Halogen Bonding with Dihalogens and Interhalogens - Halogen Bonding: Fundamentals and Applications - page 93

discussed above) collected by the Legon group [172]. The calculated bond
strengths ranged from 6.7 to nearly 85 kJ mol. Bond strength increases with

increasing gas-phase basicity of the amine, larger dipole moment, and polar-
izability of the halogen. All of the complexes can be described as “outer”-type
charge-transfer complexes in the Mulliken terminology [173]. The RHF ap-
proach was found to be unsatisfactory for calculating equilibrium geometries,
while MP2 and DFT (particularly using the BHandHLYP functionals) were
foundtobeofcomparableaccuracy.
The Legon group, in addition to their extensive work in studying com-
plexes of N bases and halogens by rotational spectroscopy, have also explored
computational strategies for studying them. In a very recent paper [13], they
used both the GAUSSIAN98W [174] and ADF2004 program packages in an
effort to accurately calculate both halogen bond strength and the nuclear
quadrupole coupling constants (QCC, as obtained from microwave data). For
the former, geometry optimizations were carried out using the BHandHLYP
density functional in conjunction with such extended standard basis set as
aug-cc-pVTZ and the small-core relativistic pseudopotential correlation con-
sistent basis sets for iodine. 2 For ADF calculations, they used the OPTX
exchange functional combined with the PBE correlation functional using an
all-electron triple-

/

+ polarization basis set of uncontracted Slater orbitals.
Scalar relativistic effects were considered using the zero order regular ap-
proximation (ZORA) [175]. The authors found good correlation between both
methods and gas phase geometries (though not solid-state N-X bond lengths
from X-ray diffraction). The experimental QCCs correlate well with those
predicted by the BHandHLYP density functional, with the exception of io-
dine. The pseudopotential used in this treatment did not reproduce the effect
of the occupied core orbitals particularly well. The less computationally ex-
pensive ZORA model was shown to give acceptable results for the QCCs,
including iodine. The halogen bond energies in these complexes were best
described as being more electrostatic than covalent.
In a particularly interesting investigation, Kusama and Sugihara [25] used
MP2(full)/LANL2DZ ∗ calculations to try to understand the influence of six
nitrogen heterocycles on the performance of some dye-sensitized solar cells.
The heterocycles have been shown to increase the open circuit voltage ( V OC )
as well as the solar energy conversion efficiency. The authors suggest this ef-
fect results from a shift in the I - /I 3 - redox electrolyte equilibrium due to the
formation of a charge-transfer complex (Eq. 3).

ζ

Equation 3

2 Basis sets were obtained from the extensible computational chemistry environment basis set
database, version 02/25/04. Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352.