Cyclic voltammetry (CV) and controlled-potential electrolysis (CPE) were employed to examine the reactions of electrogenerated ligand-reduced nickel(II) salen with benzyl bromide, 1-bromomethylnaphthalene, and alpha-bromodiphenylmethane. Cyclic voltammo-grams for nickel(II) salen in the presence of benzyl bromide or 1-bromomethylnaphthalene exhibit characteristic features for the catalytic reduction of substrates involving radical intermediates. Bulk electrolyses of benzyl bromide and 1-bromomethylnaphthalene at carbon cathodes catalyzed by nickel(II) salen were also carried out at selected potentials to afford various products. These results were compared with similar reaction involving 1-bromooctane as the substrate. Further comparison of the CVs for nickel(II) salen before and after reactions with the four different organic halides reveals that the steric effect could play an important role in the corresponding nucleophilic attack of the substrates by ligand-reduced catalyst (a radical-anion), which follows the sequence of 1-bromooctane > benzyl bromide > 1-bromomethylnaphthalene > alpha-bromodiphenylmethane in terms of reaction efficiency. Moreover, theoretical calculations using density functional theory were carried out to establish a proposed mechanism for the electrochemical reactions on the basis of previous and current studies. (C) The Author(s) 2015. Published by ECS.

For students entering Chemistry Two following a Chemistry One course, an assessment exam was given and the results were evaluated in combination with other variables to develop a predictive model that forecasts student achievement in the course. Variables considered in the analysis included student major, GPA, classification (student standing: first year, second year, etc.), Chem One grade, delay since Chem One was completed, institution type at which Chem One was completed, Chem One instructor, enrollment status in the Chem Two lab, number of repeats of Chem Two, and semester of enrollment. Our results suggest that 27−28% of variability in semester course grades can be explained on the basis of the combined influence of only six variables. We identify four general factors that we believe should be considered simultaneously in the assessment of potential student strengths and weaknesses early in the Chem Two semester: historical academic achievement, quality of prerequisite preparation, academic background, and student motivations and attitudes. By implementing a chemistry background assessment and evaluating the results in combination with relevant demographic data, instructors can be better equipped to identify students at risk—both academically and motivationally—and to address those needs and improve student success.

The cone and 1,2,3 alternate isomers of calix[6]arene bis-crown-4 were investigated computationally. Structural optimizations, energies, bond distances, and Mulliken charges were calculated by the application of the B3LYP/6-31g(d) method/basis, followed by NMR calculations via both B3LYP/6-31g(d) and HF/6-31g(d). Calculations were completed at three different levels of imposed symmetry, and two calculations investigated the chloroform solvent effects. Better NMR results were obtained from HF/6-31g(d) calculations that did not impose molecular symmetry constraints. Consideration of solvent effects improved ground state energies, but other improvements were minimal and not significant enough to justify the added computational expense of solvent calculations. Overall results are consistent with known experimental assignments and were valuable for assigning previously unknown NMR peaks. Net charges, electrostatic forces, and local dipoles - but not bond lengths - are strongly correlated to spectroscopic manifestations of steric compression. Copyright (C) 2009 John Wiley & Sons, Ltd.

Monte Carlo simulated annealing strategies, carried out on four different potential energy surfaces, are applied to benzene-cyclohexane clusters, BCn, n=3-7, 12, to identify low-energy isomers and to trace the evolution of structures as a function of cluster size. Initial structures are first heated to ensure randomization, and subsequent annealing yields optimized rigid, low-energy clusters. Five major structural isomers are identified for BC3: one assumes the form of a symmetric, modified sandwich; the remaining four lack general symmetry, assuming distorted tetrahedral arrangements. For BC4 and larger clusters, the number of low-temperature isomers is large. It is, nevertheless, feasible to classify isomers into groups based on structural similarities. The evolution of BCn structures as a function of cluster size is observed to follow one of two primary paths: The first maximizes benzene-cyclohexane interactions and places benzene in or near the BCn cluster center; the competing path maximizes cyclohexane-cyclohexane interactions and distances benzene from the cluster's center of mass. Results for BC3 and BC4 are discussed with reference to experimental results and models previously applied to interpret benzene-argon cluster spectra.

