Methyl Group Reorientation Research Articles

1H‐NMR Investigation of Methyl Group Reorientation and Cation Tumbling in Polycrystalline [X(CH3)4]I (X=N, P, As, Sb)

AbstractThe spin‐lattice relaxation time T1 of the protons in the compounds [X(CH3)4]I, (X = N, P, As, Sb), was investigated in the temperature range 100 K ≤ T ≤ 460 K. The frequency used was 34 MHz. The experiments show that the reorientational motion of the CH3‐groups at low temperatures and the reorientation of the whole cation at high temperatures are responsible for the relaxation mechanism. The relaxation behavior of the proton spin system is purely exponential in the temperature range investigated. The application of the Waugh‐Fedin‐Model to the 1H‐NMR results and the results of FIR and Raman spectroscopy show that at high temperatures a 180° reorientation step of the cation is probable.

Proton magnetic resonance investigation of solid polyamino acids

A proton magnetic resonance and relaxation investigation has been carried out on solid poly-L-alanine, poly-L-leucine, poly-L-valine and polyglycine, between 110 and 350 K. For the first three the relaxation is attributed to methyl group reorientation characterized by activation energies of 7 to 9 kJ mole −1. Significant non-exponential recovery of magnetization was encountered and ascribed to correlated motion of three-spin systems. Relaxation in polyglycine was two orders of magnitude longer.

Proton spin relaxation of butenes and butane in CaNaA zeolites

Using nmr pulse techniques the temperature dependence of proton spin relaxation times T 1 and T 2 of n-butene and butane molecules adsorbed on CaNaA zeolites with different content of Ca ++ ions has been investigated. The observed diminution of correlation times with rising degree of Ca ++ exchange can only be explained if translational motions, i.e. jumps between the supercages, dominate the proton spin relaxation of the adsorbates. Peculiarities in the microdynamical behaviour of the various n-butenes are in accordance with the model of electrostatic interactions between molecular dipole moments and electric fields in A zeolites as proposed by Tempère and Imelik in order to explain the stereoselectivity of the isomerization of n-butenes in A zeolites. Applying Torrey's well-known theory of nmr relaxation as dominated by translational jumps with arbitrary lengths, the mean time between two subsequent jumps has been estimated. In combination with measurements of self-diffusion coefficients by Kärger and Renner, these values lead to mean jump lengths which are reasonable compared with the distance of two neighbouring large zeolitic cavities. In order to interpret the methyl reorientation which dominates the longitudinal proton spin relaxation of the adsorbed hydrocarbons at lower temperatures, a model for calculating the intermolecular contribution to the relaxation rate has been discussed.

Methyl rotation in NMP-TCNQ

Comparisons of continuous wave NMR in NMP-TCNQ and the analog in which the methyl group is deuterated are used to show that methyl reorientation occurs in NMP-TCNQ at all temperatures ( T > 1.5°K). Methyl reorientation is suggested to contribute to the nuclear spin-lattice relaxation, and its effects on the specific heat are discussed.

Proton NMR Study of Molecular Motions in the Solid Phase of a Smectic A Liquid Crystal

Proton T1, T1ρ, and T1D were studied in the solid phase of diethylazoxybenzoate from 105 K to the melting point. The activation energies for the threefold reorientation of methyl groups and for the molecular self-diffusion were determined to be about 3.3 and 11 kcal/mole respectively. Between 170 and 240 K, a new slow molecular motion with an estimated activation energy of 3.7 kcal/mole was detected via the T1ρ data.

Molecular motion in polyisobutylene and poly(propylene oxide) studied by 13C nuclear magnetic relaxation

The 13C spin-lattice relaxation times of low molecular weight (up to 2500) samples of polyisobutylene and poly(propylene oxide) have been measured as a function of molecular weight, temperature and concentration in chloroform solution. For both polymers there is little dependence on molecular weight indicating a flexible conformation in the liquid state, but the relaxation time increases with increasing dilution in CHCl 3. The motion of the polymer backbone therefore depends on the microviscosity of the solution, rather than the bulk viscosity. In polyisobutylene the methyl re-orientation rate increases in parallel with the backbone re-orientation rate, showing that the two motions are interlinked. In poly(propylene oxide) the methyl re-orientation rate is independent of the backbone motion.

