Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2–Y with Y=HF, HCl, HCF3, and CO2

Abstract

Microwave rotational spectra have been observed for a number of isotopic species of the (HCN)2–HF, –HCl, –HCF3, and –CO2 trimers. The observations were made with the pulsed nozzle, Fourier transform, Flygare/Balle Mark II spectrometer. The trimers have structures which are composites of the linear (HCN)2 dimer and the HCN–Y dimers, the latter linear for Y=HF and HCl, a symmetric top for Y=HCF3, and T-shaped with C2v symmetry for Y=CO2. The rotational constants for the most abundant species of each trimer are as follows: For Y=HF and HCl, B0 is 699.204 and 467.408 MHz, respectively, and DJ is 162 and 87 Hz; for Y=HCF3, B0 is 305.742 MHz and DJ and DJK are 51 and 471 Hz; for Y=CO2, treated as a symmetric top, (B0+C0)/2 is 452.426 MHz and DJ is 1.057 kHz. Hyperfine interaction constants were determined for several species. The B0’s for each trimer were analyzed by a combination of isotopic substitution and fitting procedures to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers. The B0’s are relatively insensitive to the position of the central HCN but give r1+r2 accurately. With this limitation, r1 and r2 in the trimers are compared with the corresponding distances in the dimers, which are longer. For Y=HF, HCl, HCF3, and CO2, respectively, the shrinkages found in r1 are 0.069, 0.054, 0.030, and 0.004 Å and in r2, 0.043, 0.062, 0.042, and 0.052 Å. The shrinkage in r1 and several other properties of the trimers exhibit some correlation with the pseudodiatomic stretching force constant in the HCN–Y dimer.