An ab initio and dynamics study of the photodissociation of nitric acid HNO3

We investigated the photodissociation of HNO3 within the first (300 nm) and the third (200 nm) absorption band. The relevant S1 and S3 potential energy surfaces were calculated by taking into account the N-O single bond and N=O “double” bond distances. The striking feature of the dynamical analysis is a bifurcation of the wave packet on the S3 surface which explains the branching into the two reaction pathways with the products OH+NO2 and O+HONO found in experiments. Dissociation on the S1 surface is predicted to proceed along a single channel leading to OH+NO2, both in their electronic ground states. Corresponding author

Monitoring peptide-surface interaction by means of molecular dynamics simulation

Protein adsorption and protein surface interactions have become an important research topic in recent years. Very recently, for example, it has been shown that protein clusters can undergo a surface-induced spreading after adsorption. Such phenomena emphasize the need of a more detailed insight into protein-silica interaction at an atomic level. Therefore, we have studied a model system consisting of a short peptide, a silica slab, and water molecules by means of classical molecular dynamics simulations. The study reveals that, besides of electrostatic interactions caused by the chosen charge distribution, the peptide interacts with the silica surface through formation of direct peptide-surface hydrogen bonds as well as indirect peptide-water-surface hydrogen bonds. The number of created hydrogen bonds varies considerably among the simulated structures. The strength of hydrogen bonding determines the mobility of the peptide on the surface and the internal flexibility of the adsorbed peptide

Predissociation via conformational change: Photodissociation of N,N-dimethylnitrosamine in the S1 state

We investigated the photodissociation mechanism of N,N-dimethylnitrosamine (CH3)2NNO (DMN) by ab intio quantum chemical methods. Inspired by an earlier study we calculated two-dimensional potential energy surfaces of the S1 state of DMN in its planar and pyramidal conformations. While the planar molecular geometry appears to possess no direct dissociation channel, the pyramidal configuration is dissociative yielding the products NO + (CH3)2N. Using wave packet dynamics on the planar S1 potential energy surface the experimental absorption spectrum was well reproduced which gives indirect but strong support for the nondissociative nature of this surface. The transition from the planar to the pyramidal conformation of DMN was then investigated by an ab initio molecular dynamics method which revealed the time evolution of the geometrical parameters of the molecule up to the dissociation of the N-N bond. This occurs about 90 fs after photon excitation. The calculated minimum energy path along the N-N coordinate and the structural changes of the molecule along this coordinate provided a detailed picture of this indirect dissociation or, more specific, predissociation process via conformational change

SPECTROSCOPIC INVESTIGATION OF HSNO IN A LOW TEMPERATURE MATRIX. UV, VIS, AND IR INDUCED ISOMERIZATIONS

Author Institution: Physikalisch-Chemisches Institut der, Universit\""{a}t Z\""{u}richAn Infrared spectroscopic investigation has been performed on the trans and cis isomers of thionitrous acid (HSNO) and their D- and $^{15}NO$-isotopic modifications in argon matrices at 12 K. The substances were prepared photolytically from thionylimid (HNSO) isotopes in the matrix With UV (250 nm), VIS (585 nm), and IR irradiation the cis $\to$ trans or the trans $\to$ cis isomerization of HSNO was induced, allowing an unequivocal distinction between the closely resembling IR spectra of the trans and cis isomers. Complete set of fundamental frequencies of both rotamers were obtained and assigned by normal coordinate analysis using the transferable valence force field (TVFF) approach. Parallel to this analysis ab-initio calculations on the SCF- and CI-levels were performed to predict energy, geometry, and barrier of internal rotation for the two HSNO rotamers. These results and those of a parallel study on methylthionitrite $(CH_{3}SNO)$ are discussed and compared to the behavior of the nitrite molecules HONO and $CH_{3}ONO$