Rotational spectra and structures of the C₆H₆-HCN dimer and Ar₃-HCN tetramer

Abstract

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, D_J, and D_JK 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 χ finds its projection on the figure axis to be −4.223(4) MHz in C₆H₆-HCN and −1.143(2) in Ar₃-HCN. They correspond to average projection angles θ between the HCN and figure axes of 15.2° and 45.3°, respectively. A pseudodiatomic analysis of the rotational constants gives the c.m. to c.m. distance to be 3.96 Å in C₆H₆-HCN and 3.47 Å 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 χ(¹⁴N) similar to that of Ar-HCN; a very large projection angle θ; large centrifugal distortion including higher-order terms in H_J and H_JK; 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⁻¹). That for Ar₃-HCN is tricuspid, with large gullies, and shallow (507 cm⁻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.