Knowledge Bank

The extremely rich microwave spectrum of the title compound has been assigned and analyzed using a combination of conventional Stark modulation spectroscopy and radio frequency-microwave double resonance (RFMDR). The dipole moment has been measured and found to be approximately equally distributed along the a and c principal axes ($\mu a=2.76\pm 0.01$ D and $\mu c=2.12\pm 0.07$ D and $\mu t=3.48\pm 0.06$ D). The microwave spectra of all the singly substituted $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ were assigned using RFMDR on a sample containing a mixture of isotopically enriched $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ species. This data allowed the determination by the $rs$ method of several pertinent structural parameters associated with the carbon atom ring: $C5-C6=1.552\pm 0.002\backslash AA,C1-C4=2.194\pm 0.002\backslash AA,C1-C7=1.533\pm 0.005\backslash AA,\angle C1-C7-C4=91.3\pm 0.5\backslash AA,\angle C1-C4-C5=78\pm 1\backslash AA$. The corresponding values from a previous electron diffraction study are: $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ $C5-C6-1.530\backslash AA,C1-C4-2.50\backslash AA,\angle C1-C7=1.547\backslash AA,\angle C1-C7-C4=108.0\circ C1-C4-C5-71.5\circ$. A small value for the a-coordinate ($\sim$ 0.03 {\AA}) of $C1$ (and $C4$ by symmetry) precludes the determination of some of the parameters, by the $rs$ method, namely the $C1-C6$ bond length and several angles involving $C1$. The determination of these parameters will be discussed as well as the discrepancies between the microwave and electron diffraction results. Qualitative measurements of the relative intensities of the $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ species and the small value of the inertial defect for the $7-13C$ species indicate that the molecule possesses $Cs$ symmetry.