Showing papers on "Ab initio quantum chemistry methods published in 1983"

An effective pair potential for liquid acetonitrile

Abstract: A six site potential model has been developed for liquid acetonitrile, based on atomic interaction parameters taken from studies of other liquids and partial charges derived by ab initio calculations. When used in molecular dynamics simulations, the model yields results in good agreement with experimental data on the thermodynamics, structure, self diffusion coefficients and reorientational correlation times of the liquid.

The weakly exothermic rearrangement of methoxy radical (CH3O⋅) to the hydroxymethyl radical (CH2OH⋅)

Abstract: Although the CH3O⋅ and CH2OH⋅ radicals have long been considered critical intermediates in combustion and atmospheric processes, only very recently has the potential significance of the isomerization CH3O⋅→CH2OH⋅ been appreciated. This isomerization and related aspects of the CH3O⋅/CH2OH⋅ potential surface have been studied here using nonempirical molecular electronic structure theory with moderately large basis sets and with incorporation of electron correlation. The vibrational frequencies of CH3O⋅, CH2OH⋅ and seven other stationary points on the potential energy hypersurface have been predicted, both to compare with results from spectroscopy and to provide estimates of zero‐point vibrational corrections. In general, there is reasonable agreement with those vibrational frequencies of CH3O⋅ and CH2OH⋅ which are known from experiment. Our ab initio calculations predict that CH3O⋅ lies 5.0 kcal mol−1 higher in energy than CH2OH⋅ with a barrier to rearrangement to CH2OH⋅ of 36.0 kcal mol−1. Rearrangement of...

Rydberg states of 7Li2 by pulsed optical–optical double resonance spectroscopy: Molecular constants of 7Li2+

Abstract: Three Rydberg series of electronic states of 7Li2 have been characterized by pulsed optical–optical double resonance spectroscopy. The observed Rydberg states, which include the previously reported E 1Σ+g and G 1Πg states, have been identified as 3–10sσ 1Σ+g, 3–10dσ 1Σ+g, and 3–15dπ 1Πg. The molecular constants for several of the upper members of each of the above series have been used to deduce the ionization potential of 7Li2 and molecular constants for the X 2Σg+ state of 7Li2+. The former was determined to be T0(∞)=41 496±4 cm−1. The latter were found to be in good agreement with recent ab initio calculations.

The Structure of Liquid Formamide Studied by Means of X-Ray Diffraction and ab Initio LCGO-MO-SCF Calculations

Abstract: The molecular and liquid structures of formamide have been studied by means of X-ray diffraction. Relative stabilities of aggregates of formamide molecules have been calculated by using the ab initio LCGO-MO-SCF procedure and minimal basis sets. The atomic distances within a formamide molecule and the intermolecular N···O distance between the hydrogen–bonded molecules are determined by the X-ray diffraction experiment as follows: C=O: 1.24(1) A, C–N: 1.33(1) A, N···O (intramolecular): 2.25(2) A, and N···O (intermolecular): 3.05(5) A. The intermolecular (Remark: Graphics omitted.) bond angle (Θ) is about 120°. It is concluded from the X-ray diffraction experiment that liquid formamide mainly consists of the chain-like hydrogen–bonded structure of formamide molecules by combining through –NH2···O=CH– interactions. The conclusion is supported by the ab initio calculations. Formation of ring-dimers in the liquid formamide has not been confirmed by the X-ray diffraction study, although a possibility of the for...

Ab initio approach to the multichannel quantum defect calculation of the electronic autoionisation in the Hopfield series of N2

Abstract: The two-step formulation of the multichannel quantum defect theory, applied to molecular electronic autoionistion by Giusti-Suzor (1980) and Lefebvre-Brion (1973) is used to calculate the total and partial photoionisation cross sections in the region of the Hopfield series on N2, the 700-730 AA wavelength range. The electronic quantities necessary for this treatment are obtained from ab initio calculations. The cross sections and the photoelectron angular distribution parameters are in satisfying agreement with experiment. The results clearly show that the absorption series correspond to the (B2 Sigma u+)nd sigma g series and the apparent emission series to the (B2 Sigma u+)nd pi g series.

