Analytical applications of far infrared spectra—II

Spectrochimica

Act%

195%

pp. 105 to 905.

Pergamon

Press

Ltd.

Printed

in Northern

Irelnnd

Analytical applications of far infrared spectra-II* Spectra-structure correlations for aliphatic and aromatic hydrocarbons in the cesium bromide region F. F. Wright

Air

Development

BENTLEY Center, (Received

and E. F. Wright-Patterson 10 February

WOLPARTH Air

Force

Base,

Ohio

1958)

Abstract-The infrared absorption spectra of some 400 aliphatic and aromatic hydrocarbons have been investigated from 15 to 35 p and the characteristic absorption frequencies incorporated The classes of compounds studied were alkanes, into spectra-structure correlation charts. substituted benzenes, naphthalenes and alkenes, cyclopropanes, cyclopentanes, cyclohexanes, biphenyls. The skelefial bending frequencies of the alkanes and alkenes and the non-planar bending frequencies of the aromatic hydrocarbons are the most useful for qualitative analysis in this region. The wavelength and intensity of the out-of-plane ring frequencies of aromatic Typical infrared spectra of the molecules give some indication of the nature of the substituents. hydrocarbons are presented. lhtroducti0n

group frequencies provide a useful tool for organic qualitative analysis [l, 2, 3, 4, 51. Nearly all of the established frequencies are in the range 2-15 ,u, and it is obviously of interest to investigate the region of longer wavelengths to see whether additional ones occur there. The far infrared spectra (15-35 ,u) of over 2000 organic and inorganic substances have been investigated at this Center, and spectra-structure correlation charts have been prepared. About 400 of the compounds are hydrocarbons, and the correlations for these molecules are discussed here. The cyclic compounds and aromatic molecules in particular exhibit several absorption bands in this region, the strongest of which seems to be associated with, and characteristic of, a particular ring system. The wavelength and intensity of these bands serve to a degree to distinguish between the various types of .ring systems as well as giving some insight as to the nature of the substituents on the ring. Charactefistic ring vibrations are also observed for heterocyclic and polynuclear molecules in this region. Spectra-structure correlations for molecules containing bromine, iodine, sulfur, silicon, inorganic radicals, etc., and for functional groups, such as carboxyl, ketone, ester, ether and nitro, are also possible in this region. Spectra-structure correlations oli several of these classes of compounds will be discussed in subsequent articles. The correlations given in this study are based on the spectra of a limited number of molecules and are presented with considerable reservation since spectra in the CHARACTERISTIC

* Preaentod et the Forty-second October, 1967. [l] BARNES R. B., GORE R. C., [Z] BELL-Y L. J., The Infrared [33 COLTHUF N. B., J. Opl. Sot. [41 JONES R. N. and SANDOFRY Interscience, New York. [5] LORD R. C. and MILLER F.

1

Annual

Meeting

of the

Optical

Society

of America

in

Columbus,

STAFFOILD R. W. and WILLI~IS V. Z., And. Chem. 20,402 (1943). Speclm of Compltz Molemle~ p. 2. John Wiley, New York. Am. 40, 397 (1950). C., Claemical Applications of Spectroecopy Vol. IX, Chap. 4. Weissbergor A. AppZ.

Specb~copy

10,

116 (1956).

165

Ohio,

Series,

17

P.F.B~NTLEY&u~E.F.

WOLFARTH

cesium bromide region are,even more selisitive to small changes in structure than those in the rocksalt region. Interpretation of the far infrared spectra is further complicated by absorption bands arising from lattice vibrations of crystals where the atoms are covalently bonded together. These bands are more common in this region than in the rocksalt region. . Nevertheless, fundamental frequencies which occur between 15 and 35 ,u can be of value for empirical correlatidns as long as they are used with caution and are restricted to molecules that do not vary widely in structure from well characterized systems.

Experimental Apparatus The instrument used in these studies was a special spectrophotometer equipped with cesium bromide optics [6]. It is essentially a conventional Perkin-Elmer Model 21 double-beam optical-null spectrophotometer modified to include the double-pass system developed by COATES and SCOTTof the Perkin-Elmer Corporation. The spectrophotometer was calibrated by using atmospheric carbon dioxide and water vapor bands. The wavelength accuracy is probably better than 0.05 ,u, which is equivalent to about 1.5 cm-1 at 15.25 ,u. Naterials and pvocache The aliphatic and aromatic hydrocarbons used in this study were American Petroleum Institute samples. The substituted benzenes with substituents other than aliphatic groups were mostly commercial grade chemicals-. An effort was made to use chemically pure compounds whenever possible. The grade -of purity and the manufacturer, whenever possible, are indicated on the spectra. Cesium bromide or KRS-5 cells with a fairly long pathlength (0.5-2.0 mm) were employed for the aliphatic and aromatic hydrocarbons, while cells with a somewhat shorter pathlength were used for the more polar benzene derivatives. The spectra of the solid compounds were obtained as Nujol mulls or as cesium bromide pellets. In some instances the spectra of these compounds were recorded in benzene or other suitable solvents. Results Representative spectra are shown in the accompanying figures. In many of the spectra the transmission steadily diminishes from about 30 to 35 ,u. This is not true absorption by the sample, but is an instrumental difficulty which results from the very low energy available because of absorption by the prism and by water vapor. When true bands do occur in this region, they are often easily recognized (see Figs. 4~, 6~) although in other cases there is considerable doubt as to their reality (Figs. 5c, 6~, 6~) The absorption bands referred to as strong bands in this article are based on the spectra of compounds studied in cells with fairly long pathlengths (0.5-2.0 mm). The pathlength used for a given series of compounds is usually stated. [Li] BENTLEYF.F., WOLFARTHE.F.,Sm

N.E.and

POXVELLW. R.,

166

Speclrochim.

ActaJS,

l(1968)

Analytical

applications

of far infrared

spectra-11

Aliphatic hydrocarbons Alkanes Since the spectra of the straight chain aliphatic hydrocarbons have weak bands in the CsBr region, the spectra of these compounds were measured in 2.0 mm cells. In a series of pure hydrocarbons from n-hexane to n-hexadecane weak absorption bands were observed in the 15.6-20.6 and the 21.8-22-l ,U regions (see Fig. 1). n-Pentane has a sharp medium band at 21.35 ,u. The absorption maximum in the 18.6-20.6 ,U region of the spectra of n-hexane to n-hexadecane gradually shifts to shorter wavelengths as the chain length increases and appears to approach a limit around lS.6 ,u as shown in the spectrum of n-hexadecane. This absorption is strongest in the spectra of n-heptane and n-octane. A number of very weak bands are present in this region in the spectrum of n-nonane, and a weak broad band is present in the spectrum of n-decane. The center of the bands were used in the above correlations for the latter ‘two compounds. The greatest change in wavelength of the absorption maximum in this region with chain length occurs at S to 12 carbon atoms. In this series of compounds, absorption in the specified region (1 M-20*6 ,u) therefore gives some indication as to the number of carbon atoms present in the hydrocarbon chain. In general there are more and stronger absorption bands in the branched alkanes than in the straight chain homologs (see Figs. 2 and 3). All of the branched alkanes studied have at least one absorption band in the 17.5-20.6 p region. Data on the few molecules studied indicate that considerable differences in the absorption spectra occur with variation of the position of substituents on the hydrocarbon chain. Thirty-seven pure compounds were investigated in this Laboratory and approximately thirty others were observed in the American Petroleum Institute spectra and in the literature [7, 81. Alkenes In the infrared spectra of a series of alkenes from 1-pentene to 1-hexadecene two absorption bands were observed at 15.70-15.90 and 18.05-18.20 ,u which appear to be characteristic of unbranched 1-alkenes (R - CH = CH,) (see Fig. 4). The two strong bands are usually not observed in the branched 1-alkenes. It is interesting to note, however, that all the 4-alkyl-1-alkenes studied absorb strongly in the 16.0-16.3 ,u region and all the branched 1-alkenes observed in which the alkyl groups are substituted nearer the double bond absorb strongly in the 17.9-15.8 ,u region. The unbranched cis-2-alkenes absorb strongly in the 17.0-17.5 p and 20*521;5 ,U regions, whereas the trans-2-alkenes absorb in the 24-26 and 31-35 ,u regions (Fig. 5). As in the branched alkanes, the most of the branched- alkenes studied have at least one absorption band at 179-20.6 p. ’

