Intermolecular interactions in aniline–benzene hetero-trimer and aniline homo-trimer ions

Probing cooperativity in C-H⋯N and C-H⋯π interactions: Dissociation energies of aniline⋯(CH4)n (n = 1, 2) van der Waals complexes from resonant ionization and velocity mapped ion imaging measurements

Recent studies of the weakly bound anisole⋯CH₄ complex found a dual mode of binding, featuring both C/H⋯π and C/H⋯O noncovalent interactions. In this work, we examine the dissociation energies of related aniline⋯(CH₄)_n (n = 1, 2) van der Waals clusters, where both C/H⋯π and C/H⋯N interactions are possible. Using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity-mapped ion imaging (VMI), we derive the dissociation energies of both complexes in the ground (S₀), excited (S₁), and cation radical (D₀) states. As the amide group is non-planar in the ground state, the optimized ground state geometry of the aniline⋯CH₄ 1:1 complex shows two isomers, each with the methane positioned above the aniline ring. The observed redshift of the electronic origin from the aniline monomer is consistent with TDDFT calculations for the more stable isomer, where the methane sits on the same face as the amino hydrogens. The dissociation energies of the 1:1 complex, obtained from 2CAP measurements, are in good agreement with the calculated theoretical values from selected density functional theory methods. VMI data for the 1:1 complex gave a binding energy value overestimated by ∼179 cm^-1 when compared to the 2CAP results, indicating that dissociative ionization selectively populates an excited vibrational level of the aniline cation radical. Given that the electron donating ability of aromatic substituents trends as -NH₂ > -OCH₃ > -CH₃, it is noteworthy that the strength of methane binding also trends in this order, as found by experiment (dissociation energies in kJ/mol: 6.6 > 5.8 > 4.5) and predicted by theory (PBE0-D3/def2-QZVPPD, in kJ/mol: 6.9 > 6.0 > 5.0). For the 1:2 complex of aniline and methane, calculations predict that the more stable conformer is the one where the two methane molecules lie on opposite faces of the ring, consistent with the observed redshift of the electronic origin. Unlike the anisole-methane 1:2 complex, which shows an enhanced dissociation energy for the loss of one methane in comparison with the 1:1 complex, here, we find that the energy required to remove one methane from the ground state aniline-methane 1:2 complex is smaller than that of the 1:1 complex, consistent with theoretical expectations.

Structures and infrared photodissociation of [(aniline)-(methanol)-(water)2]. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2019;220:117119. [PMID: 31141781 DOI: 10.1016/j.saa.2019.05.024] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The structures of [(aniline)-(methanol)-(water)₂]⁺ were investigated by infrared spectroscopy coupled with linear tandem mass spectrometry. We suggest the most stable structure of [(aniline)-(methanol)-(water)₂]⁺ through infrared photodissociation spectra supported by the density functional theory calculations at the level of ωB97X-D/cc-pVQZ. Methanol and one water molecule formed hydrogen bonding with the amino group of aniline, while the other water formed hydrogen bonding with methanol. Upon infrared excitation of [(aniline)-(methanol)-(water)₂]⁺, the water molecule connected to methanol turned out to be preferentially ejected, although the total internal energy in the cluster ion was large enough to dissociate other solvent molecules. This unique dissociation feature was attributed to the significant difference in the dissociation rates as obtained by the Rice-Ramsperger-Kassel-Marcus theory calculations as well as structural restriction.

Structures of aniline(pyrrole)+, aniline(ethanol)+, and aniline-(benzene). SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2018;204:665-669. [PMID: 29982157 DOI: 10.1016/j.saa.2018.07.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Molecular structures of aniline(pyrrole)⁺, aniline(ethanol)⁺, and aniline(benzene)⁺ produced via resonance two-photon ionization at 266 nm were analyzed by infrared predissociation spectroscopy coupled with tandem mass spectrometry. Structural optimization and frequency calculation using density functional theory were carried out to suggest the most probable isomers which are in good agreement with the observed infrared absorption spectra. Intermolecular bonds in the cluster ions were formed such that the electronegative oxygen atom of the solvent molecule or the pi electron of the aromatic ring forms a hydrogen bonding to NH of aniline.

Proton transfer and isotope-induced reaction in aniline cluster ions

The proton transfer (PT) and other intraclusters reactions occurring after electron ionization of aniline clusters (PhNH2)N are investigated by the time-of-flight mass spectrometry. The mass spectra are recorded for different expansion conditions leading to the generation of different cluster sizes. Several fragment ions are shown to originate from intracluster reactions, namely, [Ph](+), [PhNH3](+) and [Ph-N-Ph](+). Reaction schemes are proposed for these ions starting with the PT process. The mass region beyond the monomer mass is dominated by cluster ions (PhNH2)n(+) accompanied by satellites with ±H and +2H. In experiments with deuterated species, new fragment ions are identified. The aniline isotopomer d5-PhNH2 yields the fragment ions (PhNH2)n⋅(N-Ph-NH2)(+). Analogical series is observed in experiments with d7-PhND2, and additional fragments occur corresponding to (PhND2)n⋅(D2N-ND-Ph-ND-ND2)(+) ions. The possible reaction pathways to these ions and the unusual isotope effects are discussed.