Model Potential Energy Surfaces Research Articles

The Jahn-Teller effect in ${\rm CH}_{\rm 3} {\rm Cl}^{\rm + } ( {{\rm \tilde X}{}^{\rm 2}{\rm E}} )$ CH 3 Cl +(X̃E2): A combined high-resolution experimental measurement and ab initio theoretical study

The energy levels of CH(3)Cl(+)X̃(2)E showing strong spin-vibronic coupling effect (Jahn-Teller effect) have been measured up to 3500 cm(-1) above the ground vibrational state using one-photon zero-kinetic energy photoelectron and mass-analyzed threshold ionization spectroscopic method. Theoretical calculations have been also performed to calculate the spin-vibronic energy levels using a diabatic model and ab initio adiabatic potential energy surfaces (PESs). In the theoretical calculations the diabatic potential energy surfaces are expanded by the Taylor expansions up to the fourth-order including the multimode vibronic interactions. The calculated spin-orbit energy splitting (224.6 cm(-1)) for the ground vibrational state is in good agreement with the experimental data (219 ± 3 cm(-1)), which indicates that the Jahn-Teller and the spin-orbit coupling have been properly described in the theoretical model near the zero-point energy level. Based on the assignments predicted by the theoretical calculations, the experimentally measured energy levels were fitted to those from the diabatic model by optimizing the main spectroscopic parameters. The PESs from the ab initio calculations at the level of CASPT2/vq(t)z were thus compared with those calculated from the experimentally determined spectroscopic parameters. The theoretical diagonal elements in the diabatic potential matrix are in good agreement with those determined using the experimental data, however, the theoretical off-diagonal elements appreciably deviate from those determined using the experimental data for geometric points far away from the conical intersections. It is also concluded that the JT effect in CH(3)Cl(+) mainly arises from the linear coupling and the mode coupling between the CH(3) deform (υ(5)) and CH(3) rock (υ(6)) vibrations. The mode couplings between the symmetric C-Cl stretching vibration υ(3) with υ(5) and υ(6) are also important to understand the spin-vibronic structure of the molecule.

Photodissociation of methyl iodide embedded in a host-guest complex: A full dimensional (189D) quantum dynamics study of CH3I@resorc[4]arene

Accurate full dimensional quantum dynamics calculations studying the photodissociation of CH(3)I@resorc[4]arene on an ab initio based potential energy surface (PES) model are reported. The converged 189D quantum dynamics calculations are facilitated by the multilayer multi-configurational time-dependent Hartree (ML-MCTDH) approach combined with the correlation discrete variable representation (CDVR) for the evaluation of potential energy matrix elements. The potential employed combines an established ab initio PES describing the photodissociation of methyl iodide in the A band with a harmonic description of the resorc[4]arene host and a bilinear modeling of the host-guest interaction. All potential parameters required in the description of the vibrations of the host molecule and the host-guest interaction are derived from ab initio calculations on the host-guest complex. Absorption spectra at 0 K and 300 K are calculated and the electronic population dynamics during the bond breaking process occurring in the first 20-30 fs after the photoexcitation is investigated. Weak but significant effects resulting from the host-guest interaction on this time scale are found and interpreted. The present study demonstrates that accurate fully quantum mechanical dynamics calculations can be preformed for systems consisting of more than 50 atoms using the ML-MCTDH/CDVR approach. Utilizing an efficient statistical approach for the construction of the ensemble of initial wavepackets, these calculations are not restricted to zero temperature but can also study the dynamics at 300 K.

Effective Hamiltonian for femtosecond vibrational dynamics

Time propagation of zero-order states of an effective spectroscopic Hamiltonian is tested against femtosecond time dependent dynamics of adiabatic wavepackets evolving on a model potential energy surface for two coupled modes of the radical HO(2) with multiple potential wells and above barrier motion. A generalized Hamiltonian which breaks the usual conserved polyad action by including extra resonance couplings (V(2:1) and V(3:1)) successfully describes the time evolution after the further addition of two "ultrafast" couplings. These new couplings are a nonresonant coupling a(1)a(2)+a(1)(†)a(2)(†) and a resonant coupling V(1:1) that functions as an ultrafast term because the system is far from 1:1 frequency resonance.

