Activation Energy For Rotation Research Articles

Electron Spin-Lattice Relaxation Processes of Radicals in Irradiated Crystalline Organic Compounds

Electron spin-lattice relaxation times (T1e) for the major radicals in γ-irradiated polycrystalline samples of glycylglycine, l-alanine, 2,4,6-tri-tert-butyl-phenol, and 4-methyl-2,6-di-tert-butyl-phenol were measured as a function of temperature using pulsed EPR. CW-saturation recovery (CW-SR) were obtained at X-band (9.1 GHz) and S-band (3.0 GHz) between about 10 and 295 K. Inversion recovery, echo-detected saturation recovery (ED-SR), and pulsed electron−electron double resonance (ELDOR) curves were obtained at X-band between 77 and about 295 K. For 2,4,6-tri-tert-butyl-phenoxy radical, which has a single-line EPR spectrum, the recovery times obtained by the three methods were in good agreement and were assigned as T1e. For the three radicals with resolved hyperfine splitting, spectral diffusion caused the recovery times observed by inversion recovery or ED-SR to be significantly shorter than T1e obtained by CW-SR or ELDOR. Spectral diffusion processes were observed directly by pulsed ELDOR experiments, and time constants for cross relaxation and nuclear relaxation were obtained by modeling the ELDOR curves. For irradiated l-alanine and for the 4-methyl-2,6-tert-butyl-phenoxy radical at some temperatures, the effects of rapid cross relaxation on CW-SR curves could not be fully mitigated even by long saturating pulses, and T1e could only be determined by ELDOR. For the radicals in γ-irradiated l-alanine, 2,4,6-tri-tert-butyl-phenol, and 4-methyl-2,6-di-tert-butyl-phenol, methyl group rotation makes significant contributions to T1e at temperatures where the rate of rotation of a methyl group is comparable to the microwave frequency. Activation energies for methyl rotation were determined by modeling the temperature dependence of T1e at X-band and S-band. In temperature ranges where methyl rotation did not dominate, T1e was dominated by Raman, direct, or local mode processes.

Order-disorder transition in pyridinium iodide: QENS study

Reorientational motion of the pyridinium cation in pyridinium iodide as studied by the high-resolution, quasi-elastic neutron scattering (QENS) technique both above and below the order–disorder transition (Tc=247 K) temperature are reported. The pyridinium ion is found to undergo reorientational motion around the axis perpendicular to its plane and is best described by a 6-fold jump rotation model in the high-temperature phase. Below the phase transition temperature, in the ordered phase, some residual disorder is observed. Reorientational time and activation energy of rotation are also obtained in the high-temperature phase.

Structure and dynamics of ND3BF3 in the solid and gas phases: a combined NMR, neutron diffraction, and Ab initio study.

The decrease in D-->A bond lengths, previously reported for some Lewis acid/base complexes, in going from the gas to the solid phases is investigated by obtaining an accurate crystal structure of solid ND(3)BF(3) by powder neutron diffraction. The B-N internuclear distance is 1.554(3) A, 0.118 A shorter than the most recent gas-phase microwave value and 0.121 A shorter than the single molecule geometry optimized (1.672 A, CISD/6-311++G(d,p)) bond length. The crystal structure also shows N-D.F-B hydrogen bonds. The effects of this change in structure and of intermolecular hydrogen-bonding on nuclear magnetic shielding (i.e., chemical shifts) and the nuclear quadrupolar coupling constants (QCC) are investigated by ab initio molecular orbital and density functional theory calculations. These calculations show that the nitrogen ((15)N and (14)N) and boron ((11)B and (10)B) chemical shifts should be rather insensitive to changes in r(BN) and that the concomitant changes in molecular structure. Calculations on hydrogen-bonded clusters, based on the crystal structure, indicate that H-bonding should also have very little effect on the chemical shifts. On the other hand, the (11)B and (14)N QCCs show large changes because of both effects. An analysis of the (10)B[(19)F] line shape in solid ND(3)(10)BF(3) yields a (11)B QCC of +/-0.130 MHz. This is reasonably close an earlier value of +/-0.080 MHz and the value of +/-0.050 MHz calculated for a [NH(3)BF(3)](4) cluster. The gas-phase value is 1.20 MHz. Temperature-dependent deuterium T(1) measurements yield an activation energy for rotation of the ND(3) group in solid ND(3)BF(3) of 9.5 +/- 0.1 kJ/mol. Simulations of the temperature-dependent T(1) anisotropy gave an E(a) of 9.5 +/- 0.2 kJ/mol and a preexponential factor, A, of 3.0 +/- 0.1 x 10(12) s(-)(1). Our calculated value for a [NH(3)BF(3)](4) cluster is 16.4 kJ/mol. Both are much higher than the previous value of 3.9 kJ/mol, from solid-state proton T(1) measurements.

