Indian Journal of ChemistryVol. 27 A. November 1988. pp. 932-936

A Unified Empirical Treatment of Carbon-13 NMR Chemical Shifts:Part II - Correlation Analysis of Data on Carbonyl Compounds

IRAGAVARAPU SURYANARAYANA*T & JOHANN GASTEIGER

Organische Chcrnisches lnstitut der Technischen Universitat, Munchen. D 8046 Garching, Fed~ral Republic ofGermany

Received 25111111' 1987; revised and accepted 18 December 1987

The '-'c NMR chemical shifts of carbonyl carbons of YO different carbonyl compounds are reproduced by alinear two parameter equation with a correlation coefficient of 0.9857 and a standard deviation of 3.07 ppm. Thetwo parameters are the total charges (both 0 and n) on the carbonyl carbon atom as calculated by the iterativepartial equalization of orbital electronegativity (PEOE method) and the effective polarizability as represented by amolecular connectivity number.

I3C NMR chemical shifts of the carbonyl carbonsof various compounds are of interest for theoreti-cal as well as experimental work. Stothers andLauterbur ' discussed the influence of alkyl substi-tution, conjugation and hydrogen bonding on theJ1C NMR chemical shifts of carbonyl carbons in anumber of compounds. They explained the ob-served downfield shift of carbonyl carbon with in-creasing methyl substitution in terms of the hyper-conjugation effect of methyl groups. This is incontrast to the opinion of Spiesecke and Schneid-er ', and Grant and Paul.', who attributed thedownfield shift to the diamagnetic anisotropy ofthe alkyl group. Jackrnann and Kelly! attributedthe downfield shift to the negative inductive effectof methyl groups. Delseth and Kentzinger ' pro-posed an additivity relationship with six parame-ters for 2R alkyl ketones, with a standard devia-tion of 2.5 ppm, and related the six parameters tothe hyperconjugation effect of methyl groups onthe polarity of the> C = 0 group. Assuming con-stant excitation energy-invariant non-polar sigmanet work and neglecting neighbouring anisotropiceffects and intermolecular dispersion effects, Ma-ciel" derived an equation relating the chemicalshift to the rt-bond polarity of the C = 0 bond interms of Taft substituent parameters for 14 carbo-nyl compounds. Sasvitsky et al.' proposed acorrelation between the i1C NMR chemical shiftsand the 1/ ---+ rt" absorption maximum for a set ofI()alkyl substituted cyclic and bicyclic ketoneswith a correlation coefficient of 0.912 and astandard deviation of I.n ppm. Yalpani et {/I.~ at-tributed the upfield shift of the carbonyl signal on

Pcrm.mcnt address: Analytical (hcrnistrv Division. RegionalResearch l.aboratorv . .lorhnt 7H:, O()6. Assam

substitution of halogens to the conformational ef-fects or the field effects of the halogens on the> C = 0 dipole in addition to the electronegativityeffect, the latter contributing to a small extent.Couperus et al? and Velichko et ai'" reported theI3C spectra for a number of esters to get informa-tion on conformational and stereochemicalaspects of esters.

In view of the conflicting explanations for thevariation of the t3C chemical shifts of carbonylcarbons, and in order to understand the factorsinfluencing the I.1C chemical shifts in general, wehave attempted to correlate the available I.1C

chemical shifts of aliphatic carbonyl carbons ofdifferent aldehydes, ketones, carboxylic acids, acidhalides and esters with the physically significantand explainable parameters like the partial atomiccharge on the carbonyl carbon atom as calculatedby the iterative partial equalization of orbital elec-tronegativity!' and the effective polarizability 1 ~ asrepresented by a molecular connectivity number,which can be calculated by inspection.

