These are a set of global and local minima of water clusters containing 2-10 waters. The geometry of each isomer was optimized using RI-MP2/aug-cc-pVDZ. The RI-MP2/CBS binding energy was calculated by extrapolating the RI-MP2/aug-cc-pVDZ//aug-cc-pVDZ, RI-MP2/aug-cc-pVTZ//aug-cc-pVDZ, and RI-MP2/aug-cc-pVQZ//aug-cc-pVDZ energies to their complete basis set limit using a 4-5 inverse 4-5 polynomial scheme that has been used extensively for water clusters. ?CCSD(T) correction using aug-cc-pVDZ basis set was added to estimate the CCSD(T)/CBS binding energy. Zero-point vibrational energy and finite temperature corrections within the ideal-gas-rigid-rotor-harmonic-oscillator (IGRRHO) model using scaled and unscaled harmonic vibrational frequencies can be found in the published article.

Date Published: 2014-10-14
Date Added: 2017-05-11
Date dateModified: 2014-10-14
Tags: h-bond, water-cluster, water, MP2
Reference: Temelso, B., Archer, K.A., Shields, G. C. J. Phys. Chem. A, 115(43), 12034-12046 (2011)
DOI: http://pubs.acs.org/doi/abs/10.1021/jp2069489
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The role of the binary nucleation of sulfuric acid in aerosol formation and its implications for global warming is one of the fundamental unsettled questions in atmospheric chemistry. We have investigated the thermodynamics of sulfuric acid hydration using ab initio quantum mechanical methods. For H2SO4(H2O)n where n = 1–6, we used a scheme combining molecular dynamics configurational sampling with high-level ab initio calculations to locate the global and many low lying local minima for each cluster size. For each isomer, we extrapolated the Møller–Plesset perturbation theory (MP2) energies to their complete basis set (CBS) limit and added finite temperature corrections within the rigid-rotor-harmonic-oscillator (RRHO) model using scaled harmonic vibrational frequencies. We found that ionic pair (HSO4–·H3O+)(H2O)n−1 clusters are competitive with the neutral (H2SO4)(H2O)n clusters for n ≥ 3 and are more stable than neutral clusters for n ≥ 4 depending on the temperature. The Boltzmann averaged Gibbs free energies for the formation of H2SO4(H2O)n clusters are favorable in colder regions of the troposphere (T = 216.65–273.15 K) for n = 1–6, but the formation of clusters with n ≥ 5 is not favorable at higher (T > 273.15 K) temperatures. Our results suggest the critical cluster of a binary H2SO4–H2O system must contain more than one H2SO4 and are in concert with recent findings(1) that the role of binary nucleation is small at ambient conditions, but significant at colder regions of the troposphere. Overall, the results support the idea that binary nucleation of sulfuric acid and water cannot account for nucleation of sulfuric acid in the lower troposphere.

Date Published: 2012-02-01
Date Added: 2017-05-12
Date dateModified: 2017-05-12
Tags: h-bond, h2so4-h2o_n, MP2, aerosol, hydration
Reference: B. Temelso, T. Morrell, R. Shields, M. Allodi, E. Wood,K. N. Kirschner, T. Castonguay, K. Archer and G. C. Shields. J. Phys. Chem. A. 119, 2209-2224 (2012)
DOI: http://pubs.acs.org/doi/abs/10.1021/jp2119026
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Using molecular dynamics configurational sampling combined with ab initio energy calculations, we determined the low energy isomers of the bisulfate hydrates. We calculated the CCSD(T) complete basis set (CBS) binding electronic and Gibbs free energies for 53 low energy isomers of HSO4–(H2O)n=1–6 and derived the thermodynamics of adding waters sequentially to the bisulfate ion and its hydrates. Comparing the HSO4–/H2O system to the neutral H2SO4/H2O cluster, water binds more strongly to the anion than it does to the neutral molecules. The difference in the binding thermodynamics of HSO4–/H2O and H2SO4/H2O systems decreases with increasing number of waters. The thermodynamics for the formation of HSO4–(H2O)n=1–5 is favorable at 298.15 K, and that of HSO4–(H2O)n=1–6 is favorable for T < 273.15 K. The HSO4– ion is almost always hydrated at temperatures and relative humidity values encountered in the troposphere. Because the bisulfate ion binds more strongly to sulfuric acid than it does to water, it is expected to play a role in ion-induced nucleation by forming a strong complex with sulfuric acid and water, thus facilitating the formation of a critical nucleus.

