FT-IR and Raman spectra of methacrylamidoantipyrine (MAAP) have been reported in the region of 4000–10 cm−1 and 4000–100 cm−1, respectively. The optimized geometric parameters, conformational analysis, normal mode frequencies, and corresponding vibrational assignments of MAAP (C15H17N3O2) have been examined by means of density functional theory (DFT) method using the Becke-3-Lee-Yang-Parr (B3LYP) exchange-correlation functional and the 6-31G++(d,p) basis sets. Vibrational assignments have been made on the basis of potential energy distribution (PED) and the thermodynamics functions, and the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) of MAAP have been predicted. Calculations are carried out with the possible seven (amide-1–5 and imide-1-2) conformational isomers of MAAP. Comparison between the experimental and theoretical results indicates that the B3LYP method provides satisfactory evidence for the prediction of vibrational wavenumbers, and the amide-1 conformational isomer is supposed to be the most stable form of MAAP.
The MAAP, which has a
Vibrational spectroscopy has been widely used as the standard tool for structural characterization of molecular systems together with DFT calculations [
Though MAAP has wide applications in science, to the best of our knowledge, there is limited information available in the literature about its spectroscopic properties. A detailed, quantum chemical study will be useful in making assignments to the fundamental normal modes and in explaining the obtained experimental data of MAAP. Furthermore, theoretically and experimentally presented data may be helpful in the context of the further studies of MAAP. For the above goals, we have reported vibrational spectra of MAAP. Vibrational frequencies with PED values, HOMO and LUMO data, structural parameters, and some thermodynamics functions of MAAP are also calculated for the most stable conformational isomer of MAAP by means of B3LYP/6-31G++(d,p) level. The results of the theoretical and spectroscopic studies are reported here.
MAAP was prepared according to the published procedure [
All the calculations were performed using Gaussian 09.A1 program [
Optimized conformational isomers and numbering of MAAP.
Therefore, after the optimization, harmonic vibrational frequencies and corresponding vibrational intensities for the amide-1 conformational isomer of MAAP were calculated by using the same method and basis set and then scaled by 0.955 (above 1800 cm−1) and 0.977 (under 1800 cm−1) [
