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We present density functional theory calculations of the vibrational and electronic properties of the misfit-layer compound (MLC) LaS–CrS2 and its isolated sublayers to identify the vibrational mod...

The C–H functionalization reaction of N-heterocycles with an unprotected N–H group is one of the most step-economic strategies to introduce functional groups without the need for installation and r...

Terpolymerizations of cyclohexene oxide (CHO), CO2, and the Michael-type monomer 2-vinylpyridine (2VP) are presented. The combination of two distinct polymerization mechanisms was enabled by the sy...

Control of stereoregularity is an integral part of a precision polymerization method and for the development of functional materials. Yttrium- and aluminum-based catalysts are known for converting ...

On the basis of experimentally reported series of novel imide-functionalized ladder-type heteroarenes (fused bithiophene imide oligomers BTIn, n = 1–5), expanded BTIn up to 24 rings with 8 imide gr...

Recently, an n → π* weak interaction has been found to be ubiquitous and significant in proteins, functional materials, medicines, and biological processes. To reveal the mechanism underlying the n...

Inorganic/organic interfaces show two phenomenologically different types of charge transfer: On inert substrates, charge is localized, leading to a coexistence of neutral and charged molecules. Conversely, on metals, which have more available charge carriers and a larger propensity to hybridize, the charge is homogeneously delocalized. In this contribution, we use hybrid density functional theory to study the adsorption of the strong electron acceptor F4TCNQ on ZnO(10-10) as a function of the substrate's doping concentration. This system undergoes a joint charge donation/backdonation reaction. Because only the former is driven by hybridization, this allows us to study the impact of hybridization and the availability of charge carriers separately. We find that here both charge-transfer types are simultaneously at work. Whereas hybridization determines the charge localization, the charge-carrier concentration determines the amount of transferred charge. Consequently, at low doping concentrations, most of the electron acceptors become slightly positively, rather than negatively, charged.

Here we report the room-temperature group transfer polymerization of conjugated polar alkenes, including linear methyl methacrylate (MMA) as well as biorenewable, cyclic γ-methyl-α-methylene-γ-buty...

Many schemes for decomposing quantum-chemical calculations of intermolecular interaction energies into physically meaningful components can be found in the literature, but the definition of the charge-transfer (CT) contribution has proven particularly vexing to define in a satisfactory way and typically depends strongly on the choice of basis set. This is problematic, especially in cases of dative bonding and for open-shell complexes involving cation radicals, for which one might expect significant CT. Here, we analyze CT interactions predicted by several popular energy decomposition analyses and ultimately recommend the definition afforded by constrained density functional theory (cDFT), as it is scarcely dependent on basis set and provides results that are in accord with chemical intuition in simple cases, and in quantitative agreement with experimental estimates of the CT energy, where available. For open-shell complexes, the cDFT approach affords CT energies that are in line with trends expected based on ionization potentials and electron affinities whereas some other definitions afford unreasonably large CT energies in large-gap systems, which are sometimes artificially offset by underestimation of van der Waals interactions by density functional theory. Our recommended energy decomposition analysis is a composite approach, in which cDFT is used to define the CT component of the interaction energy and symmetry-adapted perturbation theory (SAPT) defines the electrostatic, polarization, Pauli repulsion, and van der Waals contributions. SAPT/cDFT provides a stable and physically motivated energy decomposition that, when combined with a new implementation of open-shell SAPT, can be applied to supramolecular complexes involving molecules, ions, and/or radicals.

A practical method for the synthesis of 3-deoxy-L-ketohexoses is described. Both D- and L-ketohexoses can be transformed into rare 3-deoxy-L-ketohexoses in six steps through a group transfer process. The key step involves a radical cyclized onto a carbonyl group, followed by a fragmentation reaction, eventually resulting in the group transfer of an α-oxy carbonyl group. The process involves tin-free and environmentally benign radical conditions (TTMSS/AIBN/toluene). The acyclic form of 3-deoxy-L-fructose was prepared in only three steps from the inexpensive starting material, D-fructose. A further modification by preparing a dithioacetal derivative was accomplished, which could serve as a convenient sugar synthon for further synthetic applications. Removal of the dithioacetal protecting group results in the formation of the rare 3-deoxy-L-fructose in a total yield of 42%. This methodology could be further extended to the synthesis of other deoxy-L-ketohesoses, such as 3-deoxy-L-sorbose.

