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Computational analysis of complete gas-phase potential energy and free energy surfaces of the adenine···thymine base pair has been carried out. The study utilizes a combination of molecular dynamics simulations performed with Cornell et al. empirical force field and quenching technique. Twenty seven energy minima have been located at the potential energy surface of the adenine···thymine base pair: nine of them are H-bonded structures, eight are T-shaped dimers, and the remaining nine correspond to various stacked arrangements. H-bonded structures are the most stable while stacked and T-shaped structures are by more than 4 kcal/mol less stable than the global minimum. The global minimum and the first two local minima utilize N9−H and N3 groups of adenine for the binding, i.e., the amino group N6, and ring N1 and N7 adenine positions are not involved in the base pairing. The most stable H-bonding patterns cannot occur in nucleic acids since the N9 position is blocked by the attached sugar ring. Hoogsteen a...

The molecular geometry of monomeric and dimeric gold trifluoride, AuF3 and Au2F6, has been determined by gas-phase electron diffraction and high-level quantum chemical calculations. Both experiment and computation indicate that the ground-state structure of AuF3 has C2v symmetry, rather than 3-fold symmetry, with one shorter and two longer Au−F bonds and an almost T-shaped form, due to a first-order Jahn−Teller effect. CASSCF calculations show the triplet D3h symmetry structure, 3A‘, to lie about 42 kcal/mol above the 1A1 symmetry ground state and the D3h symmetry singlet, 1A‘, even higher than the triplet state, by about a further 13 kcal/mol. The molecule has a typical “Mexican-hat”-type potential energy surface with three equal minimum-energy structures around the brim of the hat, separated by equal-height transition structures, about 3.6 kcal/mol above the minimum energy. The geometry of the transition structure has also been calculated. The dimer has a D2h symmetry planar, halogen-bridged geometry, w...

(12/11)CASSCF and (12/11)CASPT2 ab initio electronic structure calculations with both the cc-pVDZ and cc-pVTZ basis sets find that there is a barrier to the very exothermic hydrogen shift that converts singlet methylnitrene, CH3N, to methyleneimine, H2CNH. These two energy minima are connected by a transition structure of Cs symmetry, which is computed to lie 3.8 kcal/mol above the reactant at the (12/11)CASPT2/cc-pVTZ//(12/11)CASSCF/cc-pVTZ level of theory. The (12/11)CASSCF/cc-pVTZ value for the lowest frequency vibration in the transition structure is 854 cm-1, and CASPT2 calculations concur that this a‘ ‘ vibration does indeed have a positive force constant. Thus, there is no evidence that this geometry is actually a mountain top, rather than a transition structure, on the global potential energy surface or that a C1 pathway of lower energy connects the reactant to the product. Therefore, our computational results indicate that the bands seen for singlet methylnitrene in the negative ion photoelectron...

trans-Dioxo complexes of rhenium(V) and osmium(VI) with ethylenediamine, imidazole, and oxalate ancillary ligands show luminescence maxima varying between 620 and 900 nm. The resolved vibronic progressions of the rhenium complexes have intensity distributions that cannot be rationalized with harmonic potential energy surfaces. The broadband spectra of the osmium(VI) complexes also show deviations from the band shapes expected for a harmonic ground-state potential surface. We use the large variation of luminescence energies and the vibronic features to show the influence of excited electronic states on the ground-state potential energy surface. The vibronic structure and band envelopes of the luminescence spectra are calculated using the lowest energy adiabatic surface obtained from three interacting electronic states.

An attractive manner of fighting back-electron transfer to the ground state in photoinduced electron transfer reactions is to use a system in which the donor and/or the acceptor in the ion-pair undergoes a rapid fragmentation. Intuitively, it seems that an ideal situation in this respect, leading to a unity quantum yield, should be met when fragmentation and electron transfer are concerted. Accordingly, a quantum yield below 1 would be the signature of a nonconcerted two-step mechanism. It is shown, from first principles, that a purely dissociative photoinduced electron transfer is not necessarily endowed with a unity quantum yield. The reason is that the system partitions between fragmentation and back-electron transfer in the funnel offered by the upper first-order potential energy surface combining the ground state and fragments zero-order surfaces. A semiclassical model is presented, relating the quantum yield to the electronic matrix coupling element, H. Only in the case of a completely nonadiabatic ...

We use ab initio steered molecular dynamics to investigate the mechanically induced ring opening of cyclobutene. We show that the dynamical results can be considered in terms of a force-modified potential energy surface (FMPES). We show how the minimal energy paths for the two possible competing conrotatory and disrotatory ring-opening reactions are affected by external force. We also locate minimal energy pathways in the presence of applied external force and show that the reactant, product, and transition state geometries are altered by the application of external force. The largest effects are on the transition state geometries and barrier heights. Our results provide a framework for future investigations of the role of external force on chemical reactivity.

We present an extensive theoretical study of the iron(II)−bisimine pyridine based ethylene-polymerization catalysts {[2,6-((R)NC(R‘))2−C5H3N]FeC3H7}+ (R = R‘ = H, 1a; R = 2,6-C6H4(i-Pr)2, R‘ = CH3, 1A) recently developed by the groups of Brookhart and Gibson. The study was based on density functional theory (DFT) for the “generic” model system 1a and a combined DFT and molecular mechanics approach for the “real” system 1A. It is shown that the rate-determining step for both termination and propagation in the “real” system is the capture of ethylene by 1A. The steric bulk introduced by R = 2,6-C6H4(i-Pr)2 was found to suppress ethylene capture for the termination step and increase the rate of insertion. Termination takes place on the singlet potential energy surface (PES). For propagation the singlet and triplet PES's are close in energy and spin-state change is possible. The quintet states are too high in energy to play any role in polymerization. The model system 1a was found to form an ethylene complex ...

