A rotational investigation of the complex of phthalic anhydride with water

Phthalic anhydride (PhA), a component of secondary organic aerosol, and its monohydrate have been investigated in the gas phase through chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. New rotational transitions of PhA and several 13 C and 18 O isotopologues were measured in the 2–8 GHz region and combined with previous data to refine its spectroscopic constants. A single isomer of the PhA–H₂O complex was identified in the rotational spectrum, in which water binds to PhA through a primary O–H···O hydrogen bond to a carbonyl oxygen. A secondary C–H···O interaction further stabilises the complex. Non-covalent interaction, natural bond orbital, and QTAIM analyses have been performed to assess the relative strengths of the hydrogen bond interactions. Our results provide detailed insight into the microsolvation of PhA and contribute to a molecular-level understanding of its behaviour.