Gas-phase synchrotron FTIR spectroscopy of weakly volatile alkyl phosphonate and alkyl phosphate compounds: vibrational and conformational analysis in the terahertz/far-IR spectral domain.

The high brilliance of the AILES beamline at the SOLEIL synchrotron facility has been exploited for the study of the gas-phase vibrational spectra of weakly volatile organophosphorous compounds. The propagation of the synchrotron radiation in long path length gas cells allowed improvements in the sensitivity limits and spectral coverage compared with a previous study, performed by our group with conventional thermal sources. A ppm level detection in the entire IR domain up to terahertz (THz) frequencies has been realized for dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), triethyl phosphate (TEP), and diethyl (2-methylallyl)phosphonate (DEMaP). In the present study, the assignment of the gas-phase vibrational and the conformational analysis of the two most stable conformers of DMMP and TMP have been extended to the torsional THz spectra in the 20-120 cm(-1) range. The improvement of the S/N ratio below 600 cm(-1) has permitted for the first time a gas-phase conformational analysis of the two weakly volatile and highly flexible TEP and DEMaP compounds. The experimental far-infrared (FIR)/THz spectra have been studied taking into account four low-energy conformers determined by means of high level of theory quantum chemistry calculations. Finally, due to its particularly low vapor pressure, the detection of gas-phase tributyl phosphate (TBP) in the FIR domain was unsuccessful. Nevertheless, the mid-IR/near-IR spectra of TBP recorded in a multipass cell heated to 355 K have been assigned with the harmonic vibrational predictions of the most stable conformer.