# Vibrational Spectroscopic Investigation and Conformational Analysis of Methacrylamidoantipyrine: A Comparative Density Functional Study

Authors: Özgür Alver; Mehmet Fatih Kaya; Metin Bilge; Cemal Parlak
Journal: Journal of Theoretical Chemistry
 (2013)

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## Abstract

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.


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## Body

## 1. Introduction

The MAAP, which has aπ electron rich aromatic ring, was first synthesized using antipyrine, a hydroxy radical capture and spectroscopic reagent for phenols, and used for a method described for removal of phenolic compounds with nitrophenol imprinted polymer based on π-π and hydrogen bonding interactions [1]. Then, it was used in analyzing the selective binding behavior of paraoxon and parathion compounds on surface imprinted polymers prepared using both charge transfer (MAAP) and ligand-exchange (MAAP-Gd) monomers [2]. Today, one can easily reach many papers published where it is one of the most important organic chemical product for the molecularly imprinted polymers [1–5]. For instance, it has been used in researching the biomimetic immunoassay based on molecularly imprinted polymer [3], the 8-Hydroxy-2′-deoxyguanosine (8-OHdG), which is one of the most abundant oxidative DNA lesions resulting from reactive oxygen species, and the 8-OHdG level in blood serum from a breast cancer patient [4], and new 8-OHdG imprinted quartz crystal microbalance sensor [5]. Furthermore, it is taken part in adsorption of phenol and its derivatives from water using synthetic resins and low-cost natural adsorbents [6].Vibrational spectroscopy has been widely used as the standard tool for structural characterization of molecular systems together with DFT calculations [7–14]. DFT is popular as it is a cost-effective procedure for studying the physical properties of molecules. Unlike the Hartree Fock theory, DFT recovers electron correlation in the self-consistent Kohn-Sham procedure through the functions of electron density, so it is a cost-effective and reliable method. The DFT/B3LYP model exhibits good performance particularly on vibrational frequencies and geometries of organic compounds [7–14].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.


## 2. Experimental

MAAP was prepared according to the published procedure [1] and used without further purification. FT-MIR and FIR spectra were recorded in the region of 4000–400 cm−1 and 400–10 cm−1 with Bruker Optics IFS66v/s FTIR spectrometer at a resolution of 2 cm−1. Raman spectrum was obtained using a Bruker Senterra Dispersive Raman microscope spectrometer with 785 nm excitation from a 3B diode laser having 3 cm−1 resolution in the spectral region of 4000–100 cm−1.


## 3. Calculations

All the calculations were performed using Gaussian 09.A1 program [15] on HP DL380G7 server system, and GaussView 5.0.8 [16] was used for visualization of the structure and simulated vibrational spectra. Many possible conformational isomers could be proposed for MAAP; however, the five amide and two imide conformational isomers of MAAP (Figure 1) were chosen using HF/6-31G(d,p) level by Spartan 10 program [17]. For these calculations, the possible seven conformational isomers of MAAP were first optimized in the gas phase at B3LYP level of theory using 6-31G++(d,p) basis set, and no geometric restrictions were applied. Its amide-1 conformational isomer was found more stable than the others (Figure 1). According to the theoretical calculations, the vibrational frequencies and assignments of the two conformational isomers amide-1 and amide-2 are about the same. So, it has been given the data for one of them.Figure 1
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) [10, 13]. In order to show the relative contributions of the redundant internal coordinates to each normal vibrational mode of the molecule, PED calculations were carried out by the vibrational energy distribution analysis 4 (VEDA) [18].


