Thursday, October 2, 2008
Dilute "D2O" in Benzene-d6
The 1H NMR spectrum of a mixture of H2O and D2O is a single line at about 4.8 ppm. The H2O and HDO resonances are unresolved in the spectrum due to fast chemical exchange between the isotopomers and possibly line broadening due to radiation damping. When traces of D2O are added to benzene-d6, which already contains traces of H2O, the situation is different. The resonance is shifted by more than 4 ppm to lower frequency compared to the bulk and since the water is now dilute and in small quantities, chemical exchange is slow on the NMR time scale and radiation damping is no longer a problem. The figure below shows the 500 MHz 1H NMR spectrum of dilute D2O in benzene-d6. The isotope shift between H2O and HDO and the HD coupling constant can easily be measured from the spectrum.