Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia

Magntica NuclearMagntica NuclearMagntica NuclearMagntica NuclearMagntica NuclearMagntica NuclearMagntica NuclearMagntica Nuclear

Modern Spectroscopic Methods

Revolutionized the study of Organic Chemistry

Determine the exact structure of small to medium size molecules in a few minutes.

Relaes entre Comprimento de

onda e Espectroscopia

Estados de spin nuclear em um campo magntico

< 0,1 kcal/molRdio

Vibraes das ligaes

~ 10 kcal/molInfravermelho (IV)

Eletrnica~ 100 kcal/molUV-Visvel

Molecular Energy Changes

Energia do ftonRegio Espectral

Who Invented Radio?

NikolaTesla?

Tesla coil, 1891Remote-controlled boat, 1898US patents for fundamental radio technologies, 1900

The Great Radio Controversy

http://www.pbs.org/tesla/ll/ll_whoradio.html

The Tesla Coil

Teslas vision for wireless communication

Wall Street financier J. P. Morgan

Promotional illustration for Tesla's "World System"

Tesla's tower with dome frame.

Completed in 1904.

Spin dos Ncleos Atmicos

tomos de Spin 1/2 : nmero de massa mpar.exemplos: 1H e 13C.

tomos de Spin 1: nmero de massa par.exemplos: 2H e 14N.

tomos de Spin 0: nmero de massa par.exemplos: 12C e 16O.

Element 1H 2H 12C 13C 14N 16O 17O 19F

Nuclear Spin

Quantum No 1/2 1 0 1/2 1 0 5/2 1/2

( I )

No. of Spin 2 3 0 2 3 0 6 2States

Spin QuantumSpin Quantum Numbers of Some NucleiNumbers of Some Nuclei

Elements with either odd mass or odd atomic number

have the property of nuclear spin.

The number of spin states is 2I + 1,

where I is the spin quantum number.

The most abundant isotopes of C and O do not have spin.

Estados de Energia de Prtons Submetidos a

um Campo Magntico Externo

E = a b s o r b e d l i g h t

A p p l i e d

M a g n e t i c

F i e l d

He x t

Nuclear Spin Energy LevelsNuclear Spin Energy Levels

Bo

+1/2

-1/2

In a strong magnetic

field (Bo) the two

spin states differ in

energy.

aligned

unaligned

N

S

Ressonncia Magntica Nuclear (RMN)

Nuclear Magnetic Resonance (NMR)

Ressonncia ftons de ondas de rdio podem apresentar a diferena de energia exata entre os estados de spin + and resultando na absoro de ftons enquanto os prtons tm seus estados de spin alterados.

Magntica um campo magntico forte causa uma pequena diferena de energia entre os estados de spin + e .

Nuclear nucldeos de spin (e.g. prtons) comportam-se como pequenas barras magnticas.

Nuclear ResonanceNuclear Resonance

absorption of energy by the

spinning nucleus

The The LarmorLarmor Equation!!!Equation!!!

B0 =

2

=

2

Bo

is a constant which is different for each atomic nucleus (H, C, N, etc)

E = kBo = h can be transformed into

gyromagnetic

ratio

strength of the

magnetic field

frequency of

the incoming

radiation that

will cause a

transition

1H 99.98% 1.00 42.6 267.53

1.41 60.0

2.35 100.0

7.05 300.0

13C 1.108% 1.00 10.7 67.28

2.35 25.0

7.05 75.0

Resonance Frequencies of Selected NucleiResonance Frequencies of Selected Nuclei

Isotope Abundance Bo (Tesla) Frequency(MHz) (radians/Tesla)

The NMR Experiment

The sample, dissolved in a suitable NMR solvent (e.g. CDCl3 or CCl4) is placed in the strong magnetic field of the NMR.

The sample is bombarded with a series of radio frequency (Rf) pulses and absorption of the radio waves is monitored.

The data is collected and manipulated on a computer to obtain an NMR spectrum.

The NMR Spectrometer

Bo

E

+ 1/2

- 1/2

= kBo = hdegenerate

at Bo = 0

increasing magnetic field strength

The Energy Separation Depends on BThe Energy Separation Depends on Boo

ClassicalClassical Instrumentation:Instrumentation:

The ContinuousThe Continuous--Wave NMR Wave NMR

typical before 1965;

field is scanned

A Simplified 60 MHzA Simplified 60 MHz

NMR SpectrometerNMR Spectrometer

Transmitter

Receiver

Probe

h

SN

RF

DetectorRecorder

RF (60 MHz)

Oscillator

~ 1.41 Tesla

(+/-) a few ppm

absorption

signal

MAGNETMAGNET

ModernModern Instrumentation: Instrumentation:

the Fourierthe Fourier--Transform NMRTransform NMR

requires a computer

FT-NMR

PULSED EXCITATIONPULSED EXCITATION

CH2 C

O

CH3BROADBAND

RF PULSE

All types of hydrogen are excited

simultaneously with the single RF pulse.

contains a range

of frequencies

N

S

1

2

3(1 ..... n)

Some Generalizations

NMR solvents contain deuterium

Tetramethylsilane (TMS) is the

reference.

