ABSTRACT
Potential energy curves for all states arising from the interaction of He with the 3p6, 3p54s, and 3p54p configurations of Ar have been determined using high-level electronic structure calculations. The results have been used to examine collisional energy transfer probabilities and spectral line shape parameters (shifting and broadening rate coefficients). The main focus has been on states and transitions that are of relevance to optically pumped He/Ar* laser systems. The line shape predictions were found to be in good agreement with experimental data, while there is notable disagreement for the energy transfer probabilities. The experimental data are found to be at variance with the predictions of standard two-state curve crossing models for energy transfer.
ABSTRACT
Conformation affects a carotenoid's ability to bind selectively to proteins. We calculated adiabatic energy profiles for rotating the ring end-groups around the C6C7 bond and for flexing of the ring with respect to the polyene chain. The choice of computational methods is important. A low, 4.2 kcal/mol barrier to rotation exists for a beta-ring. An 8.3 kcal/mol barrier exists for rotation of an epsilon-ring. Rotation of the epsilon-ring is sensitive to substitution at C3. In the absence of external forces neither beta- nor epsilon-rings are rotationally constrained. The nearly parallel alignment of the beta-ring to the C6C7 bond axis contrasts to the more perpendicular orientation of the epsilon-ring. Flexion of a beta-ring to the minimized epsilon-ring conformation requires approximately 23 kcal/mol; extension of the epsilon-ring to the minimized beta-ring conformation requires approximately 8 kcal/mol. Selectivity associated with beta- versus epsilon-rings is dominated by the inability of the beta-ring to flex to minimize protein/ring steric interactions and maximize van der Waal's attractions with the binding site.
