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11
NUCLEAR MAGNETIC RESONANCE IINUCLEAR MAGNETIC RESONANCE II
Instrumental AnalysisInstrumental Analysis
Dr. Nermin Salah
12th Lecture
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#
C H
Z
This represents the electron density of a C-H bond. How much
electrondensity is on the proton depends on what else is attached
to the carbon. If Zis an elelctronegative atom, the carbon becomes
electron deficient and pullssome of the electron density away from
the H. if Z is an electron donatinggroup, more electron density
ends up on the H.
+he more electron density around a proton, the more the
shield, the lower magnetic field affecting the proton
,
+he less electron density , the less the shield , the
higher magnetic field)
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BBefectiveefective = B= Bappliedapplied
− − B Blocallocal
T$e e%ective ma&netic 'el( T$e e%ective
ma&netic 'el() t$ere*+re) ,$at t$e $y(r+&en) t$ere*+re)
,$at t$e $y(r+&en
nuclei actuallynuclei actually -sense.-sense. t$r+u&$
t$e surr+un(in& electr+nict$r+u&$ t$e surr+un(in&
electr+nic
envir+nment is al,aysenvir+nment is al,ays less t$anless
t$an t$et$e actual a//lie( ma&neticactual a//lie(
ma&netic'el( 0'el( 011
S$iel(e( 3r+t+ns
In t$e classical NMR e4/eriment)In t$e classical NMR
e4/eriment) ma&netic 'el( stren&t$ must bema&netic
'el( stren&t$ must beincrease( *+r a s$iel(e( /r+t+n t+ 5i/ at
t$e same *re6uencyincrease( *+r a s$iel(e( /r+t+n t+ 5i/ at t$e
same *re6uency
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.
L+, e4ternal a//lie( 'el(L+, e4ternal a//lie( 'el( !i&$er
e4ternal a//lie( 'el(!i&$er e4ternal a//lie( 'el(
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/
3r+t+ns in a m+lecule
L+,er *re6uencyL+,er *re6uency
$i&$er *re6uency$i&$er *re6uency
(+,n'el((+,n'el(
u/'el(u/'el(
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chemical shift
=δ
=shift in Hz
spectrometer frequency in MHz= ppm
+his division gives a number independentof the instrument
used)
parts per
million
T$e C!EMICAL S!I7T T$e C!EMICAL S!I7T+he 0chemical
shift1 is a field independent value.
Chemical shift is 2 the difference in frequency beteen the
sample and
the standard over the operation frequency.
A particular proton in a given molecule ill alays come at
the same
chemical shift (constant value")
3f course, e don4t do any of this, it4s all done automatically
by the
%M' machine)
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5
Tetramet$ylsilane 8TMS Tetramet$ylsilane 8TMS
Si
C6
C6
C6
6C
+MS is added to the sample as internal standard)
+MS protons are all identical, highly shielded providing a
single
sharp pea7 alays isolated from pea7s of interest) +he +MS as
assigned δ = 0.00)
3rganic protons absorb donfield (to the left" of the +MS
signal)
+MS is inert , highly soluble in organic li$uids and
easily removed
from samples by distillation)
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89
7ACTORS A77ECTING C!EMICAL S!I7T7ACTORS A77ECTING C!EMICAL
S!I7T
7+ur maj+r *act+rs acc+unt *+r t$e res+nance/+siti+ns 8+n
t$e //m scale +* m+st /r+t+ns
9 "es$iel(in& by electr+ne&ative elements
In(uctive e%ect by electr+ne&ative &r+u/s
2 s:c$aracter 8$ybri(i;ati+n e%ect
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88
Chlorine “deshields” the proton,
that is, it takes valence electron
density away from carbon, which
in turn takes more density from
hydrogen deshielding the proton.
0highly shielded1
protons appear at upfield
(loerδ
"
0deshielded0
protons appearat donfield
(higherδ
"
(es$iel(in& m+ves /r+t+nres+nance t+ l+,er 'el( an(
$i&$er δ
NMR C!ART
C !Cl
electr+ne&ativeelement
δ: δ
δ: δ
9 "ES!IEL"ING 0= ELECTRONEGATI>E9 "ES!IEL"ING 0=
ELECTRONEGATI>E
ELEMENTSELEMENTS
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8:
C+m/+un( C!
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86
C!Cl< C!2Cl2 C!
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8;
2 s:C!ARACTER2 s:C!ARACTER
8!=0RI"IFTION O7 CAR0ON ATOM8!=0RI"IFTION O7 CAR0ON ATOM
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8
As s-character +* carb+n at+m increases) t$e electr+nic
cl+u( is$el( more closer t+ t$e carb+n an( /r+vi(es less electr+n
(ensity*+r s$iel(in& +* /r+t+ns) an( t$us t$e c$emical s$i*t)
δ) increases8s$i*te( (+,n'el(
Acc+r(in& t+ t$e ab+ve reas+nin&) t$e *+ll+,in&
tren( *+r t$ec$emical s$i*t is e4/ecte(
T$is su&&este( t$at +t$er *act+rs t$an t$e s/
c$aracter +*
carb+n mi&$t a%ect t$e c$emical s$i*t in t$is case T$e
ab+ve
(iscre/ancy can be e4/laine( by ,$at is calle(
Ma&neticAnis+tr+ ic E%ect
expected to be observedat δ > 7
But in fact, we have the following order: actually observedat
δ = 2-
(down field - higher δ
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8.
