Showing papers on "Ab initio quantum chemistry methods published in 1982"

Tautomerization of formamide, 2-pyridone, and 4-pyridone: an ab initio study

Abstract: The tautomerism of formamide, 2-pyridone, and 4-pyridone has been investigated by ab initio calculations using minimal, extended, and polarization basis sets. When the effects of geometry optimization, polarization functions, correlation energy (estimated by second-order Merller-Plesset perturbation theory), and zero-point vibration energy are combined, the following theoretical estimates are obtained: formimidic acid is 12 kcal/mol less stable than formamide, 2-pyridone is 0.3 kcal/mol more stable than 2-hydroxypyridine and 4-hydroxypyridine is 2.4 kcal/mol more stable than 4-pyridone. Only the 2-pyridone tautomerism has been observed directly in the gas phase, and theory is in good agreement with all three experimental values (0.3 f 0.3,O.l & 0.6,0.6 f 0.1 kcal/mol). In the case of 4-pyridone, the theoretical value may be closer to the actual tautomerization energy than the 7 kcal/mol in favor of hydroxypyridine obtained from indirect experiments. For the heterocycles, relative geometries of tautomers optimized with a minimal basis or semiempirical methods are as satisfactory as structural changes obtained at extended basis set levels. Relative tautomerization energies are reproduced well with the minimal or extended bases, while absolute tautomerization energies require consideration of polarization functions, correlation energy, and zero-point vibration. Tautomerism such as displayed by pyridone/hydroxypyridine plays a role in many areas of chemistry and biochemistry: e.g., the rationalization of structures, properties, and reactivities in heterocyclic chemistry;'*2 concepts and probes of aromaticity;3 measures of intrinsic stabilities vs. solvent mechanisms of enzymatic catalysis and receptor interactions;6 and possibly even mutations during DNA replicati~n.~.~ Investigations of tautom- erism of 2-pyridone date from 1907.8 Most studies since then have dealt with the equilibrium in liquid media,'v9 where the pyridone tautomer is preferred by a factor of 1000. X-ray crystallography shows that pyridone is also favored in the solid.'*I2 The dominance of the pyridone tautomer in solution, neat liquid, and solids has been shown to be the result of strong solvent effects, ion binding, and self- association^.'^^^^^^^'^ In contrast, recent IR and UV measurements have established that the two tautomers are nearly equal in energy when unassociated in the vap~r.~,'~J~ Similar gas-phase tautomerizations have since been investigated for a number of lactam/lactim pairs by using IR,I9 UV,zo pho- toelectron,21*22 ion cyclotron resonan~e,~~-~~ and mass spectros- copy.26*27 All of these gas-phase equilibria show marked dif- ferences from solution data.'*2,9,'3-'7*z8 Numerous theoretical studies with almost every available method have attempted to reproduce the tautomerization energy for pyridone/hydroxypyridine and similar heterocyclic systems2- Simulations of hydrogen bonding and solvent interactions re- produce qualitatively the shift in the equilibrium toward pyridone in condensed However, quantitative agreement with the tautomerization energy in the vapor has been difficult to obtain. Geometry optimization, basis-set flexibility, correlation energy, and zero-point vibration have been recognized as important contributors to these and related45-55 isomerization reactions. In this paper, we report an extensive series of ab initio computations on formamide, 2-pyridone, 4-pyridone, and their tautomers that take these factors into account. Method Ab initio calculations were carried out with the GAUSSIANBO series of programss6 by using minimal (STO-3G)," extended(3-21G and 6- 31G),5*~~~ and polarization (6-31G*)" basis sets. The extended basis sets are of the split-valence. type, and the polarization basis set is an extended basis augmented by a shell of six Cartesian d-type Gaussians on each non-hydrogen atom. Energies were calculated in the Hartree-Fock (HF) approximation, and correlation effects were estimated via second-order

An intermolecular potential-energy surface for (HF)2

Abstract: Molecular-beam spectroscopic data on (HF)2 are used to determine the HF⋯HF intermolecular potential-energy surface. The method used in based on the BOARS approximation of Holmgren et al.(J. Chem. Phys., 1977, 67, 4414), which is extended to diatom–diatom weakly bound complexes. Molecular constants are calculated for several proposed potential-energy surfaces and compared with experiment.The optimised potential surface is shown to be similar to those derived from ab initio calculations. It is believed accurately to represent the shape of the true surface in the region of the potential minimum and along the path of the tunnelling motion; further information is required before an accurate well depth may be obtained.

