Methoxy and Methyl Group Rotation: Solid‐State NMR <sup>1</sup>H Spin‐Lattice Relaxation, Electronic Structure Calculations, X‐ray Diffractometry, and Scanning Electron Microscopy

Beckmann, Peter A.; Mallory, Clelia W.; Mallory, Frank B.; Rheingold, Arnold L.; Wang, Xianlong

doi:10.1002/cphc.201402716

Cited by 15 publications

(32 citation statements)

References 43 publications

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“…Loose material was randomly sprinkled on carbon tape, thus achieving a variety of orientations for the particles (which may or may not be crystallites). Many crystallites of organic solids are comprised of flakes with one dimension much smaller than the other two [2,7] so this procedure potentially allows for a determination of the smallest crystallite dimension because some crystallites will be imbedded in the carbon tape in an edge-on orientation. The electron beam energy was 5 keV and the images were taken under 0.38 Torr air pressure.…”

Section: Field Emission Scanning Electron Microscopymentioning

confidence: 99%

“…Examples of clusters in other compounds with methyl groups are shown elsewhere [2,16]. These cluster models simulate the environment of the methyl group in the crystal.…”

Section: Electronic Structure Calculationsmentioning

confidence: 99%

“…Also, for this calculation, the ring atoms in all other molecules in the clusters for 1 and the five C atoms on the backbone for clusters of 2 were frozen at their X-ray determined positions in the crystal while all the other atoms in these molecules were allowed to relax. The basis set superposition error [5] was not corrected in the clusters since previous studies have shown that the basis set superposition error has little impact on the rotational barriers [2] while the computational cost is significant. All the calculations in the clusters were performed at the B3LYP/6-31G(d) level with the Grimme's D3BJ empirical correction for the London dispersion [19,20].…”

Section: Electronic Structure Calculationsmentioning

confidence: 99%

“…Solid state 1 H nuclear magnetic resonance (NMR) spin-lattice relaxation experiments [1] can be used to explore the dynamical properties of methyl (CH 3 ) groups in solids and provide information concerning interactions at the atomic, molecular, and "several molecule" (clusters of molecules) levels [2]. In these solid samples, methyl group rotation is the only motion occurring on the NMR time scale (approximately 10 -10 -10 -5 s for our experiments).…”

Section: Introductionmentioning

confidence: 99%

“…We report a variety of experiments and calculations and their interpretations regarding methyl group (CH 3 ) rotation in samples of pure 3-methylglutaric anhydride (1), pure 3-methylglutaric acid (2), and samples where the anhydride is slowly absorbing water from the air and converting to the acid [C 6 H 8 O 3 (1) + H 2 O → C 6 H 10 O 4 (2)]. The techniques are solid state 1 H nuclear magnetic resonance (NMR) spin-lattice relaxation, single-crystal X-ray diffraction, electronic structure calculations in both isolated molecules and in clusters of molecules that mimic the crystal structure, field emission scanning electron microscopy, differential scanning calorimetry, and high resolution 1 H NMR spectroscopy.…”

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confidence: 99%

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Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

Beckmann

Bohen

Ford

et al. 2017

Solid State Nuclear Magnetic Resonance

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experiments allow us to observe the temperature dependence of the parameters that characterize methyl group rotation in both compounds and in mixtures of the two compounds. In the mixtures, both types of methyl groups (that is, molecules of 1 and 2) can be observed independently and simultaneously at low temperatures because the solid state 1 H spin-lattice relaxation is appropriately described by a double exponential. We have followed the conversion 1 → 2 over periods of two years. The solid state 1 H spin-lattice relaxation experiments in pure samples of 1 and 2 indicate that there is a distribution of NMR activation energies for methyl group rotation in 1 but not in 2 and we are able to explain this in terms of the particle sizes seen in the field emission scanning electron microscopy images.

