I would appreciate hearing from anyone using the Evans method (using a
coaxial insert in a 5 mm tube) on a superconducting instrument if they are
aware of what may be different from the way the experiment worked on low
field instruments, back in the Pleistocene. We have been doing this as an
undergraduate laboratory experiment and it is the last experiment we are
migrating away from EM-390 instruments, so we can retire them. However
the experiment consistently gives an answer that is too high on a 300 MHz
instrument--for a nickel complex that has 2 unpaired electrons, in DMSO,
the answer is coming out closer to 3 unpaired electrons.
I ran the same sample on the 90, the 300, and a 600. The splitting on the
90 was about 0.17 ppm and it was 0.27 ppm on both the 300 and 600. So it
is not the case that the splitting continues to increase as the field goes
up. The temperature of the EM-390 probe is slightly above ambient, but on
the Kelvin scale, it is not a big difference, and it should actually push
the splitting down in the cooler high field probes.
David VanderVelde
Manager, High Resolution NMR Facility, Caltech
davidv_at_caltech.edu
Received on Mon Oct 19 2009 - 10:40:07 MST