The potential energy surfaces for the thermal reactions of bicyclo[3.2.0]hept-2-ene and norbornene have been explored with density functional theory at the Becke3LYP/6-31G* level. Both concerted and diradical pathways for the retro-Diels−Alder reaction of norbornene have been examined, and the activation parameters and 13C primary kinetic isotope effects predicted for the concerted pathway are in excellent agreement with experimental data. The concerted mechanism is favored over the lowest energy stepwise diradical route by 12.4 kcal/mol. For the orbital symmetry-allowed suprafacial-inversion (si) pathway of the [1,3] sigmatropic rearrangement of bicyclo[3.2.0]hept-2-ene to form norbornene, a mechanism involving a transition state which leads to a broad diradical plateau on the potential energy surface is predicted. Implications of these surfaces, which differ substantially from those obtained by semiempirical calculations, are also discussed.

We have calculated the reaction rate and kinetic isotope effects for conversion of 2-phospho-d-glycerate to phosphoenolpyruvate by yeast enolase. The potential energy surface is modeled by a combined quantum mechanical/molecular mechanical method with generalized hybrid orbitals. The dynamics calculations are carried out by semiclassical variational transition state theory with multidimensional tunneling contributions. Quantum effects are included for a 25-atom cluster consisting of the substrate and part of the protein embedded in a rigid framework consisting of the rest of the protein and water. Quantum effects are important for calculating the absolute rate constant, and variational optimization of the dynamical bottleneck location is important for calculating the kinetic isotope effects. This provides the first evidence that transition state geometries are isotope dependent for enzyme reactions.

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 C(s) 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.

The isomerization rates of a calix[4]arene in benzene and in chloroform have been calculated by using molecular dynamics simulations. The reaction coordinate that is employed is based on the unstable normal mode at the saddle point of the potential energy surface. The free energy as a function of this coordinate has been calculated by means of umbrella sampling. Comparison of the free energies in the solvents with those in vacuum reveals that both solvents destabilize the paco conformation and stabilize the transition state region. In chloroform the calix[4]arene shows a stronger preference for the cone conformation than in benzene or in vacuum. The isomerization rate has been determined by the reactive flux method. In benzene the transmission coefficient is about a third higher than in chloroform. The calculated rates are in perfect agreement with experimental data.

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.

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.

The NMR chemical shifts of the proton participating in the intramolecular hydrogen bond in a realistic model of hexabenzyloxymethyl-XDK [m-xylidenediamine-bis(Kemp's triacid)-imide] monoanion and hydrogen oxalate anion have been theoretically analyzed. Ab initio and density functional theory (DFT) calculations are fitted to a monodimensional potential energy surface where the nuclear Schrodinger equation can be solved to obtain the vibrational levels and their corresponding wave functions. Our results indicate that for hexabenzyloxymethyl-XDK monoanion, the first vibrational level appears above the transition state, and the ground vibrational state wave function has a maximum value just at the transition state region so that, as observed experimentally, the hexabenzyloxymethyl-XDK monoanion has a low-barrier hydrogen bond. Conversely, for the hydrogen oxalate anion, the ground vibrational level is well below the energy barrier separating the two minima so that the proton is most probably found at or near ...

High level ab initio calculations performed at the G2 level have been used to characterize the potential energy surface for rearrangement and fragmentation of [CH3CH2OCH2]+. Ion−neutral complexes play a crucial role in the lowest-energy decomposition leading to elimination of water. RRKM calculations on the G2 surface are found to give results consistent with the experimentally observed metastable ion elimination products and with the results of various deuterium- and 13C-labeling experiments. The consequences of producing [CH3CH2OCH2]+ by isomerization from [CH3CHOCH3]+ and of producing [CH3CH2CHOH]+ by isomerization from [CH3C(OH)CH3]+ in relation to the rearrangement/fragmentation behavior are examined and found to be nicely consistent with experimental observations. Experimental thermochemical quantities such as rate-limiting barriers and heats of formation are found generally to be in good agreement with calculated values.

The rotational potential energy surface (PES) of methyl vinyl sulfoxide (1) was calculated at MP2/6-31+G*, B3LYP/6-311+G*, B3LYP/3-21G(*), MP2/3-21G(*), PM3, and AM1. All ab initio methods gave two conformations: one with the S−O bond (a) syncoplanar to the carbon−carbon double bond and one with the lone pair in plane (b); the first one is preferred by 1.7 kcal mol-1. The energy difference is strongly basis set dependent: B3LYP/6-311+G* offers the best compromise. Semiempirical methods give a qualitatively different rotational PES. Whereas the effect methyl group in the E-position is small, Z-substitution leads to destabilization of conformation a through sterical interaction, so that conformation b is preferred by 0.4 kcal mol-1. Electron-withdrawing substituents such as ester or keto groups in the α-position destabilize conformations c and d where the lone pair is syncoplanar to the CC double bond, so that the syncoplanar orientation of the S−O bond is favored by ca. 5 kcal mol-1, depending on the sub...

Ab initio molecular orbital calculations at the G2 level of theory have been used to characterize the potential energy surface for the reactions of phosphirenium or phosphiranium ions with acetylen...