Electron spin–lattice relaxation in radicals containing two methyl groups, generated by γ-irradiation of polycrystalline solids

The effects of methyl rotation on electron spin–lattice relaxation times were examined by pulsed electron paramagnetic resonance for the major radicals in γ-irradiated polycrystalline α-amino isobutyric acid, dimethyl-malonic acid, and l-valine. The dominant radical is the same in irradiated dimethyl-malonic acid and α-amino isobutyric acid. Continuous wave saturation recovery was measured between 10 and 295 K at S-band and X-band. Inversion recovery, echo-detected saturation recovery, and pulsed electron–electron double resonance (ELDOR) data were obtained between 77 and 295 K. For the radicals in the three solids, recovery time constants measured by the various techniques were not the same, because spectral diffusion processes contribute differently for each measurement. Hyperfine splitting due to the protons of two methyl groups is resolved in the EPR spectra for each of the samples. Pulsed ELDOR data were obtained to characterize the spectral diffusion processes that transfer magnetization between hyperfine lines. Time constants were obtained for electron spin–lattice relaxation ( T 1e), nuclear spin relaxation ( T 1n), cross-relaxation ( T x1), and spin diffusion ( T s). Between 77 and 295 K rapid cross-relaxation (Δ M s=±1, Δ M I=∓1) was observed for each sample, which is attributed to methyl rotation at a rate that is approximately equal to the electron Larmor frequency. The large temperature range over which cross-relaxation was observed suggests that methyl groups in the radical and in the lattice, with different activation energies for rotation, contribute to the rapid cross-relaxation. Activation energies for methyl and amino group rotation between 160 and 1900 K (1.3–16 kJ/mol) were obtained by analysis of the temperature dependence of 1/ T 1e at S-band and X-band in the temperature intervals where the dynamic process dominates T 1e.

Conformational stability of atropisomeric 1-naphthylcarbinols and 1-(1-naphthyl)ethylamines

The stereodynamics of atropisomeric 1-naphthylcarbinols and 1-(1-naphthyl)ethylamines exhibiting two stereogenic elements, i.e. a chiral center and a chiral axis, were investigated. PM3 calculations suggest that these atropisomers populate two major conformations bearing the most sterically demanding group perpendicular to the naphthalene moiety. Due to its high conformational stability, the atropisomers of 1-(1-naphthyl)-2,2-dimethyl-1-propanol can be separated by HPLC at room temperature. Kinetic studies revealed a free energy of activation for rotation about the chiral axis of 158 kJ/mol. Incorporation of less bulky substituents into the stereogenic center of this class of atropisomers results in significantly reduced steric hindrance to isomerization. The conformational stability of N,N′-dibutyl-1-(1-naphthyl)ethylamine was determined as 75.9 kJ/mol by variable-temperature NMR spectroscopy.

Further studies on the rotational barriers of Carbamates. An NMR and DFT analysis of the solvent effect for Cyclohexyl N, N-dimethylcarbamate

The solvent effect on the Gibbs energy of activation for rotation around the (CO)–N bond in cyclohexyl N, N-dimethylcarbamate was investigated by dynamic NMR spectroscopy and density-functional theory at the B3LYP/6-311+G ∗∗ level. The experimental barriers were about 15 kcal mol −1 with no appreciable variation when the solvent polarity was changed. A reaction field model was applied to theoretically mediate the solvent effect and the results were comparable to the experimental data. An analysis, based on the Onsager solvation theory, showed that the solvent effect on rotational barriers can be understood employing the total molecular dipole moment, the difference between the dipole moments of the ground and the transition state structures, or both, as appropriate.

Structural and Dynamic Features of Bis[2-(2-furyl)indenyl]zirconium Derivatives

Treatment of 2-indanone with 2-lithio-5-methylfuran, followed by hydrolysis, dehydration, deprotonation with n-butyllithium, and transmetalation, gave bis[2-(5-methyl-2-furyl)indenyl]zirconium dichloride (6a). The dynamic 600 MHz 1H NMR spectroscopic analysis in the temperature range between 253 and 128 K revealed the presence of two metallocene conformational isomers in a 60:40 ratio. It is likely that this observed feature just originates from freezing out the indenyl−furyl rotation. The corresponding activation barrier of 6a was determined at ΔG⧧rot = 7.0 ± 0.4 kcal mol-1 by a line shape analysis. A series of analogous 5-methyl-2-furyl- and 5-methyl-2-thienyl-substituted bis(indenyl)zirconium dibromides and dichlorides show analogous conformational behavior and almost identical rotational activation energies. In the bis[2-(5-methyl-2-furyl)indenyl]ZrR2 series the metallocene rotational barrier can apparently be frozen on the 600 MHz 1H NMR time scale for R = CH2CMe3 and R = CH2Ph. The di(neopentyl)Zr c...

