Activation Energy For Rotation Research Articles

Conformational studies by dynamic nuclear magnetic resonance. Part 26. Interconversion barriers between syn- and anti-conformers of isomeric thiophenecarbaldehydes

The low-temperature 13C n.m.r. spectra (25.16 MHz) of thiophene-2- and -3-carbaldehyde [(1) and (2)] display line-broadening effects due to the exchange between their S,O-syn- and S,O-anti-conformers. In both cases the free energy of activation for rotation of the –CHO moiety was measured, the values being 10.15 and 8.5 kcal mol–1 for (1) and (2), respectively. In the case of thiophene-3-carbaldehyde (2) the signals of both conformers were detected, the ratios being 97:3 at –140 and 93:7 at –100 °C in favour of the S,O-anti-conformer. In thiophene-2-carbaldehyde (1) the minor conformer (S,O-anti) was present in too small an amount to be unambiguously detected by a conventional spectrometer, but spectra taken with a superconducting instrument (75.48 MHz) revealed ca. 1.5% of this conformer at –100 °C.

Studies of the molecular motions of large penetrant molecules in solid polymers by electron paramagnetic resonance spin-probe techniques

The determination of rotational correlation times for spin probes in polymers is reviewed, with emphasis on the relationships between rotational and translational motions. Data on the behavior of three different spin-probe molecules in polyamide and polyester films and fibers are examined in detail. It is concluded that saturation phenomena make it difficult to obtain reliable rigid limit parameters in these systems, unless very low microwave power levels can be used. At very low temperatures probe rotation appears to take place in local defects or microvoids in the glassy polymers. There is an increase in activation energy of rotation at a temperature close to the T g expected from dynamic mechanical (110 Hz) measurements, but the probe molecule rotations remain strongly hindered until significantly higher temperatures are reached. In nylons, sensitivity of probe rotations to structural differences between fibers is low in dry systems, and increases with temperature and moisture content. Even in nylon fibers containing substantial amounts of water, the probe rotation is most consistent with free-volume concepts, and is inconsistent with models of probe rotation and diffusion in solvent-filled pores or channels.

Nuclear magnetic resonance studies of amino acids and proteins. Deuterium nuclear magnetic resonance relaxation of deuteriomethyl-labeled amino acids in crystals and in Halobacterium halobium and Escherichia coli cell membranes.

We have obtained deuterium (2H) Fourier transform nuclear magnetic resonance (NMR) spectra of zwitterionic L-[beta-2H3]alanine, DL-[gamma-2H6]valine, DL-[beta, gamma-2H4]threonine, L-[delta-2H3]leucine, and L-[alpha, beta, gamma, gamma', delta-2H10]isoleucine in the crystalline solid state and have determined the deuteriomethyl group spin-lattice relaxation rates as a function of temperature. The results yield the Arrhenius activation energies (delta E) for methyl rotation, and through use of a suitable mathematical model, rotational correlation times, tau c. For alanine, valine, threonine, leucine, and isoleucine at 37 degrees C, tau c and delta E values are 780, 100, 40, 38, and 18 ps and 22, 14.0, 17.6, 15.5, and 8.6 kJ, respectively. For L-[beta-2H3]alanine in the zwitterionic lattice, a spin-lattice relaxation time (T1) minimum of 2.1 +/- 0.3 ms is observed (at 0 degree C), in excellent agreement with the 1.92-ms prediction of the mathematical model. Similar tau c and delta E measurements are reported for bacteriorhodopsin in the purple membrane of Halobacterium halobium R1 and for Escherichia coli cell membranes. Overall, our results demonstrate a great similarity between the dynamics in amino acid crystals and in membrane proteins. However, threonine exhibits a nonlinear Arrhenius behavior in bacteriorhodopsin, and in the valine-, leucine-, and isoleucine-labeled membrane samples at higher temperatures (approximately greater than 37 degrees C), there is evidence of an additional slow side-chain motion. The lipid phase state in E. coli does not appear to influence, on the average, the dynamics of the valine side chains. These results indicate that the sensitivity of the deuterium NMR technique is now adequate to study in moderate detail the dynamics of most types of amino acids in a membrane protein and that adequate sensitivity, in some instances, should be available for the study of individual amino acids in suitably labeled membrane proteins.

