Pure Rotational Spectrum and Electric Dipole Moment of CH3D

The pure rotational spectrum of CH3D in the ground vibrational and electronic state has been observed using a far-infrared grating spectrometer. This spectrum is of particular interest because the electric dipole moment μ arises entirely from an isotopic substitution. Ten distinct lines have been observed in the frequency range from 40 to 120 cm−1, and have been identified with the (J → J + 1) transitions for the rotational levels J = 5 − 14. For the (6 → 7) line, the absolute intensity and the linewidth were determined by a curve of growth method. It was found that, for this line, the dipole moment μ = (5.68 ± 0.30) × 10−3D and the broadening parameter γ0 = (0.075 ± 0.012) cm−1/amagat. This measurement constitutes the first correct experimental determination of the dipole moment of CH3D. By making further intensity measurements on other lines, it was shown that, to within 6%, μ is independent of J in the range 5 ≤ J ≤ 12. A detailed account of the experimental methods used to make the intensity measurements is given. It was also determined that the rotational constant B0 = (3.882 ± 0.002) cm−1 and the distortion constant Dj = (7.7 ± 0.3) × 10−5cm−1.