Search

Liquid mixtures, which have a phase diagram exhibiting a miscibility gap ending in a critical point of solution, have been used as solvents for chemical reactions. The reaction rate in the forward direction has often been observed to slow down as a function of temperature in the critical region. Theories based upon the Gibbs free energy of reaction as the driving force for chemical change have been invoked to explain this behavior. With the assumption that the reaction is proceeding under relaxation conditions, these theories expand the free energy in a Taylor series about the position of equilibrium. Since the free energy is zero at equilibrium, the leading term in the Taylor series is proportional to the first derivative of the free energy with respect to the extent of reaction. To analyze the critical behavior of this derivative, the theories exploit the principle of critical point isomorphism, which is thought to govern all critical phenomena. They find that the derivative goes to zero in the critical region, which accounts for the slowing down observed in the reaction rate. As has been pointed out, however, most experimental rate investigations have been carried out under irreversible conditions as opposed to relaxation conditions [Shen et al. J. Phys. Chem. A 2015, 119, 8784-8791]. Below, we consider a reaction governed by first order kinetics and invoke transition state theory to take into account the irreversible conditions. We express the apparent activation energy in terms of thermodynamic derivatives evaluated under standard conditions as well as the pseudoequilibrium conditions associated with the reactant and the activated complex. We show that these derivatives approach infinity in the critical region. The apparent activation energy follows this behavior, and its divergence accounts for the slowing down of the reaction rate. [ABSTRACT FROM AUTHOR]

A combination of solid-state 13C NMR tensor data andDFT computational methods is utilized to predict the conformationin disordered methyl α-l-rhamnofuranoside. This previouslyuncharacterized solid is found to be crystalline and consists of atleast six distinct conformations that exchange on the kHz time scale.A total of 66 model structures were evaluated, and six were identifiedas being consistent with experimental 13C NMR data. Allfeasible structures have very similar carbon and oxygen positionsand differ most significantly in OH hydrogen orientations. A concertedrearrangement of OH hydrogens is proposed to account for the observeddynamic disorder. This rearrangement is accompanied by smaller changesin ring conformation and is slow enough to be observed on the NMRtime scale due to severe steric crowding among ring substituents.The relatively minor differences in non-hydrogen atom positions inthe final structures suggest that characterization of a complete crystalstructure by X-ray powder diffraction may be feasible. [ABSTRACT FROM AUTHOR]

Rate constants for the gas-phase reactions of OH radical, NO3radical, and ozone with allyl alcohol (AAL) and allyl isocyanate (AIC) have been measured using relative rate methods at atmospheric pressure in purified air. The experimental Arrhenius expression obtained for the reaction of the OH radical with AAL is (1.68 ± 0.89) × 10−12× exp(1100/T) cm3molecule−1s−1, for T= 282−315 K; the Arrhenius expression for the reaction of OH radical with AIC is (1.94 ± 1.04) × 10−14× exp(2207/T) cm3molecule−1s−1, for T= 282−317 K, where the indicated errors are one least-squares standard deviation. All OH radical reaction rate constants have been measured relative to k(OH + α-pinene) and k(OH + 1,3,5-trimethylbenzene). The rate constant for the gas-phase reaction of OH radical with allyl alcohol-d6isotopomer (AAL-d6) has been measured at T= 298 K, and the value is 5.10 × 10−11cm3molecule−1s−1. The kinetic isotope effect is small, with k(AAL)/k(AAL-d6) = 1.32. Rate constants for the gas-phase reactions of NO3radical with AAL [relative to k(NO3+methacrolein)] and O3[relative to k(O3+ β-pinene)] have been measured, and the values are 7.7 × 10−15cm3molecule−1s−1at T= 298 K and 1.6 × 10−17cm3molecule−1s−1at T= 296 K, respectively. Rate constants for the gas-phase reactions of NO3radical and O3with AIC have been measured, and the values are 9.4 × 10−16cm3molecule−1s−1at T= 299 K and 5.54 × 10−18cm3molecule−1s−1at T= 299 K, respectively. Multireference ab initio calculations at the MRMP2/6-311G(d,p) level have been carried out for reactions of OH radical with AAL and AIC. Results indicate that prereactive hydrogen bonded complexes form in the entrance channels for these reactions. [ABSTRACT FROM AUTHOR]

The rate coefficient has been measured under pseudo-first-order conditions for the Cl CH3association reaction at T202, 250, and 298 K and P0.3−2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3were generated rapidly and simultaneously by reaction of F with HCl and CH4, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F2in He. The decay of CH3was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3concentration (Cl0/CH309−67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T202 K, the rate coefficient increases from 8.4 × 10-12at P0.30 Torr He to 1.8 × 10-11at P2.00 Torr He, both in units of cm3molecule-1s-1. A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin−orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k26.0 × 10-11cm3molecule-1s-1, independent of temperature in the range from 200 to 300 K. [ABSTRACT FROM AUTHOR]

The development of probes for rapid and selective detection of peroxynitrite in vivo is of great importance in biological science. We investigate different photoinduced electron transfer (PIET) processes of two generations of peroxynitrite probes. Each has fluorescein and phenol moieties; one is conjugated by an ether linkage while the other is conjugated via an amine linkage. Using theoretical calculations, we demonstrated that the PIET in the probe with an ether linkage occurs from the benzoic acid to the xanthene moiety. In contrast, the PIET in the probe with an amine linkage occurs from the phenol moiety to the fluorescein. This suggests that better sensitivity can be accomplished in probes with an amine linkage than with an ether linkage. Following this model, we designed two novel peroxynitrite probes and simulated their detection capabilities in the near-infrared region. [ABSTRACT FROM AUTHOR]

13C chemical shift tensor data from 2D FIREMAT spectra are reported for 4,7-di-t-butylacenaphthene and 4,7-di-t-butylacenaphthylene. In addition, calculations of the chemical shielding tensors were completed at the B3LYP/6-311G level of theory. While the experimental tensor data on 4,7-di-t-butylacenaphthylene are in agreement with theory and with previous data on polycyclic aromatic hydrocarbons, the experimental and theoretical data on 4,7-di-t-butylacenaphthene lack agreement. Instead, larger than usual differences are observed between the experimental chemical shift components and the chemical shielding tensor components calculated on a single molecule of 4,7-di-t-butylacenaphthene, with a root mean square (rms) error of ±7.0 ppm. The greatest deviation is concentrated in the component perpendicular to the aromatic plane, with the largest value being a 23 ppm difference between experiment and theory for the 13CH2carbon 11component. These differences are attributed to an intermolecular chemical shift that arises from the graphitelike, stacked arrangement of molecules found in the crystal structure of 4,7-di-t-butylacenaphthene. This conclusion is supported by a calculation on a trimer of molecules, which improves the agreement between experiment and theory for this component by 14 ppm and reduces the overall rms error between experiment and theory to 4.0 ppm. This intermolecular effect may be modeled with the use of nuclei independent chemical shieldings (NICS) calculations and is also observed in the isotropic 1H chemical shift of the CH2protons as a 4.2 ppm difference between the solution value and the solid-state chemical shift measured via a 13C−1H heteronuclear correlation experiment. [ABSTRACT FROM AUTHOR]