Transmission of substituent effects through 5-membered heteroaromatic rings

Ab initio calculations are carried out on protonation equilibria of 5-membe red heteroaromatic aldehydes (5MHAs; heteroatom Y = NH, O, PH, and S and su bstituent Z = NH2, OCH3, SCH3, CH3, N, Cl, CHO, CN, NO2) at the MP2/6-31G* level. Natural bond orbital (NBO) analyses show that the optimal localized natural Lewis structures of the protonated aldehydes, (P), are ortho (C-3) protonated (for Y = O, PH and S) and N-protonated (for Y = NH) forms in con trast to the standard structural Lewis formula for aldehydes, (R). The delo calizability of pi lone-pair on the heteroatom (n(pi)(Y)) is in the order Y = NH > O > S > PPI. The transmission efficiency of (Z) substituent effects to the carbonyl moiety run parallel to the delocalizability of n(pi)(Y) fo r R, but is dominantly influenced by the cationic charge on C-alpha (C-alph a+) for P, which is in the reverse order of the delocalizability of n(pi)(Y ). The Hammett rho values for variation of Z in the protonation are determi ned by the difference in the transmission efficiencies between P and R stat es so that simple interpretation of their magnitude is not warranted. Howev er, the magnitude of the gas-phase rho(z)(+) values decreases as the level of computation is raised from RHF/3-21G* to RHF/6-31G* and to MP2/6-31G* bu t increases again at the MP4SDQ/6-31G* level. Further decrease occurs when solvent effect (water) is accounted for by the SCRF method. Comparison of t he SCRF rho(z)(+) values with those determined in the aqueous acid solution for Y = S and CHCH shows inadequacy of accounting for the solvent effects on the rho values by a continuum model. Tr is noteworthy that semiempirical calculations, especially the AM1 method, give even lower magnitude of the gas-phase rho values.