IR laser spectroscopy of reactive intermediates and complexes trapped in superfluid helium droplets
Moradi, Christopher Pedram
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Helium nanodroplets are used to trap and stabilize a variety of reactive species including alkyl radicals, organic carbenes, and clusters containing Cl atoms. The vibrational spectra of these systems are investigated, predominantly in the C‒H stretching region, and rotational fine-structure is observed for many of the smaller systems. The ro-vibrational spectra of C3H3, C3H5, and n,i-C3H7 become increasingly congested as the degree of unsaturation of the radical is reduced, and the spectra are modeled using both low- and high-accuracy computational methods in conjunction with a local-mode Hamiltonian approach. Preliminary results of the local-mode model indicate that coupling between C‒H stretching vibrations and overtones/combinations of scissor modes lead to the observed spectral congestion. Several vibrational bands of the HCOH and HOCOH carbenes are measured in the C‒H and O‒H stretching regions. The He droplet spectra are consistent with the isolation of the trans-HCOH isomer and both the trans,trans- and trans,cis-HOCOH isomers. Vibrational frequencies, rotational constants, and permanent electric dipole moments (extracted from Stark spectroscopic measurements of HOCOH isomers) obtained for the He-solvated carbenes are compared with results of Ar matrix experiments, gas-phase results (when available), and anharmonic frequency computations. Finally, Cl atoms were generated via the thermal decomposition of Cl2 and were solvated in He droplets alongside either HCl or NH3 molecules. The Cl‒HCl van der Waals complex that aggregates in He droplets is demonstrated to have an L-shaped geometry (rather than a linear, H-bonded or Cl-bonded geometry) on the basis of both vibrational frequency of the HCl stretch and predictions of the transition dipole moment. The ClNH3 adduct, on the other hand, has a much stronger 2-center, 3-electron bond, which is confirmed computationally. There is no spectroscopic evidence for either NH3Cl or Cl‒HNH2 geometries despite both being stable minima on the CCSD(T) potential energy surface.