Intersystem crossing and dynamics in O(3P) + C2H4 multichannel reaction: experiment validates theory.

The O((3)P) + C(2)H(4) reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H + CH(2)CHO, H + CH(3)CO, H(2) + CH(2)CO, CH(3) + HCO, CH(2) + CH(2)O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4 kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross-sections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH(3) + HCO and H + CH(2)CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.

Experimental and theoretical studies of the O(3P) + C2H4 reaction dynamics: collision energy dependence of branching ratios and extent of intersystem crossing.

The reaction of O((3)P) with C(2)H(4), of importance in combustion and atmospheric chemistry, stands out as paradigm reaction involving not only the indicated triplet state potential energy surface (PES) but also an interleaved singlet PES that is coupled to the triplet surface. This reaction poses great challenges for theory and experiment, owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Crossed molecular beam (CMB) scattering experiments with soft electron ionization detection are used to disentangle the dynamics of this polyatomic multichannel reaction at a collision energy E(c) of 8.4 kcal∕mol. Five different primary products have been identified and characterized, which correspond to the five exothermic competing channels leading to H + CH(2)CHO, H + CH(3)CO, CH(3) + HCO, CH(2) + H(2)CO, and H(2) + CH(2)CO. These experiments extend our previous CMB work at higher collision energy (E(c) ∼ 13 kcal∕mol) and when the results are combined with the literature branching ratios from kinetics experiments at room temperature (E(c) ∼ 1 kcal∕mol), permit to explore the variation of the branching ratios over a wide range of collision energies. In a synergistic fashion, full-dimensional, QCT surface hopping calculations of the O((3)P) + C(2)H(4) reaction using ab initio PESs for the singlet and triplet states and their coupling, are reported at collision energies corresponding to the CMB and the kinetics ones. Both theory and experiment find almost an equal contribution from the triplet and singlet surfaces to the reaction, as seen from the collision energy dependence of branching ratios of product channels and extent of intersystem crossing (ISC). Further detailed comparisons at the level of angular distributions and translational energy distributions are made between theory and experiment for the three primary radical channel products, H + CH(2)CHO, CH(3) + HCO, and CH(2) + H(2)CO. The very good agreement between theory and experiment indicates that QCT surface-hopping calculations, using reliable coupled multidimensional PESs, can yield accurate dynamical information for polyatomic multichannel reactions in which ISC plays an important role.

Clonal population structure of Pseudomonas avellanae strains of different origin based on multilocus enzyme electrophoresis.

To assess the genetic diversity and genetic relationships of Pseudomonas avellanae, the causative agent of hazelnut decline, a total of 102 strains, obtained from central Italy (provinces of Viterbo and Rome) and northern Greece, were studied using multilocus enzyme electrophoresis (MLEE). Their allelic variation in 10 loci was determined. All loci were polymorphic and 53 electrophoretic types (ETs) were identified from the total sample. The mean genetic diversity (H) was 0.65 and this value ranged from 0.37 for the least polymorphic to 0.82 for the most polymorphic locus. The dendrogram originated from MLEE data indicated two main groups of ETs, A and B. The groups do not appear to be correlated to the geographic origin of the strains, although all the ETs from northern Greece clustered into subgroup B1. Pseudomonas syringae pv. actinidiae and P. syringae pv. theae, included in the analysis as outgroups, clustered apart. The index of association (I(A)) for P. avellanae was 0.90. The I(A) values were always significantly different from zero for the population subsets studied and no epidemic structure was found. These results would indicate that the population structure of P. avellanae is clonal either in northern Greece or in central Italy. The recent outbreaks of the bacterium in new areas of hazelnut cultivation would explain the current clonal structure that is persisting over decades.