Differing adsorption behavior of environmentally important cyanophenol isomers at the air-water interface

Vibrational sum-frequency spectroscopy and surface tensiometry have been used to study the adsorption of m- and p-cyanophenol at the air-water interface. Spectra of the cyano (CN) group under different polarization schemes are utilized to determine its hydrogen bonding environment and orientation. For both isomers, it is found that the cyano group is hydrogen bonded at the interface but that the CN orientation is independent of surface density. The average CN tilt angle (theta(0)), however, is found to differ between the isomers, such that the CN group points down toward the aqueous phase for m-cyanophenol (theta(0) = 96-106 degrees ) but points up toward the vapor phase for the p-cyanophenol (theta(0) = 65-80 degrees ). In addition, this average tilt angle is distributed over a narrow range, sigma(0) < 10 degrees for the meta isomer and sigma(0) < 16 degrees for the para isomer.     

A simple model for a two-stage chemical reaction with diffusion

We analyse the evolution of a two-stage chemical reaction betweentwo neighbouring plumes of reactants. Under the assumption thatthe plumes are approximately Gaussian we derive a system ofordinary differential equations for the total amount, the centroidand the variance of each reactant. We compare the solution ofthese equations with full numerical simulation of the reaction.Excellent agreement is obtained, with solution of the near-Gaussianmodel requiring considerably less computational effort thanthe full simulations. Of key importance is the yield of thereaction, and we discuss this feature in particular.     

Imaging cytoplasmic cAMP in mouse brainstem neurons

Background  

cAMP is an ubiquitous second messenger mediating various neuronal functions, often as a consequence of increased intracellular Ca²⁺ levels. While imaging of calcium is commonly used in neuroscience applications, probing for cAMP levels has not yet been performed in living vertebrate neuronal tissue before.     

Radio frequency magnetic field effects on electron-hole recombination

We present measurements of the spectrum (1--80 MHz) of the effect of a weak (approximately 500 microT) radio frequency magnetic field on the electron-hole recombination of radical ion pairs in solution. Distinct spectra are observed for the pyrene anion/dimethylaniline cation radical pair in which one or both of the radicals are perdeuterated. The radical pair mechanism is developed theoretically and shown to account satisfactorily for both the magnetic field effect and the associated magnetic isotope effect.     

Mid-infrared second-order susceptibility of alpha-quartz and its application to visible-infrared surface sum-frequency spectroscopy

We provide the first account of the second-order susceptibility of quartz down to 10 mum (1000 cm(-1)) and show how this data may be used along with the sum-frequency response of an amorphous gold surface to elucidate the nonlinear susceptibility of any material in the mid-infrared region. Crystalline quartz is an established material for use in second-harmonic and sum-frequency generation studies of new systems, on account of its well-characterized linear and nonlinear optical properties. Previous knowledge of its nonlinear susceptibility has been limited to its transparent region, wavelengths shorter than about 3 mum. Longer wavelength chi((2)) values for quartz are particularly important for techniques such as vibrational sum-frequency spectroscopy which are expanding into the mid-IR with the increasing availability of widely tunable infrared laser sources.     

Electron self-exchange kinetics determined by MARY spectroscopy: theory and experiment

The electron self-exchange between a neutral molecule and its charged radical, which is part of a spin-correlated radical ion pair, gives rise to line width effects in the fluorescence-detected MARY (magnetic field effect on reaction yield) spectrum similar to those observed in EPR spectroscopy. An increasing self-exchange rate (i.e., a higher concentration of the neutral molecule) leads to broadening and subsequent narrowing of the spectrum. Along with a series of MARY spectra recorded for several systems (the fluorophores pyrene, pyrene-d(10) and N-methylcarbazole in combination with 1,2- and 1,4-dicyanobenzene) in various solvents, a theoretical model is developed that describes the spin evolution and the diffusive recombination of the radical pair under the influence of the external magnetic field and electron self-exchange, thereby allowing the simulation of MARY spectra of the systems investigated experimentally. The spin evolution of the radicals in the pair is calculated separately using spin correlation tensors, thereby allowing rigorous quantum mechanical calculations for real spin systems. It is shown that the combination of these simulations with high resolution, low noise experimental spectra makes the MARY technique a novel, quantitative method for the determination of self-exchange rate constants. In comparison to a simple analytical formula which estimates the self-exchange rate constant from the slope of the linear part of a line width vs concentration plot, the simulation method yields more reliable and accurate results. The correctness of the results obtained by the MARY method is proved by a comparison with corresponding data from the well-established EPR line broadening technique. With its less stringent restrictions on radical lifetime and stability, the MARY technique provides an alternative to the classical EPR method, in particular for systems involving short-lived and unstable radicals.     

Determination of radical re-encounter probability distributions from magnetic field effects on reaction yields

Measurements are reported of the effects of 0-23 mT applied magnetic fields on the spin-selective recombination of Py*- and DMA*+ radicals formed in the photochemical reaction of pyrene and N,N-dimethylaniline. Singlet <--> triplet interconversion in [Py*- DMA*+] radical pairs is probed by investigating combinations of fully protonated and fully deuterated reaction partners. Qualitatively, the experimental B1/2 values for the four isotopomeric radical pairs agree with predictions based on the Weller equation using known hyperfine coupling constants. The amplitude of the "low field effect" (LFE) correlates well with the ratio of effective hyperfine couplings, aDMA/aPy. An efficient method is introduced for calculating the spin evolution of [Py*- DMA*+] radical pairs containing a total of 18 spin-1/2 and spin-1 magnetic nuclei. Quantitative analysis of the magnetic field effects to obtain the radical re-encounter probability distribution f (t )-a highly ill-posed and underdetermined problem-is achieved by means of Tikhonov and maximum entropy regularization methods. The resulting f (t ) functions are very similar for the four isotopomeric radical pairs and have significant amplitude between 2 and 10 ns after the creation of the geminate radical pair. This interval reflects the time scale of re-encounters that are crucial for generating the magnetic field effect. Computer simulations of generalized radical pairs containing six spin-1/2 nuclei show that Weller's equation holds approximately only when the radical pair recombination rate is comparable to the two effective hyperfine couplings and that a substantial LFE requires, but is not guaranteed by, the condition that the two effective hyperfine couplings differ by more than a factor of 5. In contrast, for very slow recombination, essentially any radical pair should show a significant LFE.