The potential energy surfaces of all eight d-aldohexoses and four d-ketohexoses have been extensively studied, employing quantum mechanical and molecular mechanical calculations. Anomeric preferences for the axial OH positions were observed for all of the hexoses studied. Several stability factors determining the potential energy surface were examined, and we found that the Hassel−Ottar effect, the delta-two effect and the cis−trans effect are not discernible on the gas-phase potential energy surfaces of the cyclic hexoses. Instead, the anomeric effect and intramolecular hydrogen bonding effects dominate. The most important finding in this study is that in the gas phase, furanose forms are more stable than pyranose forms for all of the hexoses except galactose, idose, psicose, and sorbose, in contrast to the generally greater stability of pyranose forms observed in solution. The decrease of intramolecular hydrogen bonding stabilization in solution was found to be primarily responsible for the composition ...

In this paper we have used a DFT (B3LYP) approach to investigate the potential energy surface for the reaction between ethylene and (chloromethyl)zinc chloride (ClCH2ZnCl), which represent a model system for the Simmons−Smith cyclopropanation reaction. Two reaction channels have been found: one leads to the cyclopropane product (addition channel) and the other to the propene product (insertion channel). The addition reaction has an activation energy of 24.7 kcal mol-1 and, as experimentally found, is favored with respect to the insertion, which is characterized by a larger activation energy (36.0 kcal mol-1). The addition transition state corresponds to a three-centered structure which explains the stereochemical features which have been experimentally observed for this reaction. A simple diabatic model is used to rationalize the reactivity pattern that characterizes the Simmons−Smith cyclopropanation and the different behavior observed for the reaction between singlet methylene 1CH2 and olefins.

Computational studies on ketyl anion radicals with methyl chloride and on ω-chloroalkanal radical anions, Cl(CH2)nC(H)O-• (n = 2, 3), find competing mechanisms: a dissociative electron transfer (ET) mechanism and a substitution (SUB(C)) mechanism leading to a C-alkylation product. H(CN)CO-•/CH3Cl proceeds unequivocally via the SUB(C) mechanism, and ω-chloroalkanal radical anions proceed by the ET mechanism, but the interpretation of the mechanism for H2CO-•/CH3Cl depends on the coordinate system used to explore the path. The steepest descent path in Z-matrix internal coordinates leads to the ET product at both the ROHF/6-31G* and UHF/6-31G* levels. The mass-weighted path leads to the ET product on the restricted open-shell Hartree−Fock (ROHF) surface but to the SUB(C) product on the unrestricted Hartree−Fock (UHF) surface. A valley−ridge inflection point heading in the direction of ET products was located on the mass-weighted UHF path, indicating that the potential energy surface branches toward ET produ...

The photoelectron spectrum of the dehydrocyclooctatetraene negative ion, C8H6-, is reported. The spectrum strongly resembles that previously reported for the cyclooctatetraene anion, indicating that the structure of C8H6- is very similar to that of C8H8-. Two electronic states of dehydrocyclooctatetraene are observed in the photoelectron spectrum. The lowest energy feature is assigned to singlet 1,3,5-cyclooctatrien-7-yne, while the higher energy band corresponds to a triplet state of dehydrocyclooctatetraene. The electron affinity of C8H6 is found to be 1.044 ± 0.008 eV, and the energy difference between the singlet and triplet states is 0.708 ± 0.006 eV. Vibrational activity is observed in the photoelectron spectrum and assigned using a simple potential energy surface. Stretching of the triple bond in cyclooctatrienyne is found to have a frequency of 2185 cm-1, essentially what is expected for a triple bond within an eight-membered ring. Ab initio and density functional molecular orbital calculations on...

The reaction mechanism associated with the decomposition of three α-hydroxycarboxylic acids (glycolic, lactic, and 2-hydroxyisobutyric) in the gas phase to form carbon monoxide, water, and the corresponding carbonyl compounds has been theoretically characterized by using ab initio analytical gradients at the MP2 level of theory with the 6-31G** and 6-31++G** basis sets. A detailed characterization of the potential energy surface points out the existence of three competitive reaction pathways for the decomposition process. The first pathway describes a two-step mechanism, with water elimination and formation of an α-lactone intermediate, achieved by the nucleophilic attack of the carbonylic oxygen atom of the carboxyl group (mechanism A). The second pathway is also a two-step mechanism, but in this case the formation of the α-lactone is obtained by means of the nucleophilic attack of the hydroxylic oxygen atom of the carboxyl group (mechanism B). These two pathways share a common second step in which the α...

The conversion of 4-chlorobenzoate to 4-hydroxybenzoate is carried out by first esterifying 4-chlorobenzoate by coenzyme A, and the resulting 4-chlorobenzoyl CoA serves as substrate for the 4-chlorobenzoyl CoA dehalogenase. To gain a better appreciation of the catalytic mechanism and factors controlling the catalytic efficacy of this dehalogenase, the nucleophilic aromatic substitution reaction between 4-Cl-Ph-CO-SCH3 and CH3COO- was investigated in detail in both gas phase and solution (the -CH2COO- entity of Asp145 is the enzyme nucleophile). Quantum mechanical methods (HF/6-31G*, B3LYP/6-311+G**, and PM3) were used to elucidate the gas phase reaction. The gas phase reaction profile is best described as a two-well potential surface, with the two minima corresponding to reactant-side and product-side ion−molecule complexes. There is a small overall potential energy barrier, but the free energy barrier is significant. On the HF/6-31G* potential energy surface, no minimum corresponding to the Meisenheimer ...

Statistical theory is used to calculate the F- + CH3Cl → FCH3 + Cl- rate constant versus relative translational energy Erel and CH3Cl temperature T. The calculations are performed on a potential energy surface derived from MP2 and QCISD(T) ab initio calculations with the 6-311++G(2df,2pd) basis set. At best, statistical theory only qualitatively reproduces the dependence of the experimental rate constant on translational energy and temperature. Using the height of the central barrier with respect to the pre-reaction complex as an adjustable parameter, the experimental rate constant at Erel = 0.9 kcal/mol and T = 296 K may be fit by statistical theory with a central barrier 3 kcal/mol higher than that determined from the QCISD(T) calculation. The calculation of the SN2 rate constant is insensitive to whether the unified statistical model or standard RRKM branching ratio expression is used. Also, including anharmonicity for the F-- - -CH3Cl complex does not affect the calculated rate constant. A comparison ...

