Gas phase acidities and associated equilibrium isotope effects for selected main group mono- and polyhydrides, carbon acids, and oxyacids: A G4 and W1BD study
Introduction
Gas phase acidities represent intrinsic physical properties of interest and can be exploited for a variety of applications. Correlations have been developed between electronegativity (Χ) and hardness (μ) and the gas phase acidities of second and third row compounds, suggesting that both increased electronegativity and polarizability (softness) contribute to acidity. Thus, more electronegative and softer components on the conjugate base allow it to better accommodate the additional charge following deprotonation [1]. In addition to numerous works investigating gas phase acidities of isolated groups of compounds, several studies have considered more comprehensive viewpoints, often including theoretical treatments to complement and add to the experimental database and structure–property trends (see, e.g., Refs. [2], [3], [4], [5], [6], [7], [8], [9]).
Isotopic substitution, either primary (the atoms comprising the acidic bond) or secondary (other atoms in the molecule), will also play a role in the gas phase acidity. In general, secondary hydrogen–deuterium equilibrium isotope effects (EIEs) on gas phase acidities are much smaller (typically ∼<1 kJ/mol for the α-position, and ∼<0.5 kJ/mol for the β-position) than primary EIEs [10], for which the experimental database [11] shows an effect typically on the order of ∼5 to 10 kJ/mol. However, despite the broad interest in isotope effects across all disciplines of chemistry, it appears relatively few experimental or theoretical studies have investigated EIEs on gas phase acidities [10] (of interest, we also note the following EIE solution phase studies on acidity constants [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]). Consequently, in the current work we examine the gas phase acidities of various main group hydrides, carbon acids, and oxyacids and representative isotopologues using high-level theoretical methods, providing comparison to experimental data where possible, and investigating potential periodic trends and other structure–property relationships.
Section snippets
Computational details
Calculations were conducted using the Gaussian-4 (G4) [22] and W1BD [23], [24] methods in Gaussian 09 [25] on the Western Canada Research Grid (WestGrid; project 100185 [K. Forest]) and the Shared Hierarchical Academic Research Computing Network (SHARCNET; project sn4612 [K. Forest]) of Compute/Calcul Canada. All calculations used the same gas phase starting geometries obtained with the PM6 semiempirical method [26] as implemented in MOPAC 2009 (http://www.openmopac.net/; v. 9.281). Molecular
Results and discussion
Gas phase standard state (298.15 K, 1 atm) enthalpies (ΔacidH°(g)) and free energies (ΔacidG°(g)) of acid dissociation were calculated at the G4 and W1BD levels for a range of perproteated main group hydrides (Table 1), carbon acids (Table 2), and oxyacids (Table 3) extending up to the basis set atomic number limits for the respective methods (G4, bromine; W1BD, chlorine). Experimental ΔacidH°(g) were available for comparison with all perproteated main group hydrides from the NIST database [11].
Acknowledgements
This work was made possible by the facilities of the Western Canada Research Grid (WestGrid: www.westgrid.ca; project 100185), the Shared Hierarchical Academic Research Computing Network (SHARCNET: www.sharcnet.ca; project sn4612), and Compute/Calcul Canada. We thank an anonymous reviewer for constructive comments, as well as suggestions for additional calculations on partial isotopic substitution, that improved the quality of the manuscript.
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