A Short Set of 13C-NMR D. W. Brown University of Bath. Bath, BA2 7AY, Avon. England
Many student textbooks in discussion of 13C-NMR spectrosconv stress the ereat value of the method that results from the se&tivity of t& chemical shift value to subtle alterations in environment of the individual '3C nuclei giving the familiar wide spread of resonance of over 200 ppm. This is usually followed by a short discussion of the relationship between 6 values and (1) mode of hybridization, (2) inductive effeds, (3) mesomeric effect and then by provision of tahles giving broad ranges of values for 13Catoms in various environments. The tables are of limited value for structural assimments, allowine unambiguous assignments of signals to be mide onGfor fairl; simple molecules having each nucleus in a very different environment from the others. On the other hand, there are a large number of specialist textbooks and original papers that have very detailed sets of correlation tables. Here different sets of additivity constants are defined and used to calculate the chemical shift of a given nucleus by reference to the compound type, the proximity of different functional groups, and bv takine into account a number of steric effects. Attemp& to refLe these calculations lead to the development of more sets of additivitv constants and different correction factors applicable to more closely defined chemical systems. 'I'his results in a need fur a verv. larre numlter of taltlcs of constants and correction factors, so that the whole operation becomes unwieldv and unmanaeeahle in the context of undergraduate tutohal work. ~ t u & n ttexts theretore pay less attention to I"('spectrusropv than 'H-NMK sDectroscoDv. An exception to this is the excellent Open university text in "The Nature of Chemistry, S304," course where, since the spectra are usually less complicated than proton spectra, magnetic resonance spectroscupy is introduced by treating proton dentupled 'JCNMR spectroscopy. However, the space devoted to the calculation of 6 VRIIIPS is necessarily limited and as two different bases for calculation are described this treatment can cause confusion to students. Second-year chemistry students at the University of Bath are given the following set of correlation charts during a nine-hour introductory course on NMR spectroscopy. The students have usually mastered the use of these tables after solving one or two problems and discussing the solutions in
-
a l-hr tutorial session. The charts are of narticular use to undergraduates in their final-year research projects and to new postgraduates from other universitv de~artments.The obj&t of these tables is to enable a student to Ealcu~aterapidly approximate 6 values for 13C nuclei in as wide a varietv of compoun(lias~tossible.If the agreement with mrasured v a h is within 2-3 ppm, then the authur reaards this as verv sstisfactory. Wider deviations are generaily due t o the a;thor's approximations of published additivity rules. The more crowded is the observed carbon nucleus and the more closely are placed the functional groups, the less good is the agreement with the observed value. Most of the credit for the contents of this article must go to the authors of "Tabellen zur Strukturaufklarung Organischer Verbindungen mit Spektroskopischen Methoden" ( I ) and "13C-NMR Spektroskopie in der Organischen Chemie" (2) and to those cited in these hooks. Tables not derived from the above texts have been constructed from the data ~ ~ contained ~ ~ ~ in the splendid microfiche catalog entitled "Carbon-13 NMR Suectral Data" (3) from which 6 values are taken where nossible from spectra determined in CDCl3 using TMS as internal standard.
-
~
~
~
L
~
Alkanes and Substituted Alkanes The 6 value of a carbon atom 'C can he calculated as the sum of a series of constants so that
where A is the sum of the increments allowed for various substituents depending on their positions as a,P, or y to the I3Catom in question, B is the sum of the branching corrections and -2.3 is the 6 value for methane. Increments for A are given in Table 1; those for B in Table 2. The following should he noted in using Table 2. 1) The functional groups shown may, for the purpose of selecting the steric correction increment, be regarded as corresponding ta the types of earhon atom cited. 2) In selecting the increment for the B term,for amines and ethers, regard the hetermtom as a carbonitom.
Volume 62
Number 3 March 1985
209
~
~
-
-
-
-
Table 1.
-C.sp= -CxC -C& -CI -F -Br
-I
-5' -0-
-O--CO-
Increments for 8, the Branching or Sterlc Correction Term for Alkanes and Substltuted Alkanes (4)
l3C atom
The number of Jubslihuems lother W n HI on lhe a-substituents.
obswved
1
2
3
primary secondary tertiary waternary
...
