Ab initio and DFT computer studies of complexes of trimethylphosphine and products of substituted phosphonium cations with hydroxide, chloride and fluoride anions: Phosphorus analogues of choline and acetylcholine

Theoretical calculations (DFT, MP2) are reported for up to four sets of reaction products of trimethylphosphine, (CH3)3P, each with H2O, HCl and HF together with DFT calculations on up to three sets of reaction products of substituted phosphonium cations, (CH3)3P-R+. These products comprise (a) P(III) normal complexes (CH3)3P⋯HY, (b) P(IV) 'reverse' complexes Y⋯(H-CH2)3P-R, (c) P(IV) ylidic complexes YH⋯CH2(CH3)2P-R and (d) P(V) covalent compounds Y-P(CH3)3-R for Y=HO, Cl and F and R=H, CH3, C2H5, C2H4OH and C2H4OC:OCH3. Calculations are carried out at the B3LYP/6-31+G(d,p) level in all cases and also at the MP2/6-31+G(d,p) level for systems in which R=H. Minimum energy structures are determined for predicted complexes or structures and geometrical properties, harmonic vibrations and BSSE corrected binding energies are reported and compared with the limited experimental information available. Potential energy scans predict equilibria between covalent trigonal bipyramidal P(V) forms and reverse complexes comprising hydrogen bonded or ion pair, tetrahedral P(IV) forms separated by low potential energy barriers. Similar scans are also reported for equilibria between reverse complexes and ylidic complexes for Y=OH and R=CH3, C2H5, C2H4OH and C2H4OC:OCH3. Corrected binding energies, structures and values of harmonic modes are discussed in relation to bonding The names 'pholine' and 'acetylpholine' are suggested for phosphorus analogues to choline and acetylcholine.