A test on calculated vibrational modes of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide using
The motivation for predicting or simulating vibrational spectra is to make vibrational spectroscopy a more practical tool [
In present paper, we perform a critical survey on the scaling of vibrational frequencies of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide by using two different schemes. The first scheme is based on a constant scale factor as suggested by Alecu et al. which we call “uniform scaling.” The second scheme is based on a scaling equation as recommended by Palafox et al., which we refer to as “nonuniform scaling.”
Our
For uniform scaling [
For nonuniform scaling [
First, we discuss some salient features of test molecule in condensed as well as isolated (gas) phase. The single crystal of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide is grown by slow evaporation method (melting point = 489–491 K) [
Crystal geometry of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide. Intermolecular interactions are shown by broken lines.
The measured Xray crystallographic parameters are used to model an initial structure of isolated molecule for the process of geometry optimization. After optimization, we find a sofa shaped structure in which the mean plane of dichlorophenyl ring is almost perpendicular to that of thiazole ring as shown in Figure
Molecular geometry of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide optimized at B3LYP/6–31+G(d,p) level.
In Figure
Correlation between calculated and experimental bondlengths of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide.
FTIR spectrum of test molecule is recorded by using ShimadzuModel Prestige 21 spectrometer in the region 400–4000 cm^{−1} with the sample (purity of 98%) in KBr pellet. The vibrational analysis of isolated molecule (Figure
Normal modes assignments and scaled frequencies for 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide. For comparison, observed FTIR bands are also given.
Calculated freq. cm^{−1}  Scaled freq. ( 
Scaled freq. ( 
FTIR freq. cm^{−1}  IR intensity 
Assignment of vibrational modes  Direction of polarization 

3602  3475  3459  3203  68 

Along 13N–14H 
3272  3156  3144  1 

Along 13H–11C  
3233  3119  3107  8.78 

Per R1  
3228  3114  3102  1.69 

Along 6C–5H  
3224  3110  3098  0.23 

Per R1  
3203  3090  3078  3045  3.32 

Along 2C–23H 
3137  3026  3015  0.09 

Per R1  
3093  2984  2973  3.43 

Along 11C–12O  
1746  1684  1688  1687  266.85 

Along 13N–14H 
1627  1569  1574  9.20 

Along R2  
1603  1546  1551  1550  37.68 

Per R1 
1582  1526  1531  430.27 

Per R2  
1525  1471  1477  107.34 

Along 11H–12O  
1474  1422  1428  1436  9.00  Scissoring (9H–8C–10H) + 
Along 11H–12O 
1468  1416  1423  63.64 

Along R2  
1465  1413  1420  16.52 

Per14H plane  
1459  1407  1414  26.22  Scissoring (9H–8C–10H) + 
Containing 13N–14H  
1349  1301  1309  62.92 

Along 13N–16C  
1335  1288  1296  4.25  Rocking (9H–8C–10H) + 
Along 8C–5C  
1334  1287  1295  1286  1.05 

Per R1 
1306  1260  1268  112.91 

Plane containing 17N directed along R1  
1251  1206  1215  1.41 

Per R1  
1226  1182  1192  53.76 

Along 8C–11C  
1220  1177  1186  43.98 

Along 17N–16C  
1196  1153  1163  9.49 

Per R1  
1181  1139  1149  30.16 

Along 13N–14H  
1172  1130  1140  1141  16.73 

Per R1 
1101  1062  1072  13.16 

Along 11C–12O  
1088  1049  1060  9.84  Scissoring (18H–15C–21C–17H)  Along R2  
1086  1047  1058  8.96 

Along R1  
987  952  963  0.33 

Plane containing 13N directed to R1  
975  940  952  931  7.78 


939  905  918  41.74  Rocking (9H–8C–10H)  Per R1  
909  877  889  0.06 

Along R2  
897  865  878  3.26 

Per R1  
882  850  863  856  6.68  In plane R1 bending  Along R1 
855  824  838  3.10  Ring R2 breathing (9H–8C–10H)  Along R2  
787  759  773  785  38.64 

In between 13N–14H 
783  755  769  761  23.20 

Plane containing 13N directed to R1 
767  740  754  67.40  In plane R2 bending + 
Per R1  
718  692  707  692  39.09 

Per R1 
661  637  652  11.08 

Along 16C–21H  
631  608  624  29.47 

Per R1  
624  602  617  592  8.67 

Per R1 
563  543  559  2.16 

Per R1  
559  539  555  12.84  Twisting R1 + rocking (9H–8C–10H)  Per R2  
511  493  509  11.14 