We have investigated the second hyperpolarizabilities (γ), that is, the third-order nonlinear optical (NLO) properties at the molecular scale, of the realistic Si- and Ge-disubstituted para- and meta-quinodimethanes from the viewpoint of synergy effect of the open-shell singlet nature and the donor (D)−π–donor (D) intramolecular charge transfer (ICT). It has been revealed that the disubstituted para isomers exhibit strong D−π–D nature together with the intermediate open-shell singlet nature, which leads to their significantly enhanced γ values. These results well demonstrate the validity of our recent result of the theoretical model study on para-quinodimethane with point charges, and also present a new design strategy based on the concept of open-shell NLO that the replacement of the radical site of the π-conjugated carbon framework with the heavier main group elements induces both the larger open-shell singlet nature and the D−π–D type strong ICT, both of which synergetically contribute to the further e...

This letter presents a method for the parametrization of semiempirical models for proton transfer reactions in water clusters. Two new models are developed: AM1-W, which is a reparameterization of the classic AM1 model, and AM1PG-W, which is a modified AM1-like model including a pairwise correction to the core repulsion function. Both models show good performance on hydrogen-bonding energies and on proton transfer energy profiles, which are of great importance for proton transfer reactions in large water clusters and in proteins. The parametrization method introduced is general and can be used to develop any other system-specific semiempirical models.

Using hybrid density functional theory, we investigate structural and electronic properties of small Cu-n clusters (with n = 7. For n = 5, both neutral planar and positively charged polyhedral conf ...

We investigated the oscillations found in the integral cross section of the title reaction, which are particularly evident for Ne + H2(+)(v0 = 2,j0 = 1) [essentially isoenergetic with NeH(+)(v' = 0,j' = 0) + H] at low collision energy (Ecol < 0.30 eV). We employed mainly an exact time-independent (TI) quantum dynamics method and used the best potential energy surface available. From analysis of TI initial state selected to all integral cross sections, state-to-state integral cross sections, and the corresponding differential cross sections (DCSs), we showed that the oscillations correspond to resonances. They arise from the influence of the global [Ne-H-H](+) (collinear) minimum on dynamics and probably correspond to Feshbach resonances. Besides, the forward-backward peaking DCS (which oscillates with Ecol) behavior observed could be a signature for this type of resonances. Finally, as most data on resonances in bimolecular reactions correspond to neutral systems, we hope that the present results will encourage experimentalists to re-examine this benchmark system.

The charge-transfer properties of three diazapentacene derivatives, including 5,7,12,14-tetrachloro-6,13-diazapentacene (TCDAP), 5,7,12,14-tetrachloro-6,13-diaza-6,13-dihydropentacene (TCDAHP), and 5,7,12,14-tetrafluoro-6,13-diazapentacene (TFDAP), have been studied using density functional theory. The performance of five pure GGA and seven hybrid GGA functionals and G3MP2B3 method on the reorganization energy (λ) and mobility (μ) predictions of TCDAP has been examined. Both the B3LYP functional and the G3MP2B3 method give reliable predictions for the λ value. Using the reorganization energy at the G3MP2B3 level together with the transfer integral by BHandH, BHandHLYP, and M06-2X functionals yields electron mobilities of 3.44, 3.32, and 3.29 cm2 V–1 S–1 for TCDAP, respectively, which come fortuitously close to the experimental value of 3.39 cm2 V–1 S–1. Other density functionals also give mobility predictions in agreement with the experimental value to a factor of ∼2. The TCDAHP, a −NH derivative of TCDAP...

