Department of Chemistry, University of Alberta         February 2000
NMR News 2000-01
News and tips for users of the Varian NMR systems in the Department

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. 



new macros: absf and ssf


GCOSY: now with transmitter presaturation


New macros: absf and ssf
Sometimes it is important to know the absolute frequency at a certain point in a 1D or 2D spectrum. Changing the scale of the spectrum from ppm to Hz does not provide the answer as the resulting frequencies are relative to the reference of the spectrum. The macro absf will provide absolute frequency information without making any changes to the scaling nor referencing. It works in all 1D and 2D experiments and solvents. The frequency information will be provided as a text line such as, for example: 'the absolute frequency at the cursor position is 220.8 Hz'.
No parameters will be altered
by absf it only provides the information.

The ssf macro (set satfrq), on the other hand, determines the absolute frequency at the cursor position and then actually changes the parameter satfrq to this value. This is analogous to Varian's sd command (set decoupler) except that ssf works on the transmitter and in both homo and heteronuclear cases (sd in homonuclear cases only!). Solvent presaturation by the transmitter can be used in experiments such as presat, GCOSY (only in certain solvents; see article below), GTOCSY, GHMQC and GHMBC. Associated parameters are shown below:


transmitter presaturation
(typical values)

decoupler presaturation
(typical values)


satfrq   saturation frequency

solvent & temperature dependent
use ssf to set to value at cursor position

dof   decoupler offset

solvent & temperature dependent
use sd to set to value at cursor position

power [dB]

satpwr (-16 to 0) (a)

dpwr (-10 to +10) (a)

length [sec]

satdly (1D: 3.0, 2D: 1.5) d1 = 0.01 (b)

d1 (1D: 3.0, 2D: 1.5)


satmode = 'ynn'  (c)

dm = 'ynn' (c)

(a) higher values are needed in protonated solvents such as H2O and CD3OH; (b) values smaller than 10 msec for d1 are rejected by the software; (c) the number of letters varies, however the first one is always y if presaturation is desired

Note that this terminology is used in most experiments. Other techniques may use a slightly different nomenclature. Also important is to realize that higher values of satpwr (or dpwr) reduce the solvent signal more and more but at the expense of peaks nearby that get attenuated/eliminated as well.

As a general rule, there is no reason to reduce a solvent peak more in intensity than the most intense peak in the spectrum created by the molecule of interest.

How to use ssf: acquire the spectrum first with the default values, Fourier transform it and use the cursor and ssf to reposition the presaturation frequency then type go. In a 2D case, like the GCOSY, it is not necessary to acquire the entire experiment. All that is needed is the first increment which can be processed by wft(1) and ssf applied to it. It is designed to work on a 1D spectrum that is part of a 2D.

GCOSY: now with transmitter presaturation
The GCOSY pulse sequence does not use any solvent presaturation. It is possible to make it do decoupler-based presaturation, however, this creates some artefacts. In many cases there is no need for solvent presaturation (solvents such as CDCl3, CD2Cl2, C6D6 definitely don't need it) but low concentration samples in D2O, CD3OD and to some extent DMSO may have problems. Therefore, a modified GCOSY sequence has been designed to address this problem (it appears as seqfil='aogcosy' in parameter printouts). Effective immediately, when you start a GCOSY in any of these three solvents, you will automatically get the new version with solvent presaturation set to the frequency appropriate for the spectrometer and the given solvent. This applies to all spectrometers. The temperature should be left as implemented otherwise the actual position of the solvent peak and the presaturation frequency do not match.
For temperature issues see NMR News 98-07. A special note for DMSO samples: the presaturation is set to the always present intense water impurity at ca. 3.29 ppm not the DMSO solvent peak at 2.49 ppm which is typically of lower intensity. If different presaturation point is desired use the ssf macro (see preceding article).


top of page    NMR News