Exploring alkylpyrazine stability across aggregation states: a DFT perspective

Determining the structure and properties of analytes and their ions, such as proton-bound clusters, is crucial for both the theoretical understanding and practical applications of mass spectrometry, ion mobility spectrometry, and other related chemical ionization methods. Density functional theory (DFT) calculations were utilized to investigate the conformational constraints governing the formation of stable proton-bound clusters of alkylpyrazines, encompassing monomers, dimers, and trimers. Employing the B3LYP/6-31+G(d, p) method with D3(BJ) dispersion correction, molecular properties, including electric dipole moment, polarizability, and proton affinity, were presented and compared with results from higher basis sets like aug-cc-pVTZ, demonstrating the efficiency of the chosen approach. Natural bond orbital (NBO) calculations provided insights into natural charges, charge transfer, and stability of proton-bound dimer and trimer structures, revealing a decrease in stability from monomers to trimers. Notably, protonated trimers exhibited stacked structures, aligning with experimental observations. Various factors, including structure, electric dipole moment, polarizability, charge transfer, and steric hindrance influence the stability of alkylpyrazine clusters. Additionally, proton affinity calculations indicated a linear relationship between stability and proton affinity in monomers, with constant dissociation energies observed in proton-bound dimers regardless of proton affinity variations. The study contributes to the growing understanding of the relationship between ion mobility spectra in analytical measurements and gas ions under ambient temperature and pressure conditions, particularly for compounds relevant to food system analysis.