Quantum mechanical analysis of 1,2-ethanediol conformational energetics and hydrogen bonding

A proper understanding of the conformational energetics of 1,2-ethanediol (ethylene glycol) is important to the construction of molecular mechanics force fields for the treatment of carbohydrates since these biologically important molecules have a prevalence of vicinal hydroxyl groups. In the present study, quantum mechanical analysis of the 10 unique minimum-energy conformations of ethylene glycol is performed by using 10 model chemistries ranging from HF/6-311++G(d,p) up to a hybrid method that approximates CCSD(T)/cc-pVQZ. In addition, natural bond orbital (NBO) analysis of these conformations with deletion of pairings of CO bond/antibonding and lone pair/antibonding orbitals is used to investigate contributions from the "gauche" effect to ethylene glycol conformational energetics. MP2 with the "correlation consistent" basis sets and DFT/6-311++G(d,p) do the best job of matching the approximate CCSD(T)/cc-pVQZ energies while MP2/ 6-31G(d) and Hartree-Fock both fare poorly. NBO analysis shows the conformational energies to be independent of the deletion of matrix elements associated with (i) CO bonding and antibonding orbital interactions and (ii) lone pair and antibonding orbital interactions, whereas the energetic ordering correlates with geometric parameters consistent with internal hydrogen bonds. Thus, the present results suggest that standard molecular mechanics potential energy functional forms, which lack explicit terms to account for stereoelectronic effects, are appropriate for carbohydrates.