Intramolecular interactions in diiodonaphthalenes

Article abstract of DOI:10.1021/jp0219202

The synthesis and the electronic structure of isomeric di-iodonaphthalenes is described. The electronic structure has been investigated by HeI/HeII photoelectron spectroscopy and non-Koopmans' quantum chemical calculations. The influence of the topology of iodine substitution on the electronic structure is discussed. Special emphasis is placed on elucidating the role of intramolecular iodine-iodine interactions.

Full text of DOI:10.1021/jp0219202

480
J. Phys. Chem. A 2003, 107, 480-484
Intramolecular Interactions in Diiodonaphthalenes
Igor Novak* and Huiming Jiang
Department of Chemistry and CPEC Centre, National UniVersity of Singapore, Singapore 117543, Singapore
Branka Kovacˇ
Physical Chemistry DiVision, “R. BosˇkoVic´” Institute, HR-10000, Zagreb, Croatia
ReceiVed: August 20, 2002
The synthesis and the electronic structure of isomeric di-iodonaphthalenes is described. The electronic structure
has been investigated by HeI/HeII photoelectron spectroscopy and non-Koopmans’ quantum chemical
calculations. The influence of the topology of iodine substitution on the electronic structure is discussed.
Special emphasis is placed on elucidating the role of intramolecular iodine-iodine interactions.
Introduction
The distortion of molecular structure induced by steric
overcrowding or repulsion has been of interest to chemists for
a long time. An example of a molecule where such overcrowd-
ing takes place is 1,8-diiodonaphthalene. The X-ray diffraction
analysis has revealed that the two iodine atoms are twisted out
of the rings plane by angles of 5-17° within six crystallo-
graphically independent molecules1. Despite the twist, iodine-
iodine internuclear distances which range from 3.51 to 3.54 Å
are still considerably shorter than the sum of iodine van der
Waals radii (4.30 Å). It is interesting to compare this structural
distortion with the magnitude of repulsive σ-type interactions
between I5p orbitals (lone pairs). We have therefore prepared
a series of diiodonaphthalenes in order to study such through-
space (TS) I-I interactions. UV photoelectron spectroscopy
preparation of other derivatives was achieved according to the
(UPS) allows direct probing of the energy consequencies of such
new procedures which are summarized in the Schemes 1 and
interactions and thus complements the results of X-ray
2.
study.
All of the prepared samples were characterized by IR, NMR,
The iodine lone pair bands revealed in UPS are sharp and
elemental analysis, and melting point determinations.
strong and can be readily identified through HeI/HeII measure-
The HeI/HeII photolectron spectra of the title compounds
ments. Two types of substituent effects are usually distinguished
were recorded on Vacuum Generators UV-G3 spectrometer and
theoretically: inductive and resonance. This distinction is often
calibrated with small amounts of Xe gas which was added to
difficult to make in the UPS data because both effects “operate”
the sample flow. The spectral resolutions in HeI and HeII spectra
simultaneously. However, the diiodonaphthalenes are isomeric,
were 25 and 70 meV, respectively, measured as fwhm of 2P3/2
and hence, the inductive effects can be expected to be of the
Ar+ line. For compounds 1-7, elevated sample temperatures
of 140, 120, 150, 150, 160, 150, and 140 °C, respectively, were
required in order to achieve sufficient vapor pressures in the
sample flow. The spectral bands (for the purposes of relative
same magnitude. This permits one to attribute differences in
the electronic structures of isomers mainly to resonance effects
and thus consider the two effects separately.
intensity measurements) were, when necessary, simulated by
Experimental and Theoretical Methods
Gaussian profiles, and baseline corrections were employed
(Table 1). The empirical relative intensity, RI (empir) for ith
band was calculated as
Sample Preparation. 1,8-diiodonaphthalene (1), 2,3-di-
iodonaphthalene (2), 2,7-diiodonaphthalene (3), 2,6-diiodonaph-
thalene (4), 1,5-diiodonaphthalene (5), 1,6-diiodonaphthalene
(6), and 1,7-diiodonaphthalene (7) were studied in this
work.
RIi ) {BiHeII•Σi BiHeI }/{BiHeI•Σi BiHeII
}
where Bi stands for the band intensity of ith band in HeI or
HeII spectrum and the index of summation runs through the
bands of interest.
The preparation of 1,5-diiodonaphthalene and 1,8-diiodonaph-
thalene followed the procedures reported previously.2,3 The
10.1021/jp0219202 CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/01/2003

