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

Abstract

Size-selected clusters of the tryptamine cation with N 2 ligands, TRA + -(N 2 ) n with n = 1-6, are investigated by infrared photodissociation (IRPD) spectroscopy in the hydride stretch range and quantum chemical calculations at the oB97X-D/cc-pVTZ level to characterize the microsolvation of this prototypical aromatic ethylamino neurotransmitter radical cation in a nonpolar solvent. Two types of structural isomers exhibiting different interaction motifs are identified for the TRA + -N 2 dimer, namely the TRA + -N 2 (H) global minimum, in which N 2 forms a linear hydrogen bond (H-bond) to the indolic NH group, and the less stable TRA + -N 2 (p) local minima, in which N 2 binds to the aromatic p electron system of the indolic pyrrole ring. The IRPD spectrum of TRA + -(N 2 ) 2 is consistent with contributions from two structural H-bound isomers with similar calculated stabilization energies. The first isomer, denoted as TRA + -(N 2 ) 2 (2H), exhibits an asymmetric bifurcated planar H-bonding motif, in which both N 2 ligands are attached to the indolic NH group in the aromatic plane via H-bonding and charge-quadrupole interactions. The second isomer, denoted as TRA + -(N 2 ) 2 (H/p), has a single and nearly linear H-bond of the first N 2 ligand to the indolic NH group, whereas the second ligand is p-bonded to the pyrrole ring. The natural bond orbital analysis of TRA + -(N 2 ) 2 reveals that the total stability of these types of clusters is not only controlled by the local H-bond strengths between the indolic NH group and the N 2 ligands but also by a subtle balance between various contributing intermolecular interactions, including local H-bonds, charge-quadrupole and induction interactions, dispersion, and exchange repulsion. The systematic spectral shifts as a function of cluster size suggest that the larger TRA + -(N 2 ) n clusters with n = 3-6 are composed of the strongly bound TRA + -(N 2 ) 2 (2H) core ion to which further N 2 ligands are weakly attached to either the p electron system or the indolic NH proton by stacking and charge-quadrupole forces.

IR spectrum of the protonated neurotransmitter 2-phenylethylamine: dispersion and anharmonicity of the NH3+–π interaction

“…Recent applications of this IRPD approach of tagged ions in our laboratory include hydrocarbon ions, [48][49][50][51][52][53][54][55][56][57] silicon-containing ions, [58][59][60] and biological molecules and their hydrates. [61][62][63][64][65][66] DFT calculations 67 are carried out at the B3LYP-D3/aug-cc-pVTZ level to determine the geometric, vibrational, and energetic properties of the H + PEA conformers and their H + PEA-Rg n clusters. The Grimme dispersion correction 68 is used to accurately describe the dispersion forces, which are particularly relevant for the intermolecular bonds to the Rg atoms and the intramolecular NH + -p interaction of H + PEA.…”

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

“…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. For the prototypical phenol (+) -Rg n clusters (Rg = Ar, Kr), this p -H rearrangement reaction has recently been monitored in real time by picosecond time-resolved IR spectroscopy, providing for the first time experimental data for intermolecular solvent dynamics in isolated clusters.…”

Gas-phase conformational preference of the smallest saccharide (glycolaldehyde) and its hydrated complexes with bridged hydrogen bonding

“…The uB97X-D functional contains the empirical treatment of dispersion interactions and gives reliable non-covalent interactions for many systems, i.e., dimethyl ether dimer, tryptamine + -(N 2 ) n and their hydrated complexes with satisfactory vibrational frequencies. [43][44][45][46][47] Thus, the dispersion-corrected density functional theory calculations were performed at the unrestricted uB97X-D/cc-pVTZ level to obtain vibrational energy of the GA and its hydrated complexes. The harmonic frequencies are scaled by a factor of 0.9415.…”

“…The harmonic frequencies are scaled by a factor of 0.9415. 43,44 For the hydrated complexes, natural bond orbital (NBO) analysis was also employed with the uB97X-D functional and cc-pVTZ basis set. 48 All relative energies were zero-point energy (ZPE)-corrected in the Gaussian program.…”