The Geometry of Organophosphonates: Fourier-Transform Microwave Spectroscopy and ab Initio Study of Diethyl Methylphosphonate, Diethyl Ethylphosphonate, and Diisopropyl Methylphosphonate

Published

December 1, 2004

Author(s)

R S. DaBell, R D. Suenram, R J. Lavrich, J M. Lochner, M W. Ellzy, K B. Sumpter, J O. Jensen, A C. Samuels

Abstract

The rotational spectra of diethyl methylphosphonate (DEMP), diethyl ethylphosphonate (DEEP), and diisopropyl methylphosphate (DIMP) 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., 300 cm^-1 above the ground state) may be present at room temperature (300 K) for both DEMP and DEEP. Experimentally, three conformers of DEEP are also present in the 2 K molecular beam, whereas only on conformer of DEMP is observed. In contrast, only a single low-energy conformer of DIMP is predicted by the ab initio calculations to exist at 300 K that is also rpesent in the 2 K 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 weapons agents.

Citation

Journal of Molecular Spectroscopy

Volume

228

Issue

No 2

Pub Type

Journals

Keywords

ab initio calculations, analytical chemistry, Fourier-transform microwave spectroscopy, molecular structure, organophosphonates, physical chemistry

Citation

DaBell, R. , Suenram, R. , Lavrich, R. , Lochner, J. , Ellzy, M. , Sumpter, K. , Jensen, J. and Samuels, A. (2004), The Geometry of Organophosphonates: Fourier-Transform Microwave Spectroscopy and ab Initio Study of Diethyl Methylphosphonate, Diethyl Ethylphosphonate, and Diisopropyl Methylphosphonate, Journal of Molecular Spectroscopy (Accessed July 26, 2024)