Showing papers on "Ab initio quantum chemistry methods published in 1975"

Binding energies in atomic negative ions

Abstract: This article updates a ten‐year‐old review of this subject [J. Chem. Phys. Ref. Data 4, 539 (1975)]. A survey of the electron affinity determinations for the elements up to Z=85 is presented, and based upon these data, a set of recommended electron affinities is established. Recent calculations of atomic electron affinities and the major semiempirical methods are discussed and compared with experiment. The experimental methods which yield electron binding energy data are described and intercompared. Fine structure splittings of these ions and excited state term energies are given.

The electronic structure of nitrogen dioxide. I. Multiconfiguration self‐consistent‐field calculation of the low‐lying electronic states

Abstract: Traditional spectroscopic analysis of the complex and irregular absorption spectrum of NO2 has provided a relatively small amount of information concerning the nature of the excited states. An extensive ab initio investigation has been undertaken, therefore, to provide a basis for interpretation of the experimental results. Multiconfiguration self‐consistent‐field (MC–SCF) wavefunctions have been computed for the low‐lying X 2A1, A 2B2, B 2B1, C 2A2, 4B2, 4A2, and 2Σ+g electronic states of NO2. The minima of the A 2B2, B 2B1, and C 2A2 states have all been found to be within 2 eV of the minimum of the X 2A1 ground state; for these states, C2v potential surfaces have been constructed for purposes of a spectral interpretation. The 4B2, 4A2, and 2Σ+g states are all more than 4 eV above the minimum of the ground state and have been examined in less detail. The study described here significantly improves on previous NO2 ab initio calculations in three important areas: (1) The double‐zeta‐plus‐polarization quality basis set is larger and more flexible; (2) the treatment of molecular correlation is more extensive; and (3) the electronic energies have been calculated for several different bond lengths and bond angles in each state. For the four lowest doublet states the following spectral data have been obtained: The ground state experimental constants are included in parentheses. The estimated accuracy of the various parameters is ±0.02 A for bond length, ±2° for bond angle, ±10% for the vibrational frequencies, ±0.10 D for dipole moments, and ±0.3 eV for the adiabatic excitation energies. An unusual feature has been found for the 2Σ+g state. The equilibrium geometry of this linear state has two unequal bond lengths of 1.20 A and 1.42 A and the inversion barrier is approximately 800 cm−1.

Role of ab initio calculations in elucidating properties of hydrated and ammoniated electrons

Abstract: The properties of solvated electrons are analyzed in terms of a self- consistent modified continuum model based on the techniques of ab initio molecular quantum mechanics. The model is semiclassical in spirit, employing the quantum mechanical density for the excess charge and the first solvation shell in conjunction with classical electrostatics, and is developed in a general form which can be straightforwardly applied to special cases of interest, such as the solvated mono- and dielectron complexes. The advantages and disadvantages of the technique are discussed in relation to other more empirical approaches. Computational results are presented for excess electrons (mono- and dielectrons) in water and ammonia, and the role of long-range polarization of the medium in localizing the excess charge is analyzed. The variationally determined ground states are characterized in terms of equilibrium solvation shell geometry (appreciable cavities are implied for both water and ammonia), solvation energy, photoionization energy, and charge distribution. The finding of negative spin densities at the first solvent shell protons underscores the importance of a many electron theoretical treatment. Preliminary results for excited states are also reported. The calculated results are compared with experimental and other theoretical data, and the sensitivity of the results to variousmore » features of the model is discussed. Particular attention is paid to the number of solvent molecules required to trap the excess electron. (auth)« less

Theoretical study of the H 5 + system

Abstract: Ab initio calculations have been carried out for the ground state of H 5 + in order to predict its equilibrium geometry, binding energy, enthalpy of formation, and the features of the H2 · H 3 + interaction at large and intermediate intermolecular distances. The extended basis set of Gaussian functions was carefully optimized to describe the various kinds of intermolecular interactions. Electron correlation was accounted for by means of CI calculations. Different from previous studies we find a D 2d equilibrium geometry with D e = 7.4 kcal/mol and ΔH 300 0 ≈−8.7 kcal/mol. The potential surface turns out to be extremely shallow in the vicinity of the D 2d structure which results in a great mobility of the central nucleus at room temperature.

