Factors determining methanol dipole moment value in hydrogen-bonded cluster: quantum chemistry simulation
Molecular dipole moment plays a crucial role in the definition of dielectric properties and chemical behaviour of substances. Besides, it is determined by short-range intermolecular interactions as well as by peculiarities of spatial orientation of the particles. In order to analyze molecular dipole moment distribution obtained by molecular dynamics methods, it is necessary to know how the value of the dipole moment depends on such factors as the size and geometry of cluster. Therefore, the aim of the research was to determine the dependence between the dipole moment and number of hydrogen bonds formed by a molecule.
Dipole moments of methanol clusters (CH3OH)n optimized by geometry (n varied from 1 to 4) and those of individual molecules in it were calculated in Gamess software package using both restricted Hartree-Fock method with different basis sets (6-31G, 6-31G*, 6-31G**) and second order Meller-Plesset perturbation theory. Moreover, molecular dipole moment was calculated with employing Car-Parrinello Molecular Dynamics program package based on density functional theory.
According to the data analysis it can be noted that the dipole moment of the molecule forming a single hydrogen bond is essentially independent on the size and geometry of the cluster. However, the value of the dipole moment of the molecule forming two hydrogen bonds significantly changes with an increasing number of molecules in the cluster. The dipole moment of the proton- donor molecule was shown to be lower than that of the proton-acceptor one as well as a molecule being both a donor and acceptor.
Therefore, an average dipole moment of the molecule in the cluster significantly exceeds the dipole moment of an isolated molecule. Additional polarization due to the interaction with adjacent molecules might explain this phenomenon.
It is significant that a cyclic methanol trimer has the lowest dipole moment, a chained tetramer having the largest among the configurations studied. In future, these results will allow interpreting in details the distribution of the molecular dipole moments of the sub- and supercritical methanol with the help of Car-Parrinello molecular dynamics.
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