reduced to the point where such data will be available routinely. As indicated in the preceding section, some of the structural assignments of Table VI11 are tentative. More work is required to determine the structure of the CnH2n+zN2aromatics, and to confirm the structure of the phenyl ketones. Similarly a more detailed analysis and confirmation of the other aliphatic oxygen compounds of Table VI1 seems desirable. Certain ambiguities in the analytical data for the N-alkyl indoles and carbazoles, and the pyridone derivatives, should be resolved. For the most part, however, these are minor points which cannot seriously affect the conclusions of Table VIII. Hopefully, our continuing study of the higher and lower boiling fractions from the present crude oil will answer some of these questions.
ACKNOWLEDGMENT
We are grateful for the advice and/or assistance of several people at the Union Research Center in connection with the present project: F. 0. Wood for experimental work in connection with separations and sample handling, A. E. Youngman for determining the UV spectra, R. F. Buhl and R. J. Kinsella for obtaining the IR spectra, G. C. Graham for the NMR spectra and help in their interpretation, U. Niwa for the nitrogen determinations, E. C. Schluter for the oxygen determinations, J. K. Fog0 for the sulfur determinations, and J. R. Fox and E. C. Copelin for valuable discussions and for editing the original manuscript. RECEIVED for review February 6, 1968. Accepted May 2, 1968.
Nuclear Magnetic Resonance Chemical Shifts of Oxygenated Unsaturated AI iphatics Nugent F. Chamberlain Esso Research & Engineering Co., Baytown Research and Development Dicision, Box 4255, Baytown, Texas 77520
The NMR chemical shifts for unsaturated aliphatic acids, esters, aldehydes, ketones, alcohols, and ethers are correlated with their corresponding molecular structures in a series of charts. The marked effectsof conjugation on the chemical shifts are further correlated with simple resonance hybrids and steric conformations which may simplify remembering and roughly predicting these effects. These correlations should make possible faster and more convincing structural determinations of unsaturated oxygenated aliphatics, revealing in many cases the presence or absence of conjugation and the cis-trans isomeric structure associated with it.
THE REMARKABLE effectiveness of NMR spectrometry in revealing molecular structure depends, at present, to a large extent on empirical correlations of structure with chemical shifts, coupling constants, and band shapes. Fast and convincing analyses require rapid access to precise and detailed correlations. NMR data for derivatives of aliphatic olefins have been scarce and scattered. The importance of their oxygenated derivatives in polymerization, surface activity, and natural products, however, makes it desirable to gather together the available data for this class of compounds. An extensive literature search has provided enough information for effective detailed correlations of chemical shift with structure for the unsaturated oxygenated aliphatics. The complete correlations are presented in this paper to promote greater speed and assurance in making structural analyses of compounds in this class. These correlations are presented in chart form to provide rapid access to the data and show graphically the effects of substituent position and of conjugation on the chemical shifts. This graphic presentation also facilitates the prediction of data which are not directly available. This combina-
tion of measured and predicted data provides a more complete and more useful correlation for each compound type. DATA SELECTION AND EXAMINATION
The accuracy of correlations of chemical shift with molecular structure is greatest when all nonstructural effects (solvent, concentration, temperature, hydrogen bonding, etc.) on chemical shift are eliminated or at least held constant. On the other hand, the general utility of such correlations is enhanced if they include the shifts due to the uncontrollable nonstructural effects normally encountered in analytical work. The data used in this study were selected to provide a workable compromise between these opposing characteristics. Useful- accuracy was achieved by selecting only those data measured on proton signal stabilized spectrometers or measured by the sideband technique, at room temperature, and