A study of the mode-selective trans–cis isomerization in HONO using ab initio methodology

Abstract

Ab initio calculations on the six-dimensional cis--trans double minimum potential energy surface of the electronic ground state of the HONO molecule were performed using a coupled cluster approach. An analytic fit to the data points was established. The interconversion barrier was calculated to be 4105 cm(-1). The nuclear motion problem was solved variationally using a full six-dimensional Hamiltonian in internal coordinates. The eigenstates up to about 3650 cm(-1) were tentatively assigned by harmonic quantum numbers. The assignment was based on the mean values of the internal coordinates of the six-dimensional eigenfunctions and on a comparison of the eigenenergies with those calculated by second-order perturbation theory from a full quartic force field in dimensionless normal coordinates. In cold matrices the trans- and the cis-OH nu(1) stretching modes and the first trans- and cis-NO 2nu(2) stretching overtones lead to isomerization. In the isolated molecule these modes (J=0) were found to be entirely localized. However, several overtones of the nu(4) ONO bending and nu(5) N-O stretching, which are close in energy to the OH stretch and combined with the torsional mode, were found to be strongly cis-trans delocalized.

The Case of the Weak N−X Bond: Ab Initio, Semi-Experimental, and Experimental Equilibrium Structures of XNO (X = H, F, Cl, OH) and FNO2

“…The equilibrium structure obtained in this study at the CCSD(T) level is in good agreement with the one obtained by Richter et al, 14 except for the N-O bond length. As anticipated, the agreement of the structural parameters is worse with the B3LYP results of Luckhaus.…”

“…The semi-experimental rotational constants A e , B e , and C e of F 14 Table 4 together with the experimental equilibrium structure of Degli Esposti et al 8 As expected, the agreement is excellent, confirming the accuracy of the equilibrium structure.…”

“…The correct equilibrium value should be close to 1.458 Å, that is, about 0.007 Å larger than the ab initio value. It has to be noted that this bond length is much longer than the normal single N-F equilibrium bond length, which is 1.422 Å in NH 2 F. 73 Finally, as a check, we also calculated the semi-experimental structure using the AVTZ B3LYP anharmonic force field and the experimental ground-state rotational constants (F 14 has small rotational constants, there is no isotopic substitution available for the fluorine atom, and the N atom is close to the center of mass (0.2 Å). For these reasons, the system of normal equations is not well conditioned and the derived semi-experimental structure given in Table 6 cannot be very precise.…”

Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis→trans tunneling rate

“…1 For the case of HONO, it has been suggested that kinetic or potential couplings between the excited vibrations and nearly resonant vibrational states could be responsible for the more efficient energy transfer from the excited states to the reaction coordinate upon excitation of the cis isomer. 32,33 These factors may also be relevant for acetic acid.…”

Intramolecular vibrational energy relaxation in nitrous acid (HONO)

“…The trans → cis back reaction yield is significantly less, with the ratio between the two varying with temperature 5 from 3 at 30 K to 7 at 10 K. The large ratio between the forward and backward reaction yields is suggestive since the energy reached after OH excitation of both isomers is almost identical and, after most estimates, just slightly below the barrier separating both species. 6,7 Hence, the reaction must be to some extent mode specific, and the competition between IVR and the reaction rate determines its outcome. That is, if IVR was faster than the isomerization, then the reaction quantum yield would be extremely low and the outcome of the reaction would be determined by the ratio of density of states of cis-and trans-HONO.…”