Experimental and theoretical studies of the CN-Ar van der Waals complex

The CN-Ar van der Waals complex has been observed using the B (2)Sigma(+)-X (2)Sigma(+) and A (2)Pi-X (2)Sigma(+) electronic transitions. The spectra yield a dissociation energy of D(0")=102+/-2 cm(-1) and a zero-point rotational constant of B(0")=0.067+/-0.005 cm(-1) for CN(X)-Ar. The dissociation energy for CN(A)-Ar was found to be D(0')=125+/-2 cm(-1). Transitions to vibrationally excited levels of CN(B)-Ar dominated the B-X spectrum, indicative of substantial differences in the intermolecular potential energy surfaces (PESs) for the X and B states. Ab initio PESs were calculated for the X and B states. These were used to predict rovibrational energy levels and van der Waals bond energies (D(0")=115 and D(0')=183 cm(-1)). The results for the X state were in reasonably good agreement with the experimental data. Spectral simulations based on the ab initio potentials yielded qualitative insights concerning the B-X spectrum, but the level of agreement was not sufficient to permit vibronic assignment. Electronic predissociation was observed for both CN(A)-Ar and CN(B)-Ar. The process leading to the production of CN(A,nu=8,9) fragments from the predissociation of CN(B,nu=0)-Ar was characterized using time-resolved fluorescence and optical-optical double resonance measurements.     

Microwave and ab initio studies of the Xe-CH4 van der Waals complex

An ab initio potential-energy surface of the Xe-CH4 van der Waals complex was constructed at the coupled cluster level of theory with single, double, and perturbatively included triple excitations. The recently developed small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta basis set was used for the xenon atom and Dunning's augmented correlation-consistent polarized valence triple-zeta basis set for the other atoms. The basis sets were supplemented with bond functions. Dipole moments were also calculated at various configurations. Rotational spectra of the Xe-CH4 van der Waals complex were recorded using a pulsed-nozzle Fourier transform microwave spectrometer. The isotopomers studied include those of CH4,13CH4,CD4,CH3D, and CHD3 with the five most abundant Xe isotopes. Transitions within three internal rotor states, namely, the j=0,K=0; j=1,K=0; and j=2,K=1 states, were observed and assigned. Nuclear quadrupole hyperfine structures due to the presence of 131Xe(I=3/2) were detected and analyzed. It was found that the j=1,K=0 state is perturbed by a Coriolis interaction with a nearby j=1,K=1 state. For isotopomers containing CH3D and CHD3, the j=2 states are no longer metastable and could not be observed. The spectroscopic results were used to derive structural and dynamical information of the Xe-CH4 complex.     

Spectroscopy and potential energy surface of the H2-CO2 van der Waals complex: experimental and theoretical studies

A 4-D ab initio potential energy surface is calculated for the intermolecular interaction of hydrogen and carbon dioxide, using the CCSD(T) method with a large basis set. The surface has a global minimum with a well depth of 212 cm(-1) and an intermolecular distance of 2.98 A for a planar configuration with both the O-C-O and H-H axes perpendicular to the intermolecular axis. Bound state calculations are performed for the H(2)-CO(2) van der Waals complex with H(2) in both the para and ortho spin states, and the binding energy of paraH(2)-CO(2)(50.4 cm(-1)) is found to be significantly less than that of orthoH(2)-CO(2)(71.7 cm(-1)). The surface supports 7 bound intermolecular vibrational states for paraH(2)-CO(2) and 19 for orthoH(2)-CO(2), and the lower rotational levels with J< or = 4 follow an asymmetric rotor pattern. The calculated infrared spectrum of paraH(2)-CO(2) agrees well with experiment. For orthoH(2)-CO(2), the ground state rotational levels allowed by symmetry are found to have (K(a), K(c))=(even, odd) or (odd, even). This somewhat unexpected fact enables the previously observed experimental spectrum to be assigned for the first time, in good agreement with theory, and indicates that the orientation of hydrogen is perpendicular to the intermolecular axis in the ground state of the orthoH(2)-CO(2) complex.     