A determination of the activation energy of cyclopentadienyl group rotation and molecular tumbling in [Fe(? 5-C5H5)(? 5-C5H4R)](R = CMe2Et or Bun) using carbon-13 nuclear magnetic resonance relaxation measurements

From variable-temperature 13C spin-lattice relaxation time and nuclear Overhauser enhancement measurements in [2H8]toluene solution at 25 and 100 MHz, the activation energies for ring rotation and molecular tumbling have been determined for n-butylferrocene and t-pentylferrocene.

Rotational diffusion of TEMPONE in the cytoplasm of Chinese hamster lung cells

The correlation time for rotational diffusion (tau R) of 2,2,6,6-tetramethyl-4-piperidone-N-oxide (TEMPONE) in Chinese hamster lung (V79) cells has been measured. For these cells in an isosmotic solution at 20 degrees C, tau R = 4.18 X 10(-11) s, approximately 3.6 times greater than tau R = 1.17 X 10(-11) s in water. The relationship between tau R and viscosity was investigated in a number of glycerol-water (0-50%) and sucrose-water (20-40%) solutions and a constant Stokes-Einstein volume of 44 A3 was found for TEMPONE in solutions of less than 20% glycerol and sucrose. This gives an average shear viscosity (for rotation of a small molecule) of 0.038 poise for the cytoplasm. When nonsecular terms were used in the calculation of tau R, the activation energies for rotation of TEMPONE in the above solutions correlated well with the activation energies for shear viscosity. The viscosity increases as the cell is shrunk in hypertonic solutions. It also increases with decreasing temperature with an activation energy of 3.7 kcal/mol, about the same as the activation energy for the viscosity of pure water. The rotational correlation times were carefully calculated considering inhomogeneous line broadening, non-Lorentzian line shapes, the need for accurate tensor values and nonsecular terms.

A NUCLEAR MAGNETIC RESONANCE AND MASS SPECTRAL STUDY OF SOME ALKENE COMPLEXES OF RHODIUM(I) AND IRIDIUM(I): COORDINATED ALKENE ROTATIONAL BARRIERS, APPEARANCE POTENTIALS AND ESTIMATED HEATS OF FORMATION

Abstract The activation energies (ΔG) for olefin rotation in ethylene complexes of Rh(I) and Ir(I) have been estimated by signal coalescence from their temperature dependent 1H nmr spectra in solution. The barriers decrease with increase in the electron accepting nature of the associated ligand (η5-cyclopentadienyl, pentane-2, 4-dionate, carboxylate and substituted derivatives thereof) in accordance with the expected decrease in the importance of the π-contribution to the metal-olefin bond. Barriers are substantially higher for Ir(I) than for corresponding Rh(I) complexes. The complex temperature dependent spectra of di-μ-chlorotetrakis(4-methylpenta-1, 3-diene dirhodium(I) are interpreted in terms of both intra- and intermolecular processes. The greater ΔG values found for Ir(I) compared to Rh(I) complexes are reflected in the different mass spectral fragmentation patterns for corresponding members of the two series. Appearance potentials and heats of formation are estimated for [cpRh(C2 H4)2] and [cpRh(...

The conformational behaviour of hydroxamic acids

N-Formyl-N-alkylhydroxylamines, N-formyl-α-N-hydroxyamino-acid esters and some N-acetyl analogues exist as Z/E equilibrium mixtures, as shown by H-NMR and IR. The dependence of the Z/E ratio on the nature of N-substituents, acyl groups and solvent was studied. The Z-isomer is preferred for bulky N-substituents and non-polar solvents, and for the O-protonated form. Free energies of activation for rotation about the carbonyl-nitrogen bond were calculated in a series of formylhydroxamic acids.

1H nuclear magnetic resonance spectra of some 3- and 5-substituted thiophen-2-carboxamides

The 1H n.m.r. spectra of some 3- and 5-substituted thiophen-2-carboxamides in CDCl3 are reported. 3-Substituted thiophen-2-carboxamides show intramolecular hydrogen bonding between the N–H proton and the 3-substituent and also an intermolecular hydrogen bond, except for the 3-methyl derivative, which exhibits only intermolecular hydrogen bonding. 5-Substituted thiophen-2-carboxamides exhibit intermolecular hydrogen bonding. The chemical shifts for the 4-protons of 3-substituted thiophen-2-carboxamides correlate with σp(r= 0.962; ρ= 1.39). Correlation for the 4-protons of the 5-substituted amides is poor.The activation energies of rotation of the 3-substituted amides are higher than those of the corresponding 5-substituted derivatives. For 5-substituted amides the activation energies of rotation depend on the electronic nature of the substituents, and ΔG(Tc)/σp+ shows a linear relationship (r= 0.954; ρ= 1.77).

