Journal of Theoretical Chemistry

Journal of Theoretical Chemistry / 2014 / Article

Research Article | Open Access

Volume 2014 |Article ID 125841 | https://doi.org/10.1155/2014/125841

Ambrish K. Srivastava, Anoop K. Pandey, B. Narayana, B. K. Sarojini, Prakash S. Nayak, Neeraj Misra, "Normal Modes, Molecular Orbitals and Thermochemical Analyses of 2,4 and 3,4 Dichloro Substituted Phenyl-N-(1,3-thiazol-2-yl)acetamides: DFT Study and FTIR Spectra", Journal of Theoretical Chemistry, vol. 2014, Article ID 125841, 10 pages, 2014. https://doi.org/10.1155/2014/125841

Normal Modes, Molecular Orbitals and Thermochemical Analyses of 2,4 and 3,4 Dichloro Substituted Phenyl-N-(1,3-thiazol-2-yl)acetamides: DFT Study and FTIR Spectra

Academic Editor: G. Narahari Sastry
Received12 Sep 2013
Accepted02 Dec 2013
Published29 Jan 2014

Abstract

A detailed spectroscopic analysis of two dichloro substituted phenyl-N-(1,3-thiazol-2-yl)acetamides at 2,4 and 3,4 positions of the phenyl ring has been carried out by using B3LYP method with 6-31+G(d, p) basis set within density functional scheme. The scaled theoretical wave numbers are in perfect agreement with the experimental values and the vibrational modes are interpreted in terms of potential energy distribution (PED). The internal coordinates are optimized repeatedly to maximize the PED contributions. The molecular HOMO-LUMO surfaces, their respective energy gaps, and MESP surfaces have also been drawn to explain the chemical activity of both molecules. Various thermodynamic parameters are presented at the same level of theory.

1. Introduction

Acetamide constitutes a distinguished class of biologically active molecules having an amide bond same as that between amino acids in proteins. Amides are also popular for their coordinating ability and which enable them to be used as ligands [1]. In general, a large number of natural products and drugs comprises of heterocyclic moieties containing nitrogen and sulphur atoms [2, 3] and interesting biological activities have been found to be associated specially with thiazole derivatives [4, 5]. In continuation to our previous studies on vibrational dynamics of biomolecules [6, 7], we intend here to report a detailed analysis of vibrational modes of molecules consisting of thiazole ring with acetamide ligand. More recently, such molecules have been shown to possess anti-HIV activities with suitable substitutions [8].

We present density functional based theoretical studies on two molecules, namely, 2-(2,4-dichlorophenyl)-N-(1,3-thiazol-2-yl)acetamide and 2-(3,4-dichlorophenyl)-N-(1,3-thiazol-2-yl)acetamide. The two molecules differ only in the position of two chlorines substituted on the phenyl ring. This provides us an opportunity to analyse the effect of position of substitution on vibrational properties. Keeping it in mind, we offer a complete assignment of all normal modes of vibration. The calculated vibrational spectra are compared with the observed ones. The chemical reactivity of molecules is also explained with the help of molecular orbital analysis. The thermochemistry of molecules is discussed by calculating various thermodynamical parameters.

2. Computational Method and FTIR Spectra

All the computations were carried out with Gaussian 09 program [9] and Chem3D Ultra 8.0 program [10] was used for a visual presentation of the graphics. The geometries of both molecules were optimized using a hybrid type B3LYP exchange-correlation functional with 6-31+G(d, p) basis set in the framework of DFT. The present computational scheme is very popular and extensively employed in the biomolecular studies due to reliability of its results as compared to experimental data [11]. Vibrational analyses of molecules were performed at the same level of theory. The normal mode frequencies were scaled by equation, as recommended by many studies [1214] in order to make comparison with the observed wavenumbers.

