR spectrum,
1617, 698, 115 cmꢁ1 in the Raman spectrum and at 1610, 870, 708,
557, 122 cmꢁ1 theoretically (SDD).
The spectral resolution after apodization was 2 cmꢁ1
.
Computational details
Calculations of the title compound were carried out with
Gaussian09 software [18] program using B3LYP/6-31Gꢂ,
Fig. 3. Optimized geometry (SDD) of 5-tert-Butyl-N-(4-trifluoromethylphenyl) pyrazine-2-carboxamide.

206
T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
Table 1
Optimized geometrical parameters (SDD) of 5-tert-Butyl-N-(4-trifluoromethylphenyl)pyrazine-2-carboxamide, atom labeling according Fig. 3.
Bondlengths (Å)
Bondangles (°)
Dihedralangles (°)
C1AC2
C1AN6
C1AH26
C2AN3
C2AC27
N3AC4
C4AC5
C4AH7
C5AN6
C5AC8
C8AN18
C8AO19
C9AC10
C9AC14
C9AN18
1.4185
1.3522
1.0823
1.3670
1.5326
1.3509
1.4097
1.0839
1.3581
1.5050
1.3797
1.2586
1.4165
1.4168
1.4109
1.4016
1.0819
1.4079
1.0859
1.4091
1.4951
1.3982
1.0855
1.0877
1.0210
1.4042
1.4057
1.4168
1.5476
1.5570
1.5570
1.0983
1.0966
1.0983
1.0983
1.0974
1.0946
1.0983
1.0946
1.0974
A(2,1,6)
A(2,1,26)
A(6,1,26)
A(1,2,3)
A(1,2,27)
A(3,2,27)
A(2,3,4)
A(3,4,5)
A(3,4,7)
A(5,4,7)
A(4,5,6)
A(4,5,8)
A(6,5,8)
A(1,6,5)
A(5,8,18)
A(5,8,19)
122.0
121.6
116.4
119.3
124.3
116.5
118.8
121.3
118.3
120.5
120.7
121.1
118.2
118.0
112.8
121.4
125.9
119.6
123.0
117.4
119.5
119.6
120.9
120.6
119.4
120.0
120.0
120.2
119.9
119.8
120.2
120.0
120.5
119.8
119.7
128.6
113.3
118.1
113.0
112.8
113.2
106.7
105.3
105.2
112.3
108.2
108.2
109.4
109.4
109.2
112.0
109.2
112.0
107.4
108.6
107.4
111.3
109.8
110.5
108.1
108.4D(12,13,14,9)
108.7
111.3
110.5
109.8
108.4
108.1
108.7
D(6,1,2,3)
D(6,1,2,27)
D(26,1,2,3)
D(26,1,2,27)
D(2,1,6,5)
D(26,1,6,5)
D(1,2,3,4)
D(27,2,3,4)
D(1,2,27,28)
D(1,2,27,29)
D(1,2,27,30)
D(3,2,27,28)
D(3,2,27,29)
D(3,2,27,30)
D(2,3,4,5)
D(2,3,4,7)
D(3,4,5,6)
D(3,4,5,8)
D(7,4,5,6)
D(7,4,5,8)
D(4,5,6,1)
D(8,5,6,1)
D(4,5,8,18)
D(4,5,8,19)
D(6,5,8,18)
D(6,5,8,19)
D(5,8,18,9)
D(5,8,18,20)
D(19,8,18,9)
D(19,8,18,20)
D(14,9,10,11)
D(14,9,10,15)
D(18,9,10,11)
D(18,9,10,15)
D(10,9,14,13)
D(10,9,14,21)
D(18,9,14,13)
D(18,9,14,21)
D(10,9,18,8)
D(10,9,18,20)
D(14,9,18,8)
D(14,9,18,20)
D(9,10,11,12)
D(9,10,11,16)
D(15,10,11,12)
D(15,10,11,16)
D(10,11,12,13)
D(10,11,12,22)
D(16,11,12,13)
D(16,11,12,22)
D(11,12,13,14)
D(11,12,13,17)
D(22,12,13,14)
D(22,12,13,17)
D(11,12,22,23)
D(11,12,22,24)
D(11,12,22,25)
D(13,12,22,23)
D(13,12,22,24)
D(13,12,22,25)
ꢁ0.2
ꢁ0.0
180.0
180.0
ꢁ0.0
ꢁ0.0
180.0
0.0
ꢁ180.0
ꢁ0.2
120.7
ꢁ121.1
179.8
ꢁ59.3
58.9
ꢁ0.0
C
C
C
C
C
C
C
C
C
10AC11
10AH15
11AC12
11AH16
12AC13
12AC22
13AC14
13AH17
14AH21
180.0
ꢁ0.0
A(18,8,19)
A(10,9,14)
A(10,9,18)
A(14,9,18)
A(9,10,11)
A(9,10,15)
A(11,10,15)
A(10,11,12)
A(10,11,16)
A(12,11,16)
A(11,12,13)
A(11,12,22)
A(13,12,22)
A(12,13,14)
A(12,13,17)
A(14,13,17)
A(9,14,13)
A(9,14,21)
A(13,14,21)
A(8,18,9)
A(8,18,20)
A(9,18,20)
A(12,22,23)
A(12,22,24)
A(12,22,25)
A(23,22,24)
A(23,22,25)
A(24,22,25)
A(2,27,28)
A(2,27,29)
A(2,27,30)
A(28,27,29)
A(28,27,30)
A(29,27,30)
A(27,28,31)
A(27,28,32)
A(27,28,33)
A(31,28,32)
A(31,28,33)
A(32,28,33)
A(27,29,34)
A(27,29,35)
A(27,29,36)
A(34,29,35)
A(34,29,36)
A(35,29,36)
A(27,30,37)
A(27,30,38)
A(27,30,39)
A(37,30,38)
A(37,30,39)
A(38,30,39)
180.0
180.0
ꢁ0.0
0.0
ꢁ180.0
179.9
ꢁ0.1
N18AH20
ꢁ0.1
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
22AF23
22AF24
22AF25
27AC28
27AC29
27AC30
28AH31
28AH32
28AH33
29AH34
29AH35
29AH36
30AH37
30AH38
30AH39
179.9
180.0
ꢁ0.0
ꢁ0.0
180.0
ꢁ0.2
179.8
ꢁ180.0
ꢁ0.0
0.2
ꢁ179.7
ꢁ180.0
0.1
ꢁ0.2
179.8
ꢁ180.0
ꢁ0.0
0.2
179.6
ꢁ179.8
ꢁ0.4
ꢁ0.2
ꢁ177.9
ꢁ179.6
2.7
0.2
179.5
177.9
ꢁ2.8
ꢁ31.8
ꢁ152.9
87.7
150.5
29.4
ꢁ90.0
D(12,13,14,21)
D(17,13,14,9)
D(17,13,14,21)
D(2,27,28,31)
D(2,27,28,32)
D(2,27,28,33)
D(29,27,28,31)
D(29,27,28,32)
D(29,27,28,33)
D(30,27,28,31)
D(30,27,28,32)
D(30,27,28,33)
D(2,27,29,34)
179.7
ꢁ179.6
0.4
ꢁ61.1
ꢁ180.0
61.2
178.7
59.9
ꢁ59.0
59.1
ꢁ59.7
ꢁ178.6
ꢁ63.7

T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
207
Table 1 (continued)
Bondlengths (Å)
Bondangles (°)
Dihedralangles (°)
D(2,27,29,35)
D(2,27,29,36)
D(28,27,29,34)
D(28,27,29,35)
D(28,27,29,36)
D(30,27,29,34)
D(30,27,29,35)
D(30,27,29,36)
D(2,27,30,37)
D(2,27,30,38)
D(2,27,30,39)
D(28,27,30,37)
D(28,27,30,38)
D(28,27,30,39)
D(29,27,30,37)
D(29,27,30,38)
D(29,27,30,39)
176.7
56.8
58.9
ꢁ60.6
179.4
178.6
59.1
ꢁ60.9
63.8
ꢁ56.7
ꢁ176.7
ꢁ58.9
ꢁ179.4
60.6
ꢁ178.6
60.9
ꢁ59.1
The stretching mode of NH group is generally rive rise to bands
[25,26] in the range 3500–3300 cmꢁ1. For the title compound the
NH stretching mode is observed at 3347 cmꢁ1 in the IR spectrum,
and at 3343 cmꢁ1 in the Raman spectrum and theoretical value is
3385 cmꢁ1 (SDD). In mono-substituted amides, the in-plane bend-
ing frequency and the resonance stiffened CN band stretching fre-
quency fall close together and therefore interact. The CNH
vibration where the nitrogen and the hydrogen move in opposite
directions relative to the carbon atom involves both NH bend
and CN stretching and absorbs [27] near 1500 cmꢁ1. This band is
very characteristic for mono-substituted amides. The CNH vibra-
tion where N and H atoms move in the same direction relative to
the carbon atom gives rise to a band [27] near 1250 cmꢁ1. For