Exact activation energies and phenomenological description of quantum tunneling for model potential energy surfaces. The F + H2 reaction at low temperature

Activation energies Ea calculated as the negative of the logarithmic derivatives of rate constants with respect to the inverse of absolute temperature T, are presented for three potential energy surfaces previously introduced for the reaction F+H2→HF+H in the temperature range 10<T<350K. Exact benchmark rate constants from quantum mechanical calculations on each surface have been reported [V. Aquilanti, S. Cavalli, D. De Fazio, A. Volpi, A. Aguilar, J.M. Lucas, Chem. Phys. 308 (2005) 237] and shown to deviate from Arrhenius behavior. The corresponding pronounced deviation from constancy of activation energy Ea represents a prototypical example of the role of quantum mechanical tunneling in propitiating a “sub Arrhenius” behavior. Four formulas are tested in order to provide a phenomenological description of the tunnel effect on reactivity, each introducing only one additional parameter with respect to the Arrhenius law. They correspond to: (i) the so-called Modified Arrhenius Equation, involving a linear dependence of EaversusT; (ii) the Curved Arrhenius Plot description, implying a linear dependence of Eaversus inverse T; (iii) the deformed Arrhenius law recently proposed and the corresponding inverse Ea – inverse T linear relationship recently derived from the deformed exponential distribution appearing in non – extensive statistical thermodynamics; (iv) a generalization of Mott’s law for electron and proton conduction in condensed matter, leading to an exponential relationship between Ea and T. Numerical investigation allows a discussion of merits of each formula from a temperature of 350K down to 50K. In all three cases, the deformed Arrhenius law provides a better description of the quantum mechanical trend.

Molecular dynamics method for proteins with ionization-conformation coupling and equilibrium titration

A new realization of the constant-pH molecular dynamics simulation method is proposed. Molecular dynamics simulation is performed for a protein in the most probable ionization microstate of the current conformation taking into account the potential of mean force of protein molecule in the water-proton bath in equilibrium titration conditions (MD-pH-ET). It is shown that: 1) the optimal one is the simulation of the protein in the most probable ionization for a given conformation, taking into account the correction ionization potential of mean force, which results in the librium ensemble of ionization states; 2) new method MD-pH-ET allow one to carry out an optimization of protein structure and the total free energy of a protein in the aqueous solution at constant pH, and to calculate the pH-dependent properties. Method MD-pH-ET possesses the unique features: 1) it uses the most precise and computational-effective realization of calculation of the electrostatic energy of a protein in water solution, the model of continuous dielectric media with Poisson equation and the generalized Born method with "ideal" Born atomic radii; 2) it uses the same model of the potential energy surface in the ionization-conformational phase space, both for calculating the potential energy of the protein and atomic forces and for determining the most probable ionization states; 3) it calculates the total free energy of the protein in the aqueous solution in proton reservoir under the conditions of equilibrium titration. The workability of the new method MD-pH-ET is demonstrated for the molecule of protein BPTI.

Nonadiabatic dynamics by mean-field and surface-hopping approaches: energy conservation considerations

The mean-field and surface-hopping approaches are mixed quantum-classical algorithms designed to efficiently incorporate nonadiabatic effects into molecular dynamics simulations. In this report, analytical relations underpinning the two methods are derived within the adiabatic basis to provide an implementation-level overview, with particular attention given to two avoidable sources of energy conservation deviation. It is not widely known that accuracy improvements can be obtained at negligible computational expense by allowing: (a) construction of the Schrödinger equation to account for analytic electronic phase propagation, and (b) inclusion of a Hellman–Feynman correctional term in the calculation of atomic forces. Particular emphasis on the algebraic formalism contrasts these inherent approximations, vis-à-vis the correctional approaches. The sensitivity of energy conservation with respect to phase propagation is exemplified by simulation of Xe molecular photodissociation on the repulsive Π(1/2) u excited surface. Simulations of a representative two-state avoided crossing system are presented to exemplify analytic quantities of the mean-field and surface-hopping approaches as well as to illustrate the inclusion of the Hellman–Feynman correctional term. Nonadiabatic propagation of the molecular wavefunction is observed by monitoring ground and excited state occupational probabilities, energy conservation, and nonadiabatic coupling as the system evolves along the model potential energy surfaces.

