Unraveling excited states of doped helium clusters

We discuss the use of generalized, symmetry-adapted, imaginary-time correlation functions to study the rotational spectrum of doped helium clusters within the frame of the reptation quantum Monte Carlo method. Analysis of these correlation functions allows one to enhance the computational efficiency in the calculation of weak spectral features, as well as to get a qualitative insight into the nature of the different lines. The usefulness of this approach is demonstrated by a study of the He-CO binary complex, used as a benchmark case, as well as by preliminary results for the satellite band recently observed in the IR spectrum of the CO2 molecule solvated in He nanodroplets.     

Unraveling hydridic-to-protonic dihydrogen bond predominance in monohydrated dodecaborate clusters

Hydridic-to-protonic dihydrogen bonds (DHBs) are involved in comprehensive structural and energetic evolution, and significantly affect reactivity and selectivity in solution and solid states. Grand challenges exist in understanding DHBs’ bonding nature and strength, and how to harness DHBs. Herein we launched a combined photoelectron spectroscopy and multiscale theoretical investigation using monohydrated closo-dodecaborate clusters B₁₂X₁₂²⁻·H₂O (X = H, F, I) to address such challenges. For the first time, a consistent and unambiguous picture is unraveled demonstrating that B–H⋯H–O DHBs are superior to the conventional B–X⋯H–O HBs, being 1.15 and 4.61 kcal mol⁻¹ stronger than those with X = F and I, respectively. Energy decomposition analyses reveal that induction and dispersion terms make pronounced contributions resulting in a stronger B–H⋯H–O DHB. These findings call out more attention to the prominent roles of DHBs in water environments and pave the way for efficient and eco-friendly catalytic dihydrogen production based on optimized hydridic-to-protonic interactions.

A joint gas-phase ion spectroscopic and multiscale theoretical study reveals unequivocally the predominance of the hydridic-to-protonic dihydrogen bond over the prototypical strong hydrogen bond in monohydrated dodecaborate clusters.     

Competition between cation-pi interactions and intermolecular hydrogen bonds in alkali metal ion-phenol clusters. I. Phenol dimer

The competition between ion-molecule and molecule-molecule interactions was investigated in M+(phenol)2 cluster ions for M=Li, Na, K, and Cs. Infrared predissociation spectroscopy in the O-H stretch region was used to characterize the structure of the cluster ions. By adjusting the experimental conditions, it was possible to generate species where argon was additionally bound in order to investigate cold cluster ions. The spectra showed the presence of hydrogen bonding in the colder M+(phenol)2Ar cluster ions but the absence of hydrogen bonding in the warmer M+(phenol)2 species. For the cold species, the IR spectra were compared with minimum-energy ab initio calculations to elucidate the hydrogen-bonded structures. In the dominant hydrogen-bonded configurations observed experimentally, the phenol molecules form hydrogen-bonded dimers and the alkali-metal ions bind to the phenol via a cation-pi interaction with the aromatic ring. Increasing the strength of the cation-pi interaction by decreasing the ion size forces the distance between the phenol O-H groups to increase, thus weakening the intermolecular hydrogen bond. Free-energy differences of different configurations relative to the ground state demonstrate that hydrogen-bonded structures are enthalpically favored, while non-hydrogen-bonded structures are entropically favored and are thus observed in the warm cluster ions.     

Diacetylene's weak bonding to acetylene clusters

An interaction potential previously developed for the acetylene–polyyne dimer was used to explore the interaction potential surfaces for clusters containing a diacetylene molecule and two or more acetylene molecules. Ab initio calculations were performed on the smallest clusters in order to assess the energetic and structural features predicted by the model potential. The preferred arrangements of the monomers in the clusters maximize the favorable quadrupole–quadrupole interactions between the monomers.     

