| Summary: | A blue shift in the CH stretching vibration of formic acid cyclic dimer of 6.6 ern" (symmetric) and 3 ern" (antisymmetric) is observed by high resolution Raman (symmetric) and infrared (antisymmetric) spectroscopy. This is corroborated by theoretical ab initio calculations where blue shifts in the CH stretching vibration of 12.79 ern" (symmetric) and 10.26 ern" (antisymmetric) are calculated (CP corrected MP2/6-311++G(d,p) level of theory). This is unusual due to the CH bond not playing a direct part in the bonding of the dimer. The electric dipole moment derivative curve with respect to bond length of the CH bond in formic acid is found to be unusual. The equilibrium bond length is on the negative gradient side of the maximum of the dipole, and this has been used to explain interesting behaviour observed, including the blue shift of the CH stretching vibration and how the contribution of electrostatics to the interaction energy can cause a blue shift of the stretching vibration in the spectrum. A mechanism is proposed where the electron density is transferred from the CH bond, through to the OH site where bonding does take place. This in turn causes the CH bond to have increased polarity, and therefore the bond contracts due to this interaction. Several chloroform complexes are investigated, which show either blue shifting or red shifting of the CH stretching vibration. Complexation with dimethyl ether shows an experimental red shift of the CH stretching vibration of chloroform of -1.5 ern", and a theoretical shift of -2.11 crn'. The complex of chloroform with trimethyl amine shows an experimental red shift of the CH stretching vibration of chloroform of -54 ern", and a theoretical prediction of -79.51 ern". Both of these complexes show a 1: 1 stoichiometric equation. The chloroform self dimer shows blue shifts in the CH stretching vibration, calculated to be 2.1 and 8.8 ern", experimental results are currently inconclusive. Morokuma Kitaura energy decomposition has been used to understand the energy contributions to intermolecular bonding. Electrostatic interaction and exchange repulsion have been shown to be the main contributions to bonding, but some unusual cases, for example the CH bond of formic acid cyclic dimer, have shown electrostatics to cause a blue shift. - 3 - A tuneable stimulated Raman photoacoustic spectroscopy (PARS) set up has been further applied to the trace detection of H2' and has achieved a detection limit of 6.69 ppm by volume. A non-dispersive Raman shifter method has also been investigated as a simpler alternative to the tuneable PARS set up and has achieved a less sensitive detection limit of 108 ppm by volume. Methane has also been detected qualitatively via this method, using the Raman shifter as a source of infrared light.
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