referenced to internal tetramethylsilane. Necessary exceptions were the data for acrylic acid, for which the shift measurements were less precise, and acrolein, which was referenced directly to cyclohexane and converted to TMS reference by addition of a constant. Utility was enhanced by including data for neat liquids and for solutions in CDCla and CC14. Data for acids and esters in DzO, acetone, and tetrahydrofuran were also included because they agreed within acceptable limits with data for these compounds in CDC18 and CCI4. The data selected by the foregoing criteria were examined critically to reduce errors and inconsistencies. For published spectra, all interpretations were checked and incomplete ones completed, assuming the compounds to be as claimed. For each compound, the shift of each functional group identified by NMR was plotted separately. Data for similar compounds were plotted on the same chart, permitting rapid VOL 40, NO. 8, JULY 1968
0
1317
comparison of comparable data to locate anomalies. Variations not readily explainable by structural differences were checked for errors in measurement, interpretation, or transcribing, and identifiable errors were corrected. The data were also checked against prior correlations. The data used in the final charts are thus self-consistent, and are consistent with known data for comparable functional groups in other compound types. The overall precision and accuracy of the measurements was estimated from determinations on the same compound by different investigators using different solvents. It varied with the functional group. The greatest scatter occurred in shifts of the acidic and alcoholic hydrogens, which, as is well known, vary markedly with solvent and concentration. (Chart 10.2, line 15, and Chart 10.4, line 12.) Appreciable scatter due to nonstructural factors occurred in the shifts of olefinic hydrogens (those attached directly to double-bonded carbons). The observed variation in shift for the same hydrogen in the same compound in different solvents (usually measured by different laboratories) ranged from 0.02 to 0.20 ppm, averaging 0.10 ppm. The greatest variation observed in this study, 0.39 ppm, was for the pc olefinic hydrogen of crotonaldehyde in the neat liquid us. infinite dilution in cyclohexane. Variations in shifts for hydrogens in saturated portions of the chains ranged from 0.00 to 0.12 ppm, averaging 0.05 ppm. For those saturate groups p and y to the oxygen group, an uncertainty of about 0.10 ppm in chemical shift measurement was sometimes introduced by band overlap. This uncertainty affects the accuracy rather than the precision, however, bccause measurements can be made at corresponding arbitrary points in similarly overlapped bands of different compounds. The overall precision, then, is estimated to average 0.05 to 0.10 ppm, and the accuracy is comparable. This complete study covered 93 compounds in 120 compound-solvent combinations. Eighty per cent of the data came from the literature, as indicated in the Data References. The remaining 2 0 z came from the author’s laboratory. GENERAL DESCRIPTION OF CHARTS
Separate correlations for these unsaturated compounds are needed primarily because of the marked effects of conjugation on the chemical shifts. Without conjugation, the shifts are simple linear combinations of the effects of the oxygenated group and the olefinic group, as shown in Charts 10.1 and 10.4. Conjugation introduces new influences, and requires more complex presentation of the shifts associated with it. The presentation employed in Charts 10.2, 10.3, and 10.5 is intended to show clearly the effects of conjugation on the chemical shifts of aliphatic acids, esters, aldehydes, ketones, alcohols, and ethers. Separate chart sections are presented for hydrogens in methyl groups, methylene groups, methine groups, olefinic groups, and hydrogens associated directly with oxygenated groups. Each of these groups, in turn, is located spatially with respect to the oxygenated group by a symbol directly above each data bar, and with respect to the olefin group by a symbol directly beneath the data bar. These symbols refer to the location of the entire group in question (CH8, CH,, CH, or CH=) rather than to the location of the hydrogen alone. An exception is the use of c or t to indicate the steric position of individual olefinic hydrogens. This reference method and the precise reference groups are shown in the top left corner of each chart. To assist the user, the symbols have been defined directly 13 18
