The ultraviolet spectra of some methyl-substituted aromatic hydrocarbons

Spectrochimba Acta, 1960,Vol. 16, pp. 1060to 1075. Pergamon PressLtd. Printed inX&hemIreland

The ultraviolet spectra of some methyl-substituted aromatic hydrocarbons* R. NORMAN JONES and ERNEST SPINNER? Divisionof PureChemistry,NationalResearchCouncilof Canada, Ottawa, Canada (Received

12 May 1960)

Ab&&--The ultraviolet absorption spectra of anthracene, phenanthrene, 3 : P-benzphenanthreneand several of their methyl and methylene derivativeshave been measuredat - 100°C in n-pentane solutionand at room temperaturein n-hepptane solution. The effects of the position of methyl substitution on the band envelopeshave been analysed. This work is in continuation of similar studies on methyl derivatives of 1: 2-benzanthraaenewhich have been described in a previous publication. Introduction

IN THE ultraviolet absorption spectra of condensed-ring polynuclear aromatic hydrocarbons, three distinct groups of bands can usually be distinguished below These band groups were first recognized empirically by CLAR [l] 45,000 cm-l. who called them cc-, p- and j3-systems. Subsequent theoretical work, initiated by COULSON [Z] and by KLEVENS and PLATT [3], and refined and extended by ~~OFFITT, CRAIG and others [4-B] has provided a theoretical basis for this system of band ~lassi~c~tion . These electronic band systems exhibit vibrational substructure, and in the spectra of methyl-substituted hydrocarbons the contours of the band envelopes may be modified by the position of the methyl group. Earlier publications [7, 81 from this laboratory have dealt with methyl derivatives of 1: 2-benzanthracene (I), and it has been observed that for this hydrocarbon the vibrational fine structure between 25,000 and 33,000 cm-l can be separated into two interpenetrating sets of bands. The position of methyl substitution determines the relative intensities of the two band sets, and their displacement relative to one another. It has also been observed that all the absorption to the low-frequency side of the p-band system can be fitted by one such pair of interpene~ating band systems, and it has therefore been questioned whether separate a- and p-band systems are observed in this spectrum. The present paper deals with a similar study of anthraeene (II), phenanthrene * Publishedas ContributionNo. 5799 from the Labor&o&s of the National Research Council of Canada. t National Research Council of Canada Postdoctorate Fellow. Present address: The John Curtis School of Medical Chemistry, The National University of Australia, Canberra. Australia. [l] E. CLAR, Aromatisohe .kohZenwassersto~e (2nd. E&.) p. 186. Springer, Be&n (1952). [2] C. A. COULSON, PTOO. Phys. Sm. (London) 60, 257 (1948). [3] H. B. KLEVENS and J. R. PLATT, J. Che?ra. Phys. 17, 470 (1949); J. R. PLATT, Ibid. 17, 484 (1949); 1168 (1950).

[4] W. M&F&T, J. Chew. Phys. 22, 320 (1954). [5] D. P. CRAIG and P. C. HOBBIRS, J. Chem. Sot. 539 (1955); D. P. CRAW, 2. Chew. Sot. 2302 (1955). [S] N. S. HAM and K. RUEDENBERG, J. Chew. Phya. 25, 13 (1956). [?] C. SANDORFY and R. N. JONES, Ca=. J. Chem.34,888 (1956). [S] R. N. JONESR;nd C, SANDORFY, Canadian National Research Council Bulletin No. 4 (1956).

1060

The ultraviolet spectra of some methyl-substituted aromatic hydrocarbons

(III), and derivatives,

3: 4-benzphenanthrene (IV) together with fifteen of their methyl and the bridged hydrocarbon 4 : 5-methylenephenanthrene (V). 2’

I

II

3’

6

6

Experimental The spectra were measured at room temperature in n-heptane solution and at - 100°C in n-pentane solution using a Cary model 11M spectrophotometer. The experimental technique has been described previously [7, 91. The majority of the compounds were kindly provided by Prof. M. S. NEWMAN and Dr. P.M. G. BAVIN of the Dept. of Organic Chemistry of the Ohio State University and are listed in Table 1. The low-temperature spectra reproduced in this paper have been selected to illustrate specific points in connection with the discussion, and the complete collection of curves will be published separately [lo] together with tables of the band positions, separation intervals and intensities.

Results This section is concerned with a description of the individual spectra. The term “band progression” is used to describe a set of bands separated by a repeating [9]R. N. JONESand D. S. KEIR, Can. J. Chem. 34, 1017 (1956). [lo] R. N. JONESand E. SPINNER,Canadian National Research Council Bulletin No. 8 (1960).

