Susy Lopes

Formic and acetic acids in a nitrogen matrix: Enhanced stability of the higher-energy conformer

Formic acid (HCOOH, FA) and acetic acid (CH(3)COOH, AA) are studied in a nitrogen matrix. The infrared (IR) spectra of cis and trans conformers of these carboxylic acids (and also of the HCOOD isotopologue of FA) are reported and analyzed. The higher-energy cis conformer of these molecules is produced by narrowband near-IR excitation of the more stable trans conformer, and the cis-to-trans tunneling decay is evaluated spectroscopically. The tunneling process in both molecules is found to be substantially slower in a nitrogen matrix than in rare-gas matrices, the cis-form decay constants being approximately 55 and 600 times smaller in a nitrogen matrix than in an argon matrix, for FA and AA respectively. The stabilization of the higher-energy cis conformer is discussed in terms of specific interactions with nitrogen molecule binding with the OH group of the carboxylic acid. This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative frequencies of the νOH and τCOH modes and the relative intensities of the νOH and νC=O bands.

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

Methyl 4-chloro-5-phenylisoxazole-3-carboxylate (MCPIC) has been synthesized, isolated in low temperature argon and xenon matrices, and studied by FTIR spectroscopy. The characterization of the low energy conformers of MCPIC was made by undertaking a systematic investigation of the DFT(B3LYP)/6-311++G(d,p) potential energy surface of the molecule. The theoretical calculations predicted the existence of three low energy conformers. Two of them (I and II) were observed experimentally in the cryogenic matrices. The third one (III) was found to be converted into conformer II during deposition of the matrices, a result that is in agreement with the predicted low III → II energy barrier (<0.3 kJ mol(-1)). In situ UV irradiation (λ > 235 nm) of matrix-isolated MCPIC yielded as final photoproduct the corresponding oxazole (methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate). Identification of the azirine and nitrile-ylide intermediates in the spectra of the irradiated matrices confirmed their mechanistic relevance in the isoxazole → oxazole photoisomerization.

Molecular structure and infrared spectra of dimethyl malonate: A combined quantum chemical and matrix-isolation spectroscopic study

The infrared spectra of dimethyl malonate isolated in low-temperature argon and xenon matrices were studied. Theoretical calculations, carried out at the MP4/6-31G**, MP2/6-31++G** and DFT(B3LYP)/6-311++G** levels, predict two different conformers of nearly equal internal energy, both exhibiting the methyl ester moieties in the cis (C–O) configuration. One of the conformers has C2 symmetry with the two ester groups crossed symmetrically with respect to the C–C–C plane. This structure is doubly degenerated by symmetry. The other form (gauche) belongs to the C1 point group. Four identical-by-symmetry minima on the PES correspond to this structure. The energy of this form is predicted (at the MP4 level) to be slightly lower than that of the C2 conformer. The six minima on the PES can be divided into two groups of three (one C2 isomer and two C1 forms in a group). Each structure from one group is related to its counterpart from the other group by the operation of reflection in the C–C–C plane. In each group, the conformers are separated by low energy barriers (less than 2 kJ mol−1), while conformational interconversions between the two groups imply a transition state structure with a vis-a-vis orientation of oxygen atoms and thus are associated with considerably higher energy barriers. The infrared spectra of the matrix isolated compound were found to closely match the spectrum predicted for the C1 conformer. Annealing of the matrices up to 55 K does not lead to significant changes in the spectra, suggesting that the low energy barriers separating the two conformers allow practically all molecules of dimethyl malonate to transform to the more stable gauche conformer, when they are cooled down after landing on the matrix surface. Spectra of the low temperature solid form of the compound (8 K < T < 200 K) also reveal only the presence of the C1 conformer.

Acetic acid dimers in a nitrogen matrix: Observation of structures containing the higher-energy conformer.

Acetic acid (AA) dimers are studied experimentally by infrared spectroscopy in a N2 matrix and theoretically at the MP2/6-311++G(2d,2p) level of approximation. This work is focused on the first preparation and characterization of structures containing the higher-energy (cis) conformer of AA. Nine trans-trans, fourteen trans-cis, and six cis-cis dimers are theoretically predicted. Five trans-trans and a number of trans-cis dimers are identified in the experiments, but no indication of cis-cis dimers is found. Two trans-trans dimers and the trans-cis dimers are reported for the first time. One trans-cis dimer is prepared by selective vibrational excitation of the structurally related trans-trans dimer, which converts one of the trans subunits to the cis form. Several trans-cis dimers are obtained by annealing of a matrix containing both trans and cis monomers of AA. Tunneling-induced conversion of the trans-cis dimers into trans-trans forms (including two new trans-trans forms) is observed at low temperatures.

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

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.