the title compound, the bands at 1494, 1249 cmꢁ1 (SDD) are as-
signed as CNH bending modes. The out-of-plane NH wag is as-
signed at 825 cmꢁ1 theoretically. Mary et al. [28] reported the
NH bands at 1547, 1250, 650 cmꢁ1 in the IR spectrum and at
1580, 1227, 652 cmꢁ1 theoretically for a similar derivative. Aro-
matic ring with nitrogen directly on the ring absorb at 1330–
1260 cmꢁ1 because of the stretching of the phenyl CAN bond
[27]. For the title compound, the C9AN18 stretching mode is ob-
served (SDD) at 1237 cmꢁ1. The C8AN18 stretching band is reported
in the range 1000–1200 cmꢁ1 [29] and in the present case this
band is observed at 1084 cmꢁ1 theoretically.
calculations give these rocking modes of the tBu group at 1188,
1013, 1002 and 966, 920, 906 cmꢁ1. The tBu group gives rise to five
skeletal deformations absorbing in the three regions [24]: dasCC3 in
435 85, dsCC3 in 335 80 and
q
CC3 in 300 80 cmꢁ1. These
modes normally produce bands of weak or medium intensity.
The highest (lowest) values for dasCC3 are observed [24] around
510 (355) cmꢁ1. Most of the dasCC3 modes have been assigned in
the region [24] 435 65 cmꢁ1. The SDD calculations give frequen-
cies at 496, 326 and 313 cmꢁ1 as asymmetric and symmetric defor-
mations. The bands at 290, 219 cmꢁ1(SDD) are assigned as the
rocking modes of CC3. The torsion modes
pected in the low frequency region [24]. The
modes are expected in the regions 1235 60 and 800 90 cmꢁ1
s
CH3 and
s
CC3 are ex-
tasCC3 and
tsCC3
,
respectively [24]. For the title compound the bands observed at
1244 cmꢁ1 in the IR spectrum, 1247 cmꢁ1 in the Raman spectrum
and at 1286, 1249 cmꢁ1 theoretically are assigned as
tasCC3 modes
The SDD calculations give the symmetric
tsCC3 stretching mode at
906 cmꢁ1 and the bands observed at 907 cmꢁ1 in the IR spectrum,
904 cmꢁ1 in the Raman spectrum are assigned as these modes.
These modes are not pure but contain significant contributions
from other modes.
The CF3 group possesses as many normal vibrations as methyl
and as, halogen atoms are much heavier than hydrogen, CF3 modes
are expected at considerably lower values. According to Roeges
[24] the absorption regions of ACF3 in substituted benzene are:
The methyl stretching vibrations are expected in the region
3010–2900 cmꢁ1 (asymmetric) and 2950–2850 cmꢁ1 (symmetric)
tCF 1300–1000; dCF 720–440; qCF 470–260 and sCF below
[24]. In the present case the tertiary butyl
t
asCH3 stretching vibra-
100 cmꢁ1. In the present case the bands at 1140, 1026 cmꢁ1 in
the IR spectrum, 1142, 1026 cmꢁ1 in the Raman spectrum and at
1132, 1026, 1017 cmꢁ1 (SDD) are assigned as the stretching modes
of CF3 group. The deformation bands are assigned at 674, 594, 511,
478 cmꢁ1 in the IR spectrum, 672, 601, 520, 475, 395, 372 cmꢁ1 in
the Raman spectrum and at 681, 594, 515, 480, 389, 373 cmꢁ1 the-
oretically. Iriarte et al. [30] reported the CF3 stretching values at
1244, 1213, 1140, 1128, (DFT), 1218, 1176, 1136, 1127 (experi-
mental), deformation bands of CF3 at 729 cmꢁ1 (experimental),
743 cmꢁ1 (DFT) .
The pyrazine CH stretching modes are reported in the range
3100–300 cmꢁ1 [31,32]. ꢁT1he pyrazine CH stretching modes are as-
signed at 3130, 3123 cm theoretically and experimentally bands
are observed at 3117 cmꢁ1 for the title compound. The pyrazine CH
stretching modes are reported at 3057, 3070, 3086 (IR), 3060, 3070,
3087 (Raman), 3061, 3074, 3079 cmꢁ1 (theoretical) for 2-chloro-
pyrazine and 3099, 3104 (IR), 3078, 3103 (Raman), 3096,
3100 cmꢁ1 (calculated) for 2,6-dichloropyrazine [1].
tions are observed at 3031, 2980 cmꢁ1 in the IR spectrum and at
3015, 2984 cmꢁ1 in the Raman spectrum. The calculated values
for these modes are 3038, 3032, 3014, 3006, 3005, 2999 cmꢁ1
(SDD). The symmetric stretching modes of the methyl group are
calculated to be at 2927, 2922, 2917 cmꢁ1 (SDD) and bands are ob-
served at 2934, 2910 cmꢁ1 in IR and 2918 cmꢁ1 in the Raman spec-
trum. The methyl asymmetric deformations dasCH3 absorb [24]
between 1495 and 1435 cmꢁ1. Although three methyl symmetric
bending modes are expected often only two emerge experimen-
tally. The asymmetric deformations of the methyl groups are ob-
served at 1486 and 1471 cmꢁ1 experimentally. In the present
case the bands observed at 1405, 1362 cmꢁ1 in the IR spectrum
and at 1410 in the Raman spectrum are assigned as the symmetric
deformations of the methyl group. The SDD calculations give these
modes at 1487, 1473, 1466, 1460, 1459, 1446 and 1395, 1369,
1364 cmꢁ1 as asymmetric and symmetric methyl deformations,
respectively, for the title compound. Most of the investigated mol-
ecules display the first methyl rock [24] in the region
1150 35 cmꢁ1. The other methyl rocking modes are expected in
the region [24] 1035 55, 990 50 and 925 30 cmꢁ1. The SDD
For 2-aminopyrazine-3-carboxylic acid [1] the pyrazine ring
stretching modes are observed at 1564, 1536, 1468, 1458, 1360,
1258, 1083 cmꢁ1 (Raman) and 1567, 1457, 1415, 1373, 1231,

208
T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
Table 2
Calculated (scaled) wavenumbers, IR, Raman bands and assignments of 5-tert-Butyl-N-(4-trifluoromethylphenyl)pyrazine-2-carboxamide.
