title: 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.
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan; JST, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan. Electronic address: narayanannair-radhika@ed.tmu.ac.jp.
authors.forename: Radhika
authors.initials: R
authors.lastname: Narayanan
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan; JST, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Kensuke
authors.initials: K
authors.lastname: Inomata
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan; JST, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Geetha
authors.initials: G
authors.lastname: Gopakumar
authors.affiliation: Space and Atmospheric Sciences Division, Physical Research Laboratory, Ahmedabad 380009, India.
authors.forename: Bhalamurugan
authors.initials: B
authors.lastname: Sivaraman
authors.affiliation: Computer and Information Sciences, Hosei University, 3-7-2 Kajino, Koganei 184-8584, Japan.
authors.forename: Yasunari
authors.initials: Y
authors.lastname: Zempo
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan; JST, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Masahiko
authors.initials: M
authors.lastname: Hada
keywords: DFT; Infrared spectra; Interstellar medium; Methyl acetate