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.