A Very Brief, Rapid, Simple, and Unified Method for Estimating Carbon-13 NMR Chemical Shifts The BS Method1 Ben Shoulders The University of Texas, Austin. TX 78712
Steven C. Welch2 University of Houston, Houston, TX 77004
Carbon-13 NMR has become an important and essential tool in the structural elucidation of organic molecules (1,Z). Several recent textbooks now routinely discuss 13CNMR (36). Carbon-13 NMR chemical shifts as well as the multiplicities of these chemical shifts are very useful data in determining the structural units in organic compounds (1,4,7,8). Many noise-decoupled '3C-'H NMR chemical shift assignments can be made on the basis of comnarison with reference compounds or by structure-shift correlation tables (1-3,9). Off-resonance '3C-'H decouoline . ...oermits the determination of multiplicitirs. Other terhniques that are useful in assigninp ':'C NMR chemical shifts are two-dimeniional ' C I H NMR experiments (lo), lanthanide shift NMR experiments (1, 2), labelling techniques (1, 21, and empirically derived additivity rules for calculating '3C NMR chemical shifts (I, 2). Substituent effects on13C NMR chemical shifts are additive. The estimation of 13CNMR chemical shifts based upon these empirical additive substituent effects has proven to be very useful in interpreting 'W NMR spectra. Methods for calculating 13C NMR chemical shifts have been published for alkanes (11,12), alkenes (131, alkynes (14), and substituted benzenes (15). Two elaborate and soohisticated methods ha\,e been published which pnwide renronnhly accurate ralculntion ottlC N M H chemical shifts 116.171;howe\.er, these two methods are cumbersome and complicated with extensive tables of correction data. The purpose of this paper is the introduction of a very brief, rapid, simple, and unified method for estimating 13C NMR chemical shifts called the BS Method. The method is so brief and simple that students can memorize and utilize i t to interpret '3C NMR spectra with ease. The largest factor in determining the chemical shift of a carbon atom in an NMR ex~erimentis the hvbridization of the carbon atom. This fact& is so large that ihere are three different base scales fur earh of rhe three rwes of hvbridization (sp3, spz-alkenes, sp-alkynes) presented herkin. (See Tables 1-3.) The base for sp3-hybridized carbon atoms is the same as that for the 6 scale for 13CNMR. (See Table 1.) That is, the chemical shift of TMS (tetramethylsilane) is defined as the zero point for the 6 scale. The base structure for a sp3-hybridized carbon atom is 13C[-A-B-C].,, where A, B, and C are atoms other than hydrogen atoms. (See Table 1.) The important factors to be considered for a so3-hvbridized carbon atom is the number . " of h and H atoms other than hydrogen atoms. Each A and B utom othrr than hydrogen causes a shift in the "C resonance
'This brief, rapid, simple, and unified method of estimating ''C named the BS Method in honor of the developer Ben Shoulders. Author to whom correspondence should be addressed. A simple computer program on the BS Method is also available.
NMR chemical shifts has been
Table 1. The BS Method for sp3 Carbon Atoms A. A'. B. B'.
