CHAPTER 10

Spectroscopy:Nuclear Magnetic Resonance

Fundamentals of SpectroscopySpectroscopy is the interaction of light with molecules. Since light can beeasily “watched,” we can observe how it is absorbed, reflected and diffracted byan unknown molecule. Since the structure of the molecule is the primarydeterminant of how light interacts with the molecule, we can determine thestructure of an unknown molecule by shining light on the molecule and“watching” what happens.

In general, light “interacts” with a molecule in the “ground state” by giving themolecule energy in the form of light and exciting the molecule into an “excitedstate” meaning high energy state. The difference in energy between the groundand excited state depends on molecular structure and is the basis forspectroscopic structure determination.

Energy

Ground State

Excited State

!E = h"This energy difference depends on the structural features of the molecule.

Types of light that “excite” molecules.Electromagnetic radiation can be described as a wavehaving a wavelength (λ), a frequency (ν) and a velocity (c).

Type of Light Wavelength Energy Transition

X-rays <50 nm >300 kcal/mol Core Electrons

Extreme Ultraviolet 50-200 nm 40-300 Kcal/mol Valence Electrons

Ultraviolet 200-400 nm

Visible

400-800nm

Near Infrared 1-20 um 2-35 kcal/mole Vibrations

Far Infrared 20-1000 um 0.1-2 kcal/mole Vibrations

Microwave 1-100 mm 10-4 kcal/mol Rotations

Radiowave (NMR) 0.1-1 M 10-6 kcal/mol Nuclear-spin

ΔE=hν

NMR: structure and connectivity of carbon-hydrogen frameworkIR: functional group identificationUV: conjugation of π - systemMS: molecular weight and formula

A combination of these techniques are used to identify the structureof a molecule.

Spectroscopy is a technique for analyzing the structure of molecules, usuallybased on how they absorb electromagnetic radiation. Four types are most oftenused in organic chemistry:

Nuclear Magnetic Resonance spectroscopy (NMR)

Infrared spectroscopy (IR)

Ultraviolet spectroscopy (UV)

Mass spectroscopy (MS)

Types of Spectroscopy

How to they identify a unknown molecule?

MS: What is molecular weight? (MW = 118)IR: Functional groups?

-OH present? NO-double bond? YES

UV: Is it conjugated (next to) to the phenyl group? YESNMR: What is carbon-hydrogen framework like?

Is the double bond cis or trans? trans

Structure Identification

MW = 118

OH

H2SO4

or or

MW = 136

What is “spin”? Consider the 1H nucleus which is positively charged and rotatingalong some axis. In the presence of a powerful magnetic field, the nucleus wants toalign with the field. Some want to align parallel (α) and some anti-parallel (β) tothe magnetic field.

Nuclear Magnetic Spectroscopy (NMR)Some nuclei possess spin and this property is exploited in NMRSpectroscopy. Only those nuclei possessing spin can be detected by thismethod. We will focus on the NMR of 13C and 1H, but many other nucleican be studies by NMR.

+ Magnetic moment

Axis of rotation

! "

The difference in energy betweenthe α and β states dependsdirectly on the external magneticfield strength, H0.

The α and β spin states have different energies in the magnetic field.α-spin nuclei are lower in energy than β-spin nuclei and, as predicted by the Boltzmanequation, the lower energy state is more populated. Upon excitation withelectromagnetic radiation, the α-state can be converted to the β-state and this resultsin nuclear magnetic resonance which in turn gives rise to a signal in the spectra.

Resonance occurs when the nuclei are irradiated with the proper frequency to cause a spin flip.

anti-parallelI = -1/2

parallel

I = +1/2radio frequency

!

"

h#E

Resonance

En

erg

y

0B0 (Magnetic Field Strength)

!-spin

"-spin

Electromagnetic Radiation

The difference in energy between the α and βstates depends directly on the external magneticfield strength, H0.

21,150 G 90 MHz

42,300 G 180 MHz

70,500 G 300 MHz

The energy difference is very small. Consequently the α-state is only slightly more populated thanthe β-state. This results in NMR being a relatively insensitive technique. For example, if1,000,000 hydrogens are placed in a magnetic field with a strength of 14,100 Gauss (60 MHz),500,010 will be in the lower spin state and 499,990 will be in the higher spin state. Thus out the1,000,000 hydrogens only 10 are available for excitation to the higher state.

