J. A. D. Jeffreys Department of Pure and Applied Chemistry University of Strathclyde Glasgow G I iXL, Scotland
A Rapid Method for Estimating NMR Shifts for Protons Attached to Carbon
The chemical shift of a proton bonded to carbon depends on its environment, and the idea that the value of the shift is the sum of the contributions along the other three bonds to the carbon, together with the effects of nearby unsaturation, is not new.' Previous lists of deshielding constants have concentrated on precision. The values presented here are chosen to he easy to remember and to give estimates close enough to measured shifts to allow interpretation of the spectrum of a compound of known structure. Agreement with observed values is generally within 0.3 8 for molecules which are flexible; however, the value predicted for an aldehydic proton is too low by an average of 1.3 6 . The chemical shifts derived using Table 1can be improved by extensions beyond simple summation that can he taught to advanced classes but may not be suitable for elementary ones. The extensions are considered later. Construction of Table 1
T h e elements carbon, nitrogen, oxygen, and fluorine are written in order of increasing electronegativity. The value for carbon is set to 0.7, and for each succeeding group of the Periodic Table the figure is raised by 1.0; the value for boron, -0.3, follows by extension. Elements of the second and later periods are off-set by one group, so that silicon and tin take the same value as boron, phosphorus the same as carbon, and so on. By convention, the shift for hydrogen in tetramethyl silane is zero when the value for hydrogen (0.15) is, after rounding, 0.2. The value for a benzene ring is chosen to fit the observed shift, and it holds, too, for pyridine and the diazines, but not 'Jackman, L. M., "Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry," Pergsmon Press, New York, 1959, p. 59: quoting Shoolery's Constants. See also, Emsley, J. W., Feeney, J., and Sutcliffe,L. H., "High Resolution Nuclear Magnetic Resonance Spectroscopy," Pergamon Press, Oxford, 1966, Chapter 10; O'Reilly, D. E. in "Progress in Nuclear Magnetic Resonance Speetmscopy," (Editors:Emsley, J. W., Feeney, J., and Suteliffe, L. H.), Pergamon Press, Oxford, 1978, Vol. 2, pp. 54-59.
806 / Journal of Chemical Education
Table 1. Deshlelding Values Which Allow Interpretation of the Spectrum of a Cpd. of Known Structure. Deshielding Values (Positive unless otherwise shown) (a)
B, Si, Sn H C. P N 9 0, CI. Br, i
.
F (b)
(c)
-0.3 0.2 0.7 1.7
2.7 3.7
Double bond
If the carbon atom participates If one carbon atom intervenes Benzene ring: also, pyridine, and diazines If the carbon atom participates If one carbon mtervenes Additional values, more subject to steric effects
than those above Triple band If the carbon atom participates If one carbon intervenes Four-membered ring Three-membered rlng
for the five-membered heterocycles. The value for a double hond was derived independently; it happens to equal that for benzene X 0.7. At one atom removed their effects have fallen to one fifth, and the figures are rounded off for Table 1. Further away their effects are zero, except for the cases described in the extension. The deshielding effect of a triple hond on an acetylenic proton is half that of a double bond on an ethylenic proton; hut a t one atom removed the effects of a triple bond and of a double bond are the same. The magnitudes of the effects of a four- and of a three-membered ring are the same as that for carbon, though for the two ring sizes the signs are different. Estimation of the Chemical Shift
Within a saturated compound, the 8-value for a proton is the sum of the deshielding values of the other three substituents on the carbon atom. Taking as example either of the
T a b l e 2. L a b e l l e d Protons w i t h i n S e l e c t e d Compounds. (1)
CH,OH
Methanol
(2)
Ethanol
