Rotational spectrum and structure of the linear HCN trimer

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Abstract
Microwave rotational spectra have been observed for 22 isotopic species of an HCN, hydrogen‐bonded trimer with the pulsed nozzle, Fourier transform method using the Flygare/Balle Mark II spectrometer. The 14N nuclear quadrupole hyperfine structure was analyzed and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational constants. For the normal isotopic species of (HCN)3, B0 was found to be 469.3073(1) MHz and DJ, 82.6(1) Hz. The quadrupole coupling constants χ(n) are −4.049(2), −4.251(2), and −4.375(1)MHz for n=1, 2, and 3, respectively, in HCN(1)HCN(2)HCN(3). The trimer has a linear or very near linear equilibrium structure. The B0’s are insensitive to the position and torsional oscillations of the central HCN but they determine the outer HCNs quite accurately. An isotopic substitution method gives R, the c.m. distance between the outer HCN’s, to be 8.790 Å in the 14–14–14 species. A slightly smaller value 8.788 Å is obtained from a fit of the B0’s which includes the effects of isotopic substitution on the H/D–C and C–N bond lengths in the monomer. The distance between c.m.(1) and c.m.(2) may be somewhat shorter (∼0.01 Å) than that between c.m.(2) and c.m.(3). The average c.m. separation in the trimer (4.395 Å) is halfway between that of the dimer (4.447 Å) which is known to be linear and the infinite linear chains in the solid (4.34 Å). It is shown that the smaller quadrupole coupling constants in the trimer compared to the monomer are caused by a combination of torsional and charge redistribution effects. The torsional vibrations have average angular displacements of 12.60° and 8.55° for HCN(1) and HCN(3). The charge redistribution effect upon χ in the trimer (6.5%) is about 1.8 times that in the HCN dimer. The dipole moment of the trimer was found to be 10.6 D, a value enhanced by 1.8 D over the vector sum of the three vibrating monomers (8.83 D), an enhancement about 1.7 times that of the dimer.

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