Showing papers by "Ernest R. Davidson published in 1988"
TL;DR: The potential energy surfaces of the three lowest electronic states of CH+4 are calculated in this article, where the calculated points are fit to the eigenvalues of a 3×3 matrix whose elements are a power series expansion in the normal mode displacements from the SCF tetrahedral geometry.
Abstract: The potential energy surfaces of the three lowest electronic states of CH+4 are calculated. CI calculations based on average natural orbitals from preliminary multireference singles and doubles CI calculations are found to provide a balanced description of the three states. The calculated points are fit to the eigenvalues of a 3×3 matrix whose elements are a power‐series expansion in the normal‐mode displacements from the SCF tetrahedral geometry. The equilibrium structures for the C2v, D2d, and C3v Jahn–Teller distortions are located and the vibrational frequencies for these structures are calculated. In addition, the transition state for the pseudorotation between equivalent C2v minima is located and the nature of the pseudorotation is discussed in detail.
95 citations
TL;DR: In this paper, the requirements for obtaining converged isotropic hyperfine constants, Aiso, in first row elements B-F are investigated with extended basis set CI wave functions composed of single and double excitations from a multiconfiguration reference space.
Abstract: The requirements for obtaining converged isotropic hyperfine constants, Aiso, in first row elements B–F are investigated with extended basis set CI wave functions composed of single and double excitations from a multiconfiguration reference space. Since the restricted Hartree–Fock unpaired spin density at the nucleus is zero for these elements, correlation effects account for the entire answer. Although the wave functions computed in the present study uniformly recovered 94%–98% of the empirical correlation energy, agreement with gas phase measurements of Aiso varied from a low of 55% on B to a high of 96% on N. Extension of these findings to the B2 (3Σg ) and H2 CO+ (2B2 ) molecules is also discussed. In order for small basis sets or very lightly correlated wave functions to produce good agreement with experiment, fortuitous cancellation of error between the negative core and positive valence contributions must occur. The present findings suggest relatively large basis sets and substantial correlation re...
87 citations
TL;DR: The quasidegenerate variational perturbation theory (QDVPT) as discussed by the authors is an explicitly multireference procedure and treats the entire reference space as a quasidiscrete generative space.
Abstract: In previous work on the treatment of correlation in molecular systems we have applied a multireference version of second‐order Hylleraas variational perturbation theory. The choice made for the partitioning of H treated the interactions between the correlating functions to infinite order and gave the corrections to the wave function to first order. The method was shown to be accurate in many cases, but became less so when near degeneracies occurred between the reference energy and other eigenvalues of H0. In this article we introduce an effective Hamiltonian method that is analogous to variational perturbation theory, but which is significantly more accurate when near degeneracies are important. This quasidegenerate variational perturbation theory (QDVPT) is an explicitly multireference procedure and treats the entire reference space as a quasidegenerate space. A novel method for solving the QDVPT equations is introduced that avoids explicit construction of the effective Hamiltonian. As a result, the work...
76 citations
TL;DR: In this paper, it was shown that the Xa-SW can generally predict the electronic structure and bonding of charged transition-metal complexes, and more computations of the type presented above must be performed and compared with experiment, only then would one know what molecular properties are best predicted by this technique.
Abstract: cuprate(I1) is found to have strong b,, (u-type) bonding between the copper and four ligating nitrogen atoms. The b2, and b3, out-of-plane (a-type) bonding is also strong not only for Cu-N bonds but between the biuretato ligand components as well. The in-plane a-type Cu-N bonding is found to be very weak. It manifests itself only through the interaction of the 3dX2-,,2(Cu), 4px(Cu), and 4pJCu) with the 2p,(N) and 2pJN) components of the alg, b3,, and b2\" molecular orbitals. In order to judge if the Xa-SW can generally predict the electronic structure and bonding of charged transition-metal complexes, more computations of the type presented above must be performed and compared with experiment. Only then would one know what molecular properties are best predicted by this technique. Acknowledgment. The author wishes to express his gratitude to Professor William C. Galley for his encouraging interest and valuable discussions. Financial assistance from La Direction General de L'enseignment Superieur du Quebec and the allocation of computer time by the Chemistry Department, University of Toronto, for some of the computations is acknowledged. Supplementary Material Available: Table of symmetry adapted one-electron orbitals for the bis(biuretato)cuprate(II) dianion and 10 equations for the molecular g tensor components in terms of molecular orbital coefficients (7 pages). Ordering information is given on any current masthead page. Results from ab initio calculations concerning several low-lying electronic states of trans,trans-1,3,5-hexatriene are presented and compared with experimental and previous theoretical results. The lowest excited singlet state is predicted to be the 'B, state, having essentially valencelike T-a* character. The nominally doubly excited 2]A, state is found to lie approximately 0.6-0.9 eV above the 1 'B, state. Results are also presented for several Rydberg states. The implications of the present results for current parametrizations of semiempirical T molecular orbital schemes are discussed.
