Terahertz (THz) absorption spectra of the similarly structured molecules with amide groups including benzamide, acrylamide, caprolactam, salicylamide, and sulfanilamide in the solid phase at room temperature and 7.8 K for salicylamide are reported and compared to infrared vibrational spectral calculations using density functional theory. The results of THz absorption spectra show that the molecules have characteristic bands in the region of 0.2–2.6 THz (~7–87 cm−1). THz technique can be used to distinguish different molecules with amide groups. In the THz region benzamide has three bands at 0.83, 1.63, and 1.73 THz; the bands are located at 1.44 and 2.00 THz for acrylamide; the bands at 1.24, 1.66 and 2.12 THz are observed for caprolactam. The absorption bands are located at 1.44, 1.63, and 2.39 THz at room temperature, and at 1.22, 1.46, 1.66, and 2.41 THz at low temperature for salicylamide. The bands at 1.63, 1.78, 2.00, and 2.20 THz appear for sulfanilamide. These bands in the THz region may be related to torsion, rocking, wagging, and other modes of different groups in the molecules.
Spectroscopy is a powerful technique for measuring, analyzing, and identifying various molecules. Frequencies in the far-infrared (FIR, 650–50 cm−1) and terahertz (THz) (0.1–10 THz, or 3.3–333.6 cm−1) ranges correspond to motions of the entire molecular structure, involving relatively large masses and relatively shallow potentials [
Amide groups contribute significantly to the protein structure. A variety of techniques such as IR, NMR, Raman, ultrasonic absorption, and UV/Vis spectroscopies have been used to characterize both the intermolecular and intramolecular bonding in amide compounds [
Here we have investigated the low-frequency vibrational motions of salicylamide, benzamide, acrylamide, caprolactam, and sulfanilamide in the solid state by concentrating on the spectral region between 7 and 650 cm−1 where crystal lattice vibrations, hydrogen bond bending modes and collective modes, and so forth may occur [
Salicylamide, benzamide, acrylamide, caprolactam, and sulfanilamide were obtained from commercial sources and used without further purification.
The THz absorption spectra were recorded on the THz time-domain device of Capital Normal University of China, based on photoconductive switches for generation and electrooptical crystal detection of the far-infrared light. The experimental apparatus for terahertz transmission measurements has been discussed in detail elsewhere [
The FIR spectra of benzamide, acrylamide, caprolactam, and salicylamide were measured using common used Nujol mull method, because mineral oil has no absorption in the far-IR region and the method can protect sample in solid state against wet and avoid distortion of bands or happening of ion exchange. Samples were suspended in the Nujol mull and then were daubed on a thin polyethylene window and another thin polyethylene window as background for comparison. Far infrared spectra in the range of 650–50 cm−1 were taken on a Nicolet Magna-IR 750-II Spectrometer at room temperature and at 8 cm−1 resolution, 128 scans. The optical bench was purged with dried air. The FIR spectrum of sulfanilamide was measured on a Bruker VERTEX 80v FTIR spectrometer at 4 cm−1 resolution and 32 scans. The preparation of the sample was by pressing mixed pellets with polyethylene powder.
All theoretical calculations were performed with the Gaussian 03 software package [
The molecular structures of benzamide, acrylamide, caprolactam, salicylamide, and sulfanilamide are shown in Figure
Calculated and experimental vibrational frequencies for salicylamide, benzamide, acrylamide, caprolactam, and sulfanilamide (cm−1)a.
