Use of hexafluoroacetone and fluorine nuclear magnetic resonance to

Study of crystal transformations by an evolved gas analysis technique. P. D. Garn , R. L. Tucker. Journal of Thermal Analysis 1973 5 (4), 483-497 ...
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Use of Hexafluoroacetone and Fluorine Nuclear Magnetic Resonance to Characterize Alcohols and Other Active Hydrogen Compounds Gordon R. Leader

Pennwalt Corp., King of Prussia, Pa. 19406 The use of hexafluoroacetone as an NMR rea ent is described. A dilute ethyl acetate solution o f hexafluoroacetone containing water shows a singlet ‘QF resonance for free hexafluoroacetone and a singlet for hexafluoroacetone hydrate which is 7.35 ppm upfield. Many other active hydrogen compounds (alcohols, mercaptans, amines, oximes, etc.) readily form similar adducts with hexafluoroacetone whose ‘QF spectra usually show narrow NMR lines in an 8-ppm range and with chemical shifts which are related to the type of active hydrogen compound tested, and to minor structural or steric differences between compounds of the same class. Sensitivity is enhanced because the signal detected is from six fluorine atoms for each active hydrogen in the adduct-forming compound.

A NUMBER OF METHODS utilizing NMR as a tool for classifying alcohols have been reported. Manatt (I) has shown that the 19Fspectra of the trifluoroacetic acid esters of various alcohols show characteristic chemical shift positions for the singlet trifluoromethyl group resonance which depend on whether the alcohol is primary, secondary, or tertiary. A related method involves the preparation and examination of the dichloroacetic acid esters of alcohols (2). In this case, proton NMR is used to determine the location of the singlet due to --CHC12, and this is related to the nature of the alcohol. A third method similarly utilizes trifluoromethyl sulfenyl chloride (CF8SCI)for tagging of alcohols (3). These methods have the disadvantage of requiting preliminary preparation of a derivative of the alcohol in a separate operation. The need for this is eliminated and other advantages are gained by use of the in situ method reported here. This method is based on the reaction of alcohols or other active hydrogen compounds with hexafluoroacetone (HFA) to form stable adducts. OR I

(CF&C=O

+ ROH T=? CF3-C-CF3 I

OH A similar reaction occurs with mercaptans and amines and with many other active hydrogen compounds. The reaction is usually rapid at ambient temperatures and proceeds nearly to completion with water and primary alcohols. It goes less completely when extensive branching of the active hydrogen compound causes steric hindrance (4). By the use of HFA to introduce the “probe” group =C(CF3)2, one obtains an ‘OF NMR signal response corresponding to six atoms of fluorine per active hydrogen group,

compared with only three atoms of fluorine when the trifluoroacetyl group is used as a probe. The HFA method is consequently very sensitive and requires only 1 to 5 mg of sample when working with the usual NMR sample volume of about 0.5 ml. With alcohols, the d ( C F 3 ) 2 probe is also more selectively responsive to the nature of the entity -R being tested than is the trifluoroacetyl group, as shown by the observation that chemical shift differences of HFA adducts of selected pairs of alcohols are in general substantially larger than the corresponding differences for their trifluoroacetate esters. The ‘OF NMR spectrum of a dilute solution of HFA in ethyl acetate shows a singlet due to the free ketone which is at -2.32 ppm relative to trifluoroacetic acid (TFA) as external standard. The hydrate of H F A gives a singlet at +5.03. Adducts from other active hydrogen compounds usually give bands which are effectively singlets because coupling of the (CF3)2group to protons in -OR is small (about 1 Hz or less). These singlets, with few exceptions, are located in the range -2.3 to +5.03, and are widely distributed through this range, depending on the nature of the active hydrogen compound. A similar upfield shift of the CF3 resonance for the related compound l,l,l-trifluoroacetone as a result of adduct formation with proton donor solvents has been noted by Khetrapal(5). The adducts of alcohols with HFA yield singlets in the range +2.3 to f3.8 ppm from TFA(ext) and these occur at positions within this range which are characteristic of the type of alcohol. Mercaptans give singlets in the range - 1.50 to -0.8 ppm from TFA and the chemical shifts of these also depend on whether the mercaptan is primary, secondary, or tertiary. They are also influenced by electronegative or charged groups in the vicinity of the reacting -OH or -SH site, There are similar characteristic ranges for the HFA adduct spectra of amines, amides, oximes, and other classes of active hydrogen compounds. The stereochemical characteristics of molecules containing multiple active hydrogen groups and the presence of suitably located electron donor sites for hydrogen bonding also affect the chemical shifts of the HFA adduct lines. This gives the method further usefulness in some cases for differentiation of closely related compounds or stereoisomers. Quantitative analysis by use of 19FNMR spectra of HFA adducts should be possible in the case of mixtures whose components yield individual well-separated adduct lines and which can all be converted quantitatively into the HFA adduct form.

