Measuring multiple 17O–13C J-couplings in naphthalaldehydic acid: a combined solid state NMR and density functional theory approach

Abstract

A combined multinuclear solid state NMR and gauge included projected augmented wave, density functional theory (GIPAW DFT) computational approach is evaluated to determine the four heteronuclear ¹J(¹³C,¹⁷O) couplings in solid ¹⁷O enriched naphthalaldehydic acid. Direct multi-field ¹⁷O magic angle spinning (MAS), triple quantum MAS (3QMAS) and double rotation (DOR) experiments are initially utilised to evaluate the accuracy of the DFT approximations used in the calculation of the isotropic chemical shifts (δ_iso), quadrupole coupling constants (C_Q) and asymmetry (η_Q) parameters. These combined approaches give δ_iso values of 313, 200 and 66 ppm for the carbonyl (CO), ether (–O–) and hydroxyl (–OH) environments, respectively, with the corresponding measured quadrupole products (P_Q) being 8.2, 9.0 and 10.6 MHz. The geometry optimised DFT structure derived using the CASTEP code gives firm agreement with the shifts observed for the ether (δ_iso = 223, P_Q = 9.4 MHz) and hydroxyl (δ_iso = 62, P_Q = 10.5 MHz) environments but the unoptimised experimental XRD structure has better agreement for the carbonyl group (δ_iso = 320, P_Q = 8.3 MHz). The determined δ_iso and η_Q values are shown to be consistent with bond lengths closer to 1.222 Å (experimental length) rather than the geometry optimised length of 1.238 Å. The geometry optimised DFT ¹J(¹³C,¹⁷O) coupling to the hydroxyl is calculated as 20 Hz and the couplings to the ether were calculated to be 37 (O–CO) and 32 (O–C–OH) Hz. The scalar coupling parameters for the unoptimised experimental carbonyl group predict a ¹J(¹³C,¹⁷O) value of 28 Hz, whilst optimisation gives a value of 27 Hz. These calculated ¹J(¹³C,¹⁷O) couplings, together with estimations of the probability of each O environment being isotopically labelled (determined by electrospray ionisation mass spectrometry) and the measured refocussable transverse dephasing (T₂′) behaviour, are combined to simulate the experimental decay behaviour. Good agreement between the measured and calculated decay behaviour is observed.