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

Abstract

The structure of multiply hydrogen‐bonded systems is determined with picometer accuracy by a combined solid‐state NMR and quantum‐chemical approach. On the experimental side, advanced 1H15N dipolar recoupling NMR techniques are capable of providing proton–nitrogen distances of up to about 250 pm with an accuracy level of ±1 pm for short distances (i.e., around 100 pm) and ±5 pm for longer ones (i.e., 180 to 250 pm). The experiments were performed under fast magic‐angle spinning, which ensures sufficient dipolar decoupling and spectral resolution of the 1H resonance lines. On the quantum‐chemical side, the structures of the hydrogen‐bonded systems were computationally optimised, yielding complete sets of nitrogen–proton and proton–proton distances, which are essential for correctly interpreting the experimental NMR data. In this way, nitrogen–proton distances were determined with picometer accuracy, so that vibrational averaging effects on dipole–dipole couplings need to be considered. The obtained structures were finally confirmed by the complete agreement of computed and experimental 1H and 15N chemical shifts. This demonstrates that solid‐state NMR and quantum‐chemical methods ideally complement each other and, in a combined manner, represent a powerful approach for reliable, high‐precision structure determination whenever scattering techniques are inapplicable.

15N NMR Spectroscopy in Structural Analysis: An Update (2001 - 2005)

“…The fact that the N-H distances actually measured are somewhat longer (usually 1-3 pm) than the equilibrium distances determined by neutron diffraction measurements and standard quantum chemical calculations is due to nuclear motion [120,121]. The dipole-dipole couplings are significantly affected by nuclear motion whenever the internuclear distance or the orientation of the internuclear vector is time dependent.…”

“…The dipole-dipole couplings are significantly affected by nuclear motion whenever the internuclear distance or the orientation of the internuclear vector is time dependent. Combining these effects, stretching motions have been estimated to effectively lower the dipolar coupling constant by about 1.5 %, which corresponds to an elongation of the N-H bond by about 0.5 pm, as determined from the N-H dipole-dipole coupling [120]. To a good approximation, the r IS -3 dependence is governed by a stretching vibration along the I-S bond, and the angular dependence can be attributed to perpetual motions bending away from the mean bond axis.…”

Using17O solid-state NMR and first principles calculation to characterise structure and dynamics in inorganic framework materials

“…As a result atomic positions can often be 'refined' to pm accuracy. 48,49 In this particular example, we have shown how calculations from first principles can be used to establish correlations between 17 O shifts and subtle distortions of local structure in framework materials. Since these distortions are key to understanding the complex structural chemistry of these materials, the combination of simple NMR experiments and robust quantum chemical calculations should be of great interest to materials chemistry in general.…”

Solid‐State NMR Studies on Supramolecular Chemistry

“…Spiess, Schnell, and coworkers 45 were able to demonstrate the potential of the combined use of advanced SSNMR pulse sequences, (HDOR, heteronuclear dipolar-order rotor encoding and REREDOR, rotor-encoded REDOR), and quantum-chemical calculations for investigating the multiple HB network of N -butylaminocarboxyl-6-tridecylisocitosine in the pyrimidone and pyrimidinol forms (Scheme 3).…”