JOURNAL OF MOLECULAR SPECTROSCOPY 94, 55-68 (1982)

The Methylbenzenes vis-&vis Benzene

Comparison of Their Spectra in the Valence-Shell Transitions Region

A. BOLOVINOS, J. PHILIS, E. PANTOS,’ P. TSEKERIS, AND G. ANDRITSOPOULOS

Department of Physics, University of loannina, loannina, Greece

Absolute extinction coefficient and oscillator strength values of nine gaseous benzenes (tol- uene, o-, m-, p-xylene, 1,2,4-, 1,3,5-trimethylbenzene, 2,3,5,6-tetramethylbenzene, penta- methylbenzene, and hexamethylbenzene) were recorded with moderate resolution in the region of the lower valence-shell electronic transitions (4 to 7 eV). The spectra are very similar to benzene except for a new band that appears in the heavier substituted compounds.

INTRODUCTION

This work is part of a series (l-3), where the spectroscopic properties of benzene and substituted benzenes are examined over the whole region below the first benzene ionization potential. Relative uv-vuv absorption spectra of gaseous toluene and xylenes were published years ago (4). Most reported values of extinction coefficient E and oscillator strength f are taken from solution or solid phase spectra, so the bulk of our gaseous e and f values appear for the first time.

The advantages of examining a whole series of substituted compounds have been pointed out before (3).

From the work done up to now on methylbenzenes, it is found that the effects of alkylation on the benzene r-electron spectrum are not large and in all cases the pattern of transitions to the valence states I&, ‘BIU, and ‘El,, can be readily dis- cerned. In the lower symmetries of these molecules the state symbols are not those of the benzene D6,, point group (see Table 2 of Ref. (3)), but, in order to emphasize the close relationship of these spectra to that of benzene and facilitate our analysis, we will keep this technically incorrect notation. We should point out also that the intensity of the different bands depends on the electronic wavefunctions of the respective ?r MOs whose symmetry is not seriously affected upon methyl substi- tution; so, although the topological symmetry is drastically altered, the electronic one of the ring ?r electrons is not.

From theoretical calculations (5, 6), too, one can see that methyl substitution does not cause serious changes on the energy and relative order of the different occupied MOs except for a lifting of degeneracy and a progressive decrease of the ionization energies with increasing substitution due to the inductive effect of the methyls. The inductive and hyperconjugative action of the methyls has been treated by Petrouska (7) using low-order perturbation theory. His theoretical predictions

’ Present address: Daresbury Laboratory, Warrington, England.

55 0022-2852/82/070055-14$02.00/O Copyright Q 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.

0 a

5.0

6.0

7.0

8.0

9:o

EN

ER

GY

- eV

FIG

. 1.

The

gas

abs

orpt

ion

spec

tra

of (

a) b

enze

ne,

(b)

tolu

ene,

(c

) o-

xyle

ne,

(d)

m-x

ylen

e,

(e)p

xyle

ne,

(f)

1,2,

4_tr

imet

hylb

enze

ne,

(g)

1,3,

Wri

met

hylb

enze

ne

(mes

ityle

ne),

(h

) 2,

3,5,

fSte

tram

ethy

lben

zene

, (i

) pe

ntam

ethy

lben

zene

, an

d (j

) he

xam

ethy

lben

zene

. In

stru

men

tal

band

pa

ss

LB

. =

2.5

A.

ABSORPTION SPECTRA OF METHYLBENZENES

:5 0 0

x

0 3 -- E

; -- ”

1 .-.’ ._

1 --

45 5.5 6.5 7.5 a5 ENERGY-eV

FIG. l-Continued.

BOLOVINOS ET AL.

a3 0.3 7.!J

ENERGY-c’4

FIG. l-Continued.

ABSORPTION SPECTRA OF METHYLBENZENES 59

7

I 7

+5 z W

0

-3 LL LL W

0 Y’

6.5 7.5 a5 ENERGY-eV

FIG. I-Continued.

60 BOLOVINOS ET AL.

compare well with the relative changes of ‘Bzu band intensities and ‘Blu and rElu frequency shifts observed in spectra taken mostly in solutions. Finally, high- resolution gaseous spectra of lightly substituted molecules have been taken photo- graphically and vibrational analysis of the ‘Al, - ‘B2. transition has been performed (8-12).

