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E R Davidson

Publications and source records attributed to E R Davidson.

8 recordsLinked to original sources
PubMed2001

Model studies of hydrogen atom addition and abstraction processes involving ortho-, meta-, and para-benzynes.

H-atom addition and abstraction processes involving ortho-, meta-, and para-benzyne have been investigated by multiconfigurational self-consistent field methods. The H(A) + H(B)...H(C) reaction (where r(BC) is adjusted to mimic the appropriate singlet-triplet energy gap) is shown to effectively model H-atom addition to benzyne. The doublet multiconfiguration wave functions are shown to mix the "singlet" and "triplet" valence bond structures of H(B)...H(C) along the reaction coordinate; however, the extent of mixing is dependent on the singlet-triplet energy gap (DeltaE(ST)) of the H(B)...H(C) diradical. Early in the reaction, the ground-state wave function is essentially the "singlet" VB function, yet it gains significant "triplet" VB character along the reaction coordinate that allows H(A)-H(B) bond formation. Conversely, the wave function of the first excited state is predominantly the "triplet" VB configuration early in the reaction coordinate, but gains "singlet" VB character when the H-atom is close to a radical center. As a result, the potential energy surface (PES) for H-atom addition to triplet H(B)...H(C) diradical is repulsive! The H3 model predicts, in agreement with the actual calculations on benzyne, that the singlet diradical electrons are not coupled strongly enough to give rise to an activation barrier associated with C-H bond formation. Moreover, this model predicts that the PES for H-atom addition to triplet benzyne will be characterized by a repulsive curve early in the reaction coordinate, followed by a potential avoided crossing with the (pi)1(sigma*)1 state of the phenyl radical. In contrast to H-atom addition, large activation barriers characterize the abstraction process in both the singlet ground state and first triplet state. In the ground state, this barrier results from the weakly avoided crossing of the dominant VB configurations in the ground-state singlet (S0) and first excited singlet (S1) because of the large energy gap between S0 and S1 early in the reaction coordinate. Because the S1 state is best described as the combination of the triplet X-H bond and the triplet H(B)...H(C) spin couplings, the activation barrier along the S0 abstraction PES will have much less dependence on the DeltaE(ST) of H(B)...H(C) than previously speculated. For similar reasons, the T1 potential surface is quite comparable to the S0 PES.

Journal Article↗
PubMed2001

A comparison of the influences of alkoxide and thiolate ligands on the electronic structure and reactivity of molybdenum(3+) and tungsten(3+) complexes. preparation and structures of M(2)(O(T)Bu)(2)(S(t)Bu)(4), [Mo(S(t)Bu)(3)(NO)](2), and W(S(t)Bu)(3)(NO)(py).

