Infrared spectrum of a single crystal of benzene

VoIume 14, number 2

CHEMICAL PHYSICS LElTERS

15

May

I972

INFRARED SPECTRUM OF A SINGLE CRYSTAL OF BENZENE

M.P. MARZOCCHI ar.d H. BONADEO* IstiWro di &imica Fisica deli’Wniversi& Received The conclusive

P-5012i

$4 February

arsignmerzt of the infrared active exciton

Firefxre, Fhrencey Italy

1972

components

of benzene crystal has been reached

thanks

to tile spectrum in polarized fight of a thin single crystal, whose axes directions were determined by means of a polarizing microscope. Tlte v~br~tionai spectrum of crystalfine benzene has been calculated recently by several authors [l-S]

.

The validity of these calculations is very much bound to the availability of an unquestionabIe assignment of tile crystal modes. Unfortunately, despite the large amount of experiment& work accumulated until now, some ambiguities are still left in the identification of the peaks belonging to the infrared active factor-group species. Because of the lack of single-crystal spectra in polarized light under high resolution, most of the evidence for the assignment of the factor-group components has been gained from the analysis of the spectra of oriented polycrystalline samples. The difficulty of determining the crystal plane seen by the infrared beam is the true limitation of this technique. Actually in our recent investigation of the spectra of polycrystalline films in polarized light [6], we were

liquid cooled at 5.4’C and inseminated with a small crystal. After several days large crystals of more than 3 cm length for each direction were obtained. The handiing and the cutting of the crystal were performed at -2O’C in a box cooled with dry ice. A polarizing microscope, held at -20°C in a cooled box with the ocular emerging and equipped with a universal stage, was used, as in ref. 161, to establish the orientation af the crystal zes. Once the directions of the axes were

%i( 4,)

1

able to identify with certitude the B,, components of the factor-group splittings. For the other components the best we could do was io offer two altematiye a~igr~ments, the difference between them being the interchange of all peaks assigned to the B, u and B,, symmetry species. Later, however, we were able to grow a farge single crystal of benzene and io cut it into a parallelogram with faces perpendicular to the crystal axes. The crystal was used for the stuay of the Raman spectrum [7] and the success of this investigation suggested to us :o extend the single-crystal work to the infrared spectrum.. The single crystal of benzene was grown from the *Present address: Comision National de Energia Atomica, G&a, de Investigaciones, Avda. de1 Libertador 8250, Buenos..Afres, Argentina.

i

I

1320

1300

II

-

1160

?I 30

cm-’ Fk. I.vl,(bl,) and V15(b& of benzene in polarized light. Solid line, efectric vector paraUeE to tk 4 axis. Dotted line, electiic vector ar&el to the c 3xis The peak at 1306 cm-” 1sC ‘*Cs Hg species. is due to the

Volume

14,

number 3

Table 1 Assignmentof the infrared active internal modes ofcryrtailine benzene

a)

Frequencies (cm-‘)

crystal

molecule

85%

‘&h)

Modes

c&h) -.-

403.5 404.3 418

BIU B3u

681 682

Blu

707.5

B?_U

B3u

B3u

1008.5 1009.5

B3u

1010.0

B2u

1032.5 1033.6

Bzu B3u

1034.5 1038.5

B3u

1039.0 1039.5

BIU B?,u

1142.5 i 148.0 1150.3

Blu

1311.0 1312.5 1314.5 1474.6 1475.5 1477.2 1477.7 1480.0

VII (32u)

BIU B2u

47@2d

Blu

Blu

f33u

v12(bl

lecular mode, and precisely the peaks reported at 13 14.5 and 1150 cm- 1 respectively in ref. [S] , are

u)

missing. This confirms the previous assignment to the B,, species, since these crystal modes develop transition moments along the b axis. The peaks which ocv2o(el

u)

cur at 1311.0

w(b2u)

B2u

BIU f33u

vi4(blu)

Bzu BZU B3u B2u

h9hu)

B3u

BIU

were approximatively 3 X 2.5 X 0.5 cm along t-he (I, c and b directions respectively. After several attempts we have been able to cut from this large crystal a thin sheet ( =Z80~ thick) containing the ac plane. In addition the a and c directions on the sheet were known with sufficient accuracy. The sheet was thenrfied to the sample holder of a conventional low-temperature celi. The infrared spectmm at 85°K was measured with the aid of a Perkin-Elmer model 225, equipped with an AgBr grid polarizer. The spectrum in polarized light was run with the electric vector parallel to the LIand c axes respectively. Owing to the relatively large thickness of the sheet only the weakest bands could be observed, namely the sions of the final crystal

+Jb& V14(blu), Qb& and V16(ezu) molecular modes. In fig. 1 only vt4 and v15 are shown. As expected, one of three exciton components for each mo-

B3u

a)The detailed assignment

vlg(elu)

u16(e2u)

8311

974 976 978 986.8

15 May 1972

CHEhlICAL PHYSICSLEITERS

______-..of the components

-... .-

-_-

of vla_(blu)

and

cannot be established because of the occurrence of

Fermi resonance v&h several combinations. identified, the crystal was cut to a parallelogram, whose faces were perpendicular to the crystaliographic axes. The orientation of the axes with respect to the parallelogram sides was checked anew under the microscope and the procedure was repeated until the right orientation was achieved within 2-3 degrees. The dimen-

and 1142.5

cm-1

for vL4 and vi5 re-

spectively are polarized along the LIaxis and therefore are assigned to the B,, species, whereas the peaks at 1312,5 and 1148.0cmrespecrively are polarized along c and belong to the B,, species. Clearly we can now assign all the peaks of the spectrum and not only those observed in the present work. In ref. [6], the assignment derived in this work from the single-crystal to the assignment spectrum for v14 and v15 corresponds labelled II in the text. We conclude therefore that set II of the previous work is the proper choice for the right assignment_ This choice was also proposed in ref. [5] on the basis of a much better fit between observed and calcuiated splittings with respect to set I. In table 1 the definitive assignment of the observed infrared active internal vibrations of benzene is reported. This work was supported by the Italian Consiglio Nazionale delle Ricerche. References [ 11 I. Haradn and T. Shimanouchi, J. Chrm. Phys. 46 (1967) 2708. [21 E.R. Bernstein, J. Chcm. Phys. 52 (1970) 4701.

283

Volume

14. number 2

CHEMICAL

PHYSICS LETTERS

131 N. Rich 2nd D.A. Dow% Mol. Cryk’S (I%%) $11. [4] S-H. WalmsIey, Excitons, magnons and phonons in motecular crystals, Beirut Sympium, ambridge (1968). [S 1 G. Taddei, H. Bonadeo, M.P. Martocchi and S. Catifano, J. Chem. Phys., to be published.

284.

15 hfay 1972

[61 I&&P.Muzocchi, H. Bonadeo 2nd G. Taddei, J. Chem. Phys. 53 (1970) 807. [7] H. Bonadeo, &f.P. hfanocchi, E. Casieilucci J. Chem. Phys., to be published.

and S. CaliFno,