Microwave spectra and structures of the NNO-HCN, ¹⁵NNO-HCN, and NNO-DCN complexes

A total of 60 a- and b-dipole rotational transitions were measured in the 4-18 GHz range for the NNO-HCN, ¹⁵NNO-HCN, and NNO-DCN bimolecular complexes using a pulsed-beam, Fourier transform microwave spectrometer. Spectroscopic constants (A - D_K), B, C, D_J, D _JK, eQq_aa (N of HCN), and eQq_bb (N of HCN) were obtained by fitting the observed transition frequencies with a first-order quadrupole coupling interaction Hamiltonian. The structure of the complex appears to be planar with NNO and NCH nearly parallel. It can be described with the distance R_cm between the center-of-masses of the monomer subunits, the angle θ between HCN and R_cm, and the angle φ between N₂O and R_cm. A least-squares fit to the nine rotational constants to obtain the structure parameters R_cm, θ, and φ, produced three local minimia for bent structures with standard deviations of < 25 MHz. A Kraitchman analysis was used to determine magnitudes of principal axes coordinates for the N of HCN, and the terminal N of NNO. The best nonlinear least-squares fit result (structure I, lowest standard deviation of the fit = 7.2 MHz) produced the best match to the coordinates from the Kraitchman analysis. The spectroscopic constants B, C, and eQq_aa were used in a second structural analysis to determine values for R _cm, θ, and φ. These results were compared with the above coordinates. The best least-squares fit structure parameters for the vibrationally averaged structure are R_cm = 3.253(4) Å, θ = 89.1 (5.4)°, and φ = 76.4(0.4)°. Comparisons are made with other similar weakly bound complexes.