Indian Journal of Pure & Applied Physics
Vol. 54, April 2016, pp. 269-278
FTIR, FT-Raman spectra and DFT analysis of 3-methyl-4-nitrophenol
S Jeyavijayan*
Department of Physics, Kalasalingam University, Krishnankoil 625126, India
Received 9 August 2014; revised 25 November 2014; accepted 20 May 2015
The FTIR and FT-Raman spectra of 3-methyl-4-nitrophenol (MNP) have been recorded in the regions 4000-400 cm-1 and
3500-50 cm-1, respectively. Utilizing the observed FTIR and FT-Raman data, a complete vibrational assignment and
analysis of the fundamental modes of the compound have been carried out. The optimum molecular geometry, harmonic
vibrational frequencies, infrared intensities and Raman scattering activities, have been calculated by density functional
theory (DFT/B3LYP) method using 6-31+G(d,p) and 6-311++G(d,p) basis sets. The difference between the observed and
scaled wavenumber values of most of the fundamentals is very small. A detailed interpretation of the infrared and Raman
spectra of MNP is also reported based on total energy distribution (TED). The calculated HOMO and LUMO energies show
that the charge transfers occur within the molecule.
Keywords: FTIR, FT-Raman, DFT calculations, 3-methyl-4-nitrophenol
1 Introduction
Phenol, also known as carbolic acid, is an organic
compound. It is produced on a large scale as a
precursor to many materials and useful compounds.
Phenol is also a versatile precursor to a large
collection of drugs, most notably aspirin but also
many herbicides and pharmaceuticals. It is used in the
preparation of cosmetics including sunscreens, hair
dyes and skin lightening preparations. Unlike normal
alcohols, phenols are acidic because of the influence
of the aromatic ring1. Phenol and its vapours are
corrosive to the eyes, the skin and the respiratory
tract. Phenol derivatives are interesting molecules for
theoretical studies due to their relatively small size
and similarity to biological species. In recent years,
phenol and substituted phenol have been the frequent
subjects of experimental and theoretical work because
of their significance in industry and environment.
Evans2 has extensively studied the vibrational
assignments of infrared spectrum of phenol. Huang et al.3
have elucidated halogen effect and isotope effect of
chloro phenol and Wang et al.4 have obtained the
vibrational analysis on nitro phenols. More recently,
the vibrational spectroscopy investigation using
ab-initio (HF) and DFT (B3LYP) calculations on the
structure of 3-bromo phenol have been studied by
Mahadevan et al.5 During the course of investigation
on the samples of biological and pharmaceutical
——————
*Corresponding author (E-mail: [email protected])
active compounds, the attention has been turned
towards 3-methyl-4-nitrophenol (MNP). The title
compound 3-methyl-4-nitrophenol (MNP) is one
of the most important organic intermediates, is
widely used for the manufacture of pesticides,
rubber, drugs, varnishes and dyestuffs. The
assignment of the vibrational frequencies for
substituted phenols becomes complicated problem
because of the superposition of perhaps several
vibrations due to fundamentals and substituents.
However, a comparison of the spectra with that of the
parent compound gives some definite clues about the
nature of the molecular vibrations.
The vibrational spectra of the molecule have been
studied completely and the various normal modes
with greater wave numbers accurately have been
identified. In the present study, the density functional
theory (DFT) calculations using 6-31+G(d,p) and
6-311++G(d,p) basis sets have been performed to
support the wave number assignments.
2 Experimental Details
The pure sample of MNP was obtained from
Lancaster Chemical Company, UK and used as such
for the spectral measurements. The room temperature
Fourier transform infrared spectra of the title
-1
compound was recorded in the region 4000-400 cm
-1
at a resolution of + 1 cm using BRUKER IFS 66V
model FTIR spectrometer equipped with an MCT
detector, a KBr beam splitter and globar source.
270
INDIAN J PURE & APPL PHYS, VOL 54, APRIL 2016
The FT-Raman spectrum of MNP was recorded on
a computer interfaced BRUKER IFS 66V model
interferometer equipped with FRA-106 FT-Raman
accessories. The spectrum was measured in the Stokes
region 3500-50 cm-1 using Nd: YAG laser operating
at 200 mW power continuously with 1064 nm
excitation. The reported wave numbers are expected
to be accurate within +1 cm-1.
3 Computational Details
In order to provide information with regard to the
structural characteristics and the normal vibrational
modes of MNP, the DFT-B3LYP correlation
functional calculations have been carried out. The
molecular geometry optimizations, energy and
vibrational frequency calculations were carried out for
MNP with the GAUSSIAN 09W software package6.
