4276
MANUELBALLESTER, JUAN RIERA,A N D LEONARD SPIALTER
Acknowledgments.-J. R. C. wishes to acknowledge gratefully support from h'ational Science Foundation Cooperative Fellowships and from an I.I.T. Research Institute Fellowship during the course of this work.
[CONTRIBUTION FROM
Vol sc,
The authors further wish to express thanks to the Chemistry Division of the Argonne National Laboratory for the use of their n.m.r. spectrometer during the initial phases of this study.
DEPARTAMENTO DE Q U ~ X I C A O R G ~ N I C A PATRONATO , "JUAN DE LA CIERVA,"USIVERSITY OF B A R C E L O N A , C H E M I S T R Y RESEARCH LABORATORY, ARL, W R I G H T - P A T T E R S O Y i k R FORCE BASE,O H I O ]
SPAIN, A N D
New Linear Law and Effects Relating the Ultraviolet Spectra of Substituted Benzenes and the Migration (Spectroscopic) Moment' BY
XIANUEL
BALLESTER,J U A N RIERA,'AND LEONARD SPI.\LTER3 RECEIVED~ I A R C2 3H. 1964
Ultraviolet absorption spectra in the region of 40,000 cm. (250 m u ) were obtained with high reproducibility (better than 5Y0 precision in molar absorptivity) for more than 100 benzene derivatives. These compounds, ranging from mono- t o persubstitution, contained methyl, fluoro, cliloro, bromo, iodo, mcthouy, trifluoromethyl, chloromethyl, dichloromethyl, trichloromethyl, perchloroethyl, chlorocarhonyl, perchl(~rovinyI,and/or perchlorostilbyl substituents alone or in conibinatiori. The molar absorptivities observed, ranging from 100 to 3000, correlate much better with a here-proposed linear function of the moment obtained by \-ectorial addition of substituent contributions (of the Sklar "migration moinent" type or tlie Platt "spectroscopic moment" t y p e ) rather than the currently accepted "square law." T h e effect of ortho substituents is successfully accounted for in terms of ring distortion and resonance inhibition and leads to the concept of a "constellation migration moment."
Introduction The near-ultraviolet absorption spectrum of a simple benzene derivative consists of two band systems : the secondary band, centered in the unsubstituted benzene a t about 40,000 ern.-' (250 mp), and the primary bands, a t higher wave numbers.4 T h e former is by far the better known and the most studied. I t is due to the IA1, + I B Q u or lLb transiti0n.j In the vibrationless, undistorted benzene this transition is forbidden Sklar has shown6 t h a t there are two separable contributions to the intensity of absorption in the secondary band : the migrational absorption, due to the electronic perturbation caused by the substituents, and the aibrationd absorption, due to the symmetry-breaking atomic motions. T h e former is obtained simply by subtracting the latter from the total absorption. The relevant migrational transition moment of a polysubstituted benzene is supposed to be an additive property of the substituents. Consequently, to every substituent a migration moment (Sklar) (or, equivalently, spectroscopic moment in the PlattYnomenclature) is assigned. The migration moment of a substituent is a vector in the plane of the benzene ring and perpendicular to (1) P a r t of this work has been presented before t h e Symposium on Molecular S t r u c t u r e a n d Spectroscopy, Columbus, Ohio, J u n e 11- 1 5 , 1962. T h i s paper includes portions of t h e thesis submitted by J . Kiera in partial fulfillment of t h e requirements for the degree of Iloctor in Sciences (University of Barcelona). One of t h e a u t h o r ? ( M . B ) spent 1 year as 1'isitin.g Research A4ssociatea t ARI,, 1961-1962. ( 2 ) Seccibn de Quimica Orgdnica Tebrica, Departamento de Quimica OrpLnica, P a t r o n a t o " J u a n d e la Cierva' d e Investigacibn Tecnica, Universidad de Barcelona, Spain. ( 3 ) Chemistry Research 1-aboratory, .4KI,, Wright-Patterson Air Force Base. Ohio. (4) I.. Ilouh and J . M. \'anderbelt, J , A m . Chem. Soc., 69, 2714 (1947). ( 5 ) H. H . Jaffe a n d R I . Orchin, "Theory a n d Applications of Ultraviolet Spectroscopy," J o h n Wiley a n d Sons, I n c . , S e w York. S . \'., 1962, pp. 246, 299. (6) A . I*, S k l a r , Reo. .\Jod. P h y s . , 1 4 , 232 (1942). See also ref. 7-9. ( 7 , ( a ) H. Conrad-Billroth, L. p h y s i k . C h r t n . , Bas, 135 (1934 S k l a r , J . CheTn P h y s . , 10, 1 : l j (1942); (c) T h . Fyirster. Z S a i l 1*!2 (1947) ( 8 ) J . K . P l a t t , J . C h r m . P h y s . , 19, 263 (1951)' ((4) J Petruska. zbid., 34, 1111, 1120 (1961)
the bond axis. Its direction is reversed when the substituent is shifted to the next benzene carbon atoin. T h e modulus (or intensity) of the migration moment vector can be calculated from the integrated absorptivity of the secondary band by means of expression I .
