Glory-Scattering Measurement of Water-Noble-Gas Interactions: The Birth of the Hydrogen Bond

The dawn of hydrogen and halogen bonds and their crucial role in collisional processes probing long-range intermolecular interactions

This perspective review focuses on the results of an internally consistent study developed in the Perugia laboratory, centered on the fundamental interaction components that, at large intermolecular distances, determine the formation of weak intermolecular hydrogen (HB) and halogen (XB) bonds. This investigation exploits old and novel molecular beam scattering experiments involving several gaseous prototypical systems. In particular, we focus on the kinetic energy dependence of the total (elastic + inelastic) integral cross-sections. Of particular interest is the measure of quantum interference patterns in the energy dependence of cross-sections of targeted systems and their shift compared to that of known reference systems. We interpreted these findings as interaction energy stabilization components, such as charge transfer, σ-hole, and polar flattening, that emerge at intermediate separation distance ranges and selectively manifest for specific geometries of collision complexes. Another significant observable we discuss is the absolute value of the cross-section and its dependence on permanent multipole moments of the collisional partners. Specifically, we show how the spontaneous orientation of rotationally cold and polar molecules, due to the electric field gradient associated with the interaction between permanent multipole moments, can significantly modify the magnitude of the total cross-section, even at high values of the impact parameter. We are confident that the present results can help extend the force field formulation to various interacting systems and carry out molecular dynamics simulations under conditions of application interest.

A Detailed Study of Electronic and Dynamic Properties of Noble Gas–Oxygen Molecule Adducts

In this work, the binding features of adducts formed by a noble gas (Ng = He, Ne, Ar, Kr, Xe, and Rn) atom and the oxygen molecule (O2) in its ground Σg−3, in the past target of several experimental studies, have been characterized under different theoretical points of view to clarify fundamental aspects of the intermolecular bond. For the most stable configuration of all Ng–O2 systems, binding energy has been calculated at the theory’s CCSD(T)/aug-cc-pVTZ level and compared with the experimental findings. Rovibrational energies, spectroscopic constants, and lifetime as a function of temperature were also evaluated by adopting properly formulated potential energy curves. The nature of the interaction involved was deeply investigated using charge displacement analysis, symmetry-adapted perturbation theory (SAPT), and natural bond orbital (NBO) methods. In all adducts, it was found that the charge transfer plays a minor role, although O2 is an open shell species exhibiting a positive electron affinity. Obtained results also indicate that the dispersion attraction contribution is the main responsible for the complex stability.

Distribution of Weakly Interacting Atoms and Molecules in Low-Temperature Amorphous Solid Water

Understanding the distribution and mixing of atoms and molecules in amorphous solid water (ASW) at low temperatures is relevant to the exploration of the astrochemical environment in the interstellar medium (ISM) that leads to the formation of new complex molecules. In this study, a combination of temperature programmed desorption (ΔP-TPD) experiments and Ne⁺ ion sputtering is used to determine the extent of mixing and distribution of guest atoms and molecules within thin ASW films deposited at 35 K on a Ru(0001) substrate, prior to sputtering. The mixing of krypton atoms and methyl chloride molecules within thin ASW films is directed by the physical properties of the respective species and the nature of their interaction with the host water molecules. While the Kr-H₂O interaction may be described as a weak van der Waals attraction, the CD₃Cl-H₂O interaction can be characterized as weakly hydrophobic in nature. This leads to differences in the level of homogeneity in mixing and distribution of the guest species in the ASW film. Both krypton atoms and methyl chloride molecules reveal a propensity to migrate toward the ASW-vacuum interface. The krypton atoms migrate through both diffusion and displacement by incoming H₂O molecules, while the methyl chloride molecules tend to move toward the vacuum interface primarily via displacement. This behavior results in more homogeneous mixing of Kr in ASW at 35 K compared to the dipole moment containing molecule CD₃Cl. As a general outcome of our study, it is observed that mixing in ASW at low temperatures is more homogeneous when the guest atom/molecule is inert and does not possess a constant dipole moment.

