Stephen P. Walch

A theoretical study of the potential energy surface for OH+H2

Barrier heights and transition state geometries have been calculated for the reaction OH+H2→H2O+H using large scale POL‐CI wave functions (based on GVB wave functions using basis sets of up to triple zeta valence plus double zeta polarization quality). The saddle point geometry is found to be coplanar and to resemble OH+H2 as expected because of the large exoergicity (∼16 kcal/mole) of the reaction. The OH distance of the OH moiety is essentially the same as for the OH molecule, while the HH distance of the H2 moiety is 0.10 A (∼14%) longer than for H2. The distance from the O to the near hydrogen of the H2 moiety is 0.35 A (∼36%) longer than for the H2O molecule. The HOH angle is 98° and the H2 moiety is tilted from collinearity with the O atom by 15° toward the H of the OH moiety. The calculated barrier height using a [4s3p2d/3s2p] basis set is 6.2 kcal/mole. Transition state theory calculations (including a Wigner tunneling correction) using the theoretically computed surface predict rate constants whi...

An abinitio calculation of the rate constant for the OH+H2→H2O+H reaction

The rate constants for OH+H2 reactions are calculated, using the reagent and saddle point parameters from table I and 6.2 kcal/mole barrier estimate. (AIP)

A theoretical study of the potential energy surface for O(3P)+H2

Barrier heights and transition state geometries have been calculated for the reaction O(3P)+H2→OH+H using large scale POL‐CI wave functions (based on GVB wave functions using basis sets of up to triple zeta valence plus double zeta polarization quality). A detailed study was made of the effects on the calculated barrier height and saddle point geometry of (i) basis set, (ii) choice of orbitals, and (iii) choice of reference configurations. Calculations using a [4s3p2d/3s2p] basis lead to a collinear saddle point with rHH=0.92 A and rOH=1.23 A with a corresponding barrier height of 12.5 kcal/mole. There are two surfaces which connect the reactants with the products: one of 3A′ symmetry and one of 3A″ symmetry (these correspond to the two degenerate components of the 3Π state in collinear geometries). In the transition state region, the 3A′ surface has a steeper bending curve than the 3A″ surface leading to significantly different reaction rates on the two surfaces.

A comparative study of the reaction dynamics of several potential energy surfaces of O(3P)+H2→OH+H. I

Two new potential energy surfaces for th O+H2→OH+H reaction are presented, and a detailed comparision of the saddle point properties and thermal rate constants of these and of six other O+H2 surfaces is made. The two new surfaces are (1) an extended BEBO surface and (2) a rotated‐Morse‐oscillator‐spine (RMOS) fit to the accurate ab initio POLCI surface of Walch and Dunning. In the BEBO surface, an improved end atom repulsive potential is used which leads to a much more accurate barrier estimate (11.52 kcal/mol) than with the usual anti‐Morse expression. The POLCI–RMOS surface is an essentially quantitative fit to the ab initio points, and has a barrier of 12.58 kcal/mole. The other O+H2 surfaces examined include the LEPS surface of Westenberg and de Haas (LEPS‐WDH) and of Johnson and Winter (LEPS‐JW), the diatomics‐in‐molecules (DIM) surface of Whitlock, Muckerman, and Fisher, the ab initio surface of Howard, McLean, and Lester (HML), and a fit to HML’s surface by Schinke and Lester (SL). For the LEPS‐JW ...

Calculated barrier to hydrogen atom abstraction from CH4 by O(3P)

The barrier height and transition state geometry have been calculated for the reaction CH4+O(3P) →CH3+OH using POL–CI wave functions with a valence double zeta plus polarization basis set. The saddle point geometry is found to be of C3v symmetry with the CH and OH bonds stretched by ∼0.27 A (25%) and ∼0.21 A (21%), respectively. The CH bond lengths of the CH3 group change only very slightly (<0.01 A) during the reaction, while at the transition state each CH bond is ∼13.1° out of the plane perpendicular to the CHO axis and containing the carbon atom (19.5° for CH4 and 0.0° for CH3). The calculated barrier height is 14.4 kcal/mole using a [3s2p1d/2s1p] basis set. From comparison to similiar calculations for O(3P)+H2→OH+H the basis set error in ΔEb is estimated to be ∼2.4 kcal/mole leading to a predicted barrier height of ∼12.0 kcal/mole. Including zero point corrections leads to estimated activation energies of 10.3 kcal/mole (3A′) and 10.1 kcal/mole (3A″) as compared to experimentally derived activation e...

