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

Abstract

Methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate (MCPOC) has been synthesized and isolated in cryogenic matrices (argon and xenon). FTIR spectroscopy studies on the matrix isolated compound, supported by DFT(B3LYP)/6-311++G(d,p) calculations, allow for the identification of two low-energy conformers (I and II) of the molecule, which differ from each other in the orientation of the ester group relative to the oxazole ring. In both these conformers, the ester moiety is in the s-cis configuration (O horizontal lineC-O-CH(3) dihedral: 0 degrees ). Conformer II is ca. 3.0 kJ mol(-1) higher in energy than form I in the gas phase. Two additional higher energy conformers, III and IV, with relative energies of ca. 30 and 45 kJ mol(-1), respectively, were predicted to exist by the calculations, corresponding to structures where the ester group is in an approximately s-trans arrangement. Annealing of the compound isolated in xenon at 60 K led to aggregation and simultaneous reduction of the population of I compared to that of the more polar conformer II. These results suggest the inversion of the order of stability of the two conformers in that matrix, eventually accompanied by a higher trend of conformer I to aggregate. Full assignment of the observed infrared bands to the two experimentally accessible conformers was carried out for the matrix isolated monomeric species. In addition, the infrared spectra of the neat compound in the low-temperature (10 K) amorphous and crystalline phases, as well as the infrared and Raman spectra of the crystal at room temperature were also obtained and assigned.

Conformers of β-aminoisobutyric acid probed by jet-cooled microwave and matrix isolation infrared spectroscopic techniques

“…Hence, subtle spectroscopic differences due to conformational changes can be clearly noticed, and the identity of the various conformers present in the matrix established with certainty. [28][29][30][31][32][33][34] On the other hand, when the energy barriers separating different conformers are only of a few kJ mol −1 , conformational cooling can take place during matrix deposition leading to conversion of higher energy conformers into lower energy forms. In this case, the relative populations of the conformers trapped in the cryogenic matrix may differ substantially from those existing in the gas phase prior to deposition.…”

Photochemistry and Vibrational Spectra of Matrix-Isolated Methyl 4-Chloro-5-phenylisoxazole-3-carboxylate

“…That study confirmed unambiguously the vibrational signature of both heterocycles through direct comparison of the experimental spectra of both compounds with the corresponding calculated spectra. The conformational space of MCPOC was subsequently investigated in detail both experimentally and theoretically, the compound representing an interesting case where an inversion of the order of stability of the two most stable conformers occurs upon going from the gas phase to a xenon matrix.…”

Structure and photochemical behaviour of 3-azido-acrylophenones: a matrix isolation infrared spectroscopy study

“…It is worth mentioning that the spectrum of the photoproduced oxazole (MCPOC) in both argon and xenon matrices is well-known, 40,81 so that the identification of this compound in the irradiated MACBP matrices was doubtless and band assignment followed that presented before. 81 81 On the other hand, for both the azirine and ketenimine species no experimental data for the matrix-isolated compounds were reported hitherto, so that their identification and band assignments were based on the comparison with theoretically predicted spectra obtained at the DFT(B3LYP)/6-311þþG(d,p) level of theory and on data for other molecules of the same families. 47,50,51,82e91 Moreover, these two compounds have conformationally flexible molecules and, according to the structural calculations performed on these species, several low energy conformers exist in both cases.…”