Alicyclic hydrocarbons Saturated cyclic systems have characteristic absorption frequencies arising from the bond stretching and bending modes of vibration which occur in the rocksalt region, and the out-of-plane and torsional modes which occur in the far [7] DONNEAUD [S] MCCUBBIN

M. T., Compt. Rend. 239, 1480 (1954). T. K., JR. and STINTON W. M., J. Opt. Sot.

167

Am.

40, 537

(1950).

F.F.

and E.F.

BENTLEY

WAVE

WAVE

Fig. 1. Infrarotl

spectra

from

15-35

NUMBEkS

LENGTH

,LL of normal

IN

IN

alhb?s. D, rr-heradccene.

168

WOLFARTH

CM-’

MlGRONs

A, n-pentane;

B, ~+heptane;

o, rt-decano;

Anrtlytical

applkmtions WVE

100

mo600

600

of far infrared NUMBERS

IN

spectreI1

CM-’

400

300

260

Y f 60 :: s crlw f F 40 5 ; 20 P 0

WVE ig. 2. Infrared

spectra

LENGTH

from 15-36 p of brmohed c, 2:3-dimethylpentane;

IN

MICRONS

alkanee. A, 2-methylpentane; D, 2:4-dimethylhexsne.

169

B, 2-methylhepts

me;

and E. F. WOLFARTH

F. F. BENTLEY

WVE

WME

Fig. 3. Infrtlmd

NUMBERS

LENGTH

spectra fmm 15-35 /.J of branched hexme; C, 3-methyl-3-cthylpontano;

IN

IN alkmes.

170

CM-’

MICRONS a, 2:2-dimethylbutano; D, 3:3-diethylpentme.

B, 2:2-dim&

Analytical

applications

WAVE

Fig. 4. Infrared

spectra

of far infrared

LENGTH

from 15-35 ,u of normal C, 2:4:4-trimethyl-1-pentone;

IN

spectrtiII

MICRONS

and branched alkenes. A, 1-pentene; D, 2:3-dimethyl-1-butene.

171

B, 1-nonene;

F. F. BENTLEY

W/E

Fig. 5. Infrared

speetrs

from

and E. F. WOLF~~RTH

LENGTH

IN

MICRONS

16-36 & of cis- and tram-2-alkenap. A, cia-2-pentme; o, tram-2-pontane; D, tram-2.hexene.

172

B,

&a-Z-hoxane;

-analytical

applications

of far

infrared

spectra-II

There are many articles in the literature dealing with attempts to infrared. characterize dicyclic systems, particularly cyclopropane and cyclobutane, from t,heir infrared spectra in the rocksalt region [9, 10, 11, 121. With the possible exception of cyclopropane and cycbobutane no certain means of identifying ring systems by the out-of-plane and torsional bending modes of vibration have so far been realized. The infrared and Raman spectra of cyclopropane [13, 14, 15,161, cyclobutane [17], cyclobutene [18], cyclopentane [19, 201 and cyclohexane [12, 401 have been studied by numerous investigators and for the most part these studies were extended to the cesium bromide region. The infrared spectra of liquid cyldopentane, cyclohexane and over 100 alicyclic hydrocarbon derivatives observed in this Laboratory in the cesium bromide region were found to absorb, usually quite strongly, in the 18-23 p region. The spectra of these molecules were obtained in 1 mm cells. The strong bands in this region are probably due to non-planar modes of vibration arising from the ring. The individual aliphatic.rings can be identified to a degree from the position of the bands in the 18-23 ,u region. Although the cyclopropane derivatives studied absorb in this entire region, variable bands’(weak to strong) are observed in the 18.5-20.0 ,u region. Moreover, the saturated hydrocarbons studied having a cyclopropyl group (Fig. 6) absorb strongly at 21.2-21.6 ,u while the unsaturated hydrocarbons observed absorb strongly in the 18*5-20-O ,u region. Vinylcyclopropane absorbs strongly in the 22 ,u region and is the only exception observed for this correlation. Since cyclopropane itself does not absorb in the cesium bromide region the presence of these bands may be attributed to the substituted carbon chain (see substituted alkanes). cycZoButane derivatives were not available and were not included in these studies. Strong absorption bands w&e observed in the 17.2-20.3 ,D region in the 20 cyclopentanes studied (Fig. 7). The mono-alkyl substituted cyclopentanes absorbed strongly in the somewhat smaller region of 17.2-18.9 p. In addition to cyclohexane the spectra of some seventy cyclohexane derivatives were studied in the 15-35 p region, and at least one strong band was observed in the 17.5-23 ,u region (Figs. 8 and 9). A series of several dicyclohexyl alkalies from dicyclohexyl methanes to dicyclohexyl pentanes were studied, and bands which identify certain members of the series were observed in the 15-23 ,u region (Figs. 10 and 11). For example the dicydohexyl methanes. absorb strongly in the 17.2-18.5 and 21.5-22.5 p regions, while the dicycZohex$ pentanes absorb in the 15.7-16.6 (m) and 20.0-20.4 (s) ,U regions. Similarly the low-boiling and high-boiling cyclohexylcyclohexanes are [9] 1101 ill j LIZ] [13] [14]

JOXES R. N. and S~WDORPY C., Chemical Applications of Spectroecopy p. 366. Intcncicncc, New OSBESKY G. D. and BENTLEY F. F.. J. Am. Chem. Sot. 79. 2057 11957). PLYLER E. K. and ACQUISTA N., J. .Rapenrch N&Z. Bw. iiandarcis 43, ‘37 (1949). PL~LER E. K., STR. and HUYPHREYS C. J., J. Research Nail. Bur. Standard& 38, 211 (1947). BaKEa A. W. and LORD R. C., J. Chem. Phya. 23, 1636 (1955). GUNTHARD H. H., Long R. C. and McCnsnm T. K., JR., J. Chem. Phye. 25, 768 (1956). 1151 LINNETT J. W.. J. Chem. Phue. 6. 692 (19381. il6j Lonu R. C. anti Nom B., J: &e-m. Phyiti, 656 (1956). [17] R~TEJENS G. W., JR., Frmmrm N. K., GWINN W. D. and PITZER K. S., .I. Am. Chem. Sot. 75.5634 . [18] LORD R. C. and Rm D. G., J. Am. Chem. Sot. 79, 2401 (1957). [19] CURNUTTE B. JR. and Smxm W. H., J. Mol. Spectroscopy 1, 239 (1957). [SO] MILLER F. A. and GOLOB H. R., Abstracta of Symposium on Molecular Structure and Spectroecopy, University (1955).