## 4. Results and Discussion

### 4.1. Geometrical Structures

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 1. Regarding the calculated free energies, all conformational isomers relative to the amide-1 and amide-2 forms of MAAP could be neglected for the calculation of equilibrium constant since their energy differences are larger than 2 kcal/mol [10–14]. The amide-1 conformational isomer of MAAP is more stable than amide-2 by 0.03 kcal/mol. Consequently, MAAP in the gas phase prefers the amide-1 and amide-2 conformational isomers with preference of 51% and 49%, respectively.Some of the optimized geometric parameters such as bond lengths and bond angles calculated by B3LYP/6-31G++(d,p) are listed in Table 1 for the most stable conformational isomer (amide-1) of MAAP. To the best of our knowledge, experimental data on the geometric structure of MAAP is not available in the literature. However, some geometric parameters for 4-([(1E)-(2-hydroxynaphthyl)methylidene]amino)-1,5-dimethyl-2-phenyl-2,3-dihyrdo-1H-pyrazol-3-one and p-methacryloylaminophenylarsonic acid monomer were identified in previously reported studies [19, 20]. Henceforth, the theoretical results have been compared with data for some parts of the related molecules as given in Table 1.Table 1
Some optimized geometric parameters of MAAP.
Parameters
B3LYP/6-31G++(d,p)
Amide-1
Ref. [19]
Ref. [20]
Bond lengths (Å)
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
128.2Generally, it is expected that the bond distances calculated by electron correlated methods are longer than the experimental distance. This situation can be seen in Table1 as expected. Overall, the calculated bond lengths are in good agreement with experimental results. The mean absolute deviation (MAD) and root mean square deviation (RMSD) of bond lengths are 0.022 and 0.032 Å, respectively. The biggest difference between the experimental and calculated bond distances is 0.061 Å. As can be seen in Table 1, big differences are observed in the calculated C6–N5–C7, C2–C1–N20 and C7–C1–N20 bond angles compared to experimental values. The calculated C2–C1–N20 and C6–N5–C7 bond angles are about 11° smaller than the experimental result whereas the calculated C7–C1–N20 angle is about 11° larger than the experimental value. All the other bond angles are reasonably close to the experimental data.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⋯O19 and N20-H36⋯O3. Theoretical results are N20-H36: 3.17 Å/N20-H36-O19: 25.1° and N20-H36: 2.52 Å/N20-H36-O19: 100.6°. The complete set of optimized geometric parameters for the amide-1 conformational isomer of MAAP is given in Table 3. Several thermodynamic parameters (zero point energy, entropy, etc.) calculated by B3LYP/6-31G++(d,p) method are also presented in Table  S1 (see Supplementary Material available online at http://dx.doi.org/10.1155/2013/386247). The total energy and change in total entropy for the amide-1 conformational isomer of MAAP are at room temperature.


