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Theoretical studies on the pK_a values of perfluoroalkyl carboxylic acids: Chain helicity

Sierra Rayne¹ and Kaya Forest²

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1. Ecologica Research
2. Okanagan College

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Date:

Received 06 October 2009 04:37 UTC; Posted 06 October 2009

Subjects:

Chemistry, Earth & Environment

Tags:

• perfluoroalkyl carboxylic acids; pKa; environmental fate; acidity constants

Abstract:

Semiempirical, ab initio, and density functional theory (DFT) studies were conducted on a homologous series of straight chain perfluoroalkyl carboxylic acids (PFCAs) ranging in chain length from C₁ (trifluoroacetic acid; TFA) to C₉ (perfluorodecanoic acid). Regardless of perfluoroalkyl chain length and computational method and level employed, no significant change in the electrostatic or steric characteristics of the carboxylate head group was found between C₂ and C₉. Application of a thermodynamic cycle to calculations at the B3LYP/6-311++G(3df,2p) level provided an accurate prediction of the experimental monomeric pK_a for TFA. At this – and similar – levels of theory, monomeric pK_a values of longer straight chain PFCAs are expected to display no significant increase with increasing chain length, having a relatively constant pK_a value of about 0 that is in excellent agreement with earlier predictions and with recent experimental evidence. Contrary to previous claims, perfluoroalkyl chain helicity does not appear to result in increased monomeric PFCA pK_a values at chain lengths >5 perfluorocarbons.

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This document is licensed to the public under the Creative Commons Attribution 3.0 License

How to cite this document:

Rayne, Sierra and Forest, Kaya. Theoretical studies on the pK_a values of perfluoroalkyl carboxylic acids: Chain helicity. Available from Nature Precedings <http://hdl.handle.net/10101/npre.2009.3829.1> (2009)

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