A Flipped Classroom Redesign in General Chemistry

The flipped classroom continues to attract significant attention in higher education. Building upon our recent parallel controlled study of the flipped classroom in a second-term general chemistry course (J. Chem. Educ., 2016, 93, 13–23), here we report on a redesign of the flipped course aimed at scaling up total enrollment while keeping discussion sizes small (i.e.,students), and maintaining equivalent contact hour load for faculty and workload for students. To that end, the course format featured lecture contact pushed outside of the classroom in the form of video lectures (mean duration 13 minutes) paired with online homework sets, and three parallel weekly one-hour discussion sections were held in adjoining lab rooms immediately prior to the three-hour laboratory session. As in our previous design, the discussion sections were led by teaching assistants; however, the weekly discussion meeting was shortened from 75 minutes to 50 minutes, and the primary instructor “floated” between the three parallel sessions. Two such sessions were held each week, affording a possible enrollment of 144; initial enrollment was 141, with students self-selecting into the course. We examine student performance in and satisfaction with the course using: (1) a pre-test/post-test design based on the paired questions American Chemical Society (ACS) first-term and second-term exams, (2) data on DFW (D, F, withdrawal) rates, and (3) student evaluations

Revisiting the Renner–Teller Effect in the X ̃\u3csup\u3e2\u3c/sup\u3e\u3cem\u3eΠ\u3c/em\u3e State of CCN: Pulsed Discharge-supersonic Jet Single Vibronic Level Emission Spectroscopy

The CCN radical is a prototypical system for study of the Renner–Teller effect in its degenerate X ̃2Π ground state, with a number of experimental and theoretical studies carried out over the past 50 years. Most experimental studies have focused on the low-lying vibrational structure as observed in the high-resolution spectra of hot bands, or in emission. In this work, we have used pulsed-discharge supersonic-jet single vibronic level (SVL) emission spectroscopy from selected levels of the Ã2Δ state to probe the vibronic structure of X ̃2Π up to ∼6000 cm−1 above the vibrationless level. Around 50 levels were assigned, and these were fit to a Renner–Teller Hamiltonian to derive a detailed set of spectroscopic constants. Our data are compared with the results of recent high level coupled cluster calculations

Isomerization as a Key Path to Molecular Products in the Gas-Phase Decomposition of Halons

The decomposition of halons remains controversial concerning the branching between radical and molecular products. The latter channel, where it has been found, is presumed to occur via a constrained symmetric multicenter transition state. Isomerization pathways in the gas-phase chemistry of halons have rarely been considered, despite the fact that the iso-halons, which feature a halogen−halogen bond, are widely recognized as important reactive intermediates in condensed phases. In this Letter, detailed calculations and modeling of the unimolecular decomposition of several important halons, including CF2Cl2, CF2Br2, and CHBr3, reveal that isomerization is a key pathway to molecular products. This path is important for both halons and their primary radicals as the barrier to isomerization in these compounds is typically isoenergetic with the threshold for bond fission

On Pi-Stacking, CH/Pi, and Halogen Bonding in Halobenzene Clusters: Resonant 2-Photon Ionization Studies of Chlorobenzene

Noncovalent interactions such as hydrogen bonding, π-π stacking, CH/π interactions, and halogen bonding play crucial roles in a broad spectrum of chemical and biochemical processes, and can exist in cooperation or competition. Here we report studies of the homoclusters of chlorobenzene, a prototypical system where π-π stacking, CH/π interactions, and halogen bonding interactions may all be present. The electronic spectra of chlorobenzene monomer and clusters (Clbz)n with n = 1-4 were obtained using resonant 2-photon ionization in the origin region of the S0–S1 (ππ*) state of the monomer. The cluster spectra show in all cases a broad spectrum whose center is redshifted from the monomer absorption. Electronic structure calculations aid in showing that the spectral broadening arises in large part from inhomogeneous sources, including the presence of multiple isomers and Franck-Condon (FC) activity associated with geometrical changes induced by electronic excitation. Calculations at the M06-2x/aug-cc-pVDZ level find in total five minimum energy structures for the dimer, four π-stacked structures, and one T-shaped, and six representative minimum energy structures were found for the trimer. The calculated time-dependent density functional theory spectra using range-separated and meta-GGA hybrid functionals show that these isomers absorb over a range that is roughly consistent with the breadth of the experimental spectra, and the calculated absorptions are redshifted with respect to the monomer transition, in agreement with experiment. Due to the significant geometry change in the electronic transition, where for the dimer a transition from a parallel displaced to sandwich structure occurs with a reduced separation of the two monomers, significant FC activity is predicted in low frequency intermolecular modes

Game of Frontier Orbitals: A View on the Rational Design of Novel Charge-Transfer Materials

