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

H-1 solid-state NMR methods employing fast (nu(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 H-1 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 H-1 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: Delta H = 45 +/- 4 kJ mol(-1) and Delta S = 113 +/- 11 J K-1 mol(-1). In H-1 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 x 10(16) 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.