University of Ottawa NMR Facility Web Site

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Tuesday, January 8, 2008

Proper Data Processing Can Be Very Rewarding


naresh said...

Please let me know why spin relaxation time is more for 13C nuclei which effect the integration of 13C spectra..

Glenn Facey said...


Thank you for the question. 13C T1's are generally longer and have more variability than 1H T1's. For the usually dominant dipolar mechanism, the 13C T1's are longer than 1H T1's because the relaxation rate depends on the product of gyromagnetic ratios for a given spin pair. For 13C, the dipolat relaxation rate would depend on gammaC*gammaH For 1H the relaxation rate depends on gammaH*gammaH which is 4 times greater than gammaC*gammaH.

The dipolar relaxation rates for carbon also depend on the reciprocal 6th power of the distance between the carbon and a proton. Since carbonyl carbons or quaternary carbons are far away from the nearest proton, their T1 relaxation times are very long (often tens of seconds or minutes) compared to protonated carbons where the T1's may be several seconds. This variability in 13C T1's means that in order to get a quantitative spectrum one must use very long recycle delays.

NOE's also cause a problem in obtaining quantitative 13C data.