Modeling properties of the HF dimer in argon clusters

We compare geometry configurations, vibrational properties, and electronic structures of (HF)₂ in a free state and inside argon atom shells Ar_n. For the first stage, molecular dynamics calculations for the (HF)₂ · Ar₆₂ heterocluster are performed with the help of model potentials HF(SINGLE BOND)HF, Ar(SINGLE BOND)Ar, and Ar(SINGLE BOND)HF. Then, ab initio quantum chemistry analysis is carried out for the smaller systems (HF)₂ · Ar₁₅ and (HF)₂ · Ar₆ when keeping the argon atoms closest to the trapped dimer. We conclude that the hydrogen-bonded complex (HF)₂ gains some extra stability inside the argon shells, originating primarily from a decrease of intermolecular distance R_FF. Electronic structure calculations are in accord with the changes in dynamical properties, namely, a noticeable increase in the vibrational frequency assigned to the F(SINGLE BOND)F stretching mode (+25 cm⁻¹) and decrease in rms deviations for the corresponding coordinate δ_FF. In addition to these changes, the argon atoms of the nearest solvent shell donate a small fraction of electron charge which is spent for an increase of population of the antibonding orbital σ^* of the free monomer unit and shift orbital energies primarily of the lone-pair fluorine species. These shifts are greater than the changes due to geometry alterations and the possible inaccuracies of the calculation scheme. © 1997 John Wiley & Sons, Inc.     

Inelastic scattering of unpolarized neutrons on dimer clusters

On the basis of a microscopic calculation of the inelastic neutron scattering spectra for scattering by exchange dimer clusters, we have determined the conditions for applicability of the semiphenomenological approach to determination of the scattering cross sections based on the spin model. The scattering cross sections obtained in the spin model are distinguished from the exact values by the fact that modified form factors of the SS transitions are used in the exact calculation instead of the magnetic form factors of the individual atoms. In the case of weak overlap of the one-electron wave functions for different centers, the exact scattering cross sections coincide with the cross sections calculated on the basis of the spin model.We have investigated the inelastic neutron spectrum for scattering by a dimer d²–d¹ of mixed valency. Tunneling transfer of the extra electron leads to splitting of the exchange line corresponding to the transition with S=1 in the system with localized electrons and the appearance of new lines. The spectrum consists of a series of forbidden transitions (with conservation of parity and S=1) and allowed transitions (with a change in parity and S=0, 1). The results obtained allow us to determine the parameters of Heisenberg and double exchange in mixed-valency dimers using the inelastic neutron scattering spectra.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 392–398, July–August, 1988.     

Aromatic-aromatic interactions in proteins: beyond the dimer

Aromatic residues are key widespread elements of protein structures and have been shown to be important for structure stability, folding, protein-protein recognition, and ligand binding. The interactions of pairs of aromatic residues (aromatic dimers) have been extensively studied in protein structures. Isolated aromatic molecules tend to form higher order clusters, like trimers, tetramers, and pentamers, that adopt particular well-defined structures. Taking this into account, we have surveyed protein structures deposited in the Protein Data Bank in order to find clusters of aromatic residues in proteins larger than dimers and characterized them. Our results show that larger clusters are found in one of every two unique proteins crystallized so far, that the clusters are built adopting the same trimer motifs found for benzene clusters in vacuum, and that they are clearly nonlocal brining primary structure distant sites together. We extensively analyze the trimers and tetramers conformations and found two main cluster types: a symmetric cluster and an extended ladder. Finally, using calmodulin as a test case, we show aromatic clsuters possible role in folding and protein-protein interactions. All together, our study highlights the relevance of aromatic clusters beyond the dimer in protein function, stability, and ligand recognition.     

Electronic interactions in a new pi-extended tetrathiafulvalene dimer

The first pi-extended tetrathiafulvalene (exTTF) dimer in which the two exTTF units are covalently connected by 1,3-dithiole rings has been obtained in a multistep synthetic procedure involving the Ullmann cross-coupling reaction by using copper(I) thiophene-2-carboxylate (CuTC). The electronic spectrum reveals a significant electronic interaction between the exTTF units. The electrochemical study carried out by cyclic voltammetry in solution and in thin-layer conditions, and the electrochemical simulation and spectroelectrochemical (SEC) measurements confirm the electronic communication and show that the oxidation of dimer 14 occurs as two consecutive 2 e(-) processes D(0)-D(0)-->D(2+)-D(0)-->D(2+)-D(2+). Theoretical calculations, performed at the B3P86/6-31G* level, confirm the experimental findings and predict that 14(2+) exists as a delocalized D(.+)-D(.+) species in the gas phase and as a localized D(2+)-D(0) species in solution (CH(3)CN or CH(2)Cl(2)). Oxidation of 14(2+) forms the tetracation 14(4+) which is constituted by two aromatic anthracene units bearing four aromatic, almost orthogonal 1,3-dithiolium cations.     

