A semiclassical adiabatic calculation of state densities for molecules exhibiting torsion: application to hydrogen peroxide and its isotopomers

Kramer, Zeb C.; Skodje, Rex T.

doi:10.1007/s00214-014-1530-5

Cited by 3 publications

(6 citation statements)

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“…CH 2 is a special case because of the strong coupling of its bending motion with overall rotation. We commented on this in ref , where we considered the vibrational partition function Q vib along with the rovibrational partition function Q . Typically, the anharmonicity correction factor associated with overall rotation (which can be computed as Q / Q vib Q RR ) is positive and contributes a factor of ∼(1 + Γ( T )) 3/2 to the total rovibrational anharmonicity correction, as built into our definition of Γ in eq 3 .…”

Section: Resultsmentioning

confidence: 99%

“…Here, we use Monte Carlo phase space integration − (MCPSI) to compute fully anharmonic and fully coupled, but classical, rovibrational partition functions. These are used to determine rovibrational Pitzer–Gwinn anharmonicity corrections f , which is one of several semiclassical strategies − that have been developed for improving MCPSI predictions. MCPSI is well-suited for studying rovibrational anharmonicity at high energies and temperatures, as it describes the classical limit without approximation.…”

Section: Introductionmentioning

confidence: 99%

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Anharmonic Rovibrational Partition Functions at High Temperatures: Tests of Reduced-Dimensional Models for Systems with up to Three Fluxional Modes

et al. 2019

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Curvilinear coordinate Monte Carlo phase space integration and a series of full-dimensional fitted potential energy surfaces are used to study the effectiveness of reduced-dimensional models for predicting rovibrational anharmonicity at high temperatures. Fully coupled and fully anharmonic, but classical, rovibrational partition functions Q are computed for 14 species with two or three fluxional modes (inversions or torsions) and as many as 30 degrees of freedom. These results are converted to semiclassical anharmonicity correction factors f and are analyzed alongside results obtained previously for 22 species with up to two fluxional modes. As expected, fluxional species show considerable variation in f at high temperatures; f is as small as 0.2 for acetone and is as large as 9 for methylene glycol at 2500 K. This set of full-dimensional results is used to test the accuracy of reduced-dimensional models where fluxional modes are treated as coupled to one another but as separable from the remaining nonfluxional modes. For most systems, we find that such an approximation is accurate at high temperatures, with average errors in Q of just ∼25%. For some systems, however, larger errors are found, and these are attributed to strong coupling of the fluxional modes to one or more nonfluxional modes. In particular, we identify strong coupling to low-frequency bends for some systems, and we show that by comparing curvilinear and rectilinear harmonic frequencies for the fluxional modes, we can estimate the effect of this coupling on rovibrational anharmonicity. We also quantify the accuracy of the more severe but common assumption of treating fluxional modes as separable from one another, that is, as sets of uncoupled one-dimensional inversions and torsions. This approach can work well for methyl and alkyl rotors, but it is shown to have errors as large as a factor of 7 at high temperatures for more complex systems. Finally, we note that while the present analysis focuses on the treatment of fluxional modes, the collective anharmonicity correction associated with the more numerous nonfluxional modes, although simpler to describe, comprises a significant fraction of the overall anharmonicity.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Anharmonic Rovibrational Partition Functions at High Temperatures: Tests of Reduced-Dimensional Models for Systems with up to Three Fluxional Modes

et al. 2019

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“…This system features an internal hydrogen bond in its lower-energy geometry but not its higher-energy one, leading to significant frequency changes in the nontorsional modes for the two minima. A variety of methods have been developed to incorporate adiabatic frequency changes and asymmetric torsional potentials, ,, and a comparison of these methods with the present full-dimensional results could be made in a future study.…”

Section: Resultsmentioning

confidence: 99%

Anharmonic Rovibrational Partition Functions for Fluxional Species at High Temperatures via Monte Carlo Phase Space Integrals

et al. 2018

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Monte Carlo phase space integration (MCPSI) is used to compute full dimensional and fully anharmonic, but classical, rovibrational partition functions for 22 small- and medium-sized molecules and radicals. Several of the species considered here feature multiple minima and low-frequency nonlocal motions, and efficiently sampling these systems is facilitated using curvilinear (stretch, bend, and torsion) coordinates. The curvilinear coordinate MCPSI method is demonstrated to be applicable to the treatment of fluxional species with complex rovibrational structures and as many as 21 fully coupled rovibrational degrees of freedom. Trends in the computed anharmonicity corrections are discussed. For many systems, rovibrational anharmonicities at elevated temperatures are shown to vary consistently with the number of degrees of freedom and with temperature once rovibrational coupling and torsional anharmonicity are accounted for. Larger corrections are found for systems with complex vibrational structures, such as systems with multiple large-amplitude modes and/or multiple minima.

