IR plus VUV spectroscopy of the bio-related molecules and its clusters

We report the first observation of infrared (IR) and mass spectra of neutral, aliphatic amino acid clusters: the example presented herein is for (valine)_n, n = 2-5. The clusters are generated in a supersonic expansion and their IR spectra are recorded in various fragment and (valine)_n-1H⁺ (n = 1-2) mass channels. The ions are created by single photon ionization with a VUV laser at 118 nm (10.5 eV/photon) following IR absorption in the single photon energy range 2500 to 4000 cm^-1. Mass channels obviously associated with valine clusters lose intensity and mass channels associated with valine monomers gain intensity under IR irradiation. No free OH modes identified in any of these spectra suggesting that for (valine)_n, n=2,...5, all the OH groups are hydrogen bonded. The infrared transition for the hydrogen bonded OH moiety appears as a very broad, shifted feature at ca. 3000 cm^-1 (~2800 to ~3200 cm^-1). Free and perturbed NH₂ modes can also be identified in the cluster spectra. CH modes ca. 3000 cm^-1 can be identified and appear to be coupled to the shifted and broadened OH modes of the clusters.