Date Published: 2012-04-11
Date Added: 2017-05-12
Date dateModified: 2017-05-12
Tags: h-bond, ionic, hso4-h2o_n, aerosol, MP2, hydration
Reference: D. E. Husar, B. Temelso, A. Asheworth and G. C. Shields. J. Phys. Chem. A. 116, 5151-5163 (2012)
DOI: http://pubs.acs.org/doi/abs/10.1021/jp300717j
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We have investigated the thermodynamics of sulfuric acid dimer hydration using ab initio quantum mechanical methods. For (H2SO4)2(H2O)n where n = 0–6, we employed high-level ab initio calculations to locate the most stable minima for each cluster size. The results presented herein yield a detailed understanding of the first deprotonation of sulfuric acid as a function of temperature for a system consisting of two sulfuric acid molecules and up to six waters. At 0 K, a cluster of two sulfuric acid molecules and one water remains undissociated. Addition of a second water begins the deprotonation of the first sulfuric acid leading to the di-ionic species (the bisulfate anion HSO4–, the hydronium cation H3O+, an undissociated sulfuric acid molecule, and a water). Upon the addition of a third water molecule, the second sulfuric acid molecule begins to dissociate. For the (H2SO4)2(H2O)3 cluster, the di-ionic cluster is a few kcal mol–1 more stable than the neutral cluster, which is just slightly more stable than the tetra-ionic cluster (two bisulfate anions, two hydronium cations, and one water). With four water molecules, the tetra-ionic cluster, (HSO4–)2(H3O+)2(H2O)2, becomes as favorable as the di-ionic cluster H2SO4(HSO4–)(H3O+)(H2O)3 at 0 K. Increasing the temperature favors the undissociated clusters, and at room temperature we predict that the di-ionic species is slightly more favorable than the neutral cluster once three waters have been added to the cluster. The tetra-ionic species competes with the di-ionic species once five waters have been added to the cluster. The thermodynamics of stepwise hydration of sulfuric acid dimer is similar to that of the monomer; it is favorable up to n = 4–5 at 298 K. A much more thermodynamically favorable pathway forming sulfuric acid dimer hydrates is through the combination of sulfuric acid monomer hydrates, but the low concentration of sulfuric acid relative to water vapor at ambient conditions limits that process.

Date Published: 2012-09-12
Date Added: 2017-05-12
Date dateModified: 2017-05-12
Tags: h-bond, 2-h2so4-h2o_n, MP2, aerosol
Reference: B. Temelso, T. N. Phan and G. C. Shields. J. Phys. Chem. A. 116(39), 9745-9758 (2012)
DOI: http://pubs.acs.org/doi/abs/10.1021/jp3054394
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The binary H2SO4–H2O nucleation is one of the most important pathways by which aerosols form in the atmosphere, and the presence of ternary species like amines increases aerosol formation rates. In this study, we focus on the hydration of a ternary system of sulfuric acid (H2SO4), methylamine (NH2CH3), and up to six waters to evaluate its implications for aerosol formation. By combining molecular dynamics (MD) sampling with high-level ab initio calculations, we determine the thermodynamics of forming H2SO4(NH2CH3)(H2O)n, where n = 0–6. Because it is a strong acid–base system, H2SO4–NH2CH3 quickly forms a tightly bound HSO4––NH3CH3+ complex that condenses water more readily than H2SO4 alone. The electronic binding energy of H2SO4–NH2CH3 is −21.8 kcal mol–1 compared with −16.8 kcal mol–1 for H2SO4–NH3 and −12.8 kcal mol–1 for H2SO4–H2O. Adding one to two water molecules to the H2SO4–NH2CH3 complex is more favorable than adding to H2SO4 alone, yet there is no systematic difference for n ≥ 3. However, the average number of water molecules around H2SO4–NH2CH3 is consistently higher than that of H2SO4, and it is fairly independent of temperature and relative humidity.

