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July 17, 2008

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hko

There seems to be a small tilt in the pattern. Is this an
artifact or can be drawn any information from this ´upward
roof´ ?

Arvin

Hello hko,

I had to consult a fellow expert in NMR data acquisition. Until I receive further notice, I will leave the name as anonymous.

Generally speaking, tilting in the F1 frequency domain is related to some type of evolution going on during the delays in the pulse sequence when 13C chemical shift information is being encoded into the data matrix. The most obvious example are responses that involve fluoro couplings, in which cases the tilts can be significant, ranging up to ~255 Hz, the size of the 1JCF coupling. The other source of tilting in F1 can be due to 1H chemical shift evolution during the duration of the delay. Generally, the F1 digital resolution isn't high enough to worry about that or to see it.

The tilt of the proton response is unrelated to what's going on in the F1 frequency domain of a GHMBC spectrum. Rather, the proton tile is a function of the size of the homonuclear proton-proton coupling constant and the difference between the chemical shifts of the two coupled proton resonances.

The point is illustrated on the following blog:
http://acdlabs.typepad.com/elucidation/2008/04/how-do-i-know-2.html

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