1061

R.

NORMAN

JONESand ERNESTSPIINNER

wavenumber interval, such as ao, a,, a2, a%, a*, as in Fig. 1. The term “band series” is used to designate a repeating group of successive bands such as a,, b, co in Fig. 1. CLAR’S subdivision of the spectrum into M-, p- and B-band systems will be retained, because it is useful for descriptive purposes, particularly where the nature and symmetry of the electronic and vibrational transitions are not yet known. Table 1. Sourcesand melting point characterizations of compounds studied

-

M.P. CC)

Compound

Source

-I-

Anthracene l-Methyl&nthr~cene Z-Methylanthr~cene 9-i+@thylanthracene 1: t-Dimethylanthracene 9 : IO-Dimethylanthracene Phenanthrene 4-Methylphenanthrene 9-Methylphonanthrene 9 : lo-Dimethylphenanthrene 4 : 5-Methylenephenanthrene 3 : 4-Betizphcnanthrene 5-Methyl-3: 4-benzphenanthrene &Methyl-%: 4-benzphen~nthrene 7-Methyl-3 : 4-benzphen~threne I)-Methyl-3: 4-benzphe~threne Z-Methyl-3 : 4-benzphenanthrene 1-Methyl-3: 4-benzphenanthrene 4’: 5-Dimethyl- : 4-benzphenanthrene * t z 8

216 85-86 208 91-92.5 83 179-181 98.5-99 52 91-92.5 144-144.5 114-115 6667 139-141 80&-81.5 54.5-55‘5 64.5-65 70.5-71-5 77-77.5 106-107

0 * * i 0 * * * ; : : : t

P. M. G. BAVIN, The Ohio State University. M. 8. NEWMAN, The Ohio State University. L. F. FIESER, Harvard University: Sample purified in this laboratory.

I. Anthracene The spectrum of anthracene (Figs. 1 and 2) consists of two main parts, the p-band system extending from 25,000 to 35,000 cm-l and the more intense p-band system at higher frequencies. A weak a-band system is presumed to lie beneath the p-band system [3], but has not yet been observed with certainty. by a The p-band system. The p-band system of anthracene is dominated prominent progression of six bands (a,, n = 0 ---)r5), together with two weaker progressions (b,, n = 0 + 3; c,, n = 0 -+ 2). The successive members of these progressions are separated by 1440 f 20 cm- l. The b,-a, separation is approximately 390 cm-l and the q-a, separation 790 cm-l. These spacings are most simply described by a combination of a 1440 cm-l interval (Y& with a 390 cm-l interval (Ye) as suggested by SIDMAN [ll]. CRAIG and HOBBINS [12] have shown [II] J. SIDMAN,J. Chem. Fhgs. 25, 115 (1956). [12] D. P. CRAICA and P. C. HOBBINS,J. Che9n.Sot. 2309 (1955).

1062

The &ra.vk& QM& a.

