Weak Van Der Waals Complexes Research Articles

The F19–H1 coupling constants transmitted through covalent, hydrogen bond, and van der Waals interactions

The F19–H1 coupling constants were calculated on the multiconfiguration self-consistent field (MCSCF) level in several systems, ranging from covalently bonded HF, hydrogen bonded FHF− and (HF)2 complexes to weak van der Waals complex CH4–HF. The sign of the F19–H1 coupling varies in this sequence, and its absolute value decreases. Still, it is sizable even for CH4–HF. The distance dependence of F19–H1 coupling is essentially the same in all systems under study, and the calculations for FHF− with distorted geometry suggest that the value of F19–H1 coupling is determined mainly by molecular geometry. F19–F19 coupling constants were also analyzed. F19–F19 intermolecular coupling in (HF)2 is substantial but has the opposite sign to that in FHF− and its counterpart in (H2O)2.

Ab initio molecular orbital studies of the vibrational spectra of some van der Waals complexes. Part 1. Complexes of molecular nitrogen with carbon dioxide, nitrous oxide, carbonyl sulphide and carbon disulphide

The structures, interaction energies and vibrational spectra of four weak van der Waals complexes containing molecular nitrogen, as electron donor, and the linear triatomic molecules carbon dioxide, nitrous oxide, carbonyl sulphide and carbon disulphide, as electron acceptors, have been determined by carrying out a series of ab initio molecular orbital calculations using the Gaussian 98 computer program. The calculations were performed at the second order level of Møller-Plesset perturbation theory, employing the 6-311+G(d) basis set. The results have been interpreted by correlation with some molecular properties of the electron acceptors. The predicted spectra will be used as aids in the interpretation of the matrix isolation infrared spectra of the complexes determined experimentally.

Density functional theory with an approximate kinetic energy functional applied to study structure and stability of weak van der Waals complexes

In view of further application to the study of molecular and atomic sticking on dust particles, we investigated the capability of the “freeze-and-thaw” cycle of the Kohn–Sham equations with constrained electron density (KSCED) to describe potential energy surfaces of weak van der Waals complexes. We report the results obtained for C6H6⋯X (X=O2, N2, and CO) as test cases. In the KSCED formalism, the exchange-correlation functional is defined as in the Kohn–Sham approach whereas the kinetic energy of the molecular complex is expressed differently, using both the analytic expressions for the kinetic energy of individual fragments and the explicit functional of electron density to approximate nonadditive contributions. As the analytical form of the kinetic energy functional is not known, the approach relies on approximations. Therefore, the applied implementation of KSCED requires the use of an approximate kinetic energy functional in addition to the approximate exchange-correlation functional in calculations following the Kohn–Sham formalism. Several approximate kinetic energy functionals derived using a general form by Lee, Lee, and Parr [Lee et al., Phys. Rev. A. 44, 768 (1991)] were considered. The functionals of this type are related to the approximate exchange energy functionals and it is possible to derive a kinetic energy functional from an exchange energy functional without the use of any additional parameters. The KSCED interaction energies obtained using the PW91 [Perdew and Wang, in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] exchange-correlation functional and the kinetic energy functional derived from the PW91 exchange functional agree very well with the available experimental results. Other considered functionals lead to worse results. Compared to the supermolecule Kohn–Sham interaction energies, the ones derived from the KSCED calculations depend less on the choice of the approximate functionals used. The presented KSCED results together with the previous Kohn–Sham ones [Wesołowski et al., J. Phys. Chem. A 101, 7818 (1997)] support the use of the PW91 functional for studies of weakly bound systems of our interest.

Trimesitylsilylium cation verification of a free silylium cation in solution by NMR chemical shift calculations

NMR chemical shift calculations at the SOS-DFPT/PW91/[9s6p2d/5s4pld/5s4pld/3s]//B3LYP/6-31G(d) level of theory were used to describe the trimesitylsilylium cation 1, recently synthesized in benzene solution and investigated by NMR spectroscopy. The conformation of cation 1 is characterized by mesityl rings rotated by 47° in a propeller-like form. Contrary to other silylium cations investigated, cation 1 forms a weak Van der Waals complex 3 with benzene rather than a Wheland σ-complex. The calculated 29Si NMR chemical shifts for 1 and 3 are 226 and 227 ppm, compared to the experimental value of 225.5 ppm. The agreement between calculated and measured NMR chemical shifts provides evidence that cation 1 presents the first free silylium cation synthesized.

