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An H/D kinetic isotope effect (KIE) of 80 is found at -20 °C for the oxidation of 9,10-dihydroanthracene by [Fe[IV](O)(TMCS)][sup +], a complex supported by the tetramethylcyclam (TMC) macrocycle with a tethered thiolate. This KIE value approaches that previously predicted by DFT calculations. Other [Fe[IV](O)(TMC)( anion)] complexes exhibit values of 20, suggesting that the thiolate ligand of [Fe[IV](O)(TMCS)][sup +] plays a unique role in facilitating tunneling. Calculations show that tunneling is most enhanced (a) when the bond asymmetry between C-H bond breaking and O-H bond formation in the transition state is minimized, and (b) when the electrostatic interactions in the O---H---C moiety are maximal. These two factors -- which peak for the best electron donor, the thiolate ligand -- afford a slim and narrow barrier through which the H-atom can tunnel most effectively. [ABSTRACT FROM AUTHOR]

In classical transition state theory, a transition state is connected to its reactant(s) and product(s). Recently, chemists found that reaction pathways may bifurcate after a transition state, leading to two or more sets of products. The product distribution for such a reaction containing a bifurcating potential energy surface (bPES) is usually determined by the shape of the bPES and dynamic factors. However, if the bPES leads to two intermediates (other than two products), which then undergo further transformations to give different final products, what factors control the selectivity is still not fully examined. This missing link in transition state theory is founded in the present study. Aiming to develop new methods for the synthesis of azocinoindole derivatives, we found that 2-propargyl-β-tetrahydrocarbolines can undergo ring expansion and spirocyclization under gold catalysis. DFT study revealed that the reaction starts with the intramolecular cyclization of the gold-activated 2-propargyl-β-tetrahydrocarboline with a bPES. The cyclization intermediates can not only interconvert into each other via a [1,5]-alkenyl shift, but also undergo ring expansion (through fragmentation/protodeauration mechanism) or spirocyclization (through deprotonation/protodeauration mechanism). Detailed analysis of the complex PESs for substrates with different substituents indicated that the reaction selectivity is under dynamic control if the interconversion of the intermediates is slower than the ring expansion and spirocyclization processes. Otherwise, the chemical outcome is under typical kinetic control and determined by the relative preference of ring expansion versus spirocyclization pathways. The present study may enrich chemist's understanding of the determinants for selectivities on bPESs. [ABSTRACT FROM AUTHOR]

The excited-state dynamics of phenylvinylacetylene (1-phenyl-1-buten-3-yne, PVA) have been studied using laser-induced fluorescence spectroscopy, ultraviolet depletion spectroscopy, and the newly developed method of ultraviolet population transfer spectroscopy. Both isomers of PVA (E and 2) show a substantial loss in fluorescence intensity as a function of excitation energy. This loss in fluorescence was shown to be due to the turn-on of a nonradiative process by comparison of the laser-induced fluorescence spectrum to the ultraviolet depletion spectrum of each isomer, with a threshold 600 cm-1 above the electronic origin in Z-PVA and 1000 cm-1 above the electronic origin in E-PVA. Ab initio and density functional theory calculations have been used to show that the most likely source of the nonradiative process is from the interaction of the πσ* state with a close lying πσ* state whose minimum energy structure is bent along the terminal CCH group. Ultraviolet population transfer spectroscopy has been used to probe the extent to which excited-state isomerization is facilitated by the interaction with the πσ* state. In ultraviolet population transfer spectroscopy, each isomer was selectively excited to vibronic levels in the S1 state with energies above and below the threshold for fluorescence quenching. The ultraviolet-excited populations are then recooled to the zero point levels using a reaction tube designed to constrain the supersonic expansion and increase the collision cooling capacity of the expansion. The new isomeric distribution was detected in a downstream position using resonant-2-photon ionization spectroscopy. From these spectra, relative isomerization quantum yields were calculated as a function of excitation energy. While the fluorescence quantum yield drops by a factor of 50-100, the isomerization quantum yields remain essentially constant, implying that the nonradiative process does not directly involve isomerization. On this basis, we postulate that isomerization occurs on the ground-state potential energy surface after internal conversion. In these experiments, the isomerization to naphthalene was not observed, implying a competition between isomerization and cooling on the ground-state potential energy surface. [ABSTRACT FROM AUTHOR]

Determines the potential energy surface crossings and the mechanistic spectrum for intramolecular electron transfer in organic radical cations. Use of reaction path mapping and conical intersection optimization method; Detection of second potential energy surface for diamino ethane cation.