To clarify the vibrational frequencies, it is essential to examine the geometry of any compound, as small changes in geometry can potentially cause substantial differences in frequencies. Gibbs free energy and relative stability of the optimized geometries in gas phase for seven conformational isomers of MAAP with B3LYP/6-31G++(d,p) method are given in Figure
Some of the optimized geometric parameters such as bond lengths and bond angles calculated by B3LYP/6-31G++(d,p) are listed in Table
Some optimized geometric parameters of MAAP.
Parameters | B3LYP/6-31G++(d,p) | ||
---|---|---|---|
Amide-1 | Ref. [ |
Ref. [ | |
Bond lengths ( |
|||
C1–C2 | 1.457 | 1.426 | |
C1–C7 | 1.362 | 1.369 | |
C1–N20 | 1.400 | 1.394 | 1.407 |
C2–O3 | 1.232 | 1.239 | |
C2–N4 | 1.400 | 1.395 | |
N4–N5 | 1.414 | 1.399 | |
N4–C9 | 1.420 | 1.422 | |
N5–C6 | 1.474 | 1.452 | |
N5–C7 | 1.406 | 1.362 | |
C7–C8 | 1.494 | 1.487 | |
C15–C16 | 1.508 | 1.482 | |
C15–O19 | 1.229 | 1.227 | |
C15–N20 | 1.377 | 1.359 | |
C16–C17 | 1.506 | 1.426 | |
C16–C18 | 1.342 | 1.403 | |
| |||
Bond angles (°) | |||
C2–C1–N20 | 117.8 | 128.8 | |
C7–C1–N20 | 133.0 | 122.0 | |
C1–C2–O3 | 128.2 | 132.3 | |
C1–C2–N4 | 105.0 | 104.8 | |
O3–C2–N4 | 126.7 | 122.9 | |
C2–N4–N5 | 109.8 | 109.4 | |
C2–N4–C9 | 125.1 | 127.1 | |
N4–N5–C6 | 113.1 | 119.6 | |
N4–N5–C7 | 106.7 | 107.5 | |
C6–N5–C7 | 117.3 | 128.9 | |
C1–C7–N5 | 109.3 | 109.2 | |
N5–C7–C8 | 119.2 | 121.7 | |
C16–C15–O19 | 121.3 | 122.1 | |
C16–C15–N20 | 115.9 | 115.6 | |
O19–C15–N20 | 122.8 | 122.3 | |
C15–C16–C17 | 114.8 | 116.3 | |
C15–C16–C18 | 121.7 | 120.7 | |
C17–C16–C18 | 123.4 | 123.0 | |
C1–N20–C15 | 126.2 | 121.4 | 128.2 |
Generally, it is expected that the bond distances calculated by electron correlated methods are longer than the experimental distance. This situation can be seen in Table
The MAD and RMSD of bond angles are 3.15 and 4.96°, respectively. The observed differences in bond distances and angles are not due to the theoretical shortcomings, as experimental results are also subject to variations owing to the insufficient data to calculate equilibrium structure which are sometimes averaged over zero point vibrational motion. Furthermore, it can be noted that theoretical results have been compared with available experimental data.
For the optimized geometric parameters, magnitude of dihedral angles, D (10; 11; 12; 13) = 0.70°, D (1; 7; 4; 5) = 177.49°, D (8; 7; 5; 4) = 178.46°, D (20; 1; 2; 4) = 178.68°, D (1; 20; 15; 16) = 165.90°, D (17; 16; 15; 18) = 175.60°, D (14; 9; 5; 4) = 21.60°, D (1; 2; 3; 7) = 173.40°, D (15; 16; 18; 20) = 17.85°, D (1; 20; 15; 19) = 15.35°, and D (15; 16; 17; 19) = 13.24°, indicate that a large part of molecule is nearly in the same plane. In this compound, two intramolecular hydrogen bonds can be observed as N20-H36