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8/
!romatic protons
δ " #-$ ppm
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8<
%inyl (&lefinic protons,
δ " '- ppm
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85
!cetylene protons
δ ̃≈ ).' ppm
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:9
Electronegative
oxygen atom
!ldehyde proton
δ" *-+ ppm
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:8
!="ROGEN 0ON"ING "ES!IEL"S 3ROTONS!="ROGEN 0ON"ING "ES!IEL"S
3ROTONS T$e c$emical s$i*t (e/en(s+n $+, muc$ $y(r+&en
b+n(in&is taHin& /lace 8+bserve( in $i&$
c+ncentrate( s+luti+ns
Alc+$+ls vary in c$emical s$i*t*r+m 1@ //m 8*ree O! t+ ab+ut
@1 //m 8l+ts +* ! b+n(in&
!y(r+&en b+n(in& len&t$ens t$e
O:! b+n( an( re(uces t$e valence
electr+n (ensity ar+un( t$e /r+t+n
it is (es$iel(e( an( s$i*te((+,n'el( in t$e NMR s/ectrum
&
&
&
;) =*'3GE% #3%*I%G;) =*'3GE% #3%*I%G
3- and %- Signals3- and %- Signals
/)&-e0changeable (peak for & proton
in alcohol and 1 in amines disappears
upon shaking with /)&
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::
Carb+4ylic aci(s $ave str+n&
$y(r+&en b+n(in& : t$ey*+rm (imers
&
&&
-
C-2
In met$yl salicylate) ,$ic$ $as str+n&
internal $y(r+&en b+n(in&) t$e NMRabs+r/ti+n *+r O:! is
at ab+ut 9# //m)8$i&$ly (+,n'el(8
N+tice t$at a stable B:membere( rin& is *+rme(
SOME MORE E?TREME E?AM3LESSOME MORE E?TREME E?AM3LES
Res+nance) electr+ne&ativity +* +4y&en an( t$e *+rmati+n
+*$y(r+&en b+n(in& ,it$(ra, electr+n cl+u( *r+m t$e
aci(/r+t+ns
T$us) /r+t+ns attac$e( t+ carb+4ylic aci(s are t$e
leasts$iel(e( /r+t+ns an( $ave a c$emical s$i*t +*
10-12 //m
!-
!"!-
!"!-
!-
-#
(
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:6
3ROTON INELECTRON:3OOREN>IRONMENTS
"ES!IEL"E""ON7IEL"!IG! 7REJUENC=
LARGEδ
0 - 5
3 - 5
1.5 - 3
0 – 24.5 - 7
92 99 91 D B @ # < 2 9 1
:O! :N!
CKC
!
TMS
δ
8//m
$M% &'%%()*+#'$ &H*%+ $M% &'%%()*+#'$
&H*%+
3ROTON INELECTRON:RIC!EN>IRONMENTS
S!IEL"E"U37IEL"LO 7REJUENC=
SMALLδ
9-10
RC!O
aldehde
10-12
RCOO!
acid
7-!
-C!Cl< )
aro"atic
olefinic#$2
C:C!:C
C
C:C!2:C
C:C!<
#$3
C!27
C!2Cl
C!20r
C!2I
C!2O
C!2NO2#$3 #$
C!2Ar
C!2NR2C!2S
C C:!CKC:C!2
C!2:C:O#$3
C$emical s$i*t &ives t$e electr+nic envir+nment +*
/r+t+ns
8S$iel(in& an( "es$iel(in&
di#a$$ear# u$on #ha%ingwith &2'
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:;
EQUI!LEN" !ND N#NEQUI!LEN"$%#"#NSAll +* t$e /r+t+ns in a
m+lecule ,$ic$ are in
c$emically i(entical envir+nments ,ill +*ten
e4$ibits t$e same c$emical s$i*t ie) s$+,s
+ne si&nal in NMR s/ectrum at t$e same
value +* δ.
