Hua Guo,2 and Gary E. Douberly*,1 Helium droplet infrared laser Stark spectroscopy of ClNH3: Entrance channel complex along the Cl+NH3→ClNH2+H reaction Peter R. Franke,1 Christopher P. Moradi,1 Changjian Xie,2 Matin Kaufmann,3 Hua Guo,2 and Gary E. Douberly*,1 1 Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA 2 Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA 3 Department of Physical Chemistry II, Ruhr-University Bochum, D-44801 Bochum, Germany

Motivation Unusual form of bonding (2c-3e) in a theoretically tractable system Exclusive to open-shell systems Entrance channel complexes relevant at low temperatures Interstellar chemistry Negative Temperature Dependence Contribution of QMT Atmospheric chemistry NH3 and Cl will find each other in the atmosphere. Previous Experimental Work EPR study confirmed presence of neutral radicals Two gas phase kinetics studies of the H-abstraction reaction No spectroscopic studies of the isolated complex J.B. Raynor, I.J. Rowland, and C.R. Symons, J. Chem. Soc. Faraday Trans. 87(4), 571-577 (1991). A.A. Westenberg and N. deHaas, J. Chem. Phys. 67, 2388 (1977). Gao et al., J. Phys. Chem. A 110, 6844 (2006).

Potential Energy Surface of Cl + NH3 4.08 D Take note of these dipole moments. 2.21 D Top half at G2M(CC2)//B3LYP/6-311+G(3df,2p) Bottom half at CCSD(T)/cc-pVTZ Z. F. Xu and M. C. Lin, J. Phys. Chem. A 111, 584 (2007). M. Monge-Palacios and J. Espinosa-Garcia, J. Phys. Chem. A 114, 4418 (2010). CCSD(T)/aug-cc-pVTZ

Hemibonding (2c-3e Bonding) Introduced by Pauling in 1931 Bond order of ½ Can be ~half as strong as a single bond Example: (He2)+ with 57 kcal/mol Guo found particularly strong binding energies in complexes of halogen atoms with H2O or NH3 B3LYP/aug-cc-pVTZ L. Pauling, J. Am. Chem. Soc. 52, 3225 (1931). 2970 cm-1 binding energy J. Li, Y. Li, and H. Guo, J. Chem. Phys. 138, 141102 (2013).

Experimental hνIR hνIR Laser-induced depletion spectroscopy with and without Stark field Majority of measurements made on m/z 51 (35ClNH2+) Efficient cracking of molecular chlorine Perpendicular laser polarization for Stark

NH Stretch Survey No significant NH3 signal m/z 51 No significant NH3 signal Broad * features from clusters Consider three spectral regions: 3350 cm-1 3450 cm-1 3500 cm-1 Focus on the 3450 cm-1 region. NH3 ν1 NH3 ν3 * * * *Assigned to (NH3)n≥3 clusters based on M. N. Slipchenko and A. F. Vilesov, Chem. Phys. Lett. 412, 176 (2005).

NH Stretch Survey Focus on the 3450 cm-1 region. Bands are still present with cold pyrolysis source. Bands at 3350 and 3500 cm-1 disappear. Assigned to (Cl2)nNH3 complexes * * * *Assigned to (NH3)n≥3 clusters based on M. N. Slipchenko and A. F. Vilesov, Chem. Phys. Lett. 412, 176 (2005).

NH Stretch Survey Two spectral regions to examine: 3350 cm-1 and 3500 cm-1 Focus on the features at 3500 cm-1. Antisymmetric NH Stretches K substructure is fully resolved.

Perpendicular Band Consistent with symmetric top Suggests CR2 or CR3 K=1 → K=2 transition is substantially broadened Related to the 3-fold symmetry of He/ClNH3 interaction potential Seen previously for CH3 and NH3 Not able to differentiate between CR2 and CR3 based upon differences in rotational constants alone *Assigned to (Cl2)nClNH3 complexes because they grow in intensity at higher fluxes of Cl2 through pyro. The Cl2 operating pressure is optimized for pick-up of a single Cl atom already.

HCl-NH3? C3v H-bonded complex Endeavored to rule this out m/z 52 C3v H-bonded complex Endeavored to rule this out Flow HCl into chamber; no pyro Features on m/z 51 disappear New features exclusive to m/z 52 Consistent with C3v(M) K=1 → K=2 broadened by the same mechanism A.M. Morrison, J. Chem. Phys. A 117, 11640-11647 (2013). M. N. Slipchenko and A. F. Vilesov, Chem. Phys. Lett. 412, 176 (2005).

NH Stretch Survey Two spectral regions to examine: 3350 cm-1 and 3500 cm-1 Focus on the features at 3350 cm-1 Symmetric NH Stretches The rotational structure is partially resolved.

Parallel Band Rotational Structure Well-simulated by rigid rotor symmetric top Hamiltonian a1 symmetry νs of CR2 or CR3 Rotational constants for CR2 and CR3 are similar; assignment is not definitive. Will instead exploit the greatly different dipole moments, in order to make an assignment Scanned on two mass channels: Qualitatively matches 3:1 natural isotopic abundance

Stark Spectroscopy Measured parallel band at different DC field strengths ΔM = ± 1 selection rules Simulations in pgopher reproduce Stark splitting when using a dipole moment of 4D. Calculated dipole moments: CR2: 4.08 D CR3: 2.21 D

CR3 → CR2 Isomerization? Scanned the umbrella inversion Did not succeed in locating/characterizing TS Electronic surface is displayed. Barrier may be even lower on the enthalpic surface. This may explain why no evidence for CR3 was found. Can also rationalize our findings on the basis of pre-orientation of Cl and NH3 in the droplet ΔE = 53.5 cm‒1 CR3 CR2 Unrelaxed CCSD(T)/aug-cc-pVTZ

Conclusions The IR spectrum of ClNH3 was measured in the NH stretch region, in He droplets. Two bands were assigned to the symmetric and antisymmetric NH stretching fundamentals of the hemibonding complex CR2. Vibrational bands were consistent with a symmetric top (ruling out CR1). Stark splitting is correctly reproduced, by simulation, only when a dipole moment of 4D is used (ruling out CR3 and strongly implying CR2).

We acknowledge support from the Department of Energy. Thank you for your attention.