A nuclear magnetic resonance study of molecular motion in solid diethylamine and in diethylamine clathrate deuterate

The proton magnetic resonance second moment and spin-lattice relaxation data are reported for the two solids namely pure diethylamine and diethylamine clathrate deuterate, over the temperature range 77 K to 270 K. The results indicate that in both materials the only motion which occurs at a rate great enough to affect the N.M.R. observables is methyl group reorientation and for such motion activation energies of (2·9₀±0·02) kcal mole^-1 and (2·3₄±0·02) kcal mole^-1 are obtained for pure diethylamine, and the deuterate, respectively. The strength of the dipolar interaction in the deuterate as estimated from both the second moment and the maximum in the temperature dependence of nuclear relaxation rate is consistent with a carbon-proton distance of 1·10 Å and a large degree of chemical exchange of the amine protons with the deuterons of D₂O.     

Nuclear spin relaxation and magnetic shielding tensors of atoms constituting cyanogen bromide molecule

The temperature dependence of the correlation time describing reorientation kinetics of cyanogen bromide in CDCl₃ solution has been determined on the basis of the linewidths of the ¹⁴N NMR signal. It has been found that the longitudinal spin relaxation of the ¹⁵N nucleus occurs by shielding anisotropy and spin-rotation mechanisms, whereas for the ¹³C nucleus these mechanisms are of lesser importance. In the latter case the scalar relaxation of the second kind due to carbon-bromine coupling is the predominant relaxation mechanism. The parameter values: ¹ J(¹³C—⁷⁹Br) = 349 ± 10 Hz, T ₁ (⁷⁹Br, 303 K) = 2.31 ± 0.22 × 10^?7 s, Δσ(¹⁵N) = 565 ± 16 ppm and Δσ(¹³C) = 276 ± 120 ppm have been determined from the relaxation data analysis. The shielding anisotropy parameters Δσ(¹⁵N) = 580 ± 50 ppm and Δσ(¹³C) = 274 ± 9 ppm have been independently determined using ¹³C and ¹⁵N NMR in liquid crystalline solvent. The experimentally determined shielding tensors for sp-hybridized atoms in the investigated compound and in a series of bromoacetylenes have been compared with the results of quantum mechanical calculations [GIAO, DFT B3LYP/6-311 + +G(2d,p)]. The ‘heavy atom effect’ shielding bromine-bonded carbons is of the order of — 25 ppm and concerns mainly the σ⊥ component.     

Microwave spectra of cyclohexyl bromide and cyclohexyl iodide

The microwave spectra of cyclohexyl bromide and cyclohexyl iodide have been investigated in the gas phase between 8 and 40 GHz. While for cyclohexyl bromide only low resolution spectra have been recorded, for cyclohexyl iodide high resolution spectra of both the equatorial and the axial isomer have been analyzed. The energy difference between the axial and the equatorial form is ΔG° = 0.8 ± 0.4 kcal/mole and ΔE_0,0 = 0.54 ± 0.15 kcal/mole in C₆H₁₁Br and C₆H₁₁I, respectively, the equatorial forms being more stable. Iodine nuclear quadrupole coupling constants for equatorial and axial C₆H₁₁I have been obtained. The present analysis contributes to the microwave investigation of the cyclohexyl halides.     

Electron spin relaxation studies of La@C82 in the solid state by EPR

The temperature dependence of EPR spectrum of La@C₈₂ in the powder of empty C₆₀ and C₇₀ mixed crystals was studied by EPR spectroscopy employing X- and Q-band microwave frequencies. The rigid limit spectra (at 4.2 K for the X-band and at 132 K for the Q-band) could be analyzed by static spectral simulation which yielded the EPR parameters,g _‖=2.0021,g _⊥=1.9970,^La A _⊥=7.8 MHz,^La A _‖～0 MHz and an isotropic¹³C coupling value of about 3 MHz. For higher temperatures an appreciable motional averaging effect was observed and the spectra were analyzed by using dynamic spectral simulation based on the stochastic Liouville equation, where we assumed an isotropic rotational motion with the Brownian diffusion. The calculated spectra reproduced the dominant feature of the temperature dependence of the spectra almost satisfactorily for both the X-and Q-band frequencies with the appropriate rotational correlation times. The Arrhenius plots of the correlation time gave two activation energies of 0.9 kcal/mol and 2.9–3.8 kcal/mol for the temperatures below and above 200 K, respectively.     

