An Examination of the Relationship Between Transition State Geometry in Hydron Transfer Reactions and the Temperature Dependence of the Primary Kinetic Isotope Effect

Several recent papers by Kwart feature hydron transfer reactions in which the primary kinetic isotope effect is temperature independent over wide temperature ranges. It has been asserted (without theoretical justification) that this phenomenon is associated with transition state X….H….Y bond angles which are significantly less than 180°, whereas normal temperature dependence may be associated with transition states in which the angle is close to 180°. This work employs model calculations of isotope effects, principally for [1,5] H-shifts in 1,3-pentadiene, to examine Kwarts's hypothesis. No model tested yielded a temperature-independent isotope effect of substantial magnitude. The transition state (angle = 133°) was located on the MNDO potential energy surface and the isotope effect, calculated by a new and fast computational procedure, was again temperature dependent. Model calculations of secondary hydrogen isotope effects were carried out. When bending motions of the non-reacting hydrogens were coupled to C….H stretching modes in the force constant matrix, it was found that kH/kD was greater than the equilibrium secondary isotope effect, k_H/K_D, and that k_H/k_D for hydrogen transfer exceeded k_H/k_D for deuteron transfer when simple tunnel corrections were incorporated. A novel interpretation to account for the observation of some temperature-independent isotope effects is advanced.     

Novel tapping technique for charge-coupled devices

The letter proposes a useful technique for the nondestructive tapping of charge-coupled devices, with a presentation of both theoretical and experimental results for device operation. The usefulness of the technique lies primarily in its simplicity of fabrication and operation.     

Computational fluid dynamics

Variation in the individual airway geometry makes subject-specific models essential for the study of pulmonary air flow and drug delivery. Recent evidence also suggests that early exposure to environmental pollutants has chronic, adverse effects on lung development in children from the age of ten to 18 years [1]. Thus, the capability of predicting air flow and particle deposition in the subjectspecific breathing lungs is highly desirable for understanding the correlation between structure and function and for assessing individual differences in vulnerability to airborne pollutants. Furthermore, it has been demonstrated that a strong interaction exists between lung geometry and gas properties [2]. The interaction has major implications in determining gas delivery to and clearance from the lung periphery during ventilation imaging through X-ray computed tomography (CT) using xenon gas [3]-[5] or magnetic resonance imaging (MRI) using hyperpolarized helium gas [6]-[9]. Although there is a critical need to understand these geometry?property interactions, the current state of knowledge acquired from experiments is still far from revealing the true nature of their interplays. At the same time, three-dimensional (3-D) computational fluid dynamics (CFD) simulation of air flow for the entire lung geometry remains intractable because of constraints on imaging resolution and computational power. As a result, current 3-D CFD simulations of air flow are often restricted to a few generations of branching on a fixed mesh, and most studies are based on idealized Weibel airway models. With advances in imaging and computing technologies, anatomy coupled with functional measures (ventilation and perfusion) can now be obtained via CT imaging [10], [11]. These measures provide the detail needed to interrogate the utility of CFD in providing insights into subject-specific differences in regional lung function and the underlying mechanisms of pathologic developments     

The effect of thermal exposure on the electrical conductivity and static mechanical behavior of several age hardenable aluminum alloys

Aluminum alloys 2014-T6, 2024-T3, 6061-T6, 7050-T7451, and 7075-T6 were thermally exposed at different times (1 min to 20 days) and temperatures 177–482 °C (350–900 F). This study was conducted to simulate the effects of heat damage on aluminum alloys and to determine the correlations existing between the static mechanical and electrical conductivity properties. Results indicate that at the temperatures below 260 °C (500 F) all five alloys showed clear correlations between the mechanical and physical properties.