To perform experimental FTIR spectroscopy, the title compounds were purchased from Sigma Aldrich with a purity of 98% and used as such without further purification for spectroscopic processing. The FTIR spectra were recorded by using Shimadzu-Model Prestige 21 spectrometer in the region 400–4000 cm−1 with samples in KBr pellet. The FTIR spectra of title molecules are shown in Figure 1.

3. Results and Discussions

3.1. Crystal Structure and Molecular Geometry

We have already reported X-ray crystallographic studies on these compounds [15, 16]. All these compounds belong to triclinic space group. In crystal phase, molecules are linked by pairs of N–HN hydrogen bonds. There is also a weak C–HO interaction in case of 2,4 substitution.

The crystallographic parameters were used to model initial structure for the process of geometry optimization. The optimized geometries of both molecules are displayed in Figure 2. The mean plane of dichlorophenyl ring is almost perpendicular to that of thiazole ring in both molecules thus forming a sofa shaped structure. The angular changes in hexagonal ring geometry have also been proven to be a sensitive indicator of the interaction between the substituent and the ring [17]. The structural changes in the carbon skeleton involve changes in the bond distances as well as bond angles. They are most pronounced at the place of substitution and depend on the electronegativity as well as on the σ/π, donor/acceptor character of the substituent. All bond-lengths and bond angles are nearly same in both molecules except those in phenyl ring attached with chlorine atoms at different positions. For instance, C–Cl, bond-length 1.752–1.756  in 2,4 substituted molecule is reduced to 1.744-1.745  for 3,4 substitution.

In Figure 3, we have shown a linear correlation between DFT calculated and X-ray experimental bond-lengths for title molecules. Note that C–H bond-lengths are not included here. A correlation coefficient () of greater than 0.99 in both molecules supports the fact that DFT can efficiently reproduce the experimental geometry.

3.2. Normal Mode Analysis

The calculated IR and observed FTIR spectra of both molecules are shown in Figure 4 for comparison in the wavenumber range of 1800–400 cm−1. The calculated wavenumbers are scaled by using the equation mentioned earlier. All the vibrational modes were properly assigned on the basis of the potential energy distribution (PED). Tables 1 and 2 list calculated frequencies (unscaled as well as scaled), FTIR observed frequencies, IR intensities, and assignments of all normal modes for title molecules. Both molecules contain two ring system, a phenyl ring (R1) and a thiazole ring (R2) connected by acetamide fragment (–NHCOCH2–). For the clarity of discussions, we classified their vibrations into three broad categories.


S.numberUnscaled freq. (cm−1)Scaled freq. (cm−1)FTIR freq. (intensity) (cm−1)IR intensity
(a.u.)
Mode of vibration %PED Direction of polarization

