Proton transfer (PT) in organic crystals creates localized
charges
and strong hydrogen bonding (HB), making the self-consistent field
(SCF) calculation of core-ionized and core-excited states challenging.
Today most corresponding X-ray spectral measurements are interpreted
based on empirical fitting and/or chemical intuitions. Here we present
a systematic quantum mechanical/molecular mechanical (QM/MM) study
of N 1s X-ray photoelectron (XPS) and absorption (XAS) spectra of
three isonicotinamide (IN)-based organic crystals with full (1), half (2), and no (3) protonations.
A complete picture of the structure–spectroscopy relation in
different crystal environments was provided, and assignments to three
unprotonated (pyridinic, p; amide, a
1 and a
2) sites and one protonated
(pyridinic, h) N site were clearly made. We found
that including distant residues as natural population analysis (NPA)
point charges can effectively enhance the SCF convergence of the core
states. The size of the QM part was tuned, and with some 140–170
atoms we achieved spectral convergence that can represent the infinite
crystal. At the crystal structures, simulated relative binding energies
deviate ≤0.3 eV to experiments. Simulated XAS spectra agree
well with experiments, and with molecular orbital analysis we interpreted
the π* structures as hybrid local excitation and charge transfer
states (πLE–CT
*) or pure LE states (πLE
*). Analyses on both spectra helped
understand the PT and HB nature in such organic crystals, and a debate
in XPS interpretation of 3 was resolved and its XAS assignment
corrected. Further, to model the dynamical effect of the proton in 2, XPS/XAS spectra were evaluated at snapshots with varying
N–H distances. A continuous picture illustrates the sensitive
influence of proton position to both spectra. Reduction of the N–H
distance by only 0.2 Å from the crystal structure (1.1 Å)
excellently reproduced both spectra. This perturbation phenomenologically
models effects of vibration from the equilibrium crystal structure
and environmental temperature and pressure factors.