Data Set

The data set consists of the experimental t3C

chemical shifts of aliphatic saturated carbonylcompounds, c.g., aldehydes, ketones, acids, acidhalides and esters. These different types of com-pounds were chosen so that the influence of allsaturated bonding (a) situations around carbonylgroup ean be covered to get a balanced statisticalanalysis. The sources of the experimental datafrom literatures are indicated as footnotes inTable 1. The variation of chemical shifts withchange of solvent and temperature is considerednegligihle as the chemical shift variation ohserved

SURYANARAYANA & GASTEIGER : C-13 NMR CHEMICAL SHIFTS OF CARBONYL COMPOUNDS

Table 1- The Calculated and Experimental I.'C NMR Chemical Shifts of Carbonyl Compounds

S.No. Compound b..:xp" 6,,:0111..'h .c.(O<'I'-O,aIJ Q< Q- Qlulal~' N

" 0

I H.CO.H 194.3 192.32 1.9H 0.107 O.ll()3 0.110 ·tOO

2 CH,.CO.H 199.6 . 196.70 2.90 0.117 11.002 O.IIY 5.50

3 EI.CO.H 201.H IY9.26 2.54 0.120 0.002 0.122 6.25

4 n-Pr.CO.H 201.6 200.91 0.6Y 0.120 0.002 0.122 6.625

5 II-Bu.CO.H 202.2 201.70 0.50 0.120 0.002 0.122 6.813

6 i-Pr.CO.H 204.0 201.82 2.IX 0.123 0.002 0.125 7.000

7 t-Bu.CO.H 203.Y 204.60 - 0.70 0.116 0.001 0.127 7.750

8 i-Bu.CO.H 201.1 202.60 - 1.50 0.120 0.002 0.122 7.000

Y nco-Pen.CO.H 201.1 204.00 - 2.X6 0.121 0.002 0.123 7.37510 EI~CH.CO.H 203.5 205.10 - 1.61 0.123 0.002 0.125 7.750

II ClCH,.CO.H IY3.3 IYO.55 2.75 0.135 0.000 0.135 5.500

12 CI,C.CO.H 175.Y 184.60 - XN) 0.171 -0.003 O.16X 5.500

13 Br,C.CO.H 175.Y liP.60 - 11.66 IU5X -0.002 0.156 5.50014 Me.CO.Me 205.1 200.8 4.27 0.127 0.002 O.12Y 7.00

15 Me.CO.EI 206.3 2113.6 2.67 0.1311 0.00 I 0.131 7.75

16 Me.CO.nPr 206.6 205.3 1.30 0.130 0.001 0.131 8.12517 Me.CO.nBu 206.8 206.1 0.70 0.130 0.001 0.131 101318 Me.CO.iPr 2119.1 2116.2 2.911 0.133 0.00 I 11.134 8.50019 Me.CO.IBu 211.X 2118.7 3.10 0.136 0.001 0.137 Y.25020 Me.CO.iBu 2115.R 206.7 -0.90 0.131 (1.00I 0.132 X.50021 Me.CO.neo-Pen 205.5 208.3 -2.80 0.131 0.001 0.132 X.X7522 Me.CO.C( Me ),C( Me), 211.4 213.7 - 2.30 0.136 0.001 0.137 10.375

23 Et.CO.Et 209.3 206.2 3.10 0.133 0.001 0.134 X.500

24 EI.CO.iPr 211.8 208.7 3.10 0.136 11.001 (1.137 Y.25025 EI.CO.tBu 213.0 211.8 1.30 0.138 0.001 0.139 10.00026 Et.CO.nPr 209.3 207.8 1.50 11.133 0.00 I 0.134 8.87527 Et.CO.iBu 208.0 206.2 1.80 0.134 (1.001 11.135 8.5111128 Et.CO.C(Et), 213.1 216.5 -3,40 0.139 0.000 0.139 11.125

