Knowledge Bank

The microwave rotational spectra of a large number of organic and several organometallic molecules in the molecular weight range 70-348 have been observed under conditions of low resolution. In many (more than 40) of the spectra, the prominent feature is one or more series of broad ($\sim 10-200$ MHz) but generally quite intense ``bands''. These bands are interpreted as the sum of unresolved, high K lines for particular $J\to J+1$ transitions of near-symmetric top molecules. J-values as high as 63 were observed for one of the heavier molecules. Within the measurement uncertainties, the band frequencies $\nu j$ in a series are reproduced by the simple expression $\nu j=B\ast (J+1).B\ast$The polarized phosphorescence emission and excitation spectra of riboflavin and alloxazine were measured in ethylene glycol-water (2:1, v/v) at $77\circ K$ in order to assign the lowest triplet state$\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$ will be approximately (B+C) for a near-prolate rotor. Compounds studied had asymmetry parameters in the range . For the more asymmetric compounds, the bands showed some structure and could be resolved into components under conditions of high resolution search. Rotational isomerism is thought to be the origin of the multiple series of bands found in the 1-haloalkanes ($C3$ to $C6$ compounds) and certain benzyl derivatives. Molecular constants and conformational information extracted from these band spectra will be compared to those from high resolution studies (where available) and to those calculated from assumed molecular structures.