Intramolecular single H bonding vs bifurcation in tuning the conformation of 2,2′-dihydroxybenzophenone and its derivatives: a DFT insight

Abstract

2,2′-dihydroxybenzophenones and their oxime and N-acyl hydrazone derivatives have been studied via the DFT/B3LYP-6-311++G** methodology. An almost coplanar bifurcated six-membered H bridge is found in ketones. A similar H bridge, accompanied by a seven-membered one in oximes and a nine-membered-like one in hydrazones, is also formed. While the closed (two pseudo rings) conformer is the lowest energy one in 2,2′-dihydroxybenzophenones and their oximes, the semi-closed conformer (one pseudo ring) corresponds to the lowest energy one in N-acyl hydrazones. The ΔHf of the closed conformer compared to its open counterpart is ca. 17 kcal/mol in 2,2′-dihydroxybenzophenones while that in oximes is ca. 11 kcal/mol. The energy barrier in changing from the closed to the open (no pseudo ring) conformation is <3 kcal/mol for all 2,2′-dihydroxybenzophenones and their oximes. The impact of intramolecular hydrogen bonding on the variation of ΔHf of the conformers are discussed with respect to mono- and di-p-substitution of 2,2′-dihydroxybenzophenone structure as well as to its conversion into oxime and hydrazone derivatives. ΔHf of both closed and semi-closed conformers decreases, throughout the series, unlike that of semi-closed conformer of the Br-substituted ones. A slightly decreased enthalpy due to intramolecular hydrogen bonding in 2,2′-dihydroxybenzophenones is attributed to p-substitution and a further significant decrease is noted in going from 2,2′-dihydroxybenzophenones to oximes. An enthalpy increase, on the other hand, occurs in moving from oxime to hydrazone, again with the exception of semi-closed conformer of the Br-substituted conformers.