### 4.2. Vibrational Studies of MAAP

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. [1]. The measured and calculated vibrational frequencies for MAAP along with corresponding vibrational assignments and intensities are given in Table 2.Table 2
Experimental and calculated vibrational wavenumbers (cm−1) of MAAP.
Mode
P.E.D (≥5%)
Experimental
B3LYP/6-31G++(d,p)
IR
Raman
ν
α
ν
β
I
IR
I
R
ν
1
νNH(100)
3249 s, b
3254 s, b
3588
3426
38.29
16.36
ν
2
νCH(97)
3239
3093
0.43
12.83
ν
3
νCH(100)
3094 w
3235
3090
13.53
17.90
ν
4
νCH(89)
3080 vs
3218
3073
3.23
25.29
ν
5
νCH(89)
3059 m
3207
3062
25.36
40.11
ν
6
νCH(90)
3193
3049
13.87
25.64
ν
7
νCH(93)
3044 w
3183
3040
1.14
12.12
ν
8
νCH(87)
3172
3029
2.42
13.11
ν
9
νCH(95)
3167
3025
8.58
8.38
ν
10
νCH(99)
3005 s
3154
3012
4.74
32.59
ν
11
νCH(94)
3136
2995
14.79
17.50
ν
12
νCH(96)
3132
2991
12.80
14.93
ν
13
νCH(99)
2969 s
2968 m
3111
2971
12.02
17.64
ν
14
νCH(92)
3104
2964
12.55
15.67
ν
15
νCH(91)
2919 s
2934 vs
3051
2914
18.10
52.55
ν
16
νCH(98)
3042
2905
21.81
39.66
ν
17
νCH(96)
2863 m
3028
2892
50.02
29.76
ν
18
νOC(74)
1673 vs
1683 s
1743
1703
152.72
26.56
ν
19
νOC(74)
1648 vs
1631 vs
1734
1694
363.40
52.85
ν
20
νCC(69)-5
1702
1663
40.11
111.41
ν
21
νCC(76)
1620 vs
1691
1652
100.80
34.41
ν
22
νCC(65) + δHCC(19)
1592 vs
1605 vs
1647
1609
53.05
79.10
ν
23
νCC(69) + δHCC(10)
1570 m
1633
1595
2.13
4.99
ν
24
δHNC(58) + νCN(10)
1491 vs
1556
1520
352.19
41.25
ν
25
δHCC(60) + νCC(10)
1531
1496
105.42
13.21
ν
26
δHCH(85)
1515
1481
15.71
6.95
ν
27
δHCH(84)
1500
1465
2.85
7.29
ν
28
δHCC(81)
1455 vs
1466 m, sh
1499
1465
68.70
2.59
ν
29
δHCC(60) + νCC(23)
1491
1457
7.98
1.88
ν
30
δHCH(78)
1484
1450
4.60
13.15
ν
31
δHCH(87)
1477
1443
12.74
7.51
ν
32
δHCH(77)
1446 w
1477
1443
4.71
5.30
ν
33
δHCH(80)
1456
1423
8.18
7.40
ν
34
δHCH(75)
1426 s
1454
1421
7.77
21.77
ν
35
δHCH(67) + νCC(11)
1433
1400
76.53
29.21
ν
36
δHCH(77)
1414
1382
5.79
5.84
ν
37
νCC(30) + δHCH(19)
1407 s
1392
1360
178.18
19.77
ν
38
νNC(59)
1368 s
1368 vs
1371
1339
88.20
122.68
ν
39
δHCC(63) + νCC(22)
1360
1329
1.59
2.24
ν
40
νCC(52) + δHCC(21)
1310 vs, sh
1303 s
1340
1309
78.89
32.98
ν
41
νCC(42) + δHCC(18)
1325
1295
7.14
10.34
ν
42
νNC(39)
1295 vs
1245 w, sh
1292
1263
218.19
34.06
1285 vs
ν
43
νNN(36) + νNC(11)
1200 s
1233
1205
85.54
0.69
ν
44
νNN(61) + νNC(12)
1211 m
1219
1191
43.20
36.30
ν
45
δHCC(71) + νCC(20)
1170 m
1200
1173
1.88
4.90
ν
46
δHCC(78)
1184
1157
0.60
4.78
ν
47
νNC(41) + δHCN(16) + δHCC(10)
1126 w
1164
1138
5.43
2.93
ν
48
δHCN(55)
1136 s
1156
1130
16.14
8.01
ν
49
δHCN(55) + νNC(10)
1105 m
1130
1104
14.51
7.56
ν
50
νCC(38) + δHCC(28)
1074 w
1069 w
1104
1079
12.00
0.64
ν
51
νNC(35) + δHCC(18)
1060 m
1082
1058
13.07
0.41
ν
52
δHCC(83)
1071
1047
0.12
2.18
ν
53
δHCC(26) + νNC(13)
1060
1035
4.98
1.92
ν
54
δHCC(59)
1057
1032
0.42
0.55
ν
55
νCC(45) + δCCC(17)
1026 m
1008 vs
1041
1017
18.61
24.00
ν
56
δHCC(80)
1001 w
1026
1002
6.02
2.76
ν
57
δCCC(62) + νCC(21)
973 w
1014
990
0.17
68.48
ν
58
δHCC(95)
998
975
0.06
0.48
ν
59
δHCC(95)
980
958
0.21
0.17
ν
60
δHCC(32) + νCC(22) + νNC(11)
956 vw, sh
970
948
13.72
0.75
ν
61
νCC(87)
932 s
946 m
954
932
38.98
3.92
ν
62
νNC(28) + δHCC(25)
943
921
6.80
9.61
ν
63
δHCC(79)
922
901
3.20
3.83
ν
64
νCC(28) + δHCC(27)
892 w
902 m
907
886
1.89
20.36
ν
65
τHCCN(95)
842 m
858 m
848
829
0.20
3.18
ν
66
τHNCC(43)
806 w
838
819
6.14
4.33
ν
67
τHCCC(53) + γCCCC(16)
783 w
803
785
11.75
2.48
ν
68
τHCCC(73)
763 vs
749 w
772
754
42.94
1.70
ν
69
γOCNC(81)
744 m
746
729
13.41
4.16
ν
70
νCC(35) + δCNN(20) + τHCCC(10)
708 s
705 vw
728
711
42.51
1.35
703 s
ν
71
τHCCC(84)
701
685
16.44
0.60
ν
72
δHCC(25) + γCCCC(11)
685 s
678
662
2.43
13.74
ν
73
δHCC(28) + δCNN(11)
666 m
656 m
668
653
6.92
5.11
ν
74
δCCC(60)
641 m
658
643
23.86
5.80
ν
75
δCCC(79)
630
616
1.15
7.40
ν
76
τHNCC(57)
603 s
1340
608
56.00
27.67
ν
77
γCCNN(21)
1325
590
11.65
5.02
ν
78
δCCN(30) + γCCNN(20) + νCC(10)
587 s
1292
575
25.19
15.04
ν
79
δCCO(27) + τHNCC(12) + γCCNN(10)
622
564
5.98
4.06
ν
80
γNCCC(76)
501 s
604
501
8.72
1.59
ν
81
δCCC(49)
476 w
588
478
8.40
2.76
ν
82
τCCCN(31)
436 m
577
447
19.92
15.42
ν
83
δCNC(26) + τCCNC(23)
423 m
512
417
1.68
4.29
ν
84
τCCCC(94)
409 w
397 vw
489
409
0.58
1.88
ν
85
δCCN(49)
373 s
458
383
2.79
7.76
ν
86
γCCCC(32)
427
346
0.60
11.02
ν
87
δCNN(57)
331 m
335 w
419
336
9.39
1.00
ν
88
δCCC(65)
392
305
1.22
3.69
ν
89
δCCN(49)
289 s
295 w
354
284
12.51
4.15
ν
90
δCCC(35)
261 w
275 w
344
271
1.06
1.95
ν
91
τCCCN(41)
312
244
1.68
3.36
ν
92
δCCN(38) + νNC(11)
225 vw
290
217
5.16
9.01
ν
93
τHCCC(79)
277
213
0.40