The geometry generated from the standard
geometrical parameters was minimized without any
constraint on the potential energy surface at DFT
level employing the Becke 3LYP keyword, which
invokes Becke’s three-parameter hybrid method7
using the correlation function of Lee et al.8,
implemented with 6-31+G(d,p) and 6-311++G(d,p)
basis sets. The optimized structural parameters were
used for the vibrational frequency calculations at DFT
level to characterize all the stationary points as
minima. The multiple scaling of the force constants
were performed according to SQM procedure9,10 using
selective scaling in the natural internal coordinate
representation11,12. The transformation of force field,
subsequent normal coordinate analysis and calculation
of the TED were done on a PC with the MOLVIB
program (version V7.0-G77) written by Sundius13,14.
By the use of GAUSSVIEW molecular visualization
program15 along with available related molecules, the
vibrational frequency assignments were made by their
TED with a high degree of confidence. The TED
elements provide a measure of each internal
coordinate’s contributions to the normal coordinate.
4 Results and Discussion
4.1 Molecular geometry
The optimized molecular structure of MNP is
shown in Fig. 1. The global minimum energy
obtained by the DFT structure optimization using 631+G(d,p) and 6-311++G(d,p) basis sets for MNP are
calculated as -551.32293 Hartrees and -551.44823
Hartrees, respectively. The optimization geometrical
parameters of MNP obtained by the DFT/B3LYP
method with 6-31+G(d,p) and 6-311++G(d,p) basis
Fig.1—Molecular structure of 3-methyl-4-nitrophenol
sets are listed in Table 1. Comparing bond angles
and bond lengths of B3LYP/6-31+G(d,p) and
6-311++G(d,p) basis sets for MNP, it is observed that
the geometrical parameters are found to be almost
same at B3LYP/6-31+G(d,p) and B3LYP/6311++G(d,p) levels. However, the B3LYP/631+G(d,p) level of theory, in general, slightly over
estimates bond lengths but it yields bond angles in
excellent agreement with the B3LYP/6-311++G(d,p)
method. The calculated geometrical parameters are
the bases for calculating other parameters such as
vibrational
frequencies
and
thermodynamics
properties of the compound.
According to the calculation (B3LYP/6-31+(d,p)),
the order of the bond length is C5-C6 < C1-C2 < C1C6 = C2-C3 < C4-C5 < C3-C4. From the order of the
bond length, it is clear that the hexagonal structure of
the benzene ring is slightly distorted. This can be due
to the influence of conjugation between the
substituents and the ring. The ring carbon atoms in
substituted benzenes exerts a larger attraction on the
valence electron cloud of the hydrogen atom resulting
in an increase in the C-H force constants and a
decrease in the corresponding bond length. It is
evident from the C–H bond lengths in MNP, vary
from 1.082 to 1.087Å by B3LYP/6-31+(d,p) method.
The benzene ring appears to be a little distorted
because of the NO2 group and methyl group
substitutions as seen from the bond angles C3–C4–C5