An N.M.R. study of molecular motion in solid trimethylaminegallane

Continuous wave and pulsed ¹H N.M.R. data have been obtained for solid H₃GaN(CH₃)₃ over the temperature range 63–300 K. A theoretical expression for the relaxation behaviour of a methyl group in a trimethylamine moeity undergoing various motions has been obtained to aid analysis of the data. We find the activation energy to rotation of the -GaH₃ group to be 3·6 ± 0·3 kJ/mole (0·86 ± 0·07 kcal/mole), and to a different motion in the molecule to be 21 ± 2 kJ/mole (5·0 ± 0·5 kcal/mole). In the continuous wave spectra effects due to motion of the -CH₃ groups and the whole -NMe₃ moeity may be distinguished.     

Conformational interconversion in the monovalent ions of 1,2,3,6,7,8-hexahydropyrene II

The E.P.R. spectra of the monovalent positive and negative ions of 1,2,3,6,7,8-hexahydropyrene reveal an alternating line broadening, caused by the non-synchronized motion of the two aliphatic bridges in the molecule. When this motion is described by the modified Bloch equations, using a four-sites jump model, spectra are obtained which agree quite well with the experimental ones.

The potential barrier for the conformational interconversion can be calculated from the temperature dependence of the lifetime of each configuration and is found to be 10·0±0·4 kcal mole^-1 for the radical anion and 3·5±0·4 kcal mole^-1 for the radical cation. The inversion rate at 0°C equals 2·8 × 10⁶ s^-1 for the anion and 4·2 × 10⁶s^-1 for the cation.     

Quantum effects in the low temperature reorientation of N2− in KCl and KI

The electron paramagnetic resonance (EPR) spectra of KCl:N₂^? and KI:N₂^? were studied as a function of temperature and applied uniaxial stress in the temperature range 2–35°K. The molecular reorientation rates were determined for both 60 and 90° reorientations from the motional broadening and narrowing effects in the observed spectra. Although thermally activated in the range 15–35°K for KCl:N₂^?, the parameters deduced from the data are not consistent with a simple classical model of the motion. In both hosts the 90° rotation is markedly faster than the 60° reorientation, in conflict with simple models, but explicable in terms of the suppression of the tunneling matrix elements by polarization of the host crystal. In KI:N₂^?, the 90° reorientation is still apparent at 1·7°K and the spectra clearly show the influence of the symmetry of the nuclear spin states upon the reorientation kinetics, an effect predicted by Sussman.     

An NMR investigation of the existence of halide and carboxylate co‐solute effects on the rotational barrier about the C?N bonds in urea and thiourea

Rotational barriers of the NH₂ group in thiourea and benzylurea in solution become slightly larger in the presence of anionic hydrogen‐bond acceptors. The interactions between the N? H bond and different anions were investigated in acetonitrile solution using ¹H{¹⁴N} variable‐temperature NMR. Line‐shape analyses of the spectra show a weakly anion‐dependent increase in ΔG^? of rotation, with benzoate having the strongest effect and iodide the weakest. Hydrogen‐bonding acidities of benzylurea and thiourea were also measured on the Abraham solute hydrogen‐bond acidity scale by proton NMR. Copyright © 2010 John Wiley & Sons, Ltd.     

