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The benzene potential energy hyperface was examined employing hybrid Hartree−Fock/density functional theory (B3LYP), second-order perturbation theory (MP2), and the coupled-cluster method with single, double, and perturbative triple excitations [CCSD(T)] in conjunction with DZP and TZ2P basis sets. All stationary points were characterized by harmonic vibrational frequency analyses; intrinsic reaction coordinates were calculated for all transition structures at B3LYP/DZP. Final energies were evaluated at the CCSD(T)/DZP//B3LYP/DZP level and corrected for T = 1373 K. There are three competing mechanisms for the high-temperature intramolecular topomerization of [1,2-13C2]benzene to [1,3-13C2]- and [1,4-13C2]benzene: (a) benzene ring contraction to benzvalene (ΔG ⧧ = 93.5 kcal mol-1) followed by ring opening to benzene; (b) degenerate rearrangement of benzvalene via a 1A‘‘ prefulvene TS (ΔG ⧧ = 95.0 kcal mol-1 relative to benzene) generating [1,4-13C2]benzene as a primary reaction product of [1,2-13C2]benzen...

The potential energy surface of the water trimer has been studied through the use of ab initio quantum mechanical methods. Five stationary points were located, including one minimum and two transition states. All geometries were optimized at levels up to the double-[Zeta] plus polarization plus diffuse (DZP + diff) single and double excitation coupled cluster (CCSD) level of theory. CCSD single energy points were obtained for the minimum, two transition states, and the water monomer using the triple-[Zeta] plus double polarization plus diffuse (TZ2P + diff) basis at the geometries predicted by the DZP + diff CCSD method. Reported are the following: geometrical parameters, total and relative energies, harmonic vibrational frequencies and infrared intensities for the minimum, and zero point vibrational energies for the minimum, two transition states, and three separated water molecules. 27 refs., 5 figs., 10 tabs.

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 polycyclic, 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 energy surface 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.

The formation of [C(SH)3]+ (a) by hydride abstraction from HC(SH)3 and its oxidation to the radical dication [C(SH)3]•,2+ (b) were studied to examine the potential of stabilizing carbenium ions via trithio substitution. Potential energy surfaces (PES) were explored at the HF/6-31G* level and energies were refined at the (P)MP4(full,sdtq)/6-31G* level without and with annihilation of spin contaminations. The unpaired π-electron in the radical lies well below the Fermi level and spin polarization and dynamic electron correlation become important. Open Y-conjugated structures 1 (C3h or Cs) and their rotamers 2 (Cs) are favored. Four cyclic, S−S connected, distonic, chiral stereoisomers 3b are local minima for the radical dication. The C−S rotational barriers to isomerization via 4 and automerization via 5 (two isomeric TSs) and the high energies of C3v models 6 indicate stronger S−C π-interactions in the cations 1 and 2 than in the dications. C3h-1b undergoes a Jahn−Teller distortion to Cs-1b‘ but pseudorota...

The dependence of hydrogen-bond interaction energies between identical amides (two formamides and two N-methylacetamides) on the hydrogen bond length (rO···H), the two hydrogen bond angles (θCOH an...

The geometries and force fields of phenylcarbene (PC) and cycloheptatrienylidene (CHT) in their singlet and triplet electronic states as well as of cycloheptatetraene (CHTE) and bicyclo[4.1.0]heptatriene (BCT) and the transition states for the formation and decay of the latter were evaluated by various methods. Relative single point energies were calculated at the CCSD(T)/cc-pVDZ//BLYP/6-31G* level. Finally, the effects of extending the basis set to triple-ζ quality were estimated by (R)MP2 calculations and carried over proportionally to CCSD(T). These calculations show that CHTE which has a strongly distorted allenic structure is the most stable species on that part of the C(CH)6 surface which was examined in the present study, followed by planar 3PC. The strained BCT is found to be nearly degenerate in energy with 1PC, but the high activation energy for its formation from 1PC together with the low activation energy for ring-opening to CHTE suggests that this species cannot persist under the experimental...

The potential energy surface (PES) of C 9 H 9 + has been investigated in the region of the 1,4-bishomotropylium cation (1) and its classical analogue, the cis-8,9-dihydro-1-indenyl cation (2) using second-, third-, and fourth-order MOller-Plesset perturbation theory (MP2, MP3, MP4(SDQ)) in connection with a 6-31G(d) basis set. Calculations show that 1 rather than 2 corresponds to a minimum-energy structure, contrary to recent Hartree-Fock results. Ion 1 is clearly bishomoaromatic with interaction distances of 2.1 A. Its homoaromatic character is reflected by bond and charge equalization in its seven-membered ring, by a similar equalization of calculated IGLO/6-31G(d) 13 C chemical shifts, and by MP2/6-31G(d) CC bond orders typical of aromatic delocalization of 6π electrons

