15. J. H. Ross, Anal. Chem. 25, 1288 (1953). 16. P. Blum and G. de Gaudemaris, Bull. Soc. Chem. France 996 (1954). 17. L. I. Braddock, K. Y. Garlow, L. I. Grim, A. F. Kirkpatrick, S. W.

Pease, A. J. Pollard, E. F. Price, T. L. Russmann, H. A. Rose, and M. L. Willard, Anal. Chem. 25,301 (1953).

18. G. L. Clark, W. I. Kaye, and T. D. Parks, Ind. Eng. Chem. Anal. Ed. lfl, 310 (1946).

19. B. E. Gordon, F. Wopat, Jr., H. D. Burham, and L. C. Jones, Jr., Anal. Chem. 23, 1754 (1951).

20. F. W. Matthews, G. G. Warren, and J. H. Mitchell, Anal. Chem. 22, 514 (1950).

21. L. L. Merrit, H. B. Cutter, H. R. Golden, and E. Lanterman, Anal. Chem. 22,519 (1950).

22. J. A. R. Cloutier and J. M. Manson, Appl. Spectrosc. 13, 34 (1959). 23. P. P. Williams, Anal. Chem. 31,140 (1959). 24. W. F. Huber and E. S. Lutton, J. Am. Chem. Soc. 79, 3919 (1957). 25. D. A. Lutz, and L. P. Witnauer, Anal. Chem. 29, 1780 (1957). 26. Brown and Carbridge, Nature 162, 72 (1948). 27. B. Billig and B. Greenberg, Polytechnic Inst i tute of Brooklyn

(1965). 28. "Powder Diffraction File," Joint Commit tee on Powder Diffraction

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fication of Organic Compounds (Wiley, New York, 1964), p. 200. 30. L. V. Azaroff and M. J. Buerger, The Powder Method (McGraw-Hill,

New York, 1958), p. 181.

Thc Infrared and Raman Spectra of ce and -Pinencs

H. WILLIAM WILSON*

Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208

T h e l i q u i d p h a s e i n f r a r e d a n d R a m a n s p e c t r a o f a - p i n e n e (2p6~6- tr imcthy l b i c y c l o [ 3 . 1 . 1 ] h c p t - 2 - e n c ) a n d f l - p i n c n c ( 6 ~ 6 - d i m e t h y l - 2 - m e t h y l e n e b i c y c l o [3.1.1] h e p t a n e ) h a v e b e e n s c a n n e d b e t w e e n 100 a n d 4000 c m -t a n d t h e vapor p h a s e i n f r a r e d s p e c t r a o f b o t h c o m p o u n d s b e t w e e n 625 a n d 4000 e m - l . A n u m b e r o f t h e 72 a l l o w e d m o d e s o f t h e C10H18 i s o - m e r s can be a s s i g n e d on t h e b a s i s o f g r o u p f r e q u e n c i e s . I n d e x H e a d i n g s : R a m a n s p e c t r o s c o p y ; I n f r a r e d s p e c t r o s c o p y ; P i n e n e s ; Gas p h a s e ; V i b r a t i o n a l a s s i g n m e n t s .

I N T R O D U C T I O N

The bicyclic terpenoids have been the subject of a variety of chemical and physical studies, 1-10 but little appears to have been done on the vibrational spectra of the compounds. Other than for survey scans taken pri- marily for identification purposes, ~1-14 there are only a few infrared and Raman studies that have been carried out with specific vibrational assignments in mind. ~5"~7

It has recently been shown that a- and $-pinenes, 2,6,6- tr imethyl bicyclo[3.1.1]hept-2-ene and 6,6-dimethyl- 2-methylene bicyclo[3.1.1]heptane, respectively, can be found in the natural effluent vapors of a variety of trees 18-2° and they may in part account for atmospheric haziness that is often observed on hot, humid days. 2t In addition, they may also be partly responsible for cyclic seasonal variations that have been observed in solar photometry studies of atmospheric transparency. 22"26

Whether or not ultra-long infrared pathlengths of several kilometers or even one air mass could be used to monitor the level of terpenoids in the atmosphere 27 in the 8 to 14 u region is certainly a moot point. Since, as we will discuss briefly later, there is some possibility that this could be done, it appears that vapor phase studies of the pinenes is worthwhile.

Although computer capacities of the order necessary

Received 18 August 1975; revision received 10 November 1975. * Present address: Department of Chemistry, Western Wash-

ington State College, Bellingham, WA 98225.

to do a complete normal coordinate analysis of the 26- atom compounds were not available, this work was under- taken to interpret tentatively both the liquid and gas phase infrared and Raman spectra of the a- and B-pinenes between 4000 and 30 cm -1 on the basis of known group frequencies.

