EFFECT OF RING IN IMINES AND ENAMINES TAUTOMERISM IN GAS PHASE AND SOLUTION: A COMPUTATIONAL STUDY

Abstract

ABSTRACT Tautomeric structures from imine-enamine equilibria with rings of different size were geometrically optimized at the B3LYP/6-311++G** level of theory. Then important molecular parameters, IR frequencies, NBO and dipole moment results in the gas phase and solution were extracted. The energetic results show that relative stability of enamine tautomer (versus imine tautomer) increases with the increase of ring size and polarity of solvent. Calculated frequencies show the effect on the C=N frequency (imine tautomer) is attributed to ring size. The effect of increasing ring stain by decreasing ring size is to increase the C=N frequency. In addition, variation of dipole moments and NBO charges on atoms in the solvents are studied. Keyword: DFT study, NBO analysis, PC model, imine, enamine e-mail: shabanian.m44@gmil.com INTRODUCTION The imines formation is a particularly important chemical reaction in some biological processes [1,2]. For example, the covalent binding of carbonyl-containing compounds to an enzyme usually involves the formation of imine. The imine moiety is formed by condensation of carbonyl groups of ketone or aldehyde with a primary or secondary amine. Equilibrium between imine and enamine may take place when at least one hydrogen atom is bonded with the imine nitrogen. The mole fractions of these species depend on the symmetry of the parent ketone and the substituents on the amine [3].Since the imines bonds are labile and readily hydrolyzed to the corresponding ketones, it is difficult accurately to monitor and determine which factors influents the extent of imines formations. For example, experiments using Raman and infrared spectroscopy to study the stretching frequencies of imines have been reported. By studying the kinetics of imine formation from acetophenone and substituted aniline, Lee et al. were successful in monitoring the conversion from ketones to imines [4].Tautomeric conversions [5–7] have been mostly investigated by many chemists during last decades. Recently, the study of tautomerism received renewed attention due to its importance on determination of compounds’ properties and their area of applications.In recent years, the understanding of the tautomerism progresses due to the investigation of this phenomenon became one of the major topics in theoretical chemistry. For example, tautomerism in keto–enol [8,9], imine–enamine [10,11], purines [12], pirimidines [13] and many other systems [14–15] have been studied during past two decades. Thereupon, compounds containing tautomers can be the subject of interest for the theoreticians. In title compounds, the tautomerism affects their chemical and biological activities, and any systematic publications on theoretical study of their tautomerisms have been not take place. In this connection, it seems to be important to study the complete scheme of tautomerism and the reaction pathways between different tautomers. In this line, effects of solvent on this tautomerism should also be worked out clearly [16]. In this article, molecular parameters, relative energies, NBO analysis, dipole moments and vibrational frequencies of imines and enamines with different rings have been calculated using B3LYP/6-311++G** level of theory. Furthermore, the effects of solvent have been investigated using benzene (non polar), acetone (aprotic) and water (protic) as solvents.