Manifestation of electron-nuclear dynamics in 59Co NQR spectra of a series of cobaltocenium derivatives

Abstract

A coupling equation relating the quadrupole coupling constant (e 2 Qq zz ) to the asymmetry parameter (η) of the electric field gradient for a series of cobaltocenium derivatives ( 59 Co NQR) is derived. The estimates of the correlation times of "slow" modulations of electron motions by tunneling processes lie in the range from 10 -12 to 10 -14 s. This corresponds to the interval of the characteristic times of nuclear motions.The formation of a chemical bond between a metal atom and cyclopentadienyl rings in metallocene com pounds has not been studied in detail so far. It is note worthy that conventional methods of investigation in cluding structural ones give no answer to the question about the role of dynamic effects in such systems. At the same time if there are two or more orbitally degener ate electronic levels having the same energy, the nuclear configuration can undergo a spontaneous distortion accompanied by considerable reduction of symmetry (Jahn-Teller effect 1 ). The distortion can be either static or dynamic. 2 Phenomena similar to the Jahn-Teller ef fect may also occur in electronically quasi degenerate systems where one nuclear core configuration corresponds to a number of very closely spaced (but not merged!) electron energy levels (Opic-Pryce effect 3 ), which can lead to the dipole instability that is impossible in the case of true Jahn-Teller effect. 1 The Opic-Pryce effect 3 cor responds to all intermediate situations from electronically degenerate systems to nondegenerate ones provided that the electron spin states remain unchanged. Thus, more than one configurational electron density distribution can exist in a molecular electron nuclear system. Electron tunneling assisted dynamic transitions between these con figurational states give rise to additional splitting of elec tron energy levels (so called tunneling or inversion split ting) and to subsequent onset of additional time depen dences of the electron distribution configurations. De pending on the ratio of the lifetimes of particular configu rational states and on the ratio of the times of complete electron tunneling and the characteristic times of nuclear motions, these differences either may or may not mani fest themselves as changes in the configuration of the mutual arrangement of atomic nuclei in the molecular system. Basic to the model presented in this work is analysis of the changes in the ratio of the characteristic times of nuclear motions and tunneling assisted "slow" modula tion of electron motions.Usually, the characteristic times of electron motions are assumed to be 5-10 orders of magnitude shorter than those of nuclear motions. This is the case of the well known Born-Oppenheimer approximation, 4 which ig nores the quantum coupling between electrons and atomic nuclei owing to large differences between the characteris tic times of corresponding motions (adiabatic approxi mation).However, typical electron tunneling probabilities (transparency coefficients) are of the order of 10 -5 -10 -10 , 5 which causes a significant incr...

Structural Insights from 59Co Solid‐State NMR Experiments on Organocobalt(I) Catalysts

“…From these spectra, both peaks were simulated simultaneously using the QUEST software to obtain highly precise C Q ( 59 Co) and η Q values, which are reported in Table . The C Q ( 59 Co) values in this series of Co I catalysts are similar in magnitude to the values previously reported for Co I ‘sandwich’ compounds, Co(C 2 B 9 H 11 ) 2 − and Co(C 5 H 5 ) 2 + , as well as for cobaltocenium derivatives, which, taken together, suggests that the C Q ( 59 Co) values for Co I compounds tend to be larger than those for Co III compounds…”

“…From these spectra,b oth peaks were simulated simultaneously using the QUEST software [27] to obtain highly precise C Q ( 59 Co) and h Q values, which are reported in Table 3. The C Q ( 59 Co) valuesi nt his series of Co I catalysts are similari nm agnitude to the values previously reported for Co I 'sandwich' compounds, Co(C 2 B 9 H 11 ) 2 À and Co(C 5 H 5 ) 2 + , [19] as well as for cobaltocenium derivatives, [28,29] which, taken together,s uggests that the C Q ( 59 Co) valuesf or Co I compounds tend to be largerthan those for Co III compounds. [14] Since previous studies have reported ad ependence of the 59 Co EFG tensorp arameters on the experimental temperature, [30] we explored, using NQR spectroscopy,t he temperature sensitivity of the C Q ( 59 Co) and h Q values in compound 1.W hile varying the temperature between À10 8Ca nd + 40 8C, the high frequencyN QR peak remained unchanged, whereas the low frequencyp eak shifted to as lightly lower frequency by % 140 kHzo ver the temperature range.…”