Dynamics of [n.3]paracyclophanes (n = 2 – 4) as studied by NMR. Obtaining separate Arrhenius parameters for two dynamic processes in [4.3]paracyclophane

Extending our earlier findings for [3.3]paracyclophane, NMR line shape studies of the conformational dynamics in [3.2] and [4.3]paracyclophanes are reported, of which the former is conformationally homogeneous and the latter occurs in two enantiomeric forms. For [3.2]paracyclophane, the Arrhenius activation energy E_a = 11.6 ± 0.1 kcal/mol and preexponential factor log (A/s^?1) = 12.92 ± 0.07 were found. In [4.3]paracyclophane, the conformational dynamics are quite complicated because, apart from interconversions of each enantiomer into itself proceeding via inversion of the propano bridge with rate constant k₁, the enantiomers mutually rearrange with rate constant k₂ due to inversion of the butano bridge. The determination of Arrhenius parameters from dynamic ¹H spectra of the aromatic protons for these two conformational processes (E_a = 11.2 ± 0.5 kcal/mol and log (A/s^?1) = 13.6 ± 0.5 for the former, and E_a = 9.7 ± 0.4 kcal/mol and log (A/s^?1) = 13.2 ± 0.4 for the latter) is the highlight of this work. In the investigated temperature range, in [4.3]paracyclophane, the occurrence of other conformational processes beyond those mentioned above can be excluded, because they would produce different line shape patterns than those actually observed. Copyright © 2013 John Wiley & Sons, Ltd.     

Proton Dynamics in Letovicite, (NH4)3H(SO4)2: A H and N NMR Spectroscopic Study

The improper ferroelastic phase letovicite (NH₄)₃H(SO₄)₂ has been studied by ¹H MAS NMR as well as by static ¹⁴N NMR experiments in the temperature range of 296–425 K. The ¹H MAS NMR resonance from ammonium protons can be well distinguished from that of acidic protons. A third resonance appears just below the phase transition temperature which is due to the acidic protons in the paraelastic phase. The lowering of the second moment M₂ for the ammonium protons takes place in the same temperature range as the formation of domain boundaries, while the signals of the acidic protons suffer a line narrowing in the area of T_c. The static ¹⁴N NMR spectra confirm the temperature of the motional changes of the ammonium tetrahedra. Two-dimensional ¹H NOESY spectra indicate a chemical exchange between ammonium protons and the acidic protons of the paraphase.     

Conformational analysis of 3,3‐dimethyl‐3‐silathiane, 2,3,3‐trimethyl‐3‐silathiane and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane—preferred conformers,barriers to ring inversion and substituent effects

The first conformational analysis of 3‐silathiane and its C‐substituted derivatives, namely, 3,3‐dimethyl‐3‐silathiane 1 , 2,3,3‐trimethyl‐3‐silathiane 2 , and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane 3 was performed by using dynamic NMR spectroscopy and B3LYP/6‐311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion barriers ΔG^≠ for 1 and 2 were estimated to be 6.3 and 6.8 kcal/mol, respectively. These values are considerably lower than that of thiacyclohexane (9.4 kcal/mol) but slightly higher than the one of 1,1‐dimethylsilacyclohexane (5.5 kcal/mol). The conformational free energy for the methyl group in 2 (?ΔG° = 0.35 kcal/mol) derived from low‐temperature ¹³C NMR data is fairly consistent with the calculated value. For compound 2 , theoretical calculations give ΔE value close to zero for the equilibrium between the 2 ‐Me_ax and 2 ‐Me_eq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (?ΔG° = 1.8 kcal/mol) and the predominance of the 3 ‐SiMe_{3 eq} conformer at room temperature was confirmed by the simulated ¹H NMR and 2D NOESY spectra. The effect of the 2‐substituent on the structural parameters of 2 and 3 is discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Direct Determination of Motional Correlation Times by 1D MAS and 2D Exchange NMR Techniques

One- and two-dimensional static and magic-angle spinning (MAS) exchange NMR experiments for quantifying slow (τ_c> 1 ms) molecular reorientation dynamics are analyzed, emphasizing the extent to which motional correlation times can be extracteddirectlyfrom the experimental data. The static two-dimensional (2D) exchange NMR experiment provides geometric information, as well as exchange time scales via straightforward and model-free application of Legendre-type orientational autocorrelation functions, particularly for axially symmetric interaction tensors, as often encountered in solid-state²H and¹³C NMR. Under conditions of MAS, increased sensitivity yields higher signal-to-noise spectra, with concomitant improvement in the precision and speed of correlation time measurements, although at the expense of reduced angular (geometric) resolution. For random jump motions, one-dimensional (1D)exchange-inducedsidebands (EIS)¹³C NMR and the recently developed ODESSA and time-reverse ODESSA experiments complement the static and MAS two-dimensional exchange NMR experiments by providing faster means of obtaining motional correlation times. For each of these experiments, the correlation time of a dynamic process may be obtained from a simple exponential fit to the integrated peak intensities measured as a function of mixing time. This is demonstrated on polycrystalline dimethylsulfone, where the reorientation rates from EIS, ODESSA, time-reverse ODESSA, and 2D exchange are shown to be equivalent and consistent with literature values. In the analysis, the advantages and limitations of the different methods are compared and discussed.     

