Ab initio predictions for HO2+: Theoretical Guidance for an Astronomical Detectability Study David E. Woon, Susanna L. Widicus Weaver, Branko Ruscic, and Benjamin J. McCall FD0 9 HO2+: Another Approach to the O2 Problem? Molecular oxygen (O2) is a difficult species to observe. The only observation to date the Odin study of Oph A [Larsson A&A 2007, 466, 999] found a limited abundance, just 5 x 10-8 relative to [H2]. This is much less than predicted by models, ~5-10 x 10-6 [Goldsmith, ApJ 2000, 539, 123]. It has been recognized for at least three decades [Herbst, ApJ 1977, 215, 503] that protonated forms of species that are diffi-cult to detect can be useful tracers of the parent no , A good example is N2H+, which molecules. microwa was observed well before N2 ve inactive

was finally detected. [Turner, ApJ 1974, 193, L83; Green, ApJ 1974, 193, L89; Knauth, Nature 2004, 429, 636]. large , good spectrum HO2+: Another Approach to the O2 Problem? While O2 does have observable weak dipoleallowed magnetic transitions, atmospheric spectral interference seriously impedes groundbased observations. Can HO2+ be used as a tracer 3 for O2? A 1.934 D 1.518 D MRCI/aug-ccpV5Z No rotational spectrum has yet been reported for HO2+, which currently precludes astronomical searches. A wide range of data is available for HO2 and can be used to benchmark theoretical calculations for HO2+. The best prior theory study is the work of Robbe et al. [Chem Phys 2000, 252, 9]. HO2+: Ab initio Predictions

Quantum chemical calculations were performed to evaluate the most likely pathway to the formation of HO2+ and to provide guidance for the laboratory study of its rotational spectrum. reaction dipole moment energetics rotational zero-field splitting constants tensor anharmonic spin-rotation frequencies constants Treatment: MRCI and RCCSD(T) calculations with basis sets as large as aug-cc-pV5Z. Programs used included: MOLPRO optimizations and potential energy surfaces SURFIT fitting and analysis of surfaces Formation of HO2+ from O2 and H3+ H3 + + O2 H2 + HO2+ Most likely formation pathway: H3+ + O2 H2 + HO2+ H 3 O2 + (1)

The reaction energy for this can be derived from the proton affinities of H2 and O2: H2 + H+ H3+ (2) PA(H2) = 0 H r O +(2)H+ HO + (3) PA(O ) = 2 2 rH0(3) The reaction enthalpy of (1) is: rH(1) = PA(H2) PA(O2) 2 Parallel expressions exist for Gs: gas phase basicities replace PAs. Formation of HO2+ from O2 and H3+ NIST WebBook values: PA(H2 ) = 100.93 kcal/mol

PA(O2) = 100.62 kcal/mol endothermic by 0.31 kcal/mol? However, Ruscic et al. [JPCA 2006, 110, 6592] recommended: exothermic by 0.05 PA(O2) = 100.98 0.14 kcal/ kcal/mol? mol ? which led to the collaboration with Branko Ruscic of ANL. Active Thermochemical Tables (ATcT) analysis of available data (will be described in FD10) Calculations for H3+ + O2 HO2+ + H2 Ee valence complete basis set (CBS) limit: +222.1 cm-1 Ee core-valence contribution +28.3 harmonic vibrational ZPE correction -199.5

anharmonic vibrational ZPE correction rotational ZPE correction +76.4 -63.0 H3+ [Lindsay, JMS 2001, 210, 60]: 64.121 cm-1 O2 [Cosby, JCP 1992, 97, 6108]: E01.0857 NET, Theory cm-1 1 rE00 from ATcT analysis cm-1 - +64.3 cm+509 VERY slightly endotherm ic Equilbrium Structure HO2 Treatment rOO () rOH () () RCCSD(T) Valence CBS 1.3270

0.9709 104.458 +CVDZ 1.3247 0.9701 104.549 Robbe et al. 1.337 0.968 103.90 HO2+ Treatment rOO () rOH () () RCCSD(T) Valence CBS 1.2295 1.0110 112.529