Recent density functionals and a variety of basis sets were employed in the study of three electronic states of the succinimidyl radical in C2v symmetry: 2A1 (σN), 2B1 (πN), and 2B2 (σO). The lowest energy σO state is a genuine (local) minimum, as demonstrated by the harmonic vibrational analysis; the stationary points corresponding to the two remaining states are higher-order saddle points on the potential energy surface. Similar results were obtained with the Moller−Plesset method, while the complete active space calculations predict that the πN state has the lowest total energy. The ring-opening reaction from the σO state is symmetry allowed and was calculated with the density functional theory to proceed via a true transition state. The β-scission process was found to exhibit a large secondary deuterium kinetic isotope effect, with kH/kD = 1.2. The infrared spectra of the open forms of the radical that result from β-scission reaction were calculated and compared to the calculated spectrum of β-bromop...

The rearrangement of singlet phenylnitrene (1a) to 1-azacyclohepta-1,2,4,6-tetraene (3a) has been studied computationally, using the CASSCF and CASPT2N methods in conjunction with the 6-31G*, cc-pVDZ, and 6-311G(2d,p) basis sets. Ring expansion from the 1A2 state of 1a is predicted to occur in two steps via 7-azabicyclo[4.1.0]hepta-2,4,6-triene (2a) as an intermediate. The rearrangement of 1a to 2a is estimated to have a barrier of ca. 6 kcal/mol and to be rate-determining. Azirine 2a is unlikely to be detected, because of the small calculated barrier (ca. 3 kcal/mol) to its rearrangement to 3a. At the CASPT2N/6-311G(2d,p)//CASSCF(8,8)/6-31G* + ZPE level of theory, the reaction 1a → 3a on the lowest singlet potential energy surface is calculated to be exothermic by 1.6 kcal/mol. This reaction is predicted to be ca. 19 kcal/mol less exothermic, but to have a barrier ca. 9 kcal/mol lower than the analogous ring expansion of 1A‘ phenylcarbene (1b) to cyclohepta-1,2,4,6-tetraene (3b). Factors which contribute...

The naphthylcarbene potential energy surface (PES) was examined ab initio, employing self-consistent field (SCF), second-order perturbation theory (MP2), and density functional (Becke3LYP) methods in conjunction with 6-31G*, DZ, DZP, and 6-311+G* basis sets. All stationary structures were characterized by vibrational frequency analyses at the Becke3LYP/6-31G* level; final energies were evaluated at the Becke3LYP/6-311+G*//Becke3LYP/6-31G* + ZPVE level. Cyclobuta[de]naphthalene is the global minimum on this part of the C11H8 PES. Generally, seven-membered benzocarbenes are no minima as they converge to their corresponding allenes. Both 1- and 2-naphthylcarbene have triplet ground states, but the small S−T gaps (ca. 5 kcal mol-1) allow facile rearrangements in the singlet manifold to take place. The triplet rotational barrier for the exo-methylene in 2-naphthylcarbene is relatively small (3.5 kcal mol-1) due to weak π-bonding. At low temperatures, singlet 2-naphthylcarbene equilibrates with 2,3-benzobicyclo...

The interconversion of model compounds {[(NH3)3Cu]2(μ-η2:η2-O2)}2+ (1) and {[(NH3)3Cu]2(μ-O)2}2+ (2) has been examined using multireference second-order perturbation theory with an 8-electron/8-orbital active space. At this level of theory, 1 and 2 are separated by only 0.3 kcal/mol, and the barrier to isomerization is predicted to be very low based on single-point energy calculations for intermediate structures. The flat nature of the potential energy surface along the interconversion coordinate derives from a balancing of Coulomb forces and nondynamic electron correlation. The latter effect depends critically on the significant energy change experienced by the 13au virtual orbital on passing from one isomer to the other. In addition, solvation electrostatics favor 2 over 1.

The potential energy surface for rotation of the exocyclic hydroxymethyl group of α-d-glucopyranose has been studied using ab initio quantum mechanical methods. Relevant stationary points, including for the first time rotational transition states, have been characterized by full geometry optimization using basis sets ranging in quality from 6-31G(d) to 6-311(2d,1p). Effects of dynamical electron correlation on both the geometric structures and the energy surface are also investigated using second-order Moller−Plesset perturbation theory (MP2) and density functional methods (BLYP). A total of six stationary points along the hydroxymethyl rotational surface, including three minima and three transition states, were identified. The effects of basis set augmentation and electron correlation on the relative energies are small; the relative energies for each stationary point vary by less than 5 kJ mol-1 for all levels of theory considered. Final energetic barriers to hydroxymethyl rotation ranged from 15 to 29 k...

High-Pressure Mass Spectrometric (HPMS) experiments have been carried out to probe the details of the double minimum potential energy surface for gas-phase SN2 reactions. The well depths and entropy changes associated with the formation of entrance and exit channel electrostatic complexes for the chloride and bromide adducts of methyl, ethyl, isopropyl, and tert-butyl chlorides and bromides have been determined from the temperature dependence of the equilibrium constants for adduct formation. In the cases of “symmetric” complexes associated with identity SN2 reactions, there is an increase in well depth as the size and, therefore, polarizability of the alkyl group increases. Concomitant with this is an increase in the magnitude of the negative entropy change for complex formation which is the result of an increase in the frequency of the intermolecular mode(s) of the complex arising from the increased bond strength. The data for the unsymmetrical adducts for the non-identity SN2 reactions show the same pa...

