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

Abstract

The molecular structure, vibrational spectra and photochemistry of 5-methylhydantoin (CHNO; 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.

Conformational Landscape and Polymorphism in 5-Acetic Acid Hydantoin

“…Following also the trend already noticed before for the parent hydantoin and both 1-methyl and 5-methyl hydantoins, [2][3][4] the ring internal angles with a nitrogen atom in the apex (C4-N3-C2 and C5-N1-C2) are much larger (around 113º) than those with carbon atoms in the apex (N1-C2-C3, N1-C5-C4 and N3-C4-C5; ~102-106º), pointing to relatively different s vs. p composition of the hybrid orbitals of the N and C atoms used to make the ring bonds.…”

“…The most intense band is ascribable to the anti-symmetric ring carbonyl stretching vibration (n(C=O)as), being observed at 1768 (calculated, 1775 cm -1 ). As reported before for the parent hydantoin and its 1-methyl and 5-methyl substituted derivatives [2][3][4] this band has a satellite band at ~1753 cm -1 (appearing as a shoulder of the main band) which shall be assigned to the first overtone of the n(C11-C14) vibration (whose fundamental appears at 889/886 cm -1 ) enhanced by Fermi resonance interaction with n(C=O)as. The symmetric ring carbonyl stretching vibration (n(C=O)s) is observed at 1810 cm -1 (calculated at 1817 cm -1 ), while the substituent n(C14=O) carbonyl stretching mode gives rise to the band observed at 1752 cm -1 (calculated 1775 cm -1 ) which appears also overlapped with the most intense band due to n(C=O)as.…”

“…These structural characteristics have relevant consequences for the reactivity of the hydantoin moiety, as demonstrated in the case of the previously reported photochemical preferred reaction channels exhibited by this type of molecules. [2][3][4] Hydantoins are also prone to display polymorphism, [3][4][5] though only one crystalline structure has been reported for the parent compound hitherto. 6 For 1-methyl hydantoin, two different polymorphs have been described, 3 while 5-methyl hydantoin exhibits at least four 3 polymorphs, which were found to be stable enough at room temperature to be characterized experimentally.…”

Polymorphism and Conformational Equilibrium of Nitro-Acetophenone in Solid State and under Matrix Conditions

“…For deeper insight into the hydrogen-bonding effect on the polymorphic states and isomerization we applied a wide variety of research methods (DFT, X-ray, DSC, NQR, IINS, IR and Raman) in different environments and over a wide temperature range. It is worth noting that the method of matrix isolation was unique in its ability to trace metastable states and help interpret complex reactions [ 11 , 12 , 13 , 14 ]. Moreover, investigations of objects with hydrogen bonding by IINS [ 15 , 16 , 17 , 18 ] and NQR [ 19 , 20 ] techniques are useful.…”

The crystal structure and Raman spectrum of the sodium salt of 5-Acetic acid hydantoin

“…Within the scope of our research programme on hydantoin derivatives [19][20][21][22][23][24], we have recently shown that 5AAH exhibits five different polymorphs, from which the most stable one at room temperature (polymorph I) has the peculiar characteristic of being formed by 5AAH molecules that assume a conformation identical to the highest-energy conformer for the isolated molecule [24]. In fact, this higher-energy conformer, which has an energy as large as ~40 kJ mol -1 higher than that of the conformational ground state, is selected upon crystallization from among 13 different conformers of the molecule.…”