1360034583199 (s)79 (N17–18H) Along N–H plane
2327231451 (C–H)R2 Along R1
3324331171 (C–H)R2 Along R2
4323431088 (22H–19C–25C–21H) Per R2
5322631011 (C–H)R1 Along R2
6318830643055 (w)5 (C–H)R1 Along to 3C–6C
7312130011 (22H–19C–25C–21H) Along R2
8306829507 (13H–12C–14H) Along R2
9174816901691 (s)273 (15C–16O) Opposite to C–O
101632158042 (C–C)R1 Along R2
11159915481544 (m)16 (C–C)R1 + (C–H)R1 Along 6C–9H
1215811531420 (20C–24N) + (17N–18H) Along R1
1315241477109 (19C–25C) + (17N–18H) + (C–H)R2 Per R2
141506145984 (C–H)R1 Along R2
15147314286Scissoring (13H–12C–14H ) + (20C–24N) Along R2
161461141638Scissoring (13H–12C–14H) + (17N–18H) Per R2
17141613731346 (m)15 (C–C)R1 + (C–H)R1 Along R2
181348130861 (C–H)R1 Along 19C–22H
19133913001300 (vw)1Rocking (13H–12C–14H) + (C–H)R2 Along R2
20133012913Rocking (13H–12C–14H) + (C–C)R2 Along R1
2113041267107 (20C–24N) + (17N–18H) Per R2
22129112541230 (w)3 (C–H)R1 Along R2
231227119364 (C–H)R1 + (17N–18H) Along R2
24122411903 (C–H)R1 Per R2
251205117231Twist (13H–12C–14H) + (C–H)R2 and R1Per R2
261178114630 (19C–22H) + (17N–18H) Along R2
27116311324 (1C–7H–6C–9H) Per R2
28111910901103 (m)50 (3C–8H) + (6C–9H) Along R2
291088106010Scissoring (21H–25C–19C–22H) Along R2
301066103928Breathing R2Per R1
31976954960 (w)7Twist (13H–12C–14H) + (16O–15C–17H)Along R1
329679451Twist (1C–7H–6C–9H)Along R2
339369159Rocking (13H–12C–14H) Along R2
348988793Twist (21C–25H–19C–22H)Per R2
35885867866 (w)11 (3C–8H)
368828647Breathing R2Along R1
3787185343Breathing R1In between R1 and R2
38834818823 (m)18 (1C–7H) + (6C–9H) Along R1
39786772771 (w)7 (1C–7H) + (6C–9H) + (R1)Along C–O
4075874643 (R2)
4172371211Ring R2 twistPer R2
42717706698 (m)43 (21C–25H) + (19C–22H) Along R2
436936832Ring R2 twistAlong R1
446776682 (13H–12C–14H)
456486412 (C–H)R1 Directed CH2
4662862136 (C–H)R1 + R2 + rocking (13H–12C–14H)Per R2
476236177Twist R2 + rocking (13H–12C–14H)Along R2
4857957515 (7C–1H) + (3C–8H) + (2C–11Cl) Along 2C–11Cl
495645604Rocking (13H–12C–14H) + (19C–22H) + (17N–18H) Along R1
5052452216R2 breathing + R1 twistPer R2
5151050914 (17C–18H) + (ring R2) (Along Cl) R1
524864868Ring R2 twist + (17C–18H) + (23C–25C–20C)
534524533 (3C–8H) + (6C–9H) + R(13H–12C–14H) Along 17N–18H
544374393Rocking (13H–12C–14H ) + (3C–8H) + (5C–9H) Per R2
553984023 (R1ring)Per 17N–18H
563573631 (C–C)R2 + (15C–16O) Along R2
573213281 (C–C)R2 + rocking (13H–12C–14H) Along CH2
582983071 (23C–21H) + (28C–17H) + rocking (13H–12C–14H)Along R1
592862956 (ring R2) Per R2
602672774 (11C–2Cl) + (4C–10Cl) + rocking (13H–12C–14H) Along R2
612012141 (11C–2Cl) + (4C–10Cl) + (1C–7H) Along R2
621741881 (ring R1 and R2)Per R2
631371536 (ring R1)Per R2
641051221 (ring R2)Per R1
65961141 (ring R2) Along R2
6677964 (13H–12C–14H)Per R2
6732531Rings R1 and R2 twist jointlyAlong R1
6823441Rings R1 and R2 twist jointlyAlong 4C–10Cl
6919401Rings R1 and R2 twist jointlyAlong R2

: stretching; : symmetric stretching; : asymmetric stretching; : in-plane-bending; : out-of-plane bending; : wagging; : torsion, S: scissoring; vs: very strong; s: strong; m: medium; w: weak; vw: very weak.

