Rotational spectra of CH4sdot;HCl and CD4sdot;HCl have been studied using a pulsedhyphen;nozzle Fourierhyphen;transform microwave spectrometer. TheK=0 andK=1 components of theJ=1ndash;0,2ndash;1, and 3ndash;2 transitions have been detected in 4ndash;18 GHz and assigned through the Clhyphen;nuclear quadrupole hyperfine structure characteristic for a symmetric top. The Clhyphen;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. TheK=0 transitions were observed to be split into a doublet while only one component was detected forK=1. This anomaly has been explained as it is due to twohyphen;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 freehyphen;internal rotor and a rigid symmetric top. The two observedK=0 and one observedK=1 components correspond to the three lowest states with different symmetries,A,F, andE, which correlate respectively to thej=0, 1, and 2 states of the freely rotating methane.
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