Knowledge Bank

Of 69 transitions which have been observed in the 24,000-to-25,800-Mc region of the microwave spectrum of vinyl iodide, 28 have been assigned as the $J=3\to 4$ transitions $303\to 404$, $313\to 414$, and $312\to 413$. Confirmation of this assignment was obtained from the quadrupolar hfs, Stark effect, temperature coefficient of intensity, and relative intensities of the various lines. Although first-order perturbation theory was adequate to predict the general form of the quadrupolar hfs pattern, it was necessary to take into account second-order effects to obtain a precise agreement between observed and calculated frequencies. The quadrupolar constants for the two transitions with $K-1=1$ agree reasonably well, so that on this basis the average value of these constants could be taken as $\chi aa=-1645\pm 5$ Mc and $\chi bb=+764\pm 9$ Mc. The apparent disagreement of the $\chi aa,\chi bb$ values for the $303\to 404$ transition with those for the two $K-1=1$ transitions is not surprising when one observes that the first-order quadrupolar energy terms are quite insensitive to $\chi bb$ for this transition. Of the magnetic constants, only $b$ is well determined; $a$ is rather uncertain. From the observed quadrupole coupling constants, the components of $eQV2V$ along the C-I bond are computed as $\chi zz=-1869$ Mc, $\chi yy=+988$ Mc, and $\chi xx=+882$ Mc, where $y$ and $z$ are in the molecular plane. In terms of these components, the C-I bond asymmetry is $\eta =0.057$ which is of the same order of magnitude as is found for vinyl chloride and vinyl bromide.