Matrix isolation and low temperature solid state FTIR spectroscopic study of α-furil

α-Furil [C4H3O–C(O)–C(O)–C4H3O] has been isolated in argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. The obtained spectra were fully assigned and revealed the presence in the matrices of three different conformers, all of them exhibiting skewed conformations around the intercarbonyl bond with the two C4H3O–C(O) fragments nearly planar. The three conformers differ in the orientation of the furan rings relative to the carbonyl groups: the most stable conformer, I (C2 symmetry; OC–CO intercarbonyl dihedral equal to 153.1°), has both furan rings orientated in such a way that one of their β-hydrogen atoms approaches the oxygen atom of the most distant carbonyl group, forming two H–CC–C–CO six-membered rings; the second most stable conformer, II (C1 symmetry; OC–CO intercarbonyl dihedral equal to 126.9°), has one furan ring orientated as in I, while the second furan group is rotated by ca. 180° (resulting in an energetically less favourable H–CC–CO five-membered ring); in the third conformer, III (C2 symmetry; OC–CO dihedral equal to 106.2°), both furan rings assume the latter orientation relative to the dicarbonyl group. The theoretical calculations predicted the two higher energy forms being 5.85 and 6.22 kJ mol−1 higher in energy than the most stable form, respectively, and energy barriers for conformational interconversion higher than 40 kJ mol−1. These barriers are high enough to prevent observation of conformational isomerization for the matrix isolated compound. The three possible conformers of α-furil were also found to be present in CCl4 solution, as well as in a low temperature neat amorphous phase of the compound prepared from fast condensation of its vapour onto a suitable 10 K cooled substrate. On the other hand, in agreement with the available X-ray data [S. C. Biswas, S. Ray and A. Podder, Chem. Phys. Lett., 1987, 134, 541], the IR spectra obtained for the neat low temperature crystalline state reveals that, in this phase, α-furil exists uniquely in its most stable conformational state, I.

Molecular structure and infrared spectra of dimethyl fumarate

Infrared spectra of dimethyl fumarate isolated in low-temperature argon and xenon matrixes and of the compound in the solid amorphous and crystalline states have been studied. Theoretical calculations, carried out at the MP2/6-31G** and DFT(B3LYP)/6-31G** levels, predict three planar conformers of low internal energy, all of them exhibiting the methyl ester moieties in the cis (C–O) configuration: the conformational ground state (conformer I), with the two OC–CC dihedrals equal to 0°, and forms II and III, where one or both OC–CC dihedrals are 180°. In the spectra of the matrix isolated compound, characteristic bands of all three conformers were identified. During annealing of the xenon matrix up to 60 K, conversion of the less stable conformers, II and III, into the most stable conformer, I, was observed. In the amorphous solid, these three conformers could also be identified spectroscopically. The IR and Raman spectra of the crystalline phase clearly show that in the crystal only form I is present, since no bands ascribable to other conformers could be observed.

Acetic acid–water complex: The first observation of structures containing the higher-energy acetic acid conformer

Non-covalent interaction of acetic acid (AA) and water is studied experimentally by IR spectroscopy in a nitrogen matrix and theoretically at the MP2 and coupled-cluster with single and double and perturbative triple excitations [CCSD(T)]/6-311++G(2d,2p) levels of theory. This work is focused on the first preparation and characterization of complexes of higher-energy (cis) conformer of AA with water. The calculations show three 1:1 structures for the trans-AA⋯H2O complexes and three 1:1 structures for the cis-AA⋯H2O complexes. Two trans-AA⋯H2O and two cis-AA⋯H2O complexes are found and structurally assigned in the experiments. The two cis-AA⋯ ⋅ H2O complexes are obtained by annealing of a matrix containing water and cis-AA molecules prepared by selective vibrational excitation of the ground-state trans form. The less stable trans-AA⋯H2O complex is obtained by vibrational excitation of the less stable cis-AA⋯H2O complex. In addition, the 1:2 complexes of trans-AA and cis-AA with water molecules are studied computationally and the most stable forms of the 1:2 complexes are experimentally identified.

Formic acid dimers in a nitrogen matrix

Formic acid (HCOOH) dimers are studied by infrared spectroscopy in a nitrogen matrix and by ab initio calculations. We benefit from the use of a nitrogen matrix where the lifetime of the higher-energy (cis) conformer is very long (∼11 h vs. 7 min in an argon matrix). As a result, in a nitrogen matrix, a large proportion of the cis conformer can be produced by vibrational excitation of the lower-energy (trans) conformer. Three trans-trans, four trans-cis, and three cis-cis dimers are found in the experiments. The spectroscopic information on most of these dimers is enriched compared to the previous studies in an argon matrix. The cis-cis dimers of ordinary formic acid (without deuteration) are reported here for the first time. Several conformational processes are obtained using selective excitation by infrared light, some of them also for the first time. In particular, we report on the formation of cis-cis dimers upon vibrational excitation of trans-cis dimers. Tunneling decays of several dimers have been detected in the dark. The tunneling decay of cis-cis dimers of formic acid as well as the stabilization of cis units in cis-cis dimers is also observed for the first time.