NUCLEAR MAGNETIC RESONANCE 105
- The valence s electron density at the nucleus. This is affected by the s
character of the bonding orbitals between the interacting nuclei.
Nuclei coupling to each other through spin – spin interactions may have very
similar or very different chemical shifts. The difference or similarity will affect
the appearance of the resonances associated with the coupled nuclei. Nuclei
separated by small chemical shifts are denoted with the letters A, B, and C while
sets of nuclei separated by large chemical shifts are designated A, M, and X.
Figure 3.12 Chemical shift scales and chemical shifts of some compounds. (Adapted
with permission of Nelson Thornes Ltd. from Figure 2.12 of reference 21 .)
δ scale, ppm+10 0 -50less screened than standard
low field
high frequencymore screened than standard
high field
low frequency+400^0
metal
carbenesmetal hydrides, or
protons in molecules with
shift reagents added+230 0 -460+15,100 0 -4200δ scale, ppmδ scale, ppmδ scale, ppmCS 2 CO C 6 H 6 CHCl 3 (CH 3 ) 2 COC 6 H 6 (CH 3 ) 2 COCHCl 3
CH 4(^1) H
(^13) C
(^31) P
PBr 3 Me 3 PO
H 3 PO 4
PMe 3 PBr 5 P 4
(^59) Co
Co(CN) 6 3-
Co(H 2 O) 6 3+
Co(NH 3 ) 6 3+ Co(CO) 4 -
Co(PF 3 ) 3 -
TMS
TMS