& Substituent Effects | Hot Paper| Effects of Chlorination on the Tautomeric Equilibrium of 2-Hydroxypyridine: Experiment and Theory Camilla Calabrese, [a] Assimo Maris, [a] Iciar Uriarte, [b] Emilio J. Cocinero,* [b] and Sonia Melandri* [a] Abstract: The effects of halogenation on the tautomeric and conformational equilibria of the model system 2-hydroxypyr- idine/2-pyridone have been investigated through chlorine substitution at positions 3, 4, 5, and 6. In the gas phase, the lactim syn-periplanar tautomer (OH s ) was the predominant species for all compounds over the lactam form (C =O) and the less abundant anti-periplanar lactim (OH a ). However, the population of the three species was shown to be dependent on the position of the chlorine substitution. Chlorination in position 5 or 6 strongly stabilizes the OH s tautomer, whereas the C =O form has a significant population when the ring is chlorinated in positions 3 or 4. Overall, the OH a form is the least favourable form, although the 3-substitution favours the population of this tautomer. In addition, the C =O tauto- mer is strongly stabilized in the solvent, which makes it the dominant form in some substituted species. This study has been performed by means of rotational spectroscopy in the gas phase and/or theoretical calculations in the isolated phase and in solution. Both the OH s and C =O forms of 5- chloro-2-hydroxypyridine and the OH s form of 6-chloro-2-hy- droxypyridine were experimentally observed. All transitions displayed a complex nuclear hyperfine structure owing to the presence of the chlorine and nitrogen nuclei. For all spe- cies, a full quadrupolar hyperfine analysis has been per- formed. This has provided crucial information for the unam- biguous identification of tautomers. Introduction Aromatic heterocycles play a key role in chemistry, biology, and pharmacology. To cite some examples, purines and pyrimi- dines are the building blocks of DNA and RNA, several aromat- ic heterocycles are essential components of proteins and en- zymes, and the design and creation of mimetic scaffolds for new drugs are based on the combination of different heteroar- omatic compounds. [1] In particular, prototropic tautomerism in aromatic heterocy- cles is of great interest as it represents a model for intramolec- ular proton transfer and directly affects the function/biochemi- cal behavior of this class of compounds. [2] For example, the ex- istence of the rare lactim (enol) forms of DNA base pairs has been postulated to cause mismatches in the interstrand pair- ing of nucleobases, and the stability of its tautomeric forms is, thus, a theme of great interest and debate. [3] The simplest model system with significant biological role is 2-hydroxypyridine/2-pyridone (2HP), and it has been the object of numerous studies. [4–13] Three tautomeric structures are plau- sible: two lactim forms (OH) and a lactam species (C =O). The lactim forms are named syn-periplanar (OH s ) and anti-peripla- nar (OH a ) with respect to the angle that is formed by the OH group with the C N bond, which is 0 and 1808, respectively (Figure 1). We would like to note that this is the IUPAC recom- mended nomenclature; however, in the literature, it is still very common to indicate this particular tautomeric equilibrium as keto/enol instead of lactam/lactim. In 2HP, the OH s tautomer is strongly favoured over the OH a species by a weak hydrogen bond with the nitrogen atom. In this system, the C =O tauto- mer also has a notable contribution in the equilibrium. A great number of theoretical [4–7] and experimental [8–13] studies have highlighted that the tautomeric equilibrium strongly depends on the chemical environment selected and that its behavior Figure 1. Sketch of C =O, OH s , and OH a forms of 2-hydroxypyridine/2-pyri- done systems. [a] Dr. C. Calabrese, Dr. A. Maris, Prof. S. Melandri Dipartimento di Chimica “G. Ciamician” Università degli Studi di Bologna Via Selmi 2, 40126 Bologna (Italy) E-mail : sonia.melandri@unibo.it Homepage: http ://www.ciam.unibo.it/freejet [b] I. Uriarte, Dr. E. J. Cocinero Dpto. Química Física Universidad del País Vasco (UPV/EHU) Apartado 644, 48080 Bilbao (Spain) E-mail : emiliojose.cocinero@ehu.eus Homepage: http ://www.grupodeespectroscopia.es/MW/ Supporting information (rotational spectroscopic and quantum mechanical calculations data) and the ORCID(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/chem.201604891. Chem. Eur. J. 2017, 23, 3595 – 3604  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3595 Full Paper DOI: 10.1002/chem.201604891