analyses. I n many cases independent mass spectrometric analysis for deuterium was performed, and in some cases analyses by other methods are listed for comparison. The satisfactory agreement between the methods is evident. LITERATURE CITED
(1) Jones, R. N., MacKenzie, M. A., Talanta 3, 356 (1960).
(2) Jones, R. N., MacKenzie, M. A., Ibid., 7, 124 (1960).
(3) Sickon, A., Hammons, J . H., Abstracts of Papers, 144th Meeting, ACS, p. ZOM, Los Angeles, Calif., 1963. (4) Nickon, A., Hammons, J. H., Lambert, J. L., Williams, R. O., J . Am. Chem. SOC.85, 3713 (1963). (5) Nickon, A., Lambert, J. L., I b i d . , 84, 4604 (1962). (6) Kickon, A., McGuire, F. J., Mahajan, J. R., Umezawa, B., Karang, S. A., Ibid., 86, 1437 (1964). (7) San Pietro, A,, in “Methods in Enzymology,” S. P. Colowick, S . 0. Kaplan, eds., Vol. IV, p. 473, Academic Press, New York, 1957.
(8) Thornton, V., Condon, F. E., ANAL.
CHEM.22, 690 (1950). (9) Trenner, K. R., Arison, B. H., Walker, R. W.,Ibid., 28, 530 (1956). (10) Trenner, N. R., Arison, B. H., Walker, R. W., A p p l . Spectry. 7, 166 (1953). RECEIVEDfor review April 27, 1964. Accepted July 6, 1964. Work supported by grants GM06304 and GM09693 from the Sational Institutes of Health and also by a grant from the Petroleum Research Fund administered by the American Chemical Society.
Determining Structure of Paraffinic Chains by
NMR
KENNETH W. BARTZ and NUGENT F. CHAMBERLAIN Research and Development, Humble Oil and Refining Co., Bayfown, Texas
b The NMR chemical shifts and spectral patterns of the different types of hydrogen on paraffinic chains are remarkably reliable, and are sufficiently unique to provide decisive structural information. The spectral patterns produced b y paraffinic methyl groups are uniquely characteristic of the adjacent chain members, whereas their chemical shifts are strongly influenced b y sizable segments of neighboring chain structure. O n the other hand, the spectral patterns and chemical shifts of paraffinic methylenes are primarily indicative of methylene chain length, and only secondarily indicative of chain structure. The combination of these data conveys a great deal of information concerning the detailed structure of saturated chains, including those segments of substituted paraffins which are beta or farther from the nonparaffinic substituents. This paper presents detailed charts showing the correlations of the chemical shifts vs. chain structure, and detailed figures showing the variations of spectral patterns with chain length and structure. These correlations greatly simplify the identification of pure paraffins, the characterization of saturated hydrocarbon polymers, and the determination of the structure of the paraffinic chains associated with nonparaffins.
T
HE high stability of present nuclear magnetic resonance (NMR) instruments has made possible a new order of reliability and utility of precision chemical shift data for paraffins. These data, together with spectral patterns characteristic of certain “paraffinic groups,” constitute a valuable aid to structural analysis. “Paraffinic groups” are defined by Williams and
Chamberlain (5) as alkyl groups sufficiently removed from (p and farther), and, therefore, not affected by substituents such as, for example, aromatic rings, unsaturated linkages, and heteroatoms. Consequently, the data apply not only to paraffins but also to paraffinic groups in aliphatic hydrocarbon derivatives and in alkyl substituted aromatics. However, spectral patterns of cycloparaffins, which are complex, poorly resolved, and follow different rules, overlap those of the paraffins, and their presence in a sample severely limits the use of these paraffinic data. EXPERIMENTAL
Proton chemical shifts and spectral patterns have been obtained for approximately 100 paraffins and n-alkyl benzenes, most of which are API certified samples (99.95+ % pure), with a Varian A-60 spectrometer. The optimum combination of resolution, signalto-noise, and chemical shift accuracy was obtained at 50% sample concentration in CClr. Lower concentrations (down to 10%) reduced signal-to-noise to undesirable levels with negligible change in chemical shift, whereas high concentrations reduced resolution. I n many instances carbon disulfide solvent provides better resolution, but the accompanying solvent shifts are too large to be ignored. The paraffin chemical shifts reported in Charts I to IV were measured a t 50% paraffin concentrations in CCl,. hlthough it was experimentally established that the chemical shifts of paraffins are almost insensitive to changes in CCl, concentrations, the opposite effect is observed for the paraffinic chemical shifts of n-alkyl benzenes. Therefore, the chemical shifts of the latter are not included in Charts I to IV. I n addition, chemical shifts are reported only for those spectra for which reasonable
