Intermolecular potentials, internal motions, and spectra of van der waals and hydrogen-bonded complexes

HCl trimer: HCl-stretch excited intramolecular and intermolecular vibrational states from 12D fully coupled quantum calculations employing contracted intra- and inter-molecular bases

We present fully coupled, full-dimensional quantum calculations of the inter- and intra-molecular vibrational states of HCl trimer, a paradigmatic hydrogen-bonded molecular trimer. They are performed utilizing the recently developed methodology for the rigorous 12D quantum treatment of the vibrations of the noncovalently bound trimers of flexible diatomic molecules [Felker and Bačić, J. Chem. Phys. 158, 234109 (2023)], which was previously applied to the HF trimer by us. In this work, the many-body 12D potential energy surface (PES) of (HCl)3 [Mancini and Bowman, J. Phys. Chem. A 118, 7367 (2014)] is employed. The calculations extend to the intramolecular HCl-stretch excited vibrational states of the trimer with one- and two-quanta, together with the low-energy intermolecular vibrational states in the two excited v = 1 intramolecular vibrational manifolds. They reveal significant coupling between the intra- and inter-molecular vibrational modes. The 12D calculations also show that the frequencies of the v = 1 HCl stretching states of the HCl trimer are significantly redshifted relative to those of the isolated HCl monomer. Detailed comparison is made between the results of the 12D calculations on the two-body PES, obtained by removing the three-body term from the original 2 + 3-body PES, and those computed on the 2 + 3-body PES. It demonstrates that the three-body interactions have a strong effect on the trimer binding energy as well as on its intra- and inter-molecular vibrational energy levels. Comparison with the available spectroscopic data shows that good agreement with the experiment is achieved only if the three-body interactions are included. Some low-energy vibrational states localized in a secondary minimum of the PES are characterized as well.

Effect of Temperature on the OH-Stretching Bands of the Methanol Dimer

We present a conceptually simple model for understanding the significant spectral changes that occur with the temperature in the infrared spectra of hydrogen-bound complexes. We have measured room-temperature spectra of the methanol dimer and two deuterated isotopologues in the OH(D)-stretching region. We correctly predict spectral changes observed in the gas phase for the bound OH stretch in the methanol dimer from jet-cooled to room temperature and corroborate this with experimental and theoretical results for deuterated isotopologues. The origin of the observed spectral features is explained based on a reduced-dimensional vibrational model, which includes the two high-frequency OH stretches, the two methyl torsions, and the six intermolecular low-frequency vibrations. Key to the success of the model is a new coordinate definition to describe the intrinsic large-amplitude curvilinear motion of low-frequency vibrations. Despite the deceivingly simple appearance of the room temperature bound OH-stretching fundamental band, it consists of ∼107 vibrational transitions.

Hydrogen-Bonded π Complexes between Phosphaethyne and Hydrogen Chloride

The highly labile complexes between phosphaethyne (HCP) and hydrogen chloride (HCl) with 1:1 and 1:2 stoichiometries have been generated in Ar and N₂ matrices at 10 K through laser photolysis of the molecular precursors 1-chlorophosphaethene (CH₂PCl) and dichloromethylphosphine (CH₃PCl₂), respectively. The IR spectrum of the 1:1 complex suggests the preference of a single "T-shaped" structure in which HCl acts as the hydrogen donor that interacts with the electron-rich C≡P triple bond. In contrast, three isomeric structures for the 1:2 complex bearing a core structure of the "T-shaped" 1:1 complex are present in the matrix. The spectroscopic identification of these rare HCP π-electron complexes is supported by D-isotope labeling and the quantum chemical calculations at the CCSD(T)-F12a/cc-pVTZ-F12 level of theory.

Noncovalently bound molecular complexes beyond diatom–diatom systems: full-dimensional, fully coupled quantum calculations of rovibrational states

The methodological advances made in recent years have significantly extended the range and dimensionality of noncovalently bound molecular complexes for which full-dimensional quantum calculations of their rovibrational states are feasible.

Ab initio investigation of the CO-N2 quantum scattering: The collisional perturbation of the pure rotational R(0) line in CO

We report fully quantum calculations of the collisional perturbation of a molecular line for a system that is relevant for Earth's atmosphere. We consider the N₂-perturbed pure rotational R(0) line in CO. The results agree well with the available experimental data. This work constitutes a significant step toward populating the spectroscopic databases with ab initio collisional line-shape parameters for atmosphere-relevant systems. The calculations were performed using three different recently reported potential energy surfaces (PESs). We conclude that all three PESs lead to practically the same values of the pressure broadening coefficients.

