Proton-Carbon Chemical Shift Correlations Roger S. Macomber University of Cincinnati, Cincinnati, OH 45221
Over the past 30 years nuclear magnetic resonance (NMR) spectroscopy has evolved into the most powerful tool available to chemists for investigating molecular structure. With the latest eeneration of hieh field.. nulse/Fourier transform spectrometers, i t is often possible to resolve every individual roto on and carbon sienal in even hiehlv comnlicated structures. Further, the d&elopment of s&cl"I"twd-dimensional" NMR techniaues as HIH. HIC. and CIC COSY has nermitted unambig;ous assignment df virtually every proion and carbon signal to the appropriate nucleus in a structure.' ) 13C ( 6 ~chemi) When discussing the topics of 'H ( 6 ~ and cal shifts, most monographs on NMR go to some length describing semiempirical quantitative relationships between 6 and the molecular environment of a nucleus. I t is also usually stated as obvious that there is a rough correlation between the chemicalshift of a hvdro~enandthe chemical shift of the carbon to which it ib attached. The consequence of such a correlation is seen readily in HIC COSY spectra, where most signals lie more or less along the "diagonal". It is the purpose of this brief paper to examine this correlation between 'H chemical shifts and 'F chemical shifts in somewhat more detail. To this end. 53 re~resentativeoreanic compounds were selected, with structures as simple as acetaldehyde t o complex ones such as 1 and 2, comprising 335 unambiguously assigned 'H-'3C signal pairs.2 (A list of these compounds appears in the Appendix.)
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Proton-Carbon Chemlcal Shffl Correlationsa
.
number data pts
m, ppm Hlppm C
b, ppm H
CC
vinyl formyl
55 117 62 9 27 42 23
0.0545 0.0531 0.0609 0.124 0.0413 0.0392 0.0364
0.280 0.327 -0.158 -6.55 2.30 1.02 2.67
0.819 0.849 0.616 0.942 0.675 0.637 0.697
all
335
0.0479
0.472
0.955
Type CH melhyl
melhylene melhine acetylenic ammatic
slope mand intercept barederived horn linear least-rgusresresrsarlonanalysia.CC = corre~ati~n coeniciem. ALL HYDROGENS
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.t jl.
I,.
I,.
I,.
I,.
I,.
.
7.
5.
9.
I.
4.
CARBON CHEMICAL S H I F T
Plot of 60 versus 6" tor the compounds llsted in the Appendix.
aromatic
acetvlenic
formyl
+
Each hydrogenlcarbon pair was further classified as belonging to one of the types below.
methyl 284
methylene
methine
Journal of C h e r n i ~Education l
vinyl
Linear correlation of the form 6" = m6r b was tested for each type separately, then all together. he data are summarized in the tahle. Anlot of the"al1 hvdroeens"correlation is - shown in the figure. An excellent workbook of problems involving these 2DNMR techniques (plus DEPT and Difference NOE) is available: Duddeck, H.: Dietrich, W . Structure Elucidation by Modem NMR: Sprlnger: New York, 1989. Deuteriochloroform was the solvent in virtually all the spectra.
It is clear from the data in the table and the figure that there is indeed a decent correlation of 6c with 6~ throughout the entire range of chemical shifts (correlation coefficient 0.95 for "all hvdroeens"). Interestinelv. the correlations within individual groups is somewhat & r e scattered, with coefficients raneina from 0.82-0.85 for methvl/methvlene/ methine CH data, ti0.64-0.70for v i n y l / a r o m a ~ i ~ / f oCH r~~l data. The acetylenic C H data provide the best individual correlation (0.94), though this may he due in part to the small number (9) of data points. These correlations are too scattered to allow highly accurate predictions of hydrogen chemical shifts from carbon NMR data (and vice versa). Nonetheless, t h e m and b regression coefficients for the "all hydrogen" analysis do permit approximate estimates of this sort t o he made very q&kl$, without cnlculations involving extensive tahles of shielding constants. Moreover, in cases where the chemical shift of a proton is significantly different from predictions based on the chemical shift of the carbon to which it is attached, i t may he possible to ascribe this difference to specific anisotropic (dehhielding effects. Appendix. Compounds In This Study3
Acetaldehyde (1,2); acrolein (3,4); argentilactone 6 5 ) ; b e n d dehyde (1,4); hiphenylearhoxaldehyde (3,4),4-hromoadamantanThe first letter in parentheses refers to the source reference for the '3Cspechum, the second letter to the source reference forthe 'H spectrum. Only the acetylenic C-H data for this compound were tabulated.
one (5,5);p-hromohenzaldehyde(3,4);hutanal (l,4);hutyl formate (3,4);chelidonin (5,s);chrysanthemum alcohol (cis and trans, 5,5); cinnamaldehyde(1,4); erotonaldehyde (I, 4); cyclohexane carhoxaldehyde (3,4); eyclohexyl acetylene (3,4);4,9-dibromo-adamantanone (5,5); 2,6-dichlorohenzaldehyde(1,4);dicyclopentadiene(5,s); 3,5-dimethoxyhenzaldehyde(1, 4 ) ; p-dimethylaminobenzaldehyde
(1,4);Nfl-dimethylformamide (1,6); 3,5-dimethyl-1-hexyn-3-ol(7, 7)4,2',4'-dinitro-phenyl-2-desoxy-a-D-galactopyranoside (5,5),dispiro[2.1.3.0]octane (5, 5); dodeeanal (3,4); 2-ethylhutanal (3, 4); furfural(l,4);gedunim (5,5);trans-2-hexenal(3,4);1-hexyne(1,4); p-hydroxyhenzaldehyde (1, 4); a-ionin (5,5); kainic acid (5, 5); madurensin (5,5);medicarpin acetate (5,5); 2-methyl-3-hutyn-2-01 (1,4); 1-naphthaldehyde(1, 4); p-nitrocinnamaldehyde (3,4); 1,8nonadiyne (3,4); 1-octadecyne(3, 4 ) ; 1-octyne (8, 8)
