V O L U M E 28, NO. 6, J U N E 1 9 5 6
949
Table 111. Response of Infrared Inalyzer to Various Compounds Compound Siethane E3liane Propane Butane Hexane Propene 1-Butene 1-Pentene kcetaldehyde Propionaldehyde Butyraldehyde Heptenal Acetone I-Butanone t-Methyl-Z-pentanone o-S*onanone
Arerage Sensitivity ,Scale Divisions per P.P.M.) '1.0 3 ,3 4.6 7.2 11.0
Unless the composition of the organic material in the atmosphere is known, its concentration must be expressed with reference to a standard. Thus, the records shown in Figures 13 and 14 indicate 0.4 to 2 p.p.m. of organic material calculated as butane. Because the response time of the instrument is of the order of 1 minute, the fine structure shown in Figure 14 represents I ea1 changes in concentration
1.8
ACKNOWLEDGMENT
L O 7.0 0.6 2.0 3.6 7.6 0.8 2.4 6 .3 9.5
The development of this instrument was undertaken in behalf of the Western Oil and Gas Association and brought to a conclusion under the auspices of the American Petroleum Institute. The authors are particularly indebted to Kenyon George of Applied Physics Corp. for many helpful suggestions. LITERATURE CITED
order to avoid this error the freeze-our trap was omitted completely. Although this step was undertaken with some misgivings, it appears to have been completely successful. The instrument has been operated with a minimum of supervision for 3 months on the atmosphere. The removal of the freezeout trap did not result in any gross changes in the apparent level of organic compounds, although some slight diurnal variations TVere apparent and much more fine structure present (Figure 14).
P.P.,Heddon, AI. W., Lofberg, R. T., Hoehler, R. H., ANAL.CHEM.24, 1899-902 (1952). ( 2 ) Shepherd, Xlartin, Rock, S. AI,, Howard, Royce, Stormes, John, Ibid., 23, 143140 (1951). (3) Stitt, Fred, Tjensvold. 9. H., Tomimatsu, Yoshio, Ibid., 23, 113841 (1851). (1) Mader,
RECEIVED for review August 27, 1955. Accepted March 14, 1956. Division of Petroleum Chemistry, Symposium on Industrial Waste Disposal Problems of the Petroleum Industry, 123rd Meeting, .iCS,Los A h g e l e s ,Calif., Maroh 1953.
Differential Spectrophotometric Determination of Fluoride JOHAN J. LOTHE Central
institute for lndurtrial Research, Blindern, Oslo, Norway
The differential method of analysis as applied to the indirect spectrophotometric determination of fluoride with the thorium-Alizarin Red S reagent at pH 2.80 is presented. The absorbance of the sample solution is taken against one of a set of three reference standards containing 50,100, and 200 y of fluoride per 50 ml. The coefficient of variation of the method is less than 1% in the range 50 to 200 y of fluoride per 50 ml. and less than 2% in the range 25 to 50 y . The optimum composition of the color reagent and the effect of pH and foreign ions are discussed. The color reagent is stable and can be used for at least a month.
