A THEORETICAL-STUDY OF THE SINGLET-TRIPLET ENERGY-GAP DEPENDENCE UPON ROTATION 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 gap between the singlet ground (S) and first triplet (T) states for 1,2-dihydroxyethylene as a function of the rotational angle around the central double bond (phi) and the pyramidalization angle of one CHOH group (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 standard procedures (CISD and MP2) including multiconfiguration CASSCF. For 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 examined. The S-T energy gap is most sensitive toward phi. For a twisted system, this gap may be appreciably smaller than the one found for a planar conformer. Correlated and uncorrelated wave functions show similar trends in this respect. This S-T energy gap lowering can be used to help understand the electronic structure in the enediol moiety of the Rubisco substrate: D-ribulose 1,5-bisphosphate. There, at the active site, the conformation is cis out of plane. The carboxylation/oxygenation bifunctionality of Rubisco can then be given a relatively simple explanation.