Quantum chemical studies of a model for peptide bond formation: formation of formamide and water from ammonia and formic acid

Abstract: The S/sub N/2 reaction between ammonia and formic acid has been studied as a model reaction for peptide bond formation using the semiempirical MNDO and ab initio molecular orbital methods. Two reaction mechanisms have been examined, i.e., a stepwise and a concerted reaction. The stationary points of each reaction including intermediate and transition states have been identified and free energies have been calculated for all geometry optimized reaction species to determine the thermodynamics and kinetics of each reaction. The stepwise mechanism was found to be more favorable than the concerted one by both MNDO and ab initio calculations. However, the ab initio method predicts both mechanisms to be fairly competitive with free energies of activation of about 50 kcal/mol. Despite excellent agreement between both methods in the calculated entropies and thermal energies, the minimum basis set character of MNDO leads to values of free energy of activation much higher than those obtained by the ab initio method. The basis set dependence and effect of correlation of the computed ab initio results and the relative effects of polarization and correlation were also investigated by using a number of basis sets up to 6-31G** and estimates of correlation energy by Moller-Plesset perturbationmore » theory up to fourth order. Correlation energy was found to ba a significant factor in the stabilization of transition states.« less

Ab Initio Calculations of Metalloporphyrins

Abstract: Publisher Summary This chapter explores that the metalloporphyrins that are fascinating molecules to both the coordination chemist and the biologist. They appeal to the coordination chemist for several reasons: 1. their ability to incorporate a wide variety of metals, 2. their rich stereochemistry, 3. their variety of oxidation states, and 4. their ability to bind and to activate small ligands. Recently, improvements are made in the methodology of ab initio calculations, together with the rapid development of electronic computers, have made it possible to treat these large molecules at the ab initio level. A series of calculations are represented with an organized effort to understand many properties of the metalloporphyrins. Magnesium porphine and systems related to the chlorophyll have been investigated through self-consistent field (SCF) and configuration interaction (CI) calculations with a basis of floating spherical Gaussian orbitals. The electronic structure of several metalloporphyrins has been studied through SCF and limited CI calculations.

The CNDO/S3 crystal orbital model: Definition and application to polyacetylene

Abstract: The CNDO/S3 model, a spectroscopically parametrized complete neglect of differential overlap molecular orbital method, is extended to encompass the description of radical cation states in infinite periodic quasi‐one‐dimensional molecules. Detailed model calculations of the energy bands and density of valence electron states (DOVS) are given for macromolecules of trans polyacetylene. These calculated quantities are shown to correspond directly to predictions extrapolated from sequences of CNDO/S3 calculations for finite polyenes, and to describe accurately both results of ab initio calculations and measured valence electron photoemission spectra for polyacetylene. The consequences of introducing electron–electron interactions into a noninteracting electron model are studied systematically by examining in turn the Huckel, Hubbard, CNDO–SCF, and ab initio limits of the calculated energy bands and DOVS. Comparison of the results of these calculations with measured photoemission spectra for polyacetylene revea...

Molecular interaction of dense water at high temperature

Abstract: Experimental shockwave data of water were analyzed using intermolecular potentials based on quantum mechical ab initio calculations. A central pair potential obtained by averaging the ab initio result according to the Perram and White method was found to predict accurately the shockwave thermodynamic properties at temperatures between approximately 1300 and 3000 K. At higher temperatures, the theory overestimates the single‐shock pressures and temperature by 10% to 20%. This deviation is attributed to the shock‐induced ionization of water molecules. As a result of our calculations, we suggest the molecular interaction of water becomes simpler under a high‐temperature (≳1300 K) and high‐pressure (≳8 GPa = 80 kbar) environment, except for a small ionization correction, and it is possible to carry out reasonably reliable statistical mechanical calculations under such conditions. Similar comparisons are also presented using potentials developed by Stillinger and Rahman and by Watts.

The propagation of basis set error and geometry optimization in ab initio calculations. A statistical analysis of the sulfur d-orbital problem

Abstract: By generating a sample of 100 random molecular basis sets from a normal distribution centered on the best‐energy, fully variational AO basis sets, a statistical analysis has been carried out on the distributions and pair correlations of calculated Hartree–Fock total energies, optimum bond lengths, energy components, and formal atomic charges. Dimethylsulfoxide (DMSO), the calculated geometry of which has been found extremely sensitive to basis variations in an earlier study, has been selected as model compound. The results provide a clear answer to the sulfur d‐orbital problem, since no simultaneous reproduction of experimental geometry and an adequate approximation to the variationally optimum total energy have been possible without including d‐polarization functions on sulfur.