Chemical shift in Hz from TMS vary

according to frequency of

spectrometer!

Delta values () are independent of frequency of spectrometer (ppm).

Espectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de Hidrognio

((((((((ProtonProtonProtonProtonProtonProtonProtonProton NMR NMR NMR NMR NMR NMR NMR NMR Spectrum)Spectrum)Spectrum)Spectrum)Spectrum)Spectrum)Spectrum)Spectrum)

The NMR Spectrum

The vertical axis shows the intensity of Rfabsorption.

The horizontal axis shows relative energy at which the absorption occurs (in units of parts per million = ppm)

Tetramethylsilane (TMS) in included as a standard zero point reference (0.00 ppm)

The area under any peak corresponds to the number of hydrogens represented by that peak.

CH2 C

O

CH3

EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT

PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF

PROTONS THERE ARE BY INTEGRATION.

NMR Spectrum of NMR Spectrum of PhenylacetonePhenylacetone

O Deslocamento QumicoO Deslocamento Qumico

(The Chemical(The Chemical Shift)Shift)

PEAKS ARE MEASURED RELATIVE TO TMSPEAKS ARE MEASURED RELATIVE TO TMS

TMS

shift in Hz

0

Si CH3CH3

CH3

CH3

tetramethylsilane

TMS

reference compound

n

Rather than measure the exact resonance position of a

peak, we measure how far downfield it is shifted from TMS.

Highly shielded

protons appear

way upfield.

Chemists originally

thought no other

compound would

come at a higher

field than TMS.

downfield

TMS

shift in Hz

0n

downfield

The shift observed for a given proton

in Hz also depends on the frequency

of the instrument used.Higher frequencies

= larger shifts in Hz.

HIGHER FREQUENCIES GIVE LARGER SHIFTSHIGHER FREQUENCIES GIVE LARGER SHIFTS

chemical

shift= =

shift in Hz

spectrometer frequency in MHz= ppm

This division gives a number independent

of the instrument used.

parts per

million

THE CHEMICAL SHIFTTHE CHEMICAL SHIFTThe shifts from TMS in Hz are bigger in higher field

instruments (300 MHz, 500 MHz) than they are in the

lower field instruments (100 MHz, 60 MHz).

We can adjust the shift to a field-independent value,

the chemical shift in the following way:

A particular proton in a given molecule will always come

at the same chemical shift (constant value).

Chemical Shift ()

The chemical shift () in units of ppm is defined as:

= distance from TMS (in hz)radio frequency (in Mhz)

A standard notation is used to summarize NMR spectral data. For example p-xylene:

2.3 (6H, singlet)

7.0 (4H, singlet)

Hydrogens in identical chemical environments (equivalent hydrogens) have identical chemical shifts.

All different types of protons in a molecule

have a different amounts of shielding.

This is why an NMR spectrum contains useful information

(different types of protons appear in predictable places).

They all respond differently to the applied magnetic

field and appear at different places in the spectrum.

PROTONS DIFFER IN THEIR SHIELDINGPROTONS DIFFER IN THEIR SHIELDING

UPFIELDDOWNFIELD

Highly shielded

protons appear here.

Less shielded protons

appear here.

SPECTRUM

It takes a higher field

to cause resonance.

Overview of where protons

appear in an NMR spectrum

aliphatic

C-H

CH on C

next to

pi bonds

C-H where C is

attached to an

electronega-

tive atom

alkene

=C-H

benzene

CH

aldehyde

CHO

acid

COOH

2346791012 0

X-C-HX=C-C-H

Some Specific Structural Effects on NMR Chemical Shift

10 - 12Carboxylic Acid (RCOOH)

6 - 8Aromatic (e.g. benzene)

4 - 6Alkene (R2C=CH2)

3 - 4Alkyl Halide (RCH2X)

0.8 1.7Alkyl (C H)

(ppmType of Hydrogen

Shielding The Reason for Chemical Shift Differences

Circulation of electrons within molecular orbitals results in local magnetic fields that oppose the applied magnetic field.

The greater this shielding effect, the greater the applied field needed to achieve resonance, and the further to the right (upfield) the NMR signal.

Structure Effects on Shielding

Electron donating groups increase the electron density around nearby hydrogen atoms resulting in increased shielding, shifting peaks to the right.

