Vibrational spectroscopy bent molecules

Now consider SO2 which is a bent molecule ( 2 )- Figure 3.12 shows the three normal modes of vibration all give rise to a change in molecular dipole moment and are therefore IR active. A comparison of these results for CO2 and SO2 illustrates that vibrational spectroscopy can be used to determine whether an X3 or XY2 species is linear or bent. 

Figure 5.3 Nonnal modes of vibration of linear and bent XY2 molecules (+ and — denote vibrations moving upwards and downwards, respectively, in the direction perpendicular to the plane of the paper). From Nakamoto, K., Infrared and Raman Spectra of Inorganic and Coordination Compounds , in Handbook of Vibrational Spectroscopy, Vol. 3, ChahnCTs, J. M. and Griffiths, P. R. (Eds), pp. 1872-1892. Copyright 2002. John Wiley Sons Limited. Reproduced with permission.

Theory of vibrational spectroscopy considers the energy of a vibrating molecule and the selection rules governing absorption and emission processes. We calculate the number of normal modes of vibration for linear and non-linear (bent) molecules, and view a computer simulation of the normal modes for both linear and bent triatomic molecules. 

When light traversing an optically dense medium approaches an interface with a more optically rare medium at an angle exceeding a critical value, Bent = sin (rerare/ dens), total internal reflection occurs and an evanescent wave of exponentially deca5ung intensity penetrates the rarer medium. This phenomenon is at the heart of certain spectroscopic methods used to probe biomolecules at interfaces (199). In total internal reflection fluorescence (TIRF) spectroscopy (200-202), the evanescent wave excites fluorescent probes attached to the biomolecules, and detection of the emission associated with their decay provides information on the density, composition, and conformation of adsorbed molecules. In fourier transform infrared attenuated total reflection (FTIR-ATIR) spectroscopy (203,204), the evanescent wave excites certain molecular vibrational degrees of freedom, and the detected loss in intensity due to these absorbances can provide quantitative data on density, composition, and conformation. 

---