Vibrational spectra and structure of compounds of the EtP(X)Cl2 (X=O,S, Se) series

1.	The existence of a thermodynamic equilibrium between the trans and gauche conformers in ethyldichlorophosphonate, ethyldichlorothiophosphonate, and ethyldichloroselenophosphonate has been established.
2.	The gauche conformer predominates in compounds I–III at 298°K. In the case of ethyldichlorophosphonate the trans isomer remains in the solid phase, in the thio analog the gauche isomer remains, and in the selenium analog the trans and gauche isomers remain.
3.	The experimentally observed difference in the frequencies of the conformers is close to that calculated under the assumption that their force fields are constant. Nevertheless, the frequencies of the stretching vibrations of the multiple bonds are higher in the trans isomers than in the gauche.
4.	One vibration with predominant participation of the P=X bond (P=O,P=S, andP=Se) has been isolated in the spectrum of each conformer. The characteristic nature of the vibrations for the bonds just cited decreases along the series indicated.