Ab initio calculations on the electronic spectrum of ethane

Abstract: Double-excitation Cl calculations are reported for the ground and various excited states of ethane and the positive ion. An extrapolation technique is used to obtain vertical energy differences between these states and this information is combined with calculated oscillator strengths in order to obtain a more thorough understanding of the ethane electronic spectrum. The Cl calculations find the 2 E g and 2 A 1g ionic species (as well as corresponding Rydberg series members) to have very nearly the same energy, in rather strong constrast to the results of SCF calculations for such states. It is thus argued that the strong 1400 A band system is ethane results from a combination of five distinct allowed transitions occurring in a very narrow range of wavelength, the strongest of which is the 3a 1g - 3po ( 1 A zu - 1 A 1g ) parallel polarized species, followed by a pair of perpendicularly polarized 1 E 11 - 1 A 1g excitations of 1e g -3po and 3a 1g -pπ type; despite the predominantly diffuse appearance of the upper orbital in all these transitions, however, significant valence-shell σ* character is noted in the a 2u (3po) species. The corresponding dipole-forbidden 3s Rydberg transitions from the same two valence-shell MO's are assigned to the considerably weaker absorption features observed at somewhat larger wavelengths in the ethane spectrum.

Analytical potentials for triatomic molecules from spectroscopic data V. Application to HOX (X=F, Cl, Br, I)

Abstract: Analytical potential energy functions are reported for HOX (X=F, Cl, Br, I). The surface for HOF predicts two metastable minima as well as the equilibrium configuration. These correspond to HFO (bent) and OHF (linear). Ab initio calculations performed for the HOF surface confirm these predictions. Comparisons are drawn between the two sets of results, and a vibrational analysis is undertaken for the hydrogen bonded OHF species. For HOCl, one further minimum is predicted, corresponding to HClO (bent), the parameters for which compare favourably with those reported from ab initio studies. In contrast, only the equilibrium configurations are predicted to be stable for HOBr and HOI.

Ab initio calculation of force constants for the linear molecules HCN, FCN, (CN)2 and the ion N2F+

Abstract: Force constants have been calculated from ab initio Hartree-Fock wave-functions by the force method, using 7s3p/1 and 5s2p/1 gaussian basis sets for HCN, FCN, C2N2 and FN2 +. Agreement of the quadratic and some cubic force constants with experiment is good for HCN and FCN. The influence of anharmonicity upon the l-type doubling constant of FCN is estimated. Both the experimental l-type doubling constant and the ab initio calculations indicate that the quadratic stretch, stretch coupling constant is positive in FCN, contrary to recent results of Wang and Overend, obtained from the Anderson potential function. There is good agreement for the CN, C′N′ coupling in C2N2 but the calculated CN, CC coupling, although positive, is much lower than in two recent experimental force fields. The calculated FN, NN coupling in FN2 + is small and positive. The predicted geometry of FN2 + is r NF = 1·28 A, r NN = 1·105 A. The validity of the Anderson potential function is discussed.

The N4 molecule and the N3 + ion

Abstract: The possibility of the nitrogen molecule forming a stable dimer is investigated. Through ab initio calculations in an extended contracted-gaussian basis, it is demonstrated that N4 in tetrahedral geometry and N3 + in D 3h geometry are local minimum points on their respective internuclear potential energy surfaces. The N4 molecule is considerably higher in energy than two ground-state N2 molecules and some study of the barrier for N4 decomposition has been made. The bond lengths and force constants for the two systems are shown to be similar to their hydrocarbon counterparts, C4H4 and C3H3 +, tetrahedrane and cyclopropenyl cation. Other related systems are discussed as well.

Donor properties of the three carbonyl groups of chlorophyll a: ab initio calculations and 13C magnetic resonance studies.

Abstract: The relative donor properties of the three carbonyl groups of chlorophyll a have been studied theoretically by a series of ab initio molecular fragment, floating spherical Gaussian orbital, self-consistent field calculations on ethyl chlorophyllide a and experimentally through a 13C magnetic resonance study on chlorophyll a. The approximate ground state electronic wavefunction of ethyl chlorophyllide a was perturbed by monopole and dipole point charges whose signs, magnitudes, and positions were chosen to mimic the coulombic interactions associated with carbonyl coordination to Mg. Because the polarizability of the ring V keto carbonyl binding site is substantially greater than that for the ester carbonyl binding sites, the ring V keto binding site binds with smallest binding energy for weak perturbations and with largest binding energy for strong perturbations. A comparison of 13C magnetic resonance chemical shifts in chlorophyll a monomer and dimer provides new experimental evidence that the donor-acceptor interactions that bind the chlorophyll dimer together involve a substantial participation by the ring V keto carbonyl and minimal participation by the two ester carbonyl groups, and thus are in agreement with conclusions derived from the ab initio calculations.

