# Molecular Structure and Vibrational Spectra of 2-Ethylhexyl Acrylate by Density Functional Theory Calculations

Authors: Ottman Belaidi; Tewfik Bouchaour; Ulrich Maschke
Journal: Organic Chemistry International
 (2013)

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## Abstract

The Fourier transform infrared spectra (FTIR) of 2-ethylhexyl acrylate have been measured in liquid phase. The molecular geometry, vibrational frequencies, and infrared intensities have been calculated by using density functional theory. We found two local minima representing s-cis and s-trans conformations for 2-ethylhexyl acrylate molecule. The optimized geometries at DFT//B3LYP/6-311+G** are in good agreement with electron diffraction data of methyl acrylate for the acrylic group. The two conformers were used for the interpretation of the experimental infrared spectrum of title compound. PED calculations are represented for a more complete and concise assignment. There is one band in the infrared spectrum at 1646 cm−1 that definitely indicates the conformer with s-trans arrangement of acrylic moiety to be present or not in the liquid 2-ethylhexyl acrylate.


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## Body

## 1. Introduction

Alkyl acrylates are important monomers used in the manufacture of polymers and copolymers. Acrylates give polymers exhibiting outstanding transparency and aging proprieties which have made them of interest in a wide variety of applications [1–4]. The structural data and the preferred molecular conformation of these acrylates molecules would be important for basic understanding of these polymers [5].2-Ethylhexyl acrylate is used for production of paint, adhesive, and paper coating trade. This monomer imparts flexibility and water resistance to the polymers [1, 6]. It is also used for curing polymeric materials [7–15].To our best knowledge no structural data or detailed interpretation of the vibrational spectra of 2-ethylhexyl acrylate is presented in the literature. This prompted us to look into the vibrational spectroscopy of 2-ethylhexyl acrylate more carefully.Many reports, experimental [16–21] and theoretical [20, 22], show that acrylates and related compounds exhibit rotational isomerism with the planar s-trans and s-cis heavy-atom structures being the energetically most stable conformations. However, uncertainty continues to exist regarding the relative stability of the two conformers. Gas electron diffraction studies on methyl methacrylate [20] suggested the cis/trans ratios to be equal to 2 : 1. On the other hand, the IR spectrum of methyl methacrylate in Ar low temperature matrix suggested an inverse ratio [23]. Many experimental and theoretical studies on methyl acrylate reported that the s-cis conformer is more stable than the s-trans conformer [24–29]. However, Bowles et al. [30] assumed that the s-trans conformer is more stable.


## 2. Experimental

2-Ethylhexyl acrylate (purity greater than 99%) was purchased from Sigma-Aldrich and it was used as it is without further purification. The infrared spectra of liquid films placed between the KBR windows were recorded within 4000–700 cm−1 range with a Perkin Elmer FTIR System-2000 model.


## 3. Computational

The optimizations of the stable conformers were conducted with the density functional theory using 6-311+G** basis sets. The DFT calculations were performed using Becke’s 3-parameter (local, nonlocal, Hartree-Fock) hybrid exchange functionals with Lee-Yang-Parr correlational functionals (B3LYP) [31, 32].The harmonic vibrational frequencies of the stable conformations were calculated at the same level of theories used for the calculated optimized geometries. The calculated frequencies were scaled down by the wavenumber linear scaling procedure (WLS) of Yoshida et al. [33, 34] using the following equation:
(1)νobsνcalc=1.0087-0.0000163νcalc.All the calculations were carried out with the Gaussian 03 program [35]. The vibrational assignments of the normal modes were provided on the basis of the calculated PEDs by using the program GAR2PED [36].


## 4. Geometry Optimizations and Energies

The geometries optimizations were conducted at the B3LYP/6-311+G** assuming the s-cis and s-trans conformations. All the optimized geometries are recognized as true minima due to the lack of imaginary harmonic frequencies. The results are given in Table1 for s-cis and s-trans conformers, respectively. For atoms numbering, see Figure 1.Table 1
Optimized geometries of s-cis and s-trans 2-ethylhexyl acrylate conformations by DFT//B3LYP/6-311+G**.
Internal coordinates
Level of theory DFT/6-311+G**  s-cis EHA
s-cis methyl acrylatea
DFT/6-311+G**  s-trans EHA
s-trans methyl acrylateb
Bond lengths (Å)
C1H2
1.084
1.083
1.074
C1H3
1.083
1.084
1.075
C1C4
1.331
1.341(7)
1.332
1.319
C4H5
1.084
1.084
1.075
C4C6
1.486
1.484(6)
1.484
1.484
C6O7
1.210
1.211(2)
1.211
1.191
C6O8
1.351
1.349(3)
1.348
1.325
O8C9
1.448
1.439(3)
1.448
1.417
C9H10
1.093
1.093
C9H11
1.093
1.092
C9C12
1.526
1.526
C12H13
1.098
1.098
C12C14
1.543
1.543
C14H15
1.097
1.097
C14H16
1.096
1.096
C14C17
1.531
1.531
C17C18
1.094
1.094
C17H19
1.093
1.093
C17H20
1.092
1.092
C12C21
1.549
1.549
C21H22
1.097
1.097
C21H23
1.094
1.094
C21C24
1.534
1.534
C24H25
1.097
1.097
C24H26
1.095
1.095
C24C27
1.533
1.533
C27H28
1.097
1.097
C27H29
1.097
1.096
C27C30
1.531
1.531
C30H31
1.094
1.093
C30H32
1.094
1.094
C30H33
1.094
1.094
Bond angles (°)
H2C1H3
118.049
117.591
H2C1C4
120.479
121.432
121.7
C1C4H5
122.220
121.739
122.1
C1C4C6
121.072
120.3(8)
124.970
124.2
C4C6O7
125.710
126.1(5)
123.066
122.9
C4C6O8
110.564
110.3(3)
113.320
113.7
C6O8C9
116.570
116.4(5)
116.493
116.8
O8C9H10
108.658
108.697
O8C9H11
108.313
108.339
O8C9C12
108.522
108.488
C9 C12C14
108.099
108.115
C12C14H15
108.729
108.720
C12C14H16
108.539
108.544
C12C14C17
115.410
115.400
C14C17H18
111.304
111.345
C14C17H19
110.477
110.478
C14C17H20
112.028
112.001
C9C12C21
111.382
111.384
C12C21H22
109.228
109.231
C12C21H23
109.005
109.019
C12C21C24
114.775
114.824
C21C24H25
108.695
108.731
C21C24H26
110.286
110.291
C21C24C27
113.287
113.308
C24C27H28
109.207
109.229
C24C27H29
109.217
109.208
C24C27C30
113.280
109.208
C27C30H31
111.428
111.428
C27C30H32
111.218
111.209
C27C30H33
111.189
111.202
Dihedral angles (°)
H3H2C1C4
180
−179.908
H2C1C4H5
−179.977
179.808
H2C1C4C6
0.037
0.106
C1C4C6O7
−0.166
178.652
C1C4C6O8
179.844
−1.298
C4C6O8C9
179.896
179.515
C6O8C9H10
55.606
55.506
C6O8C9H11
−61.282
−61.454
C6O8C9C12
177.303
177.169
O8C9 C12H13
55.024
54.463
O8C9C12C14
171.114
170.547
C9C12C14H15
61.844
61.568
C9C12C14H16
−53.102
−53.378
C9C12C14C17
−175.215
−175.516
C12C14C17H18
56.957
57.249
C12C14C17H19
176.491
176.782
C12C14C17H20
−63.865
−63.596
O8C9C12C21
−63.322
−63.915
C9C12C21H22
−96.883
−97.147
C9C12C21H23
18.587
18.279
C9C12C21C24
139.536
139.298
C12C21C24H25
−59.615
−59.061
C12C21C24H26
56.214
56.717
C12C21C24C27
178.823
179.360
C21C24C27H28
−58.771
−58.768
C21C24C27H29
56.758
56.753
C21C24C27C30
179.033
179.029
C24C27C30H31
179.693
−179.983
C24C27C30H32
−60.205
−59.888
C24C27C30H33
59.599
59.917
aDetermined by a joint analysis of electron diffraction data and rotational constants [21].
bDetermined by ab initio calculation at HF/6–31G** [20].Figure 1
2-Ethylhexyl acrylate molecule s-cis and s-trans conformations.In Table1, the calculated bond lengths, bond angles, and dihedral angles have approximately the same values for the s-cis and s-trans conformers. However, some changes occur in going from the s-cis to the s-trans conformation. The bond angles C1C4C6 and C4C6O8 increase by 3.8° and 2.7°, respectively, while C4C6O7 decreases by 2.5°.The X ray structural data of s-cis methyl acrylate reported by Egawa et al. [21] and ab initio structure of s-trans methyl acrylate reported by Tsuji et al. [20] are very close to those we found for 2-ethylhexyl acrylate molecule.The zero-point corrected energy of the s-cis conformation is 365.102 × 106 cal/mol showing more stability than the calculated one for the s-trans conformation. The energy for the latter conformation is 365.101 × 106 cal/mol.