and C2–C3–C4, which are calculated as 121.58° and
116.29°, respectively, by B3LYP/6-31+(d,p) method
and differ from their typical hexagonal angle of 120°.
JEYAVIJAYAN: FTIR, FT-RAMAN SPECTRA AND DFT ANALYSIS OF 3-METHYL-4-NITROPHENOL
271
Table 1—Optimized geometrical parameters of 3-methyl-4-nitrophenol obtained by DFT-B3LYP
method using 6-31+G(d,p) and 6-311++G(d,p) basis sets
Bond
length
Value (Å)
Bond angle
B3LYP/ 6- B3LYP/ 631+G(d,p) 311++G(d,p)
Value (°)
B3LYP/ 631+G(d,p)
B3LYP/ 6311++G(d,p)
Dihedral Angle
Value (°)
B3LYP/ 631+G(d,p)
B3LYP/ 6311++G(d,p)
C1–C2
1.398
1.395
C2–C1–C6
120.19
120.18
C6–C1–C2–C3
-0.24
-0.72
C1–C6
1.400
1.397
C2–C1–O7
122.46
122.45
C6–C1–C2–H9
179.68
179.04
C1–O7
1.360
1.359
C6–C1–O7
117.33
117.36
O7–C1–C2–C3
179.81
179.46
C2–C3
1.400
1.397
C1–C2–C3
122.34
122.25
O7–C1–C2–H9
-0.25
-0.76
C2–H9
1.087
1.086
C1–C2–H9
119.18
119.19
C2–C1–C6–C5
-0.04
-0.08
C3–C4
1.413
1.408
C3–C2–H9
118.47
118.54
C2–C1–C6–H18
-179.99
-179.93
C3–C10
1.509
1.508
C2–C3–C4
116.29
116.31
O7–C1–C6–C5
179.90
179.73
C4–C5
1.402
1.398
C2–C3–C10
117.95
118.24
O7–C1–C6–H18
-0.05
-0.11
C4–N14
1.465
1.470
C4–C3–C10
125.74
125.43
C2–C1–O7–H8
-0.13
-0.43
C5–C6
1.384
1.381
C3–C4–C5
121.58
121.73
C6–C1–O7–H8
179.91
179.75
C5–H17
1.082
1.081
C3–C4–N14
122.16
121.94
C1–C2–C3–C4
0.23
0.64
C6–H18
1.084
1.082
C5–C4–N14
116.24
116.32
C1–C2–C3–C10
179.87
179.71
C7–H8
0.966
0.963
C4–C5–C6
120.83
120.70
H9–C2–C3– C4
-179.69
-179.13
C10–H11
1.093
1.091
C4–C5–H17
118.43
118.50
H9–C2–C3–C10
-0.05
-0.05
C10–H12
1.092
1.091
C6–C5–H17
120.73
120.79
C2–C3–C4–C5
0.05
0.21
C10–H13
1.092
1.091
C1–C6–C5
118.75
118.80
C2–C3–C4–N14
179.97
179.95
N14–O15
1.235
1.227
C1–C6–H18
119.75
119.69
C10–C3–C4–C5
-179.56
-178.78
N14–O16
1.235
1.228
C5–C6–H18
121.49
121.50
C10–C3–C4–N14
0.36
0.96
C1–O7–H8
110.57
110.31
C2–C3–C10–H11
-2.50
-7.91
C3–C10–H11
109.66
109.70
C2–C3–C10–H12
118.22
112.64
For numbering of atoms refer Fig. 1
C3–C10–H12
111.42
111.17
C2–C3–C10–H13
-123.32
-128.76
C3–C10–H13
111.64
111.81
C4–C3–C10–H11
177.10
171.06
H11–C10–H12
108.96
108.93
C4–C3–C10–H12
-62.17
-68.37
H11–C10–H13
108.92
108.84
C4–C3–C10–H13
56.28
50.21
H12–C10–H13
106.11
106.26
C3–C4–C5–C6
-0.33
-1.01
C4–N14–O15
117.98
117.79
C3–C4–C5–H17
179.79
179.26
C4–N14–O16
118.60
118.34
N14–C4–C5–C6
179.74
179.23
O15–N14–O16
123.41
123.85
N14–C4–C5–H17
-0.12
-0.49
C3–C4–N14–O15
-174.57
-163.23
C3–C4–N14–O16
5.58
17.32
C5–C4–N14–O15
5.34
16.51
C5–C4–N14–O16
-174.48
-162.91
C4–C5–C6–C1
0.32
0.92
C4–C5–C6–H18
-179.71
-179.21
H17–C5–C6–C1
-179.80
-179.35
H17–C5–C6–H18
0.15
0.49
INDIAN J PURE & APPL PHYS, VOL 54, APRIL 2016
272
Detailed description of vibrational modes can be
given by means of normal coordinate analysis. For
this purpose, the full set of 61 standard internal
coordinates (containing 13 redundancies) for the title
compound are defined in Table 2. From these, a
non-redundant set of local symmetry coordinates was
constructed by suitable linear combinations of internal
coordinates following the recommendations of
Fogarasi et al.11,12 and are summarized in Table 3.
