Infrared Absorption Studies of Carbon-Hydrogen Stretching Frequencies In Sulfurized and Oxygenated Materials ABBOTT POZEFSKY AND NORMAN D. COGGESHALL Gulf Research & Decelopment Co., Pittsburgh, Pa. A study has been made of t h e carbon-hydrogen stretching frequencies as observed with a lithium fluoride prism for a series of sulfur-containing compounds comprising mercaptans, sulfides, and disulfides and a series of oxy genated materials comprising alcohols, ethers, ketones, acids, aldehydes, esters, and formates. The purpose has been to see if the Fox and Martin hydrocarbon assignments apply to these sulfurized and oxy genated materials, to determine if a n d when such assignable frequencies are perturbed by t h e introduction of the sulfur or oxygen atoms into t h e molecule, and to see how certain can he the interpretation of such spectra using t h e selected assignments. It has been found t h a t t h e band appearing a t approximately 2960 cm , - I is assignable to a methyl group fundamental in all cases. I n the region of approximately 2930 cm.-' both methyl and methylene groups absorb and the absorption hands are seldom resol\ed. Comparison with other bands, however, sonietinies allows t h e presence of either t j p e group to he determined. In t h e region
I
between 2850 and 2890 cm.-I the absorption must be attributed to the methyl and methylene groups. The hands were often poorly resolved, t h e methyl absorption being more intense and occurring a t the higher frequencies. KO uniformly appearing band assignable to tertiary hydrogen was found. For t h e sulfur-containing molecules the presence of the sulfur atom does not introduce significant perturhations of t h e observed frequencies. However, for the oxygenated compounds the frequencies in t h e 2960 cm.-' region allow classification b y which the attachment of ethyl, isopropyl, or sec-butyl groups to t h e functional group containing the oxygen atom may he assessed. The aldehydic C-H stretching frequency is assigned to one or t h e other of t h e two hands appearing uniformly for aldehydes a t ahout 2720 and 2820 cm.-'. The formate C-H stretching frequency is tentatively assigned a t approximately 2935 cm.-'. The results are compared to those obtained by Fox and Martin for hydrocarbons and the assignments are found to he in general agreement.
S RECEKT years a great deal of interest has centered around
thc aualitative and auantitativr determination of tvues of " _ C-H groups in organic molecules. Rose (11) investigated the 1.7-micron ahsorption of C-H groups and found near constancy of rstinction coefficient9 within a group. Hihhard :ind Cleaves (8)worked in the second ovrrtone region (8000 to 9000 cm.-l) and were ahle to wtimate "thr amount of cahain t)ranching in paraffins and the degree of su1)stitutioii on naphthrne itnd aromatic rings." Fos and Martin (3, 4))using a grdting rpectrometer, invrstigatrd thr C---H stretching funtlanirntiils of h>-drocarhon molec*ules in the 2800 to 3100-cm.-I rcgion. By :ijudicious choice of moleculcs, they sucrerdrd in corrrlatiny certain observrd frequencies x i t h different vi\)rationwl modes of the following groups:
-CH,, )CHI,
1 CH, =CH, /
:rid
30903d50 3000
EXPERIMENTAL
A Perkin-Elmer Model 12-C automatic recording spectrometer equipped with a lithium fluoride prism was used. The sulfur
2900 2850
v(CMTI) Figure 1. C-H Absorption Frequency- Diagram for Hydrocarbons (12) Dashed lines. Below base line indicate bands infrequently observed. Above base line indicate caaes where single band m i g h t split i n t o a doublet.
=CH2
The molecules were chosen in such a way that either one or two functional units predominated, and their absorption bands werr generally easily discerned. Plyler and Acquista ( 10 ) investigated 19 ryclohydrocarbons in the 3.4-micron region and extended the data on the ) C H 2 vihration frequencies. The prrsent paper reports the results of an investigation undertaken on aliphatic sulfurized and osygenated molecules. This investigation had a threcfold purpose: to see if the Fox and Martin hydrocarbon assignments apply t o these sulfurized and oxygenated materials; t o determine if a n y of the established C-H frquencies were appreciably perturbed by the introduction of the sulfur or oxygen atoms into t h e molecule and t o see how certain one could be in the interpretation of such spectra using the selected assignments,
2950
compounds were mercaptans (thiols), sulfides, and disulfides obtained from either Eastman Kodnk or Columbia Chemical; the oxygenated compounds were alcohols, ethers, ketones, aldehydes, acids, esters, and formates obtained from Eastman Kodak or Matheson. The samples !yere used as received, appropriately diluted in carbon tetrachloride. Either a 0 05- or a 0.15-mm. cell was used, although in several instances a 1.4-em. cell was employed in a n attempt t o minimize interaction effects hy studying dilute solutions. It is recognized that commercial chemicals of the highest purity may a t times contain a sufficient quantity of impurities to give spurious absorption bands, but the number of compounds investigated made any purification procedures impractical. DISCUSSION OF DATA
Figure 1 is a reproduction of a diagram previously formulated ( f a ) from the group averages given by Fox and Martin ( 4 ) . Several relative intensity changes have been made in cbonformity 1611
A N A L Y T I C A L CHEMISTRY
1612 Table I.
Obeerved Absorption Bands for Mercaptans, Sulfides, and Disulfides
Compound Mercaptans Ethyl n-Propyl Isopropyl n-Butyl Isobutyl n-Amyl Isoamyl n-Hexyl n-Heptyl n-0ct 1 n-DoJecyl n-Hexadecyl Sulfides Methyl Methyl ethyl Methyl n-butyl
2965 Cm.-1
2930 Cm.-'
2910 Cm.-1
2875 Cm.-1
2861 Cm.-l
2972 2967 2968 2965 2965 2965 2967 2965 2965 2961 2960 2960 Sh
2930 2932 2927 2932 2928 2935 2935 2933 2933 2929 2928 2928
....