ANALYTICAL CHEMISTRY
in structural terms in a number of structure diagrams drawn on each chart. At least one example of each symbol used on the chart has been included on the structure diagrams for that chart. Abbreviations used are defined in a separate box above the structure diagrams. Some items of symbolism may not be clear from the diagrams, however, making further discussion of them desirable at this point. 1. The symbol y f means “gamma and farther” from the reference group. 2. Superscripts indicate more than one reference group of the same kind. Thus az means “alpha to two identical reference groups.” 3. Symbols separated by commas signify that all apply individually, whereas groups of symbols not separated by commas signify that all apply simultaneously. Thus C U , signifies that the data block applies to either a groups or /3 groups. The symbol ap, on the other hand, signifies that the block applies only to groups which are simultaneously a to one reference group and /3 to another identical reference group. Likewise, Pt applies to groups which are simultaneously p and trans to the reference. The data are presented at three levels of reliability. Solid or striped bars indicate data actually measured from reliable spectra. Dotted bars, or portions of bars, show ranges determined by analogy to the reliable data for comparable functional groups, but not actually measured for the compounds represented. These ranges are considered reasonably accurate, although they have not been confirmed by actual measurement. Dashed and dotted lines (not bars) indicate ranges estimated from correlations but not supported directly by reliable data. The confidence which one can place in the range indicated by a given bar depends to some extent on the number of pieces of data which are included in that bar. This measure of confidence has been included by showing separately the number of compounds and the number of compound-solvent Combinations which are included in each data bar. These figures have been placed in a box at the right edge of each chart. The order of the figures in a given line in this data box corresponds to the order of the data bars in that same line in the chart. Additional important information is given in footnotes to most of the charts. These notes are referenced by circled numerals within the chart. DESCRIPTION OF INDIVIDUAL CHARTS
It is the purpose of the charts to present as many as possible of the clues which relate chemical shifts to molecular structure. These clues are numerous, varied, and sometimes subtle, and require presentation in considerable detail. Full utilization of this information requires study of the charts. The remainder of this report is intended as a guide to such study. Chart 10.1. This chart presents the chemical shift data for nonconjugated unsaturated aliphatic carboxylic acids and the acid side of their esters. In the compounds reported in this chart, the C = C group may be conjugated with another C = C , but not with the C=O. This chart has been extended to cover the data for nonconjugated unsaturated aliphatic aldehydes and ketones, also (Notes 3 and 4). Therefore, the chart really covers data for the acylalkenes as well as the carboxyalkenes. The chemical shifts of these nonconjugated compounds are usually additive combinations of the separate effects of the carboxyl (or carbonyl) and olefinic groups. Missing data can
~
-2
-3
-1
1
0
3
2
I
I I I e CARBOXYALKENES~NPNCONJUGATED t -COOH
References: n,p,y
(C-C
not Conjugated
Ill
4
1 1 1 1
1111
5
Ill1 1 1 1 1
6
Ill1 1 1 1 1
7
1111 1111
8
Ill1
9 IIIIII~II
with C = 0 )
IO
r
-
NO. COMPOUNDS IO. COMBINATIONS
! L
0
1
-
8 -COOR
---c=c
1
2
-,-A
3
6
4
-,-,-,2 1 3
I
11
6
1 5 2 2 5 2 1
6,-,2,14 6 8
2 18
&,-,M,U
7
1 -'-'-,1
0
L8
16 18
1 2 ,-
0
9
0
Two NT
NT
NT
C-CH-R
OLEFINIC
Bandsh*L+
F!q
I
TERMINAL NONCONJUG4TED OLEFINS
-0-
NONTERMINAL
1
3 13
2
4 19
R-CHmCH-R
H
-
TERMINAL NONTERMINAL
CONJUGATED OLEFINS
3 3 3 3
-3-
0
13
1d
-
@ This indicates the shift range of ketone methyls (u/Y* and a/?+; it does not apply to acids, esters, or aldehydes. @ Because of the interactions through space in these molecules, interpolation and extrapolation of shifts are not always linear. The dotted boxes and lines must therefore be used with caution.
