Rotational Tunneling and Neutron Spectroscopy:  A Compilation

Prager, M.; Heidemann, A.

doi:10.1021/cr9500848

Cited by 248 publications

(287 citation statements)

References 459 publications

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“…The plots show that the data are fitted well, although it should be emphasised that many other models might fit equally well and some care has to be taken in interpreting the results of such modelling. The activation parameters for the two methyl groups are similar and imply that they are relatively hindered [29]. The estimated tunnelling frequency from a potential barrier this size is in the 100s of kHz range, well below the Larmor frequencies involved, justifying the neglect of tunnelling terms to the relaxation [21,35].…”

Section: Methyl Group Dynamicsmentioning

confidence: 74%

“…Fitting to such expressions involves a number of assumptions. Firstly, that quantum tunnelling effects can be ignored [21,29], which is justified here by the relatively high barriers to motion that are determined below. Secondly, that an expression that is valid for molecules tumbling in isotropic solution is also valid for powder samples under magic-angle spinning [23,30].…”

Section: C Spin-lattice Relaxation Times In Solidsmentioning

confidence: 99%

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NMR characterisation of dynamics in solvates and desolvates of formoterol fumarate

Apperley

Markwell

Frantsuzov

et al. 2013

Phys. Chem. Chem. Phys.

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Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACT Solid-state NMR is used to characterise dynamics in the ethanol solvate of the pharmaceutical material formoterol fumarate and its associated desolvate. Jump rates and activation barriers for dynamic processes such as phenyl ring rotation and methyl group rotational diffusion are derived from 1D-EXSY and 13 C spin-lattice relaxation times respectively. 2 H and 13 C spin-lattice relaxation times measured under magic-angle spinning conditions are used to show that the fumarate ion in the desolvate is undergoing smallamplitude motion on a frequency scale of 100s of MHz at ambient temperature with an activation parameter of about 32 kJ mol -1 . Exact calculations of relaxation times under MAS provide a simple and robust means to test motional models in cases where relaxation rate maxima are observed, including for systems where the crystal structure of the material is unknown.

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Section: Methyl Group Dynamicsmentioning

confidence: 74%

Section: C Spin-lattice Relaxation Times In Solidsmentioning

confidence: 99%

NMR characterisation of dynamics in solvates and desolvates of formoterol fumarate

Apperley

Markwell

Frantsuzov

et al. 2013

Phys. Chem. Chem. Phys.

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“…This is physically reasonable since the site adjacent to the CH 2 D group in I is a sp 2 -hybridized carbon, while the adjacent site in the N-CH 2 D piperidine derivative is a sp 3 -hybridized nitrogen, whose lone pair engages in a hyperconjugation interaction with the deuterated methyl group [29]. Neutron spectroscopy of methyl rotors show that sp 3 hybridization of the neighbouring atom is almost always associated with strong hindering of the methyl rotation, and therefore a small or absent tunneling splitting [38]. It is therefore plausible that the CH 2 D group has much greater rotational freedom in I, as compared to the N-CH 2 D piperidine derivative studied in reference [32].…”

Section: Discussionmentioning

confidence: 99%

Long-lived nuclear spin states in rapidly rotating CH 2 D groups

Elliott

Brown

Dumez

et al. 2016

Journal of Magnetic Resonance

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Although monodeuterated methyl groups support proton long-lived states, hindering of the methyl rotation limits the singlet relaxation time. We demonstrate an experimental case in which the rapid rotation of the CH 2 D group extends the singlet lifetime but does not quench the chemical shift difference between the CH 2 D protons, induced by the chiral environment. Proton singlet order is accessed using Spin-Lock Induced Crossing (SLIC) experiments, showing that the singlet relaxation time T S is over 2 minutes, exceeding the longitudinal relaxation time T 1 by a factor of more than 10. This result shows that proton singlet states may be accessible and long-lived in rapidly rotating CH 2 D groups.