The gas-phase cyclopropanation and apparent metathesis reactivity of ligand-supported gold arylidenes with electron-rich olefins is explained by quantum-chemical calculations. A deep potential minimum corresponding to a metal-bound cyclopropane adduct is in agreement with the measured absolute energies of the cyclopropanation and metathesis channels and is also consistent with previously reported electronic effects of arylidenes and supporting phosphorus ylid ligands on the product ratios. In the gas phase, the rate-determining step for the cyclopropanation is dissociation of the Lewis-acidic metal fragment, whereas the metathesis pathway features several rate-limiting transition states that are close in energy to the final product dissociation and hence contribute to the overall reaction rate. Importantly, the presented potential energy surface also accounts for the recently reported gold-catalyzed solution-phase retro-cyclopropanation reactivity. [ABSTRACT FROM AUTHOR]

Collins' method of interpolating a potential energy surface (PES) from quantum chemical calculations for reactive systems (Jordan, M. J. T.; Thompson, K. C.; Collins, M. A. J. Chem. Phys. 1995, 102, 5647. Thompson, K. C.; Jordan, M. J. T.; Collins, M. A. J. Chem. Phys. 1998, 108, 8302. Bettens, R. P. A.; Collins, M. A. J. Chem. Phys. 1999, 111, 816) has been applied to a bound state problem. The interpolation method has been combined for the first time with quantum diffusion Monte Carlo calculations to obtain an accurate ground state zero-point energy, the vibrationally average rotational constants, and the vibrationally averaged internal coordinates. In particular, the system studied was fluoromethane using a composite method approximating the QCISD(T)/6-311++G(2df,2p) level of theory. The approach adopted in this work (a) is fully automated, (b) is fully ab initio, (c) includes all nine nuclear degrees of freedom, (d) requires no assumption of the functional form of the PES, (e) possesses the full symmetry of the system, (f) does not involve fitting any parameters of any kind, and (g) is generally applicable to any system amenable to quantum chemical calculations and Collins' interpolation method. The calculated zero-point energy agrees to within 0.2% of its current best estimate. A[sub 0] and B[sub 0] are within 0.9 and 0.3%, respectively, of experiment. [ABSTRACT FROM AUTHOR]

Examines the potential energy surface for activation of methane by platinum[sup +]. Use of guided-ion beam tandem mass spectrometer; Evidence on methane activation; Comparison between experimental thermochemistry and density functional theory.

Investigates the carbonyl insertion reaction using gradient corrected density functional theory. Two-step mechanism of the reaction; Location of stable intermediates on the potential energy surface; Location of the transition states between the reactant and the intermediates; Characterization of the transition states of the carbon monoxide attack.

Investigates the global potential energy surface for triplet C4H4 to find all possible isomers of this species, to examine isomerization pathways connecting the local minima and to study various dissociation channels. Aromatic cyclobutene and linear butyne as the most stable structures for triplet C4H4.

Abietic acid, a constituent of pine resin, is naturally derived from abietadiene - a process that requires four enzymes: one (abietadiene synthase) for conversion of the acyclic, achiral geranylgeranyl diphosphate to the polycydic, chiral abietadiene (a complex process involving the copalyl diphosphate intermediate) and then three to oxidize a single methyl group of abietadiene to the corresponding carboxylic acid. In previous work (Nature Chem. 2009, 1, 384), electronic structure calculations on carbocation rearrangements leading to abietadienyl cation revealed an interesting potential enerysurface with a bifurcating reaction pathway (two transition-state structures connected directly with no intervening minimum), which links two products - one natural and one not yet isolated from Nature. Herein we describe direct dynamics simulations of the key step in the formation of abietadiene (in the gas phase and in the absence of the enzyme). The simulations reveal that abietadiene synthase must intervene in order to produce abietadiene selectively, in essence steering this reaction to avoid the generation of byproducts with different molecular architectures. [ABSTRACT FROM AUTHOR]

Presents information on a study which calculate the potential energy surface of the C3H9 cation, with reference to chemistry. Methods used in this study; Results from the study; Discussion on the calculated potential energy surface of the C3H9 protonated propane.

