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Confirmation Of Excited-State Proton Transfer And Ground-State Heterogeneity In Hypericin By Fluorescence Upconversion, D. S. English, K. Das, K. D. Ashby, J. Park, Jacob W. Petrich, E. W. Castner Jr. Dec 1997

Confirmation Of Excited-State Proton Transfer And Ground-State Heterogeneity In Hypericin By Fluorescence Upconversion, D. S. English, K. Das, K. D. Ashby, J. Park, Jacob W. Petrich, E. W. Castner Jr.

Chemistry Publications

Fluorescence upconversion measurements of hypericin and its methylated analog, O-hexamethoxyhypericin, which possesses no labile protons, confirm excited-state proton (or hydrogen atom) transfer as the primary photophysical event in hypericin. The presence of a rising component in the time-resolved fluorescence of hypericin and the absence of such a component for the hexamethoxy analog are consistent with our assignment of excited-state proton or atom transfer as the primary photophysical process in the light-activated antiviral compound, hypericin. The results using the fluorescence upconversion technique, which detects only emission from the excited state, are in good agreement with our previous transient absorbance measurements ...


Ring Opening Of Silacyclobutane, Mark S. Gordon, Thomas J. Barton, Haruyuki Nakano Dec 1997

Ring Opening Of Silacyclobutane, Mark S. Gordon, Thomas J. Barton, Haruyuki Nakano

Chemistry Publications

Multiconfigurational self-consistent field (MCSCF) wave functions, augmented by second order perturbation theory to partially recover the dynamic correlation, suggest that the most likely route from silacyclobutane to products ethylene + silene is initial cleavage of a ring CC bond to form a trans ·CH2SiH2CH2CH2· diradical, followed by rupture of the central SiC bond. This prediction is in agreement with the available experimental results. While this trans diradical is predicted to be a minimum on the MCSCF ground state potential energy surface, the transition state separating this species from products disappears when dynamic correlation is added. Therefore, the bottleneck on this part ...


Structure, Bonding, And Heats Of Formation Of Silatitanacyclobutanes, Vassiliki-Alexandra Glezakou, Mark S. Gordon Nov 1997

Structure, Bonding, And Heats Of Formation Of Silatitanacyclobutanes, Vassiliki-Alexandra Glezakou, Mark S. Gordon

Chemistry Publications

The MP2/TZVP geometries and the standard heats of formation at 0 and 298.15 K of 1,2- and 1,3-silatitanacyclobutanes and a number of smaller Ti- and/or Si-containing alkanes are calculated using the G2 model. The G2 procedure was suitably modified to allow for treatment of first-row transition elements and was directly applied to the reference compounds, which were subsequently connected to the two rings via the appropriate homodesmic reactions. The expected accuracy should be on the order of 3 kcal mol-1. Bonding and structural characteristics are discussed in terms of Boys localized orbitals and Bader density ...


The Reactions Al(2p) + H2 → Alh2(12a‘, 22a‘) → Alh2(X2a1) Or Alh(X1Σ+) + H:  Unusual Conical Intersections And Possible Nonadiabatic Recrossing, Galina Chaban, Mark S. Gordon, David R. Yarkony Oct 1997

The Reactions Al(2p) + H2 → Alh2(12a‘, 22a‘) → Alh2(X2a1) Or Alh(X1Σ+) + H:  Unusual Conical Intersections And Possible Nonadiabatic Recrossing, Galina Chaban, Mark S. Gordon, David R. Yarkony

Chemistry Publications

The energies and derivative couplings are computed in the vicinity of the 12A‘−22A‘ seam of conical intersections for AlH2. It is shown that the reaction path for the decomposition of excited vibrational levels of AlH2(X2A1) to Al + H2 passes quite close to the minimum energy crossing point (MECP), the minimum energy point on the seam of conical intersections. Near the MECP the seam of conical intersections exhibits an unusual trifurcation with a C2v portion dividing into a branch that preserves C2v symmetry and two symmetry equivalent branches that have only Cs ...