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“…Curiously, no systematic spectroscopic studies of mesitylene (or 1,3,5-trimethylbenzene) derivatives have been undertaken whereas extensive studies of the protons tunneling in methyl groups have been published [8]. For the trichloro-tribromo-and triiodo-mesitylene (TCM, TBM, TIM) species, INS on powder samples [9] and on single crystals [10,11] has been used to establish that the three methyl groups in each molecule have a different tunneling gap, which is contrary to what is expected for a molecule with true C 3h symmetry.…”

Section: -Introductionmentioning

confidence: 99%

“…For the trichloro-tribromo-and triiodo-mesitylene (TCM, TBM, TIM) species, INS on powder samples [9] and on single crystals [10,11] has been used to establish that the three methyl groups in each molecule have a different tunneling gap, which is contrary to what is expected for a molecule with true C 3h symmetry. For TIM, for example, three excitations were observed at 15, 24 and 89 eV [8,9]. This difference in tunneling gaps is due to the difference in the intermolecular interactions exerted on the methyl groups located in non equivalent sites in the triclinic crystal cell.…”

Section: -Introductionmentioning

confidence: 99%

Vibrational spectroscopy and DFT calculations of 1,3-dibromo-2,4,6-trimethylbenzene: Anharmonicity, coupling and methyl group tunneling

Zeroual

Meinnel

Łapiński

et al. 2013

Vibrational Spectroscopy

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Abstract-The Raman, IR and INS spectra of 1,3-dibromo-2,4,6-trimethylbenzene (DBMH) were recorded in the 80-3200 cm -1 range. The molecular conformation and vibrational spectra of DBMH were computed at the MPW1PW91/LANL2DZ level. Except for the methyl 2 environment, the agreement between the DFT calculations and the neutron diffraction structure is almost perfect (deviations < 0.01 Å for bond lengths, < 0.2° for angles). The frequencies of the internal modes of vibration were calculated with the harmonic and anharmonic approximations; the later method yields results that are in remarkable agreement with the spectroscopic data, resulting in a confident assignment of the vibrational bands. Thus, no scaling is necessary. The coupling, in phase or antiphase, of the motions of symmetrical C-Br and C-Me bonds is highlighted. Our DFT calculations suggest that the torsion of methyl groups 4 and 6 is hindered in deep wells, whereas methyl group 2 is a quasi-free rotor. The failure of the calculations to determine the frequencies of the methyl torsional modes is explained as follows: DFT does not consider the methyl spins and assumes localization of the protons, whereas the methyl groups must be treated as quantum rotors.Keywords: Raman spectroscopy; IR spectroscopy; Inelastic neutron scattering; DFT calculations; 1,3-dibromo-2,4,6-trimethylbenzene 1-IntroductionA tremendous amount of work has been devoted to the spectroscopy of polysubstituted benzenes.Among those relative to penta-and hexa-substituted compounds, two extensive papers were published in 1985: one paper compared eleven polymethyl-benzenes (pMeB) [1] and the second described six chloromethyl-benzenes C 6 (CH 3 ,Cl) 6 (ClMeB) [2]. In these studies, the experimental spectra were assigned using normal coordinate calculations to determine a common multi-parameter force field. Because the molecular conformations were not accurately known the authors assumed that the aromatic ring in all molecules was a regular hexagon, that the bond angles were 120°, and the bond lengths were 1.396 Å for the carbon-carbon aromatic bond, 1.509 Å for the methyl carbonaromatic carbon bond, 1.083 Å for the aromatic carbon-hydrogen bond and 1.096 Å for the methylcarbon hydrogen bond. For the pMeB species, the Raman and infra-red measurements were performed with samples in the gaseous phase at approximately 550 K. A 46-parameter valence force field had been proposed, and the average error between the 736 observed and calculated wavenumbers was 6.4 cm -1 . This agreement suggests that the use of an excessive number of off-diagonal constants (interactions terms) allows us to compensate for the large errors done in the geometrical parameters. According to the authors in these previous studies: "there is a great stability of the internal methyl modes, in a limit of ± 5 cm -1 they are located at: 2930 cm -1 for the C-H stretching, 1445 cm -1 for the asymmetric C-H bending, 1380 cm -1 for the C-H symmetric bending, 1000 and 1060 cm -1 for the C-H rocking". For the "torsion mode" of the meth...

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Rotational Tunneling and Neutron Spectroscopy: A Compilation

Cited by 248 publications

References 459 publications

NMR characterisation of dynamics in solvates and desolvates of formoterol fumarate

NMR characterisation of dynamics in solvates and desolvates of formoterol fumarate

Long-lived nuclear spin states in rapidly rotating CH 2 D groups

Vibrational spectroscopy and DFT calculations of 1,3-dibromo-2,4,6-trimethylbenzene: Anharmonicity, coupling and methyl group tunneling

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