Abstract:The photochemical oxidation of water molecules in pyrimidine–water complexes has been explored in a combined experimental and theoretical study.
“…Extension of the ESHT reaction to describe other processes is now occurring, notably in the description of excited state H abstraction by aromatic molecules, which is the reverse process of ESHT and is demonstrated in photoexcited pyridine-H 2 O system. [80][81][82][83] These…”
A general model of excited state hydrogen transfer (ESHT) which unifies ESHT and the excited state proton transfer (ESPT) is presented from experimental and theoretical works on phenol–(NH3)n. The hidden role of ESPT is revealed.
“…Extension of the ESHT reaction to describe other processes is now occurring, notably in the description of excited state H abstraction by aromatic molecules, which is the reverse process of ESHT and is demonstrated in photoexcited pyridine-H 2 O system. [80][81][82][83] These…”
A general model of excited state hydrogen transfer (ESHT) which unifies ESHT and the excited state proton transfer (ESPT) is presented from experimental and theoretical works on phenol–(NH3)n. The hidden role of ESPT is revealed.
“…Comparison with the previously studied Pym + -(N 2 ) n clusters using the same spectroscopic and computational approach [47] illustrates the effect of solvent polarity on the interaction potential with respect to both structure (preferred binding sites and solvation network) and binding energy. We also compare our results for Pym + -W with the properties of neutral Pym-W [43,[48][49][50][51] to evaluate the effect of ionization on structure, binding motif, and interaction energy, which is fundamental to comprehend the charge-induced changes arising in solvent binding motifs of biologically relevant molecules.…”
Hydration of biomolecules is an important physiological process that governs their structure, stability, and function. Herein, we probe the microhydration structure of cationic pyrimidine (Pym), a common building block of DNA/RNA bases, by infrared photodissociation spectroscopy (IRPD) of mass-selected microhydrated clusters, $$\hbox {Pym}^{+}$$
Pym
+
-$$\hbox {W}_{n}$$
W
n
(W=$$\hbox {H}_{2}\hbox {O}$$
H
2
O
), in the size range $$n=1$$
n
=
1
–3. The IRPD spectra recorded in the OH and CH stretch range are sensitive to the evolution of the hydration network. Analysis with density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level provides a consistent picture of the most stable structures and their energetic and vibrational properties. The global minima of $$\hbox {Pym}^{+}$$
Pym
+
-$$\hbox {W}_{n}$$
W
n
predicted by the calculations are characterized by H-bonded structures, in which the H-bonded $$\hbox {W}_{n}$$
W
n
solvent cluster is attached to the most acidic C4–H proton of $$\hbox {Pym}^{+}$$
Pym
+
via a single CH...O ionic H-bond. These isomers are identified as predominant carrier of the IRPD spectra, although less stable local minima provide minor contributions. In general, the formation of the H-bonded solvent network (exterior ion solvation) is energetically preferred to less stable structures with interior ion solvation because of cooperative nonadditive three-body polarization effects. Progressive hydration activates the C4–H bond, along with increasing charge transfer from $$\hbox {Pym}^{+}$$
Pym
+
to $$\hbox {W}_{n}$$
W
n
, although no proton transfer is observed in the size range $$n\leqslant $$
n
⩽
3. The solvation with protic, dipolar, and hydrophilic W ligands is qualitative different from solvation with aprotic, quadrupolar, and hydrophobic $$\hbox {N}_{2}$$
N
2
ligands, which strongly prefer interior ion solvation by $$\uppi $$
π
stacking interactions. Comparison of $$\hbox {Pym}^{+}$$
Pym
+
-W with Pym-W and $$\hbox {H}^{+}$$
H
+
Pym-W reveals the drastic effect of ionization and protonation on the Pym...W interaction.
Graphic Abstract
“…The sustainable production of hydrated electrons requires that the electrons are obtained by photooxidation of water. Recently, it has been demonstrated in molecular beam experiments that the chromophores pyridine (Py) and pyrimidine (Pm) can catalytically photooxidize water molecules in hydrogen-bonded Py•••(H 2 O) n and Pm•••(H 2 O) n clusters via an excitedstate proton-coupled electron transfer (PCET) reaction, resulting in the formation of PyH and PmH radicals, which were detected via their electronic spectra [34,35] (here and in the following the dots indicate a hydrogen bond). As radicals, PyH and PmH possess absorbing excited states in the visible range of the spectrum.…”
Section: Introductionmentioning
confidence: 99%
“…It was shown computationally that these hypervalent radicals possess, in addition, low-lying dark excited states of 2 πσ* character which are dissociative with respect to the NH bond distance. [35,36] These 2 πσ* states can drive direct (nonstatistical) photodetachment of the excess hydrogen atom from the PyH radical. This reaction could be experimentally detected in PyH•••(H 2 O) n clusters.…”
Section: Introductionmentioning
confidence: 99%
“…As radicals, PyH and PmH possess absorbing excited states in the visible range of the spectrum. It was shown computationally that these hypervalent radicals possess, in addition, low‐lying dark excited states of 2 πσ* character which are dissociative with respect to the NH bond distance [35,36] . These 2 πσ* states can drive direct (nonstatistical) photodetachment of the excess hydrogen atom from the PyH radical.…”
Ab initio computational methods are employed to explore whether hydrated electrons can be produced by the photodetachment of the excess hydrogen atom of the heptazinyl radical (HzH) in finite-size HzH•••(H 2 O) n clusters. The HzH radical is an intermediate species in the photocatalytic oxidation of water with the heptazine (Hz) chromophore. Hz (heptaazaphenalene) is the monomer of the ubiquitous polymeric wateroxidation photocatalyst graphitic carbon nitride (g-C 3 N 4 ). The energy profiles of minimum-energy excited-state reaction paths for proton-coupled electron transfer from HzH to water molecules were computed for the HzH•••H 2 O and HzH•••(H 2 O) 4 complexes with the CASPT2 method. The results reveal that the photodetachment of the excess H-atom from the HzH radical is a barrierless reaction in these hydrogen-bonded complexes, resulting in the formation of H 3 O and H 3 O(H 2 O) 3 radicals, respectively, which are finite-size models of the hydrated electron. The computational results suggest that the photocatalytic formation of hydrated electrons from water with visible light could be possible in principle.
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