Weak Van Der Waals Complexes Research Articles

Observation of Borane-Olefin Proximity Interaction Governing the Structure and Reactivity of Boron-Containing Macrocycles.

While attractive interactions between borane and olefin have been postulated to trigger various boron-mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane-olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi-step 1,2-carboboration between a strained alkyne and 9-borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi-step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron-mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

Observation of Borane–Olefin Proximity Interaction Governing the Structure and Reactivity of Boron‐Containing Macrocycles

AbstractWhile attractive interactions between borane and olefin have been postulated to trigger various boron‐mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane–olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi‐step 1,2‐carboboration between a strained alkyne and 9‐borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi‐step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron‐mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

Theoretical Characterization of the Structure and Spectroscopy of HCNO2 Isomers and Applications.

We carried out a theoretical, fully ab initio, investigation of the stable forms of the [H,C,N,O,O] pentatomic molecular system, whose isomers are involved in fundamental combustion and atmospheric processes and are of potential interest for astrophysics. By adopting the MP2 and CCSD(T) electronic structure methods, combined with extrapolations to the complete basis set (CBS) limit, we characterized 20 low-energy isomers, excluding weak van der Waals complexes. For these molecules, we determined a set of geometrical parameters, relative energies, anharmonic vibrational frequencies, IR intensities, and fragmentation/formation energies from various atomic and/or molecular fragments. We discuss the relevance of the present findings for the search of new molecular species in astrophysical and atmospheric media and give suggestions for their possible detection in laboratory experiments. The set of data provided by the present work should facilitate the identification of these species from their gas-phase and low-temperature solid matrix spectra, whenever measured.

Photoluminescence enhancement of organic dye by graphene quantum dots

The charge transfer (CT) interaction between the blue emitting graphene quantum dots (GQDs) and an organic dye (fluorescein 27, F27) has been first studied by steady-state and time-resolved spectroscopy as a function of the pH. F27 exists in the form of different inherent fluorescent species depending on the pH of the solvent. At pH 7, the CT interaction of GQDs with F27 results in a small red shift in the absorption and fluorescence spectra of the dye. The results indicate formation of a weak van der Waals (vdW) complex in the ground state. The photoluminescence decay rate of the GQDs is generally unaffected in the mixture indicating the absence of dynamic quenching. The fluorescence intensity of F27-GQD system shows a 3-fold increase. Numerical simulations reveal that apart from the CT interaction, hotspots in the nearfield of the GQDs contribute to the fluorescence enhancement.

Non-covalent interactions and their impact on the complexation thermodynamics of noble gases with methanol.

Accurate ab initio calculations provide the reliable information needed to study the potential energy surfaces that control the non-covalent interactions (NCIs) responsible for the formation of weak van der Waals complexes. In this work, relying on the state of the art method for NCI computations, namely symmetry adapted perturbation theory (SAPT), we calculated the potential energy curves for the interaction of noble gases (Ng = He, Ne, Ar and Kr) with methanol in three different interaction sites to account for orientational anisotropy of the interaction potential. Different levels of the SAPT and basis set were employed to disclose the nature of the stabilizing forces acting upon formation of the Ng-CH3OH adducts. SAPT-derived NCIs indicate that dispersion forces are indeed the dominating component of the total energy, but also that induction and electrostatic effects are important to counterbalance the steric repulsions. By solving the Radial Nuclear Schrödinger Equation for the complexes, we also determined the rovibrational structure of the interaction wells to extract invaluable information about the thermodynamic stability of the adducts and how different temperature conditions affect the structure of the dimers. Although SAPT calculations reveal net attractive forces, these do not afford a spontaneous complexation process even at temperatures as low as 40 K.

Competition between tetrel bond and pnicogen bond in complexes of TX3-ZX2 and NH3

The complexes formed between TX3-ZX2 (T = C, Si, Ge; Z = P, As, Sb; X = F, Cl) and NH3 were studied at the MP2/aug-cc-pVTZ(PP) level. For each TX3-ZX2, two types of complex were obtained. For CX3-ZX2, NH3 is inclined to approach the σ-hole on the Z atom, forming a pnicogen bond. For TX3-ZX2 (T = Si and Ge), however, the base favors engaging in a tetrel bond with the σ-hole on the T atom although the corresponding pnicogen-bonded complex is also stable. When NH3 approaches the CX3 terminal of CX3-ZX2, weak interactions are observed that may be classified as van der Waals interactions. The relative stability of both types of complexes is not affected by the substituent X. The tetrel bond is very strong and the largest interaction energy is up to -144kJmol-1. Dispersion is dominant in the weak van der Waals complexes, while tetrel- and pnicogen-bonded complexes are dominated by electrostatic interactions, with comparable contributions from polarization.

Olefin Metathesis by Grubbs–Hoveyda Complexes: Computational and Experimental Studies of the Mechanism and Substrate-Dependent Kinetics

The potential energy surfaces for the activation of Grubbs–Hoveyda-type precatalysts with the substrates ethene, propene, 1-hexene, and ethyl vinyl ether (EVE) have been probed at the density functional theory (DFT) (M06-L) level. The energetically favored pathway of the reaction leading to a 14e Fischer carbene and styrene starts with an initiation step in which the incoming substrate and outgoing alkene ligand are both clearly associated with the ruthenium center. For these substrates, with the exception of ethene, the rate determining step is predicted to be the formation of the metallocyclobutane (MCB). We have taken the initial reactant to be a weak van der Waals complex between substrate and precatalyst. This model yields good agreement between the computed activation parameters for both the parent Grubbs–Hoveyda and Grela complex with EVE substrate, and the experimental values, reported here. The alternative model which takes the initial reactant to be two isolated molecules requires an estimate of the entropy loss on formation of the initial complex in solution which is difficult to evaluate. Our estimate of this quantity yields a barrier for the rate determining step for the interchange mechanism which is close to the value we find for the alternative mechanism in which the rate determining step is the initial dissociation of the precatalyst. The relative energetics of these two mechanisms involving different initiation steps but with similar activation barriers, could well be dependent upon the precatalyst and substrate in line with the recent experimental findings of Plenio and co-workers.

