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

Abstract

Intrinsic reaction coordinate (IRC) torsional potentials were calculated for N(2)O(4) and N(2)O(3) 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 N(2)O(4) and N(2)O(3) obtained from the CCSD(T)/CBS//B3LYP/aug-cc-pVDZ calculations are 2333 and 1704 cm(-1), respectively. For N(2)O(4) 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 N(2)O(3) 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 N(2)O(3) should be reexamined. N(2)O(4) and N(2)O(3) 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.

Theoretical study of 14N quadrupole coupling constants in some NO-containing complexes: N2O3 and FNO

“…The first microwave spectroscopic studies of N 2 O 3 [4] and FNO [5], furnishing both structure parameters and nuclear quadrupole coupling constants (NQCCs), were reported almost forty years ago in the same issue of a journal. Since then a number of experimental and theoretical studies appeared on N 2 O 3 [6][7][8][9][10][11][12][13] and FNO [14][15][16][17][18][19] (and references therein) devoted to the interpretation and determination of structures, spectroscopic properties, dipole moments, heats of formation, etc., including those providing new 14 N QCCs on the basis of high-resolution measurements [1,20], and henceforth values of the electric field gradient (EFG) at nitrogen sites in N 2 O 3 . A theoretical investigation of the EFGs in N 2 O 3 is all the more desirable, because of the encountered problems with the analysis of the complex experimental hyperfine structure of rotational spectra arising from two non-equivalent nuclei [1,20] leading to the original misassignment of the two sets of 14 N coupling constants.…”

355 nm Photodissociation of N2O3 Revealed by Velocity-Mapped Ion Imaging

“…N 2 O 3 has been characterized experimentally and theoretically. Infrared spectra of the N 2 O 3 isomers have been measured at low temperatures in the condensed phase − and in the gas phase. , Calculations have modeled the vibrational frequencies, , determined the internal rotational barriers, , and predicted the relative stabilities of the three forms of N 2 O 3 . , In cryogenic matrices, the molecule isomerizes under visible and UV radiation. − …”