A density functional study of M–C2H4 complexes (M=Li, Na, K): Singularity of the Li atom

Quantum chemical calculations on the Li–C2H4 complex have been performed with coupled-cluster and density functional methods. For both methods the electronic ground state of the complex is calculated to be 2B2, with a C2v symmetry equilibrium structure, and the calculated binding energy is quite small (around 2 kcal/mol), and therefore very much basis set dependent. The vibrational spectrum has been calculated at the harmonic approximation, including 13C/12C, 7Li/6Li, and H/D isotopic substitutions. The agreement between experimental and calculated infrared frequencies is correct, except for the low frequency symmetric Li–C stretching mode. These calculations also allow to propose an assignment for the observed C–H/C–D stretching modes. The observed blue-shift of the symmetric CH2 bending mode as well as the red-shift of the antisymmetric CH2 bending, CD2 bending, and C–C stretching modes with respect to the free ethylene have been confirmed by the density functional calculations. The Na...C2H4 complex has been found to be unstable in its 2B2 electronic state. The study of the 2A1 electronic state for both Na...C2H4 and K...C2H4 complexes show that they are at most very weak van der Waals complexes. This result confirms the conclusions of matrix isolation experiments.

Supermolecular approach to many-body dispersion interactions in weak van der Waals complexes: He, Ne, and Ar trimers

Using the diagrammatic many-body perturbation theory, various three-body dispersion terms that appear in the intermolecular Mo/ller–Plesset perturbation theory (MPPT) are identified and classified with regard to the effects of intramonomer electron correlation on the dispersion term. Via the connection with the supermolecular MPPT, it is demonstrated how the leading dispersion nonadditivities arise within supermolecular calculations that employ MPPT or coupled cluster formalisms. The numerical calculations for He3, Ne3, and Ar3 in triangular geometries fully confirm theoretical predictions. The calculated values of dispersion nonadditivity clearly show that the coupled cluster theory with single, double, and noniterative triple excitations provides the proper framework for the efficient inclusion of the intramonomer correlation effects in dispersion nonadditivity. The convergence of the two-body and three-body terms is shown to be very similar if we compare the three-body terms of an order higher than the two-body terms. This pattern is used to provide the estimates of the total nonadditivities in the three trimers within a few percent accuracy.

Infrared spectra of aluminium carbonyl complexes in solid argon

Infrared spectra of aluminium carbonyl complexes generated in argon matrices by co-condensation of aluminium atoms and carbon monoxide have been observed and analysed. AlCO, aluminium monocarbonyl, was isolated and clearly identified. It is characterized by a carbonyl force constant (Fco≈ 14 mdyn Å–1) markedly smaller than in Al(CO)2(15.5 mdyn Å–1), thus contradicting earlier studies concluding that an AlCO complex should be unbound or, at most, a weak van der Waals complex.

An ab initio investigation of the molecular structure and vibrational spectrum of the silanol–hydrogen molecular complex

The geometries and stabilization energies of various stationary points on the potential-energy surface for the weak van der Waals complex formed between H2 and silanol (H3SiOH), chosen to mimic the silica free hydroxy group, are calculated at the Hartree–Fock level with the 3-21G and 6-31G** basis sets. Electron correlation effects have been taken into account at the Møller–Plesset second-order perturbation theory (MP2) level employing the 6-31G** basis set. Harmonic frequencies, needed for the characterization of the stationary points, and the infrared and Raman intensities are evaluated at the Hartree–Fock level. The preferred mode of interaction is where the H2 molecule is directed towards the oxygen atom of the H3SiOH subunit.