CHGH OH la1 ibi
(a)
ICH,), SiCH2CH:CH.S0~! Nal+l la1 tb) IcI (dl (3)
T a b l e 3. C a l c u l a t e d a n d O b s e r v e d C h e m i c a l Shifts f o r t h e C o m p o u n d s o f T a b l e 2. Com~ound
ProtanW
Contributors to Chemical Shift
Chemical Shift (6)' Cab. Observede Notes
-
3.6 1.2 3.7 00 0.6 1.9 2.8 0.1 2.1 3.0 3.3 5.7 and 5.8 1.2 2.6 6.0 6.2 6.7 2.3 7.2 6.3
lCH,hS" la1
Sodium 2.2-Dimethyl-2-silapentane sulphonate
(4)
Teaamethyl tin
H i a i HY('*CH id) I I (bl HkN,Ct1, rcl
CHCH,
CHGH? ial
(7)
(9)
H
la1 t b )
H
IC.HJ&HBr
ih)
la)
Toluene
(8)
1
k
ldi
lcl
Oiphenylmethyi bromide
IC=CCH OH lh)
la1
1-Propyi-4-pyridyl thioether
(lo)
BPropyne-1-01
In)HIC-(Ti
I I~ , h i Hz('-0
CH0CH;CH:CN ihl
(el l a )
(1 1) 6-Methoxy propionitrile
(12)
ihl
Trimethylene oxide
C. C, C, bz. R3 C. C. BZ
protons on C ( l ) of ethanol (Tahle 2, Compound 2, protons (b)), the bonding, and the sum can be written:
..f
"'
1.1 1.7 2.9 7.1 8.4 2.5 4.3 2.6 3.4 3.6 2.7 4.7 0.7. 0.7 1.8 7.1 1.6
' In CDCI. signals (Ib~erved.but assignment leR open. b Average value: range. *003 6. Other signals not analyzed. d T s14nals ~ ~observed due to non-eauivalentoratonr 'TWO
C 0.7 Carbon ofthe methyl group
1) 2.7 Oxyeenofthe hydmrylgroup
Estimated shift for the starred proton = 3.6 For alkenes and arenes, the chemical shift of a proton on an unsaturated carbon atom is the sum of the contributions from the other two substituents, plus the effect of the a-electrons a t this carbon. Choosing toluene (Tahle 2, Compound 7) as an example, the chemical shift for the proton on a carbon atom of the ring (class (h) of this compound) is estimated thus
*H+--H
r-H 0.2 0.2
L-c RZ
6.0 Effect ofthe deiacalized r-electrons
Estimated shift for the starred proton = 7.4 The deshielding effect of a-electrons has a range greater than that of a saturated atom. Thus, for the protons of the methyl group of toluene (class (a) of this compound), the bonding diagram requires an additional note: *H-C-H
rH L C
This carbon atom is part of a benzene ring whose a-electrons contribute to the deshielding of the starred proton. The estimation of the deshielding requires an additional term:
0.7
LIZ 1.2 Effect o f l h e nearby bcnsene ring
Estimated shift for the starred proton = 2.3 Acetylenes excepted, there is a minimum of three contributions for the estimated chemical shift; from three suhstituents a t a saturated carhon atom, or from two suhstituents and immediate unsaturation. In all cases the effect of nearby unsaturation should he added. Note that nitroso- and nitrogroups contain a double bond, but N-oxides do not. Table 2 shows a selection of comoounds. while Tahle 3 lists the contributions to the chemical shift for each lahelled proton and comoares the values derived usinr the data in Tahle 1with the observed values. Volume 56. Number 12, December 1979 / 807
Table 4. Labelled Protons within Selected Compounds showing the Use of the Extensions. Part (1):
N,N-Dimethylacetamide lIe I
Id1
%H
(16) Ethyl nitrite ("I
(h)
-
I I
H I 0
CH, * I : la1
(ill ,el (61 1-Furyl-2-pyrrolyimethane
Part (ii):
I I
CHI-N H
(17)
OCH,
H (fl
(hl (18) Kheilin
Effect of accumulation of hetero-atoms (oxygen). ICI
/Cl
H
H
' , k c '
(b)
\CH, ls%l
/
l n l HC
(C)
I
I
0
(d)
7
\
CH,
CH.CHC(CH.O)&H la1 tb) lcl Id1 (20) 1,l.l-Trishydroxymethylpropane orthoformate ester
(111
(19)
4.6-Dioxacycloheplene
Part (iii):
,~,
(b)
(4 (d)
Effect of substituents on a proton attached to a bemenoid ring, or to an alkene
(el (f) 236
(21)
Pr* ton($
CHCH.ONO la) Ih)
CH.,CNICHI, lal lhl (15)
Compound
Transmission of the effects db and b r across a nitrogen or oxygen atom. 0
II
Table 5. Calculated and Observed Chemical Shins tor the Cornoounds of Table 4.
H
Id)
Q
H
(a) (b)
24
(a) (b) (c) (dl (e) (f) (g)
lhl ( e l pCyanamethyl nitrobenzene 25
(a) (b) (C)
26' (23)
Cytosine
(24)
Furoin 27
Part (iv):
Selected compounds for which more than one of the extensions apply.
.., "I?
