Photoelectron spectra of fluorotribromomethane and fluorotrichloromethane

Journal of Electron Spectroscopy and Rekted Phenomena, 6 (1975) 357-363 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

PHOTOELECTRON

SPECTRA

OF

FLUOROTRIBROMOMETHANE

AND

FLUOROTRICHLOROMETHANE

F. T. CHAU and C. A. MCDOWELL Department (Canada)

of Chemistry,

University of British Columbia,

Vancouver, British Columbia

V6T 1 WS

(First received 22 August 1974; in final form 8 November 1974)

ABSTRACT

High resolution He(I) photoelectron spectra are reported for fluorotribromomethane CFBr, and fluorotrichloromethane CFCl,. The assignments are based on CND0/2 calculations, symmetry arguments, the fine structures of the bands, and comparison with the photoelectron spectra of related compounds. Linear relationships have been found between the Pauling electronegativity values for the halogen atoms and the observed vertical ionization energies corresponding to a2 of a,’ orbitals of CFX, and related compounds CHX,, OPX,, BX, and PX, (X = F, Cl, Br, I)_ INTRODUCTION

The first four highest occupied molecular orbitals, a,, a2, e’ and e” (under C3” symmetry), of the trihalomethanes CHXs and their fluoro-substituted derivatives CFX3 (X = Cl, Br) are mainly contributed from the formally nonbonding p orbitals of the halogen atom. The relative orderings of these orbitals have been a subject of several discussions. Potts et al.’ suggested the electronic structures for chloroform and bromoform to be (e”)4(e’)4(a J 2(a2) 2 and (e”)4(a 1)2(e’)4(a2) 2 respectively in decreasing energy. In order to distinguish the first two highest occupied e orbit&, e’ and e” are chosen in such a way that they transform as different representations in the limit of a planar CX3 group. A different assignment, (e’)4(a,)Z(e”)4(a2)2, however, was given by Dixon et al. ’ for both molecules. Recently, CNDOJ2 calculations on the chloromethanes were reported by Katsumata and Kimura3 and their results give the ordering of the orbitals as (e”)4(a,)2(e’)4(a1)2S In view of this, we measured the photoelectron spectra of fluoro-substituted CHBr, and CHCl,, i.e. CFBr3, and CFCl, from which, by making use of the perhuoro effect4, ’ the energetic ordering of the nonbonding orbitals of the trihalomethane, hopefully, can be deduced.

358 EXPERIMENTAL

All fluorotrihalomethanes used here were commercial chemicals of the highest available purity and were used as received. The photoelectron spectra of these compounds were recorded using a spectrometer with 180” hemispherical analyzer (with 10 cm mean radius and 0.85 mm slit diameter). The working resolution obtained on the argon doublet is of the order of 20 meV. The observed ionization potentials for these compounds and their assignments (which are justified later) are given in Table 1. TABLE 1 OBSERVED IONIZATION POTENTIALS (eV) IN THE PHOTOELECTRON FLUOROTRIBROMOMETHANE AND FLUOROTRICHLOROMETHANE Orbital

CFCkP

CFEr9

la2 5e” 4e 5al 3e’ 4al

11.77 (1) 12.16 (1) 12.95 (1) 13.46 (1) 15.04 (2) 18.44 (2)

10.67 11.14” 11.81b 12.38 13.96 17.59

a The reproducibility of the components. INTERPRETATION

SPECTRA

OF

(1) (1) (1) (I) (2) (7)

last digit is shown inside the bracket. b Mean of the fine structure

OF THE SPECTRA

CFBr, The He(I) photoelectron spectrum of CFBr, is shown in Figure 1. Bands with W’s below 13 eV are assigned as arising from the bromine lone pair a2, e”, e’ and al orbitals (in increasing IP) owing to the intensity of these bands as well as for reasons

4e’

II

12

CFBr3

13

14

15

16

17

I8

9

IPkVlFigure 1. The He(I)

photoelectron spectrum of fluorobromomethane.

359 TABLE

2

FRANCK-CONDON

FACTORS

OF CFBr3 IN 2A1 IONIC

STATE ____

V’

0

Energy

(e

V)

FCFa

12.251b

C&d

FCF

Vibrational spacings (me V)

0.03

12.277

0.18

0.21

12.300

0.35

0.45

12.328

0.62

0.72

12.354

0.85

0.92

12.380

1.00

1.00

12.406

0.96

0.92

12.432

0.81

0.74

8

12.459

0.59

0.53

9

12.484

0.41

0.35

10

12.510

0.28

0.20

23 28 26 26 29 26 27 25 26

* The intensity of a peak is assumed to be proportional to the height of the fine structure maximum. The value listed in the Table is chosen in such a way that the intensity of the highest peak is 1 unit. b The value obtained by extrapolation.

