An ultraviolet photoelectron spectroscopic study of the electron states of iodine complexes with electron donors

Spectrochimica A('ta, Vol. 43A. No. 4, pp. 471-473, 1987

0584-8539/87 $3.(/0 + 0.00 Pergamon Journals Ltd.

Printed in Great Britain.

RAPID COMMUNICATION An

ultraviolet

photoelectron iodine

C.S.

Indian

spectroscopic

complexes

SREEKANTH,

with

M.S.

study

electron

HEGDE

AND

of

the

electron

states

of

donors +

C.N.R.

RAO

Solid State and Structural Chemistry Unit I > s t i t u t e of S c i e n c e , B a n g a l o r e - 5 6 0 012, India.

Abstract ]-lel photoelectron spectra of the vapour phase complexes of diethylether and diethylsulphide with iodine have been investigated for the firsl time. The iodine orbital ionization energy decreases on complexation while the donor lone-pair orbital ionization energy increases markedly; the shifts are considerabJy larger in the sulphide complex as expected on 1he basis of enthalpy considerations. INTRODUCTION

Interaction between electron donors and acceptors to form molecular complexes in the g r o u n d state has been investigated by employing electronic spectroscopy and a variety of other techniques [1,2]. Characteristic charge-transfer bands as well as the s h i f t e d d o n o r a n d a c c e p t o r bands have been reported in s e v e ral systems in solution phase. The nature of interaction in these complexes is interpreted in terms of Mulliken's theory [] ]. The equilibrium constants and heats of fo:_
a direct means of studying the molecular levels of the complexes in v a p o u r phase, b u t t h e s e s t u d i e s have been limited to addition compounds of the type H3B:CO or F3B:OH 9 [6,7]. We have, for the first- t i m e investigated the UV photoelectron spectra of h a l o gen complexes with electron ndonors such as diethylether and diethylsulphide in vapour phase. We wish to report these results in t h i s communication in a d d i t i o n to some p r e l i m i n a r y r e s u l t s o b t a i n ed on addition compounds of trialkylamines with BF 3 and SO 2 . EXPERIMENTAL

UV photoelectron spectra of the electron donor-acceptor complexes were recorded with a home-built spectrometer, cons i s t i n g of a HeI UV lamp, a c o l l i sion chamber a hemispherical analyser of 125mm mean diameter and a chaneltron electron multiplier. The spectrometer was differentially pumped, enabling the operation of helium lamp at 1.5 torr, the collision chamb e r at 0.5 - 0 . 1 torr. The s p e c trometer chamber is maintained at 5xi0 -5 torr. The spectrometer resolution is ]00meV at 7.5eV kinetic energy. Freshly distilled diethylsulphide (or diethylether) was taken in a small bulb with an extended neck. Freshly sublimed 12 was added ensuring that the donor was in e x c e s s . In the c a s e of the triethylamine complexes, to the freshly distilled and dried triethylamine taken in a glass ampule with a teflon

+Contribution No. 412 from the Solid State and Structural Chemistry Unit *To whom all correspondence should be addressed,

SACA) 43:4-A

471

472

C.S. SREEKANTH et al.

-

f

eS/ns

(C2Hs)zS (C2H)2S:12

12

(C2 H5)2S : 12

J

iZ ~

I 16

I 13

l 12

I 11

I 10

I 9 B E/eV

i~(CzH~lzs i 7

8

~ 6

-

Fig. 1. He] spectra of diethylsuJphide, iodine and the diethylsulphide-]2 complex in vapour phase. Inset shows the energy level diagram of the complex as well as the free donor and acceptor,

I

t

gl/2

g112

joo

I

I 1~

I 14

I 13

I 12

I 11

I0

9

H~,lzO I 8 ?

6

BE/eV

Fig. 2. Hel spectra of diethylether-I 2 complex in vapour phase.

