Meylan and Howard

Group contribution and bond contribution methods (Hine and Mookerjee 1975, Meylan and Howard 1991) ... 

A more recent study for estimating Henry s law constants using the bond contribution method was provided by Meylan and Howard (1991). In this study, the authors updated and revised the method developed by Hine and Mookeijee (1975) based on new experimental data that have become available since 1975. Bond contribution values were determined for 59 chemical bonds based on known Henry s law constants for 345 organic compounds. A good statistical fit [correlation coefficient (r ) = 0.94] was obtained when the bond contribution values were regressed against known Henry s law constants for all compounds. For selected chemicals classes, r increased slightly to 0.97. 

A rate constant for the vapor phase reaction of bis(2-chloroethyl) ether with OH radicals in the atmosphere has been estimated to be 3.2 x 10 cm /molecule-sec at 25 °C. At an atmospheric concentration of 5 x 10 OH radicals/cm , the corresponding half-life is 5 d (Meylan and Howard, 1993). 

Illustrative Example 6.4 Estimating Air-Water Partition Constants by the Bond Contribution Method Problem Estimate the Kiavi values at 25°C of (a) n-hexane, (b) benzene, (c) diethylether, and (d) ethanol using the bond contribution values given in Table 6.4. Compare these values with the experimental air-water partition constants given in Table 3.4. Note that for a linear or branched alkane (i.e., hexane) a correction factor of +0.75 log units has to be added (Meylan and Howard, 1991). 

In the following, the atom/fragment contribution method (AFC method) developed by Meylan and Howard (1995) is used to illustrate the approach. This method is similar to the CLOGP method, but it is easier to see its application without using a computer program. Here, we confine ourselves to a few selected examples of fragment coefficients and correction factors. This will reveal how the method is... 

Data from Meylan and Howard (1995) total number of correction factors derived 235. b o = ortho, m = meta,p = para substitution.c See Illustrative Example 7.2. 

Method of Meylan and Howard Meylan and Howard [9] expanded the bond contribution method of Hine and Mookerjee. Based on 345 compounds they derived bond contributions for 59 different bond types. Their method has been validated with an independent set of 74 structurally diverse compounds, obtaining a correlation coefficient of 0.96. Their method also needs correction factors for several structural-substructural features. This method has been implemented into a Henry s law constant program performing AWPC (25°C) estimations from SMILES input [15]. 

In addition to these publications, software is available that allows the user to determine vapor pressures of a wide variety of compounds at room temperature. The Texas Research Center (TRC) (1996) distributes a PC DOS/Windows database that contains experimentally derived Antoine constants for approximately 6000 chemicals from which vapor pressures at user-selected temperatures can be calculated. Another Windows-based program, MPBPVP by Meylan and Howard (1996), estimates the vapor pressure of organic compounds from their SMILES (Simplified Molecular Input Line Entry System) structure and their boiling points using the Antoine equation, the Grain-Watson method, and the Mackay method. 

Method 2 Bond Contribution Method of Meylan and Howard (1991)... 

More recently, Brennan et al. (1997) compared five methods for estimating KAW, namely the vapor pressure/solubility ratio, the group or bond contribution method, linear solvation energy methods, and molecular connectivity. The authors compared the methods by application to a common set of 150 chemicals and concluded that the Meylan and Howard (1991) bond contribution method and the molecular connectivity index method of Nirma-lakhandan and Speece (1988) are comparably accurate, having standard deviations of, 0.29 and 0.34 log units, respectively. 

Meylan and Howard (1991) essentially updated and revised this method. They list 59 bond contributions, derived from a data set for 345 chemicals. By using only the bond contributions a standard deviation of 0.45 log units was obtained, with a mean error of 0.30 log units. If correction factors are applied for certain classes, for example, for cyclic alkanes or epoxides, the standard deviation falls to 0.34 and the mean error to 0.21 units. When applied to a validation set not used to develop the correlation, the standard deviation was 0.46 log units and the mean error 0.31 log units. This method is simple and fairly accurate. It is also available as part of a suite of estimation methods (the EPIWIN Suite) from the Syracuse Research Corporation. 

