Solvent Migration in Microhydrated Aromatic Aggregates: Ionization‐Induced Site Switching in the 4‐Aminobenzonitrile–Water Cluster

Abstract

The dependence of the preferred microhydration sites of 4-aminobenzonitrile (4ABN) on electronic excitation and ionization is determined through IR spectroscopy of its clusters with water (W) in a supersonic expansion and through quantum chemical calculations. IR spectra of neutral 4ABN and two isomers of its hydrogen-bonded (H-bonded) 4ABN-W complexes are obtained in the ground and first excited singlet states (S0, S1) through IR depletion spectroscopy associated with resonance-enhanced multiphoton ionization. Spectral analysis reveals that electronic excitation does not change the H-bonding motif of each isomer, that is, H2O binding either to the CN or the NH site of 4ABN, denoted as 4ABN-W(CN) and 4ABN-W(NH), respectively. The IR spectra of 4ABN(+)-W in the doublet cation ground electronic state (D0) are measured by generating them either in an electron ionization source (EI-IR) or through resonant multiphoton ionization (REMPI-IR). The EI-IR spectrum shows only transitions of the most stable isomer of the cation, which is assigned to 4ABN(+)-W(NH). The REMPI-IR spectrum obtained through isomer-selective resonant photoionization of 4ABN-W(NH) is essentially the same as the EI-IR spectrum. The REMPI-IR spectrum obtained by ionizing 4ABN-W(CN) is also similar to that of the 4ABN(+)-W(NH) isomer, but differs from that calculated for 4ABN(+)-W(CN), indicating that the H2O ligand migrates from the CN to the NH site upon ionization with a yield of 100%. The mechanism of this CN→NH site-switching reaction is discussed in the light of the calculated potential energy surface and the role of intracluster vibrational energy redistribution.

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

“…(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. Preliminary IRPD spectra of ABN + -H 2 O (n = 1) generated by EI have been reported recently, 18 and comparison with calculations demonstrates that only the most stable NH-bound isomer is detected. Here, we extend these studies in three directions.…”

“…18,20,21 So far, three isomers of ABN-H 2 O in the neutral ground electronic state have been detected by resonance-enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF), 20,[22][23][24] and have been assigned by IR spectroscopy to the most stable NH-bound dimer and two less stable CN-bound isomers (linear and side). 23,24 Isomer-selective REMPI of the neutral NH-bound dimer produces exclusively the NH-bound isomer in the cation ground state, as detected by IR dip, 18 photoionization efficiency (PIE), 18,20 and zero-kinetic-energy photoelectron (ZEKE) spectroscopy. 21 However, IR dip spectra of ABN + -H 2 O generated by isomer-selective REMPI of the neutral CN isomer (side) are characteristic of the NH isomer, indicating that photoionization induces a CN → NH isomerization reaction with 100% yield on a timescale of less than 50 ns.…”

Investigation of the hydrated 7-hydroxy-4-methylcoumarin dimer by combined IR/UV spectroscopy

“…[45][46][47][48][49] Among these investigations rearrangement reactions within the ions have been investigated both with ns lasers 51-58 as well as time-resovled studies using ps lasers. [59][60][61] These investigations describe a migration of argon or molecules such as water, methanol, and methane in clusters with small aromatic molecules such as formanilide, 55 acetanilide, 60 4-aminobenzonitrile, 58,61 phenol, 57, 59 aminophenol, 52, 53 benzene, 51 tryptamine, 56 or phenylglcyine. 54 To the best of our knowledge no investigations on neutral or ionic 7H4MC under solvent free, isolated conditions have been performed.…”

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

“…The S 1 0 0 band of each cluster is still well resolved from the other transitions, ensuring selective excitation of the CN isomer to its S 1 0 0 origin under these conditions, as also confirmed by the previously reported hole-burning spectra. 39,43 Thus, the time-resolved spectroscopy is carried out by fixing n exc to the S 1 0 0 band of the CN isomer. ) arising from the shorter laser pulse duration.…”

“…4ABN-W(CN) holds its binding motif by excitation into S 1 , but shows the water migration from the CN to the NH 2 site upon ionization. 43 The large change in the charge distribution of the aromatic chromophore substantially destabilises the CN structure with respect to the NH isomer in 4ABN + -W. This CN -NH 2 site switching with 100% yield was inferred from the drastic change in the IR spectra before and after ionization but the dynamics of this water migration and other parameters of the reaction have not been characterized yet. To this end, we apply UV-UV'-IR ps TRIR spectroscopy to 4ABN-W(CN) to monitor the water migration dynamics in real time by the time evolution of the IR spectra measured at a variable delay time Dt after the ionization.…”