th  - 1987 - 51st ENC Conference

MK47
AN IMPROVED CHORTLE PULSE SEQUENCE
GERALD A. PEARSON
CHEMISTRY DEPARTMENT, UNIVERSITY OF IOWA
IOWA CITY, IOWA 52242
We have developed s pulse sequence which substantially
improves the accuracy and resolution of the CHORTLE (I)
experiment. (Carbon-Hydrogen correlations from One-
dimensional polaRization-Transfer spectra by LEast-squares
analysis) One can now adjust tCH, the net evolution time
for dcm prior to polarization transfer, independently o£
tM,max, the maximum net evolution time for proton chemical
shift, 6H. Using tN,max m 10 msec, CHORTLE now routinely
measures 6H with an error ~(~H) ~ I HZ, and resolves non-
equivalent geminal methylene protons whose chemical shifts
differ by only 30 Hz. Exploratory CHORTLE experiments with
longer tH,max have achieved ¢(6H) ~ 0.2 Hz in favorable
cases, and it should be possible to achieve ~(6H) s 0.02 Hz
in special cases, with a IH spectrum width ~ 5000 Hzl
In its orlElnal form, the CHORTLE experiment typically
yielded ~(6H) ~ 3 Hz, end frequently could not resolve non-
equivalent gemlnal protons whose chemical shifts differ by
less than about 70 Hz. This was because tM.max = tCH, and
tcH is restricted to values which optimize the polarization-
transfer efficiency. With tcH ~ I/2JcH, tH,max was only 3.2
meec. Tzeng(2) pointed out that it is possible to
circumvent this limitation by setting tcH = 3/2JcH, 5/2dcs,
etc., in exactly the same way that Reynolds and others TM
re-parameterized the COLOC c4) sequence. Unfortunately,
increasing tcs has three disadvantages. First of all,
errors arising from unwanted long-range polarization
transfer remain approximately constant, rather than
decreasing with increasinE tH,ma,. Secondly, the desired
polarization-transfer efficiency becomes more sensitive to
the value of Jcs; since Jcs can vary from 118 Hz to 220 Hz
or more in some samples, this will inevitably lead to loss
of sensitivity for some signals. Finally, only certain
values of ts,max are possible (ca. 10 msec, 16.5, 23, ...)
with this approach. The new pulse sequence permits one to
choose ts,max to be a truly optimum compromise between
desired accuracy and the sensitivity losses caused by IH
relaxation and IH multiplet dephasing arising from IH-IH
coupling.
1. G.A.F~Ju~SON, J.~oN.RZSON. 6,4, 48T (1~).
2. C.S.TzENo, UNIV. (:~ C~U-IF. AT RI~ERSI2* I~IVATI[ CC]I,~I~.ATION.
3. W.F.REYNOLDS, D.W.HUOMES, M.I~A,'XOK-I~, AND R.G.EN~xOUEZm
J.I'IAoN.RZ~w~N. 64, 304 (1985).
4. H.I~S~.~R, C.CW~IESI~R, J . ~ ( ~ , AND H.R.I-~I,
J.MA(Nw. REIwmw. 6"/', 331 (1~34).