ESR of pseudorotating 6Li3 and 7Li3

Abstract: ESR spectra assigned to pseudorotating 6Li3 and 7Li3 molecules have been produced by arc lamp photolysis of argon matrices doped with isotopically enriched 6Li or 7Li. The spectrum of 6Li3 consists of seven equally spaced first derivative transitions with relative intensities in good agreement with the expected values for three equivalent I=1 nuclei. The measured hf constant and g value are a=12.21(1) G and go=2.002 82(1). The absence of a stationary trimer spectrum suggests that the ground vibronic state of Li3 is relatively nonlocalized. The average isotropic spin population of the trimer ρ=0.225 is in excellent agreement with the predictions of ab initio calculations. The total isotropic spin population 3ρ≂0.68 suggests an approximately 30% p‐character for the unpaired electron wave function. A comparison of the orbital composition of M, M2, M3, M∞ (M=Li, Na, K) shows (i) a diminishing s‐character with increasing cluster size, (ii) a remarkable similarity between Na and K for all cluster sizes, and ...

Electron scattering by nitrogen molecules at intermediate energies

Abstract: The total scattering and vibrational excitation cross sections for e-N2 scattering are calculated for the first time at intermediate energies using an ab initio method. The calculations are based on the use of numerical basis functions in the R-matrix method, and exchange and charge polarisation effects are included without parametrisation. At low energies, good agreement is obtained with other ab initio calculations both for the position and width of the 2 Pi g resonance. In the energy range from 20-30 eV the vibrational cross sections are dominated by the 2 Sigma u resonance, and it is found that the resonance parameters depend strongly on the internuclear separation. Finally, the dependence of the total cross section on the target wavefunction is discussed, and it is argued that accurate results may require the inclusion of electron correlation effects in the target.

Electron affinities of the alkali dimers - Na2, K2, and Rb2

Abstract: Ab initio calculations on the ground states of the alkali dimers, Na2, K2, and Rb2, and their anions are reported. The calculations employ large Gaussian basis sets and account for nearly all of the valence correlation energy. The calculated atomic electron affinities are within 0.02 eV of experiment and the calculated adiabatic electron affinities for Na2, K2, and Rb2 are, respectively, 0.470, 0.512, and 0.513 eV.

Ab initio studies of the CO2–HF and N2O–HF complexes

Abstract: Ab initio calculations are used to confirm the geometries found experimentally by Klemperer et al. for the CO2–HF and N2O–HF complexes.

Application of transition state theory to desorption from solid surfaces: Ammonia on Ni(111)

Abstract: We have used transition state theory to derive analytical expressions for the rates of desorption of atoms and molecules (diatomic as well as polyatomic) from solid surfaces. Realistic forms for the three dimensional adsorbate‐surface interaction potentials are employed, including surface corrugation. Using potential parameters from a combination of experiment and ab initio calculations we have applied the rate expressions to evaluate the temperature programmed desorption spectra of NH3 from Ni(111). Comparing these curves to the experimental spectra leads to a bond energy of D0=21.0–3.7Θ kcal/mol, where Θ is the fraction of saturation coverage.

Theoretical equilibrium geometry, vibrational frequencies and the first electronic transition energy of HCC

Abstract: Ab initio calculations with the 6–311 G** basis set and all single and double excitations in the CI treatment have been carried out to determine the structure of the HCC radical. The theoretical geometry of R(CC) = 1·209, r(CH) = 1·067 A (or 1·205 and 1·063, respectively, if corrected for residual errors) is in excellent accordance with the experimental rotational constant. The calculated harmonic vibrational frequencies are v 1 ⋍ 3450, v 2 ⋍ 540 and v 3 ⋍ 2040 cm-1. An extremely low energy around 2000 cm-1 is obtained for the first electronic transition A 2II ← X 2∑. Results for the excited state are also given. The theoretical vibrational frequencies, with all possible errors taken into account, are inconsistent with the accepted interpretations of the few experimental results. It is shown, however, that a complete reinterpretation of the spectroscopic observations is possible, by which the present calculations fit with the recent gas phase infrared laser spectroscopic data but remain in definite contra...

Models of lanthanide crystal fields in metals

Abstract: The fitted crystal-field parameters for lanthanide atoms in metals are shown to be consistent with the usual assumption that the crystal field is a one-electron interaction. Also, the relationship between crystal-field parameters for different host crystals can be understood in terms of the superposition model. Some consequences of these results in relation to ab initio calculations are discussed.

