Hi all,
Today a user asked me about the value that we use for automatically
referencing 13C spectra in CDCl3 when TMS is not used.
I have always used 77.23ppm, but Google searches reveal values used in
the range 77.27-77.0ppm. 77.23 is the value listed in the Cambridge
Isotopes solvent table, and the 2009 Bruker Almanac.
Looking at the default 2Hlock file supplied with TopSpin3.0, I see the
shift listed as 77.0ppm, but this is intended as a basis for finding the
TMS peak at 0. I have changed my systems to look for the residual
solvent signal, unless the solvent name has a 'T' appended and then it
will reference TMS.
I know that the official IUPAC method is to use the Chi scale and
calculate from the proton TMS frequency, but for everyday use the
solvent signal is easier.
My view is that the reference should always be quoted when publishing
peak positions, and as they move with temperature, concentration and pH,
quoting to one decimal place is all that would be justified.
Does anyone have a strong justification for the value they use, or is it
mainly habit and consistency?
Thanks,
Phil.
--
Dr Phil Dennison
NMR Facility Director (949)824-6010 (office)
Department of Chemistry (949)824-5649 (lab)
University of California (949)824-8571 (fax)
Irvine, CA 92697-2025 dennison_at_uci.edu
USA
Received on Wed Feb 16 2011 - 09:52:27 MST