Hydrogen bonding versus van der Waals interactions: competitive influence of noncovalent interactions on 2D self-assembly at the liquid-solid interface

The structures of the self-assembled monolayers of various 4-alkoxybenzoic acids physisorbed at the liquid-solid interface were established by employing scanning tunnelling microscopy (STM). This study has been essentially undertaken to explore the competitive influence of van der Waals and hydrogen-bonding interactions on the process of two-dimensional self-assembly. These acid derivatives form hydrogen-bonded dimers as expected; however, the dimers organise themselves in the form of relatively complex lamellae. The characteristic feature of these lamellae is the presence of regular discommensurations or kinks along the lamella propagation direction. The formation of kinked lamellae is discussed in light of the registry mechanism of the alkyl chains with the underlying graphite substrate. The location of the kinks along a lamella depends on the number (odd or even) of carbon atoms in the alkyl chain. This result indicates that concerted van der Waals interactions of the alkyl chain units introduce the odd/even chain-length effect on the surface-assembled supramolecular patterns. The odd/even effects are retained even upon complexation with a hydrogen-bond acceptor. However, as the solvent is changed from 1-phenyloctane to 1-octanoic acid, the kinked lamellae as well as the odd/even effects disappear. This solvent-induced convergence of supramolecular patterns is attained by means of co-crystallisation of octanoic acid molecules in the 2D crystal lattice, which is evident from high-resolution STM images. The solvent co-adsorption phenomenon is discussed in terms of competing van der Waals and hydrogen-bonding interactions.     

Body-fixed complex-coordinate coupled-channel formalism for rotational predissociation of van der Waals molecules

The method of complex-coordinate coupled-channel (CCCC) formalism previously developed in the space-fixed (SF) frame is reformulated in the body-fixed (BF) frame, more appropriate for strong-coupling van der Waals complexes. The utility of the method is demonstrated through a study of the level widths and energies of rotationally predissociating atom-diatom model systems. The possible usefulness of incorporating the centrifugal decoupling approximation in the BF CCCC theory is pointed out.     

Potential-energy surface, van der Waals energy spectrum, and vibronic transitions in s-tetrazine-argon complex

The van der Waals vibrational states and the structure of the vibronic spectrum of s-tetrazine-argon complex have been studied by the ab initio methods. The potential-energy surface of the ground S(0) electronic state of the complex has been constructed by fitting the analytical many-body expansion to a large set of the interaction energy values computed using the second-order M?ller-Plesset perturbation theory combined with the standard aug-cc-pVDZ basis set. The equilibrium structure of the complex found is that with argon located above the tetrazine ring at a distance of 3.394 A. The calculated dissociation energy of 354 cm(-1) is compatible with the experiment. The van der Waals energy spectrum calculated from the potential-energy surface is explained analyzing a correlation with a simpler energy spectrum of benzene-argon. A new assignment of the S(0)-S(1) vibronic spectrum is proposed on the basis of the rigorous selection rules, vibrational energy levels in S(0) and S(1) states and vibronic transition intensities calculated from the electronic transition dipole moment surfaces.     

A combined experimental and computational investigation of the microscopic external heavy atom effect in van der Waals clusters

We present a combined experimental and computational study of the external heavy atom effect in van der Waals clusters of para-difluorobenzene (pDFB) with rare-gas atoms. Experimentally, clustering with rare-gas atoms is observed to shorten significantly the S1 fluorescence lifetime compared with that of the pDFB monomer, an effect we interpret in terms of an enhancement of the S1-T1 intersystem crossing rate. In order to test the validity of this widely held assumption, we have calculated the S1-T1 spin-orbit coupling matrix elements in the X-pDFB complexes (X=Ne, Ar, Kr) using a multiconfigurational linear response approach.     

Ab initio characterization of the Mg-HF van der Waals complex

The equilibrium structure and the three-dimensional potential energy surface of the Mg-HF van der Waals complex in its ground electronic state have been determined from accurate ab initio calculations using the coupled-cluster method, CCSD(T), in conjunction with the basis sets of triple- through quintuple-zeta quality. The core-electron correlation, high-order valence-electron correlation, and scalar relativistic effects were investigated. The Mg-HF complex was confirmed to be linear at equilibrium, with a vibrationless dissociation energy (into Mg and HF) D(e) of 280 cm(-1). The vibration-rotation energy levels of two isotopologues, (24)Mg-HF and (24)Mg-DF, were predicted using the variational method. The predicted spectroscopic constants can be useful in a further analysis of high-resolution vibration-rotation spectra of the Mg-HF complex.     