Ab initio calculation of the transition-state properties and addition rate constants for atomic hydrogen + acetylene and selected isotopic analogs

Abstract: GVB-POL-CI ab initio calculations of the geometries, energetics, and normal mode frequencies of C/sub 2/H/sub 2/, C/sub 2/H/sub 3/, and the transition state for the addition reaction of H + C/sub 2/H/sub 2/ are presented. In addition, normal mode frequencies for the isotopic variants D + C/sub 2/D/sub 2/, D + C/sub 2/H/sub 2/, and H + C/sub 2/D/sub 2/ are preented. These results are compared to experimental values for C/sub 2/H/sub 2/ and to ab initio values of Hagase and Kern, and semiempirical values of Keil, Lynch, Cowfer, and Michael. The results are also used to calculate the apparent bimolecular addition rate constant using conventional RRKM theory for chemical activation. The calculated rate constants and their isotopic variants are compared as a function of temperature and pressure to available experimental information. The agreement is little different from that obtained by Keil et al. with a similar calculation using semiempirical values for acetylene, transition-state, and vinyl radical properties. In particular, the calculated high-pressure limit of the rate constant appears to be at least 1 order of magnitude higher than the experimental limit. Several possible reasons for this discrepancy are discussed.

Ab initio computation of vibrational relaxation rate coefficients for the collisions of CO2 with helium and neon atoms

Abstract: Ab initio calculations of rate coefficients are reported for the vibrational relaxation of CO 2 molecules in collision with helium and neon atoms. Self consistent-field computations have been performed to parameterise simple three-dimensional potential energy functions which have been used in vibrational close-coupling, rotational infinite-order-sudden calculations of rate coefficients. Excellent agreement is obtained between the calculated and experimental rate coefficients for the deactivation of the (01 1 0) vibrational level in the He + CO 2 system at temperatures of 300 K and above. The ab initio predictions of rate coefficients for relaxation of CO 2 vibrational levels such as (10 0 0) and (02 0 0) should be useful in computer simulations of CO 2 lasers.

ESR investigation of matrix isolated B 16 O and B 17 O radicals: Comparison of nuclear hyperfine structure with ab initio calculations

Abstract: An extensive ESR investigation of the 2Σ B16O and B17O radicals in rare gas matrices has been conducted. Several different trapping sites in neon matrices have been found and investigated as a function of temperature over the range 3–10 K both during and after deposition. Motional averaging of the nuclear hyperfine tensor seems to be occurring for two neon sites and the argon matrix. The large isotropic boron hyperfine coupling of these sites and the 17O hfs show excellent agreement with ab initio theoretical results. BO radicals trapped in a third neon site do not appear to be undergoing motional behavior. Isotropic and dipolar hyperfine parameters for this site show reasonable agreement with theoretical results. BO radicals in one neon site which is extremely sensitive to temperature effects exhibit nearly perfect ’’single‐crystal‐like’’ preferential orientation which has not been previously observed in rare gas matrix ESR studies. The results of this BO investigation help to correct erroneous conclusio...

New semiempirical method of modeling potential energy surfaces for generalized TST and application to the kinetic isotope effects in the Cl–H–H system

Abstract: We present a test of bond‐energy‐bond‐order (BEBO)‐type potential energy surfaces for the Cl–H–H reaction system. We consider two methods, the rotated–Morse BEBO (RMBEBO) method and the Agmon–Levine‐plus‐anti‐Morse bend (AL/AB) method. Each method is treated as a one‐parameter semiempirical method. The parameter is adjusted to the energy of activation for Cl+H2 and the test of the method is the comparison of predicted kinetic isotope effects (KIE’s) to experiment. Dynamical calculations are carried out by the improved canonical variational transition state theory with Marcus–Coltrin‐path semiclassical adiabatic ground‐state transmission coefficients as well as with simpler methods. The RMBEBO surfaces are not successful, but the AL/AB method is as successful as previously employed two‐parameter methods. The anti‐Morse‐bend approximation is a general procedure, and it is tested successfully against ab initio calculations.

