Theoretical Study on the 1:1 Solvation of Triethylphosphine Oxide with the Solvent Molecule -Correlation between Empirical Solvent Parameter AN Based on 31P NMR Chemical Shift and MO Calculations -

The various empirical solvent parameters to explain the solvent effect on the chemical reactions have been advocated because the solvent effect can not be frequently interpreted by dielectric constant, solubility parameter, et al. of the bulk physical properties of the solvents. Since the solvent molecules are considered to serve to the solutes as the electron acceptor and/or electron donor, the strength of the electron acceptor and electron donor of the solvents is expressed as the acceptor number (AN) and donor number (DN) by Gutmann et al.. It has been pointed that the correlation of reaction rates and the free energies of solvation of ions with AN or DN is parallel with the correlation based on the lowest unoccupied (LUMO) or the highest occupied molecular (HOMO) eigenvalues of 8 solvent molecules (DMSO, DMF, AN, Ac, EtOH, MeOH, FA, H2O). In this study, the eigenvalues of ELUMO and EHOMO of 31 solvent molecules including the reported 8 solvent molecules have been calculated by the ab initio method. The eigenvalues of ELUMO of protic solvents increase especially as the AN values increase though the decrease of ELUMO eigenvalues is predicted. The AN values are obtained from 31P NMR chemical shift of triethylphosphine oxide (Et3PO) in neat solvents (S). The hydrogen bond interaction between Et3PO and S rather than the electron accepting to the LUMO of the solvent molecule is presumed to be the main factor in determining the solvation effects. Thus, in this work, the MO calculations on the 1:1 solvation (Et3POoS) between Et3PO and 20 solvent molecules (S) have been carried out by ab initio and semi-empirical MO methods. The correlation of the charge distribution on P and O atoms in Et3POoS, the P-O bond distance, and the hydrogen bond distance between Et3PO and S with the AN values is discussed. The solvents are divided into two groups; one is aprotic solvents with nonpolar, smaller polar (AN <10) and larger polar properties (10< AN <20), and the other is polar protic solvents (AN >20). For the aprotic solvents with AN <10 and the protic solvents with AN >20 except for H2O, the correlation coefficients R with the AN values are 0.79-0.88, resulting in the similar solvation structures in these solvents as well as in the gas phase by the MO calculations. On the other hand, the large deviation from the correlation with the AN values is found at polar aprotic (10< AN <20) solvents (DMSO, DMA, DMF, et al.) and H2O (AN=54.8). The liquid structures of polar aprotic solvents and H2O form the aggregation (cluster) among the solvent molecules based on the large dipole moment and hydrogen bond interaction, respectively. Thus, the large deviation from the linear correlation with the AN values is considered to be due to the solvation to Et3PO molecule by the aggregation (cluster) of a number of the solvents in solution.