F
LCORIDE determinations have become increasingly im-
portant in recent years, and several spectrophotometric methods are described in the literature. As fluoride does not form colored complexes, only indirect spectrophotometric procedures have been proposed to date. These procedures are based on nwasuring the bleaching of some colored metal complex by the formation of stable complexes between the metal ion and fluoride. The complexes of zirconiuni and thorium with hydroxyanthraquinones have proved to he excellent reagents for the spectrophotometric determination of fluoride. The ~irconium-~4lizarin Red S complex has been used for a long time (2-4, 10, 18). Talvitie ( 1 I ) was the first to utilize the thorium-Alizarin Red S reagent, and lately Icken and Blank ( 7 ) and Xommik (9) have published procedures based on this reagent. During an analytical investigation of fluorine compounds in this laboratory, a precise method for the determination of fluoride was needed. The procedure developed by Icken and Blank ( 7 )
was initially employed, but the repeatability turned out to be very low-only about 5%. This led to an investigation of means of increasing the precision. I n a previous paper ( 8 ) the theory of the precision of indirect spectrophotometry was presented, and it was pointed out that the maximum precision is obtained a t rather low absorbance readings, where the bleaching of the reagent is nearly complete. Taking the absorbance readings of the sample solution against a partly decolorized reference standard rather than water or a completely bleached reference solution, a significant increase in precision is achieved. Finally it was shown how the optimum concentration range could be selected by the use of known functions which give close approximations to the usual calibration curves. The present paper is concerned Tvith a precise spectrophotometric method for the determination of fluoride, based on measuring the bleaching of the thorium-Alizarin Red S complex. The method has been used successfully in this laboratory for nearly 2 years, during which period only minor improvements, especially with respect to better pH control, have been carried out. The proposed method for determining the optimum concentration range (8) is easy to carry out and should be used by laboratories which are engaged in routine determination of fluoride on a large scale. REAGENTS AND EQUIPMENT
Stock Solutions. Sodium hydroxide, 100 grams per liter. I\Ionochloroacetic acid, 472 grams per liter. illizarin Red S, 1 gram per liter. Thorium nitrate tetrahydrate, 5 grams per liter. Hydroxylamine hydrochloride 10 grams per liter. Standard sodium fluoride sodtions containing 2, 10, 50, and 300 y per ml. of fluoride. Perchloric acid, 0.0LV.
ANALYTICAL CHEMISTRY __
950 ~.
Buffer Solution. An aliquot of the monochloroacetic acid Table I. Effects of pH and Ratio of Alizarin Red S to Thorium on Absorbance at stock solution is titrated DotenThree - -.- - Fliinride . - - - - - - Concentrations tiometricallv with the sodium Ratio p n 2.01 pH 2.71b p H 2.83 b p H 3.03b .4lizarin t o -~ hydroxide "solution. Half of Thoriuma y F/30 .\TI. A AAC 9 A.4 c A 4A C A A/-; the calculated e q u i v a l e n t 160 0 0 . GO4 0.612 0.621 0.634 amount of the base is added to 0.147 0.141 0.140 0 117 400 ml. of the acid, and this .j0 0.457 0.471 0.481 0,517 0 283 0.26% 0,224 0 . '292 is diluted to 1000 ml. 200 0.165 0 188 0,219 0.293 Color Reagent. T o 500 ml. "00 0 0.664 0.674 0.686 0.702 0 162 0.158 0 145 0 . 1 6 2 of distilled wat,er are added: .j0 0.502 0 512 0 528 0.567 buffer, 40 ml. ; hydroxylamine 0.268 0.231 0,296 0 "85 200 0,206 0 22i 0.260 0.326 hydrochloride, 40 ml.; Alizarin '80 0 0.718 0.717 0.723 0,726 Red S, 200 ml.; and t,horium 0.146 0 122 0.167 0.1% nitrate, 20 ml. The solution 30 0.551 0,559 0,577 0.604 0 . 2 8 9 0 . 2 7 0 0 , 2 8 6 0.218 is diluted to 1000 ml. with 0.321 0.386 200 0.262 0,289 d i s t i l l e d water. The com0.744 0,742 0.745 0.757 400 0 ponents should be added in 0.130 0 107 0 , 1.3 0.141 0 . 