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

• Pyridine contg. water clusters, H+(pyridine)m(H2O)n, have been studied both exptl. by a quadrupole time-of-flight mass spectrometer and by quantum chem. calcns. In the expts., H+(pyridine)m(H2O)n with m = 1-4 and n = 0-80 are obsd. For the cluster distributions obsd., there are no magic nos., neither in the abundance spectra, nor in the evapn. spectra from size selected clusters. Expts. with size-selected clusters H+(pyridine)m(H2O)n, with m = 0-3, reacting with D2O 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 no. of water mols. in the cluster and not on the no. of pyridine mols. Clusters having only one pyridine mol. undergo D2O/H2O ligand exchange, while H+(pyridine)m(H2O)n, with m = 2, 3, exhibit significant H/D scrambling. These results are rationalized by quantum chem. calcns. (B3LYP and MP2) for H+(pyridine)1(H2O)n and H+(pyridine)2(H2O)n, with n = 1-6. In clusters contg. one pyridine, the water mols. form an interconnected network of hydrogen bonds assocd. with the pyridinium ion via a single hydrogen bond. For clusters contg. two pyridines, the two pyridine mols. are completely sepd. by the water mols., with each pyridine being positioned diametrically opposite within the cluster. In agreement with exptl. observations, these calcns. suggest a "see-saw mechanism" for pendular proton transfer between the two pyridines in H+(pyridine)2(H2O)n clusters. [on SciFinder (R)]

Subject headings

NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

Keyword

Pyridine Cluster Ion Atmosphere

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art (subject category)