2.36
ν
94
δCCN(37)
200 m
250
207
3.45
21.32
ν
95
τHCCC(90)
169 vw
222
169
0.51
3.26
ν
96
τCCCC(58)
218
157
1.07
5.37
ν
97
τCCNC(47)
140 s
212
141
3.32
7.37
ν
98
τHCCC(54) + τCCNC(11)
134 s
173
134
0.62
23.80
ν
99
τCCNC(27) + τHCCC(14) + δHCC(13)
111 m
160
113
4.87
17.07
ν
100
τCNCC(58)
144
77
1.35
138.91
ν
101
τCNCC(27) + τCCNC(17) + δCCN(10)
71 vw
137
68
5.61
39.27
ν
102
τCCNN(80)
60 vw
115
55
1.93
72.49
ν
103
τCNCC(56)
50 vw
79
46
2.34
115.46
ν
104
τCNCC(67)
41 vw
70
39
4.55
83.80
ν
105
τCCNC(49) + δCNC(16)
57
24
1.74
72.73
ν
α: unscaled wavenumbers, νβ: scaled with 0.955 above 1800 cm−1 and 0.977 under 1800 cm−1.  IR and R: calculated infrared and Raman intensities.  PED data are taken from VEDA4. v: very, s: strong, m: medium, w: weak, b: broad, and sh: shoulder. ν, δ, τ, and γ denote stretching, bending, torsion, and out modes, respectively.Table 3
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
117.5The theoretical and experimental vibrational spectra of MAAP are shown in Figure2. All calculated frequency values presented in this paper are obtained within the harmonic approximation. This allows us to identify vibrational motion in terms of independent vibrational modes, each of them is governed by a simple one-dimensional harmonic potential. It is difficult to determine the MAAP’s vibrational assignments in the observed spectrum due to its low symmetry. Therefore, the assignments of vibrational modes for the amide-1 conformational isomer of MAAP have been provided by VEDA 4 [18] in Table 2. The MAAP molecule consists of 37 atoms having 105 normal vibrational modes, and its most stable form belongs to the point group C1 with only identity (E) symmetry element or operation. According to the calculations, 18 normal vibrational modes of MAAP are below 400 cm−1 while 87 modes are between 4000 cm−1 and 400 cm−1.Experimental ((a), (b)) and theoretical ((c), (d)) vibrational spectra of MAAP.
(a)
(b)
(c)
(d)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 [1] where this band was found as 3260 cm−1. The corresponding scaled theoretical value for this mode is 3426 cm−1. The amide band is the most intense and is predominantly C=O stretch supports also the amide form of MAAP. Hydrogen bonding plays an important role in the broadening of the spectral bands [21]. The broadening of the NH bands is attributed to the intramolecularly bonded hydrogen to the C=O group. The CH stretching region encompasses four and six CH modes in IR and Raman spectrum, respectively. The vibrational bands at 3094, 3059 cm−1 in IR spectrum and 3080, 3044, and 3005 cm−1 in Raman spectrum are attributed to the CH stretching vibrations while the bands at 2969, 2919 cm−1 in IR spectrum and 2968, 2934, and 2863 cm−1 in Raman spectrum are assigned to the CH2 and CH3 stretchings. Corresponding scaled calculated values for these bands are found as 3090, 3073, 3062, 3040, 3012, 2971, 2914, and 2892 cm−1. In the high wavenumber region of the spectra, the anharmonicity can explain substantial differences between the experimental and calculated values. Alternatively, these differences may be due to intra/intermolecular interactions or to the laser used for Raman.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 ν1 stretch from the analysis results substantially reduced RMSD values as 15 cm−1 (IR) and 17 cm−1 (R). Furthermore, the correlation values between the experimental and calculated vibrational frequencies are found to be as 0.99898 (IR) and 0.99889 (R). Similarly, omitting the ν1 stretch from the analysis results substantially reduced correlation values as 0.99963 (IR) and 0.99960 (R). It can be seen that the B3LYP calculation is reliable for both IR and Raman spectra.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, theν42 and ν70 modes are observed as double peaks in the infrared spectrum which can be attributed to molecular interactions (Figure 2). It is also important to note that theoretical studies were conducted for an isolated molecule in the gaseous state contrary to the experimental values recorded for the presence of interactions in the solid phase.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 3 together with the HOMO-LUMO gap. The energy gap given from the ground state to first excited state is calculated at around 4.66 eV. The laser used for Raman analysis in the present study has an energy around half of this gap. Therefore, electronic excitement due to Raman laser seems unlikely.Figure 3
Frontier molecular orbitals for MAAP.