S.numberUnscaled Freq. (cm−1)Scaled freq. (cm−1)FTIR freq. (intensity) (cm−1)IR intensity (a.u.)Mode of vibration (%PED)Direction of polarization

1359834563197 (s)97 (15N–16H) Along 15N–16H plane
2327131441 (23C–19H) + (17C–20H) Along 2C–9Cl
3323431088 (23C–19H) Along 6C–8H
4322030951 (1C–7H) + (6C–8H)Along 2C–Cl
5320430801 (9C–23H) Along R2
6319130673034 (w)4 (6C–8H) + (1C–7H) Along 3C–6C
7310629861 (12H–110C–11H) Along R2
8306529478 (12H–10C–11H) Along R1
9174316851689 (vs)289 (15C–16O) Along 13C–14O
10163615831589 (w)5 (C–C)R1 Along R2
111600154913 (C-C)R1 + (C–H)R1 Along 5C–H
12158215321527 (vs)461 (18C–22N) + (15N–16H) Per R1
1315241476104 (C–C)R2 + (15N–16H) + (17C–20H) Per R2
14150614591462 (s)74 (C–H)R2 + (C–C)R2 Along R2
15147614313Scissoring (13H–12C–14H ) + (C-N)R2 Along R2
161461141638Scissoring (13H–12C–14H) + (17N–18H) Along 4C–5C
171425138241 (C–C)R1 + (C–H)R1 Along13C–15N
181349131059 (C–H)R1 + (C–C)R1 Along R2
19133112921294 (w)9Rocking (13H–12C–14H) + (C–H)R1 Along R1
20132812891Rocking (13H–12C–14H) + (C–C)R1 Per R1
2113041267107 (15C–16H) + ν(C–C)R1 Per R1
22128512481228 (w)3 (C–H)R1 Along R2
231228119415 (17C–20H) + rocking (11H–10C–12H) + (C–H)R1 Making 5C–10C with an angle 45°
24122211883 (17C–20H) + (15N–16H) + (4C–24H)R1 Per R2
25120411712Twist (11H–10C–12H) + (6C–8H)Per R1
261180114835 (17C–18H)R + 17N–18H) Along R2
27116811373 (1C–7H) + (6C–8H) Plane containing 13C and per to plane of R1
281148111850 (C–C)R1 + (C–H)R1 Along 15N–16H
291089106110 (17C–20H) + (23C–10H) Along R2
301043101735 (C–H)R1 + (C–C)R1Per R1
319779557Twist (13H–12C–14H) + scissoring (16O–15C–17H)Along R1
329709481Twist (7C–1H-6C–8H) Along R2
3394392212Rocking (13H–12C–14H) + (4C–24H) Along R2
3490989011 (C–H) + (C–C) Per R2
358988793 (C–H)R2 Per R2
3689187210Breathing R1+ (4C–24H)Along 6C–5H
378818637Breathing R2Along 15N–10H
38840824815 (m)17 (6C–8H) + (1C–7H) Along R1
39781767763 (m)23 (6C–8H) + (ring R1)Along C–O
40737725729 (w)8 (C–C)R1 + (C–H)R1
4171970843 (23C–14H) + (17C–20H) Per R2
4270869811 (C–C) + rocking (11–10C–12H) Along R2
436996893 (C–H) + (C–C) Along 13–14O
4468167210 (13H–12C–14H)
4565264417γ(C–S–C)R2 Directed CH2
4663462727 (15C–16H)R2 + R2 + rocking (11H–10C–12H)Per R2
4762461817 (C–S–C)R2 + (C–H)R2 Along R2
48598593580 (w)9Twist R1 + Rocking (13H–12C–14H)Per R2
495665625 (R2) + (15N–16H)Along R1
5052652421R1 breathing + R2 twistPer R2
5151050914 (17C–18H) + ring R2 out-of-plane bendingAlong R1
524834836 (CCC)R1+ (C–Cl)R1Along 13C–10O
534604611Ring R1 twistAlong 5C–4C
544484504 (CCC)R1 + (1C−7H) Plane containing 16H
553863911 (ring R1)Along 5C–4C
563563621 (C–C)R1 + (15N–16H) Along R2
573193273 (C–C)R1 + (13C–14) Along CH2
582953041 (ring R2)Along 4C–24H
592882973 (CCC)R1 + (C–H)R1Along 18C–15N
602432541 (11H–10C–12H)Along R2
611992121 (C–Cl)R1 + (1C–7H) Along R1
621831972 (ring R1)Per R2
631351515 (CCC) + (CH)R1Per R2
641051221 (ring R1)Per R2
65941120 (ring R2)Per R1
6676956 (11H–10C–12H) + (13C–14H)Along R2
6733541 (ring R1)Per R2
6816371 (R1)Along R1
6912341Rings R1 and R2 twist jointlyAlong 4C–10Cl