The potential energy surface (PES) of C 9 H 9 + has been explored in the region of the 9-barbaralyl cation (3) at MP2/-, MP3/-, and MP4(SDQ)/6-31G(d). Calculations show that 3 is 6.9 and 4.6 kcal/mol more stable than the nonclassical barbaralyl cation with D 3h symmetry (5) and the bicyclo[3.2.2]nona-3,6,8-trien-2-yl cation (4), respectively. The PES in the vicinity of 3 is rather flat and characterized by a spider network of reaction paths that connect 181 440 different forms of 3 with 90 720 different forms of 4 and 30 240 forms of 5. Both 4 and 5 will not be accessible to experiment, since they sit either at a transition state (TS) or in a very shallow minimum surrounded by TSs of similar energy

The potential energy surface of the cyclobutane radical cation (CB .+ ) has been explored at the QCISD-(T)/6-31G * //UMP2/6-31G * level of theory. Thereby it was found that the first-order Jahn-Teller rhombic and rectangular structures are more stable than the long-bond trapezium structure reported to be the global minimum on the CB .+ surface in previous semiempirical and ab initio SCF calculations. In agreement with ESR experiments, a rhombic structure very flexible to ring puckering was found to be the most stable one

The CH 49 * potential energy surface (PES) is investigated using level ab initio molecular orbital theory. All structures were fully optimized at Hartree-Fock (HF) and correlated levels (HF/6-31G * , MP2(full)/6-31G * , and MP2-(full)/6-31G ** ) followed by single point energy calculations at MP4sdtq/6-31G ** //MP2(full)/6-31G ** . The C s 1, C 3h 2, and C 3v 3 forms of the tert-butyl cation were investigated

A theoretical study has been performed for the thermal rearrangements connecting nitromethane, methyl nitrite, nitrosomethanol, and aci-nitromethane by using the 6-31G* basis set to optimize geometries and introducing correlation at the MP2 level. The lowest calculated rearrangement pathways from nitromethane are to methyl nitrite (..delta..H/sup double dagger/ = 73.5 kcal/mol) and aci-nitromethane (..delta..H/sup double dagger/ = 75.0 kcal/mol). Nitrosomethanol, although predicted to be only 1.6 kcal/mol less stable, is separated by two high barriers from nitromethane. An elimination transition structure connecting methyl nitrite and formaldehyde plus nitroxyl is found to have an enthalpic barrier of 44.1 kcal/mol. Fragmentation reactions have enthalpies of reaction less than calculated barrier heights, suggesting that concerted rearrangements on the CH/sub 3/NO/sub 2/ surface in general will not be observed.

The reaction F+HF→HF+F has been studied using ab initio molecular orbital theory at UHF, MP2, MP3, MP4, CCD, CCSD, CID, CISD, QCISD, QCISD(T), and CASSCF levels of theory with the 3-21G, 6-31G ** , 6-311++G ** , and D95++(3df,2p) basis sets. The UHF, MP4, CIS, CISD, CCSD, QCISD, QCISD(T), and CASSCF levels of theory predict the transition state to be a highly bent C 2υ symmetry structure, whereas MP2, MP3, and CCD levels predict the C 2υ symmetry structure to be a shallow minimum bound by up to 0.4 kcal/mol

As conjectured by Olah et al. (J. Am. Chem. Soc. 1992, 114, 5608), the protonitronium dication (HONO 2+ ) is indeed a viable molecule which can be generated in the gas phase by dissociative electron impact ionization of HNO 3 . Charge stripping of HONO .+ does not result in the formation of a detectable dication, as the vertical removal of an electron leads to a vibrationally hot HONO 2+ molecule which undergoes spontaneous Coulombic explosion to generate H + and NO 2 +

The reaction of ethylene (Et) and its radical cation (Et .+ ), which has received much attention from the experimental side, was studied at th eQCISD(T)/6-31G * //UMP2/631G * level of theory. According to these calculations the primary product of this reaction is a π complex cation (Et---Et] .+ , where the two Et molecules are linked by a single bond of ≃1.9 A and which is bound by 18.2 kcal/mol relative to Et+Et .+ , in good agreement with experiment

Correlated calculations (MP2/6-31G**) of the energies and frequencies at the saddle point, ion-dipole complex, and reactants plus additional energy calculations at selected geometries in the strong interaction region are used to parameterize a multidimensional potential energy function for the title reaction. Semiclassical variational transition-state theory is used to calculate the gas-phase rate coefficient and to determine a semiempirical value of the barrier height to reproduce the experimental value of this coefficient at 300 K. The semiempirical gas-phase barrier height is 3.1 kcal/mol. A new potential energy function with this barrier height is created and used to calculate the temperature-dependent rate coefficients and phenomenological activation energies for both CH{sub 3}Cl and CD{sub 3}Cl over the 200-1,000 K temperature range. The activation energy is predicted to show a large temperature dependence. The kinetic isotope effect is predicted to be 1.04 at room temperature.