2872 2876 2868 2875 2874 2878 2878 2878 2878 2872 -2875 Sh -2873 Sh
"-2857 Shb 2858
....
-2865 Sh
....
2992 2972 2974 2965
2922 2923 2932 2954 2928 2930 2933 -2922 Sh 2931 2931
Ethyl E t h y l n-butyl n-Propyl n-Amyl Isobutyl Disulfides Methyl Ethyl n-Propyl n-Butyl
.... .... ....
....
.... ....
.... , . ,
.... , . .
....
....
....
2860
2876 2874
2858 2865
....
2876 2877 2877
.... ....
2858 -2865 Sh
....
2873 2873
2864
2920 .... 2928 -2914 Sh 2873 2935 2910 2876 2932 -2910 Sh 2876 2933 -2910 2875 Isobutyl 2965 a indicates band positions are approximate. b Sh refers tv bands appearing as shoulders on mvre intense bands
2848
2961 2963
-2907
2991 2975 2967 2965
-
.... ....
. , .. .... .... ....
2864 -2867 Sh 2863 2863 2860 2858 2856
.... ....
....
Others
....
... .... .... .... 2840 -2844 2836 Sh -2837 -2836 Sh -2845 Sh
....
-2840
....
2820 2820
-2866 Sh -2854 Sh
..,.
....
....
....
broad minimum. The dashed vertical lines connect C-H frequencies that are assigned to the same vibrational mode of the methyl or methylene groups. I n Tables I and I1 the observed frequencies for the sulfurized and oxygenated molecules are tabulat'ed, and only those bands are listed which are unequivocally recognized. Only the first few members of each series were investigated; if the presence of the sulfur or oxygen at.om in t h e molecule is to affect drastically any of the C-H frequencies as assigned by Fox and Martin ( 4 ) for hydrocarbon molecules, the modification would be detected more readily in these small molecules. I n the following discussion, the first member of the various series is in most cases excluded because of its extreme departure from the general spectral characteristics of the remaining members. However, the data have been included in the tables and figures. 2960-CM.-1 REGION
with the results of the present investigation. I n both the 2930and 2860-cm.-l regions there is a possibility of band overlap due to the presence of methyl and methylene groups, and it was of interest to observe when two bands could be resolved in either region, In Figures 2 to 12 the displaced spectra of the various series are reprodured from the actual recording tracings, with transmitted energy as the ordinate. I n Figures 13 t o 22 graphical representations of the absorption bands which were observed in the 2800 to 31Wcm. -1 region are also presented for the various series. Theee latter figures are included to facilitate the ease of intercomparison of absorption bands for the reader. The heights of the lines represent a visual estimate of the relative intensities of the different bands which were not normalized to a unit concentration basis. An umbrella over a line designates an absorption band whioh appears either as a shoulder or with a 2
3
This frequency region was assigned to the unsymmetrical
-CH3 stretching by Fox and Martin (4), and the present study agrees with this for the following reasons: The relative intensity of this band compared to the neighboring 2930-cm.-l band decreases as the ratio of CH2/CHs groups increases; the relative intensity of this band is high for the more highly methyl-branched molecules; and the high intensity of this band compared to other absorption bands in the C-H region must be related t o the unsymmetrical -CH, stretching mode, which can be shown, by simplified calculations, to have the greatest net dipole moment change of any mode of oscillation of the isolated -CH3
groups. The unsymmetrical methyl stretching frequency for each compound studied appears in the 2960- to 2990-cm.-l range. As t h e 4B
4A
t
t
(3
W
W
W
W 2
0
0 W
z
W
5
i
(I
(3
or \ /CH2
W
n W
I-
t
5
z
2
3,
I c
4
K 4
a
I-
3000 2900 2800
v EMe1)
2900
2800
VICMTI)
Figure 2. Recorded Figure 3. SymmetriSpectra of Mercap- cal >CH2 and -CH, tans Stretching Region
3 0 0 0 2900 2800
3000 2900 2800
V (CM.-')
V(CM.-I)
Figure 4. Recorded Spectra of Sulfides
V 1CM-I)
Figure 5. Recorded Spectra of Disulfides
V O L U M E 23, NO. 11, N O V E M B E R 1 9 5 1
1613