first order interpretations could be made. All chemical shifts are referred to tetramethvlsilane as an internal standard. TKe scale used is that adopted by Tiers (4) with T M S a t 10.0 7 . The spectrometer sweep calibration was checked every day, with anisaldehyde containing tetramethylsilane, to make sure it did not deviate by more than f1/2yo. Calibration adjustments were made when necessary. DISCUSSION
M e t h y l Spectral Patterns and Chemical Shifts. Spectral patterns
of paraffinic methyl groups are uniquely characteristic of the adjacent chain members, whereas their chemical shifts are strongly influenced b y sizable segments of neighboring chain structure. These two attributes make methyl resonances extremely useful for determining structures of paraffinic chains. The three basic paraffinic methyl NMR patterns are illustrated in Figure 1. A distorted triplet pattern is observed for methyls spin-coupled to methylene groups, Figure 1, a. Methyls spin-coupled to tertiary hydrogens give rise to a characteristic doublet, Figure 1, b, and methyls attached to quaternary carbons form sharp singlet peaks, Figure 1, c. The greatest amount of structural information is obtained by considering each of these patterns, with its associated chemical shifts, separately. M e t h y l Triplets. T h e variations of methyl triplet patterns with chain length in normal paraffins and in nalkylbenzenes are illustrated in Figure 2. I n both series the shapes become invariant a t n = 4, with little change a t n = 3, b u t the shapes for n = 2 and n = 1 are sufficiently distinctive to provide useful diagnostic tests for short chain segments. T o illustrate, VOL. 36, NO. 1 1 , OCTOBER 1964
2151
O,hbnno~Paraffim: c-[c]~-c
f-Butyl Terminated /-Propyl Terminated
c-c-[cl”-c-c
Ethyl Terminated
c - [cIn- c
JL
&
I
C
9.0 9.2 1 ’ (.. I
’
9.4 ,’
I
b. n-Alkyl B o n m ~ o: - C - [ C l , - C
n=4t
I
_____
b
0
Figure 1 . patterns
9.0
C-C-[CIn-C-C I I
C
CI
c
C
9.2 I ’ [, ” I’ ’
PPM (THS.IO.0)“
Three
basic
C
paraffinic
CH3
NMR
the successful identification of 6acetoxyoctanal, I, was partially based on the close correspondence of its methyl triplet pattern CH3-CH2-CH-(CH2)4CHO OAC
I to that in Figure 2b, n = 1, which indicated a single CH2 group between the methyl and the point of attachment of the acetoxy group. The acetoxy group was thus shown to be attached to the third carbon from the methyl end of the alkyl chain. The reader should note that the carbon to which the electronegative group is attached is not counted in determining n. Methyl triplet patterns are divided into four categories according to their chemical shifts in Chart I. The chemical shifts of nonethyl terminated paraffins (Chart I, a) occur between 9.10 and 9.13 T . The remaining three categories of methyl triplets in Chart I are concerned with chemical shifts of CH3’sin ethyl appendages. Resonances of methyls located in this environment occur at slightly higher fields than those for nonethyl methyls. This upfield shift is accentuated for
Table I.
CH3’s in “isolated” ethyl groups, the resonances of which occur at the highest field position of all the methyl triplets, 9.17 to 9.27 T (Chart I, d). These methyl chemical shifts are also sensitive to the number of isolated ethyl> attached to the quaternary carbon atom (Table I, b, c, d, and e ) . For example, if the isolated CH3’s in 2,a-dimethylbutane (Table ‘I, b ) are successively replaced by ethyl groups, the CH3 triplet shifts upfield from 9.17 to 9.19, 9.24, and 9.27 T . Simultaneously, the methylene resonance shifts from 8.82 to 8.83, 8.84, and 8.88 7. The chemical shifts of the methyls attached to the quaternary carbon atoms in these compounds are affected in a similar manner as discussed in a later section. Therefore, the substitution of ethyl radicals for methyl radicals in this series of compounds increases diamagnetic shielding of all the protons. This chemical shift effect indicates that the ethyl group has greater shielding power than the methyl, or that ethyl
Chemical Shifts of Selected Isolated Methyl and Ethyl Groups
CHI Paraffin U.
C-(CHa)4
b. (CHa)a-C-CHzCHa C.
d. e. f. 8.
h.
i.
