A computational study into the reactivity of epichlorohydrin and epibromohydrin under acidic conditions in the gas phase and aqueous solution

Ab initio molecular orbital calculations have been carried out upon epichlorohydrin and epibromohydrin at the Hartree–Fock (HF) and Møller–Plesset (MP2) levels of theory to explore the reactivity of these species with respect to nucleophilic attack by water under acidic conditions in the gas phase and aqueous solution. These results suggest that nucleophilic attack occurs preferentially at the epoxy carbon atoms in both the gas phase and aqueous solution. These results are in contrast to those found for nucleophilic attack under basic conditions, where attack at the halocarbon atom is competitive with that at the epoxy carbon atoms. Copyright © 2007 John Wiley & Sons, Ltd.     

Global search for minimum energy (H2O)n clusters, n = 3-5

The Gaussian-3 (G3) model chemistry method has been used to calculate the relative deltaG(o) values for all possible conformers of neutral clusters of water, (H2O)n, where n = 3-5. A complete 12-fold conformational search around each hydrogen bond produced 144, 1728, and 20,736 initial starting structures of the water trimer, tetramer, and pentamer. These structures were optimized with PM3, followed by HF/6-31G* optimization, and then with the G3 model chemistry. Only two trimers are present on the G3 potential energy hypersurface. We identified 5 tetramers and 10 pentamers on the potential energy and free-energy hypersurfaces at 298 K. None of these 17 structures were linear; all linear starting models folded into cyclic or three-dimensional structures. The cyclic pentamer is the most stable isomer at 298 K. On the basis of this and previous studies, we expect the cyclic tetramers and pentamers to be the most significant cyclic water clusters in the atmosphere.     

Efficient single photon detection by quantum dot resonant tunneling diodes

We demonstrate that the resonant tunnel current through a double-barrier structure is sensitive to the capture of single photoexcited holes by an adjacent layer of quantum dots. This phenomenon could allow the detection of single photons with low dark count rates and high quantum efficiencies. The magnitude of the sensing current may be controlled via the thickness of the tunnel barriers. Larger currents give improved signal to noise and allow sub-mus photon time resolution.     

Carrier transport effects on the photocurrent spectra of an intrinsic GaAs/AlGaAs multiple quantum well with highly doped AlGaAs contact layers

In this study, we demonstrate how electroreflectance (ER) measurements as a function of bias, and of angle of incidence (θ₀), together with bias dependent photocurrent (PC) measurements, can be used to provide understanding of the complex electric field profile and carrier transport effects in a GaAs/Al_0.3Ga_0.7As multiple quantum well (MQW), grown inside n⁺ contact layers. The PC measurements exhibit split excitonic features, the components of which change in strength with the applied bias. The effect is explained by absorption in the front of the MQW stack, with the back of the stack acting as detector. We examine the θ₀-dependence of the ER lineshape, to determine the depth of the layers responsible for each feature. The ER and PC lineshapes and their bias dependence are explained by the unusual electric field profile across the stack. The field profile appears to be determined by tunnelling of the dark current.     

Chirality assignment of single-walled carbon nanotubes with strain

Strain-induced band gap shifts that depend strongly on the chiral angle have been observed by optical spectroscopy in single-walled carbon nanotubes (SWCNTs). Uniaxial and torsional strains are generated by changing the environment surrounding the SWCNTs, using the surrounding D2O ice temperature or the hydration state of a wrapping polymer. These methods are used as diagnostic tools to determine the quantum number q and examine chiral vector indices for specific nanotubes.     

Loose almost blowing-up and related properties

It is shown that finitely-fixed processes have -metric versions of the almost blowing-up property, the approximate-match waiting-time property, and the strong form of extremality. The proofs make use of a new result that connects, in a local almost sure way, fixed-length blocks in a process with fixed-length blocks in an expansion of the process. Supported in part by joint NSF-Hungarian Academy grant INT-9515485.     

Maintaining a high degree of research productivity at a predominately undergraduate institution as your career advances

In this perspective, two experienced academic administrators who are computational chemists discuss strategies for how to maintain an active research program at a predominately undergraduate institution as your career progresses. More responsibility equates to less time for research, so planning for research to remain a priority is essential. We all have the same amount of time, so figuring out how to use yours better is the key to remaining active. Professional organizations such as Council on Undergraduate Research, consortia of computational chemists such as Molecular Education and Research Consortium in computational chemistRY and Midwest Undergraduate Computational Chemistry Consortium, and attendance at professional conferences can help sustain your research program. Collaborations with faculty at other institutions provide a particularly effective accountability mechanism as well. Perhaps the best way to improve your productivity is to become a better mentor to your undergraduate students. Building a research group that is fun and exciting develops a culture that sustains itself and provides the momentum necessary to maintain progress toward scientific goals.