FULL PAPER DOI: 10.1002/ejic.200700228 Proton Affinities in Water of Maingroup-Element Hydrides – Effects of Hydration and Methyl Substitution Marcel Swart,[a][‡] Ernst Rösler,[a] and F. Matthias Bickelhaupt*[a] Keywords: Acidity / Basicity / Density functional calculations / Periodic table / Proton affinities / Thermochemistry / Solvent effects We have computed the proton affinities in water of arche- dration has little effect on the trends in PA values, especially typal anionic and neutral bases across the periodic table in the case of the neutral bases. However, in the case of the using the generalized gradient approximation (GGA) of den- anionic bases, hydration drastically reduces the magnitude sity functional theory (DFT) at BP86/QZ4P//BP86/TZ2P. The of the PA values. Finally, we have studied how proton affin- main purpose of this work is to provide an intrinsically con- ities in water are affected by methyl substitution at the proto- sistent set of values of the 298-K proton affinities in aqueous philic center. – solution of all anionic (XHn–1 ) and neutral bases (XHn) con- stituted by maingroup-element hydrides of groups 14–17 and (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, the noble gases (i.e., group 18) along the periods 1–6. Hy- Germany, 2007) 1. Introduction Previously, we have studied the gas-phase proton affin- ities and trends therein of anionic and neutral maingroup- Designing new (and optimizing existing) approaches and element hydrides and noble gases.[14,15] Gas-phase proton routes in chemical synthesis requires knowledge of the ther- affinities are obviously directly applicable to gas-phase mochemistry involved in the targeted reactions. In this con- chemistry[3–6] but they are also relevant for chemistry occur- text, the proton affinity (PA) of a reactant or intermediate ring in the condensed phase.[1,16] On one hand, they reveal species B often plays an important role. This thermochemi- the intrinsic basicity of the protophilic species involved and, cal quantity is defined as the enthalpy change associated thus, they shed light on how this property is affected by the [1–8] with dissociation of the conjugate acid [Equation (1)]. solvent. On the other hand, they can serve as a universal, BH+ Ǟ B + H+; ∆H = PA (1) solvent-independent framework of reference, from which the actual basicity of a species in solution might be ob- Overall reaction enthalpies and reaction barriers (and tained through correcting for the particular solvent under thus reaction rates) are related to the PA, as soon as proton consideration.[17–19] transfer occurs somewhere along the cascade of elementary Here, we focus on the proton affinities in water and how steps of a reaction mechanism. This is often the case, as the presence of the solvent changes the basicity of the pro- proton transfer is ubiquitous in chemical reaction mecha- tophilic species. The solvent environment may be taken into nisms, either as simple proton transfer (PT) or as part of a account using different approximations, e.g., through em- more complex chemical transformation, for example, base- pirical corrections (Born, Onsager), or by including explicit induced elimination reactions that may compete with nucle- solvent molecules, or through placing the solute in a dielec- ophilic substitution.[9–13] And, as chemical reactions are tric continuum (COSMO[20,21]). The empirical corrections usually carried out in the condensed phase, knowledge of give only a rough estimate for the solvation process, while the condensed-phase PA values is vital for the understanding the inclusion of explicit solvent molecules necessitates sam- and prediction of (competing) reaction mechanisms. pling of the solvents’ degrees of freedom in molecular simu- lations[22,23] (Monte Carlo, molecular dynamics). We choose [a] Theoretische Chemie, Scheikundig Laboratorium der Vrije Uni- the dielectric continuum (COSMO[20,21]) model as a reason- versiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands able compromise between accuracy and feasibility. Fax: +31-20-59-87629 The present study has three objectives. First, we aim at E-mail: [email protected] [‡] Present address: ICREA (Institució Catalana de Recerca i Es- setting up a complete description at BP86/QZ4P//BP86/ tudis Avançats) Researcher at Institut de Química Computa- TZ2P of the proton affinities (PA = ∆acidH298 of reaction cional, Universitat de Girona, 1) in aqueous solution for the anionic (B = XH –) and Campus Montilivi, 17071 Girona, Catalonia, Spain n–1 Supporting information for this article is available on the neutral (B = XHn) maingroup-element hydrides of groups WWW under http://www.eurjic.org or from the author. 