Communications: A model study on the electronic predissociation of the NeBr2 van der Waals complex

Recently, the predissociation lifetimes of the NeBr(2)(B) complex for different initial vibrational excitation (10<or=v(')<or=20) have been measured using time-resolved optical pump-probe spectroscopy [Taylor et al., J. Chem. Phys., 132, 104309 (2010)]. In the vibrational interval studied, the vibrational predissociation (VP) proceeds by the transfer of a single vibrational quantum and the lifetimes are expected to decrease smoothly with increasing v('), as predicted by the energy gap law. However, the experimental lifetimes show strong oscillations with v('), which were attributed to the occurrence of electronic predissociation into two possible dissociative electronic states of Br(2)(1(g),2(g)), based on a Franck-Condon spectator model. In this work we reproduce the experimental findings by performing full three-dimensional wave packet calculations for the competition of vibrational and electronic predissociation, including the B(0(u) (+)), 2(g), and C(1(u)) electronic states. Model potential energy surfaces were used based on previous theoretical simulations of the VP dynamics on the B state and on ab initio calculations on the NeCl(2) related system. Thus, only two parameters, the strength of the electronic couplings, are fit to achieve the excellent theoretical/experimental agreement.

The Diabatic Picture of Electron Transfer, Reaction Barriers, and Molecular Dynamics

Diabatic states have a long history in chemistry, beginning with early valence bond pictures of molecular bonding and extending through the construction of model potential energy surfaces to the modern proliferation of methods for computing these elusive states. In this review, we summarize the basic principles that define the diabatic basis and demonstrate how they can be applied in the specific context of constrained density functional theory. Using illustrative examples from electron transfer and chemical reactions, we show how the diabatic picture can be used to extract qualitative insight and quantitative predictions about energy landscapes. The review closes with a brief summary of the challenges and prospects for the further application of diabatic states in chemistry.

Enhancement and de-enhancement effects in vibrational resonance Raman optical activity

In this study, we investigate interference between several excited electronic states in resonance enhanced vibrational Raman optical activity (RROA) spectra. A gradient Franck-Condon model for the excited-state potential energy surface is applied in order to include vibronic effects in the description of the RROA intensities. Both sum-over-states and time-dependent expressions for the RROA intensities in case of close-lying excited states are given. As an example, we compare the calculated RROA and resonance Raman spectra of (S)-(+)-naproxen-OCD(3) to the experimental ones. Subsequently, we examine the excitation profiles of (S)-(+)-naproxen and study the vibration at 1611 cm(-1) in more detail in order to demonstrate how the consideration of a second excited electronic state can lead to significant changes in the RROA intensities.

Chemiluminescence from the Ba(P3)+N2O→BaO(A Σ1+)+N2 reaction: Collision energy effects on the product rotational alignment and energy release

Both fully dispersed unpolarized and polarized chemiluminescence spectra from the Ba((3)P)+N(2)O reaction have been recorded under hyperthermal laser-ablated atomic beam-Maxwellian gas conditions at three specific average collision energies E(c) in the range of 4.82-7.47 eV. A comprehensive analysis of the whole data series suggests that the A (1)Sigma(+)-->X (1)Sigma(+) band system dominates the chemiluminescence. The polarization results revealed that the BaO(A (1)Sigma(+)) product rotational alignment is insensitive to its vibrational state upsilon(') at E(c)=4.82 eV but develops into an strong negative correlation between product rotational alignment and upsilon(') at 7.47 eV. The results are interpreted in terms of a direct mechanism involving a short-range, partial electron transfer from Ba((3)P) to N(2)O which is constrained by the duration of the collision, so that the reaction has a larger probability to occur when the collision time is larger than the time needed for N(2)O bending. The latter in turn determines that, at any given E(c), collinear reactive intermediates are preferentially involved when the highest velocity components of the corresponding collision energy distributions are sampled. Moreover, the data at 4.82 eV suggest that a potential barrier to reaction which favors charge transfer to bent N(2)O at chiefly coplanar geometries is operative for most of the reactive trajectories that sample the lowest velocity components. Such a barrier would arise from the relevant ionic-covalent curve crossings occurring in the repulsive region of the covalent potential Ba((3)P)cdots, three dots, centeredN(2)O((1)Sigma(+)); from this crossing the BaO(A (1)Sigma(+)) product may be reached through mixings in the exit channel with potential energy surfaces leading most likely to the spin-allowed b (3)Pi and a (3)Sigma(+) products. The variation with increasing E(c) of both the magnitude of the average BaO(A (1)Sigma(+)) rotational alignment and the BaO(A (1)Sigma(+)) rovibrational excitation, as obtained from spectral simulations of the unpolarized chemiluminescence spectra, consistently points to additional dynamic factors, most likely the development of induced repulsive energy release as the major responsible for the angular momentum and energy disposal at the two higher E(c) studied. The results of a simplified version of the direct interaction with product repulsion-distributed as in photodissociation model do not agree with the observed average product rotational alignments, showing that a more realistic potential energy surface model will be necessary to explain the present results.

