A General Chemistry Experiment to Predict and Measure Functional-Group Stretching Frequencies

An experiment suitable for first-year students is reported. In this activity, students use a molecular modeling program to compute infrared spectra for a series of molecules. From the data obtained, students generate a group frequency chart and use it to identify unknowns. This provides students with an introduction to vibrational spectroscopy and the use of molecular modeling.     

Proton Diffusion andT1Relaxation in Polyacrylamide Gels: A Unified Approach Using Volume Averaging

The structure of polyacrylamide gels was studied using proton spin–lattice relaxation and PFG diffusion methods. Polyacrylamide gels, with total polymer concentrations ranging from 0.25 to 0.35 g/ml and crosslinker concentrations from 0 to 10% by weight, were studied. The data showed no effect of the crosslinker concentration on the diffusion of water molecules. The Ogston–Morris and Mackie–Meares models fit the general trends observed for water diffusion in gels. The diffusion coefficients from the volume averaging method also fit the data, and this theory was able to account for the effects of water-gel interactions that are not accounted for in the other two theories. The averaging theory also did not require the physically unrealistic assumption, required in the other two theories, that the acrylamide fibers are of similar size to water molecules. Contrary to the diffusion data,T₁relaxation measurements showed a significant effect of crosslinker concentration on the relaxation of water in gels. The model developed using the Bloch equations and the volume averaging method described the effects of water adsorption on the gel medium on both the diffusion coefficients and the relaxation measurements. In the proposed model the gel medium was assumed to consist of three phases (i.e., bulk water, uncrosslinked acrylamide fibers, and a bisacrylamide crosslinker phase). The effects of the crosslinker concentration were accounted for by introducing the proton partition coefficient,K^eq, between the bulk water and crosslinker phase. The derived relaxation equations were successful in fitting the experimental data. The partition coefficient,K^eq, decreased significantly as the crosslinker concentration increased from 5 to 10% by weight. This trend is consistent with the idea that bisacrylamide tends to form hydrophobic regions with increasing crosslinker concentration.     

A model for estimating (p, π+) cross sections

We have formulated a model for estimating the value of the (p, π⁺) cross section on a target nucleus by relating it to the elastic π⁺ scattering cross section on the same target.     

Investigating the Red Shift between in vitro and in vivo Urocanic Acid Photoisomerization Action Spectra

Abstract— Trans-urocanic acid (UCA) is found in the upper layer of the skin and UV irradiation induces its photoisomerization to cis -UCA. Cis -UCA mimics some of the immunosuppressive properties of UV exposure. The wavelength dependence for in vitro photoisomerization of trans-UCA (15 μM) over the spectral range 250 nm-340 nm (10 nm intervals) was determined. The action spectrum revealed that maximal cis-UCA production occurred at 280 nm, which is red-shifted by 10-12 nm from its absorption peak at 268 nm and differs markedly from the reported action spectra for cis-UCA production in mouse skin in vivo , which peaks at 300-310 nm. The reasons for the red shift between the in vitro and in vivo action spectra are not clear. There is limited evidence suggesting that the UV absorption maximum of trans- UCA red shifts from 268 nm in vitro to 310 nm on interaction with stratum corneum proteins in vivo. This phenomenon was investigated by applying trans-UCA (2.5 mg/cm²) in an oil emulsion to isolated human stratum corneum. After incubation at 37°C for 1 h, the absorption spectra of stratum corneum with UCA and with oil only were compared using a Xe arc source and a spectrora-diometer. A moderate red shift in trans-UCA absorption from ∼268 nm to 280 nm was observed. In summary, we suggest that the 10-12 nm red shift between the UCA absorption spectrum peak and the action spectrum peak in vitro may be accounted for by the wavelength dependence of quantum yields reported over the 254-313 nm range. The red shift between the in vitro and in vivo photoisomerization action spectra may result from the 10 to 12 nm red shift in the absorption of UCA in association with stratum corneum proteins, combined with increasing quantum yields over the 254-313 nm range.     

Kawasaki dynamics with infinite-range spin exchange

The essential feature of the Kawasaki model is the conserved order parameter, which places the model in class B of the Halperin, Hoheberg, and Ma classification. We have studied the energy relaxation of this model in one and two dimensions with the added feature that spin exchange may take place between any pair of sites within the system. Our results for the dynamic exponentz are indistinguishable from those for class A models, in which the order parameter is not conserved.