Barriers to rotation of the dimethylamino group in some 2‐amino‐4‐(N,N‐dimethylamino)pyrimidines

AbstractThe free energy of activation for rotation about the exocyclic CN bond of the dimethylamino group of some 6‐substituted 2‐amino‐4‐(N,N‐dimethylamino)pyrmidines has been determined using 1H NMR line shape analysis. The results are discussed in terms of the relative electron‐withdrawing and electron‐releasing effects of the substituents.

Restricted Rotation Involving the Tetrahedral Carbon. XLIII. Buttressing Effect on Rotational Barriers in Bromine-substituted 9-(2-Methoxy-4,6-dimethylphenyl)fluorenes

Abstract 9-(2-Methoxy-4,6-dimethylphenyl)fluorenes carrying one or two bromo groups on the phenyl ring were prepared and the buttressing effect on the rotational barrier about the C9–Cph bond were investigated. The free energies of activation for rotation were 26.5, 25.9, and 27.2 kcal/mol at 56 °C for 3-bromo, 5-bromo, and 3,5-dibromo compounds, respectively. The buttressing effect was enhanced when the bromo group was next to the methoxyl group relative to that given to the methyl group. The results were attributed to the fact that a methoxyl group, which gives its steric effect by the oxygen atom in the normal cases, has to take conformations, not coplanar with the phenyl ring, caused by the presence of the bromo group.

ESR studies on the lipid bilayers of separated outer and cytoplasmic membranes of a moderately halophilic bacterium.

Electron spin resonance (ESR) studies were performed on the isolated outer and cytoplasmic membranes from cells of a moderately halophilic bacterium Pseudomonas halosaccharolytica ATCC 29423, which were grown in 2 M NaCl medium at different temperatures. The cytoplasmic membrane contained more acidic phospholipids (50% or more; including most of the diphosphatidylglycerol) than the outer membrane (40%) in 25 and 30 °C grown cells, while in both membranes of 35 °C grown cells the relative content of acidic phospholipids was almost the same. With elevated growth temperature C19-cyclopropanoic fatty acids in both membranes increased.The physical properties of lipid bilayers of both membranes were studied by a spin-label method using N-oxyl spin-labelled stearic acids. Isolated membranes suspended in a medium with or without 2 M NaCl showed no appreciable differences in ESR spectra on changing the sodium chloride concentration in the medium. The lipid bilayers of both membranes gave almost uniform rotational correlation times for the first 12 carbon atoms of the hydrocarbon chains starting from the polar surface of the bilayers. The difference between rotational correlation times of either membrane and those of whole cells and liposomes of extractable lipids of this bacterium was that the rotational correlation times of both membranes gave similar absolute values regardless of change in the growth temperature and varied similarly with the temperature of ESR measurements. The surface lipid layers up to C12 had a phase transition temperature near the growth temperature of the bacterium. The rotational activation energies were 5–10 kcal/mol (1 cal = 4.1868 J) for the liquid phase and 10–25 kcal/mol for the solid phase. In general, the order parameters of both membranes decreased gradually with increasing temperature of ESR measurements, except for rapid decreases and discontinuous drops near the growth temperature with the spin-probe labelled at C12 of stearic acid (12NS). Such discontinuous decreases in order parameter of 12NS were not observed for the whole cell membrane nor for liposomes of total extractable lipids of this bacterium.

