Knowledge Bank

The rotational spectra of the 1,2-dichaicogenins, 1,2-diselenin and 2-selenathiin, were obtained with a pulsed-beam Fourier-transform microwave spectrometer. The least squares fit of the observed $\mu b$-type transitions of the $\mathrm{<mrow\; data-mjx-texclass="ORD">80</mrow>}Se80Se$ isotopomer of 1,2-diselenin to a semi rigid Watson Hamiltonian gave rotational constants of A = 2081.4262(9) MHz, B = 1659.1768(9) MHz and C = 1003.0117(4) MHz. Nuclear spin statistical weights, the presence of only $\mu b$-type transitions, and a large inertial defect of $\Delta =-43.5389u\cdot \backslash AA2$ show 1,2-diselenin has $C2$ symmetry. The six-member ring, $CH=CH-CH=CH-Se-Se$, is twisted about the Se-Se bond and substitution coordinates obtained from the $\mathrm{<mrow\; data-mjx-texclass="ORD">78</mrow>}Se80Se$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">80</mrow>}Se80Se$ isotopic moments of inertia give a Se-Se bond distance of 2.325(3) \AA. An analogous fit of the observed $\mu a$- and $\mu b$-type transitions of the $\mathrm{<mrow\; data-mjx-texclass="ORD">80</mrow>}Se32S$ isotopomer of 2-selenathiin gave rotational constants of A = 2983.8426(6) MHz, B = 2056.7288(6) MHz and C = 1325.1405(2) MHz. The large inertial defect $\Delta =-33.7140u\cdot \backslash AA2$ shows the ring of 2-selenathiin, $CH=CH-CH=CH-Se-S$, is not planar. An $rs$ Se-S bond distance of 2.205(16) \AA is calculated from substitution coordinates obtained from the $\mathrm{<mrow\; data-mjx-texclass="ORD">80</mrow>}Se32S$, $\mathrm{<mrow\; data-mjx-texclass="ORD">78</mrow>}Se32S$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">80</mrow>}Se34S$ isotopic moments of inertia. The structural parameters derived from the spectroscopic data will be compared to ab initio geometries of the 1,2-dichalcogenins.