The mechanism of excited state proton dissociation in microhydrated hydroxylamine clusters

Abstract

The dynamics and mechanism of excited-state proton dissociation and transfer in microhydrated hydroxylamine clusters are studied using NH2OH(H2O)n (n = 1-4) as model systems and the DFT/B3LYP/aug-cc-pVDZ and TD-DFT/B3LYP/aug-cc-pVDZ methods as model calculations. This investigation is based on the Förster acidity scheme and emphasizes the photoacid dissociation in the ground (S0) and lowest singlet-excited states (S1) and the interplay between the photo and thermal excitations. The quantum chemical results suggest that the intermediate complexes are formed only in the S1 state in a low local-dielectric environment (e.g., ε = 1) and that upon the S0→ S1 transition, the photon energy excites mostly NH2OH, which leads to a homolytic cleavage of the O-H bond and to dynamically stable charge-separated Rydberg-like H-bond complexes (e.g., NH2O˙-H3O(+)˙). The potential energy surfaces for proton displacement in the smallest Rydberg-like H-bond complex support the intersection of the S0 and S1 states in low local-dielectric environments, whereas in a high local-dielectric environment (e.g., ε = 78), these two states are completely separated. Based on the static results, a photoacid-dissociation mechanism that involves Rydberg-like H-bond complex formation, an H-bond chain extension and fluctuations in the local-dielectric environment is proposed. NVT-BOMD simulations confirm the static results and show that the dynamic behavior of the dissociating proton in the S1 state is not different from that of the protonated H-bond systems in the ground state, which consists of the oscillatory shuttling and structural diffusion motions. These findings allow our theoretical methods, which have been used successfully in protonated H-bond systems in the ground state, to be applied in the study of the photoacid-dissociation processes. The current theoretical study suggests effective steps as well as guidelines for the investigation of the dynamics of the photoacid-dissociation and transfer processes in the Förster acidity scheme, provided that the exciting photon does not lead to a significant change in the structure of the intermediate complex in the excited state.

Mechanisms of photoexcitation and photoionization in small water clusters

“…2 ) confirms that the outstanding feature of the S 0 → S 1 excitation is the proton transfer in H-bond. In addition, based on the spatial distribution of the LUMO, 19 structure E2-[2] with R O1–O3 = 2.82 Å ([H 3 O] + ˙⋯[OH]˙) can be considered a charge-separated Rydberg-like H-bond complex, with the HOMO–LUMO excitation energy of 8.65 eV. A similar charge-separated Rydberg-like H-bond complex ([NH 2 O]˙⋯[H 3 O] + ˙) was confirmed in a previous study to be the smallest, most active intermediate complex for proton transfer in the NH 2 OH⋯H 2 O system in the S 1 state.…”

“…A similar charge-separated Rydberg-like H-bond complex ([NH 2 O]˙⋯[H 3 O] + ˙) was confirmed in a previous study to be the smallest, most active intermediate complex for proton transfer in the NH 2 OH⋯H 2 O system in the S 1 state. 19 These intermediate complexes result from the redistribution of the electron density in the H-bond upon S 0 → S 1 excitation and are represented by electron clouds localized on the two hydrogen atoms of the non-H-bond O–H in the [H 3 O] + ˙. 9,32,33 It should be noted that because the CASPT2(4,6) geometry optimizations were performed without symmetry restrictions ( i.e.…”

“…[12][13][14] In this approach, the smallest, most active intermediate complex in a systematically selected presolvation model is used as a representative system and studied in detail. [15][16][17][18] For example, for protonated water systems, 16,17 the rate-determining step of proton transfer in the electronic ground state is characterized by the interconversion between the close-contact (O-H + /O) and shared-proton ( 19 Therefore, to study the photoexcitation and photoionization processes in gas phase (eqn (1)-(4)), the water molecule and small water clusters, (H 2 O) n ; n ¼ 2-3, were used as model systems. 2 To simplify the discussion, the water molecule and small water clusters were labeled with a three-character code, Gn- [m] and En-[m]; where G is the structure of the water cluster in the S 0 state; E is the structure of the water cluster in the S 1 state; n is the number of water molecules.…”

“…Moreover, the smallest, most active intermediate complex for photoacid dissociation in microhydrated hydroxylamine clusters in the S 1 state is the charge-separated Rydberg-like H-bond complex [NH 2 O]˙⋯[H 3 O] + ˙. 19 Therefore, to study the photoexcitation and photoionization processes in gas phase ( eqn (1)–(4) ), the water molecule and small water clusters, (H 2 O) n ; n = 2–3, were used as model systems. 2 …”

“…2) conrms that the outstanding feature of the S 0 / S 1 excitation is the proton transfer in H-bond. In addition, based on the spatial distribution of the LUMO, 19 19 These intermediate complexes result from the redistribution of the electron density in the H-bond upon S 0 / S 1 excitation and are represented by electron clouds localized on the two hydrogen atoms of the non-H-bond O-H in the [H 3 O] + c. 9,32,33 It should be noted that because the CASPT2(4,6) geometry optimizations were performed without symmetry restrictions (i.e., with C 1 symmetry), one of the non-H-bond O-H distances in the [H 3 O] + c moiety in structure E2- [2] (R O3-H5 ¼ 1.04Å) is slightly longer than the other (R O3-H4 ¼ 1.02Å). Therefore, the O 3 -H 5 Fig.…”