Francisco J. Avila

Comment on "Multiconfigurational perturbation theory can predict a false ground state" by C. Camacho, R. Cimiraglia and H. A. Witek, Phys. Chem. Chem. Phys., 2010, 12, 5058.

Prediction of the true ground state of Sc(2) with multiconfigurational perturbation theory requires a balanced active space in building the reference wave function.

Photodissociation mechanism of methyl nitrate. A study with the multistate second-order multiconfigurational perturbation theory

The photodissociation reactions of methyl nitrate CH(3)ONO(2) starting at the 193 and 248 nm photolytic wavelengths have been studied with the second-order multiconfigurational perturbation theory (CASPT2) by computation of numerical energy gradients for stationary points. In addition, energy profiles of reaction paths and vertical excitations have been investigated with the multistate extension of the multiconfigurational second-order perturbation theory (MS-CASPT2). It is found that excitation at 193 nm yields three reaction paths: (i) the so-called slow channel CH(3)ONO(2)--> CH(3)O + NO(2)--> CH(3)O + NO + O; (ii) the fast channel CH(3)ONO(2)--> CH(3)O + NO(2); and (iii) CH(3)ONO(2)--> CH(3)ONO + O. The slow channel starts at the S(4) surface, in contrast, the population of the S(3) state can lead to the fast channel or to direct atomic oxygen extrusion. The rather high relative yield of the channel leading to oxygen extrusion from methyl nitrate is explained on the basis of an S(3)/S(2) conical intersection that transfers the initial excitation localized in the npi* S(3) state to the sigmapi* S(2) state with a consequent weakening of the N-O bond. With respect to photolysis at 248 nm, it was not possible to unambiguously distinguish between S(1) and S(2) as the populated state, however, the S(2) state is suggested as mainly responsible for dissociation at this excitation energy.

Photochemistry of protonated nitrosamine: chemical inertia of NH2NOH+ versus reactivity of NH3NO+.

The photochemical behavior of the protonated simplest nitrosamine [NH2NO-H](+) has been addressed by means of the CASPT2//CASSCF methodology in conjunction with the ANO-L basis sets. The relative stability of the different tautomers, namely, (1) NH2NOH(+), (2) NH3NO(+), and (3) NH2NHO(+), has been considered, and the corresponding tautomerization transition states have been characterized. With respect to the most chemically relevant species, it has been found that NH2NOH(+) corresponds to a bound structure, while NH3NO(+) corresponds to an adduct between NH3 and NO(+) at both CASSCF and CASPT2 levels of theory. Vertical transition calculations and linear interpolations on the homolytic dissociation of NH3NO(+) in combination with previous results on neutral nitrosamine [J. Chem. Phys. 2006, 125, 164311] and neutral N,N-dimethylnitrosamine [J. Org. Chem. 2007, 72, 4741] indicate that, in acidic diluted solutions, the protonation of nitrosamine takes place on the excited surface. The N-N dissociation channels have been studied both in ground and first excited singlet state. An S1/S0 conical intersection is found to be responsible for the photostability of NH2NOH(+). On the contrary, NH3NO(+) is photochemically unstable as its first excited state is purely dissociative. The latter species is characterized by a twofold reactivity: the formation of nitrosyl cation (NO(+)) in the ground state and the photorelease of physiologically relevant nitric oxide radical (NO) in its first excited state.

An approach to the electronic structure of molecular junctions with metal clusters of atomic thickness

TD-DFT calculations predict a linear dependence of the energies of charge transfer states of Agn-pyrazine-Agn molecular junctions on the inverse of the size (1/n) of the linear metal chains. The density of charge (qeff = q/n) in the metal-to-metal charge transfer excited states (CTMM: Agnq-pyrazine-Agn-q) smoothly tunes the electronic structure of the junction, especially the metal-to-molecule charge transfer states (CT0 and CT1) and the first excited singlet of pyrazine (S1,Pz). In enlarged junctions, pyrazine bonds preferably to one of the Agn clusters and this weak adsorption produces a significant unexpected asymmetry for forward and reverse charge transfer processes.