Matrix Isolation Infrared Spectra and Photochemistry of Hydantoin Gulce Ogruc Ildiz,*,†,‡ Cláudio M. Nunes,† and Rui Fausto† †
Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal Faculty of Science and Letters, Department of Physics, Istanbul Kultur University, Atakoy Campus, Bakirkoy 34156, Istanbul, Turkey
S Supporting Information *
ABSTRACT: Hydantoin (C3H4N2O2, 2,4-imidazolidinedione) was isolated in argon matrix at 10 K and its infrared spectrum and unimolecular photochemistry were investigated. The molecular structure of the compound was studied both at the DFT(B3LYP) and MP2 levels of approximation with valence triple- and quadruple-ζ basis sets (6-311++G(d,p); cc-pVQZ). It was concluded that the minima in the potential energy surfaces of the molecule correspond to C1 symmetry structures. However, the energy barrier separating the two-equivalent-by-symmetry minima stays below their zero-point energy, which makes the Cs symmetry structure, which separates the two minima, the experimentally relevant one. The electronic structure of the molecule was studied in detail by performing the Natural Bond Orbital analysis of its electronic conﬁguration within the DFT(B3LYP)/cc-pVQZ space. The infrared spectrum of the matrix isolated compound was fully assigned also with help of the theoretically predicted spectrum. Upon irradiation at λ = 230 nm, matrix-isolated hydantoin was found to photofragment into isocyanic acid, CO, and methylenimine.
1. INTRODUCTION Hydantoins (or 2,4-imidazolidinediones) are widely used chemicals, especially as pharmaceuticals and agrochemicals, showing important applications in these ﬁelds.1−6 Though the parent compound (C3H4N2O2; Scheme 1) is optically inactive,
Condensed phase vibrational spectroscopy studies of some hydantoins and hydantoin complexes have also been reported,13−17 but practically, no information is available for this type of compounds in the gas phase or, more generally, for the isolated molecule situation. In a recent study,17 the characterization of the hydrogen bond interactions in the crystal of the parent hydantoin was performed using infrared spectroscopy as a experimental technique. The interpretation of the experimental data was supported by quantum chemical calculations (DFT/B3LYP, with the 6-31++G(d,p) basis set) performed on both the monomer and dimers of the compound. Other previously reported theoretical studies on monomeric hydantoins (and, in some cases, also on dimeric structures of the compounds) used lower theoretical levels of approximation or small basis sets.1,2,4,6 To the best of our knowledge, the most complete structural and spectroscopic study reported hitherto for a hydantoin derivative in the gas phase focused on allantoin (5ureyl-2,4-imidazolidinedione).18 In that study, the conformational space and the thermal decomposition of allantoin in the gas phase were investigated in detail by a combination of matrix-isolation infrared spectroscopy and DFT(B3LYP)/6311++G(d,p) calculations. The optimized geometries, energies, gas phase relative populations (at room temperature), and infrared spectra of the possible conformers of allantoin, as well as the barriers for conformational interconversion, were reported,18 and the compound was found to decompose
Scheme 1. Parent Hydantoin Molecule, with Atom Numbering
asymmetrically substituted hydantoins at the C5 position are enantiomeric. Very interestingly, these compounds have been shown to be ideal for chiral discrimination by preferential crystallization (a cheap method suitable for industrial separation of enantiomers), due to their trend to crystallize from a racemic mixture as conglomerates, that is, a physical mixture of enantiomerically pure crystals.7−10 In spite of their practical relevance and interesting physical and chemical properties, studies on the electronic structure, spectroscopic properties, and photochemistry of the hydantoins are scarce. In fact, most of the reported structural studies on hydantoin derivatives deal with the determination of their crystal structures and the investigation of chirallity eﬀects in the solid state.3,7−12 © 2013 American Chemical Society
Received: October 30, 2012 Revised: January 11, 2013 Published: January 11, 2013 726
dx.doi.org/10.1021/jp3107233 | J. Phys. Chem. A 2013, 117, 726−734
The Journal of Physical Chemistry A
the calculated infrared absolute intensities).26 Natural Bond Orbital Analyses (NBO) were performed accordingly to Weinhold and co-workers,27,28 using NBO 3.1, as implemented in Gaussian09. Normal coordinate analysis was performed in the internal coordinate space, as described by Schachtschneider and Mortimer,29 using a locally modiﬁed version of the program BALGA and the structural data and force constants from the B3LYP/6-311++G(d,p) calculations. The internal coordinates used in this analysis were deﬁned as recommended by Pulay et al.30
under thermal conditions into urea, isocyanic acid, NH3, and carbon. The present work aimed to shed light on the details of the electronic structure, infrared spectrum, and unimolecular UVinduced photochemistry of the parent hydantoin monomer. This information was perceived as an essential piece of data for the future investigation of other, more complex hydantoins. The selected experimental technique was matrix isolation infrared spectroscopy, which was complemented by quantum chemical calculations performed at the DFT(B3LYP) and MP2 levels of approximation with valence triple- and quadruple-ζ basis sets (6-311++G(d,p); cc-pVQZ).
3. RESULTS AND DISCUSSION 3.1. Molecular Structure of Monomeric Hydantoin. The calculations carried out for hydantoin molecule under symmetry restrictions to the Cs point group, at both B3LYP and MP2 levels of theory, yield one imaginary frequency, indicating that this structure corresponds to a ﬁrst order saddle point on the PES of the molecule. When the calculations were repeated without any symmetry restrictions, the optimizations converged to C1 symmetry minima (two symmetry equivalent geometries with no imaginary frequencies). In the B3LYP optimized C1 structures, all hydantoin ring atoms, the two oxygens, and the hydrogen atom connected to N3 (the nitrogen atom connecting the two carbonyl moieties; see Scheme 1) deviate only very slightly from planarity (