-~

(1) S. L. Manatt, J. Am. Chem. SOC.,88,1323 (1966). (2) J. S. Babiec, Jr., J. R. Barrante, and G. D. Vickers, ANAL. CHEM., 40, 610 (1968). (3) D. D. Lawson and J. D. Ingham, Space Programs Summary, No. 37-44, Vol. IV, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. (June l to July 31, 1966) p 93. (4) C. G. Krespan and W. J. Middleton, “Fluorine Chemistry Reviews,” Vol. I, Marcel Dekker, New York, 1967, p 145. 16

EXPERIMENTAL

The *9F NMR spectra were obtained at 32 “C using a Varian DP-60instrument operating at 56.44 MHz. Trifluoroacetic acid in a capillary was used as an external standard (5) C. I. Khetrapal, M. M. Dhingra, and V. B. Kartha, Proceedings

ANALYTICAL CHEMISTRY, VOL. 42, NO, 1, JANUARY 1970

of 1st International Conference on Spectroscopy, Bombay, Vol. 2,449, 1967; CA, 69,47718e (1968).

and spectra were calibrated with side bands of the TFA resonance produced by field modulation. Reagent grade ethyl acetate was used as solvent. This normally contains a sufficient amount of water as impurity (-0.273 to form with added H F A an easily detected singlet due to HFA hydrate. A stock solution of HFA (General Chemical Division, Allied Chemical Co.) was prepared by passing the gas into about 80 ml of ethyl acetate cooled to 0 “C until a weight increase of 8.3 grams was obtained, and then diluting to 100 ml with additional solvent ( O S M ) . The solution was stored a t 0 “C to prevent loss of HFA. A 0.6-rnl portion of HFA solution was placed in a 5-mm 0.d. NMR tube containing a sealed capillary of TFA. A 5-pl or 5-mg sample of the compound to be studied was then added and mixed with the solution. The tube was tightly capped and the 18F NMR spectrum was obtained. The lines due to free HFA, TFA, and HFA(H20) were readily identified, and their chemical shift positions, together with those of lines due to the alcohols or other compounds added, were determined by the usual procedures. Solvent effects due to variable HFA and HFA(H20) concentration and to the presence of the compound being tested are minimized by using the lines due to free H F A and HFA(H20) as internal standards. In the subsequent discussion, the displacement of adduct singlet lines downfield from the line due to HFA hydrate is called A . Their displacement in parts per million, upfield from the line due to free HFA is called A’. The HFAHFA(H20) separation (A A’) is 7.35 ppm in ethyl acetate. The lines due to H F A and HFA(H20) have some disadvantages as internal standards, but are useful here for showing the general location of the adduct lines for a wide variety of compounds. In use of this method, it will often be found more convenient and precise to use other internal standards appropriate to the particular class of compounds being studied-e.g., CH3NH2 for primary amines, C H 3 0 H for alcohols, etc. TFA was used as an external standard for spectrum calibration because its line is favorably located in relation to those of H F A and HFA adduct lines and no suitable internal standard at this same position was available. The line of ethyl trifluoroacetate is inconveniently close to that of HFA. The reaction of HFA with most active hydrogen compounds is rapid at 25 “C and usually allows the spectra to be observed immediately after mixing. With tertiary alcohols and mercaptans, and with certain functional groups such as -SO,NH,, the reaction is slow, and it is often necessary to wait 3 to 10 hours befme the adduct line appears. It is desirable to use an initial H F A concentration of 0.5M or less and to add only small amounts of the compound being tested, because the chemical shifts of free H F A and its adduct lines are sensitive to solvent composition. An excess of free HFA should usually be present, although, in some cases observation of the effect of an excess of the compound being studied yields useful information. Samples tested should not contain appreciable amounts of water, since this may preferentially react with all of the HFA present. The presence of basic groups in compounds tested causes broadening of the free H F A and adduct lines. This can be prevented by addition of sufficient trifluoroacetic acid to neutralize the compound. The lines due to secondary amine adducts, however, are usually observed only in the absence of acid and disappear when acid is added. Trifluoroacetic acid in solution gives a singlet at - 1.3 to - 1.8 ppm from TFA (ext) and usually does not interfere with observation of the HFA adduct lines. Other solvents such as acetone and diethyl ether can be used. Acetone, although generally suitable, gives a less stable H F A