I. E X P E R I M E N T A L SECTION

Samples of 99 % pure a- and $-pinenes were obtained commercially and used without further purification. Al- though the purity of the samples was not ascertained by gas-liquid chromatography methods, the boiling points and refractive indices of the materials matched litera- ture values and samples from several sources all gave iden- tical vibrational spectra.

The infrared spectra from 4000 to 250 cm -1 were re- corded on a Perkin-Elmer model 621 infrared spectro- photometer that was calibrated with standard 28 gases and continuously flushed with dry nitrogen during oper- ation. The liquid phase spectra were obtained from neat films pressed between CsI plates while the vapor phase spectra were recorded in air at atmospheric pressure be- tween 60 and 130°C with an NaC1 equipped hot cell assembly that has been previously described? 9

The far infrared spectra were obtained on a RI IC model FS-720 interferometer using 25 gauge mylar beam-splitters. The 1024-point double-sided interfero- grams were transformed in a PDP-12 computer and plotted on an x-y chart recorder giving about 3 to 5 cm -I resolution. The polyethylene cells used in the inter- ferometer were 1 to 2 mm thick and the spectra were calibrated with water vapor and polyethylene peaks2 °

Raman spectra were recorded using the 488.0 nm line of a CRL 52-MG laser, a Spex model 1401 monochroma- tor with an RCAC-31034 phototube and dc amplification. Spectral slit widths were of the order of 5.0 cm -1 and band centers should be accurate to within ± 3 cm -1.

Volume 30, Number 2, 1976 APPLIED SPECTROSCOPY 209

I I . R E S U L T S A N D D I S C U S S I O N

Both of the pinenes can be assigned to the C1 point group where all 72 modes of vibration are allowed in both the infrared and Raman.

To look into the possibility of using band contours as an aid in assigning the vapor phase spectra of the mole- cules, approximate rotational constants were computed for both molecules. Assuming C C , o C ~ C and CH bond distances of 1.52, 1.38 and 1.08 A, respectively, tetra- hedral angles for all but the sp 2 carbons (120°), the C4 ring (88°), and the C4 ring pucker (30°), 8148 the asym- metry factors

2 b - a - c g - (1)

a - c

where a, b, and c are the rotational constants, were found to be - 0 . 9 6 and - 0 . 6 8 for the a- and/F-compounds, re- spectively. Fig. 1 is a sketch of the a-molecule. The major axis is roughly perpendicular to the plane formed by 5 of the 6 carbon atoms in the 6-membered ring. The b axis is roughly parallel to the double bond.

Both molecules are asymmetric tops, but there is evi- dence of type A and C band contours in both spectra. The P R branch separations are calculated to be about 12.5 to 15 cm -~ and the type C contour should have a more intense Q branch. What we believe to be the 953 em -~ type A and 787 cm -1 type C bands of a-pinene are shown in Fig. 2. The measured P R separations are 12 cm -~ and 16 cm -1 respectively. The contours are not un- like those observed in the vapor phase spectra of the di- hydroxybenzenes 3. where hybridization is not so extensive.

Type B bands are observed on occasion, 35 but often they are difficult or impossible to discern 36 particularly in the hybrid contours of nonrigid rotors. Our computed P R branch separations for them are 13.5 cm-L Experi- mentally the gap may be as small as 4 to 6 cm -~ as evi- denced by a possible type B band at 1336 cm -~ (Fig. 3).

Most of the information obtained in this s tudy is to be found in Tables I and I I . The tables contain some quali-

\

/ /

?

Fro. 1. Sketch of the ~-pinene molecule showing the approximate axial configuration.

T o ~ z

I-- H

Z

I--

j '

L i / / I I 960 950 790 780

Cm-I Fro. 2. Hybrid type A and type C band contours found in the gas phase infrared spectrum of a-pinene at about 115°C in a 6-cm NaCI cell.

u? Z

1340 1330 Cm-]

Fro. 3. Hybrid type B band in ,~-pinene at 120°C in a 6-cm NaC1 cell.

tat ive intensity information in addition to the frequencies and selected assignments. I t should be remembered that intensities and frequencies are affected by phase and temperature changes 3749 and the vapor data were col- lected at elevated temperatures.

