Gas phase structure of calcium (II)-catechol complexes probed by infrared spectroscopy

BUTLER, M.; ARROYO MAÑEZ P.; MAÎTRE P.; CABRERA G. M.

Vancouver, BC

Conferencia; 60th ASMS Conference on Mass Spectrometry and Allied Topics; 2012

American Society for Mass Spectrometry

 Novel AspectFirst study of Ca(II)-catechol system in the gas phase bringing a better understanding of the complexation properties of catechol. Introduction: Catechol and catechol derivatives occur as intermediary metabolites during the degradation of naturally-occurring and synthetic aromatic compounds, being ubiquitous in biological chemistry acting whether as electron donors or complexing agents. In this work, our motivation is to understand the complexation properties of catechol. The study of catechol-metal interactions includes the comparison of different binding motifs as well as possible tautomers that might compete according to calculations. The results of this study can later be related to analogs or be used for the differentiation of isomers as already done for hydroxypyridine N-oxides [1]. IR photodissociation spectroscopy coupled with ESI, in combination with quantum chemical calculations, has proven to be an efficient tool to characterize the structure of gas-phase metal complexes. Methods: Infrared (IR) spectroscopy of gas-phase [Ca(Catechol)2(H2O)1-2]2+ ions was performed employing two experimental setups based on the coupling of a 7 Tesla Fourier transform ion cyclotron resonance (FT-ICR) tandem mass spectrometer (Bruker Apex Qe) with two IR lasers. For IR spectroscopy of the ions in the 900?1700 cm-1 spectral range, the Infrared Free Electron Laser (IR FEL) from the Centre Laser Infrarouge d?Orsay (CLIO) was used [2]. IR spectra of the ions in the 3450?3750 cm-1 spectral range were recorded coupling an optical parametric oscillator/amplifier (OPO/OPA) laser system [3]. Quantum chemical calculations were carried out at the B3LYP/6-31+G(d,p) level on the [Ca(Catechol)2(H2O)1-2]2+ ions to characterize their structure and energetics, as well as their infrared absorption spectra for band assignments. Preliminary Data: Doubly charged adducts formed from catechol and calcium(II) were produced in the gas phase using ESI, appearing as the most important ions in the mass spectra recorded. The structures of Ca(II)-catechol complexes were characterized in the gas phase by infrared photodissociation spectroscopy and quantum chemical calculations in order to determine the coordination mode (mono or bidentate) as well as the occurrence of ring hydrogen shifts. IRMPD spectra of [Ca(Catechol)2(H2O)1-2]2+ complexes exhibit 6 distinct bands in the 950-1700 cm-1 fingerprint spectral range. The most relevant ones for the structural characterization are those assignable to the stretching and bending modes of the C-O in contrast with the stretching modes of C-C and bending modes of C-H. On the other hand, gas-phase IRMPD spectra recorded in the 3450?3750 cm-1 range (OH stretching region) display few features assignable to the symmetric and asymmetric modes of vibration of water molecules and to the hydroxyl stretching modes of the catechol ligands. The comparison of experimental and calculated spectra suggests that intramolecular hydrogen bonds between the hydroxyl moieties of gas phase neutral catechol are not present in the calcium complexes. Moreover, based on the lack of C=O stretching bands, it seems clear that keto-tautomers, which are energetically competitive according to calculations, are not formed under our experimental conditions. Comparison with the IR gas phase spectrum of neutral catechol reveals the effect of adduct formation with calcium and water on the geometric structure of the ligand. [1] Butler, M.; Arroyo Mañez, P.; Cabrera, G. M. J Am Soc Mass Spectrom 2011, 22, 545-556. [2] Bakker, J. M.; Besson, T.; Lemaire, J.; Scuderi, D.; Maitre, P. J Phys Chem A 2007, 111, 13415-13424. [3] Bakker, J. M.; Sinha, R. K.; Besson, T.; Brugnara, M.; Tosi, P.; Salpin, J. Y.; Maitre, P. J Phys Chem A 2008, 112, 12393-12400.