## 5. Vibrational Analysis

The most stable conformers, s-cis and s-trans 2-ethylhexyl acrylate, belong to the C1 point group. Each conformer represents 93 normal vibrational modes. We juxtaposed the experimental IR spectrum with the computed ones for both conformers and we checked whether we can recognize them or not. Tables 2 and 3 represent the calculated and scaled fundamental wavenumbers, intensities of vibrational peaks, and potential energy distribution along the internal coordinates obtained by DFT//B3LYP/6-311+G** level of theory. The corresponding experimental wavenumbers together with assignments are also reported in these tables. The potential energy distributions are given as per the internal coordinate system recommended by Pulay et al. using DFT [37].Table 2
Frequencies and PEDs of s-cis 2-ethylhexyl acrylate.
Numberb
Observed (cm−1)
DFT/6-311+G**  Unscaled (cm−1)
Scaled (cm−1)
Intensity (km·mol−1)
Assignment (%PED)a
1
—
30.5017
30,7519
0.1860
τ (O8C9) (63); τ (C9C12) (35);
2
—
36.7273
37,02484
1.9384
τ (C9C12) (83); τ (O8C9) (17)
3
—
48.2766
48,65862
1.2435
τ (C9C12) (98)
4
—
74.7391
75,29828
0.0096
ρ (C30H3) (62); τ (C9C12) (17); τ (O8C9) (7); ρ (C14H2) (7)
5
—
93.1493
93,81827
0.7602
τ (C9C12) (84); ρ (C14H2) (15)
6
—
99.6177
100,32262
0.6102
τ (C9C12) (76); τ (C9C12) (21)
7
—
115.9962
116,78605
0.2949
τ (C9C12) (94)
8
—
133.8201
134,69244
0.7707
τ (C9C12) (43); ρ (C27H3) (31); ρ (C9H2) (15)
9
—
146.1924
147,11591
0.0469
τ (C9C12) (58); ρ (C9H2) (17); ρ (C14H2) (16)
10
—
201.0708
202,16112
1.8808
τ (C9C12) (89); δ (C6O8C9) (5)
11
—
213.9423
215,05753
2.7336
τ (C9C12) (98)
12
—
234.1345
235,27792
0.6986
ρ (C30H3) (94)
13
—
253.7618
254,91989
1.2866
ρ (C30H3) (99)
14
—
265.4468
266,60766
0.7334
τ (C9C12) (83); ρ (C30H3) (14)
15
—
285.1163
286,27176
3.5548
ρ (C30H3) (78); τ (C9C12) (18)
16
—
315.7568
316,87874
0.5814
ρ (C9H2) (89); ρ (C17H3) (6)
17
—
355.1126
356,14657
0.2263
ρ (C17H3) (36); ρ (C9C12C14) (31); ρ (C9H2) (19)
18
—
375.9666
376,93349
4.3994
ρ (C24C27C30) (27); ρ (C9C12C14) (23); ρ (C17H3) (17); δ (C21C24C27) (10); ρ (C9C12H13) (10); δ (C21H2) (8)
19
—
470.318
470,80422
0.6863
ρ (C9C12C21) (72); ρ (C17H3) (11); ρ (C9H2) (5);
20
—
512.0856
512,26637
6.2054
ρ (C9C12C14) (80); ρ (C9H2) (7); δ (C21C12C14) (6)
21
—
527.3488
527,40376
0.1504
ρ (C9H2) (87); O (C6O7) (9)
22
—
533.9978
533,99558
1.1100
ρ (C9C12C14) (77); ρ (C6O8C9) (8)
23
—
681.189
679,55184
3.1928
ρ (C9C12C14) (42); δ (O7C6C4) (42); ρ (C6O8C9) (5)
24
731 (m)
731.2122
728,8586
4.5137
ρ (C21C24C27) (99)
25
748 (sh)
779.5982
776,474
1.0039
ρ (C27H2) (65); ρ (C9C12C14) (18); ρ (C14H2) (12)
26
770 (m)
786.3101
783,07298
1.6507
ρ (C27H2) (36); ρ (C21H2) (32); ρ (C14H2) (23); ρ (C17H3) (8)
27
811 (m)
829.5925
825,59191
24.9487
ρ (C9H2) (73); O (C6O7) (21); y (C1H2) (8)
28
852 (w)
852.3514
847,92486
3.0736
ρ (C9C12C21) (82); ρ (C9H2) (16)
29
896 (w)
901.6472
896,24016
1.0870
ρ (C9C12H21) (62); ω (C24H2) (16); y (C21H2) (9); δ (C30H3) (8)
30
—
906.8585
901,34317
1.0553
ρ (C9C12H21) (86); ρ (C9H2) (10)
31
909 (w)
917.2167
911,48352
7.2928
ρ (C9H2) (58); ω (C27H2) (27); ρ (O7C6O8) (12)
32
933 (sh)
947.8927
941,49381
5.9570
δ (C14C12H3) (50); ω (C27H2) (46)
33
962 (m)
983.6705
976,45643
25.1442
ρ (C9C12H13) (83); δ (C6O8C9) (21); y (C21H2) (14); y (C27H2) (11); y (C14H2) (11)
34
984 (m)
1006.9955
999,22751
25.5334
ω (C1H2) (84); y (C14H2) (10)
35
—
1016.9241
1008,91495
5.1103
ρ (C14H2) (41); ρ (C9C12C21) (24); δ (C12C21C24) (23); ρ (C17H3) (7)
36
1015 (sh)
1023.2765
1015,11136
26.0306
ο (C4H5) (99)
37
—
1025.8451
1017,61651
1.9504
δ (C9C12C14) (98); ν (C9O8) (5)
38
1025 (sh)
1045.9956
1037,26182
27.8636
y (C24H2) (42); ν (C9C12) (42); ν (O8C9) (6)
39
—
1062.1068
1052,95957
0.4109
ρ (C14C12H13) (83); ν (C27C30) (12)
40
—
1065.9864
1056,73835
0.4863
ν (C14C17) (83); ν (C24C27) (11)
41
—
1083.5238
1073,81387
1.6531
