Rotational spectrum and structure of the Ne-HCN dimer

Microwave rotational transitions have been observed at low J (0–3) for several isotopic species of the Ne–HCN dimer using the Balle/Flygare Mark II Fourier transform spectrometer with a pulsed nozzle as the source. For 20Ne–HC 14N, the main K=0 transitions give rotational constants B̄, DJ, and HJ of 2772.816 and 1.280 MHz and 1.173 kHz. The 14N nuclear quadrupole constant increases linearly with J(J+1) at a slope Dχ of −12.7 kHz from a value for χa(14N) of −0.957 MHz at J=0. The pseudodiatomic approximation for B̄ and χa(14N) leads to a value of 3.89 Å for the Ne to HC 14N center-of-mass (c.m.) distance R, and to 46.8° for the ‘‘average’’ bend angle θ of HC 14N. Some of the K=0, J=1→2, and J=2→3 transitions exhibit one or two weak satellites ∼30 MHz away, usually below, but also both above and below. The J=1→2 low frequency satellites for 20Ne–HC 14N and 20Ne–HC 15N, nominally 111→212, are symmetrical doublets with splittings of 305 and 439 kHz, respectively. The 14N hyperfine structure (hfs) is identical for the two 20Ne–HC 14N components as is the Stark effect for 20Ne–HC 15N. The molecular mechanics for clusters (MMC) model was used to calculate potential energy surfaces for Rg–HCN dimers, giving stabilities of 21, 37, 85, and 108 cm−1 with He, Ne, Ar, and Kr as the rare gas. A qualitative comparison of the experimental properties for the dimers with Ne, Ar, and Kr as the rare gas is based on the surfaces. The extremely mobile internal dynamics of Ne–HCN are attributed to its potential surface, which is both very shallow and isotropic.