Methyl Group Rotation and 1H and 2H Zeeman Relaxation in Organic Solids

We have measured the solid state 1H and 2H Zeeman relaxation rates R at room temperature in two methyl-deuterated samples of 1,9-dimethylphenanthrene. The 1H dipolar rate (d for 1H dipolar and 9 for 9-methyl group) and the 2H quadrupolar rate (q for 2H quadrupolar and 1 for 1-methyl group) were measured in 1-(trideuteriomethyl)-9-methylphenanthrene. The 1H dipolar rate and the 2H quadrupolar rate were measured in 1-methyl-9-(trideuteriomethyl)phenanthrene. Models are developed for both and (m = 1 or 9) due to methyl group rotation. In a large class of simple dynamical models for spin relaxation in the solid state, the ratios / are independent of the dynamics (except for the mass difference between the 1H and 2H nuclei). In the present case, these ratios are also independent of m. In addition, the ratios / (k = d and q) are dependent only on the activation energy for methyl group rotation and another related dynamical parameter, but they are independent of the interaction being modulated. Because many parameters factor out, these ratios can be predicted with few or no adjustable parameters, depending on the sophistication of the theoretical model. The agreement between theory and experiment is good, even for the simplest theoretical models. These agreements give one a greater confidence in the models for quadrupolar and dipolar relaxation, but particularly the latter, which, because of spin diffusion, are more difficult to test.

Order–disorder transition in pyridinium iodide: quasi-elastic neutron scattering study

Reorientational motion of pyridinium cation in pyridinium iodide across the phase transition ( T c=247 K) temperature is studied using a high-resolution quasi-elastic neutron scattering (QENS) technique. A clear evidence of the order–disorder transition with respect to the pyridinium ion reorientation is observed. In the high-temperature phase the pyridinium ion is found to undergo a sixfold jump rotation around the axis perpendicular to its plane. Weak quasi-elastic broadening is observed below the phase transition temperature, at T=240 K, indicating presence of residual disorder. Reorientational time and activation energy of rotation are also obtained in the high-temperature phase.

A QSPR model for the prediction of the gas-phase free energies of activation of rotation around the NC(O) bond

A novel approach to predict the gas-phase rotational activation energies of amides is presented. The quantitative structure property relationship (QSPR) treatment, using the statistical package, comprehensive descriptors for structural and statistical analysis (CODESSA), resulted in a three-parameter equation with R 2=0.982 for the Δ G ≠ gas, of a set of 24 N, N-dialkylamides.

Organometallic Computational Exercise: Semiempirical Molecular Orbital Calculations on (C6H6)Cr(CO)3 and (B3N3H6)Cr(CO)3

A student exercise employing semiempirical molecular orbital calculations in the area of organometallic chemistry is presented. PM3 level calculations are applied to the ground-state structures of C6H6)Cr(CO)3 and (B3N3H6)Cr(CO)3. The calculations are generally successful in computing the known features of the ground-state structures. Comparison of the geometries computed for free and complexed C6H6 and B3N3H6 allow discussion of the reasons for the changes observed. The structures of the transition states for rotation of the rings with respect to Cr(CO)3 are also computed. These results permit calculation of the activation energies for the ring rotations. The activation energies computed at the PM3 level are 0.4 and 18.3 kcal/mol for (C6H6)Cr(CO)3 and (B3N3H6)Cr(CO)3, respectively. These results highlight the differences between the two molecules. Students are encouraged to comment on structural changes in the rings, on the phenomenon of ring rotation in these systems and others such as ferrocene, on the question of the aromaticity of the borazine ring, and on the effect of π back-bonding on the positions of the carbonyl stretching bands in the both complexes.

Ab initio study of the barrier to internal rotation in simple amides. Part 2. N, N-dimethylacetamide and N, N-dimethyl-2-(mono-, di- and tri-) halogenacetamides

The free energies of activation for rotation about the amide C–N bond in R–C(O)N(CH 3) 2 (R=CH 3, CH 2F, CHF 2, CF 3 and CCl 3) were calculated at the MP2(fc)/6-31+G ∗//6-31G ∗ level and compared with the NMR gas-phase data. The results of calculations indicate that the repulsion between the substituent R and methyl group in ground state and the conformation of R are largely responsible for the difference in the free energies of the studied amides. The anti transition state is the preferred one in all the studied compounds.

