Experiment 5: IR/Raman spectroscopy and molecular modeling

Infrared (IR) and Raman are powerful optical spectroscopy based methods for detecting vibrational and rotational modes of molecules. In the following, we will concentrate on molecular vibrations and the effect of molecular rotations is discussed elsewhere (see Experiment #9). Both IR and Raman spectra can be used to identify unknown molecules, determine their concentrations, and to study bond strengths in molecules. For identifying purposes, most spectrometers include a database of IR/Raman spectra and, for small molecules, an on-line database have been compiled [1]. Even though both IR and Raman measure molecular vibrations, they are complementary methods for two reasons; they have different selection rules (i.e., some transitions can only be seen in IR whereas others in Raman) and the requirements for physical characteristics of the samples are different. In this experiment, both IR and Raman methods are applied to determine vibrational frequencies of CCl4, CHCl3, CDCl3 and CH2Cl2 in neat solutions. Molecular rotations are most often quenched in liquids and hence only vibrations will be observed. For an overview of IR and Raman methods, see Refs. [2–5]. The latter two references also contain an extensive compilation of reference data. Due to development of modern computers, theoretical methods based on quantum mechanics have become increasingly important tools in modern chemistry.Typically these methods are based on solving of the time-independent Schrödinger equation for electrons in atoms or molecules. Among the properties that computational models can predict for the electronic ground and excited states are: molecular energies and geometries, charge distributions, and various optical and magnetic resonance spectra. In this experiment, the Hartree-Fock method is used to optimize the geometries and predict the vibartional frequencies and IR/Raman intensities of CCl4, CHCl3, CDCl3 and CH2Cl2. The Hartree-Fock method constitutes the lowest level first principles method (“ab initio”). These results can be compared with the corresponding experimental spectra and assign the peaks to specific normal modes.