+

Figure 1. Chemical shifts of alcohol and mercaptan adducts with hexafluoroacetone in ethyl acetate A . Mercaptan mixture. 1, TFA (ext.).

2, free HFA, A’ = 0.3, CsHsCHzSH, A’ = 0.68. 4, TZ-C~H~SH, A’ = 0.92. 5, Iso-C3H7SH,A’ = 1.00. 6, tert-CaH&H, A’ = 1.51 B. Alcohol and amine mixture. 1, TFA (ext). 2, n-C3H,NHz A = 3.96. 3, CHIOH, A = 2.68. 4, C2HjOH, A = 2.45. 5, Iso-C3H70H,A = 1.80. 6, tert-C4H,0H, A = 1.2.7, HFA(HzO), A = 0

stock solution and may in some instances introduce complications as a result of the participation of the acetone itself in adduct-forming reactions. When the solvent is changed, the chemical shifts of lines due to free HFA and HFA(H20), measured relative to external TFA, show considerable variation, but separation of these lines was found to lie in the range 6.8 to 7.5 ppm for ten solvents tested. The A and A ’ values for individual compounds are also different, and hence those given here apply only when the solvent is ethyl acetate. The use of alternative solvents may be desirable in some cases to achieve better resolution of lines when studying a mixture of compounds whose A values are close together. The chemicals used were in all cases from commercial sources and were used without further purification.

RESULTS Figure 1, A , shows the I9FNMR spectrum of a solution of HFA in ethyl acetate (0.5M) to which were added 3 to 10-pl amounts of benzyl mercaptan, 1-propanethiol, 2-propanethiol, and tert-butyl mercaptan, and Figure 1, B, shows thespectrum of a solution of ethyl acetate (0.5M) to which were added 3- to 10-111 amounts of n-propylamine, methanol, ethanol, 2-propanol, and tert-butyl alcohol. The chemical shifts, expressed as A values for H F A adducts of alcohols, glycols, and phenols, are given in Table I. The chemical shifts of HFA adducts of mercaptans, expressed as A ’ values, are given in Table 11. The HFA adduct lines of primary and secondary amines differ in that the lines due to primary amines have normal sharpness and are located upfield from the TFA reference line, whereas the lines due to secondary amines are broadened and are located near to or downfield from the line due to free HFA. Hence H F A adduct lines for primary amines are given in Table I11 expressed as A values, and those for secondary amines are given as A ’ values. Chemical shifts of H F A adduct lines for some simple compounds, and representatives of other classes of active hydrogen

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

17

compounds which have been studied, are given in Table IV. For convenience, these are all expressed as A values, even though some of the adduct lines are closer to the free HFA singlet.

Table I. Chemical Shifts Relative to HFA(H20) for Alcohol Adducts of HFA in Ethyl Acetate 1. Primary alcohols Methanol Ethanol 1-Propanol 1-Butanol 2-Methylpropanol 1-Pentanol 2-Mercaptoethanol(OH) 2-Mercaptopropanol(OH) 2-Methyl-2-nitropropanol

2.

3.

4.

5.