Structure, Synthesis, and Properties of Some Persubstituted 1,2‐Dintroethanes. In Quest of Nitrocyclopropyl‐Anion Derivatives

AbstractAttempts to deprotonate nitrocyclopropane led to solutions which showed strong ESR. signals (Fig. 1)and from which 1‐nitro‐ l′‐nitroso‐bicyclopropyl (3) and 1,1′‐dinitro‐bicyclopropyl (2) were isolated. The activation energy for rotation about the central C, C‐bond of 2 is estimated to be about 12 kcal/mol (1 H‐NMR. spectra in Fig. 2). In contrast, the oven‐chain analogue 2,3‐dimethyl‐2,3‐dinitrobutane (1) shows a methyl singlet down to −70° C. Low‐temperature X‐ray analyses of 1, 2, 3, and also of 1,1′‐dinitro‐bicyclobutyl (4) show that all four molecules have gauche‐conformations but reveal striking structural differences between the openchain and the cyclic derivatives (Fig. 4–6): the central C, C‐bond is long in 1 (1.575 Å), short in 2 (1.479 Å); the C, N‐bonds are long in 1 (1.549 Å), short in 2 (1.488 Å); the orientation of the nitro groups is bisected in 2 and perpendicular in 1. The crystal structure of the nitro‐nitroso compound 3 is isomorphous with that of the dinitro compound 2 and thus disordered (Fig. 15–16). The effect of the nitro group as π‐electron acceptor on the molecular conformations and bond lengths is discussed. From analysis of the anisotropic vibrational parameters of 2 the root‐mean‐square librational amplitude of the nitro groups about their C, N‐bonds is estimated to be about 5.8° at 95 K, corresponding to a rotational barrier of about 9 kcal/mol, i. e. the same order of magnitude as the NMR. estimate of about 12 kcal/mol for C, C‐rotation.

Inelastic neutron scattering studies of benzene-containing organometallic complexes. Part 1. Complexes [Cr(CO)3(η6-C6H6)] and [ Mn(CO)3(η6-C6H6)]Br

Inelastic neutron scattering spectra (0–875 cm–1) of [Cr(CO)3(η6-C6H6)](1) and [Mn(CO)3(η6-C6H6)]Br (2) have been measured. In addition, low-frequency i.r. (60–600 cm–1) and Raman (0–l 000 cm–1) spectra of (2) are reported for the first time. Those low-frequency modes of both compounds which involve significant benzene-ligand displacement have been assigned. From the assignments of the benzene-ligand torsion (ν22) in complexes (1) and (2) to bands at 88.5 and 116.5 cm–1 respectively, values for the activation energy for benzene-group rotation have been calculated to be 27.5 and 46.5 kJ mol–1 respectively. In the case of complex (1) this value has been compared with that determined from n.m.r. measurements (17.6 kJ mol–1).

Ultrasonic studies of rotational isomerism in various methylpentanes

Ultrasonic attenuation and velocity have been measured in 2-methylpentane, 3-methylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,2,4-trimethylpentane and 2,3,4-trimethylpentane from 233 to 293 K. The contribution to the attenuation from shear processes has been estimated from viscosity measurements and the corrected data have been analysed in terms of rotational isomerism. The conformational energetics so obtained are compared with those predicted on the basis of non-bonded van der Waals' interactions between neighbouring groups. In general, there is agreement between the theoretical and experimental energy differences between stable rotational states, but observed activation energies for rotation are less than those predicted theoretically.

Kinetic study of the conversion of presolvated to solvated silver atoms in polycrystalline ice: Activation energy of water molecule rotation with D-defect annihilation

The kinetics of the conversion of presolvated silver atoms formed in polycrystalline ice at 4 K to the solvated state has been studied in the temperature range of 38 to 49 K. The kinetics are approximately first order and give an activation energy of 0.09±0.01 eV. This is lower than the 0.24 eV activation energy for diffusion of orientational defects by water molecule rotation in ice and is identified with water molecule rotation together with D-defect annihilation in the silver atom solvation shell. (AIP)

The dynamics of ring rotation in ferrocene, nickelocene and ruthenocene by incoherent quadi-elastic neutron scattering

Incoherent quasi-elastic neutron scattering has been used to study the reorientational motions of the cyclopentadienyl rings in ferrocene, nickelocene and ruthenocene. The results for ferrocene show that the activation energy for ring rotation drops above the 164 K phase transition to 4.4 ± 0.5 kJ mol−1 (which is approximately half its low temperature value) but the rings still appear to jump between only five orientations on the observable time scale. At room temperature, the rings in nickelocene appear to behave the same as in ferrocene but in ruthenocene they reorientate much less frequently and resemble those in ferrocene below 164 K.

First observation of amino acid side chain dynamics in membrane proteins using high field deuterium nuclear magnetic resonance spectroscopy.

