Methyl Group Reorientation Research Articles

Molecular dynamics in solid l-adrenaline by proton NMR

Proton NMR measurements of the spectrum, second moment, spin-lattice relaxation time T 1 and dipolar relaxation time T 1D were carried out on polycrystalline l-adrenaline at 14 and 25 MHz between 55 and 400 K. Between 70 K and 250 K relaxation is dominated by C 3 reorientation of the single methyl group in each molecule, characterized by an activation energy 8.3±0.3 kJ/mole. Below 70 K tunnelling assisted relaxation is significant, characterized by an excitation energy of 1.9±0.2 kJ/mole. Above 250 K an additional molecular motion becomes significant, with activation energy above 28 kJ/mole, attributed to conformational motion of the methylene group in the ethylamine side chain.

Structures and molecular dynamics of binary mixtures of n-alkanes: C 46:C 38 and C 46:C 44

The lamellar packing and molecular dynamics of n-alkane binary mixtures C 46:C 38 and C 46:C 44 have been investigated by employing NMR, DSC and X-ray powder diffraction techniques. Different methods of crystallization resulted in different structures for the same binary mixture. Three relaxation mechanisms were present in all samples, viz. threefold reorientation of methyl groups, defect motions of chain-ends and translational-rotational diffusion of chains prior to melting.

Structures and molecular dynamics of solution-grown and melt-grown samples of n-heptatriacontane

The crystal structures and molecular dynamics of solution-grown and melt-grown polycrystalline samples of n-heptatriacontane have been investigated by employing NMR, DSC and x-ray powder diffraction techniques. The solution-grown sample has an orthorhombic structure, while the melt-grown sample is a mixture of a monoclinic and an orthorhombic structure. Three relaxation mechanisms are present in all samples, namely, threefold reorientation of methyl groups, defect motions of chain-ends between all- trans and defect orientations and twofold chain reorientations. In the high-temperature phases an additional relaxation process due to translational - rotational diffusion of chains is present.

Molecular motions in solid saccharides studied by NMR spectroscopy

Analyses of spin-lattice relaxation time and second moment measurements of the polycrystalline saccharides methyl α-D-mannopyranoside, methyl β-D-arabinopyranoside, methyl β-L-arabinopyranoside and β-D-allose have revealed, apart from methyl group reorientations in the methyl pyranosides, the jump motion of a hydroxyl proton in a hydrogen bond in β-D-allose and trans-gauche reorientations of CH 2OH groups in methyl α-D-mannopyranoside and β-D-allose.

Molecular motions and phase transitions in solid tris(n‐propylammonium) hexabromobismuthate (III)

AbstractThe proton NMR second moment and spin‐lattice relaxation time for polycrystalline tris(n‐propylammonium) hexabromobismuthate (n‐C3H7NH3)3BiBr6 (nPBB) are measured from 70 to 290 K. The results are interpreted in terms of the methyl and ammonium group reorientations for which the activation parameters are determined. In addition linear thermal expansion measurements are carried out in the temperature range 270 to 295K. A distinct anomaly in the thermal dilation occurs at 281 K indicating a first‐order phase transition.

1H solid-state nuclear magnetic resonance study of the mobility of the tetrapropylammonium template in a purely siliceous MFI-type zeolite

Dynamics of tetrapropylammonium (TPA) cation occluded during the synthesis in a siliceous MFI zeolite is investigated by 1H broad-line nuclear magnetic resonance (NMR) methods. Second moments M 2, spin-lattice relaxation times T 1 and T 1 ϱ are measured in a large temperature domain. To allow comparison, similar measurements are also reported in bulk tetrapropylammonium bromide (TPABr). Whereas methyl reorientation at low temperature and tumbling of the cation in the plastic phase are observed in crystalline TPABr in accordance with published studies, a new slower motion which could not be identified is observed below the phase transition. Such a motion is much more clearly shown by the existence of a minimum of T 1 ϱ in a quenched sample. Our measurements in the zeolite demonstrate that the TPA template exhibits a larger and more complex mobility below 378 K. Beyond the fast methyl reorientation, the results disclose a motion which probably involves the entire propyl arms inside the channels. So the zeolite framework seems to make such a kind of motion easier. On the contrary, even at 450 K, the highest temperature investigated, the tumbling which would necessitate exchange of the propyl arms between the channels is not observed. A slow motion, responsible for a decrease of T 1 ϱ above 350 K, could not be identified. While a simple correlation time is sufficient to describe the relaxation time dependences in TPABr, a distribution (such as Williams-Watts) is required to account for those in the zeolite.

Molecular motions in solid estradiol studied by nuclear magnetic resonance spectroscopy

The temperature dependence of the proton nuclear magnetic resonance (NMR) relaxation time T 1 and second moment M 2 of polycrystalline β-estradiol hemihydrate (1,3,5-estratriene-3,17β-diol) is measured at frequencies of 14 and 25 MHz. Below 260 K relaxation is found to be dominated by C 3 reorientation of the single methyl group in each molecule, characterized by an activation energy of 9.4 ± 0.1 kJ mol . Above 260 K another relaxation mechanism becomes evident, characterized by an activation energy of 22 ± 2 kJ mol and ascribed to motion of the water molecules in the solid.