Date Published: 2015-04-15
Date Added: 2017-05-12
Date dateModified: 2017-05-12
Tags: h-bond, h2so4-ma-h2o_n, aerosol, MP2, hydration
Reference: D. J. Bustos, B. Temelso and G. C. Shields. J. Phys. Chem. A. 118(35), 7430-7441 (2014)
DOI: http://pubs.acs.org/doi/abs/10.1021/jp500015t
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An efficient mixed molecular dynamics/quantum mechanics model has been applied to the water cluster system. The use of the MP2 method and correlation consistent basis sets, with appropriate correction for BSSE, allows for the accurate calculation of electronic and free energies for the formation of clusters of 2 to 10 water molecules. This approach reveals new low energy conformers for (H2O)n=7,9,10. The water heptamer conformers comprise five different structural motifs ranging from a three-dimensional prism to a quasi-planar book structure. A prism-like structure is favored energetically at low temperatures, but a chair-like structure is the global Gibbs free energy minimum past 200 K. The water nonamers exhibit less complexity with all the low energy structures shaped like a prism. The decamer has 30 conformers that are within 2 kcal/mol of the Gibbs free energy minimum structure at 298 K. These structures are categorized into four conformer classes, and a pentagonal prism is the most stable structure from 0 to 320 K. Results can be used as benchmark values for empirical water models and density functionals, and the method can be applied to larger water clusters.

Date Published: 2010-09-30
Date Added: 2017-09-15
Date dateModified: 2017-09-15
Tags: water,water-cluster, h-bond, MP2, h2o, h2o_n
Reference: Robert M. Shields, Berhane Temelso, Kaye A. Archer, Thomas E. Morrell, and George C. Shields. J. Phys. Chem. A, 2010, 114 (43), pp 11725–11737
DOI: http://pubs.acs.org/doi/abs/10.1021/jp104865w
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The abundance of α-fetoprotein (AFP), a natural protein produced by the fetal yolk sac during pregnancy, correlates with lower incidence of estrogen receptor positive (ER+) breast cancer. The pharmacophore region of AFP has been narrowed down to a four amino acid (AA) region in the third domain of the 591 AA peptide. Our computational study focuses on a 4-mer segment consisting of the amino acids threonine-proline-valine-asparagine (TPVN). We have run replica exchange molecular dynamics (REMD) simulations and used 120 configurational snapshots from the total trajectory as starting configurations for quantum chemical calculations. We optimized structures using semiempirical (PM3, PM6, PM6-D2, PM6-H2, PM6-DH+, PM6-DH2) and density functional methods (TPSS, PBE0, M06-2X). By comparing the accuracy of these methods against RI-MP2 benchmarks, we devised a protocol for calculating the lowest energy conformers of these peptides accurately and efficiently. This protocol screens out high-energy conformers using lower levels of theory and outlines a general method for predicting small peptide structures.

