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

Abstract

Ϫ 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(nϩd)Z (n ϭ D, T, Q) basis sets where the ϩd corresponds to the inclusion of tight d functions for the second row atoms P and S. In order to achieve near chemical accuracy (Ϯ1 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 are in good agreement with the respective experimental values of 323.0 Ϯ 4.9 and 302.3 Ϯ 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 pK a (H 2 CO 3 ) ϭ 6.3, pK a (H 3 PO 4 ) ϭ 2.5, and pK a (HNO 3 ) ϭ -2.4 which are within one pK a of the respective experimental values of 6.4, 2.1, and -1.4. We predict the pK a of H 2 SO 4 in aqueous solution to be in the range of -6 to -8, significantly lower than the estimated experimental values in the range of -3.

Structures, fragmentation, and protonation of trideoxynucleotide CCC mono- and dianions

“…, in good agreement with their own experimental observation that the gas-phase acidity of phosphoric acid lies between those of HCl and HBr [35]. Our own theoretical estimates (Table 2) compare favorably with this finding and a very high level ab initio value of Alexeev et al [36] of PA ϭ 1376 kJ mol Ϫ1 , compared to our "best" estimate (G3) of PA ϭ 1376 kJ mol Ϫ1 . The proton affinity seems to be quite unaffected by dimethylation; our calculations at the RI-MP2/ Table 2).…”

Superacidity of P(OH)3 and SO(OH)2 derivatives of cyclopentadiene and vinylcyclopentadiene in the gas phase: A computational DFT analysis

“…This is further supported by knowing that Morris et al determined the Δ G acid of H 3 PO 4 between 318.1 to 327.9 kcal mol −1 , while NIST Database reports the value of Δ H acid = 330.5 ±5 kcal mol −1 . Also, the G4‐computed Δ H acid and Δ G acid values for H 3 PO 4 are 329.1 and 322.1, respectively, while Alexeev et al reported values of 328.8 and 322.2 kcal mol −1 using MP2 calculations.…”

Gram-negative bacterial lipid A analysis by negative electrospray ion trap mass spectrometry: Stepwise dissociations of deprotonated species under low energy CID conditions

“…In the gas phase, the phosphate groups being the lipids A most acidic sites [21], a distant double deprotonation can occur regioselectively on these sites to give rise to the formation of the doubly charged [M PP − 2H] 2− lipid A (i.e., m/z 897.6), as displayed in the ESI mass spectrum (Fig. 2 The lack of resolution for the signal at m/z 897.6 as compared to the m/z 792.5 ion can be explained by considering that this doubly charged ion is somewhat unstable and may decompose promptly by application of the axial modulation during the analytical scan [27][28][29][30] into the ion trap analyzer.…”

“…Lipid A contains one or two phosphoryl group(s), considered to be very acidic in the gas phase [21]. The phosphate substitute(s) the N-acetylglucosamine disaccharide at the C-1 and/or C-4 position(s).…”