A guided-ion beam tandem mass spectrometer is used to study the reactions of Pt(+) with methane, PtCH(2)(+) with H(2) and D(2), and collision-induced dissociation of PtCH(4)(+) and PtCH(2)(+) with Xe. These studies experimentally probe the potential energy surface for the activation of methane by Pt(+). For the reaction of Pt(+) with methane, dehydrogenation to form PtCH(2)(+) + H(2) is exothermic, efficient, and the only process observed at low energies. PtH(+), formed in a simple C-H bond cleavage, dominates the product spectrum at high energies. The observation of a PtH(2)(+) product provides evidence that methane activation proceeds via a (H)(2)PtCH(2)(+) intermediate. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies in eV (kJ/mol) of D(0)(Pt(+)-H) = 2.81 +/- 0.05 (271 +/- 5), D(0)(Pt(+)-2H) = 6.00 +/- 0.12 (579 +/- 12), D(0)(Pt(+)-C) = 5.43 +/- 0.05 (524 +/- 5), D(0)(Pt(+)-CH) = 5.56 +/- 0.10 (536 +/- 10), and D(0)(Pt(+)-CH(3)) = 2.67 +/- 0.08 (258 +/- 8). D(0)(Pt(+)-CH(2)) = 4.80 +/- 0.03 eV (463 +/- 3 kJ/mol) is determined by measuring the forward and reverse reaction rates for Pt(+) + CH(4) right harpoon over left harpoon PtCH(2)(+) + H(2) at thermal energy. We find extensive hydrogen scrambling in the reaction of PtCH(2)(+) with D(2). Collision-induced dissociation (CID) of PtCH(4)(+), identified as the H-Pt(+)-CH(3) intermediate, with Xe reveals a bond energy of 1.77 +/- 0.08 eV (171 +/- 8 kJ/mol) relative to Pt(+) + CH(4). The experimental thermochemistry is favorably compared with density functional theory calculations (B3LYP using several basis sets), which also establish the electronic structures of these species and provide insight into the reaction mechanism. Results for the reaction of Pt(+) with methane are compared with those for the analogous palladium system and the differences in reactivity and mechanism are discussed.

The potential energy surface of the methyl chloride radical anion has been studied by ab initio molecular orbital theory. The following calculated results are obtained, different from the reported C-Cl dissociation potential curves: (1) A shallow least energy point (stationary point) is found within C 3υ symmetry in cases where dynamical correlation effects are taken into consideration

Photolysis of p-tolyldiazirine (6) in the inner phase of a hemicarcerand with four butane-1,4-dioxy linker groups (5) in C(6)D(5)CD(3) at 77 K yields the 5-methylcycloheptatetraene hemicarceplex 5 circle 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(48)-5). Hemicarceplex 5 circle 3b is not formed if the photolysis is carried out in CDCl(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(2). Upon heating solutions of d(48)-5 circle 3b in C(6)D(5)CD(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(48)-5 circle 3b in the temperature range 70-102 degrees C, the activation parameters for the 3b to 1b and 3b to 18 rearrangements are calculated (3b to 1b: DeltaG(373)++ = 27.3 +/- 1.4 kcal/mol, DeltaH(373)++ = 26.4 +/- 1.0 kcal/mol, TDeltaS(373)++ = -0.9 +/- 1.0 kcal/mol; 3b to 18: DeltaG(373)++ = 27.8 +/- 1.4 kcal/mol, DeltaH(373)++ = 19.7 +/- 1.0 kcal/mol, TDeltaS(373)++ = 8.1 +/- 1.0 kcal/mol). These values are compared with those calculated by Geise and Hadad at the B3LYP/6-311+G** 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 enthalpy-entropy compensation observed for the 3b to 18 rearrangement is discussed and interpreted as a result of a hemicarcerand and solvent reorganization along the reaction coordinate. It is taken as indirect evidence for the intermediacy of 2-methylbicyclo[4.1.0]hepta-2,4,6-triene in the 3b to 18 rearrangement.

national science foundation (grant che-9528843 (b.k.c.) and che-9314865 and che-9634576 (w.t.b.)

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(2,6)]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(2,6)]deca-3,8-dien-10-ones 2 has been studied. The ground state (S(0)) and the three lowest excited states ((3)(pi pi*), (1)(n pi*), and (3)(n pi*)) potential energy surfaces (PESs) concerning the prototypical rearrangement of 1a (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)(pi pi*) PES and the other on the (1)(n pi*) PES. Both involve initial C-C gamma-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(0) PES through a (3)(pi pi*)/S(0) surface crossing or a (1)(n pi*)/S(0) conical intersection, each one lying in the vicinity of the corresponding diradical minimum. Once on the S(0) PES, the ring-closure to 2a occurs with virtually no energy barrier. The viability of both pathways was experimentally studied by means of triplet sensitization and quenching studies on the photorearrangement of the substituted Pauson-Khand cycloadduct 1b (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)(n pi*) PES, the reaction product is formed through the (3)(pi pi*) state because the energy barrier involved in the initial C-C gamma-bond cleavage of the enone is much lower in the (3)(pi pi*) PES than in the (1)(n pi*) PES.