Gas-phase reactivity of lactones: structure and stability of their Cu+complexes

Are benzoylium and nitrilium ions produced from substituted 2-arene-2-oxazolines during mass spectrometry? A study based on density functional theory calculations, quantum theory of atoms in molecules, and electrospray ionization tandem mass spectrometry

RATIONALE

Oxazolines are important compounds for drug development, synthesis, and pharmaceutical applications. Interest in analyzing and developing methods to characterize reaction products from these small heterocyclics has led us to study the gas-phase reactivity and fragmentation of seven 2-arene-2-oxazolines compounds using computational chemistry combined with mass spectrometry.

METHOD

Protonation sites were investigated using computed proton affinity, gas-phase basicity, and some quantum chemistry descriptors of reactivity; the B3LYP/6-31+G(d,p) computational model was used. Fragmentation mechanisms were suggested by employing data from collision-induced dissociation (CID), energy-resolved plots from MS/MS spectra, multiple-stage experiments, and survival-yield method.

RESULTS

Protonation studies based on quantum theory of atoms in molecules (QTAIM) and computational thermochemistry were useful to describe the reactivity of the investigated 2-arene-2-oxazolines, which can be protonated at the nitrogen atom. Three major fragmentation pathways were identified for the protonated molecules: formation of (a) benzoylium or (b) nitrilium ions through elimination of 71 and 72 u from the protonated molecules, respectively, and (c) elimination of 54 u from [M+H]⁺ . These pathways were exploited by the density functional theory calculations combined with QTAIM studies.

CONCLUSIONS

Our results can help in identifying 2-arene-2-oxazoline derivatives using electrospray ionization tandem mass spectrometry (ESI-MS/MS), which can be applied for monitoring reactions through the identified diagnostic ions (product ions). Also, we can suggest that benzoylium and nitrilium ions emerge during fragmentation under CID conditions.

Single-Step Fabrication of a Dual-Sensitive Chitosan Hydrogel by C-Mannich Reaction: Synthesis, Physicochemical Properties, and Screening of its Cu2+ Uptake

Uncovering the value of waste materials is one of the keys to sustainability. In this current work, valorization of chitosan was pursued to fabricate a novel modified chitosan functional hydrogel using a process-efficient protocol. The fabrication proceeds by a one-pot and single-step C-Mannich condensation of chitosan (3% w/v), glutaraldehyde (20 eq.), and 4-hydroxycoumarin (40 eq.) at 22 °C in 3% v/v acetic acid. The Mannich base modified chitosan hydrogel (CS-MB) exhibits a dual-responsive swelling behavior in response to pH and temperature that has not been observed in any other hydrogel systems. Combining the pre-defined optimal swelling pH (pH = 4) and temperature (T = 22 °C), the CS-MB was screened for its Cu2+ adsorption capacity at this condition. The CS-MB achieved an optimal adsorption capacity of 12.0 mg/g with 1.2 g/L adsorbent dosage after 36 h with agitation. The adsorption of Cu2+ on the surface of CS-MB was verified by EDS, and an overview of the adsorption sites was exhibited by FT-IR. The simply fabricated novel CS-MB hydrogel under investigation presents a unique response to external stimuli that exhibits a promise in heavy metal removal from aqueous media.

Directionality of Cation/Molecule Bonding in Lewis Bases Containing the Carbonyl Group