In addition to the tabular material a few other points are worthy of note. The infrared CH stretching region is typically complex, but the = C H stretching modes can apparent ly be distinguished at 3031 cm -1 in the a-com- pound and 3083 and 3012 cm -1 in the ~-pinene. In all instances, the infrared bands are weak and, in fact, the 3012 cm -1 feature is a shoulder on the more intense - - C H stretching region. The bands would have little value in either identifying or analyzing for the pinenes in the gas phase, especially at low concentrations. The higher frequency modes are at 3029 cm -1 (a) and 3072 cm -~ (B) in the Raman spectra and they too are weak in com- parison with the nearby - - C H absorption.

There is a major difference in the infrared intensities of the C ~ C stretching modes a t 1651 cm -1 (a) and 1646 cm -1 (~), respectively. The weak a-band is essentially featureless, in line with its primarily type B character but the medium-strong hybrid AB fl-band has a dis- cernable Q branch.

The corresponding polar Raman bands are, along with polarized bands near 650 cm -~, the strongest features be- low 2000 cm -1 in the Raman spectrum and they are po- tentially good bands for analytical purposes.

210 Volume 30, Number 2, 1976

T A B L E I . O b s e r v e d i n f r a r e d a n d R a m a n b a n d f o r a - p i n e n e . ~ T A B L E I I . O b s e r v e d i n f r a r e d a n d H a m a n b a n d s f o r /~.

p i n e n e , a Infrared (cm -t)

Liquid Vapor (neat)

R a m a n (era-l) : l iquid (neat) Approximate assignments

3031 m A 3029 st 2995st A 2991 sh

2988 st 2965 2952 2925 st A 2926

2929 st

2881 st A 2882 st 2872 st

2839 sh 2837 st

2720w 2721 w

2660w 2658 w 1789w 1769w 1732 vw 1651 w A 1658w

1771 vw 1562w 1472 br st A 1469 st 1452 sh st 1447 st 1442 sh st 1436 st 1384 st A 1381 st 1378 st A 1374 st 1370 st A 1364 st

1336 m B (?) 1334 m 1326 m 1305 vw

1267 m-st B (?) 1263 m 1248 vw

1215m-st A I219m-s t 1206 m A 1203 m-st 1183 br 1181 w 1173 br 1166 br 1162 m l 1 2 6 m - s t A 1124 m-st

1099 vw 1090 st A 1085 in 1062m 1062 m

1041 w 1033 vw

1017m-st A 1014 m-st 996w 958 sh

953 st A 952 m-st

928w A 905 vw 886m-s t C 882 m A

842 br vw

814 vw 787 st C

927w 904 vw 885 st 877 sh w

853 vw 840w

786 st 770 m

619w 565w-m 482 vw 464 vw 424 390 v w 333w 305 st 250 vw

183w 138m-s t

3029 : C H s t r e t c h i n g

2989 CHa asymmetr ic s t retching 2954 CH~ asymmetr ic s t re tching 2925 vat (p) C H i asymmetr ic s t re tching 2919 (p) 2893 tert.-CH s t re tching (?) 2878 st CHs symmetr ic s t retching

2837 vat (p) C H t symmetr ic s t retching 2743

2713 w

1657 vs t (p) C = C stretching

1469 m-st 1443 st 1433 vat

1372 w-m C H i and CH2 scissoring C H (in-plane) deformations

1349 vw

1325 m (p) 1305 w 1265 m 1247 vw 1219 w-m 1205 w CHs and CH2 wagging 1181 w-m C H in-plane deformations

1165 m 1125 m

1083 m CH2 twist ing 1061 C - - C stretching 1041 m (p) Ring deformation

1013 w (p) 997 vw

953 m-st 949 m 927 m-sh 905 m-st (p) CH2 rocking 887 w C - - C stretching to ca. 850 em -I

(Nonplanar) ring - - C H defor- mation

(Planar) ring deformation 843 st C----C ring coupled deformation 821 w

789 w st (p) 773 st (p) 697 vs t (p) C : C ring coupled deformation 617 m 561 st (p) 479 m 461 m 423 388 w (Non-planar) ring deformation

331 vw 305 w 259 m 205 w

CHs torsion (?)

138 m-st

a Abbrevia t ions used: (p), strongly polarized; st, strong; w, weak; vw, very weak; m, medium; br, broad; sh, shoulder.

