Spectrometric Determination of Proton (CH) in Organic Compounds by Integrated Intensity Measurements in the vCHSecond Overtone Region Arthur S. Wexler Dewey and Almy Chemical Division, W. R. Grace & Co., 62 Whitternore Avenue, Cambridge, Mass. 02140
A near infrared procedure has been developed for estimation of carbon-linked proton content of organic compounds by measurement of the total integrated intensity assignable to CH oscillators absorbing in the vCH second overtone region (8000-9000 cm-l) where as many as 6-8 overlapping bands may be observed, as in the alpha-olefins. With few exceptions, the integrated intensity per oscillator (and therefore of aliphatic, aromatic, ethylenic, and other protons) is nearly constant and is close to 2.6 cm-2 1 mole-’. On this basis it is possible to estimate the carbonlinked proton content of organic compounds with speed (-8’) and precision (RSD -1%) bya nondestructive method. Because the intensities per proton in the exceptional cases (pendent methyl and proton vicinal to certain “chromophores” such as carbonyl) were also characteristic, it is possible to utilize the intensity data in support of proposed assignments of structure, as in detection of methyl branching. The effect of chromophoric groups in shifting frequencies as well as intensities is also discussed and some assignments for vinylene, vinylidene, and formate protons are suggested. ONE of the most important applications of infrared spectrometry is the quantitative estimation of organic functional groups such as OH, C=O, and CH by means of integrated absorption intensities. The subject of the relationships between intensities and structural parameters has been reviewed on several occasions (1, 2). Total aromatic proton content can be determined by integrated intensity measurements in the 1650-2000 cm-’ summation band region (3). Total alkane proton content can be determined by integrated intensity measurements in the 2800-3000 cm-1 C-H stretching vibration region ( 4 , 5 ) . Recently we attempted to extend the near infrared peak intensity method of Rose (6) and others (7, 8) to the determination of olefinic proton content of some vinyl hydrocarbons and esters. The method failed, because of extensive peak overlap of vinyl and aromatic CH oscillators in compounds like styrene and because of loss of intensity resulting from an interaction with carbonyl or carboxy groups in compounds such as vinyl acetate. We were able to reproduce the results of Rose with hydrocarbons, obtaining nearly the same absorptivities for methyl, methylene, methylidene, and aromatic protons at 1.19, 1.21, 1.23, 1.14 microns, respectively. (1) T. L. Brown, Chem. Reu., 58,581 (1958). (2) A. S. Wexler, Appl. Spectrosc. Reo., 1,29 (1967). (3) A. S. Wexler, ACS Division of Fuel Chemistry, Vol. - . Preprint, 11, No. 4, 185 (1967). (4) S. A. Francis. J. Chem. Phvs., 18,861 (1950). (5j A. S. Wexler; Spectrochim. Acta; 21, 1725 (1965). (6) F. W. Rose, J . Res. Nut. Bur. Stand., 20,129 (1938). (7) R. H. Hibbard and A. P. Cleaves, ANAL.CHEM., 21,486 (1949). (8) A. E. Evans, R. H. Hibbard, and H. S. Powell, ibid., 23, 1604 .
