The fundamentals: the IR spectrum of the adamantane cation, C(10)H(16)(+), has been derived by resonant IR photodissociation of weakly bound C(10)H(16)(+)⋅L(n) clusters. The analysis of the IR spectrum provides the first spectroscopic characterization of this fundamental cycloalkane carbocation in the gas phase and direct evidence for the Jahn-Teller distortion in the (2)A(1) ground electronic state.

The solvation of aromatic (bio-)molecular building blocks has a strong impact on the intermolecular interactions and function of supramolecular assemblies, proteins, and DNA. Herein we characterize the initial microsolvation process of the heterocyclic aromatic pyrrole cation (Py) in its A ground electronic state with nonpolar, quadrupolar, and dipolar ligands (L = Ar, N, and HO) by infrared photodissociation (IRPD) spectroscopy of cold mass-selected Py-L (n ≤ 3) clusters in a molecular beam and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. Size- and isomer-specific shifts in the NH stretch frequency (Δν) unravel the competition between various ligand binding sites, the strength of the respective intermolecular bonds, and the cluster growth. In Py-Ar, linear H-bonding of Ar to the acidic NH group (NHAr) is competitive with π-stacking to the aromatic ring, and both Py-Ar(H) and Py-Ar(π) are observed. For L = N and HO, the linear NHL H-bond is much more stable than any other binding site and the only observed binding motif. For the Py-Ar and Py-(N) trimers, the H/π isomer with one H-bonded and one π-bonded ligand strongly competes with a 2H isomer with two bifurcated nonlinear NHL bonds. The latter are equivalent for Ar but nonequivalent for N. Py-HO exhibits a strong and linear NHO H-bond with charge-dipole configuration and C symmetry. IRPD spectra of cold Py-HO-L clusters with L = Ar and N reveal that Ar prefers π-stacking to the Py ring, while N forms an OHN H-bond to the HO ligand. The Δν frequency shifts in Py-L are correlated with the strength of the NHL H-bond and the proton affinity (PA) of L, and a monotonic correlation between Δν of the Py-L(H) dimers and PA is established. Comparison with neutral Py-L dimers reveals the strong impact of the positive charge on the acidity of the NH group, the strength of the NHL H-bond, and the preferred ligand binding motif.

Life is believed to have its origin in aqueous environments, and 70 % of our body consists of water. The essential components of biological systems have to interact in aqueous solutions with water molecules by intermolecular forces, such as hydrogen bonds, dispersion forces, and hydrophilic/hydrophobic interactions. [1] Proteins are one of the most important biological supramolecules and offer at the CO and NH sites of the -CONH-linkages of the peptide chain attractive hydrogen-bonding sites, in which H 2 O can act either as a proton donor or a proton acceptor, respectively. The solvation of a protein has a strong effect on its molecular shape, and as a consequence the fluctuations of the water network on the surface have important influence on its folding properties and catalytic function. [2] Most fundamentally, when a protein starts its folding motion, the water network hydrogen-bonded to the protein has to rearrange and thus affects the dynamics. Therefore, up-to-date quantum chemical simulations on protein folding and its functions include water molecules explicitly. [2h,l,m] A deeper understanding of these phenomena at the molecular level requires the characterization of the dynamical processes of individual water molecules interacting with the protein. However, most experiments yield only indirect dynamical information averaged over water molecules in the first hydration layer and thus only a tentative and often controversial interpretation of the underlying mechanisms. [2a,e-g,i,k,n] Measurements visualizing the motion of a specific water molecule in a real biological environment are challenging, and so far no experimental data have been reported yet. Such dynamical experiments need to distinguish between each single water molecule, which can bind to numerous different binding sites of the protein and readily exchange their role with other H 2 O molecules in the same or higher hydration solvation layers. This inherent complexity of the hydrated protein has so far prevented measurements of the migration of individual water molecules in solution, and therefore nearly all information about such processes relies on theoretical approaches. [2a,f-h,l-o] Although quantum chemical simulations for such complex systems have substantially progressed in recent years because of rapid computer developments, their accuracy is still rather limited and experimental benchmark data for model systems are highly requested for calibration purposes. To this end, we have developed in the past decade an experimental strategy for the investigation of dynamical intermolecular processes, [3] which typically occur on the picosecond (ps) time scale. This approach involves the generation of molecular clusters isolated in supersonic beams and the characterization of their dynamics using ps time-resolved IR spectroscopy. The fruitful combination of spectroscopy and quantum chemistry currently provides the most direct and most detailed access to intermolecular interactions. [1] IR spectroscopy is particularly sensitive to structura...