molecule increases in molecular weight and the methyl group is moved away from the sulfur or oxygen atom, the observed frequency decreases and approaches a value of approximately 2960 cm.-l. As the ratio of CH2 t o CH, groups is increased, the intensity of this band relative t o that a t approximately 2930 em.-’ decreases. A consequence of the experimental data is that a methyl group is still distinguiehable in liquid solution when the ratio of CH2 t o CHBgroups has reached the value of 15 t o 1 (spectrum of hexadecyl mercaptan not shown). I t was suspected that unsymmetrically substituted molecules such as methyl n-propyl sulfide and methyl n-amyl ketone might exhibit two absorption bands in this region, one due to the methyl group adjacent to the sulfur or oxygen atom and the other to the more remote methyl group. However, only in the case of methyl %-butyrate were t n o bands observed (2970 and 2957 cm. - l ) . Except for methyl n-butyrate, it was not possible to distinguish a splitting of this methyl band into components that could be characteristic of the different methyl attachments in the molecule. Except for the first members, the unsymmetrical methyl stretching absorption band is easily recognized b y its frequency and intensity
Table 11. Compound Alcohols Rleth, 1 Ethyl n-Propyl Isopropyl n-Butyl Isobutyl eec-Butyl n-HeDtvl LerL-Buiyl B,p’-Dihydroxyethyl ether Allyl alcohol Ethylene glycol Ethers Diethyl n-Propyl Isopropyl Methyl isobutyl Ketones
I n the sulfurized molecules when an ethyl group is attached to the sulfur atom, the observed frequency is usually in the 2970 to 2980-cm.-l interval (ethyl n-butyl sulfide is a n exception). The remaining sulfurized molecules all absorbed in the 2960 t o 2969 interval. However, for the oxygenated molecules, it is convenient to consider the compounds in relation t o the following general formula:
R ixiCH2&HXR” \$-hereX represents the functional group which is defined as -OH for alcohols, -0for ethers, -COfor ketones, -COOH for acids, -CHO for aldehydes, HCOOfor formates, and -COOfor esters. For some of these R” is obviously nonexistent. The oxygenated compounds are divided into three classes with an assigned frequency interval as follon-s:
A. R=R’=H giving the ethyl group (2980 to 2990 ( m - 1 ) . B. R=CH3 and R’-H or R=R’=CH3 giving, respectively, the isopropyl and sec-butyl groups (2973 to 2979 cm-I). The tert-butyl group is also considered a nwniber of this class,
Observed Absorption Bands for Oxygenated Aliphatic
3000 Cm.-1
rioii
3007
....
2960 Cm. - 1
2930 Cm. -1
a-2983 Snb 2976 2968 2976 2966 2961 2974 2961 2976
....
2948 2927 2939 2933 2935 2932 2931 2933 2435 2931 2924 2939
2981 2968 2976 2965
2935 2939 2935 2932
2964 2983 2968 2974 2965 2961 2983 2961
2925 2941 2939 2939 2937 2933 2943 2935
.... ....
Methyl n-amyl .... Diethyl ..,. Diisobutyl ... Acids Formic 3081 ... 2944 Acetic -3027, -3012 Sh 2987 2928 Propionic .... 2989 2948, -2929 2972 n-Butyric .... 2939 2979 Isobutyric .... 2939 n-Valeric .... 2967 2935 2968 Isovaleric 2935 Aldehydes Acetaldeh de 3005 2961 .... Propionalxehyde 2987 .... 2946 +Butyraldehyde .... 2939 2969 Isobutyraldehyde 2936 2976 Isovaleraldehyde 2968 2936 n-Heptaldehyde 2961 2933 Esters Methyl acetate -3028 Sh. 3001 2955 -2930 Sh .... E t h y l acetate 2987 2941 n-Propyl acetate 2972 2943, -2930 Sh .... Isopropyl acetate 2985 2941. -2927 Sh .... n-Butyl acetate .... 2965 2937 Isobutyl acetate 2969 .... -2939 Sh Methyl propionate 2989 2952 E t h y l propionate 2987 2946 n-Propyl propionate 2975 2946, -2926 Sh Isopropyl propionate 2987 2944. -2926 Sh hlethyl-n-butyrate 2970,2957 -2939 Sh 2972 E t h y l n-butyrate 2938 n-Propyl n-butyrate 2972 2939 Formates Methyl 3106, 3034, 3005 2957 2934 Ethyl 3107 2987 2931 n-Propyl 3104 2972 2931 n-Butyl 3104 2965 2935 Isobutyl 3104 ‘ 2968 -2934 2931 Sh, 2928 arc-Butyl 2978 3104 a indicates band poeitions are approximate. 6 Sh refers to bands appearing as ahoulders on more intense bands. c These frequencies a r e definitely methylene.
-
2910 Cm. - 1 -2918
.... . ,.
Sh
....
-2Uii
....
sh
&i:! ....
2880 Cm.-’
....
-2886 2880 2882 2878 2876 2883 -2876 2875
....
Molecules 2860 C m . - l
2836
, . .
.... ....
.... ....
....
..,. 2863
....
.... . . .
.... ....
28750 2871 C 28780
..
2867 2878 2878 2874
2859 ..,.
...
....
iiio
2008 2914
....
2967 2902
.... ....
.... .... .... .... .... 2914
.... .... ....
2901
....
-2907 2907
-2iQ7
iiii ....
2912
....
-2tiii s h -2910 .... -2900 Sh -2912 Sh -2904
....
....
2907
2884 2878 2878 2876 2875 2884 2874
.... ....
2891 2882 2880 2877 2877 2897 2882 2876 2878 -2873 Sh
....
2876 2884 2884 2876 2878 2886 2886 2884 2886 2880 2878 2882 -2gsQ Sh 2885 2878 2878 2884
Others
....
-2897 Sh, 2810, 2778 2801
....
,
2853
....
....
- i s 6 9 Sh 2864
... ...
2864
....
-?Si6 Sh -2850 Sh
-2&
2781
.... ....
.... -iiii ....
... .... ....
.... ....
Sh
....
....
.... .... ....
2863 2847 2855 -2863
-2853 Sh
....
