color was noted with 1,1diethylurea, but no method was established because a sample of eufficient purity was not obtained. The solution of p-dimethylaminobenzaldehyde in isopropyl alcohol (color solution A) was an extremely sensitive spray reagent for detection of certain ureas and thioureas on paper chromatograms. Brilliant yellow spots on a white background were obtained a t room temperature. The urea in perspiration will give a spot if the papers are touched with bare hands. Although no attempt was made to apply the reaction to samples other than wheat flour, other materials including feeds containing added ureas should work equally well, particularly if they can be dried and extracted with isopropyl alcohol. If feed pigments interfere, Jongen and Berkhaut’s (8) method may be used to clarify and decolorize the sample in one step. Diacetylmonoxime. T h e reaction of diacetylmonoxime with urea is not understood. Natelson, Scott, and Beffa (11) found the active reagent to be diacetyl, not diacetylmonoxime. Diacetylmonoxime produced a greater color intensity than diacetyl in the present study. Appreciable amounts of
the volatile diacetyl probably are lost before reacting. X slow production of diacetyl by the hydrolysis of the oxime probably permits a more favorable reaction. Fearon (4) stated that the reaction was positive for compounds containing the system RKHCONHR’, where R is hydrogen or a simple aliphatic radical and R‘is not an acyl radical. Contrary to Fearon’s statement, a positive reaction was obtained with acetylurea and acetylthiourea in the present study. No color was produced with 1,3diphenylurea; 1,l-diphenylurea; ethylenethiourea; 1,3-diphenylthiourea, or 1,l-diphenylthiourea. A positive reaction was obtained with 1,l-diethylurea but no method was established because of the difficulty in obtaining a sample of sufficient purity. The reaction with diacetylmonoxime apparently is positive for compounds containing the system RNHCO(S)XR’R”,where R is hydrogen or a simple aliphatic radical; R’ is hydrogen, a simple aliphatic radical, or a phenyl group; and R ” is less complex than a phenyl group. Fearon (4) reported that diacetylmonoxime reacts with protein, but in the present study no reaction was noted with the water-soluble extracts of flour.
LITERATURE CITED
( 1 ) Barrenscheen, H. K., Biochem. 2. 140,426-34 (1923). ( 2 ) Brown, H. H., ANAL. CHEM.3 1 , 1844-6 il9.W) \ - - - - /
( 3 ) Cline, R. E., Fink, R. M., Zbid., 2 8 , 47-52 (1966). 14) Fearon, W. R., Biochem. J . 33, 902-7 (1939). ( 5 ) Finney, K. F., Trans. Am. Assoc. Cereal Chemists 12, 127-42 (1954). ( 6 ) Finney, K. F., unnumbered mimeo-
graph publication, presented at Fertilizer Conference K.S.U., Manhattan, Kan., December 1950. ( 7 ) Finney, K. F., Meyer, J. W., Smith, F. W,, Fryer, H. C., Agron. J . 49, 341-7
(1957). (8) Jongen, G. H., Berkhaut, H. W., Chern. Weekblad 52, 909-10 (1956): C.A. 51, 5635 (1957). ( 9 ) Kawerau, E., Sei. Proc. Roy. Dublin SOC. 42, 63-70 (1946); C.A. 40, ,5085 (1946). (10) LehIar, R. L., Bootzin, David, ANAL. CHEM.29, 1233-4 (1957). (11) Natelson, Samuel, Scott’, M. L. Beffa, Charles, Am. J . Clin. Pathol. 21, 275-81 (1951). (12) Rosenthsl, H. L., ANAL.CHEM.27, 1980-2 (19%). ( 1 3 ) Watt, G. W., Crisp, J. D., Ibid., 26, 452-53 (1954). Received for review April 7, 1964. Accepted July 24, 1964. Cooperative in-
vestigation between the Crops Research Division, ARS, LSDA, and the Department of Flour and Feed Milling Industries, Kansas State University, Manhattan Kan. Contribution No. 437, Department of Flour and Feed Milling Industries.
