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The solution structure and reactivity of decavanadate
Author(s) -
Comba Peter,
Helm Lothar
Publication year - 1988
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19880710605
Subject(s) - chemistry , reactivity (psychology) , kinetics , kinetic energy , scrambling , crystallography , reaction intermediate , reaction mechanism , computational chemistry , stereochemistry , catalysis , organic chemistry , medicine , linguistics , philosophy , physics , alternative medicine , pathology , quantum mechanics
Abstract The oxygen‐exchange reaction of V 10 O 28 6−with bulk water has been followed by time‐dependent 17 O‐NMR spectroscopy (buffered solutions, pH ∼ 5.5, [V 10 ] total ∼ 0.17 m , T = 298 K). It is shown that all seven structurally different sites of O‐atoms are kinetically similar but, in contrast to earlier studies, not identical (6 h ⩽ ‘ t 1/2 ’ ⩽ 11 h). The kinetic similarity of the various structural sites implies the some (but not full) O scrambling is involved. Two possible mechanisms with a ‘half‐bonded’ and an ‘open’ intermediate are discussed in detail to interpret the experimental results. A computer simulation of the exchange reaction based on these models is presented. It is shown that the ‘half‐bonded‐intermediate’ mechanism is consistent with the experimental data and the following parameters are calculated: formation of the intermediate: k 1 = 5.8 · 10 −3 s −1 , k −1 = 6.7 · 10 −2 s −1 , [intermediate] ∞ ≈ 8%; all activated O‐atoms exchange within the lifetime of the intermediate (τ ∼ 15 s), and the calculated exchange rate of the intermediate ( k 2 ⩾ 0.60 s −1 ) is consistent with earlier assumptions ( k 2 ≈ 0.5 s −1 ). It is shown that a simulation based on the ‘open‐intermediate’ mechanism results in kinetic parameters which are not consistent with the kinetics of the formation of cyclic metavanadates ((VO 3 − ) n , n = 4,5) from decavanadate, since the required formation rate is by a factor ∼ 10 2 too fast, and the equilibrium concentration of metavanadates is by a factor of ∼ 2 too large (under the conditions of the O‐exchange experiments of decavanadate ( T = 298 K, [V 10 ] total ≈ 0.17 m , pH ∼ 5.55) the total amount of metavanadates present is ∼ 8%, with [(VO 3 − ) 4 ]/[(VO 3 − ) 5 ] ∼ 4:1; a qualitative analysis of the kinetics of the formation of metavanadates ( v o kinetics; the exact mechanism of the back‐reaction (at least second‐order) is not known with certainty) leads to k 1 ⩾ 4·10 −5 s −1 ). O exchange of decavanadates via equilibrated metavanadates would lead to full scrambling of the O sites and is not consistent with the observed differences in the exchange rates. From the qualitative kinetic parameters of the metavanadate formation kinetics, it can be concluded that any contribution of an ‘open’ or an ‘metavanadate’ mechanism is of the order of 1–2% at most.