The phenyl cation is known to have two lowenergy minima, corresponding to 1 A 1 and 3 B 1 states, the ®rst of which is more stable by ca. 25 kcal/mol. The minimum energy crossing point between these two surfaces, located at various levels including a hybrid method ®rst described here, lies just above the minimum of the triplet, 0.12 kcal/mol at the CCSD(T)/cc-pVDZ// B3LYP/SV level, and there is signi®cant spin-orbit coupling between the surfaces at this point. On the basis of these results, the lifetime of the triplet is expected to be very short.

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New density functional theory and ab initio computations on the [Fe(CO)5] system are reported. Careful exploration of basis set and correlation effects leads to "best" values for the difference in energy deltaE(1,3) between ground state 3[Fe(CO)4] and the singlet excited state of ca. 8 kcal mol(-1), and for the bond dissociation energy BDE(3) of [Fe(CO)5] with respect to ground state fragments 3[Fe(CO)4] + CO of ca. 40 kcal mol(-1). A modified form of the B3PW91 functional is used to explore the potential energy surface for the spin-forbidden recombination reaction of CO with 3[Fe(CO)4]. A Cs-symmetric minimum energy crossing point (MECP) between the reactant (triplet) and product (singlet) potential energy surfaces is found, lying 0.43 kcal mol(-1) above the reactants. The rate coefficient for recombination is computed using a non-adiabatic form of transition state theory, in which the MECP is treated as the critical point in the reaction. Semi-quantitative agreement with experiment is obtained: the predicted rate coefficient, 8.8 x 10(-15) cm3 molecule(-1) s(-1), is only six times smaller than the experimental rate. This is the first computation from first principles of a rate coefficient for a spin-forbidden reaction of a transition metal compound.

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The molybdenum chalcogenide cations MoX n + (X = O, S; n = 1−3) are studied by a combined experimental and theoretical approach. The monoligated species MoO+ and MoS+ both have (4Σ-) ground states that formally arise from spin-pairing of Mo+ (6S) with O (3P) and S (3P), respectively. Similarly, the bent triatomic MoX2 + cations exhibit doublet ground states (2A1). The trichalcogenides MoO3 + and MoS3 + also have doublet ground states and exhibit similar C 3 v -symmetrical structures; however, distinct energetic differences are found in that MoO3 + is much less stable than MoS3 +, due to the necessity to ionize a strong Mo−O double bond in neutral MoO3. Sequential addition of chalcogenides to molybdenum goes hand in hand with an increase of the formal oxidation state of the metal. As a result, the ionization energies (IEs) increase with the electronegativity and the number of the chalcogenide atoms added:  IE(MoO) = 7.9 ± 0.3 eV, IE(MoO2) = 8.7 ± 0.3 eV, IE(MoO3) = 11.7 ± 0.3 eV, IE(MoS) = 7.7 ± 0.3 eV, IE(MoS2) = 8.6 ± 0.3 eV, and IE(MoS3) = 8.9 ± 0.3 eV. Thermochemical considerations in conjunction with the ion/molecule reaction bracketing technique and theoretical results provide reevaluated values for the bond dissociation energies (in kcal/mol):  Mo+−O 118 ± 2, OMo+−O 131 ± 5, O2Mo+−O 62 ± 17, Mo+−S 88 ± 14, SMo+−S 100 ± 14, and S2Mo+−S 88 ± 14. Notable differences are observed in the gas-phase reactivity of the MoX n + cations. In general, the molybdenum sulfides are less reactive than the corresponding oxides. The monoligated MoX+ cations promote C−H bond activation of hydrocarbons, while the MoX2 + cations and also MoS3 + are somewhat less reactive. The high-valent transition-metal oxide MoO3 + is the most reactive species and is even capable of activating methane.

The kinetic-energy dependence of the V++CS2 reaction is examined using guided ion-beam mass spectrometry. Several different ion sources are used to systematically vary the V+ electronic state distributions and elucidate the reactivities of both the ground and excited state V+ cation. The cross section for VS+ formation from ground state V+(5D) exhibits two endothermic features corresponding to the formation of ground state VS+(3Σ−) and excited state VS+(5Π). The thresholds for these two processes are in good agreement with theoretically determined excitation energies. The cross section for spin-forbidden formation of ground state VS+(3Σ−) exhibits an unusual variation with kinetic energy that is attributed to the energy dependence of the surface-crossing probability. From the thresholds associated with the formation of VS+ and V(CS)+, D0(V+–S)=3.72±0.09 eV and D0(V+–CS)=1.70±0.08 eV are derived. Further, circumstantial evidence for formation of a high-energy isomer of V(CS)+ is obtained.

The accurate prediction of enzyme kinetics from first principles is one of the central goals of theoretical biochemistry. Currently there is considerable debate [1][2][3] about the applicability of transition state theory (TST) to compute rate constants of enzyme-catalyzed reactions. Classical TST is known to be insufficient in some cases, but corrections for dynamical recrossing and quantum mechanical tunneling can be included. [1,2,4] Many effects that go beyond the framework of TST have been proposed, particularly focusing on the possible role of protein dynamics and conformational effects on the enzyme activity. Unfortunately, the overall importance of these effects for the effective reaction rate is difficult (if not impossible) to determine experimentally. However, if one could compute the quasi-thermodynamical free energy of activation with chemical accuracy (i.e. 1 kcal mol À1 ), comparison with the effective measured free energy of activation would directly show the importance of other effects.Combined quantum mechanical/molecular mechanical (QM/MM) methods have become an important tool for computational modeling of enzyme-catalyzed reactions. Only the substrate(s) and relevant residues in the active site are treated quantum mechanically, the rest of the protein is described at the empirical MM level. This lowers the computational expense, making it possible to treat large enzyme systems and the surrounding solvent, and to sample phase space. Nevertheless, the number of QM atoms is still relatively large, and until now only low levels of QM theory, such as semiempirical methods or density functional theory (DFT), have been feasible. Semiempirical methods, though applicable to large systems, are generally not accurate enough because computed free energies of activation may have an error of ten or more kcal mol À1 . DFT (especially with the B3LYP functional [5] ) offers improved accuracy but still lacks key physical interactions (e.g., dispersion). Often, DFT underestimates barrier heights by several kcal mol À1 , which cannot be systematically improved. Thus, when theoretical barriers do not agree with those from experiment, it is not clear whether the discrepancy arises from deficiencies in the electronic structure theory and the sampling, in the experimental observations, or in the underlying theoretical framework of QM/MM and TST.Consequently, there is a need for high-level electronic structure calculations for reliable predictions of enzyme reactivity. The ab initio electron correlation methods MP2 (Møller-Plesset second-order perturbation theory), CCSD (coupled-cluster theory with single and double excitations), and CCSD(T) (CCSD with a perturbative treatment of triple excitations) provide a well-established hierarchy that converge reliably to give high accuracy, and rate constants of gasphase reactions involving only a few atoms can be predicted with error bars comparable to those found experimentally. [6,7] However, the computational expense of these methods increases very rapidly with the number o...