Phenylalkylamines of the general formula C6H5(CH2)nNH2 (n = 1-4) have been delivered to the gas phase as protonated species using electrospray ionization. The ions thus formed have been assayed by IRMPD spectroscopy in two different spectroscopic domains, namely, the 600-1800 and the 3000-3500 cm(-1) regions using either an IR free electron laser or a tabletop OPO/OPA laser source. The interpretation of the experimental spectra is aided by density functional theory calculations of candidate species and vibrational frequency analyses. Protonated benzylamine presents a relatively straightforward instance of a single stable conformer, providing a trial case for the adopted approach. Turning to the higher homologues, C6H5(CH2)nNH3(+) (n = 2-4), more conformations become accessible. For each C6H5(CH2)nNH3(+) ion (n = 2-4), the most stable geometry is characterized by cation-π interactions between the positively charged ammonium group and the aromatic π-electronic system, permitted by the folding of the polymethylene chain. The IRMPD spectra of the sampled ions confirm the presence of the folded structures by comparison with the calculated IR spectra of the various possible conformers. An inspection of the NH stretching region is helpful in this regard.

Auf verschlungenem Pfad: Die Bewegung eines einzelnen Wasserliganden um eine Peptidbindung in Acetanilid wurde mit zeitaufgelöster IR‐Spektroskopie in Echtzeit untersucht. Ausgelöst durch Photoionisation wird der Wasserligand von der CO‐Seite der Peptidbindung freigegeben und an der NH‐Seite derselben Peptidbindung nach einer Migrationsphase von 5 ps eingefangen (siehe Bild).

The structure and dynamics of the highly flexible side chain of (protonated) phenylethylamino neurotransmitters are essential for their function. The geometric, vibrational, and energetic properties of the protonated neutrotransmitter 2-phenylethylamine (H(+)PEA) are characterized in the N-H stretch range by infrared photodissociation (IRPD) spectroscopy of cold ions using rare gas tagging (Rg = Ne and Ar) and anharmonic calculations at the B3LYP-D3/(aug-)cc-pVTZ level including dispersion corrections. A single folded gauche conformer (G) protonated at the basic amino group and stabilized by an intramolecular NH(+)-π interaction is observed. The dispersion-corrected density functional theory calculations reveal the important effects of dispersion on the cation-π interaction and the large vibrational anharmonicity of the NH3(+) group involved in the NH(+)-π hydrogen bond. They allow for assigning overtone and combination bands and explain anomalous intensities observed in previous IR multiple-photon dissociation spectra. Comparison with neutral PEA reveals the large effects of protonation on the geometric and electronic structure.

Vibrational and electronic spectra of protonated naphthalene (NaphH(+)) microsolvated by one and two water molecules were obtained by photofragmentation spectroscopy. The IR spectrum of the monohydrated species is consistent with a structure with the proton located on the aromatic molecule, NaphH(+)-H(2)O. Similar to isolated NaphH(+), the first electronic transition of NaphH(+)-H(2)O (S(1)) occurs in the visible range near 500 nm. The doubly hydrated species lacks any absorption in the visible range (420-600 nm) but absorbs in the UV range, similar to neutral Naph. This observation is consistent with a structure, in which the proton is located on the water moiety, Naph-(H(2)O)(2)H(+). Ab initio calculations for [Naph-(H(2)O)(n)]H(+) confirm that the excess proton transfers from Naph to the solvent cluster upon attachment of the second water molecule.

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.