ACD/pK_a DB

Technical Information, Page 3

• Varieties of pK_a 
• Microscopic vs. Macroscopic pK_a Constants 
• How Does ACD/pK_a DB Predict pK_a Values?  
• Calculations 

Below is a brief overview of how the ACD/pK_a DB software works. It is not necessary to know these details in order to use the prediction software. We believe, however, that an understanding of the background processes will give you greater confidence in the predictions generated.

Databases of Experimental pK_a Values

Two reference databases are provided with ACD/pK_a DB:

• The database of aqueous experimental values contains ~16,000 records with experimental values collected under different temperatures and ionic strengths in purely aqueous solutions
• A smaller database of experimental pK_a values for ~2000 compounds is also provided in non-aqueous solvents such as acetone, methanol, and pyridine

In both databases, each experimental value is accompanied by a reference to the original literature source. The reference databases can be searched directly but cannot be modified by end-users.

Database of Hammett-Type Equations

The Hammett-type equations used in ACD/pK_a DB calculations have been parameterized to cover the most popular ionizable functional groups. Each functional group has been characterized by several equations involving different types of substituent constants to achieve the most accurate calculation. All equations for a given functional group have been ranked according to their reliability (number of correlated structures, correlation coefficient, and standard deviation) and reliability of available substituent constants.

Database of Electronic Substituent Constants (σ)

There are many variants of the original electronic substituent constant, sigma (σ). The internal training set of ACD/pK_a DB contains >3,000 carefully derived experimental electronic constants.

Estimation of Electronic Substituent Constants

Although the parameter database of the internal training set contains a wide array of σ values, in some cases no reliable constant is available. When the required substituent constant is not available from the experimental database, it is calculated by one of four algorithms that describe electronic effect transmissions through molecular systems.

More information is available upon request from physchem@acdlabs.com