29 i-Pr.CO.iPr 215.1 211.8 3.30 0.138 (WOO 0.138 «WOO30 i-Pr.CO.tBu 217.1 214.4 2.70 0.141 (1.(100 0.141 10.75031 i-Pr.CO.iBu 212.4 212.0 0.40 0.136 (1.(101 0.137 10.00032 n-Bu.CO.nBu 208.6 211.3 -2.50 0.133 0.001 0.134 9.62533 (-Bu.CO.tBu 217.1 216.9 0.20 0.144 0.000 0.144 11.50034 (-Bu.CO.nPr 213.6 213.2 0.40 0.139 (1.(100 0.13Y 10.37535 i-Bu.CO.iBu 20M 212.5 -4.10 0.134 0.00 I 0.135 «WOO36 Me.CO.CH,Cl 200.7 196.9 3.80 0.145 0.000 0.145 7.00037 Me.CO.CHCI, 193.6 192.9 0.70 0.1.63 -0.002 0.161 7.00038 Me.CO.CCI, 186.3 188.7 -2.40 0.181 -0.1)03 0.178 7.00039 Me.CO.CClBr, 186.8 190.9 -4.10 0.172 - o.om O.16Y 7.00040 Me.CO.CCl,Br IX6.6 190.0 -3.40 0.176 -o.om 0.17 3 7.110041 Me.CO.CHClBr 193.9 193.9 0.00 0.158 -0.001 0.157 7.00042 Me.CO.CH,Br 199.0 197.9 1.10 0.141 0.000 11.14i 7.00043 Me.CO.CH.MeJ 202.7 200.7 2.00 0.143 (WOO 0.143 7.751144 Me.CO.CHMeBr, 200.3 200.7 - O.4{1 0.143 (l.OOO 0.143 7.75045 Me.CO.CHMeCI 201.2 IYY.7 1.50 0.148 -0.001 0.147 7.75046 Me.CO.CMeBr 193.9 197.5 -3.60 0.157 - 0.001 0;156 7.75047 Me.CO.CMeClBr 193.8 196.7 -2.YO 0.161 -0.002 0.15Y 7.75048 Me.CO.CMeCI, 194.2 195.7 - 1.50 0.165 -0.002 0.163 7.75049 t-Bu.CO.CHCI, 201.1 200.8 (1.30 0.172 -(l.O02 0.160 '1.25050 t-Bu.CO.CHBr, 201.4 203.0 -1.60 0.171 - 0.002 O.16Y Y.25051 i-Pr.CO.CH,Cl 205.8 202.5 3.30 0.150 - 0.00 I O.14Y X.50052 i-Pr.CO.CCLMe, 211.1i 207.6 3.40 0.156 -0.001 0.155 10.00053 CICH,.CO.CH,Cl 194.Y 193.2 1.70 0.162 - 0.002 0.160 7.00054 CI~HC.CO.CHCI, 183.4 185.0 -1.60 0.198 -0.005 0.IY3 7.00055 Cl~C.CO.CCI. 175.9 176.9 -1.00 0.234 -0.008 0.226 7.00056 Me.CO.Cl 171.0 173.5 - 2.50 0.219 -0.006 0.213 5.50057 Et.CO.Cl 174.7 176.3 -1.60 0.221 -0.006 0.215 6.25058 CICH,.CO.CI 167.7 169.11 -2.10 0.236 - 0.0011 0.22X 5.50059 CI:CH.CO.CI 165.5 165.6 -0.10 0.254 - O.OO') 0.245 5JOO

(COil/d.)

-

933

INDIAN 1. CHEM., VOL. 27A. NOVEMBER 1988

Tahle I-The Calculated and Experimental nC NMR Chemical Shifts of Carbonyl Compounds - Contd.

S.No.

60616263646566676H6':17071727:.74 .7576777H7':1HOHIH2H3H4H5H6H7HHH990

Compound

CI,C.CO.C1H.CO.OHMe.CO.OHEt.CO.OHi-Pr.CO.OHI-Bu.CO.OHCH~Cl.CO.OHCHCI,.CO.oHCCI,.CO.OHH.CO.OMeMe.CO.OMeEt.CO.oMePr.CO.OMePen.CO.OMeHex.CO.OMei-Pr.CO.OMeCH,CI.CO.OMeCHCI~.CO.OMeCCI,.CO.OMeCH,Br.CO.oMeCHBr,.CO.OMeCBr,.CO.OMeMeHBrC.CO.OMeMeBr ,C.CO.OMeCH,BrCHBr.CO.OMeMe.CO.OEtC1,C.CO.OEtMe.CO.OPrMe.CO.OiPrMe.CO.OiBuMe.CO.OtBu

163.5166.7171.1173.4177.1178.':1166.8163.3160.1160.7170.0173.3173.0172.1173.4175.7167.8164.4160.0166.6164.3161.816':1.2166.2167.0170.4161.1169.9167.1169.9170.2

161.4162.9167.3169.8172.6175.1163.3158.9154.':1165.7169.8172.4174.0175.5175.7175.4165.9161.7157.5166.9163.9160.7169.7172.4169.4171.5159.1172.3173.5173.9174.8

t1( b.x; b""J2.103.803.803.604.503.703.504.405.20

- 5.000.200.901.00

- 3.40-2.30

0.301.902.702.50

-0.300.401.10

-0.50-5.90-2.40- 1.10- 2.00-2.4-6.00- 4.00- 4.60

0.2730.2460.2560.2590.2620.2650.2740.2930.3110.24':10.25':10.2620.2620.2620.2620.2640.2770.2950.3140.2720.2860.3000.2750.2640.2760.25':10.3140.2590.2590.2590.259