Higher fields afford higher sensitivity. The energy difference between the two states (and alsothe population difference between them) is dependent upon the magnetic field. Higher field = greaterenergy difference = lower energy level more populated = more sensitivity.

NMR active nuclei resonate at different frequencies. For example, when using a 11.74 Teslamagnet, a hydrogen nuclei resonates at 500 MHz, and a carbon nuclei at 125 MHz. Spectrometers arereferred to by the frequency at which a hydrogen nuclei would resonate if placed in the instrument.

In comparison with other techniques, NMR is slow. In practice what this means is that thespectrum that is obtained represents an average of all of conformations and orientations which arepresent in equilibrium at that temperature. Detecting distinct conformers (e.g., two interconvertingchair conformations) requires special experimental conditions.

More Facts about Resonance

NMR Spectroscopy Facilities at OSU

Solution:• 250 MHz• 400 MHz (2)• 500 MHz• 600 MHz (2)• 800 MHz

Solid State:• 300 MHz (2)• 400 MHz (2)

Many nuclei undergo magnetic resonance.

In general, nuclei composed of an odd number of protons (1H and itsisotopes, 14N 19F, and 31P) or an odd number of neutrons (13 C) showmagnetic behavior.

If both the proton and neutron counts are even (12C or 16O) the nucleiare non-magnetic.

An NMR ExperimentThere are two ways in which an NMR Experiment can be conducted.

Fixed Magnetic Field Strength / Vary Rf1. Place the protons in a magnetic field.2. Apply a radio frequency (Rf) until an absorption is noted.3. Record the radio frequency needed for absorption to be

observed.

Fixed Radio Frequency/Vary Magnetic Field Strength 1. Place the protons in a magnetic field. 2. Apply a set radio frequency (Rf) 3. Vary magnetic field strength (H0) until absorption is observed.

Absorbance

H0 = 21,500 Gauss

90 MHz

Fixed Field Strength

Vary Rf

Rf

Absorbance

21,500 Gauss

Rf = 90 MHz

Fixed Radio Frequency

Vary H0

Rf

ShieldingThe chemical shift of a particular hydrogen depends upon the electron density thatsurrounds it. The motion of the electrons creates a magnetic field which acts in opposition to theexternal magnetic field. Thus the electrons “shield” the nucleus from the applied magnetic field. Thedegree of shielding determines where in the spectra the resonance for a particular hydrogen willappear. Hydrogens can be described as shielded or deshielded relative to each other.

H0HInduced

Heff = Ho - Hinduced

C H e- cloud

The degree of shielding of a nucleusdepends upon its surrounding electrondensity.

Adding electrons increases shielding.

Removing electrons causes deshielding.

The NMR Spectrum and Chemical ShiftNMR Spectra are usually recorded by reporting the position of peaks relative to a standard compound.Tetramethylsilane TMS is usually the standard. It’s protons are shielded relative to most protons dueto the low electronegativity of silicon. A given proton will undergo resonance at a field strength thatdiffers from that of the standard (TMS). This difference is known as the chemical shift.

Chemical shifts (d) are measured in ppm and alwaysgiven relative to standard. They are defined by thefollowing formula:

= (i–s)/o x 106 ppm

where i = frequency of the hydrogen of interest (in Hz)s = frequency of the standard (in Hz)o = operating frequency of the magnet (in Hz)

Tetramethylsilane (TMS, (CH3)4Si) is the mostcommon internal standard, and is assigned achemical shift of 0.00 ppm.

ppm 010

UpfieldDownfield

Chemical shift values are independent of thefield strength of the magnet.

100 Hz

!s

!1

100 MHz Spectrum

200 Hz

!s

!1

200 MHz Spectrum

1 ppm 1 ppm

Upfield and Downfield?

TMS

increasing field

increasing Rf frequency

10 0

Lets irradiate a proton with 180 MHz exactly. The change of α－β energystates requires 42,276 gauss at the proton.Due to shielding, the applied field must be somewhat higher than 42,276gauss to allow the effective field to have the resonance value of 42,276gauss. So more shielding means higher field necessary forresonance.

