Rotational spectrum and internal rotation of a methane–HCl complex

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.