[Mack]

This is the problem I cannot seem to grasp despite many tries. I cannot help contradicting myself in my attempts to answer it, especially on the last two. I understand that higher electronegativity leads to a lower electron density and thus a more positive number, but I still seem to get messed up somehow.  If anyone could help explain this that would be much appreciated.

Explain the chemical shifts (ppm values) for the Carbons in the following compounds in terms of the electronegativities/sizes of the attached atoms.

 CH3F--70
 CH3OH--50
 CH3I--(-23)
 CH2I2--(-62)

[fledarmus]

Start by looking at a table of electronegativities. What is the electronegativity of carbon? fluorine? oxygen? iodine? How much electron density will be pulled away from the carbon atom for each of the molecules you show?

[Dan]

Electronegativity won't provide a satisfactory answer here. Iodine shows anomalous behaviour, often referred to as the "heavy atom effect". I do not know/remember the explanation of this effect, and don't have time to look it up right now, but Mack: "heavy atom effect" should give you something to look for in textbooks/google.

This paper looks interesting. I have not had time to read it all, and to be honest I am not at all confident that I will understand the detail, but this line stuck out:

"The number of different, sometimes mutually contradictory, explanations suggested for the observed trends is remarkable."

From CEJ 1998, 4(1), 118-126

[Mack]

Thank you!, after your suggested reading I discovered the heavy atom effect was exactly what I was missing and how I was getting contradicting answers compared to the numbers.