EXPERIMENTAL DETAILS

The experimental setup and substances used have been described elsewhere

(I, 2).

RESULTS AND DISCUSSION

Figures la to j show the spectra of toluene, o-, m-, pxylene, 1,2,4-, 1,3,5-tri- methylbenzene, 2,3,5,6_tetramethylbenzene, pentamethylbenzene, and hexameth- ylbenzene. Benzene (I) is included for comparison. The ordinate axis gives the molar extinction coefficient 6 in absolute units (liter/cm l mole) and the instrumental band pass (I.B.) is 2.5 A. Spectra with smaller I.B. were also taken in order to investigate possible additional structure.

Table I contains the O-O and E,, energy positions as well as the f values of the observed transitions. Most of the references we include on the theoretical posi- tions of the lBlu and ‘El,, bands are not clear whether they speak of O-O or t, frequencies. Also, practically all the f values taken from the literature are from condensed phase measurements. Our energy position errors are ;50.005 eV for ‘A,, - ‘BZU transitions and SO.01 eV for most of ‘Al, - ‘Br,, ‘EIU. The relative error of the c values is ;SS% in the peaks and higher in the valleys of the spectra. The relative error of the f values is 515, 512, and 510% for the ‘&,,, lBlv, and ‘Elu bands, respectively. The calculation of the f values of overlapping bands was carried out by approximating each side of the strong band system (e.g., ‘El,) with a tail extending under the overlapping transitions. We then decomposed the spec- trum into separate bands and intergrated each one of them.

The main features of the methylbenzene spectra resemble those of benzene, i.e., the known benzene transitions ‘A,, - ‘&,, ‘B,,, and lElu can be easily traced on the basis of their energy position and E and f values.

‘Bzu Bands

The first valence transition becomes progressively more diffuse as the number of methyl substituents increases and spectra taken with higher resolution do not reveal more structure. Thus, apart from band congestion effects, band broadening due to an increased rate of radiationless transitions is very possible.

The IAl, - ‘Bzv transition becomes allowed in compounds of D2,,, C,,, and C, symmetry (3). This is reflected in the higher f values, the richer vibronic structure, and the appearance of the O-O transition. The O-O origin is present in all molecules of such symmetry and it is usually the strongest band. This implies the absence of any serious shift of the excited electronic potential surfaces.

The O-O transition is shifted to the red (Table I) upon methylation. These red

TA

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En

ergy

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+

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4.518

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5.83

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4.462

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62 BOLOVINOS ET AL.

FIG. 2. Vibrational analysis of the ‘A,, - ‘& transition of gaseous (a) toluene (I.B. = 0.8 A), (b) o-xylene (LB. = 1.25 A), (c) m-xylene (I.B. = 1.25 A), and (d) p-xylene (LB. = 1.25 A).

shifts as well as the changes in the f values for the different molecules are mostly in good agreement with Petrouska’s theoretical predictions (7).

In Figs. 2a to d we present spectra of this transition for toluene and the xylenes taken with higher resolution. The change in the c values of the spectral peaks from the ones of Fig. 1 are due to the effect of the different resolution (I). The assignment of several vibronic transitions is made according to earlier analyses of photograph- ically recorded spectra (8-10). Table II gives the energies of the observed vi- brations in the excited and ground state as well as their symmetry (in Wilson’s notation (22)). In o-xylene many of the transitions to the red of the assigned ones are due to nonbonded interactions between the adjacent methyl groups (10).

‘B,,, Bands

‘Blu bands are more or less broad or diffuse and they overlap seriously with ‘E,,. They are also red-shifted as we can see from Table I.

The O-O transition is easily seen in toluene and p-xylene. On the other hand, the

ABSORPTION SPECTRA OF METHYLBENZENES 63

l-

z

w -3

0 -

L

112

w

0

VI

* 450 475 5.00 5

ENERGY-eV

FIG. 2.-Continued.

peaks at 5.89 and 5.83 eV of o- and m-xylene, respectively, are not expected to be the O-O transitions, because they show strong deviation from Petrouska’s cal- culations. On the contrary the weak shoulders at -5.78 and -5.74 eV, respec- tively, are in agreement with these calculations and we tentatively assign them as O-O ones.