Nuclear Magnetic Resonance Investigation of Intramolecular Motions in Methylpyranosides

AbstractProton spin‐lattice relaxation times T1 and T1 as well as the proton second moment have been measured for polycrystalline methyl α‐D‐glucoside, methyl β‐D‐glucoside and methyl β‐D‐galactoside over a wide temperature range. Analyses of the experimental data have yielded information about intramolecular reorientations in the solid state. C3 reorientations of methyl groups take place in all the samples. Potential energy calculations have shown that the CH2OH group can execute trans‐gauche jumps between two potential minima separated by 120. Analyses of the T1 and second moment data have confirmed the presence of this conformational motion. A weak relaxation mechanism, related to the dynamics of an OH group, has been revealed in methyl β‐D‐galactoside.

The pressure dependence of molecular dynamics measured by NMR

NMR techniques have been developed to study methyl group reorientation and hydrogen bond dynamics as a function of hydrostatic pressure. This paper briefly reviews some earlier work and presents new results of the pressure dependence of the reorientation of two distinct methyl groups in methyl ethyl ketone.

Proton NMR study of molecular dynamics and phase transitions in dimethylammonium hexachloro plumbate [NH2(CH3)2]2PbCl6

Abstract Proton spin lattice relaxation time (T1) measured as function of temperature in the range 409–77 K shows phase transitions at 313K, 262K, 206K and 167K in dimethylammonium (DMA) hexachloro plumbate. T1 behaviour in the different phases is explained in terms of DMA dynamics including spin rotation, DMA flip motion, torsional oscillations and reorientation of inequivalent methyl groups. The computed second-moment (M2) variation with temperature is explained in terms of the molecular dynamics of the DMA ion and the methyl groups.

Molecular Motion in Solid Tetraethyl-and Tetrabutylammonium Perchlorates

Abstract The proton NMR second moment and spin-lattice relaxation time for polycrystalline tetraethyl-and tetrabutylammonium perchlorates have been measured over a wide range of temperatures. Solid-solid phase transitions related to the onset of cation tumbling were found for both compounds and confirmed by DSC. In the low temperature phases methyl-group reorientation was evidenced. For tetrabutylammonium cation a dynamic nonequivalence of one methyl group is found. The geared motion of the alkyl chains related with the onset of successive CH2-group reorientations is suggested.

Proton NMR study of molecular motion in solid cortisone

Polycrystalline cortisone (17,21-dihydroxy-4-pregnene-3,11,20-trione, C 21H 28O 5) has been investigated by proton NMR methods between 56 and 400 K at 14 and 25 MHz. Reductions in second moment and two very well-resolved minima in the spin-lattice relaxation time at both frequencies are attributed to reorientation of the two methyl groups on carbons 18 and 19. The data are very well fitted over the entire temperature range to the Kubo-Tomita dipolar relaxation theory using the same parameters at both frequencies. Activation energies E a characterizing the hindrances to the two methyl reorientations were 5.9 and 15.5 kJ/mol, an unusually large difference. The relaxation constants were 6.4 and 7.9 × 10 8 s −2.

Molecular motions and phase changes in the perovskite-type compound (C10H21NH3)2ZnCl4

A virgin sample of polycrystalline (C10H21NH3)2ZnCl4 shows two phase transitions, at 284 K and 363 K. In the low-temperature phase threefold reorientations of methyl and NH3 groups dominate the nuclear magnetic resonance relaxation results. The motion of the NH3 groups can be described as a three-fold jump among three potential minima of which one is shallower than the other two. In the intermediate temperature phase a defect motion of chain ends has been identified. In the high-temperature phase the spacing between adjacent layers of ZnCl4 tetrahedra is increased by ∼ 16% due to the non-intercalation of the chains. In this phase the chains experience a high degree of mobility, which is partly due to the diffusion of chain defects along the chain. On cooling the sample from the high-temperature phase, it transforms to the intermediate phase, but the transition to the low-temperature phase is absent.

Electronic structure, methyl group reorientation and reactions of radical cations of 1,2,4-trimethylcyclohexanes: an EPR study