Theoretical Study Of Van Der Waals Complexes Of Al Atom With N2, Galina Chaban, Mark S. Gordon Aug 1997

Theoretical Study Of Van Der Waals Complexes Of Al Atom With N2, Galina Chaban, Mark S. Gordon

Chemistry Publications

Al–~N2) x Van der Waals complexes have recently been produced in molecular beam experiments and studied with threshold laser photoionization.1 The ionization potential measured for the Al–N2 complex (46 821 cm21 ), combined with Al atom ionization potential (48 279 cm21 ) and Al1–N2 dissociation energy measured by Bouchard and McMahon (19246175 cm21 ) 2 leads to the estimation of the Al–N2 complex binding energy at 4666175 cm21 ~Ref. 1!. The purpose of our study is to estimate the Al–N2 and Al1–N2 binding energies using theoretical calculations and to compare them with the experimental results.


Potential Energy Surfaces For Dissociation Reactions Of High-Energy Isomers Of N2o2, Galina Chaban, Mark S. Gordon, Kiet A. Nguyen Jun 1997

Potential Energy Surfaces For Dissociation Reactions Of High-Energy Isomers Of N2o2, Galina Chaban, Mark S. Gordon, Kiet A. Nguyen

Chemistry Publications

The kinetic stability with respect to dissociation to two NO molecules was studied for several high-energy isomers of N2O2 using multiconfigurational wave functions. All of these isomers are 50−80 kcal/mol higher in energy than 2NO. Three N2O2 isomers (a four-membered D2hisomer, a planar C2v isomer, and a bicyclic C2v isomer) are found to be kinetically stable:  the estimated barriers to dissociation are about 40 kcal/mol for the D2h isomer and about 20 kcal/mol for each of the other two isomers. Reaction paths for their dissociation were determined ...


The Effect Of Micelles On The Steady-State And Time-Resolved Fluorescence Of Indole, 1-Methylindole, And 3-Methylindole In Aqueous Media, K. D. Ashby, K. Das, Jacob W. Petrich May 1997

The Effect Of Micelles On The Steady-State And Time-Resolved Fluorescence Of Indole, 1-Methylindole, And 3-Methylindole In Aqueous Media, K. D. Ashby, K. Das, Jacob W. Petrich

Chemistry Publications

3-Methylindole (skatole) is a component of animal waste and is, consequently, a primary component in odor problems arising in livestock management, notably swine production. The ability to probe and to exploit the interactions of 3-methylindole with micelles has important implications for monitoring and controlling odor problems. The effect of a surfactant (Brij-35) on the fluorescence properties of indole, 1-methylindole, and 3-methylindole in aqueous solutions is reported. Steady-state fluorescence spectra reveal a blue shift in the emission as the surfactant concentration is increased, while the absorption spectra are practically unaffected. Time-resolved fluorescence measurements reveal shorter average lifetimes for 3-methylindole (3-MI) as ...


Deuterium Isotope Effect On The Excited-State Photophysics Of Hypocrellin:  Evidence For Proton Or Hydrogen Atom Transfe, K. Das, D. S. English, Jacob W. Petrich May 1997

Deuterium Isotope Effect On The Excited-State Photophysics Of Hypocrellin:  Evidence For Proton Or Hydrogen Atom Transfe, K. Das, D. S. English, Jacob W. Petrich

Chemistry Publications

A deuterium isotope effect of 1.4 is observed in the excited-state proton or atom transfer of hypocrellin A in methanol and methanol-O-d. This is the most direct and convincing evidence to date that excited-state intramolecular proton or atom transfer occurs in hypocrellin. It is consistent with previous proposals (e.g., Chem. Rev. 1996, 96, 523) that such excited-state intramolecular processes occur generally in the class of polycyclic quinones possessing light-induced antiviral activity, of which hypocrellin and the related compound hypericin are members. The transfer reaction in these compounds is discussed in terms of being in the adiabatic limit ...


