1. The Rotational Spectrum of the Water–Hydroperoxy Radical (H 2 O–HO 2 ) Complex Kohsuke Suma, Yoshihiro Sumiyoshi, and Yasuki Endo Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo

2. Background Free radical processes of atmospheric species in troposphere HO 2 and OH play central role in atmosphere Their water complexes should play important roles by changing the reactivities of radical monomers.

3. Chaperon effect HO 2 + HO 2 → H 2 O 2 + O 2 Recombination of HO 2 Sink of atmospheric HO 2 Source of atmospheric H 2 O 2 HO 2 + HO 2 → H 2 O 2 + O 2 (1) HO 2 + H 2 O ⇄ HO 2 –H 2 O(2) HO 2 + HO 2 –H 2 O → products(3) HO 2 –H 2 O + HO 2 –H 2 O → products(4) Complex formation of H 2 O–HO 2 † k Water enhances this reaction † E. Hamilton Jr., R. Lii, Int. J. Chem. Kinet 9, 875 (1977).

4. Experiment Water + O 2 (10 %) / Ar H 2 O–HO 2 1.5 kV 4.5 atm Unlike the case of Ar–HO 2 Suma et al, JCP 122 184302 Pulsed discharge Production of H 2 O–HO 2 Observation FTMW Spectroscopy Double Resonance Spectroscopy

5. Equilibrium geometry by ab initio calculation RCCSD(T)/aug-cc-pVTZ

6. Internal rotation Intermolecular potential energy surface /cm –1 B3LYP/aug-cc-pVDZ TS1 E E* Barrier (TS1) 200 cm –1

7. Internal rotation TS2 E (12)* Barrier (TS2) 200 cm –1 Intermolecular potential energy surface /cm –1 B3LYP/aug-cc-pVDZ

8. The group G 4 Index table of the group G 4 E(12)E*(12)* A + 1 1 1 1 A – 1 1–1–1 B – 1–1–1 1 B + 1–1 1–1 A–A– B + / B – C1C1 G4G4 B + / B –  ortho (A + ) = B + / B –  nuc  total A +  para (B + ) = B +  vib

9. Observed spectra Coupling scheme of angular momenta A + state B + / B – state J = 2.5 – 1.5 140 signals 370 signals I 1 ： H nucleus of HO 2 I 2 ： H nuclei of water I 1 = ½ I 2 = 0 para (A + ) = 1 ortho (B + / B – ) F 1 = J + I 1 F = F 1 + I 2

10. Stick diagram A + B + / B –

11. Energy levels and observed transitions FTMW Double resonance a-typeb-type A + 277 B + / B – 85 11 Observed lines Same rotational transitions were observed for the A + and B + / B – state.

12. Molecular constants A + (para)B ± (ortho) A 32896.577(7)32723.731(5) B 5655.548(4)5654.616(3) C 4829.438(3)4832.029(3) Δ N 0.0375(1)0.03741(6) Δ NK 0.021(1)0.0816(8) Δ K 0.12395 (Fix)0.12395 (Fix) δ N 0.0064(1)0.0062(1) δ K 0.20230 (Fix) 0.20230 (Fix) ε aa –557.23(3)–553.69(2) ε bb –409.99(2)–409.76(1) ε cc –0.91(2)–0.98(1) |ε ab + ε ba |/2 155.00(5)154.68(4) Δ N S 0.0029(6)0.0031(4) Δ NK S 0.050(6)0.060(4) HO 2 side a F –27.51(2)–27.49(9) T aa 28.49(1)28.51(1) T bb –17.58(2)–17.59(2) T ab –11.8(2)–11.6(1) Water side a F ––0.040(6) T aa –4.870(7) T bb ––2.384(9) T ab –1.11(9) in MHz Hyperfine constants σ=11 kHz (A + ) =12 kHz (B + / B – ) A + (para)B ± (ortho)

13. Molecular structure ab initio Experiment r e TS1 TS2 A + B + / B – A /MHz31806 32552 34064 3289732734 B /MHz5968 5875 5074 56565655 C /MHz5077 4976 4509 48294832 ΔI / uÅ 2 –1.0360.00 –2.337–0.092–0.229 Inertial defects: small negative values →Vibrationally averaged structure is nearly planar. →Large amplitude motion Large deviation (5%) Rotational constants Ab initio (r e ) Experiment

14. RCCSD(T)/aug-cc-pVTZ Features Hydorgen bond （ R O1 － H3 ） is short. -cf. water dimer 2.019 Å Non-linear hydrogen bond O2 － H3 ･･･ O1 (ab initio 1.795 Å) Molecular structure ←experimental (B+C)

15. Binding energy Exp ab initio* Δ N 37.5(1) 39.5 δ N 6.4(1)6.2 Centrifugal distortion constants (kHz) *B3LYP/aug-cc-pVTZ The ab initio PES reproduces the centrifugal distortion constants well. →supports the accuracy of D e. Binding energy (D e kcal/mol) H 2 O–H 2 O5.0 † H 2 O–OH5.6 † † H 2 O–HO 2 9.4 † † † † W. Klopper et al. PCCP. 2, 2227 (2000). † † Aloisio et al. Acc. Chem. Res. 33, 825, (2000) † † † This work (RCCSD(T)/aug-cc-pVTZ)

16. Hyperfine constants Fermi coupling constants in MHz H 2 O−HO 2 HO 2 T xx 31.328.3 T yy −20.4−16.8 T zz −10.9−11.6 Magnetic dipole interaction tensor* in MHz *Principal axis values H 2 O−HO 2 *Ohshima et al. JACS 127 1108 H 2 O–HO 2 –0.040(6)–27.51(2)–27.52(8) H 2 O–OH* 0.940(5)–8.226(6) –74.04 water complexes monomer H 2 OHO 2 /OH HO 2 /OH → Very small induction effect for the electronic structure

17. Conclusion First observation of the H 2 O–HO 2 radical complex in gas phase. Large binding energy of H 2 O–HO 2, 9.4 kcal/mol, obtained by ab initio calculation is supported by close agreement of the centrifugal distortion constants. Molecular constants provide accurate transition frequencies for remote sensing.