-2895
....
2846 -is62
....
Sh
.... ....
....
.... -2853
....
-2860
.... ....
-2893
....
2848
-2b62
.... .... .... ....
-2897
2840
Sh Sh
.... .... .. .... ....
ANALYTICAL CHEMISTRY
1614
with the Fox and Martin ( 4 ) assignment for hydrocarbons. In Table I11 the averages of the frequencies observed in each absorption region are tabulated along with the averages found by Fox and hlartin in their hydrocarbon study. The average oxygenated frequency in this region is 2974 em.-' compared t o a sulfurized average of 2966 em.-' and a hydrocarbon average of 2962 em.-'. Thus the sulfur atom does not produce a perturbation great enough to cause frequency shifts that can be distinguished as unique evidence for the presence of the sulfur atom. If care is exercised, it is possible to use the oxygenated frequencies 'as a means of detecting the probable position of the oxygen functional group in the molecule. The effects of the introduction of the sulfur and oxygen atoms into the molecule can be seen by noting the unsymmetrical methyl stretching frequencies for molecules where the ethyl group is attached t o the functional group. The range for the oxygenated compounds is 2980 to 2990 cm.-', and for the sulfurized molecules i t is 2970 t o 2980 ern.-'. Thus the C-13 force comtants are
C. Other structures which are not included in either A or B (2960 t o 2972 cm.-I). I n the ether, ketone, and ester series it is possible for the molecule to fall into more than one class, depending on R". When this situation arises, the simplest classification is to be used. For example, isopropyl propionate falls into classes A and B, but is considered of class A. Using these classifications for the oxygenated materials it was found that of thirteen compounds which contained a n ethyl group attached to the functional group, class A, ten absorbed in the assigned interval of 2980 to 2990 em.-'. Of nine class B type molecules, eight absorbed in the assigned interval of 2973 to 2979 em.-'. The remaining compounds all fall in class C and all absorb in the 2960- t o 2972-cm.-l interval. The use of these three classes is somewhat arbitrary, but it does give increased orderliness to the data. If the first member in each series is excluded, the vibrational mode under discussion provides frequency values for the sulfurized and oxygenated rompounds in reasonably good agreement
7
6C
6B
6A
CH~CHZOH
t
t
G
u
W
Y W 2
HOCHpCHpOH
W
a
a c
n
w
W
I-
t
[HOCH2CH2]20
t
E
?I
4
4
e
c
I-
CH2nCHCH2OH
2800
300 2900 2800
3
3000 2900 28C Y
Figure 6.
Recorded Spectra of Alcohols
Figure 7.
(CM?
I
Recorded Spectra of Ethers
B
A
HCOOH
CH3CH2COOH
t
t
z
f
K W
K W
Y
W z
W
W
w
t
> a
W
a
a
I-
CH3(CH2)2COOH
a I-
k
t
I-
t
m
Ln
3,
C H 3 (CH2)3 COOH
4
4
W
4
c
I-
(CH3)2CHCH2COOH
I-
3100 3000 2900
2800
3 0 0 0 2900 2800 2700
v lCM.-')
8
Figure 8.
Recorded Spectra of Ketones
9
Figure 9. Recorded Spectra of Acids
10 Figure 10. Recorded Spectra of Aldehydes
1615
V O L U M E 23, N O . 11, N O V E M B E R 1 9 5 1 Table 111.
correlated with the high intensity free OH band a t 3524 em.-', and so is probably the monomeric C-H stretching frequency ('7); the 2864-cm.-] band is then believed t o be the dimeric C-H frequency. This latter conclusion is supported by the ratio of the optical density of the 2944-cm.-' band to that a t 2864 cm.-l in various concentrations of carbon tetrachloride. This ratio increased with decreased concentration, indicating that more monomer was being formed a t the expense of the dimer.
Band Frequency Averages
(First members not included) 2960 2930 2910 Cm.-l C m - 1 C m . - i 2930 2910 Sulfurized 2966 2936 Oxygenated 2974 2909 2934 2912 Hydrocarbons (F a n d 11) 2962 2926 4
2880 Cm.-' 287.5 2880 2872
2860_1 Cm. 2861 2861 2853
i
greater when the more electronegative oxygen atom is present. This effect also appears for the isopropyl group. The relative intensity of this band with respect t o the approxlmately 2930-cm. -1 band increases greatly with branching and this fact is sometimes useful in determining if a n unknown mol >tule is branched or straight chain.
