Raphy Poupko

ESR and NMR in Aqueous and Methanol Solutions of Copper(II) Solvates. Temperature and Magnetic Field Dependence of Electron and Nuclear Spin Relaxation

ESR line‐shape measurements of Cu(ClO4)2 solutions in methanol and water at X‐ and Q‐band frequencies are reported. In both frequencies, above room temperature, the over‐all linewidth increases with increasing temperature, while at low temperatures the slopes level off and (at Q band) even change signs. The results are quantitatively interpreted in terms of two main electronic relaxation mechanisms, viz., modulation of the spin rotation and anisotropic g‐tensor interactions. From the magnetic parameters of the complexes determined in frozen solution and the field dependence of the linewidth, the correlation times are determined. It is found that in both solvents the anisotropic g tensor is modulated by the fast hopping of the complex distortion axis rather than by the molecular tumbling of the complex. Assuming a random jump of the distortion axis between the three octahedral principal directions, the kinetic parameters of this hopping process are calculated to be τi(25°C)=1.6× 10−11 sec−1, ΔEi=1.2 kcal/m...

Deuterium NMR and molecular ordering in the cholesteryl alkanoates mesophases

Deuterium NMR of the smectic, cholesteric, and the blue phases of two members of the cholesteryl alkanoate series, viz. cholesteryl nonanoate and cholesteryl myristate, are reported. Deuterium spectra of probe molecules dissolved in these liquid crystals are also discussed. The measurements were made at 63 kG (42 MHz). At this high field both the smectic and cholesteric phases can be fully or partially aligned with the director oriented predominantly perpendicular to the magnetic field. The results provide information on the order parameter in the various phases and on the ordering process of the helical axis. It is found that the smectic mesophase forms well oriented domains, but that the cholesteric phase consists of two types of domains: (i) uniformly oriented domains with the helical axis parallel to the magnetic field and (ii) randomly oriented domains apparently with focal conic texture. The relative abundance of the two types of domains depends on the thermal history of the sample and other paramet...

Orientational Order in the Lyomesophases of the Disodiumchromoglycate-Water Systme by 2D, 17O and 23Na NMR

Abstract Deuterium, oxygen-17 and sodium-23 NMR measurements in the mesophase region of the disodiumchromoglycate (DSCG)-Water system are reported. The results indicate that, depending on the temperature and concentration of the DSCG, at least three different mesophases can exist in the system: At temperature below -4°C a relatively highly ordered smectic-like phase exist (phase III) which was not reported previously. Above - 4°C two phases, labeled N and M appear. The dividing line between these two phases is at approximately 18 wt.% DSCG and depends slightly on temperature. Phase N which prevails on the low concentration side of the dividing line, appears to be nematic-like as manifested by the fact that it reorients in a magnetic field, while phase M which appears on the high concentration side of the dividing line seems to be smectic-like. There is no discontinuity in the ordering characteristics of the water solvent in the N-M transition region, suggesting that these two phases have similar ordered s...

Dynamic deuterium NMR in liquid crystalline solvents: Ring inversion of cyclohexane‐d12

The application of deuterium NMR spectroscopy of molecules dissolved in liquid crystalline solvents to study intramolecular dynamic processes is demonstrated using the ring inversion kinetics of cyclohexane‐d12. The spectra of C6D12 dissolved in phase V and in hexyloxyazoxybenzene were studied over the temperature range −36 to +115 °C, and were found to exhibit a very pronounced line shape variation with temperature. At low temperatures (<−10 °C) the spectrum consists of two symmetric doublets due to the axial and equatorial sites. The doublet splittings are due to the quadrupole interactions of the corresponding deuterons. Upon increasing the temperature the lines broaden until finally the spectrum transforms into a single doublet with a spacing corresponding to the mean quadrupole splitting of the axial and equatorial deuterons. In an Appendix the line shape theory for two interacting nuclei of spin I = 1 which undergo mutual exchange is presented, and this theory is used to quantitatively interpret the...

Deuterium two−dimensional exchange nuclear magnetic resonance by rotor−synchronized magic angle spinning

The method of two‐dimensional exchange spectroscopy under condition of magic angle sample spinning (MAS) and synchronization of the mixing time with the rotor period is extended to spin I=1 nuclei. Theoretical equations are derived for the cross peak intensities as a function of the magnetic and kinetic parameters of the system and the method is demonstrated on a number of deuterated compounds. Dimethylmalonic acid–d6 is first used to illustrate the effect of rotor synchronization by the complete absence of cross peaks when no exchange takes place. The method is then applied to two dynamic systems, viz. dimethylsulfone–d6 and thiourea–C6D12 inclusion compound. The experimental results are compared with simulations as well as with analogous experiments on nonspinning samples. Since chemical shift effects are often negligible in deuterium NMR the time domain sampling can in principle be reduced to the number of the desired spinning sidebands, resulting in considerable time savings. The main advantage of the...

