Activation Energy For Rotation Research Articles

The synthesis, molecular structure and fluxional properties of the 1-phenylpropyne complex [WI 2(CO){Ph 2P(CH 2)PPh 2}( η2-MeC 2Ph)

Reaction of [WI 2(CO){Ph 2P(CH 2)PPh 2}( η 2-MeC 2Ph)] mono(1-phenylpropyne) complex [WI 2(CO){Ph 2P(CH 2)PPh 2}( η 2-MeC 2Ph)] ( 1) in high yield. The structureof 1 has been determined by single crystal X-ray diffraction. The structure can best be described as a distorted octahedron with the two phosphorus atoms of the Ph 2P(CH 2)PPh 2 ligand trans to an iodo- and a 1-phenylpropyne ligand in the equatorial plane and the other iodo and carbonyl groups in the axial sites. The two bond lengths of the two W-I and those of the two W-P bonds are quite different reflecting the nature of the trans-ligands. Variable temperature 31P NMR studies have been carried out on 1, which has enabled the activation energy for alkyne rotation, to be calculated.

Molecular motions of nitroxyl radical spin probes in X-zeolites. Dependence on zeolite cation and spin probe chemical functional group

The nitroxyl radical spin probes, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 4-oxo-TEMPO (TEMPONE) and 4-amino-TEMPO (TEMPAMINE) were examined by continuous wave (CW) electron paramagnetic resonance (EPR) in a series of cation-exchanged X-type faujasite zeolite supercages. The spectra in Li–X, Na–X and K–X between 10 and 300 K were fitted by varying the rotational correlation constants, the homogeneous linewidth, and the principal nitrogen nuclear hyperfine coupling matrix element ( A zz ) for the axis near the C–N–C plane normal. The TEMPONE activation energies for rotation, 12.1–3.1 kJ mol −1, decrease monotonically with cation size, suggesting an attractive interaction with the cation that depends on cation Lewis activity. TEMPO and TEMPAMINE have activation energies for rotation, 6.7–12.8 and 2.8–10.0 kJ mol −1, respectively, that increase with cation size, suggesting non-bonded repulsion of the molecules by the cations. We propose that the attraction of the cation to TEMPONE is via its carbonyl π electrons, an interaction found for the larger phenalenyl π systems in previous work (D.C. Doetschman, D.W. Dwyer, J.D. Fox, C.K. Frederick, S. Scull, G.D. Thomas, S.G. Utterback, J. Wei, Chem. Phys. 185 (1994) 343). In contrast, the amino group of TEMPAMINE is sp 3 hybridized and has essentially no π character. It is well known from X-ray diffraction studies, in which the NO group is found to project out of the C–N–C plane, and from the magnitude of the A zz nuclear hyperfine coupling element that the NO group has relatively little π character. The activation energies of the TEMPO rotation are consistently larger than those of TEMPAMINE. We propose that the volume of TEMPO is effectively larger than that of TEMPAMINE because TEMPO is undergoing rapid piperidine ring inversion which does not occur in TEMPAMINE. Preliminary pulsed EPR results for TEMPO are presented that appear to indicate a second type of molecular motion associated with the motion with the piperidine ring inversion.

Molecular Flexibility of N-Acyl Heterocycles Studied Using 1 3C NMR Spectroscopy and Computational Chemistry

The 13C NMR spectra of a series of N-acyl six-member ring nitrogen heterocycles have been measured as a function of temperature and the results interpreted in terms of restricted amide rotation and ring inversion. Slow rotation is observed on the NMR time scale around the N-CO bond whilst the ring inversion is fast at ambient temperature. High temperature experiments have been undertaken to investigate the free energy of activation for the amide rotation and low temperature NMR experiments were used to probe the ring inversion process. The experimental results have been compared with theoretical calculations using molecular orbital, molecular mechanics and molecular dynamics approaches and good agreement is observed in all cases.