be filled in from charts for aliphatic olefins and aliphatic acids, ketones, and aldehydes. Most of the data for methyl and methine groups have been obtained by this analogy, as indicated by the dotted boxes and lines. There are some accidental equivalences in chemical shifts which must be taken into account in the interpretation of the spectra of these compounds. Since these equivalences would not be predicted from a simple additive combination of the separate effects of the olefin and carboxyl groups, they are attributed to mild interactions between the two groups. These data d o not permit a separation between inter- and intramolecular interactions. Because of these interactions, CH, groups a to carboxyl and @ to olefin have essentially the same chemical shifts as CH2 groups a to olefin and p to caroboxyl (Lines 5 and 6 and Note 1). Somewhat surprisingly, CH2 groups a: to carboxyl and y or farther from olefin have this same chemical shift range (Line 7). The chemical shifts of CH, groups p to olefin and those p to carboxyl usually cannot be either distinguished from each other or measured accurately a t 60 Mc. This uncertainty is due to the overlap of their bands with adjacent bands (Note 2). The two terminal olefinic hydrogens, C=CH2,usually have
measurably different chemical shifts except in symmetrical 1,l-disubstituted ethylenes. The difference in the shift is so small for isolated vinyl groups, however, that they are presented as one bar on the chart (Line 11). Nonterrninal hydrogens in vinyl groups (C=CH-R) resonate a t a little lower field than d o other nonterminal hydrogens (R-CH= CH-R), as indicated in Line 12. Chart 10.2. This chart presents the data for conjugated unsaturated aliphatic carboxylic acids and the acid side of their esters. This chart covers only those compounds or those portions of compounds in which all of the double bonds are conjugated, C = C with C=O and with a second C = C if present. The data for olefins not conjugated with carboxyls are included in Chart 10.1. Two important effects of conjugation on the chemical shifts may be observed in this chart. They are the superposition of conjugative effects on inductive effects, and the enhancement of the normally small cis-trans steric effects. I n nonconjugated systems, the inductive effect always causes the resonance of a given hydrocarbon group to move downfield as the hydrocarbon group approaches the electronegative group. For instance, the &values of a CH2are in the order a: > @ > r+with respect to a carboxyl (Chart 10.1, Lines 5, 6, and 7). Conjugative effects in the a:-@ unsaturated acids, however, VOL 40, NO. 8, JULY 1968
1319
‘-a
-4
-1
-2
I I CARBOXYALKENES,
0
1
I
1 1 1 I I I1 I
2 3 4 I mpm[l I I I I nTplrTlrrllTI11I , I I
CONJUGATED (C=C conjugattd wlth
-
+ -COOH
References:
a,P,y,c,t
I
bo,
and with second C=C
6
7
1 1 , I I I I , I I I I ] I I I , , II I I
4 0 r
* n I,
whe.n p r t r e n t ) .
NO. COMPOUNDS NO. COMBINATIONS
72Ct
4 5
and -COOR
5 5
-9-
-c=c c = cis
= Terminol
W
z
1
&,A
2
%,&a .2 2 2
3
12 16
T
-
4
5
6
MONO-OLEFINIC, MONOCARBOXYLIC
TERMINAL
7 MONOCARB.
ROOC NONTERMINAL
OICARBOXY LIC
Pcr
B*ct----l
RW
I
8 MONO-OLEFINIC
TERMINAL
NONTERMINAL
i
9 DICARE,
I
LO
0
z
LL
W
I&
1 2
13
L4
0
I
14
11
13.
I
I
1).
12
I 10
I I I I I I I I I I I I I I I 1 t I I I I I I I 1 I 1 I I I 1 I I I I I I I I I I I I I 1 I I I I I I I I I I 1I I 1 I 1 1. I 8 7’ 6 5 4 3 d
9
I
I
I I I I I II I 1 1 I I
’
2
I 1 1 I1 I
1
I ob:
5
-
Chart 10.2. Generala H1 NMR chemical shifts for unsaturated aliphatic acids and their esters (Jan. 1967) @ Solvents used in obtaining these data were: CCL, CDCI,, acetone, D,O, tetrahydrofuran, and none (neat liquid). Chemical shifts of groups closely associated with carboxyl changed markedly in some other solvents. Benzene shifted these values 0.1 to 0.5 ppm upfield. CF,COOH shifted these values 0.1 to 0.4 ppm downfield. Salts of these acids in D 2 0exhibited an upfield shift of 0 to 0.5 ppm. @,@ The relative shift between groups cis and t v m s to the carboxyl is enhanced enough to be diagnostic of the steric position. Note also that the shifts of groups p and y t to carboxyl are indistinguishable, and that nonterrninal olefinic hydrogens p to carboxyl resonate at lower field than those 01 to carboxyl. @ Steric position with respect to the additional C=C in diolefinic acids also appears to contribute to chemical shift differences. This effect is not shown in other applicable sections of this chart because it is too small to be reliable. @ Because of the interactions through space in these molecules, interpolation and extrapolation of shifts are not always linear. The dotted boxes and lines must therefore be used with caution.
