On the tautomerism of maleimide and phthalimide derivatives
Journal of Molecular Structure: THEOCHEM, (1994)Abstract
The tautomerism of maleimide and phthalimide and their derivatives was examined by means of ah initio MO theory. The molecular geometry of the various tautomers was completely optimized employing the 6-31G∗ basis set and the semiempirical AM 1 method, respectively. Correlation energy effects were estimated using the MP2 formalism. The solvent influence on the tautomeric equilibria was considered by means of the quantum chemical PCM formalism within the ab initio MO theory and the Cramer-Truhlar method (C.J. Cramer and D.G. Truhlar, J. Am. Chem. Soc., 113 (1991) 8305) incorporated into the semiempirical AM1 calculations.The results indicate the diketo tautomers of maleimide and phthalimide as the most stable ones both in the gas phase and in solution. The stepwise replacement of the oxygen atoms by nitrogen causes significant changes. Whereas the lactam-lactim tautomerism can still be neglected in all compounds, the amino-imino tautomerism gains considerable importance, in particular in the maleic and phthalic imidine systems, where the amino-imino forms are of comparable stability with the diimino forms.