Spin–lattice relaxation in cross relaxed systems: Anisole

Spin−lattice relaxation has been monitored as a function of temperature in liquid anisole and its partially deuterated analogues C6D5OCH3 and C6H5OCD3, as pure liquids and in solution in C6D5OCD3. Conventional pulse and Fourier transform techniques have been used. The high resolution spectrum of C6H5OCH3 was found to contain three groups of lines, one associated with the methyl protons, one associated with the meta phenyl protons, and one associated with the ortho and para phenyl protons. The two groups of aromatic lines were found to have, within experimental error, identical spin−lattice relaxation times and activation energies. The contribution of intermolecular interactions to the relaxation mechanism was found to decrease with increasing temperature and was ? 36% for T ? 312°K. A method is discussed for determining the cross relaxation rate between the differently relaxing methyl and phenyl spins. This method utilizes average T1 values for C6H5OCH3, C6D5OCH3, and C6H5OCD3 and yields for anisole a positive cross relaxation rate as a function of temperature. The cross relaxation is found to proceed primarily by intramolecular interactions which cause methyl and phenyl spins to flip in the same sense. Correlation times for over−all molecular tumbling are calculated and compared to results from dielectric relaxation experiments. The comparison indicates that the over−all molecular tumbling may be near to the limit of large diffusive steps discussed by Ivanov [Sov. Phys.−JETP 18, 1041 (1964)]. A correlation time for the threefold methyl reorientation is also calculated. Activation energies were found for the species C6H5OCH3, C6D5OCH3, and C6H5OCD3 to be 2.77±0.08, 2.78±0.23, and 3.32±0.26 kcal/mole, respectively.

Molecular reorientation and self-diffusion in the plastic solid hexamethyldisilane studied using nuclear magnetic resonance and radiotracer techniques

Proton nuclear magnetic relaxation times, T1, T1ρ and T2, linewidth and radiotracer self-diffusion coefficients have been measured as a function of temperature in high purity samples of hexamethyl-disilane. The activation enthalpies for methyl group reorientation and molecular reorientation about the Si—Si axis in the low temperature phase and molecular tumbling in the high temperature plastic phase were determined from the T1 and T1ρ measurements. These values are in general agreement with the results of a similar, but less extensive, n.m.r. study made by Albert et al. The Torrey theory was found to give the most satisfactory analysis of the translational self-diffusion contribution to the relaxation times in the plastic phase. The diffusion coefficients obtained yielded curved Arrhenius plots and a single activation enthalpy does not describe this motion. Similar curvature was not found in the radiotracer measurements. However, these were made over a smaller temperature range. The difference between the radiotracer and n.m.r. measurements is slightly higher than expected from a consideration of correlation effects and this could be due to self-diffusion in hexamethyldisilane proceeding predominantly by a mechanism which involves relaxed vacancies. The results are compared with those for other plastic crystals where pulsed n.m.r. and radiotracer data are available.

NMR Investigation of1HSpin-Lattice Relaxation and Methyl-Group Reorientation in Polycrystalline [X(CH3)4]I, X=N, P, As, Sb

NMR Investigation of¹<i>H</i>Spin-Lattice Relaxation and Methyl-Group Reorientation in Polycrystalline [<i>X</i>(<i>CH</i>₃)₄]<i>I, X</i>=<i>N, P, As, Sb</i>

Deuteron Relaxation Study of the Reorienting Methyl Groups in p-Azoxyanisole

The deuteron spin-lattice relaxation times were measured in the nematic and isotropic phases of p-azoxyanisole(CD3)2. An apparent activation energy for the methyl group reorientation in the isotropic phase is obtained. The quadrupole coupling constant along the C—D bond is found to be 147 ± 8 kHz.

A Proton Spin–Lattice Relaxation Study of Molecular Motion in Several Addition Complexes of Trimethylamine and Trimethylphosphine

The spin–lattice relaxation times of the solid complexes of trimethylamine with I2, ICl, Br2, BCl3, and BBr3 and the relaxation null times of trimethylphosphine – boron trichloride and – boron tribromide have been measured over a range of temperature by pulsed proton magnetic resonance. A minimum in T1 corresponding to reorientation of the methyl groups about the C—N bonds is observed in each complex, although in the case of Me3, NBr2 the minimum corresponds to a combination of methyl group reorientation and reorientation of the whole Me3N moiety. The complexes with the boron trihalides show a second minimum in T1. These minima have been analyzed in terms of either of two possible motions, reorientation of NMe3 about its threefold axis, or that same motion combined with isotropic molecular tumbling. We favor the interpretation involving the simpler motion. Activation energies have been measured for all the motions. The barriers to reorientation of methyl groups about the C—N bond in the moiety Me3N appear to increase as the thermal stability of the complex decreases.

