Tautomerization of Nucleobase Model Compounds: The 4-Pyridinol and 4(1H)-Pyridinone Monomers and Their Dimers Jeffrey R. Reimers,* ,† Lachlan E. Hall, †,‡ and Noel S. Hush †,§ School of Chemistry and Department of Biochemistry, UniVersity of Sydney, NSW 2006, Australia ReceiVed: NoVember 10, 1999; In Final Form: February 28, 2000 Nucleobases are important in many biochemical pathways, and one of their key features is that low-energy tautomerization processes can cause large changes in their chemical properties. Similar effects are also seen for photovoltaic molecules such as quinacridones, except that, in these systems, tautomerization is achieved through intermolecular proton transfer. An excellent model for both of these systems is the tautomerization between the 4-pyridinol and 4(1H)-pyridinone monomers and their dimers. Indeed, 4-pyridinol is known to be the most stable monomer in the gas phase, while chemically diverse 4(1H)-pyridinone is the most stable monomer both neat and in solution in polar solvents. We evaluate the energetics of gas-phase tautomerization of both monomers and dimers of these molecules using B3LYP, HCTH, SCF, MP2, MP4, QCISD, CCSD, and CCSD(T) methodologies. For the monomers, estimates of the CCSD(T)/aug-cc-pVTZ energies are obtained, while for the dimers, estimates of basis-set-superposition-error-corrected CCSD/aug-cc-pVDZ energies are obtained. Vibrational analyses are performed at the B3LYP/cc-pVDZ level to determine zero-point energy corrections and OH- and NH-stretch vibrational frequency changes. The hydrogen-bond energies show a clear preference for 4(1H)-pyridinone-containing dimers, and the dimer in which 4-pyridinol donates a hydrogen bond to 4(1H)-pyridinone is calculated to be only slightly higher in energy than the 4-pyridinol dimer. 1. Introduction Tautomerization is a very important property in biological systems that is particularly relevant for nucleobases, 1-4 and many studies have been performed to investigate the tautomer- ization and association properties of the simplest model compounds, 5 the hydroxy pyridines. These compounds are also model systems for the study of the properties of larger molecules such as quinolones, 6 anthraquinones, 7 pyridonophanes, 8 and quinacridones, 9,10 many of which are of interest for their optical and photovoltaic 7,11,12 properties. Our concern here is with 4-pyridinol and 4(1H)-pyridinone, which are hydroxy (HYD) and oxo (OXO) tautomers, respec- tively. The HYD form is, in fact, a lactim and is fully aromatic, whereas the OXO form is a lactam and is thus not expected to be aromatic. Chemically, however, the OXO form exhibits more aromatic than lactam character, but spectroscopically, the OXO form exhibits typical lactam (or quinone) properties 13 and absorbs at a much lower energy than does the HYD form. 14 In quinacridones, tautomerization moves the main absorption band between the UV and visible regions, and hence, these molecules can function as a chemically controllable color switch. 15 Furthermore, these molecules self-associate in head-to-tail chains, 9 and hence, it is possible to envisage 15 their use in the construction of a micro electrochemical solid-state color display device. For such systems, 4(1H)-pyridinone is an excellent model compound, not only because of its analogous tautomeric and optical properties but also because of its associative properties, with its prevalent OXO tautomer being known to associate into chains of up to >30 monomer units in aqueous solution. 16,17 To direct the synthesis and hence explore the synthetic flexibility of quinacridones 10 and the like, it is important to understand the energetics of tautomerization of 4(1H)-pyridinone and its oligomers. Here, we investigate monomers and dimers of this compound through extensive ab initio and density-functional computation. An important aspect of the tautomerization of 4(1H)- pyridinone is its sensitivity to environment. In the gas phase, this molecule is known to exist to within experimental detection limits purely in its HYD form. 14,18 However, in aqueous solution, it exists in its OXO form. This is easily understood in terms of solvation effects, using either cluster 19-22 or reaction-field 23 methodologies, with the HYD form becoming the most impor- tant form in solvents of low polarity. 17,23,24 Related systems such as quinolones are interesting in that it is possible to obtain widely varying nonequilibrium gas-phase compositions by varying the mode of production of the vapor. 25 These effects are not of concern to us here, however, and we concentrate on the determination of the gas-phase equilibrium properties of 4(1H)- pyridinone and its dimers. * To whom correspondence should be addressed. E-mail: reimers@ chem.usyd.edu.au. † School of Chemistry. ‡ Current address: Molecular Electronics Research, 393 Darling St., Balmain 2041, Australia. § Department of Biochemistry. 5087 J. Phys. Chem. A 2000, 104, 5087-5092 10.1021/jp9939827 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/27/2000