VIBRATIONAL-SPECTRA, CONFORMATIONS AND FORCE-CONSTANTS OF DIALKYLPHOSPHITES (RO)2P(O)H

Molecular spectroscopy and molecular mechanics were used to study the conformations of dialkylphosphites (RO)2P(O)H (R = C2H5, n-C3H7, i-C3H7, n-C4H9, and i-C4H9). It is shown that in the liquid state, owing to internal rotation around the P-O bonds, three conformations of dialkylphospite molecules arise: symmetrical gauche',gauche (-GG), where both RO groups are parallel to the plane including P = O and P-H bonds, asymmetrical gauche,gauche (GG) and trans,gauche (TG). When cooled to freezing point, dialkylphosphites form several crystal modifications, each crystal consisting of different conformers. It is found that the stretching vibration frequency v-PH depends on the molecular conformation: for -GG conformers v-PH is about 2470 cm-1, for GG conformers v-PH is about 2440 cm-1 and for TG conformers v-PH is below 2430 cm-1. In spectra of dialkylphosphites in the liquid state, all three bands merge into one unresolved broad band. It is also shown that the conformational sensitivity of vPH and some other vibrations is accounted for by the difference between the force fields of the conformers.