8/10/2019 L6 13C NMR
1/37
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
8/10/2019 L6 13C NMR
2/37
8/10/2019 L6 13C NMR
3/37
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
8/10/2019 L6 13C NMR
4/37
NOE difference s ectra
4
8/10/2019 L6 13C NMR
5/37
pec roscopy
Suvarn S. Kulkarni
Department of Chemistry
CH 521 Lecture-6
8/10/2019 L6 13C NMR
6/37
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
8/10/2019 L6 13C NMR
7/37
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?
8/10/2019 L6 13C NMR
8/37
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.
8/10/2019 L6 13C NMR
9/37
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
8/10/2019 L6 13C NMR
10/37
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.
8/10/2019 L6 13C NMR
11/37
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.
8/10/2019 L6 13C NMR
12/37
13C chemical shift ranges
8/10/2019 L6 13C NMR
13/37
Standard 13C chemical shift values
8/10/2019 L6 13C NMR
14/37
8/10/2019 L6 13C NMR
15/37
-
8/10/2019 L6 13C NMR
16/37
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.
8/10/2019 L6 13C NMR
17/37
8/10/2019 L6 13C NMR
18/37
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.
8/10/2019 L6 13C NMR
19/37
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
8/10/2019 L6 13C NMR
20/37
8/10/2019 L6 13C NMR
21/37
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.
8/10/2019 L6 13C NMR
22/37
8/10/2019 L6 13C NMR
23/37
-
Increase in s character u s u on w e- w raw ng group
With angular distortion
8/10/2019 L6 13C NMR
24/37
8/10/2019 L6 13C NMR
25/37
This may be observed in 1H decoupled spectra
8/10/2019 L6 13C NMR
26/37
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
8/10/2019 L6 13C NMR
27/37
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
8/10/2019 L6 13C NMR
28/37
Interpretation of 13C
Diethyl phthalate
decoupled
coupled
2J couplings
2J and 3J couplings
8/10/2019 L6 13C NMR
29/37
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.
8/10/2019 L6 13C NMR
30/37
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
8/10/2019 L6 13C NMR
31/37
INV
3
RSE
1 2
GATED
H
DE
OUP
I
NG
13
8/10/2019 L6 13C NMR
32/37
Some13Cspectra
Look for correct number of C atoms
8/10/2019 L6 13C NMR
33/37
13 1
33
Pyrolle-2-carboxaldehyde
8/10/2019 L6 13C NMR
34/37
1H and 13C-NMR of 1,2,2-
r c oropropane
34
8/10/2019 L6 13C NMR
35/37
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
8/10/2019 L6 13C NMR
36/37
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
8/10/2019 L6 13C NMR
37/37
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