CHEMICAL SHIFT REFERENCING USING OFFSET VALUES

PROTON AXIS
 
 

Definitions:




* 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.
 
 

Standard Values:




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

 
 

Referencing:




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).
 


NITORGEN AXIS

Definitions:




* 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.
 
 

Standard values:




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

Freq15N(0) = 0.101329118 x Freq1H(0)

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

 
 

Referencing:




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


CARBON AXIS

Definitions:




* 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.
 
 

Standard values:




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

Freq13C(0) = 0.251449530 x Freq1H(0)

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

 
 

Referencing:




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



 
 


APPENDIX




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

tof(0):
 

dof(0),  dof2(0):
 
 
 
dof2(x)-dof2(0) = Freq13C(x)-Freq13C(0)     (A-2)
Freq1H(0) is the exact frequency value at which the transmitter irradiates the sample, when the transmitter is placed on the position corresponding to the reference peak of the DSS sample.
 

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. 


...created by Rajan Paranji
...updated on 4/29/2002