Accurate ab initio potential energy surfaces of Ar–HF, Ar–H2O, and Ar–NH3

A Rigid Bender Study of the Bending Relaxation of H2O and D2O by Collisions with Ar

The bending relaxation of H2O and D2O by collisions with Ar is studied at the Close Coupling level. Two new 4D PES are developed for these two systems. They are tested by performing rigid rotor calculations as well as by computing the D2O-Ar bound states. The results are compared with available theoretical and experimental data. Propensity rules for the dynamics are discussed and compared to those of H2O colliding with Ne or He. The bending relaxation cross sections and rates are then calculated for these two systems. The results are analysed and compared with available experimental data.

Insight into the Binding of Argon to Cyclic Water Clusters from Symmetry-Adapted Perturbation Theory

This work systematically examines the interactions between a single argon atom and the edges and faces of cyclic H2O clusters containing three-five water molecules (Ar(H2O)n=3-5). Full geometry optimizations and subsequent harmonic vibrational frequency computations were performed using MP2 with a triple-ζ correlation consistent basis set augmented with diffuse functions on the heavy atoms (cc-pVTZ for H and aug-cc-pVTZ for O and Ar; denoted as haTZ). Optimized structures and harmonic vibrational frequencies were also obtained with the two-body-many-body (2b:Mb) and three-body-many-body (3b:Mb) techniques; here, high-level CCSD(T) computations capture up through the two-body or three-body contributions from the many-body expansion, respectively, while less demanding MP2 computations recover all higher-order contributions. Five unique stationary points have been identified in which Ar binds to the cyclic water trimer, along with four for (H2O)4 and three for (H2O)5. To the best of our knowledge, eleven of these twelve structures have been characterized here for the first time. Ar consistently binds more strongly to the faces than the edges of the cyclic (H2O)n clusters, by as much as a factor of two. The 3b:Mb electronic energies computed with the haTZ basis set indicate that Ar binds to the faces of the water clusters by at least 3 kJ mol-1 and by nearly 6 kJ mol-1 for one Ar(H2O)5 complex. An analysis of the interaction energies for the different binding motifs based on symmetry-adapted perturbation theory (SAPT) indicates that dispersion interactions are primarily responsible for the observed trends. The binding of a single Ar atom to a face of these cyclic water clusters can induce perturbations to the harmonic vibrational frequencies on the order of 5 cm-1 for some hydrogen-bonded OH stretching frequencies.

Vibrationally excited intermolecular potential energy surfaces and the predicted near infrared overtone (vOH = 2 ← 0) spectra of a H2O-Ne complex

The ab initio intra- and inter-molecular potential energy surfaces (PESs) for the H₂O-Ne system that explicitly incorporate the intramolecular overtone state (v_OH = 2) of H₂O are presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster theory [CCSD(T)-F12] level with an augmented correlation-consistent triple zeta basis set and an additional bond function. The vibrationally averaged three-dimensional intermolecular potentials for |00⁺〉, |02⁺〉, |02^-〉 and |11⁺〉 are obtained analytically by fitting to the multi-dimensional Morse/Long-range potential function form. These fits to 46 980 points have a root-mean-square (RMS) discrepancy of 0.12 cm^-1 for interaction energies less than 1000.0 cm^-1. With the vibrationally averaged PESs for the H₂O-Ne, we employed the combined radial discrete variable representation/angular finite basis representation method and Lanczos algorithm to calculate rovibrational energy levels (J = 0-10, n_s ≤ 2). The predicted infrared transitions and intensities of the para- and ortho-H₂O-Ne complex are in good agreement with the available experimental data for |02^-〉 ← |00⁺〉, |02⁺〉 ← |00⁺〉 transitions. In particular, the RMS discrepancy for |02^-〉∑^e(0₀₀,0) ← |00⁺〉∑^e(1₀₁,0), including P and R branch patterns, is only 0.045 cm^-1, which is comparable with the experimental values. These results will provide reliable theoretical guidance for the future infrared overtone spectroscopy of clusters.