is

a

Q-0

spectra of some ~e~hy~-s~bst~t~~d

~~~~6~~~~~~

eke

n,

p~~~66~~~

is

arorrzatio hydrocarbons BS&gxHA

k?

B$-) +

WYz7 %EE b,

p~o~~ess~o~tu a0 + nor%+ Q and tkte c, progression to at- + nag -+ 2~~. The p-band systems of the m~thy~anthracenes are similar (Table 2) though substitution at position 2 lowers the band intensities. The relative intensities of the a, b and c progressions 81‘8not appreciably changed and there are no regular

34

WriVENUMkXR

Fig. 2. Ultraviolet

spectrum

X 10m3

of anthraoene,

32

39

88

26

24

km-9

suggested system of vibr&onal

Imalysis,

displeoements in the b,-a, or ~~4, spacings comparable with those observed for methyl substitution in 1: Z-benxanthracene 17-j. Earlier studies of substituted anthracenes have shown ?&at a substitute&, at position 9 produces a eonsidexx.bl,y larger bath~~hrom~~ shift of the Q~ band than does sub~t~tut~o~~ at other ~~~t~~ns and it was inferred that the p-band

R. NORMAN JOKIM and

ERNEST SPINNER

system is polarized along the shorter in-plane axis [13j. This was subsequently confirmed by polarization measurements of CRAIG and HOBBINS on anthracene crystals [ 121. The O-O shifts for the methylanthracenes given in Table 3 are more accurate than the earlier data, and show the same effect. CRAIG and HOBBINS also observed the a, b and c band series in anthracene crystals at -250% but Table 2. Band separations in the spectra of anthracene and derivatives in n-pentane solution at - 100°C

Compound

Band separation

(cm-l) *

y2 fa,+I-a,f

% @PO)

f%i-zof

J_

Anthracene I-Methylanthracene 2-Methylanthracene Q-Methylanthracene 1: 3.Dimethylanthracene 9: LO-Dimethylanthracene

I

390 390 360 370 360 370

1440 1430 1460 1420 1450 1430

I

1370 1310 1390 1350 1360 1320

1070 780 800 740

* These axe averaged values for all the progressions. The individual values are given in reference [lo].

at -140’ only the a and b bands were resolved. Our solution spectra on the other hand did not reveal the additional weak bands (C, P, and Q,) which these investigators found on the low-frequenoy edge of the O-O band for crystalline anthracene at -Z50°C. These might form part of the missing u-band system. The @band system. The absorption of anthracene above 35,000 cm-l is much Table 3. Positions of the a0 and q, bands in the spectra of anthracene and derivatives in n-pentsne solution at - 100°C p-System

/I-System

I

Compound _Ix;;:yi:I Anthracene l-~ethyl~t~~ene 2-~ethy~~t~aoene Q-~othylant~acene I : 3-D~ethyl~thra~ene 9: IO-Dimethylanthracene

26,870 26,340 26,440 25,830 26,230 25,040

I

/

230 130 740 340 1530

39,460 39,140 39,100 38,910 38,820 38,300

1

/

320 360 550 640 1160

* Displacement from position in anthracene.

more intense (Fig. 1) and the most prominent band (x0) is the O-O transition of the /?-band system [ 121. At room temperature the vibrational structure is not resolved, three peaks (x0, x1, zO) and two inflexions (yO, yI) are observed. but at -100°C The x~-x,, spacing (Q) is 1370 cm-l and is presumably associated with the same [13] R. N. JOXES, Chem. Rear.41, 353 (1947).

1064

type of vibrational ex&tation as ~a in the p-band system. The yo--zo spacing is about 300 cm-l but too indefinite fox accurate measuremeut~ Methyl substitution has little effect on the x and y bands but modifies the x,-band considerably the z,-band is not, observed, (Table 2). In g-methyl- and 9 : IO-dimethylanthracene while in l-methyland 1:3-dimethylanthracene it is intensified. The z,+rO separation varies over the range from 740 cm-l in 1: 3-dimethylanthracene to 1070 em-l in anthracene; possibly the z,-band is not part of the same electronic band system as the z- and y-bands, and in the g-substituted anthracenes it may be

32 WAVENUMBER

x IO-3

30

20

icm-‘1

et c-d. [fh], in studies of the spectrum submerged beneath the x, band. &?RNExDEE of antbracene vapor, noted a shoulder near 40,800 cm-l w&h may correspond to the z,-band in solution. All the b-band system is strongly displaced to lower frequency with change from the vapour to solution phase.

Phenanthrene

II.

The spectra of several methylphenanthrenes were measured by ASKEW 1151 and the spectrum of phenanthrene in ethanol solution at room temperature, at - 100°C in n-pentane is shown in Fig. 3. The M-, p- and @band systems are all Our proposed system for the observed and show vibrational substructure. identification of the vibrational structure is summarized diagrammati~a~y in Fig. 4. 5%~ a&& sysbem. In the a-band system of phenanthrene two prominent progressions a,@ = 0 + 3) and b,Jn = 0 - 2) occur as well as weaker c-, d- and all with an interval of approximately 1400 cm-l. At room e-progressions, temperature in n-heptane only the a- and b-progressions are resolved; on cooling, the others appear and the cc,-bands intensify relative to the b,-bands. If the a,,,-band is identified with the O-O excitation, these progressions could result from a 1400 cm-l vibration ( YJ in combination with intervals of 670 cm-l ( YJ> 800 cm-l (YJ and 1050 cm-1 (Y&. The bands designated e, and e, are assigned more tentatively to combinations of Ye + vg and vr + yS + pp. These intervals