(dl (e) (f) (a) (b) (c) (a) (bl
(c) (d) (e) (f) (g)
.9 2as
(a) (b)
lhl (25)
Procaine
(26)
4-Methoxy-3-pymmlin-2-0ne
(C)
(dl
(e) (f) (g)
(27)
Safrale
(28)
2.4-Dinitrophenylhydrazone of ethyl 4.4difluoro-3dxobulanoate
808 1 Journal of Chemical Education
Contributors to Chemical Shin
H. H. N. (db) H, H. C H. C. 0. (dbl H. c. N, db. (db. db) ' C, C, DB, db C. C, DB, db C, C, DB, db C, N. DB C, 0, DB H. H. C, db H. H. 0. ( b 4 H. H, 0. (bz) C, C, DB, db C. C, DB, b r C, 0.DB H, C, 0, db H, 0. 0 (as N) C. C. DB H. H. C H, C. C H, C. 0 0 , 0 (as N). 0 (as C)
Chemical Shift (6)' Caic. Observed2
2.9 1.1 4.4 5.0 6.4 6.4 6.4 6.6 7.6 1.9 4.3 4.3 6.4 6.6 7.6 4.4 4.6 5.6 1.1 1.6 3.6 5.1
Notes
1.2 3.9 5.5
. . H. H. C. bZ C, C. C. bz C, C. BZ, - 0 C, C. BZ, - p C, C, BZ. -m C, C. BZ. -0, - p C, N, BZ, -m, -m C. C. 0. db, db C. C. DB, db C, C, DB, db C, C. DB, db C, C. DB. db. +o C, 0, DB C. 0.DB H. H, C H. C. N H. C. N H. C. 0. (db) C. C. BZ, -0. +m C. C. BZ, +o, -m H. H, 0, (db) H. C. N. db, (dbl C. C, DB. db. - 0 H. C. C, db, bz H. C, DB
2.3 3.3 6.7 7.0 7.3 6.3 6.2 5.7 6.4 6.4 6.4 7.2 7.6 7.6 1.1 2.6 2.6 4.4 6.8 6.0 3.9 4.2 5.7 3.6 5.1
C, C, DB H, 0, 0 (as N), (bz) C. C, BZ. -0, -m C, C, BZ, - 0 , - m C, C. BZ, -m. - p H. H. C H. C. C. db. db H, C. 0. (db) C, F, F (as N), db C. C. BZ. +m, +m C. C. BZ, +o, + p C. C. BZ. +o. +o
5.6 5.6
2.3 3.2 6.6 6.7 7.1 5.9 7.7 5.8 6.4 6.4 6.6 7.3 7.4 7.6 1.1 2.6 2.6 4.1 6.6 7.8 3.6 3.9 5.1 3.3 5.0. 5.0 5.9 5.9
0
6.7 6.9 1.1 3.2 4.4 6.9 7.6 8.5 8.8
1.3 3.7 4.3 6.3 8.0 8.4 9.1
In CDClsunless othewise stated. Mmpoued is not Mmed in me index in reference 2:Suchmmmuedn can be lacatedmvs: (NO. in mis paper). NO. inreference2417). 525: (20).493; (2%. 10s: (28). 293 oo from the 1.r.n. ,m om from me wrroe r.ng AS me tho & m e oonor n me snor rang are mdroen3en'. their e l f e ~ b a r e revarrla , lranrmmaa arrorr ,he nlrogon alam m r o - n m ar oxacnmman m w r r r a cnroonotom.so I:% ~.wm.calsn s ir not allartea by menearby benzene ring. u m e m e cases of me protons~b) a~ (clanachedto saturated c s m n atom*. 'In alkaline D20. '+oCom the ketone function. 'TWOsignals abrervsd due to non-equivalem protons. 9 Almough the carbon atom to which the* protons are bonded is linked by two paths to the ring, as there is only one benzene ring involved it is counted only once. Avetaee value: range. +0.03 6.
Table 6. The Precision of the Method Error
la1 x l o
Number of Signals 94 175 158 101 64 Afi
Cumulative Percentage of Examples Included 13 36 57 71 79 85
and for these two the rule is that the first atom has its full weight; the second atom counts as nitrogen; the third atom counts as carhon; giving, in summary, twu oxygen atoms contribute 4.4, three, 5.1; two fluorine atoms contribute 5.4, three. - - ~ 6.1. ~ (3) Suhstituents in derivatives of benzene and similar rings mav nroduce chanees. with both magnitude and sirn, in the chemical shift of protons on other carbon atoms o f t h e ring; alkenes are considered a t the end of this paragraph.
~. ~. . ~
~
~
The magnitude, other than zero, depends only on the location of the substituent:
The sign depends on the nature of the group. If the atom next the ring has a positive charge, whether real as, ex., -NH:r (+I, or-NO.,. -.or furrnal as., e.e.. .,. -CN. C O - X . -CX=NY (X. Y = anything), the sign is positive; if the atom attached to the ring is oxygen or nitrogen that can be directly protonated, e.g. -NH2, or -OCH:], the sign is negative. For all other groups the effect is zero. Note that amides and esters are urotonated on the oxygen of the carbonyl group, so that the acetoxyeroun. .. for examole. has zero effect. The effects of several substituents are additive. For alkenes, only a suhstituent Zto a hvdroren atom counts, and the effect is that appropriate to the-01th;-position in a derivative of henzene; more distant suhstituents in a cnniugated system are ignored. In some cases a hetero-suhstituen
