NMR + Carbon Environments
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What are the two main types of NMR used in A - level chemistry?
Carbon - 13 NMR and Proton (¹H) NMR
What does NMR tell us about an unknown molecule? (nmbr. diff. chmcl. envrmnts. prsnt.)
The number of different chemical environments present in the molecule
What does the position of a peak in a carbon - 13 NMR spectrum tell you? (info. atms. bnded. - C)
Information about the atoms bonded to the carbon in that environment
When are two carbon atoms in the same environment? (sme. set - nghbrng. atms. + sme. nmbr. bnds. awy. - idntcl. atms.)
When they have the same set of neighbouring atoms and are the same number of bonds away from identical atoms
What is meant by a “unique atom” in NMR analysis?
An atom that is different from all others in the molecule
What is meant by a “carbon environment” in NMR? (C w/ unq. set - ngbring. atms.)
A carbon environment is a carbon atom with a unique set of neighbouring atoms
How does the presence of a unique atom affect carbon environments? (C - diff. nmbr. bnds. awy. unq. atm. = diff. envrmnts.)
If carbons are a different number of bonds away from a unique atom, they must be in different environments
Why can’t rotation around double bonds be used to identify equivalent environments? (rot. only 3 rnd. sngle. bnds. - sgma. bnds. - dble. bnds. cntn. pi bnds. - prvnt. 3 rot.)
Rotation can only occur freely around single bonds because they are sigma bonds, double bonds contain pi bonds which prevent free rotation
How do you predict the number of peaks in a ¹³C NMR spectrum?
Count the number of different carbon environments in the molecule
State the relationship between the number of peaks and environments in ¹³C NMR
Each peak represents a different carbon environment
What is the standard reference compound used in carbon - 13 NMR?
Tetramethylsilane (TMS)
Why does TMS produce a single peak?
It has only one carbon environment
Why does the TMS peak not interfere with sample peaks? (C - hgh. e- dnsty. - bnd. w/ Slcn. - apprs. lwr. chem. shft. > mst. org. cmpnds.)
Its carbons have high electron density due to bonding to silicon, so it appears at a lower chemical shift than most organic compounds
Give two practical reasons why TMS is used as a standard (lw. bp. - esy. rmvl. + chem. inrt.)
Low boiling point (easily removed) and chemically inert
Why does TMS produce a strong signal? (4C - sme envrmnt. - 1 intse. pk.)
All four carbon are in the same environment, giving one intense peak