B3LYP/6-31Gꢂ B3PW91/6-31Gꢂ
(cmꢁ1
IRI
92.21
B3LYP/SDD
(cmꢁ1
IR
Raman
(cmꢁ1
)
Assignmentsa
t
)
RA
t
(cmꢁ1
)
IRI
RA
t
)
IRI
RA
305.36
t
(cmꢁ1
)
t
3389
3156
3127
3123
3114
3108
3076
3031
3026
3005
2998
2997
2992
2932
2927
2922
1635
1613
276.24
42.56
116.69
39.08
115.07
89.88
46.42
90.16
10.41
260.82
49.22
46.87
15.85
330.46
3.67
3389
3158
3140
3133
3128
3120
3092
3054
3050
3032
3026
3024
3019
2937
2934
2929
1628
1615
121.56
5.47
312.49
51.44
110.25
31.88
105.12
70.13
38.58
80.19
10.07
227.16
34.87
37.70
14.72
389.98
3.23
3385
3149
3130
3123
3113
3105
3076
3038
3032
3014
3006
3005
2999
2927
2922
2917
1610
1598
109.64
5.29
3347
–
3343
3156
–
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
NH(99)
CH(97)
CH(99)Pz
CH(99)Pz
CH(98)
5.61
9.78
3.54
2.60
2.54
10.35
27.52
1.58
78.63
8.07
47.92
103.96
36.03
105.79
68.98
38.81
81.01
10.02
227.66
41.21
41.92
10.62
377.30
3.15
9.78
4.56
2.24
2.29
9.89
36.94
2.63
100.65
6.24
43.36
11.50
32.22
42.36
24.90
145.87
136.18
10.29
3.99
2.10
–
–
–
3105
3070
–
3031
–
3117
3117
3082
3062
–
–
3015
–
2.38
CH(97)
CH(99)
10.02
40.11
2.60
105.31
8.70
asCH3(94)
asCH3(92)
asCH3(100)
asCH3(72)
asCH3(99)
asCH3
sCH3(88)
sCH3(100)
sCH3(87)
CO(66)
–
–
40.05
7.92
51.27
7.37
–
2980
2934
–
2910
1613
1599
2984
–
–
2918
1617
1599
26.71
36.11
19.31
135.06
48.00
29.52
40.73
22.71
152.82
92.42
18.29
465.76
268.09
44.10
841.63
52.49
28.51
912.27
22.22
Ph(53),
CO(16)
Ph(72),
1585
1540
1527
1514
249.66
88.67
489.81
8.25
204.25
953.87
477.84
23.26
1587
1543
1524
1506
138.02
83.69
566.20
5.82
94.16
990.00
714.37
3.05
1572
1527
1510
1494
167.83
90.89
84.36
1004.88
597.96
0.97
1580
1531
–
1582
1523
–
dNH(16)
dCH3(20),
t
t
Pz(56)
Ph(61),
533.37
12.33
dNH(21)
dNH(57),
–
–
t
Ph(22)
dasCH3(76)
tPz(63)
dasCH3(88)
dasCH3(86)
dasCH3(89)
dasCH3(87)
dasCH3(85)
dCHPz(46),
1506
1495
1492
1486
1480
1478
1466
1454
48.45
4.46
9.38
13.17
0.50
1.55
7.04
72.78
27.70
24.22
23.84
24.34
6.25
1497
1489
1476
1468
1461
1461
1447
1447
99.27
52.45
15.19
14.70
8.16
2.48
61.10
1.97
71.38
2.13
1487
1481
1473
1466
1460
1459
1446
1440
97.63
18.26
14.38
14.51
0.90
4.79
0.24
61.45
7.39
54.20
17.96
14.86
14.11
20.09
4.86
1486
–
–
1471
–
–
–
–
–
–
–
–
–
–
–
18.14
15.10
19.69
14.63
42.10
5.90
0.02
54.61
60.12
54.01
1445
t
Pz(20)
dsCH3(91)
tPh(41),
1412
1410
4.56
109.27
3.35
70.22
1405
1395
91.14
8.46
48.32
10.23
1395
1395
106.37
6.44
54.22
12.42
1405
–
1410
–
dCH(36)
1386
1381
1336
1325
1309
1295
1293
1275
12.03
9.07
105.72
0.11
28.18
527.63
33.82
42.48
7.10
2.27
402.50
16.44
9.88
132.71
7.30
14.21
1368
1364
1353
1312
1303
1289
1282
1281
27.05
18.45
115.82
8.41
45.05
0.12
1.62
0.45
292.89
103.98
4.87
13.65
118.87
140.80
1369
1364
1334
1309
1291
1286
1270
1269
21.94
17.18
120.14
10.25
22.54
0.39
3.23
0.41
337.38
73.06
3.61
4.95
125.14
128.51
–
–
–
dsCH3(94)
dsCH3(96)
1362
1328
–
1293
–
1336
1312
1291
–
–
1270
t
t
t
t
Ph(65)
Ph(18), dCH(63)
Pz(66)
asCC3(57), tPz(17)
519.11
45.24
28.08
575.98
–
dCHPz(62)
tC2C27(47),
1268
dCH3(28)
dNH(48), tasCC3(37)
1261
1250
1211
1195
1193
1186
1152
1146
1105
1092
1084
1035
1031
1028
1006
18.85
65.65
6.21
364.74
342.25
32.40
10.36
124.40
0.25
1257
1246
1224
1199
1193
1186
1137
1130
1093
1039
1023
1019
1016
1003
997
16.14
62.96
15.27
2.46
30.37
36.50
59.43
38.09
4.64
194.89
103.59
1.56
2.81
140.86
54.95
147.85
417.35
68.74
16.22
1.98
116.04
32.23
12.11
29.97
1.93
2.92
5.83
10.89
20.32
11.62
1249
1237
1207
1188
1183
1177
1132
1126
1084
1026
1017
1015
1013
1002
989
0.68
63.15
9.52
280.22
399.36
48.01
16.76
144.33
0.61
33.47
19.37
29.97
1.85
5.74
3.79
11.19
2.72
25.56
1244
–
1213
1190
–
1162
1140
1120
1070
1026
–
1247
–
–
1192
–
–
1142
1124
1069
1026
–
t
t
C9N18(50), dCHPz(24)
Pz(53), dCH3(18)
2.41
3.09
dCH3(84)
dCH(73)
56.68
15.11
36.70
77.89
36.86
111.78
236.56
125.00
0.74
64.79
17.63
54.83
52.57
3.62
188.36
1.33
91.52
4.07
t
t
t
t
t
t
Pz(67)
CF3(53), dCH(32)
Pz(79)
CF3(16),
CF3(69)
CF3(71)
7.82
33.30
24.74
10.12
21.15
5.54
3.66
9.23
6.97
tPz(14), tC8N18(47)
–
–
–
–
–
–
–
987
dCH(46),
dCH3(72)
dCH3(67)
dPh(56),
tCF3(22), dCF3(16)
4.28
69.79
27.10
134.95
t
Ph(18)
999
993
960
957
956
924
924
905
44.42
1.16
1.60
0.22
0.39
4.67
1.42
0.33
2.76
1.37