I
(1951). 1868
0
ANALYTICAL CHEMISTRY
In the course of this investigation a close correlation was observed between the integrated absorption intensity over the entire 3vCH second overtone region (7500-9500 cm-l) and the carbon-linked proton content of alkylated aromatics and unsaturates. Subject to certain restrictions (presence of pendent methyl and carbonyl groups, with resultant decreases in intensity), it was found possible to estimate rapidly the carbon-linked proton content of organic substances with excellent precision and reasonable accuracy from 3vCH integrated intensity measurements. EXPERIMENTAL
All spectrophotometric measurements were made with a Beckman DK-2 double beam recording spectrophotometer similar to the instrument described by Kaye (9). Wavelength drive gears were reversed to provide expansion of the wavelength scale. Slit potentiometer gain was set near 100 divisions to keep the apparent slit width and resolution constant, the nominal slit reading being 0.02 mm. Resolution was sufficient to resolve partly the benzene doublet (10) near 1.140 and 1.145 microns, a result superior to that reported ( 7 ) for an instrument cited as having a resolution of 22 cm-’. Because the main objective in these studies was the establishment of correlations between 3vCH (second overtone) integrated intensities and proton content of organic compounds with a high degree of precision, deliberate efforts were maintained to secure good reproducibility of measurement. Operating controls were set as follows: Scanning range Scanning speed Time constant Wavelength-chart traverse ratio Ordinate scale
7500-9500 cm-1 3 cm-1 per second 0 . 2 (2-second period)
12-14 cm-l per mm “Absorbance”
The majority of materials studied were pure reference compounds of known structure. In most cases single scans were made of undiluted liquid in 1-cm cells, yielding areas in the range 25-50 sq cm. Solids were run at 0.5-1M concentration in carbon tetrachloride (chloroform was used for amides) in 10-cm cells. No significant changes of intensities with dilution were observed with most hydrocarbons tested. Base lines were drawn from suitable minima near 1.08 and 1.28 microns, as shown in Figure 1, which is a facsimile reproduction of a highly aromatic petroleum solvent run undiluted in a 1-cm cell. Because of nonlinearity of DK2 wavelength scale it was necessary either to replot the curves on a linear wavenumber scale for calculation of integrated intensities, which is a slow tedious process, or else convert measured areas (with a polar planimeter) to integrated areas by multiplying by suitable conversion factors ranging in value from 0.31 to 0.34 in these studies. The wavelength ranges for each conversion factor are indicated in Figure 1. These conversion factors were computed as follows: (9) Wilbur Kaye, Spectrochim. Acru, 6,257 (1954). (10) M. Tuot and P. Barchewitz, BUN. SOC. Chem. Fr., 1950, 851.
E
A
L!lL F
ALI
0.0
I
1008
*
1
1,lO
1
1
1.14
I
I
1.18
.
I
I
1.22
I
1.28
,
I
1.30
,
L2
I.!
I
d
l
I
1
-, .
1.2
1.1
1.3
Figure 2. Absorption frequencies 5 M in proton; 5-cm path cell
=
cm-l/mrn x 0.00394 absorbance/mm X 6.5 sq mm/ planimeter division
=
0.325
0.01 cm-1 for the range studied. Integrated Area (7500-9500 cm-I) moles/liter x cm
a
.