TRAPDOR double-resonance and high-resolution MAS NMR for structural and template studies in zeolite ZSM-5

A combination of ²⁷Al magic-angle spinning (MAS)/multiple-quantum (MQ) MAS, and ²⁷Al–{¹⁴N} TRAnsfer of Population in DOuble-Resonance (TRAPDOR) nuclear magnetic resonance (NMR) was used to study aluminium environments in zeolite ZSM-5. ²⁷Al–{¹⁴N} TRAPDOR experiments, in combination with ¹⁴N NMR were employed to show that the two tetrahedral peaks observed in the ²⁷Al MAS/3Q-MAS spectra of as-synthesized ZSM-5 are due to aluminium atoms occupying crystallographically inequivalent T-sites. A ¹³C–{²⁷Al} TRAPDOR experiment was used to study the template, tetrapropyl ammonium bromide (TPABr), in the three-dimensional pore system of ZSM-5. The inequivalency of the methyl groups of TPA was observed in the ¹³C–{²⁷Al} TRAPDOR spectra of as-synthesized ZSM-5 and the motion of the methyl end of the propyl chain appeared to be more restricted in the sinusoidal channel than in the straight channel.     

1H ENDOR investigations on gamma-irradiated betaine calcium chloride dihydrate in the incommensurate phase

The electron nuclear double resonance (ENDOR) technique is applied to study the influence of the incommensurate modulation on the proton hyperfine structure (hfs) tensors of the N(CH₃)₃ radical in betaine calcium chloride dihydrate (BCCD). The temperature dependence of the edge singularities due to the increasing of the incommensurate (IC) modulation amplitude could be observed in the¹H ENDOR of the CH₃ groups for the first time. The rotation pattern of the edge singularities observed in the¹H spectra in the IC phase of BCCD can be explained by a libration around the C₁-N axis with a tilt angle of about ±7°.     

The stabilization of unusual conformers in guest-host systems: Solid-state NMR investigations of 2-methylhexadecane in urea and thiourea

Solid-state nuclear magnetic resonance (NMR) spectroscopy is employed for the first time on urea and thiourea inclusion compounds (UICs and TICs) containing branched alkyl chains. In the present work,²H and¹³C NMR as well as X-ray diffraction studies of two selectively deuterated 2-methylhexadecanes in UIC and TIC are presented. An analysis of the derivedT ₁ data reveals significant differences between UICs and TICs, which can be attributed to differences in the motional features of the guest species. It is found that four different motional contributions have to be considered, namely, chain rotation, chain wobbling,trans-gauche isomerization and methyl group rotation. 2-Methylhexadecane in UIC exists in an almost all-trans conformation (gauche amount not more than 5%) and undergoes fast chain rotation (6-site jump process, activation energyE _A=16.7 kJ/mol). The analysis of the²H NMR spectra of 2-methylhexadecane-1,1′,2-d₇ in urea proves that the branched chain end exists in an eclipsed conformation. TheT ₁ data of 2-methylhexadecane-3-d₂ in thiourea can be reproduced by an overall rotation (E _A=9.8 kJ/mol) and atrans-gauche isomerization with torsional jumps around the C-3-C-4 bond (E _A=11.0 kJ/mol,gauche population=15%). As for the corresponding UIC, the²H NMR spectra of 2-methylhexadecane-1,1′,2-d₇ in TIC can be only explained by the existence of an eclipsed conformation at the branched chain end.     