An Ab Initio Study Of The Reaction Mechanism Of Co++Nh3, Tetsuya Taketsugu, Mark S. Gordon May 1997

An Ab Initio Study Of The Reaction Mechanism Of Co++Nh3, Tetsuya Taketsugu, Mark S. Gordon

Chemistry Publications

To investigate the mechanism for N–H bond activation by a transition metal, the reactions of Co+(3F,5F) with NH3 have been studied with complete active space self-consistent field (CASSCF), multireference configuration interaction (MR-SDCI), and multireference many body perturbation theory (MRMP) wave functions, using both effective core potential and all-electron methods. Upon their initial approach, the reactants yield an ion–molecule complex, CoNH+3(3E,5A2,5A1), with retention of C3ν symmetry. The Co+=NH3 binding energies are estimated to be 49 (triplet) and 45 (quintet) kcal/mol. Subsequently, the N–H bond is activated, leading to an intermediate ...


Hypericin, Hypocrellin, And Model Compounds:  Primary Photoprocesses Of Light-Induced Antiviral Agents, D. S. English, K. Das, J. M. Zenner, W. Zhang, George A. Kraus, R. C. Larock, Jacob W. Petrich May 1997

Hypericin, Hypocrellin, And Model Compounds:  Primary Photoprocesses Of Light-Induced Antiviral Agents, D. S. English, K. Das, J. M. Zenner, W. Zhang, George A. Kraus, R. C. Larock, Jacob W. Petrich

Chemistry Publications

The excited-state photophysics of the light-induced antiviral agents hypericin and hypocrellin are compared with those of the hexa- and tetramethoxy analogues of hypericin. The results are consistent with the interpretation of the primary photoprocess in hypericin and hypocrellin as that of excited-state intramolecular proton or atom transfer.


A Direct Route To Biologically Active Kainic Acid Analogs, Hiroshi Maeda, George A. Kraus Apr 1997

A Direct Route To Biologically Active Kainic Acid Analogs, Hiroshi Maeda, George A. Kraus

Chemistry Publications

The synthesis of kainic acid, acromelic acid, and related compounds such as domoic acid has been the subject of considerable investigation.1 The observation by Shirahama that 1a and 1b have potent neurophysiological activity has spawned intense synthetic attention toward C-4 aryl analogs.2 Control of the C-3/C-4 stereochemistry is important, since the isomer bearing the opposite stereochemistry exhibits little biological activity. Most of the synthetic routes to C-4 aryl analogs begin from 4-hydroxyproline.3 Most notable among these routes is a recent contribution by Baldwin and co-workers wherein the crucial C-3/C-4 stereochemistry was introduced by a hydroxyl-directed ...


Photophysics And Biological Applications Of 7-Azaindole And Its Analogs, Alexandre V. Smirnov, D. S. English, R. L. Rich, J. Lane, L. Teyton, A. W. Schwabacher, S. Luo, R. W. Thornburg, Jacob W. Petrich Apr 1997

Photophysics And Biological Applications Of 7-Azaindole And Its Analogs, Alexandre V. Smirnov, D. S. English, R. L. Rich, J. Lane, L. Teyton, A. W. Schwabacher, S. Luo, R. W. Thornburg, Jacob W. Petrich

Chemistry Publications

7-Azaindole is the chromophoric moiety of 7-azatryptophan, which is an alternative to tryptophan as an optical probe of protein structure and dynamics. The great power of the 7-azaindole chromophore is that it is red shifted both in absorption and emission from tryptophan, that its fluorescence decay is single exponential in water under appropriate conditions, and that its emission is sensitive to solvent. In addition, 7-azatryptophan can be incorporated into synthetic peptides and bacterial protein. In this article, the interactions of 7-azaindole with its environment are discussed. Special attention is directed to the difference in its fluorescence properties in water as ...


