Interaction of the Benzemium Ion with Inert Ligands: IR Spectra of C_6H_7~+-L_n Cluster Cations (L=Ar,N_2,CH_4,H_2O)

摘要

IR photodissociation spectra of mass-selected clusters composed of protonated benzene (C_6H_7~+) and several ligands L are analyized in the range of the C-H strtch fundamentals The investigated systems include C_6H_7~+-Ar C_6H_7~+-(N_2)_n (n=1-40,C_6H_7~+-(CH_4)_n (n=1-4) and C_6H+7~+-H_2O.The complexes are produced in a supersonic plasma expansion using chemical ionization The IR spectra display absorptions near 2800 and 3100 cm~-1 whicl are attributed to ghe aliphatic and aromatic C-H stretch vibratins rspectively of the benzemium ion that is the sigma complex of C-6H_7~+ The C_6H_7~+-(CH_4)_n clusters show additional C-H ligands Both the frequencies and the relative intensities fo the C_6H_7~+ absorptions are nearly independent of the choice and number of ligands suggesting that the benzenium ion in the detected C_6H_7~+-L_n clusters is only weakly pertubed by the microsol vation process Analysis of photofragmentatio branching ratios yield estimated ligand binding energies of the order of 800 and 950cm~-1 (approx=9.5 and 11.5 kJmol~-1) for N_2 and CH_4 respectively The interpretation of the experimental data is supported by ab initio calculations for C_6H_7~+-Ar and C_6H_7~+-N_2 at the MP2/6-311G(2df,2pd) lewvel Both the calcuphysical implications of the deduced IR spectrum of C_6H_7+ are briefly discussed.