Spectroscopy, lifetime, and charge-displacement of the methanol-noble gas complexes: An integrated experimental-theoretical investigation

An integrated experimental-theoretical investigation was employed to determine rovibrational energies, spectroscopic constants, lifetime as a function of temperature in gas phase complexes of methanol with noble gas (NgHe, Ne, Ar, Kr, Xe, and Rn). Beside that, a parallel effort has been addressed to theoretically characterize the nature of intermolecular interactions determining the dissociation energy and equilibrium distance of the formed adducts. Dynamics and lifetime results reveal that, except for the CH₃OH-He aggregate, all other methanol-Ng compounds are sufficiently stable under thermal conditions. Their lifetimes are larger than 1 ps for the temperature of the bulk in the range between 200 and 500 K. In addition, the current lifetime results suggest that the aggregates formed by methanol and Ng are globally more stable than corresponding complexes formed by water with Ng. From the point of view of the CCSD(T)/aug-cc-pVTZ level calculation, in all compounds, the electron densities of Ng partners are weakly polarized in the presence of CH₃OH molecule. The charge-displacement curves and NBO analysis indicate that the charge transfer from Ng to methanol molecule, in general, plays a minor role, being appreciable only in the aggregate involving Ar. Finally, it was verified from the SAPT2 + (CCD)-δMP2/aug-cc-pVTZ calculations and NCI analysis that the dispersion is the essential long-range attractive contribution to the interaction energy for all studied complexes. This feature strongly suggests that these compounds are held bonded substantially by van der Waals forces. Then non-covalent intermolecular bonds are effectively formed in the gas phase, which is disturbed by small stabilizing charge-transfer contributions.

Leading Interaction Components in the Structure and Reactivity of Noble Gases Compounds

The nature, strength, range and role of the bonds in adducts of noble gas atoms with both neutral and ionic partners have been investigated by exploiting a fine-tuned integrated phenomenological–theoretical approach. The identification of the leading interaction components in the noble gases adducts and their modeling allows the encompassing of the transitions from pure noncovalent to covalent bound aggregates and to rationalize the anomalous behavior (deviations from noncovalent type interaction) pointed out in peculiar cases. Selected adducts affected by a weak chemical bond, as those promoting the formation of the intermolecular halogen bond, are also properly rationalized. The behavior of noble gas atoms excited in their long-life metastable states, showing a strongly enhanced reactivity, has been also enclosed in the present investigation.

Complexes of helium with neutral molecules: Progress toward a quantitative scale of bonding character

The complexes of helium with nearly 30 neutral molecules (M) were investigated by various techniques of bonding analysis and symmetry-adapted perturbation theory (SAPT). The main investigated function was the local electron energy density H(r), analyzed, in particular, so to estimate the degree of polarization (DoP) of He in the various He(M). As we showed recently (Borocci et al., J. Comput. Chem., 2019, 40, 2318-2328), the DoP is a quantitative index that is generally informative about the role of polarization (induction plus charge transfer [CT]) and dispersion in noncovalent noble gas complexes. As further evidence in this regard, we presently ascertained quantitative correlations between the DoP(He) of the He(M) and indices based on the electron density ρ(r), including the molecular electrostatic potential at the HeM bond critical point, as well as the percentage contributions of induction and dispersion to the SAPT binding energies. Based also on the explicit evaluation of the CT, accomplished through the study of the charge-displacement function, we derived a quantitative scale that ranks the He(M) according to their dispersive, inductive, and CT bonding character. Our taken approach could be conceivably extended to other types of noncovalent complexes.

Ab Initio Molecular Dynamics Investigation of the Electronic and Structural Stability of Anionic O2-(H2O)n, n = 1-16 Clusters

We report an ab initio molecular dynamics investigation of the electronic and structural stability of water molecules binding to a nucleation O₂^- particle, O₂^-(H₂O)_n with n = 1-16, to ascertain the factors that create particularly stable species. Our results compare well with previous experimental and theoretical reports for clusters with less water content, find three new geometries for species with 7, 9, and 10 water molecules, and determine that 8, 11, 13, and 15 water molecules form remarkably stable structures around O₂^-. These special clusters correspond to well-defined compact structures formed by cubes and four-member rings made of water's hydrogen bonds interacting with a negative kernel formed by O₂^- with five water molecules, O₂^-(H₂O)₅, in which the negative charge is localized in the first four water molecules, while the fifth molecule provides geometrical stability. We assess the clusters' energetic stability based on dissociation energies, analyze electron detachment energies to understand its geometrical evolution, and investigate its charge distribution based upon isosurfaces of the highest occupied molecular orbital (HOMO). This research can help provide theoretical insight into the starting steps of nucleation of water clusters around ionic particles.

