Calculated Spectroscopy and Atmospheric Photodissociation of Phosphoric Acid

Abstract

Detection of phosphine (PH(3)) gas in the upper troposphere suggests that the biogeochemical P cycle also includes an atmospheric component that consists of volatile phosphorus-containing molecules. A reasonable end product for the oxidation of PH(3) in the atmosphere is phosphoric acid (H(3)PO(4)). We propose that H(3)PO(4) may be photodissociated into HOPO(2) and H(2)O in the stratosphere, where H(3)PO(4) is likely to be present in gaseous form. We have calculated the energy barrier of this reaction and show that in addition to electronic transitions, OH-stretching overtone transitions can also provide the necessary energy. OH-stretching fundamental and overtone transitions were calculated with the use of an anharmonic oscillator local mode model. The probability of overtone induced photodissociation was estimated with molecular dynamical reaction coordinate simulations. Electronic transitions were calculated with the equation of motion coupled cluster singles doubles method. We have calculated the photodissociation rate constants for absorption of visible, UV, and Lyman-alpha radiation at altitudes from 20 to 100 km. We show that at altitudes between 30 and 70 km, the photodissociation of H(3)PO(4) is likely to proceed via absorption in the UV region by electronic transitions.

Fundamental and overtone vibrational spectroscopy, enthalpy of hydrogen bond formation and equilibrium constant determination of the methanol–dimethylamine complex

“…The difference between the use of a Morse potential approximation for the potential and a full potential with a numeric solution of the 1D Schrodinger is at most a few wavenumbers. 70,71 For the hydrogen bonded OH-stretching vibration in the water dimer the difference was less than 1 cm À1 and for the MeOH-DMA complex we find a difference of 6 cm À1 . The dimensionless oscillator strength of a transition from the ground vibrational state to an excited vibrational state is calculated using the transition frequency and the transition dipole moment matrix.…”

Phosphoric acid – a potentially elusive participant in atmospheric new particle formation

“…Phosphoric acid exists in both a C 1 and C 3 conformation. At the CCSD(T)/aug-cc-pV(T+d)Z level of theory [75] the C 3 conformation has been identified to be 1.5 kcal/mol lower in energy than the C 1 conformation and we will only consider this conformer in the cluster sampling. The following sampling routine is applied: As DFT can be prone to errors in the electronic binding energy, the single point energies of each conformation is evaluated at the CCSD(T)-F12a/VDZ-F12 level of theory.…”

Formic Acid Catalyzed Gas‐Phase Reaction of H2O with SO3 and the Reverse Reaction: A Theoretical Study

“…These values are closely consistent with the calculated value of 36.0 kcal mol À1 . [19] Therefore, these test calculations show that both of the theoretical methods produce consistent results for the binding and activation energies of the reaction between SO 3 and H 2 O. From the geometrical point of view, according to the microwave spectrum of gaseous sulfuric acid, [70] the equivalent S=O (1.422 ), SÀO (1.574 ), and OÀH (0.970 ) bond lengths agree well with the calculated geometrical parameters of S=O (1.423 , 1.421 ), SÀO (1.588, 1.594 ), and OÀH (0.970, 0.968 ) at the MP2/aug-cc-pv(T+d)z and B3LYP/cc-pv(T+d)z levels, respectively.…”

“…As the ratio of the rate constant [18] of the reaction SO 3 + HO 2 ···H 2 O to that of SO 3 + H 2 O···H 2 O is 10 6 and the ratio between the HO 2 ···H 2 O complex and the H 2 O···H 2 O complex is 10 À6 , the process competes well with the water dimer to lead to formation of sulfuric acid. In addition, since the barrier of unimolecular decomposition of sulfuric acid into SO 3 and H 2 O is high (36.0 kcal mol À1 [19] ), direct decomposition of H 2 SO 4 into SO 3 and H 2 O is improbable. It was proposed that H 2 SO 4 and its hydrate are photolyzed to SO 3 via overtone excitation of the OH stretch in sulfuric acid.…”