Two isomeric radical cations of 1,2,4-Me3-cyclohexane, r-1,t-2, t-4-Me3-c-C6+ and r-1, t-2, c-4-Me3-c-C6+, have been studied at cryogenic temperatures adopting the halocarbon matrix technique. Both isomer cations show the same EPR hyperfine (hf) pattern with a triplet of triplets with a1H= 60 G (2 H) and a2H= 32 G (2 H) at 4.2 K. The hf parameters coincide with those of the cis and trans isomers of the 1,2-Me2-c-C6 cation, implying a similar electronic ground state structure in which the main part of the unpaired electron resides in the C-1–C-2–σ-bond. The EPR spectra show a reversible temperature change between ca. 77 and 180 K. The temperature-dependent spectra can be explained by a dynamic effect in which the methyl groups on C-1 and C-2 rotate. Adopting a three-site jump model the rate constants were evaluated to be ca. 5.0 × 106 s–1(100 K) to ca. 5.0 × 107 s–1(180 K) with an activation energy of 1.1 kcal mol.–1 This low activation energy is compared to methyl rotation in radical cations of propane+ and butane+, and is discussed in terms of the unpaired electron distribution within the framework of carbon atoms. Furthermore, it is shown that secondary and tertiary alkyl radicals are predominantly formed by thermal reactions of r-1, t-2, t-4-Me3-c-C6+ and r-1, t-2, c-4-Me3-c-C6+, respectively, in the CF3–c-C6F11 matrix.

Molecular Motion in Solid Tetrapropylammonium Bromide and Iodide

Abstract The proton NMR second moment and spin-lattice relaxation time for tetrapropylammonium bromide and iodide have been measured over a wide temperature range. A solid-solid phase transition related to the onset of cation tumbling was found for both salts and confirmed by DTA. In the low temperature phases methyl group reorientation was evidenced. For iodide a dynamic nonequivalence of the methyl groups and the onset of ethyl groups motion was also discovered

Temperature dependence of methyl reorientation at intermediate hindering potentials.

Spin conversion and spin-lattice relaxation measurements are performed on four methyl compounds, chosen so as to have almost equally high hindering potentials for the methyl reorientation. The relaxation data are analyzed to give two characteristic parameters of this motion: ${\mathrm{\ensuremath{\omega}}}_{\mathit{t}}$, the frequency of the coherent tunneling, and ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}}$, the correlation time of the thermally assisted methyl reorientation. The temperature dependence of both parameters is discussed and a comparison is made for the four samples studied.

Defect chain motions in the low temperature phase of multiple mixtures of n-alkanes by means of nuclear magnetic resonance spin–lattice relaxation time measurements

Nuclear magnetic resonance spin–lattice relaxation time measurements in the laboratory and rotating frames, supported by differential scanning calorimetry and x-ray diffraction measurements, are reported for a series of multiple mixtures consisting of Gaussian chain length distributions of n-alkanes. The results revealed three types of molecular motions, viz., threefold reorientations of methyl groups, gauche defect motions of the chain ends of the long chains and twofold screw motions of the short chains in these mixtures. The main factor influencing the latter two motions is the width of the chain length distribution.

Proton relaxation NMR study of polycrystalline progesterone

Polycrystalline progesterone (4-pregnene-3,20-dione, C 21H 30O 2) has been investigated by proton NMR methods between 80 and 350 K. A reduction in dipolar second moment is ascribed to methyl group reorientation. Minima in the spin-lattice relaxation time found in measurements at five frequencies from 7 to 200 MHz are attributed to reorientation of two of the three methyl groups in each molecule, characterized by activation energy E a = 10.9 ± 0.8 kJ/mol and τ 0 = (2.3 ± 0.2) × 10 −13, s. Additional relaxation at lower temperatures is attributed to reorientation of the third methyl group with E a approximately 3.4 kJ/mol. Measurements were also made of relaxation in the rotating frame.

A relaxation time study of molecular motions in the glass and crystalline phases of 1,6-hexanediol dimethacrylate

Proton relaxation time measurements ( T 1 and T 1ρ) have been done on 1,6-hexanediol dimethacrylate (HDDMA) as a function of temperature between 77 and 296 K. It has been found that HDDMA can exist in either a glass, crystalline, or mixed glass-crystalline phase, depending on the sample temperature and thermal history. The low-temperature motion in the glass and crystalline phases is characterized by methyl group reorientation. An excellent fit to the low-temperature T 1 data can be made using a Cole-Davidson (CD) distribution of cutoff correlation times. A broader distribution is observed in the glass phase than in the crystalline phase. In the melt and in the glass phase at high temperatures, whole-molecule reorientation is observed. The T 1 data in this range can be fit using a single Arrhenius correlation time. However, in order to simultaneously fit both the T 1 and T 1ρ data, a CD distribution is required with a cutoff correlation time described by a temperature-dependent activation energy. Additional unresolved motions are observed in the crystalline phase at high temperatures.

NMR study of solid lactic acid (2-hydroxypropanoic acid)

Polycrystalline l-lactic acid ((S)-2-hydroxypropanoic acid) has been investigated by pulsed proton magnetic resonance methods between 77 K and the melting point, 329 K, at 14 and 25 MHz. A reduction of dipolar second moment is ascribed to methyl group reorientation. This motion is also responsible for spin-lattice relaxation; its correlation time is characterized by an activation energy 12·2 ± 1 kJ mol-1 and time factor (3·5 ± 1) × 10-13 s. Below 100 K quantum-mechanical tunnelling contributes to reduction of the second moment and to spin-lattice relaxation, characterized by an excitation energy 6·2 ± 1 kJ mol-1.