Anisotropic effects affect chemical sites
The delta of the proton of acetylene (CH ≡ CH) is measured to be 2.88, the delta of the proton of ethylene (H2C = CH2) is 5.84, and the delta of the proton of ethane (CH3CH3) is 0.96. The electronegativity of the carbon bond is sp > sp2 > sp3 ore. From the perspective of the induction effect, their delta values should be increased in the order of: delta ethane < delta ethylene < delta acetylene, but it is inconsistent with the above experimental facts. In addition, the resonance peaks of the aldehyde proton and the aromatic ring proton appear in a very low magnetic field, which cannot be explained by the induction effect.
The abnormal chemical shifts of alkyne protons, aromatic protons and aldehyde protons are caused by the anisotropic effect of these groups. The so-called anisotropic effect is the anisotropic small magnetic field generated by the electron cloud ring of the chemical bond, which can affect the chemical shift of the proton through space.
Under the influence of an external magnetic field, the electron circulation of chemical bonds in some molecules is anisotropic (asymmetric), and the resulting induced magnetic field against the external magnetic field is also anisotropic. This anisotropic effect can cause the external magnetic field in the adjacent proton region to increase or decrease. Enhancing the anisotropic effect of the external magnetic field will cause the proton to produce a resonant peak in the lower magnetic field (paramagnetic de-shielding effect). Reducing the anisotropic effect of the external magnetic field will cause the proton to produce a resonant peak in the higher magnetic field (diamagnetic shielding effect). Molecules containing π bonds (such as aromatic compounds, olefins, and compounds containing carbonyl groups), the anisotropic effect is an important factor affecting the chemical shift.
(1) Anisotropic effect of alkyne bond
When the intramolecular alkyne bond is parallel to the external magnetic field (when the alkyne bond is perpendicular to the external magnetic field, the electron circulation of the π bond is greatly restricted), the electron circulation of the π bond is in an axisymmetric orbit, and the induced magnetic field generated by the electron ring generates an diamagnetic magnetic field near the bond axis. Although the induction effect of the sp-carbon bond reduces the electron cloud density near the alkyne proton, the alkyne proton is still subject to a high shielding effect due to the anisotropy effect. Therefore, the delta value of the alkyne proton is small, δ 2~ 3. However, the electron circulation of the alkyne bond π. The electron circulation generates a paramagnetic magnetic field around the bond axis, so the protons in these regions will
(2) The anisotropic effect of double bonds
Double bonds also produce anisotropic effects on their neighboring protons. For example, the signal of aldehyde protons appears in a very low magnetic field (δ 9~ 10), which is caused by the anisotropic effect of carbonyl groups. In the external magnetic field, when the plane formed by carbonyl groups is perpendicular to the external magnetic field (when the plane formed by carbonyl groups is parallel to the external magnetic field, the circulation of π-bond electrons is limited), and the circulation of π-bond electrons generates a paramagnetic field near the aldehyde protons, and the aldehyde protons are de-shielded, so the signal appears in the low magnetic field region.
Protons in the plane below the double bond are affected by the diamagnetic shielding effect. Therefore, the chemical shift of the proton signal in these regions will be reduced.
Like the aldehyde protons mentioned above, alkene protons are also affected by paramagnetic fields and resonate at lower magnetic fields.
(3) Anisotropic effect of benzene ring
In the external magnetic field, the π bond electron circulation of the benzene ring is parallel to the benzene ring, and the magnetic field caused by the electron circulation is perpendicular to the benzene ring. The outer peripheral region of the benzene ring is a paramagnetic shielding region, and the upper or lower regions of the benzene ring are diamagnetic shielding regions.
Aromatic ring π electrons are delocalized electrons, and the anisotropic effect produced by the electron circulation at the aromatic ring mouth is stronger than that of the ethylene double bond and the conjugated double bond. Therefore, the delta value of the proton in the unshielded region of the aromatic ring (benzene ring δ = 7.26) is greater than the delta value of the ethylene proton.
(4) Anisotropic effect of single bond
The valence electrons (a-electrons) of the C-C single bond can also produce anisotropic effects, but they are much weaker than those produced by π electron circulation.
The bond axis of the C~ C single bond is the axis of the unshielding cone. Therefore, when the hydrogen on the carbon is replaced by alkyl groups one by one, the remaining hydrogen is subject to stronger and stronger unshielding effect, and the resonance signal shifts to a low field.
Overall, anisotropic effects exist in various types of compounds.