6 1 8 0.050 .iO 0 591 0 GO1 the order given. The color 0.2>9 0.242 0.226 0.192 reagent is filtered after 2 days 200 0.332 0.359 0.392 0,438 and can be used for a t least a a Expressed as milliliters of .llizarin Red 9 $tack solution per 1000 ml. of color reagent: 20 ml. of thorium stock month. solution used tlirougliout. Reference Standards. These b Total concentration of monocliloroacetic acid was 0.035 in all experiments. c Absorbance diffrrencw between 0- and 50-7 and between 50- and 200-7 samples. contain 50, 100, and 300 y of fiuoridc per 50 ml. and are . prepared by adding 200 ml. of the color reagent to 10 and 20 ml. of the 50 y per ml. fluoride standard and 10 ml. RESULT s of the 300 y per ml. fluoride standard. Each is then di1:ited to 500 ml. with distilled water. The reference standards are Effect of pH. The effect of pH on the absorbance for the p H filtered after 2 days and are stable for several months. range 2.0 to 4.5 was studied for three fluoride concentrations Apparatus. Beckman Model B spectrophotometer equipped n-ith 10-mm. Corex cells; Beckman Model H pII meter. (Figure 1). I n these experiments the concentrations of thorium a n d Alizarin Red S were the same as in the standard procedure. PROCEDURE The great p H sensitivity of the reaction is evidenced by the curves. A4region of near p H independence is found only for the Careful p H control is of utmost importance for accurate results 0-7 series. FurtherniorP, the absorbance is most sensitive to of analysis. The samples are therefore neutralized with 0.05N perchloric acid using p-nitrophenol as indicator. fluoride in the low p H region, but here the p H effect is greatest. Suitable aliquots of the neutralized samples are placed in 50Year p H 4.5 the reaction becomes almost insensitive to fluoride nil. volumetric flasks, 20 ml. of the color reagent are added, the Factorial Experiment. In order to get further insight into the samples are diluted to volume with distiiled water, and the flasks reaction, a more detailed study of the repeatability and of the are shaken. rhange of absorbance with pH, time, and fluoride concentration \\as planned for a more limited p H range. For practical reasons, a consideration of the stability of the color reagent with time was also included. I n selecting the optimum color reagent, the independent variables are ratio of Alizaiiii Red S to thorium and the p H of the reagent. As these two variables are probably not independent and the effect may be a function of the fluoride concentration, a factorial experiment was set up for four p H values and four ratios of alizarin to thorium. The 16 color reagents were allowed to stand for 20 houis and xere well shaken before 20-ml. samples A- ori;/som: \\'ere withdran-n. The absorbances of 0, 50, and 200 y of fluoride B=50yF/50ml ion per 50 ml. xere measured 30, 60, and 120 minutes after mixCJO" ing. Duplicate analyses \\'ere run a t each condition. T o c = 100 y ml avoid too many figures, only thc average absorbances for the 60-minute period and the absorbance differences betn-een the 0and 50- y, and the 50- and 200- y samples are reproduced in Table I. The other observations are discussed in the text. The effect of p H on the absorbance is approximately linear over the limited pH range investigated here and is apparently independent of the Alizarin Red S-thorium ratio. Figure 1. Effect of p1-I on absorbance The change of absorbance with fluoride concentration depends 0. PH adjusted with hydrochloric acid both on the p H and on the ratio of alizarin to thorium. Bn 0. p H adjusted with monochloroacetic acid buffers approximate measure of these effects is given by the absorbance differences of Table I. It is evident that an optimum alizarinThe spectrophotometer is balanced at zero absorbance with thorium ratio may be found, the optimum value depending slightly one of the reference standards. The choice of reference standard on the fluoride concentration and on the pH. is dictated by the anticipated amount of fluoride in the samples. The results from the study of the change of absorbance with After some practice the operator will be able to choose the correct time are not reuroduced here. For the 50--t series a clear-cut reference standard from a mere visual observation of the color of the solutions. effect was observed. The absorbance did not change at the After 60 f 10 minutes the absorbances of the solutions in 10lotvest pH value (pH 2.61) during the time period investigated, mm. cells are read at 525 mp. whereas it decreased somewhat a t the higher p H values. The '4 set of fluoride standards is always run with each group of decrease with time was slightly more pronounced a t the higher samples. ~
~
~~
- ".