... ...
-1.1 -2.5 -9.5 -15.0
increments
increments Substituent
Table 2.
lncrements for A, the Shleldlng Term for Alkanes and Substituted Alkanes ( 1 )
a
B
y
9.1 4.4 19.5 22.1 31.0 70.1 18.9 -7.2 21.4 49.0 56.5
9.4 5.6 6.9 9.3 10.0 7.8 11.0 10.9 2.6 10.1 6.5
-2.5 -3.4 -2.1 -2.6 -5.1 -6.8 -3.8 -1.5 -2.5 -6.2 4.0
e
a
r
... ...
Carbon emivalem
3) 0 and N-alkyl groups in esters and amides are counted as y-substituents in calculating the value of A for
primary secondw terliary
-+Hs,
' X =OH. -NH1.
SH m haw".
+
+
+ +
A
+
+
+ 9.4
a( = -2.3 + (9.1 x 2 + 9.4
+
10.6 7.2 -1.5 -7.9 -1.6 +1.5
0 a'
7'
+
+
+
4 0 1 H . 4 0 ~--NO* ~ . -CHO. --CONH2,-CH2X' -4OR
increments
8'
6' = -2.3 + A B = -2.3 (2a1 'a +B+y)+B = -2.3 (9.1 X 2 49.0 0.0) (-3.7) = 70.6 (observed 68.8)
-15.0 -25.0
Table 3. Increments for A and B lor the General Alkene 'C==C-C,I-C#I Structure (5) -CgC,-
a T h e above points are illustrated by t h e following calculations.
-3.4
- 7.2
Functional moue
Position of s~bslitution
(1) CH&HziCH(OH)CH3
-3.7 -8.4
-1.5
4
Table 4.
Substituent X
TYW 01 disubstiMion
lnnements
a,& cis a.a gem a',a' gem 0.B gem
-1.1 -4.6 +2.5 -2.3
B
Increments lor A I w the General Alkene Derlvatlve Structure ( I , 6 ) X-CH(ff)=CH,(fl)
Increments
a
8
Increments a i3
Substituent
10.1 0.0) (-2.5) = 32.9 (observed 32.3)
ai = 2.3 + (9.1 X 3 + 61.6
+ 30.3 + 0.0)
+
(-8.4 X 3 - 1.5) = 90.2 (observed 94.9)
+
+
(9.1 x 3 22.5 ai = -2.3 9.4 0.0) (-15.0) = 41.9 (observed 44.3)
+ +
+
+
+
F o r Table 4, two examples a r e
6' = -2.3 (9.1 22.6 3.0 - 2.5 - 2.5) = 27.4 (observed 27.8)
+
+
+
S i = -2.3 (9.1 X 2 49.0 18.8) (-3.7 X 2) = 76.7 (observed 77.0)
+
+
Alkenes and Alkene Derivatives
T h e 6 value of a carbon atom 'C can be calculated as t h e s u m of a series of constants such t h a t
6' = 122.8 + A
CHZ='C(CH3)CONHz
+
B = 122.8 +(a+a)+B = 122.8 (13.4 9.6) (-4.8) = 141.0 (observed 139.2) 6' = 122.8 a 8 = 122.8 (13.4 8.0) (0.0) = 144.2 (observed 144.0)
+
(E)-CHsiCH=CHCOOEt
+ + +
+
+
+
+
Note that 'C in the part-structure (Z)-R-CH=CH-'CC is shielded by between 4 and 9 ppm. Alkynes
T h e 6 value of a carbon atom 'C can b e calculated a s the s u m of a series of constants such t h a t where A is the sum of t h e increments allowed for alkyl-substituents a, and y to "2, B is the correction term for the type of disubstitution and 122.8 is t h e observed 6 value for ethene. T h e increments for A and B a r e given in Tables 3 a n d 4. An example of the application of Table 3 is
6'
0,
(E)-CH3CH='CHCH2CHs
+B = 122.8 + ( a + B + a') = 122.8 + (10.6 + 7.2 - 7.9)
210
Bum'C=CCOCH3
6' = 122.8 + A
= 132.7 (observed 132.7)
Journal of Chemical Education
= 71.9
+A
where 71.9 is the 6 value for ethyne. Increments in A are given in Table 5 a n d two sample calculations a r e
p-CICsH4-C=C-CH3
+ + + + + + +
6' = 71.9 (a P a') = 71.9 (6.9 4.8 4.0) = 87.6 (observed 87.0) Si = 71.9 (12.7 - 5.7) : = 78.9 (observed 79.6)
Table 5.
