Theoretical study of the mechanism for spin‐forbidden quenching process O(1D) + CO2(1Σ) → O(3P) + CO2(1Σ)

The mechanism of the spin‐forbidden quenching process O(¹D) + CO₂(¹Σ) → O(³P) + CO₂(¹Σ) was investigated by ab initio quantum chemistry methods. The calculations showed the singlet potential surface [O(¹D)+CO₂] is attractive where a strongly bound intermediate complex CO₃ is formed in the potential basin without a transition state, whereas the complex CO₃ that is formed on the triplet surface [O(³P)+CO₃] must overcome a barrier. The complex channel was documented by searching minimum energy intersection points in the region of the bound complex CO₃ and calculating spin‐orbit coupling at the point. A direct channel was proposed by a study of cross point of singlet and triplet PESs with different collision angles and calculations of spin‐orbit coupling at those cross points in a nonbound region of the [O(¹D)+CO₃] system. The mechanism of the energy transfer is discussed on the basis of the theoretical results. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Conformational Study of Heteropentapeptides Containing an α‐Ethylated α,α‐Disubstituted Amino Acid: (S)‐Butylethylglycine (=2‐Amino‐2‐ethylhexanoic Acid) within a Sequence of Dimethylglycine (=2‐Aminoisobutyric Acid) Residues

Heteropentapeptides containing the α‐ethylated α,α‐disubstituted amino acid (S)‐butylethylglycine and four dimethylglycine residues, i.e., CF₃CO‐[(S)‐Beg]‐(Aib)₄‐OEt ( 4 ) and CF₃CO‐(Aib)₂‐[(S)‐Beg]‐(Aib)₂‐OEt ( 7 ), were synthesized by conventional solution methods. In the solid state, the preferred conformation of 4 was shown to be both a right‐handed (P) and a left‐handed (M) 3₁₀‐helical structure, and that of 7 was a right‐handed (P) 3₁₀‐helical structure. IR, CD, and ¹H‐NMR spectra revealed that the dominant conformation of both 4 and 7 in solution was the 3₁₀‐helical structure. These conformations were also supported by molecular‐mechanics calculations.