Detection of a higher energy isomer of the CO-N2O van der Waals complex and determination of two of its intermolecular frequencies

Infrared spectra in the carbon monoxide CO stretch region (B2150 cm(-1)) and in the n3 asymmetric stretch region of N2O (B2223 cm(-1)) are assigned to the previously unobserved O-bonded form of the CO-N2O dimer ("isomer 2''). This van der Waals complex has a planar skewed T-shaped structure like that of the previously observed C-bonded form ("isomer 1''), but with the CO rotated by 180 degrees. The effective intermolecular distance between the centers of mass is 3.51 for iso 2 as compared to 3.88 A fforomer1. In addition to the fundamental band, two combination bands are observed for isomer 2, yielding values for two intermolecular vibrational modes: 14.502(5) cm(-1) for the coupled disrotatory motion or the uncoupled CO rock and 21219(5) cm(-1) for the out-of-plane rock. We show that the published ab initio study on this system is inadequate in predicting the intermolecular frequencies for isomer 2 of CO-N2O.