Dynamic NMR and NOE - Dynamic NMR and Nuclear Overhauser...

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Dynamic NMR and Nuclear Overhauser Effects
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Spectroscopic Time Scales - UV/Vis OH NO 2 O NO 2 + H pKa~7 “Slow exchange” on the UV/Vis time scale
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Spectroscopic Time Scales - NMR OH NO 2 O NO 2 + H pKa~7 “Fast exchange” on the NMR time scale Signal averaged
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Why Different Time Scale? • NMR and UV/Vis operate at different frequencies. • Heisenberg’s Uncertainly principle Δ E τ h/2 π , In order to measure small difference in energy( Δ E), the life time, τ , must be sufficiently long. Δ E = h Δν ˀ τ Δν 1/2 π For A B, k ex ˿ 1/ τ k ex If k ex > Δν fast exchange If k ex < Δν slow exchange ν A ν B Δν = ν A −ν B
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Phenol - Phenolate (UV/Vis) OH NO 2 O NO 2 + H pKa~7 318nm 395nm ν = c/ λ , c = 3 x 10 8 ms -1 9.4 x 10 14 Hz 7.6 x 10 14 Hz ˀ Δν = 1.8 x 10 14 Hz k protonation = 10 9 ~ 10 10 s -1 (diffusion control) k protonation << Δν Slow exchange Only 1 nm difference in λ max Δν = 2 x 10 12 Hz No chemical reaction can achieve fast exchange on the UV/Vis time scale
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This note was uploaded on 01/18/2012 for the course CHEM 460AB taught by Professor Sasaki during the Fall '11 term at University of Washington.

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Dynamic NMR and NOE - Dynamic NMR and Nuclear Overhauser...

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