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Reaction Pathways in Solid-State Processes. 1. Carbon-13 NMR and X-ray Crystallography of Fluorobullvalene
journal contribution
posted on 1996-08-28, 00:00 authored by K. Müller, H. Zimmermann, C. Krieger, R. Poupko, Z. LuzCarbon-13 MAS−NMR and X-ray diffraction experiments
on solid fluorobullvalene are reported. The
compound crystallizes in the orthorhombic space group Pnam
with four symmetry related molecules per unit cell.
The crystal consists entirely of isomer 4 (in which the fluorine
is bound to the bridgehead carbon). Temperature
dependent 1D and 2D NMR exchange experiments reveal the occurrence of
two independent dynamic processes,
both preserving the crystal order, but on a completely different time
scale. The faster of the two processes involves
3-fold jumps about the molecular (pseudo) C3
symmetry axis. Line shape analysis of dynamic 1D MAS
spectra
yields an Arrhenius rate equation with a pre-exponential factor,
AJ = 6.0 × 1017
s-1, and an activation energy,
EJ
= 21.7 kcal mol-1. The mechanism of this process
was confirmed by a rotor synchronized 2D exchange
experiment
performed with a mixing time of 20 ms. This spectrum exhibits auto
cross peaks between spinning side bands of
the same types of carbons, but no hetero cross peaks linking different
types of carbons. Two-dimensional exchange
spectra recorded on a much longer time scale (of the order of seconds)
exhibit, in addition, hetero cross peaks
between the main and spinning side bands of different types of carbons.
These cross peaks can only result from
Cope rearrangement involving other isomers of fluorobullvalene as
intermediates. It is argued that the dominant
mechanism of this process involves the sequence: isomer 4 → isomer
1 → isomer 3 → isomer 1 → isomer 4, where
isomers 1 and 3 serve as transient intermediates. Magnetization
transfer experiments provide the following estimates
for the kinetic parameters of this process, AC
= 4.6 × 109 s-1,
EC = 14.5 kcal
mol-1.