Prediction of vapour pressures for halogenated diphenyl ether congeners
from molecular descriptors
Öberg, T.
ESPR - Environ Sci & Pollut Res (OnlineFirst) 9, 405-411 (2002)
Abstract
Background, Aim and Scope --- Polychlorinated diphenyl ethers (PCDE) and
polybrominated diphenyl ethers (PBDE) have both been identified as environmental
contaminants. The physical properties are important in determining the
distribution and fate of organic contaminants in the environment. The purpose of
the present investigation was to characterise halogenated diphenyl ethers using
computationally derived descriptors, and to develop calibration models for the
vapour pressure from published experimental data. --- Methods --- Experimental
data for vapour pressures were obtained from the literature. The chemical
structure of each PCDE and PBDE congener was optimised prior to descriptor
generation. The data analysis was performed using principal component analysis (PCA)
and partial least squares regression (PLSR). The calibration models were
validated with external test sets. --- Results and Discussion --- All congeners
of PCDEs and PBDEs were characterised by 795 molecular descriptors and two
principal components could account for about two thirds of the variance within
each group. Bilinear calibration models were developed that could explain 99.4%
of the variance in the external validation test sets. Vapour pressures were
subsequently predicted for all congeners that were adequately described by these
calibration models. The type and number of halogen atoms in the molecule were
the main factors influencing the vapour pressures of halogen substituted
diphenyl ethers, but the variations in substitution pattern was also shown to be
a significant factor. --- Conclusions --- The molecular descriptor patterns of
halogenated aromatic compounds such as diphenyl ethers can be described and
interpreted using principal component analysis (PCA). The major sources of
variation in the descriptor spaces for PCDEs and PBDEs are the same as those
contributing to the differences in vapour pressure, similar to what has
previously been reported for the PCBs. The bilinear calibration models for
vapour pressure presented here, has a standard error of prediction that is lower
than what is reported as the experimental uncertainty or observed as deviations
between experimental investigations. The estimated prediction errors are
expected to be within the reported boundaries when the models are applied to new
objects within the same molecular descriptor space, and model predictions can
hence extend the current database of experimental values. --- Recommendations
and Outlook --- The results from this investigation and others show that the
establishment of quantitative structure- property relationships (QSPR) is a
viable approach to estimate physical properties for halogenated diphenyl ethers.
It is easy to foresee an increased need for using QSPR estimation methods in the
future, for evaluation of the environmental fate for organic pollutants. Despite
method developments and automation, it is unlikely that laboratory
determinations can cope with the pace that new pollutants are identified.
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