A theory for nonisothermal unimolecular reaction rates We consider the relation between observed pump-probe signals in the femtosecond regime and the kinetics of unimolecular reactions, that is, the exponential decay of reactants and the exponential rise of the product population, respectively. It is shown that the signals cannot be fully accounted for within standard approaches of unimolecular decay, conventionally used in the past, since interference effects between the quasi-bound vibrational states within the bandwidth of the pump laser cannot be neglected. When these effects are included, all features of the signals can be accounted for. We apply this theoretical treatment of coherent interference to examine the dynamics and kinetics of the quasi-bound transition configurations, and relate them to the decay rates of individual quasi-bound resonance states. The signals show multi-exponential behavior, reflecting the different decay rates of the resonance states, with an average rate constant within the bandwidth of the pump laser which can be extracted directly from the signals. The persistence of coherence is evident in the observed signals. The predissociation of NaI is used as a prototype for numerical illustration.

Articles you may be interested inOn the role of coherence in the transition from kinetics to dynamics: Theory and application to femtosecond unimolecular reactions A theory for nonisothermal unimolecular reaction rates Response to "Comment on 'On the relation between unimolecular reaction rates and overlapping resonances'" [A method for computing unimolecular reaction rate constants in the IVR-limited regime is presented. It is based on Kramers' energy diffusion theory, with the reaction coordinate taken as the subsystem and the rest of the vibrational modes as the bath. Applications to some bond fission reactions demonstrate that the method accurately predicts the rate constants for wide range of energies by using the result of a dynamical calculation of the reaction rate at a single energy to determine the friction coefficient. Examination of the energy exchange in the reaction coordinate provides a qualitative understanding of the validity of the approach for treating unimolecular reactions. Thus, the method provides a practical means of calculating reaction rates in the IVR-limited regime at considerable savings of computer time than that required by standard classical trajectory calculations.

Successful regeneration of biological tissues in vitro requires the utilization of three-dimensional (3D) scaffolds that provide a near natural microenvironment for progenitor cells to grow, interact, replicate, and differentiate to form target tissues. In this work, a rapid aqueous photo-polymerization route was developed toward the fabrication of a variety of polymer hydrogel 3D inverted colloidal crystal (ICC) scaffolds having different physical and chemical properties. To demonstrate the versatility of this technique, a variety of polymer hydrogel ICC scaffolds were prepared, including (1) polyacrylamide (pAAM) scaffolds, (2) poly(2-hydroxyethyl methacrylate) (pHEMA) scaffolds, (3) poly(2-hydroxyethyl acrylate) (pHEA) scaffolds, and composite scaffolds including (4) pAAM-pHEMA scaffolds, (5) pHEMA-pMAETAC [poly(2-methacryloyloxy) trimethyl ammonium] scaffolds, and (6) pHEA-pMEATAC scaffolds. Templates for scaffolds incorporated both uniform sized (104 microm diameter) and nonuniform sized (100 +/- 20 microm diameter) closely packed noncrosslinked poly(methyl methacrylate) beads. Human bone marrow stromal HS-5 cells were cultured on the six different types of scaffolds to demonstrate biocompatibility. Experimental results show that cells can remain viable in these scaffolds for at least 5 weeks. Of the six scaffolds, maximal cell adhesion and proliferation are obtained on the positively charged composite hydrogel pHEMA-pMEATAC and pHEA-pMAETAC scaffolds.

The rapid Friedel-Crafts chromogenic acylation of alkene groups at ambient temperatures using a 25:1 mixture of 98% acetyl chloride and 70% perchloric acid is shown to have all the properties needed to serve as a potential quality control reagent that can be used to routinely discriminate among steroid analogs. Although ostensibly a non-selective reagent, from these and prior applications in terpenes and polyunsaturated acid esters, it is seen that the reaction is capable of discriminating bewteen geometric isomers and even enantiomers. The selectivity towards acylation of the alpha- over the beta-position at C-17 makes the method adaptable to screening for anabolic steroids. Reactions at that position produce the more unusual results, including a positive color reaction for alpha-methyltestosterone even though there is no alkene functional group in the vicinity of C-17. For molecules with more than one alkene, concurrent acylations are independent one from the other and, in the absence of any interferences, their spectral properties are found to be additive.

There is a renewed interest in the development of chemical and biological agent sensors due to the increased threat of weapons deployment by terrorist organizations and rogue states. Optically based sensors address the needs of military and homeland security forces in that they are reliable, rapidly deployed, and can provide continuous monitoring with little to no operator involvement. Nomadics has developed optically based chemical weapons sensors that utilize reactive fluorescent chromophores initially developed by Professor Tim Swager at MIT. The chromophores provide unprecedented sensitivity and selectivity toward toxic industrial chemicals and certain chemical weapon agents. The selectivity is based upon the reactivity of the G-class nerve agents (phosphorylation of acetylcholinesterase enzyme) that makes them toxic. Because the sensor recognizes the reactivity of strong electrophiles and not molecular weight, chemical affinity or ionizability, our system detects a specific class of reactive agents and will be able to detect newly developed or modified agents that are not currently known. We have recently extended this work to pursue a combined chemical/biological agent sensor system incorporating technologies based upon novel deep ultraviolet (UV) light emitting diodes (LEDs) developed out of the DARPA Semiconductor UV Optical Sources (SUVOS) program.