Issue in Honor of Prof Nicolò Vivona ARKIVOC 2009 (viii) 212-221 17 O vs 13 C NMR chemical shifts as effective indicators of π-electron-density distribution in hindered methyl benzoates Massimo Maccagno, a Andrea Mele, b Roberta Musio, c Giovanni Petrillo, a Fernando Sancassan, a * Oronzo Sciacovelli, c and Domenico Spinelli d * a Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, I-16146 Genova b Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘Giulio Natta’ del Politecnico, Via Mancinelli 7, I-20131 Milano c Dipartimento di Chimica, Via Orabona 4, I-70126 Bari d Dipartimento di Chimica ‘G. Ciamician’, Via Selmi 2, I-40126 Bologna E-mail: domenico.spinelli@unibo.it Dedicated to Prof. Nicolò Vivona on his 70th birthday Abstract 17 O NMR Substituent Chemical Shift (SCS) values on the carbonyl and methoxy oxygens in methyl 2,6-dimethyl-4-X-benzoates 2’ have been measured in acetonitrile-d 3 and compared with those of the 2,6-unsubstituted methyl 4-X-benzoates 2, the former exhibiting very little sensitivity to the effect of 4-substituents and not appearing to be related to π-electron density changes in such sterically hindered systems. Conversely 13 C NMR SCS values of the methoxy carbon of the ester function in either 2 and 2’, although modest in size, exhibit a very good dual substituent parameter (DSP) correlation with the polar and resonance effect of 4-substituents and are therefore effective indicators of local π- electron densities. Moreover, 13 C NMR SCS values of the methoxy carbon of 2’ exhibit an excellent linear correlation with those of 2,6-unsubstituted benzoates 2. Once more this agrees with our previous conclusion that the resonance interaction between the aromatic ring and the ester group is essentially related to a π-polarization effect, not involving charge-transfer between the two moieties and therefore not sensitive to steric hindrance to their coplanarity. Keywords: 17 O and 13 C NMR spectroscopy, alkyl benzoates, substituent effect ISSN 1551-7012 Page 212 © ARKAT USA, Inc.