: stretching; : symmetric stretching; : asymmetric stretching; : in-plane-bending; : out-of-plane bending; : wagging; : torsion, S: scissoring; vs: very strong; s: strong; m: medium; w: weak; vw: very weak.
3.2.1. Phenyl Ring (R1) Vibrations

The phenyl ring vibrations contain the C–H stretching modes in the region 3100–3000 cm−1, which is the characteristic region for the identification of C–H stretching vibrational modes [18]. In this region, the bands are not much affected by the nature of the substituents. The scaled C–H modes are found between 3096–3060 cm−1 which are observed at 3055 cm−1 for 2,4 substituted molecule polarized along 3C–6C and at 3034 cm−1 for 3,4 substitution. These modes are purely stretching having PED greater than 90%. Most of the C–H stretching modes are found to be weak due to charge transfer from hydrogen to carbon atom. Other C–H modes coming from bending (in-plane and out-of-plane), breathing and twisting of ring are found in the region below 1500 cm−1 having medium to weak intensities.

The C–C ring stretching vibrations are expected within the region 1650–1200 cm−1 [19]. Most of these ring modes are affected by the substitution to aromatic ring. Scaled frequencies for C–C stretching modes at B3LYP/6-31+G(d, p) are 1580 cm−1, 1548 cm−1, and 1477 cm−1 for 2,4 and 1583 cm−1, 1549 cm−1, and 1476 cm−1 for 3,4 substitution on the ring. Some other C–C modes associated with bending, twisting and breathing vibrations are also found to lie in lower frequency region as well as overlapped with other modes. Most of C–C modes are comparatively stronger than C–H modes.

3.2.2. Thiazol Ring (R2) Vibrations

The vibrations of thiazole ring associated with C–H stretching are calculated in between 3145–2950 cm−1 for title molecules which are essentially independent on the substitutions in the phenyl ring. These values are in agreement with observed bands in thiazoles and its derivatives [20]. The most intense bands corresponding to C–N stretching in thiazole are calculated at 1532-1531 cm−1. These vibrational modes are found to be coupled with N–H stretching and polarized along phenyl ring. The same bands are observed at 1544 cm−1–1527 cm−1 in the FTIR spectra. Other weak modes of thiazole ring vibration associated with breathing and twisting of rings are calculated below 1500 cm−1 and those specially associated with S atom are found near 600 cm−1.

3.2.3. Fragment (–NHCOCH2–) Vibrations

Amides show somewhat strong and broad band in the range between 3500 cm−1 and 3100 cm−1 for N–H stretch as well as a stronger band between 1700 cm−1 and 1650 cm−1 for C=O stretching [21]. Carbonyl absorptions are very sensitive and both the carbon and the oxygen atoms move during the vibration having nearly equal amplitude. The scaled values, 3458 cm−1–3456 cm−1 for N–H stretch and 1690 cm−1–1685 cm−1 for C=O stretch agree with literature values. In FTIR spectra, the corresponding band for C=O stretching are observed at 1691 cm−1–1689 cm−1 while N-stretching frequencies are lowered to 3199 cm−1–3197 cm−1. The differences between calculated and observed N–H stretch band are attributed to the presence of intermolecular H-bond in condensed phase which are absent in the isolated state of molecules.

In acetamide fragment, the stretching of methylene (–CH2) group and bending (scissoring and rocking) always occur in 3000 cm−1–2850 cm−1 and below 1500 cm−1, respectively [20]. These vibrations are more common and so are not much of significance. The stretching mode polarized along to ring R2 corresponding to –CH2 is calculated at 2950 cm−1 for 2,4 substitution and 2947 cm−1 for 3,4 substituted molecule.