0<tt

-0.0]1-0.008- (U)09-0.009- 0.010- 0.010-0.011-0.012-0.014- 0.009-0.009-0.009-0.009-O.OW- o.oro- 0.010-0.011-0.012-0.014-O.OIU-0.012-0.013-0.011- 0.010-0.011-0.009-0.014- 0.009-(l.009- 0.009- 0.009

0.2620.2380.2470.2500:2520.2550.2630.2810.2':170.2400.2500.2530.2530.2520.2520.2540.2660.2830.3000.2620.2740.2870.2640.2540.2650.2500.3000.2500.2500.2500.250

N

5.5004.5006.0006.7507.5008.2506.0006.0006.0005.2506.7507.5007.5008.1568.2038.2506.7506.7506.7506.7506.7506.7507.5007.5007.5007.1257.1257.3137.5007.6887.875

(a) The expt. values are taken from the ref. indicated in the bracket against the compound number:1(6); 2-10 & 14-35 (5); 36,37,39-48(8); 13,49-54 (values obtained by us); 69-75,85.87-90 (9); 76-84(10); 86(17):11.12,3R.55-61, and 62-68 with H.bonding correction b - 7 ppm (12)

(b) values calculated by Eq.2.(c) in electron volts.

Tahle 2 - Results of Multilinear Regression Analysis of I3CNMR Chemical Shifts of Carbonyl CarbonsIn = Number of points. R = regression coefficient. s = standard deviation, CII'C, and C, are the coefficients of Eqs I and 2]

all carbonylcompounds

all withH. bondingcorrectionwithoutfluorocompounds

Parameters

0"0"o.,O",NO",NOltHal' N

O".NO".NQltllal·N

0".O".NO,,,,,,,.N

n9'1999'19':199':1'1':1999'19l)()

l)()

l)()

R0.91620.91420.'11610.'16790.96650.9679

0.97900.97760.97900.985'1O.985HO.l)H57

CII7.117.197.114.394.484.39

3.653.773.653.063.073.07

C,236.37201.83239.33198.27168.2H200.40

19':1.05167.75201.HO199.16167.30201.911

C,- 240.642928.74

- 262.07- 213.532614.63

- 232.40

-222.792727.61

- 242.48

- 227.172797.64-247.l0

4.294.234.304.364.294.374.394.314.40

carbonyl compounds with partial atomic chargesto derive Eq.l, since the charge values containstructural information. Charge (Q) as a single par-ameter gave a correlation coefficient (R)0.9161 and standard deviation (s) 7.1 ppm forall the carhonyl carbons.b = C" + C1Q ... (1)

for acetone between CCI~ and CDC1] is only Ippm which is less than the standard error. Similaris the case with temperature variation of ]{)0c.

Results and DiscussionMultiple linear regression analysis (MLRA) was

performed on the I.1C chemical shifts of different

()}4

SURYANARAYANA & GASTEIGER : C-J3 NMR CHEMICAL SHIFfS OF CARBONYL COMPOUNDS

%Q.

Q.22O~If)~!!: 210xIf)

ctu200~~~ 190<!!i~ 180ii&LIQ.

~170

160

150 160 110 180 190 200 210 220CALCULATED CHEMICAL SHIFTS IN PPM

Fig. I - Correlation of calculated and experimental chemical shifts of carbonyl compounds

Addition of connectivity number, which repre-sents effective polarizability, as a second parame-ter markedly improved the correlation to R =0.9679 with s = 4.39 ppm (Table 2). Among the99 compounds which were studied originally, car-boxylic acids and fluoro substituted carbonylcompounds were found to deviate to an extent of7-10 ppm.