Ab initio potential energy surface and microwave spectrum of the NH3-N2 van der Waals complex

We present a five-dimensional intermolecular potential energy surface (PES) of the NH₃-N₂ complex, bound state calculations, and new microwave (MW) measurements that provide information on the structure of this complex and a critical test of the potential. Ab initio calculations were carried out using the explicitly correlated coupled cluster [CCSD(T)-F12a] approach with the augmented correlation-consistent aug-cc-pVTZ basis set. The global minimum of the PES corresponds to a configuration in which the angle between the NH₃ symmetry axis and the intermolecular axis is 58.7° with the N atom of the NH₃ unit closest to the N₂ unit, which is nearly parallel to the NH₃ symmetry axis. The intermolecular distance is 7.01 a₀, and the binding energy D_e is 250.6 cm^-1. The bound rovibrational levels of the four nuclear spin isomers of the complex, which are formed when ortho/para (o/p)-NH₃ combines with (o/p)-N₂, were calculated on this intermolecular potential surface. The computed dissociation energies D₀ are 144.91 cm^-1, 146.50 cm^-1, 152.29 cm^-1, and 154.64 cm^-1 for (o)-NH₃-(o)-N₂, (o)-NH₃-(p)-N₂, (p)-NH₃-(o)-N₂, and (p)-NH₃-(p)-N₂, respectively. Guided by these calculations, the pure rotational transitions of the NH₃-N₂ van der Waals complex were observed in the frequency range of 13-27 GHz using the chirped-pulse Fourier-transform MW technique. A complicated hyperfine structure due to three quadrupole ¹⁴N nuclei was partly resolved and examined for all four nuclear spin isomers of the complex. Newly obtained data definitively established the K values (the projection of the angular momentum J on the intermolecular axis) for the lowest states of the different NH₃-N₂ nuclear spin isomers.

Mode-specific vibrational predissociation dynamics of (HCl)2 via the free and bound HCl stretch overtones

Velocity-map ion imaging has been used to study the vibrational predissociation dynamics of the HCl dimer following infrared (IR) excitation in the HCl stretch overtone region near 1.77 Å. HCl monomer predissociation products were detected state-selectively using 2 + 1 resonance-enhanced multiphoton ionization spectroscopy. The IR action spectrum shows the free HCl stretch (2ν₁), the bound HCl stretch (2ν₂), and a combination band involving the intermolecular van der Waals stretching mode (2ν₂ + ν₄). Fragment speed distributions extracted from ion images obtained for a range of HCl(v = 0, 1; J) levels following vibrational excitation on the 2ν₁ and 2ν₂ bands yield the correlated product pair distributions. All product pairs comprise HCl(v = 1) + HCl(v = 0) and show a strong propensity to minimize the recoil kinetic energy. Highly non-statistical and mode-dependent HCl product rotational distributions are observed, in contrast to that observed following stretch fundamental excitation. Predissociation lifetimes are also mode-dependent: excitation of the free HCl leads to τ_VP = 13 ± 1 ns, while the bound stretch has a shorter lifetime τ_VP ≤ 6 ns. The dimer dissociation energy determined from energy conservation (D₀ = 397 ± 7 cm^-1) is slightly smaller than the previously reported values. The results are discussed in the context of previous observations for (HF)₂ and (HCl)₂ after excitation of HX stretch fundamentals and models for vibrational predissociation.

Benchmark Experimental Gas-Phase Intermolecular Dissociation Energies by the SEP-R2PI Method

The gas-phase ground-state dissociation energy D0( S0) of an isolated and cold bimolecular complex is a fundamental measure of the intermolecular interaction strength between its constituents. Accurate D0 values are important for the understanding of intermolecular bonding, for benchmarking high-level theoretical calculations, and for the parameterization of dispersion-corrected density functionals or force-field models that are used in fields ranging from crystallography to biochemistry. We review experimental measurements of the gas-phase D0( S0) and D0( S1) values of 55 different M⋅S complexes, where M is a (hetero)aromatic molecule and S is a closed-shell solvent atom or molecule. The experiments employ the triply resonant SEP-R2PI laser method, which involves M-centered ( S0 → S1) electronic excitation, followed by S1 → S0 stimulated emission spanning a range of S0 state vibrational levels. At sufficiently high vibrational energy, vibrational predissociation of the M⋅S complex occurs. A total of 49 dissociation energies were bracketed to within ≤1.0 kJ/mol, providing a large experimental database of accurate noncovalent interactions.

