Structures and conformations of organophosphorus molecules

Abstract

The molecules of formula CH 3XP(Y)Z 1Z 2 with X = CH 2, O, and S; Y = O, S, BH 3, or simply the lone pair of non-bonded electrons; and Z 1Z 2 = CH 3, F, Cl, or possibly OR, potentially have two or more conformers present at ambient temperature. From the “ gauche effect” one would predict that the most stable conformer should be the one with the maximum number of interactions between the adjacent electron pairs and/or polar bonds. Although this effect usually predicts the most stable conformations for the compounds of the second row elements, i.e., C, N and O, it has not been extensively tested on the third row elements, i.e., Si, P and S. Rotational or vibrational studies, or a combination thereof, may provide sufficient data by which one can adequately test the theory as to its applicability to phosphorus compounds for which there has not previously been reliable data for the most part on the conformer stabilities. Until very recently, the vibrational spectroscopies, i.e., far infrared and gas phase Raman, had not been applied to conformational studies because of the lack of suitable instrumentation. However, in the past decade, the advances in instrumentation in both laser Raman and Fourier transform infrared spectroscopy have given researchers far easier access to the entire spectral region, i.e., 4000 to 10 cm −1. Additionally, the ability to routinely record the spectra of molecules in the gaseous state, where they are free from the intermolecular forces in the liquid or intramolecular forces in a solvent which may affect the true relative conformer stabilities, enables one to study the conformational changes of the molecules upon going from the gaseous state to the liquid and solid states. Also, under favorable circumstances, it is possible to calculate from the low frequency Raman and far infrared data the potential functions for the asymmetric torsions from which the relative energies of the conformers can be obtained along with the barriers to interconversion. Therefore, the combined systematic studies of both the rotational and vibrational spectra have yielded accurate structural parameters, conformational stabilities and, in some cases, potential functions for internal rotation for these organophosphorus molecules.