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Monday, July 28, 2008

T2 vs T2*

The T2 relaxation time is the exponential decay constant for transverse magnetization (i.e. magnetization in the xy plane). In principle, one should be able to measure the T2 relaxation time by applying a 90 degree pulse to create transverse magnetization and measuring the decay constant of the FID. In reality however, the decay rate of the FID is also affected by such things as magnetic field homogeneity, unresolved coupling, temperature gradients.....etc. Because of these effects, the decay constant of the FID is called T2* rather than T2. T2* is an instrumentally dependant parameter and it determines the line width of an NMR resonance. T2, on the other hand, is a physically meaningful parameter independent of field inhomogeneity, J coupling and other factors. It is measured with a 90-tau-180-tau-FID pulse sequence as a function of tau. T2 is always greater than or equal to T2*. The figure below compares T2 to T2* for the proton resonance of CHCl3 for the lineshape sample in a reasonably well shimmed 300 MHz magnet. The line shape specifications were 0.3 Hz (at 50%), 2.9 Hz (at 0.55 %) and 6.2 Hz (at 0.11%). Even in a well shimmed magnet, the T2 for CHCl3 is nearly 19 times longer than the T2*.

10 comments:

Luke O'Dell said...

So what about T1 and T1-rho?

Glenn Facey said...

Hi Luke,

?

See BLOG entries for October 31/07 and November 7/07.

T1 and T1-rho may be the subjects of future BLOG posts.

cheers,
Glenn

Ластоногий said...

Hi Glenn,
You rote that T2 time is measured with 90-tau-180-tau-FID pulse.
Why there is an FID in the end? Maybe you meant 90-tau-180-tau-ECHO?

Glenn Facey said...

I wrote that T2 is "...measured with a 90-tau-180-tau-FID pulse sequence ....." And yes, it would be more correct to say " ....measured with a 90-tau-180-tau-ECHO pulse sequence...." .

Anonymous said...

Dear Glenn,

what is the effect of hybridyzation on T1 relaxation.

Glenn Facey said...

Anonymous,
T1 relaxation depends on many things. It depends on the origin and magnitude of the local magnetic fields experienced by nuclei as a result of their neighbours. It depends on the spectral density function evaluated at the Larmor frequency which in turn depends on temperature and molecular size etc.. It is difficult to answer your question without knowing more information (nucleus, molecular size, specific molecules, etc...). I'm not even sure if there is a simple answer.
Glenn

Anonymous said...

What is the effect of T1 relaxation on H nuclei of a compound having sp3 , sp2, and sp hybridized protons attached to carbons. And whether the effect is prominent in case of Carbon nuclei for the same or hydrogen ?

Glenn Facey said...

Anonymous,
The T1 relaxation rate for 1H and 13C in hydrocarbons is typically dominated by the dipolar mechanism. For protons, this depends on the reciprocal 6th power of the distance between protons and the correlation time for molecular rotation. The proton T1 gets longer at shorter correlation times (faster motion). For 13C, the relaxation rate depends on the reciprocal 6th power of the distance between the carbon and its attched protons as well as the correlation time. In general, the 13C T1's should be shorter for CH2 vs CH carbons. Since methyl groups rotate very quickly the 13C T1 tends to be longer as the correlation times are shorter. In conclusion, there is no simple answer to your questions.

Glenn

Anonymous said...

Thanks for your valuable time.

Can you please suggest any book or article for above discussion?

Glenn Facey said...

Annonymous,
Here are two good references a s a place to start.

James Keeler. Understanding NMR spectroscopy. 2nd Ed. Wiley, 2010. Chapter 9.

Malcolm Levitt. Spin Dynamics - Basics of Nuclear Magnetic Resonance. 2nd Ed. Wiley, 2008. Chapter 20.

Glenn