Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2-Ar, a T-shaped trimer

Microwave rotational spectra have been observed for the four ¹⁴N/¹⁵N isotopic species of an (HCN)₂-Ar trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. Thirteen J→J' asymmetric top transitions were measured in the 2.5 to 10 GHz region for the parent ¹⁴N,¹⁴N trimer and nine for each of the other three species. The ¹⁴N nuclear quadrupole hyperfine structure was analyzed for the transitions and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational and quartic centrifugal distortion constants. For the ¹⁴N/¹⁴N trimer these are (in MHz) for A`, B`, and C`: 2013.5993(10), 1759.2756(3), 932.3709(2); and for τ₁, τ₂, τ_aaaa, τ_bbbb, and τ_cccc: -0.3017(1), -0.0660(1), -0.1700(9), -0.013 71(1), and -0.0088(1). The inertial defect is 3.7881 amu Ų. The zero-point vibrationally averaged geometry is planar and T shaped with the structure of the linear HCN dimer remarkably unperturbed by presence of the Ar atom. The substitution NṡṡṡN distance in the cluster is 4.453 Å compared with 4.461 Å in the free dimer. The argon is attracted by the π electrons of the two -C≡N groups, being equidistant from them. The ArṡṡṡN(1) and ArṡṡṡN(2) distances are 3.671 and 4.316 Å, respectively.