A reinvestigation of the structure and torsional potential of N2O5 by gas-phase electron diffraction augmented by ab initio theoretical calculations

Gaseous N2O5 consists of two NO2 groups bonded to a bridging O-atom to form a nonlinear N-O-N moiety. The NO2 groups undergo slightly hindered internal rotation around the bonds to the bridge so that instantaneous composition of the gaseous system is characterized by molecules with all combinations of torsion angles. In an earlier investigation. an attempt was made to determine the coefficients for an empirical form of the double-rotor torsional potential. and the bond lengths and bond angles measured subject to assumptions that the structure of the O-NO2 groups was invariant to torsion angle and that these groups had C-2 nu symmetry. The system has now been reinvestigated in terms of a more realistic model in which this symmetry restriction was relaxed. account was taken of structural changes in the NO2 groups with torsion angle as predicted by ab initio theory at the B3LYP/6-311 + G* level. and a more convenient form of the torsional potential was assumed. The most stable conformation has C-2, symmetry with torsion angles tau (1) (defined as not less than (N-O-N=O-4)) equal to tau (2) (defined as not less than (N-O-N-O-6)) equal to 33.7 : because of the broad potential minimum in this region, the uncertainty in these angles is difficult to estimate, but is probably 3 similar to 4. The results for the bond lengths and bond angles for the most stable conformation are r(g)(N-O) = 1.505(4)Angstrom, r(g)(N-O) = 1.188(2)Angstrom, not less than (a)(N-O-N) = 112.3(17), not less than (a)(O=N=O) = 134.2(4), < not less than (a)(O-N=O)> = 112.8(2)degrees. The difference between the symmetry-nonequivalent O-N-O angles is estimated to be ca. 6.7 with the larger angle positioning the two N=O bonds on different NO2 groups nearest each other. These average values are similar to those obtained in the original study. The main difference is found in the shape of the torsional potential. which at tau (1)/tau (2) = 0/0 has a saddle point in the present work and a substantial peak in the earlier. The implication of the torsion-angle findings for electron-diffraction investigations of this type is discussed.