and the presence of a splitting at 6: (which is absent in C!, H,-HDO) provide vibronic level evidence that the water molecule is on the sixfold axis undergoing internal rotation about that axis. Rotational band contour analysis of the 6: transitions of the isotopomers confirms this picture and als.2 determines a ground state center-of-mass separation between C, H, and D, 0 of 3.32 f 0.07 A, very close to that predicted by ab initio calculations. R2PI scans of the van der Waals structure in the isotopic series C, H,-H, 0, C, H,-HDO, and C, H,-D, 0 provide tentative assignments for three of the six van der Waals modes in the complex. In C,H,-(H, 0) Z, rotational band contour analysis of the origin transition provides a best-fit structure in which the two water molecules reside on the same side of the benzene ring at a H, O-H, 0 separation close to that in the free water dimer. Qualitatively, the structure of the 1:2 cluster is thus one which maximizes the strength of the water-water hydrogen bond at the expense of a somewhat poorer interaction of the second water molecule with the benzene ring in an off-axis geometry. Several intriguing features of the structure are suggested by our analysis, but are near the limit of our ability to distinguish from band contour fitting. Among these features are (i) the on-axis water molecule is pulled slightly in toward the ring from that in the 1: 1 complex; (ii) the water dimer prefers an orientation bisecting a CC bond in the benzene ring; (iii) the water-water separation is-0.2 A less than that in the free water dimer; and (iv) the water dimer axis is tilted by about lo" relative to the plane of the benzene ring. Finally, the van der Waals structure in C, H6-(H, 0), and C, H,-(D, 0) 2 suggests the possibility of large amplitude motion in these complexes as well. We postulate that this motion involves a hindered rotation of the on-axis water molecule.
General chemistry students in four different courses spanning a 12-year period were given a paper-pencil reasoning test entitled An Inventory of Piagetian Developmental Tasks (IPDT). A subset of twelve questions from this test, representing visualization abilities, were used to investigate spatial perceptual skills of students with regards to gender and course level. These findings indicate statistically significant differences in test scores based on gender, showing that males scored higher than females. A longitudinal study, comparing data from the early 1980's to data from the 1990's reveals an eroding of IPDT scores in students enrolled in the general chemistry course at Southeast Missouri State University. Gender-related scores over the same time period show a decrease in the gender gap due to a lowering of male scores. Possible explanations of results and further areas of study are suggested in the context of spatial perception research.
The benzene-HCI/DCl and toluene-HClIDCI complexes have been studied using both fluorescence and multi photon ionization detection. These complexes are prototypical of 1r hydrogen-bonded complexes involved in the chemically important proCeSS of electrophilic aromatic attack. Laser-induced fluorescence (LIF) etalon scans of the 66 rotational band contour are used to determine the SI state geometry of the benzene-HCI as one in which HCl is on the sixfold axis with a center-of-mass separation of 3.64±0.03 A. The lack of significant van der Waals' intensity points to the complex having a hydrogen-bonded geometry similar to that found in the ground state. Dispersed fluorescence scans are used to put crude bounds on the So and SI binding energies of the benzene-HCI complex of 1.8<,Do <, 3.8 kcallmol and 1.5<,D6<,3.5 kcallmol. The fluorescence lifetimes of bound levels of the complexes are factors of 7-12 times shorter than the corresponding levels of the free molecules. In contrast, the C 6 H 6 -CH 3 CI complex, which has a similar geometry and binding energy, has a fluorescence lifetime nearly as long as C 6 H 6 • We argue that the differences observed are consequences of the hydrogen bonding interactions present in benzene-HCI and toluene-HCl. Two-color multiphoton ionization experiments in which the delay between the So-S 1 laser and the ionization lasers is continuously scanned give evidence that the hydrogen bonding interactions lead to enhanced intersystem crossing to the triplet state. One-color RE2PI scans show that fragmentation of the [benzene-HCI] + and [toluene-HCI] + ionic complexes proceeds nearly quantitatively (;;;.98%) from the hydrogen-bonded Sl state. This fragmentation occurs by virtue of the repulsive geometry formed for the ionic complex in Franck-Condon excitation from the SI state.
Aggregation is an important area of scientific investigation because of the consequences of this process for many aspects of protein and peptide chemistry. Previous studies of the aggregation of the βA4 peptide fragment, β(12−28), and synthetic analogues in low pH aqueous solution show that replacing either or both phenylalanines with glycine reduces the tendency of this peptide to form aggregates. In this investigation, several β(12−28) analogues have been synthesized in which the phenylalanine residues 19 and/or 20 have been substituted with the nonnative amino acid, naphthylalanine, to produce the peptides [napAla19,20], [napAla19,Gly20], and [Gly19,napAla20] and allowing the aggregation behavior of these peptides to be examined in aqueous solution at low pH with both NMR and fluorescence spectroscopy. The NMR chemical shift, diffusion coefficients and relaxation times as well as rotational correlation times measured with both NMR and fluorescence spectroscopy are concentration dependent providing evidence that [napAla19,20]β(12−28) forms soluble aggregates. Similar results obtained for [napAla19,Gly20]β(12−28) and [Gly19,napAla20]β(12−28) suggest that these peptides have a greatly reduced tendency to aggregate. In addition, [napAla19,20]β(12−28) produces excimer fluorescence emission in a concentration-dependent manner with essentially no excimer detected in the fluorescence spectra of the singly substituted naphthylalanine analogues. Fluorescence lifetimes were measured, and unlike naphthylalanine, the free amino acid, the excimer fluorescence decay of [napAla19,20]β(12−28) does not exhibit a rise time component, suggesting a ground-state preassociation of the peptides through naphthyl π−π interactions that stabilize the aggregates. Fluorescence spectroscopy, due to its concentration sensitivity, permits measurements of peptide solutions at much lower concentration than NMR, allowing direct measurement of the peptide monomer. However, NMR spectroscopy, through the measurement of nuclear relaxation times, can provide complementary information about the differential regional mobility of the peptide. The application of both NMR and fluorescence spectroscopy to the analysis of these naphthyl-substituted peptides produces a more complete picture of their aggregation behavior than could be obtained using either method alone. An advantage of using the combination of these methods is that their different time scales make them sensitive to different ranges of molecular motion.
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