Infrared multiple photon dissociation action spectroscopy of sodium cationized halouracils: Effects of sodium cationization and halogenation on gas-phase conformation

• The Na+(xU) complexes are examined via IRMPD action spectroscopy techniques.
• IRMPD spectra are compared to theoretical IR spectra of the low-energy conformers.
• Na+ binds to O4 of the canonical diketo tautomers in the ground-state conformers.
• Ground-state conformers are the dominant contributors to the IRMPD spectra.
• Excited minor tautomers of Na+(6ClU) also contribute to the IRMPD spectrum.

The gas-phase structures of sodium cationized complexes of 5- and 6-halo-substituted uracils are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical electronic structure calculations. The halouracils examined in this investigation include: 5-flourouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, and 6-chlorouracil. Experimental IRMPD action spectra of the sodium cationized halouracil complexes are measured using a 4.7 T Fourier transform ion cyclotron resonance mass spectrometer coupled to the FELIX free electron laser (FEL). Irradiation of the mass selected sodium cationized halouracil complexes by the FEL was carried out over the range of frequencies extending from 950 to 1900 cm−1. Theoretical linear IR spectra predicted for the stable low-energy conformations of the sodium cationized halouracils, calculated at B3LYP/6-31G(d) level of theory, are compared with the measured IRMPD action spectra to identify the structures accessed in the experiments. Relative stabilities of the low-energy conformations are determined from single-point energy calculations performed at the B3LYP/6-311+G(2d,2p) level of theory. The evolution of IRMPD spectral features as a function of the size (F, Cl, Br, and I) and position (5 versus 6) of the halogen substituent are examined to elucidate the effects of the halogen substituent and noncovalent interactions with sodium cations on the structure of the nucleobase. Present results are compared with results from energy-resolved collision-induced dissociation and IRMPD action spectroscopy studies previously reported for the protonated and sodium cationized forms of uracil, and halo-, methyl-, and thioketo-substituted uracils. The present results suggest that only a single conformer is accessed for all of the 5-halouracil complexes, whereas multiple conformers are accessed for the Na+(6ClU) complex. In all cases, the experimental IRMPD action spectra confirm that the sodium cation binds to the O4 carbonyl oxygen atom of the canonical diketo tautomer in the ground-state conformers, and gains additional stabilization via chelation interactions with the halogen substituent in the complexes to the 5-halouracils as predicted by theory.

The gas-phase structures of sodium cationized complexes of 5- and 6-halouracils are examined via infrared multiple photon dissociation action spectroscopy and theoretical calculations. Present results indicate that although binding of a sodium cation preferentially stabilizes minor tautomers of the halouracils, the most stable structures still involve binding of Na+ to the O4 carbonyl oxygen atom of the ground-state canonical diketo tautomer. Sodium cationization is found to stabilize minor tautomers of 6ClU to a greater extent than for any of the 5-halouracils such that minor tautomers also contribute to the measured IRMPD spectrum of the Na+(6ClU) complex.Figure optionsDownload high-quality image (122 K)Download as PowerPoint slide