Novel N,O-chelated naphthyridine-BF(2) complexes with push-pull structures have been synthesized and characterized. Spectral investigations on these complexes reveal that photoinduced intramolecular charge transfer occurs and results in a large Stokes shift, which is further supported by density functional theory based theoretical calculations.

A significant number of different exchange correlation functionals, ranging from generalized gradient approximations to double hybrids, has been tested on a difficult playground represented by proton transfer reactions. In order to have a complete picture of their performances, both energetics and structural features have been compared and the obtained ranking compared with those issued from the standard test for kinetics (i.e., the DBH24/08 set). Among all of the functionals, the B97X, BMK, B1LYP, and PBE0-DH approaches are those providing a good error balance on all four trials. Beyond these figures, the obtained results allow for some general considerations, such as those on the role of Hartree-Fock exchange in reaction barriers or the relation between structure and energetics. © 2012 American Chemical Society.

Weak, medium, and strong charge-transfer (CT) complexes containing various electron donors (C(2)H(4), C(2)H(2), NH(3), NMe(3), HCN, H(2)O) and acceptors (F(2), Cl(2), BH(3), SO(2)) were investigated at the CCSD(T)/complete basis set (CBS) limit. The nature of the stabilization for these CT complexes was evaluated on the basis of perturbative NBO calculations and DFT-SAPT/CBS calculations. The structure of all of the complexes was determined by the counterpoise-corrected gradient optimization performed at the MP2/cc-pVTZ level, and most of complexes possess a linear-like contact structure. The total stabilization energies lie between 1 and 55 kcal/mol and the strongest complexes contain BH(3) as an electron acceptor. When ordering the electron donors and electron acceptors on the basis of these energies, we obtain the same order as that based on the perturbative E2 charge-transfer energies, which provides evidence that the charge-transfer term is the dominant energy contribution. The CCSD(T) correction term, defined as the difference between the CCSD(T) and MP2 interaction energies, is mostly small, which allows the investigation of the CT complexes of this type at the "cheap" MP2/CBS level. In the case of weak and medium CT complexes (with stabilization energy smaller than about 15 kcal/mol), the dominant stabilization originates in the electrostatic term; the dispersion as well as induction and δ(HF) terms covering the CT energy contribution are, however, important as well. For strong CT complexes, induction energy is the second (after electrostatic) most important energy term. The role of the induction and δ(HF) terms is unique and characteristic for CT complexes. For all CT complexes, the CCSD(T)/CBS and DFT-SAPT/CBS stabilization energies are comparable, and surprisingly, it is true even for very strong CT complexes with stabilization energy close to 50 kcal/mol characteristic by substantial charge transfer (more than 0.3 e). It is thus possible to conclude that perturbative DFT-SAPT analysis is robust enough to be applied even for dative-like complexes with substantial charge transfer.

We model the triplet-triplet energy-transfer experiments from the Closs group [ Closs , G. L. ; et al. J. Am. Chem. Soc. 1988 , 110 , 2652. ] using a combination of Marcus theory and either Boys or Edmiston-Ruedenberg localized diabatization, and we show that relative and absolute rates of electronic excitation transfer may be computed successfully. For the case where both the donor and acceptor occupy equatorial positions on a rigid cyclohexane bridge, we find beta(calc) = 2.8 per C-C bond, compared with the experimental value beta(exp) = 2.6. This work highlights the power of using localized diabatization methods as a tool for modeling nonequilibrium processes.

Recent observations on VV transfer in H(2) have shown interesting results. For nonresonant processes, comparison of the experimental rate constants with the results of previous semiclassical calculations, quantum oscillators/classical rotors coupled via classical collisions, showed the theoretical rate constants to be too slow by a factor of 3 or more. The semiclassical rate constant of the resonant VV process (v = 1 + v = 0 --v = 0 + v = 1) was also found to be too slow, by more than an order of magnitude, compared with the experimental rate. Further, the semiclassical model predicted the value of k(1,1--0,2) to exceed that of k(1,0--0,1), but the experimental data shows it to be a factor of approximately 2 less. In this work we employ an accurate interaction potential for the H(2)-H(2) system, and treat both rotation and vibration of the diatoms as coupled quantum-mechanical degrees of freedom. These new calculated results are in better overall agreement with the near-resonant experimental values, but the calculated rate constants are a factor of 2 to 3 larger than experiment for the nonresonant processes.