I ! HCOOCH3
t
> HCOOCHpCH3
Allyl alcohol, ethylene glycol, and p,p'-dihydroxyethyl ethar do not contain methyl groups, and there is correspondingh. no methyl band in the spectra of these compounds in this region. Allyl alcohol contains a terminal olefinic group, and a reasonably strong absorption band occurs a t 3086 em.-' in agreement with the Fox and SIartln assignment for this type of group. This 1)and is the unsymmetrical =CH2 stretching frequency, with possibly the 2989-cm.-' frequency due to the less intense sym-
a
E Y
0
L W
/CH2CH3 HCOOCH 'CH3
(A
metrical stretching. The = -H grouping appears t o account for the 3014-cm.-' band. Acids of low molecular weight generally do not have as well defined absorption in the C-H region as the acids of higher niolecular weight ; in more concentrated solutions the strong hydrogen bonding that occurs results in a broad band which partially overlaps the C-H region (Z), and the hydrocarbon group absorptions are superimposed on this bound hydroxyl background. When acetic and propionic acids were run in dilute solutions of carbon tetrachloride (-0.01 mole per liter in a 1.4cm. cell), the hands were weak and rather broad, indicating that hydrogen bonding with dimer formation ( 5 ) was still occurring a t these high dilutions (see Figure 9; acetic acid not shown). Cnder the present experimental conditions the monomeric acid molecules were probably never the only species present. However, interaction through hydrogen bonding should not be expected to shift the methyl frequencies appreciably, except possibly in acetic acid. Formic acid was run in verv dilute solution, in which both the monomer and dimer forms might be expected. The rather broad hand a t 3081 em. -1 is assigned to the hydroxyl frequency of the dimer (for spectral data in the vapor phase see 6). A much more intense and sharp absorption band appeared a t 3524 em.-', which is the monomer hydroxyl ahsorption. The remaining bands of formic acid nil1 be briefly discussed a t this point. An intense band appears a t 2944 em.-' and aweaker one at 2864 cm-1. The former with its high intensity must he
Y (CM-l)
A
Figure 12.
n Recorded Spectra of Formates
2930-CM. -1 REGION
For hydrocarbon molecules Fox and llart,in found t \vo absorpt,ion h a n d s i n t h i s r e g i o n , one a t 2934 em. chaIartcrC istic of methyl groups and t h r other at 2926 em.-' charactrristic of methylene groups (KY Figure 1). The latter frequenvy arises from the, unsymmetrical strrtching mode
m t
Y (CM-')
Methyl acetatr contains two methyl groups in different cnvironments, and it is pomible that the 2955- and 3001-cm.-' bands can he attribut,ed to an unsymmetrical strrtrhing mode of each of these methyl groups (see Figure 11, 9)..Icetic acid and methyl formate rrpresent modifications of methyl acetate wherein onr of the methyl groups has hrrn replaced hy a hydrogen. However, the observed spectrum of :iwtir arid \ v a ~t.oo diffuse to permit reasonahle comparisons t o he niadr; methyl formate has a vrrv clran spectrum in this rpgion. SIethvl formate atworhs a t 3005 and 2957 m i . - 1 ( w e Figure 12, A ) , while methyl arrtate has equivalent tmids :it 3001 and 2955 cm-I. The apprarance of these hands i n I)oth compounds appears to caontradict an assignmrnt of thrni to thc diffrrent methyl groups.
1
t
>
3 1 0 0 3 0 0 0 2900 2800
3100 3 0 0 0 2900 2800
W L
CH~(CH~)~COOCHJ
W
CH~(CH~)ZCOOCH~CH~
0
W 0
L
k
3, 4
c
3000 2900 2800 Y ICM:')
I00 2900 2800
loo
Y (CM:Il
Figure 11.
Recorded Spectra of Esters
2900 2800
of the )CH! group; the origin / of the former frequency has not bern definite]? established. It !vas of interest to see i f these two hands could be r ~ solvcd when both groups appear in t h r same molecule, and in addition t o determine the effrct of the sulfur and oxygen atoms. In thri folloning discussion the series first member is not included unlcsE specifically mentioned. All compounds showed a t least one absorption band which fell
1616
ANALYTICAL CHEMISTRY
in the 2922- to 2948-cm.-l interval. The methyl and methylene absorption bands apparently overlap to such an extent that their resolution was generally not achieved under the experimental conditions of this investigation. I n the sulfurized molecules methyl n-butyl sulfide has a doublet in this region and n-propyl sulfide shows two overlapping bands (see Figure 4). These are the only two instances in the sulfur-containing compounds where two bands were definitely observed. I n the oxygenated series only propionic acid and sec-butyl formate showed two distinct bands of appreciable intensity in this region. I n the esters a slight shoulder sometimes appeared on the low frequency side of the more intense band. This slight shoulder appeared in four esters, and it does not seem possible to assign it definitely to a methyl or methylene group or to correlate its appearance with some other type of hydrocarbon structure. Thus in general only one absorption band was found; sometimes a weak shoulder appeared on the low frequency side of this more intense band. I
[HOCH2CH2]20 I
CH2'CHCH20H
I
I 1
1
The difficulty that might arise in an attempted interpretation of a single band that appears in this region for an unknown material is illustrated by the following. Isopropyl alcohol has a weak but d e h i t e band a t 2933 cm.-I, and isobutyl alcohol has the same type of band a t 2932 cm.-l. On the other hand, secbutyl alcohol absorbs much more strongly than the above two molecules a t 2931 cm.-l (see Figure 6). Here is asituation xhere a molecule containing no methylene groups has a weak band, and EO does a molecule containing only one methylene group, but an isomer of the latter still containing only one methylene group absorbs much more strongly. tert-Butyl alcohol has no methylene groups and a weak band appears a t 2935 cm.-1 This indicates the uncertainty one might have in a n attempt to identify a lone methylene group in a n unknown material by the frequency and intensity of the absorption in the 2930-cm. -1 region. -4reaaonably strong band must be exhibited to indicate this type of group. As the ratio of CH, to CH8 increases, the intensity of the 2930cm. --I band increases relative to that a t 2960 cm.-' and identification of >CH2 groups becomes more positive. The 2930-cm.-l bands do not offer a unique means of detecting unambiguously the presence of either methyl or methylene groups in the compounds of lower molecular weight. The intensity of this band is weak relative to the 2960-crn.-I region for the more highly methyl-branched materials examined and positive evidence for the presence of a methylene group might not be attainable from the spectra in this region. However, the straightrchain compounds, wherein the CH2 to CHs ratio is high (order of magnitude of approximately 5 to l),show stronger absorption relative to the 2960-cm.-l band. The spectrum of such a material represents good evidence for the presence of the methylene group, whose absorption intensity obscures any methyl absorption--e.g., n-heptyl mercaptan (Figure 2) The average absorption frequencies observed for the oxygenated and sulfurized molecules in this region are tabulated in Table 111. The oxygenated materials absorbed a t 2936 cm.-1, compared to a sulfurized average of 2930 em.-'. The oxygenated functional groups appear to strengthen the force constants of t h e C-H bonds, but not enough to cause any serious deviation from the Fox and Martin hydrocarbon assignment. The intensity of the 2930-cm.-l band in formates appears visually to be greater relative to the 2960-cm.-' band than do the
