Matrix Isolation FTIR Spectroscopic and Theoretical Study of Dimethyl Sulfite

Sulfidated microscale zero-valent iron/reduced graphene oxide composite (S-mZVI/rGO) for enhanced degradation of trichloroethylene: The role of hydrogen spillover

Atomic hydrogen (H*) has long been thought to play an important role in the dechlorination of trichloroethylene (TCE) by carbon-supported zero-valent iron (ZVI), which offers an alternative pathway for TCE dechlorination. Herein, we demonstrate that the reductive dechlorination of TCE by sulfidated microscale ZVI (S-mZVI) can be further enhanced by promoting the formation of H* through the introduction of reduced graphene oxide (rGO). The completely degradation of 10 mg/L TCE can be achieved by S-mZVI/rGO within 24 h, which was 3.3 times faster than that of S-mZVI. The change in the distribution of TCE degradation products over time suggests that the introduction of rGO leads to a change in the dechlorination pathway. The percentage of ethane in the final products of TCE degradation by S-mZVI/rGO was 34.3 %, while that of S-mZVI was only 21.9 %. The electrochemical tests confirmed the occurrence of hydrogen spillover in the S-mZVI/rGO composite, which promoted the reductive dechlorination of TCE by H*. Although the S-mZVI/rGO composite had stronger hydrogen evolution propensity than S-mZVI, the S-mZVI/rGO composite still exhibited higher electron utilization efficiency than S-mZVI thanks to the increased utilization of hydrogen.

Towards complete elucidation of structural factors controlling thermal stability of IL/MOF composites: effects of ligand functionalization on MOFs

In this work, we incorporated an ionic liquid (IL), 1-n-butyl-3-methylimidazolium methyl sulfate ([BMIM][MeSO4]) into two different metal organic frameworks (MOFs), UiO-66, and its amino-functionalized counterpart, NH2-UiO-66, to investigate the effects of ligand-functionalization on the thermal stability limits of IL/MOF composites. The as-synthesized IL/MOF composites were characterized in detail by combining x-ray diffraction, scanning electron microscopy, Brunauer-Emmett-Teller analysis, x-ray fluorescence, infrared spectroscopies (FTIR), and their thermal stability limits were determined by thermogravimetric analysis (TGA). Characterization data confirmed the successful incorporation of the IL into each MOF and indicated the presence of direct interactions between them. A comparison of the interactions in [BMIM][MeSO4]-incorporated UiO-66 and NH2-UiO-66 with those in their 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])-incorporated counterparts showed that the hydrophilic IL, [BMIM][MeSO4], interacts with the 1,4-benzenedicarboxylate (BDC) ligand of the UiO-66, while the hydrophobic IL, [BMIM][PF6], is interacting with the joints where zirconium metal cluster coordinates with BDC ligand. The TGA data demonstrated that the composite with the ligand-functionalized MOF, NH2-UiO-66, exhibited a lower percentage decrease in the maximum tolerable temperature compared to those of IL/UiO-66 composites. Moreover, it is discovered that when the IL is hydrophilic, its hydrogen bonding ability can be utilized to designate an interaction site on MOF's ligand structure, leads to a lower reduction in thermal stability limits. These results provide insights for the rational design of IL/MOF composites and contribute towards the complete elucidation of structural factors controlling the thermal stability.

Laser desorption single-conformation UV and IR spectroscopy of the sulfonamide drug sulfanilamide, the sulfanilamide-water complex, and the sulfanilamide dimer

We have studied the conformational preferences of the sulfonamide drug sulfanilamide, its dimer, and its monohydrated complex through laser desorption single-conformation UV and IR spectroscopy in a molecular beam. Based on potential energy curves for the inversion of the anilinic and the sulfonamide NH₂ groups calculated at DFT level, we suggest that the zero-point level wave function of the sulfanilamide monomer is appreciably delocalized over all four conformer wells. The sulfanilamide dimer, and the monohydrated complex each exhibit a single isomer in the molecular beam. The isomeric structures of the sulfanilamide dimer and the monohydrated sulfanilamide complex were assigned based on their conformer-specific IR spectra in the NH and OH stretch region. Quantum Theory of Atoms in Molecules (QTAIM) analysis of the calculated electron density in the water complex suggests that the water molecule is bound side-on in a hydrogen bonding pocket, donating one O-HO[double bond, length as m-dash]S hydrogen bond and accepting two hydrogen bonds, a NHO and a CHO hydrogen bond. QTAIM analysis of the dimer electron density suggests that the C_i symmetry dimer structure exhibits two dominating N-HO[double bond, length as m-dash]S hydrogen bonds, and three weaker types of interactions: two CHO bonds, two CHN bonds, and a chalcogen OO interaction. Most interestingly, the molecular beam dimer structure closely resembles the R dimer unit - the dimer unit with the greatest interaction energy - of the α, γ, and δ crystal polymorphs. Interacting Quantum Atoms analysis provides evidence that the total intermolecular interaction in the dimer is dominated by the short-range exchange-correlation contribution.