The intermolecular Pauson-Khand reaction (PKR), a carbonylative cycloaddition between an alkyne and an alkene, is a convenient method to prepare cyclopentenones. Using norbornadiene as alkene, a myriad of tricyclo[5.2.1.0[sup2,6]]deca-4,8-dien-3-ones 1 can be easily prepared. The mechanism of the photochemical rearrangement of these adducts 1 into tricyclo[5.2.1.0(sup26]]deca-3,8-dien-10-ones 2 has been studied. The ground state (S[sub0]) and the three lowest excited states (³(ππ*), ¹(nπ*), and ³(nπ*)) potential energy surfaces (PESs) concerning the prototypical rearrangement of la (the cycloadduct of the PK carbonylative cycloaddition of norbornadiene and ethyne) to 2a have been thoroughly explored by means of CASSCF and CASPT2 calculations. From this study, two possible nonadiabatic pathways for the photochemical rearrangement arise: one starting on the ³(ππ*) PES and the other on the ¹(nπ*) PES. Both involve initial C-C γ-bond cleavage of the enone, which leads to the formation of a bis-allyl or an allyl- butadienyloxyl diradical, respectively, that then decays to the S[sub0] PES through a ³(ππ*)/S[sub0] surface crossing or a ¹(nπ*)/S[sub0] conical intersection, each one lying in the vicinity of the corresponding diradical minimum. Once on the S[sub0] PES, the ring-closure to 2a occurs with virtually no energy barrier. The viability of both pathways was experimentally studied by means of triplet senzitization and quenching studies on the photorearrangement of the substituted Pauson-Khand cycloadduct lb (R = TMS, R' = H) to 2b. Using high concentrations of either piperylene as a triplet quencher, or benzophenone as a triplet sensitizer, the reaction rate significantly slowed down. A Stern-Volmer type plot of product 2b concentration vs triplet quencher concentration showed an excellent linear correlation, thus indicating that only one excited state is involved in the photorearrangement. We conclude that, though there is a nonadiabatic pathway starting on the ¹(nπ*) PES, the reaction product is formed through the ³(ππ*) state because the energy barrier involved in the initial C-C γ-bond cleavage of the enone is much lower in the ³(ππ*) PES than in the ¹(nπ*) PES. [ABSTRACT FROM AUTHOR]

The decay paths on the singlet excited-state surface of 9H-adenine and the associated energy barriers have been calculated at the CAS-PT2//CASSCF level. There are three fundamental paths for the photophysics: two paths for the ¹Lb state which are virtually barrierless at the present level of theory and correspond to formation of the (n,π*) intermediate and direct decay to the ground state and a third path for ground-state decay of the (n,π*) state with an activation barrier of approximately 0.1 eV. The ¹La state, which has the largest oscillator strength, either decays directly to the ground state or contributes indirectly to the excited-state lifetime by populating the two other states. The results are used to interpret the photophysics in terms of an excited-state plateau for the ¹Lb state that corresponds to the short-lived excited- state component (approximately 0.1 ps) and a well (i.e., a proper minimum) for the (n,π*) state that gives rise to the long component (1 ps or more). The direct decay to the ground state of the ¹Lb state is probably the decay channel invoked to explain the experimental wavelength dependence of the relative amplitudes of the two components. In addition to that, the excited-state component in the nanosecond range detected in the time-resolved photoelectron spectrum is proposed to be a triplet (π,π*) state formed after intersystem crossing from the singlet (n,π*) state. [ABSTRACT FROM AUTHOR]

The way in which enzymes influence the rate of chemical processes is still a question of debate. The protein promotes the catalysis of biochemical processes by lowering the free energy barrier in comparison with the reference uncatalyzed reaction in solution. In this article we are reporting static and dynamic aspects of the enzyme catalysis in a bimolecular reaction, namely a methyl transfer from S-adenosylmethionine to the hydroxylate oxygen of a substituted catechol catalyzed by catechol O-methyltransferase. From QM/MM optimizations, we will first analyze the participation of the environment on the transition vector. The study of molecular dynamics trajectories will allow us to estimate the transmission coefficient from a previously localized transition state as the maximum in the potential of mean force profile. The analysis of the reactive and nonreactive trajectories in the enzyme environment and in solution will also allow studying the geometrical and electronic changes, with special attention to the chemical system movements and the coupling with the environment. The main result, coming from both analyses, is the approximation of the magnesium cation to the nucleophilic and the hydroxyl group of the catecholate as a result of a general movement of the protein, stabilizing in this way the transition state. Consequently, the free energy barrier of the enzyme reaction is dramatically decreased with respect to the reaction in solution. [ABSTRACT FROM AUTHOR]