The nature of bonding in PdC

Abstract: The nature of bonding in PdC is described through all electron ab initio calculations. The Hartree‐Fock‐Rotthaan formalism is employed. (AIP)

Anisotropy of the nuclear spin–spin coupling tensor in water, ammonia, and methane molecules

Abstract: The anisotropy of the nuclear spin–spin coupling tensor relative to H–H and X–H (X = O, N, C) interactions is investigated through extended ab initio calculations on the molecules H2O, NH3, and CH4. Spin–orbit, spin–dipolar, and Fermi contact‐spin–dipolar cross terms are accounted for in the coupled Hartree–Fock perturbative scheme. The major role played by the Fermi contact‐spin–dipolar cross term is ascertained. A large discrepancy emerges between theoretical and experimental anisotropy for the CH coupling, which evidences the need for further careful analyses of the measured data.

Analysis of the far infrared H2-He spectrum

Abstract: Previous measurements of the far infrared absorption due to H2-He collisions at the temperatures of 77, 195, and 292 K are analyzed. The spectra are fitted by a semiempirical line shape representing the isotropic induced overlap component and combined anisotropic quadrupolar and overlap components. The experimental spectral moments are evaluated and compared with theory for several induced-dipole and potential models. From the isotropic contribution, the range and strength of the induced dipole is evaluated and compared with the results of ab initio calculations. Fitting parameters are obtained with physically plausible temperature dependences which allow simple and accurate representation of the spectra and of their moments at temperatures different from those of the measurements.

Effects of Polar Substituents on Carbon-Silicon Multiple Bonds

Abstract: The results of ab initio calculations on the effects of fluorine and lithium substitution on silaethyne and silaethylene and their isomers are presented. It is found that fluorine has a strong stabilizing effect when attached to silicon. This results in the predictions (a) that a distorted 1-fluoro-1-silaethyne is the most stable structure of HCSiF and (b) that 1,1-difluoro1-silaethylene is much lower in energy than 1,2-difluoromethylsilylene. Approximate correlation corrections are added to study the HC~iF FHC=Si and HC==SiLi LiHC=Si potential energy surfaces.

CNDO/2‐FPP atom‐in‐molecule polarizabilities

Abstract: The finite perturbation (FP) method of obtaining molecular and atom‐in‐molecule polarizabilities from the semiempirical CNDO/2 algorithm has been extended by the addition of parametrized polarization orbitals (PO) (2p for H, 3d for C, N, O, F, P, S, Cl, and 4d for Se and Br) The new parameters in CNDO/2‐FPP are a screening constant for the off‐diagonal core Hamiltonian terms, the PO orbital exponent, and, for the heavier elements, the PO electrongativities and the resonance integral The parametrization preserves the charge distribution and the occupied molecular orbitals, and is designed to fit 13 model molecules; it obtains extra polarizability from the new hybrid contributions to the dipole moment and from reshuffling the virtual molecular orbitals Excellent molecular and atom‐in‐molecule polarizabilities are obtained, for 47 other molecules, in agreement with available experimental data, and in competition with the best ab initio calculations

Comparison of electron gas and abinitio potentials for the N2–N2 interactions. Application in the second virial coefficient

Abstract: A detailed potential for the interaction between two rigid N2 molecules is given in the form of a spherical expansion. The interaction energy is found as the sum of the so‐called Hartree–Fock part of the electron gas expression including the Rae correction and the ’’ab initio’’ dispersion energy in the multipole expansion. Potential surface cuts computed with this expansion agree to a large extent with a similar potential completely based on ab initio calculations. Comparison of the experimental second virial coefficient curve with the curves obtained from a four dimensional quadrature using both ab initio and electron gas potentials demonstrates the usefulness of these potentials, and underlines the importance of the anisotropic contributions.

Associative detachment in collisions between negative halogen ions and hydrogen atoms

Abstract: The associative detachment reaction is studied theoretically in the thermal energy range for Cl- and F- ions colliding with hydrogen atoms. The interaction of the outer low-energy electron with the hydrogen halide molecule is described in the zero-range-potential approximation, which allows an exact treatment of the collision process. In the case of F- projectiles, the molecular data needed are taken from the ab initio calculations of Segal and Wolf (1981); for the Cl- projectile, these molecular data are partly fitted to reproduce the experimental data. The relative population of the final levels, or equivalently the sharing of the exothermicity of the reaction between the various degrees of freedom, is thoroughly investigated; it appears that most of the available energy goes into the vibration of the final molecule. Comparison with the experimental data of McFarland et al. (1975) and Zwier et al. (1980) is presented.