173

York.

(1953).

Ohio

State

F. F. BENTLEY W\VE

and

E. F. WOLFARTH

NUMBERS

IN

CM-’ 3w

WAVE

Fig. 6. Infrared

LENGTH

IN

spectra from 15-35 ,u of cyclopropane cyclopropane; c, B-cyclopropylbutme;

174

MICRONS

derivatives. A, Z-cyclopropylpropane; D, Z-ycZopropyl-3-mothylbutane.

Amlytic~l

applications WAVE

WWE Fig. 7. Infrared spectra from pentane; B, methylgclopentane;

E-35

of far infrared NUMBERS

LENGTH

~1 of cyclopentane c, isopropylcyclopentane;

175

IN

IN

spectra-II

CM-’

MICRONS and

alkyl

substituted derivatives. D, I-fans-3dimethylcycZopentane.

A, cycle-

Fig. 8. Infrared hexane (The

epectra two cell

WIVE LENGTH IN MICRONS from 15-35~ of cyclohexane and alkyl substituted derivatives. A, thkknegsea should be interchanged); B, methylc?/cZohnxanc; c, ethj hexane; D, n-decylcyclohexane.

176

Analytical

applications

WVE

WVE Fig. 9. Infrared cyclohexanc;

spectra from 1535 B, tiopropylcyclohexane;

of far

NUMBERS

LENGTH

infrared

IN

IN

/J of mono and di-alkyl o ,l:l-dimethylcyclohexm~e;

spectreI1

CM-’

MICRONS substituted

cycZ&exanes.

A, n-propyi-

D, 1-tie-2-dimethylcyclohexane.

F. F. BENTLEY

WIVE

Fig.

10. Infrared

cyclohmane

spectra

(low-boiling);

from

15-35

and

NUMStRS

IL

I?. wOL??ARTH

IN

CM-’

WAVE LENGTH IN MICRONS p of 6onm cyclohoxano derivatives.

B, 1-methyl-2-cyclohoxylcyclohexarlo hoxano: D, dicyclohexylmathane.

(high-boiling);

A, 1.methyl-a-cyclohoxyl. c, c~~cZobexylcyclo.

A.nalytical

applica;tions

WAVE

infrared

of far

NUMBERS

IN

BpectrtiII

GM-’

;60

f E 40

WAVE Fig.

11. Infrared spectra B, l:l-dicyclohoxylpropane;

from

E-35

LENGTH

IN

~1 of l:l-dicyclohexyl o, l:l-dicyclohexylbut~nno;

MICRONS alkanes. A, l:l-dic@ohexylethane; D, l:l-dicyclohexylpentane.

F. F. BENTLEY

and

E. F. WOLFARTH

readily distinguished from their infrared spectra in the cesium bromide region (Fig. 10). The low boiling cyclohexylcyclohexane isomers absorb strongly in the 19.2-20.4 and 21.8-23.3 p regions, while the high boiling cycbohexylcyclohexane isomers absorb strongly in the 169-16.3, 17.2-17.5 and 21.8-23.3 ,U regions. Differences are also observed in the rocksalt spectra of these high- and low-boiling molecules, as would be expected since they are geometrical isomers [Zl, 221.

Aromatic hydrocarbons The substituted benaenes are uniquely suited for study in the far infrared. Benzene itself has no infrared-active fundamental below 67 1 cm-l, but the reduced symmetry of the derivatives allows certain additional ring frequencies to become spectra are fairly simple and may be active in the CsBr region. The resulting correlated empirically with molecular structure. There are well-known bands in the,rocksalt region which are determined by the number and position of the substituents, but they are relatively insensitive to the nature of the substituent group [23]. The long-wavelength bands are somewhat more sensitive to the nature of the substituents because the vibrations involve appreciable motion of the bonds linking the substituent groups to the ring [24]. As a result the observed changes in the position and intensity of the absorption are useful in distinguishing between very similar molecules.. A few studies have been made of the infrared spectrum of benzene in the long STRONG [25] obtained the spectrum of this .molecule in the wavelength region. 6-35 ,U region by the reststrahlen technique in 1931. BARNES [26] and co-workers studied the absorption of liquid benzene from 40-135 ,u underhigh resolution and observed weak bands with maxima at 46.3, 59.5, 71.5, 85.5 and 11-5.0 ,u. They interpreted these as difference bands. Their observations supported the plane, symmetrical, hexagonal model of benzene. Probably the most complete data on benzene and its deuterium derivatives are due to INGOLD and co-workers [27-311. MILLER and CRAWFORD [32] also observed the spectra of deuterated benzenes in the potassium bromide region. One weak absorption band at 24.85 ,U (403 cm-l) is observed in the infrared spectrum of liquid benzene between 16 and 35 ,u. A weak Raman line is also observed at this frequency (404 cm-l) in liquid benzene. LORD and ANDREWS [33] [Sl] [22]

1231 [24] [25] [26] [27] [ZS] (291 [30] [31] [32] [33]

LAXXECK J. H., HIPSEER H. F. and FENX V. O., Injm-red Spectra of 47 Dicyclio Hydrocarbons, Notional Advisory Committee for Aeronautics, Tech. Note 3164 (lQ64). SERIJAN H. T., GOODMAN I. A. and YANILUJSKAS W. J., InfraredSpecLm oj59 Dicyclic Hydrocarbona. National Advisory Committee for Aeronautics, Tech. Noto 2567 (1954) (See also Tech. Note 3154 (1954) by Laam~crc, HIFSHER and FENN) . JONES R. N. and SANDORFY C., Chemical Applicationa of Speclroecopy p. 399. Interscience, New York. PLYLERE. K., Discusuiotw. Famday Sot. 9, 100 (1950). STRONG J., Phye. Rev. 37, 1665 (1931). BARNES R. B., BENEDICT W. S. and LEWIS C. M., Phya. Rev. 47, 129 (1936). BAIZEY C. R., CARSON S. C., GORDON R. R. md INGOLD 0. Ii., J. Chem. Sot. 288 (1946). BAILEY C. R., C-ON S. C. and.Iimom C. K., J. Chem. Sot. 252 (1946). B-Y C. R., GORDON R. R., HALE J. B., HERZBELD N., INGOLD C. K. md POOLE H. G., J. Chem. Sot. 2QQ (1946). BAILEY C. R., HALE J. B., HEIUBELD N., INGOLD C. K., LECEIE A. H. md POOLE H. G., ,7. Chetn. SOC. 268 (1946). BAILEY C. R., HALE J. B., INGOLD C. K. and THO~~~SON J. W., .7. C&m. Sot. 931 (1936). MILLER F. A. tmd CRAWFORD B. L., JR., J. Chem. Phye. 14, 292 (1946). LOED R. C. md ANDREWS D. H., J. Phys. Chem. 41, 149 (1937).