3.3. Molecular Orbital Analysis

The highest occupied molecular orbital (HOMO) represents ability to donate an electron while lowest unoccupied molecular orbital (LUMO) denotes ability to accept it. The HOMO-LUMO plots for title molecules are shown in Figure 5. Evidently, the HOMOs of both molecules are located on the thiazole ring including amide fragment while the LUMOs are contributed mainly by phenyl ring system. The transition from HOMO → LUMO in these molecules indicates charge transfer to phenyl ring. The energy difference () between HOMO and LUMO describes the chemical reactivity of molecule. The smaller , 4.89 eV in 2,4 substituted molecule as compared to 4.94 eV in 3,4 substitution may suggest that former is chemically more reactive than the latter one. The chlorine atoms attached to adjacent carbons on phenyl ring for 3,4 substituted molecule make it less reactive.

Figure 5 also plots the molecular electrostatic potential (MESP) surface for title molecules. The MESP, a map of electrostatic potential on uniform electron density, is used to visualize charge or electron density distribution within the molecule. The importance of MESP lies in the fact that it simultaneously displays molecular size, shape as well as positive, negative, and neutral electrostatic potential regions in terms of colour grading. In present context, colour code varies between positive region for red and negative for blue regions with an isovalue of 1.00000 eV. Evidently, the electronegative region lies in the vicinity of carbonyl group of amide fragment. On the basis of MESP plot, it can be asserted that an electrophile will be attracted towards the negative region of amide fragment in both molecules.

3.4. Thermochemical Analysis

Thermochemical properties of molecules are dominated by molecular vibrations as electronic contribution becomes negligible due to absence of free electrons, especially at the room temperature. Various thermal parameters for title molecules are calculated and reported in Table 3. These parameters are related to one another via standard thermodynamic relations and can be useful for the estimation of chemical reaction paths. Zero point energy (ZPE) is also given. Thermal energy of 3,4 substituted molecule is slightly higher than that with 2,4 substitution while heat capacity is smaller. The entropy values are closely related to the geometry of molecules. The substitution of chlorines at 3,4 position of ring leads to an increase in entropy value as compared to 2,4 substitution. The increase in thermal energy and entropy values may indicate the enhancement in molecular vibrations due to steric repulsion generated with chlorine atoms substituted at two consecutive (3,4) positions of the phenyl ring.


Parameters2-(2,4-Dichlorophenyl)-N-(1,3-thiazol-2-yl)acetamide2-(3,4-Dichlorophenyl)-N-(1,3-thiazol-2-yl)acetamide

Electronic energy (Hartree)−1927.348−1927.345
ZPE (kcal/mol)108.137
108.118
Thermal energy (kcal/mol)117.690117.696
Thermal enthalpy (kcal/mol)118.281118.287
Thermal free energy (kcal/mol)79.18778.675
Constant volume heat capacity (cal/mol-K)56.50956.438
Entropy (cal/mol-K)131.123132.861

4. Conclusions

We have performed a theoretical density functional calculations on dichloro substituted phenyl-N-(1,3-thiazol-2-yl)acetamides at 2,4 and 3,4 positions of the phenyl ring. The calculated structural parameter shows a good correlation with corresponding experimental data showing the validity of calculations. We have presented complete vibrational mode assignments using same computational scheme. Normal modes are discussed in detail and compared with those observed by FTIR spectroscopy. The chemical reactivity of molecules is discussed by HOMO-LUMO as well as MESP analysis thus exploring the effect of substituent positions. Calculated various thermodynamic parameters are very useful in determining chemical reaction paths.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

Ambrish K. Srivastava gratefully acknowledge the Council of Scientific and Industrial Research (CSIR), India, for providing financial assistance in the form of Junior Research Fellowship.

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Copyright © 2014 Ambrish K. Srivastava et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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