Conformational Behavior of Dimethyl 5-Methyl-1H,3H-pyrrolo[1,2-c][1,3]thiazole-6,7-dicarboxylate 2,2-Dioxide Isolated in Low-Temperature Matrixes

Abstract

The structure of dimethyl 5-methyl-1H,3H-pyrrolo[1,2-c][1,3]thiazole-6,7-dicarboxylate 2,2-dioxide (PTD) was investigated in low-temperature noble gas matrixes (Ar, Kr, Xe), amorphous solid, and the crystalline state by infrared spectroscopy and computational methods. The geometry of PTD conformers is defined by the orientation of two methyl ester groups, which may adopt pseudo-trans or pseudo-cis positions in relation to the pyrrolo-thiazole system. For both methyl ester groups, the latter arrangement was predicted by the calculations to be energetically the most favorable in the isolated molecule. The envelope form of the thiazolidine ring is present in all conformers, with the sulfur atom placed in the apex position, while the pyrrole ring is almost planar. Three types of conformers differing in the orientation of the methyl ester groups relative to the pyrrolo-thiazole system (cis/cis, trans/cis, cis/trans) were identified in the matrixes. The cis/cis forms were found to be the most stable ones in both gaseous state and argon matrixes. On the other hand, the more polar trans/cis forms were found to be stabilized in the more polarizable krypton and xenon matrixes as well as in the neat amorphous and crystalline phases. On the basis of annealing experiments, performed in argon and xenon matrixes up to 35 and 68 K, respectively, conformational changes preceding the aggregation of the compound are suggested.

Molecular structure and infrared spectra of the monomeric 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide (methyl pseudosaccharyl ether)

“…Indeed, most of the standard computational methods and basis sets have been shown to be unable to properly treat this moiety, leading to considerable discrepancies between the experimental and theoretically predicted values for the S@O bond lengths and for the vibrational frequencies associated with this bond [22][23][24][25]. The underestimation of the theoretically predicted frequencies for the S@O stretching vibrations was noticed previously for different types of compounds containing the S@O bond, from simple molecules, as dimethyl sulfate [26], dimethyl sulfite [27] or the SSO À anion [28], to more complex species, like 5-methyl-1H,3H-pyrrolo [1,2c] [1,3]thiazole-6,7-dicarboxylate 2,2-dioxide [29]. Although a good agreement of the experimental and predicted frequencies was achieved for both dimethyl sulfate and sulfite when the B3LYP method was used together with the split valence quadruple-f basis set (aug-cc-pVQZ) [26,27], calculations on pseudosaccharins at this level of theory are currently unpractical due to its excessive computational cost.…”

Conformational Space and Vibrational Spectra of Methyl 4-Chloro-5-phenyl-1,3-oxazole-2-carboxylate

“…Examples of inversion of the relative order of stability of conformers upon deposition of a compound in a matrix can be found in recent literature, e.g., for dimethylglycine methyl ester, 70 methyl cyanoacetate, 71 and 5-methyl-1H,3H-pyrrolo [1,2-c] [1,3]thiazole-6,7-dicarboxylate 2,2-dioxide. 69 The last compound is in fact a case where both selective aggregation and inversion of the order of stability of conformers were found to take place simultaneously.…”

Using heavy atom rare gas matrix to control the reactivity of 4-methoxybenzaldehyde: A comparison with benzaldehyde

“…A relatively common situation is the alteration of the relative populations of the possible conformers of a given molecule due to stabilization of the more polar forms in more polarizable matrices. [1][2][3][4][5][6][7][8] Matrix-solute interactions may also change the relative heights of the barriers for conformational interconversions, hence modifying the ways of conformational isomerization of an isolated species. 5,9 Interactions with the matrix media have also been found to lead, for example, to different tunneling decay rates of higher energy conformers into more stable forms in different matrices.…”