While the strongly bound D3h AH3+ cations are favored energetically for CH3+ and SiH3+, Cs HA+···H2 complexes are computed to be the global minima for A = Sn and Pb. The structures, relative energies, and dissociation limits (Do) of the AH3+ cations (A = C, Si, Ge, Sn, and Pb) were studied at the Becke3LYP density functional level [6-311++G(2d,2p) basis sets for C, Si, and Ge, (quasi)relativistic effective core potentials with TZ+2P valence basis sets for Sn and Pb]. The SiH3+ potential energy surface computed for calibration at the CCSD(T) level is in very good quantitative agreement with the Becke3LYP results. The AH3+ → HA+···H2 rearrangement illustrates the rare case where two transition states are connected directly (one high-lying, 45−58 kcal mol-1 vs D3h AH3+, and one lower in energy, 0−37 kcal mol-1) without an intervening minimum. Due to the appreciable Do values for both the AH3+ and the HA+···H2 species, they should be experimentally verifiable.

Density-functional theory has been applied to the study of the mechanism of the Diels−Alder reaction of butadiene and ethylene. Both synchronous concerted and two-step diradical mechanisms were studied at the Becke3LYP/6-31G* level. The lowest energy stepwise pathway has a free energy of activation 7.7 kcal/mol above that of the concerted path. Spin correction of the spin-contaminated diradical transition structure energy reduces this energy difference to 2.3 kcal/mol. A study of the H2 potential energy surface suggests that the spin-projection procedure overcorrects the energies of diradical species; the diradical energies likely fall between the corrected and uncorrected values. Thus, the free energy of concert for the Diels−Alder reaction is predicted to be between 2.3 and 7.7 kcal/mol, in excellent agreement with thermochemical estimates. Energies of reaction and geometries of the reactants and product are in good agreement with available experimental results. Calculated secondary kinetic isotope effe...

The potential energy surface corresponding to the reaction of the cobalt cation with ethane, which represents a prototype of the activation of C−C and C−H bonds in alkanes by transition metal cations, has been investigated by employing the recently suggested density functional theory/Hartree−Fock hybrid method B3LYP combined with reasonably large one-particle basis sets. The quality of this approach has been calibrated against experimentally known Co−R+ binding energies of possible exit channels and against CCSD(T) energy calculations. The performance of the chosen model is satisyfing with respect to the description of relative energies, although some nonuniform deviations between calculated and experimental data have been found for absolute binding energies. The calculated barriers for the initial insertion steps of Co+ into C−C and C−H bonds are undoubtedly found to be considerably less energy demanding than the activation barriers connected with the reductive elimination of H2 and CH4, respectively. Th...

A potential energy surface for the reaction of vinyl radical with molecular oxygen has been studied using the ab initio G2M(RCC, MP2) method. The most favorable reaction pathway leading to the major CHO+CH{sub 2}O products is described. The C{sub 2}H{sub 3}O+O products can be formed by elimination of the oxygen atom from C{sub 2}H{sub 3}OO via TS 23, which is by 7.8 kcal/mol lower in energy than the reactants, but by 6.5 kcal/mol higher than TS 9`. The hydrogen migration in 1` gives rise to another significant product channel: C{sub 2}H{sub 3}+O{sub 2} {yields} 1` {yields} TS 25` {yields} C{sub 2}H{sub 2}+O{sub 2}H, with TS 25` lying below C{sub 2}H{sub 3}+O{sub 2} by 3.5 kcal/mol. Multichannel RRKM calculations have been carried out for the total and individual rate constants for various channels using the G2M(RCC, MP2) energetics and molecular parameters of the intermediates and transition states. The computed low pressure reaction rate constant is in quantitative agreement with experiment. At atmospheric pressure, the title reaction is dominated by the stabilization of vinylperoxy radical C{sub 2}H{sub 3}OO at room temperature. In the 500-900 K temperature range, the CHO+CH{sub 2}O channel has the highest rate constant, and at T >= 900 K,more » C{sub 2}H{sub 3}O+O are the major products. At very high temperatures, the channel producing C{sub 2}H{sub 2} + O{sub 2}H becomes competitive. 15 refs., 3 figs., 4 tabs.« less

The conformational behavior of the synthetic glycosidase inhibitor C-lactose (1) has been studied in different solvents (water, N,N-dimethylformamide, dimethyl sulfoxide, pyridine) using NMR spectroscopy and molecular mechanics calculations. The obtained results have been compared to those previously obtained for its natural analogue, methyl α-lactoside (2). It is shown that the conformational behavior of C- and O-lactoses is only similar around the glycosidic bond, but not around the aglyconic bond. In addition, the extent of flexibility around the β(1→4) linkage is much larger for C-lactose (1) than for methyl α-lactoside, about 23% of the complete potential energy surface of 1 is appreciably populated, and several energy minima coexist in solution. The obtained results indicate that β-linked C-glycosides are fairly flexible compounds and that even variations of the solvent may heavily affect their conformational behavior. Finally, we report on the use of 2D transferred NOE experiments to study the reco...

