Soft XAS as a tool for locating protons in organic crystal structures: A Classical Electrostatic model of Hydrogen Bonding and Brønsted Proton Transfer

Authors: Paul Edwards (University Of Leeds; Diamond Light Source) , Joanna Stevens (Cambridge Crystallographic Data Centre) , Elizabeth Shotton (Diamond Light Source) , Sven Schroeder (University Of Leeds)
Co-authored by industrial partner: No

Type: Conference Paper
Conference: XAFS2021
Peer Reviewed: No

State: Published (Approved)
Published: July 2021

Abstract: Using standard X-ray diffraction (XRD) techniques for crystal structure refinement often leaves the precise location of protons ambiguous. Core level spectroscopies can be used as a tool for proton location refinement due to the direct sensitivity of the core electron binding energy to the acceptor-proton distance. We have demonstrated how both Near Edge X-ray Absorption Fine Structure (NEXAFS) and X-ray Photoelectron Spectroscopy (XPS) can be used as a diagnostic tool for proton transfer in hydrogen bonds through a consistent core level shift of +2 eV in the nitrogen acceptor 1s emission where proton transfer is observed.1,2 We have studied a range of organic crystals consisting of both salts and cocrystals, where the core level shift appears to be a universal effect.1,2 This core level shift can be linked directly to the electrostatic effect of a shorter N-H distance. Using a model based entirely on classical electrostatics, we have modelled the effect of the proton position on the 1s -> π* transition of N 1s electrons, to understand the contribution of the electrostatic interaction to the core level shift observed with changing proton position. From this classical electrostatic model, we observe a continuous change in core electron binding energy with change in proton position. A shift of +2 eV is observed between N-H distances of 1 Å - 2 Å, as observed experimentally 1,2, with structures containing centred protons resulting in an intermediate shift, fully in keeping with a continuum.1 This indicates that the primary interaction involved is electrostatic, further suggesting that the effect observed in XPS and NEXAFS is direct electrostatic sensitivity to the proton position. Future work will investigate the use of NEXAFS spectroscopy with additional information from Density Functional Theory calculations to more accurately refine proton positions in organic crystals than the current standard methods used in XRD.

Subject Areas: Chemistry


Technical Areas:

Added On: 09/08/2021 13:05

Discipline Tags:

Chemistry Organic Chemistry

Technical Tags:

Spectroscopy X-ray Absorption Spectroscopy (XAS)