Analysis for Deuterium in Organic Compounds by Combustion-infrared Spectrometry JAMES L. LAMBERT, JAMES H. HAMMONS, JOSEPH A. WALTER, and ALEX NICKON Department o f Chemistry, The Johns Hopkins University, Baltimore, Md. 2 7 2 7 8
b A method is described for the quantitative analysis of deuterium in compounds by combustion and infrared spectrometry of the derived water sample, Combustion is effected conveniently by a dynamic combustion train, modified to remove acidic oxides and halogen from the combustion gases. The memory effect of this system has been reduced to an acceptable level, Analysis on the milligram scale is possible. Results agree well with analyses by mass spectrometry and the falling-drop method.
T
AND CONDON first described an infrared method for the determination of deuterium oxide in water (8). This method depends on the fact that HOD has an absorption band a t 2520 cm.-’ (3.97 microns) that can be observed through the overlapping spectrum of HOH. The intensity of the HOD band provides a direct measure of the total deuterium content of the water
HORNTON
2 148 *
ANALYTICAL CHEMISTRY
sample a t high isotopic dilution, where the concentration of DOD is negligible. Trenner, Arison, and Walker developed this technique for the determination of deuterium in organic compounds by assay of the water formed on combustion. In their early work (10) these investigators used the conventional Pregle microanalytical type of combustion train for the oxidation of the organic compound. They found the usefulness of this dynamic combustion technique limited by a serious memory effect attributed mainly to the barium carbonate used to remove acidic oxides. Subsequently they developed a static oxidation method in which the organic compound was heated at 750’ to 800’ with a large excess of copper oxide in a sealed quartz tube (9). The water sample was then recovered and purified in a 7-acuum distillation train. Jones and MacKenzie ( 1 ) further refined the method and suggested a number of procedural modifications regarding the methods of sample handling and of
water collection. These latter workers overcame the difficulties associated with temperature changes on the spectrometry of deuterium-enriched water by employing differential analysis against natural-abundance water. The static combustion method entails considerable inconvenience and the disadvantages concomitant with complex techniques. The quartz combustion ampule must be carefully prepared and heated to avoid explosive loss; and to recover and purify the water sample obtained from combustion, a complex ampule-breaker/distillation apparatus is required together with careful manipulation. We have simplified the method by returning to the dynamic combustion train technique for the combustion of the deuterium-enriched compound. In our method water obtained from combustion of compounds containing nitrogen, sulfur, and halogen was purified in the combustion train by use of suitable absorbents which removed acidic nitrogen, sulfur oxides, and halo-
gen before the water sample was collected. Our combustion train design gave rise to only a slight memory effect. This memory effect was rendered negligible by the convenient procedure of conditioning-combustions (10). I n addition, our method allows use of a spectrophotometer whose operating parameters cannot be assumed to remain unchanged, because of the constant and general use of the instrument, over the period of days, weeks, or possibly months separating deuterium analyses. These modifications have led to no sacrifice of the accuracy or the microscale of analysis possible by this technique.
7 I
I I
I 1 I
I I I I
I I
I I I I
50.8 cm. I
I I
I
I
I I I
I EXPERIMENTAL
Apparatus for Combustion and Water Collection. A block diagram of the combustion train is given in Figure 1, and the components are described. When compounds containing sulfur were burned, a special combustion tube was used t h a t contained copper ovide (20-cm. column) and barium carbonate (5-cm. column at the down-stream end of the tube). To allow for dehydrative shrinkage the barium carbonate was heated a t 700' under a stream of dry oxygen for 24 hours before i t was used to pack the combustion tube. Barium carbonate placed directly in the conibustion tube and heated to about 700' gives rise to negligible memory effect and retains its effectiveness in removing sulfur oxides from the combustion gases. Kitrogen oxides were removed from the stream of combustion gases by an absorption tube containing 600 mg. of Hopcalite (Mines Safety Appliances Co.; Braddock, Thomas & Meade Streets, Pittsburgh, Pa.) placed after the combustion tube. The Hopcalite tube was heated to 170' while in use. Between runs, absorbed nitrogen oxides were driven off and the Hopcalite was regenerated by heating the material to about 335' and sweeping with dry oxygen for about one hour. Similarly, a n absorption tube containing silver wire and heated to about 400' was used to remove halogen from the stream of combustion gases. The purified gases from combustion were swept into a small V-shaped borosilicate tube, Figure 2, and the bottom of the tube was packed in dry ice. Water could be collected as a small drop in the bottom of the V-tube. Deuterium enriched water samples were kept frozen in V-tubes tightly stoppered with rubber and/or Teflon caps for several weeks before infrared analysis with no change in isotopic concentration. The resistance heaters for the combustion apparatus were wired in series with each oven heater, trimmed so t h a t a t equilibrium the desired temperature was maintained. This arrangement simplified the system since no voltage regulators were required. To maintain a constant overall current, a ballast heater was added to or subtracted from the series by the same switch t h a t
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: E I
I
117. I U
Figure 1.