Date Published: 2014-04-05
Date Added: 2017-05-12
Date dateModified: 2017-05-12
Tags:
Reference: B. Temelso, K. A. Alser, A. Gauthier, A. K. Palmer and G. C. Shields J. Phys. Chem. B 118(17), 4514-4526 (2014)
DOI: http://pubs.acs.org/doi/abs/10.1021/jp500017b
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Broadband rotational spectroscopy of water clusters produced in a pulsed molecular jet expansion has been used to determine the oxygen atom geometry in three isomers of the nonamer and two isomers of the decamer. The isomers for each cluster size have the same nominal geometry but differ in the arrangement of their hydrogen bond networks. The nearest neighbor O[BOND]O distances show a characteristic pattern for each hydrogen bond network isomer that is caused by three-body effects that produce cooperative hydrogen bonding. The observed structures are the lowest energy cluster geometries identified by quantum chemistry and the experimental and theoretical O[BOND]O distances are in good agreement. The cooperativity effects revealed by the hydrogen bond O[BOND]O distance variations are shown to be consistent with a simple model for hydrogen bonding in water that takes into account the cooperative and anticooperative bonding effects of nearby water molecules.

Date Published: 2014-10-14
Date Added: 2017-05-11
Date dateModified: 2017-05-11
Tags: h-bond, water, MP2, water-cluster
Reference: C. Pérez, D. P. Zaleski, N. A. Seifert, B. Temelso, G. C. Shields, Z. Kisiel, B. H. Pate. Angewandte Chemie 53(52), 14368-14372 (2014)
DOI: http://onlinelibrary.wiley.com/doi/10.1002/anie.201407447/abstract
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We have studied the structure and stability of H3O+(H2O)8 clusters using a combination of molecular dynamics sampling and high-level ab initio calculations. 20 distinct oxygen frameworks are found within 2 kcal/mol of the electronic or standard Gibbs free energy minimum. The impact of quantum zero-point vibrational corrections on the relative stability of these isomers is quite significant. The box-like isomers are favored in terms of electronic energy, but with the inclusion of zero-point vibrational corrections and entropic effects tree-like isomers are favored at higher temperatures. Under conditions from 0 to 298.15 K, the global minimum is predicted to be a tree-like structure with one dangling singly coordinated water molecule. Above 298.15 K, higher entropy tree-like isomers with two or more singly coordinated water molecules are favored. These assignments are generally consistent with experimental IR spectra of (H3O+)(H2O)8 obtained at ∼150 K.

Date Published: 2013-10-01
Date Added: 2017-05-31
Date dateModified: 2017-05-31
Tags: h-bond, ionic, MP2
Reference: B. Temelso, T. Koddermann, K. N. Kirschner, K. Klein and G. C. Shields Comp. Theor. Chem. 1021, 240-248 (2013)
DOI: https://doi.org/10.1016/j.comptc.2013.07.039
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We report the results of a broadband rotational spectroscopic study of corannulene, C20H10, all of its singly substituted 13C isotopologues, and a complex of corannulene with one molecule of water. Corannulene is a polycyclic aromatic hydrocarbon (PAH) with a curved structure that results in a large dipole moment. Observation of 13C isotopic species in natural abundance allowed us to precisely determine the molecular structure of corannulene. Our experimental and theoretical results indicate that the water molecule rotates freely around its C2 axis and that dispersion interactions are the dominant contribution to the binding.

Date Published: 2017-04-25
Date Added: 2017-09-01
Date dateModified: 2017-09-01
Tags: h-bond, hydration, MP2, interstellar, spectroscopy
Reference: C. Perez,A. L. Steber, A. M. Rijs, B. Temelso, G. C. Shields, J. C. Lopez, Z. Kisiel, M. Schnell. Phys. Chem. Chem. Phys., 2017,19, 14214-14223
DOI: http://pubs.rsc.org/En/content/articlelanding/2017/cp/c7cp01506b
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Polycyclic aromatic hydrocarbons (PAHs) are key players in reaction chemistry. While it is postulated that they serve as a basis for ice grains, there has been no direct detection of PAHs in astronomical environments. We aim to investigate the hydration of PAHs to set a foundation for the future exploration of potential ice formation pathways. We report results from chirped pulse Fourier transform microwave spectroscopy and quantum-chemical calculations for the PAH acenaphthene and acenaphthene complexed with up to four water molecules. The acenaphthene–(H2O)3 complex is of particular interest as the elusive cyclic water trimer was observed. It appears in a slightly distorted configuration when compared with the pure water trimer. This is explained by hydrogen-bond net cooperativity effects. Binding energies for the complexes are presented. Our results provide insight into the onset of complex aggregation that could be occurring in extraterrestrial environments as part of ice grain formation.

