Fl~orene-9-~'C, fl~orenone-9-~'C, acenaphthenone-ll-"C, a~enaphthenone-l2-~~C, 1-methylcydopentanoll-13C and l-methylcycl~pentene-l-~~C were synthesized to obtain J(CC) values between the natural carbons and the labeled carbons. Each of these compounds possessed at least one asymmetric dual-path coupling, i.e., coupling between the labeled carbon and another carbon via simultaneous two-and three-bonded coupling paths. Model "C-labeled compounds were Synthesized where necessary to give expected values of the constituent mono-path couplings. Values of these dual-path couplings (2+3'6 suggested that the observed value is the (at least approximate) algebraic sum of the two constituent J values.
naphthoic acid (3), and [carboxyl-13C]-9,10-dihydro-9-anthroic acid (4) were synthesized and studied by proton NMR spectroscopy to obtain all homoallylic carbon-proton couplings. The ratio Jch/Jhh is 0.6, as previously observed for other systems. It is observed that these carbon-proton couplings are just as sensitive as proton-proton couplings to the degree of puckering of the dihydro ring. A comparison of theoretical and empirical J values is consistent, whereby the ratio ./eq-cq/Vx-cq (for Thh) decreases, and the ratio -axVax-eq (for Tch) increases throughout the series 2-4, showing the extent of puckering of 3 is intermediate between that of 2 and that of 4. Thus are generated three categories-"flat", "flattened boat", and "true boat"-to compare the geometries of 2-4. Considerations of other types of coupling constants (vicinal and allylic) in the series 2-4 and in other compounds supports the designation of these separate categories.It is clear that 1,4-dihydrobenzene and its monosubstituted derivatives are planar,2"5 and that 9,10-dihydroanthracenes are puckered with 9-substituents axial.6 However, some uncertainty shrouds the NMR conformational analysis of dihydroaromatic compounds in general, because the ratio of the two proton-proton homoallylic coupling constants Veis/ Vtrans (see 1) remains close to 1 throughout a series of com-1 pounds.2•7 Indeed, it has been proposed that this ratio of homoallylic coupling constants should not be used as a conformational tool.26 This hypothesis is somewhat surprising, because a strong geometrical dependence of Vhh had been previously shown (in single-path olefins).8In the present account it is shown that part of the past difficulty in utilizing the ratio Vc¡s/Vtrans arises from the fact that in proton-proton couplings this ratio uses axial-equatorial and equatorial-equatorial couplings. Perhaps better suited to the conformational analysis are axial-axial and axial-equatorial couplings, whose Vc¡s/Vtrans ratio does not remain so close to 1. Since the substituent is axial, one must then use couplings to the substituent. In the present study the substituent was labeled with 13C, and proton couplings to this labeled carbon were used. It is then realized, from data generated from this ancj from past studies, that there is a consistent trend in the ratio Veis/Vtrans (both Vax-ax/Vax-eq and Veq-eq/Vax-eq) throughout a series of dihydroaromatic compounds. Results and Preliminary RemarksSpectral Analysis. The compounds synthesized in this study are 2-4. Compounds 2 and 3 were deuterated as shown so as ¿í X=H, Y=D 3a X=H, Y=D b.
Significant increases in the overall efficiency of an isotopic segregation scheme may be achieved by irradiation of a lowpressure mixture of reactant in an excess of effective coreagent and by cooling the sample and reaction vessel. However, a greater increase in selectivity and end product enrichment could be realized by a more judicious choice of IR laser frequency or frequencies. Our technique allows us to rapidly assay relative isotopic dissociation probabilities and choose appropriate wavelengths to provide both high selectivity and high yields. Our studies only require microscopic quantities of isotopically enriched reagent (i.e., a few micrograms). In a system analogous to those of Ritter and Freund one should observe macroscopic isotopic segregations consistent with an average selectivity of a > 100 for the dissociation of12CF2Cl2 at 10.7 /am (the P(30) line at 934 cm-1) and of 1 /a * 30 for carbon-13 enriched product species at 9.5 gm (the P(8) line at 1057 cm-1). These represent substantial increases over the values of 5 and 1.04 reported previously.Very few attempts have been successful in the selective segregation of rare isotopes into product species using IR multiphoton dissociation techniques. As demonstrated by the work of Ritter and Freund, the low carbon-13 dissociation selectivities result in negligible product array enrichments; it is then preferable to dissociate >90% of the sample with selectivity for carbon-12 decomposition.By using real-time LEF diagnostics to determine collisionfree dissociation probabilities for each isotopic species, one can very rapidly assess optimum IR wavelengths for the selective multiphoton dissociation of rare isotopic species. This allows one to design experiments to selectively perturb those isotopic species of lesser abundance. Provided it is possible to scavenge the initially formed intermediates without encouraging secondary photolysis, scrambling, or nonselective thermal processes, one may realize a maximum isotopic enrichment of 13i8p,max = Rp/Ro = 1 / a0 for the products.
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