ω (C9H2) (83); ω (C21H2) (15)
42
1057 (m)
1089.4787
1079,60965
2.2238
ρ (C9H2) (84); ω (C27H2) (14)
43
—
1150.5508
1138,98319
1.9252
ρ (C9C12H13) (96)
44
1116 (w)
1168.5589
1156,46723
11.8316
δ (C12C21C24) (67); ρ (C14C12H13) (9); ρ (O8C9C12) (7); δ (C14C12C21) (67)
45
1060 (sh)
1185.6606
1173,06145
0.0857
δ (C9C12C14) (63); ρ (C30H3) (26); δ (C12C14C17) (6)
46
1191
1205.1044
1191,9167
626.1232
ρ (C6C4H5) (87)
47
—
1243.7853
1229,3901
0.3621
y (C21H2) (94); y (C27H2) (5)
48
—
1258.6429
1243,77093
1.0505
ρ (C9H2) (76); ω (C21H2) (23)
49
—
1281.7118
1266,0853
0.7178
y (C14H2) (55); ω (C21H2) (44)
50
—
1300.3301
1284,08198
1.3337
ω (C21H2) (93)
51
—
1317.9028
1301,05761
11.1581
ω (C24H2) (24); ρ (C9C12H13) (17); y (C14H2) (13); ω (C27H2) (7); ρ (C6O8C9) (5)
52
1272
1324.5992
1307,52384
68.3227
y (C27H2) (37); ρ (C9C12H13) (29); y (C21H2) (22); y (C14H2) (6)
53
1295
1328.2386
1311,03753
6.0034
y (C27H2) (93)
54
—
1338.1682
1320,62195
0.5711
y (C24H2) (70); y (C27H2) (28)
55
1340
1361.0156
1342,66291
1.3124
ω (C21H2) (78); ω (C9H2) (21)
56
—
1381.2224
1362,1423
0.4732
ρ (C9C12C21) (78); ω (C21H2) (21)
57
1357
1394.2309
1374,67547
3.3349
ω (C9H2) (95)
58
—
1404.5719
1384,63467
4.5032
ω (C24H2) (95); ω (C21H2) (5)
59
1381
1410.9338
1390,75996
7.3272
ω (C21H2) (97)
60
—
1414.1585
1393,86422
2.3703
ω (C21H2) (99)
61
—
1418.0649
1397,62426
0.4645
ω (C21H2) (93); ω (C14H2) (6)
62
1408
1437.761
1416,57487
83.1633
ω (C9H2) (49); ω (C21H2) (26); δ (CH2) (20)
63
—
1486.1635
1463,09161
1.0310
δ (C21H2) (40); δ (C24H2) (20); δ (C12C21C24) (12); δ (C14H2) (7)
64
—
1488.3957
1465,235
1.0310
δ (C27H2) (29); δ (C21H2) (28); δ (C24H2) (18); δ (C12C21C24) (14); δ (C14H2) (10); δ (C27H2) (10)
65
—
1492.293
1468,97685
1.3617
δ (C21H2) (67); δ (C27H2) (11); δ (C14H2) (7)
66
—
1499.9113
1476,28987
8.3925
δ (C30H3) (88); δ (C27H2) (10)
67
—
1501.7672
1478,07111
4.3738
δ (C21H2) (29); δ (C30H3) (28); δ (C24H2) (15); δ (C21C12H13) (11); δ (C14H2) (6)
68
—
1503.707
1479,93276
3.5078
δ (C27H2) (31); δ (C21H2) (29); δ (C9H2) (18)
69
1464
1509.3473
1485,34511
9.9123
δ (C17H3) (27); δ (C27H2) (26); δ (C21H2) (10); δ (C12C21C24) (8); δ (C24H2) (7); δ (C12C14C21) (6)
70
—
1510.6836
1486,62726
10.8575
δ (C21H2) (83); δ (C27H2) (10)
71
—
1515.4657
1491,21508
6.0184
δ (C24H2) (42); δ (C21C24C27) (25); δ (C27H2) (13); δ (C21H2) (10)
72
1637
1687.5503
1655,81242
26.9151
ν (C1C4) (87); δ (C1H2) (7)
73
1727
1770.955
1735,24092
243.4946
ν (C6O7) (80); δ (O8C9H12) (7); δ (C21H2) (5); ν (C4C6) (5)
74
—
2997.3914
2877,02371
2.7860
ν (C21H22) (45); ν (C14H2) (33); ν (C12H13) (14)
75
—
2999.8142
2879,23075
1.7988
ν (C21H2) (76); ν (C24H2) (14)
76
—
3002.0159
2881,23622
13.8469
ν (C27H2) (33); ν (C24H2) (32); ν (C30H31) (22); ν (C21H2) (6)
77
—
3005.8916
2884,76609
14.6381
ν (C21H2) (74); ν (C14H2) (14); ν (C12H13) (5)
78
2861
3014.5745
2892,67245
45.7319
ν (C12H13) (63); ν (C30H31) (31)
79
2875
3018.0275
2895,81595
55.9324
ν (C30H31) (98)
80
—
3023.8407
2901,10723
22.2942
ν (C17H19) (58); ν (C17H18) (29)
81
—
3026.1436
2903,20306
16.4278
ν (C17H19) (32); ν (C14H2) (29); ν (C17H18) (15); ν (C30C31) (8); ν (C21H2) (6)
82
—
3034.8324
2911,10906
35.1134
ν (C14H2) (96)
83
—
3041.9158
2917,55246
16.0910
ν (C27H2) (38); ν (C9H2) (20); ν (C14H2) (19); ν (C21H2) (6); ν (C24H) (6)
84
—
3043.1697
2918,6929
14.5616
ν (C27H2) (47); ν (C9H2) (26); ν (C24H2) (10); ν (C14H2) (7)
85
—
3062.906
2936,63657
46.5673
ν (C21H2) (55); ν (C27H2) (25); ν (C24H2) (9); ν (C12H13) (5)
86
2931
3078.8255
2951,10087
85.8987
ν (C27H2) (62); ν (C27H30) (10); ν 2955,03236 (C30H3) (9); ν (C9H2) (6); ν (C17H19) (6)
87
—
3083.1541
2955,03236
45.5349
ν (C30H31) (99)
88
—
3084.5244
2956,27682
35.1235
ν (C17H19) (67); ν (C17C19) (67); ν (C17C18) (17); ν (C30H31) (15)
89
—
3087.9105
2959,3517
19.5239
ν (C9H2) (63); ν (O8C9) (16); ν-(C12H13) (11); ν (C17C19) (6)
90
2961
3093.1423
2964,10191
