The rotational spectrum of the benzene-hydrogen bromide complex

Ground-state rotational spectra of six symmetric-top isotopomers, C6H6 ⋯ H79 Br, C6H6 ⋯ H81 Br, C6D6 ⋯ H79 Br, C6D6 ⋯ H 81Br, C6H6 ⋯ D 79Br and C6H6 ⋯ D 81Br, of the benzene-hydrogen bromide complex were observed and analysed to give the rotational constant B0, the centrifugal distortion constants DJ and DJK, and the Br nuclear quadrupole and spin-rotation coupling constants Xaa and Mbb in each case. It is concluded that in the zero-point state the complex has effective C6v symmetry, with HBr oriented so that H lies closest to the benzene ring and undergoes a circular motion which allows it to sample the π-electron density of the ring.