Rotational tunneling of methyl groups in pentamethylbenzene studied by NMR relaxation resonance (original) (raw)
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Anomalous proton spin‐lattice relaxation in organic compounds containing methyl groups
Concepts in Magnetic Resonance Part A, 2019
Temperature measurements of proton spin‐lattice relaxation time performed for acetates ((CH3COO)2Ba, (CH3COO)2Cd, and (CH3COO)2Ca) and acetyl halides ((CH3CO)2O, CH3COBr and CH3COCl) are fitted to a Haupt equation. It is impossible to fit the temperature dependence of T1 protons using the BPP equation. Of importance is the assumption that complex C3 molecular motion of methyl protons takes place. An understanding of the correlation functions of complex C3 reorientation allow for the calculation of the relaxation time, T1, and the second moment of the NMR resonance. The spectral densities are calculated applying Woessner theory of complex motion and assuming a tunneling correlation time implemented from solving the Schrödinger equation. The acceptance of the tunneling correlation time resulting from the Schrödinger equation elucidates the reduction in the second moment at 0 K. The fitting leads to an excellent agreement between the experimental results of T1 temperature dependences a...
1H NMR study of internal motions and quantum rotational tunneling in (CH3)4NGeCl3
Magnetic Resonance in Chemistry, 2008
is prepared, characterized and studied using 1 H NMR spin lattice relaxation time and second moment to understand the internal motions and quantum rotational tunneling. Proton second moment is measured at 7 MHz as function of temperature in the range 300-77 K and spin lattice relaxation time (T 1) is measured at two Larmor frequencies, as a function of temperature in the range 270-17 K employing a homemade wide-line/pulsed NMR spectrometers. T 1 data are analyzed in two temperature regions using relevant theoretical models. The relaxation in the higher temperatures (270-115 K) is attributed to the hindered reorientations of symmetric groups (CH 3 and (CH 3) 4 N). Broad asymmetric T 1 minima observed below 115 K down to 17 K are attributed to quantum rotational tunneling of the inequivalent methyl groups.
(2008-MRC) 1H NMR study of internal motions and quantum rotational tunneling in (CH3)4NGeCl3
(CH3)4NGeCl3 is prepared, characterized and studied using 1H NMR spin lattice relaxation time and second moment to understand the internal motions and quantum rotational tunneling. Proton second moment is measured at 7 MHz as function of temperature in the range 300–77 K and spin lattice relaxation time (T1) is measured at two Larmor frequencies, as a function of temperature in the range 270–17 K employing a homemade wide-line/pulsed NMR spectrometers. T1 data are analyzed in two temperature regions using relevant theoretical models. The relaxation in the higher temperatures (270–115 K) is attributed to the hindered reorientations of symmetric groups (CH3 and (CH3)4N). Broad asymmetric T1 minima observed below 115 K down to 17 K are attributed to quantum rotational tunneling of the inequivalent methyl groups. Copyright © 2007 John Wiley & Sons, Ltd.
Methyl tunnelling rotation in the n-alkanes
Chemical Physics Letters, 1987
The tunnelling rotation frequencies of the terminal methyl groups in thirteen n-alkanes have been measured at 4.2 K using the technique of dipole-dipole driven nuclear magnetic resonance. In all the even-numbered alkanes measured, the frequency is 300 k 4 kHz. In the odd-numbered alkanes, with more than nine carbon atoms, it is 340+ 4 kHz. Heptane and nonane have two kinds of methyl groups with tunnel frequencies in the ranges of 300 k 4 kHz and I40+ 6 kHz. The dominant source of the hindering barrier is intramolecular in origin and the variations reflect small intermolecular contributions dependent on the crystal structure.
Protons Relaxation and Temperature Dependence Due To Tunneling Methyl Group
Tunneling frequency and temperature dependence of proton spin lattice relaxation time T1, are depend upon the height and the shape of the hindering barrier of methyl rotation and carry information on the group is molecular environment are reported for some samples containing tertiary-butyl group.The temperature rang was 4-300k.Data has been analyzed to provide estimates for the magnitude of the three fold potential barrier to reorientation of all methyl groups in these materials. At low temperature the motion of the tertiary-butyl protons can usually be neglected. All protons of the samples relax as a single system.In one or two cases tunneling is observed for the first time in Tert-butyl. The T1 results are used to evaluate tunnel frequency in other cases. The result suggest the importance of collective motion of methyl group in tert-butyl
Rotational tunneling of methyl groups in a strong magnetic field: a path-integral approach
Canadian Journal of Physics, 1989
The magnetic-flux splitting of the E levels of the methyl group in a threefold potential is analyzed using path-integral techniques. The results are in agreement with the conclusions obtained previously using the traditional methods of wave mechanics. The influence of the magnetic-flux splitting on the spin-lattice relaxation in single crystal samples is also discussed.
