The geometry of organophosphonates: Fourier-transform microwave spectroscopy and ab initio study of diethyl methylphosphonate, diethyl ethylphosphonate, and diisopropyl methylphosphonate

The rotational spectra of diethyl methylphosphonate (DEMP), diethyl ethylphosphonate (DEEP), and diisopropyl methylphosphonate (DIMP) in supersonic expansions have been acquired using Fourier-transform microwave spectroscopy. Spectroscopic constants have been determined for five distinct conformers of the three molecules. Experimental data have been compared to ab initio calculations performed for each species. For both DEMP and DEEP, the calculations indicate the presence of several low-energy conformers (i.e., ?∼400 cm⁻¹ above the ground state) may be present at room temperature (300 K) for both DEMP and DEEP. When entrained in a supersonic expansion, the rotational temperatures of the samples are much colder (∼2 K); nonetheless, spectra from three conformers of DEEP are still observed experimentally, whereas only one conformer of DEMP is observed. In contrast, only a single low-energy conformer of DIMP is predicted by theory, and is present in the molecular beam. The relative abundance of low-energy conformers of DEMP and DEEP is attributed to the flexibility of the ethoxy groups within each molecule. The presence of multiple DEEP conformers in the supersonic beam indicates a more complex potential energy surface for this molecule that is directly related to conformational shifts of the PCH₂CH₃ group. Conversely, the absence of low-energy conformers of DIMP is attributed to steric hindrance between isopropoxy groups in the molecule. The internal rotation barrier for the PCH₃ group in DEMP and DIMP is compared to that found in DMMP and several phosphonate-based chemical weapon agents.