Pure Rotational Spectrum and Electric Dipole Moment of CH3D

Abstract

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 $\mu$ 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 $\text{(J\hspace{0.17em}→\hspace{0.17em}J\hspace{0.17em}+\hspace{0.17em}1)}$ transitions for the rotational levels $\text{J\hspace{0.17em}=\hspace{0.17em}5\hspace{0.17em}−\hspace{0.17em}14}$ . 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 ${\text{μ\hspace{0.17em}=\hspace{0.17em}(5.68\hspace{0.17em}±\hspace{0.17em}0.30)\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−3}}\mathrm{D}$ and the broadening parameter ${\gamma }_0\text{\hspace{0.17em}=\hspace{0.17em}(0.075\hspace{0.17em}±\hspace{0.17em}0.012)}{\mathrm{cm}}^{\text{−1}}/\mathrm{amagat}$ . 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%, $\mu$ is independent of $J$ in the range $\text{5\hspace{0.17em}≤\hspace{0.17em}J\hspace{0.17em}≤\hspace{0.17em}12}$ . A detailed account of the experimental methods used to make the intensity measurements is given. It was also determined that the rotational constant $B_0\text{\hspace{0.17em}=\hspace{0.17em}(3.882\hspace{0.17em}±\hspace{0.17em}0.002)}{\mathrm{cm}}^{\text{−1}}$ and the distortion constant $D_j{\text{\hspace{0.17em}=\hspace{0.17em}(7.7\hspace{0.17em}±\hspace{0.17em}0.3)\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−5}}{\mathrm{cm}}^{\text{−1}}$ .