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

Abstract

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, τaaaa, τbbbb, and τcccc: −0.3017(1), −0.0660(1), −0.1700(9), −0.013 71(1), and −0.0088(1). The inertial defect is 3.7881 amu Å2. 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 N⋅⋅⋅N 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 Ar⋅⋅⋅N(1) and Ar⋅⋅⋅N(2) distances are 3.671 and 4.316 Å, respectively.