Microwave Determination of the Molecular Structures and Nuclear Couplings of the Methyl Halides

Accurate measurements on the pure rotation spectra of methyl chloride, bromide and iodide have been made with multiples of a 10 mc/sec. frequency monitored by station WWV. The measurements, with existing infra-red data, allow a complete determination of the structures of these molecules. The $I_{\mathrm{B}}$ values in g/${\mathrm{cm}}^2$×${10}^{-40\mathrm{}}$ are: ${63.11}_{38}$ for ${\mathrm{C}}^{12}$ ${\mathrm{H}}_3$ ${\mathrm{Cl}}^{35}$, ${64.10}_{09}$ for ${\mathrm{C}}^{12}$ ${\mathrm{H}}_3$ ${\mathrm{Cl}}^{37}$, ${87.68}_{35}$ for ${\mathrm{C}}^{12}$ ${\mathrm{H}}_3$ ${\mathrm{Br}}^{79}$, ${88.01}_{80}$ for ${\mathrm{C}}^{12}$ ${\mathrm{H}}_3$ ${\mathrm{Br}}^{81}$, ${111.8}_{43}$ for ${\mathrm{C}}^{12}$ ${\mathrm{H}}_3$ ${\mathrm{I}}^{127}$ and ${117.8}_{48}$ for ${\mathrm{C}}^{13}$ ${\mathrm{H}}_3$ ${\mathrm{I}}^{127}$. The molecular dimensions obtained are: for methyl chloride, $d_{\mathrm{CH}}=1.109\mathrm{}$, $d_{\mathrm{CCl}}=1.779\mathrm{}$, < HCH = 110° 0′; for methyl bromide, $d_{\mathrm{CH}}=1.104\mathrm{}$, $d_{\mathrm{CBr}}=1.936\mathrm{}$, < HCH = 110° 15′; for methyl iodide, $d_{\mathrm{CH}}=1.100\mathrm{}$, $d_{\mathrm{CI}}=2.139\mathrm{}$, < HCH = 110° 58′. The nuclear quadrupole couplings, $eQ\left(\frac{{\partial }^{2}V}{\partial z^2}\right)$, are: for ${\mathrm{Cl}}^{35}$, - 75.13 mc/sec.; for ${\mathrm{Cl}}^{37}$, - 59.03 mc/sec.; for ${\mathrm{Br}}^{79}$, 577.0 mc/sec.; for ${\mathrm{Br}}^{81}$, 482.0 mc/sec.; and for ${\mathrm{I}}^{127}$, - 1934 mc/sec. Second-order effects in the hyperfine structure were observed for methyl bromide and methyl iodide.