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We have calculated XH-stretching (where X=O, C, F, Cl) fundamental and overtone transitions for three diatomics and a few small molecules using a local mode model. The potential energy curves and dipole moment functions are calculated using the recently developed explicitly correlated coupled cluster with single doubles and perturbative triples theory [CCSD_T_-F12] with the associated VXZ-F12 (where X=D, T, Q) basis sets. We find that the basis set convergence of calculated frequencies and oscillator strengths obtained with the explicitly correlated method is much more rapid than with conventional CCSD(T) and the Dunning type correlation consistent basis sets. Furthermore, CCSD(T)-F12 frequencies and oscillator strengths obtained with the VTZ-F12 and VQZ-F12 basis sets are found to be in excellent agreement with the CCSD(T) complete ba...
We have optimized the lowest energy structures and calculated interaction energies for the H2O–H2O, H2O–H2S, H2O–NH3, and H2O–PH3 dimers with the recently developed explicitly correlated CCSD(T)-F12 methods and the associated VXZ-F12 (where X=D,T,Q) basis sets. For a given cardinal number, we find that the results obtained with the CCSD(T)-F12 methods are much closer to the CCSD(T) complete basis set limit than the conventional CCSD(T) results. In general we find that CCSD(T)-F12 results obtained with the VTZ-F12 basis set are better than the conventional CCSD(T) results obtained with an aug-cc-pV5Z basis set. We also investigate two ways to reduce the effects of basis set superposition error with conventional CCSD(T), namely, the popular counterpoise correction and limiting diffuse basis functions to the heavy atoms only. We find that...
We have used different computational methods, including B3LYP, CCSD(T)-F12 and CBS-QB3, to study and compare the addition–elimination reaction of the nitrate radical NO₃ with four sulfur-containing species relevant to atmospheric chemistry: hydrogen sulfide (H₂S), dimethyl sulfide [(CH₃)₂S], dimethyl sulfoxide [(CH3)₂SO] and sulfur dioxide (SO₂). We find that the reaction with (CH₃)₂SO to give NO₂ + (CH₃)₂SO₂ has a very low barrier, and is likely to be the dominant oxidation mechanism for (CH₃)₂SO in the atmosphere. In agreement with previous experimental data and computational results, we find that the reaction with H₂S and SO₂ is very slow, and the reaction with (CH₃)₂S is not competitive with the hydrogen abstraction route. The differences in reaction energetics and rates between the four species are explained in terms of stabilizin...
We have identified the dimethylamine-trimethylamine complex (DMA-TMA) at room temperature in the gas phase. The Fourier transform infrared (FTIR) spectrum of DMA-TMA in the NH-stretching fundamental region was obtained by spectral subtraction of spectra of each monomer. Explicitly correlated coupled cluster calculations were used to determine the minimum energy structure and interaction energy of DMA-TMA. Frequencies and intensities of NH-stretching transitions were also calculated at this level of theory with an anharmonic oscillator local mode model. The fundamental NH-stretching intensity in DMA-TMA is calculated to be approximately 700 times larger than that of the DMA monomer. The measured and calculated intensity is used to determine a room temperature equilibrium constant of DMA-TMA of 1.7 × 10⁻³ atm⁻¹ at 298 K.
We investigate the OH-stretching overtone dynamics of fluorosulfonic acid (FSO₃H) and chlorosulfonic acid (ClSO₃H) using classical trajectory simulations. The initial nuclear coordinates and momenta for the trajectories are sampled from anharmonic correlation-corrected vibrational self consistent field (CC-VSCF) wavefunctions. We consider both OH-stretching overtone states and combination states containing a mix of OH-stretching overtones and SOH-bending or OSOH-torsional modes. Our molecular dynamics simulations confirm that photodissociation of these sulfonic acids to form the corresponding hydrogen halides and sulfur trioxide (HF + SO₃ and HCl + SO₃), is possible via highly vibrationally excited states on a picosecond timescale. Hydrogen-hopping events are also observed in the trajectories whereby the hydrogen atom of the excited OH...
Detection of phosphine (PH₃) 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 PH3 in the atmosphere is phosphoric acid (H₃PO₄). We propose that H₃PO₄ may be photodissociated into HOPO₂ and H₂O in the stratosphere, where H₃PO₄ 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 coord...
The vibrational overtone spectra of propargyl alcohol (prop-2-yn-1-ol, PA), allyl alcohol (prop-2-en-1-ol, AA), propargyl carbinol (but-3-yn-1-ol, PC) and allyl carbinol (but-3-en-1-ol, AC) were recorded with intracavity laser photoacoustic spectroscopy (ICL-PAS) in the ΔvOH = 3, 4 and 5 regions for propargyl alcohol and allyl alcohol and in the ΔvOH = 4 and 5 regions for propargyl carbinol and allyl carbinol. Local mode anharmonic oscillator calculations were performed with explicitly correlated coupled cluster methods to guide spectral assignment. Atoms in molecules (AIM) and non-covalent interactions (NCI) calculations were carried out to analyze the interactions between the OH-group and the π-electrons of the carbon–carbon multiple bonds. We ascertain the effect of the carbon chain length and saturation on the conformation and spec...
We have studied the oxidation of SO₂ to SO₃ by four peroxyradicals and two carbonyl oxides (Criegee intermediates) using both density functional theory, B3LYP, and explicitly correlated coupled cluster theory, CCSD(T)-F12. All the studied peroxyradicals react very slowly with SO₂ due to energy barriers (activation energies) of around 10 kcal/mol or more. We find that water molecules are not able to catalyze these reactions. The reaction of stabilized Criegee intermediates with SO₂ is predicted to be fast, as the transition states for these oxidation reactions are below the free reactants in energy. The atmospheric relevance of these reactions depends on the lifetimes of the Criegee intermediates, which, at present, is highly uncertain.
The water nitric oxide complex has been studied computationally. We consider the four lowest energy structures of the H₂O−NO complex: two from both symmetries ²A′ and ²A′′. We use the coupled cluster method with correlation consistent basis sets in all ab initio calculations. Vibrational transitions have been calculated using a model that describes the complex as two individually vibrating monomer units: H₂O and NO. We use the variational method to solve the vibrational problem. The OH-stretching energy levels and transition intensities are calculated up to the second and NO-stretching to the third overtone region. We also study NO-stretching vibronic transitions (A²Σ+ ← X²Π). We use an isolated local mode approach to calculate energies and oscillator strengths of the vibronic transitions. The results for the complex are compared to th...
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