2.01
0.11
1.89
0.33
11.07
11.94
987
982
969
967
944
934
928
916
15.81
179.29
1.72
7.25
0.24
8.31
3.14
1.25
4.52
2.99
0.39
1.84
0.01
0.28
8.36
9.50
982
976
967
966
943
930
920
906
15.80
201.00
2.12
28.86
0.14
9.77
2.46
1.04
3.98
5.71
0.47
4.77
0.01
0.31
9.82
11.12
–
979
–
958
–
927
–
–
–
–
962
–
934
–
904
dPz(41), tPz(29)
c
c
CH(85)
CHPz(81)
dCH3(95)
c
c
t
CH(82),
CHPz(68)
C12C22(49), dCH3(32)
sPh(11)
907
dCH3(32),
sCC3(52)
t

T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
209
Table 2 (continued)
B3LYP/6-31Gꢂ
B3PW91/6-31Gꢂ
B3LYP/SDD
IR
Raman
Assignmentsa
t
(cmꢁ1
)
IRI
21.41
111.17
63.56
4.83
1.90
1.43
27.33
17.78
1.73
0.01
8.75
0.32
4.63
5.15
14.79
3.65
RA
13.31
1.89
4.76
12.88
15.14
3.33
0.25
t
(cmꢁ1
)
IRI
RA
t
(cmꢁ1
)
IRI
RA
t
(cmꢁ1
)
t
(cmꢁ1
)
879
866
845
838
819
812
789
759
735
704
699
641
638
608
578
567
871
867
851
833
818
818
800
750
744
712
686
626
624
597
573
564
128.70
19.15
79.41
5.62
1.29
870
861
847
825
814
810
792
745
739
708
681
626
624
594
571
563
115.37
18.32
86.32
5.61
1.56
17.64
0.49
9.88
1.98
16.90
0.63
62.84
1.03
0.84
8.92
5.07
25.37
3.49
0.94
0.73
–
–
843
–
–
809
781
749
729
–
674
630
–
–
dNH(37), dCO(48)
16.87
0.39
8.75
9.35
10.91
0.81
59.13
1.09
0.76
8.32
5.32
24.03
2.92
0.88
0.72
859
842
–
c
c
c
t
CH(66)
CH(71),
NH(51),
CC(42),
c
t
t
NH(14)
CC(13)
C2C27(22)
CC(27)
Ph(48)
CC(29)
3.11
3.21
3.75
3.56
–
812
780
–
737
698
672
635
–
sPz(52), c
c
dPz(29), t
11.89
22.37
2.65
20.53
22.99
2.97
NH(37), t
48.32
1.80
s
c
t
Ph(65), dCN(13),
c
c
CC(11)
CC(17)
2.57
4.68
4.67
21.69
2.53
0.97
0.66
4.58
1.05
4.21
0.65
3.80
0.75
5.00
CO(43),
s
Pz(11),
CF3(42), dPh(28)
dPh(77)
dPz(78)
dCF3(32), dPh(31)
4.98
4.56
594
–
566
601
578
–
25.56
4.18
21.92
4.91
c
CN(22), Ph(19),
s
cCC(12)
0.65
s
Pz(37), cCC(34)
566
531
0.80
0.13
1.08
1.90
560
519
0.30
0.05
0.61
1.93
557
515
0.17
0.05
0.80
2.22
538
511
–
s
CO(46), dCC(25)
520
dCF3(45), dCC(22)
dasCC3(66)
dCF3(41),
dCH3(43), dCC(22)
s
s
504
486
449
425
415
397
392
382
376
350
331
322
301
285
279
263
252
236
196
195
160
130
125
1.03
1.02
2.34
18.20
0.16
2.71
19.43
1.65
0.51
16.00
1.84
1.04
0.02
0.45
5.70
5.60
0.50
0.15
1.38
0.95
0.80
0.91
2.95
9.32
1.14
1.41
0.13
0.09
1.39
4.35
2.59
1.35
0.48
1.40
3.94313
0.01
0.56
0.13
0.92
0.41
0.33
1.55
3.76
0.54
1.49
0.75
498
483
442
422
409
391
385
373
370
341
326
0.95
2.81
1.44
17.46
0.13
3.18
20.79
1.70
0.78
14.71
1.62
4.12
0.01
0.10
4.73
7.15
0.69
0.10
1.28
1.04
0.73
0.93
3.37
8.22
0.96
1.37
0.16
0.05
1.56
5.18
2.67
1.39
0.44
1.10
496
480
441
424
409
389
383
373
370
341
326
0.91
2.42
1.62
16.73
0.08
2.98
20.47
0.98
0.86
16.07
1.62
4.32
0.01
0.02
6.42
5.14
0.66
0.10
1.32
1.03
0.79
0.93
3.25
7.82
0.99
1.48
0.17
0.04
1.55
5.44
2.59
1.60
0.49
1.10
–
0.01
0.43
0.26
1.14
0.41
0.93
3.27
1.32
0.86
1.39
0.79
490
478
444
418
408
–
–
–
–
–
–
306
–
–
–
–
–
–
–
501
475
–
428
–
395
–
372
–
–
333
dCH3(33),
286
–
–
258
–
222
195
–
165
–
sPh(35)
Pz(72)
Ph(81)
dCF3(56),
dCF3(44), dCN(30)
dCF3(46), dCH3(22)
dCH3(29), Pz(24), dCF3(15)
dCO(23), dNH(22), dCF3(21)
dCH3(36), dasCC3(37)
dsCC3(44)
s
s
s
dCC(38),
s
s
dCC(36), dNH(11), dCF3(16)
dCC(32), CC(44)
dCC(39), dPh(32)
dCC(22),
dCC(17),
sPz(32)
s
1.32
298
313
1.16
290
0.01
0.44
0.11
1.26
0.36
0.78
3.58
1.05
0.80
1.39
0.76
CC3(81)
CC3(43), dCC(10)
CC(41), dCC(29)
280
271
256
249
225
191
189
157
130
121
278
270
256
248
219
191
190
157
129
122
s
CC(37)
Ph(33), dCF3(16),
CC(71)
sCC(23)
–
–
–
–
c
s
CC(21), cNH(17)
115
s
s
s
s
s
CO(47)
CN(57),
CC(30),
CN(38),
Pz(25)
86
67
54
2.62
0.35
0.59
0.43
0.20
0.95
84
68
53
2.78
0.49
0.87
0.31
0.36
0.42
84
68
53
2.97
0.39
0.83
0.31
0.38
0.45
–
–
–
87
–
–
c
CC(11)
s
CN(29), sPz(20)
46
35
20
14
0.06
1.27
0.01
0.13
0.31
6.27
0.03
0.32
44
32
20
13
0.06
1.07
0.02
0.21
0.29
4.59
0.02
0.77
44
32
21
10
0.05
1.10
0.02
0.23
0.30
4.57
0.02
0.85
–
–
–
–
–
–
–
–
dCCN(27), dNH(21), dCC(21)
s
s
s
CC(61)
CN(29), cNH(11), sCC(21)
CC(59), dCC(11)
a
% Of PED is given in the brackets with the assignments. Abbreviations-
t
-stretching; d-in-plane deformation;
c
-out-of-plane deformation;
s
-twisting; s-symmetric; as-
asymmetric; Ph-benzenering; Pz-pyrazinering; IRI – IR intensity; RA – Raman activity.