Compounds | Experiment | Calculation | Vibrational assignments | |
---|---|---|---|---|
FIR | THz-TDS | 6-311++G(3df, 2pd) | ||
Salicylamide | 526 | 538 |
| |
515 | 509 |
| ||
456 | 439 | In-plane wagging of ph, t(C=O-NH2 and COH) | ||
419 | Out-of-plane folding of ph, | |||
385 | 383 |
| ||
296 | 275 |
| ||
247 | Out-of-plane wagging of whole molecule, especially ph ring | |||
161 | 184 |
|
||
146 | ||||
108 | 141 |
| ||
94 | ||||
80 | ||||
54 (55*) | ||||
48 (49*) | ||||
41* | 30 | t(ph-C=O-NH2) | ||
|
||||
Benzamide | 503 | In-plane wagging of ph and amide | ||
415 | Out-of-plane folding of ph ring | |||
412 | 414 | Out-of-plane folding of ph ring, |
||
383 | 375 | In-plane rocking of amide group, | ||
341 |
|
|||
251 | 217 | In-plane rocking of ph ring, | ||
178 | ||||
151 | 152 |
|
||
110 | ||||
90 | ||||
58 | ||||
54 | 54 | 57 |
|
|
38 | ||||
|
||||
Acrylamide | 508 | 470 | t(CH=CH2), tNH2 | |
468 |
| |||
314 | 277 |
| ||
185 | 190 |
|
||
122 | 114 |
| ||
67 | 67 | |||
48 | ||||
|
||||
Caprolactam | 503 | 508 |
|
|
488 | 437 |
| ||
398 | 402 |
| ||
337 | 323 |
|
||
323 | 301 |
| ||
258 | 258 |
| ||
195 | 158 |
| ||
129 | 107 |
| ||
87 | ||||
69 | 71 | |||
56 | 55 | |||
41 | ||||
|
||||
Sulfanilamide | 73 | 85 | Out-of-plane wagging of ph, |
|
67 | ||||
59 | ||||
54 | 15 | Out-of-plane wagging of ph, tNH2 |
The molecular structures of acrylamide, benzamide, salicylamide, caprolactam, and sulfanilamide. (a) Acrylamide; (b) benzamide; (c) salicylamide; (d) caprolactam; (e) sulfanilamide.
Acrylamide
Benzamide
Salicylamide
Caprolactam
Sulfanilamide
THz absorption spectra of acrylamide, benzamide, salicylamide, caprolactam, and sulfanilamide.
FIR spectra of acrylamide, benzamide, salicylamide, caprolactam, and sulfanilamide in the 650–50 cm−1 region.
All of the structures of benzamide, acrylamide, caprolactam, salicylamide and sulfanilamide have amide groups. The difference of benzamide, and salicylamide is that there is one more OH group in the structure of salicylamide, and both of the two molecules have benzene ring (ph). The THz absorption spectra of benzamide, acrylamide, caprolactam, salicylamide and sulfanilamide in Figure
To clarify the bands in the THz region, second derivatives have been performed using Omnic 5.0 software for the THz spectra of the samples, and the results show that main bands and some relatively minor bands are observed in the second derivatives results (shown in Figure
The second derivatives results for the THz spectra of the samples using Omnic 5.0 software.
The FIR spectra of benzamide, acrylamide, caprolactam, salicylamide and sulfanilamide, after automatic baseline correction shown in Figure
For salicylamide, it has many bands in the region: 608, 564, 526, 515, 456, 421, 385, 296, 160, 146, 108, and 93 cm−1. For benzamide, its bands are as follows: 635, 529, 412, 382, 251, 177, 151, 110, 89, and 54 cm−1; for acrylamide, it has several bands in the region: 619, 508, 314, 185, 122, and 67 cm−1; for caprolactam, it has relatively more bands in the region: 581, 503, 488, 398, 337, 323, 258, 195, 129, 87, 69, and 56 cm−1. Sulfanilamide has the bands located at 641, 626, 563, 541, 497, 450, 414, 365, 302, 224, 132, 88, 72, and 58 cm−1 in its FIR spectrum. The FIR and THz results are consistent for the samples in the 100–50 cm−1 region as shown in Table
For benzamide and salicylamide, there is one OH difference. Their FIR spectra have various peak positions and relative intensities; only several peak positions are close, such as 529(benzamide)/526(salicylamide); 382/385; 151/146; 110/108; 89/93 cm−1. Other bands are located at different positions. For acrylamide and caprolactam, the bands at 508/503, 314/323, 185/195, 122/129, and 67/69 cm−1 are similar in some extent. Caprolactam has relatively more bands in the FIR region. Sulfanilamide has two strong bands located at 563 and 541 cm−1, and the band at 365 cm−1 is relatively strong, and other bands are weak. The five molecules have different characteristic bands in the FIR region.
