CH 9-4: NMR-I Chemical Equivalence and
Chemical Shift
(1) Chemical Equivalence of Hydrogen atoms.
(a)
(b)
(b)
(a)
The number of equivalent H atoms, or groups of H atoms
corresponds to the number of “signals” in the NMR
spectrum.
Chemical Equivalence = Number of Signals
3 signals
3 signals
2 signals
2 signals
1 signal
2 signals
(2) The NMR Spectrum: Chemical Shift Scale
(a)
(b)
(b)
(a)
(a)
(b)
• The “Chemical Shift” scale is defined as ppm, and ranges
from 0 - 13 ppm.
• The “Chemical Shift” is a measure of how far a proton
signal is relative to “zero” as defined by the TMS reference.
CH3
H3 C
Si
CH3
CH3
Tetramethyl silane
"TMS"
“downfield”
“upfield”
H-1 NMR Chemical Shift Data
Chemical Shift Data: 0 – 4.5 ppm
Many protons give signals in the
region from 0 – 4.5 ppm; not
diagnostic for protons on specific
functional groups.
Chemical Shift Data: 4.5 – 13 ppm
• The region from 4.5 – 13 ppm is very diagnostic for
protons on specific functional groups.