X-Ray crystallographic analysis and photochemical reaction of asymmetrically substituted α,β-unsaturated thioamides

Photochemical reaction of asymmetrically substituted N-benzyl-N-isopropyl-α,β-unsaturated thioamides both in solution and in the solid state was investigated. All thioamides exist in an equilibrium between two rotamers owing to the rotation of the C(S)–N bond. The free energy of activation for the bond rotation was estimated by temperature-dependent 1H NMR spectroscopy. The free energy of activation lies in a range 18.4–19.5 kcal mol–1. Irradiation in benzene solution proceeded to give hydrogen abstraction by alkenyl carbon from both benzyl and isopropyl groups, leading to β-thiolactam and 1,3,5-dithiazinane products, respectively. Hydrogen abstraction from only the isopropyl group took place in the solid-state photolysis, giving an isomeric β-thiolactam product.

Proton mobility in the solid inorganic hydrates of acids and acid salts

The method for the calculation of the proton-transfer frequency (nt) and its activation energies (Et) was suggested. Results of the calculations were presented. The experimental data on the activation energy of proton-containing group rotation and protonconductivity values for some hydrates of inorganic acids and acid salts were compared with the calculated ones.

Dynamics of phase transitions in a liquid crystalline compound (TBDA) probed by Raman spectroscopy

The precise value of linewidths obtained after correcting for the slitwidth effect for five Raman bands associated with the core of a liquid crystalline compound, terepthal-bis-decyl aniline (TBDA), have been studied in the temperature range of 12 - 472 K. The variation of the linewidth with temperature confirms two already reported phase transitions at 346 and 427 K. However, the phase transition at 427 K is not purely first order as reported earlier, but of mixed order in nature. From the measured linewidths of the and modes below room temperature, the estimated activation energy for rotation along the bond closely matches with the phase transition temperature suggesting that the rotation about the bond is closely linked with this transition.

Chemistry of cyclic aminooxycarbenes

A series of oxazolidin-2-ylidenes and one tetrahydro-1,3-oxazin-2-ylidene, generated by thermolysis of Δ3-1,3,4-oxadiazolines in benzene at 90 °C, were intercepted by insertion into the OH bond of phenols. In two cases the initial products rearranged to N-(2-aryloxyethyl)-N-methylformamides. The activation energy for rotation about the amide CN bond of those ultimate products was measured as 20.4 kcal/mol. The aminooxycarbenes reacted with two equivalents of methyl or phenyl isocyanate to give spiro-fused hydantoins. Major products from the reactions of the N-carbonyl carbenes with dimethyl acetylenedicarboxylate or with methyl propiolate were 2-oxazolines resulting from apparent acyl transfers from N to C in the proposed dipolar intermediates; minor products of 1:2 (carbene:trap) stoichiometry were also observed. Keywords: nucleophilic carbene, aminooxycarbene, oxadiazoline, amide rotation, oxazolidine.

Cationic Self-diffusion in Solid Choline Perchlorate Studied by NMR

Abstract The 1H spin-lattice and spin-spin relaxation times, and the second moment of the 1H NMR linewidth of choline Perchlorate, [(CH3)3NCH2CH2OH]ClO4, were measured in its highest-temperature solid phase, i. e. above 275 K. X-ray powder patterns taken at ca. 380 K revealed that in this phase the crystal has a CsCl-type cubic structure (a = 6.326(4) Å and Z = 1). From 1H NMR experiments it was found that the cations in this phase undergo isotropic rotation and translational self-diffusion. From the 1H T1, measurements, the activation energies of the cationic rotation and self-diffusion were evaluated to be 21.4 ± 0.4 and 62 ± 3 kJ mol-1 , respectively.

Deuterium and Carbon‐13 NMR Study of Molecular Rotational Diffusion in Two Star‐Hydrocarbon Oils

AbstractRelaxation rates of carbon‐13 magnetization of the C9 carbon positions in two poly‐n‐octyl star‐hydrocarbon oils, 9‐n‐octyl‐n‐heptadecane and 9,10‐di‐n‐octyl‐n‐octadecane, and of deuterium magnetization in 9‐n‐octyl‐n‐heptadecane‐9d, and 9,10‐di‐n‐octyl‐n‐octadecane‐9d1, labelled with deuterium at the same carbon position, have been measured over wide ranges of temperature and at two field strengths. In this paper simulation of the observed relaxation rates within the assumptions of the Lipari and Szabo picture of molecular rotational diffusion, and dipolar and quadrupolar relaxation mechanisms, determined molecular rotational diffusion constants, activation energies, and Lipari and Szabo order parameters and correlation times for internal molecular motion at the labelled positions. The appropriate quadrupolar coupling constants could also be determined from the measured data. The effect of the rate of internal molecular isomerization was discussed and it was shown that in general this had to be included in the theoretical picture in order to derive a consistent set of motional parameters that generated all observable relaxation rates of magnetization.

