Rotational spectra and structures of the C<SUB>6</SUB>H<SUB>6</SUB>-HCN dimer and Ar<SUB>3</SUB>-HCN tetramer

A comparative study has been made of the rotational properties of C6H6-HCN and Ar3-HCN, observed with the Balle/Flygare pulsed beam, Fourier transform microwave spectrometer. C6H6-HCN is found to be a prolate symmetric top and Ar3-HCN an oblate one, both with the H in the middle. The rotational constants B0, DJ, and DJK of the parent species are 1219.9108(4) MHz, 1.12(3) kHz, and 18.32(8) kHz for C6H6-HCN, and 886.4878(1) MHz, 10.374(2) kHz, and 173.16(1) kHz for Ar3-HCN. Rotational constants are reported for the isotopic species C6H6-H13CN,-HC15N, and 13CC5H6-HC15N, and for Ar3-HC15N and -DCN. Analysis of the 14N hyperfine interaction &#967; finds its projection on the figure axis to be &#8722;4.223(4) MHz in C6H6-HCN and &#8722;1.143(2) in Ar3-HCN. They correspond to average projection angles &#952; between the HCN and figure axes of 15.2&#176; and 45.3&#176;, respectively. A pseudodiatomic analysis of the rotational constants gives the c.m. to c.m. distance to be 3.96 &#197; in C6H6-HCN and 3.47 &#197; in Ar3-HCN. While the rotational properties of C6H6-HCN are "normal," those of Ar3-HCN display a long list of "abnormalities." They include a J-dependent &#967;(14N) similar to that of Ar-HCN; a very large projection angle &#952;; large centrifugal distortion including higher-order terms in HJ and HJK; splitting of the K=3 transitions into J-dependent doublets; and the ready observation of an excited vibrational state. These behavioral differences are related qualitatively to the interaction surfaces for the two clusters, calculated with the molecular mechanics for clusters (MMC) model, and discussed. The potential minimum for C6H6-HCN is smooth, circular, steep except for a flat bottom, and deep (1762 cm&#8722;1). That for Ar3-HCN is tricuspid, with large gullies, and shallow (507 cm&#8722;1). In addition to the dispersion forces, the dominant interaction forming C6H6-HCN is between the benzene quadrupole moment and the HCN dipole moment, a strong 4-2 potential. That in Ar3-HCN is polarization of the spherical Ar by the HCN dipole and quadrupole moments, a weak 0-2,4 potential