180

Analytical

applic&iona

of f&r in&red

speck-II

attributed the Raman and infrared inactive C - C - C nonplanar bending vibration, y2,,(e2J, to this frequency (Herzberg’s designation). Since this vibration is infrared-inactive it is interesting to note that the presence of this absorption band in the infrared spectrum of liquid benzene represents a breakdown in the selection rules. This view is supported by the absence of the band in benzene vapor. Substituted

benzenes

With the exception of benzene itself the substituted benzenes have been more extensively studied in the long wavelength region than other aromatics. LECOMTE and co-workers [3&37] studied the infrared and Raman spectra of hundreds of mono-, di- and tri-substituted benzenes in the 7-20 ,u region. BARCHEWITZ and PARODI [38] studied the reflection and absorption spectra of monosubstituted benzenes in the 17-55 ,U region by the method of residual rays. Recently PLYLER [24] has drawn attention to the specificity of the infrared absorption of substituted benzenes beyond 15 ,u, and the nature of these vibrations has been discussed by PITZER and SCOTT[39]. RANDLE and WHIFFEN [40] have discussed the characteristic vibrational frequencies of substituted benzenes in considerable detail. WILMSHURST and BERNSTEIN [41] have given assignments for some of the of some of the vibrational substituted benzeaes. GARG [42] has made calculations frequencies of monosubstituted benzenes and correlated the vibrations of these molecules with those of its parts. The infrared and Raman spectra of substituted fluorobenzenes have been studied recently in the 2-38 p region by NIELSEN and co-workers [43-491. The above studies were chiefly concerned with the assignments for the infraredand Raman-active frequencies of the substituted benzenes and little attention was given to the application of the out-of-plane bending modes to qualitative analysis. The far infrared absorption spectra of over sixty alkyl substituted benzenes and over 1.00 other substituted benzenes &h various functional groups were studied in this laboratory. As there are very few data on the infrared absorption of the alkyl substituted benzenes in the cesium bromide region (Figs. 12-24), these compounds will be discussed here in some detail. The other substituted benzenes are included merely to demonstrate the effect of various functional groups on the wavelength of absorption bands which appear to be due to out-of-plane bending vibrations (v~,-,after HERZBERG). [34]

[36] [36] [37]

[38] [39] [40]

[41] [42] [43] [44] [46] [46] [47] [48] [49]

LECOMTE LECOMTE LEOOXTE LECOIUTE

J., J., J., J.,

Compl. Rend. 204, 1186 (1937). Corn@. Red 206, 1568 (1938). J. Phy6. Radium 8, 489 (1937). J. Phya. Radium 9, 13 (1938). P. and PARODI M., Cm@. Rend.

BAROHEWITZ 207, 903 (1938). Prrmn I(. S. and Soom D. W., J. Am Chm. Soo. 95, 803 (1943). RANDLE R. R. and W-EN D. H., ItfoZecuZm Specl~oscopy p. 111. Institute WILMSHWST J. R. and BERNSTELN H. J., Can. J. Chem. 35, 911 (1967). GARO, S. N., J. Sci. Research 4, 83 (1953-1954). FERGUSON 1. E., COLLINS FERGUSON E. E., HIIDSON FER~USON E. E., HUDSON FERQUSON E. E., HTJDSON FEROUSON E. E., OLSEN NIELSEN J. R., LIAXO -. SXITH D. C., FERGUSON E.

2

R. R. R. R.

L., N~SEN J. R. and Samit D. L., NEILSEN J. R. and SIUTH D. L., NEILSEN J. R. and SXITH D. L., NEDEN J. R. and SXITH D. L., NEWEN J. R. and S~H D. and SITE D. C., Dticu.wions Fam&y E., HIJDSON R. L. and NEILSXN J.

181

of Petroleum,

Phya. 21, Uhem. Phy.s.21, Chem. Phye. 21, Chem. Phya. 21, Ohem. Phy8.21, Sot. 9, 177 (1950). R., J. Chem. Phya. 21, C., C., C., C., C.,

J. J. J. J. J.

Chem.

1470 1457 1464 1727 1730

London (1963). (1963). (1963). (1963). (1953).

1475 (1953).

(1955).

F. F. BENTLEY

and E. F. WOLFARTE

The isomeric alkyd benzenes are characterized by fairly consistent absorption bands in various regions of the long-wavelength spectrum. At least one absorption band, usually strong, is observed in the 1’7.0-23 ~1region in all the alkyl benzenes studied. In most instances this absorption band is attributed to the out-of-plane ring deformation vibration analogous to the benzene vibration yqo. This reduced symmetry in the monosubstituted benzenes (D,, + C,,) splits the degeneracy of the benzene-forbidden ~~~~~~~~vibration into two modes, a2 and b,, the latter being allowed and observed as shown in Figs. 12-14. The same argument applies to 1 : 2-dialkyl; 1 : 3-dialkyl; J : 2 : 3-trialkyl and 1 : 2 : 3 : Gtetra-alkyl benzenes, all of which have approximately C,, symmetry and one strong absorption band in the 15-23 ,u region. In like manner the reduced symmetry of the 1 : 4dialkyl and 1 : 2 : 4 ‘: B-tetra alkyl benzenes (D,, + V,) splits the forbidden ezu into a, The 1 : 4dimethyl and 1 : 2 : 4 : B-tetra methyl (forbidden) and b,, (allowed). benzenes have V, symmetry and absorb in the 20-23 p region as shown in Figs. 19 and 20 respectively. In the case of 1 : 2 : 4-trimethyl benzene, the symmetry reduction is D,, + C,. Here the ‘doubly degenerate forbidden e2,, sp lits into two a” modes of vibration. Both are allowed, and two strong absorption bands are indeed observed in the IS and 23 ,u region (Fig. 20). Finally, in the case of 1 : 3 : Ei-trimethyl benzene, symmetry reduction is Do, --f D,, and the forbidden eZu now becomes e”, still doubly degenerate, and still forbidden. Hence the medium strong absorption at 19.5 p (Fig. 21) cannot be the analog of vZo. There are other fundamentals of benzene below 700 cm-l (yq, vg. and ~rs) whose analogs in the substituted benzenes may be responsible for additional bands such as the one just mentioned. However vZo seems to dominate the absorption. Shifts in the wavelength and relative intensity of the vZo band, which can usually be recognized from its high intensity, give some indication as to which alkyl benzene is present. Absorption arising from the out-of-plane ring deformation modes of vibration is also useful in indicating the possible isomeric benzenes where the substituents are not alkyl groups. These molecules absorb in wider but definite regions of the spectrum covered by the cesium bromide prism. Mono-substituted

alkyl benzenes (Figs.

12-14)

Usually one strong absorption band was observed in the infrared spectra of the monoalkylbenzenes in the cesium bromide region. It is located in the 17.5-22.0 p region and is apparently due to an out-of-plane ring deformation vibration of symmetry class b,. This absorption band broadens as the straight chain alkyl substituents increase in size, and in the neighborhood of the n-decyl benzene derivative unresolved shoulders appear. This effect is probably due’to absorption by the side chain. As the molecular weight of the alkyl group increases the position of the absorption band is shifted to shorter wavelengths. Branching on the carbon linked to the phenyl ring also shifts this band to shorter wavelengths. In the n-alkyl mono-substituted benzenes, and the branched alkyl monosubstituted benzenes where the branching is not on the cc-carbon atom, the position of the above band is beyond 20 p. In those compounds studied where the branching 182

Analytical

applications

WAVE 700

600

of far infrared

NUMBERS

500

400

spectraGI1

IN 350

C M-’ 300

250

100

40 L P H P 4o

I I I I I I I I I I I I I I I I I I I

I 5 40 Y H 20 III

II

II

1lI.l

IIIIIII

I

n

6) ”

25 WAVE

Fig.

12.

