Proton spin relaxation and methyl and hydroxy group motion

Proton spin-lattice relaxation rates R = T1−1 have been measured in powdered samples of 4-methyl-2,6-di-tertiarybutyl phenol between 77 and 170 K. Deuteration of the hydroxy proton leads to a large change in R in the vicinity of the 120-K R maximum and in the high-temperature region. The data are successfully, although probably not uniquely, fitted and the interpretation suggests that in addition to the motion of the hydroxy proton playing a significant role in the relaxation, the two pairs of t-butyl methyl groups also contributing are inequivalent. The departure, at low temperatures, from an ω−2dependence of R where ω is the nuclear Larmor angular frequency is also investigated

Nonexponential 1H Spin-Lattice Relaxation and Methyl Group Rotation in Molecular Solids

We report a quantitative measure of the nonexponential 1H spin-lattice relaxation resulting from methyl group (CH3) rotation in six polycrystalline van der Waals solids. We briefly review the subject in general to put the report in context. We then summarize several significant issues to consider when reporting 1H or 19F spin-lattice relaxation measurements when the relaxation is resulting from the rotation of a CH3 or CF3 group in a molecular solid

Solid state proton spin-lattice relaxation in polycrystalline methylphenanthrenes. IV. 1,4-dimethylphenanthrene

We present and model the NMR frequency (8.50, 22.5, and 53.0 MHz) and temperature (97–300 K) dependence of the solid state 1H spin-lattice relaxation process in polycrystalline 1,4-dimethylphenanthrene. The solid state gives rise to a situation where methyl group rotation is the only motion on the NMR time scale and the relaxation rates due to the rotations of the 1- and 4-methyl groups are conveniently well-separated in temperature. At these low NMR frequencies, both the slow- and fast-motion limits are observed for the rotation of both methyl groups which allows for a more stringent test of the models. The relaxation is nonexponential as expected when it is caused by methyl group rotation in which case the initial relaxation rate is modeled. Parameters characterizing stretched-exponential fits of the relaxation process are also used both to quantify the degree of nonexponential relaxation and indicate that the observed relaxation is indeed due to the rotation of the two methyl groups. The results are compared with several other polycrystalline methylphenanthrenes and dimethylphenanthrenes. These systems allow for an investigation into how intramolecular and intermolecular interactions between methyl groups and neighboring atoms on the same and neighboring molecules determine the barriers to methyl group rotation

A review of polytypism in lead iodide

Lead Iodide (PbI2) is an important inorganic solid for both basic scientific research and possible technological applications and in this brief review we discuss the structure of PbI2. Although the basic structure is a simple I-Pb-I layered structure with a [PbI6]4- near-octahedron being the basic building block, there are many ways of stacking the layers which results in many polytypes. We present 20 of the 23 entries for the structure of PbI2 from the Inorganic Structural Database and order them by polytype. This represents more than 80 years of crystallographic research in the structure of this compound. We present a simple way to view the 2H, 4H, 6H, and 6R polytypes and extend the procedure to higher-order polytypes. We note a relationship, not generally appreciated, between the distortion of the near [PbI6]4- octahedrons and the polytype. We suggest that the significance of vacancies has only recently been appreciated. We suggest that small discrepancies in structure determination are probably due to different distributions of vacancies and that there are, in practice, very many structures for macroscopic or even mesoscopic samples of a given polytype when vacancies are considered. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Nuclear magnetic resonance spectroscopy and relaxation in molecular solids containing isopropyl groups. Part 2—The large two-phase temperature region in 1,4-di-isopropylbenzene

We report a proton n.m.r. study of the large, long-lived two-phase temperature region in 1,4-di-isopropylbenzene (DIB). The proton spectra are characterized by a narrow line superimposed on a 42 kHz broad line. The narrow line shows the chemical shift structure of DIB. When the sample is in the solid state the narrow lines are sharp (\u3c30 Hz) near the melting point of 256 K and they broaden to ca.1.5 kHz at ca. 170 K where they disappear. A variety of selective and non-selective Zeeman relaxation experiments as well as a Goldman–Shen exchange experiment were performed. It is concluded that the two sets of spins do not communicate on any timescale shorter than the spin–lattice relaxation times. The most likely interpretation of these results is that macroscopic pockets of molecules undergoing liquid-like motions are maintained in the solid, even 90 K below the melting point

Proton spin relaxation and methyl and hydroxy group motion

Proton spin-lattice relaxation rates R = T1−1 have been measured in powdered samples of 4-methyl-2,6-di-tertiarybutyl phenol between 77 and 170 K. Deuteration of the hydroxy proton leads to a large change in R in the vicinity of the 120-K R maximum and in the high-temperature region. The data are successfully, although probably not uniquely, fitted and the interpretation suggests that in addition to the motion of the hydroxy proton playing a significant role in the relaxation, the two pairs of t-butyl methyl groups also contributing are inequivalent. The departure, at low temperatures, from an ω−2dependence of R where ω is the nuclear Larmor angular frequency is also investigated

The electron-methyl group spin-spin interaction

The nuclear spin symmetry conversion transition, whereby a methyl group changes tunnelling state (A↔E) and total nuclear spin (ΔI = ± 1), is made resonant by the flip of an unpaired electron spin (Δms = ± 1). The coupling between an unpaired electron in a free radical and a methyl group in a nearby molecule is via the inter-molecular spin-spin interaction. The matrix elements and transition probabilities for this transition are calculated explicitly. The motivation behind the calculation is to aid in the interpretation of electron spin relaxation experiments in γ-irradiated 4-methyl-2,6-t-butylphenol where these resonant transitions have a profound effect. The results presented have also been useful in the interpretation of nuclear magnetic resonance experiments in the same substance

A review of polytypism in lead iodide

Lead Iodide (PbI2) is an important inorganic solid for both basic scientific research and possible technological applications and in this brief review we discuss the structure of PbI2. Although the basic structure is a simple I-Pb-I layered structure with a [PbI6]4- near-octahedron being the basic building block, there are many ways of stacking the layers which results in many polytypes. We present 20 of the 23 entries for the structure of PbI2 from the Inorganic Structural Database and order them by polytype. This represents more than 80 years of crystallographic research in the structure of this compound. We present a simple way to view the 2H, 4H, 6H, and 6R polytypes and extend the procedure to higher-order polytypes. We note a relationship, not generally appreciated, between the distortion of the near [PbI6]4- octahedrons and the polytype. We suggest that the significance of vacancies has only recently been appreciated. We suggest that small discrepancies in structure determination are probably due to different distributions of vacancies and that there are, in practice, very many structures for macroscopic or even mesoscopic samples of a given polytype when vacancies are considered. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim