Study of the methanol trimer potential energy surface

Abstract

The potential energy surface of methanol trimer has been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G (d,p) and B3LYP/6-311+G (d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies for the most stable ( CH 3 OH ) n (n=1,3) clusters were calculated in the framework of the G2(MP2,SVP) theory. For these and all the other structures the final energies were also obtained using the B3LYP/6-311++G (3df,2p) approach. Three local minima have been located. The global minimum corresponds to a cyclic structure with two methyl groups on one side of the O–O–O plane and the third one on the other side. The bowl conformer, where the three methyl groups are on the same side of the O–O–O plane, is predicted to be only 0.8 kcal/mol less stable than the global minimum. The third local minimum, where one of the monomers behaves as a biacceptor is predicted to lie much higher in energy. Other stationary points associated with a systematic flipping of the methyl groups have been also located. These stationary points, which are transition states or saddle points of higher order, are very close in energy to the global minimum, indicating that the potential energy surface of the methanol trimer is very flat and very similar to that reported before for the water trimer. The calculated enthalpies of association for the dimer and the trimer of methanol are in very good agreement with the experimental values when estimated at the G2(MP2,SVP) level, while they are slightly underestimated when the B3LYP approach is used. The most stable trimer has three different O–H stretching frequencies, in agreement with the experimental evidence. The calculated frequency shifts are only in rough agreement with the experimental values. Cooperative effects are significant in the methanol trimer. They are reflected in larger frequency shifts, greater lengthening of the O–H bonds, shorter O⋯O distances than in the dimer. The G2(MP2,SVP) calculated additive interaction energy is also significantly large.