I 0
~~
~
540
~
V O L U M E 2 8 , NO. 6, J U N E 1 9 5 6
95 1
ratios of alizarin to thorium. For the 200--/ series a similar effect was observed, though i t was less pronounced. The duplicate samples showed excellent repeatability, except for the lowest p H value. A closer inspection revealed that this n-as probably due to low stability of the color reagent a t this pH. The color reagent's were stored for a 7-day period and the stability was checked both visually for precipitate and spectrophotometrically for a change of the absorbance value. All four reagents at p H 2.61 were unstable, as a precipitate settled. At, pH 2.71 only the reagent with the lowest concentration of alizarin n-:ts nnstable. The other reagents showed no change of absorba n w . I n short, the stability is better t,he higher the pH and the higher the ratio of alizarin to thorium. Selection of Optimum Color Reagent. The factorial experiment shows that the pH of the color reagent should not be below 2.7 as tlie thorium-.lliznrin Red S complex becomes unstable. Too high p H values should be avoided, as the method becomes rather insensitive to fluoride, and a t the same time the absorbance readings are somewhat time-dependent. It was finally decided to use a monochloroacetic acid buffering system of a composition that would give the highest possible buffering capacity, which occurs a t p H 2.80. An effort t o improve the pH control by increasing the concentration of monochloroacetic acid from 0.03.\to O.OSAr in the final solution failed, as the color reagent became unstable. The use of monochloroacetic acid buffers is well known in the titrimetric determination of fluoride with thorium using A41izarinRed S as indicator ( 1 , e), and Xijmmik (9) utilized the same buffer in the corresponding spectrophotometric method. The use of 200 ml. of the .ilizarin Red S stock solution and 20 nil. of the thorium stork solution per 1000 ml. of color reagent was chosen because the corresponding ratio was found to give the most sensitive reagent to fluoride. This ratio is the same as proposed by Icken and Blank ( 7 ) . The main differences betn.een the present reagent and the one pmposed by Icken and Blank are the close pH control in the present method by the use of the monochloroacetic acid buffer and the use of a more dilute color reagent to ensure stability. As an added precaution t o obt,ain a stable reagent,, 500 ml. of xvater is added before the thorium-..ilizarin Red 8 coniplex is formed. The present reagent can be used for at least a month, whereas that of Icken and Blank must be made u p for each determination. The change of absorbance with time for the present reagent is reproduced in Table 11. The color becomes stable only after
about 2 hours of development. For practical reasons it has, however, been standard practice in this laboratory t o read the absorbance after 60 i 10 minutes. Methods based on the thorium-..llizarin Red S reagent are sensitive to pH, and the presence of the monochloroacetic acid buffer alone does not guarantee a close pH control. T o achieve the high degree of precision attained in this laboratory, it is also necessary to neutralize all samples to p-nitrophenol with 0 OX\perchloric acid.
Table 11.
EFfert of Development Time on ibsorbance Fluoride. v / 5 0 LI1 50a Abqorbance
20a
Minutes 10
0 0 0 0
092 089 087 086 0 086
30 60 90 150
0 0 0 0 0
____
200 i -
0 0 0 0 0
010
008 004 002 001
110 108 1Oi 1Oi 106
Reading%taken against 50-y reference standard. b Readings taken against 300-7 reference standard.
(1
Evaluation of Optimum Concentration Range. I n a previous paper (8) it vias shown that the photometric error of analysis could be estimated by the use of known functions which give close approximations to the usual calibration curves of indiicct spectrophotomctiy. .\ convenient function is
where zo = tot,al concentration of fluoride, micrograms per 50 ml A , = nhsorbance of sample solution, referred to water The most convenient w i y of calculating the constants a, 8, and -( is probably by plotting the difference (xo - -~,idz),,p us. ( 4 2 ) e z pfor some assumed values of the constant y , The best approximation of this plot to a straight line in the higher ahsorbance region gives the corresponding values of the constants CY and from the slope of the liiie and its intersection with the A , = 0 axis. "calibration' curve is then calculated from this set of constants and co:npared with the experimental data. Curve 1 of Figure 2 gives the calculated calibration curve for the determination of fluoride and should be compared with the experimental points. The agreement is good up to about 300 -/ and is sufficient for the evaluation of the photometric error. A\
t I
io0
300
200
400
MICROGRAMS OF FLUORIDE PER 50ML. Figure 2. 1. 2. 3.