53 citations
TL;DR: In this paper, the authors used ab initio CI calculations for three low-lying singlet states of all-trans-butadiene and -hexatriene and obtained good agreement with experimental 0-0 transition energies for the 1 1 B u, states.
51 citations
TL;DR: Hylleraas variational perturbation theory is applied through second order in energy to estimate the correlation energy in several molecular systems as discussed by the authors, which leads to equations nearly identical to the multireference linearized coupled cluster method of Laidig and Bartlett.
Abstract: Hylleraas variational perturbation theory is applied through second order in energy to estimate the correlation energy in several molecular systems. The specific choices for H0 and V which are made lead to equations nearly identical to the multireference linearized coupled‐cluster method of Laidig and Bartlett. The results obtained are in virtually exact agreement where comparisons have been made. Results from test calculations are presented for BeH2, CH2, and C2H4. In addition, the utility of perturbation theory for selecting correlating configurations is examined. This procedure is found to be quite accurate while significantly reducing the size of the system of linear equations to be solved.
50 citations
TL;DR: In this article, the binding energies and momentum distributions of the four valence orbitals of H 2 S have been measured by high-resolution electron momentum spectroscopy and compared on a quantitative basis with target Hartree-Fock approximation calculations using SCF wavefunctions.
47 citations
TL;DR: In this article, the C-H stretching frequencies of an F-center laser operating at 10 GHz (0.3 cm-I) were compared with other gas-phase measurements.
Abstract: Molecular beams of monomer HCN and its clusters, generated by supersonic helium expansions, were crossed with the output of an F-center laser operating in the C-H stretching region. The laser was scanned in its low-resolution mode, A v l j 2 = 10 GHz (0.3 cm-I), from 3100 to 3400 cm-I while the molecular beam energy was monitored with a liquid helium cooled bolometer. Excitation spectra of monomer HCN and vibrational photofragmentation spectra of HCN clusters were recorded. Bands associated with unresolved vibration-rotation envelopes were assigned to dimer and trimer transitions. Features corresponding to tetramers and higher polymers were observed but remain unassigned. Under some expansion conditions a photofragmenting solidlike hydrogen cyanide feature was observed. A band corresponding to one of the bound C-H stretching vibrations in the linear trimer was studied under high resolution (15 MHz). For the few comparisons possible, our C-H stretching frequencies are in agreement with other gas-phase measurements. Our results are also consistent with matrix isolation studies when allowances are made for the large solvent shifts possible. Three theoretical approaches were used to predict trends in the C-H stretching frequencies on hydrogen bonding. These are normal-coordinate analysis, a classical electrostatic model, and ab initio quantum mechanics calculations. A critique of these theoretical models is presented. Finally, the observed vibrational predissociation lifetime of the linear trimer is discussed in terms of simple propensity rules.
36 citations
TL;DR: The results of ab initio calculations that examine vertical and nonvertical transitions to several low-lying states of all-trans-octatetraene are presented in this paper.
Abstract: The results of ab initio calculations that examine vertical and nonvertical transitions to several low-lying states of all-trans-octatetraene are presented. It is found that the lowest vertical excitation is to a valence A-A* 'B, state, and the nominally doubly excited 2 ' 4 state occurs approximately 0.4 eV higher at the geometry of the ground state. Using estimated excited-state equilibrium geometries, we find that the 2 ' 4 state is indeed the lowest singlet excited state, having a (HI transition energy of 4.15 eV. The 0-0 transition energy for the 1 lB, state is calculated to be 4.56 eV. The present results are compared with past theoretical and experimental results.
31 citations
TL;DR: In this paper, the influence of α-carboxylate sur le mecanisme du depart de l'anion chlorure du chloroacetate was investigated.
Abstract: L'influence du groupe α-carboxylate sur le mecanisme du depart de l'anion chlorure du chloroacetate a ete etudiee en phase gazeuse
30 citations
TL;DR: In this article, the stabilization energy of a 3-substituted ethyl cation was shown to be related to the electronegativity of the substituents.