1068 cmꢁ1 (theoretical). For pyrazine, the ring stretching modes
are reported at 1485, 1411, 1128, 1065 (IR), 1579, 1522, 1015 (Ra-
man) and 1597, 1545, 1482, 1412, 1140, 1063, 1009 cmꢁ1 theoret-
ically [1]. For the title compound the pyrazine ring stretching
modes are observed at 1531, 1293, 1213, 1162 cmꢁ1 in the IR spec-
trum, 1523, 1291 cmꢁ1 in the Raman spectrum and at 1527, 1481,
1291, 1207, 1177 cmꢁ1 theoretically (SDD). The ring breathing
mode of the pyrazine ring is observed at 1120 cmꢁ1 in the IR spec-
trum, 1124 cmꢁ1 in the Raman spectrum and at 1126 cmꢁ1 theo-
retically. The ring breathing mode of pyrazine is reported at
1015 cmꢁ1 [1]. In the Raman spectrum of 2,6-dichloropyrazine
and 2-chloropyrazine the ring breathing mode is reported at
1131 and 1049 cmꢁ1 [1]. For the title compound the in-plane and
out-of-plane CH bending modes are assigned at 1440, 1270 and
967, 930 cmꢁ1 (SDD) theoretically. Corresponding bands observed
at 927 in the IR spectrum and at 1445, 934 cmꢁ1 in the Raman
spectrum which is in agreement with literature [1].
The benzene ring possesses six ring stretching modes, of which
the four with the highest wavenumbers (occurring near 1600,
1580, 1490 and 1440 cmꢁ1) are good group vibrations [24]. With
heavy substituents, the bands tend to shift to somewhat lower
wavenumbers. In the absence of ring conjugation, the band at
1580 cmꢁ1 is usually weaker than at 1600 cmꢁ1. In the case of
C@O substitution, the band near 1490 cmꢁ1 can be very weak.
The fifth ring stretching mode is active near 1315 65 cmꢁ1, a re-
gion that overlaps strongly with that of the CH in-plane deforma-
tion. The sixth ring stretching mode or the ring breathing mode,
appears as a weak band near 1000 cmꢁ1 in mono-, 1,3-di and

210
T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
1,3,5-trisubstitued benzenes [24]. In the otherwise substituted
benzenes, however, this mode is substituent sensitive and difficult
the bond lengths of the two CAC bonds, C2AC1 = 1.4185 and
0
C5AC4 = 1.4097 ÅA in the pyrazine ring and are also smaller than
0
to distinguish from the ring in-plane deformation [24]. The
t
Ph
that of the normal CAC single bond of 1.54 ÅA [40]. The CAN bond
0
modes are expected in the range 1280–1630 cmꢁ1 for para substi-
tuted phenyl rings [24] and the modes observed at 1599, 1580,
1328 cmꢁ1 in the IR spectrum, 1599, 1582, 1336 cmꢁ1 in the Ra-
man spectrum and at 1598, 1572, 1510, 1395, 1334 cmꢁ1 theoret-
lengths C8AN18 = 1.3797 and C9AN18 = 1.4109 ÅA are also shorter
0
than the normal CAN single bond of 1.49 ÅA, which confirms this
bond to have some character of a double or conjugated bond [41].
At N18 position, the angles C8AN18AH20 is 113.3°, C9AN18AH20
is 118.1° and C8AN18AC9 is 128.6°. This asymmetry of angles at
N18 position indicates the weakening of N18AH20 bond resulting
in proton transfer to the oxygen atom O19 [42]. The CCF angles
lie in the range 112.8–113.2° and the FCF angles in the range
105.2–106.7°, which are in agreement with reported literature
ically (SDD) are assigned as tPh modes. The ring breathing mode of
the para substituted benzenes with entirely different substituents
[33] has been reported in the interval 780–880 cmꢁ1. For the title
compound, this is confirmed by the band in the infrared spectrum
at 781 cmꢁ1 and at 792 cmꢁ1 theoretically, which finds support
from computational results. For para substituted benzene, the ring
breathing mode was reported at 804 and 792 cmꢁ1 experimentally
and at 782 and 795 cmꢁ1 theoretically [34,35].
[30]. The CF bond lengths are reported as 1.4068, 1.3284, 1.3251,
0
1.3284 ÅA theoretically [43,44] and for the title compound the CF
0
lengths are in the range 1.4042–1.4168 ÅA.
The CH in-plane deformation bands of the benzene ring are ex-
pected above 1000 cmꢁ1 [24] and in the present case, the bands
observed at 1140 cmꢁ1 in the IR spectrum, 1312, 1142 cmꢁ1 in
the Raman spectrum and at 1309, 1183, 1132, 1015 cmꢁ1 theoret-
ically are assigned as these modes. The out-of-plane CH deforma-
tions of the phenyl ring [24] are observed between 1000 and
700 cmꢁ1. Generally, the CH out-of-plane deformations with the
highest wavenumbers have weaker intensity than those absorbing
at lower wavenumbers. The CH out-of-plane vibrations are ob-
served at 979, 843 cmꢁ1 in the IR spectrum, 859, 842 cmꢁ1 in the
Raman spectrum and at 976, 943, 861, 847 cmꢁ1 theoretically.
Due to C(CH3) substitution in the pyrazine ring the C1AC2 bond
0
0
length (1.4185 ÅA) is greater than the C4AC5 (1.4097 ÅA) bond length.
The methyl susbtituent affects all the CN and CC bond lengths of
the pyrazine ring of the title compound in comparison with the
corresponding bonds of pyrazine [38]. At the C2 position of the pyr-
azine ring, there is asymmetry between the angles, C1AC2AC27
(124.3°) and N3AC2AC27(116.5°) due to steric hindrance between
H26 and the tBu group.
For benzamide derivatives, Noveron et al. [45] reported the
bond lengths C9AN18, C8AO19, C8AN18, C8AC5 and N18AH20 as
0
1.3953, 1.2253, 1.3703, 1.4943, 0.733 ÅA, whereas the correspond-
The strong
c
CH occurring at 840 50 cmꢁ1, is typical for 1,4-disub-
ing values for the title compound are 1.4109, 1.2586, 1.3797,
0
stitution, and the band observed at 843 cmꢁ1 in the IR spectrum is
assigned to this mode. The SDD calculations give this mode at
847 cmꢁ1. The substituent sensitive modes and other deformation
bands of the phenyl and pyrazine ring are also identified and as-
signed (Table 2). Most of the modes are not pure, but contain sig-
nificant contributions from other modes.
