Knowledge Bank

The infrared absorption spectra of the eight compounds $CH2DX$ and $CHD2X$, where $X=F,Cl,Br,I$, each of very high isotopic purity, have been obtained throughout the fundamental regions. A double-pass prism instrument was used with $CaF2,NaCl$, and KBr prisms, while a grating instrument was used above $2000cm-1$. All 72 fundamentals were observed in the gas phase and band centers assigned. Convenient criteria for determining band centers of perpendicular components were found, involving red and blue shifts of the two strongest central rotational lines. Several cases of Fermi resonance were observed and qualitative methods for determining the amounts present were developed. In $CHD2F$ a case of Coriolis resonance, between fundamentals of different species, is judged to be present. The general quadratic and Urey-Bradley force constants recently proposed for MeX were used to calculate the harmonic fundamentals of $CH2DX$ and $CHD2X$. Several semi-empirical methods of determining individual anharmonicities are applied to these calculated harmonic frequencies, with the overall agreement with experiment reasonably good. A definition of normal mode mixing upon isotopic substitution, through the L matrices, is used to establish a correlation diagram for each system $CH3X-CH2DX-CHD2X-CD2X$. For MeF the first-order theorem ""[FIGURE]"" $$\frac{\partial\ \omega_{k}}{\partial\ m_{i}}\leq 0$$ where $\omega k$ is the kth harmonic frequency and $mi$ the mass of atom i, cannot be simultaneously satisfied for every normal mode by any number of frequency interchanges within the diagram; at least one violation will always occur.