ESI-MS of Cucurbituril Complexes Under Negative Polarity

Area of Science:

• Supramolecular Chemistry
• Analytical Chemistry
• Mass Spectrometry

Background:

• Electrospray ionization mass spectrometry (ESI-MS) is crucial for studying host-guest interactions.
• Both positive and negative ESI modes are employed, depending on the analytes.
• Cucurbit[n]urils (CB[n]) are macrocyclic hosts known for complexation.

Purpose of the Study:

• To investigate the formation of host-guest complexes between CB[n] and various cations using negative ESI-MS.
• To explore the gas-phase behavior and structural characterization of these complexes.
• To evaluate the utility of negative ESI-MS for analyzing supramolecular assemblies.

Main Methods:

• Negative electrospray ionization mass spectrometry (ESI-MS) was utilized.
• Cucurbit[n]urils (CB[n], n=7, 8) were complexed with amine, styryl pyridine, and styryl pyridine dimer cations.
• Analysis focused on ion pairing, charge neutralization, and spectral simplicity.

Main Results:

• Unexpected formation of CB[n] host-guest complexes observed under negative ESI.
• Non-complexed CB[n] formed double-charged halide adducts.
• 1:1 and 1:2 CB[n]-guest complexes were identified, with guest charge states determined by halide ion pairing for neutralization.
• Negative ESI distinguished between different complex stoichiometries and structures.

Conclusions:

• Negative ESI-MS is a powerful and sensitive technique for studying host-guest supramolecular interactions.
• It offers simpler spectra compared to positive ESI and enables determination of gas-phase guest charge states.
• This method provides insights into the structural nuances of supramolecular complexes.