A THEORETICAL-STUDY OF THE SINGLET-TRIPLET ENERGY-GAP DEPENDENCE UPONROTATION AND PYRAMIDALIZATION FOR 1,2-DIHYDROXYETHYLENE - A SIMPLE-MODEL TO STUDY THE ENEDIOL MOIETY IN RUBISCO SUBSTRATE

A quantum chemical study is reported of the variation of the energy ga p between the singlet ground (S) and first triplet (T) states for 1,2- dihydroxyethylene as a function of the rotational angle around the cen tral double bond (phi) and the pyramidalization angle of one CHOH grou p (partial derivative). RHF and UHF levels of theory are used; the ab initio calculations are made with three types of basis sets: 3-21G, 4 -31G, and 6-31G. Correlation effects are estimated with different sta ndard procedures (CISD and MP2) including multiconfiguration CASSCF. F or the sake of completeness, potential energy surfaces for the singlet and triplet states are reported. AM1 calculations are also presented. The potentiality of different approaches to describe the gap is exami ned. The S-T energy gap is most sensitive toward phi. For a twisted sy stem, this gap may be appreciably smaller than the one found for a pla nar conformer. Correlated and uncorrelated wave functions show similar trends in this respect. This S-T energy gap lowering can be used to h elp understand the electronic structure in the enediol moiety of the R ubisco substrate: D-ribulose 1,5-bisphosphate. There, at the active si te, the conformation is cis out of plane. The carboxylation/oxygenatio n bifunctionality of Rubisco can then be given a relatively simple exp lanation.