given below. The first sharp peak in the spectrum reflects the nonbonding character of the a2 orbital from which the electron is removed. The ionization potential of this orbital is shifted to a higher value by 0.2 eV with respect to bromoform. The fourth band (with vertical IP 12.38 eV) exhibits well resolved vibrational structure starting at 12.28 eV. The result of Franck-Condon factor calculations (Table 2) by using the method developed by Coon et aL6 indicates that the fine structure arises from a single progression of v3* rather than from a composite of v2 and v3. The C-F and C-Br bonds, and the BrCBr angle are found to be increased by 0.11 and 0.017 A, and 3 O, respectively, during the ionization process. The second and third bands in the spectrum, relating to the loss of electrons from the e orbitals, consist of well-resolved doublets with separations 0.25 and 0.21 eV respectively. The splitting of the first band, 0.25 eV, compared to those observed for 0.25 and SPBr,‘, 0.22 eV. Single quantum PBr, ‘, 0.2; CHBr, ‘, 0.14; OPBr,‘, * The vibrational modes ~1 (C-F stretch), vz (C-Brs symmetry stretch), and YQ(C-Brs metry bending) have values 1069, 398 and 218 cm-l respectively in neutral molecule7.

sym-

360 excitations of v3 (213 cm-‘) and v 1 (874 cm- ‘) are also observed in the first as well as the second E band respectively. The result of a molecular orbital calcuIation carried out by using Boyd’s programme (see ref. 9) shows that there is a weak C-Br antibonding character in the e” orbital while there is weak C-Br bonding character in the e’ orbital though these two highest occupied e orbitals are essentially nonbonding. An increment in the value of v3 indicates that the first E band is derived from the e” orbital while a decrease in v 1, again, shows than the second E band comes from e’ orbital. Recently we remeasured the photoelectron spectrum of chloroform”‘. The right hand shoulder of the second E band exhibits two well resolved progressions in vZ, the C-Bra symmetric stretching mode with values slightly less than that of neutral molecule. The same result was obtained for deuterated chloroform. This supports our assignment that the second PE band of CFBr, arises from the removal of an electron from the e’ orbital. The last two photoelectron bands of CFBr, in the 13-18 eV region are readily assigned to the e’ and al orbitals according to the MO calculations. It is interesting to note that the e’ orbital is destabilized by about 0.8 eV with respect to the corresponding band of CHCl,. This reflects the antibonding character of the e’ orbital for the CF bond. The observation parallels the result of MO calculations on this molecule. CFCI, The photoelectron spectrum of CFCl, is shown in Figure 2. The relative areas of the first bands reflect the degeneracies of the levels invdlved and thus their ordering is likely to be a, e, e, a; these orbitals contain a major contribution from the chlorine p orbitals. The first band at 11.77 eV can readily be assigned as arising from the a2 nonbonding orbital. The next two bands are fairly broad and asymmetrical and no splitting nor any vibrational structure could be resolved. The assignment of these two

4 1

1

12

I3

I

14

I

15

1

16

I

17

18

19

20

;

IP(eV)-

Figure 2.

The

He(l) photoelectron spectrum of fluorotrichloromethane.

361 bands is somewhat ambiguous. Assuming that the orbital ordering to be the same as that of CFBr,, the experimental IP’s at 11.16 and 12.96 eV correspond to the e” and e’ orbitals. However, another assignment that e’ is associated with a higher IP than e” cannot be definitely ruled out. In the fourth band, the vibrational spacings w 1011, 641 and 303 cm-l can be deduced. This suggests that all the three totally symmetric vibrational modes v 1, v2, and v3 (1085,535 and 350 cm- ’ respectively in ground state 1 l) are excited during the ionization process. The last two bands relate to electron loss from e’ and a 1 orbitals in contrast to CHCl, and CFBr,. The e’ orbital again shifts to lower energy with respect to CHCIS presumably for the same reasons as we offered in explaining the downward shift for the corresponding e’ orbital in CFBr,. The peak with an ionization potential at 19.98 eV comes from low energy scattered electrons in the spectrometer itself. DISCUSSION AND CONCLUSION From the analysis of the fluorotribromomethane spectrum, the e’ orbital is found to have a higher bonding energy than the e” orbital in this molecule. Following the assignment on the experimental ionization potentials of CHBr, given by Potts et al.l, the e’ orbital is stabilized by ~0.9 eV while the e” orbital is destabilized by ~0.7 eV from CHBr, to CFBr,. The trends and the magnitudes of these shifts in energies in nonbonding orbitals as influenced by the perfluoro effect seem to be contrary observations for other molecules4v 5. Because of this, Dixon et al. ‘s assignment2 on CHBr, is preferred. When similar arguments are applied to CHCI,, the relative order of e’ and e” orbitals is found to agree with that proposed by Dixon et aL2 and by later unpublished work in this laboratory_ The correlation diagram (Figure 3) shows that the energy levels of CFBr, and CFCl, have the same pattern with the former being uniformly shifted towards lower energies by x 1 eV. The same observation applies to chloroform and bromoform, and phosphoryl chloride and bromide 5. This reflects the fact that the first five or six highest occupied orbitals are mainly built up from halogen p atomic orbitals, In Figure 4, the vertical IP’s of the nonbonding orbitals la2 or 1a2’ of CFX, ’ *, CHX3’,2andOPX38, 12andBX,landPXJ1(X = F, Cl, Br or I) are plotted against the Pauling electronegativity values of halogen atoms. Straight lines are seen to be obtained. The predicted IP’s for la, orbitals of CF13 and OPI,, and for la,’ orbital of PI, are 9.45, 10.28 and 9.36 eV respectively. Following arguments similar to those by Kimura et al. ’ 3, the gradients of these lines in the plots indicate the contributions of halogen p atomic orbitals in the trihalo compounds. It is interesting to note that the gradients in the two series CHX, and OPX, (Figure 4) are nearly the same. It should be noticed that our assignments of the first four nonbonding orbitals * For CF+ the vertical IP of la2 is considered the same as that of tl which under CaV symmetry splits into as and e.

