Barak Hirshberg

Formation of Carbonic Acid in Impact of CO2 on Ice and Water.

A new mode of formation is proposed for carbonic acid in the atmosphere. It involves impact of vibrationally excited gas-phase CO2 molecules on water or ice particles. This is a first mechanism that supports formation on ice as well as on liquid water surfaces. Results of ab initio molecular dynamics simulations are presented on collisions of CO2 with (H2O)n clusters (n = 1, 4, 8, 12). Efficient formation of carbonic acid is seen with product lifetimes exceeding 100 ps. The reaction is feasible even for collision of CO2 with a single water molecule but in a different mechanism than for larger clusters. For clusters, the transition state shows charge separation into H3O(+)···HCO3(-), which transforms into neutral carbonic acid as the product, hydrated by the remaining waters. Possible atmospheric implications of the results are discussed.

Trapping and Structural Characterization of the XNO2·NO3ˉ (X = Cl, Br, I) Exit Channel Complexes in the Water-Mediated Xˉ + N2O5 Reactions with Cryogenic Vibrational Spectroscopy

The heterogeneous reaction of N2O5 with sea spray aerosols yields the ClNO2 molecule, which is postulated to occur through water-mediated charge separation into NO3- and NO2+ ions followed by association with Cl-. Here we address an alternative mechanism where the attack by a halide ion can yield XNO2 by direct insertion in the presence of water. This was accomplished by reacting X-(D2O)n (X = Cl, Br, I) cluster ions with N2O5 to produce ions with stoichiometry [XN2O5]-. These species were cooled in a 20 K ion trap and structurally characterized by vibrational spectroscopy using the D2 messenger tagging technique. Analysis of the resulting band patterns with DFT calculations indicates that they all correspond to exit channel ion-molecule complexes based on the association of NO3- with XNO2, with the NO3- constituent increasingly perturbed in the order I > Br > Cl. These results establish that XNO2 can be generated even when more exoergic reaction pathways involving hydrolysis are available and demonstrate the role of the intermediate [XN2O5]- in the formation of XNO2.

Autocorrelation of electronic wave-functions: a new approach for describing the evolution of electronic structure in the course of dynamics

ABSTRACT We introduce a new approach for analysing changes in electronic structure in the course of ab initio molecular dynamics simulations. The analysis is based on the time autocorrelation function of the many-body electronic wave-function. The approach facilitates the interpretation of dynamical events that may not be easily revealed by consideration of nuclear configurations alone. We apply the method to several illustrative examples: the shared proton vibration in the F−(H2O) complex, representing changes in strength of non-covalent interactions; proton transfer in the water dimer cation, as an example for chemical reactions in weakly bound systems; and the intramolecular proton transfer in malonaldehyde. In all cases, we observe distinct features in the time autocorrelation function when chemical changes occur. The autocorrelation function serves as an effective reaction coordinate, incorporating all degrees of freedom, including electronic ones. The method is also sensitive to changes in the electronic wave-function not accompanied by significant nuclear motions. GRAPHICAL ABSTRACT