M(2)(O(t)Bu)(6) compounds (M = Mo, W) react in hydrocarbon solvents with an excess of (t)BuSH to give M(2)(O(t)Bu)(2)(S(t)Bu)(4), red, air- and temperature-sensitive compounds. (1)H NMR studies reveal the equilibrium M(2)(O(t)Bu)(6) + 4(t)BuSH <==> M(2)(O(t)Bu)(2)(S(t)Bu)(4) + 4(t)BuOH proceeds to the right slowly at 22 degrees C. The intermediates M(2)(O(t)Bu)(4)(S(t)Bu)(2), M(2)(O(t)Bu)(3)(S(t)Bu)(3), and M(2)(O(t)Bu)(5)(S(t)Bu) have been detected. The equilibrium constants show the M-O(t)Bu bonds to be enthalpically favored over the M-S(t)Bu bonds. In contrast to the M(2)(O(t)Bu)(6) compounds, M(2)(O(t)Bu)(2)(S(t)Bu)(4) compounds are inert with respect to the addition of CO, CO(2), ethyne, (t)BuC triple bond CH, MeC triple bond N, and PhC triple bond N. Addition of an excess of (t)BuSH to a hydrocarbon solution of W(2)(O(t)Bu)(6)(mu-CO) leads to the rapid expulsion of CO and subsequent formation of W(2)(O(t)Bu)(2)(S(t)Bu)(4). Addition of an excess of (t)BuSH to hydrocarbon solutions of [Mo(O(t)Bu)(3)(NO)](2) and W(O(t)Bu)(3)(NO)(py) gives the structurally related compounds [Mo(S(t)Bu)(3)(NO)](2) and W(S(t)Bu)(3)(NO)(py), with linear M-N-O moieties and five-coordinate metal atoms. The values of nu(NO) are higher in the related thiolate compounds than in their alkoxide counterparts. The bonding in the model compounds M(2)(EH)(6), M(2)(OH)(2)(EH)(4), (HE)(3)M triple bond CMe, and W(EH)(3)(NO)(NH(3)) and the fragments M(EH)(3), where M = Mo or W and E = O or S, has been examined by DFT B3LYP calculations employing various basis sets including polarization functions for O and S and two different core potentials, LANL2 and relativistic CEP. BLYP calculations were done with ZORA relativistic terms using ADF 2000. The calculations, irrespective of the method used, indicate that the M-O bonds are more ionic than the M-S bonds and that E ppi to M dpi bonding is more important for E = O. The latter raises the M-M pi orbital energies by ca. 1 eV for M(2)(OH)(6) relative to M(2)(SH)(6). For M(EH)(3) fragments, the metal d(xz)(),d(yz)() orbitals are destabilized by OH ppi bonding, and in W(EH)(3)(NO)(NH(3)) the O ppi to M dpi donation enhances W dpi to NO pi* back-bonding. Estimates of the bond strengths for the M triple bond M in M(2)(EH)(6) compounds and M triple bond C in (EH)(3)M triple bond CMe have been obtained. The stronger pi donation of the alkoxide ligands is proposed to enhance back-bonding to the pi* orbitals of alkynes and nitriles and facilitate their reductive cleavage, a reaction that is not observed for their thiolate counterpart.

Journal Article↗
PubMed2001

UDFT and MCSCF descriptions of the photochemical Bergman cyclization of enediynes.

Several singlet and triplet potential energy surfaces (PES) for the Bergman cyclization of cis-1,5-hexadiyne-3-ene (1a) have been computed by UDFT, CI, CASCI, CASSCF, and CASMP2 methods. It is found that the first six excited states of 1a can be qualitatively described as linear combinations of the configurations of weakly interacting ethylene and acetylene units. Although the symmetry relaxation from C2nu to C2 makes cyclization of the 13B state Woodward-Hoffmann allowed, it also increases the probability of competing cis-trans isomerization. Hydrogen atom abstraction is another plausible pathway because the terminal alkyne carbons possess a large radical character. In view of the competing processes, we conclude that the Bergman cyclization along the 13B path is unlikely despite its exothermicity (Delta = -42 kcal/mol). Calculations on cyclic analogues of 1a lead to similar conclusions. A less exothermic, but more plausible pathway for photochemical cyclization lies on the 2(1)A PES (Delta = -18 kcal/mol). Compared to the 1(1)A(1) and 1(3)B states, the 2(1)A state has less in-plane electron repulsion which may facilitate cyclization. The resulting p-benzyne intermediate has an unusual electronic structure combining singlet carbene and open-shell diradical features. Deactivation of the 2(1)A state of 1a is a competing pathway.

Journal Article↗
PubMed1991

Psychosocial adjustment among pediatric cancer patients: a multidimensional assessment.

Identified types and frequencies of psychological difficulties manifested by pediatric oncology patients and child-, family-, and illness-related correlates of adjustment. Parents of 48 children with cancer, 4 to 17 years of age, completed the Personality Inventory for Children (PIC). Analysis of mean PIC scores indicated that the children had a high frequency of somatic concerns and problems in academic functioning. Similar mean PIC profiles were obtained for children across gender, age, and diagnostic groups. Overall, 52% of the children had profiles with two or more clinically significant problem areas. Children's adjustment was associated with gender, social competence, and parental coping. Boys exhibited significantly more problems than did girls. Children whom teachers rated as less socially competent and whose parents reported few effective coping responses exhibited greater difficulties in adjustment.