2-Phenylethanol Allyl alcohol 2-Propyne-1-01 Benzyl alcohol Glycolic acid Trifluoroethanol Glycols and related compounds Ethylene glycol Ethylene glycol monomethyl ether Tetrahydrofurfuryl alcohol 1,3-Propanediol 1,4-Butanediol 1,4-Butenediol 2-Methyl-2-nitro-I,3-propanediol 1,2-Propanediol Pri. (OH) Sec. (OH) Glycerol Pri (OH) Sec. (OH) Ethanolamine (OH) Diethanolamine (OH) 3-Amino-1-propanol (OH) 2-Methylaminoethanol (OH) Dimethylaminoethanol Secondary alcohols 2-Propanol 2-Butanol 1,3-Dichloro-2-propanol Cyclohexanol Cyclopentanol 1,4-Cyclohexanediol Hexafluoro-2-propanol 2,2,4,4-Tetramethyl-l,3-~yclobutanediol Tertiary alcohols tert-Butyl alcohol tert-Amyl alcohol Diacetone alcohol Citric acid Phenols Phenol Hydroquinone Resorcinol o-Cresol o-Chlorophenol p-Cresol p-Chlorophenol 4-Chloro-3-methylphenol

A PPm

2.68 2.45 2.57 2.58 2.62 2.57 2.50 2.56 2.67 2.55 2.54 2.47 2.80 2.27 2.12

2.36 (1,36)’2 1.51 1.49 2.46 2.60 2.53 2.54 2.52 1.70b

Alcohols. Table I shows that A for common primary alcohols lies in the range 2.45 to 2.70, for secondary alcohols in the range 1.75 to 1.95, and for tertiary alcohols in the range 1.2 to 1.4. Compounds listed at the end of each group in the table show that there are numerous exceptions. These compounds illustrate some of the structural features which can change A from the normal values and must be taken into account in the use of HFA as an NMR reagent. Phenols show A values of about 2.7, and for benzyl alcohol, A = 2.8. The phenyl group evidently causes a downfield shift from the expected adduct line position. A similar downfield shift due to a phenyl group close to the active hydrogen site is also observed for the adduct lines due to phenyl mercaptan and benzyl mercaptan. The low A values observed for CF3CH20Hand HOOCCH20H illustrate neighboring groups which cause a n upfield shift of the adduct line position. In HSCH2COOC2H6,the -COOC~HS group also causes a n upfield shift of the mercaptan adduct line. A double or triple bond attached to -CH20H, however, appears to have little effect. A nitro group in the p position also has relatively little effect. Effect of Hydrogen Bonding. Compounds containing multiple OH or NH2 groups of the same or different types will frequently show (with excess HFA) narrow adduct lines at the expected position for each type as illustrated by the data in Tables I, 11, and 111. A compound such as ethylene glycol combines with HFA at both sites when HFA is in excess, but with the glycol in excess, part of the glycol molecules present have unreacted -OH groups. This causes a n additional adduct line to appear at A = 1.36 which can be attributed to the hydrogen-bonded structure:

2.46 (1.97) 1.71 2.38 2.74 (1.95) [2.8410 2.55 2.50 [2.91] 0.57 [3.1] 1.80 1.94 1.88 1.78 2.00 1.87, 1.72 1.99 2.49, 2.59 1.20 1.41 1.35 3.08 2.73 2.70 2.74 (2.61) 3.07 3.04 2.71 2.66 2.65

a A values in parentheses are for an additional HFA adduct line which appears when compound equivalent concentration exceeds that of HFA. Center of complex pattern. c A values in brackets are for HFA adduct line position when TFA sufficient to neutralize the basic group is present.

18

DISCUSSION

Such hydrogen bonding does not occur to an OH site which has itself reacted with HFA. Similar hydrogen bonding can occur even in the presence of excess HFA with the compounds ethylene glycol monomethyl ether and tetrahydrofurfuryl alcohol and is reflected in a similar upfield shift of the adduct line. Ethanolamine, with excess HFA, shows adduct lines of equal size at A = 2.38 for OH and A = 3.60 for NH2, but when the ethanolamine is in excess, an additional line appears at A = 2.95 which can be attributed to a hydrogen-bonded monoadduct. Glycerol, with deficient HFA, shows a third line at A = 1.97 which can be attributed to HFA attached at a primary site and hydrogen bonded to the adjacent unreacted secondary OH. Ring formation through hydrogen bonding also can account for the marked difference shown by the NH2 adduct line of 2-aminopyridine as compared with those for the 3- and 4-isomers. Similar hydrogen bonding can also explain the pairs of adduct lines which are observed for hydrogen peroxide and hydrazine (Table IV). In each case, the diadduct line, for molecules with HFA attached a t both ends, is observed when there is an excess of HFA, and the monoadduct h e , with hydrogenbonded -(CF3)2C-OH at only one group, is observed when HFA is deficient. At intermediate concentrations, both adduct lines are observed. The monoadduct line is in both cases upfield from that due to the diadduct.

