Vibrational spectra and possible conformers of dimethylmethylphosphonate by normal mode analysis

Abstract A careful assignment of the i.r. and Raman spectra of CH 3 P(O)(OCH 3 ) 2 , CD 3 P(O)(OCH 3 ) 2 , CH 3 P(O)(OCD 3 ) 2 and CD 3 P(O)(OCD 3 ) 2 and a comparison with those of the parent molecules CH 3 OP(O)Cl 2 and CD 3 OP(O)Cl 2 suggest that only one conformer exists in the liquid state. This conformer is probably close to the most stable one calculated by CNDO/2 and ab initio STO-3G methods, with one CH 3 O group trans and the other cis to the PO group. The characteristic modes of the CP(O)(OC) 2 skeleton are analysed through a force field calculation. It is shown that the stretching vibrations of the single bonds PC and PO are strongly coupled. As already observed for carboxylic esters, it is shown that only mixed skeletal modes and mostly the bending modes are sensitive to the conformation of the methoxy groups around the PO single bond. On the other hand the v (PO) mode does not vary for the different conformers, and the extra components appearing in its vicinity, as well as in that of the v (PC) main vibrational mode, are best understood as being due to Fermi resonances with harmonic or combination tones coupling with these modes.

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