The theoretical study of substituent effect on intramolecular hydrogen bond strength in conformers of acyloine indole derivatives

The intramolecular hydrogen band (IMHB) formation in a α-acyloinindoles was investigated using density functional theory and Moller-Plesset second order methods, atoms in molecules (AIM) and natural bond orbital (NBO) theories. It was found that the cis isomer which involved intramolecular hydrogen bond is more stable than both of the two trans isomers that couldn’t accommodate the intramolecular hydrogen bonding. The effect of electron donating (ED) and electron withdrawing (EW) substituents of phenyl ring on the strength and nature of IMHB was also studied. As was expected, the calculations suggested that ED groups increase the strength of IMHB and relative stability of the related conformers, while the EW groups decrease the IMHB power and result in decreasing of stability energy. The Hammet plot was investigated for the substituent effects on the relative stability energy of conformers. Although the correlation was not well linear, but it approved the regular decrease in relative stability of the conformer by decreasing the electron donating power of the substituent.

Keywords:

Intramolecular hydrogen bond , acyloine indole , conformation , AIM , NBO