To the best of our knowledge, the vibrational wavenumbers and assignments of MAAP in the middle and far infrared regions of the spectrum have not been reported in the literature. But, a few selected bands of MAAP were reported by Ersöz et al. [
Experimental and calculated vibrational wavenumbers (cm−1) of MAAP.
Mode | P.E.D (≥5%) | Experimental | B3LYP/6-31G++(d,p) | ||||
---|---|---|---|---|---|---|---|
IR | Raman |
|
|
|
| ||
|
|
3249 s, b | 3254 s, b | 3588 | 3426 | 38.29 | 16.36 |
|
|
3239 | 3093 | 0.43 | 12.83 | ||
|
|
3094 w | 3235 | 3090 | 13.53 | 17.90 | |
|
|
3080 vs | 3218 | 3073 | 3.23 | 25.29 | |
|
|
3059 m | 3207 | 3062 | 25.36 | 40.11 | |
|
|
3193 | 3049 | 13.87 | 25.64 | ||
|
|
3044 w | 3183 | 3040 | 1.14 | 12.12 | |
|
|
3172 | 3029 | 2.42 | 13.11 | ||
|
|
3167 | 3025 | 8.58 | 8.38 | ||
|
|
3005 s | 3154 | 3012 | 4.74 | 32.59 | |
|
|
3136 | 2995 | 14.79 | 17.50 | ||
|
|
3132 | 2991 | 12.80 | 14.93 | ||
|
|
2969 s | 2968 m | 3111 | 2971 | 12.02 | 17.64 |
|
|
3104 | 2964 | 12.55 | 15.67 | ||
|
|
2919 s | 2934 vs | 3051 | 2914 | 18.10 | 52.55 |
|
|
3042 | 2905 | 21.81 | 39.66 | ||
|
|
2863 m | 3028 | 2892 | 50.02 | 29.76 | |
|
|
1673 vs | 1683 s | 1743 | 1703 | 152.72 | 26.56 |
|
|
1648 vs | 1631 vs | 1734 | 1694 | 363.40 | 52.85 |
|
|
1702 | 1663 | 40.11 | 111.41 | ||
|
|
1620 vs | 1691 | 1652 | 100.80 | 34.41 | |
|
|
1592 vs | 1605 vs | 1647 | 1609 | 53.05 | 79.10 |
|
|
1570 m | 1633 | 1595 | 2.13 | 4.99 | |
|
|
1491 vs | 1556 | 1520 | 352.19 | 41.25 | |
|
|
1531 | 1496 | 105.42 | 13.21 | ||
|
|
1515 | 1481 | 15.71 | 6.95 | ||
|
|
1500 | 1465 | 2.85 | 7.29 | ||
|
|
1455 vs | 1466 m, sh | 1499 | 1465 | 68.70 | 2.59 |
|
|
1491 | 1457 | 7.98 | 1.88 | ||
|
|
1484 | 1450 | 4.60 | 13.15 | ||
|
|
1477 | 1443 | 12.74 | 7.51 | ||
|
|
1446 w | 1477 | 1443 | 4.71 | 5.30 | |
|
|
1456 | 1423 | 8.18 | 7.40 | ||
|
|
1426 s | 1454 | 1421 | 7.77 | 21.77 | |
|
|
1433 | 1400 | 76.53 | 29.21 | ||
|
|
1414 | 1382 | 5.79 | 5.84 | ||
|
|
1407 s | 1392 | 1360 | 178.18 | 19.77 | |
|
|
1368 s | 1368 vs | 1371 | 1339 | 88.20 | 122.68 |
|
|
1360 | 1329 | 1.59 | 2.24 | ||
|
|
1310 vs, sh | 1303 s | 1340 | 1309 | 78.89 | 32.98 |
|
|
1325 | 1295 | 7.14 | 10.34 | ||
|
|
1295 vs | 1245 w, sh | 1292 | 1263 | 218.19 | 34.06 |
1285 vs | |||||||
|
|
1200 s | 1233 | 1205 | 85.54 | 0.69 | |
|
|
1211 m | 1219 | 1191 | 43.20 | 36.30 | |
