CIS TRANS ISOMERS AND NMR

 

Cis and trans coupling appear differently on ¹H NMR spectrum

Here are a couple of terms to know:

• Vicinal - Coupling between hydrogens on adjacent carbons. 
• Geminal - Coupling between nonequivalent hydrogens on the same carbon atom. 

Coupling Constants Around a Double Bond

Type of coupling	Name	Range (Hz)	Typical (Hz)
	Vicinal, cis	6-14	10
	Vicinal, trans	11-18	16
	Geminal	0-3	2
	None	4-10	6
	Allylic	0.5-3.0	2

When alkynel hydrogen atoms are not symmetrically substituted on a double bonded carbon, the hydrogens of a cis and trans isomer will yield a different shift on the NMR spectrum. Because the coupling constant is smaller in a cis isomer than in a trans isomer, the NMR spectrums of the two isomers are different conveying the hydrogens in a cis isomer to be slightly more upfield to-- the right of the spectrum-- and trans hydrogens to be more downfield to the left.                                                  

Sometimes coupling will lead to very complicated patterns as a result of the J values that vary widely due to the relationship between the hydrogens involved. When this occurs, information can still be derived to determine the structure of a molecule by looking at the number of signals, the chemical shift of each one, integration, and splitting patterns similarly to identifying alkane NMR. 

Alkenyl carbons are deshielded in ¹³C NMR

For background information on ¹³C NMR, please refer to ¹³C Nuclear Magnetic Resonance from the previous chapter. Compared to alkane carbons with one bond, alkene carbons show a relatively low field shift on the ¹³C NMR spectrum and absorb about 100 ppm lower field. Also, in broad-band decoupled ¹³C NMR, sp² carbons absorb as sharp single lines so with these two methods, it is easy to determine the presence of a double bond in ¹³C NMR spectrum.

Here are the common ¹³C Chemical Shift Ranges:

Note that the carbon-carbon double bonds are found in the range between 100-170 ppm. Carbon atoms on alkenes that are attatched to another carbon group are found more downfield than carbon alkenes attatched to hydrogens.

Let's try a ¹H NMR practice problem with C₄H₇Cl:

 Remember from previous sections that to solve an NMR spectrum with double bonds, we must know the Degrees of Unsaturation. From this, we get degrees of unsaturation= (9-7)/2=1 so there is one pi bond or ring in our molecule. Next we must look at the integration of the NMR spectrums.

• ppm= 1.8 with 3H reveals a CH₃attached to an unsaturated functional group
• ppm= 4.0 with integration of 2H is a CH₂ most likely attached to the Cl
• ppm = 4.9 and 5.1 are singlets of 1H and must be our two alkene hydrogens

There are three ways to attach the discovery we made, but only one of them are the correct answer. Since the coupling of the two alkene hydrogens are small whereas vicinal hydrogens tend to be large, we conclude that the hydrogens are geminal and appear on the same carbon. This leaves the two other groups to be located on the other alkene carbon.

Stereo Chemical Information from coupling constant

Distinction between cis and trans isomers

Trans coupling is greater than for cis. From the value the isomers can be identified as in the following example.

• For coupling the two hydrogens must have different chemical shift.
• Trans coupling (range :11 to 19 Hz) is greater than cis coupling (range: 5 to 14 Hz).
• ³J_HH means coupling between two hydrogens separated by a distance of three bonds (vicinal coupling). If it is coupling between hydrogens at the same carbon then it is ²J_HH (geminal coupling)

In some molecules the cis-trans, distinction is made even from the position of the absorption, as in cis and trans stilbenes. In trans case the hydrogens are deshielded by two aromatic rings while in the cis case it is by one ring.

Variation of dihedral angle with coupling constant

Dihedral angle: Consider the following Newman and Sawhorse projections for ethane.

• Looking from the front H_b is eclipsed by Ha the angle between the plane containing H_a and H_b is zero (they are in the same plane).
• The second conformation has H_b at a different spatial position, they are not in the same plane. The plane containing H_b is 60⁰ to the right of the plane containing H_a. This is the dihedral angle, in the first case the dihedral angle is zero.
• In the third case though the two atoms are in the same plane H_b has undergone a rotation of 180⁰ and that is the dihedral angle.
• The dihedral angle is used when two are separated by three bonds.
• Martin Karplus was the first to study the variation of coupling constant ³J_HH with dihedral angle.
• The magnitude of the coupling constant between hydrogens of adjacent carbon atoms depend directly on the dihedral angle.
• This magnitude is greatest when this is zero or 180⁰ and smallest when it is 90⁰.
• When it is zero side-side overlap of the two C-H bond orbitals will be maximum, when it is 180⁰ back lobes overlap is maximum leading to higher “J”. When it is 60⁰ C-H bond orbitals overlap is at a minimum since they are perpendicular with each other leading to lower “J”.

Stereochemistry of cyclohexane derivatives

The information from the above can be used to assign the configuration of the anomeric carbon in cyclic sugars. In the case of α- glucose at the anomeric carbon the equatorial proton is has a chemical shift of δ 5.2 which is down field by 0.5 for the corresponding axial proton in β-glucose.

 Coupling Constants : Cis-Trans Isomers of Olefins

The magnitude of the ³J(H,H) coupling constant in olefins follows the general rule J_trans > J_cis and this is a convenient way to distinguish between cis and trans isomers:

 

 

³J is also influenced by the olefin's substituents, the distinction is meaningful only for one by direct comparison of the isomers or for olefins with very similar substitution pattern.

( Z ) - hex - 2 - en - 1 - yl acetate

(Z)-hex-3-en-1-yl acetate

 ( E ) - hex - 2 - en - 1 - yl  acetate

(2E)-Hex-2-en-1-yl acetate

(E)-2-Hexenyl acetate

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