4.2 Vibrational assignments
From the structural point of view, the molecule is
assumed to have C1 point group symmetry and hence,
all the calculated frequency transforming to the same
symmetry species (A). The molecule consists of
18 atoms and expected to have 48 normal modes of
Table 3—Definition of local symmetry coordinates of
3-methyl-4-nitrophenol
Table 2—Definition of internal coordinates of 3-methyl-4nitrophenol
No
Symbol
Type
Pi
PI
Ri
14
15
16
17,18
Ui
Ti
Qi
Vi
CH
C2–H9, C5–H17, C6–H18
CH (methyl) C10–H11, C10–H12, C10–H13
CC
C1–C2, C2–C3, C3–C4, C4–C5,
C6–C1, C3–C10
CO
C1–O7
CN
C4–N14
OH
O7–H18
NO (nitro)
N14–O16, N14–O15
In-plane bending
19–24
i
25–27
i
28–30
i
31,32
33
34,35
36,37
38,39
40
41–46
i
i
is
i
i
i
i
CH
C3–C2–H9, C1–C2–H9, C1–C6–
H18, C5–C6–H18, C4–C5–H17,
C6–C5–H17
CCH
C3–C10–H11, C3–C10–H12, C3–
(methyl)
C10–H13
HCH
H11–C10– H12, H11–C10–H13,
H12–C10–H13
CNO
C4–N14–O16, C4–N14–O15
ONO
O14–N14–O15
CCC
C4–C3–C10, C2–C3–C10
CCN
C5–C4–N14, C3–C4–N14
CCO
C6–C1–O7, C2–C1–O7
COH
C1–O7–H8
Ring [CCC] C1–C2–C3, C2–C3–C4, C3–C4–
C5, C4–C5–C6, C5–C6–C1, C6–
C1–C2
i
50–55
i
56
57
58
59
60
i
i
i
i
i
61
i
Definitiona
P1, P2, P3
(P4 + P5 + P6) / 3 , (2P4 + P5 + P6) / 6 ,
(P4 – P2) 2
7–13
14
15
16
17
CC
CO
CN
OH
NO2ass
R7, R8, R9, R10, R11, R12, R13
U14
T15
Q16
(V17 + V18) / 2
18
NO2ass
19
Rtrigd
H9–C2–C1–C3, H18–C6–C1–C5,
H17–C5–C6–C4
Ring [CCC] C1–C2–C3–C4, C2–C3–C4–C5,
C3–C4–C5–C6, C4–C5–C6–C1,
C5–C6–C1–C2, C6–C1–C2–C3,
CN
N14–C4–C3–C5
CC
C10–C3–C2–C4
CO
O7–C1–C2–C6
OH
H8–O7–C1–(C2, C6)
t CH3
(H11, H12, H13)–C10–C3–(C2 –
C4)
t NO2
(O15, O16)–N14–C4–(C3, C5)
(V17 - V18) /
2
20
Rsymd
( 19 -  20 +  21 -  22 +  23 -  24)/ 6
(- 19 –  20 + 2  21 -  22 -  23 + 2  24)/ 12
21
22-24
Rasymd
bCH
( 19 -  20 +  22 -  23) / 2
(25 - 26) / 2 , (27 - 28) / 2 , (29 - 30) /
2
Out-of-plane bending
47–49
Type
CH
CH (methyl)
Definition
Stretching
1–3
4–6
7–13
No
1–3
4–6
CH
a
25
CH3sb
26
CH3ipb
27
CH3opb
(-33 - 34 - 235) / 6
(33 - 34) / 2
28
CH3ipr
(231 - 32 - 33) /
29
CH3opr
30
NO2rock
(32 - 33) / 2
(37 - 38) / 2
31
NO2twist
(37 + 38) /
32
NO2sciss
33
bCN
34
bCC
35
bCO
36
37–39
40
41
42
43
44
bOH
CH
CN
CC
CO
OH
tRtrigd
45
tRsymd
46
tRasymd
47
48
tCH3
tNO2
(31 - 32 - 33 + 34 + 35 + 36) /
6
6
2
(239 - 38 - 37) / 2
(40 - 41) / 2
(42 - 43) / 2
(44 -  45) / 2
46
47, 48, 49
50
51
52
53
(54 - 55 - 56 +  57 - 58 + 59) /
6
(54 - 56 - 57 - 59) / 2
(-66 + 267 - 68 - 69 + 270 - 71) 12
72
73
The internal coordinates used here are defined in Table 2
JEYAVIJAYAN: FTIR, FT-RAMAN SPECTRA AND DFT ANALYSIS OF 3-METHYL-4-NITROPHENOL
273
Table 4—Observed FTIR, FT Raman and calculated (unscaled and scaled) frequencies (cm-1), IR intensity (km mol-1), Raman Activity
(Å4 amu-1), reduced masses (amu) and and force constant (m dyne Å-1) and probable assignments (characterized by TED) of 3-methyl-4nitrophenol using B3LYP method (contd.)