Intramolecular Interactions in Diiodonaphthalenes
J. Phys. Chem. A, Vol. 107, No. 4, 2003 481
SCHEME 1: 1,6-; 2,6-; 2,7-Diiodonaphthalene
TABLE 1: Vertical Ionization Energies (Ei ( 0.03 eV),
Calculated Orbital Energies (GF/eV), Band Assignments,
and HeII/HeI Relative Band Intensity Ratios (RI) for
Orbitals in Iodonaphthalenesa,b
SCHEME 2: 2,3-Diiodonaphthalene
MO
compound band
Ei
(8.00)
8.53
9.13
9.38
9.78
10.38
10.83
11.15
11.58
11.95
(8.14)
8.58
9.17
9.70
10.08
10.27
10.63
11.25
GF/eV
7.62
8.18
8.86
8.93
9.52
assignment
RI
1 (C2V)
X
A
B
C
D
E
F
a2 (π)
b2 (σI)
b1 (π)
b1 (πI)
a2 (πI)
a1 (σI)
b1 (π)
a1 (σ)
a2 (π)
b2 (σ)
a2 (π)
b1 (π)
b2 (σI)
a2 (πI)
b1 (π)
a1 (σI)
a2 (πI)
b2 (σ)
1.96
0.66
1.30
1.30
0.76
0.61
0.93
0.93
0.93
0.93
1.10
1.10
0.52
0.55
0.9
Calculations. All of the calculations were performed with
Gaussian 98 program.4 The DFT calculations with the hybrid
B3PW91 functional and the TZP basis set were performed first
in order to get fully optimized molecular geometry. Subse-
quently, single-point ROVGF calculations at B3PW91 geometry
were performed in order to obtain ionization energies. The
ROVGF method goes beyond Koopmans’ approximation and
is often used for the assignment of photoelectron spectra.5
10.08
10.81
11.04
11.69
11.88
7.90
G
H
I
2 (C2V)
X
A
B
C
D
E
F
8.25
Results and Discussion
8.77
9.43
Photoelectron Spectra. The photoelectron spectra are shown
in Figures 1-4, and their analysis is summarized in Figure 5
and Table 1. The spectra indicate that the density of ionic states
is large, and the information obtained from several empirical
and theoretical considerations must be used simultaneously if
one is to arrive at a reliable assignment.
9.79
9.90
0.9
0.75
1.25
10.48
11.32
G
H, I 11.73
11.74, 11.88 a1 (σI), b1 (π) 1.11
3 (C2V)
4 (C2h)
5 (C2h)
6 (Cs)
X
A
B
C
(8.21)
8.62
9.47
9.70
8.01
8.26
9.38
9.42
a2 (π)
b1 (π)
b2 (σI)
a2 (πI)
1.48
1.48
0.68
0.58
The assignments are based on the following considerations.
The interpretation of the spectra of diiodonaphthalenes (Figures
1-3) relies on the comparison with the HeI spectrum of
naphthalene6 and HeI/HeII spectra of 1-iodonaphthalene (Figure
4). The spectral region 8-13 eV contains, besides ionizations
from five ring π-orbitals (π), the two out-of-plane iodine lone
pairs (πI) and two in-plane iodine lone pairs (σI). The relative
intensity of bands corresponding to ionizations from the orbitals
with large I5p character decreases most prominently on going
from HeI to HeII radiation. This is due to different energy
dependence of the photoionization cross-sections for I5p and
C2p orbitals.7 The iodine 5p cross-section decreases three times
as much as C2p on going from HeI to HeII. The iodine σI
orbitals can acquire C2p character only by weak interaction with
the energetically remote ring σ orbitals (thus σI orbitals are prone
to show a strong drop in the relative HeII band intensity). The
iodine πI orbitals can, on the other hand, interact strongly with
energetically close ring π orbitals and thus acquire C2p character
(leading to a modest drop in relative HeII band intensity). The
sharp, narrow bands correspond to strongly localized (less
bonding) orbitals as is suggested by the Franck-Condon
principle.
The assignment of naphthalene spectrum is well established
and can be used to analyze the 1-iodonaphthalene spectrum. In
the latter’s spectrum (Figure 4), the bands at 9.29 and 9.74 eV
show a pronounced decrease in relative intensity on going from
HeI to HeII radiation. Thus, the two bands can be assigned to
ionizations from the two iodine lone pairs; the 9.29 eV band is
broader and can be attributed to the out-of-plane lone pair πI
orbital, whereas the 9.74 eV band is narrower which means that
the ionized orbital has less bonding character and can thus be
described as σI orbital. The five ring localized π orbitals can
then be attributed to different bands on the basis of comparison
with UPS of naphthalene (Figure 5). The assigned spectrum of
1-iodonaphthalene can be utilized as the starting point for the
assignment of the spectra of diiodonaphthalenes.
D, E 9.97
9.42, 9.88 a1 (σI), b1 (πI) 0.58
F
G
10.70
11.23
10.58
11.31
b1 (π)
b2 (σ)
1.17
1.16
H, I 11.80
11.72, 11.96 a1 (σ), a2 (π) 1.16
X
A
B
C
D
(8.11)
8.77
9.31
9.70
9.92
7.88
8.54
9.10
9.40
9.42
au (π)
au (π)
bg (πI)
bu (σI)
ag (σI)
1.14
1.51
0.50
0.36
0.36
E,F 10.5
10.18, 10.45 au (πI), bg (π) 1.18
11.27 (11.15) 11.28 ag (σ) 1.58
11.97, 11.97 bu (σ), bg (π) 1.26
7.85 1.23
G
H, I 11.95
X
(8.13)
au (π)
A, B 9.08
8.78, 8.92 au (π), bg (πI) 1.07
9.28, 9.42 ag (σI), bu (σI) 0.56
C,D
E
F
G
H
I
X
A
B
C
D
E
9.73
10.33
10.78
11.2
9.77
10.80
11.06
11.75
12.11
7.90
au (πI)
bg (π)
ag (σ)
bg (π)
bu (σ)
a′′ (π)
a′′ (π)
a′′ (πI)
a′ (σI)
a′ (σI)
a′′ (πI)
a′′ (π)
a′ (σ)
0.62
0.97
1.40
1.40
1.40
1.25
1.17
0.57
0.57
0.57
0.73
1.33
1.28
11.66
12.08
(8.12)
8.77
8.52
9.32
9.16
9.65
9.34
9.92
9.42
10.27
10.70
11.27
9.88
F
G
10.68
11.29
H,I 11.72
11.68, 11.87 a′ (σ), a′′ (π) 1.36
7 (Cs)
X
A
B
C
D
E
(8.07)
8.75
7.87
8.52
a′′ (π)
a′′ (π)
a′′ (πI)
a′ (σI)
a′ (σI)
a′′ (πI)
a′′ (π)
a′ (σ)
1.30
0.96
0.59
0.59
0.59
0.59
1.61
1.31
9.35
9.20
9.60
9.35
9.84
9.36
10.17
10.77
11.27
9.75
F
G
10.73
11.29
H, I 11.67
11.64, 11.84 a′ (σ), a′′ (π) 1.31
a The values in brackets correspond to adiabatic ionization energies.
b The bands were simulated by asymmetric Gaussian band shapes as
suggested in ref 13, and the variable bandwidths were in the range
0.1-0.3 eV.

