Occurrence Magnetic inequivalence

1 occurrence

1.1 paired h-c-c-h fragments

1.1.1 ortho-disubstituted benzenes
1.1.2 para-disubstituted benzenes
1.1.3 other aromatics
1.1.4 non-aromatic systems


1.2 h2c-ch2 fragments
1.3 other nuclei

occurrence

two (or more) chemically equivalent (symmetry-related) spins have same chemical shift, have different coupling relationship same coupling partner magnetically inequivalent coupling criterion. occurs in molecules bearing 2 (or more) chemically distinct groups of symmetry-related nuclei, 1 element of symmetry relating them. commonly, 2 chemically inequivalent pairs of hydrogen nuclei (protons) involved, although other magnetically active nuclei show phenomenon, , spin system labelled aa′bb′ system. additional coupling partners may present, 2 a/a′ , b/b′ signals (at different chemical shifts) said show magnetic inequivalence between symmetry-related , a′ (or b , b′) pairs @ same chemical shift. if chemical shift difference (νa−νb) large compared largest coupling constant, spin system may designated aa′xx′.

paired h-c-c-h fragments

magnetic inequivalence may occur 2 symmetry-related ha-c-c-hb fragments (where different subscripts indicate chemical inequivalence) may or may not contiguous. in order distinguish resulting coupling relationships, symmetry-related pair labelled ha′-c-c-hb′.

ortho-disubstituted benzenes

aromatic region (cdcl3, 300 mhz); starred peaks impurities

h-3 , h-6 in 1,2-homodisubstituted benzene related mirror plane of symmetry bisecting 1,2 , 4,5 c-c bonds. therefore chemically equivalent (and magnetically equivalent chemical shift criterion) but, because have different spatial , connectivity relations h-4 (with 3-bond vs. 4-bond couplings of different strengths), magnetically inequivalent coupling criterion. same true respect coupling relationships h-5. similarly, h-4 , h-5 chemically equivalent magnetically inequivalent owing different coupling relationships h-3 (or h-6).

a classic example showing highly complex splitting of 1,2-dichlorobenzene. 2 signals mirror-symmetrical. in 1,2-diaminobenzene (ortho-phenylenediamine), 2 signals have same chemical shift, resultant signals form complex multiplet.

para-disubstituted benzenes

h-2/6 signal close-up (cdcl3, 300 mhz); h-3/5 signal identical

h-2 , h-6 in 1,4-heterodisubstituted benzene related mirror plane of symmetry passing through c-1 , c-4. therefore chemically equivalent (and magnetically equivalent chemical shift criterion) but, because have different spatial , connectivity relations h-3 (with 3-bond vs. 5-bond coupling constants of different strengths), magnetically inequivalent coupling criterion. same true respect coupling relationships h-5. similarly, h-3 , h-5 chemically equivalent magnetically inequivalent owing different coupling relationships h-2 (or h-6).

an example provided 4-nitroaniline. although each signal retains gross doublet shape predicted first-order analysis, close-up view of each reveals additional peaks.

other aromatics

any 4-substituted pyridine, pyridine itself, 1-substituted pyrazinium ion, diazine, 1-substituted or unsubstituted pyrrole , related aromatic heterocyclics (phospholes, furan, thiophene, etc.) unsubstituted or 1-substituted cyclopentadienes , 1-substituted cyclopentadienides have same symmetry framework para-disubstituted or ortho-homodisubstituted benzenes, , present chemically equivalent magnetically inequivalent pairs of protons. in heterocycles , in five-membered rings in general, however, j values can smaller in benzenes , manifestation of magnetic inequivalence may subtle.

the rarer seven-membered , higher ring systems may show same symmetry property, can linked , fused aromatic ring systems such biphenyls, naphthalenes , isoindoles. similarly, 1-h benzimidazoles have appropriate symmetry if n-deprotonated or n-protonated, or result of rapid tautomerization of neutral form (for instance, in dmso-d6) signals resemble of 1,2-dichlorobenzene.

non-aromatic systems

the occurrence of symmetry-related pairs of ha-c-c-hb fragments not limited aromatic systems. instance, magnetic inequivalence found in 1,4-homodisubstituted butadienes. might expected in molecule such symmetrical 2,3,4,5-tetrasubstituted pyrrolidine, less rigid , less flat sp frameworks tend show weak long-range couplings (through 4 or more bonds) not manifest sign of magnetic inequivalence. reich gives several additional examples of magnetic inequivalence in non-aromatic h-c-c-h pairs.

h2c-ch2 fragments

ch2 region (cdcl3, 300 mhz); starred peaks impurities

magnetic inequivalence may occur h2c-ch2 fragments subdivided 2 groups of 2 in either geminal relationships via mirror plane along c-c bond, i.e. haha′c-chbhb′, or in vicinal relationships via mirror plane bisecting c-c bond, i.e. in hahbc-cha′hb′, or via rotational axis of symmetry (a c2-axis), i.e. hahbc-chb′ha′. coupling constants differ because of geometry (cis vs. trans) or connectivity (2-bond vs. 3-bond) , level of complexity depend on differences. conformational dynamics may reduce or obliterate difference between cis , trans couplings, if fast compared nmr timescale. there may additional couplings other nuclei.

the ethylene fragment in 2-substituted dioxolanes can show high level of complexity if substituent large. symmetrical norbornanes , rigid compounds (e.g. 7-oxabicyclo[2.2.1]heptane) show complex signals ethylene fragments, made more complicated additional splitting bridgehead protons. reich gives several additional examples of magnetic inequivalence in acyclic , cyclic systems containing h2c-ch2 fragments.

with other nuclei

any pair of symmetry-related x-c-c-y fragments (where x , y different magnetically active nuclei) xyc-cxy (cis or trans) , x2c-cy2 fragments may show magnetic inequivalence when heteronuclear coupling constants (jxy or jxy) non-negligible. in principle, magnetically active nuclei may disposed on non-carbon atoms.

a classic example h-nmr spectrum of 1,1-difluoroethylene. single h-nmr signal made complex jh-h , 2 different jh-f splittings. f-nmr spectrum identical. other 2 difluoroethylene isomers give complex spectra.