An orbital and electron density analysis of weak interactions in ethanol-water, methanol-water, ethanol and methanol small clusters

A computational study of (ethanol)(n)-water, n = 1 to 5 heteroclusters was carried out employing the B3LYP∕6-31+G(d) approach. The molecular (MO) and atomic (AO) orbital analysis and the topological study of the electron density provided results that were successfully correlated. Results were compared with those obtained for (ethanol)(n), (methanol)(n), n = 1 to 6 clusters and (methanol)(n)-water, n = 1 to 5 heteroclusters. These systems showed the same trends observed in the (ethanol)(n)-water, n = 1 to 5 heteroclusters such as an O---O distance of 5 ? to which the O-H---O hydrogen bonds (HBs) can have significant influence on the constituent monomers. The HOMO of the hetero(clusters) is less stable than the HOMO of the isolated alcohol monomer as the hetero(cluster) size increases, that destabilization is higher for linear geometries than for cyclic geometries. Changes of the occupancy and energy of the AO are correlated with the strength of O-H---O and C-H---O HBs as well as with the proton donor and/or acceptor character of the involved molecules. In summary, the current MO and AO analysis provides alternative ways to characterize HBs. However, this analysis cannot be applied to the study of H---H interactions observed in the molecular graphs.     

Compliant fields for molecular interactions: Water dimer and formic acid dimer

The redundancy-free internal valence compliance constants of open-chain water dimer and formic acid cyclic dimer have been determined by the combined use of the CNDO /Force method and the compliance constant formalism. The final compliant fields of these dimers have been refined with the help of experimental frequency data.     

Probing weak intermolecular interactions in self-assembled nanotubes

Extreme confinement affects the physical properties of fluids, but little quantitative data is available. We report on studies of a bisurea compound that self-assembles into nanotubes to probe solvent confinement on the angstrom scale. By applying a statistical model to calorimetric data obtained on solvent mixtures, we show that the thermodynamic stability of the nanotubes is an extremely sensitive function of the solvent composition because solvent interactions inside and outside of the nanotubes are different. We are able to measure energetic effects as small as 0.01 kT and relate them to the differences in molecular structure of the solvents.     

Unusually weak binding interactions in tetrazole-amidine complexes

[reaction: see text] Tetrazoles frequently replace carboxylic acids in pharmaceutical drugs. However, while the binding modes of tetrazolate and carboxylate anions in amidinium complexes turns out to be similar, the association constant of the former is 2-3 orders of magnitude smaller in DMSO. Crystal structures revealed that the N...H-N hydrogen bonds in amidinium tetrazolates are bent (162 degrees and 169 degrees ) and noticeably longer (N...N 2.96 A) than corresponding hydrogen bonds in both amidinium carboxylates and ammonium tetrazolates.     

Theoretical investigation of the weak interactions of rare gas atoms with silver clusters by resonance Raman spectroscopy modeling

The interactions of rare gas atoms (Rg = Ar, Kr, and Xe) with small neutral and cationic silver clusters have been investigated by density functional methods and the effect of these weak interactions on the resonance Raman spectra of the complexes has been evaluated. The resonance Raman technique that depends on the properties of ground and excited state, seems deeply sensitive to the weak rare gas–metal cluster interactions, and the use of inert gases has been proven to be an excellent approach to recognize the ability of this technique to detect extremely weak interactions. In this work, for , and complexes the IR, normal and resonance Raman spectra have been calculated and the effect of rare gas–cluster stretching vibration ( ) on the pattern and the relative intensities of different spectra have been investigated. The resonance Raman spectra for the weakly interacted complexes (with the interaction energies less than ?2.0 kcal/mol) exhibit the vibration with the detectable intensity that its intensity increases by going from Ag₆–Ar to Ag₆–Xe complex. Moreover, the resonance Raman spectra (based on the excited state gradient approximation) for high intensity nearly degenerate excited states, proved the effect of accumulation of the excited state charge density on the relative intensity of vibration.     