NMR analysis of restricted internal rotation in 2‐substituted‐2,3‐dihydro‐3‐o‐tolyl(chlorophenyl)‐4(1H)‐quinazolinones

AbstractSteric interactions between aryl and heterocyclic moieties in 2‐substituted‐2,3‐dihydro‐3‐o‐tolyl(chlorophenyl)‐4(1H)‐quinazolinones 1a‐j produce sufficient restriction to rotation about the aryl CN bond that the presence of torsional isomers may be detected at room temperature. Diastereomeric population and free energy of activation for rotation have been calculated by 1H nmr spectra. Probably due to a preferred axial position of R2 substituent no dramatic variation both in A/B ratio and in ΔG≠ value has been observed for 1a‐f. The comparison between 1a and 1j ΔG≠ values allows to formulate a hypothesis on the structure of the transition state.

Interaction of water with clay minerals as studied by 2H nuclear magnetic resonance spectroscopy

The interaction of water with a series of Ca 2+/Na + , Ca 2+/NH 4 +, and NH 4 +/Na +-exchanged hectorite and saponite clays in water- and clay-dominated systems was studied using 2H NMR. Variabletemperature Nuclear Magnetic Resonance (NMR) (in the range of 189–299 K) was also used to determine the frequency of exchange and the rotation activation energy between water molecules hydrating the cations and the free water of the tactoid. The 2H quadrupolar splitting and spin-lattice relaxation time ( T 1) are related to the ratio of clay to adsorbed water in these systems. In water-dominated systems (where clay/water ratio ≪ 1), motional averaging occurs between water hydrating the cations and the “free” or bulk water of the tactoid. In these systems, the spin-lattice relaxation time ( T 1) is dominated by the relaxation of the “free” water on the external surface of the tactoids and has a value close to that for D 2O (0.436 s), with the rotational correlation time ( τ c) approaching 2.93 × 10 −12 s. A more recent article places τ c at 1.95 × 10 −12 s. The clay systems in this study have clay/ water ratios (=0.0238 g-clay/mL-D 2O), and the quadrupolar splittings are of the order of 100 Hz or less. Splittings of <100 Hz indicate that approximately 1 water molecule is bound to the clay for every 1.7 × 10 3 “free” water molecules. In clay-dominated systems with extremely low water contents (clay/water ratio > 1 g-clay/mL-D 2O), less motional averaging occurs. Rotational correlation times are slower and greater residual quadrupolar splittings are also observed. Increased residual quadrupolar splittings are observed with increasing clay/ water ratios. For a Na-exchanged hectorite sample (5.31 g-clay/mL-D 2O), the quadrupolar splittings are of the order of 12 kHz, and for a Na-exchanged saponite (4.05 g-clay/mL-D 2O), the quadrupolar splittings are of the order of 11 kHz. Rotational activation energies (E a) of 37.8 kJ-mole −1 (9.0 kcal · mole −1) for adsorbed water molecules on Na-exchanged hectorite and 27.2 kJ · mole −1 (6.5 kcal · mole” −1) for Na-exchanged saponite were calculated from spin-lattice relaxation ( T 1) measurements. These values compare with calculated activation energies of rotation (8.5 kcal · mole −1) for water of hydration adsorbed to a Na-exchanged vermiculite.

Solid State Dynamics of Tricarbonyl(eta-1,5-cyclohexadienylium)iron Tetrafluoroborate and Tricarbonyl(eta-1,5-cycloheptadienylium)iron Tetrafluoroborate.

