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Rate coefficients have been measured for the H + HC3N (cyanoacetylene) reaction at T=200, 250, and 298 K and at P=0.5, 1.0, and 2.0 Torr He using a discharge-flow mass spectrometry apparatus. The reaction was monitored under pseudo-first-order conditions with the H atom concentration in large excess over the HC(sub 3)N concentration.

The ground- and excited-state electronic structures of the solar cell sensitizer bis(4,4'-dicarboxylato-2,2'-bipyridine)bis(isothiocyanato)ruthenium(II) are examined. Density functional theory calculations are used for this purpose.

It has been possible to observe ketene products from the photochemical reaction of O(3P) atoms with m-xylene, toluene, p-xylene and mesitylene in solid argon matrices at 12 K. It is concluded that the methyl groups act as ortho/para directors in the electrophilic aromatic addition reactions.

A study of the transition metal complexes has been conducted by means of the femtosecond visible electronic absorption spectroscopy. Results include identification of the optical marker for the thermalized excited state as a unique absorptive feature.

The oxygen atom addition to dimethylacetylene (DMA) and the oxygen molecule and ozone molecule reactions with DMA are reported.

Research was conducted to evaluate charge transfer distances for several members of a family of mixed valence systems that are known to exhibit both interesting ultrafast electron-transfer behavior and promising second-order nonlinear optical (NLO) behavior. Electroabsorption studies of the systems indicate that the metal-to-metal or intervalence charge-transfer transitions are related to the change in dipole moment.

A study was conducted to analyze suppression the specific rate of hydrolysis of organic halides in three water-based liquid mixtures near their equilibrium consolute points. The solvent was made at its consolute composition to start a kinetics run. The reaction mixture was then placed in a test tube designed to accommodate an immersion probe. Experimental results indicated the occurrence of a slowdown phenomenon that correlated with a shift in the critical temperature caused by a change in the initial reactant concentration.

The [super 13]C chemical shift tensors measured from two-dimensional (2D) FIREMAT on 4,7-di-t-butylacenaphthene and 4,7-di-t-butyl-acenaphthylene is described along with a series of calculations on both compounds. The experiments have revealed a break in symmetry caused by the crystal packing that has given rise to additional spectral lines beyond those that would predict on the basis of molecular symmetry in an isolated molecule.

The rate coefficient measurement under pseudo-first-order conditions for the Cl + C[H.sub.3] association reaction at T = 202, 250 and 298 K and P = 0.3-2.0 Torr helium was conducted by using the technique of discharge-flow mass spectrometry with low-energy electron-impact ionization and collision-free sampling. The findings reveal a high-pressure-limit rate coefficient that is about a factor 4 lower, than that obtained in previous studies.

Steady-state and time-resolved X-ray absorption spectroscopy (XAS) is performed on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption fine structure.

The rate of carbon dioxide evolution in the aniline catalyzed decomposition of acetone dicarboxylic acid in a mixture of isobutryic acid and water near its consolute point is measured. The observation of a critical effect in the rate of decomposition of acetone dicarboxylic acid proves that kinetic critical phenomena are observable in heterogenous reactions.

The rates of the S(sub N)1 hydrolysis reactions of 2-chloro 2-methylbutane in isobutyric acid + water and 2-bromo-2-methylpropane in trietylamine + water are measured near their respective consolute points. In case of both reactions the specific rate of hydrolysis slowed at temperatures above the critical solution temperature and accelerated at temperatures below it.

Steady-state and time-resolved fluorescence measurements are conducted on newly synthesized Frechet-type dendrimers with naphthalene peripheral groups in homogeneous and sodium dodecyl (SDS) micellar solutions. It is observed that geometry and conformation are very important qualities of a desired dendrimer for the study of photophysical properties of attached chromophores.

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]

In a fluid mixture near a critical point, there are long-range fluctuations in the component concentrations that exceed the range of the intermolecular forces. If the components are linked by a chemical reaction, then the fluctuations in the concentrations of the reactants and products have their origin in the fluctuation in the extent of reaction.

Low-symmetry ruthenium II (Ru) polypyridil compounds tend to exhibit a systematic shift of the metal-to-ligand charge transfer (MLCT) absorption envelope. This MLCT transition represents successive substitutions of dea for dmb, which in turn, renders dea-based chromophore as a lowest-energy species. It was also proven that localization of the MLCT state persists in all complexes where Ru ligand exists, making the dea-based excited state unique to these compounds.

Differences in electronic structures of the three oxidation states of the ligand 3,6-di-tert-butylorthoquinone (3,6-DTBQ) are analyzed using nonlocal gradient-corrected density functional theory. Optimized structures are derived at the B3LYP/6-31G* and BLYP/6-31G* levels and show that neutral 3,6-DTBQ contains two equivalent C-O double bonds as well as a nonaromatic six-membered carbon ring. The single bonds of the ligand become shorter while its double bonds become longer upon one- and two-electron reduction to its semiquinone and catechol oxidation states, respectively.

