Highly correlated ab initio study of the far infrared spectra of methyl acetate

Abstract

Highly correlated ab initio calculations (CCSD(T)) are used to compute gas phase spectroscopic parameters of three isotopologues of the methyl acetate (CH 3 COOCH 3 , CD 3 COOCH 3 , and CH 3 COOCD 3 ), searching to help experimental assignments and astrophysical detections. The molecule shows two conformers cis and trans separated by a barrier of 4457 cm −1 . The potential energy surface presents 18 minima that intertransform through three internal rotation motions. To analyze the far infrared spectrum at low temperatures, a three-dimensional Hamiltonian is solved variationally. The two methyl torsion barriers are calculated to be 99.2 cm −1 (C-CH 3 ) and 413.1 cm

Electron impact dissociation of amorphous cis-methyl acetate ice analogs

“…Several other new bands appeared in the spectra between 1890 cm À1 and 2850 cm À1 region ( Figure 2b) and are listed in Table 1 (Table 1). Though several new bands appeared in the spectra by depositing CH 3 COOCH 3 molecules at 85 K, we found no match between the suggested band positions for trans-CH 3 COOCH 3 molecule by Senent et al [7]. Instead, the recorded IR spectra was found to be in good agreement with the band positions reported by Sivaraman et al [6] and therefore the icy layers grown by molecular deposition on to the cold substrate, kept at 85 K, was found to contain only cis-CH 3 COOCH 3 molecules.…”

Vacuum Ultraviolet and Infrared Spectra of Condensed Methyl Acetate on Cold Astrochemical Dust Analogs

“…The spectrum of this 'annealed ice' was found to contain seven new bands at 3022 cm -1 , 2987.9 cm -1 , 2738.9 cm -1 , 2490.5 cm -1 , 1984.2 cm -1 , 1844.2 cm -1 and1457.2 cm -1 ( Figure 3). While most of these new bands are left unassigned, the 1457.2 cm -1 band may correspond to the (O)CH 3 bending vibration (Senent et al 2013). However, the fundamental, overtone and combination bands that were discussed earlier for the methyl acetate ice at 15 K and 120 K were also observed to have shifted by few cm -1 (Figure 3)…”

“…However, the phase change temperature could be between 110 K and 120 K because the spectra recorded at 120 K was observed to show a phase change that had completed and the spectrum at 110 K was found to match with the spectra of the amorphous ice at lower temperatures. Transformation from cis to trans conformer of methyl acetate by annealing cannot happen due to the large barrier 4457 cm -1 (Senent, et al 2013) involved in this transformation. Also the band positions for trans-methyl acetate calculated in Senent et al (2013) were not observed.…”

“…Transformation from cis to trans conformer of methyl acetate by annealing cannot happen due to the large barrier 4457 cm -1 (Senent, et al 2013) involved in this transformation. Also the band positions for trans-methyl acetate calculated in Senent et al (2013) were not observed. Therefore by comparing the VUV and IR spectra recorded at different temperatures we notice that the ice formed at 10 K was amorphous cismethyl acetate ice.…”

Theoretical study of the infrared frequencies of crystalline methyl acetate under interstellar medium conditions

“…Methyl acetate has cis and trans conformers (based on the direction of the MeO bond with respect to the C=O bond), which have different energies (ΔH° ≈ 8.5 ± 1.0 kcal mol −1 ) [5]. Even though the conformers can interconvert through internal rotation of the C-O single bond, the corresponding torsional barrier is relatively high (4457 cm −1 ) [6]. Hence, it is possible to analyze the vibrational spectra of the stable cis-conformer without considering conformer interconversion.…”