Spin Labels as the Instrument for Analysis of Topochemistry of Polymer-Immobilized Ziegler Catalytic Systems

A spin labeling technique was used to investigate the topochemical characteristics of polymer carriers and immobilized metal complexes. Functionalized polyethylenes (PE) such as PE–grafted-polyallylamine (1), PE–grafted-polydiallyl-amine (2), and PE–grafted-poly-4-vinylpyridine (3), obtained by grafting polymerization of the corresponding monomers, were used as polymer carriers. Metal-containing polymers were obtained by immobilization to 2 with either TiCl4 (2/Ti) or Al(C2H5)2 Cl (2/Al). The stable nitroxyl radical 2,2,6,6-tetramethyl-4-(2′-oxy-4′,6′-dichlorotriazine)piperidine-1-oxyl $$(\dot R_1 )$$ was used for spin labeling 2 to give $$2/\dot R_1 $$ , while 2,2,5,5,tetramethyl-3-(N-acetoamidiiodine)-pyrrollidine-1-oxyl $$(\dot R_2 )$$ was used for spin labeling 3 to give $$3/\dot R_2 $$ . Radical 2,2,6,6-tetramethyl-4-hydroxy-pyperidine-1-oxyl $$(\dot R_3 )$$ was bonded to 2/Ti and 2/Al to give $$2/Ti/\dot R_3 $$ and $$2/Al/\dot R_3 $$ , respectively. The accessibility of the functionalized polymers 1–3 to interact with the spin labels and the immobilized metal complexes was examined. Estimation of the effective distances between the spin labels and the dynamic behavior of nitroxyl radicals in the functionalized polymer matrixes and metal-containing polymers revealed several important features of spin-labeled systems. Metallation of a functional covering makes the polymer more accessible for spin labeling and has a considerable effect on the dynamic characteristics of polymer matrix. A change of the rotational activation energy of the spin labels from 15 kcal/mol in $$2/\dot R_1 $$ to 44 kcal/mol for $$2/Ti(Al)/\dot R_3 $$ in the temperature range 100–150°C was observed.

Solid state molecular motion in sucrose octapalmitate as studied by deuterium NMR spectroscopy

Sucrose octapalmitate-d 11, d 24 and d 248 have been synthesized. Using 2H NMR T 1 and analyses of the temperature dependence of the lineshapes, a detailed description of the solid state molecular motional modes is presented. Activation energies for methyl and methylene group rotation in the fatty acyl chains have been determined. The sucrose moiety is found to be static on the solid state deuterium NMR timescale.

A solid state NMR and molecular orbital study of hydroxylammonium chloride

Deuterium and nitrogen-15 NMR spectroscopy has been used to measure the 2H quadrupolar coupling and 15N chemical shift tensors in solid hydroxylammonium chloride, NH3OH+Cl-, (HAC). In addition, the NH3 and OH dynamics have been investigated by variable temperature 2H line shapes and T1 measurements. The Arrhenius activation energy for NH3 rotation is 22.5 ± 1.8 kJ/mol with a pre-exponential factor of 8 ± 3 × 1012 s-1 from line shapes and 21.3 ± 2 kJ/mol with an infinite temperature correlation time, τinf,, of 5.0 ± 0.4 × 10-14 s from the T1 analysis. The latter value corresponds to a pre-exponential factor of 6.7 ± 0.5 × 1012 s-1, if a three-site exchange is assumed. There was no evidence for OH reorientation up to 405 K, indicating a rather strong OH···Cl hydrogen bond. Previously reported inconsistencies between crystal structure and molecular orbital derived N-O bond lengths are cleared up by performing geometry optimizations with large basis sets and taking electron correlation into account. The internal rotational potential for the isolated HA cation is calculated to be 5.8 kJ/mol at the MP2/6-31G** level, with the trans geometry preferred. Calculations that employ the neutron diffraction geometry and include the Cl- anions that surround the HA+ cation yield an upper limit for the activation energy for NH3 group rotation of 62 kJ/mol. Analysis of the deuterium spectrum and T1 data yield nuclear quadrupolar coupling constants of 160 ± 5 kHz and 194 ± 5 kHz (η = 0.50 ± 0.05) for the ND3 and OD deuterons, respectively. Density functional calculations of the deuterium and nitrogen-14 nuclear quadrupolar coupling constants at the B3LYP level show that it is necessary to include the influence of the surrounding chloride anions. We have also shown that it is possible to obtain accurate proton chemical shifts from the deuterium MAS spectrum of solid HAC-d4.Key words: solid state NMR, molecular dynamics, nitrogen 15 chemical shift anisotropy.