Relationship between the C═O-X⁺ (X = H, Li, Na, K, Al, Cu) angle and covalent characteristic of the X⁺-M (M = CH₂O, CH₃CHO, acetone, imidazol-2-one (C₂H₂N₂O), cytosine, γ-butyrolactone) was investigated, theoretically. The calculated electron densities ρ at the bond critical points revealed that the covalency of the M-X⁺ interaction depended on the nature of the cation and varied as H⁺ > Cu⁺ > Al⁺ > Li⁺ > Na⁺ > K⁺. The alkali cations tended to participate in electrostatic interactions and aligned with the direction of the molecule dipole or local dipole of C═O group to form linear C═O-X geometries. Because of overlapping with lone-pair electrons of the sp² carbonyl oxygen, the H⁺ and Cu⁺ formed a bent C═O-X angle. Al⁺ displayed an intermediate behavior; the C═O-Al angle was 180° in [CH₂O/Al]⁺ (mainly electrostatic), but when the angle was bent (146°) under the effect of local dipole of an adjacent imine group in cytosine, the covalency of the CO-Al⁺ interaction increased. The C═O-X angles in M/X⁺ adduct ions were scanned in different O-X bond lengths. It was found that the most favorable C═O-X angle depended on the O-X bond length. This dependency was attributed to variation of covalent and electrostatic contributions with O-X distance. In addition, the structures of [CH₂S/X]⁺ and [CH₂Se/X]⁺ were studied, and only bent C═S-X and C═Se-X angles were obtained for all cations, although the dipole vectors of CH₂S and CH₂Se coincide with the C═S and C═Se bonds. The bending of the C═S-X and C═Se-X angles was attributed to the covalent characteristic of S-X and Se-X interactions due to high polarizability of S and Se atoms.

Evaluation of the enthalpy of formation, proton affinity, and gas-phase basicity of gamma-butyrolactone and 2-pyrrolidinone by isodesmic reactions

The knowledge of thermochemical parameters such as the enthalpy of formation, gas-phase basicity, and proton affinity may be the key to understanding molecular reactivity. The obtention of these thermochemical parameters by theoretical chemical models may be advantageous when experimental measurements are difficult to accomplish. The development of ab initio composite models represents a major advance in the obtention of these thermochemical parameters, but these methods do not always lead to accurate values. Aiming at achieving a comparison between the ab initio models and the hybrid models based on the density functional theory (DFT), we have studied gamma-butyrolactone and 2-pyrrolidinone with a goal of obtaining high-quality thermochemical parameters using the composite chemical models G2, G2MP2, MP2, G3, CBS-Q, CBS-4, and CBS-QB3; the DFT methods B3LYP, B3P86, PW91PW91, mPW1PW, and B98; and the basis sets 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d), 6-311+G(d), 6-311G(d,p), 6-311+G(d,p), 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ. Values obtained for the enthalpies of formation, proton affinity, and gas-phase basicity of the two target molecules were compared to the experimental data reported in the literature. The best results were achieved with the use of DFT models, and the B3LYP method led to the most accurate data.

Density Functional Theory Study of the Hydrogen Bond Interaction between Lactones, Lactams, and Methanol

The structure and relative stability of methanol complexes with various cyclic ketones, lactones, lactams, and N-methyl lactams from three- to seven-membered rings have been investigated using the density functional theory method. The geometries, harmonic frequencies, and energies were calculated at the B3LYP/6-311+G(d,p) level. Three stable structures, cis-a, cis-b, and trans, with respect to the ring oxygen (nitrogen) atom, were found to be local minima of the potential energy surface. For lactones and N-methyl lactams, the most stable structure is trans; it is stabilized, as in cyclic ketones, through the conventional hydrogen bond (HB) interaction between the basic carbonyl oxygen and the acidic methanolic hydrogen and an unconventional HB interaction between the methanolic oxygen and the CH hydrogen, in the alpha position of the carbonyl group. For unsubstituted lactams, the cis-a structure, stabilized through a HB interaction between the NH group and the methanol oxygen in addition to the conventional HB interaction, is the most stable. The topological properties of the electron density ratify the existence of conventional (N,O-H. . .O) and unconventional (C-H. . .O) hydrogen bonding. A good correlation was found between the HB distances and the electron density at the HB critical point. The unsubstituted lactams yield more stable complexes with methanol than N-methyl lactams, lactones, and cyclic ketones. In the most stable complexes, both components behave simultaneously as a HB donor and as a HB acceptor.