6

1065

R.

NORMANJONES and EBNE~TSPINNER

can be correlated with the Raman-active ground-state vibrations of 1346 cm-l, 1037 cm-1 and 733 cm-l. Similar intervals occur in the spectra of the methyl derivatives (Table 4), but there are considerable differences in the relative intensities of the bands.

1 32 WAVENUMBER

x lO’3

30

28

h-‘)

Fig. 4. Ultraviolet spectrum of phenanthrene, suggested system of vibrational analysis.

In phenanthrene the b,-peaks are almost as strong as the an-peaks, but for all the methyl derivatives the @,-peaks are considerably stronger. This is shown for 9methylphenanthrene in Fig. 5. This would seem to be a true enhancement Table 4.

Band separ&ions in the spectra of phen~threne in n-pentane solution at - 100°C

and derivatives

Band separation (cm-l) * Compound

3

y3 be%J

--~~ Phenanthrene 4-Methylphenanthrene SMethylphenanthrene 9 : IO-Dimethylphen&nthrene

670

(800)

1050

__I_1400

(r,-o,) -I-

360

I 560

-

(1090)

1380

680

,

-

1040

1400

710

,

-

1140

1380

/ 1060 I

1430

2280

1220

-

1390

2320

(1050)

1410

2380

1210

1380

2370

1180

370

I / 380 / 1 360

I

1030

/ 1010

+ These sre avsra8ed values for all progressions. The individual values axe given in reference [lo]. Figures in parentheses are for inflexions.

of the j’-value of the @,-bands and not merely an increase in E,, at the expense of the band width. The positional displacements of the b,-a, spacing vary over the range 560 to 710 cm-l compared with 200 to 774 cm-r in the methyl-1 : 2benzanthracenes. The a-band system of 9: lo-dimethylphenanthrene (Fig. 6) shows anomalies which may be associated with the strain resulting from the crowding of the At room temperature the spectrum is of normal type, but at methyl groups. - 100°C the stronger bands split into doublets with the more intense component on This could result the high-fre~ueno~ side. The separation is about 170 cm-r. 1066

from a

“hot” band ~so~~a~ed with a grumxd state v~b~t~o~~~ I~el of f’70 cm-‘, but when the spectrum was me~xu.+ed at - %86PGin a glass of iso~~t~e~~o~rnal pentane the relative intensities were not significantly changed. The p-band system of phenanthrene oonsists of eight Tha p-band system. bands which appear to form two groups of four each (og, pa, (zo, r,) repeated after sn interval of 2280 cm-l (Q) (ol, pl, pl, rI), as shown in Figs,, f31and 4. The p-o,

q-o and P-O spacings rtre 360 cm-X (us), 1060 em-’ (~~‘6)and f&30 CM-~ (u,), respective/y. The spacings and relative intensities are much the same in the spectra of the??methyl derivatives (Table 4). The intervals Ye and q probably correspond with, P) and v4 in the a-band @y&em, and with the Raman-ective ground-state vibrations of 1037 cm-l and 1346 cm- l. The 360 cm-l vibration appears to have no counterpart in the cc-band system, and although the 1430 cm-1 separation occurs in this series it does not appear to play a dominant rola in forming a progre~&3n intervaL The 2280 orn~-~intezvat is most nrmrsual and it is diEicuft to it co&d resuft from a o -+ 2, correlate with any simple ~~~~~~-~~a~e vihrs%ion; X067

o-+4... excitationof a non-totally-symmetrical vibration. As will be shown later, the prominent 1420 em-x progression interval reappears in the p-band system of 4 : 5-methylenephena~~thre~e (V). !l%e B-band system. This system exhibits less fine struoture. The spectrum of phenanthrene at --1OO” (Fig. 3) shows a single progressian of four bands (z,, n = 0 --+ 3) with a spacing of approximately 1220 cm-1 (Y&. The separation decreases slightly as the progression is ascended. The spectra of the methyl derivatives are similar though the structure is less well resolved. O&W bands. The phenanthrenes all show an additional band (u) lying between the p- and P-band systems. Methyiation effects this band differently from either

0.6

!i

40."

,.

1 42

.. . . 40

.I 36

I, 36

,

34

t1 32

30

28' '

of its neighbors and it may be associated with a separate electronic transition. It resembles the L-band in the spectra of the 1: 2-benzanthracencs [7, 81. The spectrum of 4 : 5-methylenqh7henanthrene. The spectrum of 4 : 5-methylenephensnthrene (V) shown in Fig. 7 exhibits considerably more fine structure. This effect has been observed previously in other compounds containing a methylene bridge, such as 1’ : 9-methyLme: 2-benzanthracene [7, 161 and fluorene (161. In the a-band system there are numerous sharp bands which oan be associated with combinations of the intervals 160 cm- I, 470 cm-l, 1020 cm-l, 1390 cm-l and 1490 cm--I [to]. These bands fall into groups which can be identified with the a-, b- and c-progressions of the phenanthrene spectrum, though only fto remains as a single band and the other bands are split into several components. It is probable that the spectra of the other phenanthrene compounds would split up in a similar fashion at lower temperature or under other conditions favoring increased resolution of the vibrational structure. The p-band system of 4 : 5methylenephenanthrene also differs in that the r0 and rr bands are missing and the a? p, q-band series is repeated twice with the recurrent spacing in 1420 cnl-iv The @-band system exhibits three bands with a spacing of 650 cm-l. The u-band is fully resolved and there is a o1ea.rindication of a weaker v-band accompanying it. jfB] R, N. JOXES,J. Am C%em.Sm. 6’7, 2127 (f945f. 1068

