7 October 1994
ELSEVIER
CHEMICAL
PHYSICS
LETTERS
Chemical Physics Letters 228 (1994) 431-435
Ultraviolet photoelectron spectroscopy of complexes of bromine
with n-donors in the vapor phase *
S.P. Ananthavel a, S. Salai Cheettu Ammal b, P. Venuvanalingam b,
J. Chandrasekhar ‘pd,M.S. Hegde a
Solid State and Structural Chemistty Unit, Indian Institute of Science, Bangalore 560 012, India
b Department of Chemistry, Bharathidasan University, TiruchirappaIIi 620 024, India
‘Department of Organic Chemistry, Indian Institute ofscience, Bangalore 560 012, India
’ Jawaharlal Nehru Centrefor Advanced Scientific Research, Indian Institute of Science Campus, Bangalore 560 012, India
l
Received 15 July 1994; in fmal form 2 August 1994
AbStl?lCt
He I photoelectron spectra of gas-phase complexes formed by Br2 with diethyl ether and diethyl sulphide have been studied
and interpreted using ab initio MO calculations. The key spectral features are assigned to the highest occupied molecular orbitals
derived from Br, and the donor molecule whose ionization potentials are shifted to lower and higher binding energies, respectively. All-electron and effective core potential calculations yield a CZv structure for EtsO...Brz and a C, form for Et$...Brr. The
nature and magnitude of the interactions are found to be comparable as reflected by the computed binding energy and quantum
of charge transfer.
1. Introduction
Electron
donor-acceptor
(EDA) complexes of
halogens with various donors have long been of interest [ 11. Primarily electronic absorption spectroscopy has been used especially since n-donors and I2
mixed in non-polar solvents have a characteristic
charge transfer (CT) band in the visible region.
However, similar n-o” complexes of other halogens,
viz. Brz and Clz could not be studied by conventional
UV spectroscopy mainly because the CT bands are
shifted to the vacuum UV region [ 21. UV photoelectron spectroscopy (UVPES ) , a powerful tool for obtaining orbital ionization energies of molecules, represents an attractive methodology for investigating
these complexes. Relatively weak interactions such as
in n-donor BF3 [ 31, hydrogen-bonded molecules
( Hz0 )2 [ 4 1, and van der Waals molecules [ 5 ] can
be probed employing UVPES. Valuable insights are
obtained by combining UVPES studies with supporting MO calculations.
As part of our continuing program to study the
electron states of molecular complexes by employing
WPES and electron energy loss spectroscopy
(EELS) [ 6 1, we considered it worthwhile to study
the complexes of Br2 with n-donors. In the present
study, we report the He I photoelectron spectra of the
EDA complexes of Brz with diethyl ether and diethyl
sulphide in the gas phase. We also report ab initio MO
calculations of Br2 complexes with the above ndonors.
* Contribution No. 1027 from Solid State and Structural Chemistry Unit.
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved
SSDIOOO9-2614(94)00964-3
432
S.P. Ananthavdet al. /Chemical Phyws Letters 228 (1994) 431-435
2. Experimental and computational details
Ultraviolet photoelectron spectra of the EDA complexes were recorded with a home-built spectrometer, consisting of a He I UV lamp, a 3 mm diameter
collision chamber and a channeltron electron multiplier [ 7 1. Differential pumping enabled operation of
the He I lamp at 1.5 Torr, a sample pressure of 0. I0.5 Torr and a vacuum of 5 x 10e5 Torr in the rest of
the spectrometer. The resolution of the spectrometer
is 100 meV fwhm at 19.7 eV.
Diethyl ether and diethyl sulphide, freshly distilled
to remove water, were used. Samples of the mixtures
were prepared in vacuum by co-condensation of the
acceptor with a small excess of the donor in a glass
ampoule fitted with a teflon tap. Samples of these
mixtures were warmed slightly by means of a heating
tape and admitted to the spectrometer through a
variable leak valve. Initially the UVPES of only the
donor molecule was observed. After excess donor was
removed by continuous pumping, photoelectron
spectra of both the monomers and the complex appeared. Spectra were recorded a number of times to
establish the peak position.
Ab initio MO calculations on the monomers and
the complexes were carried out using the GAUSSIAN 92 [ 81 package. Geometry optimization was
carried out with the all-electron 3-21G basis [ 91 as
well as with the LANLIDZ basis which employs an
effective core potential (ECP) for Br in conjunction
with the Dunning-Huzinaga double zeta basis [ lo].
The optimized structures were confirmed to be true
minima through vibrational frequency calculations.