All carboxylic acids deviate almost constantlyto an extent of 7.0 ppm and this deviation wasconsidered earlier by Lauterbur et al:' to be dueto the dimerization of acids through hydrogenbonding. By correcting the carboxylic acid chemi-cal shifts for hydrogen bonding, the correlationimproved significantly and gave a correlation co-cfficiera of 0.9790 with a standard deviation of3.65 ppm. In this correlation, the fluoro com-pounds show the most deviation. Deletion of thefluoro compounds reduced the standard deviationto 3.07 ppm with a regression coefficient of0.9859. The plot of the calculated chemical shiftswith the experimental shifts is shown in Fig.I.The deviation of the fluoro compounds may bedue to either the high electronegativity of the flu-orine or the back donation of the fluorine lonepair of electrons to carbon as attributed by Ditch-field and Ellis 1\ giving partial double bond char-acter, and thereby causing an upfield shift of thecarbonyl carbon. The same problem was encoun-tered in our earlier studies of halogen substitutedrncthanes!'.

The I'C chemical shifts of carbonyl carbonscan be calculated using Eq.2:

6 = 201.90 - 247.10Q + 4.40N ... (2)

The charge on the carbonyl carbon can be cal-culated by the partial equalization of electronega-tivity method, in which the atoms arc character-ized by their orbital electronegativity and the to-pology of the molecule. N, the effective polariza-bility was calculated by a bond counting ansatz,The number of bonds per sphere were summedto give N, while allowing for an attenuation by afactor of 0.5 for each sphere further from thereactive centre.

Thus, this equation reproduces the 11C chemi-cal shifts of 90 carbonyl carbons of different alde-hydes, ketones, acid halides, carboxylic acids andesters (Table 1), just with two parameters, the par-tial atomic charge on the carbonyl carbon and theeffective polarizability. As mentioned in our earli-er work 14, these results further support the viewthat polarizability influences 11C chemical shiftsconsiderably as it has an intrinsic relationshipwith magnetic anisotropy.

The slope of the dependence of the 11C chemi-cal shift on the charge of the carbon (-220 ppm/electron), is in the expected range. The negativesign and the magnitude of the coefficient of thecharge is in agreement with the results of Flis-zar ", arrived at using ab initio calculations. Thenegative sign of the coefficient indicates that the

INDIAN J. CHEM., VOL. 27A, NOVEMBER 1988

increase in the charge shifts the signal to downfield.

AcknowledgementOne of the authors (I S N) is grateful to

Deutsche Akademische Austauschdienst for aPost-doctoral Fellowship and CSIR, New Delhifor depuation from RRL, Jorhat. The computeraccess was provided by Leibniz-Rechenzentrum,Munchen. Programming assistance by Dr M G ~Hutchings, B Christoph and H Saller is gratefullyacknowledged.

ReferencesI Stothers J B & Lauterbur P C. ClIll J Chem, 42 (1964)

1563.2 Spiesecke N & Schneider W G. J chem Phys, 35 (1971)

722.3 Grant 0 M & Paul E G, J Am chem Soc, 86 (1964)

2984.4 Jackmann L M & Kelly 0 p, J chem Soc. 8 (1970)102.5 Oelseth C & Kintzinger J P, Helv chim Acta, 59 (1976)

466.

936

6 MacielGE,J chemPhys, 42 (1965) 2746.7 Savitsky G B, Namakawa: K & Zweifel G, J phys Chern,

69 (1965) 3105.8 Yalpani M, Modavai B & Koshdal E, Org Magn Res, 12

(1972) 254.9 Couperus P A, Claugue A 0 H & Von Daugen J P C M,

Org Magn Res, 11 (1978) 590.10 Velichko F K, Dostovalova V I, Vinogradova L V &

Freidlina R H, Org Magn Res, 13 (1980) 442.11 GasteigerJ & Marsili M, Tetrahedron, 36 (1980) 3219.12 Gasteiger J & Hutchings M G, Tetrahedron Len, (1983)

2537;J chem Soc, Perkin II, (1984) 559.13 Oitchfield R & Ellis P 0, Topics in I3C NMR spectros-

copy, Vol I, Edited by G C Levy (Wiley Interscience,New York) 1974.31.

14 Gasteiger J & Suryanarayana I. Magn Res in Chern, 23(1985) 156.

15 Beraldin M T, Vantheir E & Fliszar S, Tan J Chern, 60(1982) 106.

16 Spectral data for structural determination of organic com-pounds. edited by Petsch Clerc Seibl Simon (SpringerVerlag, Berlin) 1983, C170.

17 IlC NMR spectroscopy, edited by G C Levy (John Wiley,New York) 1980, 140.