The ultraviolet spectra of scme lnethyi-substituted aromatic hydrocarbons

The lowest frequency band of each band system may be The O-O met~yZ shijk tentatively identified with the O-O transition [f7]. The displacements of these bands with methyl substitution are summarized in Table 5. Methyl substitution does not always lower the band frequency, thus the displacement appears negative for the x,-band of 4-methylphenanthrene and for the a,,-band of 4: li-methylenephenanthrene. The latter compound also shows the maximum positive shift These data serve to emphasize that for the first band of the p-band system. methyl substitution effects are apt to differ for the CC-,p- and p-band systems. Table 5. Positions of the ac, oe and x0 bands in the spectra of phenanthrene and derivatives in n-pentane solution at - 100°C I

or-System Compound ~_-..._.

i

(cm-l)

j

28,930

__--

i

28y6Qo 28,740

1

28,540? 29,020

/

@System

~. Shift* (cm-l)

aa

Phenanthrene 4-Methylphenanthrene Q-Methylphenanthrene 9 : 10-Dimethylphenanthrene 4: &Methylenephenanthrene

p-System

/__-

-, ! !

240 190 390t -90

A_

00

(cm-l)

-_

Shift* (cm-l)

34,080 33,600 33,700 33,470 33,280

* Displacement from position in phenanthrene. t An alternative assignment would be 28,350 cm-l with shift of 570 en-l

III.

! j _I

480 380 610 800

-__

!.

I

(cm-l)

Shift * (cm-l)

39,650 39,700 39,340 39,090 39,490

-50 310 560 160

X0

-

L

(see Fig. 5).

3 : 4-Ben.zphe?z.nnthrene

The spectra of 3 :4-benzphenanthrene and its monomethyl derivatives have been measured previously by BADGER and WALKER [IS] in ethanol solution at room temperature. The low-temperature spectra in n-pentane show more fine structure, but the resolution is still poor, particularly for the a-band system. The a-, p- and /?-band systems are crowded together, and both the CL-and p-systems fall on the steeply rising shoulder of the stronger ,&system (Figs. 8 and 9). a,-, b,The u-band system. In the N-band system of 3:4_benzphenanthrene, and e,-progressions are observed, the strongest band being a,. The progression interval (us) is 1350 cm-l and the b,--a, and c,--o, spacings are 400 cm-l (Q) and 680 cm-l (P%), respectively. The general pattern of this absorpt,ion resembles that of the u-band system of phenanthrene, though the spacings are diminished. For the methyl derivatives the C%-CS, spacing varies over the range 670-850 cm-l. The principal progression spacing (Ye) is reduced to about 1000 cm-l for 5-methyl and 4’ : 5dimethyl: 4_benzphenanthrene, but, with these exceptions, y2 and ~a are little affected by methyl substitution (Table 6). The spectrum of &methyl3 :4-benzphenanthrene shows an additional small band 350 cm-l below a, [lo]. and its methyl derivatives the The p-band system. In 3 : 4-benzphenanthrene p-band system consists of six bands which lie on the rising shoulder of the /?-band system. The spacings show no obvious progressional pattern, but the bands show [17] D. P. CRAIG, Revs. Pure and Appl. Chem. (Australia) 3, 207 (1953). [lS] G. M. BAI)GER and I. S. WALKER, J. C&m. Sot. 3238 (1954).

1069

R. NORMAN

JONESEII~ERNESTSPIINNER

p-BAND SYSTEM _

6-

0.16-

4-

O.OE-

Z-

\

I

IIl,,,II

34 WAVENUMBER

Fig. 8. Ultraviolet

-I

32

x 10m3

km-‘)

spe&rum of 3:4-benzphenanthrene

I’I

30

at -100°C

in n-pentane solution.

I”“,““‘1

I”“““‘1

I2

4.8 t

IO 8-

3.2

6-

2.4

4-

I.6

2-

-i 34 WAVENUMBER

Fig. 9. Uttraviolet

spectrum

i

32 tom3

of 3:4-benzphenanthrene, analysis.

30

(cm-‘)

suggested system of vibrational

some similarity to the first six bands of the p-system of phenanthrene (cf. Figs. 3 and 8), and the 3: 4-benzphenanthrene bands have therefore been labelled oO, p,,, If this analogy with phenanthrene is valid, the additional bands q1 (IO?To, 01. and r, are submerged beneath the stronger p-system absorption. The spacings for 3:&benzphenanthrene are p,-o,, 510 cm-l (Y& qo-oo, 1020 cm-l (Ye), and ~~-0~ 1310 cm-l (Q). In four of the methyl-3:Cbenzphenanthrenes the qo- and r,-bands are not io70

The ultraviolet spectra of some methyl-substituted

aromatic hydrocarbons

fully resolved at - lOO”C, and in this they resemble the spectra of the other comThe o,-band is usually broad and sometimes pounds at room temperature. unsymmetrical due to overlap with pI. The ox-o0 spacings ( vt) for the methyl3 : 4-benzphenanthrenes fall in the range 2330-2610 cm-l compared with 2250-2380 cm-l for the methylphenanthrenes. The /?-band system. There is more fine structure in the @-band systems of the 3 :4_benzphenanthrenes than in the P-systems of the other hydrocarbons Table 6. Band separations for the cx-and p-band systems in the spectra of 3 : 4-benzphenanthrene and derivatives in n-pentane solution at -- 100°C Band position