As far as vibrational structure is concerned we have to mention the following: In toluene there appears a progression of the totally symmetric 12((~,) normal vibration (Fig. 1 b) with a frequency value of -900 cm-‘. In p-xylene, with the help also of spectra taken in Nz and Kr matrices (23, 24), the observed structure can be attributed to the ~(LQ and 7a(cu,) normal vibrations (Fig. le).

The broadness and diffuseness of the remaining spectra prevented us from any further assignments of O-O transitions and vibronic structure. Finally, the absence or weakness of the O-O transition and the shape in general of the Franck-Condon envelope suggest that the geometry of the ‘Blu state is different from the ground state one.

64 BOLOVINOS ET AL.

Energy

TABLE II

and Symmetry of the Observed Vibrations in the ‘Al, - ‘B,, Transition

of Toluene and o-, m-, and p-Xylene

Vibrational Vibrational Corresponding Compound Vibration frequency frequency

(excited state) (ground state) smQ..Y ~fe&22

cm-’ an-’

A 456 522 a, 6a

B 528 623 b2 6b

Toluene C 751 788 al ’

D 932 1005 a, 12

E 964 1030 a, 8a

F 1189 1208 a1 13

A 507 582 a, 6a

B 692 733 Ql ’

o-xylene C

D

939

1195

1052

1222

6;s

a, 7a

A 470 515 or 536 b2 or a, 6b or 6a

B 675 724 a1 1

m-xylene C 965 999 a1 12

D 1145 1249 a, 13

E 1250 1264 or 1375 b2 or 6:s j or

p-xylene

A 367 457

B 552 648

C 775 209

D 802 021)

E 1185 1204

“s 6a

b3p 6b

blE! 10a

*g ’

ari 7a

‘El,, Bands

‘El” bands are the most intense since they correspond to the allowed ‘A,, - lEIu transition of benzene. They are also red-shifted and in some of them we may assign a O-O transition (Table I). A theoretical prediction (14) of O-O positions and split- tings might imply that our assigned O-O and c,,,.~ energy positions for the xylenes and 1,2,4_trimethylbenzene could be the two nondegenerate O-O origins expected from the lowering of molecular summetry. Yet, work on Kr matrix (23) spectra of toluene and m-xylene did not reveal such a doublet while it did so for p-xylene.

ABSORPTION SPECTRA OF METHYLBENZENES 65

p-xylene is the only molecule among the methyl-, halogen-, and azabenzenes, where the splitting of ‘El,, has been observed with enough certainty. Thus, with the assumption of two electronic origins O-O,, 0-Ob, which have a separation of -420 cm-’ in our gas spectra, we can analyze the ‘Elu band system in terms of progressions of 1 ((Y,) and 7a(a,) vibrations which have frequency values of -760 and - 1200 cm-’ in the excited state and 829 and 1204 cm-’ in the ground state, respectively. These progressions can be seen on Fig. le.

Some vibrational structure is seen also in m-xylene, 1,2,4-, and 1,3,5-trimethyl- benzenes. In m-xylene there appears one vibronic transition due to the 12((~~) normal vibration with a frequency value of -950 cm-’ in this excited state. In 1,2,4-trimethylbenzene the peak at 6.57 eV is -700 cm-’ away from the O-O band and may be attributed to a C-H out of plane bending vibration (27). In mesitylene a weak transition at 6.38 eV and a strong one at 6.45 eV can be attributed to 16a(c”) and 12(ar’,) normal vibrations with frequency values -410 and - 1000 cm-’ in this excited state and 454 and 995 cm-’ in the ground state, respectively.

The A,, - E,, O-O transition of benzene is taken to be the peak at 6.87 eV (24, 25). The values given in Table I for the same transition in the substituted molecules are in good agreement with Petrouska’s calculations (7). Actually we were helped by these calculations, too, in choosing some of them instead of others existing in

2500 2400 2300 2; co

FIG. 3. The ‘A,, - C transition of (a) 2,3,5,6_tetramethylbenzene, (b) pentamethylbenzene, and (c) hexamethylbenzene.