Abstract: In this study, we calculate the stabilization of /3-substituted ethyl cations (R = H, Li, BeH, BH,. CH,, NH,, OH, F, Na, MgH, A1H2, SiH,, PH2, SH, CI) and a-substituted methyl cations (R = H, Li, CH,, NH,, OH, F, Na, SiH,, PH,, SH, CI) in order to obtain a relationship between the nature of the substituent and the degree of stabilization of the cation. Results show that the stabilization energy is related to the electronegativity of the @ substituents, but not the CY substituents. The rotational barrier of the @-substituted ethyl cation is linearly related to the Mulliken population of the 2p(C+) orbital. We found that the stabilization energy is linearly related to the ionization potential of the aand PLA @-substituted radicals, and the HOMO energies of the PLA /3-substituted radical are linearly related to the corresponding ionization potentials. Trends in the stabilization by secondand third-row substituents are discussed. A wide variety of theoretical results have been presented which examine substituent effects in carbocation sy~tems.I-’~ Simple theoretical models have been studied as a means of interpreting and a-acceptors. For the case of carbocations the a-acceptor function is of less importance than for radicals or carbanions.I2 experimental results obtained for larger, more complex-carbocations. As a result, a relatively clear qualitative picture has emerged of the factors that can affect the stability of carbocations.‘, 99. 1291. (1) Hoffmann, R.; Radom, L.; Pople, J. A,; Schleyer, P. v. R.; Hehre, W. (2) Apeloig, Y.; Schleyer, P. v. R.; Pople, J . A. J . Am. Chem. SOC. 1977, J , ; Salem, L. J , Am. Chem, sot, 1972, 94, 6221, One way of assessing the stability of a carbocation is to use an isodesmic reaction such as eq 1, where SE+ (SE+ is the SE+ RCH2CH2’ + HCH2CH3 RCHZCH, + HCH2CH2’ (1) “stabilization” energy of the cation) is defined as AE.IZ (Here we use a @-substituted ethyl cation as an example; an analogous equation would be used for methyl cations.) When SE+ is greater than zero the cation RCH2CH2+ is more stable than the corresponding unsubstituted one, HCH2CH2+. It has been determined that two distinct types of substituents are capable of stabilizing the cation center. The first consists of substituents more electropositive than H (e.g., BeH, BH,, AIH,) that can act as cr-donors *Author to whom correspondence should be addressed. (3) Apeloig, Y.; Schleyer, P. v. R.; Pople, J. A. J . Am. Chem. SOC. 1977, (4) Pross, A.; Radom, L. Aust. J . Chem. 1980, 33, 241. ( 5 ) Radom, L.; Pople, J . A,; Schleyer, P. v. R. J . Am. Chem. SOC. 1972, ( 6 ) Creary, X.; Merhrsheikh-Mohammadi, M. E.; Eggers, M. D. 1. Am. (7 ) Pople, J . A.; Apeloig, Y.; Schleyer, P. v. R. Chem. Phys. Lett. 1982, (8) Hopkinson, A. C.; Lien, M. H. Can. J . Chem. 1985, 63, 3582. (9) Lien, M. H.; Hopkinson, A. C. J . Mol. Struct. (Theochem) 1985, 121, (10) Dorado, M.; M6,O.; YBRez, M. J . Am. Chem. SOC. 1980, 102,947. ( I 1 ) Radom, L.; Poppinger, D.; Haddon, R. C. Carbonium Ions; Olah, G . (12) Hehre, W. J . ; Radom, L.; Schleyer, P. v. R.; Pople, J . A. Ab Initio ( 1 3) Rdriquez, C. F.; Hopkinson, A. C. J . Mol. Struct. (Theochem) 1987, 99, 5901.
TL;DR: The 12C31P and 13C 31P diatomic radicals have been generated by the laser vaporization reaction between carbon and phosphorus which were pressed into a pellet to form the laser target as discussed by the authors.