Ab initio study of the barrier to internal rotation in simple amides: 1. N,N-dimethylformamide and N,N-dimethylcarbamic halogenides

Free energies of activation for rotation about the amide C–N bond in X–C(O)N(CH3)2 (X=H, F, Cl and Br) were calculated at the MP2(fc)/6-31+G*//6-31G* and MP2(fc)/6-311++G**//6-311++G** levels and compared with NMR gas-phase data. The results of calculations indicate that the repulsion between X and methyl group in ground state and the repulsion between X or oxygen and nitrogen lone pair in transition states (TS) are largely responsible for the difference in the free energies of the studied amides. For X=H (DMF), the anti TS is more stable; for the cases X=Cl, Br, the syn TS is more stable, while for the case X=F the two transition states are energetically almost equivalent.

Internal rotation dynamics of nitromethane at low temperatures

A detailed Raman spectroscopic study of nitromethane in the temperature interval from room temperature to 15 K was undertaken. The crystal structure was stable from 15 to 242 K. In the interval 243–244 K a new phase was observed before the transition into the liquid state took place. These observations were supported by DSC measurements which indicated the existence of an intermediate phase between the stable crystal form and the liquid phase. The activation energy for the internal rotation of the methyl group was determined by measuring the FWHM of the symmetric CH 3 stretching band from 15 to 235 K. The activation energy found was 25±1 meV, which was in very good agreement with the energy difference between the ground energy level and the barrier top as determined by previous neutron and Raman scattering experiments [D. Cavagnat, J. Chim. Phys. Phys. Chim. Biol. 82(2–3) (1985) 239].

Mobility of Spin Probes in Micelles of Cationic Gemini Surfactants Having Dodecyl Groups.

The mobility of nonionic spin probes in gemini surfactant (alkanediyl-α, ω-bis (dimethyldodecylammonium bromide)) micelles was investigated by means of electron spin resonance (ESR) measurements to compare with that of conventional surfactant (dodecyltrimethylammonium bromide and ethyldodecyldimethylammonium bromide) micelles. The rotational correlation times, τR, of nitroxide spin probes in aqueous surfactant solutions were calculated from ESR spectra and plotted against the surfactant concentrations. As a result, τR abruptly increased in a concentration region, which is believed to correspond with critical micelle concentration (CMC). CMC's determined by means of the spin probe technique were almost the same as those estimated by the electrical conductance measurements. For the conventional surfactants, the τR values became constant below and above CMC, whereas for one of the gemini surfactants they continued to increase above CMC. Furthermore, activation energies for rotation, EaR, were determined from the temperature dependence of τR. EaR was dependent on the kind of the surfactants, suggesting that the mobility of the spin probes changes significantly with micelle structure. Thus the mobility of the spin probes reflects the discrepancy in the microenvironments between the gemini and conventional surfactant micelles.

X-Ray crystallographic analysis and the photochemical reaction of N-benzyl-N-methylmethacrylthioamide in solution and the solid state

Photochemical reaction of asymmetrically substituted N-benzyl-N-methylmethacrylthioamide both in solution and in the solid state was investigated. The thioamide exists in an equilibrium between two rotamers owing to rotation about the C(S)–N bond. The free energy of activation for the bond rotation was estimated as 22.7 kcal mol–1 by temperature-dependent 1H NMR spectroscopy. The thioamide crystallized in a chiral fashion, and the photoreaction in the solid state gave the optically active β-thiolactam whereas both the β-thiolactam and the corresponding thiazolidinethione were obtained by solution-phase photochemistry.

Pressure and Temperature Effects on the Hydrogen-Bond Structures of Liquid and Supercritical Fluid Methanol

The proton spin-lattice relaxation times and proton chemical shifts for the hydroxyl and methyl protons in methanol were measured at liquid and supercritical densities using capillary high-pressure NMR spectroscopy. The pressure range for the proton nuclear relaxation measurements was between 50 and 3500 bar over a temperature range of 50--3500 bar and a temperature range of 298--773 K. Attempts were made to separate the contributions of the dipolar and spin-rotation interactions to the spin-relaxation processes at each thermodynamic condition over methanol densities ranging from liquid to supercritical fluid. An average number of hydrogen bonds per molecule in methanol and the apparent activation energy of the methyl group internal rotation have been extracted from the experimental relaxation data. The extracted quantities show a moderate pressure dependence in addition to temperature effects, which suggest that molecular packing effects on hydrogen-bonded methanol are important at higher pressures. A comparison between methanol and water at similar thermodynamic conditions was also made to obtain new insight into these two important supercritical solvents.