(CHa)z--C-(CH2CHa)z CHs-C-(CHzCHa)a C-(CHZCHd4 (CH3)3-C-CH2-CH2-CHs ( CH~)~-C-(CHZ-CH~-CH~)~ (CH~)~-C-(CH~)~-CHS (CHa)a-C(CH2)4-CHs
2 152
ANALYTICAL CHEMISTRY
Singlet 9.06 9.13 9.19 9.25 , . .
9.12 9.17 9.13 9.12
Triplet , . .
9.17
9.19 9.24 9.27 9.12 9.12 9.10 9.10
CH2 Multiplet ... 8.82 8.83 8.84 8.88 8.81
8.82 8.79 8 77
causes less deshielding than methyl [the “methyl effect” described by Cavanaugh and Dailey (Z)]. Substitution of propyl for ethyl, Table I, b and f, causes only a slight change in the methyl singlet resonance but it markedly affects the triplet resonance. Simultaneously, the methylene resonance shifts slightly downfield. An analogous chemical shift behavior is observed for paraffins c and g in Table I. These results indicate that the isolated methyl groups are exerting the controlling influence on these shifts, and that this influence does not extend past groups which are y to a methyl ( p to the t-butyl group). It is assumed that this effect is related to the “crowding” and “shielding” effects of t-butyl and internal geminal methyl groups which are discussed later. Note, however, that the observed methyl triplet shift in comparing f to b and g to c is in the wrong direction assuming that methyls are deshielding groups. No explanation of this anomaly is offered a t this time. Further increases in the length of the isolated paraffinic chain (Table I, f, h, and i) have no effect on the methyl singlet position and little effect on the methyl triplet and the methylene positions. This shows that the chemical shifts, as well as the spectral patterns, become invariant a t chain lengths of about 4 methylene groups. In addition to having unique chemical shifts, “isolated” ethyl groups give rise to characteristic and easily recognizable methyl and methylene resonance patterns (Figures 3 and 6, row 1, column 3). These three unique features of an
11
Chart
Chart 1
CHEMICAL SHIFTS OF [ C H ~ ] - C ~ H
CHEMICAL SHIFTS OF [CH3]-CHeMETHYL T W P L E T S Q
METHYL DOUBLETSO
CHEMICAL WIFT. I
10. RANGE 3.
SINGLE
METHYL
BRANCHES
c I X045
NO. RANGE
2 9.13
S.
3 9.089.12
31
I
43 9.13-
9.10. 9.13
9.16
9.139.1 4
3 9.199.24 ISOPROPYL
9.\49.t 6
dl &-C-!-
,,
R
6-r
4 9.w-
9.09
( 7 9.44-
9.14
R: m
b
Chemical shift8 a n measured from the middle peak of the trlplrt.
R : m>O
L179.27
CHEMICAL SHIFT. T
@
G
T E RY INALS
01
a1 righl
Q p) [C]*-b- C - R
R : m>4
I 9.15 5 9.169.17
8 9.09 9.24
= Alkyl mdlcalr, CmHzm,t, with m*O.
R, Rl, R,,
0 Chemical ,shifts are mawred from thu daublet center.
“isolated” ethyl group serve as a diagnostic test for its presence in a paraffinic group. The methyl pattern of isolated propyl is almost always masked by other methyl resonances, if present, but isolated propyls can be identified from their characteristic methylene pattern illustrated in Figure 6, row 2, column 3. Methyl Doublets. As illustrated in Figure l b , methyls spin coupled to tertiary protons give rise to well defined doublets, in each of which the high field peak approximates two thirds the height of the low field peak and the separation between peaks is
R I
R‘- C - C -C I R” The sfactrum shown is for 3,3-Dlethylpentane IR= R = R”=€t), but this pattern appears to be characteristic of the ethyl group CH3’s when R , R : R” ore other alkyl groups (not hydrogen).
In
8.5
b
I
3
I s
3
0
I
L
I
I
I
I
D l 1 1
8
0
0
1 %I
I a
1 ,
a
I,
8.8 9.0 9.2: 9.4 9.6 9.8 10.0 PARTS PER MILLION REFERRED TO INTERNAL TETRAMETHYLSILANE AS 10.0.
Figure 3’. Distorted triplet pattern of methyls in isolated ethyls
@
R , R,, R0,R3
@
R l Included 1-butyl and internal gem methyl group.