Electron withdrawing groups decrease the electron density around nearby hydrogen atoms resulting in decreased shielding, (deshielding) shifting peaks to the left.

DESHIELDING AND ANISOTROPYDESHIELDING AND ANISOTROPY

Three major factors account for the resonance

positions (on the ppm scale) of most protons.

1. Deshielding by electronegative elements.

2. Anisotropic fields usually due to pi-bonded

electrons in the molecule.

3. Deshielding due to hydrogen bonding.

Generalizations

electrons shield nucleus

electronegativity: withdraws electrons to

deshield nucleus

downfield (deshielding) = left side of

spectrum

upfield (shielding) = right side of

spectrum

delta values increase from right to left!

DESHIELDING BY DESHIELDING BY

ELECTRONEGATIVE ELEMENTSELECTRONEGATIVE ELEMENTS

highly shielded

protons appear

at high field

deshielded

protons appear

at low field

deshielding moves proton

resonance to lower field

C HClChlorine deshields the proton,

that is, it takes valence electron

density away from carbon, which

in turn takes more density from

hydrogen deshielding the proton. electronegativeelement

DESHIELDING BY AN ELECTRONEGATIVE ELEMENTDESHIELDING BY AN ELECTRONEGATIVE ELEMENT

NMR CHART

- ++++

- ++++

ElectronegativityElectronegativity Dependence Dependence

of Chemical Shiftof Chemical Shift

Compound CH3X

Element X

Electronegativity of X

Chemical shift

CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si

F O Cl Br I H Si

4.0 3.5 3.1 2.8 2.5 2.1 1.8

4.26 3.40 3.05 2.68 2.16 0.23 0

Dependence of the Chemical Shift of CH3X on the Element X

deshielding increases with the

electronegativity of atom X

TMSmostdeshielded

Substitution Effects on Substitution Effects on

Chemical ShiftChemical Shift

CHCl3 CH2Cl2 CH3Cl

7.27 5.30 3.05 ppm

-CH2-Br -CH2-CH2Br -CH2-CH2CH2Br

3.30 1.69 1.25 ppm

most

deshielded

most

deshielded

The effect decreases

with incresing distance.

The effect

increases with

greater numbers

of electronegative

atoms.

ANISOTROPIC FIELDSANISOTROPIC FIELDSDUE TO THE PRESENCE OF PI BONDS

The presence of a nearby bond greatly affects the chemical shift.

Benzene rings have the greatest effect.

Secondary magnetic field

generated by circulating electrons deshields aromatic

protons

Circulating electrons

Ring Current in BenzeneRing Current in Benzene

Bo

Deshielded

H Hfields add together

C=C

HH

H H

Bo

ANISOTROPIC FIELD IN AN ALKENEANISOTROPIC FIELD IN AN ALKENE

protons are

deshielded

shifted

downfield

secondary

magnetic

(anisotropic)

field lines

Deshielded

fields add

Bo

secondary

magnetic

(anisotropic)

field

H

H

C

C

ANISOTROPIC FIELD FOR AN ALKYNEANISOTROPIC FIELD FOR AN ALKYNE

hydrogens

are shielded

Shielded

fields subtract

HYDROGEN BONDINGHYDROGEN BONDING

HYDROGEN BONDING DESHIELDS PROTONSHYDROGEN BONDING DESHIELDS PROTONS

O H

R

O R

HHO

R

The chemical shift depends

on how much hydrogen bonding

is taking place.

Alcohols vary in chemical shift

from 0.5 ppm (free OH) to about

5.0 ppm (lots of H bonding).

Hydrogen bonding lengthens the

O-H bond and reduces the valence

electron density around the proton

- it is deshielded and shifted

downfield in the NMR spectrum.

O

C

O

R

H

H

C

O

O

RCarboxylic acids have strong

hydrogen bonding - they

form dimers.

With carboxylic acids the O-H

absorptions are found between

10 and 12 ppm very far downfield.

O

OO

H

CH3

In methyl salicylate, which has strong

internal hydrogen bonding, the NMR

absortion for O-H is at about 14 ppm,

way, way downfield.

Notice that a 6-membered ring is formed.

SOME MORE EXTREME EXAMPLESSOME MORE EXTREME EXAMPLES

NMR Correlation ChartNMR Correlation Chart

12 11 10 9 8 7 6 5 4 3 2 1 0

-OH -NH

CH2F

CH2Cl

CH2Br

CH2I

CH2O

CH2NO2

CH2Ar

CH2NR2CH2S

C C-H

C=C-CH2CH2-C-

O

C-CH-C

C

C-CH2-C

C-CH3

RCOOH RCHO C=C

H

TMS

HCHCl3 ,

(ppm)

DOWNFIELD UPFIELD

DESHIELDED SHIELDED

Spin-Spin Splitting

Non-equivalent hydrogens will almost always have different chemical shifts.