66 BOLOVINOS ET AL.

TABLE III

Expected Energy Values EC, Which Have a Term Value of 2.90 eV

Compound I.P. E E

eV eV

Toluene 8.825 5.92

o-xylene 8.55 5.65

m-xylene 8.55 5.65

p-xylene 8.445 5.54

1,3,5- 8.40 5.50 trimethylbenzene

1,2,4- 8.59 5.69 trimethylbenzene

NOM. The IP values are from Ref. (2).

longer wavelengths. Specifically the transition at 6.56 eV in o-xylene is a Rydberg one (26), the transition at 6.47 eV in m-xylene might be a Rydberg (2) one and it is not seen in cold matrices (23), and the transition at 6.37 eV in 1,2,4-tri- methylbenzene might also be a Rydberg one (2). In 2,3,5,6_tetramethylbenzene we can distinguish three transitions at higher wavelengths (6.09, 6.21, and 6.36 eV) than the assigned O-O one. The first of them at 6.09 eV might by a Rydberg one (2), while the others are not assigned.

New Bands

The systematic examination of the whole series of methyl-substituted benzenes has not been as fruitful in revealing bands not seen in benzene as in the fluoro- benzene spectra (3). Only in tetra-, penta-, and hexamethylbenzene a broad and rather weak band C is observed on the red side of ‘B,,. A shoulder at -5.5 eV in mesitylene and a weaker one in 1,2,4_trimethylbenzene may be of the same nature. There exists also the possibility that this shoulder is the O-O transition in the case of 1,2,4_trimethylbenzene while for mesitylene the O-O transition is forbidden. This C band had been observed earlier in hexamethylbenzene (28). The oscillator strength of the C bands are included in Table I and their error is -30%. We present these bands in Figs. 3a to c after subtracting the overlapping ‘A,, - ‘Blu contribution.

The C bands are absent in the solution spectra of these molecules (29). This implies that they may be Rydberg or charge transfer (CT) bands. Their f values are compatible with either designation. Theoretical calculations in toluene (25) predict several CT states below 7.0 eV. On the other hand, the possibility for the C bands to be of ‘EZg ~-a* parentage is not very strong, since they should not disappear in solutions. Also they would be symmetry forbidden in tetra- and hexa- methylbenzene. Nevertheless the possibility of a 3s Rydberg band assignment might be strengthened for the following reasons.

The first ionization potential (IP) of tetra- and pentamethylbenzene has been found (2) to be 8.32 and 8.30 eV, respectively; thus, the corresponding term values

ABSORPTION SPECTRA OF METHYLBENZENES 67

for the C bands of these two molecules are -2.90 eV. The 3s Rydberg transition of benzene, which has been seen (2, 30-33) at 6.33 eV, has a similar term value of 2.92 eV. If this similarity of term values also holds for the other compounds we would expect that similar bands could appear at the energies E, given in Table III. Actually all the compounds, except toluene, do show very weak shoulders on the red side of ‘Blu band and in the region of the predicted energies. In toluene this energy value falls on top of ‘Blu and is not expected to be seen. Yet, we should not forget that a one-photon 3s Rydberg transition is symmetry forbidden in p-xylene, 1,3,5trimethylbenzene, tetra-, and hexamethylbenzene (2). So, if these bands have any Rydberg character, they should have to be either of a Rydberg-valence con- jugate type or the symmetry would not be that good for these molecules or molecular states.

Multiphoton ionization (30) and high-resolution electron impact (31) spectros- copy would be quite helpful in giving definite assignments. These methods could also clarify the nature of the bands we have already mentioned.

Rydberg Bands

The analysis of the Rydberg bands has been carried out elsewhere (2). We observe that the background in this region is higher compared to that of benzene, while in the fluorobenzenes the opposite is observed (3). It may be that this increased absorption is due to the contribution of several c - ?r*, and CT transitions which are either forbidden in benzene or are situated at lower wavelengths.

Note added in proof After this work was accepted for publication, solution spectra of hexa-, penta-, tetra-, and 1,3,5-trimethylbenzenes (American Petroleum Institute, Research Project 44) came to our notice, where the C bands do appear. After this we would suggest that these bands might be rydberg or CT or ‘Et valence-type bands and the discussion about their possible rydberg nature continues to hold, since rydberg states can exist in solutions (21) in some cases.

ACKNOWLEDGMENT

One of us (A.B) acknowledges the National Hellenic Research Foundation (E.I.E.) for a research scholarship.

RECEIVED: December 4, 198 1

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68 BOLOVINOS ET AL.

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