Infrared spectra of amino acid zwitterions isolated in alkali halide matrices

A new spray sampling method (DSD) exploited in the measurement of infrared absorption of amino acids and dipeptides has resulted in a third category of spectra. They display qualitative similarities to the resolved bands obtained in the vapor and low-temperature inert-gas isolated matrix, and on the other hand have some common vibrational frequencies with the solution and alkali halide pressed pellet spectra. Comparisons of group frequencies obtained by the DSD and pellet methods, and presented for a number of amino acids and dipeptides, are informative of structural differences in the molecules obtained by the two methods. As well, results are presented for the measured and predicted dependence on the alkali halide matrix. It is argued that the sum of evidence presented supports the conclusion that zwitterions isolated as monomers are produced by the DSD sampling method. The method is not exclusive to the amino acids. Other applications include purine and water.