ATOMIC AND MOLECULAR PHYSICS
DFT, FT-IR AND FT-RAMAN INVESTIGATIONS
OF 2-CHLORO-5-NITROBENZYL ALCOHOL
S. RAMACHANDRAN, G. VELRAJ*
Department of Physics, Periyar University, Salem - 636 011, Tamilnadu, India.
*E-mail: gvelraj@yahoo.co.uk
Received June 15, 2012
The experimental and theoretical study on the structure and vibrations of 2-Chloro-5Nitrobenzyl Alcohol (2C5NBA) are presented. The Fourier transform infrared spectra
(4000-400cm-1) and the Fourier transform Raman spectra (4000-100cm-1) of the title
compound in the solid phase have been recorded. The geometry was optimized with C1
symmetry constraints using the DFT/B3LYP method with 6-31G(d) basis set. The
vibrational frequencies determined experimentally were compared with DFT gradient
calculations which was obtained theoretically employing the B3LYP/6-31G(d) basis set
method for the optimized geometry of the compound. The vibrations obtained from
DFT method were found in good agreement with the experimental data. The study is
extended to the HOMO - LUMO analysis to calculate the energy gap (∆), Ionization
potential (I), Electron Affinity (A), Global Hardness (η), Chemical Potential (µ),
Global Electrophilicity (w). The calculated HOMO and LUMO energy reveals shows
that the charge transfers occurring within the molecule. In addition, thermodynamic
properties were obtained in the range of 100-700 K.
Key words: DFT, FT-IR, FT-Raman, 2-chloro-5-nitrobenzyl alcohol, Vibrational analysis,
HOMO LUMO.
1. INTRODUCTION
Benzyl Alcohol (BA) is commonly used in pharmaceutical products as an
antimicrobial preservative at levels of 3-5% [1]. It is well established when
exposed to air, BA oxidizes slowly to benzaldehyde and subsequently to benzoic
acid. The chemical oxidation of benzyl alcohol to benzaldehyde has been widely
studied [2-4]. The use of aromatic alcohols has been restricted to their antimicrobial
activity. That is probably because these molecules have been reported to cause
aggregation of proteins, and are hence not ideal candidates as protein stabilizers.
Protein aggregation not only has an adverse effect on the functionality of a protein
but can also trigger a severe immune response in some patients [5-7]. BA is
considered to be one of the least toxic preservatives and can be used up to 2% v/v
Rom. Journ. Phys., Vol. 58, Nos. 3–4, P. 305–317, Bucharest, 2013
306
S. Ramachandran, G. Velraj
2
for parenteral applications [8]. It also possesses potential antioxidative properties
[9]. BA, also referred to as benzene methanol or phenylmethanol, is a naturally
occurring organic compound that is commonly found in many plants, fruits and
teas. Due to its polarity and low toxicity, BA is used as a general solvent and as a
carrier solvent for flavoring substances in some foods and beverages and also as a
preservative in cosmetic industries. Benzyl alcohol is mainly used in flavor
industries, perfumery, artificial essences, cosmetics, paints, lacquers, soap and
industrial solvents [10]. It is used as a local anesthetic [11, 12]. However, BA and
m-cresol have been shown to accelerate aggregation by binding to proteins and
favoring the formation of the aggregation prone molecular population. This was
observed as the lower melting temperature of proteins like recombinant human
interleukin-1 receptor antagonist (rhIL-1ra), recombinant human growth hormone
(rhGH), and monoclonal antibody in the presence of BA [13-15]. It has also been
shown that the effect of benzyl alcohol-induced aggregation was pH-dependent; an
acceleration in the rate of aggregation of recombinant human granulocyte colony
stimulating factor (rh GCSF) was seen at pH 7.0, whereas, a high positive charge
on the same protein at pH 3.5 resulted in strong protein-protein electrostatic
repulsion, thus, inhibiting BA induced aggregation [16]. BA and its substituted are
found in the natural products and used for mechanistic investigation [17-19].