Theoretical predictions of the molecular structure of sulphur tetrafluoride

Abstract: The equilibrium geometry of the SF4 molecule has been investigated by means of ab initio electronic structure theory. Self-consistent-field (SCF) calculations employing a minimum basis set (MBS) incorrectly predict SF4 to have a square- pyramidal (C4V) structure. The correct qualitative prediction of a C2V geometry is obtained with a basis of nearly double zeta quality. Comparable calculations on H2S and SF2 suggest that it may only be for hypervalent molecules containing sulphur that minimum basis sets are inappropriate. However, polarization functions seem to be necessary for an accurate prediction of S-F bond distances.

The use of two-centre expansions in the scattering of electrons by N2 molecules

Abstract: The use of a two-centre prolate spheroidal coordinate system in ab initio calculations of scattering of slow electrons by N2 molecules is studied. The speedier convergence of the resulting multipole expansion of the static potential near the nuclei, compared with that of its one-centre form, is demonstrated. Eigenphase sums calculated using two-centre coordinates also converge more rapidly.

Ab initio calculations including solvent effects of the structure of pyrazine, 4-nitropyridine and dicyanobenzenes ion pairs

Abstract: The potential surfaces of pyrazine, 4-nitropyridine, 1,2-, 1,3-, 1,4- dicyanobenzene radical anions have been calculated within theab initio and electrostatic molecular potential formalism, including the effect of the solvent. These potential surfaces suggest models of ion pairs with alkali cations, which are in agreement with those deduced from related ESR spectra.

Rotational isomerism and structure of penta-1,4-diene: Raman spectrum and ab initio calculations

Abstract: Several points on the potential energy surface of penta-1,4-diene as a function of the two torsional angles around the C—C single bonds have been computed using a SCF MO LCAO scheme with a basis of contracted gaussian functions. Three stable conformations are predicted for the molecule at very similar total energies and a fourth metastable form is found at a higher energy. The stable isomers belong to the point groups Cs, C2 and C1; the metastable conformation has C1 symmetry. Following a very simple mechanism suggested by the analysis of the theoretical results, the isomerization paths are also investigated, and estimates of the appropriate barrier heights obtained.The Raman spectra of the liquid and of the solid phase have been recorded and show the existence of three conformers. The results prove that the three isomers have Cs, C2 and C1 symmetry, and have nearly equal enthalpies.

Calculation of electric polarizabilities within the CNDO framework including polarization functions in the basis set

Abstract: A semi-empirical method within the CNDO framework is applied to the calculation of the electric polarizabilities for H2O, H2CO, NH3, CH3OH, C2H2, C2H4 and N2. The approach allows for the inclusion of different valence shell orbitals on the same atom and yields results which agree well with experiment. The main features of elaborate ab initio calculations are also reproduced. In most cases the polarization functions have been optimized and account well for the polarization of the molecule. It is shown that a judicious choice of these functions is intimately related with the environmental anisotropy of bonds and is indirectly connected with internal field effects.The polarization of the hydrogen atoms is an important aspect of molecular polarization, particularly when the symmetry of the molecule increases and the heavy atoms lie in a near-isotropic field of surrounding hydrogen atoms. This hydrogen polarization is well accounted for by the inclusion of 2p hydrogen orbitals obtained from a variational calculation on the hydrogen atom polarizability. In less symmetric molecules the addition of polarization functions to other atoms is necessary. In our work we used carefully chosen 3d functions and this is discussed in detail for H2O, C2H2, C2H4 and N2. The environmental anisotropy of the hydrogen atoms plays a role in the evaluation of polarizability anisotropies and is taken into consideration by the exclusion of one or two p orbitals centred on hydrogen atoms.Given the small number of basis functions we have used and the simplicity of the semi-empirical approach, the results are encouraging and may easily be extended to larger molecules. As the features of more elaborate ab initio calculations are reproduced rather well it is possible that such calculations would benefit from consideration of these results, in particular from the choice of orbitals and their exponents.

Low-lying electronic states of the ethynyl free radical

Abstract: Ab initio calculations on the low-lying electronic states of C2H have been performed. Its ground and first excited states are found to be X2Σ+ and A2Π respectively. Evidence is given in support of the assignment of the observed 10 000 A band system to the transition between these states.