where E and E\, are the observed maximum and the vibrational absorptivities, respectively, a t frequency V , k is a factor which can be considered constant, and m, is the migration moment of the i-substituent The summation indicated is vectorial addition. If one assumes that the half-width and shape of the "smoothed" secondary bandlo are constant, then the integrated absorptivity is proportional to emax a n d , hence, expression I can be replaced by the simpler and more convenient expression 11"
where emax represents the observed m a x i m u m a b s o r p h i t y of the smoothed curve, and k' is a proportionality coiistant. The migration moments of different substituents may vary not only in magnitude but also in sign (or direction). Their sign and value indicate, respectively, the direction and value of the resonance effect of the substituent. The migration moment of hydrogen is zero. The induct;\-e effect seems to be relatively unimportant. l ? (10) T h e secoiidary hand usually has a vibrational structure with maxima a n d shoulders. T h e "smoothed" curve is obtained by averaging them o u t so t h a t t h e over-all shape a n d integrated a b w r p t i v i t y are preserved. T h i s c a n usually be reproduced within acceptable limits (11) T h e validity of such substitution has been checked for over 70 carefully selected benzene derivatives T h e d a t a fit t h e expression
f edv
=
3240e,,,,
where e is t h e molar absorptivity a t frequency P. T h e deviation- frttm t h i i expression are within 101,7~. (13) P e t r u s k a , b y comparison of t h e intensity with t h e frequency shlity has concluded t h a t t h e migration moment arises principally from the i n t l u c ~ tivc effect 5 T h e l o w value of t h e moment f o r K€Xi*, 1 . 1 3 the parallelism with
RELATION BETWEEN
Oct. 20, 1964
S P E C T R A .\ND ~ I I G R A T X O Xh \ I O M E N T
Results and Discussion The Square Law.-Although the Sklar-Forster theory permits one to predict certain trends concerning the variation of the secondary band absorptivity with type and nature of substitution, Ballester, et al., have reported16 that the calculated quantitative values can on occasion be seriously divergent from the experimental ones. I t has been assumed67ht h a t the vibrational contribution to the absorptivity on the secondary band varies insignificantly with the position, and relatively little with the nature and number, of the substituents. Platt assumed the vibrational absorptivity (e,) was equal to 130 units, i.c., 10 units greater than that of benzene itself. In Table I the maximuni absorptivities of 13 substituted benzenes with theoretically predicted zero resultant migration moment are listed. These absorptivity values should, therefore, be t, for these compounds. The vibrational contribution, although it seems to increase with the number of substituents, their masses, and bond strength in many cases, can indeed, as will be seen later, be assumed constant. TABLE 1 (SMOOTHED l--lIg + lBlu BAND) EI'ITH THEORETICALLY ZERO ItESULTANT MIGRATION MOMEST
OBSERVED T-ALUES FOR MOLECLTLES
OF
llerivative
Benzene Mesitylene Hernirnell iteiie 1,3,5-Trifluorobenzene 1,3,S-Trichlorobenzene 1,3,5-Triisopropj.lbenzeiie
110"
180'
1,3,5-Tri-t-hutvlbenzeiie
170' 125d 160 180' 1701
1,3,5-Tris(trifluorornethyl jbenzene 1 ",4H,iiH-SonacIilororiiesitylene
i
1100
q5?,
