aularmn2

Prof. Moacir Geraldo Pizzolatti 02/04/2014

1

Information in a NMR Spectra

ObservablePeak

Position

Splitting

Intensity

Shape

Name

Chemical shifts(δ or ppm)

Coupling constant(J) Hz

Integral

Line wide

Quantitative

δ = νob –νr/ νr(Hz)

Peak separation(Hz)

Relative area

Information

Chemical environment

Neighboring nucleus

Nucleus equivalent

MotionChan. Chem.

An NMR spectrum is a plot of the radio frequency appliedagainstabsorption.

•A signal in the spectrum is referred to as a resonance.

•The frequency of a signal is known as its chemical shift.

•Splitting of a signal is referred to as a spin-spin coupling(J)

•The signal intensity is referred as integral

Information in a NMR Spectra


2

Chemical Shift

•Chemical shiftarises from circulation of electrons

surrounding the nucleusunder the influence of

the applied magnetic field

•This creates a small magnetic fieldthat opposes to (B0).

•So, the nucleus is exposed to an effective fieldthat is

usually smaller than the external field.

•The magnitude of the field developed internallyis

directly proportional to the applied external field.

Bo

H1

O

Bo1 Bo2

H2Bo1

Bo

Bo2

C

Circulation of electrons surrounding the nucleuscreates a small magnetic fieldthat opposes the (B0).

Chemical ShiftNuclear Shielding


3


Chemical Shift –δ or ppm = υυυυ0000

Circulação de e- no micro ambiente –Momento Magnetico

Alteração do Campo Magnetico no microambiente do Núcleo

Blindagem = σ − geralmente positiva

Mum campo magnetico externo constante:

Diminuindo a blindagem resulta numa maior frequência de ressonância


4

The resonance condition,thus becomes :


O deslocamento químico

Chemical Shift –δ or ppm = υυυυ0000

Tetramethylsilane


5

In the 90 MHz 1H NMR spectrum (B0 = 2.11 T) the

signal of TMS appears at exactly 90 000 000 Hz

90 000 237 Hz (CH3Br)

90 000 441 Hz (CH2Br2)

90 000 614 Hz (CHBr3)

Chemical ShiftScale -Location of an nmr signal

Reference compounds

The TMS

and

The δ-Scale


•chemically unreactive

•easily removed

•single sharp nmr signal

•does not interfere


6

observing frequency of 300 MHz these intervals are:

calculate the chemical shifts:

δ-ScaleChemical Shift

90 000 237 Hz (CH3Br)

90 000 441 Hz (CH2Br2)

90 000 614 Hz (CHBr3)



7



The nmr resonance signals depend on external magnetic field strength and frequency


8

Chemical ShiftThe Solvent

Chemical ShiftThe Equivalent Nucleous


9

Chemical ShiftThe Equivalent Nucleous –molecular symmetry



10




11


1 – Inductive effect

Chemical Shift

Factors who affect the chemical shift


12

2- Anisotropic effect

π-Electron Functions - Anisotropy

Chemical Shift

π-Electron Functions - anisotropy

Chemical Shift


13


Chemical Shift


Chemical Shift


14

3 - Solvent Effects

Chemical Shift

Chemical Shift


15

Chemical Shift

Chemical Shift

4 - Hydroxyl Proton Exchange

and the Influence of Hydrogen Bonding

•A wide range over which this chemical shift may be found

The OH proton signal is seen at

• 2.37 δ in 2-methyl-3-butyne-2-ol

• 3.87 δ in 4-hydroxy-4-methyl-2-pentanone

•Rapid OH exange with the deuterium


16

Chemical Shift

Hydroxyl Proton Exchange

Chemical Shift

The Influence of Hydrogen Bonding

Hydrogen bondingshifts the resonance signal of a

proton to lower field( higher frequency)

i) The chemical shift of the hydroxyl hydrogen of

an alcohol varies with concentration

2-methyl-2-propanol,

in dilute solutions (< 1.0 δ ).

In concentrated solution (near 2.5 δ).


17

Chemical Shift


ii) The more acidic OH group of phenol generates

a lower-field resonance signal, which shows a

similar concentration dependence to that of

alcohols.

Chemical Shift


iii) Dimeric association - the OH of carboxylic acids

displays a resonance signal at 10.0 to 13.0 δ and

is often broader than other signals.


18

Chemical Shift


iv) Intramolecular hydrogen bonds, especially those

defining a six-membered ring, generally

display a very low-field proton resonance

Chemical Shift

The Influence of Temperature


19

Chemical Shift

The Influence of Solvent

Signal Strength

Integral


20

Coupling Constant - J



21


"First-Order" arrangement of lines

spin-coupled nuclei have very different chemical sh ifts

(i.e. ∆ν is large compared to J)



22




23


If the coupled nuclei have similar chemical shifts, the

splitting patterns are distorted (second order beha vior)


"First ad second-Order" arrangement of lines

If the ratio of ∆ν to J (both in Hz)

decreases to less than 10 a significant

distortion of this expected


24




25




26




27




28


αααααααααβ βααβ βααβ βααβ βα

ββββββββ

ααααααααααααααβ αβα βααααβ αβα βααααβ αβα βααααβ αβα βαααββ βαβ ββααββ βαβ ββααββ βαβ ββααββ βαβ ββα

ββββββββββββCH3 CH2


1) Nuclei having the same chemical shift do not exh ibit spin-splitting. They may actually be spin-coupled, but the splitting cannot be observed directly.2) Nuclei separated by three or fewer bonds, will u sually be spin-coupled and will show mutual spin-splitting of the resonance signals , provided they have different chemical shifts. Longer-range coupling may be obser ved in molecules having rigid configurations of atoms.3) The magnitude of the observed spin-splitting dep ends on many factors and is given by the coupling constant J and is the same fo r both partners in a spin-splitting interaction and is independent of the ext ernal magnetic field strength.4) The splitting pattern of a given nucleus (or set of equivalent nuclei) can be predicted by the n+1 rule, where n is the number of neighboring spin-coupled nuclei with the same J. If there are 2 neighboring, spin-coupled, nuclei the observed signal is a triplet ( 2+1=3 ); if there ar e three spin-coupled neighbors the signal is a quartet ( 3+1=4 ). In all cases the cen tral line(s) of the splitting pattern are stronger than those on the periphery. The inten sity ratio of these lines is given by the numbers in Pascal's triangle. Thus a doublet has 1:1 or equal intensities, a triplet has an intensity ratio of 1:2:1, a quartet 1:3:3:1 etc.


29



30



31


Coupling Constant - JSystem spins: AB – AX – A nB – AnX-- ABC – ABX – AMX

AMX system

J = 18; 11 and 1 Hz


32




33



34


35


36


37

EPICATEQUINA

Flavan-3-ol


38


39


40


Spin Decoupling


41


42

Apigenina-7-Metoxi


43

XANTONAS

XANTONAS – NOEdifDeterminação da posição da metoxila


44

Cumarina - RMN 1H , J

J = 9,1 Hz ?


45


46


47


48


49

KAEMPFERITRIN


50


51

aularmn2

Documents