Infrared

spectra

from

LENGTH

15-35 p of benzene B, methylbenzene;

30 IN

40

MICRONS

end mono-substituted o, ethylbenzene.

183

35

alkylbenzenea.

A, benzene;

WAVE

NUMBERS

IN

600 1 ._-

‘.. ” . .:,,... / i , .! i i) j 1 1 :I( j ;., ; : .j !I! ; / ’ :.! )

;_

h j i j .I./ / , -

. .,

_.-. _.... _ ,j- .I. .I

WAVE Fig.

13. Infrared

spectra

from

LENGTH

IN

‘.-... ‘./.j:. .

1II .:1:I:: .!1: .-

MICRONS

15-35 p of mono-substituted akylbemenes. a, n-butylbmzene; a, n-dacylbenzene.

IS4

A,

n-propylbenzene;

Amlyticd

applications

of far

imthmd

WAVE NUMBERS

IN

spectr*II

CM-’

II~I~l1lIII-TTrlIIiIIIIIIIIIIIIIIIrIIIIIIlllIllllIlllIII 14

’

8 260 2 k 5 60

g40 5 ti a L

20

PI

’

,‘,,,, III1

1 I\\ ,,,,I ,,,,[,I ,, 111T+rllll111ll1111111111111111111111111111111

WAVE Fig.

14. Infrared

spectra

/,,,I

) ,I

LENGTH

IN

,,,,,,,,,I,

I,,,,,,

MICRONS

from 15-36 p of mono-substituted alkylbenzenes. B, sec.-butylbemeno; c, tert.-butylbenzene.

185

,,/

A,

tiobutylbenzene;

, ,

F.F.

BENTLEY

andE.F.

WOLFARTE

is on.the a-carbon atom the position of the band is at 18-19 ,u, with another weak band in the 21.5 ,U region. Another absorption band of medium intensity at 16.7-19-2 ,u in the spectra of It is present in the monoalkyl benzenes was useful in identifying these molecules. the spectra of all the mono-substituted benzenes with n-alkyl groups or branched groups where the branching is not on the carbon atom alpha to the phenyl ring. The band shifts to shorter wavelengths as the molecular weight of the alkyl groups increases, and it is split into two incompletely resolved bands in the higher homo-

BENZENE BENZENE

METHYL

.

ETHYL

.

w-PROPYL ,

.

II-BUTYL n-DECY

I,

L

II

ISO-BUTYL iso-PROPYL

”

SEC-GUTY

L

”

rrar-BUTY

L

”

2-METHYL-2-PHENYL

BUTANE

i2

1’6 WAVE

Fig.

16. Tabulated

band9

from

12-22

LENGTH

,u of some

IN

mono-substituted

MICRONS

alkylbenzenes.

logs. In toluene the band is of medium intensity, but it is fairly strong in the higher monoalkyl derivatives. Very weak, but consistent, absorption bands were also observed in the 15.9-16.3 and 24.5-25.0 p regions. The infrared spectra of methyl, ethyl, n-propyl, n-butyl, n-decyl, isopropyl, sec.-butyl and tert.-butyl benzene derivatives are shown in Figs. 12, 13 and 14. As can be seen from the spectra of these compounds the wavelength shift of the strong absorption arising from the out-of-plsne ring deformation vibration is appreciable. This is illustrated pictorially in a chart, Fig. 15, for a few of the mono-substituted benzenes. It is interesting to note that absorption due to the non-planar CR bending vibrations in the 13*0-13-7 p region also shifts to shorter wavelengths, but to a smaller degree. The value of using characteristic absorption frequencies in the long wavelength region as a complement to the rocksalt region is uniquely demonstrated by the infrared spectra of the mono-alkyl substituted benzenes. For example the familiar G-H stretching and in-plane ring vibrations, which give rise to absorption in the 3.25 and 6.25 p region, suggest the presence of an aromatic ring. Strong absorption arising from the non-planar bending vibrations in the 13 and 14 p region further 186

Analytical

applications

of far infrared

spectra-II

suggest the presence of a mono-substituted benzene. Thus, the number and position of the alkyd substituents are determined through the use of the group correlations in the rocksalt spectra, but little information as to the nature of the substituents is WVE NUMBERS

WL\VE Fig.

16. Infrared spectra from A, l:Z-dimethylbonzene;

15-35

LENGTH

p of o&m, B, 1:3-dim&h

IN

IN

CM-’

MICRONS

meta and y lb enzene;

para di-substituted c, 1:4-dimethylhen~ene.

alkylbemenes.

revealed. By observing the intensity and Iwavelength of absorption arising from the ring deformation vibrations in the 17-f&22*0 ,U region, however, valuable information as to the nature of the individual alkyl substituent, and consequently the identity of the mono-substituted benzene, is obtained. This example indicates the power of group frequencies in the combined rocksalt and cesium bromide regions in determining molecular structure without reference to spectral files. 187

F. F. BENTLEY

and E. F. WOLFARTE

O&o substituted alkyd bemzenes (Figs. 16, 17) The o&o substituted alkyl benzenes are characterized by medium absorption in the 16.9-17.3, 19.7-20.7 and 29-32 p regions and by strong absorption in the 21~G23.0 ,U region for the few molecules studied. These bands increase in intensity as the molecular weight of the alkyl groups increases. Another weak band at 24.4-24.7 p was observed to decrease in intensity as the size of the alkyl groups increased. The band at 21xC23 p is the strongest band in the spectra of the o&o to the out-ofisomers in the cesium bromide region. This band may be attributed plane ring deformation vibration of class b,. It shifts to shorter wavelengths as the molecular weight of the alkyl groups increases and appears to broaden. The weak to medium band in the 29-32 ,u region also shifts in about the same manner. m-Alkyl benzenes (Fig. 18) Fairly consistent absorption in the 18.7-19.7 (m), 22.2-23.3 (s) and 31-35 (w) ,u As in the ortho alkyl benzenes the strong region identify the m-alkyl derivatives. band at 22.2-23.3 ,LL,which has been tentatively assigned to the out-of-plane class b,, shifts to shorter wavelengths and increases in intensity as the molecular weight of the shift is not as great as for the of the alkyl groups increases. The magnitude o&o derivatives. However, the wavelength of the absorption band is not appreciably affected by branching in the alkyl groups. p-alkyb benzenes (Fig. 19) Strong absorption bands in the 17.5-20.5 ,U region and very consistent, weak absorption bands in the 155-15.6 ,u region were observed in the spectra of the few p-alkyl benzenes studied. The strong absorption band at 17.5-20.8 ,u: which may be attributed to the out-of-plane ring deformation vibration of species bz,, shifts to shorter wavelengths as the molecular weight of the alkyl groups increases. This band splits into a medium and strong band in the higher n-alkyl homologs. Branching in the alkyl carbon atom linked to the phenyl ring also shifts the absorption band to shorter wavelengths. 1:2;BTrialkyl benzenes (Fig. 20) In the infrared spectra of the few 1:2:3-trialkylbenzenes studied, very consistent absorption was observed in the 15.3-15.5 (m) and 18.7-18.9 (s) ,u regions. The latter band is the strongest band in the region studied, and on the basis of wavelength and intensity this band may be assigned to the out-of-plane ring deformation vibration of symmetry type b,. In contrast to the direction of shift for this band in the mono, ortho, meta and para tisubstituted alkyl benzenes, the strong band shifted very slightly to longer wavelengths as the moledular weight of the alkyl groups increased. Most of the absorption bands in the spectra of the 1:2:3-trialkylbenzenes were clustered in the 18-22 ~1region and were of medium intensity. In the infrared spectra of the 1:2:3:5-tetra-alkylbenzenes studied, strong absorption was observed at slightly shorter wavelengths in the 18.3-18.5 p region. Apparently the fourth substituent on the phenyl ring of these isomers does not 188

Analytical

applications

of far infrared

WAVE NUMBERS

spectr+II

ml-’

IN

4

i

3po

250 I

_ -

(4 100

w 4 60 2

25

WAVE Fig.