The Hine and Mookerjee method has lasted well but is now outdated, and the similar Meylan and Howard correlation supercedes it. This method has the significant advantage of transparency and simplicity thus it is preferred by many over the more complex and less intuitively satisfying connectivity/polarizability methods. If time is available, and espe-... 

Table 4.1, reproduced from Meylan and Howard (1991), lists the 59 bond contributions, and Table 4.2 lists correction factors. These groups and factors when combined, yield the log (base 10) water to air partition coefficient. Table 4.3 compares the two methods and can be used to guide the method selection process. The original paper gives specimen calculations... 

Comparison of Mean Class Errors for the Meylan and Howard Method and Nirmalakhandan and Speece Method. Copyright by Pergamon Press. Reproduced with permission. 

In this appendix, H and KAW are estimated for four of the benchmark chemicals using reported data on solubility and vapor pressure and the Meylan and Howard method. 

One method used often relies on structure-reactivity relationships (Atkinson, 1986,1987, 1988 Kwok and Atkinson, 1995). This empirical estimation method has been shown to provide good agreement (generally to within a factor of 2) between experimentally measured and calculated room temperature rate constants for 90% of 485 organic compounds (Kwok and Atkinson, 1995). It is the basis of the Syracuse Research Corporation s "Atmospheric Oxidation Program" [see Meylan and Howard (1993) for a discussion of an earlier version of this program]. The general approach of this estimation method has been described (Atkinson, 1986,1987,1988 Kwok and Atkinson, 1995). 

Reaction of DEHP vapor with hydroxyl radicals in the atmosphere has been predicted, with an estimated half-life of about 6 hours using the Atmospheric Oxidation Program (Meylan and Howard 1993). The atmospheric half-life, however, is expected to be longer for DEHP adsorbed to atmospheric particulates. Based on the estimated half-life alone, extensive transport of DEHP would not be expected and concentrations in Antarctic snow would not be predicted. Nonetheless, DEHP appears to be present in urban and rural atmospheres (see Section 6.4), and its transport might be mainly in the sorbed state. Data confirming this degradation pathway have not been located. Direct photolysis and photooxidation are not likely to be important (Warns 1987). 

Mackay et al. (2000) recommend the bond contribution method of Meylan and Howard (1991), one of two methods of predicting the Henry s law constant used by the SRC program HENRYWIN. These are developed from the work of Hine and Mookeijee (1975). HENRYWIN also predicts the Henry s law constant based on a group contribution method. 

The log Kqw values were calculated by Andreas Kaune using the LOG KOW Program of Meylan and Howard [232a, b]. 

Duffythe group contribution approach of Klopman and the neural network model of Huuskonen. " The PCCHEM program used at the US Environmental Protection Agency (EPA) incorporates three different equations. All of them are similar to GSE but have different coefficients to predict aqueous solubility depending on the range of log P values. " Meylan and Howard used a database of 817 (RMSE = 0.62) compounds to derive a heuristic equation ... 

The Meylan-Howard hydrophobic model is derived from an atom/fragment contribution method providing 150 hydrophobic atomic and fragmental constants fi measuring the lipophilic contributions of atoms and fragments in the molecule, together with 250 correction factors [Meylan and Howard, 1995,1996, 2000 KOWWIN - Syracuse Research Corporation, 2008]. 

Up to now there has only been a theoretical estimation of the rate constants for the atmospheric gas-phase reaction of these compounds with OH radicals (Meylan and Howard, 1993). According to the estimate both 3M2N and 5M2N should have the same rate constant of... 

Little information was found in the available literature concerning the transformation of wood or coal tar creosote components in the atmosphere. Some volatile coal tar constituents may undergo oxidation by vapor phase reaction with photochemically produced hydroxyl radicals, with calculated half-lives of 2 hours to 10 days based on experimental and estimated rate constants of 1.12-103xl012 cm/molecules-second at 25 °C and using an average atmospheric hydroxyl radical concentration of 5x10s molecules/cm3 (Atkinson 1989 Meylan and Howard 1993). Rates may be slowed since some components will exist as... 

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