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.

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.

The wide range of variability of the reduction potential (E(0)) of blue-copper proteins has been the subject of a large number of studies in the past several years. In particular, a series of azurin mutants have been recently rationally designed tuning E(0) over a very broad range (700 mV) without significantly altering the redox-active site [Marshall et al., Nature, 2009, 462, 113]. This clearly suggests that interactions outside the primary coordination sphere are relevant to determine E(0) in cupredoxins. However, the molecular determinants of the redox potential variability are still undisclosed. Here, by means of atomistic molecular dynamics simulations and hybrid quantum/classical calculations, the mechanisms that determine the E(0) shift of two azurin mutants with high potential shifts are unravelled. The reduction potentials of native azurin and of the mutants are calculated obtaining results in good agreement with the experiments. The analysis of the simulations reveals that only a small number of residues (including non-mutated ones) are relevant in determining the experimentally observed E(0) variation via site-specific, but diverse, mechanisms. These findings open the path to the rational design of new azurin mutants with different E(0).

In this paper, the perturbed matrix method (PMM) is used in combination with basic statistical mechanics, to develop a general theoretical method to model chemical reactions and related molecular processes in complex systems, i.e., liquids, biochemical systems, macromolecules, etc. The main feature of this approach consists of the explicit treatment of the coupling between the reaction center and the fluctuating atomic-molecular environment, providing a rigorous statistical mechanical description of the chemical event. A special attention is dedicated to the approximations and assumptions necessary to use such a theoretical procedure in combination with simulation data.

The singlet and triplet potential energy surfaces of a series of p-X-substituted aryl cations (X ‫؍‬ H, CN, CH 3 , F, OH, NH 2 ) are investigated computationally at the B3LYP/6-31G(d) level of theory. The first four species are found to be ground state singlets, the last has a triplet ground state, and the spin states of the OH derivative are almost isoenergetic. The minimum energy crossing points (MECPs) between the two surfaces are found to lie very little above the higher of the two minima in all cases, and the spin-orbit coupling is significant at those points. Therefore, it is expected that aryl cations will rapidly convert to their most stable spin state, and that in cases of near degeneracy such as for p-HO-C 6 H 4 ؉ , the states may interconvert rapidly enough to both be accessible in thermal reactions.

We report a combined experimental and theoretical study of the xenon monohalide radicals XeX • ͑XϭF, Cl, Br, and I͒ together with their cationic and anionic counterparts XeX ϩ and XeX Ϫ. In brief, the XeX ϩ cations are characterized by reasonably strong chemical bonds with significant charge-transfer stabilization, except for XϭF. In contrast, the neutral XeX • radicals as well as the XeX Ϫ anions can mostly be described in terms of van der Waals complexes and exhibit bond strengths of only a few tenths of an electron volt. For both XeX • and XeX Ϫ the fluorides ͑XϭF͒ are the most strongly bound among the xenon halides due to significant covalency in the neutral radical, and to the large charge density on fluoride in the XeX Ϫ anion, respectively. Mass spectrometric experiments reveal the different behavior of xenon fluoride as compared to the other halides, and in kiloelectron-volt collisions sequential electron transfer according to XeX ϩ →XeX • →XeX Ϫ can be achieved allowing one to generate neutral XeX • radicals with lifetimes of at least a few microseconds for XϭF and I.

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Conjugated organic polymers based on substituted thiophene units are versatile building blocks of many photoactive materials, such as photochromic molecular switches or solar energy conversion devices. Unraveling the different processes underlying their photochemistry, such as the evolution on different electronic states and multidimensional structural relaxation, is a challenge critical to defining their function. Using femtosecond stimulated Raman scattering (FSRS) supported by quantum chemical calculations, we visualize the reaction pathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initial wavepacket dynamics of the reaction coordinates occurs within the first ≈1.5 ps, followed by a ≈10 ps thermalization. Subsequent slow opening of the thiophene ring through a cleavage of the carbon-sulfur bond triggers an intersystem crossing to the triplet excited state. Our work demonstrates how a detailed mapping of the excited-state dynamics can be obtained, combining simultaneous structural sensitivity and ultrafast temporal resolution of FSRS with the chemical information provided by time-dependent density functional theory calculations.