Combustion train, schematic
(1) Oxygen input and 1 10-volt a x . current plug. (2) Needle valve for oxygen flow. ( 3 ) Overpressure release volve (5-cm. mercury column over a flne fritted disk). ( 4 ) Gas absorption tube ( 2 0 0 X 2 0 mm.) pocked with Ascar'te. ( 5 ) Flow meter. An oxygen flow of between 10 and 2 0 cc./minute was used. ( 6 ) Pre-combustion unit to purify oxygen. The oxygen was passed over copper oxide heated to approximately 700' in a quartz tube. (7)Gas absorption tube (200 X 15 mm.) packed with Ascar te and Anhydrone. (8) One-way valve (mercury layer over a flne fritted dirk) to prevent back diffusion of water vapor. ( 9 ) Vycor microcombustion tube packed with a (10) Traveling combustion furnace. 25-cm. column of copper oxide and heated to about 700'. This small furnace could be heated to 800' and moved along the combustion tube at a rote of 2 mm./minute. (1 1 ) Main combustion ovei. (1 2 ) Absorption tube for the removal of halogen. (1 3) Absorption tube for the removal of nitrogen oxides. If the compound under analys:s contained no halogen or nitrogen the two absorption tubes (12) ond ( 1 3 ) were disconnected, and the gas flow from the combustion oven was discharged directly into the water collecting apparatus. ( 1 4 ) W a t e r collecting apparatus. (1 5) Drying tube filled with Drierite. ( 1 6 ) Electric resistance heoler to compen. sate for the traveling combustion oven. ( 1 7) Electric switches
turned the traveling combustion furnace off or on. A similar compensating resistance was used with the Hopcalitetube oven to obtain absorption temperature or regeneration temperature. Spectrometric Technique. The spectra were measured on a PerkinElmer Model 21 spectrophotometer equipped with a sodium chloride prism, and our technique was based on t h a t described by Jones and MacKenzie ( I ) . Because we observed a considerable variation in spectrometer sensitivity over periods of several months, we adopted the following procedure for opening the slits to obtain nearly reproducible operation conditions. The reference cell was filled with natural-abundance water (tap water distilled once from a tin still) and placed in the reference beam. The spectrometer was set at 2800 cm.-1, the reference beam shutter only was opened, and the 0.03-volt test signal was turned on. The slits were opened manually until a pen displacement of 3oy0 transmission was recorded. Slit widths that ranged from 150 to 250 microns were required to obtain this displacement depending on the condition of the spectrometer. A calibration curve was taken a t the time of each group of analyses. A series of solutions containing 0.05- to 0.70(in steps of approximately 0.05) atom % excess deuterium were used for the calibration spectra. These solutions
3 mm.
t- - - - - - - g 0 m Figure 2. sample
4
V-tube for collecting water
were prepared in two independent sets by gravimetric dilution of deuterium oxide (D greater than 99.5y0) with natural-abundance water. The determination of a calibration curve simultaneously with analysis of the unknowns was of minor inconvenience and ensured greater accuracy. Analytical Procedure. DILUTION OF DEUTERIUM- ENRICHED COMPOUNDS. T h e spectrophotometric analysis is limited to water samples having atom excess deuterium concentrations of less than 0.7% ( I ) . If the deuterium-enriched compound contains more than this concentration of excess deuterium (which is generally the case) the compound must be diluted. VOL. 36,
NO.
