Strong intramolecular hydrogen bonding in protonated β-methylaminoalanine: A vibrational spectroscopic and computational study

Strong intramolecular hydrogen bonding in confined amino acids

Intramolecular hydrogen bonding is evaluated in three different amino acids encapsulated in C₆₀ fullerene in the context of electron density analysis. While conventional intramolecular hydrogen bonding in isolated amino acids are dominated by electrostatic character, it is shown that strong intramolecular hydrogen bonding can be formed in confined amino acids so that in two cases covalent intramolecular hydrogen bonding is appeared in the confined species. Also, results show that zwitterionic amino acids are stable in confined state, where no implicit or explicit solvation is applied. Covalent character for intramolecular hydrogen bonding in amino acids have not yet been reported.

A vibrational spectroscopic and computational study of the structures of protonated imidacloprid and its fragmentation products in the gas phase

Infrared multiple photon dissociation (IRMPD) spectroscopy experiments in the 600-2000 cm^-1 region and computational chemistry studies were combined with the aim of elucidating the structures of protonated imidacloprid (pIMI), and its unimolecular decomposition products. The computed IR spectra for the lowest energy structures for pIMI as well as for protonated desnitrosoimidacloprid, corresponding to the loss of NO radical (pIMI-NO), and protonated imidacloprid urea corresponding to the loss of N₂O (pIMIU) were found to reproduce the experimental IRMPD spectrum quite well. The complex IRMPD spectrum for protonated desnitroimidaclpride (pDIMI), resulting from the loss of NO₂ radical from pIMI, was explained as a contribution from several computed structures, including those involving simple loss of NO₂ radical and some isomerization. However, based on a comparison of the computed IR spectrum for the lowest energy structure of pDIMI and the IRMPD spectrum, it was concluded that the lowest energy structure is a minor contributor to the experimental spectrum. This observation is rationalized as being due to the energy requirement for isomerization to the lowest energy structure, being substantially higher than that for simple loss of NO₂ radical. Experimental mass spectrometry fragmentation results indicated that the loss of N, O₂, H was the result of a loss of NO radical followed by loss of OH radical. A comparison of the experimental IRMPD and computed IR spectra revealed that following NO radical loss, the structure entailing a hydride shift from the methylene bridge to the guanidine moiety followed by OH radical elimination, generated the best match with the experimental IRMPD spectrum. This was consistent with the computed potential energy surfaces showing this structure as having the lowest energy requirement.

An IRMPD spectroscopic and computational study of protonated guanine-containing mismatched base pairs in the gas phase

Infrared multiple photon dissociation spectroscopy has been used to probe the structures of the three protonated base-pair mismatches containing 9-ethylguanine (9eG) in the gas phase. Some of these protonated base-pairs have been identified in RNA.