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

Abstract

Identification of methyl acetate in the interstellar medium (ISM) and its spectroscopic studies have prompted us to investigate the structure of crystalline methyl acetate using numerical calculations. Here, we present a theoretical study of the structure of crystalline methyl acetate and its isotopologues and compare the calculated infrared (IR) spectra with the available experimental data. The optimized structure and vibrational properties were calculated using SIESTA software at 0 K. In the optimization process, the Perdew-Burke-Ernzerhof functional and conjugate gradient methods were used with double zeta polarization basis functions. After optimization of the periodic structure, the vibrational frequencies and normal modes were calculated within the harmonic approximation. Using the calculated results, we refine the mode assignments of experimental work on crystalline methyl acetate and determine the low frequency modes (below 650 cm(-1)). To investigate the accuracy of the pseudopotential and confirm the IR frequencies, we performed molecular calculations using a periodic model of methyl acetate and its isotopologues using SIESTA and compared them with results obtained from Gaussian 09 (all electron method) calculations. Finally, we assigned the vibrational modes of crystalline CD3-COO-CH3 and CH3-COO-CD3, for which experimental data are not available in the crystalline phase under ISM conditions. For all of the calculation methods, the IR vibrational modes of molecular and crystalline methyl acetate and its isotopologues were in good agreement with the available experimental data and predict the unavailable values.

Infrared Spectra of Gas Hydrates from First-Principles

“…We used the revised version of the Perdew–Burke–Ernzerhoff (revPBE) exchange-correlation (XC) functional under the general gradient approximation (GGA), in all cases. This particular GGA XC functional has been shown to perform very well for gas hydrate material properties despite its shortcomings. − GGA XC functionals have also been used in many other recent studies investigating vibrational properties from first-principles. − We also tested a van der Waals (vdW) XC functional (DRSLL), recently made available for SIESTA, for the empty sII hydrate structure. This resulted in only small differences in the IR spectrum when compared to using the GGA functional, not significant enough to warrant the much larger associated computational cost.…”

“…The convolution was done using a half width at half-maximum (hwhm) value of 70 cm –1 for the OH stretching bands and 30 cm –1 for all other bands. These values were chosen in order to obtain the best representation for the band peaks of our computed spectra and also based on previous studies. ,, …”

“…These values were chosen in order to obtain the best representation for the band peaks of our computed spectra and also based on previous studies. 40,43,46 In this work, we focused on sII gas hydrates with hydrocarbon guests (i.e., methane, ethane, propane, and isobutane), which are the main components found in natural gas. Structure II gas hydrates have a cubic lattice structure.…”