Microsolvation of the indole cation (In⁺) in a nonpolar environment: IR spectra of In⁺–L_ncomplexes (L = Ar and N₂, n ≤ 8)

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

“…2 shows the IRPD spectra of TRA + -(N 2 ) n with n = 1-6 recorded in the range of the indolic NH stretch fundamental (n NH ) by monitoring the dominant TRA + -(N 2 ) m fragment channel, denoted as n -m. The IRPD spectrum of the TRA + -N 2 dimer (n = 1) shows two transitions with maxima at 3395 AE 1 and 3447 AE 1 cm À1 , which are assigned to the n NH transitions of the H-bound and p-bound isomers, respectively, by comparison to those observed for the related In + -N 2 isomers, with n NH = 3379 and 3450 cm À1 for In + -N 2 (H) and In + -N 2 (p), respectively. 46 The blue shaded band profile of the intense n NH band of the H-bound isomer, with a large width of B25 cm À1 and a band origin close to the P-branch head, is typical for the excitation of a proton donor stretch vibration and thus confirms the isomer assignment. 32,37,[53][54][55] The computed structures of the relevant TRA + -N 2 isomers are illustrated in Fig.…”

“…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.…”

“…It is, however, well known that p-bonded N 2 ligands hardly affect the properties of the N-H and O-H bonds in aromatic molecules. 46 Therefore, n NH of the p-bound isomer is very close to that of bare TRA + , with a minor blueshift of r5 cm À1 . The calculations predict shifts of + 1 cm À1 for both TRA + -N 2 (p top ) and TRA + -N 2 (p bottom ).…”

“…52 Harmonic frequencies are scaled by a factor of 0.9453, obtained by matching the experimental and calculated NH stretch frequencies of In + (n NH = 3454 cm À1 ). 46,51 All binding and relative energies are corrected for vibrational zero-point energies. While the oB97X-D/ cc-pVTZ data agree semi-quantitatively with the experimental data, initial calculations at the B3LYP/cc-pVDZ, CAM-B3LYP/ aug-cc-pVDZ, M06-2X/(aug-)cc-pVTZ, and oB97X-D/cc-pVDZ levels yielded much less satisfying results.…”

“…The weak H-bonds in In + -(N 2 ) n clusters with n r 8 have been investigated previously. 46 For In + -N 2 , two stable isomers, namely the H-bound and p-bound structures, are identified, in which N 2 is either H-bonded to the NH group (global minimum) or attached to the aromatic p-electron system, respectively. While in the H-bonded In + -N 2 dimer, the N 2 ligand binds linearly to the NH group, in the larger In + -(N 2 ) n clusters two N 2 molecules are H-bonded to the NH group in the aromatic plane leading to a bifurcated double H-bonded motif, and additional N 2 ligands are attached to the aromatic ring of In + .…”