While the Et20...Br2 minimum was calculated to have
Clv symmetry, the corresponding structure of
EtzS...Brz had one imaginary frequency. The minimum energy structure for the latter was found on relaxing the constraints to C, symmetry, with pyramidal coordination at S.
3. Results and discussion
The He I photoelectron spectra of Br2, ( C2H5)20
and their complex are shown in Fig. 1. Bromine exhibits features at 10.55, 10.91, 13.09 and 14.60 eV.
The first two peaks are derived from the 5cgMO and
split due to spin-orbit coupling while the remaining
14
13
BindIng
12
11
10
J9
Energy(eV)
Fig. 1. He1 spectra of (A) Br,, (B) diethyl ether and (C) the
complex between ether and bromine.
peaks are due to ionization from the A, and a, MOs,
respectively. The first ionization energy of diethyl
ether at 9.60 eV is due to the n-type oxygen lone pair.
The next peak at 11.08 is due to the o-type oxygen
lone pair and the features at 11.92-l 5.15 are due to
occ, 5ccH2,%-H, and bco. The ionization energies of
bromine and ether obtained in the present study
match well with the literature values [ 111. The additional features observed in the spectrum shown in
Fig. lc are attributed to those due to complex formation between bromine and diethyl ether. Four prominent peaks at 9.81, 10.25, 10.52 and 12.07 are found.
The He I photoelectron spectra of Brz, diethyl sulphide and the complex between these two are given
in Fig. 2. The He I spectrum of diethyl sulphide shows
features at 8.50, 10.75, 11.60 and 12.67 eV due to nand o-type lone pairs on sulphur, ocs and ?tCH3ionization, respectively. The spectrum of the complex is
different from those of Brz and diethyl sulphide. The
additional features in the spectra of EtzS-Brz include
an intense peak at 8.80 eV and peaks at 10.40 and
10.80 eV.
433
S.P. Ananthavel et al. /Chemical Physics Letters 228 (1994) 431-435
(4
Br
15
14
13
12
Binding
11
Energy
10
9
8
Br
(eV)
Fig. 2. He I spectra of (A) Br2, (B) diethyl sulphide and (C) the
complex between bromine and diethyl sulphide.
UVPES studies of amine-Br, complexes have been
reported by Utsunomiya et al. [ 12 1. Their spectra
contain peaks due to Hz0 and also features due to
HBr. When Hz0 impurity was present in the donors,
we also observed peaks due to HBr in the photoelectron spectra of donor-bromine mixture and could not
get the spectra due to the D-A complexes. Therefore
the additional features observed in their study of
amine-Br, complexes may not entirely be due to the
EDA complexes.
In order to assign the various observed bands and
to understand the nature of intermolecular interactions, ab initio MO calculations have been done on
the complexes of Br, with diethyl ether and diethyl
sulphide. The optimized geometries of the complexes
with bond lengths and bond angles obtained using 32 1G and LANLl DZ basis sets are given in Fig. 3. The
LANLlDZ results are given in parentheses. In the
case of the EtzO...Brz complex, the minimum energy
geometry corresponds to the Czv form (Fig. 3a). The
structure is that of a weak n-dc complex involving
the o-type lone pair on oxygen. The 0-Br distance is
2.808 A (2.937 A) which is less than the sum of the
van der Waals radii of 0 and Br. The Br-Br distance
3.056
3.133)
/
\
W
2
(100.1)
' (96.1)
a.
l.693
(1.879)
,,, 1.532
(1.530)
Fig. 3. Optimized structures of (a) Br,-diethyl ether complex
and (b) Br,-diethyl sulphide complex. (Bond lengths in A, bond
angles in deg). Basis sets are 3-21G and LANLlDZ (in
parentheses).
2.454 A (2.476 A) in the complex is only slightly increased compared to the distance 2.443 8, in the bromine molecule. The binding energy of the complex is
computed to be 4.39 kcal/mol, with the 3-2 1G basis
set, but is reduced to 2.44 kcal/mol in the ECP calculation (Table 1).
The Cav form of the EtzS...Brz complex is computed to have one inversion mode about S with an
imaginary frequency. The true minimum has C, symmetry with significant pyramidalization at S (Fig.