The dynamic behavior of [(C(6)H(7))Fe(CO)(3)]BF(4) (I) and [(C(7)H(9))Fe(CO)(3)]BF(4) (II) in the solid state has been investigated principally by NMR spectroscopy. High-resolution variable-temperature (1)H and (13)C NMR spectra indicate that both complexes have a solid state phase transition above which there is rapid reorientation of the cyclodienylium rings and fast exchange of the carbonyl groups. The transition occurs between 253 and 263 K for I and between 329 and 341 K for II. The presence of the phase transition is confirmed by differential scanning calorimetry (DSC). (57)Fe Mössbauer spectroscopy supports the notion that complex I is highly mobile at room temperature, while II is relatively static. The activation energy for the cyclodienylium group rotation in the high-temperature phase of I is estimated from (1)H spin-lattice relaxation time measurements to be 17.5 kJ mol(-)(1). Static (13)C NMR measurements of the solid complexes in the high-temperature phase indicate that the (13)C chemical shift anisotropies are only 20-30 ppm. This is significantly less than that expected to result from motion of individual groups and thus suggests that rotation of the whole molecule is involved. A single-crystal X-ray structural determination of complex II, at 295 K, showed that the complex is tetragonal (space group P4(1), a = 10.610(1) Å, c = 21.761(3) Å, V = 2449.7(5) Å(3), rho(calc) = 1.734 g cm(-)(3)), with eight cycloheptadienyl cations and eight tetrafluoroborate anions per unit cell. In addition, powder X-ray diffraction studies of both I and II confirm that at low temperatures both complexes have a tetragonal unit cell, which transforms to a cubic unit cell above the phase transition. The powder patterns, recorded above the phase transition, support the proposal that the complexes are undergoing whole-molecule tumbling in their dynamic regimes.

Water electret relaxation at dispersed silica surfaces

The behavior of water adsorbed at highly dispersed silica surfaces at low temperatures has been studied by thermally stimulated depolarization (TSD) and quantum chemical methods. Three relaxation maxima of the TSD spectra are observed without high-temperature treatment of the samples, and are interpreted as connected with uni-, bi- and tridimensional water clusters. Dynamic simulation of different relaxation processes of water molecules, which bind to silica surfaces directly or through hydrogen bonds with other adsorbed molecules, shows that the relaxation time decreases as the water cluster grows. The activation energy of rotation is a maximum for molecules with donor-acceptor bonds Si←OH 2 in the complexes ▪.

Dielectric Properties and Dynamic Simulation of Water Bound to Titania/Silica Surfaces

Water adsorbed on titania/silica (TS) surfaces has been studied by dielectric spectroscopy (DS) in the 3-10 MHz frequency range at T = 120-300 K, thermally stimulated depolarization (TSD) at the same temperatures, 1 H NMR, and quantum chemical methods. The TSD spectra ofTS have only one relaxation maximum after sample heating at 400 K. The activation energies of depolarization (TSD) and polarization (DS) decrease at TiO 2 content increasing in TS. According to the TSD, DS, and 1 H NMR data, water adsorbed on the TS surfaces essentially in the interface region forms more weakly bound clusters than on the silica surfaces. Simulation of vibrational-rotational relaxation of the water molecules attached to the TS surfaces shows that the activation energy of rotation is a maximum for the molecules having donor-acceptor bonds, Me-OH 2 (Me = Si, Ti), in the complexes HO-Me-OH 2 + mH 2 O and it is minimum for one water molecule linked to the Ti-O(H)-Si bridge through a hydrogen bond.

A multinuclear NMR study of N-(chlorodimethylsilylmethyl) amides

A series of N-(chlorodimethylsilylmethyl) amides of the type RCONR′(CH 2Si(CH 3) 2Cl), where R=H, R′=CH 3; R = CH 3, R′ = CH 3; R = H, R′ = C 6H 5; and R = CH 3, R′ = p-XC 6H 4 (X = H, OCH 3, Cl, Br, CF 3, NO 2) were prepared by amination or transsilyation. The 1H, 13C, 14N, 17O, and 29Si NMR spectra were obtained and used t to determine the structure of the compounds. The 14N and 17O shifts are indicative of the amide structure reported previously. The 17O and 29Si shifts can be related to the extent of dative interaction between the carbonyl oxygen and the silicon and correlate with the carbonyl stretching frequencies. The 1H spectra of most of the acetanilides in CDCl 3 contain a broad peak for the Si(CH 3) 2 protons. At lower temperatures this peak separates into two sharp peaks of unequal intensities. The two peaks can be attributed to rotamers, with the large peak at higher frequencies being due to the rotamer with the CH 2Si(CH 3) 2Cl group cis to the carbonyl oxygen. In toluene, CH 2Cl 2 and THF, the Si(CH 3) 2 protons appear as a sharp peak which does not broaden upon heating to the boiling point of the solvent. The free energies of activation for rotation about the carbon-nitrogen bond were determined by the approximate method of Shanan-Atidi and Bar-Eli and show a rough correlation with the sigma constant of the substituent.