Journal of Magnetic Resonance (1969), 1981
Carbon-13 spin-lattice relaxation rates were determined for 1,3,5-triphenylbenzene in CDC13 over a temperature range of WC, and the results interpreted in terms of the molecular dynamics. Dynamic models appropriate to the problem, including the effects of anisotropic motion and internal jumping between two or four states, were derived and the results evaluated comparatively for several different models. Similarities in the activation energies based on temperature-dependent measurements for all carbons suggests that either fortuitously similar barriers characterize both overall and internal motion, or that all relaxation rates depend primarily on a single type of motion, with the individual T, differences reflecting geometric factors. The latter interpretation leads to a rapid internal jump motion with a range of-6O", as would be predicted on the basis of theoretical conformational energy calculations. Application of a four-state jump model in which the jump rates are assumed to depend on the 0 and 90" theoretically determined rotational barriers allow an estimation of the rates of internal motion consistent with the relaxation data. The physical basis for complex terms which occur in the bistable (and tristable) jump models is discussed.
Journal of the Chemical Society, Faraday Trans, 1983
It is shown that in certain situations 13C n.m.r. spin-lattice relaxation time and n.0.e. studies may be used to separate 'H relaxation times into intra-and inter-molecular components. The approach, which applies only when the carbon and hydrogen atoms are considered subject to the same overall molecular motion, does not depend on the theoretical evaluation of any parameters intrinsically involved in the characterization of the relaxation times. However, it is shown that simple geometrical calculations can facilitate the evaluation of equivalent rotational correlation times from both 13C and 'H intramolecular relaxation rates and that the constituents of the latter can be readily isolated. The overall approach is illustrated by studies of the aryl carbon and hydrogen nuclei of 1,3,5-trimethylbenzene dissolved in carbon tetrachloride and in mixtures of cyclohexane and tetramethyl silane. Nuclear spin-lattice relaxation times, T1, are known' to be composed of several contributions according to T;' = T;,~D-k T ; ,~R-k T;,&a-k TT,bc (1) where the respective contributions arise from dipole-dipole, spin rotation, chemical shift anisotropy and scalar coupling mechanisms. The four terms individually contribute to varying extents to the relaxation of different nuclides. For the most commonly studied 'H and I3C nuclei T1,DD is often the dominant contribution to the observed relaxation time. 1*2 Despite extensive efforts to describe Ti,DD theoretically it remains difficult to predict this term with precision for other than the most simple systems. One underlying reason for this is that T1 may have both intra-and inter-molecular contributions according to and it often proves difficult t o isolate the intra-and inter-molecular components necessary to test the relevant theories. Fundamental investigations of have often been facilitated by studies of 'H when the intra-and inter-molecular terms can be separated by two procedures. The first involves dilution studies using the perdeuterated analogue of the subject compound so that is obtained from the extrapolated relaxation time at infinite d i l~t i o n .~ This method depends on the availability and also the cost of the deuterated compound. The second procedure is to conduct dilution studies analogous to the first approach but with diluents such as C C 4 or CS; that provide negligible intermolecular dipolar contributions to the T1 of the subject compound. The disadvantage of this procedure is that the value of Tl,intra that results is 102 1
Physical Review Letters, 1992
Spin correlation effects are often observed in inelastic neutron scattering and nuclear magnetic resonance (NMR) when tunneling is important. ln studies of proton-proton exchange couplings observed in the 'H NMR of transition-metal polyhydrides, we have had to deal with exchange degeneracies in large collections of identical spins. This Letter presents a method that easily handles such problems with low symmetry, eliminates ad hoc assumptions with respect to the feasibility of any particular permutation of particles, and is readily implemented in computer calculations.
NMR study of rotational tunneling in the partially deuterated methanes
Physical review, 1983
We have observed the deuteron NMR absorption line at 46 MHz in phase III of the partially deuterated methanes in the temperature range 40 mK to 4 K. For CHD3 the line shape depends strongly on the temperature up to 700 mK. A model is developed for the tunneling states of CHD3 which reproduces both the derivative NMR line shape and the temperature dependence of the line area. We find that the line shape is determined by the interaction of the deuteron quadrupole moment with the gradient of the electric field of the molecular carbon-deuteron bond. The coupling constant for this interaction, e qg/h, is measured to be 158 8 kHZ, and the broadening of the NMR line is found to be consistent with the strength of the intramolecular dipole-dipole interactions. The data imply an absence of spin-species conversion in CHD3 on a time scale of 10 h. Our tunneling-level model for CHD3 is also compared with thermodynamic measurements. For CH2Dz in the temperature range 40 to 60 mK, the NMR line is compared with the predictions of an energy-level scheme deduced from neutron scattering and thermodynamic measurements. We conclude that the tunneling of the deuterons around the C3 symmetry axis in CHD3, and around the C& symmetry axis in CH2Dq, proceeds at a rate fast compared with e qg/h'. No absorption lines of satisfactory signalto-noise ratio were observed for CH3D.