Variable temperature (from− 55 to− 150° C) studies of the infrared spectra (3500–400cm− 1) of ethylphosphonic difluoride, CH3CH2P (O) F2 and ethylphosphonothioic difluoride, CH3CH2P (S) F2 dissolved in liquid xenon or krypton have been recorded. From these data, the enthalpy differences have been determined to be 76±9cm− 1 (0.91±0.11 kJ/mol), for CH3CH2P (O) F2 with the trans conformer the more stable rotamer and 53±7cm− 1 (0.63± ...