Accurate heats of formation and acidities for H3PO4, H2SO4, and H2CO3 from ab initio electronic structure calculations

Abstract

AbstractAtomization energies and heats of formation at 0 and 298 K for H2CO3, HCO, H2SO4, HSO, H3PO4, and H2PO were calculated using coupled cluster theory including noniterative, quasi‐perturbative triple excitations (CCSD(T)) with large basis sets. Optimized geometries and harmonic vibrational frequencies were calculated at the MP2/aug‐cc‐pVTZ level. Atomization energies were obtained by extrapolating CCSD(T) valence energies to the complete basis set limit (CBS) using the aug‐cc‐pV(n+d)Z (n = D, T, Q) basis sets where the +d corresponds to the inclusion of tight d functions for the second row atoms P and S. In order to achieve near chemical accuracy (±1 kcal/mol) in the thermodynamic properties, a core/valence correction, a Douglas‐Kroll‐Hess scalar relativistic correction, and a first‐order atomic spin‐orbit correction were included. The calculated heats of formation of the neutral molecules at 0 K are ΔH(H2CO3) = –275.2, ΔH(H2SO4) = –167.9, and ΔH(H3PO4) = –268.8 kcal/mol. The only experimental value is ΔH(H2SO4) = –172.4 ± 2, a difference of about 4 kcal/mol. The calculated gas acidities (ΔG) at 298 K are 332.2, 322.2, and 304.6 kcal/mol for H2CO3, H3PO4, and H2SO4, respectively. The calculated values for H3PO4 and H2SO4 are in good agreement with the respective experimental values of 323.0 ± 4.9 and 302.3 ± 2.6 kcal/mol. Solution acidities are computed using the fully polarizable continuum model. Using an electron density contour value of 0.001 a.u. to define the cavity for the neutral molecules and of 0.0022 a.u. for the anions, we calculate pKa(H2CO3) = 6.3, pKa(H3PO4) = 2.5, and pKa(HNO3) = –2.4 which are within one pKa of the respective experimental values of 6.4, 2.1, and –1.4. We predict the pKa of H2SO4 in aqueous solution to be in the range of –6 to –8, significantly lower than the estimated experimental values in the range of –3. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005