IR spectra of protonated benzaldehyde clusters, C7H7O+–Ln (L=Ar,N2;n≤2): Ion-ligand binding motifs of the cis and trans oxonium isomers

Abstract

Infrared photodissociation (IRPD) spectra of mass-selected protonated benzaldehyde (C7H7O+,BZH+) and its weakly bound clusters with Ar and N2 produced in an electron impact source are recorded in the C–H and O–H stretch ranges. The experimental results are supported by ab initio and density functional calculations. Analysis of the IRPD spectrum of the BZH+ monomer is consistent with the presence of the cis and trans isomers of the oxonium ions, which is confirmed by the cluster spectra. No signature of the less stable carbenium ions is detected. Frequency shifts in the IRPD spectra of dimers and trimers provide information about the preferred intermolecular ligand binding site (𝜋-bonding versus H-bonding) and the corresponding interaction strength. H-bonding to the OH group of the oxonium ions of BZH+ is found to be favored over π-bonding to the aromatic ring for both Ar and N2. There are significant differences in the microsolvation structure and energetics of the cis and trans oxonium isomers of BZH+ due to the rather different acidities of their OH groups and isomer-dependent effects arising from steric hindrance. The large positive partial charge of the protonated formyl group implies that the cluster growth of the larger clusters continues by further solvation of the protonated substituent rather than the aromatic ring