Accurate Vertical Electron Detachment Energies and Multiphoton Resonant Photoelectron Spectra of Biochromophore Anions in Aqueous Solution

We introduce a new methodology for calculating vertical
electron
detachment energies (VDEs) of biologically relevant chromophores in
their deprotonated anionic forms in aqueous solution. It combines
a large-scale mixed DFT/EFP/MD approach with the high-level multireference
perturbation theory XMCQDPT2 and the Effective Fragment Potential
(EFP) method. The methodology includes a multiscale flexible treatment
of inner (∼1000 water molecules) and outer (∼18000 water
molecules) water shells around a charged solute, capturing both the
effects of specific solvation and the properties of bulk water. VDEs
are calculated as a function of system size for getting a converged
value at the DFT/EFP level of theory. The XMCQDPT2/EFP approach, adapted
for calculating VDEs, supports the DFT/EFP results. When corrected
for a solvent polarization contribution, the XMCQDPT2/EFP method yields
the most accurate estimate to date of the first VDE for aqueous phenolate
(7.3 ± 0.1 eV), which agrees well with liquid-jet X-ray photoelectron
spectroscopy data (7.1 ± 0.1 eV). We show that the geometry of
the water shell and its size are essential for accurate VDE calculations
of aqueous phenolate and its biologically relevant derivatives. By
simulating photoelectron spectra of aqueous phenolate upon two-photon
excitation at wavelengths resonant with the S0 →
S1 transition, we also provide interpretation of recent
multiphoton UV liquid-microjet photoelectron spectroscopy experiments.
We show that its first VDE is consistent with our estimate of 7.3
eV, when experimental two-photon binding energies are corrected for
the resonant contribution.