The integrated area was obtained by multiplying the area under the absorption band (measured with a polar planimeter) by the appropriate conversion factor. At least two planimeter measurements totalling 500-1000 divisions were made on each curve or section. If readings differed by more than 1% (rare) several replicates were averaged. The relative standard deviations for 6 to 8 planimeter scans were usually in the range 0.6-1%. The principal source of error of intensity measurement appeared to be the slit width variation. The extreme range of slit width obtained by adjustment of potentiometer gain in the 1.2-micron region corresponded to intensity variations The possible variations in daily and long range of = t t S % . performance were closely observed with a standard reference substance, n-octane of 99 mole purity, run undiluted in a special 1-cm cell. Care was taken to maintain the same nominal slit width of 0.02 mm (actually closer to 0.01 mm) during all runs. The observed variations in areas were less than 1% within a day's run and nearly 1.0% (r. s. d.) over a 3-month period. The apparent integrated intensity of the n-octane was found to be 48.5 =k 0.5 cme2 1 mole'', a value which is suggested as a standard for other instruments in the interest of data transferability. This value is not necessarily either the true integrated intensity or the absolute intensity because of the approximations and uncertainties involved in making measurements on a nonlinear wavelength scale with no corrections being applied for slit width or dilution effects. These uncertainties are, in essence, resolved by calibration with a reference substance of similar composition such as n-octane. Wavelength Assignments in the 3vCH Region. The assignment of 3vCH intensities is aided by a knowledge of absorption frequencies (illustrated in Figures 2 and 3), which are reproductions of spectra of carbon tetrachloride solutions of selected compounds at 5-cm path lengths and 5M aliphatic or aromatic proton concentration. A peak near 1.21 microns is evidence of methylene proton as in dotriacontane (Figure
( F ) Pentene-1 (G) Acetaldehyde5 (H) Methyl formateb (I) Vinyl formateb (J) Di-n-propylamine
Dotriacontane Cyclopentane (C) 2,2,3-Trimethylbutane ( D ) n-Pentane ( E ) cis-Pentene-2 b
Integrated intensities were calculated as follows: Integrated intensity = (cm-2 1 mole-')
U
I
Figure 1. Aromatic petroleum distillate, undiluted, 0.5-cm cell
&
* I
1.34
WAVELENGTH IN MICRONS
Factor
I
A A J
G
Not including aldehydic proton Not including formate proton
2 4 , shifted to 1.195 microns in cyclopentane (Figure 2B). Ring strain and ring vibration in cycloalkane rings may explain the frequency shift and the higher intensities observed in the alicyclics. Peaks near 1.195 microns (strong) and 1.15 microns (weak) are evidence of the methyl group (symmetric and asymmetric modes?) but the weaker peak may be confused with aromatic absorption which overlaps. A peak near 1.23 microns in 2,2,3-trimethylbutane (Figure 2C) and other branched hydrocarbons such as 2,3,4-trimethylpentane may be evidence of methylidene proton. Both the methyl and methylene proton absorption are partly resolved in n-pentane (Figure 20), but in higher
-
m
1
'
:
'
I
E
C
A
0.02
r om
L IL U
1,2
13
1.2
1.1
WAVELENGTH
1.3
1.1
1.2
1.3
I N MICRONS
Figure 3. Absorption frequencies 5 M in proton; 5-cm cell path ( A ) Phenola ( B ) Styrene (0 Diphenylmethand a
(D) Naphthalene ( E ) Chlorobenzene
Not including hydroxylic proton
* Not including methylene proton
VOL. 40, NO. 12, OCTOBER 1968
1869
Table I. 3vCH Proton Integrated Intensity Assignments
Type of proton Aldehydic proton (aliphatic) Aldehydic proton (aromatic) Methylene proton (alicyclic) Methylene proton (aliphatic) Methylidene proton (aliphatic) Methyl proton (terminal) Methyl proton (pendent) Vinylidene proton Vinylene proton Aromatic proton CH oscillator adjacent to Group I carbonyld CH oscillator adjacent to Group I1 carbonyl"
Peak wavelength (microns)
Half band width (cm-l)
1 .24-1.27 1.24-1.26 1.21 1.21 1.23 1.195 1.195 1.15 1.17 1.145
300 300 200 230
... ..
Peaka
Intensity per proton Integrated*
0.015 0.01 0.01 0.01 0.01 0.014 0.008 0.015
... ...
125 100
...
3.5 3.0 3.2 2.8 2.8 2.6 1.7 2.4 2.4 2.2-2,7 1.6
...
