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NMR studies of the reorientational motions of cyclopentadienyl ligands in solid cis ‐ and trans ‐[(η 5 ‐C 5 H 5 ) 2 Fe 2 (CO) 2 (μ 2 ‐CO) 2 ]
Author(s) -
Aime Silvio,
Botta Mauro,
Gobetto Roberto,
Orlandi Alessandra
Publication year - 1990
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260281311
Subject(s) - cyclopentadienyl complex , chemistry , crystallography , activation energy , relaxation (psychology) , spin–lattice relaxation , dipole , stereochemistry , anisotropy , proton , organic chemistry , catalysis , psychology , social psychology , physics , quantum mechanics
Abstract Chemical shift anisotropies and spin‐lattice relaxation times, T 1 , were determined for bridging and terminal carbonyls in c's and trans isomers of [(η 5 ‐C 5 H 5 ) 2 Fe 2 (CO) 2 (μ 2 ‐CO) 2 ]. The relaxation path of the 13 CO resonances occurs through the dipolar interaction with the cyclopentadienyl protons modulated by the reorientational motion of the C 5 H 5 (Cp) rings around their five‐fold axes. Wide‐line proton spin‐lattice relaxation times, T 1 , at variable temperature allow the evaluation of the activation energies associated with the internal rotation of the cyclopentadienyl rings. In the cis isomer, the two crystallographically non‐equivalent Cp ligands rotate at different rates ( E a = 7.2 and 15.8 kJ mol −1 , respectively). Carbon‐13 spin‐lattice relaxation times in the rotating frame of the cyclopentadienyl carbons of the cis isomer support this view. Finally, good agreement is found between our work, the results obtained from crystallographic data and theoretical calculations on the energy barriers for the rotation of the Cp rings.