Incrementslor A for the General Structure ( 8 )
-cgc,-
-C.s&+ --CHIOH --COCHa -4Hr -CWH*
Incrementsfor A for the General Structure 0
Increments S~bstlNenls
T&le 7.
'c=c-c,~-C-p
a
a'
B
8'
6.9 11.1 31.4 12.7 10.0
-5.7 1.9 4.0 6.4 11.0
4.8
2.3
II
-c,-CrC,-
'C-c,-cB--c,Increments
Substit~ent -C.s&+ -CsHs -CHSH2 -2-furanyl
A.
Ae
6.5 -1.2 -0.8 -12.0
2.6 0.0 0.0
A.. *
1.0
... ... ...
...
When C,C# = W=m-
Table 6. Increments lor A for the General Structure
where A is the sum of the increments for substituents in the a , (3, and y positions (Table 7) and 193.0is an assumed 6 value for methanal. Two examples are Substituent X A H 8 -Et -CPr tau
-CWHp -C-H -CsHs -CHO -COCHa -C02H -C02-C02R -CONH2 -CN -CI -OH -OCHs -0Gdb -OCOCH3 -NH2 -NlCHd2 -NO2 -SH -S03H
lncremenls At
AP
AJ
A4
9.3 15.8 20.3 22.4 7.6 -6.1 13.0 8.6 9.1 2.1 7.6 2.1 5.4 -15.4 62 26.9 31.4 29.1 23.0 18.7 22.4 20.0 2.2 15.0
0.8 -0.4 -1.9 -3.1 -1.8 3.8 -1.1 1.3 0.1 1.5 0.8 1.2 -0.3 3.6 0.4 -12.7 -14.4 -9.5 -6.4 -12.4 -15.7 -4.8 0.7 -2.2
0.0 -0.1 0.1 -0.2 -1.8 0.4 0.5 0.6 0.0 0.0 0.0 0.0 -0.9 0.6 1.3 1.4 1.0 0.3 1.3 1.3 0.8 0.9 0.4 1.3
-2.9 -2.6 -2.4 -2.9 -3.5 -0.2 -1.0 5.5 4.2 5.1 2.6 4.4 5.0 3.9 -1.9 -7.3 -7.7 -5.3 -2.3 -9.5 -11.6 5.8 -3.1 3.8
6' = 193.0 + A = 193.0 a y = 193.2 (observed 192.4)
+ +
6' = 193.0 = 193.0 = 211.2
+ 2a + 2P + 13.0 + 5.2
(observed 212.1)
Carboxylio Aclds and Esters
The 6 value of the carhonyl carbon can be calculated in the usual way from the equation where 166 is the assumed 6 value for methanoic acid. Values for A are given in Table 8, and two sample calculations are given below
+ + +
6' = 166 a 0 X =166+11+3-5 = 175 (observed 172.7)
CH3COCH2CH2'COCHzCH2CH8
11
0 CH&H"OOMe
Table 8. Benzenold Aromatics
The 6 value of a carbon atom C can he calculated as the sum of a series of constants such that
6'=166+a+X =166+5-5 = 166 (observed 165.5)
Inaements for A for the General Structure 0
-c,-c,$-c,-c-0-x
II
Increments
6 = 128.5 + A
where A is the sum of the increments allowed for the substituents at positions 1,2,3, and 4 (Table 6) and 128.5 is the 6 value for benzene. Two examples are 6' =128.5+Al+2XA3 = 128.5 2.1 2 X 0.9 = 132.4 (observed 132.2)
+
+
Substituents
a
0
Y
X
-C.$ -CsHs --CH=CHs
11
3 1
-1
-5 -8 -9
6
...
...