Concerning the Role of σ-Hole in Non-Covalent Interactions: Insights from the Study of the Complexes of ArBeO with Simple Ligands

The structure, stability, and bonding character of some exemplary LAr and L-ArBeO (L = He, Ne, Ar, N₂, CO, F₂, Cl₂, ClF, HF, HCl, NH₃) were investigated by MP2 and coupled-cluster calculations, and by symmetry-adapted perturbation theory. The nature of the stabilizing interactions was also assayed by the method recently proposed by the authors to classify the chemical bonds in noble-gas compounds. The comparative analysis of the LAr and L-ArBeO unraveled geometric and bonding effects peculiarly related to the σ-hole at the Ar atom of ArBeO, including the major stabilizing/destabilizing role of the electrostatic interactionensuing from the negative/positive molecular electrostatic potential of L at the contact zone with ArBeO. The role of the inductive and dispersive components was also assayed, making it possible to discern the factors governing the transition from the (mainly) dispersive domain of the LAr, to the σ-hole domain of the L-ArBeO. Our conclusions could be valid for various types of non-covalent interactions, especially those involving σ-holes of respectable strength such as those occurring in ArBeO.

Explicitly correlated ab initio potential energy surface and predicted rovibrational spectra for H2O-N2 and D2O-N2 complexes

An ab initio intermolecular potential energy surface (PES) for the van der Waals complex of H₂O-N₂ that explicitly incorporates the intramolecular Q₂ bending normal mode of the H₂O monomer is presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster theory [CCSD(T)-F12] with an augmented correlation-consistent triple zeta basis set and an additional bond function. Analytic five-dimensional intermolecular PESs for ν₂(H₂O) = 0 and 1 are obtained by fitting to the multi-dimensional Morse/long-range potential function form. These fits to 40 890 points have the root-mean-square (rms) discrepancy of 0.88 cm^-1 for interaction energies less than 2000.0 cm^-1. The resulting vibrationally averaged PESs provide good representations of the experimental microwave and infrared data: for microwave transitions of H₂O-N₂, the rms discrepancy is only 0.0003 cm^-1, and for infrared transitions of the A₁ symmetry of the H₂O(ν₂ = 1 ← 0)-N₂, the rms discrepancy is 0.001 cm^-1. The calculated infrared band origin shifts associated with the ν₂ bending vibration of water are 2.210 cm^-1 and 1.323 cm^-1 for H₂O-N₂ and D₂O-N₂, respectively, in good agreement with the experimental values of 2.254 cm^-1 and 1.266 cm^-1. The benchmark tests and comparisons of the predicted spectral properties are carried out between CCSD(T)-F12a and CCSD(T)-F12b approaches.

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.

Dynamics and spectroscopy of van der Waals complexes composed of ammonia and noble gases

In this work, we calculate the rovibrational energies and spectroscopic constants for the systems formed by ammonia (NH₃) and noble gases (Ng=He, Ne, Ar, Kr and Xe). For the spectroscopic constant calculations, we used two different methods: Dunham and another one that use rovibrational energies (here calculated by discrete variable method). In both cases, we used the improved Lennard-Jones potential energy curves (PECs). These PECs, which describe very well van der Waals systems, were built using the dissociation and equilibrium distance obtained from experiments of crossed molecular beams. The spectroscopic constant results, obtained by both methods were in excellent agreement with each other for all NH₃-Ng studied systems. Also in relation to NH₃-He system, we realize that although this system has a relatively small dissociation energy, it has one vibrational level. Finally, the spectroscopic constants and fundamental rovibrational energy results were used to verify the stability of each system through the lifetime decomposition.

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.

Ab initio study of the CO–N2 complex: a new highly accurate intermolecular potential energy surface and rovibrational spectrum

We present a highly accurate ab initio intermolecular potential-energy surface and rovibrational spectrum for the CO–N₂ complex.

Theoretical Study of the Pyridine-Helium van der Waals Complexes

In this study we evaluate a high-level ab initio ground-state intermolecular potential-energy surface for the pyridine-He van der Waals complex, using the CCSD(T) method and Dunning's augmented correlation consistent polarized valence double-ζ basis set extended with a set of 3s3p2d1f1g midbond functions. The potential is characterized by two symmetric global minima of -93.2 cm(-1) that correspond to geometries where the distance between the helium atom and the pyridine center of mass is 3.105 Å and the angle with respect to the pyridine c rotational axis is 3.9°. Six local minima can be observed for geometries with the helium atom in the plane cotaining the pyridine molecule. To further analyze the nature of the intermolecular interactions in the complex, we use symmetry-adapted perturbation theory (SAPT). Additional consideration of the pyridine-He2 complex provides a better insight into many-body nonadditive contributions to intermolecular interactions in systems with more helium atoms.

