The molecule in question has the following structure:

(source: sigmaaldrich.com)

The 13C NMR is given below:


My assignments are:

23.700 ppm, -CH3

65.569 ppm, CH adjacent to the -OH

66.138/66.183 ppm; 67.924/67.977 ppm, substituted Cp carbons with protons attached

68.344 ppm; Cp carbons

94.857 ppm; Quaternary C

Are these assignments correct? The Cp carbons usually appear between 80-95 ppm but they are shifted down for some reason (I'm not sure why).

Additionally, why are the substituted Cp carbons (non-quaternary) doublets? I thought it may be due to the presence of staggered/eclipsed forms of the molecule but this would affect all the carbon environments.

  • 1
    $\begingroup$ Because of the chiral centre outside your ring, C2/C5 on the top Cp ring are diastereotopic (and C3/C4 too). Based on a tiny bit of internet digging, I think the shifts around 65–80 ppm are perfectly normal for ferrocenes - it surprises me more that C1 on the top Cp has such a high shift. $\endgroup$ – orthocresol Mar 20 '18 at 20:15
  • $\begingroup$ @orthocresol: I agree. For example, substituted $\ce C_p$ carbons in this reference (!citeseerx.ist.psu.edu/viewdoc/…) are reported as: $\delta$ $68.6, 69.4, 83.6$. $\endgroup$ – Mathew Mahindaratne Mar 20 '18 at 21:37
  • $\begingroup$ Thanks, I completely forgot about the diastereotopic effect. One more question - in this case, I assigned the 68.344 ppm peak to the Cp protons purely because it had a much larger peak than the others (i.e. the 65.599 ppm peak). Is this a valid way to assign this peak even though 13C NMR integration isn't really a thing? $\endgroup$ – chemistrystudent Mar 20 '18 at 23:00
  • $\begingroup$ I think it is perfectly fair, especially in the absence of other data. You can't do 13C integration quantitatively, unless you modify the usual pulse sequence, but qualitatively there is still a link: more carbons = stronger signal. In particular it is the rate of relaxation that messes up the integration (faster relaxation = stronger signal) but between the top and bottom Cp rings I personally wouldn't expect there to be significant differences, except for the quaternary carbon of course. [Also some NOE enhancement due to decoupling but again I don't expect any real differences.] $\endgroup$ – orthocresol Mar 20 '18 at 23:59

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