1,213 research outputs found
Methyl reorientation in methylphenanthrenes. II. Solid-state proton spin-lattice relaxation in the 1-CH3, 9-CH3, and 1-CD3, 9-CH3 systems
We report proton Zeeman relaxation rates R as a function of temperature T at 8.5 and 53 MHz in polycrystalline 1,9-dimethylphenanthrene (1,9-DMP) and l-trideuteriomethyl-9-methylphenanthrene (1, 9-DMP[1-d3]). The data are interpreted using a Davidson-Cole spectral density for intramolecular reorientation and the implications of this are discussed. R vs Tâ1data for 1,9-DMP[1-d3] are used to determine the parameters that characterize the reorientation of the 9-methyl group. By assuming that the parameters characterizing the dynamics of the 9-methyl group are the same in 1,9-DMP and 1,9-DMP[1-d3], we subtract out the R vs Tâ1 contribution of the 9-methyl group in 1,9-DMP to determine the parameters that characterize the dynamics of the 1-methyl group. We find that the barrier for reorientation of the 9-methyl group is larger than the barrier for the 1-methyl group and this is discussed in terms of the various contributions to the barrier
Methyl Reorientation in Methylphenanthrenes: 1. Solid-State Proton Spin-Lattice Relaxation in the 3-Methyl, 9-Methyl, and 3,9-Dimethyl Systems
We have investigated the dynamics of methyl group reorientation in solid methylâsubstituted phenanthrenes. The temperature dependence of the proton spinâlattice relaxation rates has been measured in polycrystalline 3âmethylphenanthrene (3âMP), 9âmethylphenanthrene (9âMP), and 3,9âdimethylphenanthrene (3,9âDMP) at Larmor frequencies of 8.50, 22.5, and 53.0 MHz. The data are interpreted using a DavidsonâCole spectral density which implies either that the correlation functions for intramolecular reorientation are nonexponential or that there is a distribution of exponential correlation times. Comparing the fitted parameters that characterize the relaxation data for the three molecules shows that the individual contributions to the relaxation rate from the 3â and 9âmethyls in 3,9âDMP can be separated and that the parameters specifying each are similar to the equivalent group in the two single methylphenanthrenes. The 9âmethyl group is characterized by effective activation energies of 10.6±0.6 and 12.5±0.9 kJ/mol in 9âMP and 3,9âDMP, respectively, whereas the 3âmethyl group is characterized by effective activation energies of 5.2±0.8 and 5±1 kJ/mol in 3âMP and 3,9âDMP, respectively. The agreement between the fitted and calculated values of the spinâlattice interaction strength, assuming only intramethyl proton dipoleâdipole interactions need be considered, is excellent. A comparison between experimentally determined correlation times and those calculated from a variety of very simple dynamical models is given, and the results suggest, as have several previous studies, that at high temperatures where tunneling plays no role, methyl reorientation is a simple, thermally activated, hopping process. We have also analyzed many published data in methylâsubstituted phenanthrenes, anthracenes, and naphthalenes (14 molecules) in the same way as we did for the phenanthrene data presented here, and a consistent picture for the dynamics of methyl reorientation emerges
Methyl Reorientation in Methylphenanthrenes: 1. Solid-State Proton Spin-Lattice Relaxation in the 3-Methyl, 9-Methyl, and 3,9-Dimethyl Systems
We have investigated the dynamics of methyl group reorientation in solid methylâsubstituted phenanthrenes. The temperature dependence of the proton spinâlattice relaxation rates has been measured in polycrystalline 3âmethylphenanthrene (3âMP), 9âmethylphenanthrene (9âMP), and 3,9âdimethylphenanthrene (3,9âDMP) at Larmor frequencies of 8.50, 22.5, and 53.0 MHz. The data are interpreted using a DavidsonâCole spectral density which implies either that the correlation functions for intramolecular reorientation are nonexponential or that there is a distribution of exponential correlation times. Comparing the fitted parameters that characterize the relaxation data for the three molecules shows that the individual contributions to the relaxation rate from the 3â and 9âmethyls in 3,9âDMP can be separated and that the parameters specifying each are similar to the equivalent group in the two single methylphenanthrenes. The 9âmethyl group is characterized by effective activation energies of 10.6±0.6 and 12.5±0.9 kJ/mol in 9âMP and 3,9âDMP, respectively, whereas the 3âmethyl group is characterized by effective activation energies of 5.2±0.8 and 5±1 kJ/mol in 3âMP and 3,9âDMP, respectively. The agreement between the fitted and calculated values of the spinâlattice interaction strength, assuming only intramethyl proton dipoleâdipole interactions need be considered, is excellent. A comparison between experimentally determined correlation times and those calculated from a variety of very simple dynamical models is given, and the results suggest, as have several previous studies, that at high temperatures where tunneling plays no role, methyl reorientation is a simple, thermally activated, hopping process. We have also analyzed many published data in methylâsubstituted phenanthrenes, anthracenes, and naphthalenes (14 molecules) in the same way as we did for the phenanthrene data presented here, and a consistent picture for the dynamics of methyl reorientation emerges
The Quenching of Isopropyl Group Rotation in Van Der Waals Molecular Solids
X-ray diffraction experiments are employed to determine the molecular and crystal structure of 3-isopropylchrysene. Based on this structure, electronic structure calculations are employed to calculate methyl group and isopropyl group rotational barriers in a central molecule of a ten-molecule cluster. The two slightly inequivalent methyl group barriers are found to be 12 and 15 kJ mol(-1) and the isopropyl group barrier is found to be about 240 kJ mol(-1), meaning that isopropyl group rotation is completely quenched in the solid state. For comparison, electronic structure calculations are also performed in the isolated molecule, determining both the structure and the rotational barriers, which are determined to be 15 kJ mol(-1) for both the isopropyl group and the two equivalent methyl groups. These calculations are compared with, and are consistent with, previously published NMR (1)H spin-lattice relaxation experiments where it was found that the barrier for methyl group rotation was 11 +/- 1 kJ mol(-1) and that the barrier for isopropyl group rotation was infinite on the solid state NMR time scale