17. Infrared

spectra

30

LENGTH

from 15-35 ,u of 07tho B, 1-methyl-Z-ethylbenzone;

disubstituted

189

IN

35.

MICRONS

alkylbemenes. c, I:2-diethylbonzeno.

A, l:Z.dimethylbe~ene;

40

F.F.BENTLEY

mdE.F.

WAVE

WOLFISH

NUMBERS

IN

CM-’

0 (A) 100

0 (B) 100

0

(C)

'5

.

20

25 MVE

Fig.

13. I&-d

gpectra

30

‘LENGTH

from 16-36 ,u of meta dimbstituted B, I-methyl-3.ethylbenzene;

190

IN

35

MICRONS

alkylbenzenes. c, I:3-diethylbemene.

A, 1:3-dimethylbemne;

40

Andyticd

appliobions

of far infrared

WAVE NUMSER,S IN

.sljectreII

CM-’

2 60 9 E

40

ii z 20 E!

i i iYi i 1 i i Ii i 1 i i 1 1 i i 1 i i 1 i i i 1 i i i 1 i i i i i i i i ; i J I ’ ’ ’ 25I ’ ’ ’ ’ 30’ ’ I ’ ’ 35.’ ’ ’ ’ ’ 40’ MVE Fig.

19. Infrared

spectra,

LENGTH

from 15-35 p of para n, l-methyl-4-ethylbenzene;

disubstituted

IN

MICRONS

alkylbenzenes. or l:4-diethylbenzene.

191

A, l:4-dimethylbonzcnc;

F. F. BENTLEY

WAVE Fig. 20. Infrared spectra from 15-35 bomene; B, 1:2:3:5-tetramcthylbanzenc;

and E. F. WOLFABTH

LENGTH

.IN

MICRONS

p of tri- and t&-a-substituted 0, 1. .2:4-trimothylboqme;

192

slkylbanzoncs. A, 1:2:3-trim&h+ D, 1:2:4:5-tctramethylbenzene.

Analytical

appreciably derivatives

applications

of far infrared

spectra-II

affect the position of this absorption band. also absorb strongly in the 30 to 35 ,U region.

The 1:2:3- and 1:2:3:5-

1:2:4-Alkyl belzxenes (Fig. 20) Two strong absorption bands at 17.0-18.9 and 22.7-23.0 ,u, which may be tentatively designated as arising from a” type vibrations, identify the 1:2:4The latter absorption band is the strongest and most consistent trialkylbenzenes. band in the cesium bromide spectrum for these molecules. A medium absorption band was observed in the spectrum of 1:2:4-trimethylbenzene at 31.3 ,a, but the other alkyl derivatives were not studied in this region. Although the 1:2:4:5-tetra-alkylbenzenes belong to the same symmetry class as the 1:4dialkylbenzenes, they will be discussed here because the 1:2:4-tri- and 1:2:4:5-tetra-substituted alkyl benzenes appear to absorb strongly at approximately the same wavelength in the cesium bromide region. Only one absorption band was observed for 1:2:4:5tetramethylbenzene (Fig. 20) in the region covered by the double-pass Model 21 spectrophotometer. As in the case of the 1:2:3:5and 1:2:3-derivatives studied the position of this band (22.45 ,u) was at slightly higher frequencies in the spectrum of 1:2:4:5-tetramethylbenzene than in the It is interesting to note that the same relationspectra of 1:2:4-trialkylbenzenes. ship holds for the 1:2:4:5- and 1:2:4fluorobenzenes [45, 461 with the exception that the latter compounds absorb at somewhat higher frequencies in the 22 p region. 1:3:5-Trialkyl benzenes (Fig. 21) The 1:3:5trialkylbenzenes are characterized by a strong absorption band at 19.5-20.5 ,u (six compounds). This is fairly consistent in wavelength, but shifts to longer wavelengths as the molecular weight of the alkyl groups increases or as they become branched. It cannot be an analog of the yZO vibration since the forbidden es11class is resolved into species e”, still doubly degenerate and not active in the infrared. Strong absorption was also observed in all the alkyl benzenes studied in the 25-35 ,u region except 1:3:5-trimethylbenzene. This band appears to shift to shorter wavelengths as the alkyl groups increase in size. General correlations for the substituted alkyl benzene5 It has been found from a study of the Raman spectra of benzene derivatives that, of the disubstitution products, the 1:4-substituted benzenes are decidedly less rich in lines than the 1:2- and 1:3-benzenes, and that, of the tetrasubstituted molecules, the 1:2:4:5-benzenes are less rich in lines than the 1:2:3:4- and the 1:2:3:5-substituted benzenes [50]. This is true because the 1:4- and 1:2:4:5substituted benzenes are more symmetrical than the others and therefore a number of’Raman lines are forbidden for them, whereas for the others all Raman lines are allowed. The number of absorption bands in the infrared spectra of the isomeric alkyl benzenes in the cesium bromide region follows the same pattern observed in the Raman spectra. Fewer absorption bands were present in the spectra of the 1:4, [50]

ECERZBER~

G., Infrared

and

Raman

Spectm

of Polyalomic

193

Moleculw

p. 367.

Van

Nostrand,

New

York.

F. F. BENTLEY and E. F. Womuw~

WAVE

NUMEERS

IN

CM-’

loo

,

1 I

I

I

I

I,

I

I

I

I

I

I

I

I

I

I,

I,

I

I

I,

I

I,

I,

I I I I I I I I I I I I I I I II

I

I,

I

I

I,

I,

I

I,

I,,

(B)

WAVE Fig.

21. Infrared

LENGTH

IN

MICRONS

spectra from 15-35 /A of trisubetituted allcylbsnzenes. B, 1:3-dimethyl-6-ethylbenzcne; o, l-methyl-3:5-diethylbenzene,

194

,

,

,

I

I,

I

I I I I I I I I I I I I I I I I I I I I I I I I

A, 1:3:5-trimethylbcnzone;

Anelytical

applications

of far

infrared

spectr+II

1:3:5 and 1:2:4:5 than the other derivatives. The 1:2, 1:2:3 and 1:2:kderivatives were fairly rich in absorption bands. The intensity of the absorption bands is another clue to the structure of these molecules. The mono-, o-, m- and p-alkylbenzenes absorb strongly in the cesium bromide region, particularly in the case of the. out-of-plane ring deformation vibration. The para-isomer absorbs somewhat more strongly than the ortho- and m&a-isomers. Fairly strong absorption was also observed in the infrared spectra of the 1:2:4- and 1:2:4:5-derivatives, while the 1:2:3- and 1:3:5-derivatives exhibit fairly weak absorption in this region. WAVE

CM-1

500

400

s

I

0:I

IN

NUMBERS

600

I

(35)

-

0\

(22) 5

‘I Q / \ 0

I

(9)

-

(27) s

I

:I 0

-I

(271

&I

(6)

R 16

17 16 WAVE Fig. 22. Wavelength ring deformation

I9

(3)

20

LENGTH position vibrations

21

22

23

24

IN MICRONS of absorption arising from of substituted benzenes.