@
@
a
alkyl radicals, C,Hemtl, aith valuer of m w s h n . When m : 0, R *H. -
Chemical shift of CH3 doublet produced by triisoprapylmethane. Non-equivalent methyls in isopropyl groups. They produce t m separate dcublets, OM in the “I” block nnd one in the “h” block.
approximately 6 c.p.s. The doublet patterns in monomethyl paraffins are somewhat masked by the superposition of a methyl triplet pattern as is shown in Figure 4. Slight pattern variations with increasing chain length are observed for the lower homologs of the 2- and 3-methyl series, but pattern invariance is attained a t n = 3. The methyl pattern of 4-methylheptane resembles those of the 3-methylparaffin series. However, as this was the only compound in this series available to us, nothing can be said about its pattern invariance with increasing chain length. The chemical shifts of such methyls are conveniently categorized in Chart I1 into those arising from single methyl branches and those arising from isopropyl terminals. Each category is further subdivided according to the influence of adjacent groups. The resonance positions of “normal” methyls of these two types, those which are not influenced by neighboring groups, are shown in Chart 11, b and e. The very narrow ranges of these chemical shifts seem adequately documented by a reasonable number of compounds of differing over-all structures. The shifts of ‘‘normal” isopropyl terminals occur from 9.11 to 9.14 T , whereas those of “normal” single methyl branches occur a t slightly higher field, 9.13 to 9.16 2.
The effects of neighboring groups on the chemical shifts of methyl doublets are defined by a smaller number of compounds, but they are given additional support by the fact that the phenomma observed are consistent for both categories of methyl doublets and for both categories of methyl singlets (next section). When the methyls of single methyl branches or isopropyl groups are located on a carbon beta to t-butyl cir internal geminal methyl, the resultani, doublet resonances are shifted downfield significantly (Chart 11, a and d ) . Methyl branches on carbons beta to two such bulky groups exhibit an accentwted downfield shift (Chart 11, a, Rz = neopentyl). This phenomenon, reported for methyl singlets by Edwards and Chrtmberlain (S), is attributed to extreme “crowding” of the methyl groups in such structures. As indicated by Stuart and Briegleb models, such “crowding” hinders rotation of the groups involved and probably causes changes in the bond angles within the system. These effects may change the relative lifetimes of the various rotational conformers and the net effect of the anisotropies of the numerous carbon-carbon bonds. On the other hand, the doublet resonances from isopropyl groups located alpha to t-butyl or internal VOL. 36, NO. 1 1 , OCTOBER 1964
2153
2- Methyl Poroffinr: C-C- [C],8.8
_ . , .9.4. .
9.0 1 9 . 2
I C
1
geminal methyl groups are shifted upfield significantly (Chart 11,f ) . Stuart and Briegleb models show that in such cases there is little steric hindrance to rotation and no significant bond angle distortion. The upfield shift may be due simply to the net effect of the anisotropies of the additional neighboring carbon-carbon bonds. This upfield shift is not observed for resonances of single methyl branches unless the methyls are flanked by t-butyl or internal geminal methyl groups on both sides (Chart 11, c). No explanation for this difference is offered a t this time. It is important to note that in two specific cases the simultaneous occurrence of both shielding and crowding structures in the same molecule resulted in essentially no shift of the methyl doublet from its "normal" position (Chart 11, a, Rz = t-butyl). I t is emphasized that the crowding and shielding phenomena just described apply specifically to structures which have t-butyl groups or internal geminal methyls. Data are not available to show whether these effects also apply to structures involving quaternary carbons carrying more than two chains longer than methyl. Methyls in isopropyl groups attached to a tertiary carbon carrying a single methyl branch are magnetically nonequivalent because of the asymmetric environment, and produce separate doublets (Chart 11, 9 ) . Although the chemical shift ranges of these two doublets overlap slightly, they are distinguishable. The complete resonance patterns are usually complex, as indicated in Figure 5 , making precise determination of the shifts difficult. Williams and Chamberlain ( 5 ) considered the resonance patterns of Figure 5 to be analogous to the special ABX, patterns calculated by h n e t (1). This would be true if the resonances of the tertiary hydrogens were separated by less than about 8 C.P.S. It has since
C
.-/ I
l l .9.0, . 119.2 , . , l .9.4. .
9.0. "1 9 , ."2 ' ' '9.4 ,
I . I9.0 I I jIib.2
9.4
1
Ii
3 -Methyl Paroffins: C - C - C
- [C],
-C
4 - Methyl Heplone
C NMR CONDITIONS
Frequewy: 60 Mc. Scan Rote: 0.5cprlSec R F. Arnplihlde. 0.1mG lnrrtrument:VorlooA-60 Sample Temp.: 32' C Solvent: CCb Concentration: x)Vcl.% Scoles ors ports per miiiion referred to inter.
nul tetfomethylrilone os 10.0.
Figure 4.
-
CHI NMR patterns for monomethyl paraffins Chart
In
CHEMICAL SHIFTS OF METHYL SIMLETS No. a.