1.505, 1.021 ÅA. The C@O and CN bond lengths [46] in benzamide,
acetamide and formamide are respectively, 1.2253, 1.2203,
0
0
1.2123 ÅA and 1.3801, 1.3804, 1.3683 ÅA. According to literature
[47,48] the changes in bond lengths in C@O and CN are consistent
with the following interpretation: that is, hydrogen bond decreases
the double bond character of C@O bond and increases the double
bond character of CAN bond. At C9 position the angels C10AC9AN18
is increased by 3.0° and C14AC9AN18 is reduced by 2.6° from 120°
and this asymmetry reveals the interaction between the amide
moiety and the phenyl ring. The0 CC bond lengths in the phenyl ring
Geometrical parameters and first hyperppolarizability
To the best of our knowledge, no X-ray crystallographic data of
the title compound has yet been established. However, the theoret-
ical results obtained are almost comparable with the reported
structural parameters of similar molecules. In the case of 2-amino-
lie between 1.3982 and 1.4168 ÅA and the CH bond lengths between
0
1.0819 and 1.0877 ÅA. The CC bond length of benzene [49] is 1.3993
0
and benzaldehyde [50] 1.3973 ÅA.
pyrazine-3-carboxylic acid the bond lengths C5AC8, C8AO19
,
The first hyperpolarizability (b0) of this novel molecular system
is calculated using B3LYP/6-31Gꢂ method, based on the finite field
approach. In the presence of an applied electric field, the energy of
a system is a function of the electric field. First hyperpolarizability
is a third rank tensor that can be described by a 3 ꢃ 3 ꢃ 3 matrix.
The 27 components of the 3D matrix can be reduced to 10 compo-
nents due to the Kleinman symmetry [51]. The components of b
are defined as the coefficients in the Taylor series expansion of
the energy in the external electric field. When the electric field is
weak and homogeneous, this expansion becomes
0
C5AN6, C5AC4 are1.479, 1.212, 1.333, 1.4079 ÅA (XRD) and 1.492,
0
1.186, 1.315, 1.4092 ÅA (ab initio calculations) [12,36]. In the pres-
ent case, the corresponding values are1.505, 1.2586, 1.3581 and
1.4097. For pyrazine ring [37] the CN bond lengths are 1.339 and
0
1.331 ÅA. For 2-chloropyrazine [1] CN bond lengths are in the range
0
1.335–1.312 ÅA. For the title compound, the pyrazine bond lengths
C1AC2, C1AN6, C5AN6, C5AC4, C4AN3 and C2AN3 are 1.4185,
0
1.3522, 1.3581, 1.4097, 1.3509, 1.367 ÅA, respectively. For a similar
derivative Mary et al. [28] reported the corresponding values as
0
1.3917, 1.2996, 1.3229, 1.3840, 1.322 and 1.3116 ÅA. Endredi et al.
X
X
X
X
1
2
1
6
1
24
E ¼ E0 ꢁ
l
iFi ꢁ
a
ijFiFj ꢁ
bijkFiFjFk ꢁ
c
ijklFiFjFkFl þ...
[38] reported the bond lengths C2AC1, C1AN6, C5AN6, C5AC4 and
0
C4AN3 as 1.391, 1.331, 1.331, 1.331 and 1.331 ÅA for pyrazine, 1.4,
i
ij
ijk
ijkl
0
1.327, 1.333, 1.387 and 1.334 ÅA for 2-methylpyrazine, 1.41,
0
1.331, 1.33, 1.385 and 1.331 ÅA 0 for 2,3-dimthylpyrazine, 1.396,
where E0 is the energy of the unperturbed molecule, Fi is the field at
the origin, li ij, bijk and ijkl are the components of dipole moment,
1.327, 1.335, 1.396 and 1.335 ÅA for 2,5-dimethylpyrazine, and
,
a
c
0
1.399, 1.326, 1.332, 1.399 and 1.332 ÅA for 2,6-dimethylpyrazine.
polarizability, the first hyper polarizabilities, and second hyperpo-
For pyrazinamide, Chis et al. [39] reported bond lengths, C5AN6,
larizibilites, respectively. The calculated first hyperpolarizability of
C4AC5, C4AN3, C2AN3, C1AC2, C1AN6, C5AC8, C8AN18, C8AO19
,
the title compound is 3.97 ꢃ 10ꢁ30 esu. The CAN distances in the
0
C4AH7, N18AH20 as 1.341, 1.4, 1.337, 1.338, 1.397, 1.336, 1.510,
calculated molecular structure vary from 1.3509 to 1.4109 ÅA which
0
0
1.357, 1.226, 1.086, 1.010 ÅA and these results are in agreement
with the present study.
are intermediate betwee0n those of a CAN single bond (1.48 ÅA) and a
C@N double bond (1.28 ÅA). Therefore, the calculated data suggest an
The CN bond length in the pyrazine ring of the title compound
extended p-electron delocalization over the pyrazine ring and car-
C1AN6 = 1.3522, 0
C4AN3 = 1.3509 ÅA are much shorter than the normal CAN single
bond that is referred to 1.49 ÅA. The same results are shown for
C5AN6 = 1.3581,
C2AN3 = 1.3670
and
boxamide moiety [52] which is responsible for the nonlinearity of
the molecule. We conclude that the title compound is an attractive
object for future studies of non linear optical properties.
0

T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
211
In order to investigate the performance of vibrational wave-
numbers of the title compound, the root mean square (RMS) value
between the calculated and observed wavenumbers were calcu-
lated. The RMS values of wavenumbers were calculated using the
following expression [53].
sult in a loss of occupancy from the localized NBO of the idealized
Lewis structure into an empty non-Lewis orbital. For each donor (i)
and acceptor (j) the stabilization energy E(2) associated with the
delocalization i ? j is determined as
2
ðFi;jÞ
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Eð2Þ ¼ Eij ¼ qi
D
u
n
ðEj ꢁ EiÞ
X
u
t
1
2
RMS ¼
ð
tci alc
ꢁ
tei xpÞ :
n ꢁ 1
i
qi, donor orbital occupancy; Ei, Ej, diagonal elements; Fij, the off
diagonal NBO Fock matrix element.
The RMS error of the observed IR and Raman bands are found to
18.42, 16.36 for B3LYP/6-31Gꢂ, 15.38, 14.23 for B3PW91/6-31Gꢂ
and 9.25, 9.27 for B3LYP/SDD methods, respectively. The small dif-
ferences between experimental and calculated vibrational modes
are observed. This is due to the fact that experimental results be-
long to solid phase and theoretical calculations belong to gaseous
phase.
