Tom, I have read your post and think I know the solution. The answer is
probably one of chemical exchange of a small solvent molecule (ligand) with
the organometallic compound. The chemical exchange process is occurring at
a slow rate so that it does not interfere with the timing (cycling) of the
lock signal detection. However, you are probably stimulating the exchange
process using the H1-decoupler, increasing sample heating and ligand
collision rates enough to loose the lock signal. Try a pulse train like
1-3-3-1 or cool you sample down 10 degrees to try and avoid this effect.
Alan Kook
alan_kook@dell.com
----- Original Message -----
From: vnmr1 <vnmr1@ultranmr.chem.ualberta.ca>
To: <ammrl@wwitch.unl.edu>
Sent: Friday, April 30, 1999 1:58 PM
> Hi group;
>
> I have a question for the group-maybe someone else has seen this problem
and
> knows how to cure it. We have a Unity 500 with a 5mm bbprobe. Sometimes
> we have trouble with the deuterium lock signal only when the H1 decoupler
is
> turned on and only on samples containing organo-metallic compounds with a
> positive charge and an anion-say BF4-. This problem occurs in solvents
with
> relatively low boiling points, eg, acetone or methylene chloride. If the
> decoupler is turned on then the lock signal decreases by about 60% and
returns
> to normal if its turned off. I assume some type of local heating occurs
> causing eddy currents in the sample causing the lock to drop. Standard
> organic compounds don't have this problem. Has anyone else seen this
> phenomenon and cured the problem?
>
> Thanks in advance.
>
> Tom
>
> Tom Nakashima
> Chemistry Department
> University of Alberta
> Edmonton Alberta
> Canada T6G 2G2
>
> e-mail address:
>
> vnmr1@ultranmr.chem.ualberta.ca
>
>