Photolysis of p-tolyldiazirine (6) in the inner phase of a hemicarcerand with four butane-l,4dioxy linker groups (5) in C[sub 6]D[sub 5]CD[sub 3] at 77 K yields the 5-methylcycloheptatetraene hemicarceplex 5⊙3b in 41% together with innermolecular reaction products resulting from an insertion of transient p-tolylcarbene (1b) into an acetal C-H or linker C-O bond of 5 and from the addition of 1b to an aryl unit of 5. The yield of incarcerated 3b increased up to 67% if 6 is photolyzed inside a hemicarcerand with deuterated spanners and butane-1,4-dioxy linker groups (d[sub 48]-5). Hemicarceplex 5⊙3b is not formed if the photolysis is carried out in CDCI[sub 3]. Incarcerated 3b is stable at room temperature in the absence of oxygen and is characterized by 1D and 2D NMR spectroscopy. In the presence of oxygen, 3b reacts quantitatively to yield toluene and CO[sub 2]. Upon heating solutions of d[sub 48]-5⊙3b in C[sub 6]D[sub 5]CD[sub 3], 3b rearranges to 1b and m-tolylcarbene (18). Both tolylcarbenes immediately react with the surrounding host. From a product analysis and the measured rate constants for the thermal decomposition of d[sub 48]-5⊙3b in the temperature range 70-102 °C, the activation parameters for the 3b to 1b and 3b to 18 rearrangements are calculated (3b to 1b: ΔG[sub 373] = 27.3 ± 1.4 kcal/mol, ΔH[sub 373] = 26.4 ± 1.0 kcal/mol, TΔS[sub 373] = -0.9 ± 1.0 kcal/mol; 3b to 18: ΔG[sub 373] = 27.8 ± 1.4 kcal/mol, ΔH[sub 373] = 19.7 ± 1.0 kcal/mol, TΔS[sub 373] = 8.1 ± 1.0 kcal/mol). These values are compared with those calculated by Geise and Hadad at the B3LYP/6-311+G[sup **] level of theory (Geise, C. M.; Hadad, C. M. J. Org. Chem. 2002, 67, 2532-2540). The slightly higher inner phase activation free energy of the 3b to 18 rearrangement is explained through steric constraints imposed by the surrounding hemicarcerand on the transition state. The... [ABSTRACT FROM AUTHOR]

Provides information on the hormone melatonin, an indole derivative with a flexible peptide-like side chain attached at the C3 position. Use of a combination of two-color resonant two-photon ionization and laser-induced fluorescence excitation; Observation of cis-amide melatonin conformers in the molecular beam.

Studies the electronic absorption spectra and the laser-induced fluorescence spectra of supersonic-jet-cooled 1,3-benzodioxole molecules. Analysis of spectra; Calculation of potential energy spectra; Comparison of the potential energy change along the puckering coordinate for the ground and excited electronic states.

Presents the far-infrared and Raman spectra of 1,3-benzodioxole vapor. Dispersed fluorescence spectra, vibrational potential energy surface and anomeric effect of 1,3-benzodioxole; Transitions between the various ring-puckering energy levels in the ground and excited ring-flapping states; Nonplanarity of bicyclic ring system.

Studies the potential energy surface of the benzene dimer by ab initio methods with inclusion of correlation energy. Survey of previous studies on benzene dimer; One-electron properties; Structure; Stabilization energies; Energy barriers among the most stable minima; Accuracy of calculated stabilization energy; Zero-point energy; Stabilization energy at the energy minima.

Discusses the potential energy surface (PES) of the C9H9+ ion. Ab initio calculations; Focus on PES areas that host the potentially homoaromatic 1,4-bishomotropylium cation and its classical counterpart; Analysis of the 9-barbaralyl cation that undergoes unique rearrangements; Detailed description of a double-bifurcation reaction mechanism; Importance of symmetry rules.

Studies the F + HF reaction using ab initio molecular orbital theory at the different levels of theory . Shallow depression in potential energy surface; Total and relative energies for reactants and transition states; Geometries of F-H-F; Potential energy curves; Vibrational frequencies; Highly bent transition state; More.

Studies the potential energy surface (PES) of the cyclobutane radical cation (CB.+) relevant to Jahn-Teller (JT) distortions. Computational methods; Calculation of the equilibrium structure of neutral cyclobutane at the MP2/6-31G* level; Results of Jahn-Teller surface of CB.+; Mono-, di- and tetramethyl derivatives of CB.+.

Investigates the potential energy surface of the C4H9+ carbocation using high-level ab initio molecular orbital theory. Various forms of the tert-butyl cation; Structures and stabilities; Infrared spectra.

Examines the dependence of hydrogen-bond strength of intermolecular hydrogen bonds on the hydrogen-bonding distance, the two hydrogen-bond angles and the intermolecular dihedral angle. Determination of the potential energy surface of intermolecular amide hydrogen bonding using ab initio methods.

Studies nitrogen oxide potential energy surface using the density functional theory with nonlocal corrections and Hartree-Fock theory with perturbative electron correlation. Heats of formation; Experimental O-O bond distances and bond dissociation energies; Estimated activation barriers.

Reports on the use of ab initio molecular orbital calculations at the UMP2, QCISD and QCISD(T) levels of theory with the 6-31G(d) and 6-31G(d,p) basis sets to investigate the relevant parts of the C4H10.+ ground-state potential energy surface. Theoretical results; Mass spectrometry experimental findings.

