15min:

PETER GRONER, Department of Chemistry, University of Missouri - Kansas City, Kansas City, MO 64110; CHARLES W. GILLIES, Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY; JENNIFER Z. GILLIES, Department of Chemistry, Siena College, Loudonville, NY.

The rotational spectra of eight isotopomers of dimethyl diselenide, CH₃SeSeCH₃, were recorded between 6 and 18 GHz with a pulsed-beam Fourier-transform microwave spectrometer. All rotational transitions of the symmetric isotopomers with C₂ symmetry, CH₃⁷⁸Se⁷⁸SeCH₃ and CH₃⁸⁰Se⁸⁰SeCH₃, are split into four components because of the interaction with the internal rotation of the methyl groups. The rotational transitions of the asymmetric isotopomers CH₃⁷⁶Se⁸⁰SeCH₃, CH₃⁷⁷Se⁸⁰SeCH₃, CH₃⁷⁸Se⁸⁰SeCH₃, CH₃⁷⁸Se⁸²SeCH₃, and CH₃⁸⁰Se⁸²SeCH₃ are split into five components. The spectra for these isotopomers (100 to 125 frequencies each) were fit with experimental precision (1.5 kHz) to the effective rotational Hamiltonian for molecules with two periodic internal motions . The fits required eight rotational and quartic centrifugal distortion constants and five or nine internal rotation and tunneling parameters for the symmetric or asymmetric isotopomers, respectively. For CH₃⁸⁰Se⁸⁰SeCH₃, the principal results were: A = 5156.15730(41) MHz, B = 1481.13557(56) MHz, C = 1420.13993(60) MHz, = 0.013930(49), = 36.554(52)^\circ, = 39.093(14)^\circ, resulting in I = 3.0816(80) uŲ. A similar fit was performed for ¹³CH₃Se⁸⁰Se⁸⁰CH₃ where fewer lines were measured due to the low intensity of the spectrum recorded in natural abundance. Substitution parameters were calculated in the CH₃⁸⁰Se⁸⁰SeCH₃ isotopic frame using the CH₃⁷⁸Se⁸⁰SeCH₃ and ¹³CH₃⁸⁰Se⁸⁰SeCH₃ moments of inertia to give r_s(Se-Se) = 2.332 Å, r_s(Se-C) = 1.957 Å, (CSeSe) = 99.41^\circ and (CSeSeC) = 85.25^\circ.