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Unraveling the protonation site of oxazole and solvation with hydrophobic ligands by infrared photodissociation spectroscopy
Chatterjee, Kuntal; Dopfer, Otto
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 NH⋯L 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 NH⋯L2 (2H isomer) and linear NH⋯L 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.
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