Infrared (em -1)

Liquid Vapor (neat)

R a m a n (em-l): l iquid (neat) Approximate assignments

3083 st A 3072 st 3072m-st 3 O l 2 w s h 3003 st A 2991 st A 2981 st A 2981 st 2981 st 2972 2971 2961

2952

= C H I stretching : C H s stretching

(p) C H I a symmet r i c s t re tching

2947 2937 - - C H s and - - C H s asymmetr ic s t retching

2925 2926 2929 st 2890 st 2897 sh 2899 sh 2883 st A

2872 st 2872 st - - C H i and - -CH2 symmetr ic s t retching

2848 sh 2759 w 2732 w 2724 w 2665 w

1765 1759 w

1646 m A 1641 st 1644 vs t (p) C = C st re tching 1595 w 1593 vw 1481 sh 1475 st 1475 m 1474 st A 1469 st 1464 st A 1457 st 1458 m-st 1442 A 1442 st 1438 m-st

1434 vw 1432 st C H I and CHs scissoring

1411 w 1411 st C H (in-plane) deformations 1392 w 1397 w

1387 st A 1383 st 1387 w 1372 st A 1396 st 1368 vw 1316 1315 m 1318 w

1291 m 1291 m 1289 w 1284 m 1259 m-st 1260 m

1252 m 1254 m-st

1231 w 1233 w CHa and CH~ wagging

1216 w 1218 m C H (in-plane) deformations 1204 m B (?) 1202 w 1202 m

1195 w I186 vw 1184 w 1185 m 1143 A I142 st 1147 m

I105 A 1103 st 1103 w C H i twis t ing 1081 m 1082 w C - - C s t re tching

1056 A 1054 m Ring deformations 1048 1048 msh 1049 in

1010 w 1011 m-st (p) 988 w 991 vw

973 w A 972 m 974 w 952 vw 953 m 952 w

954 w 926 w A 924 m 928 m 905 wsh 903 In 906 w (p) CH2 rocking

880 st C 874 st 881 m-st (Planar) ring deformations 852 wsll A 852 m-st 855 m-st C - - C stretching (to ca. 850

em-l) 822 w 824 w (p) (Non-planar) ring - - C H defor-

mat ions 782 sh 786 m 790 vw C : C ring coupled deformations 765 C 764 w 767 st

716 m 717 m (p) 666 w 642 m 647 vat (p) 603 m 605 m 561 w 525 m-st 525 w 473 w 475 st (Non-planar) ring deformations 455 wm 459 w 416 wm 417 w 392 vw 387 w 352 w 357 w-m

172 m 290 w CH~ torsion (?) 172 m 169 m

a Abbreviations used: (p), strongly polarized; st, strong; vst, very strong; w, weak; vw, very weak; m, medium; br, broad, sh, shoulder.

APPLIED SPECTROSCOPY 211

Although there is a wealth of methyl and methylene modes falling below 1500 cm -~, there is little hope of sorting them out into meaningful assignments. Two possible exceptions in the infrared spectra are strong type C bands at 880 cm -~ in ~-pinene and 787 cm -~ in the a-compound. The dipole change along the major axes that would result in C contours must involve the double-bond carbons so they are undoubtedly out-of- plane ring deformation vibrations that are coupled with the C--~-C bonds.

The 787 and 880 cm -~ a- and ~-pinene bands are typ- ical of those found in a variety of monoterpenoids ~4 and they offer one of the best opportunities for.the infra- red study of the hydrocarbons in the atmosphere through the 8 to 14 ~ by very long path spectroscopy. We have measured the integrated infrared intensities of the bands in the vapor phase and found that they are of the order of 10 cm mole-~. 4° With this value and a path length of 2 km, which is apparently common in atmospheric work, 4z either of the bands could appear in spectra as absorption peaks where the transmittance is about 50 to 40 % at a temperature of 300°K. The vapor pressure of the pinenes is about 4 to 5 torr at this temperature. There are of course a multitude of factors affecting such a technique such as wind conditions, atmospheric homogeneity, inter- fering substances, and so forth, but the qualitative moni- toring method at least appears to be theroetically pos- sible.

Polarized Raman bands at 667 cm -~ (a) and 647 cm -~ (B) are strong but they are near 667 cm -~ CO2 absorption and they may well be of limited value in remote studies.

The far infrared spectra of the pinenes were not studied in the gas phase, but a good correlation was found be- tween the Raman and infrared interferometer liquid phase spectra. In general the frequencies fit with known CH3 torsional modes and out-of-plane hydrocarbon deformation vibrations.

A C K N O W L E D G M E N T S

We would like to express our sincere appreciation to Dr. D. F. Eggers, Jr. of the University of Washington and Dr. J. R. Durig of the University of South Carolina for the use of their Raman and far infrared interferome- ter equipment for this study.

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212 Volume 30, Number 2, 1976