An exchange-Coulomb model potential energy surface for the Ne–CO interaction. II. Molecular beam scattering and bulk gas phenomena in Ne–CO mixtures

Four potential energy surfaces are of current interest for the Ne-CO interaction. Two are high-level fully ab initio surfaces obtained a decade ago using symmetry-adapted perturbation theory and supermolecule coupled-cluster methods. The other two are very recent exchange-Coulomb (XC) model potential energy surfaces constructed by using ab initio Heitler-London interaction energies and literature long range dispersion and induction energies, followed by the determination of a small number of adjustable parameters to reproduce a selected subset of pure rotational transition frequencies for the (20)Ne-(12)C(16)O van der Waals cluster. Testing of the four potential energy surfaces against a wide range of available experimental microwave, millimeter-wave, and mid-infrared Ne-CO transition frequencies indicated that the XC potential energy surfaces gave results that were generally far superior to the earlier fully ab initio surfaces. In this paper, two XC model surfaces and the two fully ab initio surfaces are tested for their abilities to reproduce experiment for a wide range of nonspectroscopic Ne-CO gas mixture properties. The properties considered here are relative integral cross sections and the angle dependence of rotational state-to-state differential cross sections, rotational relaxation rate constants for CO(v=2) in Ne-CO mixtures at T=296 K, pressure broadening of two pure rotational lines and of the rovibrational lines in the CO fundamental and first overtone transitions at 300 K, and the temperature and, where appropriate, mole fraction dependencies of the interaction second virial coefficient, the binary diffusion coefficient, the interaction viscosity, the mixture shear viscosity and thermal conductivity coefficients, and the thermal diffusion factor. The XC model potential energy surfaces give results that lie within or very nearly within the experimental uncertainties for all properties considered, while the coupled-cluster ab initio surface gives results that agree similarly well for all but one of the properties considered. When the present comparisons are combined with the ability to give accurate spectroscopic transition frequencies for the Ne-CO van der Waals complex, only the XC potential energy surfaces give results that agree well with all extant experimental data for the Ne-CO interaction.

Systematic Study of Spin Crossover and Structure in [Co(terpyRX)2](Y)2 Systems (terpyRX = 4′-alkoxy-2,2′:6′,2′′-terpyridine, X = 4, 8, 12, Y = BF4−, ClO4−, PF6−, BPh4−)

A family of spin crossover cobalt(II) complexes of the type [Co(terpyRX)(2)](Y)(2) x nH(2)O (X = 4, 8, 12 and Y = BF(4)(-), ClO(4)(-), PF(6)(-), BPh(4)(-)) has been synthesized, whereby the alkyl chain length, RX, and counteranion, Y, have been systematically varied. The structural (single crystal X-ray diffraction) and electronic (magnetic susceptibility, electron paramagnetic resonance (EPR)) properties have been investigated within this family of compounds. Single crystal X-ray diffraction analysis of [Co(terpyR8)(2)](ClO(4))(2), [Co(terpyR8)(2)](BF(4))(2) x H(2)O, and [Co(terpyR4)(2)](PF(6))(2) x 3 H(2)O, at 123 K, revealed compressed octahedral low spin Co(II) environments and showed varying extents of disorder in the alkyl tail portions of the terpyRX ligands. The magnetic and EPR studies were focused on the BF(4)(-) family and, for polycrystalline solid samples, revealed that the spin transition onset temperature (from low to high spin) decreased as the alkyl chain lengthened. EPR studies of polycrystalline powder samples confirmed these results, showing signals only due to the low spin state at the temperatures seen in magnetic measurements. Further to this, simultaneous simulation of the EPR spectra of frozen solutions of [Co(terpyR8)(2)](BF(4))(2) x H(2)O, recorded at S-, X-, and Q-band frequencies, allowed accurate determination of the g and A values of the low spin ground state. The temperature dependence of the polycrystalline powder EPR spectra of this and the R4 and R12 complexes is explained in terms of Jahn-Teller effects using the warped Mexican hat potential energy surface model perturbed by the low symmetry of the ligands. While well recognized in Cu(II) systems, this is one of the few times this approach has been used for Co(II).