Calculated frequencies (cm-1)
Species Observed wave
C1
number (cm-1)
FTIR
TED%
among
B3LYP/6-31+G(d,p)
B3LYP/6-311++G(d,p)
types of
FT UnscaledScaled Reduce Force
IR Raman UnscaledScaled Reduced Force
IR Raman coordinat
es
Raman
d mass Constants intensity activity
mass Constants intensity activity
A
3396(s) 3392(vw) 3819
3567 1.06
9.16
87.84 162.17 3827
3574
1.06
9.20
91.42 152.41
A
3091(ms) 3090(vw) 3246
3032 1.09
6.78
3.53
3223
3010
1.09
6.69
3.00
91.17
A
2991(ms)
-
3221
3008 1.09
6.67
1.22 103.65 3203
2992
1.09
6.59
0.90
93.46
A
2946(w)
-
3171
2962 1.09
6.45
15.75 100.48 3154
2946
1.09
6.38
14.11 98.22
A
2902(vw)
-
3133
2926 1.10
6.38
17.65 61.00
3117
2911
1.10
6.31
16.26 55.68
A
-
2892(vw) 3130
2923 1.10
6.35
4.42
3110
2905
1.09
6.27
5.04
A
2836(vw)
3058
2856 1.03
5.71
16.82 180.27 3047
2846
1.03
5.66
14.36 190.23
A
1604(ms) 1604(ms) 1656
1607 6.96
11.26
175.61 46.48
1648
1599
6.94
11.13
152.30 39.06
-
-
84.42
66.38
65.86
A
1590(s)
1639
1590 6.20
9.82
138.66 46.21
1631
1582
5.96
9.35
128.36 51.42
A
1519(s) 1525(vw) 1583
1536 9.39
13.88
181.67 16.11
1570
1523
9.13
13.36
213.04 14.02
A
1479(s) 1482(w)
1525
1480 2.81
3.86
34.01
8.76
1520
1474
2.76
3.76
38.49
8.62
A
1460(s)
-
1500
1455 1.39
1.85
37.06
1.43
1496
1451
1.36
1.80
36.01
1.29
A
1433(w)
-
1479
1435 1.04
1.34
8.04
8.57
1476
1431
1.06
1.37
8.45
7.17
A
1424(s)
-
1456
1413 2.15
2.69
38.12
4.52
1452
1408
2.08
2.58
41.21
4.61
A
1385(vs) 1380(vs)
1425
1383 1.22
1.47
2.89
15.65
1421
1378
1.21
1.45
4.30
13.04
1356(vw) 1375
1334 6.94
7.74
207.80 150.95 1362
1321
8.55
9.35
301.67 224.04
A
-
A
1322(s)
1368
1327 7.93
8.75
144.38 138.17 1357
1316
6.27
6.80
28.75 32.49
A
1261(s) 1260(ms) 1330
1290 2.50
2.60
108.45 63.06
1325
1285
2.28
2.36
82.47 53.22
-
A
1248(ms)
1290
1251 1.84
1.80
182.81 22.50
1284
1245
2.03
1.97
196.85 29.29
A
1200(vw) 1196(ms) 1197
1161 1.45
1.23
67.38 18.55
1197
1161
1.35
1.14
146.48 18.27
1190
1155 1.59
1.33
130.05 1.09
1190
1154
1.80
1.50
52.90
1.28
1122(vs) 1125(vw) 1170
1135 1.62
1.30
1166
1131
1.61
1.29
6.65
14.27
A
A
-
-
1165(w)
9.86
14.83
OH
(99)
CH
(98)
CH
(97)
CH
(95)
CH3ss
(92)
CH3ips
(91)
CH3ops
(93)
NO2ass
(90)
CC
(89)
CC
(87)
CC
(88)
CC
(86)
CC
(85)
CH3ipb
(82)
NO2ss
(81)
CC
(80)
CH3sb
(83)
CC
(80)
CN
(78)
CO
(76)
bOH
(72)
bCH
(73)
(contd.)
INDIAN J PURE & APPL PHYS, VOL 54, APRIL 2016
274
Table 4—Observed FTIR, FT Raman and calculated (unscaled and scaled) frequencies (cm-1), IR intensity (km mol-1), Raman Activity
(Å4 amu-1), reduced masses (amu) and and force constant (m dyne Å-1) and probable assignments (characterized by TED) of 3-methyl-4nitrophenol using B3LYP method
Species Observed wave
C1
number (cm-1)
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
Calculated frequencies (cm-1)
TED%
among
B3LYP/6-31+G(d,p)
B3LYP/6-311++G(d,p)
types of
FTIR
FT UnscaledScaled Reduce Force
IR Raman UnscaledScaled Reduced Force
IR Raman coordinat
Raman
mass Constants intensity activity
d mass Constants intensity activity
es
1074(ms) 1093 1061 3.65
2.57
67.44 39.22 1092 1059 3.66
2.57
66.03 41.31 bCH
(70)
1022(w) 1056 1025 1.51
0.99
2.96 0.08 1054 1022 1.52
1.00
2.95 0.55 CH3opb
(71)
1010(ms)
1036 1005 1.52
0.96
3.04 8.70 1034 1003 1.52
0.96
2.62 7.89
bCH
(70)
963(w)
986
957 1.30
0.75
0.41 0.20
982
953
1.39
0.79
0.73 1.25 CH3opr
(69)
952(ms)
975
946 3.23
1.81
13.90 11.95 970
941
2.89
1.60
15.87 10.02 CH3ipr
(71)
838(ms) 856
831 1.63
0.70
44.67 0.00
857
831
9.07
3.93
21.39 13.67 Rtrigd
(68)
830(vs)
847
822 9.34
3.95
19.82 15.15 852
826
1.64
0.70
42.82 0.21 Rasymd
(69)
818(vs)
838
813 1.31
0.54
0.02 0.13
836
811
1.33
0.55
0.65 0.50 Rsymd
(70)
741(vs)
751
729 7.35
2.44
17.01 0.70
753
730
7.01
2.34
11.10 0.77 NO2sciss
(70)
732(ms) 728(w)
744
722 6.35