The Poisson-Boltzmann (PB) equation is widely used for modeling solvation effects. The computational cost of PB has restricted its applications largely to single-conformation calculations. The generalized Born (GB) model provides an approximation at substantially reduced cost. Currently the best GB methods reproduce PB results for electrostatic solvation energies with errors at ~5 kcal/mol. When two proteins form a complex, the net electrostatic contributions to the binding free energy are typically of the order of 5 to 10 kcal/mol. Similarly, the net contributions of individual residues to protein folding free energy are < 5 kcal/mol. Clearly in these applications the accuracy of current GB methods is insufficient. Here we present a simple scaling scheme that allows our GB method, GBr6, to reproduce PB results for binding, folding, and transfer free energies with high accuracy. From an ensemble of conformations sampled from molecular dynamics simulations, five were judiciously selected for PB calculations. These PB results were used for scaling GBr6. Tests on the binding free energies of the barnase-barstar, GTPase-WASp, and U1A-U1hpII complexes and on the folding free energy of FKBP show that the effects of point mutations calculated by scaled GBr6 are accurate to within 0.3 kcal/mol of PB results. Similar accuracy was also achieved for the free energies of transfer for ribonuclease Sa and insulin from the crystalline phase to the solution phase at various pH’s. This method makes it possible to thoroughly sample the transient-complex ensemble in predicting protein binding rate constants and to incorporate conformational sampling in electrostatic modeling (such as done in the MM-GBSA approach) without loss of accuracy.

We address the problem of charge transfer through DNA-gold junctions from a theoretical and numerical perspective. The geometry and the electronic structure of the DNA fragment is described on an atomistic level making use of an extended Su-Schrieffer-Heeger Hamiltonian. The emerging potential energy surfaces exhibit the characteristics of small polaron formation and can be analyzed to obtain the energy parameters relevant to Marcus' theory of charge transfer and the corresponding interbase hopping rates. At stationarity, the resulting master equations lead to a maximum current of 5 nA per A-DNA double strand upon the application of a potential of +/- 2 V, a value comparable to recent experimental findings. In addition, the overall shape of the I-V curves and their pronounced dependence upon the DNA sequence is reproduced.

The group transfer polymerization (GTP) of acrylates with early d-block and f-block metallocenium ester enolates suffers from side reactions, which cause the process not to be living, with a lack o...

In line with the charge transfer (DeltaNmax = -mu/eta) proposed by Parr et al. (Parr, R. G.; Szentpály, L. V.; Liu, S. J. Am. Chem. Soc. 1999, 121, 1922), we propose an electrophilicity-based charge transfer (ECT) descriptor in this paper and validate it through the interaction between a series of chlorophenols and DNA bases. Application of ECT can be extended to the interaction of any toxin with the biosystem.

The main features of double group transfer reactions have been studied under the density-functional theory framework. It is found that a wide range of structure-types and processes including type II-dyotropic reactions and the Meerwein-Ponndorf-Verley reduction take place via highly synchronous in-plane aromatic transition structures. Actually, the orbital topology of these saddle points is equivalent to that which corresponds to a D2h-symmetric aromatic molecule such as pyrazine.

In this work, we address the phenomenon of charge transport in DNA using a simple, but chemically specific, approach that is intimately related to the Su-Schrieffer-Heeger (SSH) model. The emerging potential energy surface for hole transport is analyzed using Marcus' theory of charge transfer. Our results are fully compatible with the conjecture of charge transfer in DNA via two competing mechanisms, and the computations provide the corresponding charge-transfer rates both in the short-range superexchange and in the long-range hopping regime as the output of a single atomistic theory. Finally, the model allows the computation of the transport properties of systems containing modified bases and of more complex arrangements of base pairs as an additional element of verification.

