Photodissociation of small carbonaceous molecules of astrophysical interest

Astronomical observations have shown that small carbonaceous molecules can persist in interstellar clouds exposed to intense ultraviolet radiation. Current astrochemical models lack quantitative information on photodissociation rates in order to interpret these data. We here present ab initio multi-reference configuration-interaction calculations of the vertical excitation energies, transition dipole moments and oscillator strengths for a number of astrophysically relevant molecules: C3, C4, C2H, l- and c-C3H, l- and c-C3H2, HC3H, l-C4H and l-C5H. Highly excited states up to the 9'th root of each symmetry are computed, and several new states with large oscillator strengths are found below the ionization potentials. These data are used to calculate upper limits on photodissociation rates in the unattenuated interstellar radiation field by assuming that all absorptions above the dissociation limit lead to dissociation.Comment: Full tables, rates and cross sections are posted at http://www.strw.leidenuniv.nl/~ewine/phot

Quantitative first principles calculations of protein circular dichroism in the near-ultraviolet

Vibrational structure in the near-UV circular dichroism (CD) spectra of proteins is an important source of information on protein conformation and can be exploited to study structure and folding. A fully quantitative theory of the relationship between protein conformation and optical spectroscopy would facilitate deeper interpretation and insights into biophysical and simulation studies of protein dynamics and folding. We have developed new models of the aromatic side chain chromophores toluene, p-cresol and 3-methylindole, which incorporate ab initio calculations of the Franck-Condon effect into first principles calculations of CD using an exciton approach. The near-UV CD spectra of 40 proteins are calculated with the new parameter set and the correlation between the computed and the experimental intensity from 270 to 290 nm is much improved. The contribution of individual chromophores to the CD spectra has been calculated for several mutants and in many cases helps rationalize changes in their experimental spectra. Considering conformational flexibility by using families of NMR structures leads to further improvements for some proteins and illustrates an informative level of sensitivity to side chain conformation. In several cases, the near-UV CD calculations can distinguish the native protein structure from a set of computer-generated misfolded decoy structures

Theoretical Prediction of the Magnetic Circular Dichroism Spectrum in Cl2Cl_{2}: The 11Πu←X(1Σg+)1 ^{1}\Pi_{u} \leftarrow X( ^{1}\Sigma _{g}^{+}) Electronic Transition.

1. M. Brith, M.D. Rowe, O. Schnepp, and P.J. Stephens, Chem. Phys. 9, 57 (1975). 2. P.J. Stephens, Adv. Chem. Phys. 35, 197 (1976).""Author Institution: US Army Ballistic Research Laboratory, SLCBR-IB-I, Aberdeen Proving GroundA newly implemented method based on first-order perturbation theory will be used to calculate the electronic magnetic circular dichroism (MCD) spectrum in $Cl_{2}$. Specifically, the $1 ^{1}\Pi_{u} \leftarrow X( ^{1}\Sigma _{g}^{+})$ transition will be calculated via ab initio techniques including state-averaged CASSCF plus Cl. Brith et $al.^{1}$ have predicted that the MCD intensity for this transition arises from a combination of both the ${\cal A}$-term and ${\cal B}$-term as defined by $Stephens^{2}$. The experimental work was supported by a crude theoretical treatment. It is the purpose of this study to investigate the origin of the intensity for the MCD signal of the $1 ^{1}\Pi_{u} \leftarrow X( ^{1}\Sigma _{g}^{+})$, and determine the relative contributions of the ${\cal A}$- and ${\cal B}$-terms

Ab Initio CI study of the Magnetic Circular Dichroism Spectrum of Acetylene for the X−>B~(1Bu)X -> \tilde{B}(^{1}Bu) and X−>C~(1Πu)X -> \tilde{C}(^{1}\Pi u) Electronic Transitions

$^{1}$ D. R. Yarkony. J. Chem. Phys. 86, 1642 (1987); and references therein.Author Institution: US Army Ballistic Research Laboratory, SLCBR-IB-I; Chemical Research. Development, and Engineering Center, SLCBR-IB-I; Department of Chemistry, The Johns Hopkins UniversityA method developed recently for studying spin.orbit (SO) Interactions (1) is here applied to calculating MCD spectra. The method can be briefly described as a first-order perturbation technique which involves solving the set of linear equations: $(\bar{H^{o}}- E)\phi^{1}_{1}- \tilde{H}^{p}\Phi^{o}_{1}=(\tilde{H}^{P} some perturbing Hamiltonian)$ over Configuration State Functions (CSF's) to obtain the first-order correction $\gamma^{1}$; to the electronic state $\Phi_{i}$. This partially alleviates the impossible task of attempting to calculate explicitly the infinite manifold of perturbing states represented in the usual expression for a first-order perturbation. [FIGURE] The MCD B term, which acquires it's intensity through such perturbations, can be calculated via eq. I thus avoiding the need to calculate explicitly the large number of excited states involved in the summation of eq. 2. Results are presented for this new application of the method to the singlet $X -> B(^{1}Bu)$ and $X -> C(^{1}Hu)$ transitions in acetylene