14N nuclear quadrupole coupling and methyl internal rotation in 3-methylpyrrole investigated by microwave spectroscopy

Abstract

The molecular structure of 3-methylpyrrole in the gas phase has been determined using a combination of high-resolution spectroscopy and quantum chemical calculations. The rotational spectrum was recorded using a molecular jet Fourier transform microwave spectrometer covering the frequency range from 2.0 to 26.5 GHz. The experimental data were analyzed using the programs XIAM and BELGI-Cs-hyperfine. Because the internal rotor axis accidentally lies along the principal a-axis of inertia, the rho axis system and the principal axis system coincide, enabling a direct comparison of the fits. With the program XIAM, the rotational constants A = 8631.1629(12), B = 3342.19750(43), and C = 2445.73846(42) MHz were obtained. Torsional splittings due to internal rotation of the methyl group were observed, leading to the determination of the V3 potential of 245.92445(31) cm−1. Hyperfine splittings arising from the nuclear quadrupole coupling of the 14N nucleus could be resolved, and the quadrupole coupling constants χaa = 1.4159(49) and χbb − χcc = 4.1622(86) MHz were found.