THEORETICAL-STUDY ON SALICYLALDEHYDE AND 2-MERCAPTOBENZALDEHYDE - INTRAMOLECULAR HYDROGEN-BONDING

The molecular structures and the intramolecular hydrogen bonding for s alicylaldehyde and 2-mercaptobenzaldehyde have been investigated with both ab initio and density functional theory methods. We have consider ed the several possible conformations with respect to the rotation of two functional groups in a given molecule not only to understand the c onformational behaviors but also to estimate the energy of intramolecu lar hydrogen bonding. The optimized geometrical parameters for salicyl aldehyde at the B3LYP levels and the computed H-1 NMR chemical shifts for 2-mercaptobenzaldehyde at the B3LYP/6-31+G optimized geometry are in good agreement with those of previous experimental data. The resul ts show that the inclusion of electron correlation at the B3LYP levels is more crucial in comparing the relative stability among the conform ers of 2-mercaptobenzaldehede than among the conformers of salicylalde hede. The hydrogen-bonding energies are estimated by comparing the mol ecular energies between two different conformations either with a hydr ogen bond or with no hydrogen bond of a given molecule. These energies for salicylaldehyde and 2-mercaptobenzaldehyde are computed;to be abo ut 9 and 2 kcal/mol at the B3LYP levels, respectively.