25 the

In general, strong absorption resulting from the out-of-plane ring deformation is more useful in indicating the individual alkyl benzene isomers. The position of this absorption band for the various isomers studied is shown in Fig. 22. In the monoalkylbenzenes this absorption covers a fairly wide range of the cesium bromide spectrum, but as the substitution increases the absorption occurs in a narrower The para-isomer absorbs at shorter wavelengths than the region of the spectrum. o&o- and nzeta-isomers and is less consistent in wavelength. The meta-isomer absorbs at slightly longer wavelengths than the o&o-isomer and is the most consistent. In the trialkylbenzenes the position of this band was very consistent for the few molecules studied. The 1:2:4-derivatives absorbed at longer wavelengths than the 1:3:5- and 1:2:3:- derivatives respectively. The 1:2:3.5--and 1:2:3:4-tetra-alkylbenzenes studied absorb in approximately the same spectral region as the respective trisubstituted derivative, but at slightly. shorter wavelengths. In general, shifts within the characteristic regions give some indication as to the nature of the alkyl substituents linked to the aromatic ring. Diphenyl methanes (Figs. 23, 24) Some interesting correlations were observed in the spectra of a series of diphenyl methanes. The infrared spectra of these molecules have many strong absorption 195

F. F. BENTLEY and E. F. WO~ARTFI

WAVE

NUMBERS

IN

GM-’

0 (4 100

0 PI .lOO-

30

WAVE Fig.

23. Infrared spectra phenylmethane;

from B,

LENGTH

16-36 p of methyl 3-methyldiphenylmethmm;

196

IN substituted

35

40

MICRONS diphenylmethmes.-a, o, P-met~yl~phenylmeth~ne.

Z-methyldi.

An~Iytic~l

applications

WAVE ,700

600’

500

of far infrared

NUMBERS

IN

spectr+II

CM-’ 300

400

250

100

0 (4 100

C(B)

WAVE Fig. 24. Infrared

8

LENGTH

spectra from 15-36 /.J of ethyl methane; B, 2-ethyldiphenylmethae;

IN

MICRONS

substituted diphenylmethanes. o, 4-ethyldiphenylmethce.

197

A, 3-ethyldiphenyl-

F. F. BENTLEYR&E.F.

WOLIMGTE

bands, several of which are fairly constant in wavelength. The infrared spectra from 15 to 35 p of 2-, 3- and 4-methyldiphenylmethane and 2-, 3- and 4- ethyldiphenylmethane are shown in Figs. 23 and 24. Two weak, but very consistent, absorption bands are observed in the spectra of the diphenylmethanes in the 1.4.9-15.1 ,u [22] and 24.6-24-g ,u regions. The former band is masked by the strong absorption due to the non-planar CH bending vibrations of the phenyl ring and is observed as a shoulder if thin cells bands are used. These bands appear to be analogous to the weak absorption observed in the 15.9-16.3 Fnd 24.5-25.0 ,u regions in the spectra of the monoalkyl substituted benzenes. The diphenyl methanes studied are characterized by strong absorption in the 16-O-1.7.0, 15.0-19.1 and 20.3-23.3 ,u regions. The first absorption band is the most consistent in wavelength, and in the 2-alkyl diphenyl methanes weak side bands are observed on the high-frequency side of this band. Only a weak absorption band is observed for 4-methyldiphenyl methane in the 18.0-19.1 p region, which is the only exception to this correlation. The absorption band in the 20.3-23.3 ,u region apparently arises from the out-of-plane bending vibration of the phenyl ring. It is usually the strongest band in ,the cesium bromide spectra of these molecules and is usually split into two bands. The position of the absorption band shifts to shorter wavelengths as the molecular weight of the alkyl groups increases, and in the case of the n-alkyl diphenyl methanes the wavelength separation of the two bands gradually increases. The para-isomers absorb on the short-wavelength side of the indicated region and the qneta-isomers at the long-wavelength side. The ort7Lo-isomers assume the intermediate position at slightly shorter wavelengths than the meta-isomers.

Polynuclear hydrocarbons The infrared spectra of about fifty polynuclear hydrocarbons, most of which were of high purity, were studied in the cesium bromide region. The infrared spectra of naphthalene, anthracene and biphenyl and the few 2-alkyl-naphthalenes studied were not very rich in absorption bands. The only, strong band in their spectra appeared quite consistently at 21.2-21.5 ,u. On the other hand the infrared spectra of 1-alkyl naphthalenes, substituted biphenyls and the 1:2:3:4tetrahydronaphthalenes were very rich in absorption bands. Anthracene and phenanthrene derivatives were not included in this investigation. It is interesting to note, however, that the strong absorption band apparently arising from an out-ofplane bending vibration of the ring systems is present in the spectra of the parent compounds (Fig. 25). Naphthalene hydrocarbons (Pigs. 25, 26)

There has been a considerable amount of experimental work on the infrared and Raman spectra of naphthalene. This effort is summarized in two recent papers [51, 521. Assignments for the out-of-plane vibrational frequencies of naphthalene, based on calculations using force constants transferred from benzene, were [511 ~~PINCOTT E. R. and O'REILLY l3. J., J. Chem. WI MCCLELLAN A. L. and PIMENTSL G. C.. J. Chem.

Phya. Phys.

198

23, 238 (1955). 23, 246 (1965).

Analytical

applications

WAVE

of far infrared

NUMBERS

IN

spectr*II

CM-’

(B) LOO

,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

,,,,,,,,,,J

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

WAVE Fig.

26. Infrared A,

spectra from naphthalene;

LENGTH

15-35 p of naphthalene I), 1-methylnaphthalene;

199

IN

2;4cvfP~T,n~L~N~

MICRONi and mono substituted c, 2-methylnaphthnlene.

akylnaphthalene~

F.F.

J~ENTLEY

and

E. F. WOLFAXTR

0 w

WAVE LENGTH Fig.

20. Infrared

spectra lene (pure);

from 15-35/c of mono B, 1-n-amylnaphthaleno;

IN

MICRONS

substituted allcylnaphthalenes. c, I-tiopropylnaphthrtlene

200

A, I-ethylnaphtha. (pure).