1 1 , OCTOBER 1964
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In most cases the deuterium-enriched compound was diluted with the naturalabundance compound. The approximate deuterium contents of the compounds were known, and the dilution factors were adjusted to give water samples containing about 0.3 atom % excess deuterium. For example: mass spectrometric analysis of a sample of deuteriocamphenilone (CsHI40) indicated that the mclecule contained an average of about 2.3 atom excess deuteriums, 16.4 atom % excess deuterium ( 5 ) . The sample waij converted to the crystalline semicarbazone (C10H17NSO),and the deuterium-enriched camphenilone semicarbazone (0.002114 gram, molecular weight 197.5 for species containing 2.3 deuteriums, 13.5 atom % excess deuterium) was diluted with natural-abundance camphenilone semicarbazone (0.099186 gram) giving a dilution factor of 48.4. Analysis of a 2-mg. sample of deuteriumenriched compound would he practically impossible. Dilution increases the sample size, in this case to about 100 mg., which nermits convenient analvsis in triplicate. The deuterium-enriched and diluent camphenilone semicarbazone samples were dissclved in methanol to ensure homogeneous mixing, and the solvent was evaporated. Techniques for the dilution of liquid samples and of volatile compounds have been described by Jones and MacKenzie ( I , 2). Some of our deuterated solids, the norbornanes (3, 4),were so volatile that they could not be recovered free of solvent after dilution. We diluted them, therefore, with a liquid n-alkane of most nearly the same volatility and subjected the solution itself to combustion. Errors introduced by this method were small, for values obtained were in good agreement with values obtained by independent methods. COMBCSTIOK OF THE DILUTEDDEUTERIUM - EZRICHEDCOMPOL-ND.For convenience in collecting the water sample and filling the microcell we burned sufficient deuterium-enriched compound to yield 15 to 20 mg. of water. Approximately 7 mg. of water was sufficient to fill the microcell. Routinely, 40 minutes were allowed for sample combustion and collection of the derived water sample. If the Hopcalite absorption tube had been used, it was regenerated for one hour before a second nitrogen-containing sample was burned. RESULTS
Memory Effect. A slight memory effect was observed in the combustion train. For example, a sample of deuterium-enriched compound containing 0.284 atom yo excess deuterium was burned. Then a sample of natural-abundance water was swept through the train and collected. of this water sample showed t h a t it had picked up a small amount of deuterium (about 0.04 atom yo excess) from the combustion train. Similarly, the first sample of deuterium21 50
ANALYTICAL CHEMISTRY
Table I.
Representative Analyses of Deuterated Organic Compounds
Compound Camphenilone-d semicarbazone Camphenilone-d semicarbazone Camphenilone-d semicarbazone Camphenilone-d semicarbazone Camphenilone-d semicarbazone Camphenilone-d semicarbazone Camphenilone-d 1) as semicarbazone 2) as tosylhydrazone t-Butanol-d (Merck & Co. of Canada) endo-d-Norbornane 1) octane as diluent 2) heptane as diluent em-d-Norbornane" l-d-?iorbornaned d-Sorbornanec 6-d-2-exo-Xorbornyl acetate 2-em-d-2-endo-Sorborneol
6-d-2-ero-Sorborneol 6-d-2-Xorcamphor
Deuterium content as atoms excess D per molecule by: Molecular Independent formula Combustion-infrareda analysis 0 . 614b CloH1,N30 0,612; 0,621 CloH17S30 1,06; 1.11; 1.12; 1.12 1.18b ClOH1,?,0 2.38; 2,36 2.27b CIoH17h 3 0 2.67; 2.59; 2,62 2.57b C10H17S30 1,94; 1.95 CloH,7?;30 1,74; 1.66; 1 . 6 3 CioHi7S30 2.61; 2.53 C I ~ H ~ ~ X ~ 2.61; O & ?2.69 CaHloO 1.06; 1.00; 1.00 0.96; 0.98 GHiz 0 . 9 7 ; 0 . 9 4 ; 1.00 0,98b 1.01; 0.97; 0.99 0.98b C7H12 0.93; 0.96 0.98b C7Hiz 0.94; 0.97; 0.97 0.98b C7H1Z 1.01; 1.02 1.08b CsH1*02 0.91;0.91;0.91;0.91 C.rH,?O 0.91: 0.92 C;HI;O 0.74; 0.76 0 . 7 7 ;0 . 7 7
