Knowledge Bank

Fundamental IR transition strengths arise from vibrationally induced non-zero dipole moment derivatives along a specific normal mode coordinate. A conventional picture of what leads to strong vs. weak intensities in these fundamental vibrations is the bond dipole model, in essence arising from vibrational displacements of partially charged atoms. Additional transition moment intensity can arise from vibrationally induced changes in bond polarization (which we figuratively call charge-sloshing'') that can either interfere constructively or destructively with the bond dipole contributions. This charge-sloshing'' effect proves to be remarkably strong in the series of halomethyl radicals CH$2$F and CH$2$Cl, high resolution IR spectra of which have been acquired in the CH$2$ symmetric and asymmetric stretch regions. Theoretical analysis of the data indicates competition between bond-dipole and ``charge-sloshing'' contributions, which results in i) greatly enhanced symmetric vs. asymmetric stretch intensities, ii) a near vanishing of the CH$2$ asymmetric stretch transition moment in CH$2$Cl, and iii) even a sign reversal of the symmetric and asymmetric stretch transition moments from simple bond dipole expectations.