We have carried out a combined density functional theory and molecular mechanics study of aryl group transfer reactions from [MeB(C6F5)3]- to the metal center of L2MMe+ (M = Ti, Zr). This reaction,...

Hartree−Fock and density functional theory calculations are used to study the 1,3-intramolecular proton-transfer reactions in HXNY → XNYH (X,Y = O,S). Energy and a number of reactivity descriptors such as chemical potential, hardness, and electrophilicity index have been studied along the proton-transfer reaction path. It has been found that the profiles of most properties of the 1,3-reordering HXNY → XNYH can be described as a combination of the corresponding profiles of symmetric reactions in which X = Y. A good linear correlation between potential energy and the electrophilicity index in the HONO → ONOH reaction has been found, indicating that any change of these properties during the reaction are well represented in terms of changes in both the electronic chemical potential and molecular hardness.

High-level calculations (G2+) have been used to characterize the gas phase potential energy surfaces of the nonidentity proton transfer reactions of a series of simple anions (H - ,C H 3 - ,N H 2 - ,O H - ,F - , SiH3 - ,P H 2 - , SH - ,C l - , HCC - , and NC - ) with their conjugate acids. Only the combinations that give reactions that are exothermic by less than 20 kcal/mol were considered. The surfaces for the nonidentity reactions appear to be hybridizations of those of the corresponding identity reactions, and a simple averaging approach provides a reasonable representation of the nonidentity surface. Measures based on the geometry or electron density indicate that the position of the transition state (“early” or “late”) is better correlated with the properties of the proton donor/acceptor than the exothermicity of the proton transfer process in these systems.

It was found that 2,6-dithiopurine isolated in low-temperature argon and nitrogen matrixes adopts only one tautomeric form: dithione-N7H. The UV (λ > 345 nm) induced reactions converting this tautomer into dithiol-N3H and dithiol-N7H isomers were observed in Ar and N2 matrixes. The first of these reactions represents a novel type of photoinduced double-proton transfer, whereas the second is analogous to that previously observed for 2,4-dithiouracil (Lapinski, L.; Nowak, M. J.; Kolos, R.; Kwiatkowski, J. S.; Leszczynski, J. Spectrochim. Acta A 1998, 54, 685). The assignment of the form initially present in the matrix to the dithione-N7H isomer and of the photoproducts to the dithiol-N3H and dithiol-N7H isomers was based on the good agreement between the experimental IR matrix spectra and the spectra theoretically simulated at DFT(B3LYP)/6-31G(d,p) level.

Valence bond SCF calculations have been carried out on identity proton transfers of the type X−H + X- → X- + H−X and X−H+ + X → X + X−H+ for systems where X = F-, Cl-, Br-, OH-, SH-, NH2-, PH2-, CH3-, SiH3-, OH2, NH3, and PH3. A hybrid consisting of three contributing structuresreactantlike, productlike, and ionic (X- H+ X- or X H+ X)gives reasonable results, though additional structures afford a slight further decrease in energy. Energies somewhat below Hartree−Fock and reasonable weights are obtained. Calculated barriers agree well with those obtained by high-level ab initio calculations with correlation. Insights into the factors determining barrier height are obtained.