5
...
Amldes
The 6 value for the amide carbon atom can he calculated in the usual way from the equation
\
NO*
6' = 165 + A 6'
= 128.5 + Aa = 128.5 - 9.5 s 119.0 (observed 119.0)
Aldehydes and Ketones (saturated and or,&unsaturated)
where 165 is an assumed 6 value for the methanamide. A increments are given in Table 9: note that the svn-carbon atoms (designated >)are shieldedhy 3-5 ppm with respect to the anti-carbon atoms (a'). An example is C6H5NHLCOCH2CH$2H3
6'
The 6 value of the carbonyl carbon atom can he calculated as the sum of a series of constants such that
= 165 = 165
+ a + 6 + a' + 7.7 + 4.5 - 4.5
= 172.7 (observed 172.1)
Volume 62
Number 3
March 1985
211
Table 9.
Incrementsfor the General Structure
Examples ot the Variation ot 6 Values for Nltrlles In Dmerent Environments
Table 10.
6
6
S ~ b s i i t ~ s n l value
Substiwnt
117.7 120.8 123.7
-Me
--Et -Pr
+
-CH&I -vinyl
-+Hr
+
+
= -2.3 (9.1 22.0 28.3 (-9.5 - 3.7 - 3.7) = 56.0 (observed 59.8)
+
Nltrlles
The 6' value for the R-C=N nitrile-carbon atom is fairly constant. The examples listed in Table 10 indicate the way in which the 6 value varies in different environments. More Complex Examples
A severe limitation of the additivity rules described here is that they cover only benzenoid aromatics. However, additivity rules have been published for many other polycyclic benzenoid and heterocyclic systems2 including pyridine (12), pyrrole (13,14), furan (15,16), and thiophene (15,17). The use of the tables has been illustrated so far by reference to very simple compounds. Fair predictions (& 4 ppm) of 6 values can he made for carbons in the more complicated structures likely to he encountered in undergraduate projects and postgraduate research work.
For I.
value
115.7 117.2 118.7
6 Substiwnt
112.2 111.7 118.3
-pCeH4N02
-2furanyl --cinnamyl
+ 9.4 X 2 + 2.0 + -2.5
value
X 2)
- ++ -+ ++ - + + -
6=-2.3+(2a'+2a2+2P'+$+B3+5y1+y2)
+
(40 30 40 20 40 20 40 10) = -2.3(18.2 98.0 18.4 10.1 11.3 - 12.5 - 6.2) (-15.0 - 8.4 - 8.4 - 1.5) = 101.1 (observed 97.6)
+
Literature Clted (1) PrFtach, E..CI~X,T., kihl.*,andSimon, W.."TebeUen zur Stmkt"raulWm"gorganiwher Verbiidung~n mit Spektrak0pisch.n Methoden:' springer-Vdar, Bedin-Heidelburg, 1916. (2) KleinpDter,E..andBarsdorf,R.Y'SC-NMRSpsLhoeLopieindddwaehmChemie," Akadernie-Verlag, Berlin. 1981. (3) Brernser, W., Emst, L., h k e , B.. Gerbart.. R.. and Hardt, A,. "Carbon-13 NMR Spectral Data." Verlag-Chemie. Weinheim. 1981. (4) Grant. U.M..PaulE. G..J. Amer. Chem. Soe., 86.2984 (1964). (5) Dorrnan,D. E.,Jautelat, M.,and Roberu. J. D., J. Org Chem., 36.2157 (1971). (6) Schrarnl. J., Coil. C2ech. Cham. Camm., 41,3063 11976) and Yonernao, T., J M w . Res., 13.153 (19741. I71 de Hasn, J. W., and Van de Ven, L. J. M., 018.M a g Re8 5,147 (1973). (8) Habald, W., Radeplia, &and Klase, D., J. Phrct. Chom.,318,519 11916). I91 Levy, C. G.,Nelson, G.L.,and Cargioli, J. D..J A m r . Chem. Sac., 94,3089 (19721. (10) Buehansn, G.W.. Rpyrs-Zamora, C., and Clarke, D. E., Con. J. Chem., SJ, 3985 $1971, $111 K#e