Ultraviolet photoelectron spectroscopic study

tap, dry gaseous SO 2 (or BF 3 ) was added at liqufd nitrogen temperatures. This mixture was slowly w a r m e d to room t e m p e r a t u r e to allow the reaction to occur. This process was repeated until SO 2 (or BF 3 ) was slightly in excess. The vapours of the donoracceptor mixtures were allowed into the collision chamber with the help of a needle valve. The m i x t u r e of 12 with d i e t h y l s u l p h i d e (or diethylether) was heated uniformly to 350K. The initial spectrum was contaminated by the excess of the donor which p u m p e d away to give the s p e c t r u m of pure complex. In the case of triethylamine-SO~ ( or -BF 3), the initial spectrum was t~at of SO 2 (or BF3 ], which rapidly pumped away, yielding the spectrum of the addition compound. RESULTS

AND

DISCUSSION

In Fig. I we show the HeI UV photoelectron spectra of diethylsulphide, :iodine and the complex in the vapour phase. We readily see from the spectra that the ionization energies of the ~ q 3 / 2 and ~g1/2 orbitals of 12 at 9 / 3 6 and 9.9eV are lowered considerably (to 8.75 and 9.3eV respectively) in the complex. Furthermore, the i o n i z a t i o n energy of the n$ lone-pair orbital of the sulphide at 8.4eV increases to ] 0.4eV; the ionization energy of n lone-pair orbital of sulp h i d e S also appears to be shifted to a higher energy. The ~3/2 , H u l / 2 and ~ erbital ionizahlon energies of g I? show a decrease by 0.2-0.4eV. It-is indeed significant that we observe a marked change (0.6eV) in the energies of the ~. ~ 2 and ~gI/2 orbitals of iodine g3±~,is is what we w o u l d expect, since the iodine m o l e c u l e accepts charge from the donor molecule. Accordingly, the n orbital gets shifted by 2eV s Assuming K o o p m a n 's theorem we can c o n s t r u c t a molecular orbital level diagram as shown in inset of Fig.1. Since the ground state wave function of complex would correspond to the shifted n orbital, we estimate the excit s ed state of the complex to be 4.27eV above this level noting that the charge-transfer transition in the vapour phase occurs at 290nm [3,4]. A study of the d i e t h y l e t h e r iodine complex has also yielded interesting results (Fig.2) . The ~g3/2 and ~gI/2 orbitals of 12 are shifted only by O. 2eV in this complex while the energy of the lone-pair n o orbital seems

473

to be shifted from 9.6 to 10.5eV. The change in the no orbital ( ~0.9eV) is smaller than that of the sulphur lone-pair orbital in Et2s-I 2 complex. Since the Et20-I 2 complex is much weaker (-A H = T 6 k J / m o l ) than the Et2S-I 2 complex (- A H = 3 7 k J / m o l ) , the observed shifts in the orbital energies of the donor and the acceptor molecules are consistent. Our priliminary studies on the triethylamine-SO 2 complex show that the ionization energy of the n i t r o g e n lone-pair orbital of the amine increases from 8.1eV to 9.7eV, the i o n i z a t i o n energies of the n and ns orbitals of S02 being ~ o w e r e d by 2.1eV. These shifts are to be compared with those found in triethylamine-BFq complex where the nitrogen lone = pair ionization energy increases from 8.1eV to 12eV in the complex. The relative changes in lone-pair energies of these two addition compounds are consistent with their enthalpies of formation. Acknowledgements : The authors thank the Department of Science and T e c h n o l o g y and the U n i v e r s i t y Grants Commission for support of this research. REFERENCES

[I] R.S. MULLIKEN and W.B. PERSON, 'Molecular Complexes', P. 142, Wiley Interscience, New York (1969). [2] C . N . R . P.C. trosc.

RAO, S.N. BHAT and DWIVEDI, Appl. SpecRev. 5, I, (1971).

[3] M. TAMRES in 'Molecular Complexes' Vol.1, P.49, (edited by R. Foster, Elek Science, London (1973). [4] C.N.R. RAO, and S.N.BHAT, trosc. 33, 554

G.C. CHATURVEDI J. Molec. Spec(1970).

[5] P.V. KAMATH, M.S. HEGDE and C.N.R. RAO, J. Phys. Chem. 90, 1990 (1986). [6]

I.H. HILLER in 'Molecular Interactions' Vol.2 (edited by W.J. Orville-Thomas and H. Ratajczak) John Wiley, New York (1981).

[7] M . C . and Phys.

DURRANT, M.S. C.N.R. RAO, J. (in press).

HEGDE Chem.