Microsolvation of the 4-aminobenzonitrile cation (abn~+) in a nonpolar solvent: IR spectra of ABN~+-L_n (L=Ar and N _2, n≤4)

摘要

IR photodissociation (IRPD) spectra of mass-selected cluster ions of 4-aminobenzonitrile (ABN~+) with up to four Ar and N_2 ligands are recorded over the spectral range of the N-H stretching vibrations (ν_(s/a)) of ABN~+ in its ~2B_1 ground electronic state. ABN~+-L_n clusters are produced in an electron impact cluster ion source, which predominantly generates the most stable isomer of a given cluster ion. Vibrational frequency shifts of ν_(s/a) provide information about the sequential microsolvation process of ABN~+ in a nonpolar solvent. In ABN~+-(N _2)_n, the first two ligands fill a first subshell by forming hydrogen bonds to the acidic protons of the amino group, whereas further ligands bind more weakly to the aromatic ring (π bonds). Although the preferred cluster growth sequence in ABN~+-Ar_n is similar, several isomers are observed because the hydrogen bonds are only slightly stronger than the π bonds. Quantum chemical calculations at the M06-2X/aug-cc-pVTZ level confirm the cluster growth sequence derived from the IR spectra and provide further details of the intermolecular potential. The calculated binding energies of D_0(H)=532 and 895 cm~(-1) for hydrogen-bonded and D_0(π)=512 and 530 cm~(-1) for π-bonded Ar and N_2 ligands are consistent with the observed photofragmentation branching ratios. Comparison between ABN~+-L _n and the corresponding clusters with the aniline cation demonstrates that the NH protons of the amino group become slightly more acidic upon H→CN substitution at the para position. Comparison between charged and neutral ABN~((+))-L dimers indicates that ionization switches the preferred ion-ligand binding motif from π to hydrogen bonding.