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 90deg-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.