Potential energy surface and bound states of the NH3-Ar and ND3-Ar complexes

A new, four-dimensional potential energy surface for the interaction of NH3 and ND3 with Ar is computed using the coupled-cluster method with single, double, and perturbative triple excitations and large basis sets. The umbrella motion of the ammonia molecule is explicitly taken into account. The bound states of both NH3-Ar and ND3-Ar are calculated on this potential for total angular momentum values from J = 0 to 10, with the inclusion of Coriolis interactions. The energies and splittings of the rovibrational levels are in excellent agreement with the extensive high-resolution spectroscopic data accumulated over the years in the infrared and microwave regions for both complexes, which demonstrates the quality of the potential energy surface.

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.

Rovibrational energies and spectroscopic constants for H2O-Ng complexes

In this work, rovibrational energies and spectroscopic constants for the water -Ng complexes (Ng = He, Ne, Ar, Kr and Xe) were calculated through two different approaches (by solving the Nuclear Schrödinger equation and by applying the Dunham's method) and using two different potential energy curves (PEC). These PEC were determined using potential parameters obtained through molecular beam scattering experiments and accurate theoretical calculation, respectively. It was found that the theoretical rovibrational energies are in a good agreement (only for the lowest numbers of vibrational states) with those obtained through experimental PEC. Another important conclusions was regarding the calculated first two rovibrational energies for the H 2 O-Ar system, that are in a good agreement with the experimental data.

A new ab initio intermolecular potential energy surface and predicted rotational spectra of the Ne-H2S complex

We report a new three-dimensional ab initio intermolecular potential energy surface for the Ne-H(2)S complex with H(2)S monomer fixed at its experimental average structure. Using the supermolecular approach, the intermolecular potential energies were evaluated at CCSD(T) (coupled cluster with single and double and perturbative triple excitations) level with large basis sets including bond functions. The full counterpoise procedure was employed to correct the basis set superposition error. The planar T-shaped global minimum is located at the intermolecular distance of 3.51 Å with a well depth of 71.57 cm(-1). An additional planar local minimum was found to be separated from the global minimum with an energy barrier of 23.11 cm(-1). In addition, two first-order and one second-order saddle points were also located. The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to evaluate the rovibrational energy levels for eight isotopic species of the Ne-H(2)S complexes. The rotational transition frequencies for the eight isotopomers were also determined for the ground and first vibrational excited states, which are all in very good agreement with the available experimental values.

Theoretical studies of potential energy surface and rotational spectra of Xe-H(2)O van der Waals complex

In this work we report an ab initio intermolecular potential energy surface and theoretical spectroscopic studies for Xe-H(2)O complex. The ab initio energies are calculated with CCSD(T) method and large basis sets (aug-cc-pVQZ for H and O and aug-cc-pVQZ-PP for Xe) augmented by a {3s3p2d2f1g} set of bond functions. This potential energy surface has a global minimum corresponding to a planar and nearly linear hydrogen bonded configuration with a well depth of 192.5 cm(-1) at intermolecular distance of 4.0 A, which is consistent with the previous determined potential by Wen and Jager [J. Phys. Chem. A 110, 7560 (2006)]. The bound state calculations have been performed for the complex by approximating the water molecule as a rigid rotor. The theoretical rotational transition frequencies, isotopic shifts, nuclear quadrupole coupling constants, and structure parameters are in good agreement with the experimental observed values. The wavefunctions are analyzed to understand the dynamics of the ground and the first excited states.