|
|
1170 m | 1200 | 1173 | 1.88 | 4.90 | |
|
|
1184 | 1157 | 0.60 | 4.78 | ||
|
|
1126 w | 1164 | 1138 | 5.43 | 2.93 | |
|
|
1136 s | 1156 | 1130 | 16.14 | 8.01 | |
|
|
1105 m | 1130 | 1104 | 14.51 | 7.56 | |
|
|
1074 w | 1069 w | 1104 | 1079 | 12.00 | 0.64 |
|
|
1060 m | 1082 | 1058 | 13.07 | 0.41 | |
|
|
1071 | 1047 | 0.12 | 2.18 | ||
|
|
1060 | 1035 | 4.98 | 1.92 | ||
|
|
1057 | 1032 | 0.42 | 0.55 | ||
|
|
1026 m | 1008 vs | 1041 | 1017 | 18.61 | 24.00 |
|
|
1001 w | 1026 | 1002 | 6.02 | 2.76 | |
|
|
973 w | 1014 | 990 | 0.17 | 68.48 | |
|
|
998 | 975 | 0.06 | 0.48 | ||
|
|
980 | 958 | 0.21 | 0.17 | ||
|
|
956 vw, sh | 970 | 948 | 13.72 | 0.75 | |
|
|
932 s | 946 m | 954 | 932 | 38.98 | 3.92 |
|
|
943 | 921 | 6.80 | 9.61 | ||
|
|
922 | 901 | 3.20 | 3.83 | ||
|
|
892 w | 902 m | 907 | 886 | 1.89 | 20.36 |
|
|
842 m | 858 m | 848 | 829 | 0.20 | 3.18 |
|
|
806 w | 838 | 819 | 6.14 | 4.33 | |
|
|
783 w | 803 | 785 | 11.75 | 2.48 | |
|
|
763 vs | 749 w | 772 | 754 | 42.94 | 1.70 |
|
|
744 m | 746 | 729 | 13.41 | 4.16 | |
|
|
708 s | 705 vw | 728 | 711 | 42.51 | 1.35 |
703 s | |||||||
|
|
701 | 685 | 16.44 | 0.60 | ||
|
|
685 s | 678 | 662 | 2.43 | 13.74 | |
|
|
666 m | 656 m | 668 | 653 | 6.92 | 5.11 |
|
|
641 m | 658 | 643 | 23.86 | 5.80 | |
|
|
630 | 616 | 1.15 | 7.40 | ||
|
|
603 s | 1340 | 608 | 56.00 | 27.67 | |
|
|
1325 | 590 | 11.65 | 5.02 | ||
|
|
587 s | 1292 | 575 | 25.19 | 15.04 | |
|
|
622 | 564 | 5.98 | 4.06 | ||
|
|
501 s | 604 | 501 | 8.72 | 1.59 | |
|
|
476 w | 588 | 478 | 8.40 | 2.76 | |
|
|
436 m | 577 | 447 | 19.92 | 15.42 | |
|
|
423 m | 512 | 417 | 1.68 | 4.29 | |
|
|
409 w | 397 vw | 489 | 409 | 0.58 | 1.88 |
|
|
373 s | 458 | 383 | 2.79 | 7.76 | |
|
|
427 | 346 | 0.60 | 11.02 | ||
|
|
331 m | 335 w | 419 | 336 | 9.39 | 1.00 |
|
|
392 | 305 | 1.22 | 3.69 | ||
|
|
289 s | 295 w | 354 | 284 | 12.51 | 4.15 |
|
|
261 w | 275 w | 344 | 271 | 1.06 | 1.95 |
|
|
312 | 244 | 1.68 | 3.36 | ||
|
|
225 vw | 290 | 217 | 5.16 | 9.01 | |
|
|
277 | 213 | 0.40 | 2.36 | ||
|
|
200 m | 250 | 207 | 3.45 | 21.32 | |
|
|
169 vw | 222 | 169 | 0.51 | 3.26 | |
|
|
218 | 157 | 1.07 | 5.37 | ||
|
|
140 s | 212 | 141 | 3.32 | 7.37 | |
|
|
134 s | 173 | 134 | 0.62 | 23.80 | |
|
|
111 m | 160 | 113 | 4.87 | 17.07 | |
|
|
144 | 77 | 1.35 | 138.91 | ||
|
|
71 vw | 137 | 68 | 5.61 | 39.27 | |
|
|
60 vw | 115 | 55 | 1.93 | 72.49 | |
|
|
50 vw | 79 | 46 | 2.34 | 115.46 | |
|
|
41 vw | 70 | 39 | 4.55 | 83.80 | |
|
|
57 | 24 | 1.74 | 72.73 |
Complete set of optimized geometric parameters for amide-1 conformational isomer of MAAP.