Adaptation, Psychological↗
PubMed1980

Asymptotic behavior of atomic and molecular wave functions.

The asymptotic form of bound-state wave functions is derived by analytic continuation of asymptotic scattering-state wave functions. The result is also regorously derived by using an approach that is independent of scattering theory. One aspect of the result is that the N electron wave function becomes the lowest accessible exact wave function for the remaining N - 1 electrons when one electron is far away from all the nuclei. This shows that the recently developed extended Koopmans' procedures are in principle exact for the first ionization energy.

Journal Article↗
PubMed1976

Multicomponent analysis in clinical chemistry by use of rapid scanning fluorescence spectroscopy.

To be useful in the clinical laboratory, multicomponent fluorescence analysis requires both the rapid measurement of the fluorescence intensity at a variety of excitation and emission wavelengths and the unambiguous reduction of the data by efficient algorithms. The Video Fluorometer, which exploits the multi-channel capability of a low-light-level television sensor to simultaneously acquire excitation and emission spectra, can meet the first requirement. For example, a complete set of emission and excitation spectra for perylene can be obtained in less than 2 s at concentrations of 10(-10) mol/liter. To meet the second need, we present two types of data-reduction strategies: (a)a least-squares fit to the data, with use of the spectra of previously determined compounds likely to be present; and (b)a determination of the eigenvalues and eigenvectors fo the fluorescence matrix, from which the number of components and the possible spectra of each can be estimated. Computer simulations of the least-squares fitting algorithms show that five strongly overlapping components can be determined in the presence of noise with an accuracy of better than 5%. Also, a fluorescent sample containing two species with very similar but unknown spectral properties can be resolved to obtain the spectrum of each.

Anthracenes↗
PubMedDate not supplied

Dimolybdenum bis((S,S,S)-triisopropanolaminate(3-)): a blue compound with an unusual Mo-Mo triple bond.

Mo2(OtBu)6 and Mo2(NMe2)6 each react with (S,S,S)-triisopropanolamine (2 equiv) in benzene to yield dimolybdenum bis((S,S,S)-isopropanolaminate(3-)), Mo2[(OC-(S)-HMeCH2)3N]2 (M identical to M), as a blue crystalline solid. Cell parameters at -160 degrees C: a = 17.389(6) A, b = 10.843(3) A, c = 10.463(3) A, beta = 125.28(1) degrees, Z = 2 in space group C2. The molecular structure involves an Mo2 unit inside an O6N2 distorted cubic box. The Mo2 axis is disordered about three positions with occupancy factors of ca. 45%, 45%, and 10%. Despite this disorder, the molecular structure is shown to contain a central Mo identical to Mo unit of distance 2.15(3) A coordinated to two triolate ligands which each have two chelating arms and one that spans the Mo identical to Mo bond. The local Mo2O6N2 moiety has approximate C2h symmetry, and the Mo-N distances are long, 2.4 A. The 1H and 13C(1H) NMR spectra recorded in benzene-d6 are consistent with the geometry found in the solid-state structure. The blue color arises from weak absorptions, epsilon approximately 150 dm3 mol-1 cm-1, at 580 and 450 nm in the visible region of the electronic absorption spectrum. Raman spectra recorded in KCl reveal pronounced resonance effects with excitation wavelengths of 488.0, 514.5, and 568.2 nm, particularly for the 322 cm-1 band, which can probably be assigned to nu(Mo identical to Mo). The electronic structure of this compound is investigated by B3LYP DFT calculations, and a comparison is made with the more typical ethane-like (D3d) Mo2(OR)6 compounds is presented. The distortion imposed on the molecule by the triisopropanolaminate(3-) ligands removes the degeneracy of the M-M pi molecular orbitals. The HOMO and SHOMO are both M-M pi and M-O sigma* in character, while the LUMO is M-M pi* and the SLUMO is predominantly M-O sigma* with metal sp character. The calculated singlet-singlet transition energies are compared with those implicit in the observed electronic spectrum.

Journal Article↗