-
change this to y t ,8 for saturated groups (Chart 10.2, Lines 2, 3 , 4 , and 5) and p > a for olefinic groups (Lines 8, 12, and 14). The enhancement of the relative shift between groups cis and trans to the carboxyl is great enough to make these shifts diagnostic of the steric position. Whereas in Chart 10.1, Line 11, this relative shift was too small to be of general value (0.1 ppm), in Chart 10.2 the corresponding relative shift is about 0.6 ppm and the data bars are well separated (Lines 7 to 14). Even for saturated groups closely associated with these conjugated systems the relative cis-?runs shift is 0.2 to 0.4 ppm (Lines 1, 2, and 4). The second conjugated olefinic group in diolefinic acids also appears to contribute to cis-trans chemical shift differences. These differences are much smaller than those produced by the carbonyl group, however, and are clearly discernible only in Line 14 (Note 4). These conjugative effects can be correlated with a simple 1320
ANALYTICAL CHEMISTRY
two-part model. This model is to be used only as a device for correlating the data. The explanation of the origin of relative chemical shifts is, in general, not well understood ( I ) . The first part of the model assumes the existence of the resonance hybrid shown in Figure 1 (2).
B
‘R
The changes in relative shifts of a, 6, and y groups are attributed to the effects of the charge separation, and the upper limits of these changes are estimated from the shifts for related (1) J. I. Musher, “Advances in Magnetic Resonance,” J. W. Waugh, Ed,, Academic Press, New York, 1966, pp 177-224.
(2) R. T. Morrison and R. N. Boyd, “Organic Chemistry,” Allyn and Bacon, Boston, 1959, pp 451-4 and 728-44.
organic ions. The organic ions are taken as the limiting case of complete charge separation with realistic charge localization (or delocalization). Predictions from this part of the model correlate with observations as shown in Table I. The predictions are brought into line with the observations by making the reasonable assumption that charge separation is incomplete in this model. The second part of the model correlates the marked enhancement of the relative cis-trans chemical shifts with the anisotropy attributed to the carbonyl group (3) and the magnetic field which arises from it. (Other books also discuss this general phenomenon, but Jackman seems to have been first to describe it specifically for the carbonyl.) Stuart and Briegleb models show that the /?c and yc groups are held much closer to the carbonyl than are the P t and y t groups, even when reasonably free rotation is permitted. The greater downfield shift of the cis groups is then attributed to the greater influence of that region around the carbonyl group which induces downfield shifts. This situation is represented in two dimensions in Figure 2, which shows those
0“
(Figure 2) rotational conformers in which the cis groups will be most strongly influenced by the carbonyl ( 4 ) . Additional evidence that electron density distribution may be the dominant factor in the chemical shifts of these compounds is afforded by the solvent effects discussed in Note 1 to this chart. Trifluoracetic acid solvent introduces additional downfield shift in the resonances of groups closely associated with the carboxyl groups. This can be attributed to a positive charge acquired by the carboxyl as the result of protonation. Likewise, the upfield shift of similar groups in the DzO solutions of salts of these acids can be attributed to the acquisition of a negative charge by the carboxyl group because of ionization of the salts. Benzene, as a solvent, produces preferential shifting of the resonances of groups closely associated with the carbonyl or olefin groups. This leads to greater scatter in the shiftstructure correlations and makes benzene a n undesirable solvent for use in measuring accurate shifts. Although the data for the nonterminal olefinic hydrogens in diolefinic dicarboxylic acids (Line 14) appear to be quite detailed and informative, it must be remembered that they have been collected from only one or two compounds. The data bars must, therefore, be used with caution. The chemical shift of the acidic hydrogen (Line 15) is related to the strength of hydrogen bonding, which is a function of the solvent and concentration. In water, this shift is coincident with that of the water because of rapid exchange. Chart 10.3. This chart presents the chemical shift data for the conjugated unsaturated aliphatic aldehydes and ketones. It includes data for only those compounds in which the C=C is conjugated with C=O and with a second C = C when present. Data for C==C groups which are not conjugated with C=O are presented in Chart 10.1.
