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Selective Spin Decoupling (Double Resonance)Intense irradiation of a proton at its resonance frequency in aspin coupled system removes the protons coupling effect.

A powerful tool forDetermining connectivityof protons through bonds

and assigning proton peaks.Overlapping peaks can besimplified by irradiating

.coupling partner.

1

& detect the loss of couplingin all the coupling protons

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Nuclear Overhouser Enhancement (NOE)

It is a through space effectUses much weaker irradiation

proton w ic is c oser in space to t e irra iate one is a ectewhether it is coupled to it or not. Effective distance is upto 4 .If it is cou led it retains some cou ling.NOE is usually less than 20%. So use NOE difference (subtract)

3

o ar za on o near y nuc e resu s n ncrease n o u a on o g er

energy level in nearby non-irradiated protons and subsequent T1 relaxationto lower energy state results in enhancement of intensity of nearby Hs

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NOE difference s ectra

4

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pec roscopy

Suvarn S. Kulkarni

Department of Chemistry

CH 521 Lecture-6

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NMR In a magnetic field, a sample can

frequencies governed by the

Absorption is a function ofcertain

A plot of frequencies Vs. peak

e e

6

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Differences between 1H and 13C

Large chemical shift range than 1H

no. of C atoms due to longer T1and NOE For a given D solvent the multiplicities of

solvent si nal differ in 13C and 1H. E.g. CDCl3

started in 1970s. Why?

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Carbon-13 NMR Carbon-12 (12C) has no magnetic spin. - =

1/2), but is only 1.1% of the carbon in a. ~ .

1

H; the overall sensitivity is ~ 1/5700. v ous y tec n que s not use u Larger sample and longer time required For carbon a technique called Fourier

trans orm s ectrosco is used.

8

Complements 1H NMR.

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Fourier Trans orm NMR Radio-frequency pulse given.

uc e a sor energy an precess sp nlike little tops.

A i i has the nuclei lose energy.

Free induction decay (FID) is converted to.

9

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Scale difference You will notice at once that the scale is

1 ,

about 10 p.p.m. instead of the 200 p.p.m.

This is because the variation in the chemical

nucleus by the electrons around it.

ere ev a y e c a ge o ethe distribution of two electrons around ay roge uc eu a a o e e g

valence electrons around a carbon nucleus.

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Carbon

Home work

Chemical

Table of approximate chemical shifts values for 13C-NMR. Most of these values for a carbon atom are

11

about 1520 times the chemical shift of a proton if

it were bonded to the carbon atom. Some deviations.

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13C chemical shift ranges

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Standard 13C chemical shift values

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-

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S in-S in S littin It is unlikely that a 13C would be adjacent13 . ,

splitting by carbon is negligible.

protons and adjacent protons. arge CH va ues or - - z ,& 2JCH 3JCH values for 13C-C-1H & 13C-C-C-1H (0-60 Hz) couplings (1H coupled spectra)

These com lex s littin atterns with

16

great overlaps are difficult to interpret.

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Proton Spin

To simplify the spectrum, a technique called as

proton broadband decoupling is used. Protons are continuously irradiated over aroa requency range, so t ey are rapi y

flipping and are not able to couple. e car on nuc e ee an average o a e

possible proton spin states.

,single, unsplit peak because carbon-hydrogens littin was eliminated.

18

Cholesterol gives 27 singlets in 1H decoupled13C spectrum. Compare the simplicity.

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Differences Between 1H and13

C Techni ue The gyromagnetic ratio of 13C is one-fourth of thatof 1H. So, resonance frequency is about one-fourth

that of hydrogen, 15.1 MHz instead of 60 MHz.75.6 instead of 300. ea areas are not proport ona to num er o

carbons.- 1 .

of Carbon. If T1 is short quat C are missed!

Carbon atoms with more hydrogens absorb morei h Th

19

enhancement for quaternary C is practically zero!

Intensit increase is a nonlinear fn of no. of Hs

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decoupling

Considerable time saving than continuous broadbanddecoupling (in which decoupler is always on) s emp oye o re a n a eas par o an s

maintain CH coupling The broadband H decou er is ated switched on

during the relaxation delay process and gated offduring the brief acquisition period.

us s ow process w c as u up ur ngthe lengthy delay period, decays only partially duringthe brief ac uisition eriod.

Coupling (fast process) is immediately established andremains throughout the acquisition.

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-

Increase in s character u s u on w e- w raw ng group

With angular distortion

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This may be observed in 1H decoupled spectra

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SolventsIn1HNMR CDCl sin let at 7.26 arises

In13CNMR

There is no uestion of this

due

to

CHCl3impurity.

peak

in13

C

as

all

the

protonsaresaturated

DMSOd6(CD3SOCD3)gives

aquintetat2.49dueto

CDC 3givesatrip etat77.0

(32Hz)in13C(2x1x1+1=3)

couplingwithD(2nI+1=5)

39.7(21Hz)by2x3x1+1=7

D analog of

Pascal trian le for H

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CDCl In13

C D has spin = 1

= 1, 2nI+1 = 3), where the spins have quantumnos of -1 0 +1 In CDCl3 solution, the molecule can have D

with an one of the three s in states and allare equally probable. The triplet results from splitting of 13C peak

by D Therefore we see three different chemical

s i ts or t e atom in D 3 ratio 1:1:1

13C-D coupling constant is 32 Hz

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Interpretation of 13C

Diethyl phthalate

decoupled

coupled

2J couplings

2J and 3J couplings

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decoupled spectra because.

may not return to the eqm boltzmann .

the signal do not achieve full amplitude

. e var es amongs nuc e anso signal intensities vary.

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Inverse Gated

pro on ecoup ng

This technique is used for quantitative analysis of13

C H Decoupler is gated off at the beginning of there axa on e ay ga e on a e s ar o acqu s on

Thus, coupling (fast process) is not allowed in acquisition

of acquisition & a low level of NOE is thus maintained We get a 13C spectrum consisting of singlets whose

intensity corresponds to the no. of 13C it represents. By using long relaxation delays Rd > 5T1 and by using

.

I3

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INV

3

RSE

1 2

GATED

H

DE

OUP

I

NG

13

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Some13Cspectra

Look for correct number of C atoms

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13 1

33

Pyrolle-2-carboxaldehyde

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1H and 13C-NMR of 1,2,2-

r c oropropane

34

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A C H O com ound has stron infrared

absorption at 3300 to 3400 cm-1

The1

HNMR s ectrum has three sin lets at 0.9 , 3.45 and 3.2; relative areas

3:2:1. The 13C NMR s ectrum showsthree signals all at higher field than 100m. Su est a structure for this

compound.

(CH3)2C(CH2OH)2

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A C H O com ound has a stron

infrared absorption at 1150 cm-1

, but noabsor tion at 3300 to 3400 cm-1. It's1H NMR spectrum shows a singlet at

3.55 m. The 13C NMR s ectrum showsone signal at 66.5 ppm. Suggest astructure for this com ound.

, - oxane

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A C H N com ound shows infrared

absorption at 3300 cm-1

. It's1

H NMRs ectrum has three si nals: sin lets at1.0 (6H), 1.4 (1H) and 2.7 (4H) ppm. The

13C NMR s ectrum has three si nals, allat fields higher than 100 ppm. Suggest astructure for this com ound.

(CH3)2C(CH2)2NH 4 membered cyclic