V O L U M E 23, N O . 1 1 , N O V E M B E R 1 9 5 1
1617
equivalent bands of analogous compounds of other series. The increased intensity of the formate bands in this region compared to other series presented a new question. Possibly this could be caused by the C-H stretching vibration of the formate group. I n formic acid a C-H frequency was found t o occur a t 2944 cm.-l in dilute carbon tetrachloride solution where the monomeric molerule appears t o prdomjnate (see Figure 9). This
I I
I
I II
r
',
,*'
i
I;
,a
',
I I
i
I I
1
I
,
I
I
II
,
,
,
'
1 -
1
value compares to 2940 cm. -1 found by Bonncr and Hofstadter (1) for monomeric formic acid vapor. Methyl acetate has its most intense absorption band a t 2955 cm.-1 (Figure 11, A ) , and this must be assigned to the methyl group. Methyl Formate haa two very intense bands a t 2957 and 2934 cni.-l. If the high frequency be assigned to the methyl group by analogy with methyl acetate, then the other might well be the formate C-H stretching frequency. The 2957-cm.-' component disappears in the remaining formates and might conceivably be a shifted first member methyl band. I n Figure 6, C, it is seen that sec-butyl alcohol exhibits one absorption band in this region while sec-butyl formate exhibits two (see Figure 12, A ) . This might also be taken as further evidence that the formate C-H frequency is a t approximately 2935 cm.-l. The evidence presented above indicates that the formate C-H frequency could possibly occur in the 2930-cm.-l region. However, this possibility has not been shown to be incontrovertibly true, and further experimental data are necessary either to confirm or refute this. If this assignment is correct, in all but sec-butyl formate a new near-degeneracy occurs, with the result t h a t only one band is observed. Three groupings would then absorb in the same frequency range, and resolution of these was generally not realized. The generally slightly high frequencies observed for the esters as compared to the other classes of materials are possibly due to the hydrocarbon groups on the functional group. These are on the average smaller than in the other series and more affected by the functional group. Allyl alcohol, B,o'-dihydroxyethyI ether, and ethylene glycol all contain methylene groups and absorb a t 2924, 2931, and 2939 cm. -1, respectively (see Figure 6, C). 2850 TO 2890-CM.-1 REGION
HCOOH CH3COOH CH3CH2COOH CH3lCH2)2COOH (CH3)2CHCOOH CH3(CH2)3COOH (CH3)2CHCH2COOH 2800
Figure 17 (upper). Figure 18 (center). Figure 19 (lower).
2700
Figure 20.
I n this region the symmetrical methyl and methylene C-H stretching frequencies appear. The methylene absorption occurs a t approximately 2853 cm.-', while that of the methyl group appears a t approximately 2872 cm. -l. Thew assignments are the result of the Fox and Martin investigation. It was observed that usually two bands could be found in this region for the straighbchain molecules, one a t approximately 2876 cm.-l and the other a t approximately 2860 em.-'. I n the branched-chain molecules usually only one band appears. It was of interest to see when two bands appear and when only one band appears, and to check the assignments. I n Figure 3 the 2800 t o 2900-cm.-l region of mercaptan molecules is reproduced. For the straight-chain mercaptans two bands were always ob2900 3000 served. Up t o n-butyl mercaptan the higher frequency band was the more intense. As the number of methylene groups inV (Ct4-I) creases, the lower frequency component begins to play a more Observed Frequencies of Ethers dominant intensity role, until in n-amyl mercaptan both compoObserved Frequencies of Ketones nents are approximately of the same intensit>. I n n-heptyl Observed Frequencies of Acids mercaptan the low frequency component is definitely the more intense and can be correlated with the high intensity of the 2930 cm.-1 band (see Figure 2); the high relative intensities of these two bands can be associated with a large CHBto CH3 ratio. The last three spectra in Figure 3 are of branched-chain molecules and it is seen that only one band is found in the isopropyl and isobutyl mercaptans. I n isoamyl mercaptan a faint indication of methylene absorption is evident M a weak shoulder on the low frequency side of the more intense band. Thus the more intense high frequency band is assigned to the methyl group and 2900 3000 the low frequency band to the methylene group. 2800 I n the alcohol series the same general type of w (CM.") behavior would be expected to exist as in the Observed Frequencies of Aldehydes mercaptan series. I n n-butyl alcohol an a b s o r p
1618
ANALYTICAL CHEMISTRY
tion band appears at 2878 cm.-' with enough dissymmetry on its low frequency side to suggest the possibility of another band (sre Figure 6, A ) . I n 1-heptanol t h r 2876-cm -1 absorption is no77 the weaker component; the 2863-cm. -1 component is somen hat more intense. It appears that by analogy Kith the mercaptan series, the methyl absorption accounts for the single absorption band generally observed a t approximately 2878 cm.? and that the methylene absorption, when observable, occurs a t approximately 2863 cm.-l. Comparing the analogous mercaptan and alcsohol molecules, it is seen that the methylene absorption appeare I inore frequently in the mercaptan series than for the alcohols. In the alcohols of lower molecular weight this could occur by an increased force constant for the C-H groups adjacent to the functional group-i.e., the methylene groups would be most affected. Thus the methylene frequency for the first few members of the alcohol series nould be shifted toward higher values, with the result of sufficient overlap with the methyl band so that only one band could be found. In l-heptano1 the effect of the hydroxyl group on the methylene force constants is limited to the first and possibly second methylenes. The remaining methylenes are practically unaffectr I . overlap is not so pronounced and good resolution of two bands is again observed. These conclusions qeern to applv for the other ouygenated materials.
