* Proton resonance at the extreme down-field position of DSS compound is used as the mark for zero position in the proton chemical shift scale.
 

* ppm1H(x) = {[tof(x)-tof(0)]/Freq1H(0)} x 1e6             ...(1)

tof(x) - transmitter offset frequency for a given resonance 'x'
 

tof(0) - transmitter offset frequency that corresponds to the 'zero' position of the chemical shift axis
 

Freq1H(0) - Absolute frequency in MHz, if the transmitter were to be positioned on the 'zero' position of the chemical shift axis.
 
 

At a given lock frequency setting, values of Freq1H(0) and tof(0) are absolute and remain constant. They change a little over time when the magnetic field strength changes.
 

At 25 deg. C, using a DSS sample, the following values were computed for the two spectrometers at CABM.
 

	INOVA 500	INOVA 600
Freq1H(0)	499.7042185 MHz	599.8365394 MHz
tof(0)	-2525.4 Hz	-3075.6 Hz

 
 

For any given data set, on the proton axis, if the transmitter offset, called tof(x) is known, the corresponding ppm value of that position can be simply computed using equation (1).
 

* ppm15N(x) = {[dof(x)-dof(0)]/Freq15N(0)} x 1e6            ...(2)
 

dof(x)- decoupler offset frequency in Hz for a given position 'x' on the 15N axis.
 

dof(0) - decoupler offset frequency in Hz that corresponds to the 'zero' position of the 15N chemical shift axis.
 

Freq15N(0) - Absolute frequency in MHz, if the decoupler were to be positioned on the 'zero' position of the chemical shift axis.
 
 

Freq15N(0) is related to Freq1H(0) by the following relation.
 
 

The standard values computed for the two CABM machines at 25 deg. C are the following.
 

	INOVA 500	INOVA 600
Freq15N(0)	50.63458772 MHz	60.78090748 MHz
dof(0)	-4966.08 Hz	-5413.02 Hz

 
 

For a given data set, with the value of dof(x) for the 15N axis use equation (2) to calculate the corresponding ppm value.
 

* ppm13C(x) = {[dof2(x)-dof2(0)]/Freq13C(0)} x 1e6         ...(3)
 

dof2(x)- Second decoupler offset frequency in Hz for a given position 'x' on the 13C chemical shift axis.
 

dof2(0) - Second decoupler offset frequency in Hz that corresponds to the 'zero' position of the 13C chemical shift axis.
 

Freq13C(0) - Absolute frequency in MHz, if the second decoupler were to be positioned on the 'zero' position of the 13C chemical shift axis.
 
 

Freq13C(0) is related to Freq1H(0) by the following relation.
 
 

The standard values computed for the two CABM machines at 25 deg. C are the following.
 
 

	INOVA 500	INOVA 600
Freq13C(0)	125.6503909 MHz	150.8286159 MHz
dof2(0)	-12249.6 Hz	-13882.8 Hz

 
 

For a given data set, with the value of dof2(x) for the 13C axis use equation (3) to calculate the corresponding ppm value.
 
 
 
 
 

The values of tof(0), dof(0) and dof2(0) are estimated in the following manner:
 

tof(0):
 

• A DSS sample is inserted and a 1D 1H spectrum is recorded.
• The cursor is moved on to the resonance that is on the extreme right of the spectrum (extreme downfield position). 'nl' command in vnmr is used to accurately place the cursor on this resonance peak.
• A 'movetof' command is entered.
• The 'tof' value is queried. This value is the tof(0) value in Hz.

 

• A three channel experiment is setup. Here Channel 1 corresponds to 1H; Channel 2 corresponds to 15N; Channel 3 corresponds to 13C. The transmitter offset is set to tof(0) that we obtained above. The offset values dof(x) and dof2(x) can be any arbitrary values corresponding to some resonance position 'x' on their respective chemical shift axes. These values are noted down.

• After the above procedure a 'spcfrq' command is entered.

 
• 'spcfrq' command lists a set of frequencies in MHz, which we call here for convenience as, Freq1H(0), Freq15N(x), Freq13C(x).

 
• These frequencies and offsets obey the following equations.

 

 
 
 
 
 

dof(x)-dof(0) = Freq15N(x)-Freq15N(0)         (A-1)

 
dof2(x)-dof2(0) = Freq13C(x)-Freq13C(0)     (A-2)

• One uses equations (A-1) and (A-2), to compute the dof(0) and dof2(0) values. These are offset values that appear under 'standard values' in the main document.

 

In equations A-1 and A-2, definitions of Freq15N(0) and Freq13C(0) are the same as we have seen before. These values are given under the 'standard values' shown before.
 

Freq15N(x) is the exact frequency value at which the first deocupler irradiates the sample, when the decoupler is placed on a position corresponding to some arbitrary resonance 'x' on the Nitrogen axis. 'dof(x)' is the associated offset value.
 

Freq13C(x) is the exact frequency value at which the second deocupler irradiates the sample, when the decoupler is placed on a position corresponding to some arbitrary resonance 'x' on the Carbon axis. 'dof2(x)' is the associated offset value.