2.07
3.50 14.36 745
723
6.34
2.07
3.60 14.64 bCC
(68)
644(ms)
667
647 3.57
0.93
5.02 0.32
687
666
3.45
0.96
9.33 0.36 NO2wag
(65)
622(vs) 622(vw) 631
612 7.12
1.67
17.08 0.47
631
612
7.00
1.64
16.15 0.61
bCN
(66)
575 4.52
0.93
4.12 7.20
597
579
4.13
0.86
4.52 5.69
bCO
580(vw) 592
(65)
561 3.72
0.73
0.27 1.79
567
550
4.06
0.77
0.24 2.85 NO2rock
565(vw) 577
(65)
513 5.04
0.83
7.65 4.85
528
512
4.97
0.81
7.15 4.73
518(w) 514(vw) 528
CH
(64)
445(vw) 452
439 2.95
0.35
4.66 0.17
450
437
3.00
0.35
9.20 0.49
CH
(62)
428(vw)
429
417 5.64
0.61
7.45 1.85
433
420
5.17
0.57
7.05 1.34
CH
(63)
388(vw) 389
378 1.11
0.09 119.19 1.96
364
353
1.19
0.09 105.49 1.75 tRtrigd
(60)
366(vw) 361
350 8.12
0.62
0.06 2.35
359
348
5.82
0.44
6.04 2.05 tRasymd
(61)
325(vw) 328
319 3.48
0.22
0.88 0.27
325
315
3.59
0.22
0.80 0.27 tRsymd
(62)
300(vw) 303
294 5.69
0.30
2.12 0.90
292
283
5.31
0.26
3.42 0.97
CC
(60)
268(w)
249
242 6.50
0.23
3.12 0.36
244
237
7.77
0.27
3.38 0.40
CN
(62)
219(ms) 241
234 1.14
0.03
0.40 0.70
221
214
1.17
0.03
0.15 1.02
CO
(60)
202(w)
199
193 2.96
0.06
0.30 0.74
197
191
2.55
0.05
0.47 0.55
OH
(59)
115(w)
103
103 5.86
0.03
0.84 0.60
102
102
5.92
0.03
0.82 0.72 NO2twist
(58)
85(w)
82
82 8.06
0.00
0.13 0.34
40
40
8.19
0.01
0.35 0.94 CH3twist
(58)
JEYAVIJAYAN: FTIR, FT-RAMAN SPECTRA AND DFT ANALYSIS OF 3-METHYL-4-NITROPHENOL
vibrations. All the vibrations are active both in the
Raman scattering and infrared absorption. The
detailed vibrational assignment of fundamental modes
of MNP along with the calculated IR and Raman
frequencies and normal mode descriptions
(characterized by TED) are reported in Table 4. The
observed and calculated FTIR and FT-Raman spectra
of MNP are shown in Figs 2 and 3, respectively.
Fig.2—Comparison of observed and calculated IR spectra of
3-methyl-4-nitrophenol (a) observed, (b) calculated with
B3LYP/6-31+G(d,p) and (c) calculated with B3LYP/
6-311++G(d,p)
275
The main focus of the present investigation is the
proper assignment of the experimental frequencies to
the various vibrational modes of MNP in
corroboration with the calculated harmonic
vibrational frequencies at B3LYP level using the
standard 6-31+G(d,p) and 6-311++G(d,p) basis sets.
Comparison of the frequencies calculated by
DFT-B3LYP method with the experimental values
reveals the over estimation of the calculated
vibrational modes due to neglect of anharmonicity in
real system. The results indicate that the B3LYP/6311++G(d,p) calculations approximate the observed
fundamental frequencies much better than the
B3LYP/6-31+G(d,p) results. The vibrational analysis
obtained for MNP with the unscaled B3LYP force
field is, generally, somewhat greater than the
experimental values. These discrepancies can be
corrected either by computing anharmonic corrections
explicitly or by introducing a scaled field or directly
scaling the calculated wavenumbers with proper
factor5. A tentative assignment is often made on the
basis of the unscaled frequencies by assuming the
observed frequencies so that they are in the same
order as the calculated ones. Then, for an easier
comparison to the observed values, the calculated
frequencies are scaled by the scale to less than 1, to
minimize the overall deviation. A better agreement
between the computed and experimental frequencies
can be obtained by using different scale factors for
different regions of vibrations. For that purpose, a
scale factor of 0.934 (up to 1700 cm-1) and 0.97
(below 1700 cm-1) for all the fundamental modes
except the torsional mode was used to compute the
corrected wavenumbers at DFT level and compared
with the experimentally observed frequencies. The
resultant scaled frequencies are also listed in Table 4.
