Knowledge Bank

The $m=0$, ground internal rotation state of the $C6H6-H2O,-D2O$, and --HDO dimers has been characterized by $Blake1$ at high J (4 to 10). We report extension of the studies to low J (0 to 4), to the first excited $(m=1)$ internal rotation state, to the hfs exhibited by some of the transitions, and to several other isotopic species $(H218O,C6H5D,C513CH6)$. The $m=0$ and 1 rotational spectra of the dimers with $C6H6$ are fitted by the $expression2$ $v=2(J+1)[B-DJKK2-DJmm2-DJKmKm-HJKmM2K2]-4(J+1)3DJ$ which for $m=0$ is the standard description of a symmetric top. The fit indicates that the dimers with $C6H6$ are coaxial rotors of six- and two-fold symmetry with a near-zero effective barrier to internal $rotation.3$ The dimers with $C6H5D$ or $C5\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH6$ have $m=0$ transitions fitted as semi-rigid, slightly asymmetric tops while the $m=1$ transitions exhibit features implying that the D and $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ substitution affect the barrier. The geometrical and dynamic structures of the dimers will be discussed.