Strong Hydrogen Bonded Molecular Interactions between Atmospheric Diamines and Sulfuric Acid

We investigate the molecular interaction
between methyl-substituted N,N,N′,N′-ethylenediamines, propane-1,3-diamine,
butane-1,4-diamine,
and sulfuric acid using computational methods. Molecular structure
of the diamines and their dimer clusters with sulfuric acid is studied
using three density functional theory methods (PW91, M06-2X, and ωB97X-D)
with the 6-31++G­(d,p) basis set. A high level explicitly correlated
CCSD­(T)-F12a/VDZ-F12 method is used to obtain accurate binding energies.
The reaction Gibbs free energies are evaluated and compared with values
for reactions involving ammonia and atmospherically relevant monoamines
(methylamine, dimethylamine, and trimethylamine). We find that the
complex formation between sulfuric acid and the studied diamines provides
similar or more favorable reaction free energies than dimethylamine.
Diamines that contain one or more secondary amino groups are found
to stabilize sulfuric acid complexes more efficiently. Elongating
the carbon backbone from ethylenediamine to propane-1,3-diamine or
butane-1,4-diamine further stabilizes the complex formation with sulfuric
acid by up to 4.3 kcal/mol. Dimethyl-substituted butane-1,4-diamine
yields a staggering formation free energy of −19.1 kcal/mol
for the clustering with sulfuric acid, indicating that such diamines
could potentially be a key species in the initial step in the formation
of new particles. For studying larger clusters consisting of a diamine
molecule with up to four sulfuric acid molecules, we benchmark and
utilize a domain local pair natural orbital coupled cluster (DLPNO-CCSD­(T))
method. We find that a single diamine is capable of efficiently stabilizing
sulfuric acid clusters with up to four acid molecules, whereas monoamines
such as dimethylamine are capable of stabilizing at most 2–3
sulfuric acid molecules.

本研究采用计算方法，探究了甲基取代的N,N,N′,N′-乙二胺类、丙-1,3-二胺、丁-1,4-二胺与硫酸之间的分子相互作用。我们采用三种密度泛函理论（Density Functional Theory, DFT）方法，即PW91、M06-2X与ωB97X-D，结合6-31++G(d,p)基组，对上述二胺及其与硫酸形成的二聚团簇的分子结构进行了研究。我们使用高精度显式关联CCSD(T)-F12a/VDZ-F12方法获取准确的结合能，并对反应吉布斯自由能进行计算，同时与涉及氨以及大气相关一元胺（甲胺、二甲胺、三甲胺）的反应数值进行对比。研究发现，硫酸与本次研究所涉二胺之间形成复合物的反应自由能，与二甲胺相当甚至更具优势。含有一个或多个仲氨基的二胺可更高效地稳定硫酸复合物。将碳骨架从乙二胺延长至丙-1,3-二胺或丁-1,4-二胺，可进一步提升与硫酸形成复合物的稳定性，最大稳定能提升幅度可达4.3 kcal/mol。二甲基取代丁-1,4-二胺与硫酸团簇的形成自由能可达惊人的-19.1 kcal/mol，表明此类二胺可能是新粒子形成初始步骤中的关键物种。为研究包含一个二胺分子与最多四个硫酸分子的更大团簇，我们对域定域对自然轨道耦合簇（Domain Local Pair Natural Orbital Coupled Cluster, DLPNO-CCSD(T)）方法进行了基准测试并加以应用。结果表明，单个二胺可高效稳定包含最多四个硫酸分子的团簇，而二甲胺等一元胺最多仅能稳定2~3个硫酸分子。