Unveiling the Collision-Induced Dissociation Pathways of Protonated PO3G Dimer: An Integrated Energy-Resolved MS and Quantum Chemical Study

Poly(trimethylene ether) glycol (PO3G) is a relevant biobased polyether whose combination of terminal hydroxyl groups and ether linkages enables multiple competing gas-phase dissociation pathways, yet its energy-dependent collision-induced dissociation (CID) remains poorly understood at the mechanistic level. Here, the protonated PO3G dimer (m/z 135) was investigated by energy-resolved mass spectrometry (ERMS) and high-level quantum chemical calculations, focusing on the main first-order fragments at m/z 117, 77, and 59. The lowest energy pathways identified presented barriers of 164.7, 176.7, and 187.3 kJ/mol, respectively, a progression qualitatively consistent with the experimental fragmentation trends. The calculations further suggest that kinetically accessible structures may differ from the thermodynamically most stable products. In particular, a protonated epoxide intermediate provides a lower-energy route to m/z 59 than the formation of protonated allyl alcohol previously suggested by automated spectral prediction approaches. A semiquantitative energy transfer model was used to assess the compatibility of the calculated barriers with the experimentally accessible collision-energy range. Within the limitations of this qualitative treatment, the combined results constrain plausible first-order dissociation pathways and provide a mechanistic framework for future quantitative studies based on precursor internal-energy distributions and RRKM kinetics.