David Yaron

Second order optical nonlinearity in octupolar aromatic systems

We investigate the second‐order nonlinear optical properties of octupolar molecules, as opposed to traditional charge transfer dipolar molecules. We consider in particular the case of hexasubstituted aromatic rings, trianiline, trinitrobenzene, and tri‐amino‐,tri‐nitro‐benzene (TATB).

Water hydrogen bonding: The structure of the water–carbon monoxide complex

Rotational transitions between J≤3 levels within the K=0 manifold have been observed for H2O–CO, HDO–CO, D2O–CO, H2O–13CO, HDO–13CO, and H217O–CO using the molecular beam electric resonance and Fourier transform microwave absorption techniques. ΔMJ=0→1 transitions within the J=1 level were also measured at high electric fields. A tunneling motion which exchanges the equivalent hydrogens gives rise to two states in the H2O and D2O complexes. The spectroscopic parameters for H2O–CO in the spatially symmetric tunneling state are [∼(B0) =2749.130(2)MHz, D0=20.9(2)kHz, and μa=1.055 32(2)D] and in the spatially antisymmetric state are [∼(B0) =2750.508(1)MHz, D0=20.5(1)kHz, and μa=1.033 07(1)D]. Hyperfine structure is resolved for all isotopes. The equilibrium structure of the complex has the heavy atoms approximately collinear. The water is hydrogen bonded to the carbon of CO; however the bond is nonlinear. At equilibrium, the O–H bond of water makes an angle of 11.5° with the a axis of the complex; the C2v axi...

The ChemCollective—Virtual Labs for Introductory Chemistry Courses

A collection of online activities emphasizes the design and interpretation of experiments. Chemistry concepts are abstract and can be difficult to attach to real-world experiences. For this reason, high-school and college chemistry courses focus on a concrete set of problem types that have become canonized in textbooks and standard exams. These problem types emphasize development of the core notational and computational tools of chemistry. Even though these tools may form the underlying procedural knowledge base from which the “real stuff” can be approached, when taught out of contexts that show their utility or that draw connections to core ideas of science, they can appear as a disconnected bag of tricks (1).

Absolute infrared transition moments for open shell diatomics from J dependence of transition intensities: Application to OH

A general approach to the determination of the dipole moment function and of the absolute vibrational transition moments for diatomic molecules is presented. This method utilizes the variation of intensity with J within a vibrational transition, together with permanent dipole moment information, to extract the absolute transition moments. An essential feature of the model is its use of algebraic expressions for calculating vibration–rotation line intensities. These expressions can be rapidly evaluated in a least squares fit which determines the dipole moment function. This approach is general in that it is not limited to 1Σ state molecules, nor to the simplest of Hund’s case couplings of spin, orbital and mechanical angular momentum. It is also not limited to molecules with essentially linear dipole moment functions. The model is successfully applied to the OH molecule which violates each of these restrictions. In the accompanying work we report experimental measurements of relative infrared absorption intensity measurements for OH v=1←0 transitions and the extraction of an experimental μ(r) using the approach presented here.

Comparison of density matrix renormalization group calculations with electron-hole models of exciton binding in conjugated polymers

By analogy with inorganic semiconductors such as GaAs or Si, electron-hole models may be expected to provide a useful description of the excited states of conjugated polymers. Here, these models are tested against density matrix renormalization group (DMRG) calculations. The DMRG method is used to generate nearly-exact descriptions of the ground state, 11Bu optical gap state, and the band gap of the Pariser-Parr-Pople (PPP) Hamiltonian of polyenes with between 2 and 40 carbon atoms. These are compared with both bare electron-hole (singles configuration interaction theory and the random phase approximation) and dressed electron-hole (second and third order Green’s function) methods. For the optical gap, only second-order Green’s function results were obtained. When an unscreened (Ohno) electron-electron interaction potential is used, the dressed electron-hole methods work well for the band gap. The difference between the band gap predicted by bare and dressed electron-hole methods increases with chain leng...