The assignment of the spectra in the FIR and THz region is difficult. Fortunately, there are some calculation results for benzamide, salicylamide, and sulfanilamide [
Low-frequency vibrational modes of the molecules below 100 cm−1. (a) 57 cm−1 for benzamide; (b) 30 cm−1 for salicylamide; (c) 15 cm−1 for sulfanilamide; (d) 85 cm−1 for sulfanilamide.
57 cm−1 for benzamide
30 cm−1 for salicylamide
15 cm−1 for sulfanilamide
85 cm−1 for sulfanilamide
According to the DFT calculation results (shown in Table
For salicylamide, 30 cm−1 band is assigned to twisting of ph–CONH2; 141 cm−1 band is assigned to out-of-plane vibration of whole molecule; 184 cm−1 band is assigned to wagging of NH2; 247 cm−1 band is assigned to out-of-plane wagging of whole molecule, especially ph ring; 275 cm−1 band is assigned to rocking of NH2, rocking of ph; 383 cm−1 band is assigned to in-plane vibration of ph and torsion of (C=O–NH2 and COH); 419 cm−1 band is assigned to out-of-plane folding of ph and stretching of NH; 439 cm−1 band is assigned to in-plane wagging of ph; twisting of (C=O–NH2 and COH); 509 cm−1 band is assigned to rocking of NH2, rocking of (C=O and OH), in-plane wagging of ph, and so forth. For acrylamide, 114 cm−1 band is assigned to wagging of NH2 and wagging of CH=CH2; 190 cm−1 band is assigned to wagging of NH2; 277 cm−1 band is assigned to rocking of (CH=CH2) and rocking of (C=O–NH2); 468 cm−1 band is assigned to rocking of NH2 and wagging of (CH=CH2); 470 cm−1 band is assigned to twisting of (CH=CH2) and twisting of NH2; 613 cm−1 band is assigned to twisting of NH2 and twisting of CH2, and so forth. For caprolactam, 107 cm−1 band is assigned to wagging of CH2, stretching of C=O, and out-of-plane vibration of the ring; 158 cm−1 band is assigned to rocking of CH2 and in-plane vibration of the ring; 258 cm−1 band is assigned to rocking of CH2, wagging of CH2, stretching of NH, stretching of C=O, and so forth; 301 cm−1 band is assigned to stretching of NH and in-plane vibration of the ring; 323 cm−1 band is assigned to rocking of CH2; 402 cm−1 band is assigned to in-plane vibration of the ring, and so forth. In most of the cases whole molecule is involved in the vibration modes. The calculation results are similar in some extent to the results in [
The FIR and THz characteristic bands of five molecules with amide groups are observed using FIR and terahertz time-domain spectroscopy. The five molecules exhibit several bands in the THz region, which may be related to torsion, rocking, wagging, and other modes of different groups in the molecules according to calculation and the assignments in the references. THz spectra are effective method to distinguish different molecules with amide groups. In addition, it is also the basis for investigation on hydration of amide group, and so forth.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the financial support by National Natural Science Foundation of China for the Grants (21001009 and 50973003), the State Key Project for Fundamental Research of MOST (2011CB808304), National High-tech R&D Program of China (863 Program) of MOST (2010AA03A406), and the Scientific Research Project of Beijing Municipal Commission of Education and Beijing Natural Science Foundation (Grant no. KZ201310028032). Limin Yang is one of the corresponding authors.