This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011 New J. Chem., 2011, 35, 1433–1439 1433 Cite this: New J. Chem., 2011, 35, 1433–1439 The effect of benzoannulation on the transition state and the proton transfer equilibrium in di(2-pyridyl)methane derivativeswz Borys Os´mia$owski,* a Tadeusz M. Krygowski, b Justyna Dominikowska c and Marcin Palusiak c Received (in Montpellier, France) 11th February 2011, Accepted 22nd March 2011 DOI: 10.1039/c1nj20108e The tautomeric properties of di(2-pyridyl)methane and its benzoannulated derivatives were studied using a computational approach (M05/6-31G(2d,p)). Our analysis showed that the degree of cyclic p-electron delocalization in benzene and pyridine rings is directly connected to the effect of resonance present in quasi-rings formed by intramolecular hydrogen bonds of the N–HN type. This direct relation can be explained using two concepts, namely, the concept of Clar’s aromatic sextet and the Leffler–Hammond concept originally developed for the explanation of the energy relation between ground-state and transition-state structures corresponding to proton transfer in H-bridges. Application of these two different concepts allows us to explain in detail the role of intramolecular hydrogen bonding in polycyclic aromatic hydrocarbons containing N heteroatoms. Introduction Stepwise benzoannulation to benzene allows one to construct a huge number of benzenoid hydrocarbons. They present a great variety of physicochemical properties, 1–4 which are mostly dependent on the mobile nature of the p-electron struc- ture and p-electron delocalization. 5–9 Analyses of resonance energy-like characteristics describing whole molecules show a great diversity in this property, 10,11 with even similar molecules, such as tetracene and its isomers, 12 differing in energy by 14 kcal mol 1 . It is important to mention that diversity in p-electron delocalization observed in individual rings of a given benzenoid hydrocarbon is sometimes very large, with systems ranging from antiaromatic to fully aromatic. This has been shown using various measures of p-electron delocalization (generally known as aromaticity indices). 10,13–26 The references mentioned lead to the well-documented conclusion that rings in a given molecule of a benzenoid hydrocarbon may have strongly different p-electron delocalization. To some degree, it may be said that, depending on the kind of benzoannulation (i.e. how many rings are annulated and in what position), the parent ring may have substantially different p-electron properties. Consider a very simple example: two rings are annulated to the benzene ring (denoted, say, by A), on both sides of A. As a result we obtain anthracene or phenanthrene with A in a central position. We see that p-electron delocalization in ring A in both cases differs substantially. In the phenanthrene molecule the ring A has a localized double bond and its aromaticity is much lower than in two lateral rings, whereas in anthracene all rings have comparable aromatic characters, as shown by values of different aromaticity indices. 27,28 The problem of the mode of benzoannulation in this case is also associated with specific features. Greater stability is observed for kinked linear polyacenes rather than for straight linear ones, as shown recently for phenanthrene and anthracene, 29 and even for infinite chains of these two kinds of systems. 30 It was also shown recently 31–34 that if the annulated ring is not a benzene but the so-called quasi-aromatic ring in which CHCHCH frag- ment is replaced by O–H–O or O–Li–O fragments (or similar), then these quasi-rings function in a similar way as the benzene rings, being more similar in the case of O–Li–O and less similar for O–H–O. The purpose of this paper is to investigate how the quasi-ring formed as a result of H-bond in two positions of proton location depends on the kind of benzoannulation to the pyridine ring participating in H-bond complexation. We also consider how the location of the proton in the transition state (TS) depends on the aromaticity of both the p-electron system participating in the H-bond and the opposite relation – how the hetero-ring and the complete p-electron moieties depend on the position of a Faculty of Chemical Technology and Engineering, University of Technology and Life Sciences, Seminaryjna 3, PL-85-326 Bydgoszcz, Poland. E-mail: borys.osmialowski@utp.edu.pl b Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland c Department of Theoretical and Structural Chemistry, University of Lo ´dz ´, Tamka 12, 91-403 Lo ´dz ´, Poland w Electronic supplementary information (ESI) available: Cartesian coordinates, computed total energies of optimized structures, aromaticity indices, densities of total electron energy at the ring critical points, and definitions of aromaticity indices. See DOI: 10.1039/c1nj20108e z This paper is dedicated in memory of our friend, Prof. Zvonimir B. Maksic (1938–2011) in recognition of his outstanding contribution to theoretical organic chemistry. NJC Dynamic Article Links www.rsc.org/njc PAPER