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

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 that definitely indicates the conformer with s-trans arrangement of acrylic moiety to be present or not in the liquid 2-ethylhexyl acrylate.


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][2][3][4].The structural data and the preferred molecular conformation of these acrylates molecules would be important for basic understanding of these polymers [5].
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][17][18][19][20][21] and theoretical [20,22], show that acrylates and related compounds exhibit rotational isomerism with the planar s-trans and s-cis heavyatom 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][25][26][27][28][29].However, Bowles et al. [30] assumed that the s-trans conformer is more stable.

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.
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: ] obs ] calc = 1.0087 − 0.0000163] calc .
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].

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 Table 1 for s-cis and s-trans conformers, respectively.For atoms numbering, see Figure 1.
In Table 1, 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 C 1 C 4 C 6 and C 4 C 6 O 8 increase by 3.8 ∘ and 2.7 ∘ , respectively, while C 4 C 6 O 7 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 × 10 6 cal/mol showing more stability than the calculated one for the s-trans conformation.The energy for the latter conformation is 365.101 × 10 6 cal/mol.

Vibrational Analysis
The most stable conformers, s-cis and s-trans 2-ethylhexyl acrylate, belong to the C 1 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].

The CH Stretching
Vibrations Region (3200-2800 cm −1 , Figure 2).In this region, the calculations reveal the existence of 20 vibrational normal modes for both the s-cis and strans conformers.These normal modes are of approximately the same energies and intensities for both conformers.In the IR spectrum in this region (Figure 2(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 C 1 H and C 4 H 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.

The C=O and C=C Stretching and CH Bending Vibrations
(1800-1200 cm −1 , Figure 3).In the IR spectrum (Figure 3(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 C 17 H 3 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  form of oleic acid, many peaks reported in the region 1370 to 1220 cm −1 for the CH 2 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 CH 2 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.

The CO Stretching and CH Bending
Vibrations (1200-700 cm −1 , Figure 4).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 CH 2 rocking, C 1 H 2 wagging, CCH rocking O 8 C 9 , and C 1 H 2 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 CH 2 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 Organic Chemistry International 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).  2 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 strans 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 .

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.

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 Tables