4. .I.
I
Calibration curve and error curve for spectrophotometric determination of fluoride
+
+
"Calibration curve," so = -272 A 142 27/A 1% photometric error. reference standard water 1% photometric error, reference s t a n d a r d 300 y of fluoride 1% photometric error, reference s t a n d a r d 100 y of fluonde 1% photonietric e r r o r , r r f r r r n r p standard 50 7 of fluoride
t
I 0
8 20
I
I
I
50
100
200
SODIUM SULFATE, mghOmL Figure 3.
Effect of sulfate
Readings taken against water
I
ANALYTICAL CHEMISTRY
952 The error curve for the case that the reference solution is water is now easily calculated Lyith the help of the following equation (8):
The lowest relative error occurs at rather high fluoride concentrations, around 200 y per 50 m l ! where the absorbance is only 0.23 (curve 2, Figure 2). The next step is to calculate the error of differential spectrophotometry, in which the readings are taken against a partly bleached reference standard, the absorbance of which, referred to water, is A*. The corresponding error curves are very simply obtained by dividing the error curve, referred to r a t e r , point by point by loA*. The set of error curves of Figure 2 shows the great impiovement in precision obtained bv the use of reference standards. For 40 y of fluoride per 50 ml., for instance, the relative error of analysis per 1% photometric error is 14% when measuring against water and is reduced to 4.2% with the 50-e! reference standard. However, the photometric error of analysis gives only an estimate of the lon-er limit of the actual error of analysis, vhich sometimes is dominated by other factors. I n the present method of fluoride determination it might well be that the p H control limits the precision aftei the introduction of the differential technique. An approximate comparison was made for three fluoride concentrations, assuming a pH control within 4rO.01 pH unit and a reading of the photometric scale of the instrument within 0.ZC4 intensity units The reading of the instrument
I
Figure 4.
Effect of aluminum ion
Readings taken against water
scale is the limiting factor for the 50-y sample w-hen measuring against water, whereas thr uncertainty arising from the pH control probably limits the precision when the photometric error is reduced by using the 50-7 reference standard (Table 111). Table 111. Per Cent Relative Error for 0.01 pH Error and For lower fluoride concentrations one may assume that the 0.2% Photometric Error photometric error aln-ays will be dominating, and the use of Photometric Error 6 reference standards will consequently give better resulta of P 1% Ref. s t d . Ref. soln. y F/5O h l l . Errora 50 y F water analysis. 1.8 6.1 20 1.8 Repeatability. The results from a test of the repeatability 1 . 1 2.2 50 0.7 1.0 0.6 200 of the method are given in Table IV. Interference. Any test for salt interference should be cona Determined experimentally. b Calculated from error curves. ducted at different fluoride concentrations. The effects of perchlorate, chloride, and sulfate in the present method Table IV. Consecutive Analyses of Fluoride Standards have been investigated for 0, Sample 20 y F Sample 200 1 F. Sample 50 y F, 20, 50, and 200 y of fluoride. R e f . s t d . 50 y F Ref. s t d . 300 y F Ref. s t d . 