Infrared Resonance Enhanced Photodissociation (IR- REPD) Spectroscopy used to determine solvation and structure of Ni + (C 6 H 6 ) n and Ni + (C 6 H 6 ) n Ar complexes Department of Chemistry, University of Georgia Athens, GA J.B. Jaeger, T.D. Jaeger and M.A. Duncan

Introduction Dibenzene chromium - E.O. Fischer and W. Hafner in 1955 aromatic  -bonded sandwich structure similar to ferrocene Collision Induced Dissociation (Armentrout) Equilibrium Mass Spec (Dunbar) Ion Mobility (Bowers) Photodissociation (Freiser) Electronic Spectroscopy/Photodissociation (Duncan) Photoelectron Spectroscopy (anions) IR Matrix Deposition, observed multiple decker “sandwiches” (Kaya and Nakajima) Magnetic Deflection multiple decker “sandwich” (Nakajima and Knickelbein) IR spectra of alkali cation-benzene-water complexes in O-H stretch region (Lisy)

IR spectra of transition metal cation-benzene complexes- fingerprint region only using free electron laser with ion trap mass spectrometer (Duncan, Meijer) Limited to small complexes (two benzene ligands or less) due to mass selection constraints Broad laser linewidth (~10 cm -1 ), multiple photon processes, possibility of thermally and/or electronically excited ions (ArF ionization) yielded wide spectral features (FWHM cm -1 ) Comparison to theory was qualitative at best More recently IR spectra of V + (C 6 H 6 ) n and Si + (C 6 H 6 ) n complexes in the C-H stretch region ( cm -1 UGA better linewidth and colder ion source Previous IR Studies on Gas Phase Transition Metal-Cation Benzene Complexes

Production of larger Ni + (benzene) n clusters Ions direct from jet; pulse extracted Mass gate – pulsed deflection plates before reflectron give total mass selection Species intersected by IR output of OPO/OPA (LaserVision; cm -1 ; 0.5 cm -1 res; 1-15 mJ/pulse; 5 nsec) Ability to obtain IR-PD spectra in C-H stretch region by monitoring photofragment yield vs IR wavelength

Mass spectrum is dominated by Ni + (C 6 H 6 ) n ions Smaller masses in between are Ni + (C 6 H 6 ) n (H 2 O), Ni + (C 6 H 6 ) n Ar, Ni 2 + (C 6 H 6 ) n

D 0 = 2.52 eV and 1.52 eV (CID,Armentrout) ~ 20,300 cm -1 and 12,200 cm -1 IR active C-H stretch of benzene near 3100 cm -1 At least 7 photons would be needed for benzene elimination! No dissociation observed for Ni + (C 6 H 6 ) and Ni + (C 6 H 6 ) 2 Solution: Rare gas tagging Ni + Ar binding energy 4436 cm -1 Ni + (benzene) and Ni + (benzene) 2 are bound strongly

No Fermi triad observed for the 1,1,2 complex The perturbation of the Ni + is enough to remove the degeneracy of the triad modes The recommended frequency for the 12 mode in free benzene is 3063 cm -1 in the absence of the Fermi triad. Blue shift of 30 and 36 cm -1 respectively for doublet

30 and 36 cm cm -1 Similar blue shift for V + (C 6 H 6 )Ar complexes The perturbation of the cation is enough to remove the degeneracy of the triad modes Dopfer and coworkers found blue shift of ~30 cm -1 C 6 H 6 + (L) complexes. Due to stiffening of C-H bonds upon electron removal from the HOMO e 1g orbital

DFT calculated by S. Klippenstein using B3LYP functional with G(d,p) basis set Theory calculated without argon The benzene ligand has two types of nonequivalent hydrogens yielding two main IR active modes 3089 & 3093 cm -1 Good agreement between predicted vibrational spectrum and the observed doublet The angle of the benzene ring distortion is exaggerated for demonstration, actually ~1-2 degrees Smaller band not evident within our sensitivity

The 1,2,1 spectrum shows a single mode with red shoulder shading centered at about the same position as 1,1,2 DFT predicts two doublet spin states close in energy DFT incorrectly predicted the lowest energy ground state to be 2 A C 1 The 2 B 2g D 2h structure appears to be a much better match for our spectrum

Spectrum changes dramatically at the 1,3 cluster size S/N increases from 1,2,1 to 1,3. This indicates the efficiency of photodissociation due to external ligands not bound directly to the Ni + The “core” mode is still evident in the 1,3 complex blue line New multiplet of modes are first evidence for the presence of the Fermi triad modes

The 1,3,1 spectrum is a little sharper than the 1,3 spectrum, however the spectral position is relatively unchanged The high S/N for the 1,3 spectrum suggests efficient photodissociation is achieved without Ar tagging This means that the loss of the third benzene occurs easily; it must be bound externally Benzene dimer is bound by ~ cm -1 The appearance of Fermi triad (occurs when benzene is unperturbed) and the photodissociation efficiency both suggest that benzene ligands are beginning to pile up on the outside of the complex

Ligands not bound directly to Ni + Appearance of Fermi triad suggest unperturbed benzene ligands High s/n indicates IR photodissociation spectra occurs efficiently Benzene dimer bound by ~ cm -1 The larger complexes have essentially the same bands The similarity between the NIST spectrum and our spectra suggest solvation following the third benzene ligand

Conclusions First IR-REPD spectra produced for Ni + (C 6 H 6 ) n and Ni + (C 6 H 6 ) n Ar complexes The perturbation of the benzene ring by the Ni + removes the degeneracies of the Fermi triad The blue shifted modes observed in the smaller complexes’ spectra are most likely from a stiffening of the C-H bonds due to charge removal from the benzene ligand The 1,1,2 spectrum agrees with calculated 2 B 2 ground state structure where the benzene ring is distorted in a C 2v manner DFT incorrectly predicts the ground state of the 1,2,1 complex as a 2 A state with C 1 symmetry, but the observed spectrum instead matches better with the distorted 2 B 2g D 2h structure The larger clusters show spectra similar to liquid benzene, showing a Fermi triad of frequencies and therefore solvation after the addition of three benzenes