ROTATIONAL SPECTRUM AND STRUCTURE OF THE LINEAR HCN-HCCH DIMER

The ground state rotational spectra have been observed for five isotopic species of the linear, hydrogen bonded dimer, HCN-HCCH, using a Fourier transform Balle-Flygare microwave spectrometer. The B(O) and D(J) rotational constants for the parent species are 1625.381 MHz and 3.08 kHz, respectively. Analysis of the spectra gives substitution values of 4.651 angstrom, 17.1-degrees and 7.8-degrees, respectively, for the c.m. to c.m. distance R between monomers, and for the average amplitudes theta and phi of the HCN and HCCH torsional vibrations. The D(J) leads to a stretching force constant k(s) of 4.4 N/m and a well depth of 660 cm-1. The substitution and N-14 quadrupole interaction data are employed to assess the small charge redistribution effects caused by formation of the dimer. The results are compared with those for HCN-HCN and HCN-HF. Corresponding ab initio and model calculations indicate that the charge redistribution is simple electrical polarization. A T-shaped HCN/HCCH dimer was reported earlier. The properties of the two isomers are compared. It is found that formation of the linear form is favored by using neon first run as the carrier gas, but it is strongly relaxed to the T-shaped isomer by addition of a small amount of Ar (6%).