The lowest singlet and triplet states of cyclopropyne and silacyclopropyne have been investigated using ab initio electronic structure models. Employing DZP and TZ(2df,2pd) basis sets, optimum geometries and harmonic vibrational frequencies have been obtained with the following methods: restricted Hartree-Fock or self-consistent-field (SCF), two-configuration self-consistent-field (TCSCF), single and double excitation configuration interaction (CISD) and coupled cluster (CCSD), and CCSD incorporating perturbative estimates of connected triple excitations [CCSD(T)]. Although silacyclopropyne has been observed via matrix isolation, cyclopropyne remains a high-lying saddle point on the C{sub 3}H{sub 2} potential energy surface. Structural and electronic differences between these two molecules are explored. The triplet states of cyclopropyne and silacyclopropyne are minima on their potential energy surfaces and lie higher in energy than the corresponding singlet states, by 10 and 40 kcal mol{sup -1}, respectively. 60 refs., 7 figs., 1 tab.

The anomalous fluorescence of azuleneemission from S2 rather than S1was first recognized by Beer and Longuet-Higgins 40 years ago. Femtosecond laser studies and spectroscopic line width measurements have now established that radiationless decay from S1 to the ground state takes place in less than 1 ps. In this paper we show how such ultrafast S1 decay can be explained by relaxation through an unavoided S1/S0 crossing (i.e., a conical intersection). The S1 relaxation dynamics of azulene from its Franck−Condon structure are modeled semiclassically using a hybrid quantum-mechanical/force-field potential (MMVB) which reproduces the structure of the ab initio CASSCF potential energy surface. This simulation suggests that the S1 → S0 decay takes place in femtoseconds before a single oscillation through the crossing is completed.

A global contour plot is described for reactions involving stepwise or concerted addition of two ethylenes to form cyclobutane. The relevant isomers of the various species and of the reaction paths, with plains or valleys, minima, saddle points, and domes or conical intersections, are described. Two collective asymmetric coordinates are introduced as axes for the plot, which presents an overview of the system and which complements the usual 2-dimensional cuts of the many-dimensional potential energy surface. Other global coordinates are also introduced. The plot involves a pointwise minimization of the potential energy with respect to the coordinates not used as axes. A permutation symmetry can be used to derive the various coordinates. Free energy and entropy (or number of states) curves versus a reaction coordinate are discussed.

The lower excited states for the molecules aminobenzonitrile (ABN) and (dimethylamino)benzonitrile (DMABN) have been studied as a function of the twisting and wagging motion of the amino group. Theoretical calculations have been performed using the complete active space (GAS) SCF method in combination with multiconfigurational second Order perturbation theory (CASPT2): Basis sets of the ANO-type (C,N/3s2p1d and H/2s) were employed. Ground state geometries were optimized at the CASSCF level. The excitation energies were computed as function of a twist angle, where the amino group is rotated with respect to the benzonitrile plane, and for two values of the wagging angle (0 and 21 degrees). The influence of the wagging angle in the nontwisted molecules was also analyzed. The results fully confirm the twist intramolecular charge transfer (TICT) model proposed to explain the dual fluorescence phenomena occurring in DMABN. The absence of the low frequency part of the fluorescence spectrum in ABN in explained by the shape of the potential energy surface along the isomerization path, due to the large energy gaps among the interacting states, which prevents the amino group from rotating into the TICT state. Calculated transition energies (absorption and emission), structural, and electrical properties of the ground and excited states are in agreement with available experimental information.

N-alkoxyheterocycles can act as powerful one-electron acceptors in photochemical electron-transfer reactions. One-electron reduction of these species results in formation of a radical that undergoes N-O bond fragmentation to form an alkoxy radical and a neutral heterocycle. The kinetics of this N-O bond fragmentation reaction have been determined for a series of radicals with varying substituents and extents of delocalization. Rate constants varying over 7 orders of magnitude are obtained. A reaction potential energy surface is described that involves avoidance of a conical intersection. A molecular basis for the variation of the reaction rate constant with radical structure is given in terms of the relationship between the energies of the important molecular orbitals and the reaction potential energy surface. Ab initio and density functional electronic structure calculations provide support for the proposed reaction energy surface.

The working mechanism of a geometrically overcrowded, chiral stilbene light-driven molecular rotary motor [(2R,2R)-2,2',7,7'-tetramethyl-1,1'-bis(indanylidene), 3] has been investigated by a potential energy surface (PES) study. The reaction paths of the two photoinitiated cis-trans (or E/Z) isomerization processes, namely, (P,P)-stable-cis→(M,M)-unstable-trans-3 and (P,P)-stable-trans→(M,M)-unstable-cis-3, have been explored at the CASPT2//CASSCF level of theory. The minimal energy reaction paths (MEPs) of these two processes are nearly parallel on the PESs, separated by a ridge of high inversion barrier. The MEPs have a remarkably steep slope, which drives C═C bond rotation unidirectionally. The asymmetric bias on the excited-state MEPs is caused by the substituents on the "fjord" region as well as by the phenyl moieties. The overall photoisomerization reaction can be described as a three-state (1B→2A→1A) multimode mechanism: The molecule excited to the 1B state first crosses one of the sloped 1B/2A seams, and then follows two cooperative torsional reaction modes to cross preferentially one of the two 2A/1A conical intersections to reach the isomerized ground-state product.

The portion of the C[sub 2]H[sub 2]O potential energy hypersurface that includes oxirene, formylmethylene, and ketene has been studied with ab initio methods incorporating high levels of electron correlation and basis sets that include up to f and g functions. Our best geometries were determined at the CCSD(T)/6-3 UG(df,p) level of theory. Single-point energies were then determined by using CCSD(T) and BD(T) calculations with the cc-pVTZ basis set augmented with additional f and g polarization functions. Our main conclusion is that there is little or no barrier separating formylmethylene from oxirene, the potential energy surface linking these two species begin extremely flat. On the other hand, a more significant barrier (21-23 kJ mol[sup [minus]1]) separates formylmethylene or oxirene from ketene. These results strongly support the previously postulated intermediacy of oxirene in the Wolff rearrangement. Ketene is found to lie 325 kJ mol[sup [minus]1] lower in energy than oxirene. Our predicted energy threshold for the scrambling of carbon atoms in ketene (348 kJ mol[sup [minus]1]) is in close agreement with a recent experimental value (340 kJ mol[sup [minus]1]). 42 refs., 5 figs., 3 tabs.