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

Some neighboring substituents permitting hydrogen bonding t o the attached -C(CF3)20H can also have intrinsic shielding or deshielding effects which may augment or oppose the effect of hydrogen bonding, and may be large or small in comparison with it. This is illustrated by HOOCCH20H, A = 2.27 (predominant H-bonding effect), and citric acid, A = 3.08 (predominant deshielding effect). A neighboring basic group, such as that present in dimethylaminoethanol, is strongly hydrogen-bonded t o H F A attached a t -OH, as shown by the abnormally low A value for this compound. In acid solution, the amine group is protonated, and in this form has a deshielding effect as shown by the increase of A to 3.1. With 2-methylaminoethanol, the secondary amine group does not function as a n electron donor site for hydrogen bonding because of its own reaction with HFA, and the OH adduct line is found a t the expected positicn. When acid is added, however, the attached H F A a t the secondary NH site is replaced by a proton and the OH adduct line is shifted from A = 2.5 t o A = 2.9. Analogous effects on A for compounds containing a COOH group near a reacting OH or NH, site are found t o result from neutralization of the COOH group by a suitable base. Glycolic acid and glycine, when partially neutralized with pyridine, show additional adduct lines at A = 1.07 and 2.02, respectively, which can be attributed t o the corresponding anions. The complete conversion of compounds of this kind into the anion form presents experimental difficulties in many cases because of the severe line broadening, especially of the free H F A and HFA(H20) lines, which occurs in basic solution. The OH or NH, groups as neighbors to another active hydrogen group usually exercise their shielding effect while themselves combined with HFA, and, in this form, cause a small decrease in A for H F A attached at a nearby -OH o r -NH2 group. Complex Adduct Spectra. Complex adduct line patterns were observed for seven compounds, all of which contained -OH or -NH2 groups attached a t a n asymmetric carbon. An example of such a pattern is that obtained with CH3CHOHCHrOCH3which consists of two quartets (54 = 8.8Hz), one a t A = -0.4 and the other at A = 1.78. With CH3CHOHCH,OH, a normal adduct singlet for primary OH is observed at A = 2.52 and apparent triplets at A = 1.51 and 1.88 for adducts of the secondary OH. These complex patterns can be attributed t o structural features of the adduct which make individual CF3 groups in the attached H F A portion nonequivalent. The coupling between fluorine atoms of these groups gives a first-order spectrum when the chemical shift difference is large, as in the first case cited above, or a more complex pattern approaching that of a n A3B3 spin system when the chemical shift difference is small relative to 5 as in the second case. In the limiting case where the degree of nonequivalence of the CF3groups is very small, this effect may appear only in broadening of the adduct line, as in the case of 2-butanol and 2-butanethiol. Effect of Ring Size and Isomerism. The A observed for cyclopentanol is considerably higher than that for cyclohexanol, indicating a n influence of ring size. A continuation of this shift to lower field is found in the case of 2,2,4,4tetramethylcyclobutane-l,3-diol, which gives two adduct lines of equal size at A = 2.48 and 2.58. The two lines are indicative of the presence of cis and trans isomers in equal amounts. Cyclohexane-1 ,4-diol can likewise exist in cis and trans forms, and here also two lines of approximately equal intensity are observed at A = 1.72 and 1.87. A similar sensitivity t o such steric differences has been noted for the classifica-

Table 11. Chemical Shifts Relative to HFA for Mercaptan Adducts of HFA in Ethyl Acetate

Primary mercaptans Ethanethiol 1-Propanethi01 1-Butanethiol 2-Methyl propanethiol 2-Mercaptoethanol (SH) 3-Mercaptopropanol (SH) Ethyl thioglycolate Benzyl mercaptan 2. Secondary mercaptans 2-Propanethiol 2-Butanethiol 3. Tertiary mercaptans fert-Butyl mercaptan 4. Aromatic mercaptan Benzenet hiol 1.

