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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 21, Number 9
BKCSDE 21(9)
September 20, 2000 

Transmission of Substituent Effects through 5-Membered Heteroaromatic Rings
Ikchoon Lee,* Soon Ki Rhee, Chang Kon Kim, Dong Soo Chung, Chan Kyung Kim
Ab initio calculations are carried out on protonation equilibria of 5-memberedheteroaromatic aldehydes (5MHAs;heteroatom Y = NH, O, PH,and S and substituentZ = NH2, OCH3, SCH3, CH3, H, Cl, CHO, CN,NO2) at the MP2/6-31G* level. Naturalbond orbital (NBO) analyses show that the optimal localized natural Lewis structures of the protonated aldehydes,(P), are ortho (C3) protonated (for Y = O, PH and S) and N-pro-tonated (for Y = NH) forms in contrast to the standard structural Lewis formula for aldehydes, (R). The delo-calizability of πlone-pairon the heteroatom (nπ(Y)) is in the order Y = NH > O > S > PH. The transmission efficiency of (Z) substituent effects to the carbonyl moiety run parallel to the delocalizability of nπ(Y) for R,but is dominantly influenced by the cationic charge on Cα(Cα+) for P, which is in the reverse order of thede-localizability of nπ(Y). The Hammett ρvalues for variation of Z in the protonation are determined by the dif-ference in the transmission efficiencies between Pand R stateat simple interpretation of their magnitude is not warranted. However,the magnitude of the gas-phase ρz+ values decreases as the level ofcomputation is raised from RHF/3-21G* to RHF/6-31G* and to MP2/6-31G* but increases again at the MP4SDQ/6-31G* level. Further decrease occurs when solvent effect (water) is accounted for by the SCRF method. Comparison of the SCRF ρz+values with those determined in the aqueous acid solution for Y = S and CHCH shows inadequacy of accounting for the solvent effects on the ρvalues by a continuum model. It is noteworthy that semiempirical calculations, especially theAM1 method, give even lower magnitude of the gas-phase ρvalues.
882 - 890
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