3b). The bromine interacting with S makes an angle
S.P. Ananthavel et al. /Chemical PhysicsLetters 228 (1994)431-435
434
Table 1
Calculated total energies (hartrce) and complexation energies (kcal/mol)
Molecule
Symmetry
3-21G//3-21G
LANLlDZ//LANDLlDZ’
Br2
D oab
C2”
C2”
C2”
- 5120.07838
-230.86173
-551.98972
-5350.94712 (4.39)
-5672.07567 (4.74)
-25.82995
-232.08084
-167.12451
-257.91468 (2.44)
- 192.95446 (3.37)
(CzH,)zG
(C2%)2S
(CzH,)zO-Brr
( C2I-b )2S-J3r2
G
a Dunning/Huzinaga valence double-zeta on C, 0 and H, Los Alamos ECP+DZ on S and Br.
Table 2
Vertical ionization energies, calculated orbital energies and assignments of bromine, diethyl ether, diethyl sulphide and the complexes
Molecule
Br2
(C2W20
(CrH,)sG-Brr
(C2Hs)2.5
(C2HS)2S-Brz
I (eV)
Assignment
-e (eV)
3-21G
LANLIDZ”
10.55, 10.91
13.09
14.60
11.06
13.12
13.38
11.43
13.41
13.58
9.60
11.08
11.92-15.15
10.95
12.13
13.11-15.83
11.15
12.28
13.28-15.91
9.81
10.25, 10.52
12.07
11.46
10.44, 10.45
12.49
11.58
10.83, 10.85
12.15
8.50
10.75
11.60
12.67
9.08
11.05
11.86
13.64
8.97
10.99
12.00
13.61
8.80
10.4, 10.8
9.37
10.47, 10.48
9.34
10.81, 10.83
’ Dunning/Huzinaga valence double-zeta on C, 0 and H, Los Alamos ECP+DZ on S and Br.
of 99.5” with the CSC unit. The interaction evidently
involves donation from the x-type lone pair on S to
the 6” orbital of Br,. Interestingly, approach of an
electrophile towards sulphides has been shown to be
preferred precisely in the same direction both in gasphase structures [ 131 and crystal structures of sulphur-containing compounds [ 14 1. The S-Br distance at the 3-21G level is computed to be 3.056 A
(LANLlDZ: 3.133 A). The Br-Br distance of 2.472
8, (2.449 A) indicates greater involvement of the dc
orbital in the donor-acceptor interaction in the sulphide compared to that in the ether complex. The
calculated binding energy is also slightly higher for
the sulphide complex: 4.75 and 3.37 kcal/mol at the
3-2 1G and ECP levels, respectively (Table 1).
The observed PE spectra can be interpreted using
ab initio orbital energies in conjunction with Koopmans’ theorem [ 15 1. The computed orbital energies
of Br2, Et*0 and EtzS are generally consistent with
observed vertical ionization energies (Table 2) and
follow the expected sequence of orbitals. However,
the values are not in quantitative agreement, with the
ionization energies of Et20 being overestimated by
more than 1 eV. After taking this factor into account
while assigning the peaks of the Et*O...Br* complex,
the first new feature at 9.8 1 eV is attributable to the
S.P. Ananthavel et al. /Chemical Physics Letters 228 (1994) 431-435
oxygen lone pair of ether. The peaks at 10.25 and
10.52 eV must be due to ionization from the nearly
degenerate highest occupied MOs based on the Brl
unit, split due to spin-orbit coupling. The peak at
12.07 eV would then correspond to ionization from
the x, orbital. In effect, the oxygen lone pair is shifted
by 0.20 eV to a higher binding energy as a result of
complex formation while the Br2 orbitals are shifted
to lower binding energies. The same trends are noted
in the computed orbital energies at both levels employed (Table 2 ) . The shifts in the ionization potentials in the Et$...Brz complex follow a similar pattern. The first peak at 8.80 eV can be assigned to the
sulphur lone pair. The MO is found to be more bound
as a result of complex formation, both in the observed IEs and the computed orbital energies. In contrast the bromine based MO become less bound in
the complex (Table 2 ) .
The shifts in binding energies found in the two
complexes are characteristics of donor-acceptor interaction [ 61. Although many factors may determine
the magnitude of the shift in ionization energies, the
most important one in such complexes appears to be
charge transfer. The calculated magnitude of charge
transfer from ether and diethyl sulphide to bromine
using a Mulliken population analysis are 0.04 and
0.07, respectively.
4. Conclusions
Photoelectron spectra of complexes of diethyl ether
and diethyl sulphide with Brz have been obtained. Ab
initio calculations enable assignment of key features
of the spectra. The shifts in ionization energies and
computed orbital energies as well as the calculated
geometries and charge distribution are characteristic
of a weak n-dc interaction in the two systems.
435
Acknowledgement
SSCA thanks UGC (New Delhi) for a research
fellowship.
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