Asymmetric molecules spectra properties

Vibrational sum-frequency spectroscopy (VSFS) is a second-order non-linear optical technique that can directly measure the vibrational spectrum of molecules at an interface. Under the dipole approximation, this second-order non-linear optical technique is uniquely suited to the study of surfaces because it is forbidden in media possessing inversion symmetry. At the interface between two centrosymmetric media there is no inversion centre and sum-frequency generation is allowed. Thus the asymmetric nature of the interface allows a selectivity for interfacial properties at a molecular level that is not inherent in other, linear, surface vibrational spectroscopies such as infrared or Raman spectroscopy. VSFS is related to the more conunon but optically simpler second harmonic generation process in which both beams are of the same fixed frequency and is also surface-specific. 

The most common protic solvent is water. It is also one of the most complex from the point of view of vibrational spectroscopy because of its highly structured nature. Since water is a triatomic, non-linear molecule it has three vibrational modes, which are illustrated in fig. 5.13. The Vj mode is the symmetrical stretch V2 is the bending mode and V3 is the asymmetrical stretch. All three vibrational modes for water are active in the infrared because they involve changes in the dipole moment. Activity in the Raman spectrum requires that the polarizability of the molecule changes during vibration. Analysis of this aspect of molecular properties is more difficult but it shows that all three modes are also Raman active. A summary of the frequencies of these vibrations for H2O, and the isotopes D2O, and HOD determined from gas phase spectra are given in table 5.7. 

Inspired by efficient and stereospecific metal-free enzymatic processes, synthetic chemists have devoted much effort in recent years to the development of a broad spectrum of short peptide-based asymmetric catalysts, which mimic various qualities of enzymes. The structural diversity available with short peptide sequences and the fact that peptides offer to the reactants a more strictly defined asymmetric scaffold, compared to single amino acids, makes this class of molecules particularly promising for the development of a broad spectrum of organic catalysts with fine-tunable structural and electronic properties. 

---