Multiconfigurational wave functions were used to study the inversion processes of bicyclobutane (C4H6) and its isoelectronic congener bicyclodiazoxane (N20 2). The barriers are about 50 ( 47) and 40 (32) kcal/mol, respectively, as calculated with multireference CI (second-order multireference perturbation theory). Multiconfigurational descriptions ofthese systems with simpler GVB wave functions were also carried out. Good agreement between GVB and MCSCF is obtained for geometries. The GVB energetics are not reliable, but relative energies obtained at GVB geometries, using higher levels of theory, provide a reasonable representation of the potential energy surface.

A detailed molecular understanding of hydrocarbon combustion processes is recognized as an important goal. The specific system studied (via ab initio theoretical methods) in this work is the reaction between the ethyl radical (CH[sub 3]CH[sub 2][sup [sm bullet]]) and molecular oxygen (O[sub 2]). Theoretical methods used include restricted open-shell Hartree-Fock (ROHF), configuration interaction including single and double excitation (CISD), coupled cluster including single and double excitations (CCSD), and a perturbational treatment of connected triple excitations [CCSD(T)]. Basis sets up to double-[zeta] plus polarization (DZP) quality were used. Theoretical results indicate that there are two distinct but energetically proximate intramolecular hydrogen-transfer transition states on the potential energy surface. One of the structures corresponds to a [sup 2]A[double prime] electronic state (C[sub s] symmetry), the other to a [sup 2]A electronic state (no symmetry); this latter structure is derived from a [sup 2]A[prime] planar structure at which the energy Hessian possesses two imaginary vibrational frequencies. At the best level of theory used, DZP CCSD(T) including zero-point vibrational energy corrections, the [sup 2]A[double prime] transition state lies 4.5 kcal mol[sup [minus]1] above the reactants (CH[sub 3]CH[sub 2] + O[sub 2]). 44 refs., 3 figs., 7 tabs.

The internal dynamics of the hydrogen bonding network of the water trimer are investigated by tunable far-infrared laser spectroscopy. New intermolecular vibrations have been measured at 87.1 [(HzO),] and 98.1 cm-’ [(DzO),]. Symmetry restrictions produce an exact oblate symmetric rotor pattern in the spectrum, even though theory predicts the trimer structure to be an asymmetric near-planar ring. In addition, each rovibrational transition is split into a quartet. A group theoretical treatment identifies two classes of structural rearrangements to account for these effects. There is considerable current interest in the spectroscopy and dynamics of small water clusters. Experimental and theoretical investigations of these species are motivated by the quest for a detailed understanding of the intermolecular forces and dynamics of the hydrogen bonding networks that operate in the condensed phases of water and in many biological systems.’ Numerous spectroscopic? ab initio,3 and empirical studies4 have addressed the intermolecular dynamics and potential energy surface of the water dimer. A similarly detailed characterization of the water trimer will enable a quantitative comparison with the dimer that could contribute significantly to the understanding of macroscopic systems. For instance, comparison of the water dimer and trimer ab initio intermolecular potential energy surfaces (IPS) predicts that non-pairwise additive forces (“three-body” interactions that can occur in the trimer but not the dimer) contribute 10% of the total binding energy of the trimer. Other predicted “three-body” effects are shorter 0-0 distances and higher average intermolecular vibrational frequencies in the trimer than in the dimer. Similarities in internal dynamics of the dimer and trimer can suggest mechanisms for important processes, such as proton transfer, that occur in the condensed phases of water. Tunable far-infrared laser vibration-rotation-tunneling spectroscopy (FIRVRTS) has emerged as a powerful new tool for addressing such subjects.’ Pugliano and Saykally’ (PLS) recently reported the first detailed experimental study of the cyclic water trimer. In that work, an intermolecular vibration of (DzO), was measured near 89.6 cm-1. This band displayed a strongly perturbed nearsymmetric top rotational pattern with each rovibrational transition split into a quartet. Crude estimates of the 0-0 distances were made by assuming three point masses of 20 amu and adjusting their separations for optimal agreement with the reported rotational constants. The spectral splittings were interpreted as resulting from isomerization tunneling among 96 identical frameworks (48 pairs of enatiomers) via three pathways: (1) “flipping” Aktract published in Aduance ACS Abstracts, March 1, 1994. (1) Saykally, R. J.; Blake, G. A. Science 1993, 259, 1570 and references therein. (2) Fraecr,G.T.Int.Reu.Phys. Chem. 1991,10,189andreferencestberein. ( 3 ) Smith, B. J.; Swanton, D. J.; Pople, J. A,; Scbaefer, H. F., III; Radom, L. J. Chem. Phys. 1990, 92, 1240. Niesor, U.; Corongin, G.; Clementi, E.; Kneller, G.; Bbattacbaraya, D. J. Phys. Chem. 1990,94, 7949. (4) Reimen, J. R.; Watts, R. 0.; Klein, M. L. Chem. Phys. 1982,64,95 and references therein. Cieplak, P.; Kollman, P.; Lybrand, T. J. Chem. Phys. 1990,92,6755. Jorgensen, W. L.; Chandrasekhar, J.; Madura, J.; Impey, R.; Klein, M. J. Chem. Phys. 1983, 79, 926. Townsend, M.; Morse, M.; Rice, S . A. J. Chem. Phys. 1983, 79, 2496. ( 5 ) F’ugliano, N.; Saykally, R. J. Science 1992, 257, 1937. OOO2-7863/94/1516-3507$04.50/0 of a single free hydrogen from one side of the ring to the other; (2) a motion that effectively results in a Cz rotation of a single monomer about its symmetry axis; and (3) a concerted motion that reverses the sense (yclockwise” or “counterclockwise” [cwH.s.0~) of the hydrogen bonding network around the ring. That work precipitated a number of sophisticated theoretical calculations of the trimer structure, vibrational frequencies, and interconversion tunneling dynamics.611 Of these, the calculations by Fowler and SchaefeI.6 are done at the highest level. All highlevel ab initio calculations agree that the lowest energy structure is that shown in Figure 1, and that the flipping motion is nearly free; moreover, all disagree with the crudely estimated 0-0 distances. Wales’’‘’ elegant treatment of the tunneling dynamics identified three low-energy reaction paths on the twelvedimensional IPS and estimated and associated splittings within a high barrier approximation. Schiitz et al.” carried out a detailed treatment of just the three flipping coordinates, giving special attention to the implications of the very low barrier to flipping. The highest-level ab initio calculations performed to date are those of Fowler and Schaefer.6 In this paper, we report the measurement of two new intermolecular vibrations of FIRVRTS. The Berkeley tunable far infrared spectrometer systems employed in this work have been described in detail elsewhere.13 A total of 284 VRT transitions rotationally assigned to a C-type band of (H2O)o and 57 transitionsassigned toan a-type bandof (DzO)3 weremeasured. The transitions of each isotopomer were fit to a symmetric top hamiltonian and the determined molecular constants are listed in Table 1. Portions of the (H2O)s and (D2O)s data are displayed in Figure 2.