Microwave investigation of the CO-CH4 van der Waals complex

Rotational spectra of eight isotopomers of the weakly bound van der Waals complex CO-CH4 were recorded in the frequency range from 4 to 19 GHz using a pulsed molecular beam Fourier transform microwave spectrometer. For the isotopomers containing methane monomers of Td symmetry, namely, 12C16O-12CH4, 12C16O-13CH4, 12C16O-12CD4, 13C16O-12CH4, and 13C18O-12CH4, three rotational progressions were observed that correlate to the jm=0, 1, and 2 rotational levels of free methane. For those containing partially deuterated methane monomers with C3V symmetry, namely, 12C16O-12CH3D and 12C16O-12CHD3, only two progressions were recorded, correlating to the jk=0(0) and 1(1) rotational levels of free CH3D and CHD3, respectively. The van der Waals bond distance R, intermolecular stretching frequency nus, and the corresponding stretching force constant ks were derived from the obtained spectroscopic results. The results obtained for the jm=0 ground state are compared to the previous infrared and millimeter wave data. A 17O nuclear quadrupole coupling constant was determined from the resolved hyperfine structure of 13C17O-12CH4 and was used to obtain angular information about the carbon monoxide subunit. A Coriolis interaction was deduced from the irregular spectral pattern involving levels with jm=1. Qualitative information about the extent of the perturbation was obtained from a comparison of spectroscopic constants of different isotopomers.     

The rotational spectrum and structure of the phosphine-hydrogen cyanide complex

The rotational spectrum of the weakly bound complex (PH₃, HCN) in its vibrational ground state has been observed by the technique of pulsed-nozzle, Fourier-transform microwave spectroscopy. The isotopic species (PH₃, HC¹⁴N), (PH₃, DC¹⁴N) and (PH₃, HC¹⁵N) exhibit spectra of the symmetric-top type from which accurate values of the spectroscopic constants B₀, D_J, D_JK and X_aa(¹⁴N) have been determined. For (PH₃, HC¹⁴N) the appropriate values are: B₀ = 1553.3709(1) MHz, D_J = 3.306(3) kHz, D_JK = 256.9(6) kHz and X_aa(¹⁴N) = ?4.3591(14) MHz. The geometry of the complex established from the spectroscopic constants is one of C_3v symmetry at equilibrium, with the HCN molecule lying along the C₃ axis of PH₃ and oriented so that it forms a hydrogen bond to the P atom. The effective distance from P to the C nucleus is r(P ? C) = 3.913 Å.     

Modeling chemical reactions on surfaces: The roles of chemical bonding and van der Waals interactions

Chemical reactions on surfaces play central roles in heterogeneous catalysis, and most reactions involve the formation and/or the cleavage of bonds. At present, density functional theory (DFT) has become the workhorse for computational investigation of reaction mechanisms, but its predictive power has been severely limited by the lack of appropriate exchange-correlation functionals. Here, we show that there are many cases where the chemical bonding and van der Waals (vdW) interactions both play a key role in chemical reactions on surfaces. After briefly introducing some DFT methods and basic theory in chemical reactions, we first demonstrate that DFT can help to understand the mechanisms of “classic” reactions that mainly dominated by covalent bonding and vdW forces, as exemplified in electrocatalytic reduction of CO₂ and the fabrication of 2D materials on metal substrates. We next show that DFT calculations can help to uncover the tautomerization reactions of molecules on metal surfaces, wherein the hydrogen bonding and vdW forces would largely affect the reaction process. More importantly, we show that in some cases, the vdW interactions can become the decisive effect that determines the adsorption configuration, energy hierarchy, and the potential-energy surface of chemical reactions, yielding distinct pathways and products. Additionally, we highlight the importance of more realistic conditions, such as surface defects, finite coverage, and temperature effects, in accurate modeling of chemical reactions. Finally, we summarize some challenges in modeling catalysis, which include many-body dispersive correction, strong correlation effect, and non-adiabatic approximations.     

