Journal of Molecular Structure (Theochem), 262 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR (1992) 7-20 Elsevier Science Publishers B.V., Amsterdam A comparative study of the procaine-H+ structures. Interpretation of the procaine hydrochloride spectra by AM1 M. Alcolea Palafox Departamento de Q&mica Fisica I (Espectroscopia), Facultad de Ciencias Quimicas, Universidad Complutense, Madrid-28040 (Spain) (Received 22 August 1991; in final form 27 January 1992) zyxwvutsrqponmlkjihgfedcbaZ Abstract The structure of the procaine cation was determined by the AM1 semi-empirical method. A comparison with crystallographic data was made. The frequencies and intensities of the vibrations were also computed by AM1 and compared with IR and Raman experimental frequencies. Correction factors of the vibrations for AM1 calculations were used. The charge and electron distributions on the structure were determined. INTRODUCTION Local anesthetics are drugs that block conduction in nerve tissue when applied in appropriate concentrations. Many compounds have this property but often cause permanent neural damage or else are too toxic following systematic absorption from their site of application. The confor- mational aspects of these anesthetics have been subjects of special interest for a long time, in particular the geometrical arrangement of the lipophilic and hydrophilic centers. Maps of the conformational energy were con- structed by quantum mechanical methods, to indicate its variation as a function of rotations about the appropriate torsion angles [l]. Although it is obvious that such a study of molecules in a vacuum is not necessarily related directly to their pharmacological activities, numerous examples have shown that at least some degree of correlation between the two may reasonably be expected. The naturally occurring alkaloid cocaine was the first local anesthetic used clinically. A chemical search for a substitute led to the discovery of Correspondence to: M. Alcolea Palafox, Departamento de Quiimica Fisica 1 (Espectrosco- pia), Facultad de Ciencias Quimicas, Universidad Complutense, Madrid-28040, Spain. 0166-1280/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.