## 4.1. Geometrical Structures

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 1. Regarding the calculated free energies, all conformational isomers relative to the amide-1 and amide-2 forms of MAAP could be neglected for the calculation of equilibrium constant since their energy differences are larger than 2 kcal/mol [10–14]. The amide-1 conformational isomer of MAAP is more stable than amide-2 by 0.03 kcal/mol. Consequently, MAAP in the gas phase prefers the amide-1 and amide-2 conformational isomers with preference of 51% and 49%, respectively.Some of the optimized geometric parameters such as bond lengths and bond angles calculated by B3LYP/6-31G++(d,p) are listed in Table 1 for the most stable conformational isomer (amide-1) of MAAP. To the best of our knowledge, experimental data on the geometric structure of MAAP is not available in the literature. However, some geometric parameters for 4-([(1E)-(2-hydroxynaphthyl)methylidene]amino)-1,5-dimethyl-2-phenyl-2,3-dihyrdo-1H-pyrazol-3-one and p-methacryloylaminophenylarsonic acid monomer were identified in previously reported studies [19, 20]. Henceforth, the theoretical results have been compared with data for some parts of the related molecules as given in Table 1.Table 1
Some optimized geometric parameters of MAAP.
Parameters
B3LYP/6-31G++(d,p)
Amide-1
Ref. [19]
Ref. [20]
Bond lengths (Å)
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
128.2Generally, it is expected that the bond distances calculated by electron correlated methods are longer than the experimental distance. This situation can be seen in Table1 as expected. Overall, the calculated bond lengths are in good agreement with experimental results. The mean absolute deviation (MAD) and root mean square deviation (RMSD) of bond lengths are 0.022 and 0.032 Å, respectively. The biggest difference between the experimental and calculated bond distances is 0.061 Å. As can be seen in Table 1, big differences are observed in the calculated C6–N5–C7, C2–C1–N20 and C7–C1–N20 bond angles compared to experimental values. The calculated C2–C1–N20 and C6–N5–C7 bond angles are about 11° smaller than the experimental result whereas the calculated C7–C1–N20 angle is about 11° larger than the experimental value. All the other bond angles are reasonably close to the experimental data.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⋯O19 and N20-H36⋯O3. Theoretical results are N20-H36: 3.17 Å/N20-H36-O19: 25.1° and N20-H36: 2.52 Å/N20-H36-O19: 100.6°. The complete set of optimized geometric parameters for the amide-1 conformational isomer of MAAP is given in Table 3. Several thermodynamic parameters (zero point energy, entropy, etc.) calculated by B3LYP/6-31G++(d,p) method are also presented in Table  S1 (see Supplementary Material available online at http://dx.doi.org/10.1155/2013/386247). The total energy and change in total entropy for the amide-1 conformational isomer of MAAP are at room temperature.