B3LYP/6-31G++(d,p) | |||||||
---|---|---|---|---|---|---|---|
Parameters | Amide-1 | Parameters | Amide-1 | Parameters | Amide-1 | Parameters | Amide-1 |
Bond lengths (Å) | Bond angles (°) | Dihedral angles (°) | Dihedral angles (°) | ||||
C1–C2 | 1.457 | C2–C1–C7 | 108.9 | C7–C1–C2–O3 | −173.4 | N4–C9–C10–H27 | 0.6 |
C1–C7 | 1.362 | C2–C1–N20 | 117.8 | C7–C1–C2–N4 | 3.4 | C14–C9–C10–C11 | −0.2 |
C1–N20 | 1.400 | C7–C1–N20 | 133.0 | N20–C1–C2–O3 | 1.9 | C14–C9–C10–H27 | −179.2 |
C2–O3 | 1.232 | C1–C2–O3 | 128.2 | N20–C1–C2–N4 | 178.7 | N4–C9–C14–C13 | 179.5 |
C2–N4 | 1.400 | C1–C2–N4 | 105.0 | C2–C1–C7–N5 | −0.5 | N4–C9–C14–H31 | −1.4 |
N4–N5 | 1.414 | O3–C2–N4 | 126.7 | C2–C1–C7–C8 | 178.3 | C10–C9–C14–C13 | −0.7 |
N4–C9 | 1.420 | C2–N4–N5 | 109.8 | N20–C1–C7–N5 | −174.9 | C10–C9–C14–H31 | 178.4 |
N5–C6 | 1.474 | C2–N4–C9 | 125.1 | N20–C1–C7–C8 | 4.0 | C9–C10–C11–C12 | 0.9 |
N5–C7 | 1.406 | N5–N4–C9 | 119.5 | C2–C1–N20–C15 | 139.6 | C9–C10–C11–H28 | −179.5 |
C6–H21 | 1.098 | N4–N5–C6 | 113.1 | C2–C1–N20–H37 | −19.6 | H27–C10–C11–C12 | 179.9 |
C6–H22 | 1.091 | N4–N5–C7 | 106.7 | C7–C1–N20–C15 | −46.5 | H27–C10–C11–H28 | −0.6 |
C6–H23 | 1.091 | C6–N5–C7 | 117.3 | C7–C1–N20–H37 | 154.3 | C10–C11–C12–C13 | −0.7 |
C7–C8 | 1.494 | N5–C6–H21 | 111.4 | C1–C2–N4–N5 | −4.9 | C10–C11–C12–H29 | 179.4 |
C8–H24 | 1.089 | N5–C6–H22 | 109.1 | C1–C2–N4–C9 | −158.0 | H28–C11–C12–C13 | 179.8 |
C8–H25 | 1.096 | N5–C6–H23 | 108.4 | O3–C2–N4–N5 | 171.9 | H28–C11–C12–H29 | −0.1 |
C8–H26 | 1.095 | H21–C6–H22 | 109.5 | O3–C2–N4–C9 | 161.2 | C11–C12–C13–C14 | −0.2 |
C9–C10 | 1.402 | H21–C6–H23 | 109.7 | C2–N4–N5–C6 | 135.1 | C11–C12–C13–H30 | −179.4 |
C9–C14 | 1.402 | H22–C6–H23 | 108.6 | C2–N4–N5–C7 | 4.7 | H29–C12–C13–C14 | 179.6 |
C10–C11 | 1.395 | C1–C7–N5 | 109.3 | C9–N4–N5–C6 | 70.1 | H29–C12–C13–H30 | 0.4 |
C10–H27 | 1.083 | C1–C7–C8 | 131.4 | C9–N4–N5–C7 | 20.5 | C12–C13–C14–C9 | 0.9 |
C11–C12 | 1.398 | C1–C7–N5 | 109.3 | C2–N4–C9–C10 | −50.7 | C12–C13–C14–H31 | −178.2 |
C11–H28 | 1.086 | N5–C7–C8 | 119.2 | C2–N4–C9–C14 | 129.1 | H30–C13–C14–C9 | −179.9 |
C12–C13 | 1.397 | C7–C8–H24 | 109.9 | N5–N4–C9–C10 | 158.6 | H30–C13–C14–H31 | 1.0 |
C12–H29 | 1.086 | C7–C8–H25 | 111.7 | N5–N4–C9–C14 | −21.6 | O19–C15–C16–C17 | 31.4 |