IR‐induced photorotamerization of allyl alcohol in low‐temperature matrices and ab initio calculations

Abstract: The infrared regions νOH, τOH, and 950–900 cm−1 of allyl alcohol (2‐propen‐l‐ol) were studied in low‐temperature Ar, Kr, Xe, N2, and CO matrices. A fast IR‐induced process and a similar slow dark process were found. It is concluded that the photoprocess is a rotamerization (obviously Cg→Gg′), since the process was similar in all five matrix media and also for other allyl compounds. The photoprocess was found to obey the first order rate law. Radiation from 9000 to below 2600 cm−1 was found to be active in initiating the process, especially the radiation corresponding to the νOH and methylenic νCH2 regions. The process is very fast in xenon. A possible mechanism for the process is discussed. To aid the interpretation of the results, ab initio calculations were carried out. All five conformers of allyl alcohol were optimized on the 4‐31G level, and the corresponding semirigid potential energy curves of the torsional coordinates were constructed. Some calculations were carried out also on the 6‐31G**, MP/4‐3...

The Insertion of Singlet Silylene into Ethylene

Abstract: Ab initio calculations at the minimal, split valence and extended basis set levels, both with and without electron correlation corrections, were used to study the insertion of singlet silylene into ethylene to form silacyclopropane. The least motion C2v path is symmetry forbidden and an artificial barrier of about 36 kcal/mol is found within C2v symmetry. Relaxation of symmetry to C, eliminates the symmetry forbiddenness of the reaction. The 3-21G and 6-31G* SCF calculations still predict a small barrier along this symmetry allowed path; however, the addition of second order correlation corrections results in the prediction of an insertion process with no barrier. The overall ΔE for the reaction is predicted by MP2/6-31G*//3-21G to be 47.9 kcal/mol.

Potential Energy Curves for Dissociative Recombination

Abstract: The direct dissociative recombination (DR) of a diatomic molecular ion, AB+, with an electron is described by AB++e where AB* → AB* A + B is a repulsive state of the neutral molecule which dissociates directly to A and B. The dissociation prevents the emission of an electron by AB* and accounts for the high rate of electron recombination with molecular ions compared to atomic ions. This paper presents a discussion of ab initio calculations of potential energy curves for the AB* and AB+ states in (1). Results for recent large scale calculations for O2 are presented along with additional results for H2 and He2.These calculations provide answers to the following questions: What are the identities of the molecular states responsible for DR? What are the translational energies and states of the resulting atoms? How do the atomic state quantum yields vary with ion vibrational excitation and electron temperature?

Experimental Versus Theoretical Electron Densities. Small Molecules, Large Molecules, and Salts

Abstract: High quality ab initio calculations of electron deformation densities and comparisons with available experimental results are presented for small molecules. AHF (approximate Hartree-Fock) basis set and electron correlation effects are assessed. The high quality of the 4-31G + BF (bond functions) basis set for the computation of deformation densities is established for small molecules. Comparison of 4-31G + BF electron densities with all available experimental data is made for some selected large organic molecules. Errors and problems in the experimental and theoretical methods are discussed. Crystal effects such as hydrogen bonding and ion effects in salts are estimated. It is shown that all the effects considered (i.e., near HF basis set, correlation, hydrogen bonding and ion effects) are small compared to possible experimental uncertainties. Thus, at present, economical 4-31G + BF calculations of isolated molecules are sufficiently accurate for comparisons with any type of experimental determin...

Autoionization spectra of Li2 and the X2Σ+g ground state of Li+2: Experimental and theoretical investigations

Abstract: Autoionizing Rydberg levels of Li 2 molecules in a supersonic molecular beam are populated by stepwise excitation with two tunable pulsed dye lasers. The observed autoionization spectra show severe perturbations. Based on calculations of quantum defects and a perturbation treatment of l -uncoupling a tentative assignment of Rydberg series up to n = 32 is proposed. The convergence limits of these series yield a value of IP = 41475 cm −1 for the adiabatic ionization potential and a vibrational constant ω e = 263 cm −1 for the X 2 Σ + g ground state of Li + 2 . The experimental results are compared with ab initio calculations combined with a core polarization potential, which yield the potential curve. the dissociation energy, the quadrupole moment and the vibrational frequency for the X 2 Σ + g ground state of Li + 2 , in the excellent agreement with experimental findings.