The Group - Our Research
Research Activities
Experimental
Determination of the Kinetics and Mechanism of Chemical Reactions in the
Atmosphere
The growing problem of environmental quality
deterioration due to the release of substances capable of inducing changes in
the composition of atmosphere constitutes one of the hottest issues worldwide.
The laboratory is participating in global efforts pointing towards the
assessment of the problem by carrying out a study of the tropospheric
degradation of chemical compounds initiated by the so-called "atmospheric
detergents" (OH and Cl). The kinetics and the mechanism of chlorine atom
reactions with halogen-, sulfur- and nitrogen- containing compounds, related
with environmental problems, are studied by the very low pressure reactor (VLPR)
technique, in collaboration with the Laboratory of Chemical Kinetics at the
Department of Chemistry in the University of Crete. Earlier studies were
focused on brominated and iodinated compounds, and the clarification of the
effects of the halogen-atom adducts formation in the overall reaction
mechanism. Recent studies involve the reaction mechanism and kinetics of Cl
atoms with fluorinated ethers and alcohols as well as fluorinated alkenes,
which are proposed as possible alternatives of chlorofluorocarbons (CFC), since
the latter are very well known to be potentially harmful to stratospheric ozone
and contribute significantly to the global warming. 
Development
of Theoretical Model Chemistries for the Accurate Determination of the
Thermodynamic Properties of Chemical Compounds
The availability of high performance electronic
computers is continuously growing as an effective tool in achieving accurate
thermodynamic properties of chemical compounds by electronic structure
quantum-mechanical calculations. The knowledge of accurate thermodynamic
properties permits the prediction of the molecular reactivity and gas-phase
reaction mechanisms, directly applicable in the study of atmospheric
degradation processes. The laboratory has extensively examined the reliability
for a plethora of model chemistries employing inexpensive density-functional
theory (DFT) methods and infinite basis extrapolation methods in conjunction
with coupled-cluster theory - CCSD(T) -, mainly for molecules containing all
four halogen atoms (F, Cl, Br, and I). The superiority of the B3P86 functional
in providing very accurate bond dissociation energies was the outcome of a relatively
recent work, as well as that of the B3PW91 and B3LYP functionals is the
determination of accurate enthalpies of formation and ionization potentials.
Further work aiming at reducing the cost of CCSD(T) calculations in large
molecules as well as providing approximations for the scalar-relativistic and
core/valence correlation effects is currently in progress.

Design
of Environmentally Friendly Molecules as Refrigerants and Fire-suppressing
agents
The correlation of theoretically calculated
molecular properties for extensive sets of compounds with kinetic parameters of
their reactions with OH radicals and Cl atoms enables the construction of
empirical expressions which may be used to predict their tropospheric
reactivity. Furthermore, these expressions may suggest the optimal molecular
structural features of molecules in order to be environment friendly. In recent
studies, the correlation of calculated C-H bond dissociation energies and
vertical ionization potentials with experimentally determined rate parameters
for a series of fluorinated ethers was performed. The expressions derived
permit the prediction of room-temperature rate coefficients with Cl atoms with
an order of magnitude accuracy, using computationally affordable DFT
calculations. Furthermore, it was shown that for fluorinated ethers and
alcohols (proposed as alternatives to freons), high atmospheric reactivity
(shortening their lifetimes and their consequent contribution to global
warming) can be achieved by the presence of the -OCH3 or -CH2OH
groups attached to a fluorinated alkyl chain whose structure can be freely
tuned to attain the optimal physical properties.


|