Microwave spectra of 13CH3 12CD3 and 12CH3 13CD3 and the C–C bond length of ethane

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Abstract
We observed the J=2←1, K=1 transitions of two doubly labeled ethanes 13CH3 12CD3 and 12CH3 13CD3, which were contained in natural abundance in a CH3CD3 sample. The measured frequencies were subsequently made more accurate using enriched samples. Assuming the centrifugal distortion constants DJ and DJK, which were determined for 12CH3 12CD3, we calculated the B0 constants to be 16 088.428±0.030 and 16 251.914±0.030 MHz for 13CH3 12CD3 and 12CH3 13CD3, respectively. By combining these with the B0 of 12CH3 12CD3(16 474.438±0.030 MHz), we derived the rs distance of C–C to be 1.526131±0.000083 Å, where the error was due to uncertainties of the rotational constants. We also calculated the rz(C–C) distance to be 1.5312 Å in 12CH3 12CD3 by transferring Kuchitsu’s rz(C–H), rz(C–D), ϑz(CCH), and ϑz(CCD). We evaluated the dipole moments of the two species from the Stark effects to be 0.01067±0.00010 D and 0.01094±0.00011 D for 13CH3 12CD3 and 12CH3 13CD3, respectively, which were to be compared with 0.01078±0.00009 D of 12CH3 12CD3.

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