0.012 0.0040.005 0.003
150 300 300
0.8
1 mole-' cm-1. cm-2 1 mole-'. c Includes absorption peaking at 1.095, 1.15, 1.29 microns. Group I : ketones, esters, aldehydes, amides, lactones. e Group I1 : acids, acyl halides, anhydrides. a
hydrocarbons above dodecane the methyl group absorption is evident only as a shoulder or inflection point on the predominant methylene proton absorption. A peak near 1.17 microns appears to be evidence of vinylene proton, as in cis-pentane-2 (Figure 2E), confirmed by the presence of a single peak centering at this wavelength in cyclooctatetraene. A peak at 1.15 microns and additional weaker peaks at 1.10, 1.12, and 1.34 microns are assignable to vinylidene proton, as in pentene-1 (Figure 2F). Some approximate assignments for olefinic 3vCH absorption have been previously suggested by Goddu (11) and others (12). Aldehydic protons absorb at 1.24-1.27 microns in acetaldehyde (Figure 2G) and other aldehydes [clearly distinguishable in aromatic aldehydes (13) such as benzaldehyde]. The methyl proton absorption is shifted by about 0.02 micron in compounds with adjacent carbonyl or carboxy groups, as in acetaldehyde, acetic acid, methyl acetate, and methyl formate (Figure 2H). Peak intensities are also markedly decreased in these compounds. A peak near 1.19 microns in methyl and in vinyl formate (Figure 21) appears to be associated with formate proton. This peak is not resolved in the undiluted state. Peak shifts are observed in vinyl proton absorptions in compounds with adjacent chromophores such as carbonyl, nitrile, and halogens, the shift being of the order of 0.020.03 micron. A peak near 1.26-1.27 microns in sec- (Figure W )and tertamines may be assignable to combinations of HC and N C stretching modes. Primary amines display several barely resolved peaks on the long wavelength portion of the 1.19micron methylene proton absorption which increased somewhat the apparent integrated intensity. Evidence of the N H group is also seen by a peak at 1.05 microns (3vNH). Overlapping absorption is also observed in hydroxylic compounds and this absorption is clearly delineated in phenol (Figure 3 4 as shown by peaks at 1.19 and 1.22 microns (1.23 and 1.25 microns in o-bromophenol). Mixing of vibrations due to coupling of aromatic and vinyl CH oscillators appears to occur in styrene (Figure 3B),
(11) R. F. Goddu, ANAL.CHEM., 29, 1790(1957). (12) R. T. Holman and P. R. Edmondson, ibid.,28,1533 (1956). (13) R. M. Powers, J. L. Harper, and H. Tai, ibid., 32,1287 (1960).
1870
ANALYTICAL CHEMISTRY
as shown by the intensified narrow peak at 1.145 microns. Otherwise the presence of a relatively narrow band (half band widths of 125-150 cm-1) in this region is evidence of aromatic proton absorption as shown by diphenylmethane (Figure 3C), naphthalene (Figure 30), and chlorobenzene (Figure 3E). The lowest peak wavelengths observed were those for pyrrole and furaldehyde which absorb near 1.115 microns. Evidently there is a correlation between 3vCH frequency and chemical resonance in compounds with sp2hybrid orbitals. RESULTS 3vCH integrated intensity values per proton are presented in Table I. Values per proton for unbranched hydrocarbons with aliphatic chains were in the narrow range of 2.7-2.8 for n-paraffins, olefins, halogens, alcohols, ethers, and amines. Slightly lower values, 2.5-2.6, were obtained for alkylbenzenes up to C12 (triethyl benzene and n-hexylbenzene). These results imply a constant group intensity per methylene proton. The group value for methylene proton obtained by averaging intensity differences between adjacent members of a homologous series was 2.77 i 0.12. A lower intensity was obtained with compounds containing a pendent methyl group :
H
CHI
I I wwCwwwvwwOr.ww,w,w C.-w I nn
CH3
Analyses of the data for a large number of reference hydrocarbons yielded intensity assignments of 1.7 per pendent methyl proton and 2.6 for terminal methyl protons (the status of aromatic ring methyl is not certain but the intensity may be slightly lowered). Assuming an intensity of 2.8 for both methylene and methylidene protons, it was possible to calculate intensities for branched hydrocarbons in good agreement with the observed values. A lowering of intensity was also observed in compounds with CH oscillators adjacent to a chromophoric group such as carbonyl or carboxy.
