Complexes of gold and imidazole formed in helium nanodroplets

Abstract

We have studied complexes of gold atoms and imidazole (C₃N₂H₄, abbreviated Im) produced in helium nanodroplets. Following the ionization of the doped droplets we detect a broad range of different Au_mIm_n⁺ complexes, however we find that for specific values of m certain n are "magic" and thus particularly abundant. Our density functional theory calculations indicate that these abundant clusters sizes are partially the result of particularly stable complexes, e.g. AuIm₂⁺, and partially due to a transition in fragmentation patterns from the loss of neutral imidazole molecules for large systems to the loss of neutral gold atoms for smaller systems.

Fig. 1. The molecular structure of imidazole. The labels next to each C and N atom are used to denote the locations of bond sites when discussing our results.

Fig. 2. Cationic mass spectrum of ionized helium nanodroplets containing gold and imidazole. Series of Au_mIm_n⁺ with fixed values of m and 0 ≤ n ≤ 15 are indicated by their peak heights and grouped by color. The pure imidazole clusters (gray line) are predominantly protonated and follow a smooth size distribution. The extra proton likely originates from the dissociation of an imidazole unit in a larger neutral precursor cluster upon charge transfer from He⁺. Clusters containing gold are not observed to be protonated. The inset shows a zoom in of the mass spectrum where four peaks (the main peak in each isotopologue series) are labeled, demonstrating the high resolution of the current setup.

Fig. 3. Integrated intensities of Au_mIm_n⁺ complexes arranged by fixed values of m. Vertical lines indicate the most abundant number of imidazole molecules in each panel. The statistical error bars are in most cases smaller than the data points.

Fig. 4. Proposed structures of AuIm_n⁺, n = 1, 2, 3, and lowest dissociation energies from M06/def2-TZVP//M06/def2-SVP calculations.

Fig. 5. Proposed structures of Au₂Im_n⁺, n = 1, 2, 3, 4, and lowest dissociation energies from M06/def2-TZVP//M06/def2-SVP calculations.

Fig. 6. Proposed structures of Au₃Im_n⁺, n = 1, 2, 3, 4, and lowest dissociation energies from M06/def2-TZVP//M06/def2-SVP calculations.

Fig. 7. Proposed structures of Au₄Im_n⁺, n = 1, 2, 3, 4, 5, and lowest dissociation energies from M06/def2-TZVP//M06/def2-SVP calculations.

Fig. 8. Lowest dissociation energies determined by our structure calculations at M06/def2-TZVP//M06/def2-SVP level of theory. The lines connecting the data points are to guide the reader. The corresponding charged fragments are given in Fig. 9.

Fig. 9. Matrix indicating the charged product that remains when Au_mIm_n⁺ dissociates along the lowest energy pathway (not considering intermediate barriers). Columns represent fixed values of m and rows values of n. Cells shaded red indicate products that have loss at least one Au atom (and any number of imidazole), while blue cells indicate the loss of a single neutral imidazole ring. The striped cell for the Au₂Im₃⁺ parent indicates two dissociation energies separated by less than 0.1 eV. We expect all systems larger than those shown here to fragment mainly through the loss of neutral imidazole.