Low-temperature isomeric energies, structures, and properties of benzene-cyclohexane clusters are investigated via Monte Carlo simulations. The Monte Carlo strategy is first documented and then applied to (C(6)H(6))(C(6)H(12)) and (C(6)H(6))(C(6)H(12))(2) using four different potential energy surfaces. Results identify a single parallel-displaced dimer isomer. MP2 optimizations and frequency calculations support the Monte Carlo dimer structure and identify the van der Waals mode observed in vibronic spectra. Caloric simulations identify two temperatures where structural transitions occur and imply an experimental temperature below 10 K for dimers in cold supersonic expansions. The (C(6)H(6))(C(6)H(12))(2) studies identify eight independent trimer isomers: three form parallel-stacked (sandwich) arrangements with the two cyclohexane moieties related through a D(6)(h) transformation. The remaining five trimer isomers are trigonal, with no overall symmetry. Caloric studies indicate that the sandwich and trigonal isomeric classes coexist independently below 60 K, consistent with trimer vibronic spectra that contain two independent van der Waals progressions.

The experimental B2u ← A1g000 spectrum of (C6H6)13 was analyzed within the weak-interaction model using minimum energy structures calculated from six different potential energy surfaces. The coexistence of two isomers—both of C3 symmetry and with nearly equal populations—is supported. Structures predicted by two of the six potential energy surfaces are strongly favored. The transition dipole of benzene moieties within the cluster has a magnitude of ∼0.23 Debye. Weak transition dipole–dipole interactions fall between −1.95 and +2.24 cm−1 and site shifts of ligand molecules range from −160.3 to −153.8 cm−1. Although the weak-interaction transition dipole–dipole model falls short of unambiguously determining isomeric structures of benzene-13, it establishes a solid foundation on which modeling can be based for determining structures of larger, high-symmetry, molecular clusters.

One color resonant two-photon ionization spectra have been measured for benzene–(cyclohexane)n clusters, n=1–14. Van der Waals progressions dominate the n=1–3 spectra. Minimum energy structures were calculated for these three clusters via Monte Carlo simulated annealing. The van der Waals modes are analyzed and discussed with reference to cluster structures. Results support generalized cluster aufbau principles that predict maximization of nearest neighbor molecule–molecule interactions.

Monte Carlo computations have been carried out using six different potential energy parameter sets in order to investigate the low-energy structure(s) of (benzene)(13). Improved energies were identified for three previously published structures, and their resulting symmetries were identified. Each of the computed structures at 0.01 K is unique; however, each possesses at least one symmetry element: a C-3 rotational axis, a center of inversion, or both (i.e., an S-6 axis). From simulations at 1 K, it is hypothesized that there are two competing structures, separated by similar to0.2 kJ/mol. Composite coordinates are derived for both structures with 95% confidence limits. These results can be used in conjunction with experimental data to identify the precise low-energy structure(s) of (benzene)(13).

Low-temperature Monte Carlo computations have been carried out to investigate minimum-energy structures of the (C6H6)(13) cluster. The simulations have identified a new cluster structure Of C-3 symmetry that is distinct from previously identified structures. The newly identified isomer is found to occupy an isolated region of the potential energy surface; this finding strengthens the hypothesis that distinct isomeric forms coexist within experimental cluster beams.

Size-resolved benzene–argon clusters, (C6H6)Arn, n=13–40, were generated by supersonic expansion and studied by ultraviolet resonant two-photon ionization spectroscopy through benzene’s B2u←A1g601 transition. The size-specific sequence of cluster spectra reveals six features that can be isolated, allowing for an independent analysis of each feature’s evolution. In the n=13–40 range, each cluster spectrum is well described by the sum of between two (n=13) and five (n=17–24,26–29) Gaussian features. Only two spectral features (D,E) span the 28-cluster sequence, while each of the other four features appears over a limited consecutive cluster size range (A:n=14–39; B:n=17–29; C:n=14–24; F:n=26–40). The evolution of the spectral shift, width, and relative amplitude is traced for each feature. The observations are rationalized through a model that has been previously developed for the discussion of benzene–argon cluster data. We report for the first time a “high shift” spectral feature, observed at a spectral shift of −80 cm−1, which approaches the bulk shift more closely than anything previously reported.

