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Showing papers by "Ernest R. Davidson published in 2003"


Journal ArticleDOI
TL;DR: In this article, the authors explored several different methods that partition the 3D space of a molecule into its atomic components using Hermitian one-electron position-space projection operators, PA.
Abstract: Several different methods have been explored that partition the 3-D space of a molecule into its atomic components using Hermitian one-electron position-space projection operators, PA. Previously, we defined PA so that hard interatomic boundaries between atoms are observed. This idea has been extended to allow softer boundaries that have a region of overlap between atoms that can be controlled through an iterative process. Functions that determine the shape of the atomic volume are also discussed. The atomic electron populations of 17 different systems are presented as a function of these two factors. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 384–394, 2003

66 citations


Journal ArticleDOI
TL;DR: Clark and Davidson as discussed by the authors proposed a semi-empirational approach for computing expectation values in the Heisenberg spin model for polynuclear transition metal complexes using the intermediate neglect of differential overlap Hamiltonian parameterized for optical spectroscopy.
Abstract: The local spin formalism (Clark, A. E.; Davidson, E. R. J Chem Phys 2001, 115, 7382–7392) for computing expectation values 〈SA· SB〉 that appear in the Heisenberg spin model has been extended to semiempirical single determinant wave functions. An alternative derivation of expectation values in restricted and unrestricted cases is given that takes advantage of the zero differential overlap (ZDO) approximation. A formal connection between single determinant wave functions (which are not in general spin eigenfunctions) and the Heisenberg spin model was established by demonstrating that energies of single determinants that are eigenfunctions of the local spin operators with eigenvalues corresponding to high-spin radical centers are given by the same Heisenberg coupling constants {JAB} that describe the true spin states of the system. Unrestricted single determinant wave functions for transition metal complexes are good approximations of local spin eigenfunctions when the metal d orbitals are local in character and all unpaired electrons on each metal have the same spin (although spins on different metals might be reversed). Good approximations of the coupling constants can then be extracted from local spin expectation values 〈SA· SB〉 energies of the single determinant wave functions. Once the coupling constants are obtained, diagonalization of the Heisenberg spin Hamiltonian provides predictions of the energies and compositions of the spin states. A computational method is presented for obtaining coupling constants and spin-state energies in this way for polynuclear transition metal complexes using the intermediate neglect of differential overlap Hamiltonian parameterized for optical spectroscopy (INDO/S) in the ZINDO program. This method is referred to as ZILSH, derived from ZINDO, Davidson's local spin formalism, and the Heisenberg spin model. Coupling constants and spin ground states obtained for 10 iron complexes containing from 2 to 6 metals are found to agree well with experimental results in most cases. In the case of the complex [Fe6O3(OAc)9(OEt)2(bpy)2]+, a priori predictions of the coupling constants yield a ground-state spin of zero, in agreement with variable-temperature magnetization data, and corroborate spin alignments proposed earlier on the basis of structural considerations. This demonstrates the potential of the ZILSH method to aid in understanding magnetic interactions in polynuclear transition metal complexes. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

44 citations


Journal ArticleDOI
TL;DR: The isotope effect method reported should also prove useful for studying the mechanism of other sulfuryl group transfers, including sulfatase and sulfotransferase reactions, as well as sulfate hydrolyses under other conditions.
Abstract: In this communication, we report the first determination of 34S kinetic isotope effects (KIEs) for the hydrolysis of sulfate monoesters The method involves the conversion of the inorganic sulfate, acquired at partial extent of reaction, to SO2, followed by isotope ratio determination by mass spectrometry The KIEs determined for p-nitrophenyl sulfate and p-acetylphenyl sulfate are 10154 (±00002) and 10172 (±00003), respectively These results, together with previous peripheral 18O KIE values, are inconsistent with an associative mechanism The isotope effect method we report should also prove useful for studying the mechanism of other sulfuryl group transfers, including sulfatase and sulfotransferase reactions, as well as sulfate hydrolyses under other conditions

26 citations


Journal ArticleDOI
TL;DR: In this article, diagonal linear inequalities for three and four-body reduced density matrices were derived, which gave a different insight into the previous derivations of linear inequalities of the two-body reduction matrix.
Abstract: Diagonal linear inequalities are derived for three- and four-body reduced density matrices. These give a different insight into the previous derivations of linear inequalities for the two-body reduced density matrix. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

25 citations


Journal ArticleDOI
TL;DR: It is shown that the exact ground-state wave function for an arbitrary two-body Hamiltonian in second-quantized, finite basis set, form cannot generally be represented by a generalized coupled-cluster operator with real finite matrix elements acting on an arbitrary initial trial function.
Abstract: We show that the exact ground-state wave function for an arbitrary two-body Hamiltonian in second-quantized, finite basis set, form cannot generally be represented by a generalized coupled-cluster operator with real finite matrix elements acting on an arbitrary initial trial function.