Hydrogen bonding of the ammonium ion

Hydrogen bonding of ammonium ions in crystals is discussed, using evidence based upon the NH bond length, vibrational frequencies, the activation energy for rotation, theoretical results on ammonium-water clusters and nuclear quadrupole coupling constants. The high symmetry environments found in crystals appear to inhibit hydrogen bond formation.

A noble ionic plastic phase of cesium formate studied by 1H, 2H and 133Cs NMR and electrical conductivity measurements

1H, 2H and 133Cs NMR spin-lattice relaxation times T 1, 1H spin-spin relaxation times T 2, spin-lattice relaxation times in the rotating frame T 1 ϱ , and the electrical conductivity of cesium formate were measured in the high-temperature cubic phase obtainable between 316K and the melting point (536K). The observed relaxation data were attributable to anionic overall rotation and self-diffusion of both ions. The activation energies of the anionic rotation and self-diffusion were evaluated, by applying the simple Bloembergen-Purcell-Pound theory, to be about 30 and 53 kJ mol −1, respectively, while that of the cationic diffusion was 45 kJ mol −1, in good agreement with 47 kJ mol −1 for the ionic diffusion determined by the conductivity measurements. From these results together with the thermal and structural data reported, it was shown that cesium formate forms an ionic plastic phase above 316 K.

Comparisons of activation energies for dimethyl sulfoxide rotations in the inner phase of seven carcerands

The syntheses and characterizations of nine new carceplexes are reported, the ΔG values for rotations of Me2SO in the inner phases of seven of them are determined and found to be unaffected by changes in bulk solvent.

Dynamics of benzene and pyridine guest molecules in their tri-ortho-thymotide inclusion compounds. Solid-state 2H NMR Studies

Dynamic properties of perdeuteriated benzene (C6D6) and perdeuteriated pyridine (C5D5N) guest molecules in their tri-ortho-thymotide (TOT) inclusion compounds have been studied via variable-temperature wide-line solid-state 2H NMR spectroscopy. The temperature dependences of the quadrupole echo 2H NMR lineshape and the 2H NMR spin–lattice relaxation time for C6D6/TOT in the temperature range 96–293 K are interpreted in terms of in-plane rotation (nearest-neighbour 2π/6 jump model) of the benzene molecule, together with restricted wobbling of the molecular plane. The 2H NMR spin–lattice relaxation time measurements suggest that there are two dynamically inequivalent types of benzene molecule (included in structurally different types of tunnel within the TOT host structure), with their activation energies for in-plane rotation estimated to be 4.0 and 5.9 kJ mol–1. For C5D5N/TOT in the temperature range 90–363 K, three types of motion have been identified: (i) in-plane libration; (ii)π jumps of each pyridine molecule about its two-fold symmetry axis; (iii) restricted wobbling of the molecular plane. On the basis of 2H NMR lineshape analysis of 2H NMR spin–lattice relaxation time measurements, the activation energy for motion (ii) is estimated to be ca. 17-20 kJ mol–1.

Internal Rotation of the Methyl Groups in the t-Butyl Radical as Studied by ESR

Abstract The ESR spectrum of the t-butyl radical in a rigid matrix is observed at temperatures between 6.6 and 70 K. At temperatures below about 50 K equally spaced nineteen lines are observed which is associated with the tunneling rotation of the three methyl groups. By the analysis of the spectral pattern at a low temperature of 6.6 K the pairwise interaction potential of the internal rotation, if any, is deduced to be proportional to cos θi cos θj where θi and θj are the internal rotation angles of two methyl groups. By the simulation of the spectral change with temperature the thermal activation energy for the internal rotation is estimated to be 6 ± 2 kJ mol−1.