56.1433
ν (C17H19) (38); ν (C9H2) (26); ν (C17H20) (12); ν (C14H2) (10); ν (C17H18) (9)
91
3038
3142.8663
3009,20422
5.1952
ν (C1C4) (45); ν (C1H2) (38); ν (C1H3) (9)
92
—
3185.092
3047,44188
3.2650
ν (C1H2) (95)
93
3104
3235.9105
3093,38372
2.3617
ν (C4H5) (67); ν (C1H2) (22); ν (C1H3) (6)
aAssignments and potential energy distribution (PED) (contributing ≥ 5%) for vibrational normal mode. Types of vibration: ν: stretching; δ: deformation; ο: out-of-plane bending; ω: wagging; y: twisting; ρ: rocking; τ: torsion; w: weak; m: medium; sh: shoulder.
bNormal mode number.Table 3
Frequencies and PEDs of s-trans 2-ethylhexyl acrylate.
Numberb
Observed (cm−1)
DFT/6-311+G**  Unscaled (cm−1)
Scaled (cm−1)
Intensity (km·mol−1)
Assignment (%PED)a
1
—
29.5569
29,79981
1.3888
τ (C21C24) (95)
2
—
31.2922
31,54848
1.2915
τ (C21C24) (100)
3
—
45.3002
45,66086
0.0227
τ (C21C24) (91); τ (O8C9) (6)
4
—
69.075
69,59818
0.0113
τ (C21C24) (98)
5
—
91.6561
92,31657
0.7304
ρ (C14H2) (64); τ (C9C12) (19); τ (C24C21) (15)
6
—
106.795
107,53821
0.0788
τ (C12C14) (92)
7
—
116.406
117,19786
0.4606
τ (O8C9) (90); τ (C21C24) (7)
8
—
128.002
128,84855
0.7652
τ (C21C24) (99)
9
—
137.7311
138,62015
0.0737
τ (C24C21) (94)
10
—
188.6176
189,67867
2.4795
τ (C24C21) (53); O (O8C9) (44)
11
—
196.4898
197,56995
2.2280
ρ (C9H2) (82); τ (C9C12) (13)
12
—
234.6
235,74391
1.7535
ρ (C30H3) (90); τ (C21C24) (7)
13
—
249.5865
250,74252
3.2507
ρ (C30H3) (78); τ (C12C21) (19)
14
—
262.3636
263,52416
1.2259
ρ (C9H2) (72); ρ (C17H3) (22)
15
—
274.1636
275,32362
0.5126
τ (C12C21) (96)
16
—
335.2283
336,31302
0.4554
δ (C21C12C14) (93)
17
—
354.6136
355,649
0.1655
δ (C21C12C24) (95)
18
—
406.411
407,25451
2.5402
δ (C9C12C14) (92)
19
—
471.3032
471,78287
1.0320
ρ (C9C12C14) (70); δ (C9C12C21) (12); ρ (C14H2) (11)
20
—
509.8943
510,09251
0.4733
y (C1H2) (86); O (C6O8) (12)
21
—
514.0157
514,18098
7.3067
ρ (C12C14C17) (38); ρ (C9C12C21) (15); ρ (C12C21) (14);ρ (C14C12H13) (13); ρ (C1H2) (8); δ (C12C21C24) (7);
22
—
550.859
550,70531
2.2808
ρ (C9C12C14) (78); ρ (C6O8C9) (9); ρ (C1H2) (6);
23
—
636.0175
634,9572
1.9146
ρ (O8C9C12) (97);
24
727 (m)
728.4338
726,12214
4.3677
ρ (C9C12C21) (92);
25
—
778.027
774,92902
0.6843
ρ (C9C12C21) (98);
26
770 (m)
785.397
782,17532
1.8732
ρ (C21H2) (59); ρ (C14H2) (36);
27
811 (m)
832.1364
828,08904
21.5147
y (C1H2) (94); ρ (C9H2) (5);
28
852 (w)
850.1904
845,80503
3.4731
ρ (C9C12C21) (47); ρ (C9H2) (27); ρ (C1H2) (15); ν (C14C17) (5);
29
896 (w)
901.7561
896,3468
0.9206
ρ (C9C12C21) (73); ρ (C21C24C27) (14);
30
909 (w)
906.5632
901,05403
1.3451
ρ (C9C12C21) (46); ρ (C9H2) (35); ρ (C12C14C17) (9);
31
933 (sh)
917.0283
911,29911
7.6343
ρ (O7C6O8) (86); ρ (C9H2) (10);
32
962 (m)
947.0255
940,64585
6.0649
ρ (C6O8C9) (99);
33
—
972.6775
965,71834
7.0484
ρ (C6O8C9) (99);
34
984 (m)
1001.2212
993,59199
34.4957
ω (C1H2) (52); ν (C1C4) (15); δ (C4C6O8) (15); ν (C12C21) (15)
35
1015 (sh)
1017.1676
1009,15249
7.0006
ν (C12C21) (92)
36
—
1021.9635
1013,83071
3.5045
ν (C21C24) (85); δ (C9C12O14) (9)
37
1025 (sh)
1029.9052
1021,57589
18.5130
y (C1H2) (97)
38
—
1040.755
1032,15388
0.3554
ν (C21C24) (70); ν (C9C12) (14); y (C21H2) (5)
39
—
1060.8287
1051,71458
5.5936
y (C21H2) (58); ρ (C14C12H13) (29); ρ (C14H2) (7)
40
—
1064.0573
1054,85945
18.8097
ν (C21C24) (88); ν (C14C17) (8)
41
1057 (m)
1076.7989
1067,26727
192.6571
ν (C9O8) (82); ν (C14C17) (13)
42
1060 (sh)
1089.5069
1079,6371
4.6500
ρ (C9H2) (40); δ (C9C12C21) (30); ω (C27H2) (25)
43
1116 (w)
1150.6075
1139,03825
1.4631
ρ (C9C12H13) (51); δ (C9C12C21) (42)
44
1191
1168.2621
1156,17915
3.2253
ρ (C14C12H13) (72); ω (C21H2) (16); ρ (O8C9C12) (9)
45
—
1185.0265
1172,44634
0.1695
δ (C9C12C14) (94)
46
—
1242.923
1228,55525
1.7298
y (C21H2) (89)
47
—
1258.238
1243,37912
1.1546
ρ (C9C12C21) (35); ω (C21H2) (35); ω (C9H2) (27)
48
—
1280.1017
1264,52842
77.9146
