| Abstract
| - High-level ab initio calculations were performed onfive of the most stable tautomers of 6-thioguanine (6TG)at the second- and fourth-order levels of correlated Møller−Plessettheory using Pople's type of valencedouble-ζ basis sets with additional sets of polarization and (in somecases) diffusion functions. At the highestapplied level of theory (MP4(SDQ)/6-31++G(d,p)//MP2/6-31G(d,p) +0.9(HF/6-31G(d,p) ZPE)), the orderof stability of the tautomers was found to be N9(H)trans>N9(H)cis> N1,7(H) > N1,9(H) > N7(H).When the ZPE corrections were not employed, all calculationsutilizing structures minimized at the correlatedMP2 level returned a stability order of N1,7(H) >N9(H)trans> N9(H)cis> N1,9(H) ≫ N7(H).Calculationscarried out using the self-consistent reaction field (SCRF) model tosimulate aqueous solution behavior yieldeda stability order (with correction for ZPE) of N9(H)trans>N1,9(H) > N9(H)cis> N1,7(H) ≫ N7(H).CNDO/S calculations on the excited states of 6TG tautomers showedhigher oscillator strengths and lowerexcited-state energies for thiones than for thiols; the latter wasconfirmed experimentally via UV absorbanceand luminescence spectroscopic studies on 6TG and two model compoundsfor specific tautomers. Theseexperimental spectra confirmed the existence of at least two types of6TG thione chromophores.
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