Microwave measurements and theoretical calculations on the structures of NNO-HCl complexes

Pulsed-beam Fourier transform microwave spectroscopy was used to measure a and b dipole transitions for the N₂O-H³⁵Cl, N ₂O-H³⁷Cl, N₂O-D³⁵Cl, and ¹⁵NNO-H³⁵Cl van der Waals complexes. The observed transition frequencies were fit to determine the spectroscopic constants A-D_K, B, C, D_j, D_JK, eQq_aa(Cl), and eQq_bb(Cl). The structure of the complex appears to be a planar asymmetric top with a centers-of-mass separation R_c.m.- ≈ 3.51 Å. The angle θ between R_c.m. and the HCl axis is approximately 110°. The angle φ between the N₂O axis and R_c.m. is approximately 77°. The structure was fit using a weighted least squares fit to B and C isotopic rotational constants with R _c.m. θ, and φ as the adjustable parameters, and this procedure yielded three local minima with standard deviations less than 5 MHz. Principal axis coordinates for the Cl, H, and terminal N atoms in the complex were determined with single isotopic Kraitchman analysis to aid in the selection of the "best" structure. In a second structural analysis R _c.m. θ, and φ values were determined from the spectroscopic constants B, C, and eQq_aa(Cl). The "best fit" structure parameters for N₂O-HCl are R_c.m. = 3.512(2) Å, θ = 110(9)°, and θ = 77(2)°. Ab initioa calculations of N₂O-HCl structures using GAUSSIAN86 with MP2 yielded three energetically stable equilibrium conformations. One of the bound structures is very similar to the present experimental vibrationally averaged structure.