1 st -order spin-spin coupling. 2 nd -order spin-spin coupling. ν/j < 10

Transcription

1 1 st -order spin-spin coupling We observe 1 st -order NMR spectra when the frequency difference between the chemical shifts of any given pair of nuclei is much larger than the value of the coupling constant between them ν/j > 10 and any set of chemically equivalent nuclei is also magnetically equivalent. 1 st -order NMR spectra exhibit a number of simple characteristics: Multiplicities that result from coupling reflect the 2nI + 1 rule (I = nuclear spin quantum number e.g. I = ½); The intensities of spin-spin multiplets correspond to Pascal s triangle for I = ½; Nuclei with the same chemical shift do not split each other, even when the coupling constant between them is not zero; Spacings between adjacent components of a spin-spin multiplet are equal to J; Spin-spin multiplets are centred on the resonance frequency; 2 nd -order spin-spin coupling We observe 2 nd -order NMR spectra when the frequency difference between the chemical shifts of any given pair of nuclei is small compared to the value of the coupling constant between them ν/j < 10 and/or any set of chemically equivalent nuclei is not magnetically equivalent. Nuclei are chemically equivalent if they can be interchanged by a symmetry operation of the molecule. Nuclei that are interchangeable by a rotation (C n ) are said to be homotopic. Nuclei related only by a mirror plane are termed enantiotopic. Chemically equivalent nuclei are isochronous (same chemical shift) but the converse is not necessarily true. Nuclei are magnetically equivalent if they are isochronous and if all the coupling constants for couplings to any other nucleus are equal for each nucleus (isogamous coupling). The example spin systems consist of two different magnetically active nuclei and F; A = and X = F

2 Enantiotopic and Diastereotopic Protons 3 C C 3 Enantiotopic protons: no rotational symmetry but superimposed by inversion (i) Diastereotopic protons of methylene groups 3 C CO 2 O chiral molecule O 2 C CO 2 O achiral molecule plane makes the two 1 ( 2 ) protons chemically equivalent; no plane through the two C 2 groups, thus the protons of each C 2 group are diastereotopic; Diastereotopic protons can not be placed into the same chemical environment Staggered Rotamers non-chiral anti anti rotamer: 1 and 2 as well as 3 and 4 are enantiotopic interchanged through a plane of symmetry; all other rotamers incl. gauche: no symmetry, 1 and 2 as well as 3 and 4 are diastereotopic; the chemical shifts of 1 and 2 ( 3 and 4 ) for all rotamers other than anti are not equivalent; butrapid rotation gives one chemical shift for 1 and 2 and another for 3 and 4 ; 4 3 gauche 1 4 Staggered Rotamers chiral centre next to methylene group and 2 are not chemically equivalent as they cannot be interchanged by a symmetry operation; no plane, axis or inversion center not interchanged by rapid rotation (chemical environment is always different) averaged chemical shift is not identical

3 Spectrum of 1-Chloro-4-nitrobenzene AA XX spectrum Magnetic Equivalence If chemical shift equivalent nuclei couple equally to other nuclei then they are magnetically equivalent! magnetic equivalent if symmetrically disposed with respect to each nuclei in the spin system. 1 NO 2 1 ' 1 and 1 are chemically equivalent and, thus, have the same chemical shift but 1 and 1 couple differently to 2 and 2 ; 2 2 ' J 12 J 1 2 = 7-10 z, J 1 2 J 12 = 1 z they are magnetically not equivalent and the AA XX spectrum is complex Aliphatic AA BB Spectrum

4 Aliphatic AMX Spectrum of Styrene Aromatic AA BB Spectrum of 1,2-dichlorobenzene Aliphatic A 2 B 2 Spectrum of 2-Chloroethanol

5 AB spin systems Analysis of AB spin systems = (4C 2 J 2 ) The ratio of intensities between larger inner and smaller outer peaks is (1+J/2C)/(1-J/2C) Geminal couplings Coupling constants can have positive or negative values and their determination might aid the analysis of complex spectra. Lone pairs of electrons, for example, can donate electron density and make 2 J more positive as shown below. Coupling of magnetically equivalent protons des not appear in the NMR spectrum but the coupling constants can be determined by deuteriation or from 13 C satellite signals. Geminal ( 2 J) couplings are usually negative and reach values of up to 30 z. Geminal protons attached to double and triple bonds can have positive coupling constants. F O O -3.2 z -1.3 z O

6 Vicinal Couplings Vicinal ( 3 J) couplings are often positive and usually reach values of up to 20 z. Vicinal couplings ( 3 J) depend on the dihedral angle 1-6 z 8-13 z 0-5 z Vicinal couplings ( 3 J) depend on the dihedral angle

7 Long-Range Couplings All couplings between protons that are more than 3 bonds apart are called long-range couplings ( 4 J, 5 J, etc.). Their coupling constant can reach values between z if both sets of protons are connected to the same π-electron system. 5 4 J J O 1.7 z t.-bu 1.45 z N 0.9 z 4 J C 3 CO