The local mode model of high energy XH stretching vibrations will be introduced and briefly reviewed. I will describe how this model can be used to calculate overtone intensities with simple vibrational wavefunctions and ab initio dipole moment functions. Comparison to more detailed calculations will allow us to understand the role of electron correlation in accounting for spectral intensities. I will describe how we measure these very weak spectra by listening to the molecules absorb light. We will look in detail at the spectra of molecules with a methyl group. As the methyl
group rotates, the CH bond length changes. A model will be presented that will explain the role of internal methyl rotation in CH-stretching overtone spectra.