Dear Friends,
Thanks to those who replied to my inquiry below about DOSY. Some AMMRlers
provided some hints to the problem, which led to my further inspection and
examination of the DOSY results. After repeating the experiments with more
gradient points, it is clear that the protons on the middle 4 carbons of B
are not exactly at the diffusion of A, but roughly at an averaged diffusion
rate of A and B, because these protons overlap with some signals of A in
terms of 1H chemical shifts. DOSY processing uses the inverse Laplace
transformation and it cannot seem to differentiate two close diffusion rates
at a single chemical shift. As a result, only one diffusion rate will be
given for multiple components in the same sample at the chemical shifts
where overlapping takes place. The problem is also mentioned and discussed
in this book on page 102: NMR Studies of Translational Motion - Principles
and Applications, by W.S. Price, 2009, University Press, Cambridge, UK. I'll
still welcome inputs on the inverse Laplace transformation as to how to
improve its resolution in the diffusion dimension of DOSY.
Happy Summer!
Cheers,
Jerry
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****** Original Inquiry ******
I have the following question for those of you who are familiar with DOSY or
diffusion in general:
For a reaction between two molecules A and B, where A is a bulk molecule
with a molecular weight of about 2000 while B is an 8-carbon diamine
(H2N-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-NH2) and presumably the middle part
(carbons 3-6) of B will interact with A, we were expecting from DOSY one
diffusion coefficient if they form AB, assuming a molar ratio A:B=1:1 and
the reaction completes, and two separate coefficients if they did not react
at all (arguably there could be more coefficients if equilibrium).
Surprisingly, the 1H DOSY showed that A plus the protons on the middle 4
carbons of B occurred at roughly the diffusion coefficient of A while the
protons on the 4 end carbons of B appeared at the diffusion coefficient of
B. Since B did not break into pieces during the reaction, how could it occur
at two different diffusion rates? We are puzzled by the results. I'll
appreciate if you could shed light on this observation.
Cheers,
Jerry
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Jerry Hu, Ph.D. Email:
jghu_at_mrl.ucsb.edu
Project Scientist Tel:
(805)893-7914
Materials Research Lab, UCSB Fax: (805)893-7914
Santa Barbara, CA 93106
Received on Tue Jun 14 2011 - 06:33:39 MST