Uridine and ribothymidine derivatives, bearing different substituents at C-5 and enriched (Cu 50%) with 1 7 0 in the 0-4 and 0-2 carbonyls, have been studied via "0 NMR in both acetonitrile and aqueous solvents. The solvent shift differences between acetonitrile and water at 0-4 (30-42ppm) and 0-2 (13-16ppm) vary significantly from each other, but the chemical shift changes induced by changing the substituent at C-5 correlated well only with the 0-4 shifts and the electron-withdrawing ability of the substituent. Examination of the I7O shifts of model compounds reconfirms the predominance of keto tautomers for both carbonyls. The significance of the solvent shifts and substituent shifts are discussed with respect to the electronic structure of the nucleoside base rings, and with respect to the hydrogen-bonding abTties of the carbonyl groups. Other nucleoside derivatives studied include those in which the "0 enrichment is in the ring linking the base to the sugar moiety in a pyrimidine cyclonucleoside, in the sugar hydroxy groups and in the phosphodiester linkage of a highly strained ring system in a nucleoside cyclic monophosphate.
* For Parts 1 and 2 of this series, see Refs 3 and 4, respectively.