Noncovalent Complexes of the Noble-Gas Atoms: Analyzing the Transition from Physical to Chemical Interactions

The bonding character of the noncovalent complexes of the noble-gas (Ng) atoms ranges from nearly purely dispersive contacts to interactions featuring appreciable contributions of induction and charge transfer. In this study, we discuss a new quantitative index that seems peculiarly informative about these diverse bonding situations. This index was termed as the degree of polarization (DoP) of Ng, as it measures, in essence, the Ng polarization promoted by the binding partner. The definition of the DoP(Ng) relies on the analysis of the local electron energy density H(r), and its physical meaning was best appreciated by studying also the charge-displacement function and the molecular electrostatic potential of the investigated benchmark species, that include nearly 60 Ngs complexes of different bonding character. The DoP(Ng) appears of general applicability, and is also positively correlated with other bonding character indices. © 2019 Wiley Periodicals, Inc.

Krypton-methanol spectroscopic study: Assessment of the complexation dynamics and the role of the van der Waals interaction

The Kr-CH₃OH (Krypton-Methanol) system has several technological applications, such as the determination of diffusivity coefficients, their use in the development of detectors and combustion techniques among others. We report an extensive theoretical study concerning the stability of such complex. A mix between molecular dynamics, electronic structure calculations and solution of the nuclear Schrodinger equation lead to investigation of spectroscopic constants, lifetime of the complex and its Quantum Theory Atom in Molecules (QTAIM) properties. The study of the Potential Energy Curves (PEC) suggested three configurations to be stable as their potential well were able to harbor 9 vibrational levels. Properties from the curves also allowed us to obtain the lifetime of the complex, whose values were >1 ps regardless of the conformation. Furthermore, topological investigations of the charge density profile of the complex, in the scope of QTAIM properties, show that van der Waals type interactions takes place between the noble gas and the methanol molecule. These features are in consonance to the experimental fact that this complex is stable.

Quantum Interference of Glory Rescattering in Strong-Field Atomic Ionization

During the ionization of atoms irradiated by linearly polarized intense laser fields, we find for the first time that the transverse momentum distribution of photoelectrons can be well fitted by a squared zeroth-order Bessel function because of the quantum interference effect of glory rescattering. The characteristic of the Bessel function is determined by the common angular momentum of a number of semiclassical paths termed as glory trajectories, which are launched with different nonzero initial transverse momenta distributed on a specific circle in the momentum plane and finally deflected to the same asymptotic momentum, which is along the polarization direction, through post-tunneling rescattering. Glory rescattering theory based on the semiclassical path-integral formalism is developed to address this effect quantitatively. Our theory can resolve the long-standing discrepancies between existing theories and experiments on the fringe location, predict the sudden transition of the fringe structure in holographic patterns, and shed light on the quantum interference aspects of low-energy structures in strong-field atomic ionization.

Experimental and theoretical investigations of H2O-Ar

We have used continuous-wave cavity ring-down spectroscopy to record the spectrum of H₂O-Ar in the 2OH excitation range of H₂O. 24 sub-bands have been observed. Their rotational structure (T_rot = 12 K) is analyzed and the lines are fitted separately for ortho and para species together with microwave and far infrared data from the literature, with a unitless standard deviation σ=0.98 and 1.31, respectively. Their vibrational analysis is supported by a theoretical input based on an intramolecular potential energy surface obtained through ab initio calculations and computation of the rotational energy of sub-states of the complex with the water monomer in excited vibrational states up to the first hexad. For the ground and (010) vibrational states, the theoretical results agree well with experimental energies and rotational constants in the literature. For the excited vibrational states of the first hexad, they guided the assignment of the observed sub-bands. The upper state vibrational predissociation lifetime is estimated to be 3 ns from observed spectral linewidths.

Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions

London dispersion forces are the weakest interactions between molecules. Because of this, their influence on chemical processes is often low, but can definitely not be ignored, and even becomes important in cases of molecules with large contact surfaces. Hierarchically assembled dinuclear titanium(IV) helicates represent a rare example in which the direct observation of London dispersion forces is possible in solution even in the presence of strong cohesive solvent effects. Hereby, the dispersion forces do not unlimitedly support the formation of the dimeric complexes. Although they have some favorable enthalpic contribution to the dimerization of the monomeric complex units, large flexible substituents become conformationally restricted by the interactions leading to an entropic disadvantage. The dimeric helicates are entropically destabilized.

Diverging effects of isotopic fractionation upon molecular diffusion of noble gases in water: mechanistic insights through ab initio molecular dynamics simulations

Atmospheric noble gases are routinely used as natural tracers to analyze gas transfer processes in aquatic systems. Their isotopic ratios can be employed to discriminate between different physical transport mechanisms by comparison to the unfractionated atmospheric isotope composition. In many applications of aquatic systems molecular diffusion was thought to cause a mass dependent fractionation of noble gases and their isotopes according to the square root ratio of their masses. However, recent experiments focusing on isotopic fractionation within a single element challenged this broadly accepted assumption. The determined fractionation factors of Ne, Ar, Kr and Xe isotopes revealed that only Ar follows the prediction of the so-called square root relation, whereas within the Ne, Kr and Xe elements no mass-dependence was found. The reason for this unexpected divergence of Ar is not yet understood. The aim of our computational exercise is to establish the molecular-resolved mechanisms behind molecular diffusion of noble gases in water. We make the hypothesis that weak intermolecular interactions are relevant for the dynamical properties of noble gases dissolved in water. Therefore, we used ab initio molecular dynamics to explicitly account for the electronic degrees of freedom. Depending on the size and polarizability of the hydrophobic particles such as noble gases, their motion in dense and polar liquids like water is subject to different diffusive regimes: the inter-cavity hopping mechanism of small particles (He, Ne) breaks down if a critical particle size achieved. For the case of large particles (Kr, Xe), the motion through the water solvent is governed by mass-independent viscous friction leading to hydrodynamical diffusion. Finally, Ar falls in between the two diffusive regimes, where particle dispersion is propagated at the molecular collision time scale of the surrounding water molecules.

Interaction of O2 with CH4, CF4, and CCl4 by Molecular Beam Scattering Experiments and Theoretical Calculations

Gas phase collisions of O2 by CH4, CF4, and CCl4 have been investigated with the molecular beam technique by measuring both the integral cross section value, Q, and its dependence on the collision velocity, v. The adopted experimental conditions have been appropriate to resolve the oscillating "glory" pattern, a quantum interference effect controlled by the features of the intermolecular interaction, for all the three case studies. The analysis of the Q(v) data, performed by adopting a suitable representation of the intermolecular potential function, provided the basic features of the anisotropic potential energy surfaces at intermediate and large separation distances and information on the relative role of the physically relevant types of contributions to the global interaction. The present work demonstrates that while O2-CH4 and O2-CF4 are basically bound through the balance between size (Pauli) repulsion and dispersion attraction, an appreaciable intermolecular bond stabilization by charge transfer is operative in O2-CCl4. Ab initio calculations of the strength of the interaction, coupled with detailed analysis of electronic charge displacement promoted by the formation of the dimer, fully rationalizes the experimental findings. This investigation indicates that the interactions of O2, when averaged over its relative orientations, are similar to that of a noble gas (Ng), specifically Ar. We also show that the binding energy in the basic configurations of the prototypical Ng-CF4,CCl4 systems [ Cappelletti , D. ; Chem. Eur. J. 2015 , 21 , 6234 - 6240 ] can be reconstructed by using the interactions in Ng-F and Ng-Cl systems, previously characterized by molecular beam scattering experiments of state-selected halogen atom beams. This information is fundamental to approach the modeling of the weak intermolecular halogen bond. On the basis of the electronic polarizability, this also confirms [ Aquilanti , V. ; Angew. Chem., Int. Ed. 2005 , 44 , 2356 - 2360 ] that O2 can be taken as a proper reference partner for simulating the behavior of some basic noncovalent components of the interactions involving water. Present results are of fundamental relevance to build up the force field controlling the hydrophobic behavior of prototypical apolar CX4 (X = H, F, Cl) molecules.

