Interaction of Ionic Biomolecular Building Blocks with Nonpolar Solvents:  Acidity of the Imidazole Cation (Im⁺) Probed by IR Spectra of Im⁺−L_n Complexes (L = Ar, N₂; n ≤ 3)

Abstract

The intermolecular interaction between the imidazole cation (Im+ = C3N2H4+) and nonpolar ligands is characterized in the ground electronic state by infrared photodissociation (IRPD) spectroscopy of size-selected Im+-Ln complexes (L = Ar, N2) and quantum chemical calculations performed at the UMP2/6-311G(2df,2pd) and UB3LYP/6-311G(2df,2pd) levels of theory. The complexes are created in an electron impact cluster ion source, which predominantly produces the most stable isomers of a given cluster ion. The analysis of the size-dependent frequency shifts of both the N-H and the C-H stretch vibrations and the photofragmentation branching ratios provides valuable information about the stepwise microsolvation of Im+ in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between various intermolecular binding motifs (H-bonding and pi-bonding) and their interaction energies, and the acidity of both the CH and NH protons. In line with the calculations, the IRPD spectra show that the most stable Im+-L dimers feature planar H-bound equilibrium structures with nearly linear H-bonds of L to the acidic NH group of Im+. Further solvation occurs at the aromatic ring of Im+ via the formation of intermolecular pi-bonds. Comparison with neutral Im-Ar demonstrates the drastic effect of ionization on the topology of the intermolecular potential, in particular in the preferred aromatic substrate-nonpolar recognition motif, which changes from pi-bonding to H-bonding. .

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

“…31,32 However, the calculations show that such a binding motif is less stable for t-AA for imidazole. 29 Thus, the measured shift for AH + = t-AA + of (Fig. 7 and 10 1634 (13) 1640 (14) 1636 (10) 1635 (14) 1638 (8) (14) 1448 (12) 1447 (14) 1450 ( (8) 1435 (5) 1432 (8) 1435 (13) 1434 (8) b CH3 (asym) P 1424/1428 1424 1426 (52) 1431 (50) 1425 (49) 1428 (42) 1428 (52) b CH (ph, n 19b ) Q 1386 1386 1379 (44) 1376 (42) 1375 (45) 1375 (41) 1378 (43) b CH3 (sym) R 1355…”

“…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.…”

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

“…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 ). To this end, we review the current knowledge on these systems relevant for the present work.…”

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

“…Indeed, inspection of the IR spectra of imidazole + -(N 2 ) n clusters reported in ref. 43 also exhibits the spectroscopic signature of the bifurcated binding motif for n Z 2 (although not identified as such in ref. 43), supporting this hypothesis.…”