Microwave spectra and molecular conformation of methoxy difluorophosphinoxide

Abstract Two closely spaced a -type low resolution microwave band series were observed for the normal, 13 C and d 3 isotopic forms of methoxy difluorophosphinoxide. The lower frequency series of the three isotopic species was found and assigned using a pulse beam Fabry—Perot cavity Fourier transform microwave spectrometer. The a - and b -type spectra of the three species were fitted to a Watson hamiltonian which gave A =4568.215(1) MHz, B =2500.471(9) MHz and C =2453.863(9) MHz for the normal species. Stark effect measurements of CH 3 OP(O)F 2 led to the determination of the electric dipole component μ a =2.699(2) D and μ b =1.522(2) D. Moments of inertia and electric dipole data show that the lowest energy conformation has C s symmetry. The data are most consistent with an assignment of the observed conformer to the trans configuration. The second a -type low resolution microwave band series not observed in the pulsed beam cavity spectrometer arises from a methoxy torsional excited state fo the trans conformer or a second higher energy conformation. The absence of A–E torsional splittings in the a - and b -type spectra gives a lower limit of ∼2500 cal mol −1 for the V 3 methyl internal rotation barrier.