Recent microwave studies of N 2 O-containing van der Waals complexes, Ar-N 2 O (1, 2), HCCH-N 2 O (1, 3), CO 2 -N 2 O (4), and 15 N 2 -N 2 O (5), have shown that upon complexation, the electric field gradients at the nitrogen nuclei of N 2 O are perturbed from their values in the free molecule. The major cause of this perturbation appears to be the response of the N 2 O electron distribution to the electric field of its binding partner, although the results for Ar-N 2 O indicate other factors must be operative as well. In our attempts to understand this effect, it is essential that we know the structure of the complex. Although structures have been easily obtained for most of the complexes mentioned above, experimental data on a single isotopic modification of CO 2 -N 2 O could not resolve an ambiguity concerning the structure of this complex.
CO 2 -N 2 O was first studied in the infrared in the region of the 3 asymmetric stretch of the CO 2 monomer (6). Based on the structural similarities between the two homodimers, (N 2 O) 2 (7, 8) and (CO 2 ) 2 (9, 10), a slipped parallel structure was assumed for the heterodimer CO 2 -N 2 O. Two possibilities are consistent with such a geometry, one with O in N 2 O closest to C in CO 2 and another with the terminal N in N 2 O closest to C in CO 2 , but they cannot be distinguished using rotational constants alone. Ab initio calculations were presented in connection with the infrared study which argued for the minimum energy structure being that with O in N 2 O closest to C in CO 2 .