Microwave Spectrum of 1-Cyano-3-fluoro-but-1-ene.

The rotational spectrum of the 1-cyano-3-fluoro-but-1-ene has been recorded with a pulsed-nozzle microwave Fourier transform spectrometer over the range 6-20 GHz. The frequencies were fitted to the Hamiltonian of Watson (A-reduction, I(r) representation). The resulting rotational constants are A = 7493.404(1) MHz, B = 1211.9831(2) MHz, and C = 1096.0908(1) MHz. By comparing the experimental rotational constants with those obtained by ab initio calculations, we found without ambiguity that the stable conformation for the molecule is the one with the fluorine atom lying in the C&bond;CCN plane (CF-eclipsed conformer). Copyright 2000 Academic Press.