+he protons in this case are said to be+he protons in this case
are said to be
chemically e$uivalentchemically e$uivalent
On t$e +t$er $an() m+lecules ,$ic$
$ave sets +* /r+t+ns ,$ic$ are
c$emically (istinct 8$ave (i%erentc$emical
envir+nments *r+m +ne
an+t$er &ive rise t+ (i%erent
abs+r/ti+n si&nals *r+m eac$ +t$er
Chemically none$uivalent protonsChemically none$uivalent
protons
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:
Inte&rati+n K (eterminati+n +* t$e area un(er a
/eaH
INTEGRATION O7 A 3EA INTEGRATION O7 A 3EA
N+t +nly (+es eac$ (i%erent ty/e +* $y(r+&en &ive
a(istinct /eaH in t$e NMR s/ectrum) but ,e can als+ tellt$e
relative numbers +* eac$ ty/e +* $y(r+&en by a
/r+cess calle( inte&rati+n
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:/
NMR s/ectrum can reveal t$e number +* /r+t+ns assi&ne(
*+r eac$si&nal
Inte ral lines n+ +* r+t+ns in eac$ si nal
M+(ern s/ectr+metersaut+matically /rint +ut t$einte&rals as
numbers +nt$e s/ectrum
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:<
S3IN:S3IN S3LITTINGS3IN:S3IN S3LITTING
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:5
O*ten a &r+u/ +* $y(r+&ens ,ill a//ear as a
multi/letrat$er t$an as a sin&le /eaH
Multi/lets are name( as *+ll+,s
Sin&let 8s Juintet 86uin"+ublet 8( Si4tet 8si4
Tri/let 8t Se/tet 8se/tJuartet 86 Multi/let 8m
T$is $a//ens because +* interacti+n ,it$
nei&$b+rin&$y(r+&ens an( is calle(
S3IN:S3IN S3LITTING
S/in:s/in c+u/lin&
S&I%-S&I% S&>I++I%GS&I%-S&I%
S&>I++I%G
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Chapter 86 69
%one$uivalent protons on ad?acent carbons,
+,+,)-3ribromoethane
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68
T!E ORIGIN O7 T!E ORIGIN O7
S3IN:S3IN S3LITTINGS3IN:S3IN S3LITTING
!O IT !A33ENS
S/in:s/in s/littin& arises because $y(r+&en +n
a(jacent carb+n at+ms can -sense. +nean+t$er
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Chapter 86 6:
/oublet
+ ad4acent proton
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Chapter 86 66
3riplet
) !d4acent 5rotons
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6;
C C
! !
!
C C
! !
!
t,+ nei&$b+rs
n9 K <tri/let
+ne nei&$b+rn9 K 2(+ublet
sin&let(+ublettri/let
6uartet6uintetsi4tetse/tet
MULTI3LETSt$is $y(r+&ens /eaHis s/lit by its t,+
nei&$b+rs
t$ese $y(r+&ens ares/lit by t$eir sin&le
nei&$b+r
@here n is the number of 6789%!:613 protons on
Ad?acent carbon atoms
n 9n 9
RULERULE
E?CE3TIONS TO T!E n9 RULEE?CE3TIONS TO T!E n9 RULE
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6
E?CE3TIONS TO T!E n9 RULEE?CE3TIONS TO T!E n9 RULEIM3ORTANT
3r+t+ns t$at are e6uivalent by symmetry usually (+ n+t s/lit+ne
an+t$er
C ! C !? = C!2 C!2? =
n+ s/littin& i* 4 K y s/littin& i* 4 ≠ yt,+
tri/lets
9
2 3r+t+ns in t$e same &r+u/ usually (+ n+t s/lit +ne
an+t$er
C
-
-
- +r C
-
-
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6.
3ASCALS TRIANGLE3ASCALS TRIANGLE
(+ublet
tri/let
sin&let9
9 9
9 299 < < 9 6uartet
Intensities +* multi/let /eaHs
INTENSITIES O7 MULTI3LET 3EASINTENSITIES O7 MULTI3LET 3EAS
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6/
NMR s/ectrum in(icates t$e carb+n sHelet+n
S/in:s/in s/littin& &ives t$e number +*
e6uivalent/r+t+ns +n a(jacent carb+n at+ms
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6<
IN7ORMATION E CAN GET 7ROM NMR S3ECTRUM
Number +* 9!:NMR si&nals K Num'er o( )in*s o(+rotons.
C$emical s$i*t &ives t$e electronic
environment +*/r+t+ns
8S$iel(in& an( "es$iel(in&
NMR s/ectrum can reveal t$e num'er o( +rotons assi&ne(
*+r eac$ si&nal 8inte&ral lines n+ +* /r+t+ns in
eac$ si&nal
NMR s/ectrum in(icates t$e car'on
s)eleton S/in:s/ins/littin& &ives t$e number +*
e6uivalent /r+t+ns +na(jacent carb+n at+ms
Summary
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65
Ty/ical >alues Ty/ical >alues
ote that the#e are t$ical value# and that thereare lot# of
ece$tion#
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%esources an* re(erences
;9
"e,t'oo) 3rinci/les +* Instrumental Analysis)
Skoog, Holler, Nieman %ecommen*e* (urther rea*in-3rinci/les
+* instrumental analysis) @t$ e( by
Skoog, Holler, Nieman. C$a/ter 9D
E4tra res+urces are available +n t$e intranet
%elevant /e' sites
htt+///.chemui*e.co.u)analsismenu.html
http://www.chemguide.co.uk/analysismenu.htmlhttp://www.chemguide.co.uk/analysismenu.htmlhttp://www.chemguide.co.uk/analysismenu.htmlhttp://www.chemguide.co.uk/analysismenu.html
