Spectroscopic Identification of Carbenium and Ammonium Isomers of Protonated Aniline (AnH⁺):  IR Spectra of Weakly Bound AnH⁺−L_n Clusters (L = Ar, N₂)

Abstract

Infrared photodissociation (IRPD) spectra of mass-selected clusters composed of protonated aniline (C6H8N+ = AnH+) and a variable number of neutral ligands (L = Ar, N2) are obtained in the N-H stretch range. The AnH+ -Ln complexes (n < or = 3) are produced by chemical ionization in a supersonic expansion of An, H2, and L. The IRPD spectra of AnH+-Ln feature the unambiguous fingerprints of at least two different AnH+ nucleation centers, namely, the ammonium isomer (5) and the carbenium ions (1 and/or 3) corresponding to protonation at the N atom and at the C atoms in the para and/or ortho positions, respectively. Protonation at the meta and ipso positions is not observed. Both classes of observed AnH+-Ln isomers exhibit very different photofragmentation behavior upon vibrational excitation arising from the different interaction strengths of the AnH+ cores with the surrounding neutral ligands. Analysis of the incremental N-H stretch frequency shifts as a function of cluster size shows that microsolvation of both 5 and 1/3 in Ar and N2 starts with the formation of intermolecular H bonds of the ligands to the acidic NH protons and proceeds by intermolecular pi bonding to the aromatic ring. The analysis of both the photofragmentation branching ratios and the N-H stretch frequencies demonstrates that the N-H bonds in 5 are weaker and more acidic than those in 1/3, leading to stronger intermolecular H bonds with L. The interpretation of the spectroscopic data is supported by density functional calculations conducted at the B3LYP level using the 6-31G* and 6-311G(2df,2pd) basis sets. Comparison with clusters of neutral aniline and the aniline radical cation demonstrates the drastic effect of protonation and ionization on the acidity of the N-H bonds and the topology of the intermolecular potential, in particular on the preferred aromatic substrate-nonpolar ligand recognition motif.

Structures and IR/UV spectra of neutral and ionic phenol–Arn cluster isomers (n≤ 4): competition between hydrogen bonding and stacking

“…[3][4][5][7][8][9][10][11] Neutral PhOH-Rg dimers prefer p-bonding because dispersion dominates the attraction, whereas PhOH + -Rg cations prefer H-bonding because the additional charge-induced polarisation forces provide substantial further stabilisation. 12,13 This charge-induced p -H switch is a general phenomenon for acidic aromatic molecules interacting with nonpolar ligands, 4 and has been established for a variety of aromatic molecules with acidic OH or NH functional groups, including phenol, [12][13][14][15][16][17][18] resorcinol, 19 naphthol, 20 aniline, [21][22][23] indole, 24 and imidazole. 25 The present work characterizes the structures, binding energies, and vibrational and electronic spectra of various isomers of neutral and ionic PhOH (+) -Ar n clusters with n r 4 by quantum chemical calculations in three different electronic states, namely the neutral ground and first excited singlet states (S 0 , S 1 ) and the cation ground state (D 0 ).…”

Infrared spectra of protonated polycyclic aromatic hydrocarbon molecules: Azulene

“…13 The first technique employs modern, relatively low-intensity, optical parametric oscillator laser systems in the frequency range 800-4000 cm −1 to drive one-photon IRPD of AH + -L n cluster ions. 12 This approach is based on the evaporation of one or more of the weakly bound ligands upon resonant absorption of a single photon ͑messenger technique͒, 14 and has been applied to a variety of AH + -L n cluster ions, including A = benzene, [15][16][17][18] phenol, 12,19 fluorobenzene, 20 para-halogenated phenols, 21 toluene, 17 pyridine, 18 aniline, 22 imidazole, 23 various amino acids and peptides, 24 and neurotransmitters. 25 The IRPD method can also be applied to break weak chemical bonds in certain AH + isomers, 11,26 but usually fails to dissociate common AH + ions because the energy of a single IR photon is insufficient to break strong covalent bonds.…”

Diastereo-specific conformational properties of neutral, protonated and radical cation forms of (1R,2S)-cis- and (1R,2R)-trans-amino-indanol by gas phase spectroscopy

“…S3 in the ESI †) immediately reveals that the Ar ligand substantially affects the spectrum. [83][84][85][86][87][88] Hence, Ar complexation must be taken into account in the calculations. To this end, geometries and vibrational frequencies of the cis-H + AI I/II -Ar clusters ( Fig.…”

“…It turns out that the preferred Ar binding site for both conformers is indeed the NH 3 + group with a NHÁ Á ÁAr contact. [83][84][85][86][87][88] For cis-H + AI I -Ar, the preferred Ar binding site is the NH group pointing toward the phenyl ring (Ar@NH I ), because Ar benefits from the additional attractive dispersion interaction with the aromatic p-electrons. 89 The second and third most stable isomers correspond to Ar binding to the NH group pointing away from the molecule (Ar@NH II ) and to that involved in the NHÁ Á ÁO H-bond (Ar@NH III ).…”

“…Finally, we note that the DFT calculations do not take into account the Ar tagging, which may induce additional minor spectral shifts and splittings and stabilise different AI + conformers to a different degree. [88][89][90] However, the analysis of the frequencies in terms of conformer families allows for disentangling the rich IRPD spectrum, providing a tentative but fully consistent assignment. In contrast to the neutral and protonated species it is perhaps not surprising to detect higher energy species for the radical cation.…”