Far Ultraviolet Absorption Spectra of Small Ring HydrocarbonsBarbara B. Loeffler, Elspeth Eberlin, and Lucy W. Pickett Citation: The Journal of Chemical Physics 28, 345 (1958); doi: 10.1063/1.1744120 View online: http://dx.doi.org/10.1063/1.1744120 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/28/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of Ring Size on the Far Ultraviolet Absorption and Photolysis of Cyclic Ethers J. Chem. Phys. 30, 351 (1959); 10.1063/1.1729951 The Far Ultraviolet Absorption Spectra of Selected Isomeric Hexenes J. Chem. Phys. 22, 1266 (1954); 10.1063/1.1740362 The Far Ultraviolet Absorption Spectra of the Isomeric Butenes J. Chem. Phys. 22, 599 (1954); 10.1063/1.1740132 The Far Ultraviolet Absorption Spectra of Simple Alkyl Amines J. Chem. Phys. 21, 311 (1953); 10.1063/1.1698878 Far Ultraviolet Absorption Spectra of Some Aliphatic Ketones J. Chem. Phys. 8, 444 (1940); 10.1063/1.1750685

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THEORY OF SEPARATED ELECTRON PAIRS 345

while for a heteropolar bond almost all that matters is the amount of charge transferred-the quantity QI.

The argument in this paper has been concerned in the main with the case in which each electron pair is in a singlet state and the molecule is in its ground state, but triplet states of the pairs (which enter the exact molecular wave functions)36 and excited states of the molecule also can be handled. A single electron can be treated, as can a group of three.37

That electron pairs in practice are not often separated in the technical sense of the present paper is proper cause for concern. However, the model often should be a good approximation, and its treatment here has followed a rigorous variational pattern. Furthermore, the way is open for systematic incorporation of improvements.3s

In purely theoretical treatments of molecules inte-

36 See Sec. VI of reference 6. 37 J. M. Parks, Ph.D. Thesis, Carnegie Institute of Technology,

1956. 38 See Sec. V of reference 6, and reference 10.

THE JOURNAL OF CHEMICAL PHYSICS

grals conventionally are broken down to integrals in­volving bare nuclei. In treatments of large molecules with many electrons an alternative procedure seems more desirable, referring matters where possible to integrals involving neutral atoms or ions of small charge. As has been demonstrated in this and a preceding paper, 6 one then can treat electrons one pair at a time or one group at a time, from the outside in, so to speak. Furthermore, purely theoretical and semiempirical theories alike then can be put in the same language, and this language is closer to the everyday language of ordinary chemists.

Extensive quantitative calculations with the equa­tions of this paper and appropriate extensions and generalizations of them have been carried out by the authors on the formaldehyde molecule, H2CO, yielding information about the charges transferred in and be­tween the various bonds in several molecular and ionic states, their effects on one another, and so on.37 The result~ will be presented elsewhere.

VOLUME 28, NUMBER 2 FEBRUARY, 1958

Far Ultraviolet Absorption Spectra of Small Ring Hydrocarbons*

BARBARA B. LOEFFLER,t ELSPETH EBERLIN,t AND Lucy W. PICKETT

Carr Chemistry Laboratory, Mount Holyoke College, South Hadley, Massachusetts (Received October 23, 1957)

Molar extinction coefficients for the far ultraviolet absorption of cyclobutene, 1-methylcyclobutene, and methylenecyclobutane were measured between 40 000 cm-1 and 64 000 cm-1• These spectra show the characteristic absorption of 7r electrons with oscillator strengths of 0.3 and with band positions which are explicable on the basis of molecular structure. Photodecomposition to give ethylene was observed in the case of methylenecyclobutane and 1-methylcyclobutene.

INTRODUCTION

T HE measurement of the far ultraviolet absorption spectra of alicyclic hydrocarbons has been ex­

tended to the four membered ring, cyclobutene, and two related compounds, methylenecyclobutane and I-methylcyclobutene. The purpose of the investigation was to determine, first, if ring size and ring strain have significant effect on the absorption due to the 7r elec­trons and, second, if the spectra follow the generaliza­tions arrived at through study of the butenes, pentenes, and hexenes; namely that (1) the oscillator strength of such a transition is about 0.3-0.4, (2) that the band maximum depends on the relation of the direction of the dipole moment of the normal molecule to the transition moment, and (3) that the beginning of

* This work was supported by the National Science Foundation (NSF-G752) and by the Esso Research and Engineering Company.

t Present address: Bell Telephone Laboratories, Murray Hill, New Jersey.

:\: Present address: American Cyanamid Company, Stamford, Connecticut.

absorption is dependent on the number of alkyl groups substituted on the doubly bonded carbon atoms.