Atomic reorientation rates in liquid chalcogenide glasses: NMR in Se and As2Se3

Temperature dependence of the NMR spectrum in liquid Se indicates that atomic reorientation in this molten ring-chain structure glass is governed by a distribution of correlation times with a median of τ^∗ = 5 × 10⁻⁵sec at 220°C. In liquid layer structure glasses at equivalent viscosity, atomic reorientation rates are not sufficient to narrow observed NMR spectra. SeSe and AsSe bonds in glassy Se and As₂Se₃ are distinguishable on the basis of their chemically shifted Se⁷⁷ NMR spectra.     

Molecular motion in supercooled glycerol

Quasi-elastic Rayleigh scattering of 14·4 keV photons has been measured on supercooled liquid glycerol at -30°C and 0°C by employing the Mössbauer effect. Total scattered intensity, quasi-elastically scattered intensity I _q and energy width of I _q(k, ω) have been determined for k=0·6 to 4·2 Å^-1. The molecular motion is modelled as: random-walk diffusional motions for the centre-of-mass translation and for the orientation of independent rigid molecules, plus fast-bounded translational jitter (vibration). The model parameters are evaluated. The temperature dependence of the translational diffusion constant corresponds to an activation energy of 12 kcal/mol. Comparison is made especially with N.M.R. results for rotational motion. The effect of orientational jitter (libration) is considered and its possible influence on nuclear magnetic relaxation is pointed out.     

2H NMR study of phase transition and hydrogen dynamics in hydrogen bonded organic antiferroelectric 55DMBP-H2ca

Hydrogen dynamics in one-dimensional hydrogen bonded organic antiferroelectric, co-crystal of 5,5’-dimethyl-2,2’-bipyridine (55DMBP) and chloranilic acid (H₂ca), was investigated by use of ²H high resolution solid-state NMR. The two types of hydrogen bonds O-H …N and N⁺-H …O^? in the antiferroelectric phase were clearly observed as the splitting of the side band of the ²H MAS NMR spectra of the acid-proton deuterated compound 55DMBP-D ₂ca. The temperature dependence of the spin-lattice relaxation time was measured of the N⁺-H and O-H deuterons, respectively. It was suggested that the motion of the O-H deuteron is already in the antiferroelectric phase in the fast-motion regime in the NMR time scale, while that of the N⁺-H deuteron is a slow motion. In the high-temperature paraelectric phase, the both deuterons become equivalent and the fast motion of the deuterons in the NMR time scale is taking place with the activation energy of 7.9 kJ mol^?1.     

The repulsive energy in sodium chloride and potassium chloride crystals

The repulsive energy in sodium chloride and potassium chloride crystals has been evaluated and related to the lattice constant. The accuracy aimed at was ±0·2 kcal mole^-1 but owing to some unidentified experimental error the accuracy claimed is only ±0·6 kcal mole^-1 as compared with previous analyses in which the uncertainty is about ±2 kcal mole^-1. The improvement has been achieved by critical evaluation of the experimental data, by the elimination of inaccuracies from the thermodynamic formulae and by carrying out the analysis at several specified temperatures.     

Deuterium NMR studies of guest motions in urea inclusion compounds of 1,6-dibromohexane with analytical evaluation of spectra in the fast motion limit

²H NMR (nuclear magnetic resonance) spectroscopy, in conjunction with X-ray diffraction experiments, was used to characterize the guest motions of 1,6-dibromohexane in its urea inclusion compound. These motions are characterized by alkyl chain jumps between two conformations, each approximately gauche to the terminal bromines, which remain stationary. In this distorted urea channel, one conformer is heavily preferred, but thermally activated population of the unfavorable conformer leads to reversible, temperature-dependent changes in the unit cell parameters. Although rapid motions of the guest chain give rise to ambiguities in the interpretation of the²H NMR spectra, fortuitous temperature-independent spectral features of guests containing deuterium at the α, β and γ positions indicate that the guest motion resembles a two-site jump with unequal probabilities. Analytical lineshape calculations on the three sets of²H NMR spectra indicate a single jump mechanism in which the range of jump angles is narrowly prescribed. This NMR model provided a starting point for successful solution and refinement of the crystal structures at 213 and 298 K, which had been complicated by motional disorder.