In the present work, the DFT frequency calculations for 2C5NBA have been
verified both experimentally and theoretically. Therefore, the present investigation
was to study the vibrational spectra of the molecule completely with wave number
accuracy. The Density Functional Theory (DFT) calculations have been performed
to support wave number assignments, thermodynamic properties and HOMO
LUMO properties. This study may provide useful information about the molecular
geometry, HOMO LUMO and thermodynamic property of 2C5NBA.
2. EXPERIMENTAL METHODS
The compound 2C5NBA (99%) was purchased from Sigma-Aldrich
Company and used as such for the spectral measurements. The room temperature
Fourier Transform IR spectra of the title compounds were measured in the 4000400cm−1 region at a resolution of ±2cm-1 using BRUKER Tensor 27FT-IR
spectrometer equipped with a KBr beam splitter and globar source. The Fourier
Transform-Raman spectra of 2C5NBA were recorded on a BRUKER IFS-66 V
model interferometer equipped with an FRA-106 FT-Raman accessory. The spectra
were recorded in the region 4000-100cm−1 stokes region using the 1064nm line of
Nd:YAG laser for the excitation operating at 200mW power. The reported wave
numbers are expected to be accurate within ±2cm-1.
3
DFT, FT-IR and FT-Raman investigations
307
3. COMPUTATIONAL DETAILS
The molecular structure optimization of the title compound and
corresponding vibrational harmonic frequencies were calculated using the DFT
with Beckee-3-Lee-Yag-Parr(B3LYP) with 6-31G(d) basis set using GAUSSIAN
03 program package without any constraint on the geometry. The optimized
geometrical parameters, true rotational constants, fundamental vibrational
frequencies, IR intensity, Raman activity, electronic polarizabiliy, atomic charges,
dipole moment and other thermodynamic parameters were calculated using the
Gaussian 03 package [20]. By combining the results of the GAUSSVIEW [21]
program. We know that DFT potentials symmetrically overestimate (2.5%) the
Vibrational wavenumbers. These discrepancies are corrected either by computing
anharmonic corrections explicitly [22] or by introducing a scaled field or by
directly scaling the calculated wavenumbers with a proper factor. We calibrated the
vibrational wavenumbers calculated by B3LYP method. After scaling with a
scaling factor, the deviation from the experiment is more reliable. Analytic
frequency calculations at the optimized geometry were done to confirm the
optimized structures to be an energy minimum and to obtain the theoretical
vibrational spectra. The total energy distribution (TED) was calculated by using the
scaled quantum mechanics (SQM) method [23] and parallel quantum solution
(PQS) [24] program and the fundamental vibrational modes were characterized by
their TED.
The Raman activities (SRa) converted to relative Raman intensities (IRa)
using the following relationship derived from the intensity theory of Raman
scattering
f (ν o − ν i ) 4 Si
Ii =
ν i [1 − exp(− hcν i / kT )]
(1)
Where ν o is the laser exciting wavenumber in cm−1 (in this work, we have
used the excitation wavenumber ν o = 9398.5cm−1, which corresponds to the
wavelength of 1064nm of a Nd: YAG laser), ν i the vibrational wavenumber of the
ith normal mode (cm−1), while Si is the Raman scattering activity of the normal
mode ν i . f (is a constant equal to 10−12) is a suitably chosen common
normalization factor for all peak intensities. h, k, c and T are Planck and Boltzmann
constants, speed of light and room temperature in Kelvin. However, the defined
coordinates forms complete set and matches quite well with the motions observed
using GAUSSVIEW program. In order, to analyze the very low frequency
vibrations, the FT-IR and FT-Raman spectrum was taken in the range of 4000100cm-1 in the solid phase.
308
S. Ramachandran, G. Velraj
4
4. RESULT AND DISCUSSION
4.1. MOLECULAR GEOMETRY
The structure and the scheme of numbering the atoms of 2C5NBA are shown
in Fig.1. The geometry of the molecules under investigation is considered to
possess C1 point group symmetry. The 48 fundamental modes of vibrations of each
compound are distributed into the irreducible representation under C1 symmetry as
Γ3N-6vib= 48. All vibrations are active in both IR and Raman. All the frequencies
are assigned in terms of fundamental, overtone and combination bands. The
optimized geometrical parameters (bond length and bond angles) by DFT/B3LYP
with 6-31G(d) as the basis set are tabulated in Table 1.
