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THE STERIC THRESHOLD AND CHEMICAL SHIFT

What causes chemical shift?

The fundamental

causes are local magnetic fields and electron screening of magnetic fields. That is to say if

two given protons are in exactly the same magnetic field as felt at the nuclei, the two protons will have exactly the same chemical shift. The magnetic

fields found at a given carbon atom and at a proton attached to this carbon atom will usually be very similar due to the very short bond between the carbon atom and the hydrogen atom. Thus a given

carbon atom and an attached hydrogen atom will usually be shifted in the same direction by nearby magnetic fields. Shifting due to electron First electron

screening occurs in two ways.

density may change due to either induction or due to resonance and cause direct changes in chemical shift. Therefore increasing electron density at a

given carbon atom will cause a chemical shift to higher field. A hydrogen atom attached to this

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carbon atom will also receive increased electron density due to induction from the carbon atom and also be shifted to higher field. The second way

that screening may affect chemical shift is by the distortion of the electron cloud. A spherical

electron cloud as found in an S orbital should give the maximum screening and therefore maximum chemical shift to higher field. If however the

electron cloud is distorted away from spherical symmetry the effective electron screening will be diminished as will the chemical shift. Now imagine

that two different methyl groups are sterically jammed together. The hydrogen atoms on the outside

of the methyl groups will be the first to interact. There will be a nonbonded repulsion of the electron clouds on the hydrogen atoms. effects. There will be two

First, the electron cloud on the hydrogen Second, electron density

atoms will be distorted.

will be repelled from each of the hydrogen atoms down the hydrogen-carbon bond and thus to the carbon atom. This has the effect of shifting the

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proton resonance to lower field and shifting the carbon resonance to higher field. All other

mechanisms shift proton resonance and carbon resonance in the same direction. Thus in the

absence of steric compression (or steric crowding) The chemical shift of a given proton should be proportional to the chemical shift of the carbon atom to which it is attached. In fact for many Here

compounds this is not a bad generalization. we are concerned with resonance's that do not follow this generalization. That is, we are

concerned with steric compression.

We are thus

most concerned with above average and below average steric compression. We will find that often two

isomers may be distinguished by the fact that one of then is more stericly compressed than the other in a predictable way. Since the normal chemical

shift range for proton resonance is 10 PPM and the normal chemical shift range for carbon resonance is 200 PPM, we chose the proportionality constant between proton resonance and carbon resonance to be

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20.

Since TMS is the zero of reference for both

protons and carbon and TMS apparently is less crowded that the average molecule, we have a small offset. These considerations lead to the following

relationship for the averaged sterically compressed carbon-hydrogen bonded pair.

C = (H-0.5)*20 or H = C/20 +0.5

Some pairs will be more sterically compressed than the average and others will be less than average. This deviation from the average is reproducible and easily correlated with structural features. A few

examples of molecules that have only average steric compression and thus have C/H HETCOR resonances close to the steric threshold line are methyl ethyl ketone (org1c), 1-propanol (org16c), 3-pentanone (org2c), cyclohexanone (org3c), and tetrabutyl ammonium bromide (u11c).

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O CH3 H3C H3C OH H3C

O CH3

(org1c)

O

(org16c)

(org2c)

(+)

N

Br

(-)

(org3c)

(u11c)

A few examples of molecules that have predominately less steric compression than average are 3,5-dimethyl-1-adamantanol (u25c), -pinene

OH H3C CH2 CH3 H3C

CH3 H3C

CH3 H3C H 3C CH 3 O C C F2 H O CH 3

(u25c)

Br

(u55c)

(s97c)

F2 C CF 3

(x53c)

(s59cc)

(s97c), camphene (u55c), 5-bromonorbornane (x53c), and HFC (s59cc).

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Notice that the last three examples are derivatives of norbornane and that all of these examples are of multi-ring compounds.

A few examples of compounds that have predominately more steric compression than average are glucose pentaacetate (s19d), phenanthrene (u47c), -butyrolactone (u37c), and hexamethylbenzene (u17c). This above average

steric compression is due to a variety of causes, which we should discuss individually.

CH2Ac H Ac Ac H Ac O H Ac H3C H O O H3C

CH3 CH3

CH3 CH3

(s19d)

(u47c)

(u37c)

(u17c)

In general steric repulsion is the interaction of the outer electrons of the atoms involved. There is however a difference in the interaction of

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two hydrogen atoms and that of a hydrogen atom with a heavy atom. I think of hydrogen atoms as small,

compact, and hard (i.e. non-polarizable), while larger atoms have more electrons distributed over a larger volume and are thus are softer (i.e. more polarizable).

A few generalizations about the steric threshold follow. In general olefins are less

sterically compressed than average while aromatic compounds are more compressed than average. olefins are generally found below the steric threshold and aromatics resonances are found above the steric threshold. Conjugated olefins are Thus

generally more compressed than nonconjugated olefins. Cis nonconjugated olefins are generally

less compressed than the corresponding trans olefins. olefins. Alkynes are even less compressed than However, this is observable only in the In cyclohexanes and

case of terminal alkynes.

norbornanes the equatorial protons are more

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compressed than their corresponding gyminal axial protons. In naphthylenes and related condensed

aromatics the positions that are peri are more compressed than the positions that are not peri. The position on the sidechain attached to an aromatic ring is generally more compressed than average while the substitution of a proton on an aromatic ring by carbon generally lessens the steric compression of the aromatic ring. In

general when we have an effect in the BS Method we will see increased steric compression and where we have a third methylene effect in the BS Method we see decreaded steric compression. Now let us look at a few specific examples. related group of compounds that is informative is composed of cymene (n79c), terpinene (n89c), and terpinene (n75c). A

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CH3

CH3

CH3

C H3C H CH3 H3C

C H CH3 H3C

C H CH3

(n79c)

(n75c)

(n89c)

In all three of these cases the positions that are to the isopropyl methyl groups are sterically crowded. These positions are circled in the Also notice that all of the

structural drawings.

groups that are to SP2 carbons are sterically crowded. Another instructive set of molecules includes naphthylene (n87c), 1,2-dihydronaphthylene (n95c), rubrene (n111c), and anthracene (n61c).

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(n87c)

(n95c)

(n61c)

(n111c)

In naphthylene the inner positions are sterically crowded due to a peri interaction while the outer positions are only slightly more crowded than benzene. This spectrum was taken in C6D6 so that In rubrene

you may use the solvent for reference.

(n111c) we find the internal position that corresponds to the position in naphthylene is more crowded than the position of naphthylene.

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The aromatic ring of 1,2-dihydronaphthylene (n95c) is less crowded than naphthylene and the two methylene positions are both more crowded than average, however, the methylene next to the aromatic ring is the most crowded. Similarly the

olefinic position in 1,2-dihydronaphthylene next to the aromatic ring is the more crowded of the two olefinic positions. The central position of

anthracene (n61c) is more crowded than the internal position of naphthylene due to two peri interactions.

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