Department of Chemistry, University
of Alberta
June 1999
NMR News 99-04
News and tips for users of the Varian NMR systems in the Department
Editor: Albin.Otter@ualberta.ca
http://nmr.chem.ualberta.ca
There are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.
Other content of this NMR News is no longer meaningful and has been removed April 2010.
Contents
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FAQ 99-04.1 why can we not record APT, DEPT and other direct 13C experiments on i300? |
FAQ 99-04.1 why can we not record APT, DEPT and other direct 13C experiments on i300?
What experiments can be carried out on a spectrometer depends whether certain hardware is present or not, e.g. gradients, wave form generators etc. Equally important is the type of probes that are available for a given system. The subject of probe design is quite complicated but with some simplification at least two types of probes should be distinguished:
a) indirect
detection probes (also called inverse probes)
These probes are built in such a way that the inner
coil, i.e. the one closest to the sample, is the proton coil. Consequently, they
provide high sensitivity and also otherwise high quality proton data. In order to
apply pulses to heteronuclei such as 13C, a second coil is installed but further away from
the sample tube. This results in a longer pulse length for 90 degree carbon pulses
compared to proton pulses and, much more importantly, reduced sensitivity for direct
measurements of heteronuclei. The probe installed on the i300 is of this type, providing
excellent proton data and, through indirect detection, also 13C information. An
example for indirect detection is GHMQC
and/or GHSQC where in essence the 13C satellites (1%) are
recorded while 99% of the signals (protons attached to 12C) are filtered out. The added bonus of
GHMQC
over 1D 13C is that correlations are obtained and not only
isolated 13C signals with no relationship to the proton spectrum.
The disadvantage is that the 13C dimension can only be obtained in a
two-dimensional experiment (the F1 dimension in GHMQC). The sensitivity gain in doing
indirect vs. direct carbon detection overcompensates in general the extra time needed due
to the 2D nature of the GHMQC experiment.
Pictures of the inside of the i300 probe are available when taking an i300 lab tour on the web: simply click on the
probe sticking out of the magnet in the lab photo.
b) heteronuclear probes, commonly called broadband probes
In this case the inner coil is optimized to record
heteronuclei. 'Broadband' indicates that a wide range of nuclei with
very different frequencies is covered. Examples on a 7 Tesla magnet such as
i300: 13C is at 75 MHz, 15N at 30 MHz and 31P at 121 MHz. These frequencies are
also known as Larmor frequencies and are an intrinsic property of the nuclei for
a given magnetic field. As i300 has no such probe, it is not possible to record
heteronuclei directly in an efficient way and hence no APT or other direct 13C
spectrum can be recorded.