Millimeter-Wave Rotational Spectrum, Barrier to Internal Rotation, and DFT Calculation of o-Tolunitrile

The millimeter-wave rotational spectra of o-tolunitrile (C6H4CH3CN) have been investigated in the ground torsional state in the frequency range 50.0–75.0？GHz. Many high-J rotational lines with large A-E splitting due to internal rotation of the methyl top have been assigned. A least squares analysis of the A-E splitting of 92 transitions resulted in the determination of accurate values of internal rotation parameters. The observed parameters were compared with the previously reported experimental values and DFT calculation results. 1. Introduction The microwave rotational spectra of molecules having a methyl top attached to a planar frame shows a characteristic splitting corresponding to A and E symmetry of the torsional states. These A-E doublets may be closely or widely spaced depending on the height of the potential barrier and transitions studied. Hence, an analysis of the observed A-E splitting would lead to the determination of the barrier height hindering the internal rotation. Microwave spectroscopic techniques have been widely employed to study the effect of internal rotation of the methyl group on the rotational spectra of the molecules concerned. Extensive studies on the microwave [1–5] and electronic [6–8] spectra of toluene and substituted toluenes have been reported in the literature. In contrast, not much microwave spectroscopic work has been reported on tolunitriles. Fujii et al. [9] have investigated the fluorescence excitation and dispersed fluorescence spectra of o-, m-, and p-tolunitrile in supersonic jets. By analyzing the low-frequency bands due to internal rotation of the methyl group they have successfully determined the height of the potential barrier hindering the internal rotation for m- and p-tolunitrile. However, in the case of o-tolunitrile, the absence of low-frequency bands prevented them from determining the potential barrier. In an earlier communication, Jaman et al. [10] have reported the rotational constants and a value of the potential barrier from an analysis of the A-E splitting of the microwave spectrum of o-tolunitrile in the ground torsional state. Later, Hansen et al [11] have investigated the high-resolution microwave spectrum of o-tolunitrile by using molecular beam FT microwave spectroscopic technique and determined rotational, centrifugal distortion constants, 14N nuclear quadrupole hyperfine constants, as well as . The internal rotation parameters reported in [10, 11] were in good agreement with each other. In the present communication, we have extended the analysis of the microwave spectra of