EXPERIMENTAL

Cyclobutene was prepared in this laboratory! by a synthesis described by Cope and co-workers.2 This in­volved the reduction of cyclooctatetraene to 1,3,S-cyclo­octatriene and the decomposition of an adduct of the latter compound with dimethylacetylenedicarboxylate.

The first cyclobutene obtained contained 8% of 1,3-butadiene as identified by the characteristic ultraviolet spectral bands of the latter. A second run was made with distillation into a liquid nitrogen cooled receiver since it was believed that the diene might have resulted from overheating the cyclobutene in the reaction vessel, but the spectral evidence showed but

1 The preparation was carried out by Barbara Bowen Loeffler under the direction of Dr. Mary L. Sherrill.

2 Cope, Haven, Ramp, and Trumbull, J. Am. Chern. Soc. 74, 4867 (1952).

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346 LOEFFLER, EBERLIN, AND PICKETT

4.0

3.5

A. I·METHYLCYCLOBuTENE

B. CYCLOBUTENE

55

WAVE NUMBER X !O-', eM·'

FIG. 1. Absorption curves for cyc10butene and l-rnethylcyclobutene vapor.

little improvement in purity. A recent paper by Lord and Rea3 indicates a similar experience.

The cyclobutene was purified by the addition of dry cuprous chloride. The mixture was allowed to stand for five days at low temperatures (-15°C to -.40°C) then cooled to -80°C for five hours. The cyclobutene was then removed by vacuum distillation in three fractions. The first fraction distilled at temperatures up to -18°C and showed no evidence of butadiene in the spectrum. The infrared spectra were determined and found to agree with the report by Roberts and Sauer4 and more recently by Lord and Rea.S A small amount of 1,3-butadiene, estimated as 0.6%, was found in the second fraction.

The samples of methylenecyclobutane (bp 41.0-42.0° Cat 739 mm) and I-methylcyclobutene (bp 37.1°C at 750 mm)5 were obtained through the courtesy of J. D. Roberts, E. R. Buchman, and D. Semenow of California Institute of Technology. The compounds were vacuum distilled into the sample tube attached to the absorption cell and kept sealed under vacuum at all times. The spectral measurements were made by an earlier de-

'" o -'

4.0

35

;0

25

50

A. METHYLENE CYCL08UTANe:

8. 2-METHYL-f-BUTEN£

55 60

WAVE NUMBER X IO-~ eM-"

FIG. 2. Absorption curves for rnethylenecyclobutane and 2-methyl-l-butene vapor.

3 R. C. Lord and D. G. Rea, J. Am. Chem. Soc. 79, 2401 (1957). 4 J. D. Roberts and C. W. Sauer, J. Arn. Chem. Soc. 74, 3192

(1952). • Shand, Schomaker, and Fischer, J. Am. Chem. Soc. 66, 636

(1944).

scribed technique6•1 using a fluorite prism vacuum spectrograph and a flowing vapor sample. The range of vapor pressures used was cyclobutene 0.02-0.98 mm; methylenecyclobutane 0.024-0.154 mm; 1-methylcyclo­butene 0.030-0.388 mm.

RESULTS

The absorption curves of cyclobutene and its methyl derivative are shown in Fig. 1 while methylenecyclo­butane is given in Fig. 2 with a comparable acyclic alkene.8 The curves were studied in relation to the following points.

A. Intensity and Oscillator Strength of Bands

The absorption maximum in all cases is at a molar extinction coefficient of approximately 10000. This is characteristic of acyclic alkenes as has been observed in this laboratory and by Jones and Taylor9 in a study involving many such olefins. The oscillator strengths of these and other cyclic compounds studied in this laboratory are shown in Table I.I0

These are subject to the usual uncertainties of over­lapping bands and the consequent lack of knowledge of the short wavelength limits since bands attributed to sigma electrons are found near 1600 A. The values for these three compounds are somewhat lower than the average value, j=0.36, found for twenty hydro­carbons with one double bond. It is of interest that this is dose to the value given by the free electron approxi­mationll as well as being of the order of that found by molecular orbital calculations.12

B. Band Positions

In the series cyclobutene, cyclopentene,lO and cydo­hexene,lO the last two are comparable both in the beginning of the absorption and in the general position of the band maxima although diffuse vibrational struc­ture obscures their exact position. In cyclobutene the

TABLE 1. Oscillator strengths of cyclic alkenes.

Compound

cyclobutene methylenecyclobutane l-methylcyclobutenc cyclohexene cyclopentene

• See reference 10.

Wave no. range in em-I.