Excited-State Photophysics Of Hypericin And Its Hexamethoxy Analog:  Intramolecular Proton Transfer As A Nonradiative Process In Hypericin, D. S. English, W. Zhang, George A. Kraus, Jacob W. Petrich Apr 1997

Excited-State Photophysics Of Hypericin And Its Hexamethoxy Analog:  Intramolecular Proton Transfer As A Nonradiative Process In Hypericin, D. S. English, W. Zhang, George A. Kraus, Jacob W. Petrich

Chemistry Publications

The excited-state photophysics of the light induced antiviral agent, hypericin, are compared with those of its methylated analog, hexamethoxyhypericin. This comparison is instructive in understanding both the ground- and the excited-state properties of hypericin. That the hexamethoxy analog has no labile protons that can be transferred, that it cannot protonate its own carbonyl groups, that it has a reduced fluorescence quantum yield and lifetime with respect to hypericin, and that it exhibits no stimulated emission or, more specifically, rise time in stimulated emission completely support our emerging model of the hypericin photophysics. The results are consistent with the presence of ...


Solvent Dependence On The Intramolecular Excited-State Proton Or Hydrogen Atom Transfer In Hypocrellin, K. Das, D. S. English, Jacob W. Petrich Mar 1997

Solvent Dependence On The Intramolecular Excited-State Proton Or Hydrogen Atom Transfer In Hypocrellin, K. Das, D. S. English, Jacob W. Petrich

Chemistry Publications

Hypocrellin and its analog, hypericin (Figure 1), are naturally occurring quinones that have been used for centuries as folk medicines in the orient and the occident and that have attracted much interest because of their light-induced toxicity toward the human immunodeficiency virus (HIV).1The structural similarities of hypocrellin and hypericin would seem to suggest that hypocrellin exhibits excited-state and antiviral behavior similar to that of hypericin. Although they both execute excited-state proton or atom transfer2,3between the keto and enol oxygens (Figure 1), there are many important differences between them. Hypocrellin absolutely requires oxygen for antiviral activity ...


Structural Analysis Of The Fivefold Symmetric Surface Of The Al70pd21mn9 Quasicrystal By Low Energy Electron Diffraction, M. Gierer, M. A. Van Hove, A. I. Goldman, Z. Shen, S.-L. Chang, Cynthia J. Jenks, C.-M. Zhang, Patricia A. Thiel Jan 1997

Structural Analysis Of The Fivefold Symmetric Surface Of The Al70pd21mn9 Quasicrystal By Low Energy Electron Diffraction, M. Gierer, M. A. Van Hove, A. I. Goldman, Z. Shen, S.-L. Chang, Cynthia J. Jenks, C.-M. Zhang, Patricia A. Thiel

Chemistry Publications

The atomic structure of the fivefold symmetric quasicrystal surface of Al70Pd21Mn9 was investigated by means of a dynamical low energy electron-diffraction analysis. Approximations are developed to address the high structural complexity of quasicrystals, yielding average layer-by-layer structural and compositional information. A mix of several relaxed bulklike terminations is found, consistently favoring dense Al-rich outermost atomic layers. These results can be understood in terms of principles known to govern surface structures of simpler, crystalline metals.


A Bioctahedral Nbiv Cluster With Bridging Sulfides: [Nb2(Μ-S)2cl4(Thf)4], M. Yoon, V. Young, Gordon J. Miller Jan 1997

A Bioctahedral Nbiv Cluster With Bridging Sulfides: [Nb2(Μ-S)2cl4(Thf)4], M. Yoon, V. Young, Gordon J. Miller

Chemistry Publications

The title compound, tetrachloro-l~2Cl,2n2Cl-tetrakis - (tetrahydrofuran-O)- 1 n 20,2n 20-di-#-thioxo- 1:2naS-dinio - bium(IV)(Nb--Nb), [Nb2C14S2(CnHsO)4], was obtained by the reaction of NbC14(thO2 (thf is tetrahydrofuran) with S(SiMe3)2 (hexamethyldisilathiane) in tetrahydrofuran. The compound forms edge-sharing bioctahedral clusters with bridging sulfide ligands and terminal chloride and thf ligands. All thf ligands coordinate in the equatorial plane formed by the Nb--S--Nb---S cycle. Two independent half dimers are found in the asymmetric unit about independent inversion centers located at the midpoints of the Nb---Nb bonds. Distance ranges are: Nb---Nb 2.865(1)-2.869(1 ...


