Isotope exchange in reactions between D₂O and size-selected ionic water clusters containing pyridine, H⁺(pyridine)_m(H₂O)_n

Abstract

Pyridine containing water clusters, H(+)(pyridine)(m)(H(2)O)(n), have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. In the experiments, H(+)(pyridine)(m)(H(2)O)(n) with m = 1-4 and n = 0-80 are observed. For the cluster distributions observed, there are no magic numbers, neither in the abundance spectra, nor in the evaporation spectra from size selected clusters. Experiments with size-selected clusters H(+)(pyridine)(m)(H(2)O)(n), with m = 0-3, reacting with D(2)O at a center-of-mass energy of 0.1 eV were also performed. The cross-sections for H/D isotope exchange depend mainly on the number of water molecules in the cluster and not on the number of pyridine molecules. Clusters having only one pyridine molecule undergo D(2)O/H(2)O ligand exchange, while H(+)(pyridine)(m)(H(2)O)(n), with m = 2, 3, exhibit significant H/D scrambling. These results are rationalized by quantum chemical calculations (B3LYP and MP2) for H(+)(pyridine)(1)(H(2)O)(n) and H(+)(pyridine)(2)(H(2)O)(n), with n = 1-6. In clusters containing one pyridine, the water molecules form an interconnected network of hydrogen bonds associated with the pyridinium ion via a single hydrogen bond. For clusters containing two pyridines, the two pyridine molecules are completely separated by the water molecules, with each pyridine being positioned diametrically opposite within the cluster. In agreement with experimental observations, these calculations suggest a "see-saw mechanism" for pendular proton transfer between the two pyridines in H(+)(pyridine)(2)(H(2)O)(n) clusters.

Insights into the dynamics of evaporation and proton migration in protonated water clusters from Large‐scale Born–Oppenheimer direct dynamics

“…We also observe a putative contradiction between the conclusions from experimental[58] and computational[36] studies of proton transfer in protonated water clusters and the fact that the proton solvation shell in clusters in general is stable. [59] This contradiction may be resolved by realizing that proton transfer resulting in the reformation of a similar solvation structure is a fast and a rare event which does not affect the results of time‐unresolved experiments.…”

“…It is notable that the former mechanism appears to be responsible for proton transfer in some systems, as for example in pyridine‐containing water cluster. [58] This is, however, only possible if the proton is localized, which is not the case for the protonated pure water clusters studied here.…”

Proton Migration in Clusters Consisting of Protonated Pyridine Solvated by Water Molecules

“…[14] Besides, the PA of aw ater dimer is 808 kJ mol À1 (8.4 eV), [15] and it has been predicted, theoretically,t hat the PA of water clusters in- creasesw ithi ncreasing size of the clusterw ith bulk water, reachingP A= 1130 kJ mol À1 (11.7 eV). [1,17,18] Besides,U ggeruda nd co-workers have shown [1] that the preference for N-protonationo fP yi sa lso retained in the PyH + (H 2 O) n clusters for n = 1-6, that is, for all sizes, no single potentiale nergy minimum was located in which the protonisa ttached to water. [1,17,18] Besides,U ggeruda nd co-workers have shown [1] that the preference for N-protonationo fP yi sa lso retained in the PyH + (H 2 O) n clusters for n = 1-6, that is, for all sizes, no single potentiale nergy minimum was located in which the protonisa ttached to water.…”

“…[1][2][3][4][5][6][7][8] Besides their relevance in atmospheric chemistry, [1] microhydrated protonated pyridine PyH + (H 2 O) n moieties are targeted here to study solvation effects in the frame of an investigation on the radiation action at the nanoscale. Besidest he fragmentation channel associated with the evaporation of water molecules, the charged-fragment mass spectrum shows competition between the production of the PyH + ion (or its corresponding charged fragments) and the production of H + (H 2 O) or H + (H 2 O) 2 ions.…”

“…[1][2][3][4][5][6][7][8] Besides their relevance in atmospheric chemistry, [1] microhydrated protonated pyridine PyH + (H 2 O) n moieties are targeted here to study solvation effects in the frame of an investigation on the radiation action at the nanoscale. [1][2][3][4][5][6][7][8] Besides their relevance in atmospheric chemistry, [1] microhydrated protonated pyridine PyH + (H 2 O) n moieties are targeted here to study solvation effects in the frame of an investigation on the radiation action at the nanoscale.…”