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

Rotational spectrum of cyclohexyl sarin

“…This is part of an ongoing effort to examine rotational spectra of chemical warfare agents and simulant compounds. Additional studies have included the spectra of the warfare agents sarin [4] and soman [5], nerve agent simulants including dimethyl methylphosphonate (DMMP) [6] and related organophosphonates [7], and blister or mustard agent simulants such as ethyl sulfide [8], hydroxyethyl ethyl sulfide [9], thiodiglycol [10], and chloroethyl ethyl sulfide [11]. One objective of these studies is to establish FTMW spectroscopy as a quantitative tool for the detection of chemical agents (and other substances) in air.…”

“…Furthermore, ab initio studies help establish whether multiple conformers will be present at room temperature. For several agents and simulants, more than one conformer has often been observed in the 1 K molecular beam including, for example, soman [5], diethyl ethylphosphonate [7], ethyl sulfide [8], hydroxyethyl ethyl sulfide [9], thiodiglycol [10], and chloroethyl ethyl sulfide [11]. Such observations imply that the interconversion barriers between the different conformers of these molecules exceed 400 cm À1 [17].…”

“…As a consequence of this smaller R group, and the concomitant reduction in steric hindrance with the CH 3 AP methyl rotation, its rotation barrier is approximately 16% smaller (677.0 ± 0.4 cm À1 ) [4]. Comparisons of the methyl torsional barriers for other organophosphonate molecules have been discussed elsewhere [7].…”

“…Dipole moment values do not follow this experimental trend. However, previous work associated with species exhibiting multiple low-energy conformers illustrates that calculated dipole moments may vary significantly due to the shallow potential energy well [7,22]. Consequently, Geometry 4 (2) cannot be removed as a possible choice for the GF-1 (GF-2) geometry solely on the basis of dipole moment values, and thus an unambiguous assignment is not possible.…”

Rotational spectra, nuclear quadrupole hyperfine tensors, and conformational structures of the mustard gas simulent 2-chloroethyl ethyl sulfide

“…Instead of a full structure determination for large molecules, we assign experimental rotational spectra to their corresponding conformational structures by matching the experimental rotational constants to those calculated from ab initio optimized geometries. Comparison of only one set of experimental rotational constants to theoretical predictions may not conclusively identify the conformational structure, especially if several conformations predict similar rotational constants for the primary isotopic species [5]. Conclusive assignments may be made in these cases, however, using additional rotational constants from a limited number of isotopic species or experimental dipole moment data from Stark effect measurements.…”