Abstract:
Group contribution method is one of the main methods for predicting the physicochemical properties of organic compounds, the position of each group is not taken into consideration by classical group contribution method yet. In this work, atomic connectivity group contribution (ACGC) method is developed for predicting critical properties of organic compounds for the first time. Herein, a new group defining method, namely atomic adjacent group (AAG) method, is proposed to describe the relationship between core atom and its adjacent atoms. For distinguishing isomers effectively, the shape factor (SF) is used to describe the effect of molecular shape on group, and atomic connectivity factors (ACF) are defined for describing the position of each group in a molecule. ACGC models using a general formula are developed for predicting three properties of 710 organic compounds. The external verification and internal verification methods, usually ignored by the traditional group contribution method, were further utilized during the modelling process. Compared with AAG model, ARE decreased by 6.82-42.57 % when SF was considered and, ARE decreased by 24.19-62.25 % when both SF and ACF were applied as using the ACGC method. Accordingly, SF and ACF are effective in improving the group contribution method and ACGC method is accurate in calculating the properties of organic compounds.
KEYWORDS: Atomic connectivity group contribution (ACGC) method; Group contribution (GC) method; Atomic connectivity factors (ACF); Critical properties; Shape factor (SF)
Introduction
The physical and transport properties of compounds play an indispensable role in many chemical engineering applications. Critical properties (critical temperature T c,critical pressureP c,critical volume V c) are important properties of substances in chemistry and chemical engineering. For example, when calculating thermodynamic properties and transfer parameters of matter by corresponding state method, critical properties of matter need to be used. When the corresponding state method or state equation method is used to solve the PVT relation of pure matter and mixtures1, 2, the knowledge of critical properties is needed. Critical properties are essential property parameters for understanding the phase behavior of pure components and mixtures3. Critical properties are also needed in situations where critical phenomena are studied for high-pressure operations such as supercritical extraction4, 5 and oil drilling6. In view of the importance of critical properties, a great deal of effort has been expended on collecting, sorting and evaluating them bit by bit. Unfortunately, data on experimental critical properties of organic compounds are limited. Experimental measurements of critical properties are sometimes laborious, expensive, and even difficult to measure2. On the other hand, with the increase of organic compound data, some experimental data cannot be found from the database, so it becomes crucial to develop mathematical models to provide reasonable estimates of these properties.
As a simple pen-and-paper structure-property relationship (SPR) method7-9, group contribution (GC) method is based on the principle of group addition10. The group contribution method is widely used to predict various thermodynamic properties of organic compounds, such as critical temperature11-20, critical pressure13, 21-24 , critical volume17, 25 and normal boiling point10, 26. Among GC approaches, Joback27, C-G (Constantinou and Gani)28 and M-G (Marrero and Gani)29 methods are classical GC methods. Joback27 is the first-order group technique and the principal advantages of this method are the simplicity and generality. And yet the combination of first-order groups in one molecule may result in some isomeric molecules and thus the properties of these molecules calculated by first-order group contribution method is the same. The first-order groups are insufficient in describing the portions of the molecular structure and in distinguishing isomers. Based on the first-order group technique, Constantinou and Gani28developed the second-order group technique, which further considers the influence of the first and second nearest neighbors of the group under consideration. In C-G method, 12 first order level groups and 41 second order level groups were used for about 300 compounds. Average relative error (ARE) is in the range of 0.85-2.89 % for critical properties and decreased by 22-48 % compared with first order level group models. In order to better describe the molecular structure part and distinguish isomers, Marrero and Gani29 developed a group contribution (M-G) method at three levels. In M-G method, 124 first order level groups, 79 second order level groups and 32 third order level groups were used for about 800 compounds with critical properties. The use of third level provided more structural information about compounds and improved the accuracy and applicability of GC method. For example, ARE for critical properties decreased by 5-12 % compared with second order level group models. Hukkerikar et. al.30presented revised model parameters for M-G method (M-G+) at three levels for 18 pure component properties with about 12000 compounds. 130 first order level groups, 90 second order level groups and 31 third order level groups were used for about 900 compounds with critical properties. The critical properties of ARE are higher than MG method.
The hypothesis of classical GC method is that a group has an equal contribution value at any molecule. The position of each group plays an important role in the properties of compounds, while it is also not considered by the traditional group contribution methods. In addition, no uniform rule for group division was developed in the above classical GC methods, which also brought inconvenience to the division of complex compounds. Also, traditional group contribution methods usually do not carry out external verification and internal verification, so the stability of the model is not verified. Sun and Sahinidis13 proposed a method to identify functional groups in molecular structures and to verify the stability of the model by dividing the training and test sets. A total of 74 functional groups were used to describe the critical properties of about 860 organic compounds. TheR 2 of critical temperature, critical pressure and critical volume are 0.96, 0.95 and 0.99 respectively, and the ARE are 2.02 %, 4.09 % and 3.10 % respectively.
The aim of proposed atomic connectivity group contribution (ACGC) method is to establish a general and simple group contribution method for calculating phase transition properties of organic compounds. A general formula containing group, shape factor and localization factor is used to predict the different properties of organic compounds. The present work is completed for the calculation of the critical properties. Based on the relationship between core atom and its adjacent atoms,atomic adjacent group (AAG) with unified dividing rules is defined. In order to describe the influence of the molecular shape of compounds such as branched chains and rings on the calculation of properties, the shape factor (SF) is defined. The atomic connectivity factors (ACF) are defined for describing the position of each group. The predictive ability and stability of the model are verified by external and internal validation.
Methodology