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

Abstract

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 NH 2 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).

The sodium cation-bound dimer of theophylline: IRMPD spectroscopy of a highly symmetric electrostatically bound species

“…5 suggesting that IX O(2)/O (6) and X O(2)/O (2) are the dominant isomers displayed in the IRMPD spectrum, which is also consistent with the predicted relative free energies (298 K) obtained by electronic structure calculations ( Table 2). Although calculated IR intensities will not be compared here to IRMPD intensities due to the complex nature of the IRMPD process, the match between the calculated harmonic frequencies of both isomers IX O(2)/O (6) and X O(2)/O (2) with the IRMPD spectrum of the SCBD of theophylline is very good, with a MAD of only 10 and 11 cm −1 , respectively.…”

“…1 and the three most favourable sites of protonation [4] with respect to free energy (298 K) as N(9), O(2) and O (6). The cyclic amide nitrogen atoms, N(1) and N(3) are not Lewis basic due to the fact that their lone pairs are highly delocalized about the ring system and carbonyl oxygen atoms.…”

“…5 with spectral assignments available in Table 3. Although isomer VII O(2)/N(9) is significantly higher in free energy (298 K) than isomers VIII O(6)/O (6) , IX O(2)/O (6) and X O(2)/O (2) , its spectrum is also included in Fig. 5 demonstrating calculated vibrational signatures of a dimer possessing an inefficient sharing of Na + , specifically engaged in a C(2) O· · ·Na + · · ·N(9) interaction between the theophylline molecules.…”

“…Although many excellent IRMPD studies have yielded detailed descriptions of alkali metal cation interactions with amino acids and peptides [6][7][8][9]11,13,14,46], methylxanthines demonstrate another unexplored example of small biologically relevant molecules possessing the necessary attributes amenable to IRMPD spectroscopy experiments in the spectral region of 1000-1900 cm −1 . Here the IRMPD spectrum of the sodium cationbound dimer (SCBD) of theophylline is presented; in compliment to recent work showing the IRMPD spectra of both protonated theophylline and the PBD of theophylline and ammonia [4].…”

Glycine Dimers: Structure, Stability, and Medium Effects

“…For instance, a high-pressure mass spectrometric and density functional theory investigation of the thermochemical properties and structure of protonated dimers and trimers of glycine was reported by Raspopov and McMahon. [38] Atkins et al [39] carried out a IR multiple photon dissociation spectra study on proton-and sodium-ion-bound glycine dimers.…”