Presents a rudimentary map of the photoisomerization potential energy surface for tetraphenylethylene in alkane solvents. Line shapes of the vertical and relaxed excited states emissions at 294 K in methylcyclohexane; Temperature dependencies of the vertical and relaxed states emission quantum yields and of the time resolved fluorescence decays.

Defines the potential energy surface for a gas-phase carbonyl addition-elimination reaction through the synthesis, isolation, and characterization of intermediates in an exothermic carbonyl displacement reaction. Chemical and photochemical experiments to determine the structure and reactivity of the intermediates; Reaction of CN- with 3,5-difluorobenzoyl chloride.

Examines the structure and potential energy surface of a three-helix bundle protein. Ranges of helical contents of the trapped states; Thermodynamic stabilities; Effects of interhelix interactions; Effects of metal complexation.

Looks at a study which examined the ring opening reaction of the cyclobutene radical cation, using density function calculations. In-depth look at the electrocyclic ring-opening reaction; Details on polyene radical ions' excited states; Methodology used to conduct the study; Results of the study.

Investigates the structures, stabilities and isomerization patterns of C5H9+ ions arising from the gas-phase protonation of spiropentane. Use of nuclear-decay, radiolytic FT-ICR techniques; Consistency of experimental and theoretical results; Effects of solvation and ion-pairing.

Discusses the 1,4-bishomotropylium cation and its classic analogue. Structure; Stability; Magnetic properties; Bonding; Investigation of qualification of cation as bishomoaromatic compound; Utilization of Moller-Plesset perturbation theory.

Studies the ketenyl radical HCCO (a4A'') energy hypersurface using ab initio quantum mechanical techniques. Rate constant measurements; Attractive potential of the reaction of CH(a4 sigma -) with CO; Vanishingly small energy barrier; Energetics and geometries of reactants and products; More.

Studies the reaction of ethylene (Et) and its radical cation (Et.+) at the QCISD(T)/6-31G* level of theory. Computational methods; Et + Et.+ cycloaddition reaction; Rearrangememt to 1-butene radical cation; Revised potential energy surfaces; Reliability of theoretical models.

Presents a study which employed the use of the Hartee-Fock/density functional theory, second-order perturbation theory and the coupled-cluster method to examine the benzene potential energy hyperface. Mechanism for the topomerization of benzene; Thermochemical corrections and thermal topomerization of benzene; Details on theories used in study.

Reports on the evaluation of the geometries and force fields using relative energies and infrared assignment of several chemical compounds. Includes phenylcarbene; Cycloheptatrienylidene; Cycloheptatetraene; Bicycloheptatriene.

Studies the potential energy of the water trimer through the use of ab initio quantum mechanical methods. Location of five stationary points; Harmonic vibrational frequencies; Infrared intensities for the minimum; Zero point vibrational energies for the minimum.

We report high-resolution photoelectron spectra of the simplest carbanions, CH3- and CD3-. The vibrationally resolved spectra are dominated by a long progression in the umbrella mode (v2) of ·CH3 and ·CD3, indicating a transition from a pyramidal C3v anion to the planar D3h methyl radical. Analysis of the spectra provides electron affinities of ·CH3 (0.093(3) eV) and ·CD3 (0.082(4) eV). These results enable improved determination of the corresponding gas-phase acidities: ΔacidH0K°(CH4) = 414.79(6) kcal/mol and ΔacidH0K°(CD4) = 417.58(8) kcal/mol. On the basis of the photoelectron anisotropy distribution, the electron is photodetached from an orbital with predominant p-character, consistent with the sp³-hybridized orbital picture of the pyramidal anion. The double-well potential energy surface along the umbrella inversion coordinate leads to a splitting of the vibrational energy levels of the umbrella mode. The inversion splittings of CH3- and CD3- are 21(5) and 6(4) cm-1, respectively, and the corresponding anion umbrella vibrational frequencies are 444(13) and 373(12) cm-1, respectively. Quantum mechanical calculations reported herein show good agreement with the experimental data and provide insight regarding the electronic potential energy surface of CH3-. [ABSTRACT FROM AUTHOR]