It has been shown that the rate of symmetrical hydride transfer reaction varies with the hydride affinity of the (identical) donor and acceptor. In that case, Marcus theory of atom and group transfer predicts that the Brønsted alpha depends on the location of the substituent, whether it is in the donor or the acceptor, and the tightness of the critical configuration, as well as the resemblance of the critical configuration to reactants or products. This prediction has now been confirmed for hydride transfer reactions between heterocyclic, nitrogen-containing cations, which can be regarded as analogues of the enzyme cofactor, nicotinamide adenine dinucleotide (NAD+). A series of reactions with substituents in the donor gives Brønsted alpha of 0.67 +/- 0.03 and a tightness parameter, tau, of 0.64 +/- 0.06. With substituents in the acceptor alpha = 0.32 +/- 0.03 and tau = 0.68 +/- 0.08. The reactions are all spontaneous, with equilibrium constants between 0.4 and 3 x 10(4), and the two sets span about the same range of equilibrium constants. The two tau values are essentially identical with an average value of 0.66 +/- 0.05. These results can be semiquantitatively mimicked by rate constants calculated for a linear, triatomic model of the reaction. Variational transition state theory and a physically motivated but empirically calibrated potential function were used. The computed rate constants generate an alpha value of 0.56 if the hydride affinity of the acceptor is varied and an alpha of 0.44 if the hydride affinity of the donor is varied. The calculated kinetic isotope effects are similar to the measured values. A previous error in the Born charging term of the potential function has been corrected. Marcus theory can be successfully fitted to both the experimental and computed rate constants, and appears to be the most compact way to express and compare them. The success of the linear triatomic model in qualitatively reproducing these results encourages the continued use of this easily conceptualized model to think about group, ion, and atom transfer reactions.

The study of ab initio potential energy curves along the proton-transfer coordinate in XH-NH 3 (X = F, Cl, Br) hydrogen-bonded complexes is reported. Equilibrium geometries, harmonic vibrational frequencies, one-dimensional energy profiles, and anharmonic proton stretching frequencies have been calculated at B3-LYP/ 6-311++G(d,p), MP2/6-311++G(d,p), MP2/6-311++G(2df,2pd), and CCD/6-311++G(2df,2pd) levels of theory. The results have been compared to more accurate CCSD(T)/6-311++G(2df,2pd) values. It has been found that the structures and most of the harmonic vibrational frequencies of the complexes agree well, but the proton stretching frequencies differ significantly. These differences are enhanced when anharmonicity along the proton transfer coordinate is taken into account, and even small inaccuracies in the PES cause large errors in the computed frequencies. These results show that the proper treatment of electron correlation is extremely important for the correct description of anharmonic energy profiles in hydrogen-bonded complexes.

A theory for proton-coupled electron-transfer (PCET) reactions (Cukier, R. I.; Nocera, D. Ann. Rev. Phys. Chem. 1998, 49, 337) is generalized to also apply to hydrogen-atom transfer (HAT) reactions...

The gas-phase identity methyl transfer reactions, X- + CH3X ⇌ XCH3 + X-, have been investigated with X = H, F, Cl and Br at the MP2, B3LYP, QCISD and QCISD(T) levels by geometry and energy optimizations using the 6-311++G(3df,2p) basis sets at each level. Energy barriers, Δ , Δ , ΔH⧧ and ΔG⧧, are reported relative to both the reactants (ΔG⧧) and ion−dipole complex levels (Δ ). The electron correlation energy (−Ecorr) decreases in the MP2, QCISD and QCISD(T) results as the size (number of electron) of the system becomes larger (X = F → Cl → Br). The MP2 and QCISD methods underestimate the electron correlation effects relative to the highest level QCISD(T) results, which are, in general, in good agreement with the available experimental values. The lowest and highest activation barriers obtained with X = F and H, respectively, are found to be the consequences of the strong electrostatic interaction energies in the TS (ΔEes ≪ 0 and ΔEes ≫ 0, respectively), in contrast to small differences between nucleophile...