(cm-l) *

Compound y5

3 : 4.Benzphenanthrene l-Methyl-3 : 4-benzphenanthrene 2-Methyl-3 : 4-benzphenanthrene 5-Methyl-3 : 4-benzphenanthrene 6-Methyl-3 : 4-benzphenanthrene 7-ilIethyl-3 : Q-benzphenanthrene S-Methyl-3 : il-benzphenanthrene 4’: 5-Dimethyl-3: 4-benzphenanthrene

____--_-_ 400 i (680)

“6 @o-%J)

1350

1020

1 1310

2440

(700)

(

1330

1010

1260

2590

/ (750)

)

1380

(1130)

1320

2560

j

990

2610

/

340

;

670

430

1 /

830

1

850

;

1330

; (380)

* . , . I32Ot.. I 1000 ] (1190) I I . ‘ . 1140t. *

680

i

(1230)

/ (480)

. ‘ . . 12sot. .

2460

-

1

1050

1 (220)

. . . . 1270t.

2570

(570f

(220) -/

i j

i

(420)

(PO-“0)

450

~

1360 I

2380 2430

* Figures in parentheses are for inflaxions. t Asymmetrical blended band.

considered in this paper. Five bands (x0 -+xq) are observed for the low-temperature spectrum of 3 : 4-benzphenanthrene between 41,000 and 35,000 cm-l. These would appear to constitute a single progression but the spacings show a tendency to increase as the progression is ascended (Table 7) indicative of negative anharmonicity. All the methyl-3:4-benzphenanthr~ne show this effect, but it is most noticeable for the 4’:ti-dimethyl compound for which the intervals increase from 1100 cm-l for x,-x, to 1900 cm-l for x4-x3. Other bands. The 3 : 4-benzphenanthrene spectra also show two bands (u and v) on the low-frequency side of x0. The spacings suggest these are not part of the p-system (viz. the missing (I~-and r,-bands), but are analogues of the U- and v-bands of phenanthrene and are labelled accordingly. There is also an additional band at the high-frequency end of the spectrum (band-z) which is manifestly part of another electronic transition. The O-O methyl sh@‘L As for phenanthrene, the a,-, oO- and s,-bands are tentatively identified with the O-O transitions of the three electronic states. 1071

Table 7. Band separations for the /&band system in the speotra of 3:4-benzphen~t~ono and derivatives in n-pentane solution at - 100°C

_-~-

Band separat.ious (cm-‘)* Compound

/-_ x1-xo --

3 : &Benzphenanthrene l-Methyl:3 : 4-benzphenanthrene 2.Methyl-3 : 4-benzphenanthrene 5*&I&@-3 : 4-benzphenanthrene 6-MeLhyl-3 : 4-benzphenanthrene 7-NethyL3 : 4-benz~henanth~ne 8-MethyI-3 : 4-benz~henanth~ene 4’ : Ei-Dimethyl- : 4-~nzphenanth~en0

1330 1270 1360 3220 1280 F380 1310 1110

T

j

xZ-x1

x$-q

4%

1250 I.570 1380 (1310) 1510 lS30 1360 (1660)

1390 1300 1350 (1260) 1310 1340 1330 (1300)

1600 1490 1610 (1450) 1490 1420 1400 (I~ffO~

* Figures in parentheses am for inflexions.

Table 8. Positions of the a,,, o0 c~nd x0 hands in the spectra of 3: 4-bennzphenanthrene and derivatives in n-pentane at - 100°C

T

p-System Compound

a0

___3 : 4-~en~~he~nth~~~~~ fXdhyL3 : 4-benzphenEldhrene 2-Methyl-3 : 4-benzphenanthBxx? 5-Methyl~3 : 4-benzphenanthrone 6.Methyl-3 : 4-benzphenunthrena 7-Mathy1-3 : 4-benzphenanthrreno 8-Methyl-3 : 4-benzphenanthrone 4’ : fi-Dimeihyf-3 : Q-benzphen5nthrene

(cm-l) -_._

Shift * (cm-l 1

T

--

I

Shift* (em-l)

X0

(CXll-‘f 35,380

i

26,570

300

26,690

Shift* ftmP1)

--

_L

26,810

/3-System

-__~

100

35,150

230

180

320

35,140

340

26,630

240

760

34,760

620

26,660

210

35,160

220

26,770

100

100

35,160

220

26,730?:

140t

340

34,880

500

25,930

940

1340

34,x 10

129if

I

-10

* Displtlcsment from position in 3:4*be~~~~e~~th~ue, 1 An &ernative assignment of LE@ would be 26,450IX-~ with shift of 420 cm-“.

Tho displacements of these bands with methyl substitution are summarized in Table 8. For the monomethyl derivatives the oO and x,, displacements are maximal for substitution at position 5, and for this compound a planar structure is impossible without distortion of the C-CH, bond angle; BADGER and WALKER [lS] have associated the large O-O shift for this compound with the steriu effect and in co~fo~rn~ty witPhthis we observe a still larger methyl shift for the highly hindered 4’ : 5-dimethyl eom~onnd. The sequence of these methyl shifts for the various

band systems has been computed by BAD~~ER, and ~A~~~~ and by PETEES [193 ~o~~~~~~r-or~~~~l and valence-bond theory, but the agreement with the experimental results is not always satisfactory.

from

Discussion The vibrational

