Group frequencies methine

Protons are magnetically equivalent if they have the same chemical shift and are coupled equally to otiier equivalent nuclei in die molecule. This is similar to chemical equivalence but is a more rigorous definition of equivalence. For example, the methyl protons of isobutane are chemically and magnetically equivalent since they absorb at die same frequency and are all coupled equally to the methine proton (which should be split into a 10-line multiplet ). Likewise die two mediyl groups of /i-xylene are chemically and magnetically equivalent because they are coupled equally (J = 0) to the aromatic protons both ortho and meta to diem. 

Carbon resonances arising from both nonprotonated and proto-nated aromatic carbons may appear at the same frequency under proton decoupling. Yet these two resonances could possess very different relaxation behavior and in a solid could evolve very differently due to local proton dipolar fields which attenuate with the carbon-proton distances as 1/rcH When the spin locking pulse for proton nuclei is turned off, carbons with directly bound protons such as methines and methylenes rapidly dephase in the local proton fields and their spectral response is rapidly diminished. The rapid internal motion of CH3 groups greatly decreases the effectiveness of methyl protons. Nonprotonated carbons are only dephased by remote and therefore... 

It is possible to observe NOEs for protons that are bonded to sp3, sp2 or sp carbons. The effect is best observed for methine (R3C-H) protons, though it can be demonstrated for methylene (R2CH2) protons also. It is less easy to observe for methyl (CH3) protons, because methyl protons contribute mutually to the dipole-dipole relaxation of each other. A methine proton, being bonded to three carbons that do not assist in dipole-dipole relaxation, shows an NOE more readily. Therefore, in the case of a closely separated proton, Ha, and a methyl group C(Hb)3 in a molecule, irradiation at the methyl proton frequency results in an enhancement of the Ha absorption, whereas irradiation of Ha will not usually result in a meaningful enhancement of the methyl group absorption. However, as indicated later in this section, it is possible to obtain small NOEs for certain methyl groups. 

The structure of arenaine (84) was revealed from the interpretation of its n.m.r. ( H and C) spectra. Examination of the C noise resonance decoupled and single frequency decoupled spectra showed the presence of three non-proto-nated carbons (carbonyl, guanidine, and a-amino-tetrahedral types), three methine (olefinic methine, aminomethine, and one other) carbons, three methylenes (one olefinic and two saturated ones which appear at highly shielded and deshielded positions, thus implying that they are part of a R3CCH2CH2 unit), and two methyl groups. This information alone allows formulation of the... 

The most direct evidence for back coordination in iron(II) methine complexes comes from the observation of IR C=N stretching frequencies in complexes of ligands containing aliphatic or alicyclic imine or cizine groupings (see Table 5). The substantial decrease of v (C=N) (ca. 100 cm i) on complexation with iron(II) can, with some confidence, be taken as a qualitative indication of back coordination. However, at present it is difficult to assess the extent of r-electron delocalization in the iron(II) diimine ring from the observed IR shifts. 

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