4.2.1 O-H vibrations
Fig.3—Comparison of observed and calculated Raman spectra
of 3-methyl-4-nitrophenol (a) observed, (b) calculated with
B3LYP/6-31+G(d,p) and (c) calculated with B3LYP/
6-311++G(d,p)
Hydrogen bonding alters the frequencies of the
stretching and bending vibration. The O-H stretching
bands move to lower frequencies usually with
increased intensity and band broadening in the
hydrogen bonded species. Hydrogen bonding if
present in five or six member ring system would
reduce the O-H stretching band to 3200-3550 cm-1
region16. The O-H in-plane-bending vibration in
phenol, in general, lies in the region 1150-1250 cm-1
and is not much affected due to hydrogen bonding
unlike the stretching and out-of-plane deformation
frequencies. The O-H out-of-plane deformation
vibration in phenols lies in the region 290-320 cm-1
276
INDIAN J PURE & APPL PHYS, VOL 54, APRIL 2016
for free O-H and in the region 517-710 cm-1 for
associated O-H16. In MNP, the FTIR and FT-Raman
bands appeared at 3396 and 3392 cm-1 are assigned to
O-H stretching modes of vibration, respectively, which
are further supported by the TED contribution of 99%.
The in-plane and out-of-plane bending vibrations of
hydroxy group for MNP have been identified at 1165
and 202 cm-1 in FT-Raman, respectively.
4.2.2 C-H vibrations
Aromatic compounds commonly exhibit multiple
weak bands in the region 3100-3000 cm-1 due to
aromatic C-H stretching vibrations17. The bands due
to C-H in-plane ring bending vibrations, interact
somewhat with C-C stretching vibrations, are
observed as a number of sharp bands in the region
1300 -1000 cm-1. The C-H out-of-plane bending
vibrations are strongly coupled vibrations and occur
in the region 900 - 667 cm-1. Hence, the infrared
bands appeared at 3091, 2991 and 2946 cm-1 and the
Raman band found at 3090 cm-1 in MNP have been
assigned to C-H stretching vibrations and these modes
are confirmed by their TED values. The in-plane and
out-of-plane bending vibrations of C-H group have
also been identified for MNP and presented in
Table 4. The theoretically computed values for C-H
vibrational modes by B3LYP/6-311++G(d, p) method
give excellent agreement with experimental data.
4.2.3 NO2 group vibrations
The characteristics group frequencies of nitro
group are relatively independent of the rest of the
molecule, which makes this group convenient to
identify. Aromatic nitro compounds have strong
absorptions due to the asymmetric and symmetric
stretching vibrations of the NO2 group at 1625-1540 cm-1
and 1400-1360 cm-1, respectively18. Hydrogen
bonding has little effect on the NO2 asymmetric
stretching vibrations19,20. In the title compound, the
infrared and Raman bands both at 1604 cm-1 have
been designated to asymmetric stretching mode of
NO2 group. The strong bands at 1385 cm-1 in IR and
1380 cm-1 in Raman spectra have been assigned to
symmetric stretching mode of nitro group and found
to be in good agreement with TED output. The
scissoring mode of NO2 group has been designated to
the band at 741 cm-1 in IR spectrum. The band at
565 cm-1 in Raman is attributed to NO2 rocking mode.
The wagging and twisting vibrational modes of NO2
for MNP are also observed at 644 cm-1 in FTIR and
115 cm-1 in Raman spectrum.
4.2.4 C–C vibrations
The bands between 1400 and 1650 cm-1 in benzene
derivatives are due to C-C stretching vibrations21.
Therefore, the C-C stretching vibrations of the title
compound are found at 1590, 1519, 1479, 1460, 1433,
1261 cm-1 in FTIR and 1525, 1482, 1356, 1260 cm-1 in
the FT-Raman spectrum and these modes are confirmed
by their TED values. The C-C in-plane and out-of-plane
bending vibrations of the title compound were well
identified in the recorded spectra within their
characteristic region. These bending modes show
consistent agreement with the computed B3LYP results.
4.2.5 C-N vibrations
The identification of C-N stretching vibration is a
difficult task since, it falls in a complicated region of
the vibrational spectrum22. The IR band appeared at
1248 cm-1 in MNP has been designated to C-N
stretching vibration. The IR and Raman band appeared
at 622 cm-1 in MNP has been designated to C-N
in-plane bending vibration. The C-N out-of-plane
bending vibration is observed at 268 cm-1 in Raman.