bands have been considered above for the individual compounds. The spectra all exhibit some common features as well ss individual differences, and in this concluding section WQ shall review them collectively in relation to the spectra of 1: Z-benzanthracene and naphthalene. The absorption of phenanthrenc and 3 : 4-benzphen~nthrene below 35,000 cm-i consists of distinct p- and x-band systems associated with different electronic transitions. In this region of the anthracene spectrum only the p-band system is distinctly identified though the two additional wes,k bands noted by CRtlra and Hoasr~s in the spectrum of the crystal could be part of a missing a-band system. Several investigators have made detailed studies of the spectrum of naphthalene, the most recent measurements being those of MCCLURE [20] on highly dispersed solid solutions of naphthalene in durene, and of CRAIG and collaborators [2l, 221 for the vapor and crystalline sttttes. For technical reasons these investigations have dealt only with the a-band system and the beginning of the p-band system near 34,000 cm-l. Superficial observation of the spectrum of 1: Z-benzanthracene suggests some analogy with that of naphthalene, with the two bands of lowest frequency constituting the x-band system, followed closely by the p-system, and PLATT and KLEVEBS have assigned the electronic transitions in these terms [3f_ Numerical analysis of the vibrational spacings indicates that more probably all the absorption of f : 2-benzanthraoene between 25,000 and 35,000 cm-l constitutes a single electronic system with two sets of interpenetrating vibrational systems. Alternntively there could be two electronic band systems but, unlike the usual pand a- systems, both would hsve to be of comparable intensity, I3etween 35,000 and 45,000 cm-i one intense band system is observed for anthracene, phenanthrene cznd 3 : 4benzphenanthrene with poorly resolved vibrational structure. The spectrum of 1 :&benzanthracene in this region is superficially similar to these, but there is again evidence of two interpenetrating sets of vibrational levels. For naphthalene the electronic system analogous to the above is displaced upwards near 45,000 cm-1- These spectra also exhibit a few additional bands vvhich, from their intensities and the directions and magnitudes of their displ~~en~ents on methyl substitution, are thought to involve separate These include the a- and v-bands of phenanthrene states of electronic excitation. and 3:4_benzphenanthrene, and the a-band of 1: 2_benzanthracene, all of which ocour on the low-frequency edge of the p-band system. In addition there is the anomalous z-band in the spectrum of anthracene. In general the introduction of a methyl substituent induces only a minor displacement on the vibrational band spacings ((200 cm-i), though it can cause

1073

~nsjde~ab~e change in the relative i~t~~sjtjesof the VjbratjQ~al subthese changes art-3 sensitiveto the r)Q&iQR of metby substjt~ltjon* More signiticanl changes fxx!ur where the jntrodu~tion of the methyl substituent causes steric strain. Increase in the resolution of the vibrational structure when a methylene bridge is present is partioularly striking. The work of MUCLURE,and of CRAIG and collaborators has shown that the a-band system of the naphthalene spectrum is long-axis pol,arized with respect to the electronic excitation, but coupling occurs with the excited-state vibration levels to give rise to three distinot iuterpenetrating series of bands. One of these is long-axis polarized while the other two involve short-axis polarized vibrations. The ele~tro~~~~l~~sport-axis polarized p-bauff. system of na~hth~~ene is simpleq and for this &l the a&i-Fe vibrational ex~~~~t~~~sare a&3.3shor%-sxis ~o~a~~~~, The detailed analyses of the n~~h~ale~e s~~~~rurnprovide a model for the in~~~ta~io~~ of the spectra of the more wmpI5x ~~~*~~~~~~~ arc~mra;tic hydracarbons, but these speotra cannot be analysed until adequate techniques have been worked out to determine the directions of the excitation ‘vectors for both the electronic and vibrational transitions; it will &XI be necessary to obtain the spectra under conditions where t,he vibrational structure is more completely resolved. In this p&per we are only attempting to deal with the partially resolved vibrational structure in a descriptive fashion. 11 is, however, relevant to speoulate abont the possible influence of the molecular symmetry on these spectra by anallogy with the more perfectly understood ~~~ld~ti~nsobt~i~~~g in ~a~btha~e~e and anthr~~~e* Because of the ~~~-s~n~~~etry of ~~~1~~~~~~~~an3 ~nthr~~ene~ aud %&e Q2.- ~mmetry of p~~~a~t~ene and 3 : ~-be~~~b~na~~t~e~~e~ it can re~onab~~ be assumed that all the in-plane efectronir: and vibrational expiration vectors will be polarized along one or other of the principal in-plane symmetry axes, fVIx+-VId), and that the intcracttion of these electronic and vibrational states will be controlled by rigid selection rules. The observations on the spectra of the methylanthracenes, m&thylphenanthrerres and methyl-3 : 4benzphenanthrenea lead to the conolusion that fox these hydrocarbons methyl substitution does not appreciably affect the spectrosoopically active energy levels. Nevertheless, the position of methyl substitutiou does ai%& the relative inteusities &he vibrational bands considerably and this may ~~~i~at~‘tfiat it reduces the ~~l~~~l~r s~~m~~~y ~~~~~~~~~y $0 refax a very

jnte~~ty

struatnre,and

9ome

of&e

f;hiike

se&&itx3 f&e

c&her

rrzk3ss, ~~~~~~~~~

axfoma%c