An exchange-Coulomb model potential energy surface for the Ne–CO interaction. I. Calculation of Ne–CO van der Waals spectra

Exchange-Coulomb model potential energy surfaces have been developed for the Ne-CO interaction. The initial model is a three-dimensional potential energy surface based upon computed Heitler-London interaction energies and literature results for the long-range induction and dispersion energies, all as functions of interspecies distance, the orientation of CO relative to the interspecies axis, and the bond length of the CO molecule. Both a rigid-rotor model potential energy surface, obtained by setting the CO bond length equal to its experimental spectroscopic equilibrium value, and a vibrationally averaged model potential energy surface, obtained by averaging the stretching dependence over the ground vibrational motion of the CO molecule, have been constructed from the full data set. Adjustable parameters in each model potential energy surface have been determined through fitting a selected subset of pure rotational transition frequencies calculated for the (20)Ne-(12)C(12)O isotopolog to precisely known experimental values. Both potential energy surfaces provide calculated results for a wide range of available experimental microwave, millimeter-wave, and midinfrared Ne-CO transition frequencies that are generally far superior to those obtained using the best current literature potential energy surfaces. The vibrationally averaged CO ground state potential energy surface, employed together with a potential energy surface obtained from it by replacing the ground vibrational state average of the CO stretching dependence of the potential energy surface by an average over the first excited CO vibrational state, has been found to be particularly useful for computing and/or interpreting mid-IR transition frequencies in the Ne-CO dimer.

Use of the virial theorem in modeling the potential energy surfaces for triatomic collinear reactions

The new model potential energy surface for triatomic collinear reactions, WT(R), is presented. The surface is generated via the integral virial theorem from the “normalized” electronic kinetic energy surface, TM(R), satisfying along each uniform scaling path the condition (A): S0∞[TM(SR)—T(∞)] ds = Vnn(R); Vnn is the nuclear repulsion energy, s is a scale factor, and ∞ corresponds to the separated atoms limit. The TM(R) surface is approximated by the Morse function rotating around the united atom axis. For a given Rs = sR, the corresponding WT(Rs) cut has the form of the modified Morse function: WT(Rs) = (a/2cRs){exp[-2c(Rs — b)]-4exp[-c(Rs — b)]}, parameters of which can be determined from the condition (A) and the coordinates and energy of the corresponding point on the zero-virial path (ZVP, locus of minima along the uniform scaling directions). Illustrative examples of the TM(R) and WT(R) surfaces, generated from the BEBO ZVP for the symmetrical (H—H + H H + H—H) and unsymmetrical (F—H + H F + H—H) reactions are presented. These preliminary results suggest that the present model offers an efficient way to use the virial theorem in an a priori determining the valleys curvature variations during reaction.

Theoretical study of proton tunneling in the excited state of tropolone

Ab initio CIS/6-311++G(d,p) calculations of geometry and vibrational frequencies have been carried out in the A state of tropolone. The grids of potential energy surfaces along the coordinates of high frequency tunneling vibration and the low-frequency coupled vibration have been calculated. Two-dimensional model potentials, formed from symmetric mode coupling potential and squeezed double well potential, have been fitted to the calculated potential energy surfaces and used to analyze proton dynamics. The tunneling splittings for different vibrationally excited states have been calculated and compared with the available experimental data. The model potential energy surfaces, based on the CIS/6-311++G(d,p) calculations, give good estimation of the tunneling energy splittings in the vibrationally ground and excited states of tropolone and explain monotonic decrease in tunneling splittings with the excitation of low-frequency out-of-plane modes and increase in the tunneling splittings with the excitation of low-frequency planar modes.

Narrow resonances in complex potential energy surfaces

Several studies of the vibrational resonances of diatomic molecular anions have shown evidence of very narrow resonances. Here we present an analytical model for the complex potential energy surface of diatomic anions where we study the appearance of very narrow resonances together with very broad resonances. We find two different mechanisms by which the eigenvalues of the model Hamiltonian begin to separate into narrow and broad resonances. In one of these mechanisms the eigenvalues go through a branch point where whereas in the other mechanism they go through a turning point in the complex energy plane where . At these special points in the spectrum two different conditions on the wavefunctions can be formulated depending on the mechanism involved.

