Showing papers by "Ernest R. Davidson published in 1979"

A theoretical study on the potential surfaces of the lower electronic states of HCO

Abstract: The results of ab initio potential‐energy surface calculations are presented for the lowest four doublet states of HCO. A conical Jahn–Teller intersection was found between the lowest two 2A′ surfaces. The equilibrium geometries of the lowest two quartet states were also calculated. Discussion is primarily focused on the assignment of the hydrocarbon flame bands. The A bands of the hydrocarbon flame bands are assigned as a ?2A′→?2A′ transition which concurs with Dixon’s speculation. As for the ? state which is the origin of the B bands, the most probable state is the bent second 2A″ state which is described by the configuration (6a′)2(7a′)2(1a″)1.

Singlet-Triplet Energy Separations in Some Hydrocarbon Diradicals

Abstract: A diradical is literally a molecule with two unpaired electrons. We prefer the somewhat broader definition of a diradical as a molecule in which two electrons occupy a degenerate or nearly degenerate pair of orbitals. Group theory predicts a symmetry enforced degeneracy for a pair of orbitals, occupied by a total of two electrons, in the most symmetrical molecular geometry of methylene (CH2), cydobutadiene (CBD), and trimethylenemethane (TMM) (see Figure 1). Thus, we consider all three of these molecules to be diradicals. CH2 is the simplest molecule containing a divalent carbon atom. Although a very large number of other carbenes have been �!udied (1-3), in this review we confine our discussion to the parent hydro­ carbon. CBD yields to cyclopropenyl anion (4, 5) the position of being the smallest fully conjugated cyclic hydrocarbon containing 4n pi elec­ trons; but the former annulene has received more attention. Therefore, our discussion of molecules that are diradicals by virtue of being antiaromatic annulenes focuses on CBD. We have also chosen to discuss another four pi electron system, TMM, as representative of the class of diradicals that are fully conjugated hydrocarbons but for which no classical Kekule structures can be written (6). TMM is the simplest such molecule and, again, the best studied.

Ab initio theory of the polarizability and polarizability derivatives in H2S

Abstract: Ab initio finite-perturbation self-consistent-field (SCF) and configuration interaction (CI) calculations utilizing an extensive basis of cartesian gaussian orbitals are reported for the electric dipole polarizability tensor, α, and the polarizability derivative tensor, α′, of H2S. Particular attention was devoted to the selection of an appropriate molecular orbital basis and configuration expansion for determining electron correlation contributions to the polarizability. The common practice of truncating configuration expansions based on a perturbation theory correction to the energy is shown to be inadequate for this property. Contrary to the predictions of such an approach, we conclude that electron correlation has little effect on α and α′ in H2S. Our calculation yielded a nearly isotropic polarizability tensor, whose trace, α , is ca. 5% below the experimental result. The magnitude of the derivative ( α ′) along the symmetric stretch is also in good agreementwith experiment. It is positive and roughly an order of magnitude larger than the components along the other two modes. The derivative anisotropy parameter is found to be positive for all three normal modes. Theoretical dipole moment derivatives have also been evaluated and are compared to those determined from the infrared spectrum. Recent data from Rayleigh and Raman scattering experiments on H2O are discussed in light of our results for H2S.

The singlet and triplet state rotational potential surfaces for dihydroxycarbene

Abstract: Potential surfaces have been computed for rotation of the OH groups in C(OH)2 for both the lowest singlet and triplet state. The minima on the singlet surface occur at planar geometries, which represent maxima on the triplet surface. This result is interpreted in terms of different modes of pi electron donation from oxygen being favored in the two states. Evidence is presented that shows pi electron donation is chiefly responsible for making dihydroxycarbene a ground state singlet.

ESR matrix isolation investigation of the aluminum hydride radical cation−AlH+

Abstract: The AlH+ radical cation has been trapped in a neon matrix at 4 K and studied via electron spin resonance spectroscopy (ESR). The magnetic parameters are Al: A∥=1009(3), A⊥=877(3) and H: A∥=A⊥=283(3) MHz; g∥=2.002(1) and g⊥=2.000(1). The isotropic and dipolar hyperfine components were calculated from ab initio CI and SCF type wave functions for comparison with experimental results. AlH+ is the first paramagnetic molecular cation studied via rare gas matrix isolation ESR spectroscopy.

An L2 calculation of the 1s and 2s photoionization cross sections of Ne

Abstract: Discrete Gaussian orbitals are used to approximate the continuum for neon photoionization. Results within 2% of the exact numerical static exchange answer are obtained for both the 1s and 2s subshells by Stieltjes imaging. An alternative to the Tchebycheff method for obtaining the cross section from the moment distribution is given.

A modification of the langhoff imaging technique

Abstract: P. W. Langhoff1 has introduced Stieltjes and Tchebycheff imaging techniques for smoothing a discrete L2 approximation to the continuous photoionization cross section. These techniques are discussed at length in Langhoff’s accompanying presentation.