Fig. 1 – Molecular structure of 2C5NBA along with numbering of atom.
4.2. VIBRATIONAL ASSIGNMENT
The observed FT-IR and FT-Raman spectra along with the theoretical
infrared and Raman spectra of 2C5NBA are shown in Figs. 2-3. The observed and
calculated frequencies using DFT/B3LYP with 6-31G(d) basis set along with their
probable assignments of 2C5NBA are summarized in Table 2.
4.2.1. C-H vibrations
The aromatic C-H stretching vibrations were normally found between 3100
and 3000 cm-1 [25]. Accordingly, in the title compound, asymmetric and symmetric
stretching vibrations were observed at 3090cm-1 and 2901cm-1 in the FT-IR
spectrum whereas, in the FT-Raman, it is at 3087cm-1 and 2896cm-1, respectively.
The in-plane bending and out-of-plane bending vibrations of the aromatic C-H
group have also been identified for the title compound and they are presented in
Table 2.
5
DFT, FT-IR and FT-Raman investigations
309
Table 1
Optimized geometric data for 2C5NBA using B3LYP/6-31G(d)
Bond length
C1–C2
C1–O12
C1–H13
C1–H14
C2–C3
C2–C7
C3–C4
C3–Cl11
C4–C5
C4–H15
C5–C6
C5–H16
C6–C7
C6–C8
C7–H17
N8–O9
N8–O10
O12–H18
Value( Å )
1.509
1.424
1.100
1.099
1.405
1.395
1.396
1.752
1.390
1.084
1.391
1.082
1.392
1.470
1.083
1.230
1.230
0.969
Bond angle
C2–C1–O12
C2–C1–H13
C2–C1–H14
O12–C1–H13
O12–C1–H14
H13–C1–H14
C1–C2–C3
C1–C2–C7
C3–C2–C7
C2–C3–C4
C2–C3–Cl11
C4–C3–Cl11
C3–C4–C5
C3–C4–H15
C5–C4–H15
C4–C5–C6
C4–C5–H16
C6–C5–H16
C5–C6–C7
C5–C6–N8
C7–C6–N8
C2–C7–C6
C2–C7–H17
C6–C7–H17
C6–N8–O9
C6–N8–O10
O9–N8–O10
C1–O12–H18
Value (˚)
108.1
108.8
109.4
111.2
111.3
107.7
122.4
120.0
117.5
121.9
120.3
117.7
119.8
119.7
120.4
118.4
121.5
119.9
121.8
119.1
119.0
120.3
120.4
119.1
117.6
117.5
124.7
107.7
Transmittance %
(b )
(a )
4000
3500
3000
2500
2000
W a v e n u m b e r (c m
1500
-1
1000
)
Fig. 2 – FT-IR spectrum: (a) Observed; (b) 6-31G(d).
500
S. Ramachandran, G. Velraj
Raman Intensity (Arbitr. Units)
310
6
(b )
(a )
4000
3500
3000
2500
2000
1500
1000
500
-1
W a v e n u m b e r (c m )
Fig. 3 – FT-Raman spectrum: (a) Observed; (b) 6-31G(d).
4.2.2. C-Cl vibrations
The C-Cl stretching gives generally strong bands in 770-505cm-1 region [26].
The sharp FT-IR and FT-Raman bands at 600cm-1 and 612cm-1, respectively
observed in the spectrum of the title compound are assigned to C-Cl is stretching
vibration.
4.2.3. C-C vibrations
The bands between 1650-1400cm-1in benzene derivatives were assigned to
C-C stretching modes [27]. Accordingly, in the present study, the carbon-carbon
vibrations of the title compound were observed at 1584 and 1714cm-1 in the FT-IR
spectrum and the FT-Raman spectrum observed at 1612cm-1 and 1420cm-1.