IIesaiiiethylbenzene 225% Hesafluorobenzene 200 2,4,6-Tricliloroinesit~lene 180h Hexaethylbeiizene 915' Hexacli!orobenzene d2jk a - H , a ' - H , a "-H-Sonachlorornesitylene 310h . k P . I . Project 11, Shell Development Co., 1913, p. 40. * Zhid , Shell Development Co., 1947, p 164. c I b i d . , Shell Development Co., 1947, p 162. I h i d . , Duke University, 1933, p . 495. e Zbi'd., Standard Oil Co , 1955, p. 620. 1 Ibid., Standard Oil Co.,18.53, p . 622 0 C. rl. Cooper and F. \V Soegel, J . Chew Phys , 20, 1903 (193%). M. Railester, J . Castafier, and E. Guardiola, Amies reo/ soc e s p n 6 . , f i s q z i i i i i . (Madrid), 56B, 723 ( 1 R f i O i . 1 ?vI.C..-l. Research Project, Allied Chemical Corp., 1961, p. 36. .-l P.1. Project 14, Sinclair Research 51 Ballester niid J. Castafier, Laboratories, Iiic , 1952, p 488 ./. .Am Chew?. .S01-., 82, 1259 ( 196Oi.
The best comprehensive test for the Sklar-Forster theory is presented in Fig. 1. Thirteen pairs of homosubstituted benzenes have been selected. The second molecule of each pair should have a resultant migration moment twice as large as that of the first. =ICT a f t s o i b l : t h e hyi,erconjiigati,rn of t h e alkyl groups ' 8 a n d t h e s h a r p decrease of t h e mrjment u i t h t h e angle between t h e plane of t h e benzene ring a n d t h a t of t h e gr lum and the saturation elyect in the rcgion betwzern that d z e r c vibrational perturbation is predominant (emax :3,jl)) nnd thc zisz~alupper limit of ahsorptitlities for the secondary band (3 x IIP). App1ications.~-The present study has been of decisive help in making band assignment for two import a n t groups of benzene derivatives.?‘ I t has also facilitated the characterization of benzenoid cornpounds and helped rule out several structural possibilities and choose the correct one. I t has been used to detect contamination in new benzene deriva t’ives. In a particular case an apparent erroneous literature citation was corrected.2i It may be emplujed in the study of the extent of resonance in resonance-inhibited benzene derivatives. Moreover, it is probably one of the most sensitive and simplest techniques known for the detection of distortion of the benzene nucleus and steric effects of substituents. The preceding considerations show t h a t the concept of migration moments as presented here is not only a correlation of fundamental significance but also a useful tool. Experimental Two instruments were used: a Hilger “rvispek” Model H700 spectrophotometer and a Cary Moticl 11 hlS spcctrophotometer. Their calibrations were checkctl with benzene of spectroscopic quality and with m-bisjtrichlorornethyl)benzc~ie. Matched pairs of stoppered quartz cells were employed. Benzene-free cyclohexane was used. Most of the chlorinated suhstances were prepared in the Barcelona laboratory in a high state of purity. T h e rest of the substances were high-quality chernicals purcliased from Matheson Coleman and Bell, Eastrnan Kodak, or K and K laboratories. The purity of most of liquid substances was checked by vapor phase chromatography o n a Barber-Coleman Model 61C gas chroinatograph.
Acknowledgments.--The present work was initiated and developed in the Barcelona laboratory uIider Contract *AF 61(052)-141 with the Office of Aerospace Research, United States Air Force. I t was extended with the inclusion of crucial data, concepts, and supplemental experimental data a t .ARL. 51. B. wishes to express his appreciation to the Ohio State University Research Foundation for an appointment which made possible his one year stay a t ;’RL. The authors are also indebted to Dr. Juan Castafier, Cniversity of Barcelona, for his valuable assistance, and to Professor Ralph S. Becker, University of Houston and a referee for their criticism of the manuscript.