0.95 0.92

0.93 0.87 1.18 1.03 1.46 0.68 1

.oo

0.96 1.51 0.04

Table 111. Chemical Shifts for HFA Adducts of Amines in Ethyl Acetate

Aliphatic primary amines Methylamine N-Propylamine Isopropylamine Benzylamine a-Methylbenzylamine Ethanolamine (NH2) 3-Amino-1-propanol (NH2) Monoisopropanolamine (NH2) /3-Ethoxyethylamine 3-Methoxypropylamine Ethylenediamine (AS EDA.2HFA) AS EDA-HFA) Glycine

1.

2. Aromatic primary amines

A, PPm

4.11 3.96 3.48 3.93 3.52 3.60(2.95p 3.85 3.86 2.70 3.63 3.74 2.55 [3. SIb 3.38 A, PPrn

Aniline 3.74 o-Chloroaniline 3.03 p-Chloroaniline 3.61 p-Aminophenol (NH2) 3.74 p-Aminobenzoic acid 3.51 o-Aminobenzoic acid methyl ester 3.28 Sulfanilamide (NH,) 3.50 2-Arninopyridine 11.051 3-Aminopyridine [3.20] 4-Aminopyridine [3.42] 3. Secondary amines Dirnethylamine -1.85 Diet hylamine -0.67 Di-N-propylamine -0.86 Diet hanolamine +O. 16 N- Methylaniline -0.60 2-Methylaminoethanol -1.22 Piperidine -2.06 Morpholine -1.70 Piperazine -1.77 A values in parentheses are for an additional HFA adduct line which appears when compound equivalent concentration exceeds that of HFA. * A values in brackets are for HFA adduct line position when TFA sufficient to neutralize the basic group is present. (1

tion method based on trifluoroacetate esters (1). Aromatic position isomers can also be distinguished, as illustrated by the differing A values for o- and p-cresol. Oximes form adducts with H F A whose lines fall in the range A = 3.7 to 4.0, and these are also sensitive to syn-anti stereochemical differences as illustrated by the two adduct

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

0

19

Table IV. Chemical Shifts of HFA Adduct Lines for Miscellaneous Compounds in Ethyl Acetate Compound Hydrogen sulfide Ammonia (or NH4+) Hydrogen peroxide AS HFA.HzOz AS (HFA),.HOz Hydrazine AS HFA‘N2H4 AS (HFA)z.NzHp Methyl hydrazine 1,I-Dimethylhydrazine 2,4-Dinitrophenylhydrazine

Hydroxylamine OH NHz Dimethylglyoxime Acetophenone oxime Methyl ethyl ketoxime Diethylhydroxylamine Acetamide Acrylamide Chloroacetamide Benzamide Sulfanilamide (SOzNHZ) p-Toluene sulfonamide Methane sulfonamide Methyl thiourea Phenyl urea Methyl urea

A, PPm

4.84 0.86 5.08 5.55 2.90 4.55 5.26 2.41 4.65 4.11 6.29 4.01 3.93 3.70,3.80 2.74 0.62 0.85 1.27 1.43 1.74 1,76 1.60 1.61 0.36 -0.05

lines (of unequal size) which are observed for methyl ethyl ketoxime. Acetophenone oxime, which exists as a single isomer, shows only a one-line adduct spectrum. Evidence that the sample used (m.p. 58 t o 60 “C) consisted of a single isomer was obtained from its proton NMR spectrum which shows only a single CHI resonance. Steric Hindrance. The effect of steric hindrance on adduct formation is apparent in the observed rapid and complete conversion of primary alcohols to H F A adducts as contrasted with the slow and incomplete conversion of tert-butyl alcohol which occurs under the same conditions. With further increase in steric hindrance, as in the compound CsH6C(CH&OH, adduct formation with H F A was not observed. Similar effects of steric hindrance can be expected to operate when H F A adduct formation is attempted with other active hydrogen compounds. Mercaptans. Table I1 shows that the A ’ values for primary and secondary mercaptans fall in the same range and hence are not easily distinguished by this method. The shielding effect of substituents near the mercapto group affects A ’ as much as does the degree of substitution on the carbon to which -SH is attached. Hence even though a primarysecondary distinction cannot be made on the basis of A ’ for an unknown compound, it is often possible to detect closely related mercaptans separately in a mixture because of different shielding effects and resulting different A ’ values for each compound present. The A ’ value for a single tertiary mercaptan which was tested is significantly larger than those for ordinary primary and secondary mercaptans. An unusually low A ’ value is observed for phenyl mercaptan because of the downfield shift caused by the phenyl group, as observed also for phenol itself. The presence of alcohol impurities in a mercaptan is readily shown by the appearance of HFA adduct singlets in 20