4.2.4. Nitro group vibrations
The characteristic 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 1655cm-1 and 1603cm-1
respectively. The Hydrogen bonding has little effect on the NO2 asymmetric
stretching vibrations [28, 29]. The FT-IR band at 1789cm-1 has been designated to
scissoring modes of NO2. The FT-Raman band has been designated to the rocking
mode of nitro group. The FT-IR and FT-Raman bands at 794cm-1and 805cm-1 in
2C5NBA have been designated to scissoring modes of the nitro group. The very
weak FT-Raman band at 211cm-1 is assigned to NO2 torsion mode.
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DFT, FT-IR and FT-Raman investigations
311
Table 2
Detailed assignments of fundamental vibrations of 2C5NBA
Experimental frequency(cm-1)a
Modes no
υ1
υ2
υ3
υ4
υ5
υ6
υ7
υ8
υ9
υ10
υ11
υ12
υ13
υ14
υ15
υ16
υ17
υ18
υ19
υ20
υ21
υ22
υ23
υ24
υ25
υ26
υ27
υ28
υ29
υ30
υ31
υ32
υ33
υ34
υ35
υ36
υ37
υ38
υ39
υ40
υ41
υ42
υ43
υ44
υ45
υ46
υ47
υ48
a
FT-IR
3889vw
3509s
3090m
2901m
1789w
1714w
1584m
1513vs
1340vs
1184s
1027vs
903s
794s
740s
600w
491m
-
FT-Raman
3087m
2896m
1612w
1575m
1514m
1420m
1347s
1194s
1096s
1070s
1038s
925s
805m
712m
612m
482wv
336vw
296vw
211vw
149vw
112vw
-
Calculated frequency (B3LYP)(cm-1)
Vibrational Assignment
6-31G(d)
Raman
with TED maximum (%)b
IR Intensity
wavenumbers
Intensity
3752
20.031
5.893
υsOH (100)
3256
2.167
3.691
υasOH (98)
3245
1.901
4.67
υsOH (98)
3231
0.923
5.414
υCH (88)
3049
31.073
8.298
υCH (86)
3013
52.313
5.916
υCH (84)
1673
91.128
85.038
υasCH (61)
1640
70.603
24.691
υsCH (55)
1616
82.416
11.038
δNO2 (63)
1547
2.231
0.666
υCC (77)
1512
36.598
15.544
υCC (83)
1484
18.607
2.063
δCH (89)
1442
6.421
371.897
δCH(84)
1397
371.379
6.324
wCH (79)
1369
14.660
2.241
υ CC (53)
1301
10.196
15.532
υRing (58)
1287
37.327
79.130
υCN (67)
1246
28.020
80.183
τCH (74)
1219
31.938
9.730
υCN(52)
1165
4.199
24.092
βOH (63)
1132
8.287
22.082
ρNO2 (82)
1076
122.833
47.785
βCH (67)
1062
43.288
2.395
βCH (63)
1015
4.838
3.78
υCO (78)
982
0.467
5.837
υCO (54)
945
31.312
8.743
ρCH (67)
933
31.464
8.51
υCN (55)
853
12.158
38.814
γCH (71)
819
18.493
7.016
γCH (66)
754
25.193
2.511
γCH (61)
740
11.511
41.53
δNO2 (81)
708
2.030
13.792
βRing (77)
615
10.046
8.908
τRing (81)
577
15.810
29.573
υCCl (47)
542
6.460
6.594
βCH (51)
522
9.385
6.704
γCN(59)
465
1.810
80.531
βRing (42)
391
1.661
25.62
τRing (47)
352
0.555
47.713
βCN (36)
315
0.402
18.418
βRing (52)
277
2.918
13.374
τRing (65)
245
1.690
87.97
βCCl (36)
206
110.287
17.466
τNO2 (51)
193
2.248
136.556
γOH (41)
136
1.860
33.089
βCC (38)
94
3.615
104.361
τRing (37)
58
0.057
108.820
τRing (54)
55
0.157
5.893
τNO2 (36)
vs-very strong; s-strong; m-medium strong; w-weak; vw-very weak.