base = 0 ppm
Care atoms other than H. base structure: '3C[-A-B-C]~
A = B = 7.5 oom
6p3 Corrections
[Cadel
1. C-Me group not anached to a -CH*-: -2.4 ppm. 2. C-Me group anached to a -CHr: -0.4 ppm 3. C(13) is a CH, group with a C carbon atom: -0.4 ppm 4. C(13) is a -CH,- and the lhird atom other than H in the chain: 2.5 ppm. 5. C(13) is tertiary: -1.5 ppm X #of B atoms 6. C(13) is quaternary: -4.4 ppm X #of B atoms 7. C(13) is aliylic or transallylic: 5 ppm 8. C(13) is cisallylie: -1.25 ppm 9. C(13) is prapargylic: -10 ppm 10. C(13) is in a cyciopropane ring: -32.6 ppm 11. C(13) is in a cycbbutane ring: -6.7 ppm 12. C(13) is in a cycbpentane ring: -3.5 ppm 13. C(13) is in a cyclohexane ring: -2.2 ppm 14. C(13)IS In a cycloheptane ring: -0.6 ppm 15. C(13)is on all axis of symmetry: -2.7 ppm 16. A-sp30xygenatoms: 37.5 ppm X #of oxygen atoms 17. A-sp3nitrogen atoms: 22.5 ppm X # of nitrogen atoms 18. A l p 3 bromine atoms: 22.5 ppm X # of bromine atoms 19. A-s~~chlorine atoms: 30 o, . m X tl,. of chlorine atoms 20. B-& oxygen atoms of an aldehyde or ketone: 6.1 ppm 21. Gsp3oxygen atoms: -3.2 ppm X # of oxygen atoms 22. C-sp2oxygen atoms: -5.0 ppm X # of oxygen atoms
i~oj LC01
[Csp20]
of +7.5 ppm. Thus using these factors alone the estimated chemical shifts for each of the carbon atoms of orooane is +15 ppm. The experimental values are 15.6 p i m for the methyl carbon and 16.1 ppm for the methylene carbon. Additional small corrections can be made for a more accurate estimation of 13Cchemical shifts. See Table 1for corrections and Table 2 for examples of their applications. The first of these small corrections is the 7 effect. The y effect results from steric interactions between the 14-nonbonded '% atom and the C atom (or y atom) 1,2. In the case of I3C atoms the y effect is seen primarily with a methyl group (7)or oxygen atom (8)in the C position ( y position). The 'Tatom will experience a shift of -2.5 ppm if there is a C methyl group that is not attached to a methylene group (-CH2-). Carbons 1 and 5 of compound 1 exemplify this correction. The 13Cresonance will be shifted -0.4 ppm if the C atom is a methyl group attached to a methylene group. See carbons 4 and 8 in compound 1. In either case the C methyl group will also be shifted -0.4 ppm. See carbons 1,7, and 8 in compound 1.The 'X NMRchemical shifts calculated by the method of Lindeman and Adams (1, 12) are listed for comparison purposes. Volume 64
Number 11 November 1987
915
Table 2.
Selected Examples lor sp3-HybridizedCarbon Atoms (base = 0 ppm)
.J.-.
J.44 calc'd P P ~
Carbon
BS Method
I 2 3 4 5 6 7 6
1A 16 (C-MeNA) Me 2A 28 3rdM 3A+2B+T 3rdM 2A 3 8 (C-MeAT) 2A 28 (C-MeNA) 3rdM 2A 1B IA+lB+Ms 1A 26 IC-MeAt)+ Me
-
+ + + +
+
+ + + + +
+
+
+ +
12.1 32.5 34.5 39.6 30.0 22.5 14.6 21.7
L&A Memod ( l a cab'd (exp) P P ~ P P ~ 10.9 29.6 34.6 36.2 29.7 22.9 13.9 19.1
(11.3) (29.7) (34.7) (36.5) (29.5) (23.3) (14.1) 119.31
Carbon 1 2 3 4 5 6
calc'd P P ~
BS Method 1A+2B+Me 3A+lB+T+AO 2A 3 8 (C-MeAT) 2A 2B (C-MeNA) CO 2 A + 1B lA+lB+Me
+ + + +
+
+ 3rdM
+ 3rdM
(exp) P P ~
21.1 66.0 39.6 26.8
(23.3) (67.2) (39.2) (26.3)
22.5 14.6
(22.9) (13.9)
OH
Carbon 1 2 3 4 5
calc'd P P ~
BS Method 1A+3B+Me 4A 16 (C-MeNA) 2A 5 B + 3rdM 3A 1B (C-MeNA) 1A+2B+Me
+ + + + +
+ +T
(1
29.6 30.6 55.0 26.0 22.1
(exp) P P ~ (29.9) (30.4) (53.5) (25.3) (24.7)
Carbon 1 2 3 4 5 6
calc'd P P ~
BS Method IA+IB+Me+CO 2A+2B 3A 28 (C-MeAt) 2A 3 8 (C-MeAT) ZA+lB+CO l A + IB+Me
+ +
+ +
+T+ A0 + 3rdM
(exp) P P ~
11.4 30 71.6 39.6 19.3 14.6
(9.9) (30.3) (72.3) (39.4) (19.4) (14.0)
calc'd
X=H. Y=OH ppm
X=OH. Y=H ppm
67.1 35.3 29.6
(70.1) (35.4) (25.4)
(61.7) (33.0) (20.7)
49.6 33.5 26.9
(47.0) (31.6) (27.2)
(47.9) (32.1) (27.1)
x
3
Carbon 1 2 3 4 5 6
BS Melhod 2A+IB+AO 2A 28 3rdM 2A 26 (C-MeAT) 2A+2B+3rdM 2 A + 1B IA+lB+Me
