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Issue 14, 2010
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Infrared vibrational spectra as a structural probe of gaseous ions formed by caffeine and theophylline

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Abstract

Ionic hydrogen bond (IHB) interactions, resulting from the association of ammonia and the two protonated methylxanthine derivatives, caffeine and theophylline, have been characterized using infrared multiphoton dissociation (IRMPD) spectroscopy and electronic structure calculations at the MP2/aug-cc-pVTZ//B3LYP/6-311+G(d,p) level of theory. The proton-bound dimer (PBD) of caffeine and ammonia exhibits a low binding energy and was found to be elusive under the experimental conditions due, most probably, to collision-induced dissociation of the complex with helium buffer gas before IRMPD irradiation. The IRMPD spectrum of a PBD of theophylline and ammonia was obtained and revealed bidentate IHB formation within the complex, which greatly increased the binding energy relative to the most stable isomer of the PBD of caffeine and ammonia. The IRMPD spectra of the protonated forms of caffeine and theophylline have also been obtained. The spectrum of protonated caffeine showed dominant protonation at the N(9) site, whereas the spectrum of protonated theophylline showed a mixture of two isomers. The first protonated isomer of theophylline exhibits protonation at the N(9) site and the second isomer demonstrated protonation at the C(6) carbonyl oxygen. The protonated carbonyl isomer of theophylline cannot be produced as a result of direct protonation and is thus suggested to be a consequence of proton-transport catalysis (PTC) initiated by the electrostatic interaction between water and N(9) protonated theophylline. Calculated anharmonic spectra have been simulated at the B3LYP/6-311+G(d,p) level of theory. It is shown that calculated anharmonic frequencies significantly outperform calculated harmonic frequencies in providing simulated IRMPD spectra in all cases.

Graphical abstract: Infrared vibrational spectra as a structural probe of gaseous ions formed by caffeine and theophylline

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The article was received on 08 Oct 2009, accepted on 10 Feb 2010 and first published on 04 Mar 2010


Article type: Paper
DOI: 10.1039/B921102K
Citation: Phys. Chem. Chem. Phys., 2010,12, 3431-3442

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    Infrared vibrational spectra as a structural probe of gaseous ions formed by caffeine and theophylline

    R. A. Marta, R. Wu, K. R. Eldridge, J. K. Martens and T. B. McMahon, Phys. Chem. Chem. Phys., 2010, 12, 3431
    DOI: 10.1039/B921102K

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