Abstract: The 12C31P and 13C31P diatomic radicals have been generated by the laser vaporization reaction between carbon and phosphorus which were pressed into a pellet to form the laser target. This method is applicable to a wide range of nonmetallic systems for generating new reactive intermediates which cannot be done with more conventional approaches. The radicals were isolated in neon and argon matrices at 4 K for detailed ESR investigations. The magnetic parameters (MHz) for neon were: g∥=2.0009(3); g⊥=1.9902(3); A∥(31P)=145.0(3); A⊥(31P)=−269.0(2); A∥(13C)=580.0(3); and A⊥(13C)=422.0(3). The argon A and g tensors were virtually indistinguishable from these neon results. Extensive ab initio theoretical calculations were conducted for CP which yielded nuclear hyperfine A values in close agreement with the experimental results. Valence orbital spin populations extracted from the calculated CI wave functions are compared with those determined directly from the ESR hyperfine parameters for 13C and 31P. The observe...
TL;DR: In this paper, the electronic structure of hydridocolbalt tri and tetra-carbonyls was investigated by means of ab initio methods in two different Gaussian basis sets.
Abstract: The electronic structure of hydridocolbalt tri‐ and tetra‐carbonyls was investigated by means of ab initio methods in two different Gaussian basis sets. Hartree–Fock, configuration‐interaction, and MCSCF calculations were performed on a number of conformations and electronic states of both compounds. Pyramidal distortions of the symmetric C3v forms of HCo(CO)3 were investigated, and Jahn–Teller distortions of the triplet states were described at the MCSCF level of theory. The interdependence of correlation effects and molecular geometry was examined in both basis sets. It is concluded that dispersion forces play a large role in metal–ligand bonding and need to be included in the qualitative model.
TL;DR: In this article, high momentum resolution experimental momentum profiles (XMPs) of the valence orbitals of H2CO have been measured by electron momentum spectroscopy (EMS), and good quantitative agreement is obtained between the measured XMPs and the momentum distributions calculated from a near-Hartree-Fock wavefunction except for the outermost 2b2 orbital.
TL;DR: In this paper, the potential energy surfaces for the three states of interest were modeled using a functional form of the potential that is based on normal mode distortions from the tetrahedral configuration and it was shown that this form of potential, which includes quadratic terms in the normal mode displacements, is not able to correctly describe the region containing the van der Waals Cs minimum structure.
Abstract: The three states that correspond to the triply degenerate 2T2 state of SiH+4 at the tetrahedral configuration are studied using configuration‐interaction (CI) calculations. The potential‐energy surfaces for the three states of interest are modeled using a functional form of the potential that is based on normal‐mode distortions from the tetrahedral configuration. It is shown that this form of the potential, which includes quadratic terms in the normal‐mode displacements, is not able to correctly describe the region containing the van der Waals Cs minimum structure. However, using single‐root CI calculations, the ground and excited states of the Cs, C2v, and SiH+2 +H2 structures are then studied in detail.
TL;DR: In this paper, the binding energies and momentum distribution of the four valence orbitals of CO have been investigated by high-resolution electron momentum spectroscopy and compared on a quantitative basis to theoretical calculations using SCF wavefunctions ranging in quality from minimal basis to a recently published 136-GTO wavefunction which is essentially at the Hartree-Fock limit.
01 Jan 1988
TL;DR: Potential energy surfaces (PES) have been used to explain the equilibrium structures and spectra of molecules, as well as chemical reactions between molecules as discussed by the authors, which has led to increased interest in detailed studies of polyatomic systems.
Abstract: Potential-energy surfaces (PES) are used to explain the equilibrium structures and spectra of molecules, as well as chemical reactions between molecules. Improvements in both experimental and theoretical techniques have led to increased interest in detailed studies of polyatomic systems. This in turn has led to a need for understanding the potential-energy surfaces of polyatomic systems.
TL;DR: In this article, the molecular geometry and electronic structure of formylcobalt tricarbonyl, (eta-ethylene)hydridocobalt tricarbonyls, and (ethyl cobalt) tricarbinearls were investigated by means of ab initio Hartree-Fock and configuration-interaction calculations in a Gaussian basis set.
Abstract: The molecular geometry and electronic structure of formylcobalt tricarbonyl, (eta-ethylene)hydridocobalt tricarbonyl, and ethylcobalt tricarbonyl were investigated by means of ab initio Hartree-Fock and configuration-interaction calculations in a Gaussian basis set. Several stable conformations of each of the complexes were found, and the geometry, frontier orbitals, and the character of chemical bonds of these conformations were examined. Two intramolecular hydrogen migration reactions were investigated, and their energy barriers were found to depend considerably on the choice of the initial and terminal sites of the migrating hydrogen. The effect of electron correlation upon the molecular geometry was examined in some detail in the case of the eta-ethylene complex.