The OH radical is the most important radical in combustion and in the atmosphere, and methanol is a fuel and antifreeze additive, model biofuel, and trace atmospheric constituent. These reagents are also present in interstellar space. Here we calculate the rate constants, branching ratios, and kinetic isotope effects (KIEs) of the hydrogen abstraction reaction of methanol by OH radical in a broad temperature range of 30-2000 K, covering interstellar space, the atmosphere, and combustion by using the competitive canonical unified statistical (CCUS) model in both the low-pressure and high-pressure limits and, for comparison, the pre-equilibrium model. Coupled cluster CCSD(T)-F12a theory and multi-reference CASPT2 theory were used to carry out benchmark calculations of the stationary points on the potential energy surface to select the most appropriate density functional method for direct dynamics calculations of rate constants. We find a significant effect of the anharmonicity of high-frequency modes of transition states on the low-temperature rate constant, and we show how tunneling leads to an unusual negative temperature dependence of the rate constants in the range 200 K > T > 100 K. The calculations also demonstrate the importance of the extent of stabilization of the pre-reactive complex. The capture rate for the formation of the complex is the dominant dynamical bottleneck for T < 100 K, and it leads to weak temperature dependence of the rate below 100 K in the high-pressure-limit of the CCUS model. We also report the pressure dependence of branching ratios (which are hard to measure so theory is essential) and the KIEs, and we report an unusual nonmonotonic variation of the KIE in the high-pressure limit at low temperatures. [ABSTRACT FROM AUTHOR]

The slippery potential energy surface of aryl nitrenes has revealed unexpected and fascinating reactions. To explore such a challenging surface, one powerful approach is to use a combination of a cryogenic matrix environment and a tunable narrowband radiation source. In this way, we discovered the heavy-atom tunneling reaction involving spontaneous ring expansion of a fused-ring benzazirine into a seven-membered ring cyclic ketenimine. The benzazirine was generated in situ by the photochemistry of protium and deuterated triplet 2‑formylphenylnitrene isolated in an argon matrix. The ringexpansion reaction takes place at 10 K with a rate constant of ∼7.4 × 10-7 s-1, despite an estimated activation barrier of 7.5 kcal mol-1. Moreover, it shows only a marginal increase in the rate upon increase of the absolute temperature by a factor of 2. Computed rate constants with and without tunneling confirm that the reaction can only occur by a tunneling process from the ground state at cryogenic conditions. It was also found that the ringexpansion reaction rate is more than 1 order of magnitude faster when the sample is exposed to broadband IR radiation. [ABSTRACT FROM AUTHOR]

Dual-level dynamics calculation with variational transition state theory including multidimensional tunneling has been performed on the isomerization reaction of cyclic ozone → normal (open) ozone, which was believed to be the stability-determining reaction of the elusive cyclic ozone molecule under thermal condition. The high-level potential energy surface data were obtained from the calculation using the MRCISD+Q theory with the aug-cc-pVQZ basis set, while the low-level reaction path information was obtained using the hybrid density functional theory B3LYP with the cc-pVTZ basis set. The calculated results showed very significant tunneling effects below 300 K (a factor of 200 at 300 K and over 107 at 200 K). Because of the strong tunneling effects and the potential energy surface crossing of the 1A1 and 1A2 states, the isomerization reactions were found to be significantly faster than previously believed. The half-life of the cyclic ozone was estimated only 10 s at 200 K and 70 s below 100 K, which might partly explain the unsuccessful attempts for its experimental identification. The kinetic isotope effects (KIEs) for various 18O substitution reactions were also calculated as a function of temperature and were as high as 10 at very low temperature. Because of the large KIEs, the experimental identification of the cyclic 18O3 seems more promising. [ABSTRACT FROM AUTHOR]

We report the first preparation of a stable aminothioketyl radical, CH3C•(SH)NHCH3 (1), by fast electron transfer to protonated thioacetamide in the gas phase. The radical was characterized by neutralization-reionization mass spectrometry and ab initio calculations at high levels of theory. The unimolecular dissociations of 1 were elucidated with deuterium-labeled radicals CH3C•(SD)NHCH3 (1a), CH3C•(SH)NDCH3 (1b), CH3C•(SH)NHCD3 (1c), and CD3C•(SH)NHCH3 (1d). The main dissociations of 1 were a highly specific loss of the thiol H atom and a specific loss of the N-methyl group, which were competitive on the potential energy surface of the ground electronic state of the radical. RRKM calculations on the CCSD(T)/aug-cc-pVTZ potential energy surface indicated that the cleavage of the S-H bond in 1 dominated at low internal energies, Eint < 232 kJ mol-1. The cleavage of the N-CH3 bond was calculated to prevail at higher internal energies. Loss of the thiol hydrogen atom can be further enhanced by dissociations originating from the B excited state of 1 when accessed by vertical electron transfer. Hydrogen atom addition to the thioamide sulfur atom is calculated to have an extremely low activation energy that may enable the thioamide group to function as a hydrogen atom trap in peptide radicals. The electronic properties and reactivity of the simple aminothioketyl radical reported here may be extrapolated and applied to elucidate the chemistry of thioxopeptide radicals and cation radicals of interest to protein structure studies. [ABSTRACT FROM AUTHOR]