the region 2.3 to 3.8 ppm above TFA(ext) in addition to the mercaptan adduct line. Amines. Most primary amines, both aliphatic and aromatic, give adduct singlets in the range A = 3.4 to 4.1, as shown in Table 111. The adduct for -NH2 attached to a secondary carbon is upfield from that for -NH, attached to a primary carbon, as in the case of alcohols. Aromatic amines with electronegative substituents also show differences for ortho and para isomers which parallel those for phenols. The A values for amines are influenced by deshielding effects of nearby electronegative groups or protonated basic groups and, as with alcohols, can show marked changes when a neighboring group is favorably located for hydrogen bonding with the -C(CF&OH attached at an -NH2 site. The secondary amines tested usually gave moderately broadened adduct lines which lie close to or downfield from the line due to free H F A ( A ’ = 0 to -2.1). They can best be observed by using very small amounts of the amine (2 t o 3 pl), since appreciably larger amounts also cause broadening of the free H F A and H F A hydrate lines, indicating a catalytic effect on reactions of the adduct-forming equilibria. When a strong acid, such as trifluoroacetic acid, is added in amounts sufficient to neutralize the secondary amine, the secondary amine adduct lines usually disappear. Exceptions to this general behavior have been observed for some secondary amines in which it is possible for the H F A adduct at )NH to be stabilized by hydrogen bonding. Adduct lines due to ammonia, primary amines, and amides are unaffected o r shifted only slightly by the addition of small amounts of a strong acid. The A ’ values observed for secondary amines show considerable variation with structure, and may have some value for identification of individual secondary amines. The broad lines produced reduce somewhat the sensitivity of the H F A reagent for detection of secondary amine groups, and, since the bonding of H F A to secondary amine groups appears to be weak, it is sometimes found, with mixtures of closely related secondary amines-e.g., piperidine and piperazine-that only a single adduct band is observed because of exchange. Amides and Ureas. Table IV shows that A values for amides, sulfonamides, and monosubstituted ureas fall in the range 0 to 1.8 and are responsive to shielding effects of neighboring atoms, as observed previously for other functional groups. Many other active hydrogen compounds which have not been tested can also be expected to have distinctive H F A adduct line positions. However, the data given show the wide scope of this method, and its ability to give well separated 19F NMR signal responses from compounds whose very similar behavior in other properties may make them difficult to distinguish or t o determine in the presence of each other by existing techniques. Coupling in HFA Adducts of Fluoroalcohols. The protons in the normal alkanol adducts with H F A are weakly coupled to (CFs),C== and cause only slight broadening of the adduct NMR line. The H F A adduct lines for CHBOHand CH3SH can be resolved into quartets with J about 1 Hz. When fluorine atoms are present near the active site in the alcohol or other active hydrogen compound being studied, these can show larger couplings with the HFA moiety, and their own chemical shifts are also changed as a result of adduct formation. The adducts of the fluoroalcohols CF8CH20Hand (CF& CHOH, which have structures A and B furnish examples of this behavior.

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

(6) CF 3

(a)

/ CF3-CH2-0-C

I\

CF3

O H CF3 A

CF3 H

\I

/

/

c-o-h

CFs

OH CF3

B

In A , the CF3triplet (in dilute acetone solution) is found at -2.44 ppm from TFA(ext), whereas the corresponding triplet of unreacted CF3CH20H occurs at -0.12 ppm. In B, C F s groups (a) give a band at -4.03 ppm compared with a doublet (J = 6.6 Hz) at - 1.42 ppm for CF3 groups in (CF3)2CFOH. The band observed for (a) in the adduct (9 lines) results from coupling of (a) with the methine proton( J = -6 Hz) as well as coupling between fluorines (a) and (6) with J = 2.5 Hz. The latter coupling is also apparent in the septet due to fluorines (b)at $3.31 ppm (the usual HFA adduct band for this alcohol). In dilute solutions containing CF3CHz0H and HFA, the

