Nuclear magnetic resonance downfield signals

Alkanes are characterized spectroscopically by strong infrared (IR) absorption in the C-H stretching region between 3950 and 2850cm, lack of ultraviolet (UV) absorption above 200 nm, H nuclear magnetic resonance (NMR) signals below 5 2.0 ppm downfield from tetramethylsUane (TMS) = 0.00, and NMR signals below about 5 50 ppm downfield from TMS = 0.00. Their mass spectra usually involve sequential loss of methylene units (-CH2-, miz = 14). 

Nuclear magnetic resonance spectroscopy gives precise information on complexation in solution. Equilibrium is rapidly established on an NMR time scale, hence only an average spectrum is observed and it is difficult to determine the spectrum of a pure complex. When complexation of a sugar or polyol with a diamagnetic ion occurs, all of the signals shift downfield. Equation (11.1) allows the variation of the shielding constant Ao- of the proton to be calculated when the nucleus is subjected to an electric field E whose projection on the C-H bond is... 

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