Host-Controlled Stepwise CO2 Growth on Maleic Anhydride Revealed by Rotational Spectroscopy

Gas-phase aggregation of CO2 with organic compounds provides molecular-level benchmarks for understanding how weak interactions guide early cluster growth─a process relevant to CO2 recognition, uptake, and organization in chemically diverse environments. Here, clusters of maleic anhydride with one to three CO2 molecules were characterized by Fourier transform microwave spectroscopy in combination with quantum-chemical calculations. One isomer was identified in the pulsed jet for each cluster size. The dimer adopts a nearly coplanar structure in which CO2 binds at a carbonyl edge through a dominant C···O tetrel bond assisted by a weak cooperative C-H···O hydrogen bond. Growth to n = 2 and 3 is accompanied by pronounced structural reorganization rather than simple retention of the dimer geometry, with the initially bound CO2 substantially repositioned as distinct binding regions are sequentially occupied. These findings show that a compact, electronically differentiated host can redirect stepwise CO2 growth away from simple site-by-site addition, providing a molecular-level basis for understanding the early stages of CO2 recognition and uptake in organic host environments.