Journal zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of M olecular Structure, 108 (1984) 161-172 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR THEOCHEM Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands zyxwvutsrqponmlkjihgf THE PROBLEM OF THE RELATIONSHIP BETWEEN PROTON AFFINITY (INTRINSIC BASICITY) AND THE CHARGE ON THE BASIC CENTRE J. CATALAN, J. L. G. DE PAZ and M. YAGEZ Departamento de Quimica FlSica y Q&mica Cudntica, Centro Coordinado CSIC-UAM, Facultad de Ciencias, Universidad Autdnoma de M adrid, Cantoblanco, M adrid 34 (Spain) J. ELGUERO Znstituto de Quimica M ldica, CSZC, Juan de la Cierva 3, M adrid 6 (Spain) (Received 30 June 1983) ABSTRACT In opposition to all previous reports, we show in this paper that a linear relationship exists between the protonation energy, AE,, and the charge on the basic centre. The unfavourable case of pyrazoles was selected to test the model. Instead of the total charge on the basic nitrogen atom, qN,, we have used a “lone pair charge”, qLPF, which is de- fined here for the first time. INTRODUCTION zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA The relationship between substituentinduced changes in energy and charge has been the subject of considerable attention from a theoretical point of view, since these relationships would constitute a very general tool to investi- gate different chemical processes and, in particular, proton-transfer equilibria by using ab initio molecular orbital theory. However, it seems well established that the change in the net charge of the centre which undergoes protonation shows only a rough relationship to the calculated energies of the corres- ponding proton-transfer equilibrium, which makes this relationship of no practical use. This lack of correlation was first pointed out [ l] for a large set of 3- and 4-substituted pyridines and later on confirmed in an analogous study carried out by Hehre et al. [2] on a variety of nitrogen, oxygen and carbon bases. Quite recently, Fossey et al. [3] in an ab initio study of aminopyridines and Gribov et al. [4] in semiempirical calculations on the protonation of 4- and &substituted pyrazoles, reached the same conclusion. It should be indicated, however, that, in general, linear relationships have been found between the gas-phase proton affinities and (i) the electron population of the hydrogen atom attached to the basic centre [2], (ii) the energy of the nitrogen lone pair [l, 5, 61, (iii) the minimum value of the 0166-1280/84/$03.00 o 1984 Elsevier Science Publishers B.V.