In NBO analysis large E(2) value shows the intensive interaction
between electron-donors and electron-acceptors and greater the
extent of conjugation of the whole system, the possible intensive
interactions are given in Table 3. The second-order perturbation
theory analysis of Fock matrix in NBO basis shows strong intra-
molecular hyper-conjugative interactions of
p electrons. The in-
tra-molecular hyper-conjugative interactions are formed by the
orbital overlap between n(C) and rꢂ(CAN) bond orbital which re-
sults in ICT causing stabilization of the system. The strong intra-
molecular hyper-conjugative interaction of C1AN6, N3AC4 from of
n1(C2) ? pꢂ (C1AN6) which increases ED (0.36808e) that weakens
NBO analysis
The natural bond orbitals (NBO) calculations were performed
using NBO 3.1 program [54] as implemented in the Gaussian 09
package at the DFT/B3LYP level in order to understand various sec-
ond-order interactions between the filled orbitals of one subsys-
the respective bonds leading to stabilization of 100.89 kcal molꢁ1
.
Also the strong intra-molecular hyper-conjugative interaction of
C1AN6, N3AC4 from of n1(C5) ? pꢂ (N3AC4) which increases ED
(0.32213e) that weakens the respective bonds leading to stabiliza-
tion of 84.99 kcal molꢁ1. Another intra-molecular hyper-conjuga-
tive interactions are formed by the orbital overlap between n(N)
and pꢂ (CAO) bond orbital which results in ICT causing stabiliza-
tion of the system. The strong intra-molecular hyper-conjugative
interaction of C8AO19 from of n1(N18) ? pꢂ (C8AO19) which in-
creases ED (0.32658e) that weakens the respective bonds leading
to stabilization of 67.86 kcal molꢁ1. These interactions are ob-
served as an increase in electron density (ED) in CAO anti-bonding
orbitals that weakens the respective bonds. The increased electron
density at the nitrogen, carbon atoms leads to the elongation of
tem and vacant orbitals of another subsystem, which is
a
measure of the intermolecular delocalization or hyper-conjuga-
tion. NBO analysis provides the most accurate possible natural Le-
wis structure picture of ‘j’ because all orbital details are
mathematically chosen to include the highest possible percentage
of the electron density. A useful aspect of the NBO method is that it
gives information about interactions of both filled and virtual orbi-
tal spaces that could enhance the analysis of intra and inter molec-
ular interactions.
The second-order Fock-matrix was carried out to evaluate the
donor–acceptor interactions in the NBO basis. The interactions re-
Table 3
Second-order perturbation theory analysis of Fock matrix in NBO basis corresponding to the intramolecular bonds of the title compound.
Donor (i)
Type
ED/e
Acceptor (j)
Type
ED/e
E(2)a
E(j)–E(i)b
F(i,j)c
C1AN6
C1AN6
N3AC4
N3AC4
C5AC8
C8AO19
C9AC10
r
r
r
p
r
p
p
p
p
r
r
r
r
n
r
n
n
n
n
n
n
n
n
n
n
n
n
n
1.98361
1.71948
1.98230
1.69101
1.96777
1.97049
1.60844
1.97477
1.66648
1.98027
1.98887
1.98890
0.89702
1.92760
0.98894
1.91986
1.63050
1.97563
1.88860
1.98877
1.96205
1.94289
1.98882
1.96241
1.94369
1.98904
1.96241
1.94334
C5AC8
C5
C2AC27
C2
C9AN18
C5
rꢂ
n
0.06866
0.98894
0.04709
0.89702
0.03234
0.98894
0.38796
0.03234
0.30629
0.01075
0.02398
0.02410
0.36808
0.04088
0.32213
0.04088
0.32658
0.06866
0.06856
0.05079
0.12193
0.10051
0.05079
0.12193
0.10037
0.05079
0.05079
0.10051
3.65
47.93
4.07
51.59
6.19
8.09
25.75
5.04
23.61
5.17
2.39
2.47
100.89
10.74
84.99
10.05
67.86
2.70
22.15
0.85
5.06
10.49
0.85
4.92
10.28
0.75
4.59
1.23
0.18
1.22
0.19
1.06
0.22
0.28
1.08
0.29
1.14
1.46
1.47
0.10
0.84
0.13
0.86
0.25
1.07
0.67
1.44
0.53
0.53
1.44
0.53
0.53
1.44
0.77
0.52
0.061
0.103
0.063
0.104
0.072
0.055
0.076
0.066
0.074
0.069
0.053
0.054
0.113
0.085
0.114
0.084
0.117
0.049
0.110
0.032
0.047
0.067
0.032
0.047
0.067
0.030
0.053
0.063
rꢂ
n
rꢂ
n
C
11AC12
C4AC5
13AC14
C8AO19
pꢂ
pꢂ
pꢂ
rꢂ
rꢂ
rꢂ
pꢂ
rꢂ
pꢂ
rꢂ
pꢂ
rꢂ
rꢂ
rꢂ
rꢂ
rꢂ
rꢂ
rꢂ
rꢂ
rꢂ
rꢂ
rꢂ
C
C
10AC11
11AC12
C
N18AH20
C
C
22AF23
22AF24
C
C
12AC13
11AC12
LP(1)C2
LP(1)N3
LP(1)C5
LP(1)N6
LP(1)N18
LP(1)O19
LP(2)O19
LP(1)F23
LP(2)F23
LP(3)F23
LP(1)F24
LP(2)F24
LP(3)F24
LP(1)F25
LP(2)F25
LP(3)F25
C1AN6
C4AC5
N3AC4
C4AC5
C8AO19
C5AC8
C8AN18
C
C
C
C
C
C
C
C
C
12AC22
22AF25
22AF24
12AC22
22AF25
22AF23
12AC22
12AC22
12AF24
9.23
a
b
c
E(2) means energy of hyper-conjugative interactions (stabilization energy in kcalmolꢁ1).
Energy difference between donor and acceptor i and j NBO orbitals in a.u.
F(i,j) is the Fock matrix elements between I and j NBO orbitals in a.u.

212
T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
respective bond length and a lowering of the corresponding
stretching wavenumber. The electron density (ED) is transferred
from the n(C), n(N) to the anti-bonding pꢂ orbital of the CAN,
CAO explaining both the elongation and the red shift[55]. The
NAH, AC@O stretching modes can be used as a good probe for
evaluating the bonding configuration around the corresponding
atoms and the electronic distribution of the molecule. Hence the
above structure is stabilized by these orbital interactions.
The NBO analysis also describes the bonding in terms of the nat-
ural hybrid orbital n2(F23), which occupy a higher energy orbital
ꢁ0.43558a.u.) with a considerable p-character (100.0%) and low
occupation number (1.96205a.u.) and the other n1(F23) occupy a
lower energy orbital (ꢁ1.10382) with p-character (21.61%) and
high occupation number (1.98877a.u.).Also n2(F24), which occupy
a higher energy orbital (ꢁ0.43640a.u.) with considerable p-charac-
ter (99.99%) and low occupation number (1.96241a.u.) and the
other n1(F24) occupy a lower energy orbital (ꢁ1.10464a.u.) with
p-character (21.60%) and high occupation number (1.98882a.u.).
Again n2(F25), which occupy
a
higher energy orbital
(ꢁ0.42919a.u.) with considerable p-character (99.73%) and low
occupation number (1.96241a.u.) and the other n1(F25) occupy a
lower energy orbital (ꢁ1.10506a.u.) with p-character (20.74%)
and high occupation number (1.98904a.u.). Thus, a very close to
pure p-type lone pair orbital participates in the electron donation
to the rꢂ(CAC) orbital for n(F) ? r ꢂ(CAC) interactions in the com-
pound. The results are tabulated in Table 4.
