Infrared Multiple Photon Dissociation Spectra of Proton- and Sodium Ion-Bound Glycine Dimers in the N−H and O−H Stretching Region

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 112; no. 41; pp. 10220 - 10225
• Main Authors: Atkins, Chad G, Rajabi, Khadijeh, Gillis, Elizabeth A. L, Fridgen, Travis D
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.10.2008
• Subjects: A: Kinetics, Spectroscopy; Computer Simulation; Dimerization; Glycine - chemistry; Hydrogen Bonding; Ions - chemistry; Models, Chemical; Osmolar Concentration; Photons; Sodium - chemistry; Spectrophotometry, Infrared - methods
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp805514b

The proton- and the sodium ion-bound glycine homodimers are studied by a combination of infrared multiple photon dissociation (IRMPD) spectroscopy in the N−H and O−H stretching region and electronic structure calculations. For the proton-bound glycine dimer, in the region above 3100 cm−1, the present spectrum agrees well with one recorded previously. The present work also reveals a weak, broad absorption spanning the region from 2650 to 3300 cm−1. This feature is assigned to the strongly hydrogen-bonded and anharmonic N−H and O−H stretching modes. As well, the shared proton stretch is observed at 2440 cm−1. The IRMPD spectra for the proton-bound glycine dimer confirms that the lowest energy structure is an ion−dipole complex between N-protonated glycine and the carboxyl group of the second glycine. This spectrum also helps to eliminate the existence of any of the higher-energy structures considered. The IRMPD spectrum for the sodium ion-bound dimer is a much simpler spectrum consisting of three bands assigned to the O−H stretch and the asymmetric and symmetric NH2 stretching modes. The positions of these bands are very similar to those observed for the proton-bound glycine dimer. Numerous structures were considered and the experimental spectrum agrees with the B3LYP/6-31+G(d,p) predicted spectrum for the lowest energy structure, two bidentate glycine molecules bound to Na+. Though some of the structures cannot be completely ruled out by comparing the experimental and theoretical spectra, they are energetically disfavored by at least 20 kJ mol−1.