These assignments are also supported by the TED values.
4.3 HOMO, LUMO analysis
The highest occupied molecular orbitals (HOMOs)
and the lowest unoccupied molecular orbitals
(LUMOs) are named as frontier molecular orbitals
(FMOs). The FMOs play an important role in the
electric and optical properties, as well as in UV–Vis
spectra and chemical reactions 23. The atomic orbital
HOMO and LUMO compositions of the frontier
molecular orbital for MNP computed at the B3LYP/
6-31+G(d,p) are shown in Fig. 4. The calculations
Fig.4—Atomic orbital HOMO and LUMO compositions of the
frontier molecular orbital for 3-methyl-4-nitrophenol
JEYAVIJAYAN: FTIR, FT-RAMAN SPECTRA AND DFT ANALYSIS OF 3-METHYL-4-NITROPHENOL
indicate that the title compound have 40 occupied
MOs. The LUMO: of π nature, (i.e. benzene ring) is
delocalized over the whole C-C bond. By contrast, the
HOMO is located over OH group; consequently the
HOMO → LUMO transition implies an electron
density transfer to C-C bond of the benzene ring and
NO2 group from OH group. Moreover, these three
orbitals significantly overlap in their position of the
benzene ring for MNP. The HOMO–LUMO energy
gap of MNP was calculated at B3LYP method using
6-31+G(d,p) and 6-311++G(d,p) basis sets and
presented in Table 5, which reveals that the energy
gap reflects the chemical activity of the molecule. The
LUMO as an electron acceptor represents the ability
Table 5—HOMO-LUMO energy values of 3-methyl-4nitrophenol calculated by B3LYP method
Parameters
HOMO energy (a.u)
LUMO energy(a.u)
HOMO-LUMO energy
gap(a.u)
Method/Basis set
B3LYP/631+G(d,p)
-0.26377
-0.09585
0.16792
B3LYP/6311++G(d,p)
-0.26421
-0.09536
0.16885
Table 6—Thermodynamic parameters of 3-methyl-4-nitrophenol
calculated at B3LYP method
Parameters
Total energy(thermal), Etotal
(kcal mol-1)
Heat capacity, Cv (kcal mol-1
k-1)
Entropy, S (kcal mol-1 k-1)
Total
Translational
Rotational
Vibrational
Vibrational energy, Evib (kcal
mol-1)
Zero point vibrational
energy, (kcal mol-1)
Rotational constants (GHz)
A
B
C
Dipole moment (Debye)
μx
μy
μz
μtotal
Method/Basis set
B3LYP/631+G(d,p)
90.647
B3LYP/6311++G(d,p)
90.340
0.0362
0.0363
0.0982
0.0410
0.0301
0.0271
88.869
0.0963
0.0409
0.0301
0.0252
88.562
84.666
84.342
2.222
0.801
0.591
2.241
0.801
0.596
4.958
2.414
0.025
5.515
4.817
2.390
0.080
5.378
277
to obtain an electron, and HOMO represents the
ability to donate an electron. Moreover, a lower
HOMO–LUMO energy gap explains the fact that
eventual charge transfer interaction is taking place
within the molecule.
4.4 Other molecular properties
Using the DFT/B3LYP with 6-31+G(d,p) and
6-311++G(d,p) basis set calculations, several
thermodynamic properties like heat capacity, zero
point energy and entropy of MNP have been
calculated and presented in Table 6. The difference in
the values calculated by both the basis sets is only
marginal. Scale factors have been recommended24 for
an accurate prediction in determining the zero-point
vibration energy (ZPVE) and the entropy (Svib). The
variation in the ZPVE seems to be insignificant. The
total energy and the change in the total entropy of the
compound at room temperature are also presented.
5 Conclusions
The optimized geometries, harmonic vibrational
wavenumbers and intensities of vibrational bands of
MNP have been carried out using the B3LYP method
with the standard 6-31+G(d,p) and 6-311++G(d,p)
basis set calculations for the first time in this
investigation. The theoretical results were compared
with the experimental vibrations. The DFT based
quantum mechanical approach provides the most
reliable theoretical information on the vibrational
properties of MNP. The assignments of most of the
fundamentals provided in the present work are
believed to be unambiguous. The TED calculation
regarding the normal modes of vibration provides a
strong support for the frequency assignment.
Therefore, the assignments proposed at higher level of
theory with higher basis set with only reasonable
deviations from the experimental values seem to be
correct. The calculated HOMO and LUMO energies
show that the charge transfer occurs within the
molecule. Furthermore, the thermodynamic and total
dipole moment properties of the compound have been
calculated in order to get insight into the compound.
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