Publication costs assisted by the U S . Office of Naval ResearchThe infrared spectra have been obtained for a number of 1:l hydrogen bonded complexes isolated in a nitrogen matrix at 15'K. The complexes studied and their absorptions (successively, us and Yb, given in cm-l) are as follows: (CH3)2O -HCl, 2280,525; (CH&N -HC1, 1595, 1370; (CH3)sN -HBr, 1870,1600; H3N HBr, 1390, 1211; (CH3)zCO -HBr, 1700; HzO HBr, 2310, 410. For each acid, the value of us in the complex depends systematically, though not monotonically, upon base strength, as measured by proton affinity. We define the normalized proton affinity difference, A = [PA(B) -PA(X-)]/[PA(B) + PA(X-)I, and find that plots of both Au,/uSo and Yb against A correlate all of the systems studied (including H3N a HCl and HzO -HC1). We call such a plot a vibrational correlation diagram and we associate the trends with an increasing extent of proton transfer as A and base strength increase. According to this interpretation, the proton is "completely shared" if A is near -0.23. Partial proton transfer occurs if A is less negative than -0.23, increasing progressively a8 base strength increases.
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