Per R1  
486  468  485  8.55 

Per R2  
478  461  478  4.74 

Per R2  
424  409  426  4.28 

Along 10C–11O  
404  389  407  5.09 

Per R1 
Calculated (a) and experimental (b) vibrational IR spectra of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide in the region 1800–600 cm^{−1}.
In Figures
Correlation between calculated (scaled) and experimental wavenumbers of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide. Scaling is performed by a scale factor (equation (
Correlation between calculated (scaled) and experimental wavenumbers of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide. Scaling is performed by a scaling equation (equation (
This region covers C–H stretching vibrations of phenyl ring and methylene group along with N–H as well as C=O stretching modes of amide group. N–H stretching mode is observed experimentally at 3203 cm^{−1} and corresponding scaled frequencies are calculated at 3475 cm^{−1} and 3459 cm^{−1} by uniform scaling and nonuniform scaling, respectively. The apparent discrepancy between experimental and theoretical frequencies is a consequence of intermolecular hydrogen bonding present in condensed phase (see Figure
The C=O stretching observed at 1687 cm^{−1} is scaled at 1684 and 1688 cm^{−1}, respectively. Similarly, C–C stretching of phenyl ring at 1550 cm^{−1} corresponds to scaled values of 1546 cm^{−1} and 1551 cm^{−1}. The C–H stretching associated with methylene group is found to be coupled with N–H stretching mode at 1436 cm^{−1} which is theoretically scaled at 1422 cm^{−1} and 1428 cm^{−1} by uniform scaling and nonuniform scaling schemes, respectively.
In this region, the bending modes associated with ring systems and various groups are generally observed. The inplane bending of methylene group observed at 1286 cm^{−1} and 1141 cm^{−1} is coupled with phenyl ring vibration. The uniform scaling gives these modes at 1287 cm^{−1} and 1130 cm^{−1} while nonuniform scaling calculates at 1295 cm^{−1} and 1140 cm^{−1}, respectively. Similarly, out of plane bending of methylene coupled with phenyl ring at 931 cm^{−1} in FTIR is scaled at 940 cm^{−1} and 952 cm^{−1} by our theoretical schemes.
The inplane bending of thiazole ring is observed at 856 cm^{−1} against scaled values of 850 cm^{−1} and 863 cm^{−1}. On the other hand, the bending of phenyl ring observed at 785 cm^{−1} is scaled at 759 cm^{−1} and 773 cm^{−1}. Furthermore, the coupling of vibrations of both rings is observed at 761 cm^{−1} corresponding to scaled values of 755 cm^{−1} and 769 cm^{−1}. Moreover, even lower normal modes at 692 cm^{−1} and 592 cm^{−1} associated with bending of thiazole rings are scaled at 692 cm^{−1} and 602 cm^{−1} by uniform scaling method, whereas at 707 cm^{−1} and 617 cm^{−1} by nonuniform scaling scheme.
The above discussion suggests that scaled wavenumbers by both schemes successfully explain observed bands in case of title molecule, giving a good correlation with experimental frequencies (see Figures
The difference between scaled and experimental wavenumbers for 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide.
We have performed a test on the scaling of calculated vibrational bands in case of 2(2,6dichlorophenyl)N(1,3thiazol2yl) acetamide, adopting two different scaling schemes, namely, scale factor and scaling equation. Scaled normal modes are compared with observed vibrational bands. The validity of both schemes is established by the correlation between theoretically scaled frequencies and experimentally observed frequencies with the coefficient of 0.9964. The analysis of individual modes has revealed that both schemes perform equally well for the lower range of frequencies below 1400 cm^{−1} (torsion, wagging, etc.). In case of high frequency region (above 1400 cm^{−1}) where most of the stretching modes occur, scaling equation provides more accurate vibrational bands as compared to scale factor. To be specific, C−H stretching of phenyl ring observed at 3045 cm^{−1} (in present case) is better represented by 3078 cm^{−1} (by scaling equation) rather than by 3090 cm^{−1} (by scale factor) at B3LYP/6–31+G(d,p) theory. Thus, the present assessment is supposed not only to assist further studies on the prediction or simulation of vibrational spectra of new molecules but also to assign more accurate vibrational modes for previously reported molecules in the literature.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Ambrish K. Srivastava wishes to thank Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing a research fellowship.