3
AN01
9.02
9.03
8
9.04 9.t t
to
9.E 9.t3
4
9.14
3
9.16
INTERNAL GEMINAL METHYLS
F;'
1
8.99
2
9.03
1
8.08
2
9.16, 9.M
3
9.t7 9.t9
4
9.1.
3
9.23 9.25
2
9.30
c c I 1 c-c-y-c-c
f
F:?
c-c-c-c-c-c
+
: alkyl rodlcoie, C n H e m + ( , with m b O , but t-butyl or internal gem m t h y l radical8 except where indicated.
-
@ 2 , 2 dimthylpropane ( naapntane 1, 9.065 I. @
Polyirobutyknr.
21 54
ANALYTICAL CHEMISTRY
I
C
9.23
CHEMICAL SHIFT. I
0 R, R,,
c c c I l l c-c-c-c-c
b
b
Figure 5. NMR patterns resulting from nonequivalent methyls in isopropyl groups
Chart
IV
CHEMICAL SHIFTS OF METHYLENE GROUPS@
a)
c-[~]”-c
nrr
R,=R-(-
C
bl R?-b-[C],-C
I
C
C RI. R*C-
c
C;st.e
i C-[Cl,-
8.7713 8.84
8-c - m - c
c
+- + .
c-cC
[Ch-
c
c-
_
[Ch-
c
-
-c
C
n-5
0 Chemical
shifts were measured from resmance patterns.
@
R olkyl
radical, , C m j e m t t , wlth
@
Polyisobutylene,
@
Poly-3-methylbutsne-1,
,
the centen of
the methylene N M R CWMTIONS
I
1
m>O.
8.58r.
been found, however, that in 2,2,3,4tetramethylpentane (Figure 5 , c) and analogous paraffins these resonances are separated by 50 c.p.s. or more, making the Anet calculations inapplicable. If comparable shifts apply to all configurations in Chart 11, g , the complexity of the patterns must therefore be due primarily, if not entirely, to nonequivalence of the methyls. Methyl Singlets. Methyls attached to quaternary carbon atoms give rise to sharp singlet resonances (Figure 1, c) lying between 8.88 and 9.30 7 . The chemical shifts of these singlets are conveniently divided into those arising from t-butyl groups and those arising from internal geminal methyls (Chart 111). Each category is further subdivided according to the effects cf neighbcring alkyl groups. Methyl singlets, in a manner analogous to methyl doublets, exhibit both “crowding” and shielding effects. The chemical shifts of “normal” tbutyl groups lie in the range 9.12 to 9.13 7 (Chart 111, c), whereas those for “normal” internal geminal methyls lie in the slightly higher range 9.17 to 9.19 7 (Chart III, h ) . The range for t-butyl methyls is based on a reasonable number of compounds, but the range for internal geminal methyls is not very well documented. Additional support for the latter range comes from its consistence u-ith the other ranges shown. A methyl branch on the carbon beta to or an isopropyl group beta to a t-butyl group causes a significant down-
Figure 6.
field shift in the singlet resonance (Chart 111, b ) . t-butyl or an internal geminal methyl group in the beta position causes an even greater downfield shift (Chart 111, a ) . These same configurations produce similar effects on resonance singlets arising from internal geminal methyls (Chart 111, f and 9 ) . These shifts are attributed to the “crowding” effect previously described. Bulky groups alpha to t-butyls or internal geminal methyls cause an upfield shift of the resonance singlets (Chart 111,d, e , i,j, and IC). The trends, however, are not so regularly related to over-all structure as they are for the crowding effect (note reversal of structure type in Chart 111, d and e , us. i and a). S o explanation for this reversal is offered a t this time. Another interesting effect of neighboring groups on chemical shift is illustrated in Table I, a-d. Beginning with neopentane, successive replacement of methyls with ethyls causes a progressive upfield shift of the methyl singlet. Aisimilar effect on the methyl triplet has already been discussed. Methylene Spectral Patterns and Chemical Shifts. T h e spectral patterns and chemical shifts of paraffinic methylenes are primarily indicative of methylene chain length and only secondarily indicative of chain structures. T h e y are useful for confirming
Alkyl
CH, NMR
patterns vs. chain length