ω (C21H2) (47); ρ (C9C12C21) (37); ρ (C9H2) (27);
49
1272
1283.202
1267,52616
351.6365
ω (C21H2) (60); y (C14H2) (27); y (C9H2) (10)
50
—
1299.1286
1282,92094
4.4292
y (C9H2) (72); y (C21H2) (26);
51
1295
1311.7811
1295,14505
7.2842
y (C14H2) (99)
52
—
1318.75
1301,87577
6.8204
y (C14H2) (100)
53
—
1326.4691
1309,3292
0.1479
y (C21H2) (97)
54
—
1336.2199
1318,74163
0.4090
y (C24H2) (94); y (C21H2) (5)
55
1340
1359.997
1341,68063
1.0793
ω (C21H2) (86); ρ (C21C12C14) (7); ω (C9H2) (5)
56
—
1380.0145
1360,97825
0.4801
ρ (C9C12C21) (43); δ (C9H12) (28); ω (C21H2) (25)
57
1357
1393.1134
1373,59902
2.8872
ω (C9H2) (50); ω (C14H2) (23); δ (C21C12H13) (12); ω (C21H2) (86)
58
—
1404.3849
1384,45461
0.9476
ω (C21H2) (86); ω (C9H2) (12)
59
—
1411.9509
1391,73911
0.8054
ω (C9H2) (85); ω (C21H2) (14)
60
1381
1414.5333
1394,225
5.0233
ω (C21H2) (54); ω (C9H2) (45)
61
—
1417.3104
1396,89807
0.8100
ω (C21H2) (89); ω (C9H2) (8)
62
1408
1441.4398
1420,11302
30.5726
ω (C9H2) (93); ω (C21H2) (5)
63
—
1483.1028
1460,15241
2.0326
δ (C9H2) (38); δ (C21C12H13) (18); δ (C24C21C12) (15); δ (C14H2) (13); δ (C24H2) (7); δ (C21H2) (6)
64
—
1487.6468
1464,51591
0.4300
δ (C21C12H13) (33); δ (C27H2) (28); δ (C24H2) (20); δ (C21H2) (10)
65
—
1491.6884
1468,3964
1.2432
δ (C21H2) (54); δ (C9H2) (21); δ (C27H2) (11)
66
—
1500.0764
1476,44833
2.4692
δ (C24H2) (32); δ (C21C12H13) (24); δ (C27H2) (8); δ (C30H3) (8); δ (C21H2) (6)
67
—
1500.718
1477,06413
7.0795
δ (C30H3) (75); δ (C9H2) (11); δ (C27H2) (6)
68
—
1502.5266
1478,79993
8.9950
δ (C9H2) (74); δ (C17H3) (21)
69
1464
1508.1671
1484,21269
9.8400
δ (C27H2) (26); δ (C30H3) (22); δ (C21H2) (14); δ (C17H3) (14); δ (C24H2) (11)
70
—
1511.6225
1487,52807
10.5533
δ (C21H2) (73); δ (C21C12H13) (6); δ (C24H2) (5); δ (C17H3) (5)
71
—
1514.069
1489,8752
5.6625
δ (C24H2) (30); δ (C21H2) (29); δ (C21C24C27) (20); δ (C27H2) (9)
72
1619
1678.7098
1647,38009
28.7096
ν (C1C4) (52); ν (C4C6) (22); δ (C9C12H13) (11); δ (C21H2) (8)
73
1727
1766.8533
1731,34006
315.5882
δ (C9C12C21) (63); ν (C4C6) (16); δ (C21H2) (6); ν (C6O7) (6)
74
—
2995.932
2875,69419
2.0873
ν (C24H2) (35); ν (C14H2) (34); ν (C21H2) (25)
75
—
2998.467
2878,00355
0.6759
ν (C24H2) (40); ν (C12H13) (29); ν (C21H2) (12); ν (C14H2) (10)
76
—
3001.649
2880,90203
13.5187
ν (C21H2) (92)
77
—
3005.3152
2884,24116
13.2101
ν (C24H2) (87); ν (C24H2) (8)
78
2861
3013.385
2891,58948
58.5964
ν (C21H2) (45); ν (C30C32) (36); ν (C12H13) (12)
79
2875
3017.8939
2895,69433
47.2871
ν (C30C32) (96)
80
—
3023.6248
2900,91073
21.1397
ν (C14H12) (46); ν (C17H19) (32); ν (C17H18) (17)
81
—
3026.0401
2903,10887
15.9088
ν (C12H13) (30); ν (C17H19) (27); ν (C17H18) (13); ν (C30C32) (12)
82
—
3034.55
2910,85214
32.9164
ν (C21H2) (66); ν (C14H2) (28)
83
—
3040.4085
2916,18149
4.0314
ν (C21H2) (55); ν (C30H32) (27); ν (C12H13) (7)
84
—
3043.7899
2919,25696
26.3738
ν (C21H2) (98)
85
—
3060.9167
2934,82854
50.3404
ν (C21H2) (85); ν (C30C32) (12)
86
2931
3078.6333
2950,92629
82.4131
ν (C30C32) (95)
87
—
3083.3975
2955,25341
44.1245
ν (C30C32) (93)
88
—
3084.2781
2956,05315
36.9537
ν (C17H19) (67); ν (C17H18) (18); ν (C21H2) (9)
89
—
3088.9498
2960,29541
15.1201
ν (C21H2) (52); ν (C9H2) (14); ν (C12H13) (14); ν (C14H2) (8); ν (C17H19) (7)
90
2961
3093.0704
2964,03663
59.5099
ν (C17H19) (36); ν (C14H2) (28); ν (C17H20) (10); ν (C21H2) (10); ν (C17H18) (8); ν (C9H2) (6)
91
3038
3146.2567
3012,27655
5.2018
ν (C1C4) (50); ν (C1H2) (31); ν (C1H3) (10)
92
3069
3185.7999
3048,08243
3.4136
ν (C4H5) (57); ν (C4C6) (23); ν (C1C4) (14)
93
3104
3237.707
3095,00628
4.4771
ν (C1H2) (70); ν (C1H3) (11); ν (C4H5) (7)
aAssignments and potential energy distribution (PED) (contributing ≥ 5%) for vibrational normal mode. Types of vibration: ν: stretching; δ: deformation; ο: out-of-plane bending; ω: wagging; y: twisting; ρ: rocking; τ: torsion; w: weak; m: medium; sh: shoulder.
bNormal mode number.


