Accurate Ab Initio Calculations Of Intermolecular Potential Energy Surfaces And The Applications

Ab initio calculations using bond function basis sets are carried out to studytheintermolecular potential energy surfaces (PES), along with the equilibrium structuresand molecular properties, of three weakly bounded intermolecular systems. Details ofthestudies on the three intermolecular systems are as follows:(1) Ar–ethane intermolecular potential energy surface using bond functionbasis setsThe intermolecular potential energy surface of argon with ethanewascalculated at thelevels of second-order M ller-Plesset perturbation theory (MP2) and coupled-clustertheory with single, double, and non-iterative triple configurations (CCSD(T)) using a seriesof augmented correlation-consistent basis sets. Two sets of bond functions, bf1(3s3p2d)and bf2(6s6p4d2f) respectively, were added to the basis sets to show a dramatic andsystematic improvement in the convergence of the entire PES. The PES of Ar-ethane ischaracterized by a global minimum at a near T-shaped configuration with a well depth of0.611kcal mol-1, a second minimum at a collinear configuration with a well depth of0.456kcal mol-1, and a saddle point connecting the two minima. It is shown that an augmentedcorrelation-consistent basis set with a set of bond functions, either bf1or bf2, caneffectively produce results equivalent to the next larger augmented correlation-consistentbasis set, that is, aug-cc-pVDZ-bf1≈aug-cc-pVTZ, aug-cc-pVTZ-bf1≈aug-cc-pVQZ.Very importantly, the use of bond functions improves the PES globally, resulting accuratepotential anisotropy. Finally, MP2method is inadequate for accurate calculations because itgives a potentially overestimated well depth and, more seriously, a poor potentialanisotropy.(2) The pyridine dimer intermolecular potential energy surface using bondfunction basis setsSections of the potential energy surface of the pyridine dimer in eight majorconfigurations were calculated at second-order M ller-Plesset approximation (MP2) using various large basis sets augmented with bond functions. The basis set superposition error(BSSE) was considered by the counterpoise method. The study shows that bond functionscan effectively give accurate description of π-π interactions. The intermolecular energies atthe MP2potential minima of the eight configurations were recalculated using thecoupled-cluster method including single, double, and perturbative triple configurations,CCSD(T), with the aug-cc-pVDZ basis set augmented with the bond functions {6s6p4d2f}.Two most stable geometries were found to be antiparallel-displaced, with the CCSD(T)interaction energies of-3.05and-2.95kcal/mol, respectively. The CCSD(T) interactionenergies of other configurations were found to be-1.05,-2.30,-1.42,-2.29,-1.79, and-1.83kcal/mol, respectively. Dispersion is largely responsible for attraction in the pyridinedimer and, as a result, electron correlation beyond MP2is necessary as in the case ofheteroaromatic molecules.(3) Prediction of aqueous pKa values of hydroxybenzoic acidMolecular geometry, binding energy, and harmonic frequencies of hydrogen-bondedcomplexes of various hydroxybenzoic acids with ammonia were calculated using densityfunctional theory. The bond length and stretching frequency of the acidic hydroxyl group,the hydrogen bond length and binding energy, are shown to be highly correlated with oneanother and are linearly correlated with known experimental aqueous pKavalues of thehydroxybenzoic acids. The predicted pKavalues are shown to be consistent from differentmolecular properties and are in good agreement with available experimental values. Thisstudy suggests that calculated molecular properties of hydrogen-bonded complexes alloweffective and systematic prediction of pKavalues.