ZEKE Photoelectron
Spectroscopy of <i>p</i>‑Fluorophenol···H<sub>2</sub>S/H<sub>2</sub>O Complexes and Dissociation Energy Measurement
Using the Birge–Sponer
Extrapolation Method
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Abstract
In
this work we have shown that the Birge–Sponer extrapolation
method can be successfully used to determine the dissociation energies
(<i>D</i><sub>0</sub>) of noncovalently bound
complexes. The O–H···S hydrogen-bonding interaction
in the cationic state of the <i>p</i>-fluorophenol···H<sub>2</sub>S complex was characterized using zero kinetic energy (ZEKE)
photoelectron spectroscopy. This is the first ZEKE report on the O–H···S
hydrogen-bonding interaction. The adiabatic ionization energy (AIE)
of the complex was determined as 65 542 cm<sup>–1</sup>. Various intermolecular and intramolecular vibrational modes of
the cation were assigned. A long progression was observed in the intermolecular
stretching mode (σ) of the complex with significant anharmonicity
along this mode. The anharmonicity information was used to estimate
the dissociation energy (<i>D</i><sub>0</sub>) in the cationic
state using the Birge–Sponer extrapolation method. The <i>D</i><sub>0</sub> was estimated as 9.72 ± 1.05 kcal mol<sup>–1</sup>. The ZEKE photoelectron spectra of analogous complex
FLP···H<sub>2</sub>O was also recorded for the sake
of comparison. The AIE was determined as 64 082 cm<sup>–1</sup>. The intermolecular stretching mode in this system, however, was
found to be quite harmonic, unlike that in the H<sub>2</sub>S complex.
The dissociation energies of both the complexes, along with those
of a few benchmark systems, such as phenol···H<sub>2</sub>O and indole···benzene complexes, were computed
at various levels of theory such as MP2 at the complete basis set
limit, ωB97X-D, and CCSD(T). It was found that only the ωB97X-D
level values were in excellent agreement with the experimental results
for the benchmark systems for the ground as well as the cationic states.
The dissociation energy of the (FLP···H<sub>2</sub>S)<sup>+</sup> complex determined by the Birge–Sponer extrapolation
was about ∼18% lower than that computed at the ωB97X-D
level