Table I. Observed us. Predicted Conjugative Effects, Based on Figure 1
Group =CH-COOR
-CH=C-COOR
-C==C(CH,)-COOR CH,-C=C-COOR
Observed A shift
Predicted A shift
0.2-0.4 ppm downfield from unperturbed olefinic Ha 0.7-1.2 ppm downfield from unperturbed olefinic Ha None 0.2-0.3 ppm downfield from corresponding groups in Chart
0.4 ppm or less downfield from unperturbed olefinic Hb 3-5 ppm downfield for complete charge separationc Little or none 0.5-1.3 ppm downfield for complete charge separationC
10.1 a
0 c
Compared with Reference (5). By analogy to CH resonance in saturated aliphatic acids. From data for unsaturated carbonium ions (6).
The conjugative effects in these compounds are similar to those for the conjugated unsaturated acids but are not quite so pronounced. On the unsaturate side of the carbonyl, the chemical shifts of saturated groups y t to the carbonyl are indistinguishable from those of saturated groups p to the carbonyl (Lines 1 and 4). F o r the nonterminal olefinic groups, chemical shifts of those a to the carbonyl are about the same as for those p to the carbonyl (Lines 11, 12, and 15 and Note 1). The difference in chemical shift between groups cis and trans to the carbonyl is also increased by the conjugation, but less than it is for the unsaturated acids. This difference is not as diagnostic for the steric configuration of ketones and aldehydes as it is for acids and esters. These conjugative effects are correlated by models similar to those of Figures 1 and 2. The resonance hybrid for the aldehydes and ketones is shown in Figure 3. (Figure 3)
In this case only one oxygen atom is available to help separate the charges, whereas two are available in acids and esters. It therefore seems reasonable that the charge separation would be less for aldehydes and ketones and its effect on their chemical shifts would be less, as is observed. The lesser enhancement of the relative cis-fvans chemical shift may conceivably be attributed to a lower electron density on the carbonyl which results in a weaker magnetic field being generated by the carbonyl. Data to support or reject such a proposal are not available, however. This chart is not as complete as is desirable because of the scarcity of data. The lines for the olefinic hydrogens of diolefinic aldehydes and ketones contain data for only one to three compounds. F o r this reason these data bars must be used with caution. No data at all were available for unsaturated compounds containing two carbonyl groups. The chemical shifts for such compounds may be estimated from the appropriate sections of Chart 10.2 with one exception, The shifts of nonterminal olefinic hydrogens a to ketone or
(3) L. M. Jackman, “Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry,” Pergamon Press, New York, 1959, p
124. (4) L. M. Jackman and R. H. Wiley, J . Chem. SOC.,1960,2886.
C. Stehling and K. W. Bartz, ANAL.CHEM.,38, 1467 (1966). (6) N. C. Deno, Chem. Eng. News, Oct. 5 , 1964, 88-100. ( 5 ) F.
VOL 40, NO. 8, JULY 1968
1321
4 0
ACYLALKENES, CONJUGATED (C=C conlugated with C-0, and with second C=C +
-CHO
and
when present)?
-COR
References:
a,B,7,8,c,t
7
NO. COMPOUNDS
d
YO. COYLIINATIOKS
E
LA2
1
3 15
---c=c
-
T E Terminnl
4 6
-8-
4 4
2
L,1
3
1 4 -,-,1 4
4
3 1
LA-,-@ 5 1 2 -,-11
6
7
8
9
-_-__ 1
..
I1
/&
0
f,
&,T
,Q$\
0-c,
MONO-OLEFINIC
DlOLEFlNlC
/D
BtNT
I ALO.
ALDEHYDIC H ACROLEIN@
-ALoEHYo~C
BE
;c-0 ALO. 11
TERMINAL
NONTERMINAL
PC
-I-
6
6
7 7.
7 10 -4-
0
1
3
2
5
OLEFINIC H PCYCBt yt 8
I I I I----
7
Bc
Y BC
1 1 1 1 , 1 1 1 1
TERMINAL
P
-,-1-1-
OIOLEFINICO
a
‘I!
Pt-I
l17‘
Bc
