Abstract
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.
Highlights
We have recently started a research program on the study of derivatives of hydantoin, focusing on their structure and electron distribution characteristics, vibrational spectra, photochemistry and thermal properties.[1,2,3,4] The parent hydantoin and its 1-methyl and5-methyl derivatives were the first targeted compounds, mostly due to their intrinsic structural simplicity, including the fact that they can exist only in a single stable conformation.Those previous studies[1,2,3,4] allowed us to establish the basic properties of the hydantoin moiety
The structural analysis showed that the studied hydantoins have a planar or quasi/planar (5-methyl hydantoin4) ring, sharing common geometrical and electronic features: to mention just some of the most important ones, (i) the relative lengths of the two distinct C=O bonds in the ring are dictated by the degree of p electronic delocalization from the nitrogen atoms, being longer when the C=O fragment is connected to two nitrogen atoms than when connected to a single nitrogen; (ii) the internal angles of the ring with a nitrogen atom in the apex are much larger than those with carbon atoms in the apex (~101-106o), due to different s vs. p compositions of the hybrid orbitals of the
Nand C atoms used to make the ring bonds; (iii) the p charge of the oxygen atom of the carbonyl group connected to two nitrogen atoms is more negative than that of the oxygen atom of the second carbonyl group, in agreement with its longer bond length and involvement in a more extended p delocalization; and (iv) the σ charges of the two oxygen atoms are considerably less negative than the corresponding π charges and have opposite relative values, showing that a larger π bond polarization towards the oxygen atom leads to reduce the trend for the associated σ bond to be polarized in the same direction
Summary
We have recently started a research program on the study of derivatives of hydantoin, focusing on their structure and electron distribution characteristics, vibrational spectra, photochemistry and thermal properties.[1,2,3,4] The parent hydantoin and its 1-methyl and. Nand C atoms used to make the ring bonds; (iii) the p charge of the oxygen atom of the carbonyl group connected to two nitrogen atoms is more negative than that of the oxygen atom of the second carbonyl group (connected to a single nitrogen atom), in agreement with its longer bond length and involvement in a more extended p delocalization; and (iv) the σ charges of the two oxygen atoms are considerably less negative than the corresponding π charges and have opposite relative values, showing that a larger π bond polarization towards the oxygen atom leads to reduce the trend for the associated σ bond to be polarized in the same direction 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]. In the room temperature most stable polymorph, whose structure has been refined in the present study, the conformer selected to make the crystal corresponds to the highest-energy form of the isolated molecule
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