50 y F Ref. std. 300 y F F found, F found, F found, F found, Aliquots of the fluoride standA Y A Y A 7 A 7 ards, together with the sodium 0.087 19.65 198 4 0,472 20.70 0.102 50.00 salt of the acid, were placed in 0.086 20.00 200 0 0.477 18.95 0.100 50.37 20.35 0.088 0.100 200 0 50.00 0,470 21.40 50-ml. volumetric flasks. Each 19.65 198 4 0,087 0.102 49.63 0.472 20.70 200 8 19.65 0.480 17.90 0.087 0,099 49.26 was neutralized with 0.05-V 199 2 20.35 0.474 20.00 0.088 0.101 49.26 perchloric acid using p-nitro199.2 18 95 0.475 19.65 0.089 0.101 50.74 20,oo 198 4 0.475 19.65 0,086 0.102 50.00 phenol as indicator before the 19.65 200 0 0.470 21.40 0.087 0.100 49.63 20.35 0.476 19.30 0.085 197 6 0.103 49.63 addition of the color reagent. Mean 19.86 19.97 199.2 49.85 Sulfate, which forms moderately Standard deviation +l.ll 10.44 il.0 =t0.47 strong complexes with thorium, 2.2 Coeff. of variation, 70 5.6 0.9 0.5 displays the most serious interference (Figure 3). Increasing amounts of perchlorate deTable V. Effect of Perchlorate on Determination of Fluoride crease the absorbance slightly NaClO4 OrF 20 y F 50 y F 200 > F Mg./50 Mole/ F F F F (Table V), but the effect is so Aa found, y Aa found, y AO found, y Ab found. -/ ml . liter small that it may be neglected 0 0 0.171 0.0 0.100 20.0 0.011 200 50.0 0.097 0.168 1.0 20 0.0033 0.097 21.0 0.010 50.4 0.094 201 in most work. Chloride was 50 0.0082 0.170 0.3 0.099 20.4 0.007 51.4 0.093 202 found to give the same effect 0.170 0.3 0,097 100 0.016 21.0 0.007 51.4 0.088 204 200 0.033 0.169 0.7 0.098 20.7 206 0.006 51.8 0.083 as perchlorate, calculated on a 0.167 1.3 400 0.065 0.095 21 8 0,003 52.8 0.080 207 molar basis. Of cations, only 0 Readings taken against 5 0 - y reference s t a n d a r d . b Readings taken against 300-7 reference standard aluminum has been investigated in some detail (Figure
V O L U M E 28, N O . 6, J U N E 1 9 5 6 4). With increasing concentrations of aluminum a significant increase in absorbance is observed, except for the 0-*( series. This behavior means that aluminum forms stable complexes with fluoride but not n i t h -1lizarin Red S to any extent a t the pH utilized. DISCUSSION
The thorium-Alizarin Red S reagent was originally introduced in this laboratory to determine comparatively large amounts of fluoride, around 500 y. The color reagent was similar to the one described here, except that twice the amounts of the thorium and the hlizarin Red S stock solutions were used. This reagent had to be made up fresh for each determination. As expected (8)>the precision was very good at the higher concentrations. The coefficient of variation was only O.2YGfor 10 samples of 400 y of fluoride, the readings being taken against a 500--i reference standard. The procedure described here has proved to be reliable even for amounts of fluoride of around 20 y. The stability of the reagent for a t least a month makes it convenient for laboratories engaged in routine determinations of fluoride, as it can be made up in large quantities. The main disadvantage of methods based on thorium-iilizarin Red S reagents is the great pH sensitivity. Furthermorc, the reagent is not as sensitive to small amounts of
953 fluoride, as is the thoron reagent, for example, proposed by Horton, Thomason, and Miller (6). ACKNOWLEDGMENT
The author gratefully acknowledges the valuable aid and assistance of Celia Gorl6n and Kirsten Matheson during the course of this investigation. Appreciation is expressed to A/S L d a l og Sunndal Verk for support of this study. LITERATURE CITED
Armstrong, W. D., IND.ENG.CHEM.,ANAL.ED. 8 , 384 (1936). Barr, G., Thorogood, A . L., Analyst 59, 378 (1934). Bumsted, H. E., Wells, J. C . , ANAL.CHEM.24, 1595 (1952). Danielsen, M. E.,. Univ. Bergen &bok, h‘aturvitemkap. Rekke KO.9, 1-10 (1953). Horton, A. D., Thomason. F. D.. lliller, F. J., ANAL.CHEN. 24, 548 (1952).
Hoskins, W. hl., Ferris, C. A., IND.ENG.CHEM..ANAL.ED. 8, 6 (1936).