Intermolecular dissociation energies of 1-naphthol complexes with large dispersion-energy donors: Decalins and adamantane

The ground-state intermolecular dissociation energies D₀(S₀) of supersonic-jet cooled intermolecular complexes of 1-naphthol (1NpOH) with the bi- and tricycloalkanes trans-decalin, cis-decalin, and adamantane were measured using the stimulated-emission-pumping/resonant two-photon ionization (SEP-R2PI) method. Using UV/UV holeburning, we identified two isomers (A and B) of the adamantane and trans-decalin complexes and four isomers (A-D) of the cis-decalin complex. For 1NpOH·adamantane A and B, the D₀(S₀) values are 21.6 ± 0.15 kJ/mol and 21.2 ± 0.32 kJ/mol, those of 1NpOH·trans-decalin A and B are 28.7 ± 0.3 kJ/mol and 28.1 ± 0.9 kJ/mol, and those of 1NpOH·cis-decalin A and B are 28.9 ± 0.15 kJ/mol and 28.7 ± 0.3 kJ/mol. Upon S₀ → S₁ electronic excitation of the 1NpOH moiety, the dissociation energies of adamantane, trans-decalin, and the cis-decalin isomer C change by <1% and those of cis-decalin isomers A, B, and D increase only slightly (1%-3%). This implies that the hydrocarbons are dispersively adsorbed to a naphthalene "face." Calculations using the dispersion-corrected density functional theory methods B97-D3 and B3LYP-D3 indeed predict that the stable structures have face geometries. The B97-D3 calculated D₀(S₀) values are within 1 kJ/mol of the experiment, while B3LYP-D3 predicts D₀ values that are 1.4-3.3 kJ/mol larger. Although adamantane has been recommended as a "dispersion-energy donor," the binding energies of the trans- and cis-decalin adducts to 1NpOH are 30% larger than that of adamantane. In fact, the D₀ value of 1NpOH·adamantane is close to that of 1NpOH·cyclohexane, reflecting the nearly identical contact layer between the two molecules.

Full quantum calculation of the rovibrational states and intensities for a symmetric top-linear molecule dimer: Hamiltonian, basis set, and matrix elements

The rovibrational energy levels and intensities of the CH₃F-H₂ dimer have been obtained using our recent global intermolecular potential energy surface [X.-L. Zhang et al., J. Chem. Phys. 148, 124302 (2018)]. The Hamiltonian, basis set, and matrix elements are derived and given for a symmetric top-linear molecule complex. This approach to the generation of energy levels and wavefunctions can readily be utilized for studying the rovibrational spectra of other van der Waals complexes composed of a symmetric top molecule and a linear molecule, and may readily be extended to other complexes of nonlinear molecules and linear molecules. To confirm our method, the rovibrational levels of the H₂O-H₂ dimer have been computed and shown to be in good agreement with experiment and with previous theoretical results. The rovibrational Schrödinger equation has been solved using a Lanczos algorithm together with an uncoupled product basis set. As expected, dimers containing ortho-H₂ are more strongly bound than dimers containing para-H₂. Energies and wavefunctions of the discrete rovibrational levels of CH₃F-paraH₂ complexes obtained from the direct vibrationally averaged 5-dimensional potentials are in good agreement with the results of the reduced 3-dimensional adiabatic-hindered-rotor (AHR) approximation. Accurate calculations of the transition line strengths for the orthoCH₃F-paraH₂ complex are also carried out, and are consistent with results obtained using the AHR approximation. The microwave spectrum associated with the orthoCH₃F-orthoH₂ dimer has been predicted for the first time.

Intermolecular interactions of sulfur trioxide and sulfuric acid dimers studied by the DFT method

Density functional theory with 6-311++G** basis sets has been applied to the investigation of intermolecular interactions of sulfur trioxide and sulfuric acid dimers. Counterpoise procedure was used for the basis set superposition error (BSSE) correction. The corrected binding energy for (HSO4) 2 is −71.37 kJ/mol at the B3LYP/6-311++G** level, and −37.79 and −39.04 kJ/mol for two H2SO4/SO3 binary complexes (denoted as dimers B and C) respectively. Dimers B and C can be spontaneously produced when gaseous sulfur trioxide mixes with sulfuric acid, which is in agreement with the experimental fact that sulfur trioxide is prone to dissolve in sulfuric acid. The O…S interacting is stronger than the O…H interacting in the heterodimer of H2SO4/SO3. Electron density at bond critical points and natural bond orbital analysis were performed to probe the origin of the intermolecular interactions.

Double resonance rotational spectroscopy of He-HCO. Phys Chem Chem Phys 2019;21:3440-3445. [PMID: 30191208 DOI: 10.1039/c8cp04532a]

The ground state of He-HCO+ is investigated using a recently developed double resonance technique, consisting of a rotational transition followed by a vibrational transition into a dissociative state. In order to derive precise predictions for the rotational states, the high resolution infrared predissociation spectroscopy of the v1 C-H stretching mode is revisited. Eleven pure rotational transitions are measured via the double resonance method. A least squares fit of these transitions to a standard linear rotor Hamiltonian reveals that the semirigid rotor model cannot fully describe the loosely bound He-HCO+ complex. The novel double resonance technique is compared with other action spectroscopic schemes, and some potential future applications are presented.

