**Guillermo Caballero-García, Gustavo Mondragón-Solórzano, Raúl Torres-Cadena, Marco Díaz-García, Jacinto Sandoval-Lira and Joaquín Barroso-Flores \***

Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM, Unidad San Cayetano, Carretera Toluca—Atlacomulco km 14.5, Personal de la UNAM, Toluca 50200, Mexico; gcaballerog91@gmail.com (G.C.-G.); gustavms93@gmail.com (G.M.-S.); torres.cadena.raul@gmail.com (R.T.-C.); diaz.marco95@outlook.com (M.D.-G.); jsandovalira@gmail.com (J.S.-L.) **\*** Correspondence: jbarroso@unam.mx; Tel.: +52-722-276-6610 (ext. 7754)

Academic Editor: Steve Scheiner

Received: 16 October 2018; Accepted: 25 December 2018; Published: 26 December 2018

**Abstract:** The theoretical calculation of p*K*a values for Brønsted acids is a challenging task that involves sophisticated and time-consuming methods. Therefore, heuristic approaches are efficient and appealing methodologies to approximate these values. Herein, we used the maximum surface electrostatic potential (*V*S,max) on the acidic hydrogen atoms of carboxylic acids to describe the H-bond interaction with water (the same descriptor that is used to characterize σ-bonded complexes) and correlate the results with experimental p*K*a values to obtain a predictive model for other carboxylic acids. We benchmarked six different methods, all including an implicit solvation model (water): Five density functionals and the Møller–Plesset second order perturbation theory in combination with six different basis sets for a total of thirty-six levels of theory. The ωB97X-D/cc-pVDZ level of theory stood out as the best one for consistently reproducing the reported p*K*a values, with a predictive power of 98% correlation in a test set of ten other carboxylic acids.

**Keywords:** p*K*a; hydrogen bond; maximum surface potential; σ–hole
