Elsevier

Chemical Physics Letters

Volumes 519–520, 5 January 2012, Pages 40-44
Chemical Physics Letters

Experimental and theoretical investigation of the parabanic acid molecule following VUV excitation and photodissociation

https://doi.org/10.1016/j.cplett.2011.11.052Get rights and content

Abstract

Photodissociation experiments have been performed for the parabanic acid (C3H2N2O3) molecule in vapor phase using time-of-flight mass spectrometry and synchrotron radiation in the VUV photon energy range. Electron ion coincidence (PEPICO) spectra and partial ion yields have been recorded as a function of the photon energy covering the 11–21 eV valence range region. The resulting photoionization products as well as proposed fragmentation pathways leading to those species are presented and discussed. Electronic structure computations for the neutral and ionic species were also carried out at the B3LYP/aug-cc-pVTZ level of theory.

Highlights

► Experimental and theoretical investigation of parabanic acid molecule. ► Photoelectron photoion coincidence spectra in the 11–21 eV range. ► VUV photoionization and dissociation mechanisms of parabanic acid molecule.

Introduction

The investigation of the photo-excitation and dissociative photoionization processes in biomolecules and related compounds has received increasing attention in recent years [1], [2], [3], [4], [5], [6], [7], [8], [9] due to their relevance to several fields, including biochemistry, photochemistry and astrophysics [10], [11], [12], [13], among other applications. In spite of the different investigations concerning imidazole derivatives, no comprehensive and accurate description on the photoionization, dissociation dynamics as well as the ionic and radical species produced by the ionizing photons is yet available in the literature.

Imidazole derivative compounds represent an important class used in pharmaceutical, medicinal and cosmetic industry. Besides, these species present chemical structures which are similar to those of some important molecules of biological interest, for instance, amino acids and DNA bases. In this context, parabanic acid, C3H2N2O3, (also known as oxalylurea or imidazolidine-2,4,5-trione) may be considered an historic compound, since some important research groups in organic chemistry were involved in its synthesis and characterization. In addition, its biological interest results from the fact that parabanic acid is the final product of the oxidation of a number of fundamental biological compounds such as uric acid, guanine, uracil and alloxanic acid [14], [15], [16], [17], [18], [19]. Parabanic acid is also important in polymer science (employed in the preparation of Fe complexes and sensors) [20], [21] and also in astrobiology, as it has even been found in the Murchison meteorite [22]. More recently, the thermophysical and thermochemical investigation have been reported for crystalline parabanic acid using differential scanning calorimetry and combustion calorimetry [23].

In the present work, in order to investigate the photoionization of this molecule, in vapor phase, we have used time-of-flight mass spectrometry (TOF-MS) in the electron-ion coincidence mode (PEPICO) and synchrotron radiation in the VUV range as ionizing agent. These techniques allowed us to obtain important experimental evidences to elucidate the ionic dissociation pathways of this molecule as a function of the photon energy. Relative abundances and branching ratios have been determined for the molecular ion and the ionic fragments from the analysis of the corresponding peak shapes in the PEPICO mass spectra.

Our present results provide the first report on the vapor phase valence-shell excitation and photodissociation of the parabanic acid molecule following valence level excitation and ionization. The resulting photoionization products as well as the most possible fragmentation pathways leading to those species are presented and discussed.

High level DFT calculations at B3LYP/aug-cc-pVTZ level of theory have been performed for this molecule in order to obtain additional information on the geometry, electronic structure and energies. The electronic structure computations for the radical cation were also carried out with the unrestricted implementation UB3LYP/aug-ccpVTZ, which has helped in our analysis for this molecule.

Section snippets

Experimental set-up

The experiments were performed at the Brazilian Synchrotron Light Laboratory (LNLS) located in Campinas, Brazil. Vacuum ultraviolet (VUV) photons in the range from 11.2 to 22 eV from the toroidal grating monochromator (TGM) beamline [24] were used in the present studies. This beamline provides a photon flux in the order of 1011 photons/s and a resolving power (EE = 500–700) in the 11–21 eV range.

The measurements have been undertaken using a gas phase harmonics filter, which has been described in

Computational details

The quantum chemical computations were carried out using the Gaussian 09 package [27]. Calculation procedures are described in the literature [23], [28]. Geometrical parameters for the neutral parabanic acid molecule have been previously reported [23]. In the present the work we carried out additional geometry optimizations and electronic structure computations for neutral and cation species at the B3LYP/aug-cc-pVTZ level of theory. The basis aug-cc-pVTZ basis set is Dunning’s correlation

Results and discussion

A representation of the structure with the (U)B3LYP/aug-cc-pVTZ optimized geometry of the neutral molecule (1) and the radical cation (1)radical dot+ of parabanic acid is depicted in Figure 1, both with C2v symmetry (energy minima). The main bond distances for the neutral (1) and the ion (1)radical dot+ are also shown. The double arrows in (1)•+ from Figure 1 show the elongation – outward double arrow – or shrinkage – inward double arrow – of the chemical bonds after removal of an electron in (1) and nuclear

Summary and conclusions

Time-of-flight mass spectrometry in coincidence mode (PEPICO) and synchrotron radiation have been employed to the investigation of the parabanic acid molecule. PEPICO mass spectra have been recorded in the 11.2–21 eV valence region of the vapor phase parabanic acid. The relatively high photo-stability of this molecule has been confirmed from our results and is mainly due to strong bonding resulted from its closed molecular structure. We have performed high level DFT calculations for this

Acknowledgements

The authors wish to acknowledge the staff of the Brazilian Synchrotron Light Laboratory (LNLS) for the support during the experiments. This work was financially supported by the Brazilian agencies CNPq and FAPESP, Spanish Project MICINN/CTQ2009-13 652, EU projects NMP4-SL-2008-213 669-ENSEMBLE, FP7-ICT-2009-4-248 855-N4E and FP7-ICT-2009-4-248 909-LIMA. JZD gratefully acknowledges the support of the CSIC-Programa Movilidad (PA1002640).

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