Applying density functional theory (DFT) calculations to the rational design of catalysts for complex reaction networks has been an ongoing challenge, primarily because of the high computational cost of these calculations. Certain correlations can be used to reduce the number and complexity of DFT calculations necessary to describe trends in activity and selectivity across metal and alloy surfaces, thus extending the reach of DFT to more complex systems. In this work, the well-known family of Brønsted-Evans-Polanyi (BEP) correlations, connecting minima with maxima in the potential energy surface of elementary steps, in tandem with a scaling relation, connecting binding energies of complex adsorbates with those of simpler ones (e.g., C, 0), is used to develop a potential-energy surface for ethanol decomposition on 10 transition metal surfaces. Using a simple kinetic model, the selectivity and activity on a subset of these surfaces are calculated. Experiments on supported catalysts verify that this simple model is reasonably accurate in describing reactivity trends across metals, suggesting that the combination of BEP and scaling relations may substantially reduce the cost of DFT calculations required for identifying reactivity descriptors of more complex reactions. [ABSTRACT FROM AUTHOR]

The ene reaction between singlet oxygen (102) and simple alkenes has been reported as the first experimentally supported example of a potential-energy surface (PES) featuring a valley-ridge inflection that contributes to the selectivity of product formation (J. Am. Chem. Soc. 2003, 125, 1319-1328). That PES, based on gas-phase ab initio calculations and experimental kinetic isotope effects, has shaped the current 1O2 ene mechanism by advocating a concerted "two-step no-intermediate" mechanism. Our current investigation of the ene reaction between 1O2 and tetramethylethylene in water, DMSO, and cyclohexane using novel 3-dimensional potentials of mean force (3-D PMF) calculations coupled to QM/MM simulations predicts an alternative free-energy surface that supports a traditional stepwise mechanism interpretation featuring a symmetric charge-separated perepoxide intermediate. Solvent polarity dictates the stability of the intermediate and controls the activation barrier for ene product formation. Transformation of the higher order condensed-phase 3-D PMF potential-energy surface, computed following three simultaneous reaction coordinates, into a downgraded 2-D surface results in the "two-step no-intermediate" mechanism. CCSD(T) and MP4(SDQ) free-energy maps in DMSO, constructed from a 3-D grid of B3LYP geometries using the CPCM solvent model, reproduce the QMIMM results and confirm that when solvent is taken into account the gas-phase bifurcation reaction pathway converts into a stepwise mechanism. This manuscript provides new insight into the biologically and synthetically important 1O2 ene reaction and highlights a new multidimensional approach for constructing potential-energy surfaces. [ABSTRACT FROM AUTHOR]

Penicillin-binding protein 5 (PBP 5) of Escherichia coil hydrolyzes the terminal D-AIa-D-Ala peptide bond of the stem peptides of the cell wall peptidoglycan. The mechanism of PBP 5 catalysis of amide bond hydrolysis is initial acylation of an active site serine by the peptide substrate, followed by hydrolytic deacylation of this acyl-enzyme intermediate to complete the turnover. The microscopic events of both the acylation and deacylation half-reactions have not been studied. This absence is addressed here by the use of explicit-solvent molecular dynamics simulations and ONIOM quantum mechanics/molecular mechanics (QM/MM) calculations. The potential-energy surface for the acylation reaction, based on MP2/6-31+G(d) calculations, reveals that Lys47 acts as the general base for proton abstraction from Ser44 in the serine acylation step. A discrete potential-energy minimum for the tetrahedral species is not found. The absence of such a minimum implies a conformational change in the transition state, concomitant with serine addition to the amide carbonyl, so as to enable the nitrogen atom of the scissile bond to accept the proton that is necessary for progression to the acyl-enzyme intermediate. Molecular dynamics simulations indicate that transiently protonated Lys47 is the proton donor in tetrahedral intermediate collapse to the acyl-enzyme species. Two pathways for this proton transfer are observed. One is the direct migration of a proton from Lys47. The second pathway is proton transfer via an intermediary water molecule. Although the energy barriers for the two pathways are similar, more conformers sample the latter pathway. The same water molecule that mediates the Lys47 proton transfer to the nitrogen of the departing D-Ala is well positioned, with respect to the Lys47 amine, to act as the hydrolytic water in the deacylation step. Deacylation occurs with the formation of a tetrahedral intermediate over a 24 kcal·mol-1 barrier. This barrier is approximately 2 kcal·mol-1 greater than the barrier (22 kcal·mol-1) for the formation of the tetrahedral species in acylation. The potential-energy surface for the collapse of the deacylation tetrahedral species gives a 24 kcal·mol-1 higher energy species for the product, signifying that the complex would readily reorganize and pave the way for the expulsion of the product of the reaction from the active site and the regeneration of the catalyst. These computational data dovetail with the knowledge on the reaction from experimental approaches. [ABSTRACT FROM AUTHOR]