Ab initio investigation of the structures and stabilities of the donor-acceptor complexes H 3PBF 3 and Me 3PBF 3

The structures of the complexes H 3PBF 3 and Me 3PBF 3 are determined from ab initio electronic-structure calculations. In contrast to gas-phase experiments, H 3PBF 3 is found to give only a weak van der Waals complex with a P—B distance of 349 pm (SCF) and 322 pm (MP2), whereas the experimental value is 192 pm. The corresponding dissociation energies are 8.0 and 12.7 kJ/mol, respectively. These results are confirmed by coupled-pair functional calculations. The methods employed are checked by calculations on Me 3PBF 3, where deviations from experiment are in the expected range.

An ab initio molecular orbital study of silanol–hydrogen complexes: a model for the interaction of hydrogen with silicate hydroxyl groups

The geometry of the weak van der Waals complex formed between H2 and silanol is determined by high level ab initio molecular orbital methods and is shown to provide a good model for H2 adsorption complexes on silicates and zeolites.

Finite-field many-body perturbation theory. VIII. Multipole polarizabilities of the CO molecule

The dipole and quadrupole polarizability functions of CO are calculated by using the finite-field form of the complete fourth-order many-body perturbation theory. At the experimental equilibrium distance R CO = 2.132 au the calculated values of α X,X, α Z,Z, C X,X and C Z,Z are equal to 12.02, 15.59, 26.01, and 48.03 au. Both the basis set dependence of the calculated polarizabilities and the convergence of the corresponding correlation perturbation series are investigated in relation to calculations of potential energy surfaces for weak van der Waals complexes of CO.

An investigation of NO3 as a possible intermediate in the oxidation of nitric oxide

AbstractThe lowest potential energy surfaces of NO3 have been investigated using CASSCF and contracted CI methods, to determine if NO3 is an intermediate in the oxidation of nitric oxide. This reaction is observed to be termolecular where an intermediate is almost certainly formed. The calculations, which were performed in C2v symmetry, show that the form of NO3 which has a near D3h geometry should not be possible to reach from ground state NO and O2. The energy of this isomer of NO3 is calculated to be 25 kcal/mol higher in energy than separated NO and O2. Furthermore, NO + O2 is separated from NO3 by very high barriers. The C2v constrained barrier is as high as 145 kcal/mol, which makes it very unlikely that NO3 could be formed even along less symmetric pathways. The origin of the barriers are analyzed, and it is shown that a special type of mechanism is involved for the state with the lowest barrier. In this mechanism, which was observed recently also for NiH2, the occupation in each symmetry changes only by one unit in each step of the reaction. The only isomer of NO3 which could be formed is a weak van der Waals complex with a very long bond distance of 6.5 a.u. N2O2 does, however, in this respect seem to be a more likely intermediate with its shorter bond distance of 4.5 a.u. Calculations around the D3h equilibrium further show that 2B2 is the ground state of this isomer of NO3 with 2A2 and 2B1 slightly higher in energy.

Structures and bonding of ArHF and ArHO

Ab initio molecular orbital calculations show that both ArHF and ArHO are weak van der Waals complexes with linear ground state structures.

LCAO–MO–SCF Calculations Using Gaussian Basis Functions. IV. The Helium Adduct of Lithium Hydride, HeLiH

Population analyses of wavefunctions calculated for LiH indicated that the Li-atom site was severely electron deficient, thus favoring complexing to a species with an unshared electron pair, even on a rare-gas atom. Calculations were performed for linear HeLiH as a function of HeLi distance at several fixed LiH distances. The minimum EHF in each case was found at an HeLi distance of about 4.0 bohr, and in each case the system is bound relative to the separated products He+LiH. With the HeLi distance fixed at the equilibrium position 4.0 bohr, variation of the LiH distance gave an EHF minimum at 3.05 bohr, close to the equilibrium distance of 3.015 bohr in LiH itself. The Hartree–Fock binding energy of HeLiH relative to He+LiH was estimated to be 0.092 eV. A calculation of the zero-point vibration energy of the He–Li bond indicates that HeLiH is bound by 0.08 eV with respect to dissociation along the He–Li bond. The HeLiH system appears to be intermediate between a true compound and an extremely weak van der Waals complex. The higher rare-gas complexes of LiH are expected to be more stable.