Circulation of tt electrons

Ring current (Section 15.8) The circulation of tt electrons induced in aromatic rings by an external magnetic field. This effect accounts for the downfield shift of aromatic ring protons in the lH NMR spectrum. 

The circulation of tt electrons in benzene creates an induced magnetic field that, at the position of the protons, reinforces the applied magnetic field. This reinforcement causes the protons to be strongly deshielded and to have a relatively high frequency (S 7) absorption. 

Effect on chemical shifts of induced field from circulation of TT-electrons. (a) Deshielding of vinylic protons, (b) Shielding of acetylenic protons, (c) Deshielding of aromatic protons. 

The effect of the induced circulation of tt electrons on a vinylic hydrogen (Figure 13.10) is opposite that on an acetylenic hydrogen. The direction of the induced magnetic field in the tt bond of a carbon-carbon double bond is parallel to the applied field in the region of the vinylic hydrogens. The induced magnetic field... 

The circulation of tt electrons in benzene creates an induced magnetic field that, at... 

Ring current Electric field associated with circulating system of tt electrons. 

We learned in Section 9.6 that circulations of a electrons of C — H bonds cause the protons of alkanes to be shielded from the applied magnetic field of an NMR spectrometer and, consequently, these protons absorb at lower frequency. We shall now explain the high-frequency absorption of benzene protons on the basis of deshielding caused by circulation of the TT electrons of benzene, and this explanation, as you will see, requires that the tt electrons be delocalized. 

FIGURE 13 9 (a) The TT elec trons of acetylene circulate in a region surrounding the long axis of the molecule (b) The induced magnetic field associated with the tt electrons opposes the applied field and shields the protons... 

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to 5/)-hybridized carbon. This is because the tt electrons circulate around the triple bond, not along it (Figure 13.9a). Therefore, the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 3.9b). Acetylenic protons typically have chemical shifts near- 8 2.5. 

FIGURE 13.9 (a) The TT electrons of acetylene circulate in a region surrounding the long axis of the molecule. 

Figure 15.14 The origin of aromatic ring-current. Aromatic protons are deshielded by the induced magnetic field caused by delocalized tt electrons circulating in the molecular orbitals of the aromatic ring.

These different environments explain why the 11 NMR spectrum contains two peaks in a 2 1 ratio. The less intense signal, that for the interior protons, is more shielded than the signal for the outside protons. This results from the magnetic field induced by the circulating tt electrons of this aromatic ring, which reinforces the applied field in the region of the outside protons but opposes it in the interior of the ring. 

Long-range deshlelding can occur in aromatic and other molecules with delocalized TT electrons. For example, when the plane of the benzene molecule is oriented perpendicular to B, circulation of the tt electrons produces a ring current (see Fig. lb). This ring current induces a secondary field at the protons that is aligned parallel to B and thus increases... 

Circulation of electrons—specifically, tt electrons—about nearby nuclei generates a field that can either oppose or reinforce the applied field at the proton, depending on the proton s location (Fig. 13.4). If the induced field opposes the... 

Let us begin with acetylene. The molecule is linear, and the triple bond is symmetrical about the axis. If this axis is aligned with the applied magnetic field, the tt electrons of the bond can circulate at right angles to the... 

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