+ + + +
6
calc'd P P ~
+ CO
60.0 32.5 26.4 32.5 22.5 14.6
(exp) P P ~ (61.9) (32.6) (25.6) (32.0) (22.6) (14.2)
The I3C atom will experience a shift up to -7.5 ppm when the C atom is a sp3 oxygen atom. The magnitude of this shift is inversely dependent upon the steric proximity of the C oxygen atom to the '% atom. A good approximation for nonrigid molecules is -3.2 pprn times the number of C-sp3 oxvgen atoms. Carbon 3 in comuound 3. carbon 4 in compound 4, and carbons 1 and 5 in compound 5 exemplify this correction. However, when I3C is in a more rigid structure; such as, carbon 3 of 4-tert-hutylcyclohexanol~6) (2, 18, 19) the correction of -3.2 pprn for a C-sp3 oxygen atom is inadequate. In this case the calculated shift for carbon 3 is 29.6 ppm. This is 4.2 pprn too high for the equatorial alcohol (X = H, Y = OH) and 8.9 pprn too high for the axial alcohol (X = OH, Y = H). Therefore, in the case of an equatorial C-sp3oxygen atom a correction of -7.5 pprn is more appropriate. For an axial C-sp3 oxygen atom a correction of -12.1 ppm is a better choice. Another small y effect is seen with C-sp2 oxygen atoms. The I3C resonance will he shifted -5.0 pprn for each C-su2oxveen atom. See correction 22 in Table I. Ano1ht.r sknll biiect forsp,cnrbon aroms is seen when the "Cis hntha mc.thylenearoupand the third atom other than hydrogen from the endbf the chain. This third methylene effect produces a shift of +2.5 ppmfor the13C resonance. See carbons 2,4, and 5 in compound 1. The substitution pattern on I3C also has an effect on the chemical shift. When '3C is trisubstituted there is a shift of -1.5 pprn times the number 916
Journal of Chemical Education
Carbon
BS Method
1 2 3
3A 28 T R6 A 0 S 2A+3B+RB 2A 3 8 (C-MeNA) R6 f CO 3A+5B+T+R6+S 4A+2B+Q+S IA+3B+Me+S
4 5 6
+ + + + + + + +
of B atoms. See carbon 3 in compound 1 or carbon 4 in compound 2. Likewise, when '3C is tetrasubstituted the correction is -4.4 pprn times the number of B atoms. See carbon 2 in compound 2 or carbon 5 in compound 6. Functional groups in molecules cause changes in the chemical shift of carbon atoms in close proximity. The functional group may have a different electronegativity than carbon and thus donate or withdraw electron density from the ':>Catom. See examples in compounds 3 t o 6. The functional grout, or i t mav have maanetic . . mav he coniuaated .anisotropy, that is, the functional group may have a magnetic field that is felt at carbon atoms nearby. The functional group may also have some steric interaction. The influence of each of these factors varies with the functional group. An example, of this phenomenon can be found in alkenes. If the 'W atom is allylic or trans-allylic the resonance will be shifted +5.0 ppm; however, if the '3C atom is cis-allylic the absorption will he shifted -1.25 ppm. The latter shift results again from a y steric effect. Examples of these allylic corrections can he seen in carbons 1 and 4 in compounds 7 and 8, resoectivelv. In the case of alkvnes where the '3C atom is propargylic thr resonance is shifted -10 ppm. Ser carbon atom 3 in compound I I and rnrlrms 1 and 1 in compound 12 for rxmnples of this propnrgylic correction. Ring size has un efl'cct on therhemical shift of spl.hybrid. ized cnrhon aroms. If the sp?-hytrridizrd 'C atom is in two or
'
marre rings, only onecorrc.ction is used. Generally, the smaller or sn~allestring determines the rorrertion. The corrections for ring sizes are cyclopropane, -32.6 ppm; cyclobutane, -6.7 ppm; cyclopentane, -3.5 ppm; cyclohexane, -2.2 ppm; and cycloheptane, -0.6 ppm. See an example of this ring correction in compound 9. In those rare occasions where molecular symmetry exists in cyclic compounds there is an infrequently used correction of -2.7 ppm for13C atoms that 3.
