[Daeron13]

Greetings!

I'm new here so, I'll introduce myself: I'm Ray, a graduate student, who is studying Science of Materials in a university in Mexico. Almost 25 years old, and I love chemistry, physics and math!

So much about myself. Now, the question is... I'm doing research about molecular recognition of anions, and I'm trying to obtain the calculate spectrum of my molecule, but I'm having problems with the tables... A table (1) given by my professor, shows me a value of chemical shifts to certain proton (e.g., -I = 1.82), but another table (2) I found in books, shows me another value (e.g., -I = 1.94). When I try to calculate the chemical shifts of the proton of CH₂ in CH₃CH₂I, the calculated values using both tables are:
(1): 0.23 + 0.47 (-C(R/H)₃) + 1.82 (-I) = 2.52 ppm
(2): (0.23 + 0.62 (-R_α) + 1.94 (-I_α))*0.90^a = 2.511 ppm

Here, the difference isn't significative but... When I try to calculate the chemical shifts of the methyl proton in the same molecule, the calculated values are:
(1): 0.23 + 0.47 (-C(R/H)₃) = 0.70 ppm
(2): (0.23 + 0.62 (-R_α) + 0.90 (-I_β)) = 1.575 ppm

For the methyl proton, the chemical shifts are different!! I'm doing exactly what the professor teach us, and what the table says in the reference (for methylene proton, multiply by 0.90 after the addition, and for all protons, add the chemical shifts from α and β substituents). Am I doing something wrong? I'm attaching the table the professor gives and the table I extract from a book D:

The formula I used with professor's table was: δ = 0.23 (base value) + δ_X + δ_Y, and for book's table: δ = 0.23 (base value) + δ_α + δ_β

Professor's Table


Book's Table

[pgk]

The Shoorley rule is the table that your professor gave, and permits the estimation of methylene and methyn proton shifts; meaning protons of carbon atoms that are between or among other groups:
XCH2Y and XCH(W)X
Therefore, you cannot use the Shoorley rule, in order to estimate the shifts of methyl protons.
However, the Shoorley rule does not take in account, the β-interactions.
The second table that you found in the book, is a different approach that takes in account the β-interactions, too. That way you can estimate the shifts of both methyl and methylene (and methyn) groups. The methyl, as you did and the methylene by adding the base shift of methylene plus the α-interaction of methyl (R) plus the α-interaction of iodine.
There are also similar tables that additionally take in account the type of proton of the tetrahedral carbon and notably methyl, methylene or methyn. Also, there are similar tables of estimation of alkene protons in relation with Z- or E-, α- and β- interactions. The confusion starts by wrongly naming all these tables as Shoorley rule.
Note that estimated shift values by these rules, are approximate and indicative and slightly different from the real ones. Besides, shift values depend on the D-solvent (CDCl3, D6-DMSO, D2O, etc.), too. Anyhow, the Shoorley and similar rules, are a useful and a powerful tool for the interpretation of 1H-MNR spectra.

[Daeron13]

Ok. Then, the table my professor gave me can't help me with methyl protons so, for them, I must use the table I found in the book. Is that right?

[pgk]

Yes.

[Daeron13]

Thanks, pgk!