We have obtained the first deuterium NMR spectra of an individual membrane protein, bacteriorhodopsin in the purple membrane of Halobacterium halobium R1. Biosynthetic isotopic enrichment with [gamma-2H6]valine and high field Fourier transform operation permitted rapid data acquisition on intact membranes, including measurement of relaxation times. At some temperatures high quality spectra could be obtained in less than 1 s. [U-14C]Valine tracer studies indicate that less than or equal to 2% of valine added to the growth medium is broken down and incorporated into other membrane constituents. The NMR results indicate that the valine side chain is a rather rigid structure. Motion about C alpha-C beta is slow (less than 10(5) s-1) at growth temperature, While motion about C beta-C gamma is as expected fast (much greater than 10(5) s-1) at all accessible temperatures. The activation energy for methyl group rotation from spin-lattice relaxation data between -75 and 53 degrees C is approximately 2.4 kcal/mol, in good agreement with previous 1H NMR studies on solid alkanes. Preliminary data on [gamma-2H6]valine-labeled Acholeplasma laidlawii B (PG9) cell membranes are also presented. Our results strongly suggest that it should now be possible to observe in great detail the motions of any type of amino acid side chain in membrane proteins, including the effects of lipid composition on protein dynamics.

Elastic after-effect studies of self-interstitials in tungsten after fast neutron irradiation at 5 K

Abstract Elastic after-effects in Wsingle crystals with various orientations were measured below 50 K after fast neutron irradiation at about 5 K. Two prominent relaxation peaks were observed at 5.5 K and 17.6 K (relaxation time of about 2 min). These two peaks correspond, respectively, with the 8 K and 27 K internal friction peaks in previous high frequency measurements (vibrational frequency of about 500 Hz). The 5.5 K peak recovered monotonicly in recovery stage I(∼18 K). On the other hand, the 17.6 K peak showed a transieni growth in stage I and then steadily recovered in stage IIa(∼28 K). From these results, and their dependences on irradiation fluence and orientation, it was concluded that the 5.5 K and 17.6 K peaks are, respectively, associated with a stress-induced rotation (without migration) of free ⟨110⟩ split type interstitials and of di-interstitials with ⟨110⟩-symmetry. Anisotropy ratios and activation parameters of these defects were determined more accurately than in the previous work. Both activation energies for rotation and for migration showed a somewhat broad distribution for the 5.5 K and 17.6 K peaks. Further, it was found that a simple 1/T relation for relative peak heights broken down for the 5.5 K peak. Besides these peaks, many minor peaks were also observed, and their origins were discussed.

Effect of pressure on the rotational mobility of spin probes in polymers

The effects of pressure on the rotational mobility of three nitroxyl radicals (spin probes) in natural rubber, polyethylene, and butadiene-nitrile rubber SRN-26 have been studied. The activation volumes, activation energies, and pre-exponential factors of spin probe rotation at constant pressure and volume have been determined. The activation volumes of probe rotation (20–70 cm 3/mol) increase with increasing size of radicals and differ insignificantly from the activation volumes of the β-relaxation process. In the polar polymer SRN-26, the activation volumes of rotation of radicals are appreciably more than in the nonpolar polymers, natural rubber and polyethylene. These features are apparently due to different volumes of the kinetic chain segment controlling probe rotation. The activation volumes of radical rotation around different molecular axes differ significantly. The activation energy of probe rotation at constant volume is appreciably less than at constant pressure. It has also been shown that the energy necessary for the formation of a fluctuation hole is the main factor that determines temperature dependence of the rotational mobility of low-molecular particles in the polymer.

Application of 15N spectroscopy and dynamic NMR to the study of ureas, thioureas and their lewis acid adducts

AbstractRotational barriers and 15N chemical shifts have been measured in a number of ureas and thioureas. As anticipated on the basis of the 15N shifts, several previously unobserved rotational barriers could be detected by using lanthanide reagents or a high field spectrometer. Nearly constant effects on both the rotational activation energy and the 15N shift are produced on going from ureas to the corresponding thioureas, and correlations are found between the ΔG‡ and δ15N values. The results are discussed in terms of lone pair delocalization, and anomalies with respect to the general behaviour are tentatively explained in the light of the effect of steric torsion in crowded structures on the 15N shifts and rotation barriers.