Spin labeling monitors weak host-guest interactions

A resorcinarene bearing four TEMPO units recognizes small molecules in solutions.     

Structural consequences of weak interactions in dispirooxindole derivatives

Spiro scaffolds are being increasingly utilized in drug discovery due to their inherent three‐dimensionality and structural variations, resulting in new synthetic routes to introduce spiro building blocks into more pharmaceutically active molecules. Multicomponent cascade reactions, involving the in situ generation of carbonyl ylides from α‐diazocarbonyl compounds and aldehydes, and 1,3‐dipolar cycloadditon with 3‐arylideneoxindoles gave a novel class of dispirooxindole derivatives, namely 1,1′′‐dibenzyl‐5′‐(4‐chlorophenyl)‐4′‐phenyl‐4′,5′‐dihydrodispiro[indoline‐3,2′‐furan‐3′,3′′‐indoline]‐2,2′′‐dione, C₄₄H₃₃ClN₂O₃, (I), 1′′‐acetyl‐1‐benzyl‐5′‐(4‐chlorophenyl)‐4′‐phenyl‐4′,5′‐dihydrodispiro[indoline‐3,2′‐furan‐3′,3′′‐indoline]‐2,2′′‐dione, C₃₉H₂₉ClN₂O₄, (II), 1′′‐acetyl‐1‐benzyl‐4′,5′‐diphenyl‐4′,5′‐dihydrodispiro[indoline‐3,2′‐furan‐3′,3′′‐indoline]‐2,2′′‐dione, C₃₉H₃₀N₂O₄, (III), and 1′′‐acetyl‐1‐benzyl‐4′,5′‐diphenyl‐4′,5′‐dihydrodispiro[indoline‐3,2′‐furan‐3′,3′′‐indoline]‐2,2′′‐dione acetonitrile hemisolvate, C₃₉H₃₀N₂O₄·0.5C₂H₃N, (IV). All four compounds exist as racemic mixtures of the SSSR and RRRS stereoisomers. In these structures, the two H atoms of the dihydrofuran ring and the two substituted oxindole rings are in a trans orientation, facilitating intramolecular C—H...O and π–π interactions. These weak interactions play a prominent role in the structural stability and aid the highly regio‐ and diastereoselective synthesis. In each of the four structures, the molecular assembly in the crystal is also governed by weak noncovalent interactions. Compound (IV) is the solvated analogue of (III) and the two compounds show similar structural features.     

Ferroelectric heterobimetallic clusters with ferromagnetic interactions

Two homochiral trinuclear clusters, {(MeTp)2Fe2(CN)6Ni[(1R,2R)-chxn]2} (1) and {(MeTp)2Fe2(CN)6Ni[(1S,2S)-chxn]2} (2) [chxn = 1,2-diaminocyclohexane; MeTp = methyltris(pyrazolyl)borate], have been synthesized and structurally characterized. Ferroelectric and magnetic measurements reveal that they are rare examples of metal-organic compounds bearing ferroelectricity and intramolecular ferromagnetic interactions.     

Competing non-covalent interactions in alkali metal ion-acetonitrile-water clusters

Competitive ion-dipole, ion-water, and water-water interactions were investigated at the molecular level in M+ (CH3CN)n(H2O)m cluster ions for M = Na and K. Different [n,m] combinations for two different n + m cluster sizes were characterized with infrared predissociation spectroscopy in the O-H stretch region and MP2 calculations. In all cases, no differences were observed between the two alkali metal ions. The results showed that at the n + m = 4 cluster size, the solvent molecules interact only with the ion, and that the interaction between the ion and the large dipole moment of CH3CN decreases the ion-water electrostatic interactions. At the n + m = 5 cluster size, at least two different hydrogen-bonded structures were identified. In these structures, the ion-dipole interaction weakens the ability of the ion to polarize the hydrogen bonds and thus decreases the strength of the water-water interactions in the immediate vicinity of the alkali metal ion.