Mychel E. Varner

Dissociation Energy of the HOOO Radical

The dissociation of the hydrotrioxy (HOOO) radical to OH and O(2) has been studied theoretically using coupled-cluster methods. The calculated dissociation energy for the trans-HOOO isomer is 2.5 kcal mol(-1) including zero-point corrections. The minimum energy path to dissociation has been explored and an exit barrier has been revealed, which may help to rationalize the apparent disagreement between theory and experiment on the magnitude of the bond energy.

Infrared overtone spectroscopy and unimolecular decay dynamics of peroxynitrous acid.

Peroxynitrous acid (HOONO) is generated in a pulsed supersonic expansion through recombination of photolytically generated OH and NO(2) radicals. A rotationally resolved infrared action spectrum of HOONO is obtained in the OH overtone region at 6971.351(4) cm(-1) (origin), providing definitive spectroscopic identification of the trans-perp (tp) conformer of HOONO. Analysis of the rotational band structure yields rotational constants for the near prolate asymmetric top, the ratio of the a-type to c-type components of the transition dipole moment for the hybrid band, and a homogeneous linewidth arising from intramolecular vibrational energy redistribution and/or dissociation. The quantum state distribution of the OH (nu=0,J(OH)) products from dissociation is well characterized by a microcanonical statistical distribution constrained only by the energy available to products, 1304+/-38 cm(-1). This yields a 5667+/-38 cm(-1) [16.2(1) kcal mol(-1)] binding energy for tp-HOONO. An equivalent available energy and corresponding binding energy are obtained from the highest observed OH product state. Complementary high level ab initio calculations are carried out in conjunction with second-order vibrational perturbation theory to predict the spectroscopic observables associated with the OH overtone transition of tp-HOONO including its vibrational frequency, rotational constants, and transition dipole moment. The same approach is used to compute frequencies and intensities of multiple quantum transitions that aid in the assignment of weaker features observed in the OH overtone region, in particular, a combination band of tp-HOONO involving the HOON torsional mode.

A comparison of experimental and calculated spectra of HNO3 in the near-infrared using Fourier transform infrared spectroscopy and vibrational perturbation theory

This work combines new laboratory studies of the near-infrared vibrational spectra of HNO3 with theoretical predictions of these spectra as a means to understand the properties of this molecule at energies well above the fundamental region. Trends in overtone and combination band energy levels and intensities are compiled and examined. The theoretical calculations are in excellent agreement with the observed frequencies and intensities of the transitions in this spectral region. The calculations also serve as a valuable aid for assigning many of the transitions. This work validates the ab initio generated potential energy surface for HNO3 by comparing vibrational perturbation theory calculations to experimental spectra focused on combination band and overtone absorptions.

Analysis of the HOOO torsional potential

Torsional levels of cis and trans HOOO and DOOO, observed previously via infrared action spectroscopy [E. L. Derro, T. D. Sechler, C. Murray, and M. I. Lester, J. Chem. Phys. 128, 244313 (2008)], have been used in conjunction with ab initio theory to obtain a torsional potential energy surface for the hydrotrioxy radical. High level electronic structure calculations based on the equation-of-motion coupled-cluster method for ionized states (EOMIP-CCSD) are utilized to produce a torsional potential. Eigenvalues of the potential are computed by diagonalizing the torsional hamiltonian in a free-rotor basis. Uniform scaling of the theoretical potential by a factor of 1.35 yields vibrational frequencies in good agreement with the experiment, and allows prediction of the barrier height to isomerization of ∼340 cm(-1) and relative stability of trans-HOOO with respect to cis-HOOO of ∼70 cm(-1). Examination of the optimized nuclear coordinates with respect to the torsional angle, suggests that the central O-O bond length is strongly coupled to the torsion and is important in determining the relative stabilities of the two conformers. The scaled potential is then used to determine the torsional contribution to the partition function for atmospheric modeling of HOOO.

Vibrational overtone spectrum of matrix isolated cis, cis-HOONO.

Cis, cis-peroxynitrous acid is known to be an intermediate in atmospheric reactions between OH and NO2 as well as HOO and NO. The infrared absorption spectra of matrix-isolated cc-HOONO and cc-DOONO in argon have been observed in the range of 500-8000 cm-1. Besides the seven fundamental vibrational modes that have been assigned earlier for this molecule [Zhang et al., J. Chem. Phys. 124, 084305 (2006)], more than 50 of the overtone and combination bands have been observed for cc-HOONO and cc-DOONO. Ab initio CCSD(T)/atomic natural orbital anharmonic force field calculations were used to help guide the assignments. Based on this study of the vibrational overtone transitions of cis, cis-HOONO that go as high as 8000 cm-1 and the earlier paper on the vibrational fundamentals, we conclude that the CCSD(T)/ANO anharmonic frequencies seem to correct to +/-35 cm-1. The success of the theoretically predicted anharmonic frequencies {upsilon} in assigning overtone spectra of HOONO up to 8000 cm-1 suggests that the CCSD(T)/ANO method is producing a reliable potential energy surface for this reactive molecule.

Raman spectroscopy of solutions and interfaces containing nitrogen dioxide, water, and 1,4 dioxane: Evidence for repulsion of surface water by NO2 gas

The interaction of water, 1,4 dioxane, and gaseous nitrogen dioxide, has been studied as a function of distance measured through the liquid-vapour interface by Raman spectroscopy with a narrow (<0.1 mm) laser beam directed parallel to the interface. The Raman spectra show that water is present at the surface of a dioxane-water mixture when gaseous NO2 is absent, but is virtually absent from the surface of a dioxane-water mixture when gaseous NO2 is present. This is consistent with recent theoretical calculations that show NO2 to be mildly hydrophobic.