The potential energy surface for the interaction of uracil with one water molecule is investigated using ab initio techniques. The structures of four cyclic minima, as well as two transition-state structures, have been determined using second-order Møller−Plesset perturbation theory (MP2) and the interaction-optimized DZPi basis set. At the optimized geometries, the counterpoise-corrected interaction energies have also been computed with a slightly larger basis set containing bond functions, labeled ESPB. The MP2/ESPB calculations predict D e for the four uracil−water minima to be −40.0, −31.8, −33.5, and −26.6 kJ/mol. The barrier height between the global minimum and the adjoining local minimum (with D e = −31.8 kJ/mol) is found to be as much as 23 kJ/mol, while the barrier height between the two most stable local minima (D e = −33.5 and −31.8 kJ/mol) is only 10 kJ/mol. For the global minimum we also investigated the effect of basis set superposition error (BSSE) on the two hydrogen bond distances, as well as the effect of freezing the monomer geometries during optimization. It is found that BSSE decreases the hydrogen bond lengths by about 0.1 Å, while freezing the intramolecular geometries reduces the uracil−water interaction energy by less than 2 kJ/mol.

No abstract

The conformational landscape of neutral serotonin has been investigated by several theoretical methods. The potential energy surface was scanned by systematically varying the three dihedral angles that determine the conformation of the alkyl side chain. In addition, the two possible conformations of the phenol hydroxyl group (anti and syn with respect to the indole NH) were considered. The OH-anti stationary points located with SCF/ 6-31G* have been re-optimised with B3LYP/6-31+G*, which resulted in twelve true minima. Eleven of these have a corresponding OH-syn conformer that is 1-4 kJ mol À1 higher in energy. IR vibrational spectra of all twenty-three serotonin conformers, computed at the B3LYP/6-31+G* level of theory, are presented. The initial scan of the serotonin potential energy surface has been repeated with several computationally cheaper methods, to assess their reliability for locating the correct serotonin conformers. It is found that the semi-empirical methods AM1 and PM3 do not yield sufficiently accurate results, due to their inability to account for subtle intramolecular interactions within the serotonin molecule. On the other hand, SCF in combination with the 3-21G* basis set is ascertained to be a good alternative to SCF/6-31G* for performing the initial scan of the potential energy surface of flexible molecules.y Electronic supplementary information (ESI) available: computed harmonic hydride stretch frequencies and intensities of the serotonin (OH-anti) and (OH-syn) conformers. See