τ- torsion; γ- out of plane bending; β- in plane bending; ρ- rocking; δ- scissoring; υ- stretching;
υs- symmetric stretching; υas- asymmetric stretching; w- wagging.
b
312
S. Ramachandran, G. Velraj
8
4.2.5. O-H and C-O vibrations
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 3200-3550cm−1 region [30]. In the present study (for 2C5NBA),
the stretching vibration of the hydroxyl group is observed at 3889cm-1 and 3509cm-1
in the FT-IR spectrum. The bending vibrations of the alcohol group are also
identified and listed in Table 2. The strong band observed in the region 1300-1200
cm-1 may be due to C-O stretching vibrations for p-mono substituted phenols [31].
In the present study, strong FT-Raman bands are observed at 1070cm-1 and
1096cm-1.
4.3. HOMO-LUMO ENERGY GAP AND RELATED MOLECULAR PROPERTIES
The analysis of wave function indicates that the electron absorption
corresponds to the transition from the ground to the first excited state and is mainly
described by one electron excitation from the highest occupied molecular orbital
(HOMO) to the lowest unoccupied orbital (LUMO). The HOMO, LUMO and
HOMO-LUMO energy gap of 2C5NBA in the DFT level in 6-31G(d) basis set has
been calculated. The HOMO-LUMO energy gap reveals that the energy gap
reflects the chemical activity of the molecule. The atomic orbital compositions of
the frontier molecule orbital are sketched in Fig.4.
Fig. 4 – Atomic orbital composition of the frontier molecule for 2C5NBA.
9
DFT, FT-IR and FT-Raman investigations
313
Associated within the framework of Self-Consistency Function (SCF)
Molecular Orbital(MO) theory the ionization energy and electron affinity can be
expressed through HOMO and LUMO orbital energies as I = -EHOMO and
A = -ELUMO. The hardness corresponds to the gap between the HOMO and LUMO
orbital energies. The larger the HOMO-LUMO energy gaps the harder the
molecule [32].
The global hardness, is η=1/2(ELUMO-EHOMO). The hardness has been
associated with the stability of chemical system. The electron affinity can be used
in combination with ionization energy to give electronic chemical potential,
µ=1/2(EHOMO + ELUMO). The global electrophilicity index, w = µ2/2η is also
calculated and listed in Table 3.
Table 3
Comparison of HOMO, LUMO, energy gaps (HOMO –LUMO)
and related molecular properties of 2C5NBA(eV)
Molecular Energy
EHOMO
ELUMO
Energy gap (∆)
Ionisation Potential (I)
Electron affinity (A)
Global Hardness (η)
Chemical potential (µ)
Global Electrophilicity (ω)
B3LYP/6-31G(d)
–6.7577
–0.6054
6.1522
6.7577
0.6054
3.0761
–3.6815
2.2029
4.4. THERMODYNAMIC PROPERTIES
The thermodynamic parameters namely heat capacity, entropy, rotational
constants, dipole moments, vibration and vibrational zero point energies of the
compounds have also been computed at the DFT-B3LYP level using 6-31G(d)
basis sets and are presented in Table 4. The energy of 2C5NBA has been calculated
to -1010.86075197a.u. The thermodynamic data provide helpful information for the
further study on the title compound, when these may be used as a reactant to take
part in a new reaction. These standard thermodynamic functions can be used as
reference thermodynamic values to calculate the changes of entropies (∆ST),
changes of enthalpies (∆HT) and changes of Gibbs free energies (∆GT) of the
reaction. There is no much deviation in the thermodynamic properties calculated
from these methods. The dipole moment and its principal inertial axes strongly
depend upon the molecular conformation. The 2C5NBA has higher dipole
moments (4.0140Debye).
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S. Ramachandran, G. Velraj
10
Table 4
The calculated thermodynamic parameters of 2C5NBA employing B3LYP/ 6-31G(d) methods.
Thermodynamic parameters (298K)
B3LYP/ 6-31G(d)
SCF energy (a.u.)
