Laser-induced fluorescence of jet-cooled IBr: B3.PI.0+ .rarw. X1.SIGMA.+ excitation spectra

Abstract

activity seen for addition of DMSO parallels that observed for methanol, strongly indicating that the changes observed are again due primarily to the variation in solvent dielectric constant. DMSO has a dielectric constant (< = 47) similar to that of methanol but is devoid of hydrogen bonding. DiscussionThe conclusion drawn above, that the "dissociated" model of Schipper best fits the data for L-histidine-Tb(DPA)33", differs from that of Brittain et al., who have described the interaction of various chiral substrates including L-histidine with Ln(DPA)33" as forming 1:1 complexes.10 Our results do, however, agree with those of Schipper, who examined similar concentration data for Pfeiffer-active transition-metal systems and also concluded that the dissociated model appears to give a better fit to the experimental data than the associated model.17Although the addition of L-histidine to racemic solutions of lanthanide-DPA complexes clearly results in an equilibrium shift, exactly which enantiomer is favored has not been known until very recently. By comparison of CPL and CD spectra from an optically active crystal of Na3Eu(DPA)3-2NaC104-6H20 to that of a Pfeiffer-perturbed Eu(DPA)33" solution, we have concluded that addition of L-histidine results in an excess of the -enantiomer.19 This conclusion is in agreement with that of Brittain.11 This result implies that ( ) is negative (see eq 14, 16, and 18) and that the L-histidineinteraction, ( -), is of lower energy. A multipole-multipole calculation of this interaction is in progress.The results presented above for mixed solvent systems indicate that the chiral discriminatory interaction has a large electrostatic component. In addition, it is concluded that hydrogen-bonding effects are less important. No attempt has been made to take into account selective solvation or other complications of these mixed solvent systems,20 but such work is under way.

Jet-Cooled Fluorescence Spectroscopy of a Natural Product: Anethole

“…The current work employed standard jet-cooling techniques, and therefore a brief description will be given here. , Anethole (Eastman) was introduced into the fluorescence chamber via Series 9 General Valve (150 μm orifice) operating at 30 Hz. Samples were entrained in helium with a stagnation pressure of 2.5 bar.…”

Experimental observation of competing pathways in the relaxation of ICl* in a He supersonic expansion

“…An abrupt end in the ICl B-X vibrational progression is observed above the v 0 ¼ 3 level. This break in the spectrum is attributed to the crossing of the B 3 P 0þ state by the repulsive Z 1 and Y 0þ states at energies near the ICl(B,v 0 ¼ 3) vibrational level, 4,41,45,46 as illustrated on the right side of Fig. 1.…”