基金supported by the National Science Founda-tion ( Under grant numbers EAR9004783 and EAR-9805731to P.E.and EAR-9804835 to D.J.L.)the Petroleum Research Fund of the American Chemical Society ( Under grants ACS-PRF34810-AC8 and ACS-PRF37193-AC8to P.E. and ACS-PRF33122-B8 and ACS-PRF40948-B2 to D.J.L.) ~~