25 citations


Journal ArticleDOI
TL;DR: It is believed that rehybridization of the ring carbons destabilizes the A radical orbital and can lead to large singlet-triplet splittings.
Abstract: In an effort to find a p-benzyne (1,4-didehydrobenzene) derivative with a triplet ground state, we have investigated tetrasubstitution by −F, −NH2, −CH3, and −NO2 groups. These were predicted to reduce the singlet−triplet gap, but none led to a triplet ground state because of unexpected destabilization of one of the radical orbitals. This effect is likely the result of rehybridization of the substituted C atom, which has been observed for substituted benzene and perturbs the side σ and σ* orbital energies of the phenyl ring. The role of substituent rotation on the energy difference between the two nominally singly occupied orbitals (S and A) was then investigated. The energy of the A radical orbital was found to be much more sensitive to perturbations within the σ C−C framework than the S MO. Consequently, we believe that rehybridization of the ring carbons destabilizes the A radical orbital and can lead to large singlet−triplet splittings. To test this hypothesis, calculations on a p-benzyne with 2,6 sub...

20 citations


Journal ArticleDOI
TL;DR: The vapor phase kinetics of S1 → S0 p-difluorobenzene (pDFB) fluorescence quenching by O2 has been characterized over an O2 pressure range spanning more than 4 orders of magnitude, ranging from the single-collision regime at less than one Torr to about 37000 Torr as discussed by the authors.
Abstract: The vapor phase kinetics of S1 → S0 p-difluorobenzene (pDFB) fluorescence quenching by O2 has been characterized over an O2 pressure range spanning more than 4 orders of magnitude, ranging from the single-collision regime at less than one Torr to about 37000 Torr. pDFB was pumped to an S1 level with evib = 3310 cm-1. Non Stern−Volmer kinetics is observed. The standard Stern−Volmer model, for which the ratio of fluorescence intensity without and with added oxygen against O2 pressure is linear with an intercept of unity, fits the data only for pressures 3000 Torr, the quenching again becomes linear but with a much lower slope and higher intercept. The quenching rate constants for the low- and high-pressure regimes are 1.3 × 1011 L mol-1 s-1 = 7.7 × 106 Torr-1 s-1 and 0.13 × 1011 L mol-1 s-1 = 0.78 × 106 Torr-1 s-1, respectively. Less detailed studies showed that quenching from S1 levels with evib = 3705 and 2887 cm-1 has kinetics similar to that of the 3310 cm-1 level. A proposed ...

6 citations


Journal ArticleDOI
TL;DR: Time-dependent density functional theory shows that the photoreactivities of copper and zinc metalloenediynes derive from multi-configurational excited states involving the enediyne and pyridine pi systems.

4 citations


Journal ArticleDOI
TL;DR: In this article, the relative energies of the metal-carbon π-bonding molecular orbitals are greatly influenced by the metal and the ancillary ligand X, and the effect of oxygen is to increase the relative energy of the M-C π bond by approximately 1 eV as a result of M-O π antibonding interactions.

3 citations


Journal ArticleDOI
TL;DR: In this paper, the role of substituent rotation on the energy difference between the two nominally singly occupied orbitals (S and A) was investigated and the energy of the A radical orbital was found to be much more sensitive to perturbations within the σ C−C framework than the S MO.
Abstract: In an effort to find a p-benzyne (1,4-didehydrobenzene) derivative with a triplet ground state, we have investigated tetrasubstitution by −F, −NH2, −CH3, and −NO2 groups. These were predicted to reduce the singlet−triplet gap, but none led to a triplet ground state because of unexpected destabilization of one of the radical orbitals. This effect is likely the result of rehybridization of the substituted C atom, which has been observed for substituted benzene and perturbs the side σ and σ* orbital energies of the phenyl ring. The role of substituent rotation on the energy difference between the two nominally singly occupied orbitals (S and A) was then investigated. The energy of the A radical orbital was found to be much more sensitive to perturbations within the σ C−C framework than the S MO. Consequently, we believe that rehybridization of the ring carbons destabilizes the A radical orbital and can lead to large singlet−triplet splittings. To test this hypothesis, calculations on a p-benzyne with 2,6 sub...

3 citations