.4~1drostan-3~-01-17-one-l6-d
Caryolan-1-ol-d (6)
C7Hio0 C19H3009 Cl&O
a-Caryophyllene alcohol (6)
CljHzeO
0.79; 0 . 8 0 ; 0.84 1 .87 0.86; 0.87; 0.84 0.82: 0.87 2.7052.72; 2.75
Diketocarboxylic acid derived from or-caryophyllene alcohol
C1jH2,Oa
2.47; 2.51
0 , 84c 0.84"
1.94~'
0.846' 2 . 7 0 e ;2,768 2,75h; 2,74i
(6) a Each entry represents a separate combustion and infrared analysis. Each horizontal row of entries represents one dilution of the deuterio-sample under analysis. b Deuterium analysis performed by mass spectrometry of the volatile deuterio-compound. e Octane was used as the *diluent. Heptane was used as the diluent. e Deuterium analysis performed by J. Nemeth (303 W. Washington St., Urbana, Ill.) using the falling-drop method. 1 This deuterio-sample was kindly supplied by Jones and MacKenzie ( 1 ) . Their analysis gave this value for the sample's deuterium content. * This compound was analyzed for deuterium by combustion over copper oxide, reduction of the derived water to hydrogen and deut'erium with zinc [according to the method developed by San Pietro (?)I, and mass spectrometric analysis of the gas. Deuterium analysis performed by M. A. MacKenzie (Sational Research Council of Canada, Ottawa) using the combustioninfrared method. Mass spectrometric deuterium analysis of the volatile deuteriocompound performed by S. Meyerson (American Oil Co., Whiting, Ind.).
enriched compound (0.284 atom yo excess D ) burned after the train had been exposed to natural-abundance water gave a low analysis (0.263 atom % excess D). The second sample burned, analyzed correctly (0.282 atom y, excess D ) . Thus the first combustion had corrected the memory effect caused by exposure of the system to natural-abundance water and had established a condition of excess deuterium which nullified any small memory effect. Trenner, hrison, and Walker tested the dosing technique in their vacuum system used to recover water samples from combustion ampules and found that when samples were within 0.3% of the deuterium of the equilibrated system level, memory perturbations become subdetectable (9). SimiIarly, we found that if the combustion train was exposed only to deuterium-enriched samples of approximately the same deuterium level, then complications caused by a memory effect were nmexistent. -1s an added precaution, when a sufficiently large sample of deuterium-enriched compound was on hand, we regularly first
burned one small sample to establish a correct memory effect before samples for analysis were burned and collected. The water sample from this conditioning-combustion was not collected, Calculations and Results. Peak absorbances recorded for the standard deuterium solutions were plotted against the known atom yo eycess deuterium to obtain a calibration curve. From this curve it is possible to read directly the atom % excess deuterium corresponding to the eyperimental peak absorbance values for water samples obtained from the comi bustion of compounds under analysis. Multiplication of this observed atom yo excess deuterium value by the appropriate dilution factor gives the true atom % excess deuterium in the compound analyzed. This can be converted readily to atom excess deuterium if the molecular formula of the compound is known. A rigorous but somewhat inconvenient formula for calculatinq atom excess deuterium has been derived b j Jones and MacKenzie ( 1 ) . Table I lists the results of typical
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.8 5 , 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., 8 6 , 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.2 2 , 690 (1950). (9) Trenner, K. R., Arison, B. H., Walker, R. W.,Ibid., 2 8 , 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
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