Date Published: 2017-12-07
Date Added: 2018-01-27
Date dateModified: 2018-01-27
Tags: h-bond, hydration, MP2, interstellar, PAH, acenaphthene, spectroscopy
Reference: J. Phys. Chem. Lett., 2017, 8 (23), pp 5744–5750
DOI: http://pubs.acs.org/doi/10.1021/acs.jpclett.7b02695
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The rich potential energy surface of the water undecamer was explored with a basin-hopping algorithm using a TIP4P potential followed by extensive ab initio MP2 minimizations and CCSD(T) corrections. This protocol yielded 17, 66, and 125 distinct minima within 0.5, 1.0 and 2.0 kcal mol-1 of the complete basis set CCSD(T) global minimum, respectively. These minima were categorized into fifteen different groups based on their oxygen framework and hydrogen bonding topology. Determination of the global minimum proved challenging because of the presence of many nearly isoenergetic isomers. The predicted global minimum varied between ab initio methods, density functionals and model potentials, and it was sensitive to the choice of energy extrapolation schemes, higher-order CCSD(T) corrections and inclusion of zero-point vibrational energy. The presence of a large number of nearly degenerate structures and the isomerization between them has manifested itself in the anomalous broadening of the heat capacity curve of the undecamer in simulations around the melting region.

Date Published: 2017-09-05
Date Added: 2017-09-15
Date dateModified: 2017-09-15
Tags: h-bond, h2o_n,water, water-cluster, MP2, CCSDt
Reference: Berhane Temelso, Katurah L. Klein, Joel W. Mabey, Cristóbal Pérez, Brooks H. Pate, Zbigniew Kisiel, George C. Shields. JCTC 2017
DOI: http://doi.org/10.1021/acs.jctc.7b00938
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Sulfate aerosols' cooling effect on the global climate has spurred research to understand their mechanisms of formation. Both theoretical and laboratory studies have shown that the formation of sulfate aerosols is enhanced by the presence of a base like ammonia. Stronger alkylamine bases such as monomethylamine (MMA), dimethylamine (DMA) and trimethylamine (TMA) further increase aerosol formation rates by many orders of magnitude relative to that of ammonia. However, recent lab measurements have found that presence of ammonia and alkylamines together increases nucleation rates by another 1-2 orders of magnitude relative to the stronger alkylamines alone. This work explores that observation by studying the thermodynamic stability of clusters containing up to two sulfuric acids and two bases of the same or different type. Initial configurational sampling is performed using genetic algorithm (GA) interfaced to semi­empirical methods to find a large number of low energy configurations. These structures are then subject to quantum mechanical calculations using PW91, M06­2X and ωB97X­D functionals and MP2 with large basis sets. The thermodynamics of formation is reviewed to determine if it rationalizes why mixed base systems yield higher rates of aerosol formation than single base ones. The gas phase basicity of the bases in a cluster is the main determinant of binding strength in smaller clusters such as those in the current study while aqueous phase basicity is more important for larger particles. Besides thermodynamic considerations, the differences in aerosol formation mechanisms as a function of size, and between the gas and particle phases are discussed.

Date Published: 2018-01-19
Date Added: 2018-01-19
Date dateModified: 2018-01-19
Tags: h-bond, aerosol, MP2, H2SO4, bases, ammonia, amines
Reference: J. Phys. Chem. A, Just Accepted Manuscript
DOI: http://pubs.acs.org/doi/10.1021/acs.jpca.7b11236
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