Nonexponential nuclear spin-lattice relaxation in polycrystalline dimethyl sulfone

Spin-lattice relaxation has been monitored via pulse techniques in polycrystalline dimethyl sulfone (DMS) and a 1:1 polycrystalline solution of DMS in the perdeutero analogue. The temperature range covered for DMS was between −90.1 and +75.8°C, while for the 1:1 solution it was between −111.4 and +72.8°C. The relaxation was found to be consistently nonexponential, with exponential relaxation being approached at the lowest temperature. The extent of the nonexponentiality is intermediate between that expected from the effects of cross correlations and that expected from a simple averaging of the exponential behavior of randomly oriented single crystals. A parameter δ=τnull/Tlav is introduced, which may be used to judge the extent to which cross correlations are increasing the observed nonexponentiality beyond that expected from a simple averaging of the exponential behavior of randomly oriented single crystals. Further, it is suggested, in situations where cross correlations do not completely determine the nonexponentiality, that the initial slopes of the decay plots be used to determine correlation times for methyl reorientation since these initial slopes are independent of cross correlation effects. Correlation times for methyl reorientation in DMS have been determined and yield an activation energy of 4.22±0.07 kcal/mole.

NMR studies of molecular motions in solid tetramethylammonium bromide

The molecular motions of tetramethylammonium bromide (TMA-Br) in the solid phase were investigated by means of the temperature dependence of the proton magnetic resonance line width and of the spin lattice relaxation time at several frequencies (6, 8, 16, 30, and 59 MHz). The results indicate the presence of two motions: (1) methyl reorientation about its threefold axis ( T> 150 K) and (2) isotropic tumbling of the cation ( T > 230 K). Activation energies and frequency factors of the two motions were evaluated. The double minima, observed in the temperature variation of T 1, were attributed to the above motions. The T 1 data for the various frequencies in the whole measured temperature region were analyzed using Woessner's treatment improved by the inclusion of intermethyl interactions.

Nuclear magnetic resonance absorption and relaxation study of tert-butylamine and tert-butylamine clathrate deuterate

Proton magnetic resonance absortion (at 16 MHZ) and spin-lattice relaxation time (at 26.46 MHz) measurements have been carried out for pure tertiary-butylamine, and tertiary-butylamine clathrate deuterate from 77°K to the melting points of these compunds. The absorption line measurements on t-butylamine show that the second moment changes from values expected for a rigid lattice to a plateau value consistent with the three methyl groups and the t-butyl group rotating at temperatures near 150°K. In the case of the deuterate, t-butylamine exhibited much more freedom of motion as judged from the second moment measurements. The motional behavior ranges from methyl and t-butyl group rotation to isotropic rotation of the whole molecules. The proton spin-lattice relaxation measurements on t-butylamine exhibited a broad minimum of 16 msec, which is thought to be mainly due to methyl group reorientation. An activation energy of &amp;lt;(3.2± 0.1) kcal/mole was calculated from T1 measurements at various temperatures and this is associated with the barrier hindering methyl group reorientation. The clathrate deuterate of this amine showed a very broad minimum of (32± 1) msec which is thought to arise from three types of motion; namely, methyl and t-butyl group rotation as well as isotropic rotation of the whole molecule. The limiting activation energies in the deuterate of (1.73± 0.03) kcal mole−1 and (2.5± 0.02) kcal mole−1 and therefore expected to be determined by methyl and t-butyl rotation and by t-butyl and isotropic rotation, respectively.

A nuclear magnetic resonance study of molecular motion in solid diethylamine and in diethylamine clathrate deuterate

The proton magnetic resonance second moment and spin-lattice relaxation data are reported for the two solids namely pure diethylamine and diethylamine clathrate deuterate, over the temperature range 77 K to 270 K. The results indicate that in both materials the only motion which occurs at a rate great enough to affect the N.M.R. observables is methyl group reorientation and for such motion activation energies of (2·90±0·02) kcal mole-1 and (2·34±0·02) kcal mole-1 are obtained for pure diethylamine, and the deuterate, respectively. The strength of the dipolar interaction in the deuterate as estimated from both the second moment and the maximum in the temperature dependence of nuclear relaxation rate is consistent with a carbon-proton distance of 1·10 A and a large degree of chemical exchange of the amine protons with the deuterons of D2O.

