Solving NMR Problems

1. Check the molecular formula and degree of unsaturation. How many rings/double bonds?

2. Make sure that the integration adds up to the total number of H’s in the formula.

3. Are there any signals in the double bond region?

4. Check each signal and write down a possible sub-structure for each one.

5. Try to put the sub-structures together to find the structure of the compound.

Proton NMR Spectrum: C9H12

1H NMR Spectrum: C4H7O2Br

t2H

t2H

s3H

5.0 4.0 3.0 2.0 1.0 0

Electronegative Substituents: Shift Left

HH33C—CHC—CH22—CH—CH33 OO22N—CHN—CH22—CH—CH22—CH—CH33

0.90.9 0.90.9 1.31.3 1.01.0 4.34.3 2.02.0

– CHCH33ClCl 3.1 3.1 (one Cl) (one Cl)

– CHCH22ClCl22 5.3 5.3 (two (two

Cl’s)Cl’s)

– CHClCHCl33 7.3 7.3 (three Cl’s) (three Cl’s)

Effect is cumulativeEffect is cumulative

Hydrogens on Heteroatoms

Type of protonType of protonChemical shift (ppm)Chemical shift (ppm)

1-31-3 HH NRNR

0.5-50.5-5 HH OROR

6-86-8 HH OArOAr

10-1310-13 CC

OO

HOHO

Chemical shifts for protons on heteroatoms are variable, Chemical shifts for protons on heteroatoms are variable, and signals are often and signals are often broadbroad (not generally useful). (not generally useful).

13C NMR Spectroscopy

• Carbon-13: only carbon isotope with a nuclear spin

natural abundance of 13C is only 1.1%

(99% of carbon atoms are 12C, with no NMR signal)

• All signals are obtained simultaneously using a broad pulse of energy. The resulting “mass signal” changed into an NMR spectrum mathematically using the operation of Fourier transform (FT-NMR)

• Frequent repeated pulses give many data sets that are averaged to eliminate noise

13C signals go from 0 to 240 ppm. 13C signals: always sharp singlets.

(wider range than in 1H NMR) (1H signals: broad multiplets)

These two facts mean that in carbon-13 NMR, each separate signal is usually visible, and you can accurately count the number of different carbons in the molecule.

13C NMR Spectroscopy

Chemical shift affected by electronegativity of nearby atoms:

alkane-like range: 0 – 40 ppm (R-CH2-R)

heteroatom range: 50 – 100 ppm (O-CH2-R)

double bond range: 100 – 220 ppm (sp2 carbons)

Why does 13C NMR give singlets?13C is only 1.1% natural abundant, so most carbons are 12C,

and give no NMR signal.

No splitting seen with carbon, because carbons next to the 13C are likely to be carbon-12:

Sample of 1-Propanol:

12CH3-12CH2-12CH2-OH 12CH3-12CH2-12CH2-OH

12CH3-13CH2-12CH2-OH 13CH3-12CH2-12CH2-OH

12CH3-12CH2-12CH2-OH 12CH3-12CH2-12CH2-OH

12CH3-13CH2-13CH2-OH 12CH3-12CH2-12CH2-OH

NMR: Number of Signals for 13C NMR

How many signals should appear in the carbon-13 NMR spectrum for these compounds?

In theory:

Signals actually resolved:

O

octane

13C NMR Example

Note the wide spectral width and the sharp singlets in the spectrum below.

Also note that there is no integration with 13C NMR.

13C NMR: smaller signal to noise ratio

morescans(noise

smaller)

13C NMR Spectrum: C5H11Cl

13C NMR Spectrum: C4H7O2Br

200 150 100 50 0