Alkali Cation Chelation in Cold β‑O‑4 Tetralignol Complexes

We
employ cold ion spectroscopy (UV action and IR–UV double
resonance) in the gas phase to unravel the qualitative structural
elements of G-type alkali metal cationized (X = Li+, Na+, K+) tetralignol complexes connected by β-O-4
linkages. The conformation-specific spectroscopy reveals a variety
of conformers, each containing distinct infrared spectra in the OH
stretching region, building on recent studies of the neutral and alkali
metal cationized β-O-4 dimers. The alkali metal ion is discovered
to bind in penta-coordinate pockets to ether and OH groups involving
at least two of the three β-O-4 linkages. Different binding
sites are distinguished from one another by the number of M+···OH···O interactions present in the
binding pocket, leading to characteristic IR transitions appearing
below 3550 cm–1. This interaction is mitigated in
the major conformer of the K+ adduct, demonstrating a clear
impact of the size of the charge center on the three-dimensional structure
of the tetramer.

我们采用气相冷离子光谱法（cold ion spectroscopy）——涵盖紫外光诱导光谱（UV action spectroscopy）与红外-紫外双共振光谱（IR–UV double resonance spectroscopy）——以解析由β-O-4连接键连接的G型碱金属阳离子化（X = Li⁺、Na⁺、K⁺）四木质醇复合物的定性结构特征。基于近期针对中性及碱金属阳离子化β-O-4二聚体的研究，本研究通过构象特异性光谱法鉴定出多种构象体，每种构象体在羟基（OH）伸缩振动区均具有独特的红外光谱。研究发现，碱金属离子可结合于五配位口袋中，与至少两个β-O-4连接键所涉及的醚键及羟基形成配位作用。不同的结合位点可通过结合口袋内M⁺···OH···O相互作用的数量加以区分，对应特征红外跃迁的波数均低于3550 cm⁻¹。在钾离子（K⁺）加合物的主要构象体中，该相互作用被显著削弱，这清晰展现了电荷中心尺寸对该四聚体三维结构的影响。