### 5.1. The CH Stretching Vibrations Region (3200–2800 cm−1, Figure 2)

Comparison of the experimental IR (a) CH stretching region (3200–2800 cm−1) with the appropriate theoretical DFT//B3LYP/6-311+G** spectra ((b) and (c)) for s-cis and s-trans 2-ethylhexyl acrylate, respectively.
(a)
(b)
(c)In this region, the calculations reveal the existence of 20 vibrational normal modes for both the s-cis and s-trans conformers. These normal modes are of approximately the same energies and intensities for both conformers. In the IR spectrum in this region (Figure2(a)) there are three groups of bands, two groups of strong bands and one group of a very weak band. In the latter there are three bands of very weak intensity at 3104, 3069, and 3038 cm−1 which are well reproduced by the theoretical bands 3093, 3047, and 3012 cm−1, respectively. These bands are ascribed to the C1H and C4H stretching modes of vibrations of the vinyl group. The second group is composed of the strongest peaks at 2961 and 2931 cm−1. These two bands are well reproduced by the theoretical bands 2963 and 2953 cm−1, respectively. The third group is composed of one peak slightly asymmetric at 2875 cm−1 in the experimental IR spectrum; this band is reproduced at 2903 cm−1 in the theoretical calculations. The remaining bands below 2900 cm−1 with higher intensity in these series of CH stretching vibrational modes may be assigned to reproduce the broad shoulder at 2861 cm−1. All the theoretical peaks located between 3000 and 2860 cm−1 are ascribed to CH stretching of the methylene groups in the butyl and the ethyl sides of the 2-ethylhexyl acrylate molecule.


### 5.2. The C=O and C=C Stretching and CH Bending Vibrations (1800–1200 cm−1, Figure 3)

Comparison of the experimental IR (a) C=O and C=C stretching and CH bending region (1800–1200 cm−1) with the appropriate theoretical B3LYP/6-311+G** spectra ((b) and (c)) for s-cis and s-trans 2-ethylhexyl acrylate, respectively.
(a)
(b)
(c)In the IR spectrum (Figure3(a)), a very strong peak at 1727 cm−1 represents the C=O stretching band. The simulated peaks are at 1735 and 1731 cm−1 for the s-cis and s-trans conformations; these two frequencies contribute to the experimental C=O stretching band.There are two C=C stretching bands in the experimental spectra located at 1637 and 1619 cm−1, of medium intensity. These bands are well reproduced by the theoretical bands at 1655 cm−1 for s-cis conformer (Figure 3(b)) and 1647 cm−1 for the s-trans conformer (Figure 3(c)). The band at 1637 cm−1 may be used to definitely indicate that conformer with s-trans arrangement of acrylic moiety is present or not in the liquid 2-ethylhexyl acrylate.In the region from 1500 to 1200 cm−1, in the experimental spectrum (Figure 3), there are four bands of medium intensity located at 1464, 1408, 1295, and 1272 cm−1. The bands of weak intensity of account of three are located at 1381, 1357, and 1340 cm−1. The calculated frequencies in this region for the s-cis and s-trans conformations have close energies for each normal mode, and the differences do not exceed 5 cm−1. Based on the assignments reported by Dulce et al. [28] and our PED calculations, we have ascribed the theoretical frequencies 1485 cm−1 (s-cis) and 1484 cm−1 (s-trans) to the experimental band at 1464 cm−1 and they are representing a C17H3 symmetric bending. The calculated frequencies at 1479, 1478, and 1476 cm−1 may be contributed to the experimental band at 1464 cm−1 due to its relatively large middle width of about 75 cm−1. Mishra et al. [38] have reported the experimental and the theoretical IR spectra of γ form of oleic acid, many peaks reported in the region 1370 to 1220 cm−1 for the CH2 carboxyl-sided chain correspond approximately to those we found in our spectrum from 1400 to 1200 cm−1, and all of them may be assigned to CH2 deformation. The assignment is based on the bands intensities; we found a good correlation between the experimental and the theoretical spectra. One should point out that the very intense bands predicted at 1267 and 1264 cm−1 for s-trans form reproduce well the asymmetric shape of the experimental band at 1272 cm−1. For bands assignment, see Tables 2 and 3.