The potential energy surface for the reaction of atomic carbon with thiophene has been studied computationally. Intermediates which are energetically viable include the 2- and 3-thienylcarbenes 8 and 11, thiacyclohexa-3,5-dien-2-ylidene, 10, and thiacyclohexa-2,3,5-triene, 6. In accord with experimental data, 6 and 8 are in equilibrium. The lowest-energy pathway for rearrangement of 6 to 8, which is endothermic by 14.5 kcal/mol, involves ring opening to Z-2-penten-4-ynthial which then recloses to carbene 8. A 1,4 addition of C across the diene system in thiophene generates an ylid which rearranges with little or no barrier to cyclopentadienethione, the global minimum on this potential energy surface.

The essential features of the potential energy surface for the thermal decomposition of silane have been calculated with extended basis sets, augmented by correlation corrections. It is predicted that the transition state for the molecular elimination lies 56.9 kcal/mol above silane. For the reverse reaction, the transition state is less than 2 kcal/mol above the separated fragments, silylene and molecular hydrogen, but 4.8 kcal/mol above a long-range potential well. In the latter, the silylene-H/sub 2/ separation is 1.78 A, and the bond in H/sub 2/ has stretched by more than 0.05 A. This indicates a significant electronic interaction between the fragments even at the large fragment separation. The depth of the well is less than 1 kcal/mol at the SCF level of theory, but it increases substantially when correlation is introduced into the wave function. Since the calculated SiH bond energy is 22 kcal/mol larger than the activation energy for the molecular elimination, the homolytic cleavage of silane to form silyl radical is not expected to be an important process in the low-energy pyrolysis of silane.

International audience; Polypyridine ruthenium sulfoxide complexes are intriguing compounds which can display both photochromic and electrochromic properties. These properties are based on the Ru-S → Ru-O linkage isomerization capability of the sulfoxide group. The photoisomerization mechanism is of particular importance in order to understand the photophysical properties of such molecules. Density functional theory calculations demonstrate that the main photoisomerization mechanism is nonadiabatic for the system under study in agreement with the experimental observations. Indeed, funnels for efficient radiationless decay back to the ground state are shown to be easily accessible compared to transition states on the adiabatic triplet potential energy surface. However, we highlight for the first time that triplet metal-centered states play a central role in the photoisomerization mechanism of these compounds.

The combined C-H activation/Cope rearrangement (CHCR) is an effective C-H functionalization process that has been used for the asymmetric synthesis of natural products and pharmaceutical building blocks. Up until now, a detailed understanding of this process was lacking. Herein, we describe a combination of theoretical and experimental studies that have resulted in a coherent description of the likely mechanism of the reaction. Density functional studies on the reactions of rhodium vinylcarbenoids at allylic C-H sites demonstrate that the CHCR proceeds through a concerted, but highly asynchronous, hydride-transfer/C-C bond-forming event. Even though most of the previously known examples of this process are highly diastereoselective, the calculations demonstrate that other transition-states and stereochemical outcomes might be possible by appropriate modifications of the reagents, and this was confirmed experimentally. The calculations also indicate that there is a potential energy surface bifurcation between CHCR and the competing direct C-H insertion.

A simple treatment is used to estimate the tunneling splittings caused by rearrangements in a variety of model cluster systems, includiag (HF) 2 , benzene-Ar, benzene-Ar 2 , C 5 H 5 + , B 8 H 8 2- , and homoatomic clusters bound by the Lenaard-Jones and Morse potentials. Given sufficient generators to represent all the point group operations and feasible rearrangements, the effective molecular symmetry group is calculated along with the connectivity of the minima on the potential energy surface. This defines the secular determinant which provides the best solutions to the multiminima problem that may be written as linear combinations of localized functions. A Huckel-type approximation is then employed, assuming that the only non-zero off-diagonal Hamiltonian matrix elements are between minima which are directly linked by a rearrangement