Prediction of Microscopic Protonation Constants of Polybasic Molecules Via Computational Methods: A Complete Microequilibrium Analysis of Spermine SADEGH SALEHZADEH, YASIN GHOLIEE, MEHDI BAYAT Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran Received 14 July 2010; accepted 26 July 2010 Published online 16 November 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/qua.22927 ABSTRACT: For the first time, a simple methodology is reported for theoretical calculation of microscopic protonation constants of polybasic molecules in solution. Density functional theory study was used for complete microequilibrium analysis of spermine, H 2 N(CH 2 ) 3 NH(CH 2 ) 4 NH(CH 2 ) 3 NH 2 , a linear tetraamine with 16 known microspecies. A general thermodynamic cycle is proposed to calculate protonation microconstants of polybasic molecules using calculated micro-DG values in aqueous solution. The microscopic protonation constants were determined with considering both the most abundant and most stable conformers for all microspecies. The results show that the microscopic protonation constants derived from the most abundant conformers (i.e., linear conformers in which the intramolecular hydrogen bonding does not exist) are in good agreement with the corresponding available experimental data. V C 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3608–3615, 2011 Key words: microscopic protonation constants; spermine; thermodynamic cycle 1. Introduction T he acid-base properties of polybasic mole- cules are usually characterized in terms of macroscopic protonation constants, which are composites of the microscopic constants for the individual basic sites [1–5]. The protonation microconstants are defined as specific basicity pa- rameters of a particular basic site, in a defined protonation state for all other basic sites in the molecule. Because of the instantaneous character of the protonation reactions, microspecies are in incessant interconversion. The coexistence and short individual lifetime preclude them from direct analytical determination by any of the exist- ing, fast experimental techniques [1]. Thus, deter- mination of microscopic protonation constants, in contrast to macroscopic protonation constants, is Correspondence to: S. Salehzadeh; e-mail: saleh@basu.ac.ir Additional Supporting Information may be found in the online version of this article. International Journal of Quantum Chemistry, Vol 111, 3608–3615 (2011) V C 2010 Wiley Periodicals, Inc.