Behaviour of the XH-*-π and YX-*-π interactions (X, Y = F, Cl, Br and I) in the coronene π-system, as elucidated by QTAIM dual functional analysis with QC calculations

The dynamic and static nature of XH-*-π and YX-*-π in the coronene π-system (π(C24H12)) is elucidated by QTAIM dual functional analysis, where * emphasizes the presence of bond critical points (BCPs) in the interactions. The nature of the interactions is elucidated by analysing the plots of the total electron energy densities H b(r c) versus H b(r c) - V b(r c)/2 [=(ħ 2/8m)∇2 ρ b(r c)] for the interactions at BCPs, where V b(r c) are the potential energy densities at the BCPs. The data for the perturbed structures around the fully optimized structures are employed for the plots in addition to those of the fully optimized structures. The plots are analysed using the polar coordinate of (R, θ) for the data of the fully optimized structures, while those containing the perturbed structures are analysed using (θ p, κ p), where θ p corresponds to the tangent line of each plot and κ p is the curvature. Whereas (R, θ) show the static nature, (θ p, κ p) represent the dynamic nature of the interactions. All interactions in X-H-*-π(C24H12) (X = F, Cl, Br and I) and Y-X-*-π(C24H12) (Y-X = F-F, Cl-Cl, Br-Br, I-I, F-Cl, F-Br and F-I) are classified by pure CS (closed shell) interactions and are characterized as having the vdW nature, except for X-H = F-H and Y-X = F-Cl, F-Br and F-I, which show the typical-HB nature without covalency. The structural features of the complexes are also discussed.

Theoretical studies of atmospheric molecular complexes interacting with NIR to UV light

Theoretical structural and spectroscopic data for weakly bonded atmospheric complexes of formaldehyde interacting with Earth’s electromagnetic spectrum.

Influences of the propyl group on the van der Waals structures of 4-propylaniline complexes with one and two argon atoms studied by electronic and cationic spectroscopy

4-propylaniline complexes with one and two argon atoms formed in the molecular beam were studied in the first excited electronic state, S1, using resonance enhanced two-photon ionization spectroscopy and in the cation ground state, D0, using mass analyzed threshold ionization spectroscopy. The combination of electronic and cationic spectra of the clusters allows two conformations to be identified in both aniline-Ar1 and aniline-Ar2, which are assigned to either the gauche configuration or anti-configuration of 4-propylaniline. The gauche isomer exhibits complex bands shifted 29 cm(-1) and 89 cm(-1) from the S1 origin bands and 83 cm(-1) and 148 cm(-1) from the ionization potential assigned to the Ar1 and Ar2 complexes, respectively. For the anti-rotamer, the corresponding shifts actually become nearly additive, 53 cm(-1) and 109 cm(-1) for the S1 origin bands, and 61 cm(-1) and 125 cm(-1) for the ionization potentials. Ab initio calculations provide insights into the influences of the propyl and amino groups on the positions of the argon atoms within the clusters. In addition, the binding energy of one argon with the gauche isomer of 4-propylaniline has been measured to be 550 ± 5 cm(-1) in the D0 state, 496 ± 5 cm(-1) in the S1 state, and 467 ± 5 cm(-1) in the neutral ground state, S0.

COOPERATIVITY BETWEEN HYDROGEN BOND AND HALOGEN BOND IN HBr⋯BrX⋯HBr (X = F, Cl, Br)

An ab initio study has been carried out to investigate the structures, binding energies and atomic charge of the binary complexes HBr⋯BrX and ternary complexes HBr⋯BrX⋯HBr (X = F, Cl, Br) at the MP2/aug-cc-pvdz theoretical level. Four types of interactions are observed among these systems: hydrogen bond, halogen bond, unusual hydrogen bond and unusual halogen bond. Two types of stable structures existed in the trimers: acyclic and monocyclic. The stabilities of the complexes may be decided by three factors: the strength of the halogen bond, the nature of halogen or hydrogen bond of intermolecular interaction and the type of the trimer's structure. The absolute value of the cooperativity contribution in these triads is in range of 0.77 ∼ 21.59%, which shows that the cooperative energy is of importance in them. The nature of the intermolecular interactions has been also investigated through the electrostatic potential and the theory of atoms in molecules (AIM). The values of ∇2ρ at BCPs are positive in all complexes, indicating that the interactions are closed-shell. The ratio |Vc|∕Gc of all complexes ranges from 0.782 to 0.935 (satisfy the condition |Vc|∕Gc < 1) along with Hc > 0, revealing that these complexes are predominantly electrostatic in character.