C13–C14 | 1.396 | C7–C8–H26 | 110.4 | N4–N5–C6–H21 | −62.2 | O19–C15–C16–C18 | −144.2 |
C13–H30 | 1.086 | H24–C8–H25 | 106.6 | N4–N5–C6–H22 | 58.8 | N20–C15–C16–C17 | −147.4 |
C14–H31 | 1.084 | H24–C8–H26 | 109.3 | N4–N5–C6–H23 | 176.9 | N20–C15–C16–C18 | 37.0 |
C15–C16 | 1.508 | H25–C8–H26 | 108.9 | C7–N5–C6–H21 | 62.6 | C16–C15–N20–C1 | −165.9 |
C15–O19 | 1.229 | N4–C9–C10 | 118.9 | C7–N5–C6–H22 | −176.3 | C16–C15–N20–H37 | −7.4 |
C15–N20 | 1.377 | N4–C9–C14 | 120.9 | C7–N5–C6–H23 | −58.2 | O19–C15–N20–C1 | 15.4 |
C16–C17 | 1.506 | C10–C9–C14 | 120.2 | N4–N5–C7–C1 | −2.5 | O19–C15–N20–H37 | 173.8 |
C16–C18 | 1.342 | C9–C10–C11 | 119.5 | N4–N5–C7–C8 | 1.6 | C15–C16–C17–H32 | −175.8 |
C17–H32 | 1.093 | C9–C10–H27 | 119.6 | C6–N5–C7–C1 | −130.5 | C15–C16–C17–H33 | −54.7 |
C17–H33 | 1.094 | C11–C10–H27 | 120.9 | C6–N5–C7–C8 | 129.5 | C15–C16–C17–H34 | 63.2 |
C17–H34 | 1.097 | C10–C11–C12 | 120.7 | C1–C7–C8–H24 | −21.7 | C18–C16–C17–H32 | −0.3 |
C18–H35 | 1.086 | C10–C11–H28 | 119.2 | C1–C7–C8–H25 | 96.4 | C18–C16–C17–H33 | 120.9 |
C18–H36 | 1.086 | C12–C11–H28 | 120.1 | C1–C7–C8–H26 | −142.3 | C18–C16–C17–H34 | −121.2 |
N20–H37 | 1.014 | C11–C12–C13 | 119.5 | N5–C7–C8–H24 | 157.1 | C15–C16–C18–H35 | −1.0 |
C11–C12–H29 | 120.3 | N5–C7–C8–H25 | −84.8 | C15–C16–C18–H36 | 176.8 | ||
C13–C12–H29 | 120.3 | N5–C7–C8–H26 | 36.5 | C17–C16–C18–H35 | −176.2 | ||
C12–C13–C14 | 120.5 | N4–C9–C10–C11 | 179.6 | C17–C16–C18–H36 | 1.5 | ||
C12–C13–H30 | 120.2 | ||||||
C14–C13–H30 | 119.3 | ||||||
C9–C14–C13 | 119.6 | ||||||
C9–C14–H31 | 119.7 | ||||||
C13–C14–H31 | 120.7 | ||||||
C16–C15–O19 | 121.3 | ||||||
C16–C15–N20 | 115.9 | ||||||
O19–C15–N20 | 122.8 | ||||||
C15–C16–C17 | 114.8 | ||||||
C15–C16–C18 | 121.7 | ||||||
C17–C16–C18 | 123.4 | ||||||
C16–C17–H32 | 111.0 | ||||||
C16–C17–H33 | 110.1 | ||||||
C16–C17–H34 | 111.0 | ||||||
H32–C17–H33 | 109.3 | ||||||
H32–C17–H34 | 108.6 | ||||||
H33–C17–H34 | 106.7 | ||||||
C16–C18–H35 | 122.9 | ||||||
C16–C18–H36 | 120.8 | ||||||
H35–C18–H36 | 116.2 | ||||||
C1–N20–C15 | 126.2 | ||||||
C1–N20–H37 | 113.2 | ||||||
C15–N20–H37 | 117.5 |
The theoretical and experimental vibrational spectra of MAAP are shown in Figure
Experimental ((a), (b)) and theoretical ((c), (d)) vibrational spectra of MAAP.
