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QSPR Modelling of ILs + H2O Binary Systems
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3.1. Introduction
Quantitative Structure-Activity Relationships (QSAR) and Quantitative Structure-Property
Relationships (QSPR) are methods for the direct prediction of compound’s properties
based on its molecular structure. These models correlate experimental data with specific
parameters, named molecular descriptors, derived from the compound’s 3D structure.
Using these models it is possible to predict properties of new and hypothetical molecules,
not yet synthesised. QSAR/QSPR models are especially apt to the development of new
drugs, conception of new molecules, molecular modelling and resolution of new problems
related with chemical engineering. QSAR deals mainly with biological/chemical activity
while QSPR with physical and chemical properties.
In 1868 the first publication on the quantitative relation of physiological properties with
the chemical structure appeared 42 . In the 20 th century, during the 60’s, there was an
important progress in this field with the development of a linear free energy relationship
(LFER). These studies, along with the development of computers, allowed the appearance
of new and more complex descriptors 43 .
These models can be used as predictive instruments for the estimation of physical-chemical
properties, identifying the most favourable compounds, thus orienting the synthesis and
reducing the number of compounds to be synthesized. They can also be used as diagnostic
instruments, giving the possible information about types of interaction forces, in the
receptor nature, in the mechanism of action and in detection of exceptions that may occur.
These models show however some limitations, since they are not accurate to describe some
specific interactions between molecules, such as hydrogen bonding 43 .
For QSPR studies a quantitative description of the molecular structure is required, that is
obtained by parameters and descriptors. A parameter describes a molecular property and is
obtained from a regression model using an estimator, while a descriptor is known as a
term, index or a parameter that transmits structural information.
There have been studies of biological and physical-chemical properties such as solubility
of aqueous solutions, vapour pressure, partioning coefficients, among others, by this
approach 44-50 . Recently, QSPR studies have been used in correlations and prediction of
various properties of ILs such as melting points of quaternary ammonium, imidazolium
and pyridinium-based ILs 51 , conductivities and viscosities 52 .
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