## 4.2. Vibrational Studies of MAAP

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. [1]. The measured and calculated vibrational frequencies for MAAP along with corresponding vibrational assignments and intensities are given in Table 2.Table 2
Experimental and calculated vibrational wavenumbers (cm−1) of MAAP.
Mode
P.E.D (≥5%)
Experimental
B3LYP/6-31G++(d,p)
IR
Raman
ν
α
ν
β
I
IR
I
R
ν
1
νNH(100)
3249 s, b
3254 s, b
3588
3426
38.29
16.36
ν
2
νCH(97)
3239
3093
0.43
12.83
ν
3
νCH(100)
3094 w
3235
3090
13.53
17.90
ν
4
νCH(89)
3080 vs
3218
3073
3.23
25.29
ν
5
νCH(89)
3059 m
3207
3062
25.36
40.11
ν
6
νCH(90)
3193
3049
13.87
25.64
ν
7
νCH(93)
3044 w
3183
3040
1.14
12.12
ν
8
νCH(87)
3172
3029
2.42
13.11
ν
9
νCH(95)
3167
3025
8.58
8.38
ν
10
νCH(99)
3005 s
3154
3012
4.74
32.59
ν
11
νCH(94)
3136
2995
14.79
17.50
ν
12
νCH(96)
3132
2991
12.80
14.93
ν
13
νCH(99)
2969 s
2968 m
3111
2971
12.02
17.64
ν
14
νCH(92)
3104
2964
12.55
15.67
ν
15
νCH(91)
2919 s
2934 vs
3051
2914
18.10
52.55
ν
16
νCH(98)
3042
2905
21.81
39.66
ν
17
νCH(96)
2863 m
3028
2892
50.02
29.76
ν
18
νOC(74)
1673 vs
1683 s
1743
1703
152.72
26.56
ν
19
νOC(74)
1648 vs
1631 vs
1734
1694
363.40
52.85
ν
20
νCC(69)-5
1702
1663
40.11
111.41
ν
21
νCC(76)
1620 vs
1691
1652
100.80
34.41
ν
22
νCC(65) + δHCC(19)
1592 vs
1605 vs
1647
1609
53.05
79.10
ν
23
νCC(69) + δHCC(10)
1570 m
1633
1595
2.13
4.99
ν
24
δHNC(58) + νCN(10)
1491 vs
1556
1520
352.19
41.25
ν
25
δHCC(60) + νCC(10)
1531
1496
105.42
13.21
ν
26
δHCH(85)
1515
1481
15.71
6.95
ν
27
δHCH(84)
1500
1465
2.85
7.29
ν
28
δHCC(81)
1455 vs
1466 m, sh
1499
1465
68.70
2.59
ν
29
δHCC(60) + νCC(23)
1491
1457
7.98
1.88
ν
30
δHCH(78)
1484
1450
4.60
13.15
ν
31
δHCH(87)
1477
1443
12.74
7.51
ν
32
δHCH(77)
1446 w
1477
1443
4.71
5.30
ν
33
δHCH(80)
1456
1423
8.18
7.40
ν
34
δHCH(75)
1426 s
1454
1421
7.77
21.77
ν
35
δHCH(67) + νCC(11)
1433
1400
76.53
29.21
ν
36
δHCH(77)
1414
1382
5.79
5.84
ν
37
νCC(30) + δHCH(19)
1407 s
1392
1360
178.18
19.77
ν
38
νNC(59)
1368 s
1368 vs
1371
1339
88.20
122.68
ν
39
δHCC(63) + νCC(22)
1360
1329
1.59
2.24
ν
40
νCC(52) + δHCC(21)
1310 vs, sh
1303 s
1340
1309
78.89
32.98
ν
41
νCC(42) + δHCC(18)
1325
1295
7.14
10.34
ν
42
νNC(39)
1295 vs
1245 w, sh
1292
1263
218.19
34.06
1285 vs
ν
43
νNN(36) + νNC(11)
1200 s
1233
1205
85.54
0.69
ν
44
νNN(61) + νNC(12)
1211 m
1219
1191
43.20
36.30
ν
45
δHCC(71) + νCC(20)
1170 m
1200
1173
1.88
4.90
ν
46
δHCC(78)
1184
1157
0.60
4.78
ν
47
νNC(41) + δHCN(16) + δHCC(10)
1126 w
1164
1138
5.43
2.93
ν
48
δHCN(55)
1136 s
1156
1130
16.14
8.01
ν
49
δHCN(55) + νNC(10)
1105 m
1130
1104
14.51
7.56
ν
50
νCC(38) + δHCC(28)
1074 w
1069 w
1104
1079
12.00
0.64
ν
51
νNC(35) + δHCC(18)
1060 m
1082
1058
13.07
0.41
ν
52
δHCC(83)
1071
1047
0.12
2.18
ν
53
δHCC(26) + νNC(13)
1060
1035
4.98
1.92
ν
54
δHCC(59)
1057
1032
0.42
0.55
ν
55
νCC(45) + δCCC(17)
1026 m
1008 vs
1041
1017
18.61
24.00
ν
56
δHCC(80)
1001 w
1026
1002
6.02
2.76
ν
57
δCCC(62) + νCC(21)
973 w
1014
990
0.17
68.48
ν
58
δHCC(95)
998
975
0.06
0.48
ν
59
δHCC(95)
980
958
0.21
0.17
ν
60
δHCC(32) + νCC(22) + νNC(11)
956 vw, sh
970
948
13.72
0.75
ν
61
νCC(87)
932 s
946 m
954