2000
2900
3000
v (CM-l) Figure 21.
Observed Frequencies of Esters
It appears that the symmetrica1 stretching mode of the methyl group entails a greater net dipole moment change than the same mode of vibration of the methylene group (if one neglects the effect of the phase relationship of the symmetrical stretching mode between the individual methylene groups along the chain). This results in a high frequency band attributable to the methyl group and a weak band or shoulder on the low frequency attributable to the methylene group for the smaller molecules. FOXand Martin had determined the absorption coefficients for these two groups in this region and found them to be of the same order of magnitude for hydrocarbon molecules. I n this study it appears that this is not the case and that the methyl group absorption coefficient is appreciably greater than that of the methylene group.
[
I
1
I
l
l
I
I
1
l
l
HCOOCH2CH3 HCOO(CH2)2CH3 HCOO(CH2)3CH3 HCOOCH2CH(CH3)2
/CH2CH 3 HCOOCH, cH3
2800
2900
v Figure 22.
3000
(CM-1)
Observed Frequencies of Formates
The average methyl f r e ~ u e n c yis 2880 cni.-' for the oxygenated molecules compared to 2875 cm.-l for the sulfurized series (see Table 111). The methylene average was 2861 em.-' for both the oxygenated and sulfurized series. (The methylene average for the oxygenated series would probably be somewhat higher if the values Tor the compounds of lower molecular weight could be obtained and included in the average.) It was more difficult to detect methylene absorption in the oxygenated series. The observed frequencies in this region agree reasonably well with the Fox and Martin hydrocarbon assignments. With the proper caution these assignments can be applied to unknown materials. The absence of detectable methylene absorption does not necessarily indicate the absence of methylene groups in the molecule. The appearance 04' two bands is reasonably good evidence for the presence of both groups, but the appearance of only one band must be iriterpreted with caution.
For relatively small molecules of the type studied, a single band in this region must be interpreted in conjunction with the bands that might appear in the other two frequency regions already discussed. For examplr, in ethylene glycol, @,P'-dihydroxyethyl ether, and allyl alcohol, a single band appears a t approximately 2874 cm. --I and no band appears in the 2960 cni. -1 region. Consequently, the interpretation can be made that a methylene gro ip is giving rise to these 2874-cm.-l absorption bands. It is seen that the effect of the oxygen atoms on the C-H force constants of these three compounds is to increase them slightly to give a higher observed methylene frequrncsy. I n the acid series a meak shoulder fre uently appeared a t approximately 2855 cm.-l. This shouljer has been tentatively assigned to methylene absorption, although its appearance in isobutyric acid contradicts such an aqsignment. All the methyl esters-i.e., acetate, propionate, and h t y rate-have distinct nonoverlapping bands a t approximately 2850 cm.-' (see Figure 11). The other esters which have bands near this frequency are distinctive in that these appear as shoulders on the more intense higher frequrncy band. These approximately 2850 cm.-l bands of the methyl esters are probably not due to methylene absorption alone. OTHER REGIONS
Frequently a weak band appears a t approximately 2910 c w . - 1 , and this was assigned to the methyl group by Fox and Martin (see Figure 1). The present data offer no definite information concerning this assignment, in so far as it is not observed for all the materials examined which contain the methyl group. The same investigators report an infrequently occurring band at ap-
\ /
proximately 2893 em.-' which they ascribe to the -CH
group.