The steric effects on ligand geometry were examined using computational methods. Neutral aryl-pyridines were studied as models for ground-state ligand structure by performing ab initio calculations of HF and MP2, reduced aryl-pyridines were investigated as models for metal-to-ligand charge transfer excited states through the use of ab initio calculations of UHF, UMP2, ROHF and ROMP2 as well density functional calculations of UB-LYP and USVWN. The findings are discussed.

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 principal components of the 13 C chemical shift tensors for the ten crystallographically distinct carbon atoms of the active pharmaceutical ingredient cimetidine Form A have been measured using the FIREMAT technique. Density functional theory (DFT) calculations of 13 C and 15 N magnetic shielding tensors are used to assign the 13 C and 15 N peaks. DFT calculations were performed on cimetidine and a training set of organic crystals using both plane-wave and cluster-based approaches. The former set of calculations allowed several structural refinement strategies to be employed, including calculations utilizing a dispersion-corrected force field that was parametrized using 13 C and 15 N magnetic shielding tensors. The latter set of calculations featured the use of resource-intensive hybrid-DFT methods for the calculation of magnetic shielding tensors. Calculations on structures refined using the new force-field correction result in improved values of 15 N magnetic shielding tensors (as gauged by agreement with experimental chemical shift tensors), although little improvement is seen in the prediction of 13 C shielding tensors. Calculations of 13 C and 15 N magnetic shielding tensors using hybrid functionals show better agreement with experimental values in comparison to those using GGA functionals, independent of the method of structural refinement; the shielding of carbon atoms bonded to nitrogen are especially improved using hybrid DFT methods.

The rate of iodination of acetone has been measured as a function of temperature in the binary solvent isobutyric acid (IBA) + water near the upper consolute point. The reaction mixture was prepared by the addition of acetone, iodine, and potassium iodide to IBA + water at its critical composition of 38.8 mass % IBA. The value of the critical temperature determined immediately after mixing was 25.43 degrees C. Aliquots were extracted from the mixture at regular intervals in order to follow the time course of the reaction. After dilution of the aliquot with water to quench the reaction, the concentration of triiodide ion was determined by the measurement of the optical density at a wavelength of 565 nm. These measurements showed that the kinetics were zeroth order. When at the end of 24 h the reaction had come to equilibrium, the critical temperature was determined again and found to be 24.83 degrees C. An Arrhenius plot of the temperature dependence of the observed rate constant, k(obs), was linear over the temperature range 27.00-38.00 degrees C, but between 25.43 and 27.00 degrees C, the values of k(obs) fell below the extrapolation of the Arrhenius line. This behavior is evidence in support of critical slowing down. Our experimental method and results are significant in three ways: (1) In contrast to in situ measurements of optical density, the determination of the optical density of diluted aliquots avoided any interference from critical opalescence. (2) The measured reaction rate exhibited critical slowing down. (3) The rate law was pseudo zeroth order both inside and outside the critical region, indicating that the reaction mechanism was unaffected by the presence of the critical point.

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 13CH2 carbon delta11 component. 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 CH2 protons as a 4.2 ppm difference between the solution value and the solid-state chemical shift measured via a 13C-1H heteronuclear correlation experiment.

The rate coefficient has been measured under pseudo-first-order conditions for the Cl+CH3 association reaction at T=202, 250, and 298 K and P=0.3-2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3 were 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 F2 in He. The decay of CH3 was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3 concentration ([Cl]0/[CH3]0=9-67). 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 T=202 K, the rate coefficient increases from 8.4x10(-12) at P=0.30 Torr He to 1.8x10(-11) at P=2.00 Torr He, both in units of cm3 molecule-1 s-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 k2=6.0x10(-11) cm3 molecule-1 s-1, independent of temperature in the range from 200 to 300 K.

We have measured the rate of carbon dioxide evolution in the aniline catalyzed decomposition of acetone dicarboxylic acid in a mixture of isobutyric acid + water near its consolute point. Within a temperature interval of 1 degrees C, which included the critical solution temperature, the first-order rate constant oscillated in magnitude by about 10% as it passed through three complete cycles of slowing down followed by speeding up. Whereas we can find no ready explanation for the speeding up, we suggest that, because the mixture contained no inert components, the slowing down should belong to the Griffiths-Wheeler class of strong critical effects [Phys. Rev. A 1970, 2, 1047]. As a check on this conclusion, we have measured the rate of the SN1 decomposition of benzene diazonium tetrafluoroborate in 2-butoxyethanol + water near the lower critical solution temperature and also the rate of the acid-catalyzed decomposition of ethyl diazoacetate in isobutyric acid + water near the upper critical solution temperature. Both of these reactions evolve nitrogen. In the first reaction, 2-butoxyethanol is inert, whereas in the second, isobutyric acid is inert. In both cases, because there was one inert component, we regarded the response of the rate constant to temperature in the critical region to be representative of the Griffiths-Wheeler class of weak critical effects. Within our accuracy of measurement of about 2% in the rate constant and about 1 mK in the temperature, we could detect no effect of the critical point on the rates of either of these reactions, suggesting that a weak effect may be too small to be seen with our experimental apparatus. The successful observation of a critical effect in the rate of decomposition of acetone dicarboxylic acid proves, however, that kinetic critical phenomena are observable in heterogeneous reactions.