Abstract

As an aid in the interpretation of the infrared spectra of nearly linear, slightly asymmetric molecules, calculations have been made upon molecular models of varying degrees of asymmetry in order to determine the general characteristics of their infrared bands. ${A}_{x}$, ${A}_{y}$, and ${A}_{z}$ have been chosen as the principal moments of inertia and the atoms comprising the molecule have been taken to be coplanar such that ${A}_{x}={A}_{y}+{A}_{z}$. For convenience of calculation, a parameter $\ensuremath{\rho}$ has been chosen where $\ensuremath{\rho}=\frac{{A}_{z}}{{A}_{y}}$ and calculations have been made for ten values of $\ensuremath{\rho}$, varying in steps of $\ensuremath{\Delta}\ensuremath{\rho}=0.02$ from $\ensuremath{\rho}=0.20$ to $\ensuremath{\rho}=0.00$. In the computation of the intensities of lines, Boltzmann factors were used corresponding to a largest moment of inertia ${A}_{x}=2.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}39}$ gm \ifmmode\cdot\else\textperiodcentered\fi{} ${\mathrm{cm}}^{2}$. As in the work of Dennison on completely asymmetric molecules, of which this is essentially an extension, charts have been prepared depicting the general characteristics of the three types of bands which may occur.Type A bands. Type A bands arise from vibrations of the electric moment along the $z$-axis or least axis of inertia and may be seen to consist of $P$, $Q$, and $R$ branches. The members of the $P$ and $R$ branches are not single lines as in the symmetric rotator, but are groups of closely spaced lines spreading out as $\ensuremath{\rho}$ increases. The $Q$ branch also is complex and decreases in intensity as $\ensuremath{\rho}$ decreases finally vanishing in the limiting linear case where $\ensuremath{\rho}=0$.Type B bands. Type B bands arise from vibration of the electric moment along the $y$-axis or intermediate axis of inertia. They too may be seen to consist, not of single lines, but of groups of closely spaced lines. As $\ensuremath{\rho}$ decreases the spacing between these groups becomes coarser and coarser until finally where $\ensuremath{\rho}=0$, this type of band vanishes completely. Type B consists essentially only of one branch.Type C bands. Type C bands arise from vibrations of the electric moment along the $x$-axis or the largest axis of inertia and for the class of molecules here considered are very similar to type B bands. As in the former case the spacing between line groups becomes coarser and coarser, the band finally vanishing completely where $\ensuremath{\rho}=0$.