Millimeter-Wave Spectroscopy of Methylfuran Isomers: Local vs. Global Treatments of the Internal Rotation

Abstract

Methylfurans are methylated aromatic heterocyclic volatile organic compounds and primary or secondary pollutants in the atmosphere due to their capability to form secondary organic aerosols in presence of atmospheric oxidants. There is therefore a significant interest to monitor these molecules in the gas phase. High resolution spectroscopic studies of methylated furan compounds are generally limited to pure rotational spectroscopy in the vibrational ground state. This lack of results might be explained by the difficulties arisen from the internal rotation of the methyl group inducing non-trivial patterns in the rotational spectra. In this study, we discuss the benefits to assign the mm-wave rotational-torsional spectra of methylfuran with the global approach of the BELGI-Cs code compared to local approaches such as XIAM and ERHAM. The global approach reproduces the observed rotational lines of 2-methylfuran and 3-methylfuran in the mm-wave region at the experimental accuracy for the ground vt=0 and the first torsional vt=1 states with a unique set of molecular parameters. In addition, the V3 and V6 parameters describing the internal rotation potential barrier may be determined with a high degree of accuracy with the global approach. Finally, a discussion with other heterocyclic compounds enables the study of the influence of the electronic environment on the hindered rotation of the methyl group.

Potential and kinetic interaction of two internal tops in molecules belonging to C2v(M) molecular symmetry group

“…Internal rotation in molecules (one of the types of large amplitude motion (LAM)) continues to attract the attention of researchers today [1][2][3][4][5][6][7]. This is due to the important role that internal rotation plays in determining the thermodynamic properties of molecules [1], as well as in the formation of the fine structure of rotational and vibrational-rotational absorption bands [8][9][10].…”

“…This is also consistent with the results of works [7,35,33,34] according to which the tunneling splittings of the ground vibrational states in the trans and cis conformers of the HSSSH, HOSOH and HOOOH molecules are very small (the maximum calculated value of the tunneling splitting was found in the cis conformer of the molecule HOOOH and ranged from 3*10 -7 [34] to 1*10 -6 [33] cm -1 ). As far as we know, only in [4] the tunneling splitting of the first excited torsional state of the C 6 H 4 (OH) 2-ortho molecule was experimentally recorded, the value of which was 1.041*10 -5 cm -1 . This is in reasonable agreement with the tunneling splittings of the corresponding torsional state calculated at various levels of theory (from 0.27*10 -5 to 0.61*10 -5 cm -1 ) [36].…”