932
38.98
3.92
ν
62
νNC(28) + δHCC(25)
943
921
6.80
9.61
ν
63
δHCC(79)
922
901
3.20
3.83
ν
64
νCC(28) + δHCC(27)
892 w
902 m
907
886
1.89
20.36
ν
65
τHCCN(95)
842 m
858 m
848
829
0.20
3.18
ν
66
τHNCC(43)
806 w
838
819
6.14
4.33
ν
67
τHCCC(53) + γCCCC(16)
783 w
803
785
11.75
2.48
ν
68
τHCCC(73)
763 vs
749 w
772
754
42.94
1.70
ν
69
γOCNC(81)
744 m
746
729
13.41
4.16
ν
70
νCC(35) + δCNN(20) + τHCCC(10)
708 s
705 vw
728
711
42.51
1.35
703 s
ν
71
τHCCC(84)
701
685
16.44
0.60
ν
72
δHCC(25) + γCCCC(11)
685 s
678
662
2.43
13.74
ν
73
δHCC(28) + δCNN(11)
666 m
656 m
668
653
6.92
5.11
ν
74
δCCC(60)
641 m
658
643
23.86
5.80
ν
75
δCCC(79)
630
616
1.15
7.40
ν
76
τHNCC(57)
603 s
1340
608
56.00
27.67
ν
77
γCCNN(21)
1325
590
11.65
5.02
ν
78
δCCN(30) + γCCNN(20) + νCC(10)
587 s
1292
575
25.19
15.04
ν
79
δCCO(27) + τHNCC(12) + γCCNN(10)
622
564
5.98
4.06
ν
80
γNCCC(76)
501 s
604
501
8.72
1.59
ν
81
δCCC(49)
476 w
588
478
8.40
2.76
ν
82
τCCCN(31)
436 m
577
447
19.92
15.42
ν
83
δCNC(26) + τCCNC(23)
423 m
512
417
1.68
4.29
ν
84
τCCCC(94)
409 w
397 vw
489
409
0.58
1.88
ν
85
δCCN(49)
373 s
458
383
2.79
7.76
ν
86
γCCCC(32)
427
346
0.60
11.02
ν
87
δCNN(57)
331 m
335 w
419
336
9.39
1.00
ν
88
δCCC(65)
392
305
1.22
3.69
ν
89
δCCN(49)
289 s
295 w
354
284
12.51
4.15
ν
90
δCCC(35)
261 w
275 w
344
271
1.06
1.95
ν
91
τCCCN(41)
312
244
1.68
3.36
ν
92
δCCN(38) + νNC(11)
225 vw
290
217
5.16
9.01
ν
93
τHCCC(79)
277
213
0.40
2.36
ν
94
δCCN(37)
200 m
250
207
3.45
21.32
ν
95
τHCCC(90)
169 vw
222
169
0.51
3.26
ν
96
τCCCC(58)
218
157
1.07
5.37
ν
97
τCCNC(47)
140 s
212
141
3.32
7.37
ν
98
τHCCC(54) + τCCNC(11)
134 s
173
134
0.62
23.80
ν
99
τCCNC(27) + τHCCC(14) + δHCC(13)
111 m
160
113
4.87
17.07
ν
100
τCNCC(58)
144
77
1.35
138.91
ν
101
τCNCC(27) + τCCNC(17) + δCCN(10)
71 vw
137
68
5.61
39.27
ν
102
τCCNN(80)
60 vw
115
55
1.93
72.49
ν
103
τCNCC(56)
50 vw
79
46
2.34
115.46
ν
104
τCNCC(67)
41 vw
70
39
4.55
83.80
ν
105
τCCNC(49) + δCNC(16)
57
24
1.74
72.73
ν
α: unscaled wavenumbers, νβ: scaled with 0.955 above 1800 cm−1 and 0.977 under 1800 cm−1.  IR and R: calculated infrared and Raman intensities.  PED data are taken from VEDA4. v: very, s: strong, m: medium, w: weak, b: broad, and sh: shoulder. ν, δ, τ, and γ denote stretching, bending, torsion, and out modes, respectively.Table 3
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
117.5The theoretical and experimental vibrational spectra of MAAP are shown in Figure2. All calculated frequency values presented in this paper are obtained within the harmonic approximation. This allows us to identify vibrational motion in terms of independent vibrational modes, each of them is governed by a simple one-dimensional harmonic potential. It is difficult to determine the MAAP’s vibrational assignments in the observed spectrum due to its low symmetry. Therefore, the assignments of vibrational modes for the amide-1 conformational isomer of MAAP have been provided by VEDA 4 [18] in Table 2. The MAAP molecule consists of 37 atoms having 105 normal vibrational modes, and its most stable form belongs to the point group C1 with only identity (E) symmetry element or operation. According to the calculations, 18 normal vibrational modes of MAAP are below 400 cm−1 while 87 modes are between 4000 cm−1 and 400 cm−1.Experimental ((a), (b)) and theoretical ((c), (d)) vibrational spectra of MAAP.
