Strong Even/Odd Pattern in the Computed Gas-Phase Stability of Dicarboxylic Acid Dimers: Implications for Condensation Thermodynamics

The
physical properties of small straight-chain dicarboxylic acids
are well known to exhibit even/odd alternations with respect to the
carbon chain length. For example, odd numbered diacids have lower
melting points and higher saturation vapor pressures than adjacent
even numbered diacids. This alternation has previously been explained
in terms of solid-state properties, such as higher torsional strain
of odd number diacids. Using quantum chemical methods, we demonstrate
an additional contribution to this alternation in properties resulting
from gas-phase dimer formation. Due to a combination of hydrogen bond
strength and torsional strain, dimer formation in the gas phase occurs
efficiently for glutaric acid (C5) and pimelic acid (C7) but is unfavorable
for succinic acid (C4) and adipic acid (C6). Our results indicate
that a significant fraction of the total atmospheric gas-phase concentration
of glutaric and pimelic acid may consist of dimers.

众所周知，小型直链二羧酸的物理性质会随碳链长度呈现奇偶交替规律。例如，相较于相邻的偶数碳二羧酸，奇数碳二羧酸的熔点更低、饱和蒸气压更高。此前，这类奇偶交替现象多从固态性质角度予以解释，例如奇数碳二羧酸存在更高的扭转张力。本研究采用量子化学（quantum chemical）方法，证实气相二聚体形成过程会对该性质的奇偶交替现象产生额外贡献。鉴于氢键强度与扭转张力的共同作用，气相二聚体形成在戊二酸（C5）与庚二酸（C7）中可高效发生，但对琥珀酸（C4）和己二酸（C6）而言则并不有利。我们的研究结果表明，大气中戊二酸与庚二酸的气相总浓度中，有相当一部分可能以二聚体形式存在。