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2 Bulletin of the Polish Academy of Sciences. Chemistry

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Single-crystal neutron study on cobalt iodate tetrahydrate, a compound with H...|I(V)O3- hydrogen bonds?

Nagel R., Kümmerle E., Lutz H. D.

Bulletin of the Polish Academy of Sciences. Chemistry

2002

Vol. 50, No. 1

51-66

The crystal structure of cobalt iodate tetrahydrate Co(IO(3)(2)-4H(2)O (space group P21/c, Z=2, a=836.8(5) pm, b=656.2(3) pm, c=850.2(5) pm, and Beta=100.12°, final Rl = 0.0349 (I>i mniejszości 2 sigma(I) for 1031 unique reflections) was redetermined by single-crystal neutron diffraction studies. The structure is built up by layers parallel (100) connected by interionic O(3)I(-..'OIO(-)(2) bonds. Unequivocal interpretation of the four differently strong OH(...) OIO(-3) hydrogen bonds of the two crystallographic different hydrate H(2)O molecules of the isostructural series M(IO3)(2)-4H(2)O (M = Mg, Co, Ni) on the basis of the observed hydrogen-bond distances and assignment to the uncoupled OD stretching modes of matrix isolated HDO molecules is only possible if weak hydrogen bonds to the iodine lone-pairs of adjacent IO-(3) ions are additionally considered. The potential strength of H(...)I(V)O-(3) hydrogen bonds with respect to H(...) I distances and H(...)IO(-)(3) angles is discussed. A bond-valence concept for estimating the strength of H(...I)I(V)O(-)(3) hydrogen bonds is given.

The role of triplet excited state of hydrocarbons in catalysis by transition-metal species

Minaev B. F.

Bulletin of the Polish Academy of Sciences. Chemistry

2001

Vol. 49, No. 1

27-56

Reactions of the second-row transition metal (TM) atoms, clusters and complexes with small hydrocarbons have been considered by a simple valence bond (VB) approximation together with conventional molecular orbital approaches. Correlation diagram obtained from VB treatment of the C-H bond activation by TM atom demonstrates the involvement of the triplet excited state of hydrocarbon into the total wave function at the transition state of the catalytic process. VB approximation indicates that the activation barrier for TM atom insertion into C-H bond of a number of alkanes and alkenes is strongly determined by the "singlet-triplet" avoided crossing when the total spin of the reacting system is fixed by the low-spin term of the TM-catalyst. The energy of the triplet state of hydrocarbon determines the barrier in a simple VB approach. The same general features of spin uncoupling are typical of other TM-species and catalytic processes. The nature of barrier in chemical reaction between two closed shell reagents is often determined by avoided grossing of the ground and doubly-excited singlet states; the latter consists of two triplet excitations coupled to the singlet pairing. In order to diminish the barrier one has to activate in some way the triplet state of the reactants. The catalyst supplies its nonpaired electrons in order to activate the "singlet-triplet" avoided crossing in hydrocarbon.