Analytiml

applications

of far

infrared

speck-II

suggested by SCULLY and WHIPPEN [53]. The infrared spectra from 15-35 p of the few alkyl naphthalenes obtained in this study present some interesting correlations and demonstrate the influence of substitution on the complexity of the spectra. The I-alkylnaphthelenes are rich in bands, and absorption in the 15.5-16.2, 16-g-17.7 (m to s), 18.8-19.1 (s), 20.3-21.4 (s) and 23.0-24.9 (vs) ,u regions serves to identify these compounds. The bands in the 15.5-16.3 and 20.3-21.3 ,U regions are also present in naphthalene and other substituted naphthalenes. .The latter band shifts to shorter wavelengths as the size and complexity of the alkyl groups increases. The absorption bands in the region from 16.9-17.7 ,u increase in intensity In the few compounds studied (methyl to and are split in the propyl derivatives. n-pentyl) the very consistent absorption at 1S.G19.1 ,U decreases slightly in intensity as the molecular weight of the alkyl substituents increases. The characteristic features of the very strong absorption band in the 23.0-24.9 ,u region are analogous to those observed for strong absorption attributed to the out-of-plane bending vibrations in the substituted benzenes. In contrast to the l-alkylnaphthalenes, the 2-alkylnaphthalenes are characterized by simple spectra in the long-wavelength region. In the few compounds studied, their spectra resemble the spectrum of naphthalene. It is interesting to note that the strong absorption band in the 23-24.9 p region, which served to identify 1-alkylnaphthalene derivatives, is not present in the spectra of the 2-alkylnaphthalenes. In the few 2-alkyl derivatives studied, consistent absorption bands were observed in the 16.0-16.4 and 21.0-21.5 ,u regions and served to identify this structure. Weak, but less consistent, absorption bands were observed in the 24.5-28 ,a region. The complex spectra exhibited by the 1-alkylnaphthalenes compared to the simple spectra of the 2-alkyl-naphthalenes are illustrated in Figs. 25 and 26. The isomeric methyl naphthalenes differ only in the position of the methyl groups on the naphthalene ring, but their spectra are spectacularly different. The spectrum of 1-methylnaphthalene is rich in strong absorption bands, while the spectrum of 2-methylnaphthalene (obtained as benzene solution and Nujol mull) possesses only a few absorption bands. When recorded in cells with the same pathlength the absorption band at 21.2 ,IJ (0.50 g/ cm3 in benzene) in the spectrum of 2-methylnaphthalene is slightly more intense than the strong bands at 18.8 and 24.5 ,u (obtained as pure liquid) in the spectrum of l-methylnaphthalene. The spectra of the 1:2:3:4-tetrahydronaphthalenes were quite complex, but absorption in the 21.5-23.6 p is useful in identifying these molecules. The absorption bands in this region were very strong for 1:2:3:4-tetrahydronaphthalene, 1-alkyl-, 2-alkyl- and 6-alkyl-1:2:3:4-tetrahydronaphthalenes,. but weak for the 5-alkyl-1:2:3:4-tetrahydronaphthalenes studied. In the latter series of compounds very strong absorption bands were observed in the 29-35 p region. Biphenyl hydrocasbons (Fig. 27) A few 2-alkyl- and 3-alkyl- substituted biphenyls (twelve included in these studies and absorption bands in the 14.9-15-l [53]

SCULLY

D. B. and

WHIFFEN

D. H.,

J. Mol.

~S’pecwmo~~

201

1, 257 (1957).

compounds) were (m), 16.2-16.4 (s),

F. F. BENTLEY HU\VE

WAVE Fig.

27. Infrared spectra 2-phenylbenzsnc;

and E. F. WOLF~TH NUMBERS

LENGTH

from 16-35 p of biphonyl c, 1-methyl-3-phenylbenzene;

202

IN

IN

CM-’

MICRONS

hydrocarbons.

A, phenylbenzone;

D, 1-iaobutyl-2-phenylbonzene.

B, I-methyl-

Analyticd

ctpplications

of far infrared

spectra-II

(D -ii

4

T

203

T T-

P

I.3-DISUBST.

BENZENES

I,3.5-TRISUBST.

BIPHENYLS

Fig. 29. Spectra-structure M-S = medium

BIPHENYLS

P-A&YL

3-ALKYL

BIPHENYLS

correlation to strong.

121

II,

W

TETRAHYTIRONAPHTHALENES~II

12,

P-ALKYLNAPHTHALENES

ALKYL

in

(5,

,ZZl

,111

WI

WI

I33,

1.1

12,

I21

(5,

(6,

I-ALKYLNAPHTHALENES

NAPHT.HALENES

BENZENES

I,2,4-TRISUBST.

BENZENES

BENZENES BENZENES

I,2-DISUGST.

I,4-DISUBST.

BENZENES

BENZENES

1,3.5-TRIALKYL DIPHENYLMETHANESw

BENZENES

MONOALKYL + MONOSUBST.

BENZENES

I,2,4-TRIALKYL

BENZENES

I,4-DIALKYL

I,2,3-TRIALKYL

BENZENES BENZENES

13,

w3, ,Sl

BENZENES

BENZENES

I,2-OIALKYL

ALKYL

SUBST.

1.3- OIALKYL

MONO

ALL

\ROMATICS

I5

___16-35

16

k A A

+i

,1)

chart from p (Tho bands indicated

-

I? k

k

6

r

’ L I--

near

I6 IN

J

‘: MICRONS

I9

CM-I

hydrocarbons. 300 cm-’ by dotted

WAVE LENGTH _ aromatic tar

r

t-

,Y Y

-i

t--P-i

FREQUENCY L

V = variable, lines are often

,

-I

I

.b--

I

t

W = weak, M = medium, not real. See “Experimental”).

1

S = strong,

An+tical

applications

of far infrared

spectr~I1

174-19.3 (vs), 20.5-22.5 (vs) and 24.5-25.1 (m) ,B regions appear to be characteristic of these molecules. The absorption bands in the 16.2-16.4, 20.5-22.5 and 24.5-25.1 ~1regions are also observed in the spectra of other biphenyl derivatives. The first band is the most consistent in the spectra of the biphenyls studied and its intensity does not vary appreciably, whereas the latter absorption decreases in intensity as the alkyl groups increase in size. It is strong in the spectrum of 1-methyl-2-phenyl benzene but weak in the spectrum of 1-n-butyl-2-phenylbenzene. A cluster of very strong bands are observed in spectra of the alkylsubstituted biphenyls in the 174-19.3 and 20.5-22.5 ,U regions, and variations in these bands give some information as to the nature of the alkyl substituents. Few differences exist in these correlation bands for the 2-alkyl and 3-alkyl biphenyls, but the latter absorbs more consistently in the 21.9-22.5 p region with only weak absorption bands in the 30-35 ,u region, while the 2-alkyl biphenyls possess medium absorption bands at 29-31.6 ,u.

Spectra-structure correlation charts Spectr*structure correlation charts showing the probable positions of the characteristic absorption frequencies of the alkanes, alkenes, cycloalkanes and aromatic hydrocarbons are shown in Figs. 25 and 29. The spectra-structure charts are linear in frequency (cm-l). The name or structure of each class of compound is given in the left margin of the chart and the number of individual molecules studied in each class is given by the small numbers in parenthesis. The horizontal lines in the chart indicate the frequency range in which the characteristic absorption frequencies of a given structure are expected to occur. The small letters above the lines indicate the intensities of the characteristic absorption frequencies in relation to other absorption frequencies in the cesium bromide region. These letters, V, W, M, S, M-S, indicate variable, weak, medium, strong and medium-tostrong absorption bands respectively. Although many of the empirical assignments appear quite specific for certain groups, some of the assignments are tentative and should be used with discretion. Some of the group frequencies used in this chart were abstracted from the literature, but the bulk of the correlations were made from the unpublished work conducted at this Center. Acknowledgements-The authors wish to thank Mrs. H. MARSH, Mrs. N. E. SRP and Mr. W. R. Pow~‘& of this Centre for their assistance in recording some of the infrared spectra used iu this study. The authors also wish to thank Dr. W. J. Porrrs of the Dow Chemicd Company, and Lt. D. W. Mayo and Lt. L. A. HURRAH of this Center, for their helpful comments and criticisms of this paper and for suggesting assignments for specific absorption frequencies in the spectra of many of the substituted benzenes studied. The National Advisory Committee for Aeronautics furnished the diphenylmethanes and related aromatic hydrocarbons used in these studies.

205