Icken, J. M., Blank, B. M., ANAL. CHEM.25, 1741 (1953). Lothe, J. J., Ibid., 27, 1546 (1955). Nommik, H., Acta Polytech., C h a . M e t . Ser. 3. No. 7, 7-121 (1953).
Sanchis, J. hl., IND.ENG.CHEM.,ASAL. ED.6, 134 (1934). Talvitie, N. A,, Ibid., 15, 620 (1943). Thompson, T. G., Taylor, H. J., Ibid., 5. 87 (1933). RECEIVED for review December 6, 1955. Accepted March 16, 1956.
Carbon-Hydrogen Stretching Bands in High-Resolution Infrared Spectra of Heterocyclic Compounds W. H. TALLENT and IRIS J. SIEWERS National Heart Institute, National Institutes o f Health, Bethesda 14,
Wave lengths for carbon-hydrogen stretching bands found in high-resolution infrared spectra of a number of heterocyclic compounds are given. Comparison of these spectra with those for analogous hydrocarbons suggests that assignments for carbon-hydrogen stretching bands in hydrocarbons can be extended to heterocyclic compounds, with certain limitations. The detection and characterization of nonaromatic double bonds in compounds containing both aromatic rings and nonaromatic double bonds, by comparing spectra of these compounds with those of their dihydro derivatives, are illustrated.
D
URING the past fen- years infrared spectrophotometers with high resolving pon er in the 3- to 4micron region have become commercially available. The possibility of using C-H stretching bands found in this region for determining the nature and location of C=CH bonds, and for detecting the presence of three-membered carbocyclic rings, was recognized sometime ago. .4 number of papers and reviews contain structural assignments for bands in this region (1, 8, Q , Z O , d l ) ; of these the data pertaining to steroidal systems, as recently summarized by Jones and Herling (11)and by Cole ( 5 ) ,are perhaps the best known. Much of the work of this laboratory is concerned with nitrogen-containing compounds, and in the course of a separate study on the structure of pinidine [a new- alkaloid isolated from leaves of Pinus sabzniana Dougl. (WS)]the problem arose of determining the nature and position of an unsaturated bond present in this compound. T o use the available infrared data effectively, it was necessary to compare the C-H spectra of a number of analogous
Md.
carbocyclic and heterocyclic compounds. The information obtained, which is potentially useful in structural studies on alkaloids and related compounds, is summarized in this paper. The overlap in =CH absorption originating in an aromatic system with nonaromatic =CH absorption is also considered, and a method for detecting nonaromatic =CH absorption bands in the presence of interfering aromatic =CH bands is illustrated. SPECTROSCOPY
A Beckman IR-3 spectrophotometer (which achieves high resolution by unique optical and electronic systems without recourse to lithium fluoride or calcium fluoride prisms) was used in these studies. The spectra were obtained using sodium chloride optics and liquid samples or carbon tetrachloride solutions and cell thicknesses of 0.03 and 0.10 mm., respectively (with the exceptions given below). The spectra of pinidine and dihydropinidine are given to illustrate the type of results obtained. The vinyl hydrogen stretching absorption band in the spectrum of pinidine (3.31 microns) is relatively low in intensity as compared with that found for most of the other olefins. The s ectrum of 1-ethyl-2-ethylidenecyclohexanealso showed low vinyr hydrogen absorption. Integration %-asnot considered a feasible method for obtaining data for comparison, as in Table 111, because the absorption bands in question were very sharp and very close to adjacent and much more intense bands. Molar absorptivities were calculated in the usual manner, using transmittances recorded by the spectrophotometer. They are reported here not as physical constants for the compounds in question but merely as measures of absorbances a t given wave lengths in the spectra obtained by the method used in this work. The Beckman IR-3 is a single-beam instrument with which the pattern of variation of slit width with wave length, established in a “blank” run giving a base line (zero transmittance) for a given cell (in this case 0.40 mm. thick) and solvent (carbon tetrachloride), is repeated exactly when the spectrum of a com-
