Ionization-induced switch in aromatic molecule–nonpolar ligand recognition: Acidity of 1-naphthol⁺(1-Np⁺) rotamers probed by IR spectra of 1-Np⁺–L_ncomplexes (L = Ar/N₂, n ≤ 5)

Abstract

The interaction of the trans (t) and cis (c) rotamers of the 1-naphthol cation (1-C 10 H 8 O 1 ¼ 1-Np 1 ¼ 1-hydroxynaphthalene 1 ) with nonpolar ligands in the ground electronic state is characterized by IR photodissociation spectra of isolated 1-Np 1 -L n complexes (L ¼ Ar/N 2 ) and density functional calculations at the UB3LYP/6-311G(2df,2pd) level. Size-dependent frequency shifts of the O-H stretch vibration (Dn 1 ) and photofragmentation branching ratios provide information about the stepwise microsolvation of both 1-Np 1 rotamers in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between H-bonding and p-bonding, the estimation of ligand binding energies, and the acidity of t/c-1-Np 1 . t-1-Np 1 is predicted to be more stable than c-1-Np 1 by 9 kJ mol À1 , with an isomerization barrier of 38 kJ mol À1 . The OH group in t-1-Np 1 is slightly more acidic than in c-1-Np 1 leading to stronger intermolecular H-bonds. Both 1-Np 1 rotamers are considerably less acidic than the phenol cation because of enhanced charge delocalization. The 1-Np 1 ÀAr spectrum displays n 1 bands of the more stable H-bound and the less stable p-bound t-1-Np 1 -Ar isomers. Only the more stable H-bound dimers are identified for t/c-1-Np 1 -L 2 . Analysis of the Dn 1 shifts of the H-bound dimers yields a first experimental estimate for the proton affinity of the t-1-naphthoxy radical (B908 AE 30 kJ mol À1 ). The Dn 1 shifts of 1-Np 1 -L n (n r 2 for Ar, n r 5 for N 2 ) suggest that the preferred microsolvation path begins with the formation of H-bound 1-Np 1 -L, which is further solvated by (nÀ1) p-bound ligands. Ionization of 1-NpÀL n drastically changes the topology of the intermolecular interaction potential and thus the preferred aromatic substrate-nonpolar ligand recognition pattern.

Weak hydrogen bonding motifs of ethylamino neurotransmitter radical cations in a hydrophobic environment: infrared spectra of tryptamine+–(N2)n clusters (n ≤ 6)

“…As TRA + is expected to form weak H-bonds with nonpolar N 2 molecules, their characteristic features become accessible by direct comparison with the strong conventional H-bond observed in TRA + -H 2 O. The current TRA + -(N 2 ) n study extends our previous characterization of A + -(N 2 ) n clusters with more simple aromatic chromophores (A + ), 32 such as benzene, 33,34 phenols and naphthols, [35][36][37][38][39] anilines, [40][41][42] imidazole, 43 cyclopropenyl, 44,45 and indole (In). 46 These studies revealed that the microsolvation process of acidic aromatic ions in molecular nitrogen is dominated by the competition between two principal binding motifs, namely H-bonding to the acidic functional OH and NH groups and p-stacking to the aromatic ring.…”

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

“…44 As the binding energies and potential barriers for Ar clusters interacting with PhOH and benzene are of similar magnitude, the melting temperatures for both cluster systems are expected to be similar, too. Interestingly, the simulations reveal that at temperatures above melting and below boiling (evaporation) the less stable (20) isomer of benzene-Ar 2 is more abundant than the more stable (11) isomer due to entropy. 44 Spectroscopic 12,13,15 and theoretical studies 12,14,35,45 show that in the cationic D 0 state the H-bonded (H00) structure is the global minimum on the PhOH + -Ar potential and more stable than the p-bonded (10) local minimum.…”

“…[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 ).…”

“…Calculations at moderate theoretical levels are however consistent with this assignment. 29,31,39 Although spectra of the related benzene-Ar 2 and aniline-Ar 2 clusters exhibit also significant signals from a less stable (20) structure, this type of isomer has escaped identification in the PhOH-Ar 2 spectra for a long time owing to its weak intensity in most REMPI spectra. Only very recently, an empirical additivity model developed for the DS 1 shifts of PhOH-Ar n suggested a tentative assignment of the S 1 origin of (20) to a weak transition at DS 1 = À2 cm À1 (Fig.…”

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.…”