Conformers, infrared spectrum, UV-induced photochemistry, and near-IR-induced generation of two rare conformers of matrix-isolated phenylglycine

The conformational space of α-phenylglycine (PG) have been investigated theoretically at both the DFT/B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of approximation. Seventeen different minima were found on the investigated potential energy surfaces, which are characterized by different dominant intramolecular interactions: type I conformers are stabilized by hydrogen bonds of the type N-H···O=C, type II by a strong O-H···N hydrogen bond, type III by weak N-H···O-H hydrogen bonds, and type IV by a C=O···H-C contact. The calculations indicate also that entropic effects are relevant in determining the equilibrium populations of the conformers of PG in the gas phase, in particular in the case of conformers of type II, where the strong intramolecular O-H···N hydrogen bond considerably diminishes entropy by reducing the conformational mobility of the molecule. In consonance with the relative energies of the conformers and barriers for conformational interconversion, only 3 conformers of PG were observed for the compound isolated in cryogenic Ar, Xe, and N2 matrices: the conformational ground state (ICa), and forms ICc and IITa. All other significantly populated conformers existing in the gas phase prior to deposition convert either to conformer ICa or to conformer ICc during matrix deposition. The experimental observation of ICc had never been achieved hitherto. Narrowband near-IR irradiation of the first overtone of νOH vibrational mode of ICa and ICc in nitrogen matrices (at 6910 and 6930 cm(-1), respectively) led to selective generation of two additional conformers of high-energy, ITc and ITa, respectively, which were also observed experimentally for the first time. In addition, these experiments also provided the key information for the detailed vibrational characterization of the 3 conformers initially present in the matrices. On the other hand, UV irradiation (λ = 255 nm) of PG isolated in a xenon matrix revealed that PG undergoes facile photofragmentation through two photochemical pathways that are favored for different initial conformations of the reactant: (a) decarboxylation, leading to CO2 plus benzylamine (the dominant photofragmentation channel in PG cis-COOH conformers ICa and ICc) and (b) decarbonylation, with generation of CO plus benzonitrile, H2O and H2 (prevalent in the case of the trans-COOH conformer, IITa).

Photochemistry and vibrational spectra of matrix-isolated 5-ethoxy-1-phenyl-1H-tetrazole

A combined matrix isolation FT-IR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the molecular structure and photochemistry of 5-ethoxy-1-phenyl-1H-tetrazole (5EPT) was performed. A new method of synthesis of the compound is described. Calculations show three minima, very close in energy and separated by low-energy barriers (less than 4 kJ mol-1), in the ground-state potential energy profile of the molecule. The method of matrix isolation enabled the reduction of the number of populated conformational states in the experiment at low temperature due to the effect known as conformational cooling. As a result, the spectrum of the as-deposited matrix of 5EPT closely matches that of the most stable conformer predicted theoretically, pointing to the existence of only this conformer in the low-temperature matrixes. In this structure, the dihedral angle between the two rings, phenyl and tetrazole, is ca. 30 degrees, whereas the ethyl group stays nearly in the plane of the tetrazole ring and is as far as possible from the phenyl group. In situ UV irradiation (lambda > 235 nm) of the matrix-isolated 5EPT induced unimolecular decomposition, which led mainly to production of ethylcyanate and phenylazide, this later compound further reacting to yield, as final product, 1-aza-1,2,4,6-cycloheptatetraene. Anti-aromatic 3-ethoxy-1-phenyl-1H-diazirene was also observed experimentally as minor photoproduct, resulting from direct extrusion of molecular nitrogen from 5EPT. This species has not been described before and is now characterized by infrared spectroscopy for the first time.

Unusual Photochemical C−N Bond Cleavage in the Novel Methyl 2-Chloro-3-methyl-2H-azirine-2-carboxylate

The structure, preferred conformers, vibrational spectrum, and photochemical behavior of the novel azirine, methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) were investigated in low-temperature matrixes and in the neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in argon, krypton, and xenon matrixes, in agreement with the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) theoretical calculations. Both conformers were found to exhibit the carboxylic ester group in the cis conformation, differing in the arrangement defined by the O=C-C-Cl dihedral angle (cis and trans, for Ct and Cc forms, respectively). The Ct conformer was found to be the most stable conformer in the gaseous phase as well as in both argon and krypton matrixes, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the neat solid amorphous phase. In situ broadband UV (lambda > 235 nm) excitation of matrix-isolated MCMAC led to azirine ring C-C and C-N bond cleavages, the latter process corresponding to the most efficient reaction channel. The photochemical cleavage of the C-N bond had never been previously observed in the case of aliphatic 2H-azirines. Two electron withdrawing substituents (methoxycarbonyl group and chlorine atom) are connected to the azirine ring in the novel MCMAC azirine. The simultaneous presence of these two groups accelerates intersystem crossing toward the triplet state where cleavage of the C-N bond takes place. The primary photoproducts resulting from the C-N and C-C ring-opening reactions were also found to undergo further photochemical decarbonylation or decarboxylation reactions.

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

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.