The modified quasi-quantum treatment of rotationally inelastic NO(X)-He scattering

A modified quasi-quantum treatment (MQQT) of molecular scattering has been developed to account for the softness of the repulsive part of the anisotropic atom-molecule PES. A contour of the PES is chosen such that the barrier height is just large enough to reflect the incoming kinetic energy, directed anti-parallel to the hard shell normal at the site of impact. The resulting rotationally inelastic quantum state resolved DCSs and ICSs of He + NO(X) at Ecol = 508 cm(-1) are compared to those obtained from regular QQT and from quantum mechanically exact calculations performed on the full highest quality ab initio Vsum PES. The MQQT parity changing DCSs for Δj ≤ 4 exhibit much better agreement with the QM DCSs than is obtained using regular QQT, particularly in the forward scattered direction. The improvements upon the remaining MQQT DCSs with respect to the regular QQT were minor, due to the near incompressible hard shell character of the n ≠ 1 or 3 anisotropic Legendre polynomial terms of the PES.

Water clusters in an argon matrix: infrared spectra from molecular dynamics simulations with a self-consistent charge density functional-based tight binding/force-field potential

The present theoretical study aims at investigating the effects of an argon matrix on the structures, energetics, dynamics, and infrared (IR) spectra of small water clusters (H2O)n (n = 1-6). The potential energy surface is obtained from a hybrid self-consistent charge density functional-based tight binding/force-field approach (SCC-DFTB/FF) in which the water clusters are treated at the SCC-DFTB level and the matrix is modeled at the FF level by a cluster consisting of ∼340 Ar atoms with a face centered cubic (fcc) structure, namely (H2O)n/Ar. With respect to a pure FF scheme, this allows a quantum description of the molecular system embedded in the matrix, along with all-atom geometry optimization and molecular dynamics (MD) simulations of the (H2O)n/Ar system. Finite-temperature IR spectra are derived from the MD simulations. The SCC-DFTB/FF scheme is first benchmarked on (H2O)Arn clusters against correlated wave function results and DFT calculations performed in the present work, and against FF data available in the literature. Regarding (H2O)n/Ar systems, the geometries of the water clusters are found to adapt to the fcc environment, possibly leading to intermolecular distortion and matrix perturbation. Several energetical quantities are estimated to characterize the water clusters in the matrix. In the particular case of the water hexamer, substitution and insertion energies for the prism, bag, and cage are found to be lower than that for the 6-member ring isomer. Finite-temperature MD simulations show that the water monomer has a quasifree rotation motion at 13 K, in agreement with experimental data. In the case of the water dimer, the only large-amplitude motion is a distortion-rotation intermolecular motion, whereas only vibration motions around the nuclei equilibrium positions are observed for clusters with larger sizes. Regarding the IR spectra, we find that the matrix environment leads to redshifts of the stretching modes and almost no shift of the bending modes. This is in agreement with experimental data. Furthermore, in the case of the water monomer and dimer, the magnitudes of the computed shifts are in fair agreement with the experimental values. The complex case of the water hexamer, which presents several low-energy isomers, is discussed.

H2O–CH4and H2S–CH4complexes: a direct comparison through molecular beam experiments and ab initio calculations

Electron density redistribution upon the formation of the water–methane complex arises from polarisation and charge transfer effects.