Table
The BS Method for sp2 Carbon Atoms
are on the axis of symmetry. See compound 9 for an example of this svmmetrv correction. The effects o? other electronegative atoms are listed in Table 1. If '3C is bonded to an A - s d oxwen atom the correction is +37.5 ppm times the number bf oxygen atoms. See examples in compounds 3-6. Similarly if 13C is attached to either an A-sphitrogen or bromine atoms the resonance is shifted +22.5 ppm times the number of nitrogen or bromine atoms. If the '3C is bonded to a chlorine atom the correction is +33 ppm times the number of chlorine atoms. Finally, or when the B atom is a so2 oxveen .. ,, atom of an aldehvde ,~~~ keronr t h e position of the "C resnnance isshifted + 6.1 ppm. carhnn atom of an The base structure fur a so'-hvhridired . " alkene or aromatic hydrocarbon (19) is [B'-AC]2C='3C[-A-BIZ, where A, A', B, and B' are atoms other than hydrogen atoms. (See Table 3.) The hase value for alkenes and aromatic hydrocarbons is +121.5 ppm. In these cases both the A and B atoms cause a shift of +7.5 ppm for the t3C resonance, whereas an A' atom results in a correction of -7.5 ppm. See the examples in compounds 7 to 10 (Table 4). Again functionalgroups play an important role in the shift of the 13C resonance. When '3C atom is attached to an A-sp3 oxygen atom the shift is +16.7 ppm; however, when the A' atom is an sp" oxygen atom when the correction is -30 ppm. See compound 10 for examples of these corrections. When there is a B-spZor -sp3 oxygen atom present the resonance for the 13C is shifted -7.5 ppm; however, when a B'-sp2 oxygen atom is present the correction is +17.5 ppm. The hase structure for a sp-hybridized carbon atom of an alkyne is B'-A'-C=13C-A-B, where, A, A', B, and B' are atoms other than hydrogen atoms. The hase value for an alkyne is +72.5 ppm (see Table 5). Each A atom results in a ~
(Alkenesl
~
A. A'. B. 8' are atoms &her than H. base = 121.5 ppm base
base structure: [B'-A'-]tC='3C[-A-B]~
A = B = 7.5 ppm, A' = -7.5 ppm sp3 Corrections
[Code]
1. A-sp3 oxygen atom: 16.7 ppm 2. A'-$p3 oxygen atom: -30 ppm 3. B-sp2or sp3 oxygen atom: -7.5 ppm 4. 6'-sp2 oxygen atom: 17.5 ppm
Table 4.
1 2 3 4 5 6
[Asp301 [A'sp30] [Bol [B'sp20]
Selected Examples 01 spz-Hybrldlzed Carbon Atoms (Alkenes: base = 121.5 ppm)
Carbon
calc'd ppm
BS Method lA+lB+Me+AL 1A 1A' lAflB+lA' 2A t 2B (C-MeNA) AL 2AtlB lA+lB+Me
+
+
+
+ 3rdM
~~
(exp) P P ~
19.6 121.5 129 35
(17.7) (125.1) (131.7) (35.3)
22.5 14.6
(23.2) (13.7)
Table 5.
The BS Method for sp Carbon Atom (Alkynes) A. A', Bare atoms omer than H.
base: 72.5 ppm
base structure: B'-A'-C='3C-A-B
A = 7.5 ppm. B = 5 ppm, A' = -5 ppm
calc'd Carbon
P P ~
BS Method
(erp) P P ~
Table 6.
Selected Examples 01 sp-Hybridized Carbon Atoms (Alkynes: base = 72.5 ppm) I
-
a
11
Carbon
Carbon
calc'd P P ~
BS Method
(exp) P P ~
calc'd P P ~
BS Method
1 2 3
1A' 1A 2A
4 5
P 2A 28 3rdM 2A 1B lA+lB+Me
+ 1B + 28 + (C-MeAT) + 3rdM
+
+ +
+
6
,
z -
(exp) P P ~
67.5 85 22.1
(67.4) (82.8) (17.4)
32.5 22.5 14.6
(29.9) (21.2) (12.9)
.