Hindered rotation of water molecule in van der Waals complex

The influence of water molecule symmetry on hindered rotation has been investigated for H₂O-He,H₂O(D₂O)-CO₂ systems, where CO₂ molecule is treated as an atom. A strong coupling model has been used for calculating the energy spectra. The eigenfunctions of Morse oscillator and asymmetric top have been used as a basis set. The results show that experimentally observed effect of spin-selective water vapour codensation may be explained by the differences between binding energies of para- and orthomodifications for H₂O(D₂O) molecules.     

Spectroscopic characterization of the C2-Ne van der Waals complex

Binary complexes of C2 with rare-gas atoms (C2-Rg) have attracted theoretical interest as their potential-energy surfaces are predicted to support linear equilibrium geometries, without the local minimum for the T-shaped geometry that would be expected using a standard pair-potential model. In the present work we have explored the properties of C2-Ne using laser-induced fluorescence detection of the D 1Sigmau +-X 1Sigmag + transition. Bands of the complex were observed in association with the monomer 0-0 and 1-1 transitions. Rotationally resolved data yielded rotational constants of B'=0.099(3) cm(-1) and B"=0.100(3) cm(-1) for the excited and ground states, respectively. Analysis of the rovibrational energy-level structure for C2(D)-Ne indicates that the complex has a linear equilibrium structure with a barrier to internal rotation of approximately 15 cm(-1). Data for the ground state validate a recent high-level ab initio calculation of the potential-energy surface for C2(X)-Ne.     

Dynamics of an inclusion complex of chloroform and cryptophane-E: evidence for a strongly anisotropic van der Waals bond

The motional dynamics of a van der Waals inclusion complex of cryptophane-E and chloroform has been investigated by a combined NMR exchange and relaxation study. The kinetics of exchange of chloroform between the bulk solution and the complex was investigated by means of proton EXSY measurements. The carbon-13 relaxation of the cryptophane-E host and of the bound chloroform guest was analyzed using the Lipari-Szabo "model-free" approach. For interpretation of the carbon-13 relaxation measurements for chloroform, the chemical-exchange process of complex formation and dissociation had to be taken into account in terms of the modified Bloch equations. It was found that the complex behaves as a single molecule without any significant guest chloroform motion inside the host's cavity.     

The van der Waals equation: analytical and approximate solutions

The thermodynamic properties, enthalpy of vaporization, entropy, Helmholtz function, Gibbs function, but especially the heat capacity at constant volume of a van der Waals gas (and liquid) at the phase transition are examined in two different limit approximations. The first limit approximation is at the near-critical temperatures, i.e., for T/T _c → 1, where T _c is the critical temperature, the other limit approximation is at the near-zero temperatures, T→ 0. In these limits, the analytical equations for liquid and gas concentrations at saturated conditions were obtained. Although the heat capacities at constant volume of a van der Waals gas and liquid do not depend on the volume, they have different values and their change during the phase transition was calculated. It should be noticed that for real substances the equations obtained at the near-zero temperature are only valid for T > T _{triple point} and T ≪ T _c , which means that found equations can be used only for substances with T _{triple point} ≪ T _c .     

An application of the van der Waals density functional: Hydrogen bonding and stacking interactions between nucleobases

We apply the van der Waals density functional (vdW-DF) to study hydrogen bonding and stacking interactions between nucleobases. The excellent agreement of our results with high level quantum chemical calculations highlights the value of the vdW-DF for first-principles investigations of biologically important molecules. Our results suggest that, in the case of hydrogen-bonded nucleobase pairs, dispersion interactions reduce the cost of propeller twists while having a negligible effect on buckling. Furthermore, the efficient scaling of DFT methods allowed for the easy optimization of separation distance between nucleobase stacks, indicating enhancements in the interaction energy of up to 3 kcalmol over previous fixed distance calculations. We anticipate that these results are significant for extending the vdW-DF method to model larger vdW complexes and biological molecules.     