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Section: Field Emission Scanning Electron Microscopymentioning

confidence: 99%

“…Examples of clusters in other compounds with methyl groups are shown elsewhere [2,16]. These cluster models simulate the environment of the methyl group in the crystal.…”

Section: Electronic Structure Calculationsmentioning

confidence: 99%

Section: Electronic Structure Calculationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

Beckmann

Bohen

Ford

et al. 2017

Solid State Nuclear Magnetic Resonance

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Proton Spin‐Lattice Relaxation in Organic Molecular Solids: Polymorphism and the Dependence on Sample Preparation

et al. 2018

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We report solid-state nuclear magnetic resonance H spin-lattice relaxation, single-crystal X-ray diffraction, powder X-ray diffraction, field emission scanning electron microscopy, and differential scanning calorimetry in solid samples of 2-ethylanthracene (EA) and 2-ethylanthraquinone (EAQ) that have been physically purified in different ways from the same commercial starting compounds. The solid-state H spin-lattice relaxation is always non-exponential at high temperatures as expected when CH rotation is responsible for the relaxation. The H spin-lattice relaxation experiments are very sensitive to the "several-molecule" (clusters) structure of these van der Waals molecular solids. In the three differently prepared samples of EAQ, the relaxation also becomes very non-exponential at low temperatures. This is very unusual and the decay of the nuclear magnetization can be fitted with both a stretched exponential and a double exponential. This unusual result correlates with the powder X-ray diffractometry results and suggests that the anomalous relaxation is due to crystallites of two (or more) different polymorphs (concomitant polymorphism).

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Concomitant Polymorphism in an Organic Solid: Molecular and Crystal Structure and Intra‐ and Intermolecular Potential Contributions to tert‐Butyl and Methyl Group Rotation

et al. 2019

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We investigate the relationship between structure (crystal and molecular) and tert-butyl and methyl group dynamics in 2-(tertbutyl)-9-(4-(tert-butyl)phenyl)anthracene. Powder and singlecrystal X-ray diffraction, taken together, show that different polycrystalline samples recrystallized from different solvents have different amounts of at least four polymorphs (crystallites having different crystal structures), of which we have identified three by single crystal X-ray diffraction. The molecules in the asymmetric units of the different crystal structures differ by the dihedral angle the tert-butylphenyl group makes with the anthracene moiety. Ab initio electronic structure calculations on the isolated molecule show that very little intramolecular energy is required to change this angle over a range of about 60°which is probably the origin of the concomitant polymorphism (crystals of more than one polymorph in a polycrystalline sample). Solid state 1 H nuclear magnetic resonance (NMR) spin-lattice relaxation experiments support the powder and single-crystal X-ray results and provide average NMR activation energies (closely related to rotational barriers) for the rotation of the tert-butyl groups and their constituent methyl groups. These barriers have both an intramolecular and an intermolecular component. The latter is sensitive to the crystal structure. The intramolecular components of the rotational barriers of the two tert-butyl groups in the isolated molecule are investigated with ab initio electronic structure calculations.[a] Prof. Figure 1. The molecule 2-(tert-butyl)-9-(4-(tert-butyl)phenyl)anthracene. C atoms are black and green, H atoms in methyl groups in the plane of the neighboring aromatic ring are purple, H atoms in the out-of-plane methyl groups are cyan, and aromatic ring H-atoms are red. Of interest is the dihedral angle the 4-tert-butylphenyl group makes with the anthracene moiety defined using the four C atoms in green. For the isolated molecule in the ground state this dihedral angle is 68°(shown) or 112°(see Figure 2) but in the six independent molecules in three of the polymorphs found in the polycrystalline samples, this angle varies between 66 and 106°. See Table 2. The orientation of the two tert-butyl groups shown defines rotation angles of 0°for these groups.

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Methoxy and Methyl Group Rotation: Solid‐State NMR ¹H Spin‐Lattice Relaxation, Electronic Structure Calculations, X‐ray Diffractometry, and Scanning Electron Microscopy

Cited by 15 publications

References 43 publications

Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

Proton Spin‐Lattice Relaxation in Organic Molecular Solids: Polymorphism and the Dependence on Sample Preparation

Concomitant Polymorphism in an Organic Solid: Molecular and Crystal Structure and Intra‐ and Intermolecular Potential Contributions to tert‐Butyl and Methyl Group Rotation

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Methoxy and Methyl Group Rotation: Solid‐State NMR 1H Spin‐Lattice Relaxation, Electronic Structure Calculations, X‐ray Diffractometry, and Scanning Electron Microscopy

Cited by 15 publications

References 43 publications

Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

Proton Spin‐Lattice Relaxation in Organic Molecular Solids: Polymorphism and the Dependence on Sample Preparation

Concomitant Polymorphism in an Organic Solid: Molecular and Crystal Structure and Intra‐ and Intermolecular Potential Contributions to tert‐Butyl and Methyl Group Rotation

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Methoxy and Methyl Group Rotation: Solid‐State NMR ¹H Spin‐Lattice Relaxation, Electronic Structure Calculations, X‐ray Diffractometry, and Scanning Electron Microscopy