the struct,ural information obtained from methyl resonances. The effect of chain length on the resonance patterns of methylene groups alone is illustrated in Figure 6. The methylene multiplet pattern coalesces into a single peak a t n = 4 and n = 5 for contiguous methylene units, (CH,),, which are spin coupled a t both ends (Figure 6, columns 1 and 2 ) . The resonance of a methylene chain spin coupled a t one end only collapses to a single peak a t n = 3, whereas a methylene chain isolated at’ both ends gives rise to a singlet resonance for all values of n (Figure 6, columns 3 and 4). .Isdiscussed earlier, the characteristic met’hylene patterns produced by “isolated” ethyl (Figure 6, Col. 3, n = 1) and propyl (Figure 6, column 3, n = 2 ) groups help considerably to identify these groups in a paraffinic chain. I n spectra run a t 60 M c . ~methylene and methine multiplets generally merge so that it is not possible to separate them completely. Consequently, both patterns are presented together in Figure 7 , which illustrates their variation with methylene chain length. The chemical shifts of methylene groups are presented graphically in Chart IV. .Ilthough overlap among their shift ranges is great enough to reduce severely the diagnostic value of these data. some informative trends are VOL. 36, NO. 1 1 , OCTOBER 1964
2155
2.lw-Il-OIMETHYL PARAFFINS
c- c - [CI"- c - c I
c
1
2-METHYL PARAFFINS
-
c c -El" - c
c
I
1
C
which lie a t higher field for chains of 1 or 2 methylenes than for longer chains. Methine Spectral Patterns and Chemical Shifts. Methine protons are almost always strongly spin coupled to several other nearby protons; hence, they present complex multiplet patterns (Figure 7 ) . I n addition, the low concentration of methine protons and the merging of their bands with those of methylene groups masks their patterns and renders the determination of their chemical shifts inaccurate. Spin-spin decoupling or higher operating frequencies will be required to clarify these difficulties. I n the meantime, we can only estimate that methine chemical shifts occur between about 8.20 and 8.50 r. Application of Paraffinic Data to Hydrocarbon Polymers. The practical application of the paraffinic chemical shifts and spectral patterns is well illustrated in structural determination problems of polyisobutylene and poly-3-methylbutene-1. The NMR spectrum of polyisobutylene, Figure 8, a, presents two singlet
3-METHYL PARAFFINS
c - c - c - [Cl. - c C
---I--
n-4
Figure
1-b 7. Alkyl CH and CH1 NMR patterns vs. chain length
observed which may help confirm structural features deduced from methyl shifts. Positions of resonances produced by methylenes in normal paraffinic chains are independent of chain length (Chart IV, a ) . Chemical shifts of methylenes in "isolated" ethyls and propyls, Chart IV, b, lower line, occur upfield from those of longer, normal, "isdated" alkyl chains. The former resonances, however, are shifted downfield by the presence of another quaternary carbon group located alpha to the first one (Chart IV, b, upper line). As in the discussion of methyl resonances, the data on quaternary carbon groups are limited to t-butyls and internal geminal methyl groups. I n all other cases the best correlation of chemical shift appears to be with chain length, the chains containing only one or two methylenes producing significantly different shifts from the lonber chains. "Crowded" t-butyls or internal geminal methyls cause a downfield shift of the resonance of the single methylene between them just as they do for the resonance of the methyls themselves (Chart IV, c). On the other hand, "crowded" isopropyls cause the opposite (upfield) shift in the resonance of the methylene between them (Chart IV, e ) . This difference may be due to differences in bond angles and lifetimes
2 156
ANALYTICAL CHEMISTRY
I1
of rotational conformers, but no detailed explanation is offered a t this time. Methylene chains terminated a t one end by methyls and a t the other by a single methyl branch or isopropyl (Chart IV, d ) also produce resonances
NMR CONDITIONS SOLVENT: CC14 CONCENTRATION: '5%. ESTIMATED INTERNAL REF.: TETRAMETHYLSILANE SAMPLE TEMP.: 32. C. INSTRUMENT: VARIAN A - 6 0 FREPUENCY: 60 MC SCAN RATE: 1 CPSISEC. R. F. AMPLITUDE: 0.3 mG (DIAL READING)
peaks (in the ratio of 3 to 1) a t 8.88 and 8.58 7 , which are assigned to the geminal methyls and the isolated methylenes in 11. The repeating unit in I1 provides maximum "crowding" of
1
TMSy
I
-64
-C-C
Hydrogen Distribution I
-
CH3
e N M R CONDITIONS A - W SPECTROMETER AT €0 Mc. SWEEP WIDTH: 250 cps SCAN RATE: 0.5 c p s h s c R.F. AMPLITUDE: 0.3 mG SOLVENT: CC14 CONCENTRATION: ‘10 Vol.% TEMPERATURE: 32O C
i lM!*
22
I
/-
N M R CONDITIONS
Approx. Hydrogen Distributed No.Foucd Tkar.No. 1.7 I CH
5.3
6
15.0
5
22.0
22
CHI
CH3
22.0
A - 6 0 SPECTKOMETER AT 60 Mc SWEEP WIDTH: 250 cps SCAN RATE : 0.5 cpslsec R.F. AMPLITUDE: 0.3 m G SOLVENT; CCld CONCENTRATION: < i o Vol.% TEMPERATURE: 32’ C
I
U
I
/I\
I il
I
I
TYS I
I
8.0
I
I 81)
.