The existence and persistence of five-fold (quintuple) bonding in isomers of model RMMR molecules of quite different geometry are examined theoretically. The molecules studied are RMMR, with R = H, F, Cl, Br, CN, and CH3; M = Cr, Mo, and W. The potential energy surface of these molecules is quite complex, containing two, three, even four local minima. The structural preferences in these molecules are rationalized, and electronic factors responsible for these preferences are elucidated. The linear geometry is always a minimum, but almost never the global minimum; there is a definite preference in RMMR for either a trans-bent conformation or perturbations of the trans-bent isomer with at least one of the R groups in a bridging position about the MM bond. The potential energy surface of these RMMR molecules is relatively flat, the lowest energy conformation being that which for a given molecule attains the best compromise between maximization of the MM bonding and minimization of orbital interactions that are MR antibonding. A surprising low-symmetry Cs structure is identified, which along with the trans-bent isomer is one of the two most popular choices for the global minimum. Regardless of what isomer of the RMMR molecule is preferred, the MM quintuple bond persists. [ABSTRACT FROM AUTHOR]

The recombination of oxygen atoms with oxygen molecules to form ozone exhibits several strange chemical characteristics, including unusually large differences in formation rate coefficients when different isotopes of oxygen participate. Purely statistical chemical reaction rate theories cannot describe these isotope effects, suggesting that reaction dynamics must play an important role. We investigated the dynamics of the 18O + 32O2 → O3* → 16O + 34O2 isotope exchange reaction (which proceeds on the same potential energy surface as ozone formation) using crossed atomic and molecular beams at a collision energy of 7.3 kcal mol-1, providing the first direct experimental evidence that the dissociation of excited ozone exhibits significant nonstatistical behavior. These results are compared with quantum statistical and quasi-classical trajectory calculations in order to gain insight into the potential energy surface and the dynamics of ozone formation. [ABSTRACT FROM AUTHOR]

The potential energy surface of the rearrangement of 3-vinylmethylenecyclobutane to 4-methylenecyclohexene has been studied computationally using density functional theory (B3LYP) and complete active space ab initio methods (CASSCF and CASPT2). The parent reaction is nonconcerted and occurs through several parallel diradical pathways. Transition structures and diradical intermediates are highly comparable in energy, with no deep potential energy well on the potential energy surface. In the substituted system, stereoelectronic effects of the trialkylsiloxy group regulate torquoselectivity in the bond-breaking processes and this, combined with low barriers to cyclization, leads to a stepwise Cope rearrangement that is, nevertheless, stereoselective. [ABSTRACT FROM AUTHOR]

Recent neutron diffraction data have shown that the hydrogen atom involved in the short, strong hydrogen bond in urea-phosphoric acid migrates toward the midpoint of the hydrogen bond as the temperature increases. With the help of solid state ab initio calculations and inelastic neutron scattering, we have investigated the temperature dependence of the structural and vibrational properties of the system. The potential energy surface of the proton in the short, strong hydrogen bond and the thermal population of the energy levels therein cannot account for the observed proton migration. Ab initio molecular dynamics simulations clearly reveal the migration of the proton. This molecular dynamics result was reported recently by other authors, but they only offered a tentative explanation in terms of a resonance between high-frequency vibrations, which is not supported by the calculations presented here. We explain the proton migration in terms of phonon-driven structural fluctuations and their impact on the temperature-dependent evolution of the potential energy surface of the short hydrogen-bond proton. [ABSTRACT FROM AUTHOR]

Reaction of [Cplr(P-P)Cl][B(C6F5)4] (P-P = bisdimethydiphosphinomethane (dmpm), bisdiphenyldiphosphinomethane (dppm)) with [Et3Si][B(C6F5)4] in methylene chloride under 1 atm of hydrogen gas affords the dicationic compressed dihydride complexes [Cplr(P-P)H2][B(C6F5)4]2. These dicationic complexes are highly acidic and are very readily deprotonated to the corresponding monohydride cations. When the preparative reaction is carried out under HD gas, the hydride resonance exhibits JHD = 7-9 Hz, depending upon the temperature of observation, with higher values of HHD observed at higher temperatures. A thermally labile rhodium analogue, [CpRh(dmpm)(H2)][B(C6F5)4]2, was prepared similarly. A sample prepared with HD gas gave JHD = 31 Hz and JHRh = 31 Hz, allowing the Rh complex to be identified as a dihydrogen complex. Quantum dynamics calculations on a density functional theory (DFT) potential energy surface have been used to explore the structure of the Ir complexes, with particular emphasis on the nature of the potential energy surface governing the interaction between the two hydride ligands and the lr center. [ABSTRACT FROM AUTHOR]