Alkoxido, Amido, And Imido Derivatives Of Titanium(Iv) Tetratolylporphyrin, Steven D. Gray, Joseph Lyndon Thorman, Lisa Mary Berreau, L. Keith Woo Jan 1997

Alkoxido, Amido, And Imido Derivatives Of Titanium(Iv) Tetratolylporphyrin, Steven D. Gray, Joseph Lyndon Thorman, Lisa Mary Berreau, L. Keith Woo

Chemistry Publications

Treatment of (TTP)TiCl2 (1) [TTP = meso-5,10,15,20-tetra-p-tolylporphyrinato dianion] with excess NaOR (R = Ph, Me, t-Bu) affords the bis(alkoxide) derivatives (TTP)Ti(OR)2 [R = Ph (2), Me (3), t-Bu (4)] in moderate yield. The corresponding amido derivative (TTP)Ti(NPh2)2(5) is prepared in an analogous fashion employing LiNPh2. The disubstituted complexes 2, 3, and 5 react cleanly with (TTP)TiCl2 to afford the ligand exchange products (TTP)Ti(OR)Cl [R = Ph (6), Me (7)] and (TTP)Ti(NPh2)Cl (8), respectively. The monosubstituted complexes 68are also ...


Theoretical Study Of The Interaction Of Fe+ With Silene, Jerzy Moc, Mark S. Gordon Jan 1997

Theoretical Study Of The Interaction Of Fe+ With Silene, Jerzy Moc, Mark S. Gordon

Chemistry Publications

The open H2Si−CH2Fe+ (6A‘)/cyclic (H2Si−CH2)Fe+ (4A‘‘) and the methyl structure FeHSi−CH3+ (6A‘‘, 4A‘‘) isomers are predicted here to be the most stable FeSiCH4+ isomers. These are therefore the most likely candidates for the two distinguishable species observed in the gas-phase mass-spectrometric experiments. This conclusion is strongly supported by the computed barriers (42-50 kcal/mol) separating the relevant pairs of the FeSiCH4+ isomers, the magnitude of which is consistent with the experimental estimate. The reaction between Fe+ in the ground 6D and excited 4F states and silaethylene have been studied using multiconfigurational (CASSCF) wave functions and ...


Molecular Structures For Azatitanatranes, Frank Joseph Rioux Iii, Michael Schmidt, Mark S. Gordon Jan 1997

Molecular Structures For Azatitanatranes, Frank Joseph Rioux Iii, Michael Schmidt, Mark S. Gordon

Chemistry Publications

Ab initio geometry optimizations for a series of azatitanatranes, ZTi[−NR(CH2)2−]3N, (Z = CH3, NH2, OH, F, and N(CH3)2; R = H and CH3), have been performed. An analysis of the transannular Ti−N interaction indicates that it is significantly stronger than that found in the analogous azasilatranes. Of special significance in this calculation is the fact that theory correctly predicts that the axial Ti−N bond is shorter than the equatorial Ti−N bonds.


Bacu10p4:  A New Structure Composed Of Chains Of Edge-Shared Cu4 Tetrahedra, Dianna M. Young, John Charlton, Marilyn M. Olstead, Susan M. Kauzlarich, Chi-Shen Lee, Gordon J. Miller Jan 1997

Bacu10p4:  A New Structure Composed Of Chains Of Edge-Shared Cu4 Tetrahedra, Dianna M. Young, John Charlton, Marilyn M. Olstead, Susan M. Kauzlarich, Chi-Shen Lee, Gordon J. Miller

Chemistry Publications

A new barium copper phosphide compound, BaCu10P4, was synthesized by reacting stoichiometric amounts of the elements at 1200 °C for 24 h. BaCu10P4 crystallizes in the monoclinic space group C2/m, with unit cell dimensions a = 23.288(4) b = 3.9070(10), and c = 9.534(2) Å and β = 92.26(2)° (Z = 4). The structure can be described as consisting of chains of edge-shared Cu4 tetrahedral prisms that are knitted together by P atoms. The structure is related to BaCu8P4, which can be described in a ...