51 000--61150 47500--56900 48 500-58 450 45 600-64 300 47600-64 000

f

0.28 0.29 0.29 0.40" 0.32-

• J. T. Gary and L. W. Pickett, J. Chern. Phys. 22, 599 (1954). 7 Harrison, Gaddis, and Coffin, J. Chern. Phys. 18, 221 (1950). 8 Sernenow, Harrison, and Carr, J. Chern. Phys. 22, 638 (1954). 9 L. C. Jones, Jr., and L. W. Taylor, Anal. Chern. 27, 228 (1955). 10 Pickett, Muntz, and McPherson, ]. Am. Chem. Soc. 73,

4862 (1951). 11 N. S. Bayliss, Quart. Rev. 6, 319 (1952). 12 See, for example, R. S. Mulliken and C. Rieke, Repts.

Progr. in Phys. 8, 231 (1941); G. Berthier, J. Chem. Phys. 51, 137 (1954); M. Wolfsberg, J. Chem. Phys. 23, 793 (1955).

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A B SO R P T ION S PEe T R A 0 F SMA L L R I N G H Y D ROC ARB 0 N S 347

region of absorption is narrower and is displaced to higher frequencies.

Previous consideration of alkene spectra have indicated that the absorption in the region under con­sideration is the result of excitation of the ?r electrons and that there are at least two overlapping band systems. The first long wavelength band of low in­tensity can be observed in the monoolefins but com­parisons of the second band are made through the band maxima since the beginnings of these bands are inde­terminable because of overlap. The second band in each case is responsible for the oscillator strength of over 0.3 and is believed to represent an N -> V transi­tion. Recent studies by Gary and Pickettfi ,13 showed that in comparable compounds the band maxima14 were dependent on the direction of the dipole moment relative to the double bond. In cis-compounds where this directional effect is perpendicular to the double bond the band maxima are found at higher frequencies than in substituted l-alkenes where the directional effect is along the bond.

Because of the strained ring which forces the single bonds nearly at right angles to the double bond in cyclobutene and thus accentuates the effect perpendicu­lar to the bond in this compound, the displacement to higher frequencies with respect to the other cyclic com­pounds seems reasonable.

The other two compounds provide excellent test cases of such explanation .. Methylenecyclobutane would have its substituent effect along the line of the bond while I-methylcyclobutene would have a greater effect in the perpendicular direction, so that the latter would be expected to have a band maximum at higher fre­quencies. Further evidence is provided by a comparison with acyclic alkenes. Methylenecyclobutane is com-

TABLE II. Absorption band maxima.

cyclobutene cis-2-butene I-methy1cycIobutene 2-methyl-2-butene methylenecycIobutane 2-methyl-1-butene

Band maxima

56700 cm-1

57000 56500 56575 51900 53200

13 J. T. Gary and L. W. Pickett, J. Chern. Phys. 22, 1266 (1954). 14 Increasing molecular weight produces displacements to lower

frequencies so that only isomers are comparable in this way.

FIG. 3. Microphotom­eter record of photo­graph of methylenecy­cIobutane vapor show­ing bands of ethylene formed by photodecom­position. A. Flowing. B. Static.

57,380 59,000 eM .1

parable to 2-methyI-l-butene8 while I-methyIcyclo­butene is like 2-methyl-2-butene8 as to number of carbon atoms and distribution with respect to the double bond. Methylenecyclobutane because of the strained ring should accentuate the asymmetry in the bond direction and cause a bathochromic shift. I-methyIcyclobutene might reasonably be expected to bear the same relation to 2-methyl-2-butene that cyclobutene does to cis-2-butene. Table II shows that the effects are exactly as might be predicted, taking into account a slight bathochromic shift due to cycliza­tion. The position of the first narrow bands was shown early by Carr and Stticklen1l; to depend on the number of alkyl groups attached to the carbon atoms of the double bond and are attributed as Rydberg bands by them.l6 l\Iethylenecyclobutane has two carbon atoms so situated while 1-methylcyclobutene has three, and in conformance with this earlier generalization it may be observed from figures 1 and 2 that the first bands of I-methylcyclobutene are at lower frequencies than the first of the methylenecyclobutane band, thus con­forming to the pattern observed in acyclic alkenes.

C. Photodecomposition

Methylenecyclobutane was quite unstable under the conditions of measurement and traces of the character­istic ethylene bands at 57 340, 57 800, 58 720, and 59 180 cm-1 were noted even in the flowing vapor as may be seen in Fig. 3. This is a microphotometer tracing of a photograph of flowing and also of static vapor which had been irradiated three minutes and in which the ethylene bands show clearly. A trace of ethylene was observed when I-methylcyclobutene was irradiated in like manner.

15 E. P. Carr and H. Stiicklen, J. Chern. Phys. 4, 760 (1936). 16 E. P. Carr and H. Stiicklen, J. Chern. Phys. 7, 631 (1939).

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