This band was observed so infrequently that again nothing more can be said concerning its assignment. For the sulfides studied, a weak band occurring at approximately 2840 cm.-' was gener-
V O L U M E 23, N O , 1 1 , N O V E M B E R 1 9 5 1 ally observed. It was not found for any of the mercaptans and was observed for only one disulfide. KO explanation is given for these bands. All the formates exhibited a weak band a t approximately 3104 cm.-' (not included in Figure 22, hut visible in Figure 12), and this might he a useful frequency in formate identification. A very surprising set of bands arose in the aldehyde series. In each aldehyde two bands always appeared simultaneously, centering around 2720 and 2820 em.-' (Figure 20). Even in very dilute carbon tetrachloride solution (-0.01 mole per liter) these absorption bands were present. Thompson and Harris ( I S ) studied acetaldehyde vapor in the infrared, but did not assign a definite frequency to the aldehydic C-H stretching. Morris ( 9 ) investigated acetaldehJ.de and acetaldehyde-& vapors in the infrared and assigned their observed 2710-cm. -l band for acetaldehl-de to the symmetrical methyl stretching and an observe 12788cm.-' band to the aldehydic C-H stretching. This latter band does not appear in the infrared spectrum of the carbon tetrachloride solution. The point to he made is that two band9 appear throughout the aldehyde series, and in the compounds of higher molecular weight a symmetrical methyl stretching frequency has been assigned (approximately 2880 em. - l ) . Consequently, t,he two bands that appear in the liquid solutions must he explained as a fundamental and an overtone or combination band whose intensity is enhanced by Fermi resonance with the fundamental. The r e d t of this investigation is that one of these tw-o frequencies (which one is not definite) must be assigned to the aldehydic CH stretch whose unperturbed frequency would be expecte.1 to appear a t approximately 2775 cm.-l The other band might tent,atively he explained as the first overtone of the symmetrical methyl bending frequency at approximately 1380 em.-'. Renzaldehyde, which contains no methyl groups, also has these two bands when observed in carbon tetrachloride solution. The appearance of these hands in a spectrum is excellent evidence for the presence of an aldehydic molecule.
1619 here. I n carbon tetrachloride solution it seldom was possible to resolve the two bands. When the ratio of CH, t o CH1groups is approximately 5 t o 1, the contribution of the methylene groups to this band is appreciable and can generally be identified as such by comparison of the hand intensity with respect to the unsymmetrical methyl stretching band. Otherwise, an absorption hand in this region cannot be positively identified as due solely to a methyl or methylene group. It is suggested that the formate C-H stretching frequency occurs at approximately 2936 em.-'. I n the 2850- to 2890-cm.-' region the symmetrical methyl and methylene stretching frequencies occur. Generally only one relatively strong absorption band appears; this has been identified with the methyl group. Frequently a weak hand or shoulder appears on the low frequency side of this hand, and is believed to be the methylene absorption. As the ratio of CH, to CH3 groups increases, the intensity of the shoulder increases relative to the main band, and a t a ratio of approximately 6 to 1 become- the dominant absorber. The absorption frequencies of the first member of a series U P U ally are shifted appreciably from the mean values of the remaining members of the series, and the Fox and Martin assignments are then very uncertain when applied to these. A weak band at approximately 2840 cm.-' was generally ohserved for sulfides. S o hand was observed that could definitely be ascribed to the 7\C H group. Carbon tetrachloride solutions of aldehydes show two absorption bands at approximately 2720 and 2820 em.-'. These are assigned to the C-H stretching vibration of the aldehydic group and to some combination or overtone band whose intensity is enhanced by Fermi resonance. Sufficient differences in detail exist t o allow spectra in this region to be used for the identification of specific compounds.
SUMMARY AND CONCLUSIONS
LITERATURE CITED
The frequencies observed for the methyl and methylene groups are on the average highest in the oxygenated series, intermediate in the sulfurized series, and lowest in the hydrocarbons (see Table 111). The variations from the Fox and Martin assignments are generally not great enough to cause serious difficulty in using these assignments, hut care must be taken to stay within eqtahlished limits of applicability. Kucluding first members, the unsymmetrical methyl stretching frclquencies of the sulfurized molecules fall in the 2960- to 2976ctn-1 interval. (\Then an ethyl group is attached to the sulfur atom, the observed frequency is generally in the 2970- to 2976em.-' interval.) I n the oxygenated series the range waq 2960- t o 2990 em. - l , with further subclassification possible: an ethyl group (class ii) generally absorbing in the 2980- to 2990-cxn.-l interval; an isopropyl or sec-butyl (class B) generally ahmrbing in the 2973- t o 2979-cm. -l interval; the remaining groups absorbing in the 2960- to 2972-em. interval (class C. ). In the 2930-cni. -l region the unsymmetrical methylene stretching frequency appears; a methyl group absorption also occurs
( 1 ) Bonner. L. G., and Hofstadter, R., J . Chem. Phya., 6, 531 (1938).
Davies, Jf. M., and dutherland, G. B. B. &I.,Ibid., 6, 755 (1938). (3) Fox, J. J.. and Martin, d.E., Proc. R o y . Soc.. A167, 257 ( 1 9 3 5 ) . (2)
( 4 ) Ibid., A175, 208 (1940). (5) Herman, R. C., and Hofstadter, R., J . Chem. Phys., 6 , 534 (1938). ( 6 ) Herman, R. C., and Williams, V.,Ibid., 8, 447 ( 1 9 4 0 ) . ( 7 ) Herzberg, G., "Infrared and Raman Spectra of Polyatomic hlolecules," p. 321, New York, D. Van Nostrand Co., 1945. (8) Hibbard, R. R . , and Cleaves, A. P., ANAL. CHEM.,21, 486 (1949). J . C h e m . P h y s . , 11, 230 (1943). (9) Jforris, J. C., (10) Plyler, E. K., and .hquista, N., J . Research:lVatL Bur. ,Windards, 43, 37 (1949). (1 I ) Rose, F. IT., Ibid., 20, 129 (1938). (12) Saier, E. L., and Coggeshall, S . D., -4x.4~. CHEY., 20, 812 (1948). and Harris, G. P., Trans. Faraday Soc., 38, (13) Thompson, H. W., 37 (1942).
RECEIVED May 1 3 , 1931. Presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa.. AIarch 5 to 7, 1951.
