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Topic: Finding Unknowns Using IR, NMR, and Mass Spec  (Read 1674 times)

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Offline chalk5

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Finding Unknowns Using IR, NMR, and Mass Spec
« on: May 21, 2014, 01:37:13 AM »
Hello Experts,

I'm usually alright with stuff like this, but I'm completely at loss with this unknown.

I will show you what I know so far looking at the following structures:

IR: The peaks at 2938 and 2849 tells me that those are alkanes, and the numbers on the right don't peak much so I ignored those specs. And further more I don't see any carbonyl group nor do I see an O-H group.

Carbon NMR: CNMR shows me that there are 5 different kinds of Carbons, so I know there are at least 5 carbons in this compound.

Proton(H) NMR: It shows me that there are 13 Hydrogens total. There is a CH2 group, as well as 6 Hydrogens attached to a carbon, and 5 Hydrogens attached to a carbon right next to it.

Detailed HNMR shows that there are 7 different kinds of Hydrogens, where they are all multiplets(benzene ring?)

Mass Spec: This is where I usually just try to figure out how many things are there, by subtracting from the Exact Mass which is 176. So I know that so structure must include C5H13=73
(176-73)=103

103
-12
-12
-12
-12
-12
-12
=31 (weird number, I was hoping it would be 32 so I know there are 2 Oxygens, but not in this case)

Since I don't see any halogens (Br and Cl) and I don't see any Nitrogen either, I'm almost convinced that there must a typo in this compound, but I'm not quite sure if I am.

Could anyone help me out with this? Perhaps how to read the spectras better as well as what the compound would be like?

Much appreciated.

IR :[Imgur](http://i.imgur.com/07Bom8X)
C-NMR:[Imgur](http://i.imgur.com/5Got9H9)
H-NMR:[Imgur](http://i.imgur.com/1CfFt1X)
H-NMR Detailed: [Imgur](http://i.imgur.com/dujcrhz)
Mass Spec:[Imgur](http://i.imgur.com/PBzRoLA)

Offline MOTOBALL

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Re: Finding Unknowns Using IR, NMR, and Mass Spec
« Reply #1 on: May 21, 2014, 11:41:35 AM »
OK Chalk5,

Time for some basics.

1. The Nitrogen Rule: when the MW is EVEN (e.g.176) the N atom count is EVEN (N = 0, 2, 4, 6....) and vice versa.
When the MW is ODD (e.g. 177) the N atom count is ODD (N = 1, 3, 5...) and vice versa.

2. 176 is not an exact mass, it is NOMINAL.  An EXACT mass might be measured as 176.0123; this would allow the determination of possible elemental compositions (C, H, N, O, S, Br, Cl, I, P, Si etc) consistent with 176.0123.

3. What are the characteristics of Br and Cl atoms that enable them to be confirmed or discounted in a mass spectrum ?

So now take two deep breaths, revisit your mass spectrum and call us in the morning.

Good Luck !!

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