Dynamics of the chemical bond: inter- and intra-molecular hydrogen bond

In this discussion, we show that a static definition of a ‘bond’ is not viable by looking at a few examples for both inter- and intra-molecular hydrogen bonding. This follows from our earlier work (Goswami and Arunan,Phys. Chem. Chem. Phys.2009,11, 8974) which showed a practical way to differentiate ‘hydrogen bonding’ from ‘van der Waals interaction’. We report results fromab initioand atoms in molecules theoretical calculations for a series of Rg⋯HX complexes (Rg = He/Ne/Ar and X = F/Cl/Br) and ethane-1,2-diol. Results for the Rg⋯HX/DX complexes show that Rg⋯DX could have a ‘deuterium bond’ even when Rg⋯HX is not ‘hydrogen bonded’, according to the practical criterion given by Goswami and Arunan. Results for ethane-1,2-diol show that an ‘intra-molecular hydrogen bond’ can appear during a normal mode vibration which is dominated by the O⋯O stretching, though a ‘bond’ is not found in the equilibrium structure. This dynamical ‘bond’ formation may nevertheless be important in ensuring the continuity of electron density across a molecule. In the former case, a vibration ‘breaks’ an existing bond and in the later case, a vibration leads to ‘bond’ formation. In both cases, the molecule/complex stays bound irrespective of what happens to this ‘hydrogen bond’. Both these cases push the borders on the recent IUPAC recommendation on hydrogen bonding (Arunanet al. Pure. Appl. Chem.2011,831637) and justify the inclusive nature of the definition.

The gold-hydrogen bond, Au-H, and the hydrogen bond to gold, Au∙∙∙H-X

In the first part of this review, the characteristics of Au-H bonds in gold hydrides are reviewed including the data of recently prepared stable organometallic complexes with gold(I) and gold(III) centers. In the second part, the reports are summarized where authors have tried to provide evidence for hydrogen bonds to gold of the type Au∙∙∙H-X. Such interactions have been proposed for gold atoms in the Au(-I), Au(0), Au(I), and Au(III) oxidation states as hydrogen bonding acceptors and H-X units with X = O, N, C as donors, based on both experimental and quantum chemistry studies. To complement these findings, the literature was screened for examples with similar molecular geometries, for which such bonding has not yet been considered. In the discussion of the results, the recently issued IUPAC definitions of hydrogen bonding and the currently accepted description of agostic interactions have been used as guidelines to rank the Au∙∙∙H-X interactions in this broad range of weak chemical bonding. From the available data it appears that all the intra- and intermolecular Au∙∙∙H-X contacts are associated with very low binding energies and non-specific directionality. To date, the energetics have not been estimated, because there are no thermochemical and very limited IR/Raman and temperature-dependent NMR data that can be used as reliable references. Where conspicuous structural or spectroscopic effects have been observed, explanations other than hydrogen bonding Au∙∙∙H-X can also be advanced in most cases. Although numerous examples of short Au∙∙∙H-X contacts exist in the literature, it seems, at this stage, that these probably make only very minor contributions to the energy of a given system and have only a marginal influence on molecular conformations which so far have most often attracted researchers to this topic. Further, more dedicated investigations will be necessary before well founded conclusions can be drawn.

Defining the hydrogen bond: An account (IUPAC Technical Report)

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X–H···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

Quantum chemistry of C(3)H(6)O molecules: structure and stability, isomerization pathways, and chirality changing mechanisms

Electronic structure calculations were carried out to study the various isomers of formula C(3)H(6)O, as a part of our current quantum chemical and dynamical approaches to intra- and intermolecular kinetics for the C(n)H(2n)O (n = 1, 2, 3) molecules. The usefulness of the GRRM (global reaction route mapping) program developed by Ohno and Maeda in predicting the structure of all isomers and of the transition states connecting them is fully exploited. All the isomers are identified as local minima on the MP2/CC-PVDZ potential energy surface. Acetone is the most stable isomer. In increasing order of stability the others are propanal, 2-propenol, 1-propenol, allyl alcohol, methyl vinyl ether, cyclopropanol, propylene oxide, and oxetane. Various isomerization paths connecting them are identified. All the transition states are fully characterized using intrinsic reaction coordinate calculations. The isomerization reactions may proceed through a single step or involve an intermediate species which is either a carbene or a diradical. Special attention is devoted to propylene oxide, a favorite molecule in current photochemical and stereodynamical studies because of its chiral nature. It is a rigid molecule, and chirality switching is found to be supported by its isomers. Two different chirality switching mechanisms which are assisted by propanal and allyl alcohol are presented.