A compact supersonic source is employed with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to generate and investigate the metallocarbohedrene, Nb 8 C 12 + . Nb 8 C 12 + reacts with water and alcohols via sequential attachments and dehydrogenations leading to the initial truncation products Nb 8 C 12 -(OR) 4 + (R ) H, CH 3 , C 2 H 5 , C 4 H 9 ). Also, Nb 8 C 12 + reacts with NH 3 via sequential attachments to produce the initial truncation product Nb 8 C 12 (NH 3 ) 4 + . Nb 8 C 12 L 5-8 + peaks for ammonia and alcohols are also observed, but they grow in slowly. A measurement of the relative rate constants for the sequential addition reactions of NH 3 confirms that addition of the fifth NH 3 is at least an order of magnitude slower than the first four attachments. In reactions with acetonitrile and benzene, a maximum of four attachments of these ligands is observed. Sequential halide abstractions with CH 3 X yield Nb 8 C 12 X n + truncating at n ) 4 for X ) Cl and n ) 5 for X ) Br and I. The "titration" results and the relative rate constants for the sequential addition reactions of NH 3 provide supporting evidence that the geometric structure of Nb 8 C 12 + is the theoretically more stable T d or D 2d symmetry having two sets of four equivalent metal sites, as opposed to the T h symmetry with all eight equivalent metals. Finally, the results for Nb 8 C 12 + are compared to those previously obtained for V 8 C 12 + and Ti 8 C 12 + .
Nb6C7
+ is studied using a
Fourier transform ion cyclotron resonance (FTICR) mass spectrometer
coupled to
a compact supersonic source. Its reactions with H2O,
NH3, CH3OH, CH3CN, and
C6H6 are reported. For
each of these ligands, sequential attachment reactions occur leading to
the initial truncation products
Nb6C7L4
+.
Minor amounts of
Nb6C7L5
+ and
Nb6C7L6
+ are observed
for polar molecules, but they grow in slowly. A
measurement of the relative rate constants for the sequential addition
reactions of CH3OH, for example,
demonstrates quantitatively that addition of the fifth
CH3OH is very slow compared to the first four
additions.
Multiple excitation collisional activation (MECA) on
Nb6C7
+ yields
Nb5C6
+, and MECA on
Nb5C6
+ yields
Nb4C4
+ as predominant products.
Nb6C7
+ undergoes iodine
abstraction from one CH3I molecule to yield
the
truncation product, Nb6C7I+.
Ab initio Hartree−Fock calculations of
Nb6C7 suggest a structure consisting
of
two cubes with one carbon at the exact inversion center of the
D
2h
symmetry. This structure, in
which four
of the six niobium atoms are equivalent but different from the other
two, is consistent with the experimental
results. The structures of Nb4C4 and
Nb5C6 are also optimized
theoretically.
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