Pulsed NMR study of molecular motion in the diglycidyl ether of bisphenol‐A cured with 4,4′‐methylenedianiline

AbstractThe monomeric diglycidyl ether of bisphenol‐A cured with methylenedianiline has been studied by pulsed NMR. Values of the proton relaxation times T1, T1p, and T2 have been measured over the temperature range −160 to 200°C. The system was studied after being fully postcured at 180°C and after being cured at 100°C and at 54°C. The relaxation times are interpreted in terms of molecular motion in the cured resins, i.e., methyl group reorientation, segmental motions, and general molecular motion. The results are compared with those obtained previously by us for the uncured resin. Correlation frequencies for the segmental motions are compared with those obtained from dielectric relaxation and mechanical loss studies. There are at least two principal segmental motions present in the cured system, and the nature of these motions is found to depend on the cure temperature. These effects are discussed in terms of crosslinking and annealing of the system.

Pulsed NMR study of molecular motion in the uncured diglycidyl ether of bisphenol‐A

AbstractThe diglycidyl ether of bisphenol‐A, an uncured epoxy resin, has been studied by pulsed NMR. Values of the proton relaxation times T1, T1p, and T2 have been measured over the temperature range from −160 to 200°C. The resin was studied in its monomeric form and in two mixtures containing higher oligomers. The relaxation times are interpreted in terms of the molecular motion in the resins. The motion responsible for relaxation in the solid monomer form is thought to be methyl group reorientation at low temperatures and general molecular motion at high temperatures. The motions are characterized by activation energies of 5 kcal/mole and 33 kcal/mole, respectively. The solid mixtures exhibit similar effects to the monomer, but an additional relaxation mechanism is observed which is attributed to segmental motion. This motion is characterized by an activation energy of 12–15 kcal/mole. The self‐diffusion coefficient was measured in the liquid monomer, and the activation energy for self‐diffusion is found to be 11 kcal/mole.

Proton NMR and Piezoelectricity in Tetramethylammonium Chloride

A proton magnetic resonance investigation has been made on tetramethylammonium chloride between 77 and 567°K. The existence of a new phase is reported, making a total of five known phases, and these are numbered from high temperature down. It is shown that methyl-group reorientation occurs without noticeable tunneling, followed at slightly higher temperatures by pseudoisotropic reorientations of the cations. The correlation times for these motions have been measured in four phases over this temperature range, and expressed by Arrhenius-type relations. Ionic diffusion only occurs in phase I, and then infrequently. This study shows that phase II is piezoelectric, and that in the absence of any solvent it may remain metastable at room temperature. In the presence of small quantities of solvent a transition occurs to the common stable phase III at ∼400°K.

Correlation Effects and Molecular Tumbling in NMR Studies of Solid β-(CH3)4Si

The β phase of high purity samples (99.9% min) of tetramethylsilane (TMS), (CH3)4Si, was investigated between 77°K and its melting point (174°K) employing static and rotating frame proton spin-lattice relaxation (T1 and T1p, respectively) measurements. Minima in the T1 curve near 90°K and in T1p at 171°K correspond to methyl group reorientation and over-all molecular tumbling, respectively. Activation energies and inverse frequency factors for both motions are derived and discussed. Deviations from exponential behavior in the longitudinal relaxation were noted below 140°K and are inferred to be due to a correlation in the relative motion of the protons within each methyl group. The exponentiality of the spin-lattice relaxation above ∼140°K is associated with the onset of over-all molecular tumbling motions. It is suggested that the presence of exponential proton magnetization decays in solids containing methyl groups may sometimes serve as an indication of the presence of reorientations of the molecular frame to which the methyl groups are attached. The possibility that the existence of exponential and nonexponential decays may be used by itself to gain information on interactions and motions in solids, has to be further investigated.

Proton magnetic relaxation studies of molecular motion in acetone and acetone clathrate deuterate

The proton magnetic resonance second moment and spin lattice relaxation data are reported for acetone deuterate in the temperature range from 77 °K to 260 °K and for pure acetone from 77 °K to 180 °K. The results of acetone deuterate indicate isotropic rotation of acetone molecule in the larger cavities of D 2O from 212 °K to 260 °K. The activation energy derived from the relaxation data in the temperature range of 77 °K to ~98 °K for acetone deuterate is 0.4 kcal mole −1 which is not assigned. The activation energy for the barrier hindering methyl group reorientation in pure acetone from relaxation is found to be 1.33 kcal mole −1.