Computational investigations on the proton transfer in the isolated, monohydrated, and dihydrated forms of hypoxanthine have been performed. Ground state geometries were optimized at the HF/6-31G(d,p), HF/6311++G(d,p), and MP2/6-31G(d,p) levels of theory while those of the transition states corresponding to the proton transfer from the keto to the enol form of the molecule were characterized at the HF/6-31G(d,p), HF/6-311++G(d,p), MP2/6-31G(d,p)//HF/6-31G(d,p), and MP2/6-311++G(d,p)//HF/6-311++G(d,p) levels of theory. It is found that, in the gas phase, the molecule would exist mainly in the keto prototropic forms. The transition states corresponding to the proton transfer from the oxo to the hydroxy form for the monoand dihydrated forms were found to have a zwitterionic structure; the geometries are more easily expressed in the form of H3O + ‚‚‚HX - for the monohydrated forms and H5O2 + ‚‚‚HX - for the dihydrated forms of the molecule.

Group 15 1-alkenyl(triphenyl)onium (P, As, Sb) tetrafluoroborates were prepared via an onium transfer reaction of alkenyl(phenyl)iodonium tetrafluoroborates under mild conditions. The reaction involves base-induced reductive α-elimination of the iodonium salts, followed by nucleophilic trapping of the resulting free alkylidene carbenes with group 15 element-centered nucleophiles. The onium transfer reaction of the alkenyliodonium salts also produced group 16 1-alkenyl(diphenyl)onium (S, Se, Te) tetrafluoroborates in good yields.

The lighter rare gas complexes with Mg+ are known from both computational and spectroscopic determinations to have weakly bound 2Σ+ ground states with much more strongly bound low-lying 2Π1/2,3/2 excited states. In extensive configuration interaction calculations including large relativistic effects, we show that for Rn the picture is dramatically different with strongly avoided crossings in the 2Π curves due to charge-transfer states. The avoided crossings leave the 2Π states much more weakly bound. It was found that the lowering of the Rn ionization potential relative to Xe, producing the low-lying states, can be attributed almost entirely to the large spin−orbit splitting of the Rn+ 2P states. Variations in crossing interactions are readily explained in terms of two-component spinors.

A general theory of diffusion-assisted irreversible reactions on micelle surfaces is presented. An exact solution for the experimental observables is given in a matrix form in terms of the eigenmodes of the diffusion operator in the same geometry but without interaction potential and reaction. Photoinduced electron transfer followed by back transfer is considered in detail as a practically important example of reaction. Excellent agreement of the matrix solution with the numerical solution of the corresponding partial differential equations via a standard discretization procedure is observed for typical sets of parameter values.

14 pages, 10 figures, 3 tables.-- Printed version published Sep 23, 1998.-- Extra Addition/Correction paper published in J. Am. Chem. Soc. 120(47): 12371-12384 (1998), http://digital.csic.es/handle/10261/11376

Reactions involving two [H,C,Si]- anions with the protonated neutral analogues (HSiCH, H2CSi, and H2SiC) have been studied. Calculations through fourth-order perturbation theory (MP4) suggest possible routes for single-proton transfer between the silaacetylide [HCSi]- anion and neutral species. Various important reaction paths for proton-transfer reactions and internal rearrangements of ion−molecule complexes are discussed. Accurate estimates of the acidity of HSiCH and H2SiC are presented and discussed in light of earlier experimental studies.

Interconversion of five isomeric tautomers of formohydroxamic acid via intramolecular proton transfer has been examined by ab initio theoretical calculation. The transfer potential surfaces, the global isomeric structures, and the transition geometries of intramolecular proton transfer were determined by the MP2/6-31+G** level of calculation. The energy was further analyzed by a single point calculation, MP2/6-31++G**//MP2/6-31+G**, and the use of G2 theory. Not counting the unstable charge separating species, the order of stability of these tautomers calculated at the HF level was 1E > 1Z > 2Z > 2E, and it shifted to 1Z > 1E > 2Z > 2E at the MP2 level, where 1Z and 1E are keto forms, while 2Z and 2E are iminol forms. Further investigation using G2 theory redirects the order to be 1Z > 2Z > 1E > 2E. The strength of the intramolecular hydrogen bond and the effect of dipole moment are the two major factors to dominate the acidity of formohydroxamic acid. Judging from the transition barrier of intramolecular...