Isotopically labeled (benzene)(13) clusters, (C6H6)(C6D6)(12), generated in supersonic expansion, were studied by resonance-enhanced two-photon ionization (R2PI) spectroscopy as a function of nozzle-to-laser distance using two distinct sets of expansion conditions. In this report we present spectroscopic evidence for both evaporation and isomerization within the supersonic jet. The observed (C6H6)(C6D6)12 population initially undergoes a fluxional-to-rigid transition; this transition is followed by isomerization within a subpopulation of the ordered, rigid clusters, in which the unique C6H6 moiety migrates from the surface to the interior of an otherwise homogeneous C6D6 cluster. These experiments are unique in that analogous isomerization dynamics have never before been spectroscopically observed. The observed kinetics are generally insensitive to differences between the benzene:helium ratio in the expansion mixture.

Isotopically-labeled benzene13 clusters, (C6H6)(C6D6)12, were generated by supersonic expansion and studied as a function of nozzle-to-laser distance by resonance-enhanced two-photon ionization (R2PI) spectroscopy through the C6H6 B2u←A1g(601) transition. Because of the spectrum's simplicity, it serves as a sensitive monitor of the environment and dynamics of the C6H6 chromophore. We report for the first time evidence for isomerization within this cluster, where the C6H6 moiety migrates from the surface of a rigid cluster to the cluster's interior.

Isotopically labeled (benzene)13 clusters, (C6H6)(C6D6)12, were generated by supersonic expansion and studied as a function of nozzle-to-laser distance by resonance-enhanced two-photon ionization (R2PI) spectroscopy through the C6H6B2u←A1g601 transition. Because of the spectrum’s simplicity, it serves as a sensitive monitor of the environment and dynamics of the C6H6 chromophore. We report experimental evidence for both evaporation and isomerization dynamics. Initially, the observed (C6H6)(C6D6)12 cluster population undergoes a transition from fluxional to rigid, resulting from the evaporation of a single C6D6 molecule from (C6H6)(C6D6)13. “Solidification” is followed by isomerization, in which the C6H6 moiety migrates from the surface of ordered, rigid clusters to their interior. The “freezing” temperature of (C6H6)(C6D6)12 is inferred to be near 137 K, in good agreement with theoretical simulations [Bartell and Dulles, J. Phys. Chem. 99, 17107 (1995)].

In a recent report by Easter, Khoury and Whetten, analysis of the ultraviolet (B 2u ←A 1g (6 0 1 )) REMPI spectroscopy of cold (C 6 H 6 ) (C 6 D 6 ) n −1 clusters, n = 12–20, yielded striking conclusions: observed C 6 H 6 resonances derived almost exclusively from molecules in the cluster's interior site; and the 13-cluster is found to have only one dominant isomeric form, giving rise to a single prominent spectral feature. Here we report the evolution of the benzene- h 6 transition in (C 6 H 6 ) (C 6 D 6 ) 12 as a function of distance (time) from the nozzle in the supersonic jet expansion, and present the first experimental evidence for a nonrigid—rigid transition in a single-size molecular cluster. Initially (benzene) 13 condenses in a nonrigid state and solidification into a well-defined configuration occurs during subsequent free jet expansion.

Ultrafast relaxation in the fluorescent state of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) has been interrogated by time-resolved femtosecond absorption spectroscopy both in methanol and in ethylene glycol. Following a subpicosecond rise, each frequency-specific trace in ethylene glycol exhibits either a slower increase or a slower decrease in transmission; an additional long-term decay is seen in methanol solution. The data are generally consistent with recent solvation dynamics studies that employed different probe molecules, but the solvation time we measure for ethylene glycol is an order of magnitude shorter than previously reported.

We have traced the evolution of the benzene-h6 transition in (C6H6)(C6D6)12 as a function of time (distance from the nozzle) in a supersonic expansion. Taking advantage of the special properties of the isotopically substituted (benzene)13 resonant two-photon ionization spectrum, we present the first experimental evidence for a nonrigid–rigid transition in a single-size molecular cluster. Initially, under the conditions of our free-jet expansion, (benzene)13 condenses in a nonrigid form, then solidifies into a well-defined structural configuration.