Proton Transfer on the Edge of the Salt/Cocrystal Continuum: X-Ray Photoelectron Spectroscopy of Three Isonicotinamide Salts

Abstract

X-ray photoelectron spectroscopy (XPS) has emerged as a technique that allows for characterization and classification of hydrogen bonding and proton transfer interactions in organic crystal structures, in a way that is complementary to crystallography by X-ray or neutron diffraction. Here, we analyze the nitrogen 1s core-level binding energies (BEs) of isonicotinamide (IN) systems with proton transfer between donor and acceptor groups at short distances. We show how a careful calibration of the BE scale places these salt systems correctly on the edge of the so-called salt–cocrystal continuum. We show how performing a fitting analysis of the data that is consistent with elemental analysis, expected stoichiometry, and quantification of adventitious carbon contamination facilitates the determination of absolute BEs with accuracy and reproducibility within ±0.1 eV. The determined N 1s core-level BEs of the protonated IN acceptors suggest that the local geometric arrangements of the donor, acceptor, and proton can influence the N 1s core-level BE significantly.

Determination of H-Atom Positions in Organic Crystal Structures by NEXAFS Combined with Density Functional Theory: a Study of Two-Component Systems Containing Isonicotinamide

“… 31 In XPS measurements, this structure exhibits a shift of +1.7 eV in the N 1s BE related to one of the two nitrogen acceptors, confirming this analysis. 6 The experimental NEXAFS spectrum shown in Figure 4 b appears to confirm this structure, with an obvious increase in the photon energy of the protonated nitrogen acceptor N 1s → 2π* peak, leading to the distinctive double peak. In contrast to the other two complexes, the DFT-calculated NEXAFS spectrum ( Figure 4 a) shows some differences in the relative intensities of the peaks and an additional feature at 403.3 eV which is absent in the experimental spectrum.…”

“… 31 Using XPS, we confirmed this through an N 1s BE shift at the proton acceptor of +2 eV compared to a cocrystal, with relative N 1s emission intensities for all nitrogen moieties in line with the bulk stoichiometry, confirming that the composition within the near-surface region probed by XPS is the same as the bulk. 6 Figure 2 shows the modeled and experimental nitrogen K-edge NEXAFS spectra for this salt. It is immediately obvious that the modeled structure very closely resembles the experimental NEXAFS data.…”

“…The three nitrogen environments [amide (NH 2 –C=O), pyridinium ring (C–NH + =C), and nitro (−NO 2 )] are each associated with a distinct N 1s → π* resonance, consistent with our previous XPS analysis. 6 The nitro group is the most intense and visible at a photon energy of 403.5 eV. The protonated nitrogen of the pyridinium results in the peak at 399.7 eV, while the amide 1s → 1π* transition appears as a low-energy shoulder to the protonated nitrogen, the peak of that shoulder being at 398.9 eV.…”

“…We have previously shown that the core-level BE of electrons (determined using XPS) is directly affected by the distance between the H-atom and the proton acceptor. 6 However, the magnitude of the BE shifts also depends on the surround electrons and atoms, and therefore there is no universal dependency. 6 This means we have reached the limit of usefulness of XPS in the characterization of organic hydrogen bonded crystals.…”

“… 1 , 4 , 5 Our previous studies have consistently demonstrated how core-level spectroscopy in the form of X-ray photoelectron spectroscopy (XPS) is sensitive to the position of the H-atom through the 1s core-level binding energy (BE) shift at the proton acceptor, as shown in Figure 1 . 3 , 6 − 9 This effect is also present in near-edge X-ray absorption fine structure (NEXAFS) spectroscopy through a shift dependent upon both the core-level BE and the energy of the final state unoccupied molecular orbitals. 3 , 7 − 17 Due to the large number of unoccupied π* (where π bonding is present) and σ* orbitals, which are both highly sensitive to the molecular and electronic structures, 17 the technique has the potential to enable a structure refinement directly sensitive to the H-atom.…”