Hello-
The undergraduates here do an NMR experiment in one of their courses,
part of which is measuring proton T1s via the inversion recovery
sequence. It works quite well and I was interested in modifying the
procedure to add the measurement of T2 also, showing that, as you know,
for small molecules T1=T2. (I have been using benzene or dioxane for
the proton T1 experiments.)
I use the Carr-Purcell-Meiboom-Gill sequence (CPMGT2, Mercury-200, VNMR
6.1C) but every time I measure T2 I get a number that is less
substantially than T1, say ~80% of the T1. I have tried varying every
parameter I can think of: I have tried changing the 90 and 180 degree
pulse widths (away from the carefully calibrated values I determine); I
have changed the spacing of the 180 degree pulses, I have kept those
constant and changed the number of 180 degree pulses, I have changed the
recycle delay from 5XT1 to 10xT1, I have tried a degassed benzene sample
with a long T1 and I have tried a sample with a short T1, I have used
dummy pulses and I have not used them, I have acquired one transient per
experiment and I have acquired 8, I have interleaved the acquisitions
and not. I have used a 200 MHz instrument and a 400 MHz instrument, one
with a direct detection probe the other with an indirect detection
probe. (Obviously I expected many of these variables not to matter but
I thought I would try them.) I never get a T2 that is close enough to
the T1.
One thing I can think of that I haven't tried is a sample where the
volume is confined within the coil. Does anyone else have ideas?
Thanks in advance.
Fred Morin, PhD
NMR Facilities Manager
Department of Chemistry
McGill University
801 Sherbrooke St. West
Montreal, QC
H3A 2K6
Phone: 514-398-6219
Fax: 514-398-3797
frederick.morin_at_mcgill.ca
Received on Wed Feb 25 2009 - 05:41:16 MST