14–17 and the noble gases, i.e. group 18, and periods 1–6. 3646 © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2007, 3646–3654 Running Head: Proton Affinities in Water FULL PAPER In addition to the PA values of all bases (∆acidH298), we To the best of our knowledge, our series of in total 65 also report the corresponding 298 K reaction entropies bases has never before been studied consistently with one (∆acidS298, provided as –T∆acidS298 values), and 298 K reac- and the same method over its full range, either experimen- tion free energies (∆acidG298). We have previously shown tally or theoretically. The very consistency in the values of that this level of density functional theory (DFT)[24–32] per- the present study makes our data particularly suitable for forms well for computing gas-phase PA values.[14,15] Sec- inferring accurate trends in condensed-phase basicity across ond, we wish to evaluate directly the effect of hydration on the periodic table. the PA values by comparing the condensed-phase values to the previously obtained[14,15] gas-phase values. Third, we ex- 2. Results and Discussion amine the influence of stepwise methylation of the proto- – 2.1. Effect of Hydration on Proton Affinities philic center X in species MemXHn–m–1 and MemXHn–m (for periods 2 and 3), for example, PH3, MePH2, Me2PH, The computed proton affinities in water (and the corre- Me3P. sponding –T∆acidS298 and ∆acidG298 values) of all model – [a] Table 1. Thermodynamic acidity properties (kcal/mol) at T = 298 K for anionic bases MemHn–mX in water. Group 14 Group 15 Group 16 Group 17 Base ∆H –T∆S ∆G Base ∆H –T∆S ∆G Base ∆H –T∆S ∆G Base ∆H –T∆S ∆G Period 2 – – – – CH3 220.5 –8.9 211.5 NH2 202.9 –7.5 195.4 OH 182.9 –6.6 176.4 F 158.1 –5.7 152.4 – – – MeCH2 226.6 –8.9 217.7 MeNH 206.6 –7.5 199.1 MeO 183.6 –7.1 176.5 – – Me2CH 229.7 –8.0 221.7 Me2N 205.1 –7.0 198.1 – Me3C 229.5 –7.2 222.3 Period 3 – – – – SiH3 193.0 –8.2 184.8 PH2 185.1 –7.5 177.6 SH 164.9 –6.4 158.5 Cl 143.2 –5.4 137.8 – – – MeSiH2 201.6 –7.6 194.0 MePH 192.0 –7.6 184.4 MeS 171.2 –6.3 164.8 – – Me2SiH 208.3 –7.9 200.4 Me2P 198.2 –7.3 190.9 – Me3Si 213.4 –7.3 206.0 Period 4 – – – – GeH3 179.4 –8.3 171.2 AsH2 179.8 –7.5 172.3 SeH 160.4 –6.3 154.1 Br 140.0 –5.2 134.8 Period 5 – – – – SnH3 169.7 –8.1 161.6 SbH2 176.5 –7.5 169.0 TeH 158.7 –6.2 152.5 I 140.8 –5.1 135.7 Period 6 – – – – PbH3 150.4 –7.0 143.3 BiH2 174.7 –7.4 167.3 PoH 157.3 –6.2 151.1 At 140.3 –5.0 135.3 – + [a] Computed at BP86/QZ4P//BP86/TZ2P for the reaction MemHn–m–1XHǞMemHn–m–1X + H with n = 3, 2, 1 and 0 for groups 14, 15, 16 and 17, respectively. [a] Table 2. Thermodynamic acidity properties (kcal/mol) at T = 298 K for neutral bases MemHn–mX in water. Group 15 Group 16 Group 17 Group 18 Base ∆H –T∆S ∆G Base ∆H –T∆S ∆G Base ∆H –T∆S ∆G Base ∆H –T∆S ∆G Period 1 He 35.9 –5.9 30.0 Period 2 NH3 160.8 –7.2 153.6 OH2 127.2 –6.8 120.4 FH 88.7 –6.6 82.1 Ne 39.9 –5.7 34.3 MeNH2 164.2 –7.6 156.6 MeOH 129.9 –7.5 122.4 MeF 96.5 –6.5 90.0 Me2NH 165.2 –7.5 157.6 Me2O 131.2 –7.1 124.2 Me3N 164.7 –7.3 157.5 Period 3 PH3 143.6 –7.2 136.4 SH2 130.6 –7.5 123.1 ClH 101.5 –6.4 95.1 Ar 68.3 –5.4 62.9 MePH2 152.7 –7.4 145.4 MeSH 136.9 –7.7 129.2 MeCl 108.3 –6.3 102.0 Me2PH 159.3 –7.5 151.8 Me2S 141.9 –7.8 134.1 Me3P 164.2 –7.0 157.2 Period 4 AsH3 131.8 –7.2 124.6 SeH2 129.6 –7.5 122.1 BrH 103.9 –6.3 97.6 Kr 75.4 –5.3 70.1 Period 5 SbH3 127.7 –6.7 121.0 TeH2 133.6 –6.8 126.7 IH 111.7 –6.2 105.5 Xe 89.3 –5.1 84.2 Period 6 BiH3 110.4 –6.6 103.8 PoH2 133.0 –6.8 126.2 AtH 112.5 –6.2 106.3 Rn 91.8 –5.0 86.7 + + [a] Computed at BP86/QZ4P//BP86/TZ2P for the reaction MemHn–mXH Ǟ H + MemHn–mX with n = 3, 2, 1 and 0 for groups 15, 16, 17 and 18, respectively.