High field (upfield)Low Rf frequency

Low field (downfield)High Rf frequency

Effect of Shielding on Chemical Shift.Shielding causes a displacement of an NMR peak to the right in thespectrum (shifted upfield).

Deshielding causes a displacement to the left (shifted downfield).

CH2

CH3Cl-CH2-O-CH3

H0downfield upfield

TMS

Rf = 90 MHz

340 Hz

500 Hz

δ(CH3) = 340 x 106/ 90 x 106 = 3.77

δ(CH2) = 500 x 106 / 90 x106 = 5.55

Multiple substituents exert a cumulative effect.

The deshieldinginfluence of electronwithdrawing groupsdiminishes rapidly withdistance.

The NMR Spectrum and Chemical ShiftNMR Spectra are usually recorded by reporting the position of peaks relative to a standard compound.Tetramethylsilane TMS is usually the standard. It’s protons are shielded relative to most protons dueto the low electronegativity of silicon. A given proton will undergo resonance at a field strength thatdiffers from that of the standard (TMS). This difference is known as the chemical shift.

Chemical shifts (d) are measured in ppm and alwaysgiven relative to standard. They are defined by thefollowing formula:

= (i–s)/o x 106 ppm

where i = frequency of the hydrogen of interest (in Hz)s = frequency of the standard (in Hz)o = operating frequency of the magnet (in Hz)

Tetramethylsilane (TMS, (CH3)4Si) is the mostcommon internal standard, and is assigned achemical shift of 0.00 ppm.

ppm 010

UpfieldDownfield

Chemical shift values are independent of thefield strength of the magnet.

100 Hz

!s

!1

100 MHz Spectrum

200 Hz

!s

!1

200 MHz Spectrum

1 ppm 1 ppm

1H Chemical Shifts-ElectronegativityCH3-A A Electronegativity Chemical Shift (δ)

CH3-Li

CH3-SiMe3

CH3-H

CH3-CH3

CH3-NH2

CH3-OH

CH3-F

CH3-Cl

CH3-Br

CH3-I

Li

SiMe3

H

CH3

NH2

OH

F

Cl

Br

I

1.0

1.7

2.1

2.5

3.1

3.5

4.0

3.2

3.0

2.7

-1.0

0.0

0.2

0.9

2.3

3.4

4.2

3.1

2.7

2.2

The deshielding influence of electron withdrawing groups diminishesrapidly with distance.

Field AnisotropyR CH3

H

H2C CH2

1.1 ppm

5 ppm

3 ppm

7 ppm

• Increasing s character in bond more electronegative carbon.• What about acetylene and benzene?

HoH

Hinduced

Electrons are delocalized and can move around the ring, creating aring current and induced field.

Magnetic field is stronger near theprotons-->deshielded

Magnetic field is weaker near theprotons-->shielded

H

H

electronmovement

Induced magnetic field

Hinduced

Electron Withdrawing Groups

Ha

Hb

! Ha = 5.7

! Hb = 5.7

Ha

X

Ha

X

Ha

OCH3

Ha

NMe2

Ha

OLi

! Ha = 5.4 ! Ha = 4.8 ! Ha = 4.3

Electron Donating Groups

Resonance Effects

Ha

Hb

! Ha = 5.7

! Hb = 5.7

Ha

Hb

O

! Ha = 6.0

! Hb = 7.0

Ha

Hb

O

Magnetic EquivalenceChemically equivalent protons are magnetically equivalent and,therefore, have the same chemical shift.

Chemically inequivalent protons are usually magneticallyinequivalent and have different chemical shifts.

R

R Ha

Hb R

R Hb

Ha

R Ha

Hb R

Hb

Ha

Chemicallyequivalent

R C

Ha

Hc

Hb

Chemicallyinequivalent

Conformational interconversion may result inequivalence on the NMR time scale.

In the case of the rapid rotation of the methyl group inchloroethane, or the rapid conformation flip in cyclohexane, theobserved chemical shifts are the averages of the values thatwould be observed without the rapid rotation or flip.