482 J. Phys. Chem. A, Vol. 107, No. 4, 2003
Novak et al.
Figure 2. HeI/HeII photoelectron spectra of 4 and 5.
The alkane skeleton is much less efficient than either ethene
or aromatic systems in propagating I5p-I5p interactions. This
is obvious when one compares small numerical values of ∆nI
in diiodoalkanes vs diiodoethenes, diiodobenzenes, and di-
iodonaphthalenes. The reason is in the delocalized nature of π
electrons, which are good conduits for such interactions.
The iodine lone pair interactions are larger in diiodobenzenes
and diiodoethenes than in diiodonaphthalenes (with a single
significant exception). This can be attributed to the larger
number of intervening CC bonds in diiodonaphthalenes. This
suggestion is supported by the observation that in 2 the ∆nI is
the largest of all isomers except 1.
In addition to the number of CC bonds, topology of
substitution will also influence the magnitude of iodine lone
pair interactions. Thus, ∆nI values do not necessarily decrease
with the increasing number of bonds separating iodine centers.
For example, in 4, the interaction is larger than in 6 or 7, even
though in 4 there are five intervening CC bonds compared to
three or four in the latter molecules. The last important factor
which governs iodine lone pair interactions is steric repulsion
as is evident from the UPS of cis and trans 1,2-diiodoethenes,
1,2-diiodobenzene, and 1,8-diiodonaphthalene. The iodine lone
pair splitting (∆nI) in the cis isomer is 0.1 eV larger than in the
trans. The splitting in 1,2-diiodobenzene is smaller (1.73 eV)
than in 1 (1.85 eV) despite the general decrease of I5p-
I5pinteractions on going from diiodobenzenes to diiodonaph-
thalenes (see above). Further evidence in support of this
argument comes from the spectrum of 2. The interaction in 2
is 1.46 eV, which although smaller than in 1 (1.85 eV) is larger
Figure 1. He/HeII photoelectron spectra of 1-3.
The assignment of the spectrum of 1 (Figure 1) is based on
the consideration of relative band intensities in HeI spectrum
and HeI/HeII intensity variations. Bands at 8.53, 9.38, 9.78, and
10.38 eV exhibit HeII/HeI intensity decrease and can be
attributed to four iodine lone pair ionizations (N.B. 9.38 eV
band and the shoulder at 9.13 eV bands comprise two ioniza-
tions). The remaining bands can then be attributed to five ring
π orbitals and various σ orbitals on the basis of comparison
with naphthalene spectrum and GF calculations (Table 1 and
Figure 5).
The assignment of the spectra of the remaining diiodonaph-
thalenes 2-7 is based on the same principles as 1, and we
therefore give only the final assignments in Table 1 and Figure
5.
Intramolecular Interactions. The most interesting informa-
tion contained in the UPS data provides insight into the through-
bond (TB) and through-space (TS) intramolecular interactions
involving I5p orbitals. The total energy range of the I5p manifold
(∆nI) can be measured directly from the UPS spectra and serves
as an internal probe for the total, i.e., TS + TB interactions.
However, to fully understand the meaning of numerical values
for diiodonaphthalenes, we compare their spectra with UPS
spectra of diiodobenzenes,8 diiodoalkanes,9 and diiodoethenes.10
The analysis presented in Table 2 suggests the conclusions
discussed below.

