Solid-State 13C-NMR spectral evidence for charge transfer complex formation in aromatic diimides and dianhydrides

Solution and solid-state proton decoupled ¹³C-NMR spectra were determined on two diimides derived from 4, 4′-oxydiphthalic anhydride. Comparison of the individual diimide spectra to that of a mixture of the two diimides indicates that ordering of these materials occurs in the solid state via charge transfer complex formation. A similar study was conducted using two isomeric dianhydrides, 4, 4′-isophthaloyldiphthalic anhydride (IDPA) and 4, 4′-terephthaloyldiphthalic anhydride (TDPA). The solution spectra of these compounds are similar and are those which would be expected for these compounds. However, their solid state spectra differ from each other. The solid-state spectrum of TDPA resembles its solution spectrum, whereas, that of IDPA differs greatly from its solution spectrum and indicates charge transfer complex formation occurs with this molecule. This difference is explained in terms of the stereochemistry of the two isomeric dianhydrides.     

Spin dynamics in conducting polymers studied by μSR

We report studies of spin dynamics in the conducting polymers polyaniline and polypyrrole using both μ⁺SR and μ^-SR techniques. These measurements reveal characteristic field dependences and cutoff frequencies for the muon spin relaxation which can be related to the spin diffusion process. Clear evidence is seen for increased spin localisation at low temperatures where a crossover occurs from two or three dimensional spin diffusion to a one dimensional diffusion regime. This revised version was published online in August 2006 with corrections to the Cover Date.     

The two‐variable fragment with counting and equivalence

We consider the two‐variable fragment of first‐order logic with counting, subject to the stipulation that a single distinguished binary predicate be interpreted as an equivalence. We show that the satisfiability and finite satisfiability problems for this logic are both NExpTime ‐complete. We further show that the corresponding problems for two‐variable first‐order logic with counting and two equivalences are both undecidable.     

Effect of PEG end-group hydrophobicity on lysozyme interactions in solution characterized by light scattering

We compare protein-protein and protein-polymer osmotic virial coefficients measured by static light scattering for aqueous solutions of lysozyme with low-molecular-weight, hydroxy-terminated (hPEG) and methyl-terminated (mPEG) poly(ethylene glycol) at two solution conditions: pH 7.0 and 0.01 M ionic strength, and pH 6.2 and 0.8 M ionic strength. We find that adding PEG to aqueous lysozyme solutions makes a net repulsive contribution to lysozyme-lysozyme interactions, independent of ionic strength and PEG end-group hydrophobicity. PEG end-group hydrophobicity has a profound effect on the magnitude of this contribution, however, at low ionic strength where mPEG-lysozyme attractive interactions become significant. The enhanced attractions promote mPEG-lysozyme preferential interactions at the expense of lysozyme self-interactions, which leads to lysozyme-lysozyme interactions that are more repulsive in the presence of mPEG. These preferential interactions also lead to the preferential exclusion of diffusable ions locally around the protein, which results in a pronounced ionic strength dependence of mPEG-mediated lysozyme-lysozyme interactions.     

Microwave and UV excitation spectra of 4-fluorobenzyl alcohol at high resolution. S0 and S1 structures and tunneling motions along the low frequency -CH2OH torsional coordinate in both electronic states

Rotationally resolved electronic spectra of several low frequency vibrational bands that appear in the S(1) ← S(0) transition of 4-fluorobenzyl alcohol (4FBA) in the collision-free environment of a molecular beam have been observed and assigned. Each transition is split into two or more components by the tunneling motion of the attached -CH(2)OH group. A similar splitting is observed in the microwave spectrum of 4FBA. Analyses of these data show that 4FBA has a gauche structure in both electronic states, but that the ground state C(1)C(2)-C(7)O dihedral angle of ～60° changes by ～30° when the photon is absorbed. The barriers to the torsional motion of the attached -CH(2)OH group are also quite different in the two electronic states; V(2) ～ 300 cm(-1) high and ～60° wide in the S(0) state, and V(2) ～ 300 cm(-1) high and ～120° wide (or V(2) ～ 1200 cm(-1) high and ～60° wide) in the S(1) state. Possible reasons for these behaviors are discussed.     

Time-resolved predissociation of the vibrationless level of the B state of CH3I

The predissociation dynamics of the vibrationless level of the first Rydberg 6s (B (1)E) state of CH(3)I has been studied by femtosecond-resolved velocity map imaging of both the CH(3) and I photofragments. The kinetic energy distributions of the two fragments have been recorded as a function of the pump-probe delay, and as a function of excitation within the umbrella and stretching vibrational modes of the CH(3) fragment. These observations are made by using (2 + 1) Resonant Enhanced MultiPhoton Ionization (REMPI) via the state of CH(3) to detect specific vibrational levels of CH(3). The vibrational branching fractions of the CH(3) are recovered by using the individual vibrationally state-selected CH(3) distributions to fit the kinetic energy distribution obtained by using nonresonant multiphoton ionization of either the I or the CH(3) fragment. The angular distributions and rise times of the two fragments differ significantly. These observations can be rationalized through a consideration of the alignment of the CH(3) fragment and the effect of this alignment on its detection efficiency. Two additional dissociation channels are detected: one associated with Rydberg states near 9.2 eV that were observed previously in photoelectron studies, and one associated with photodissociation of the parent cation around 15 eV.     

Probing magnetic order in heavily doped La2-xSrxNiO4+\delta with μSR

We report μSR measurements on a series of compounds with composition La_2-xSr_xNiO_4+\delta where the net hole concentration x+2\delta is greater than 0.4. A magnetic transition is found in all the samples studied which occurs at a composition‐dependent temperature T_M. Below T_M, clear precession signals are observed in zero applied magnetic field. The possible muon sites are discussed within the context of dipole‐field calculations. This revised version was published online in July 2006 with corrections to the Cover Date.