Table 11. Accuracy of Estimation of Carbon-Linked Proton Content of Organic Compounds by 3vCH Integrated Intensity Measurements Absolute Conversion error % Compound class Numbers factor % Hydrogen5 hydrogen RSD, Z 4.1 11 3.5 14-16.7 0.6 Alicyclics 2.1 10 3.7 14-16.7 0.3 n-Paraffins 2.7 8 3.7 14-16 0.4 Alpha-Olefins 5.0 6 3.7 9-1 1 0.5 Alkyl halides 2.4 10 3.7 11-12 0.3 n- Alkanols 2.1 7 3.7 13-13.5 0.3 Aliphatic ethers 3.0 9-1 1 0.3 Monoalkyl benzenes 11 4.0 3.4 17 4.0 1c-11 0.35 Polyalkyl benzenes 7.3 2-5 0.3 Benzene derivatives 14 4.5 Petroleum solvents 3.7-4.1 0.2-0.4 Integrated area (7500-9500 ern-') X conversion factor a %Hydrogen = grams/100 ml X cm path length
The effect of a carbonyl group in ketones and esters was to lower the intensity per adjacent CH oscillator by 1 cm-2 1 mole - 1 per proton, or a total of 4 per carbonyl in these compounds. A similar lowering of intensity was observed in compounds with other electron-withdrawing groups such as nitro, nitrate, nitrile, and sulfoxide. The wavelength assignments for formate, vinylidene, and vinylene proton do not appear to have been reported previously. The product of half band width and peak height intensities is obviously not reliable for estimation of integrated intensities because of the significant contribution of components which broaden and distort the bands, as shown in Figures 2 and 3. On the basis of these intensity assignments it was possible to calculate intensities in excellent agreement (relative standard deviations of 2-4% for most classes of compounds studied) with the observed values obtained for a large number of compounds in 20 structure classes. The effect of a carbonyl group in lowering the intensity appears to be transmissible to the vinylidene proton as well as the vinylene in vinyl esters. This lowering of intensity is probably due to the reduced polarity, bond moment, and electrical anharmonicity resulting from the electron-withdrawing effect of the carbonyl chromophore, accompanied by possible resonance interactions. It appears possible to estimate the per cent carbon-linked proton for specific classes of compounds to within 0.3-0.5% absolute, employing appropriate conversion factors as shown in Table 11. The principal sources of uncertainty in the estimation of proton content by 3vCH integrated intensity measurements are the degree of pendent methyl branching, which causes a lowering of intensity, and concentration of alicyclic rings, which cause an increase in intensity. Fortunately these two opposite trends tend to average out in petroleum solvents containing constituents of moderate to high molecular weight. On the basis of the equation Hydrogen (carbon-linked)
=
Integrated Area (7500-9500 cm-l) X 4.0 (2) grams/100 ml x cm it should be possible to estimate the proton content of aromatic alkylates and moderately branched hydrocarbons with a precision of 1% and an accuracy of *0.3% absolute. Appropriate correction for the effect of carbonyl groups on pro-
ton intensity estimations in derivatives can be made by independent estimates of carbonyl content. DISCUSSION
The excellent precision (1 relative standard deviation) of the measurements of 3vCH integrated absorption intensities implies that carbon-linked proton content of unknowns for analysis can be determined by Equation 2 to within 0.20.4% absolute, subject to the following restrictions : 1) A correction is added for carbonyl content, equal to 0.04% hydrogen for each per cent carbonyl in acids, esters, and ketones. No correction is required for aldehydes. 2) The degree of pendent methyl branching is within the range of about one pendent methyl for each 5-8 carbon atoms. Two other possible applications of 3vCH integrated intensity measurements became apparent during the course of this work. It should be possible to use the intensity assignments to check proposed structure assignments in low molecular weight compounds where the intensity differences between proposed structures may be large, as in comparison of such isomers as normal heptane and 2,2,4-trimethylbutane, or of acetone and methyl vinyl ether. Any serious discrepancy between calculated and observed intensities would have to be reconciled before a proposed structure can be considered acceptable. Another application is the determination of degree of pendent methyl branching in compounds of known hydrogen content, using the equation x = 270
