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Topic: Trifluoroethanesulfonyl group: odd NMR result  (Read 450 times)

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Offline Babcock_Hall

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Trifluoroethanesulfonyl group: odd NMR result
« on: April 11, 2019, 09:39:24 AM »
Hello Everyone,

I was going over some old H-1 NMR data this week, and I noticed something strange.  I had synthesized the di-tert-butylester of the attached compound (R = C(CH3)3), and there were two signals in CDCl3.  One was a doublet near 4.40 ppm, presumably split by P-31.  The other was a quartet near 4.17 ppm, presumably split by F-19.  I then removed the tert-butyl groups in acid (R = H).  In d6-DMSO I saw a doublet near 4.36 ppm, presumably P-CH2O, and a quartet near 5.04 ppm, presumably CF3CH2S.  The splitting is about 9.6 Hz.  I am puzzled about the apparent downfield shift of the methylene group next to the trifluoromethyl group.  Thoughts?.

Offline pgk

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Re: Trifluoroethanesulfonyl group: odd NMR result
« Reply #1 on: April 12, 2019, 11:16:27 AM »
Such a downfield shift for CF3CH2SO3- methylene seems reasonable according to the Shoolery’s rule, as well as to the influence of the β-group.
http://chemistry.bd.psu.edu/halmi/shoolerytables.pdf
Also, see the table in “Confusion with different tables of Shoolery's Rule”, Chemical Forums, (2015)
https://www.chemicalforums.com/index.php?topic=80126.0
Besides, a little higher shifts of the unprotected derivative, are expected in DMSO due to the hydrogen bonding of the phosphate acid moiety with the solvent.

Offline Babcock_Hall

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Re: Trifluoroethanesulfonyl group: odd NMR result
« Reply #2 on: April 15, 2019, 01:49:04 PM »
http://chemistry.bd.psu.edu/halmi/shoolerytables.pdf
An online table gives for methylene groups
-CF3       1.14
-OSO2R 3.13

This table indicates a base value of 0.23 ppm.  I could not find other shifts to use for a Shoolery calculation.

https://www.chem.wisc.edu/areas/reich/nmr/notes-9-hmr-5-curphy-morrison.pdf
If the change from diester to free acid had shifted the -CH2P group, I would have been less surprised.  I tried doing some simple shift calculations using Curphy-Morrison additivity constants that I found at the Chemistry 605 website of Hans Reich, starting with a base value of 1.20 ppm (I tried not to use shifts from one system with a different system).

-PO3R2   0.59 (alpha) and 0.34 (beta)
-OSO2Me 2.90 (alpha)

From these values I estimated 4.69 ppm for the methylene group next to phosphorus, which is a little more downfield that experimental, but not unreasonably so IMO.  I did not find a shift value for -CF3 or for -SO2OR (however, it occurs to me that I could treat the latter as an alpha shift from a sulfone plus a beta shift from an oxygen).   ChemDraw came up with 4.01 ppm for -CH2P and 3.55 for CF3CH2-

Offline pgk

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Re: Trifluoroethanesulfonyl group: odd NMR result
« Reply #3 on: April 15, 2019, 02:24:16 PM »
The α-values for -CF3 and -SO3R, as well as the β- value for -CH2- for Curphy’s-Morrison’s rule are given in the table figuring in the post:
https://www.chemicalforums.com/index.php?topic=80126.0
(Although being slightly higher than other sources):
Thus:
Base value =   1.2   
α-CF3 =         1.2
α-SO3R =       2.77
β -CH2- =       0.01 (because the α- group is the -SO3- moiety)
------------------------
Total =           5.18

« Last Edit: April 15, 2019, 02:40:55 PM by pgk »

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