An extensive theoretical study of
the thermal decomposition
of
alkyl- and phenylureas, which are widely used in the pesticides, pharmaceuticals,
and materials industries, has been carried out using electronic structure
calculations and reaction rate theories. Enthalpies of formation and
bond dissociation energies (BDE) of 11 urea derivatives have been
calculated using different levels of theory (CBS-QB3, CCSD(T)/CBS//M06-2X/6-311++G(3df,2pd),
and CBS-QM062X) according to the size of the system. Potential energy
surfaces for the unimolecular decomposition pathways of these urea
derivatives were also systematically computed for the first time.
Several pericyclic reactions can be envisaged, as a function of the
size and the nature of the N substituents, and all of these pathways
were explored. Our calculations show that these compounds are solely
decomposed by four-center pericyclic reactions, yielding substituted
isocyanates and amines, and that initial bond fissions are not competitive.
Based on the set of urea derivatives studied, a new reaction rate
rule for their thermal decomposition was defined and involves the
nature of the transferred H atom (primary or secondary/alkyl or benzyl)
and the nature of the N-atom acceptor (primary, secondary, or tertiary).
This new reaction rate rule allows us to determine the product branching
ratios in the thermal decomposition of a given urea derivative and
its total rate of decomposition. Applications on urea derivatives
used in the chemical industry are presented and illustrate the usefulness
of this new rate rule that allows to predict the previously unknown
thermal decomposition kinetics of a large number of these compounds.
Photodynamic therapy (PDT) is a method of treating precancerous diseases and malignant neoplasms. The efficacy of PDT depends on different parameters such as light dosimetry, oxygen availability, and photophysical and physical–chemical properties of the photosensitizer. In PDT, a photosensitizer is activated using light to promote oxygen photosensitization and cellular transport plays a key role in the reach of it to the desired tissue. In particular, to know the effectiveness of the drug delivery in PDT and its dosage forms to target damaged organs, along with such characteristics as water solubility, it is important to know the ability of a substance to penetrate through cell membrane or accumulate in it. Lipophilicity is used to quantify the earlier‐described abilities. We evaluated the lipophilicity of three selected photosensitizers (PS) (protoporphyrin IX, pyropheophorbide‐a and photofrin) by means of three different methods: octanol–water distribution methods (shake‐flask), reversed‐phase high‐performance liquid chromatography (HPLC) and theoretical calculations based on density functional theory (DFT). We describe and compare the results of these various methods.
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