Determination of rotational constants using nuclear quadrupole hyperfine structure: The microwave spectrum and dipole moment of vinyl iodide, obtained using a computer-controlled spectrometer

Abstract

The microwave spectra of vinyl iodide, CH2=CHI, in its ground and first excited vibrational states, have been measured in the frequency range 20–108 GHz. The spectrometer used was a 100 kHz Stark modulated instrument, interfaced to a Digital Micro-PDP 11/23+ computer for control and for data acquisition and storage; this interfacing is described. The spectrum contains strong a-type transitions and very weak b-type transitions; all contain 127I quadrupole hyperfine structure, with several large perturbations. A procedure specially devised for analysis of such spectra, which takes advantage of the perturbations, was applied to produce accurate values of constants that might otherwise be unobtainable, and to permit assignment of the b-type transitions. Both components of the dipole moment have been measured. The centrifugal distortion constants and inertial defects have been compared with those calculated from a published harmonic force field, modified for the out-of-plane vibrations. A partial structure has been obtained.