Under the conditions assumed both by Forster in his resonance-transfer theory and by Kennard and Stepanov in their theory of a universal relation between fluorescence and absorption, the rates of transfer between two chromophores k and l are related by Fl→k/Fk→l = exp[−(ΔGk − ΔGl)/kBT)], where the energies ΔGj are the free energies of excitation relating to the vibrational and other bath states interacting with the chromophore. They are dominated by, but not identical with, the 0−0 transition energies of the two chromophores. We show that ΔGk − ΔGl can be computed reliably with Forster's overlap integral approximation only when the Kennard−Stepanov relation is obeyed for both chromophores. We examine the extent to which approximations to Forster−Dexter transfer agree with this detailed balance result and the implications of the result for multichromophore systems.

A series of steroids has been studied in which there is a dimethylphenylsiloxy (DPSO) group at the 3 position to serve as a light-absorbing antenna, a ketone group at C6 or C11 to serve as an energ...

Rate and equilibrium constants for hydride transfer from a series of 1,3-dimethyl-2-substituted phenylbenzimidazolines to a pyridinium ion, a quinolinium ion, and a phenanthridinium ion have been e...

As a model of the chemical reactions that take place in the active site of gluthatione reductase, the nature of the molecular mechanism for the hydride transfer step has been characterized by means of accurate quantum chemical characterizations of transition structures. The calculations have been carried out with analytical gradients at AM1 and PM3 semiempirical procedures, ab initio at HF level with 3-21G, 4-31G, 6-31G, and 6-31G basis sets and BP86 and BLYP as density functional methods. The results of this study suggest that the endo relative orientation on the substrate imposed by the active site is optimal in polarizing the C4-Ht bond and situating the system in the neighborhood of the quadratic region of the transition structure associated to the hydride transfer step on potential energy surface. The endo arrangement of the transition structure results in optimal frontier HOMO orbital interaction between NADH and FAD partners. The geometries of the transition structures and the corresponding transition vectors, that contain the fundamental information relating reactive fluctuation patterns, are model independent and weakly dependent on the level of theory used to determine them. A comparison between simple and complex molecular models shows that there is a minimal set of coordinates describing the essentials of hydride transfer step. The analysis of transition vector components suggests that the primary and secondary kinetic isotope effects can be strongly coupled, and this prompted the calculation of deuterium and tritium primary, secondary, and primary and secondary kinetic isotope effects. The results obtained agree well with experimental data and demonstrate this coupling.

Nitrate complexes of two different polyammonium macrocycles, the tetrahydrogen nitrate salts of 1,4,7,16,19,21-hexaaza-10,13-dioxacyclotetracosane [24]N6O2·4HNO3, 1, and 1,4,10,13-tetraaza-7,16-dioxacyclooctadecane, [18]N4O2·4HNO3, 2, have been isolated and their structures determined by X-ray crystallographic methods. Compound 1 crystallizes in the orthorhombic space group Imma with unit cell dimensions a = 22.692(3) A, b = 19.563(3) A, c = 7.005(1) A, V = 3110(1) A3, and Z = 4. Compound 2 crystallizes in the orthorhombic space group Pbca with a = 13.877(1) A, b = 15.553(1) A, c = 10.742(1) A, V = 2314.6(5) A3, and Z = 8. Full-matrix least-squares refinement resulted in R = 0.051 and Rw = 0.067 for 1 and R = 0.063 and Rw = 0.072 for 2. Compound 1 has a boat-shaped geometry, and one of the four nitrates is situated in the macrocyclic cavity. Compound 2 is relatively flat with two nitrates above and two below the plane of the molecule. Molecular dynamics simulations were performed for the two molecules usi...

Reaction of AlMe3 with the imido compounds Mo(NPh)(o-(SiMe3N)2C6H4)(CH2)4 (1) and Mo(NPh)(o-(SiMe3N)2C6H4)L (2; L = diphenylacetylene) results in the transfer of the diamide ligand to the Al, giving the unusual arene complexes 3 and 4.