13-1

Identify the reasonable structure for the unknown molecule given its IR spectrum and mass spectrum.

D

C

B

A BrHO

ClHO

Br

O

H

Cl

H3CO

H3CO

172174

93

65

39

15002000 100030004000

13-2

How many unsaturations are present in a molecule with formula C6H10O2?

A. 1B. 2C. 3D. 4E. 5

13-3

Which of the following functionalities does NOT constitute an unsaturation in a molecule?

A. Double bondB. RingC. CarbonylD. Hydroxy group

13-4

How many hydrogens are present in a molecule containing 4 carbons, 1 oxygen, and 1 nitrogen, with 2 unsaturations?

A. 6 B. 7 C. 8 D. 10 E. 11

13-5

Nuclear Magnetic Resonance

Chapter 13

13-6

13-7

13-8

Molecular Spectroscopy Nuclear magnetic resonance (NMR)

spectroscopy: a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of • hydrogen atoms using 1H-NMR spectroscopy• carbon atoms using 13C-NMR spectroscopy• phosphorus atoms using 31P-NMR spectroscopy

13-9

Nuclear Spin States An electron has a spin quantum number of 1/2

with allowed values of +1/2 and -1/2 • this spinning charge creates an associated magnetic

field• in effect, an electron behaves as if it is a tiny bar

magnet and has what is called a magnetic moment The same effect holds for certain atomic nuclei

• any atomic nucleus that has an odd mass number, an odd atomic number, or both also has a spin and a resulting nuclear magnetic moment

• the allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus

13-10

Nuclear Spin States

• a nucleus with spin quantum number I has 2I + 1 spin states; if I = 1/2, there are two allowed spin states

• Table 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds

1H 2H 12C 13C 14N 16O 31P 32SElement

Nuclear spinquantum number (I )

Number ofspin states

1/2 1 0 0 01/2 1

2 3 1 2 3 1

1/2

2 1

13-11

Nuclear Spins in B0

• within a collection of 1H and 13C atoms, nuclear spins are completely random in orientation

• when placed in a strong external magnetic field of strength B0, however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed

13-12

Nuclear Spins in B0

• for 1H and 13C, only two orientations are allowed

13-13

Nuclear Spin in B0

• the energy difference between allowed spin states increases linearly with applied field strength

• values shown here are for 1H nuclei

13-14

Nuclear Spins in B0

In an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for • 1H is approximately 0.120 J (0.0286 cal)/mol, which

corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz)

• 13C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz)

13-15

Nuclear Populations in a Magnetic Field

• Because the spin +1/2 level is lower in energy than the spin -1/2 level, there are more +1/2 spin nucei in the sample than -1/2 spin nuclei

• However, because the energy difference is so small, there is not a large difference

• The difference becomes larger with• Stronger magnetic field• Lower temperatures (see Boltzman expression)

En

erg

y

+1/2

-1/2

DEh = n DE

13-16

Nuclear Magnetic Resonance Resonance: in NMR spectroscopy, resonance is

the absorption of electromagnetic radiation by a precessing nucleus and the resulting “flip” of its nuclear spin from a lower energy state to a higher energy state

(BTW, the “nuclear” in NMR has

nothing to do with radioactivity)

13-17

Nuclear Magnetic Resonance

• if we were dealing with 1H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H. The same is true of 13C nuclei

• moving electrons near the nucleus affect the strength of the magnetic field and affect the energy difference between the two nuclear spin states, and therefore affect the difference in the resonance frequencies

• the circulation of electrons around a nucleus in an applied field is called diamagnetic current and the nuclear shielding resulting from it is called diamagnetic shielding

13-18

Nuclear Magnetic Resonance

• the difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small and is proportional to the strength of the magnetic field

• we generally describe the effect as a relative amount (makes it independent of the magnetic field strength)

• the difference in resonance frequencies for hydrogens in CH3Cl compared to CH3F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency

360 Hz300 x 106 Hz

1.2 = 1.2 ppm106=

13-19

Nuclear Magnetic Resonance

• signals are measured relative to the signal of the reference compound tetramethylsilane (TMS)

• for a 1H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS

• for a 13C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS

• Chemical shift (): the shift in ppm of an NMR signal from the signal of TMS

Tetramethylsilane (TMS)

CH3Si CH3CH3

CH3

13-20

NMR Spectrometer

13-21

NMR Spectrometer

Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio-frequency detector

The sample is dissolved in a solvent, most commonly CDCl3 or D2O, and placed in a sample tube which is then suspended in the magnetic field and set spinning

Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds

13-22

NMR Spectrum 1H-NMR spectrum of methyl acetate

• Downfield: the shift of an NMR signal to the left on the chart paper

• Upfield: the shift of an NMR signal to the right on the chart paper

13-23

Equivalent Hydrogens Equivalent hydrogens: have the same chemical

environment• a molecule with 1 set of equivalent hydrogens gives 1

NMR signal

H3C

C C

CH3

H3C CH3

CH3CCH3 ClCH2CH2Cl

Propanone(Acetone)

1,2-Dichloro-ethane

Cyclopentane 2,3-Dimethyl-2-butene

O

“Same Chemical Environment” : see Section 10.8• same connections (constitutionally equivalent)• conformationally easily converted

13-24

Conformation Interconversion

a

b

c

a

b

c

Different environments in the molecule

Rotation Rate > 109/sec (see Chapt 2)DE‡ = 14 kJ/mol

H

H

CH3

H

Different environments in the molecule

“Interconversion does not occur at ordinary temperatures” – Chapt 4DE‡ = 250 kJ/mol

13-25

con‘t

Ha

Hb

Hb

Ha

DE‡ = 45 kJ/mol

(chapt 7)

Can be distinguished at sufficiently low temperatures or with substituted cyclohexanes

Carbons can also be evaluated this way

13-26

Equivalent Hydrogens

• a molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set

CH3CHCl

Cl ClC C

CH3

H H

O

Cyclopent-anone

(2 signals)

1,1-Dichloro-ethane

(2 signals)

(Z)-1-Chloro-propene

(3 signals)

Cyclohexene (3 signals)

How many types of carbons are present?