1. 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 , 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 Bochum, Germany

2. 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), (1991).
A.A. Westenberg and N. deHaas, J. Chem. Phys. 67, 2388 (1977).
Gao et al., J. Phys. Chem. A 110, 6844 (2006).

3. 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

4. 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, (2013).

5. 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

6. 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).

7. 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).

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

9. 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.

10. 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, (2013).
M. N. Slipchenko and A. F. Vilesov, Chem. Phys. Lett. 412, 176 (2005).

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

12. 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

13. 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

14. 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

15. 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).

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