’

362

‘..,

2e’ -...

16/

;..

,.3a,

.... ‘..,

-2e’

.,.. .,..‘- 2e’

.:- 2e’ ,,_(’

”

2

4a,

=._/.

4+

:-.‘.’..*.,

.’

._.-.-.” la,

.,x,~,_

4e,,

:; -...., ‘k

2

‘. w.

3e/ “.‘C

;;,:

4a,

_,_.,

._

.. . ...(.::‘; 4e” la2

1CHCI,

CFCI,

CFBr,

Ia2

CHBr,

Figure 3. Correlation diagram for the first six highest occupied orbitals of CHXs and CFXs (X = Cl, Br). The data for CHC13 and CHBr3 are from Dixon et aLe and Potts et a1.l

160 15.0 I

01

IO

I

2.5

I

3.0 3.5 Electronegativity -

I

4.0

4.0

~ E lectronegatlvity

.

Figure 4. Plot of vertical IP’s of (a) the laz orbitals of CFX$ and CHX3lp 2, (b) the la2 orbitals of 0PX3st 12 and the laa’ orbitals of BXsl and PX$ with X = F, Cl, Br or I against the Pauling electronegativity of halogen atom.

363 of CHCI, disagrees with that based on the CND0/2 calculations3. However, the authors well recognize the resulting MO orderings of the chloromethanes depends greatly on the selection of parameter values for chlorine atom. The fact that no comparison was made between IP’s of related compounds may also lead to a different assignment. While this work was near completion, Doucet et aLI4 presented a communication in which they reported the photoelectron spectrum of CFCl,, their data are in good agreement with ours, though they did not offer a complete assignment of the observed IP’s. ACKNOWLEDGEMENTS

We thank the National Research Council of Canada for generous financial grants. We thank Professor D. C. Frost for his interest in this work. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14

A. W. Potts, H. J. Lempka, D. G, Streets and D. W. Price, Phil. Trans. Roy. Sot. London, Ser. A, 268 (1970) 59. R. N. Dixon, J. N. Murrell and B. Narayan, Mol. Phys., 20 (1971) 611. S. Katsumata and K. Kimura, Bull. Chem. Sot. Jap., 46 (1973) 1342. C. R. Brundle, M. B. Robin and N. A. Kuebler, .I. Amer. C/tern. SOL, 94 (1972) 1466. C. R. Brundle and M. B. Robin, J. Amer. Chem. Sot., 92 (1970) 5550. J. B. Coon, R. E. DeWames and C. M. Loyd, .I. Mol. Spectrosc., 8 (1962) 285. A. G. Meister, S. E. Rosser and F. F. Cleveland, J. Chem. Phys., 18 (1950) 346. J. C. Bunzli, D. C. Frost and C. A. McDowell, J. Electron Spectrosc., 1 (1972/73) 481. R. J. Boyd and M. A. Whitehead, J. Ch em. Sot. Dalton Trans., (1972) 73. F. T. Chau and C. A. McDowell, unpublished work. H. H. Classen, J. Chem. Phys., 22 (1954) 50. P. J. Bassett and D. R. Lloyd, J. Chem. Sot. Da&on Trans., (1972) 278. K. Kimura, S. Katsumata, Y. Achiba, H. Matsumoto and S. Nagakura, Bull. Gem. Sot. Jap., 46 (1973) 373. J. Doucet, P. Sauvageau and C. Sandorfy, J. Chem. Phys., 58 (1973) 3708.