The weak hydrogen bond in the fluorobenzene-ammonia van der Waals complex: Insights into the effects of electron withdrawing substituents on pi versus in-plane bonding

The fluorobenzene-ammonia van der Waals complex has been studied using a combination of two-color resonance enhanced multiphoton ionization (REMPI) spectroscopy, counterpoise corrected RICC2 ab initio molecular orbital calculations, and multidimensional Franck-Condon analysis. The experimental REMPI spectrum is characterized by a dominant, blueshifted band origin, and weak activity in intermolecular vibrational modes. RICC2 geometry optimizations and numerical vibrational frequency calculations of the neutral ground and first excited states have been performed on a number of different structural isomers of the complex using basis sets ranging from augmented double-zeta to quadruple-zeta level. Ground state basis set superposition error corrected zero-point binding energies show the in-plane sigma complex, forming a pseudo-six-membered ring connecting the fluorine atom and ortho-hydrogen, to be consistently the most stable of all six conformations considered, at all levels of theory. Comparison of computed zero-point excitation energies for the most stable pi and sigma conformers with fluorobenzene show that the sigma complex is the only conformer predicted to exhibit a spectral blueshift upon electronic excitation. The computed neutral ground and first excited state geometries and frequencies were used to perform multidimensional Franck-Condon simulations of the S(1)-S(0) vibronic spectrum for each of the most stable conformers. These simulations yielded null spectra for transitions involving the most stable of the pi complexes, pi(bridge); a spectrum rich in strong intermolecular vibrational structure for the second of the pi complexes, in complete contrast to the experimental spectrum; and for the sigma complex, a spectrum exhibiting weak intermolecular activity in line with that observed experimentally. This last simulation allowed an almost complete vibrational assignment of the intermolecular structure in the REMPI spectrum. The agreement between computational results and experiment overwhelmingly favors assignment of the spectrum to the in-plane sigma complex.     

The molecular structure, bonding, and energetics of oxovanadium phthalocyanine: an experimental and computational study

A mass spectrometric study of saturated vapor over oxovanadium phthalocyanine showed the thermal stability and monomeric vapor composition of this compound. The molecular structure of oxovanadium phthalocyanine (VOPc) was determined using a combination of gas-phase electron diffraction (GED), mass spectrometry, and quantum chemical calculations. According to GED, the VOPc molecule has C_4v symmetry. Experimental structural parameters are in good agreement with the parameters obtained by UB3LYP/cc-pVTZ calculations. The vanadium atom has a five-coordinated square-pyramidal geometry, being shifted above the plane of the four isoindole nitrogen atoms by 0.576(14) Å. The parameters of the square pyramid VN₄ are r _h1(V–N) = 2.048(7) Å, r _h1(N···N) = 2.780(12) Å. The vanadium–oxygen bond length is r _h1(V–O) = 1.584(11) Å. NBO analysis shows polar character of coordination bonds with significant covalent contribution and pronounced direct donation. X-ray crystallography and GED give different coordination bond lengths according to the different physical meaning of the parameters obtained by these methods. The enthalpy of sublimation [?H _s ^o (593–678 K)] is 53.3 ± 0.8 kcal/mol.     

Microwave and millimeter-wave spectroscopy of the open-shell van der Waals complex Ar-HO2

Pure rotational transitions of a rare gas atom-reactive open-shell triatom van der Waals complex Ar-HO2 have been observed by Fourier transform microwave spectroscopy. The transitions observed are of a type with K(a) = 0 and 1. Furthermore, by monitoring the change of the free induction decay signal of the a-type transitions, b-type transitions have been observed by a double resonance technique in the region 18-49 GHz. All these transitions provide us precise molecular constants. The r0 structure of Ar-HO2 has been determined by fixing the structure of the HO2 monomer. The determined structure is planar and almost T shaped, where the argon atom is slightly shifted to the hydrogen atom of HO2. The experimental data supplemented by high-level ab initio calculations indicate that the van der Waals bond of Ar-HO2 is relatively rigid. On the other hand, effects on the unpaired electron distribution by the complex formation are found to be fairly small, since the fine and hyperfine constants of Ar-HO2 are well explained by those of the HO2 monomer.