[marcelgluch]

While doing problems in preparation for my Organic midterm, I have been having trouble with determining the molecular formula from the molecular ion peaks. I have no problems determining the structures from NMR's if I am given the formula. So if anyone could pass along a link or maybe give me a few tips as to how to figure out formulas, it would be greatly appreciated. I am also going to talk to my professor about it tomorrow, but I feel as it's always best to get a few different perspectives.

Thanks!

-- Marcel

[zeropoint]

Tips:

1) Look for a peak around 77, 76, 75, these are often substituted benzene rings.
2) Look for losses from the rightmost peak (molecular ion) including -18, -15, -29, this indicate loss of alcohols (probably secondary, or tertiary), lose of methyl, or loss of ethyl.
3) If the molecular ion is odd, chances are that there are an odd number of Ns.
4) Look for series of peaks spaced equal distances away from each other. This indicates that you are loosing the same groups over and over, most likely carbons on a chain (ie. a fatty acid will have a series of losses of 14).

Mostly, you can only determine the structure of the simplest compounds from MS alone. I generally use it to confirm something that I already know or to suggest some possible functional groups.

Good luck.

[zeropoint]

Here are some ridiculously helpful links:

Common fragments: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/MassSpec/mspec.htm#ms2

The basics: http://www.chemguide.co.uk/analysis/masspec/fragment.html