Kreinbihl John J, Frederiks Nicoline C, Waller Sarah E, Yang Yi, Johnson Christopher J
Department of Chemistry, Stony Brook University, 100 Nicolls Road, Stony Brook, New York 11794-3400, USA.
J Chem Phys. 2020 Jul 21;153(3):034307. doi: 10.1063/5.0015094.
Atmospheric new particle formation is the process by which atmospheric trace gases, typically acids and bases, cluster and grow into potentially climatically relevant particles. Here, we evaluate the structures and structural motifs present in small cationic ammonium and aminium bisulfate clusters that have been studied both experimentally and computationally as seeds for new particles. For several previously studied clusters, multiple different minimum-energy structures have been predicted. Vibrational spectra of mass-selected clusters and quantum chemical calculations allow us to assign the minimum-energy structure for the smallest cationic cluster of two ammonium ions and one bisulfate ion to a C-symmetry structure that is persistent under amine substitution. We derive phenomenological vibrational frequency scaling factors for key bisulfate vibrations to aid in the comparison of experimental and computed spectra of larger clusters. Finally, we identify a previously unassigned spectral marker for intermolecular bisulfate-bisulfate hydrogen bonds and show that it is present in a class of structures that are all lower in energy than any previously reported structure. Tracking this marker suggests that this motif is prominent in larger clusters as well as ∼180 nm ammonium bisulfate particles. Taken together, these results establish a set of structural motifs responsible for binding of gases at the surface of growing clusters that fully explain the spectrum of large particles and provide benchmarks for efforts to improve structure predictions, which are critical for the accurate theoretical treatment of this process.
大气新粒子形成是指大气痕量气体（通常是酸和碱）聚集并生长为可能与气候相关的粒子的过程。在此，我们评估了小阳离子铵和硫酸氢铵簇中存在的结构和结构基序，这些簇已通过实验和计算研究作为新粒子的种子。对于几个先前研究过的簇，已预测出多种不同的最低能量结构。质量选择簇的振动光谱和量子化学计算使我们能够将两个铵离子和一个硫酸氢根离子组成的最小阳离子簇的最低能量结构指定为一种C对称结构，该结构在胺取代下保持不变。我们推导了关键硫酸氢根振动的唯象振动频率缩放因子，以帮助比较较大簇的实验光谱和计算光谱。最后，我们确定了一种先前未归属的分子间硫酸氢根 - 硫酸氢根氢键的光谱标记，并表明它存在于一类能量均低于任何先前报道结构的结构中。追踪这个标记表明，这个基序在较大的簇以及约180纳米的硫酸氢铵颗粒中也很突出。综上所述，这些结果建立了一组负责在生长簇表面结合气体的结构基序，这些基序完全解释了大颗粒的光谱，并为改进结构预测的努力提供了基准，这对于该过程的准确理论处理至关重要。
