Quest for the Origin of Basicity: Initial vs Final State Effect in Neutral Nitrogen Bases Zvonimir B. Maksic ´ * ,²,‡ and Robert Vianello ² Quantum Organic Chemistry Group, Rud jer Bos ˇkoVic ´ Institute, Bijenic ˇ ka 54, 10000 Zagreb, Croatia, and Faculty of Science and Mathematics, The UniVersity of Zagreb, Marulic ´ eV trg 19, 10000 Zagreb, Croatia ReceiVed: August 25, 2001; In Final Form: October 12, 2001 The problem of the origin of the intrinsic basicity of neutral nitrogen bases, as reflected in their gas phase proton affinities, is addressed and a simple solution is found. It is rooted in an intuitively appealing picture involving ionization of the base in question by pruning an electron, subsequent creation of the hydrogen atom with the incoming proton, and the formation of the homolytic chemical bond between a radical cation and the hydrogen. The role of the initial state (base) is mirrored by the ionization potential of the pruned electron given by Koopmans’ approximation, whereas the contribution of the final state (conjugate acid) encompasses the electron affinity of the proton, the relaxation energy of the produced radical cation, and finally the homolytic bond association energy of the newly formed N-H bond. This dissection of the protonation process into three sequential steps has a high cognitive value, enabling classification of bases into three categories at the same time. The first is given by compounds such as ammonia and its alkylated derivatives, the basicity of which is dictated by the initial state effect. The second grouping is formed by those molecules in which the final-state effects decisively influence their basicity values such as, e.g., in methyleneimine and its amino derivatives, whereas the last category encompasses systems exhibiting basicities governed by an interplay between the initial and final-state properties. Phosphazenes belong to the latter set of compounds. Finally, the solvent effect in acetonitrile is considered and briefly discussed within the context of the isodensity polarized continuum model (IPCM). It is shown that a correct hierarchy of basicity in the NH 3-n (Me) n series requires explicit account of the solvent effect. Although the present analysis is quite general, it should be particularly useful in discussing trends of changes in basicities of intimately related molecules. 1. Introduction There is a tremendous interest in the basicity and the accompanying proton affinity (PA) of neutral organic molecules, with a particular emphasis on the superbasic systems. 1-4 This is not unexpected in view of the important role of the proton transfer reaction in organic chemistry and biochemistry. 5,6 Modern computational chemistry can aid the experimental studies in two complementary ways: (1) to predict new molecular systems exhibiting desired properties (i.e., basicities) and (2) to interpret the data using simple and intuitively appealing chemical concepts, thus contributing to a deeper and better understanding of the protonation process. These two aspects are closely related because rationalization of the trends of changes in the proton affinity by recognizing the underlying fundamental principles enables an easier architecture and design of new superbases, for example. We address here the question of the origin of the intrinsic basicity of neutral bases. The simplest interpretation of the proton affinity of nitrogen atom in molecular environments is given by a relation to the hybridization s-character of its lone pair. The idea behind this picture is that a higher s-character implies a more negative energy of the lone pair electrons. Consequently, a higher energetic price has to be paid when a new [N-H] + bond is formed, leading ultimately to smaller PA values. 7,8 Essentially the same idea yields a correlation between the PAs and the ionization potentials (IPs). 9,10 Although these two simple and appealing models are of some value, they describe only a part of the protonation process. Hence, their applications are limited, being confined to small families of very closely related compounds. The same conclusion applies to the relationship between the electron densities of atoms to be protonated and the proton affinities. A more detailed discussion of the relation between the PAs and ESCA shifts is given at the end of the paper. It would be useful to have at one’s disposal a more general description of the protonation event, which could offer a more comprehensive and deeper understanding of the susceptibility of organic bases toward the proton. This is of importance because (over)simplified models could be misleading and yet they are used in the literature. In particular, we shall focus on nitrogen compounds, since they provide the most powerful neutral bases in organic chemistry. It will appear that the initial state (base) and final state (conjugate acid) effects can be delineated in a straightforward and transparent way. At the end of the paper we shall dwell on the problem of the solvent effect in moderately polar aprotic solvents exemplified by acetonitrile and discuss its relation to the proton affinities in the gas phase. 2. Theoretical Basis We shall analyze the intrinsic or gas-phase proton affinity by using the following equation: * Corresponding author, Fax: +385-1-4561118. E-mail: zmaksic@ spider.irb.hr. ² Rud jer Bos ˇkovic ´ Institute. ‡ University of Zagreb. B + H + f (B R H) + + (PA) R (1) 419 J. Phys. Chem. A 2002, 106, 419-430 10.1021/jp013296j CCC: $22.00 © 2002 American Chemical Society Published on Web 12/19/2001