Conformational Landscape and Polymorphism in 5-Acetic Acid Hydantoin.

The conformational space of 5-acetic acid hydantoin {5AAH; [2-(2,5-dioxoimidazolidin-4-yl)acetic acid]} was investigated by quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level of theory. A total of 13 conformers were located in the potential energy surface of the molecule, six of them bearing the carboxylic group in the cis arrangement (O═C-O-H dihedral equal to ∼0°) and the other seven possessing this group in the trans configuration (O═C-O-H dihedral equal to ∼180°). The most stable conformer (cis-I) was trapped from the gas phase into a low temperature argon matrix (10 K), and its infrared spectrum was fully assigned, also with help of results of normal coordinates' analysis based on the DFT computed vibrational data. The electronic structure of this conformer was analyzed by using the natural bond orbital (NBO) method. The investigation of the thermal properties of 5AAH was undertaken by differential scanning calorimetry (DSC), polarized light thermal microscopy (PLTM) and Raman spectroscopy, allowing identification of five different polymorphs. Very interestingly, in the room temperature stable polymorph the molecular units of 5AAH assume the geometry of the highest-energy conformer predicted by the calculations for the isolated molecule.

5-Methylhydantoin: From Isolated Molecules in a Low-Temperature Argon Matrix to Solid State Polymorphs Characterization.

The molecular structure, vibrational spectra and photochemistry of 5-methylhydantoin (C4H6N2O2; 5-MH) were studied by matrix isolation infrared spectroscopy and theoretical calculations at the DFT(B3LYP)/6-311++G(d,p) theory level. The natural bond orbital (NBO) analysis approach was used to study in detail the electronic structure of the minimum energy structure of 5-MH, namely the specific characteristics of the σ and π electronic systems of the molecule and the stabilizing orbital interactions. UV irradiation of 5-MH isolated in argon matrix resulted in its photofragmentation through a single photochemical pathway, yielding isocyanic acid, ethanimine, and carbon monoxide, thus following a pattern already observed before for the parent hydantoin and 1-methylhydantoin molecules. The investigation of the thermal properties of 5-MH was undertaken by differential scanning calorimetry (DSC), polarized light thermal microscopy (PLTM) and Raman spectroscopy. Four different polymorphs of 5-MH were identified. The crystal structure of one of the polymorphs, for which it was possible to grow up suitable crystals, was determined by X-ray diffraction (XRD). Two of the additional polymorphs were characterized by powder XRD, which confirmed the molecules pack in different crystallographic arrangements.

Synthesis, structural and spectroscopic studies of 2-oxoacenaphthylen-1(2H)-ylidene nicotinohydrazide.

The synthesis of a new hydrazone, 2-oxoacenaphthylen-1(2H)-ylidene nicotinohydrazide, and its structural and spectroscopic characterization is reported. The obtained powder was recrystallized from DMSO and ethanol that afforded small crystals used for single-crystal X-ray diffraction studies. The compound was found to crystallize in two polymorphs, depending on the crystallization conditions. One of the polymorphs (form I) crystallizes in the centrosymmetric P21/c monoclinic space group, the other (form II) crystallizes in the non-centrosymmetric, but achiral, orthorhombic space group P212121. Conformation of the molecules is similar in both polymorphs, but the network of weak intermolecular interactions determining the crystal packing is different. In form II an additional C-H⋯O bond connects molecules related by the screw-axis running parallel to the a-axis. Crystals of both polymorphs were also screened by FT-IR and Raman microscopy; a detailed analysis of the spectra and comparison with those of the isolated molecule calculated by ab-initio HF/MP2 and DFT/B3LYP methods using a correlation consistent cc-pVDZ basis set is presented. In addition, UV-vis and NMR studies were performed in solution.

Structure and photochemistry of a saccharyl thiotetrazole.

The molecular structure and photochemistry of 5-thiosaccharyl-1-methyltetrazole (TSMT) were studied by means of matrix-isolation FTIR spectroscopy, X-ray crystallography, and theoretical calculations. The calculations predicted two conformers of TSMT that differ in energy by more than 15 kJ mol(-1). The infrared spectrum of TSMT isolated in solid argon was fully assigned on the basis of the spectrum calculated (O3LYP/6-311++G(3df,3pd)) for the most stable conformer. In the crystal, TSMT molecules were found to assume the same conformation as for the isolated molecule, with each molecule forming four hydrogen bonds with three neighboring molecules, leading to a network of TSMT oligomers. Upon UV (λ = 265 nm) irradiation of the matrix-isolated TSMT, two photodegradation pathways were observed, both arising from cleavage of the tetrazolyl ring. Pathway a involves cleavage of the N1-N2 and N3-N4 bonds with extrusion of N2, leading to photostable diazirine and thiocarbodiimide derivatives. The photostability of the photoproduced diazirine under the conditions used precluded its rearrangement to the nitrile imine, as reported for 5-phenyltetrazole by Bégué et al. ( J. Am. Chem. Soc. 2012 , 134 , 5339 ). Pathway b involves cleavage of the C5-N1 and N4-N3 bonds, leading to a thiocyanate and methyl azide, the latter undergoing subsequent fragmentation to give CNH.