Since the first application of frontier molecular orbitals (FMOs) to rationalize stereospecificity of pericyclic reactions, FMOs have remained at the forefront of chemical theory. Yet, the practical application of FMOs in the rational design and synthesis of novel charge transfer materials remains under-appreciated. In this Perspective, we demonstrate that molecular orbital theory is a powerful and universal tool capable of rationalizing the observed redox/optoelectronic properties of various aromatic hydrocarbons in the context of their application as charge-transfer materials. Importantly, the inspection of FMOs can provide instantaneous insight into the interchromophoric electronic coupling and polaron delocalization in polychromophoric assemblies, and therefore is invaluable for the rational design and synthesis of novel materials with tailored properties

Reactive Pathways in the Chlorobenzene–Ammonia Dimer Cation Radical: New Insights from Experiment and Theory

Building upon our recent studies of noncovalent interactions in chlorobenzene and bromobenzene clusters, in this work we focus on interactions of chlorobenzene (PhCl) with a prototypical N atom donor, ammonia (NH3). Thus, we have obtained electronic spectra of PhCl···(NH3)n (n = 1–3) complexes in the region of the PhCl monomer S0 −S1 (ππ*) transition using resonant 2-photon ionization (R2PI) methods combined with time-of-flight mass analysis. Consistent with previous studies, we find that upon ionization the PhCl···NH3 dimer cation radical reacts primarily via Cl atom loss. A second channel, HCl loss, is identified for the first time in R2PI studies of the 1:1 complex, and a third channel, H atom loss, is identified for the first time. While prior studies have assumed the dominance of a π-type complex, we find that the reactive complex corresponds instead to an in-plane σ-type complex. This is supported by electronic structure calculations using density functional theory and post-Hartree–Fock methods and Franck–Condon analysis. The reactive pathways in this system were extensively characterized computationally, and consistent with results from previous calculations, we find two nearly isoenergetic arenium ions (Wheland intermediates; denoted WH1, WH2), which lie energetically below the initially formed dimer cation radical complex. At the energy of our experiment, intermediate WH1, produced from ipso-addition, is not stable with respect to Cl or HCl loss, and the relative branching between these channels observed in our experiment is well reproduced by microcanonical transition state theory calculations based upon the calculated parameters. Intermediate WH2, where NH3 adds ortho to the halogen, decomposes over a large barrier via H atom loss to form protonated o-chloroaniline. This channel is not open at the (2-photon) energy of our experiments, and it is suggested that photodissociation of a long-lived (i.e., several ns) WH2 intermediate leads to the observed products

The Integration of Information and Communication Technology Into Classroom Teaching

This study provides new images of teachers who are pioneers in the use of information and communication technology (ICT) in the classes they teach. This qualitative study looks at teachers in a high school that is a leader in using information technology in the classroom. The findings indicate that most of the teachers participating in this study were enthusiastic about the possibilities ICT holds for classroom use. Examples are given to illustrate how information technology is being used in this school to enhance classroom teaching in several subject areas. Concerns teachers have about the use of technology were identified as issues that could determine the success or failure of the use of ICT in high schools. These concerns include: maintenance, inequalities, the need for training, information overload, the pace of change and stress, plagiarism, business involvement, and teachers' time.Cette étude présente de nouvelles représentations d'enseignants qui sont des pionniers dans l'emploi des technologies de l'information et des communications (TIC) dans les cours qu'ils donnent. Cette recherche qualitative se penche sur des enseignements dans une école secondaire qui se classent comme chefs défile dans l'emploi des technologies de l'information dans la salle de classe. Les résultats indiquent que la plupart des enseignants qui ont participé à l'étude étaient enthousiastes face aux possibilités des TIC en salle de classe. L'auteur fournit des exemples illustrant différentes façons dont on se sert des technologies de l'information pour améliorer l'enseignement de plusieurs domaines en salle de classe dans cette école. Les préoccupations qu'ont évoquées les enseignants quant à l'emploi des technologies ont été identifiées comme des facteurs pouvant déterminer si l'emploi des TIC allait réussir ou pas dans les écoles secondaires. Parmi ces préoccupations, l'on retrouve: l'entretien, les disparités, la nécessité de la formation, la surcharge d'information, le rythme du changement, le stress, le plagiat, l'implication de la part des entreprises et l'emploi du temps des enseignants

Two\u27s Company, Three\u27s a Crowd: Exciton Localization in Cofacially Arrayed Polyfluorenes

Understanding the mechanisms of long-range energy transfer through polychromophoric assemblies is critically important in photovoltaics and biochemical systems. Using a set of cofacially arrayed polyfluorenes (Fn), we investigate the mechanism of (singlet) exciton delocalization in π-stacked polychromophoric assemblies. Calculations reveal that effective stabilization of an excimeric state requires an ideal sandwich-like arrangement; yet surprisingly, emission spectroscopy indicates that exciton delocalization is limited to only two fluorene units for all n. Herein, we show that delocalization is determined by the interplay between the energetic gain from delocalization, which quickly saturates beyond two units in larger Fn, and an energetic penalty associated with structural reorganization, which increases linearly with n. With these insights, we propose a hopping mechanism for exciton transfer, based upon the presence of multiple excimeric tautomers of similar energy in larger polyfluorenes (n ≥ 4) together with the anticipated low thermal barrier of their interconversion