triplet due to CF3 in the former appears only when its concentration exceeds that of HFA indicating that the reaction to form A goes nearly to completion. In contrast, solutions containing (CFJ2CHOH and HFA, with either component in excess, show bands due to both reactants and to the adduct. Bands which measure the concentration of each component are in this case conveniently spaced and separated in the spectrum, so that the relative amounts of each can be determined by integration. By using known initial concentrations of (CF,),CHOH and HFA in acetone and by measuring the concentration of each component at equilibrium in this way, the equilibrium constant for the reaction (CF3)zCHOH f (CF3)s C=O

i?

(CF,)?CHOC(OH)(CF,)L

was found to be 6.2 liters per mole at 32OC. A mixture containing reactants at initial 1M concentrations is thus approximately two thirds reacted. RECEIVED for review September 9, 1969. Accepted October 24,1969.

Moments Analysis for the Discernment of Overlapping Chromatographic Peaks Eli Grushka,’ Marcus N. Myers, and J. Calvin Giddings Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 The higher central moments in the form of the derived quantities, skew and excess, were examined for the information they contain on peak contamination and peak overlap. In particular, we have investigated the possibility of utilizing skew and excess in the discernment of low resolution peaks. Single and double peaks of various forms, especially bi-Gaussian and Poisson distribution, were generated mathematically. The skew and excess of double as compared with sin le peaks of the same asymmetry type confirmed the esired discrimination. Experimental results, on the other hand, showed that while the skew and the excess of double peaks are consistently different from those of single peaks, the actual recognition of a double peak i s not as clear as in the theoretical cases and may require an internal standard. The results also point out the qualitative information that may be obtained from skew and excess analysis.

1

IN A PRECEDING PAPER ( l ) , it was suggested that peak-shape factors may contain information about peak overlap and contamination, about column conditions, and about peak identity. Here we examine the first of these areas: the possible use of computer-analyzed peak-shape data to distinguish single peaks from composite peaks made up of strongly overlapping single peaks. The study is equally applicable to spectroscopy and to such methods as sedimentation and electrophoretic analysis, both the latter used to establish criteria of purity in biochemical systems. 1 Present address, Department of Chemistry, State University of New York at Buffalo, Buffalo, N. Y. 14214.

(1) E. Grushka, M. N. Myers, P. D. Schettler, and J. C. Giddings, ANAL.CHEM., 41, 889 (1969).

As in the previous work, we take the highcr central moments, and especially the skew and excess which derive from these moments, as the most fundamental expression of peak shape. I n particular we propose to investigate the use of skew and excess as parameters bearing information on peak overlap. These parameters are a) dimensionless, and b) zero for Gaussian peaks. Because of characteristic a) they are insensitive to any changes in scale and therefore reflect only intrinsic peak characteristics. Characteristic b) means that these parameters are a direct measure of non-Gaussian elements of a peak, elements that certainly increase in importance with the disengagement of single peaks under a composite envelope. At first it was believed that excess alone would best indicate composite peaks because the disengagement of equal Gaussians generates excess but not skew. This is a result of the flattening of the composite peak (measured by excess) but the failure to introduce any asymmetry (measured by skew). However, for real peaks it may be advantageous to use both parameters. Recent work has appeared on the problem of unresolved peaks (2-6). The approaches and goals, while different from our own, are of considerable importance. The problem of distinguishing between single peaks and composite peaks has many ramifications. If a reference peak ( 2 ) P. D. Klein, Scpar. Sei., 1,511 (1966). (3) R. 0. Butterfield, E. B. Lancaster, and H. J. Dutton, ibid., 1, 329 (1966). (4) H. J. Jones in “The Practice of Gas Chromatography,” L. S. Ettre and A. Zlatkis, Eds., Interscience Publishers, New York, N. Y., 1967. ( 5 ) L. Enchenyi, Abstracts 155th American Chemical Society National Meeting, San Francisco, Calif., April 1968, No, B-48. (6) V. Cejka, M. H. Dipert, S. A. Tyler, and P. D. Klein, ANAL. CHEM., 40, 1614 (1968).

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

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