Ab initio study of the torsional potential energy surfaces of N2O3 and N2O4: Origin of the torsional barriers

摘要

Intrinsic reaction coordinate (IRC) torsional potentials were calculated for N2O4 and N2O3 based on optimized B3LYP/aug-cc-pVDZ geometries of the respective 90 degrees-twisted saddle points. These potentials were refined by obtaining CCSD(T)/aug-cc-pVXZ energies [in the complete basis set (CBS) limit] of points along the IRC. A comparison is made between these ab initio potentials and an analytical form based on a two-term cosine expansion in terms of the N-N dihedral angle. The shapes of these two potential curves are in close agreement. The torsional barriers in N2O4 and N2O3 obtained from the CCSD(T)/CBS//B3LYP/aug-cc-pVDZ calculations are 2333 and 1704 cm(-1), respectively. For N2O4 the torsion fundamental frequency from the IRC potential is 87.06 cm(-1), which is in good agreement with the experimentally reported value of 81.73 cm(-1). However, in the case of N2O3 the torsional frequency found from the IRC potential, 144 cm(-1), is considerably larger than the reported experimental values 63-76 cm(-1). Consistent with this discrepancy, the torsional barrier obtained from several different calculations, 1417-1718 cm(-1), is higher than the value of 350 cm(-1) deduced from experimental studies. It is suggested that the assignment of the torsional mode in N2O3 should be reexamined. N2O4 and N2O3 exhibit strong hyperconjugative interactions of in-plane O lone pairs with the central N-N sigma(*) antibond. Hyperconjugative stabilization is somewhat stronger at the planar geometries because 1,4 interactions of lone pairs on cis O atoms promote delocalization of electrons into the N-N antibond. Calculations therefore suggest that the torsional barriers in these molecules arise principally from a combination of 1,4 interactions and hyperconjugation. (c) 2007 American Institute of Physics.