### 5.3. The CO Stretching and CH Bending Vibrations (1200–700 cm−1,  Figure 4)

Comparison of the experimental IR (a) CO stretching and CH bending region (1200–700 cm−1) with the appropriate theoretical B3LYP/6-311+G** spectra ((b) and (c)) for s-cis and s-trans 2-ethylhexyl acrylate, respectively.
(a)
(b)
(c)In this region, in the IR spectrum, there is one very strong band at 1192 cm−1 with weak shoulder at 1160 cm−1 and four medium intensity bands 1057, 984, 962, shoulder at 933 and 811 cm−1. The bands at 1116, 1025, 909, 896, 852, 770, and 727 cm−1 (shoulder at 748 cm−1) are of weak intensity (Figure 4). Based on PEDs and intensities of the calculated frequencies, we correlate the two intense bands at 1192 and 1156 cm−1 for the more stable and the less stable conformers to the band at 1191 cm−1 in the experimental IR spectrum. The medium bands in IR spectrum are well reproduced by the calculated peaks at 1079, 999, 976, 941, and 811 cm−1 for the s-cis conformer and 1067, 993, 940, 911, and 828 cm−1 for the s-trans conformer. They may be ascribed to CH2 rocking, C1H2 wagging, CCH rocking O8C9, and C1H2 twisting. The peaks predicted by DFT at 728 and 783 cm−1 in s-cis form and the peaks at 726 and 782 cm−1 in the s-trans form may be correlated to the bands at 727 and 770 cm−1 and they are assigned to the CH2 deformations of the alkyl sides of the title molecule.In the IR spectra of 2-ethylhexyl acrylate, reported in this work, there are a few bands with weak to very weak intensity which do not appear in the IR spectra of methyl acrylate reported by Dulce et al. [28] and George et al. [39]. These bands appear at 1157, 1120, 1052, and 1022 cm−1, and they are well correlated to the calculated peaks in DFT for both conformers; see Tables 2 and 3.The shoulders in the infrared spectrum at 1025, 1015 cm−1 of weak to medium intensities may be correlated to the bands 1037, 1015 cm−1 for s-cis and 1015, 988 cm−1 for the s-trans conformer. These bands are ascribed to the CH out-of-plane bending modes of the vinyl group.The remaining bands of weak intensity in the experimental IR spectrum are 909, 896, and 852 cm−1. These bands are close to the ones at 911, 996, and 847 cm−1 for the s-cis conformation of the title molecule. All the vibrational normal modes in this region are not pure and the majority of them were ascribed according to PED calculations to CH bending (for more details see Tables 2 and 3).


### 5.4. Region below 700 cm−1

This spectral region includes the bands associated with C=C–C, C–O–C, O=C–O, O–C–C, C–C–C, and C–C–C bending and torsion modes about the single bonds O–C and C–C. The frequencies and PED calculations are presented in Tables2 and 3. Comparing the 23 first normal modes for the two conformers, we notice that they have the same type of vibrational modes. However, the normal modes 16, 17, and 18 represent CCC bending in the s-trans and CH and CCC rocking modes in the s-cis conformer.Due to the lack of experimental data on this region, we compare our calculated frequencies to some experimental IR bands of related molecules presented in the literature. The bands at 356, 254, 116, and 100 cm−1 are close to the bands at 349(A′), 244(A′′), 114(A′′), and 114 cm−1 (A′′) reported by Dulce et al. [28]. From the same work the bands at 530, 244, 114, and 114 cm−1 belonging to A′′ symmetry, for the s-trans methyl acrylate in liquid state, are close to our calculated bands in DFT at 542, 244, 116, and 106 cm−1.


## 5.1. The CH Stretching Vibrations Region (3200–2800 cm−1, Figure 2)

Comparison of the experimental IR (a) CH stretching region (3200–2800 cm−1) with the appropriate theoretical DFT//B3LYP/6-311+G** spectra ((b) and (c)) for s-cis and s-trans 2-ethylhexyl acrylate, respectively.
(a)
(b)
(c)In this region, the calculations reveal the existence of 20 vibrational normal modes for both the s-cis and s-trans conformers. These normal modes are of approximately the same energies and intensities for both conformers. In the IR spectrum in this region (Figure2(a)) there are three groups of bands, two groups of strong bands and one group of a very weak band. In the latter there are three bands of very weak intensity at 3104, 3069, and 3038 cm−1 which are well reproduced by the theoretical bands 3093, 3047, and 3012 cm−1, respectively. These bands are ascribed to the C1H and C4H stretching modes of vibrations of the vinyl group. The second group is composed of the strongest peaks at 2961 and 2931 cm−1. These two bands are well reproduced by the theoretical bands 2963 and 2953 cm−1, respectively. The third group is composed of one peak slightly asymmetric at 2875 cm−1 in the experimental IR spectrum; this band is reproduced at 2903 cm−1 in the theoretical calculations. The remaining bands below 2900 cm−1 with higher intensity in these series of CH stretching vibrational modes may be assigned to reproduce the broad shoulder at 2861 cm−1. All the theoretical peaks located between 3000 and 2860 cm−1 are ascribed to CH stretching of the methylene groups in the butyl and the ethyl sides of the 2-ethylhexyl acrylate molecule.