Study of the Xe-NH3 van der Waals complex: high-resolution microwave spectra and ab initio calculations

An ab initio potential energy surface of the Xe-NH(3) van der Waals complex was constructed at the coupled cluster level of theory with single, double, and pertubatively included triple excitations. The small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta basis set was used for the Xe atom and Dunning's augmented correlation-consistent polarized valence triple-zeta basis set for the other atoms. The basis sets were supplemented with midbond functions. Rotational spectra of the Xe-NH(3) van der Waals complex were recorded using a pulsed-nozzle Fourier transform microwave spectrometer. Rotational transitions within two internal rotor states, namely, the Sigma0(0) and Pi1(1) (lower) states, were measured and assigned to the Xe-(14)NH(3) and Xe-(15)NH(3) isotopologues. For the deuterated isotopologues, only the Sigma0(0) states were observed. Two inversion components were observed for each state except for the "s" component of the Sigma0(0) state of the Xe-(14)NH(3) and Xe-(15)NH(3) isotopologues, which has a spin statistical weight of zero. Nuclear quadrupole hyperfine structures arising from the (14)N (nuclear spin angular momentum quantum number I=1) and (131)Xe (I=32) nuclei were detected and analyzed. The observed spectra suggest that the Pi1(1) (lower) state has lower energy than the unobserved Sigma1(1) state, in contrast to the case of Ar-NH(3).

Unusual halogen-bonded complex FBrδ+⋯Brδ+F and hydrogen-bonded complex FBrδ+⋯Hδ+F formed by interactions between two positively charged atoms of different polar molecules

Using ab initio calculations, the authors' predicted for the first time that the halogen-bonded complex FBrdelta+...delta+BrF and hydrogen-bonded complex FBrdelta+...delta+HF formed by the interactions between two positively charged atoms of different polar molecules can be stable in gas phase. It shows that halogen bond or hydrogen bond not only exists between oppositely charged atoms but also between like-charged atoms. That the attraction arising from the special halogen bond or hydrogen bond can exceed the electrostatic repulsion between two contact positively charged atoms stabilizes the complex. Of course, from the point of view of physics they can consider the interactions in FBrdelta+...delta+BrF and FBrdelta+...delta+HF as mainly the sum of the long range molecular interactions, namely, electrostatic, induction, and dispersion with some short-range repulsion. They found that the intermolecular electron correlation contribution representing dispersion interaction plays a crucial role in the stabilities of seemingly repulsive complexes FBrdelta+...delta+BrF and FBrdelta+...delta+HF.

Do single-electron lithium bonds exist? Prediction and characterization of the H3C⋯Li–Y (Y=H, F, OH, CN, NC, and CCH) complexes

A new kind of single-electron lithium bonding complexes H(3)C...LiY (Y=H, F, OH, CN, NC, and CCH) was predicted and characterized in the present paper. Their geometries (C(3v)) with all real harmonic vibrational frequencies were obtained at the MP2/aug-cc-pVTZ level. For each H(3)C...LiY complex, single-electron Li bond is formed between the unpaired electron of CH(3) radical and positively charged Li atom of LiY molecule. Due to the formation of the single-electron Li bond, the C-H bonds of the CH(3) radical bend opposite to the LiY molecule and the Li-Y bond elongates. Abnormally, the three H(3)C...LiY (Y=CN, NC, and CCH) complexes exhibit blueshifted Li-Y stretching frequencies along with the elongated Li-Y bonds. Natural bond orbital analyses suggest ca. 0.02 electron transfer from the methyl radical (CH(3)) to the LiY moiety. In the single occupied molecular orbitals of the H(3)C...LiY complexes, it is also seen that the electron could of the CH(3) radical approaches the Li atom. The single-electron Li bond energies are 5.20-6.94 kcal/mol for the H(3)C...LiY complexes at the CCSD(T)aug-cc-pVDZ+BF (bond functions) level with counterpoise procedure. By comparisons with some related systems, it is concluded that the single-electron Li bonds are stronger than single-electron H bonds, and weaker than conventional Li bonds and pi-Li bonds.

Rotational Spectroscopic and ab Initio Studies of the Xe−H2O van der Waals Dimer

An ab initio potential energy surface of the Xe-H(2)O van der Waals dimer was constructed at the coupled cluster level of theory with single, double, and pertubatively included triple excitations. For the Xe atom, the small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta (aug-cc-pVQZ-PP) basis set was used. Dunning's augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis set was chosen for O and H atoms. Midbond functions were used to supplement the atom-centered basis sets. Rotational spectra of the Xe-H(2)O van der Waals dimer were recorded with a pulsed-nozzle Fourier transform microwave spectrometer. Rotational transitions within two internal rotor states, namely, the 0(00) and 1(01) states, were measured and assigned. Nuclear quadrupole hyperfine structures due to the (131)Xe (I = (3)/(2)), D (I = 1) and (17)O (I = (5)/(2)) nuclei were also observed and analyzed. Information about the molecular structure and the H(2)O angular motions was extracted from the spectroscopic results with the assistance of the ab initio potential.