-1010.86075197
−1
Total energy (thermal), Etotal (kcal mol )
86.139
Heat capacity at const. volume, Cv (cal mol−1 K−1)
38.996
−1
−1
Entropy, S (cal mol K )
104.545
Vibrational energy, Evib (kcal mol−1)
84.362
Zero-point vibrational energy, E0 (kcal mol−1)
79.35406
Rotational constants (GHz)
A
1.58283
B
0.48348
C
0.38108
Dipole moment (Debye)
µx
3.8598
µy
-0.0170
µz
-1.1017
µtotal
4.0140
On the basis of vibrational analysis at B3LYP/6-31G(d) level, the standard
0
statistical thermodynamic functions: heat capacity ( C p ,m ), entropy ( Sm0 ), and
enthalpy changes ( H m0 ), for the title compounds were obtained from the theoretical
harmonic frequencies and listed in Table 5.
Table 5
Thermodynamic properties at different temperatures at the B3LYP/6-31G(d) level of 2C5NBA
T(K)
C p0 , m (cal mol-1k-1)
Sm0 (col mol-1k-1)
H m0 (kcal mol-1)
100
11.701
9.054
79.899
200
22.671
20.591
81.626
298.15
33.035
31.583
84.362
300
33.226
31.788
84.423
400
43.006
42.716
88.245
500
51.263
53.231
92.972
600
57.920
63.188
98.444
700
63.250
72.532
104.512
11
DFT, FT-IR and FT-Raman investigations
315
70
50
40
m,0
-1
-1
Heat capacity (Cp ) (cal mol K )
60
30
20
10
100
200
300
400
500
600
700
Tem perature (K)
Fig. 5 – Correlation graph of heat capacity and temperature for 2C5NBA molecule.
80
60
50
40
0
-1
-1
Entropy (Sm) (cal mol K )
70
30
20
10
0
100
200
300
400
500
600
700
Temperature (K)
Fig. 6 – Correlation graph of entropy and temperature for 2C5NBA molecule.
105
-1
Enthalpy (Hm) (kcal mol )
100
0
95
90
85
80
100
200
300
400
500
600
700
Temperature (K)
Fig. 7 – Correlation graph of enthalpy and temperature for 2C5NBA molecule.
C p0 , m = –1.51846 + 0.13452T –5.94593x10-5T2
(R2 = 0.99957)
(2)
316
S. Ramachandran, G. Velraj
12
Sm0 = –3.12514 + 0.1230T –2.10965x10-5T2
(R2 = 0.99997)
(3)
H m0 = 78.72648 + 0.00605T + 4.42841x10-6T2
(R2 = 0.99971)
(4)
From Table 5, it can be observed that these thermodynamic functions are
increasing with temperature ranging from 100 to 700 K due to the fact that the
molecular vibrational intensities increase with temperature [33]. The correlation
equations between heat capacities, entropies, enthalpy changes and temperatures
were fitted by quadratic formulas, and the corresponding fitting factors (R2) for
these thermodynamic properties are 0.99957, 0.99997 and 0.99971, respectively.
The corresponding fitting equations are as follows and the correlation graphics
from those shows in Figs. 5-7. All the thermodynamic data supply helpful
information for the further study on the 2C5NBA. All thermodynamic calculations
were done in gas phase and they could not be used in solution.
5. CONCLUSION
In the present work, we have performed both experimental and theoretical
vibrational analyses of 2C5NBA. A comparison of the results of experimental and
theoretical study has given a full description of the geometry, vibrational
frequencies, electronic properties and thermodynamic properties of this molecule.
The calculated geometrical parameters and vibrational frequencies obtained by
density functional theory calculations B3LYP/6-31G(d) methods are in good
agreement with the experimental values obtained for the investigated molecule.
The HOMO-LUMO energy gap of the title compound corresponding to 6.1522eV
by using B3LYP/6-31G(d) method respectively. The other molecular properties
such as Ionization Potential (I), Electron affinity (A), Global Hardness (η),
Chemical potential (µ) and Global Electrophilicity (w) were calculated. The
thermodynamic data provide helpful information for the further study on the title
compound. The correlations between the statistical thermodynamics and
temperature are also obtained. It is seen that the heat capacities, entropies and
enthalpies increase with the increasing temperature owing to the intensities of the
molecular vibrations increase with increasing temperature.
Acknowledgments. The first author Acknowledge with thanks to Periyar University for
providing University Research Fellow (URF).
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