Calculated shifts

The COSY did not clearly distinguish which of the two remaining structures best fits the experimental data. For further proof of which structure is appropriate, the calculated carbon shifts for the two structures is compared with the experimental results. Typically calculated carbon shifts are within 10 ppm of the experimental values. You may calculate the chemical shifts for the two structures using additivity rules. The results shown here are calculated by ACD's CNMR software.

The two candidate structures are:

Experimental	Structure A		Structure B
	Calculated	Error	Calculated	Error
202.7	202.1	0.6	202.2	0.5
131.6	133.6	2	131.5	0.1
124.0	129.2	5.2	124.1	0.1
50.9	44.2	6.7	51	0.1
36.9	35.6	1.3	37.2	0.3
27.7	26.2	1.5	27.8	0.1
25.6	24.1	1.5	22.8	2.8
25.4	21.6	3.8	21.4	4
19.8	21.2	1.4	19.8	0
17.6	17.7	0.1	16.4	1.2

This data clearly supports structure b for citronillal. Particularly the values for the carbon peaks observed at 50.9, and 124. These display the largest change in the two calculated spectra, and both values are closer to those calculated for structure b.

The calculated spectrum also is useful for distinguishing between the two CH₃ groupls on the quaternary carbon of the alkene. The CH₃ in the cis position has the smaller chemical shift.

Calculated Carbon Spectrum

Calculated Proton Spectrrum