Water Network Shape-Dependence of Local Interactions with the Microhydrated -NO₂^- and -CO₂^- Anionic Head Groups by...

We report the structural evolutions of water networks and solvatochromic response of the CH₃NO₂^- radical anion in the OH and CH stretching regions by analysis of the vibrational spectra displayed by cryogenically cooled CH₃NO₂^-·(H₂O) clusters. The OH stretching bands evolve with a surprisingly large discontinuity at n = 6, which features the emergence of an intense, strongly red-shifted band along with a weaker feature that appears in the region assigned to a free OH fundamental. Very similar behavior is displayed by the perdeuterated carboxylate clusters, RCO₂^-·(H₂O) (R = CD₃CD₂), indicating that this behavior is a general feature in the microhydration of the triatomic anionic domain and not associated with CH oscillators. Electronic structure calculations trace this behavior to the formation of a "book" isomer of the water hexamer that adopts a configuration in which one of the water molecules resides in an acceptor-acceptor-donor (AAD) (A = acceptor, D = donor) H-bonding site. Excitation of the bound OH in the AAD site explores the local network topology best suited to stabilize an incipient -XO₂H-OH^-(H₂O)₂ intracluster proton-transfer reaction. These systems thus provide particularly clear examples where the network shape controls the potential energy landscape that governs water network-mediated, intracluster proton transfer. The CH stretching bands of the CH₃NO₂^-·(H₂O) clusters also exhibit strong solvatochromic shifts, but in this case, they smoothly blue-shift with increasing hydration with no discontinuity at n = 6. This behavior is analyzed in the context of the solute-ion polarizability response and partial charge transfer to the water networks.