The high wavenumber region contains characteristic wavenumbers of NH stretching that are observed at 3249 cm−1 (IR) and at 3254 cm−1 (R) as strong broad band. The NH infrared band is consistent with previously reported data for MAAP [
Carbonyl stretchings are observed at 1673 cm−1—IR (1683 cm−1—R) and at 1648 cm−1—IR (1631 cm−1—R) as intense bands in the vibrational spectra. The former band is clearly assigned to the carbonyl band at cyclic ketone position, whereas the latter is attributed to the amide carbonyl band. The corresponding scaled theoretical values of these modes are 1703 cm−1 and 1694 cm−1. C=C stretchings are observed at 1620 cm−1—IR and at 1592 cm−1—IR (1605 and 1570 cm−1—R) as a very strong band in the vibrational spectra. The former band is assigned to the methacrylate double band, whereas the latter is attributed to the C=C band of rings. The corresponding theoretical values of these modes are 1652 cm−1 and 1609 cm−1 (1595 cm−1).
The CN stretching modes of the amide group are observed at 1491 cm−1 (IR-vs), 1295 cm−1 (IR-vs), 1285 cm−1 (IR-vs), 1245 cm−1 (R-w,sh), and 1060 cm−1 (IR-m) while the present theoretical values are 1520 cm−1, 1263 cm−1, and 1058 cm−1. The CN and NN stretch vibrations at cyclic ketone position are observed at 1368 cm−1 (IR-s), 1368 cm−1 (R-vs), 1200 cm−1 (IR-s), and 1211 cm−1 (R-m) while the present theoretical values are 1339 cm−1, 1205 cm−1, and 1191 cm−1. CC or NC stretching, CCC, CCN, CNC or HCC bending, some torsion, and out modes dominate the regions of 1000–500 cm−1 while CCC, CCN, CNC, or CNN bending and CCCN, CCNC, CNCC, CCNN, HCCC, or CCCC torsion modes are seen in the low frequency region. Similar situations have been shown in for calculations. Vibrational modes in the low wavenumber region of the spectrum contain contributions of several internal coordinates and their assignments have reduction approximation to one of two of the internal coordinates.
To make a comparison between the experimental and theoretical frequencies, we have calculated RMSD. This is used to measure the difference between values predicted by a model and those actually observed from the thing being modeled. In this study, RMSD values have been obtained as 29 cm−1 (IR) and 32 cm−1 (R). Omitting the
Regarding the calculated fundamentals, in general, the computed vibrational intensities are in agreement with the experimental results. Although the calculated vibrational intensities of some modes are about zero, these bands can be seen in the vibrational spectra. The opposite situation has also been observed. Similarly, the
The HOMO and LUMO orbitals are called the frontier orbitals and determine the way the molecule interacts with other species. The HOMO is the orbital that could behave as an electron donor, since it is the outermost orbital containing electrons. The LUMO is the orbital that could act as the electron acceptor, as it is the innermost orbital that has room to accept electrons. The transitions can be described from HOMO to LUMO. A single orbital, however, may be both the LUMO and the HOMO. The HOMO is dominated by nitrogen, oxygen, and carbon atoms. The LUMO is located over all atoms except from some CH3 and NH groups in MAAP. Frontier molecular orbital and their orbital energy are shown in Figure
Frontier molecular orbitals for MAAP.
The experimental and theoretical vibrational investigations of MAAP are performed, for the first time, by using FT-IR, Raman, and quantum chemical calculations. In conclusion, the following results can be summarized. Results of energy calculations for gas phase indicate that the amide-1 conformational isomer is the most stable conformer of MAAP. Furthermore, relative energies of the other five conformational isomers, except for the amide-2, are larger than 2.0 kcal/mol. Therefore, relative mole fractions of the five forms could be neglected, and these results suggest that the MAAP molecule prefers the amide-1 and amide-2 conformational isomers with preference of 51% and 49%, respectively, and the conformational energy barrier is independent of the imide form. The RMSD and correlation values between the experimental and calculated vibrational frequencies indicate that B3LYP/6-31G++(d,p) method is reliable, and this theoretical approximation makes the understanding of vibrational spectrum of MAAP easier.
The authors declare that no conflict of interests exists.