Benzoxazine Oligomers:  Evidence for a Helical Structure from Solid-State NMR Spectroscopy and DFT-Based Dynamics and Chemical Shift Calculations

Abstract

A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and helical conformations of trimeric and tetrameric units. In fast MAS (1)H NMR spectra, the resonances of the protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by means of DFT-based geometry optimizations and (1)H chemical-shift calculations. Also supporting these proposed structures are homonuclear (1)H[bond](1)H double-quantum NMR spectra, which identify the local proton-proton proximities in each material. Additionally, quantitative (15)N[bond](1)H distance measurements obtained by analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the N...H...O and O...H...O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more confidence than the individual measurements or calculations alone, in particular, because the microscopic structure of many disordered systems cannot be elucidated by means of conventional methods due to lack of long-range order.

Structurally Designed Synthesis of Mechanically Stable Poly(benzoxazine-co-resol)-Based Porous Carbon Monoliths and Their Application as High-Performance CO2 Capture Sorbents

“…62,63 Elemental analyses show that, at the initial reaction stage, the prepolymer contains 3.38 wt % of nitrogen, a similar content of a typical polybenzoxazine. 47 The polybenzoxazine segments subsequently form hydrogen bonds (ArÀOÀH 3 3 3 O) with the EO segments of F127 to a significant extent, which can guide the mesostructure assembly between the polymer species and the amphiphilic copolymer template. During the following curing step, the unreacted resorcinol and formaldehyde would copolymerize, or with polybenzoxazine under basic conditions via a condensation reaction.…”

Powder crystallography of pharmaceutical materials by combined crystal structure prediction and solid-state 1H NMR spectroscopy

“…For small molecules, however, an approach based on the analysis of chemical shifts would be most attractive. There are today many examples of chemical shifts being combined with density functional theory (DFT) calculations for structure validation in organic molecular compounds with respect to known structures [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] . However, there are very few examples of ab initio structure determination from powders by NMR without a structural hypothesis.…”

Determining the Geometry of Hydrogen Bonds in Solids with Picometer Accuracy by Quantum‐Chemical Calculations and NMR Spectroscopy

“…The obtained structures were finally confirmed by the complete agreement of computed and experimental This approach is particularly suited to the elucidation of NÀ H···N hydrogen-bonded structures, because it is perfectly applicable to fast MAS conditions and to heteronuclear spin systems including protons. [16] Herein, existing rotor-encoding techniques have been further improved to yield multiple NÀH distances with an accuracy of a few picometers and better.…”