Rotational spectrum and internal rotation of a methane–HCl complex

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Abstract
Rotational spectra of CH4⋅HCl and CD4⋅HCl have been studied using a pulsed‐nozzle Fourier‐transform microwave spectrometer. The K=0 and K=1 components of the J=1–0,2–1, and 3–2 transitions have been detected in 4–18 GHz and assigned through the Cl‐nuclear quadrupole hyperfine structure characteristic for a symmetric top. The Cl‐isotopic dependence of the determined rotational constants is consistent with a methane...HCl geometry, i.e., methane acts as a proton acceptor and hydrogen chloride as a donor, respectively. The K=0 transitions were observed to be split into a doublet while only one component was detected for K=1. This anomaly has been explained as it is due to two‐dimensional internal rotation of methane. An analysis considering the angular momentum coupling between the internal rotation and overall rotation of the complex gives a correlation between a free‐internal rotor and a rigid symmetric top. The two observed K=0 and one observed K=1 components correspond to the three lowest states with different symmetries, A, F, and E, which correlate respectively to the j=0, 1, and 2 states of the freely rotating methane.