## 5.2. The C=O and C=C Stretching and CH Bending Vibrations (1800–1200 cm−1, Figure 3)

Comparison of the experimental IR (a) C=O and C=C stretching and CH bending region (1800–1200 cm−1) with the appropriate theoretical B3LYP/6-311+G** spectra ((b) and (c)) for s-cis and s-trans 2-ethylhexyl acrylate, respectively.
(a)
(b)
(c)In the IR spectrum (Figure3(a)), a very strong peak at 1727 cm−1 represents the C=O stretching band. The simulated peaks are at 1735 and 1731 cm−1 for the s-cis and s-trans conformations; these two frequencies contribute to the experimental C=O stretching band.There are two C=C stretching bands in the experimental spectra located at 1637 and 1619 cm−1, of medium intensity. These bands are well reproduced by the theoretical bands at 1655 cm−1 for s-cis conformer (Figure 3(b)) and 1647 cm−1 for the s-trans conformer (Figure 3(c)). The band at 1637 cm−1 may be used to definitely indicate that conformer with s-trans arrangement of acrylic moiety is present or not in the liquid 2-ethylhexyl acrylate.In the region from 1500 to 1200 cm−1, in the experimental spectrum (Figure 3), there are four bands of medium intensity located at 1464, 1408, 1295, and 1272 cm−1. The bands of weak intensity of account of three are located at 1381, 1357, and 1340 cm−1. The calculated frequencies in this region for the s-cis and s-trans conformations have close energies for each normal mode, and the differences do not exceed 5 cm−1. Based on the assignments reported by Dulce et al. [28] and our PED calculations, we have ascribed the theoretical frequencies 1485 cm−1 (s-cis) and 1484 cm−1 (s-trans) to the experimental band at 1464 cm−1 and they are representing a C17H3 symmetric bending. The calculated frequencies at 1479, 1478, and 1476 cm−1 may be contributed to the experimental band at 1464 cm−1 due to its relatively large middle width of about 75 cm−1. Mishra et al. [38] have reported the experimental and the theoretical IR spectra of γ form of oleic acid, many peaks reported in the region 1370 to 1220 cm−1 for the CH2 carboxyl-sided chain correspond approximately to those we found in our spectrum from 1400 to 1200 cm−1, and all of them may be assigned to CH2 deformation. The assignment is based on the bands intensities; we found a good correlation between the experimental and the theoretical spectra. One should point out that the very intense bands predicted at 1267 and 1264 cm−1 for s-trans form reproduce well the asymmetric shape of the experimental band at 1272 cm−1. For bands assignment, see Tables 2 and 3.


## 5.3. The CO Stretching and CH Bending Vibrations (1200–700 cm−1,  Figure 4)

Comparison of the experimental IR (a) CO stretching and CH bending region (1200–700 cm−1) with the appropriate theoretical B3LYP/6-311+G** spectra ((b) and (c)) for s-cis and s-trans 2-ethylhexyl acrylate, respectively.
(a)
(b)
(c)In this region, in the IR spectrum, there is one very strong band at 1192 cm−1 with weak shoulder at 1160 cm−1 and four medium intensity bands 1057, 984, 962, shoulder at 933 and 811 cm−1. The bands at 1116, 1025, 909, 896, 852, 770, and 727 cm−1 (shoulder at 748 cm−1) are of weak intensity (Figure 4). Based on PEDs and intensities of the calculated frequencies, we correlate the two intense bands at 1192 and 1156 cm−1 for the more stable and the less stable conformers to the band at 1191 cm−1 in the experimental IR spectrum. The medium bands in IR spectrum are well reproduced by the calculated peaks at 1079, 999, 976, 941, and 811 cm−1 for the s-cis conformer and 1067, 993, 940, 911, and 828 cm−1 for the s-trans conformer. They may be ascribed to CH2 rocking, C1H2 wagging, CCH rocking O8C9, and C1H2 twisting. The peaks predicted by DFT at 728 and 783 cm−1 in s-cis form and the peaks at 726 and 782 cm−1 in the s-trans form may be correlated to the bands at 727 and 770 cm−1 and they are assigned to the CH2 deformations of the alkyl sides of the title molecule.In the IR spectra of 2-ethylhexyl acrylate, reported in this work, there are a few bands with weak to very weak intensity which do not appear in the IR spectra of methyl acrylate reported by Dulce et al. [28] and George et al. [39]. These bands appear at 1157, 1120, 1052, and 1022 cm−1, and they are well correlated to the calculated peaks in DFT for both conformers; see Tables 2 and 3.The shoulders in the infrared spectrum at 1025, 1015 cm−1 of weak to medium intensities may be correlated to the bands 1037, 1015 cm−1 for s-cis and 1015, 988 cm−1 for the s-trans conformer. These bands are ascribed to the CH out-of-plane bending modes of the vinyl group.The remaining bands of weak intensity in the experimental IR spectrum are 909, 896, and 852 cm−1. These bands are close to the ones at 911, 996, and 847 cm−1 for the s-cis conformation of the title molecule. All the vibrational normal modes in this region are not pure and the majority of them were ascribed according to PED calculations to CH bending (for more details see Tables 2 and 3).


## 5.4. Region below 700 cm−1

This spectral region includes the bands associated with C=C–C, C–O–C, O=C–O, O–C–C, C–C–C, and C–C–C bending and torsion modes about the single bonds O–C and C–C. The frequencies and PED calculations are presented in Tables2 and 3. Comparing the 23 first normal modes for the two conformers, we notice that they have the same type of vibrational modes. However, the normal modes 16, 17, and 18 represent CCC bending in the s-trans and CH and CCC rocking modes in the s-cis conformer.Due to the lack of experimental data on this region, we compare our calculated frequencies to some experimental IR bands of related molecules presented in the literature. The bands at 356, 254, 116, and 100 cm−1 are close to the bands at 349(A′), 244(A′′), 114(A′′), and 114 cm−1 (A′′) reported by Dulce et al. [28]. From the same work the bands at 530, 244, 114, and 114 cm−1 belonging to A′′ symmetry, for the s-trans methyl acrylate in liquid state, are close to our calculated bands in DFT at 542, 244, 116, and 106 cm−1.


## 6. Conclusions

The mid-IR spectrum of 2-ethylhexyl acrylate was measured and interpreted with support of the DFT//B3LYP/6-311+G** calculated vibrational spectra followed by potential energy distribution analysis. Assuming the s-cis and s-trans conformations, we found two stable conformations, close in their energies. The fully optimized geometries of s-cis and s-trans conformations by DFT//B3LYP/6-311+G** were compared with the experimental and theoretical data presented in the literature on the methyl acrylate molecule. The comparison shows a good agreement. The experimental vibrational spectrum is in good agreement with the theoretical spectra calculated for the two conformations. Two neighbor bands in the IR spectrum at 1619 and 1637 cm−1 may be used as characteristic bands to locate and distinguish the existence of one or both conformations.



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*Source: 348379-2013-06-02.xml*