I
,%a.
I (0.0 I-
884 90
9.5
Figure 9. NMR spectrum of 2,2,6-trimethylheptane (paraffin “A”) isolated from a paraffin mixture by preparative gas chromatography
the methyl and methylene groups, causing the resulting resonances to occur a t the lowest field positions of their respective paraffinic categories (see Charts 111, f , and IV, e ) . Therefore, the polyisobutylene NMR spectrum is in complete agreement with its known structure. Edwards and Chamberlain ( 3 ) reported that the low temperature, -73” C., acid catalyzed polymerization of 3-methylbutene-1 proceeded mainly by a 1,a-mechanism yielding polymer of repeating unit 111.
111 This structural assignment was based solely on the polymer’s NMR spectrum, Figure 8, b, which consists of two singlet peaks (in the ratio of 3 to 2) broadened somewhat by mild restriction of rotation in the polymer chain, and occurring a t 9.18 and 8.92 T. This requires that both methyl and methylene groups be isolated by quaternary carbon atoms. Although the methyl chemical shift agrees with that observed for “uncrowded” internal gem-methyls (Chart 111, h ) the CH2 resonance is slightly upfield from the position predicted in Chart IV, e. This upfield shift is attributed to the physical conformation of the large polymer molecule because the methylene resonances of higher members of the same homologous series of polymers, as reported by Edwards and Chamberlain 13), fall within the limits of Chart IV, c. Consequently, the NMR spectrum
I
I
8.0
8.5
I I
, I
I 8.11 89 90 I.*9.20
I
I PS
10.0
r
Figure 10. NMR spectrum of 3,3,6-trimethylheptane(paraffin “B”) isolated from a paraffin mixture by preparative gas chromatography
of poly-3-methylbutene-1 is consistent with proposed structure 111. Two relatively pure paraffins, “ A J J and “B,”were isolated from a complex, synthetic mixture by preparative gas chromatography. MS data assigned the molecular weight of each paraffin a t 142 (C10H2*). The NMR spectrum of “A” (Figure 9) presents a methyl singlet a t 9.13 r and a methyls doublet centered a t 9.14 7 . The former corresponds to a ‘‘normal” t-butyl methyl resonance (Chart 111, c ) and the latter to a ‘‘normal” single methyl branch or to a “normal” isopropyl terminal (Chart 11, b and e ) . The chemical shift, about 8.84 to 8.85 T , of the methylene band corresponds to “uncrowded’’ methylenes associated with a branchy chain (Chart IV, e, lower line, illustrates a similar but not identical case). From a consideration of the preceding data and a measurement of the admittedly unreliable hydrogen distribution (due to a poor signal-to-noise ratio), it is obvious that “A” is either 2,2,5- or 2,2,6-trimethylheptane. The main component in sample “.AJ’ was finally identified by comparing its NMR spectrum with that of pure 2,2,6trimethylheptane. The NhlR spectrum of “B,” Figure 10, presents a methyl singlet at 9.19 7, a methyl doublet a t 9.12 T , and a methyl triplet at about 9.20 T. The high field position of the methyl triplet immediately identifies it as a methyl in an “isolated ethyl” grouping (Chart I, d ) . However, the characteristic methyl and methylene patterns of the isolated ethyl group (Figures 3 and 6, row 1, column 3) can be observed only by a close inspection of Figure 10 because of the interference of other resonance patterns. The methyl singlet of “B” corresponds to that of a “normal” internal geminal methyl group (Chart
111, h ) . Logically then, the preceding data strongly argue that 7 of the 10 carbon atoms in “B” must be arranged in the same manner as the enclosed carbons in 3,3,6-trimethylheptane illustrated below :
I C I C-c-2-C-CtC--C I i
; c I 1 I
3,3,6-Trimethylheptane
Also the chemical shift of the observed methyl doublet in the spectrum of iiBJJ agrees nicely with that of a “normal” isopropyl terminal, Chart 11, e , indicating that “B”is actually 3,3,6-trimethylheptane. The hydrogen distribution (Figure 10) fits this compound very well and the methylene chemical shifts (8.80 T for the isolated ethyl and about 8.87 7 for the others) are reasonable (Chart IV, b and c). Further support for this structure was obtained from a detailed study of the mass spectrum of this sample. CONCLUSION
The above chemical shifts and characteristic group band patterns. have proved to be exceedingly useful in elucidating the structures of polymers, other paraffins, and the paraffinic portions of olefins, hydrocarbon derivatives, and aromatics. More specifically, the data have been applied with reasonable success in determining the repeating units of high and low molecular weight saturated and unsaturated polymers. It is expected that the continued development of this technique and its application to polymer structural problems will not only increase our VOL. 36, NO. 1 1 , OCTOBER 1964
2157
present' knowledge of these macromolecules but may also shed some light upon their mode of initiation and termination. ACKNOWLEDGMENT
thank R' K' Saunders> T' J ' Denson, and The0 Hines for running the S h I R spectra.