Dynamic frequency shift in the E.S.R. spectra of transition metal ions

The factors which determine the electron spin resonance lineshapes and shifts of transition metal complexes in solution are discussed. It is shown that for ions with S > ½ the dynamic frequency shift may have pronounced effects on the E.S.R. lineshapes, and experimental evidence for this effect in complexes of Cr3+, Fe3+ and Gd3+ is presented. A quantitative interpretation of these spectra in terms of a relaxation mechanism due to fluctuation of the quadratic zero field splitting interaction is given.

Electron spin resonance and photoreaction of Mn(II) in lettuce chloroplasts

Abstract Electron spin resonance (esr) of lettuce chloroplasts yields three types of signals: (i) a broad (~900 G) signal around g = 2.22 (apparently due to Cu 2+ complexes); (ii) an Mn 2+ spectrum around g = 2.003 consisting of six hyperfine lines ( A = 94.5 G ) of ~30 G width; and (iii) a sharp signal at g = 2.00 due to photosignals I and II. The present work is concerned with the Mn 2+ signal and its relation to the photosynthetic process. Intensity measurements were performed by comparing the intensities of the Mn 2+ signals of two identical chloroplast preparations, one of which was slightly acidified. The integrated intensity of the signal in the normal preparation was approximately one-fourth of that in the acidified sample, suggesting that only the− 1 2 ↔ 1 2 fine structure band is observed in untreated chloroplasts. This indicates that the manganese in the chloroplasts is bound in an asymmetric environment, apparently in protein complexes. The Mn 2+ signal is light sensitive, decreasing on illumination and reappearing in the dark. Typical values for the half-lives of the light and dark processes in normal chloroplasts are 0.25 and 2.1s, respectively. The effect is interpreted in terms of the photooxidation of Mn 2+ to higher oxidation states which are invisible to esr spectroscopy. In order to determine whether this process is related to photosynthesis the effect of certain reagents and treatments that are known to affect the photosynthetic system was studied. It was found that the oxygen evolution inhibitors 3-(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU) and carbonylcyanide- p -trifluoromethoxyphenylhydrazone (FCCP) as well as the electron donors, phenylenediamine and sodium ascorbate, reduce or completely eliminate the light effect on the Mn 2+ signal. Heat treatment and Tris washing caused deceleration of both the light and dark reactions. These effects indicate that the photooxidation of the Mn 2+ is related to the photosynthetic cycle, the most probable site being the water splitting apparatus of photosystem II.

Tetrabenzocyclododecatetraene a new core for mesogens exhibiting columnar mesophases

Abstract Octasubstituted derivatives of tetrabenzocyclododecatetraene are found to exhibit thermotropic mesophases. The molecules of these mesogens are highly flexible with an average four-fold symmetry. Optical and differential scanning calorimetry measurements are presented on five ester and ether derivatives. All mesophases appear to be columnar with a negative optical anisotropy. From the five compounds studied the four which are derived from aliphatic ester and ether are optically uniaxial, while the mesophase of the decylbenzoyloxy derivative could be biaxial. Miscibility measurements performed with some discotic and pyramidic mesogens of similar molecular dimensions suggest that the mesophases of the tetrabenzocyclododecatetraene core form new families of columnar mesophases.

Theory of dynamic magic angle spinning nuclear magnetic resonance and its application to carbon‐13 in solid bullvalene

Carbon‐13 magic angle spinning (MAS) spectra of bullvalene, at a spinning frequency of about 4 kHz, are presented for the temperature range −30 °C to +85 °C. At low temperatures (<−10 °C), separate center peaks are observed for the aliphatic and olefinic carbons, as well as spinning sidebands for the latter. Upon heating to room temperature and above, the peaks broaden and eventually coalesce to a single line at the weighted average chemical shift frequency. These results are interpreted in terms of two independent processes—a concerted Cope rearrangement reorientation and symmetric threefold jumps about the molecular C3 axes. In both processes, the high order of the bullvalene crystals is preserved. A quantitative analysis of the spectra yields the following kinetic equations for, respectively, the Cope rearrangement reorientation and the threefold jump processes: kC(s−1)=1.38×1014 exp(−14.5/RT); kJ(s−1)=2.53×1019 exp(−21.0/RT), where R is in kcal/mol degree. The analysis of the spectra was performed by ...

Dynamic 2H NMR by quadrupole echo train in the fast-exchange limit. Ring inversion of cyclohexane-d12 in liquid crystalline solvents

Abstract A NMR method is proposed to determine the kinetic and magnetic parameters for spin I = 1 nuclei with quadrupole interactions, undergoing exchange in the fast dynamic limit. The method consists of measuring the quadrupole echo decay rate under the effect of a sequence of π 2 pulses and is the quadrupole echo analog of the Can-Purcell-Meiboom-Gill experiment on spin I = 1 2 systems. The echo decay is exponential, and by use of the average Hamiltonian formalism, a general expression is derived for its relaxation rate as function of the pulse repetition rate and the π 2 pulse width. Expressions are also given for the limiting cases of infinitely narrow pulses at one end and spin-lock conditions which correspond to T1ϱ, on the other. The method is applied to a liquid crystalline solution of cyclohexane-d12 undergoing rapid ring inversion. The kinetic and magnetic parameters derived by the present method are in perfect agreement with those obtained earlier by complete lineshape analysis of the deuteron NMR spectrum of this system.