(a)
(b)
(c)
(d)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 [1] where this band was found as 3260 cm−1. The corresponding scaled theoretical value for this mode is 3426 cm−1. The amide band is the most intense and is predominantly C=O stretch supports also the amide form of MAAP. Hydrogen bonding plays an important role in the broadening of the spectral bands [21]. The broadening of the NH bands is attributed to the intramolecularly bonded hydrogen to the C=O group. The CH stretching region encompasses four and six CH modes in IR and Raman spectrum, respectively. The vibrational bands at 3094, 3059 cm−1 in IR spectrum and 3080, 3044, and 3005 cm−1 in Raman spectrum are attributed to the CH stretching vibrations while the bands at 2969, 2919 cm−1 in IR spectrum and 2968, 2934, and 2863 cm−1 in Raman spectrum are assigned to the CH2 and CH3 stretchings. Corresponding scaled calculated values for these bands are found as 3090, 3073, 3062, 3040, 3012, 2971, 2914, and 2892 cm−1. In the high wavenumber region of the spectra, the anharmonicity can explain substantial differences between the experimental and calculated values. Alternatively, these differences may be due to intra/intermolecular interactions or to the laser used for Raman.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 ν1 stretch from the analysis results substantially reduced RMSD values as 15 cm−1 (IR) and 17 cm−1 (R). Furthermore, the correlation values between the experimental and calculated vibrational frequencies are found to be as 0.99898 (IR) and 0.99889 (R). Similarly, omitting the ν1 stretch from the analysis results substantially reduced correlation values as 0.99963 (IR) and 0.99960 (R). It can be seen that the B3LYP calculation is reliable for both IR and Raman spectra.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, theν42 and ν70 modes are observed as double peaks in the infrared spectrum which can be attributed to molecular interactions (Figure 2). It is also important to note that theoretical studies were conducted for an isolated molecule in the gaseous state contrary to the experimental values recorded for the presence of interactions in the solid phase.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 3 together with the HOMO-LUMO gap. The energy gap given from the ground state to first excited state is calculated at around 4.66 eV. The laser used for Raman analysis in the present study has an energy around half of this gap. Therefore, electronic excitement due to Raman laser seems unlikely.Figure 3
Frontier molecular orbitals for MAAP.


## 5. Conclusion

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.(1)
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.(2)
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.



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*Source: 386247-2013-09-12.xml*