[Rutherford]

Why do the chemical shifts have a increasing value of ppm in the following sequence:
R-CH₃, R-CH₂-R, R₂-CH-R?

What phenomenon explains this?
I thought that alkyl groups are electron pushing so the shielding effect should increase in the sequence, and ppm should decrease.

[Archer]

Carbon is more electronegative than hydrogen, more Carbons = more desheilding of the remaining proton(s)

[Rutherford]

But use for example propane.

The two CH₃- groups should be electron pushing towards the CH₂- group, so according to the inductive effect it should be like this:

H₃ C CH₂  C H₃

and in the CH₂ group, it is: H C  H, thus the shielding effect increases. Where am I wrong?

[Archer]

Please see the following thread for what I believe is the correct reason for this phenomenon:

http://www.chemicalforums.com/index.php?topic=74630.msg270830#msg270830

This is discussing ¹³C NMR but it also applies to ¹H deshielding by alkyl substituents.