Intramolecular and intermolecular contributions to the barriers for rotation of methyl groups in crystalline solids: Electronic structure calculations and solid state NMR relaxation measurements
The rotation barriers for 10 different methyl groups in five methyl-substituted phenanthrenes and three methyl-substituted naphthalenes were determined by ab initio electronic structure calculations, both for the isolated molecules and for the central molecules in clusters containing 8â13 molecules. These clusters were constructed computationally using the carbon positions obtained from the crystal structures of the eight compounds and the hydrogen positions obtained from electronic structure calculations. The calculated methyl rotation barriers in the clusters (Eclust) range from 0.6 to 3.4 kcal/mol. Solid-state 1H NMR spinâlattice relaxation rate measurements on the polycrystalline solids gave experimental activation energies (ENMR) for methyl rotation in the range from 0.4 to 3.2 kcal/mol. The energy differences Eclust â ENMR for each of the ten methyl groups range from â0.2 kcal/mol to +0.7 kcal/mol, with a mean value of +0.2 kcal/mol and a standard deviation of 0.3 kcal/mol. The differences between each of the computed barriers in the clusters (Eclust) and the corresponding computed barriers in the isolated molecules (Eisol) provide an estimate of the intermolecular contributions to the rotation barriers in the clusters. The values of Eclust â Eisol range from 0.0 to 1.0 kcal/mol
Intramolecular and intermolecular contributions to the barriers for rotation of methyl groups in crystalline solids: Electronic structure calculations and solid state NMR relaxation measurements
The rotation barriers for 10 different methyl groups in five methyl-substituted phenanthrenes and three methyl-substituted naphthalenes were determined by ab initio electronic structure calculations, both for the isolated molecules and for the central molecules in clusters containing 8â13 molecules. These clusters were constructed computationally using the carbon positions obtained from the crystal structures of the eight compounds and the hydrogen positions obtained from electronic structure calculations. The calculated methyl rotation barriers in the clusters (Eclust) range from 0.6 to 3.4 kcal/mol. Solid-state 1H NMR spinâlattice relaxation rate measurements on the polycrystalline solids gave experimental activation energies (ENMR) for methyl rotation in the range from 0.4 to 3.2 kcal/mol. The energy differences Eclust â ENMR for each of the ten methyl groups range from â0.2 kcal/mol to +0.7 kcal/mol, with a mean value of +0.2 kcal/mol and a standard deviation of 0.3 kcal/mol. The differences between each of the computed barriers in the clusters (Eclust) and the corresponding computed barriers in the isolated molecules (Eisol) provide an estimate of the intermolecular contributions to the rotation barriers in the clusters. The values of Eclust â Eisol range from 0.0 to 1.0 kcal/mol
Zooming In Versus Flying Out: Virtual Residency Interviews in the Era of COVIDâ19
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163370/2/aet210486.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163370/1/aet210486_am.pd
G313.3+00.3: A New Planetary Nebula discovered by the Australia Telescope Compact Array and the Spitzer Space Telescope
We present a new planetary nebula, first identified in images from the
Australia Telescope Compact Array, although not recognized at that time. Recent
observations with the Spitzer Space Telescope during the GLIMPSE Legacy program
have rediscovered the object. The high-resolution radio and infrared images
enable the identification of the central star or its wind, the recognition of
the radio emission as thermal, and the probable presence of polycylic aromatic
hydrocarbons in and around the source. These lead to the conclusion that
G313.3+00.3 is a planetary nebula. This object is of particular interest
because it was discovered solely through radio and mid-infrared imaging,
without any optical (or near-infrared) confirmation, and acts as a proof of
concept for the discovery of many more highly extinguished planetary nebulae.
G313.3+00.3 is well-resolved by both the instruments with which it was
identified, and suffers extreme reddening due to its location in the
Scutum-Crux spiral arm.Comment: 18 pages, LaTeX (aastex), incl. 8 PostScript (eps) figures and 1
table. Accepted by ApJ (Part 1
Analysis of the Transport Process Providing Spin Injection through an Fe/AlGaAs Schottky Barrier
Electron spin polarizations of 32% are obtained in a GaAs quantum well via
electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An
analysis of the transport data using the Rowell criteria demonstrates that
single step tunneling is the dominant transport mechanism. The current-voltage
data show a clear zero-bias anomaly and phonon signatures corresponding to the
GaAs-like and AlAs-like longitudinal-optical phonon modes of the AlGaAs
barrier, providing further evidence for tunneling. These results provide
experimental confirmation of several theoretical analyses indicating that
tunneling enables significant spin injection from a metal into a semiconductor.Comment: 4 pages, 4 figures, submitted to AP
Small RNA Profile in Moso Bamboo Root and Leaf Obtained by High Definition Adapters
Moso bamboo (Phyllostachy heterocycla cv. pubescens L.) is an economically important fast-growing tree. In order to gain better understanding of gene expression regulation in this important species we used next generation sequencing to profile small RNAs in leaf and roots of young seedlings. Since standard kits to produce cDNA of small RNAs are biased for certain small RNAs, we used High Definition adapters that reduce ligation bias. We identified and experimentally validated five new microRNAs and a few other small non-coding RNAs that were not microRNAs. The biological implication of microRNA expression levels and targets of microRNAs are discussed
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