FULL PAPER A quantum theory atoms in molecules investigation of Lewis base protonation Natieli Alves da Silva | Luiz Alberto Terrabuio | Roberto Luiz Andrade Haiduke Departamento de Química e Física Molecular, Instituto de Química de S~ ao Carlos, Universidade de S~ ao Paulo, Av. Trabalhador S~ ao-Carlense, 400 – CP 780, S~ ao Carlos, S~ ao Paulo 13560-970, Brazil Correspondence Roberto Luiz Andrade Haiduke, Departamento de Química e Física Molecular, Universidade de S~ ao Paulo, S~ ao Carlos, S~ ao Paulo, Brazil. Email: haiduke@iqsc.usp.br Funding information The authors thank FAPESP for financial support (Grants 2014/23714-1 and 2010/ 18743-1, S~ ao Paulo Research Foundation). Abstract This investigation uses atomic properties derived from the quantum theory of atoms in molecules formalism to rationalize the infrared intensity of the stretching vibration that arises as a Lewis base (B) is protonated (B-H mode). Moreover, the interacting quantum atom (IQA) partition is employed to evaluate the energetics of protonation. All calculations are performed at the CCSD/ cc-pVQZ level except by the IQA analysis, which is carried out by means of the B3LYP/cc-pVQZ// CCSD/cc-pVQZ treatment. First, an efficiency scale is established for Lewis bases in terms of the electronic charge transfer potential. Next, this study shows that the intensity of the B-H stretching depends mostly on the electronic charge amount transferred to the proton. Thus, intensity data provide empirical assessment of Lewis base charge transfer efficiency. Finally, the group separa- tion observed during correlation of proton affinities and electronic charge transfer potential is explained by the interaction energy between fragments of the protonated system. KEYWORDS Lewis base, protonation, infrared intensities, charge–charge flux-dipole flux model 1 | INTRODUCTION The protonation of Lewis bases (B) leading to BH 1 species represents one of the most fundamental processes to chemistry. According to the formal definition, these bases are electron pair donors while the proton acts as an electron pair acceptor. This view also implies that there is a coordination of the base to the acid, resulting in a product known as a Lewis adduct. [1] The study of structural and energetic changes induced by protona- tion of specific sites in polyfunctional molecules is useful to understand the complex chemistry that can occur due to the interaction of differ- ent functional groups within the charged system. [2] Hence, acidity and basicity are relevant concepts to explain the chemical reactivity, the catalytic properties of different materials and biochemical processes occurring in organisms. [3–6] The proton transfer, for example, is funda- mental to the chemiosmosis and, consequently, for maintaining vital activities. [7] Hence, most of the research about acidity and basicity is now focused on molecules of biological interest, particularly, amino acids and peptides. [8–10] For instance, special attention has been devoted to the identification of localization sites of atomic charge con- centration during protonation of amino acids residues. [11] Beyond the relevance of protonation to processes that take place in aqueous solutions, one can also mention the recent interest in the detection of these BH 1 species in the interstellar medium or circum- stellar shells, where they can survive in gaseous phase for much longer periods of time because of the extreme rarefied conditions normally encountered there. Thus, protonation reactions are important to astro- chemistry because hydrogen is the most abundant cosmic element and the H 1 cation originated under ionization is the precursor of several new species. [12] Thus, the list of molecular systems already found in these environments contains several typical examples of simple proto- nated systems such as HCO 1 , HCS 1 , HOC 1 , HCNH 1 , HOCO 1 ,H 3 O 1 , H 2 COH 1 , NH 1 4 , NCCNH 1 , and HC 3 NH 1 . [13] Protonated species are intermediates in interstellar cloud chemistry models, allowing the deter- mination of neutral species abundances through of the adjustment of such models. [12] At least two different criteria could be used to evaluate the effi- ciency of a Lewis base. The most commonly chosen strength scale is a measure of thermodynamic stability for the protonated species with respect to reactants, as given by proton affinity, defined as the nega- tive of the enthalpy change for the protonation reaction. [3,14] Beyond thermodynamic measurements, few spectroscopic estimates of basicity Int. J. Quantum Chem. 2016; 1–12 http://q-chem.org V C 2016 Wiley Periodicals, Inc. | 1 Received: 6 June 2016 | Revised: 6 October 2016 | Accepted: 6 October 2016 DOI 10.1002/qua.25310