When non-equivalent hydrogens are on adjacent carbon atoms spin-spin splitting will occur due to the hydrogens on one carbon feeling the magnetic field from hydrogens on the adjacent carbon.

The magnitude of the splitting between two hydrogens (measured in Hz) is the coupling constant, J.

Pascals Triangle

Pattern Relative Peak Height

Singlet 1

Doublet 1 : 1

Triplet 1 : 2 : 1

Quartet 1 : 3 : 3 : 1

Quintet 1 :4 : 6 : 4 : 1

The n + 1 Rule

If Ha is a set of equivalent hydrogens and Hx is an

adjacent set of equivalent hydrogens which are not

equivalent to Ha:

The NMR signal of Ha will be split into n+1 peaks by Hx (where n = number of hydrogens in the Hx set).

The NMR signal of Hx will be split into n+1 peaks by Ha (where n = number of hydrogens in the Ha set).

NMR: Some Specific Functional Group Characteristics

O-H and N-H will often show broad peaks with no resolved splitting, and the chemical shift can vary greatly.

Aldehyde C-H is strongly deshielded.( = 9-10 ppm)

Carboxylic Acid O-H is very strongly deshielded. ( = 10-12 ppm)

NMR: Some Specific Functional Group Characteristics

Ortho splitting on aromatic rings is often resolved, but meta and para splitting is not.

Cis and trans Hs on alkenes usually show strong coupling, but vicinal Hs on alkenes show little or no resolved coupling.

13.8 Integration13.8 Integration

The area under a peak is proportional

to the number of protons that

generate the peak.

Integration = determination of the area

under a peak

INTEGRATION OF A PEAKINTEGRATION OF A PEAK

Not only does each different type of hydrogen give a

distinct peak in the NMR spectrum, but we can also tell

the relative numbers of each type of hydrogen by a

process called integration.

55 : 22 : 33 = 5 : 2 : 3

The integral line rises an amount proportional to the number of H in each peak

METHOD 1

integral line

integral

line

simplest ratio

of the heights

Benzyl AcetateBenzyl Acetate

Benzyl Acetate (FTBenzyl Acetate (FT--NMR)NMR)

assume CH333.929 / 3 = 11.3 per H

33.929 / 11.3

= 3.00

21.215 / 11.3

= 1.9058.117 / 11.3

= 5.14

Actually : 5 2 3

METHOD 2

digital

integration

Modern instruments report the integral as a number.

CH2

O C

O

CH3

Integrals are

good to about

10% accuracy.

Unknown A(Figure 9.20 Solomons 7th ed.)

Formula: C3H7I

IHD = 0

No IR data provided

1HNMR : 1.90 (d, 6H), 4.33 (sept., 1H)

Unknown B(Figure 9.20 Solomons 7th ed.)

Formula C2H4Cl2

IHD = 0

No IR data given

1HNMR : 2.03 (d, 3H), 4.32 (quartet, 1H)

Unknown C(Figure 9.20 Solomons 7th ed.)

Formula: C3H6Cl2

IHD = 0

No IR data given

1HNMR : 2.20 (pent., 2H), 3.62 (t, 4H)

Unknown A(Figure 14.27 Solomons 7th ed.)

Formula = C9H12

IHD = 4

No IR data given

1HNMR : 1.26 (d, 6H), 2.90 (sept., 1H), 7.1-7.5 (m, 5H)

Unknown B(Figure 14.27 Solomons 7th ed.)

Formula = C8H11N

IHD = 4

IR shows two medium peaks between 3300 and 3500 cm-1

1HNMR : 1.4 (d, 3H), 1.7 (s, br, 2H), 4.1(quart., 1H), 7.2-7.4 (m, 5H)

Unknown C(Figure 14.27 Solomons 7th ed.)

Formula = C9 H10

IHD = 5

No IR data given

1H NMR : 2.05 (pent., 2H), 2.90 (trip., 4H), 7.1-7.3 (m, 4H)

Unknown H(Figure 9.48 Solomons 7th ed.)

Formula = C3H4Br2

IHD = 1

No IR data given

1HNMR : 4.20 (2H), 5.63 (1H), 6.03 (1H)

Unknown Y(Figure 14.34 Solomons 7th ed.)

Formula = C9H12O

IHD = 4

IR shows a strong, broad, absorbance centered at 3400 cm-1

1HNMR : 0.85 (t, 3H), 1.75 (m, 2H), 4.38 (s, br, 1H), 4.52 (t, 1H), 7.2-7.4 (m, 5H)