Coulomb screening and exciton binding energies in conjugated polymers

Hartree–Fock solutions of the Pariser–Parr–Pople and MNDO Hamiltonians are shown to give reasonable predictions for the ionization potentials and electron affinities of gas-phase polyenes. However, the energy predicted for formation of a free electron-hole pair on an isolated chain of polyacetylene is much larger than that seen in the solid state. The prediction is 6.2 eV if soliton formation is ignored and about 4.7 eV if soliton formation is included. The effects of interchain interactions on the exciton binding energy are then explored using a model system consisting of one solute and one solvent polyene, that are coplanar and separated by 4 A. The lowering of the exciton binding energy is calculated by comparing the solvation energy of the exciton state to that of a single hole (a cationic solute polyene) and a single electron (an anionic solute polyene). It is argued that when the relative timescales of charge fluctuations on the solute and solvent chains are taken into account, it is difficult to ra...

Studying the effects of personalized language and worked examples in the context of a web-based intelligent tutor

Previous studies have demonstrated the learning benefit of personalized language and worked examples. However, previous investigators have primarily been interested in how these interventions support students as they problem solve with no other cognitive support. We hypothesized that personalized language added to a web-based intelligent tutor and worked examples provided as complements to the tutor would improve student (e-)learning. However, in a 2 x 2 factorial study, we found that personalization and worked examples had no significant effects on learning. On the other hand, there was a significant difference between the pretest and posttest across all conditions, suggesting that the online intelligent tutor present in all conditions did make a difference in learning. We conjecture why personalization and, especially, the worked examples did not have the hypothesized effect in this preliminary experiment, and discuss a new study we have begun to further investigate these effects.

Conjugated Polymers with Repeated Sequences of Group 16 Heterocycles Synthesized through Catalyst-Transfer Polycondensation

Periodic π-conjugated polymers of the group 16 heterocycles (furan, thiophene, and selenophene) were synthesized with controlled chain lengths and relatively low dispersities using catalyst-transfer polycondensation. The optical gap and redox potentials of these copolymers were fine-tuned by altering the heterocycle sequence, and atomic force microscopy revealed nanofibrillar morphologies for all the materials. Grazing incidence wide-angle X-ray scattering of the thiophene-selenophene copolymers indicated that the π-stacking distance increased with incorporation of the larger heteroatom (from ∼3.7-4.0 Å), while the lamellar spacing decreased (from ∼15.8-15.2 Å). The study also revealed that periodic sequences allow electronic properties to be tuned while retaining nanofibrillar morphologies similar to those observed for poly(3-hexylthiophene).

Learning stoichiometry: a comparison of text and multimedia formats

Even after repeated instruction, first year college chemistry students are often unable to apply stoichiometry knowledge to equilibrium and acid-base chemistry problems. The dynamic and interactive capabilities of online technology may facilitate stoichiometry instruction that promotes more meaningful learning. This study compares a technology-rich stoichiometry review course with a text-based study guide. The technology-rich course included: an overarching real-world story to both motivate the students and integrate ideas; the use of an exploratory virtual laboratory to support concept development and procedural practice; a variety of practice contexts; and feedback on both intermediate actions and submitted answers during student practice. The text-based study guide covered the same topics but without the dynamic interface, timely and informative feedback, or overarching storyline. Entering college freshmen volunteers were randomly assigned to either the technology-rich or the text-based materials. Analysis of post-test scores revealed a significant but small advantage for participants studying from the technology-rich course, but it was less important than the effects of SAT scores and gender. The degree of interaction with the Virtual Lab simulation was significantly directly related to post-test performance and eclipsed any effect of prior knowledge as measured by the SAT.

Water in weak interactions: The structure of the water–nitrous oxide complex

The rotational spectra of H2O–N2O, D2O–N2O, and HDO–N2O have been observed using molecular beam electric resonance techniques at both zero and nonzero electric fields. H2O–N2O is nonrigid with respect to internal rotation of the water subunit. Rotational constants in MHz for the spatially antisymmetric tunneling state are A=12 605.001(77), B=4437.978(32), and C=3264.302(32). Rotational constants for the spatially symmetric tunneling state are A=12 622.595(203), B=4437.422(47), C=3264.962(47). These together with the rotational constants of the other isotopomers are consistent with a planar, T‐shaped arrangement of the heavy atoms of the complex, with the distance between the centers of mass of the two subunits, Rc.m., equal to 2.91(2) A or a distance of 2.97(2) A from the H2O oxygen to the central nitrogen of N2O. The measured dipole moments of the two tunneling isomers are identical; μa = 1.480(2) and μb = 0.31(2) D. The values of these dipole moment components indicate an in‐plane equilibrium tilt of ab...