Reaction dynamics for O(3P)+H2 and D2. IV. Reduced dimensionality quantum and quasiclassical rate constants with an adiabatic incorporation of the bending motion

Reduced dimensionality exact quantum and quasiclassical reaction probabilities, transmission coefficients, and rate constants are presented for the O(3P)+H2(ν=0,1) and O(3P)+D2(ν=0,1) reactions on an effective potential surface given by the ab initio MOD POLCI potential energy surface reported previously plus the adiabatic ground state bending energy eigenvalue obtained from the two (nondegenerate) nonlinear potential energy surfaces reported here. The new rate constants are compared to experiment. Good agreement is found for thermal rate constants and isotope effects and for vibrationally excited rate constants.

Theoretical characterization of the potential energy curve for hydrogen atom addition to molecular oxygen

Large scale polarization configuration interaction (POL‐CI) calculations are reported on the HO2 molecule. For the structure of the stable complex the calculations predict (with experimental values in parentheses) ROH = 0.99 A(0.971 A), ROO = 1.37 A (1.335 A), and qHOO = 103°(104.1°). The calculated harmonic frequencies (3655, 1416, and 1181 cm−1) are also in good agreement with those derived from experiment. On the other hand, the H–O2 bond energy obtained in the calculations (42 kcal/mole) is too small by ∼10 kcal/mole (an inherent defect in the POL‐CI method). Although both the GVB and GVB‐CI calculations predict barriers of ∼3 kcal/mole to the addition of a hydrogen atom to molecular oxygen, the POL‐CI calculations reduce the barrier to less than 0.4 kcal/mole. Taking the computational limitations into account, there may well be no barrier at all to the addition reaction.

Theoretical studies of the O+H2 reaction

ab initio theoretical calculations of the potential energy surfaces and rates of reaction of the O+H2 reactions are reported. (AIP)

Calculated barriers to abstraction and exchange for CH4+H

Saddle point geometries and barrier heights have been calculated for abstraction and exchange in CH4+H using POL–CI wave functions with basis sets up to triple zeta valence with double zeta polarization on C and single zeta polarization on H. The saddle point for the abstraction reaction is found to have C3v symmetry (abstraction collinear with a CH bond). The calculated saddle point geometry is closer to products (CH3+H2) than to reactants (CH4+H) and has the CH and HH bonds stretched by 0.38 A (35%) and 0.18 A (24%), respectively. The CH bond lengths of the remaining three CH bonds change only very slightly (<0.01 A) during the reaction, while at the saddle point each methyl CH bond is ∼12.4° out of the plane perpendicular to the C–––H–––H axis and containing the C atom (19.47° for CH4 and 0° for CH3). The calculated barrier height is 15.9 kcal/mole (using the largest basis set). Comparison to comparable calculations for H3 indicates an error of ∼2.4 kcal/mole due to the POL–CI approximation leading to ...

Ab initio calculation of transition state normal mode properties and rate constants for the H(T)+CH4(CD4) abstraction and exchange reactions

We present ab initio (GVB–POL–CI) calculations for enough of the region about the abstraction and exchange saddle points for H(T)+CH4(CD4) to perform a full normal mode analysis of the transition states. The resulting normal mode frequencies are compared to four other published surfaces: an ab initio UHF–SCF calculation by Carsky and Zahradnik, a semiempirical surface by Raff, and two semiempirical surfaces by Kurylo, Hollinden, and Timmons. Significant quantitative and qualitative differences exist between the POL–CI results and those of the other surfaces. Transition state theory rate constants and vibrationally adiabatic reaction threshold energies were computed for all surfaces and compared to available experimental values. For abstraction, the POL–CI rates are in good agreement with experimental rates and in better agreement than are the rates of any of the other surfaces. For exchange, uncertainties in the experimental values and in the importance of vibrationally nonadiabatic effects cloud the comp...