+CVDZ 1.2272 1.0102 112.743 Robbe et al. 1.237 1.007 111.8 Anharmonic Properties Potential energy surfaces were fit with SURFIT to 84 energy calculations distributed around the equilibrium structure for a given level of theory and basis set. The potential function include 69 terms consisting of the full quintic potential and selected sextic terms. All RMS -1 Perturbation wascm used fitting errors theory were <0.8 . for anharmonic shifts:

i = i + ( xii, xij ) - anharmonicities B0 = Be iB - rotation-vibration interaction constants (similar for A and C) Rotational Constants HO2 rotational constant (error) (GHz) A0 B0 C0 Experimenta This work 610.2 73 615.9 33.51 8 33.60 31.66 8 31.64 Robbe et al.

97 (+5.72 4) 612.2 4 (+0.08 6) 33.24 3 (0.025) 31.44 a Chance, JMS 1997, 183,05 518. 7 8 HO2+ rotational constant (GHz) A0 B0 C0 This work 659.3

01 38.34 4 35.88 5 Vibrational Frequencies HO2 frequency (error) (cm-1) 1 2 3 Experimenta 3436 1392 1098 This work 3457 1406 1128

(+21) (+14) (+30) 3449 1396 1106 Robbe et al. Yamada, JCP 1983, 78, 4379; Burkholder, JMS 1992, 151, 493. a HO2+ 1 This work frequency (cm-1) 2 3 3028 1440 1068

Dipole Moment Components HO2 dipole moment (error) (D) a b Experimenta 1.412 1.541 This work 1.405 1.572 (0.007) a Saito, JMS 1980, 80, 34. (+0.03 1) HO2+ dipole moment (D) a b This work

1.518 1.934 Zero-Field Splitting Magnetic interactions between the unpaired electrons in triplet states give rise to line splitting even in the absence of an applied field. ORCA [Neese et al., Universitt Bonn] was used to compute the spin-spin (SS) and spin-orbit coupling (SOC) contributions to the D and E tensors. Molecular oxygen O2 (3g-) was used for benchmarking. See Ganyushin & Neese [JCP 2006, 125, 024103] and Neese [JCP 2007, 127, 164112] for more extensive comparisons. DSS (ESS) was computed at the CASSCF/AVQZ level, while DSOC (ESOC) was computed at the MRCI/VQZ level and involved summing over four states each of singlet and triplet symmetry. Zero-Field Splitting O2 HO2+ this work this work DSS

1.555 1.810 DSOC 2.220 5.060 3.775 6.870 ZFS (cm1 ) D ESS EXPT a 3.9 6 0.013 ESOC

0.020 E 0.033 Tinkham, Phys Rev 1955, 97, 937. a Spin-Rotation Coupling Constants = 4B <0|L|n> ( E0 En ) n0 ; = 151 cm- 1 [Barnes, JMS 1978, 72, 86] Angular momentum matrix elements: CASSCF Energies: MRCI

Basis set convergence was tested with AVDZ and AVTZ sets HO2: sum over 4 states of 2A and 2A symmetry HO2+: sum over 4 or 6 states of 3A and 3A symmetry Spin-Rotation Coupling Constants HO2 EXPT a aa bb cc a 49572 422.9 8.748 HO2+ this work this work 4 sts 6 sts 4673 -432 0 -159

1094 -467 1182 -481 -429 -476 Fink, JMS 1997, 185, 304. Part II: Astronomical Detectability of HO2+ Hang around for Susannas talk... Acknowledgments DEW: NASA Exobiology Program, grant NNX07AN33G. SLWW: UIUC Critical Research Initiative program. BR: Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, US Department of Energy, contract number DEAC02-06CH11357. BJM: UIUC Critical Research Initiative program, NSF CAREER award Kirk A.(NSF Peterson (Washington State CHE-0449592). University)

Frank Neese (Universitt Bonn) Thom H. Dunning, Jr. (University of Illinois) Configuration Diagrams for HO2+ A, 1 A 3 + A 1 A 3