Microsolvation of the 4‐Aminobenzonitrile Cation (ABN⁺) in a Nonpolar Solvent: IR Spectra of ABN⁺L_n (L=Ar and N₂, n≤4)

Abstract

IR photodissociation (IRPD) spectra of mass-selected cluster ions of 4-aminobenzonitrile (ABN(+)) with up to four Ar and N2 ligands are recorded over the spectral range of the N-H stretching vibrations (ν(s/a)) of ABN(+) in its (2)B1 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(+)-(N2)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 D0(H)=532 and 895 cm(-1) for hydrogen-bonded and D0(π)=512 and 530 cm(-1) for π-bonded Ar and N2 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.

Microhydrated aromatic cluster cations: Binding motifs of 4-aminobenzonitrile-(H2O)n cluster cations with n ≤ 4

“…L interaction. 15 In contrast, in ABN + -(H 2 O) n clusters, the H-bonded solvent network is competitive with interior ion solvation. (2) The bifunctional ABN (+) molecule offers several competing binding sites for H 2 O ligands, including the NH, CH, CN, and π binding sites, and their relative stability strongly depends on the charge state.…”

“…29 This theoretical approach describes the electrostatic, induction, and dispersion interactions of ABN + and other aromatic ions with nonpolar and polar ligands with sufficient accuracy. 15,30 Test calculations at the MP2 level reveal high spin contamination for these open-shell aromatic ions. 15 All coordinates are relaxed during the search for stationary points on the potential energy surface without any symmetry constraints, and their nature as minimum or transition state is verified by harmonic frequency analysis.…”

“…(1) Recently, we characterized ABN + -L n clusters with nonpolar and quadrupolar ligands (L = Ar and N 2 ) in the same size range (n ≤ 4) using the same experimental and computational approach. 15 Thus, direct comparison with ABN + -(H 2 O) n will reveal the substantial differences in the microsolvation of ABN + in a dipolar and nonpolar/quadrupolar solvent. ABN + -L n clusters with L = Ar and N 2 clearly prefer interior ion solvation because they cannot benefit from cooperative effects by forming a solvent cluster and because the ABN + .…”

Microsolvation of the acetanilide cation (AA+) in a nonpolar solvent: IR spectra of AA+–Lnclusters (L = He, Ar, N2; n ≤ 10)

“…In contrast, the corresponding A + -L dimer cations usually prefer H-bonds with the acidic functional group(s) due to additional charge-induced interactions arising from the excess charge. 9,13 This ionization-induced p -H switch in the preferred A (+) -L binding motif has recently been established via the combination of IR spectroscopy and quantum chemical calculations for a variety of acidic aromatic molecules interacting with rare gas (Rg) atoms, CH 4 , and N 2 , 9 including (substituted) phenols, [13][14][15][16][17][18][19][20][21][22] naphthol, 23 aniline, [24][25][26] aminobenzonitrile, 27,28 imidazole, 29,30 indole, 31 and tryptamine. 32 In some of these clusters, this ionization-induced p -H switch triggers an intermolecular isomerization reaction upon photoionization, which occurs on the picosecond timescale [33][34][35] and is usually inferred from IR spectra recorded before and after ionization using nanosecond laser systems.…”

Single water solvation dynamics in the 4-aminobenzonitrile–water cluster cation revealed by picosecond time-resolved infrared spectroscopy

“…3(a). 59,61 This band A can be seen in the TRIR Fig. 1 Strategy of picosecond time-resolved IR ion dip (ps TRIR) spectroscopy.…”