- 7 1 12
Carbon
BS Method
Calc'd P P ~
(expj
P P ~
n\o/ 10
Carbon 1
2
BS Method 1A 1A'
+ 1B + A-sp30 + A'-so30
calc'd P P ~
(exp)
153.2 84
153.2 842
P P ~
Volume
64
Number 11 November 1987
917
shift of +7.5 ppm and each B atom causes a shift +5.0 ppm; however, each A' atom results in a correction of -5.0 ppm. See compounds 11 and 12 (Table 6) for examples. In conclusion, the BS Method is avery brief, rapid, simple and unified method for estimating 13C NMR chemical shifts for a variety of organic molecules. The BS Method is easily remembered and utilized by students who are learning to interpret 13C NMR spectra. The method does, however, produce unreliable results for large molecules with a large degree of branching, steric strain, or hydrogen bonding effects. Acknowledgment
We thank the Robert A. Welch Foundation for the support of the Science of Chemistry in the State of Texas (SCW, grant number E-518). Literature Cited I. lev.G. C.: Lichter, R. L.;Nelson,G. L. Corbon-I3Nuclear MognalicRe8onanes.2nd ed.; Wiley: NervYork, 1980. 2. Wehrli, F. W.; Wirthlin. T. Interpretation ofCorbon-13Specfro; Hayden: New York, 1976.
918
Journal of Chemical Education
3. Silverstein. R.M.: Baaaler, G. C.: Morrill, T. C. Spactmmrtric lnd~nfificofionof Ormnie Compounds. 4th ed.: Wiley: NeaYmk. 1981. 4. Sternhell,S.;Kalman,J.R.OrganicStruclums fromSpmfm;Wiley:New York, 1986. Wiley: 5. Furhr. P. L.:Bunnell, C. A. Carbon-13NMRBo~edO1gonicSpactroIProblam; New Yark, 1979. 6. Bates, R. B.: Beavers, W. A. Corbon-13NMR Spectral Problem; Humanna: Clifton, NJ. 1981. 7. (a1 Grant. 0. M.; Cheney, B. V. J. Am. Cham. Soe. L967,89, 5315. (bl Cheney, B. V.: Grant.D. M.J.Am. Chem. Soe. 1967,89,5319. (cIDal1ing.D. K.:Grant,D. M. J A m . Chem. Soc. 1961.89.6612. 8. G~over,S.H.:Gufhrie.J.P.:Sfothen,J.B.:Tan,C.T.J.Mog.Reaononca1973,1O.227. 9. Prefseh. E.:Seibl, J.: Simon, W.: Clere, T.: Biemann, K. Toblesof Spect~oiData for Structure Doterminotion of Orgonic Compounds; Springer-Verlsg: New York. 1989. 10. Bax, A. Two-dimensional Nuclear Mognalie Resomnm in Liquids: Reidel: Baaton, MA, 1982. 11. Grsnt.D. M.:Peul,E.G. J.Am. Cham. Soc. 1964.86.2984. 12. Lindeman. L. P.; Adsms,J.Q.Aml. Cham. 1971.43,1245. 13. Darman,D. E.; Jaute1at.M.; Roberta, J.D. J. Org. Chem. 1971,36,2757. 1973,3B,1026.(b)Hobold. 14. (a)Onrman.D.E.:Jautelst,M.:Rnberte,J.D.J.Org.Chem. W.;Rsdeglia,R.: K1ore.D. J.Prokt. Chem. 1976,318,519. 15. Ewing, D. F. Org. Magnetic Rrsonancs 1979.12.499. 16. Cheng, H. Nlnduslriol ChemicolN~0r1984,(MayI, 58. 17. Brown. D. W. J. Chem.Edue. l985.62.209. 18. Roberts, J.D.; Weigert,F. J.;Kraachwitz, J. L.;Reich,H. J. J.Am. Chem. Soc. 1970.92. "an .""".
3
19. Whitesell. J. K.:Minton,M. A. J . Am. Chzm.Soc. 1987,109,225