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“…The planar structures for those stationary points are also presented in Figure . It is interesting that all the stationary points are found to be planar even though the H 2 O 2 equilibrium geometry is bent from the planar geometry by 114° with an inversion barrier (at the cis configuration) of 7.6 kcal/mol . The corresponding zero-point corrected energies for the barriers and intermediate complexes are presented graphically in Figure and, in tabular form in Table .…”

Section: Resultsmentioning

confidence: 99%

“…It is interesting that all the stationary points are found to be planar even though the H 2 O 2 equilibrium geometry is bent from the planar geometry by 114°with an inversion barrier (at the cis configuration) of 7.6 kcal/mol. 50 The corresponding zero-point corrected energies for the barriers and intermediate complexes are presented graphically in Figure 2 and, in tabular form in Table 2. We note that for X = F, Cl, Br, the saddlepoint geometries exhibit a plane of symmetry (C 2v ) with the two r HX bond lengths being equal.…”

Section: Resultsmentioning

confidence: 99%

Double Hydrogen-Atom Exchange Reactions of HX (X = F, Cl, Br, I) with HO₂

Church

Skodje

2018

J. Phys. Chem. A

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A novel double hydrogen atom exchange process, HX + H'O → H'X + HO for the halogen series X = F, Cl, Br, and I, is identified using theoretical methods. These concerted reactions are mediated through a stabilized five-membered planar ring transition state structure. The transition state barrier for the double exchange process is found to be significantly lower than that for the abstraction reaction of a single hydrogen atom. Density functional theory employing the M11 exchange functional is used to compute parameters of the potential energy surface and the rate coefficients are obtained using transition state theory with small curvature tunneling. For low temperatures, the exchange reaction proceeds at a rate several orders of magnitude faster than the abstraction channel, which is also calculated. The exchange process may be observed using isotope scrambling reactions; such reactions may contribute to observed isotope abundances in the atmosphere. The rate coefficients for the isotopically labeled reactions are computed. It is found that the trends in reactivity within the series of halogen reactions can be quantitatively understood using the degree of electron delocalization at the transition state. The barriers are found to fall as the electronegativity of the halogen atom decreases.

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A semiclassical adiabatic calculation of state densities for molecules exhibiting torsion: application to hydrogen peroxide and its isotopomers

Cited by 3 publications

References 40 publications

Anharmonic Rovibrational Partition Functions at High Temperatures: Tests of Reduced-Dimensional Models for Systems with up to Three Fluxional Modes

Anharmonic Rovibrational Partition Functions at High Temperatures: Tests of Reduced-Dimensional Models for Systems with up to Three Fluxional Modes

Anharmonic Rovibrational Partition Functions for Fluxional Species at High Temperatures via Monte Carlo Phase Space Integrals

Double Hydrogen-Atom Exchange Reactions of HX (X = F, Cl, Br, I) with HO₂

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A semiclassical adiabatic calculation of state densities for molecules exhibiting torsion: application to hydrogen peroxide and its isotopomers

Cited by 3 publications

References 40 publications

Anharmonic Rovibrational Partition Functions at High Temperatures: Tests of Reduced-Dimensional Models for Systems with up to Three Fluxional Modes

Anharmonic Rovibrational Partition Functions at High Temperatures: Tests of Reduced-Dimensional Models for Systems with up to Three Fluxional Modes

Anharmonic Rovibrational Partition Functions for Fluxional Species at High Temperatures via Monte Carlo Phase Space Integrals

Double Hydrogen-Atom Exchange Reactions of HX (X = F, Cl, Br, I) with HO2

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Double Hydrogen-Atom Exchange Reactions of HX (X = F, Cl, Br, I) with HO₂