Table 4
NBO results showing the formation of Lewis and non-Lewis orbitals.
Bond (AAB)
ED/energya
EDA (%)
EDB (%)
NBO
s (%)
p (%)
r
–
r
–
r
–
p
–
r
–
p
–
p
–
p
–
p
–
r
–
r
–
r
–
C1AN6
C1AN6
N3AC4
N3AC4
C5AC8
C8AO19
C9AC10
1.98361
ꢁ0.86634
1.71948
39.03
–
42.38
–
60.51
–
58.88
–
51.60
–
30.84
–
49.40
–
49.79
–
43.86
–
73.19
–
26.65
–
26.60
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
60.97
–
57.62
–
39.49
–
41.12
–
48.40
–
69.16
–
50.60
–
50.21
–
56.14
–
26.81
–
73.35
–
73.40
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.6247(sp2.27)C
30.55
36.07
0.00
69.45
63.93
100.0
100.0
65.62
68.75
100.0
100.0
67.50
65.26
100.0
100.0
100.0
100.0
64.36
64.21
100.0
100.0
72.30
0.00
79.29
78.41
79.39
78.43
100.0
–
70.78
–
100.0
–
+0.7041(sp1.00)N
0.6510(sp1.00)C
ꢁ0.34340
+0.7591(sp1.00)N
0.00
1.98230
0.7779(sp1.91)N
34.38
31.25
0.00
ꢁ0.85429
1.69101
+0.6284(sp2.20)C
0.7673(sp1.00)N
ꢁ0.33309
+0.6413(sp1.00)C
0.00
1.96777
0.7183(sp2.08)C
32.50
34.74
0.00
0.00
0.00
ꢁ0.68758
1.97049
0.6957(sp1.88)C
0.5553(sp1.00)C
ꢁ0.38216
+0.8316(sp1.00)O
1.60844
0.7029(sp1.00)C
ꢁ0.27101
1.97477
+0.7113(sp1.00)C
0.00
C
C
10AC11
11AC12
0.7057(sp1.81)C
35.64
35.79
0.00
ꢁ0.70744
+0.7086(sp1.79)C
1.66648
0.6623(sp1.00)C
ꢁ0.27614
1.98027
+0.7492(sp1.00)C
0.00
N
18AH20
0.8555(sp2.61)N
27.70
100.0
20.71
21.59
20.61
21.57
0.00
ꢁ0.67744
+0.5178(sp1.95)H
C
C
22AF23
23AF24
1.98887
0. 5163(sp3.83)C
ꢁ0.95314
1.98890
+0.8564(sp3.63)F
0.5157(sp3.85)C
ꢁ0.95198
+0.8568(sp3.64)F
n1C2
–
n1N3
–
n1C5
–
n1N6
–
n1N18
–
n1O19
–
n2O19
–
n1F23
–
n2F23
–
n3F23
–
n1F24
–
n2F24
–
n3F24
–
n1F25
–
n2F25
–
n3F25
–
0.89702
sp1.00
–
ꢁ0.14796
1.92760
sp2.42
–
29.22
–
0.00
–
28.94
–
0.00
–
64.72
–
0.00
–
78.39
–
0.00
–
0.02
–
78.40
–
0.01
–
0.02
–
79.26
–
0.27
–
0.00
–
ꢁ0.35960
0.98894
sp1.00
–
ꢁ0.16247
1.91986
sp2.46
–
71.06
ꢁ0.38400
1.63050
sp1.00
–
100.0
–
35.28
–
100.0
–
21.61
–
100.0
–
99.98
–
21.60
–
99.99
–
99.98
–
20.74
–
99.73
–
100.0
–
ꢁ0.28655
1.97563
sp0.55
–
ꢁ0.70584
1.88860
sp1.00
–
ꢁ0.27747
1.98877
sp0.28
–
ꢁ1.10382
1.96205
sp1.00
–
ꢁ0.43558
1.94289
sp99.99
–
ꢁ0.43496
1.98882
sp0.28
–
ꢁ1.10464
1.96241
sp99.99
–
ꢁ0.43640
1.94369
sp99.99
–
ꢁ0.43561
1.98904
–
–
–
–
–
–
–
–
–
–
–
–
sp0.26
–
ꢁ1.10506
1.96241
sp99.99
–
ꢁ0.42919
1.94334
ꢁ0.42681
sp1.00
–
a
ED/energy is expressed in a.u.

T. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 203–214
213
Frontier molecular orbitals
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 ꢁ1145.4 Hartree and above and below this torsion
angle, the energy rises.
The analysis of the wavefunction indicates that the electron
absorption corresponds to a transition from the ground to the first
excited state and is mainly described by one electron excitation
from the HOMO to LUMO. Both the HOMO and the LUMO are the
main orbital taking part in chemical reaction. The HOMO energy
characterizes the capability of electron giving; LUMO characterizes
the capability of electron accepting [56]. The frontier orbital gap
helps to characterize the chemical reactivity, optical polarizability
and chemical hardness–softness of a molecule [57]. Surfaces for
the frontier orbitals were drawn to understand the bonding
scheme of the title compound. Two important molecular orbitals
(MO) were examined for the title compound, the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) which are given in Figs. 4 and 5. The calculated
HOMO and LUMO energies are ꢁ8.486 and ꢁ5.21 eV. The chemical
hardness and softness of a molecule is a good indication of the
chemical stability of the molecule. From the HOMO–LUMO energy
gap, one can find whether the molecule is hard of soft. The mole-
cules having large energy gap are known as hard and molecules
having a small energy gap are known as soft molecules. The soft
molecules are more polarizable than the hard ones because they
need small energy to excitation. The hardness value of a molecule
Conclusion
The FT-IR and FT-Raman spectra of 5-tert-Butyl-N-(4-trifluoro-
methylphenyl)pyrazine-2-carboxamide were studied. The molecu-
lar geometry and wavenumbers were calculated using DFT
methods and the optimized geometrical parameters (SDD) are in
agreement with that of similar derivatives. The small differences
between experimental and calculated vibrational modes are ob-
served. This is due to the fact that experimental results belong to
solid phase and theoretical calculations belong to gaseous phase.
The calculated first hyperpolarizability is high and the title com-
pound is an attractive object for future studies of non linear optics.
The stability of the molecule arising from hyper-conjugative inter-
action and charge delocalization has been analyzed using NBO
analysis. From the NBO analysis it is evident that the increased
electron density at the nitrogen, carbon atoms leads to the elonga-
tion of respective bond length and a lowering of the corresponding
stretching wavenumber.
can be determined as
of the title molecule is 1.638 eV. Hence we conclude that the title
g
= (ꢁHOMO + LUMO)/2 [56]. The value of
g
Acknowledgment
compound belongs to hard material.
Hema Tresa Varghese would like to thank University Grants
Commission, India for a research grant.
PES scan study
Appendix A. Supplementary material
A detailed potential energy surface (PES) scan on dihedral angle
C5AC8AN18AH20 have been performed at B3LYP/SDD level to re-
veal the possible conformations of the title compound. The PES
scan was carried out by minimizing the potential energy in all geo-
metrical parameters by changing the torsion angle at every 20°
from ꢁ180° to +180° rotation around the bond. The result obtained
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.04.101.
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