Hydration Topology Governs the H-O-O-H Dihedral Angle in Hydrogen Peroxide as Unveiled by Rotational Spectroscopy

Hydrogen peroxide (H2O2) is a pivotal atmospheric oxidant whose reactivity in aqueous microenvironments, such as aerosols, is highly sensitive to humidity. This sensitivity is postulated to arise from water's ability to distort the H-O-O-H dihedral angle (ω), a key conformational coordinate governing H2O2's electronic excitation energy (S1) and O-O bond cleavage propensity. However, a direct molecular-level mechanism linking discrete hydration-shell architecture to ω and, consequently, to electronic tuning has remained elusive. Here, we employ high-resolution rotational spectroscopy to characterize the microsolvation of H2O2 by probing the H2O2-(H2O)3-5 clusters. The topology of the water hydrogen-bonding network, not merely the hydration number, strongly modulates ω. The results show that the formation of 3D cyclic and prismatic networks in H2O2-(H2O)4 reduces this angle to approximately 90°, while the small clusters (n ≤ 3) preserve ω close to the isolated-molecule value (∼115°). In contrast, this angle increases to about 130° in the pentahydrate. Consequently, the S1 energy increases for the tetrahydrates and decreases for the pentahydrate relative to that of isolated H2O2. This work reveals how the hydrogen-bonding topology within the hydration shell selectively manipulates molecular conformation and reactivity.