Conformational analysis of 2-substituted alkylphosphoryl compounds .2. H-1 NMR, IR and molecular mechanics modelling studies of the solvation of 2-hydroxypentylphosphonates

IR, molecular weight and molecular mechanics modelling studies of 2-hydroxypentylphosphonates indicate that intramolecular hydrogen bonding has a significant influence on the position of the conformational equilibrium. IR spectroscopy and elevation of boiling point shows similar degrees of intramolecular and intermolecular hydrogen bonding with a small proportion of non-hydrogen bonded molecules. Molecular mechanics (MM) modelling indicates that the intramolecular hydrogen bond may involve the ester oxygens of the phosphonate group as well as the phosphoryl oxygen. MM modelling was also used to calculate HCCH dihedral angles for the staggered conformers of diisopropyl 2-hydroxypentylphosphonate with and without docked solvent molecules. Utilisation of these angles in place of the perfectly staggered 60 degrees and 180 degrees angles made changes of up to 6% to the NMR estimate of the position of the conformational equilibrium. Conformational changes induced by different solvents, appear to operate mainly by moderating the intramolecular interactions. MM modelling indicates that the relationship between the LH NMR chemical shift difference of the diastereotopic alpha-methylene protons and the population of the major conformer go is due to specific solvation-with aromatic solvents associating preferentially with the hydrophobic face, and chloroform and methanol preferentially solvating the polar groups.