- Integrated Area g/cc X cm X
X 10
X
H
where x is the number of pendent methyl protons per 100 protons (carbon-linked). If the molecular weight is known it is then possible to compute the number of pendent methyl groups per molecule. In most cases the number of pendent methyl groups could be estimated to within one-half unit. Several other regions of the near infrared were examined for analytically useful correlations between intensity and proton content. The dispersion of data for a small number of compounds was apparently too great in the first overtone 2vCH region to be of value in quantitative estimation of total proton content. However, encouraging preliminary results were obtained for aromatic proton absorption in the 2.1micron combination band region. A tentative assignment of VOL. 40, NO. 12, OCTOBER 1968
1871
Table 111. Determination of Hydrogen Content of Petroleum Distillates by Near Infrared Spectrometry 50
z
Sp. Gr. Distillation Type of distillate 60 O F "F Commercial hexane 0.680 153 Commercial hexane 0.675 156 Commercial heptane 0.728 204 Naphthenic 0,754 255 Naphthenic 0.765 320 Naph thenic 0.766 323 Naph thenic 0.789 342 Naph thenic 0.790 342 0.873 320 Aromatic Aromatic 0.869 328 Aromatic 0.891 370 0.892 375 Aromatic Pure n-octaned 0.703 259 Pure benzene5 0.879 194 = Peak ratio of 1.19/1.145 micron absorption. b Factor = -0.11 (Peak Ratio) 4.3. Integrated area (7500-9500 cm-1) c % Hydrogen (NIR) = si100 ml X cm d Theoretical hydrogen 15.89. e Theoretical % hydrogen 7.75.
+
Peak ration 5.2 5.2 4.8 4.4 4.2 4.2 4.1 3.45 1.3 1.7 1.75 1.7 5.2
...
zHydrogen Factofi 3.73 3.73 3.77 3.82 3.84 3.84 3.85 3.92 4.16 4.11 4.11 4.11 3.73 4.0
NIRc 16.15 16.15 15.95 14.35 14.35 14.45 14.4 13.4 10.05 10.25 10.45 10.35 15.95 7.8
Pregl 15.8 15.9 15.0 14.3 14.3 14.5 14.0 13.3 10.0 10.4 10.4 10.3 16.0 1.7
Difference 0.35 0.25 0.95 0.05 0.05 -0.05 0.40
0.10 0.05
-0.15 0.05 0.05 -0.05
0.10
X Factor
z
12.5 =t1.5 (range) cmF2 1 mole-' per aromatic proton was obtained for a group of alkylated aromatics. This means aromatic proton content in such compounds could be estimated to within about 0.5% absolute. However, olefinic protons also absorb in this region, as is the case in the vCH and 3vCH vibrations In the absence of unsaturates it is possible to obtain the aromatic proton content by means of the 1.2-micron absorption and the alkane proton content by difference, utilizing the 3vCH total proton absorption in alkylated aromatics. Results of analyses for hydrogen content of commercial petroleum distillates by the near infrared and Pregl techniques are presented in Table 111. The choice of the conversion factor was based roughly on aromatic content as judged by the ratio of the absorption at 1.19 and 1.145 microns. Inspection of the data revealed an apparently linear relationship expressed by the equation
1872
0
ANALYTICAL CHEMISTRY
Factor
=
-0.11~
+ 4.3
where x is the peak ratio. The standard deviation of difference between the spectrometric and chemical methods for hydrogen content was, with the exception of commercial heptane, 0.12%. The deviation in the case of the heptane sample was probably caused by the fairly high content of naphthenics, possibly methyl cyclohexane, which has an intensified absorption of 3.2 per proton. The excellent agreement obtained in the two sets of assays justifies the use of Equation 2 for medium range petroleum distillates. A factor of 3.9 was required for mineral oils which apparently have a high degree of methyl branching. RECEIVED for review February 9, 1968. Accepted June 17, 1968.