~~~~~~~~~~~

~~~~j~~~~

bre,

1:2-

elements of ~yrn~~etr~. Bei2ause of this, the in-plane a.xes of eieotronic and vibmtiond ~ol~~i~~ti~nwill probably ‘tieaskew to a rectangular co-ordinate system set up symmetrically with respect to the anthracrene moiety (Vie). It is probable, though not necessary> that the orientation of the polarization axes will be such that the angle 8, of Vie will lie between 19,of VIo. and zero. Under these circumstances the seleotian rules governing interaction between electronic and vibrational levels in the excited state will be less rigid and the vibrational &ructure more complex. ~n~thermore, the mass effects of the methyl subs~~i~~~e~t an the ~~ibratio~allevels in the eleo~roni~~lly excited molecule, as well as the eleot~~~~i~ e&&s of the me&y1 s~~~~~e~~ on the dist~bu~~o~ of the ~e~~~~~~~e~~~

has

no

in-pbe

‘The ultraviolet spectra of some methyl-substi~,ut~ aromatic hydrocarbons

electron charge density, may be more sensitive to the position of the methyl substituent than is the case for the more symmetrical hydrocarbons. Under these conditions methyl substitution may appreciably perturb the directions of the axes of both electronic and vibrational polarization and these changes need not necessarily occur in the same direction. The greater sensitivity of the 1:2benz~nt~aeene spectrum to the effects of methyl substitution could result from causes of this kind, and similar effects should obviously be sought in other polynuclear aromatic ring systems of low symmetry, such as 3:4-benzpyrene. Acknowledgements-We wish to thank Prof. M. S. NEWMAN and Dr. P. M. (1;. BAVIN of the Ohio State University, Prof. R. SANDIN of the University of Alberta, and Prof. L. I?. FIESER of Harvard University, who kindly provided the compounds on which these investigations are based. We have also enjoyed the opportunity of fruitful discussions with Prof. C. SANDORFY of the Universit6 de Mont&al.

1075