Intramolecular Interactions in Diiodonaphthalenes
J. Phys. Chem. A, Vol. 107, No. 4, 2003 483
Figure 3. HeI/HeII photoelectron spectra of 6 and 7.
TABLE 2: Energy Range of I5p Manifolds in
Iodo-substituted Hydrocarbons (∆nI/eV)
Figure 4. HeI/HeII photoelectron spectra of 1-iodonaphthalene.
molecule
∆nI
molecule
∆nI
ICH2I
ICH2CH2I
ICH2CH2CH2I
ICH2CH2CH2CH2I
cis-C2H2I2
trans-C2H2I2
1,2-C6H4I2
1.1
1-C10H7I
0.45
1.85
1.25
1.46
0.50
1.19
0.95
0.82
0.76
0.84
0.61
1.61
1.53
1.73
1.20
1.54
1,8-C10H6I2
1,5-C10H6I2
2,3-C10H6I2
2,7-C10H6I2
2,6-C10H6I2
1,6-C10H6I2
1,7-C10H6I2
1,3-C6H4I2
1,4-C6H4I2
than in 5 (1.25 eV). This is due to spatial distances between
respective iodine atoms in the three molecules. The iodine atom
positions in 5 preclude steric repulsion and TS interaction. In
2, the I-I distance is larger (3.64 Å)11 than in 1 (3.53 Å)1 which
is reflected in the respective ∆nI values. It is possible to estimate
contributions to measured energy splitting arising from through-
space interactions. Comparing values in 1 and 5, the TS
interaction value is 0.6 eV. Comparing UPS data for 1 and 2
gives a TS difference of 0.21 eV. It is also possible to deduce
TS contribution in diiodobenzenes by comparing 1,2- with 1,4-
diiodobenzene, which gives the TS contribution of 0.19 eV. It
can then be tentatively suggested that the TS interactions in 1
are approximately three times larger than in either 1,2-
diiodobenzene or 2.
It is interesting to compare the steric repulsion in diionaph-
thalenes with the their bis-methylamino substituted analogues.
UPS has revealed that the nitrogen lone pair interactions increase
from 0.82 to 2.0 eV on going from 1,5-bis(dimethylamino)-
naphthalene to it’s 1,8-isomer.12 This increase of 1.18 eV is
twice as large as the 0.6 eV in diiodonaphthalene counterparts.
Although the nitrogen atom is smaller than iodine, the presence
Figure 5. Energy level diagram for diiodonaphthalenes.
of more strongly localized electron lone pairs in the former may
account for the difference in steric repulsion.
Conclusion
The analysis of UPS data for diiodonaphthalenes provides
interesting semiquantitative insights into the nature of steric
repulsions and TS interactions. The 1,8-diiodonaphthalene is a
prime example of strong steric interactions. The single reported
X-ray study1 suggested that such interactions are prominent.
However, because of the complexities associated with crystal
packing forces, the observed crystal structure could not provide
a clear answer regarding the extent of molecular distortion. The
UPS data collected in the gas phase are free from such

484 J. Phys. Chem. A, Vol. 107, No. 4, 2003
Novak et al.
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and TS interactions could be related directly.
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