8 research outputs found
Hydration of Atmospheric Molecular Clusters II: Organic Acid-Water Clusters
Using
computational methods, we study the gas phase hydration of
three different atmospherically relevant organic acids with up to
10 water molecules. We study a dicarboxylic acid (pinic acid) and
a tricarboxylic acid (3-methyl-1,2,3-butanetricarboxylic acid (mbtca))
that are both identified as products from Îą-pinene oxidation
reactions. We also study a 2-oxohexanediperoxy acid (ohdpa) that has
been identified as a product from cyclohexene autoxidation. To sample
the cluster structures, we employ our recently developed systematic
hydrate sampling technique and identify a total of 551 hydrate clusters.
The cluster structures and thermochemical parameters (at 298.15 K
and 1 atm) are obtained at the ĎB97X-D/6-31++GÂ(d,p) level of
theory, and the single point energy of the clusters have been refined
using a high level DLPNOâCCSDÂ(T)/aug-cc-pVTZ calculation. We
find that all three tested organic acids interact significantly more
weakly with water compared to the primary nucleation precursor sulfuric
acid. Even at 100% relative humidity (298.15 K and 1 atm), we find
that ohdpa remains unhydrated and only the monohydrate of pinic acid
and mbtca are slightly populated (4% and 2%, respectively). From the
obtained molecular structures, potential implications for the ice
nucleating ability of aerosol particles is discussed
Hydration of Atmospheric Molecular Clusters: A New Method for Systematic Configurational Sampling
We
present a new systematic configurational sampling algorithm
for investigating the potential energy surface of hydrated atmospheric
molecular clusters. The algorithm is based on creating a Fibonacci
sphere around each atom in the cluster and adding water molecules
to each point in nine different orientations. For the sampling of
water molecules to existing hydrogen bonds, the cluster is displaced
along the hydrogen bond, and a water molecule is placed in between
in three different orientations. Generated redundant structures are
eliminated based on minimizing the root-mean-square distance of different
conformers. Initially, the clusters are sampled using the semiempirical
PM6 method and subsequently using density functional theory (M06-2X
and ĎB97X-D) with the 6-31++GÂ(d,p) basis set. Applying the developed
algorithm, we study the hydration of sulfuric acid with up to 15 water
molecules. We find that the addition of the first four water molecules
âsaturateâ the sulfuric acid molecule and that they
are more thermodynamically favorable than the addition of water molecules
5â15. Using the large generated set of conformers, we assess
the performance of approximate methods (ĎB97X-D, M06-2X, PW91,
and PW6B95-D3) in calculating the binding energies and assigning the
global minimum conformation compared to high level CCSDÂ(T)-F12a/VDZ-F12
reference calculations. The tested DFT functionals systematically
overestimate the binding energies compared to coupled cluster calculations,
and we find that this deficiency can be corrected by a simple scaling
factor
Hydration of Atmospheric Molecular Clusters: A New Method for Systematic Configurational Sampling
We
present a new systematic configurational sampling algorithm
for investigating the potential energy surface of hydrated atmospheric
molecular clusters. The algorithm is based on creating a Fibonacci
sphere around each atom in the cluster and adding water molecules
to each point in nine different orientations. For the sampling of
water molecules to existing hydrogen bonds, the cluster is displaced
along the hydrogen bond, and a water molecule is placed in between
in three different orientations. Generated redundant structures are
eliminated based on minimizing the root-mean-square distance of different
conformers. Initially, the clusters are sampled using the semiempirical
PM6 method and subsequently using density functional theory (M06-2X
and ĎB97X-D) with the 6-31++GÂ(d,p) basis set. Applying the developed
algorithm, we study the hydration of sulfuric acid with up to 15 water
molecules. We find that the addition of the first four water molecules
âsaturateâ the sulfuric acid molecule and that they
are more thermodynamically favorable than the addition of water molecules
5â15. Using the large generated set of conformers, we assess
the performance of approximate methods (ĎB97X-D, M06-2X, PW91,
and PW6B95-D3) in calculating the binding energies and assigning the
global minimum conformation compared to high level CCSDÂ(T)-F12a/VDZ-F12
reference calculations. The tested DFT functionals systematically
overestimate the binding energies compared to coupled cluster calculations,
and we find that this deficiency can be corrected by a simple scaling
factor