LITERATURE CITED
(1) Anet, F. A. L., Can. J . Chem. 39,
2262 (1961). ( 2 ) Cavanaugh, J. R., Dailey, B. p.9 J . Chem. Phys. 34, 1099 (1961).
(3) Edwards, w.R.3 Chamberlain, s.F., J . Polymer Sci. A l , 2299 (1963). (4) Tiers, G. T'. D., J . Phys. Chem. 62, l l j l (1958). ( 5 ) Williams, R. B., Chamberlain, S. F.,
6th World Pet. Congress, Frankfurt, West Germany, June 19-26, 1963, Section T', Paper 17.
RECEIVEDfor revielv June 10, 1964. Accepted July 22, 1964. Presented in part at the Fourth Omnibus Conferencz on the Experimental Aspects of N M R at Pittsburgh, XIarch 1, 1963, and at Sixth World Petroleum Congress, Frankfurt, West Germany, June 19-26, 1963.
N e w Approach to Separation of Trivalent Actinide Elements from Lanthanide Elements Selective Liquid-Liquid Extraction with Tricaprylmethylammonium Thiocyanate FLETCHER L. MOORE Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b Preferential liquid-liquid extraction of the anionic thiocyanate complexes of the trivalent actinide elements with tricaprylmethylammonium thiocyanate dissolved in xylene or other solvents affords a new, improved method for their separation from the lanthanide elements. The order of decreasing extractability i s californium berkelium americium, curium ytterbium thulium europium promethium yttrium > cerium lanthanum. Various dilute acids enhance the separation factors markedly. Among the advantages of this method are the high single stage separation factors possible and the elimination of neutron hazard and corrosion problems associated with lithium chloride-hydrochloric acid systems. Several useful analytical and process applications of the method are discussed.
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was published ( 5 ) . The method is based on the preferential extraction of the trivalent actinide elements from dilute hydrochloric acid-concentrated lithium chloride solution with triisooctylamine dissolved in xylene or other diluents. Process studies along the same theme have also been reported (1). This method is currently employed successfully by workers in the field. More recently, interest has centered on an intensive search for a chloride-free liquid-liquid extraction system for this difficult separation. Among the important advantages of such a system would be the elimination of viscous,
highly corrosive solutions of lithibm chloride-hydrochloric acid. I n recent studies ( 1 4 , extractions by di(2-ethylhexy1)phosphoric acid or 2-ethylhexylphenylphosphoricacid from simple carboxylic acids and aminopolyacetic acids appear to be potentially useful for the purification of americium and curium but probably not for the heavier actinides. T o date, the only chloride-free system for the trivalent actinide-lanthanide group separation is that of the solid anion exchangethiocyanate method (!?> 3, 6 , 10. 12). Selective liquid-liquid extraction of the anionic thiocyanate complexes of the
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F
among the problems which confront the chemist in the transuranium field is the separation of trivalent actinide elements from the lanthanide fiqsion products. This separation has traditionally involved the use of ion exchange techniques (6, 10) because of the close similarity of the two groups. Workers have long sought simpler separation methods because of the disadvantages common to the solid ion exchange resins. Liquid-liquid extraction is often a preferred method of separation because of its simplicity, speed, and applicability to both tracer and macro amounts of ions. In 1961 the first solvent extraction separation of the trivalent actinide elements from the lanthanide elements OREMOST
21 58
ANALYTICAL CHEMISTRY
A M M O N I U M THIOCYANATE CONCENTRATION, M
Figure 1. Extraction of americium-24 I and europium-1 52-4 tracers from 0.2N sulfuric acid solution with 30% Aliquot 336-S-SCN-xylene
