The competing effects of π–π packing and hydrogen bonding in a hexabenzocoronene carboxylic acid derivative: A 1H solid-state MAS NMR investigation

Abstract

¹ H solid-state NMR methods employing fast (ν_R=30 kHz) magic-angle spinning (MAS) are applied to the investigation of both the solid and columnar liquid-crystalline (LC) phases of HBC-C₁₀COOH, a discotic molecule consisting of a hexa-peri-hexabenzocoronene core with six symmetrically attached alkyl chains, each chain being capped by a COOH group. In the solid phase at T=320 K, the presence of hydrogen-bonded COOH dimers is demonstrated by the observation of an auto COOH peak in the ¹H double-quantum (DQ) MAS NMR two-dimensional spectrum, with the interproton distance being determined to be 0.279±0.09 nm. Considering the COOH peak in the one-dimensional ¹H MAS spectra, both a shift to high field of the peak position as well as an initial increase followed by a subsequent decrease in the linewidth are observed upon heating. These observations are interpreted in terms of a chemical exchange process involving the making and breaking of hydrogen bonds, with the coalescence point corresponding to T=362 K. The equilibrium constant at a given temperature is calculated from the observed chemical shift, and a thermodynamic analysis yields for the opening of the hydrogen-bonded dimers: ΔH=45±4 kJ mol⁻¹ and ΔS=113±11 J K⁻¹ mol⁻¹. In ¹H DQ-filtered (DQF) MAS spectra, the intensity of the COOH peak is observed, upon heating, to reduce faster than expected from thermodynamic factors alone, and above T=380 K, no signal is detected. Using the fact that the observation of a DQ signal relies on the existence of a DQ coherence for the duration of the experiment, the kinetics of dimer opening are determined: the extracted activation energy and Arrhenius parameter equal 89±10 kJ mol⁻¹ and 4.2×10¹⁶ s⁻¹, respectively. On the transition to the LC phase, a marked narrowing of all resonances is observed, with the COOH chemical shift initially having a very high-field value, 9.0 ppm, which is indicative of free COOH groups. An analysis of DQ MAS spinning-sideband patterns shows that the dipolar coupling in the LC phase is reduced by a factor of 0.43±0.04.