Full dimensional quantum versus semiclassical reactivity for the bent transition state reaction N + N 2

The first full dimensional quantum calculations of the rate coefficient of the N + N 2 reaction performed on a model potential energy surface evidencing the bent nature of the transition state are presented. The calculated estimates of the quantum rate coefficients are compared with the corresponding values obtained from a semiclassical treatment both to analyze the validity of the used computational procedures and to single out the various contributions to reactivity. J shifting and transition state model treatments have also been considered in order to evaluate their applicability for improving the convergence of the calculations.

Exact state-to-state quantum dynamics of the F+HD→HF(v′=2)+D reaction on model potential energy surfaces

In this paper, we present the results of a theoretical investigation on the dynamics of the title reaction at collision energies below 1.2 kcal/mol using rigorous quantum reactive scattering calculations. Vibrationally resolved integral and differential cross sections, as well as product rotational distributions, have been calculated using two electronically adiabatic potential energy surfaces, developed by us on the basis of semiempirical modifications of the entrance channel. In particular, we focus our attention on the role of the exothermicity and of the exit channel region of the interaction on the experimental observables. From the comparison between the theoretical results, insight about the main mechanisms governing the reaction is extracted, especially regarding the bimodal structure of the HF(v = 2) nascent rotational state distributions. A good overall agreement with molecular beam scattering experiments has been obtained.

Accuracy of recent potential energy surfaces for the He–N2 interaction. II. Molecular beam scattering and bulk gas relaxation phenomena

A new semiempirical exchange-Coulomb model potential energy surface for the N(2)-He interaction was reported recently [A. K. Dham et al., J. Chem. Phys. 127, 054302 (2007)] and, using it, the temperature dependence of bulk gas properties of N(2)-He mixtures, such as the second virial coefficient and traditional transport phenomena, most of which depend primarily on the isotropic component of the interaction potential energy surface, was determined. Values of these properties, along with values calculated using two high-quality ab initio potential energy surfaces [C.-H. Hu and A. J. Thakkar, J. Chem. Phys. 104, 2541 (1996); K. Patel et al., ibid 119, 909 (2003)] were compared critically to available experimental data. The present paper reports on the ability of the same three potential energy surfaces to predict state-to-state and total differential cross sections, total integral cross sections, and the temperature dependence of bulk gas relaxation phenomena (including magnetic field effects on transport coefficients). While all three potential energy surfaces give total differential and higher speed integral scattering results that fall within the experimental uncertainties, integral scattering results and state-to-state differential cross section measurements consistently exceed the calculated values. All three surfaces give similar agreement with the relaxation properties of N(2)-He binary mixtures, with the semiempirical exchange-Coulomb model potential energy surface giving slightly better overall agreement with experiment than the two ab initio potential energy surfaces.

Non-RRKM Dynamics in the CH3O2 + NO Reaction System

Quasi-classical trajectory (QCT) calculations on a model potential energy surface (PES) show strong deviations from statistical Rice-Ramsperger-Kassel-Marcus (RRKM) rate theory for the decomposition reaction (1) CH3OONO* --> CH3O + NO2, where the highly excited CH3OONO* was formed by (2) CH3O2 + NO --> CH3OONO*. The model PES accurately describes the vibrational frequencies, structures, and thermochemistry of the cis- and trans-CH3OONO isomers; it includes cis-trans isomerization in addition to reactions 1 and 2 but does not include nitrate formation, which is too slow to affect the decay rate of CH3OONO*. The QCT results give a strongly time-dependent rate constant for decomposition and damped oscillations in the decomposition rate, not predicted by statistical rate theory. Anharmonicity is shown to play an important role in reducing the rate constant by a factor of 10 smaller than predicted using classical harmonic RRKM theory (microcanonical variational transition state theory). Master equation simulations of organic nitrate yields published previously by two groups assumed that RRKM theory is accurate for reactions 1 and 2 but required surprising parametrizations to fit experimental nitrate yield data. In the present work, it is hypothesized that the non-RRKM rate of reaction (1) and vibrational anharmonicity are at least partly responsible for the surprising parameters.