Asymmetrical linear structures including three-electron hemibonds or other interactions in the (ABA)-type triatomic cations: Ne3+, (He–Ne–He)+, (Ar–Ne–Ar)+, (Ar–O–Ar)+, (He–O–He)+, and (Ar–He–Ar)+

By the counterpoise geometry optimization at the level of CCSD(T)aug-cc-pVDZ, the asymmetrical linear structures with all the real frequencies were obtained for the triatomic cations of (ABA)+ type: Ne3+, (He-Ne-He)+, (Ar-Ne-Ar)+, (Ar-He-Ar)+, (He-O-He)+, and (Ar-O-Ar)+. The validity of this optimization method is confirmed by comparing with the method of the potential-energy surface for the calculations of Ne3+ and (He-Ne-He)+. Using the molecular-orbital theory, it is found that the interaction within the triatomic cations is dominated by the contribution from the first two atoms while the contribution from the third atom is small. This result is justified as a direct consequence of forming an asymmetrical linear structure. Specifically, four types of interaction within the triatomic cations are identified: three-electron sigma-type hemibond, three-electron pi-type hemibond, two-electron sigma bond, and the attraction between cation and atoms. For Ne3+, (He-Ne-He)+, and (He-O-He)+ clusters, it is shown that the electron correlation effect supports the asymmetry.

Hybrid diatomics-in-molecules-based quantum mechanical/molecular mechanical approach applied to the modeling of structures and spectra of mixed molecular clusters Arn(HCl)m and Arn(HF)m

A new hybrid QM/DIM approach aimed at describing equilibrium structures and spectroscopic properties of medium size mixed molecular clusters is developed. This methodology is applied to vibrational spectra of hydrogen chloride and hydrogen fluoride clusters with up to four monomer molecules embedded in argon shells Arn(H(Cl/F))m (n = 1-62, m = 1-4). The hydrogen halide complexes (QM part) are treated at the MP2/aug-cc-pVTZ level, while the interaction between HX molecules and Ar atoms (MM part) is described in terms of the semiempirical DIM methodology, based on the proper mixing between neutral and ionic states of the system [Grigorenko et al., J. Chem. Phys. 104, 5510 (1996)]. A detailed analysis of the resulting topology of the QM/DIM potential energy (hyper-)surface in the triatomic subsystem Ar-HX reveals more pronounced nonadditive atomic induction and dispersion contributions to the total interaction energy in the case of the Ar-HCl system. An extension of the original analytical DIM-based potential in the frame of the present model as well as the current limitations of the method are discussed. A modified algorithm for the gradient geometry optimization, along with partly analytical force constant matrix evaluation, is developed to treat large cages of argon atoms around molecular clusters. Calculated frequency redshifts of HX stretching vibrations in the mixed clusters relative to the isolated hydrogen-bonded complexes are in good agreement with experimental findings.

Design and application of a multicoefficient correlation method for dispersion interactions

A new multicoefficient correlation method (MCCM) is presented for the determination of accurate van der Waals interactions. The method utilizes a novel parametrization strategy that simultaneously fits to very high-level binding, Hartree-Fock and correlation energies of homo- and heteronuclear rare gas dimers of He, Ne, and Ar. The decomposition of the energy into Hartree-Fock and correlation components leads to a more transferable model. The method is applied to the krypton dimer system, rare gas-water interactions, and three-body interactions of rare gas trimers He3, Ne3, and Ar3. For the latter, a very high-level method that corrects the rare-gas two-body interactions to the total binding energy is introduced. A comparison with high-level CCSD(T) calculations using large basis sets demonstrates the MCCM method is transferable to a variety of systems not considered in the parametrization. The method allows dispersion interactions of larger systems to be studied reliably at a fraction of the computational cost, and offers a new tool for applications to rare-gas clusters, and the development of dispersion parameters for molecular simulation force fields and new semiempirical quantum models.