Chatterjee Kuntal, Dopfer Otto
Institut für Optik und Atomare Physik, TU Berlin, Hardenbergstr. 36, 10623 Berlin, Germany.
Phys Chem Chem Phys. 2019 Jul 10;21(27):15157-15166. doi: 10.1039/c9cp02787d.
Protonation and solvation of heterocyclic aromatic building blocks control the structure and function of many biological macromolecules. Herein the infrared photodissociation (IRPD) spectra of protonated oxazole (H+Ox) microsolvated by nonpolar and quadrupolar ligands, H+Ox-Ln with L = Ar (n = 1-2) and L = N2 (n = 1-4), are analyzed by density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level to determine the preferred protonation and ligand binding sites. Cold H+Ox-Ln clusters are generated in an electron impact cluster ion source. Protonation of Ox occurs exclusively at the N atom of the heterocyclic ring, in agreement with the thermochemical predictions. The analysis of the systematic shifts of the NH stretch frequency in the IRPD spectra of H+Ox-Ln provides a clear picture of the sequential cluster growth and the type and strength of various competing ligand binding motifs. The most stable structures observed for the H+Ox-L dimers (n = 1) exhibit a linear NHL hydrogen bond (H-bond), while π-bonded isomers with L attached to the aromatic ring are local minima on the potential and thus occur at a lower abundance. From the spectra of the H+Ox-L(π) isomers, the free NH frequency of bare H+Ox is extrapolated as νNH = 3444 ± 3 cm-1. The observed H+Ox-L2 clusters with L = N2 feature both bifurcated NHL2 (2H isomer) and linear NHL H-bonding motifs (H/π isomer), while for L = Ar only the linear H-bond is observed. No H+Ox-L2(2π) isomers are detected, confirming that H-bonding to the NH group is more stable than π-bonding to the ring. The most stable H+Ox-(N2)n clusters with n = 3-4 have 2H/(n - 2)π structures, in which the stable 2H core ion is further solvated by (n - 2) π-bonded ligands. Upon N-protonation, the aromatic C-H bonds of the Ox ring get slightly stronger, as revealed by higher CH stretch frequencies and strongly increased IR intensities.
杂环芳香结构单元的质子化和溶剂化作用控制着许多生物大分子的结构和功能。本文通过在色散校正的B3LYP-D3/aug-cc-pVTZ水平上进行密度泛函理论计算，分析了由非极性和四极配体微溶剂化的质子化恶唑（H⁺Ox）的红外光解离（IRPD）光谱，即H⁺Ox-Ln（L = Ar，n = 1 - 2；L = N₂，n = 1 - 4），以确定优先的质子化和配体结合位点。冷的H⁺Ox-Ln簇在电子碰撞簇离子源中产生。恶唑的质子化仅发生在杂环的N原子上，这与热化学预测一致。对H⁺Ox-Ln的IRPD光谱中NH伸缩频率的系统位移分析，清晰地呈现了簇的连续生长以及各种竞争配体结合模式的类型和强度。对于H⁺Ox-L二聚体（n = 1）观察到的最稳定结构呈现出线性NHL氢键（H键），而L连接到芳环上的π键异构体在势能面上是局部极小值，因此丰度较低。从H⁺Ox-L(π)异构体的光谱中，推断出裸H⁺Ox的自由NH频率为νNH = 3444 ± 3 cm⁻¹。观察到的L = N₂的H⁺Ox-L₂簇同时具有分叉的NHL₂（2H异构体）和线性NHL氢键模式（H/π异构体），而对于L = Ar，仅观察到线性氢键。未检测到H⁺Ox-L₂(2π)异构体，证实了与NH基团的氢键作用比与环的π键作用更稳定。n = 3 - 4的最稳定H⁺Ox-(N₂)n簇具有2H/(n - 2)π结构，其中稳定的2H核心离子进一步被(n - 2)个π键合配体溶剂化。N质子化后，恶唑环的芳香C - H键略有增强，这通过更高的CH伸缩频率和显著增加的红外强度得以揭示。