Chemical bonding in groups 10, 11, and 12 transition metal homodimers — An electron density study

The properties of metal–metal bonding for transition metal homonuclear diatomics from groups 10, 11 and 12 are studied within the framework of the quantum theory of atoms in molecules (QTAIM) at the coupled cluster CCSD and CCSD(T) levels of theory. A novel approximate method developed by Keith and Frisch is used to augment electron densities calculated with pseudopotentials with the missing relativistic core densities to obtain approximations to the total densities of the dimers. The calculated delocalization indices for group 10 dimers are: Ni2(1.6), Pd2(0.44, an outlier in the group), and Pt2(1.8); for group 11 dimers: Cu2and Ag2(1.01), and Au2(1.13), all covalent bonds; for group 12: Zn2(0.06), Cd2(0.08), and Hg2(0.09), all consistent with weak van der Waals complexes. The picture of bonding obtained by examining the values of the electron density at the bond critical points is consistent with the one obtained on the basis of these delocalization indices. A curious linear (instead of exponential) dependence of the delocalization index on the electron density at the bond critical point is presented here as an observation and will be investigated in more depth in later work. Several correlations between bond properties and bond dissociation energies are also explored. It is found that, with the exception of the Ni2dimer that exhibits considerable multi-reference character, there are correlations between the calculated bond dissociation energies of the studied diatomics and several bond critical point properties. These correlations are novel as they span a set of bonds between different pairs of elements, while traditionally these correlations were reported for bonds between the same pair or elements but with different substituents.

Haloboration of internal alkynes with boronium and borenium cations as a route to tetrasubstituted alkenes.

Hail boration! 2-Dimethylaminopyridine-ligated dihaloborocations [X2B(2-DMAP)](+) with a strained four-membered boracycle were used for the haloboration of terminal and dialkyl internal alkynes (see scheme). Esterification then provided vinyl boronate esters as useful precursors to tetrasubstituted alkenes. Following mechanistic studies, the scope of the haloboration was expanded simply by variation of the amine. Pin = 2,3-dimethyl-2,3-butanedioxy.

On photo-oxidation of single molecules

Fluorescence and fluorescence excitation spectra of terrylene in 9,10-dibromoanthracene matrix were studied at 5 K. Characteristic singlet oxygen phosphorescence, O 2( 1Δ g ) → O 2( 3Σ g ) with the line maximum at 1272.5 nm, was observed after excitation of terrylene molecules embedded into one of its two sites. It indicates that singlet oxygen, precursor in photo-oxidation of aromatic molecules, can be created only for those oxygen molecules which can form weak van der Waals complex with terrylene. According to quantum-chemical calculations terrylene and oxygen can form complex with the bonding energy 141 cm −1, in which oxygen molecule is located along the long axis of terrylene, ∼0.3 nm apart, with the molecular axis inclined by 110°.

Ethylene Polymerization Initiated by Tertiary Diamine/n-Butyllithium Complexes: An Interpretation from Density Functional Theory Study

The mechanism of ethylene insertions into eight tertiary diamine/n-butyllithium complexes has been studied at the BLYP/DNP level. In contrast to the cationic coordination polymerization in which a strong coordination complex between ethylene and the metal center is formed prior to ethylene insertion, there is only a weak van der Waals complex between ethylene and tertiary diamine/n-butyllithium complex. After crossing a four-membered-ring transition state, ethylene inserts into the Li-C bond. The insertion barriers for the eight reactions are in the range of 6.9-11.0 kcal/mol, comparable to those of ethylene cationic coordination polymerizations. However, the polymerization activities of ethylene anionic polymerizations are much lower than those of cationic coordination polymerizations. Comparing the energy profiles of these ethylene anionic polymerizations with those of cationic coordination polymerizations, it can be found that the transition states in the ethylene anionic polymerizations are higher in energy than the reactants, while the transition states in ethylene cationic coordination polymerizations are lower than the reactants. Therefore, ethylene anionic polymerizations need additional energy to climb the energy barriers, while the energies for overcoming the transition states in the cationic coordination polymerizations can be obtained from reactants that are higher in energy than the reactants. We reason the differences in their energy profiles could be one of the reasons for the lower activity of ethylene anionic polymerization than ethylene cationic coordination polymerization despite their comparable insertion barriers.

‘Relativistic bond’ in complexes of Cu, Ag, and Au elements with phosphine

The binding energies in CuPH 3 , AgPH 3 , and AuPH 3 complexes, as determined from one-component (scalar) CCSD(T) calculations with aug-cc-pVTZ basis set, are 32.9, 11.5, and 66.7 kJ/mol, respectively. The large binding energy in AuPH 3 is essentially due to relativistic effects. On neglecting them the interaction between Au and the lone-electron pair of PH 3 results in a weak van der Waals complex. The bonding in M ⋯ PH 3 complexes is interpreted in terms of the charge transfer from the electron lone-pair of PH 3 to the metal atom. In AuPH 3 this mechanism is supported by the large relativistic increase of the Au electron affinity.

The structure and chemical bonding in the N2CuX and N2···XCu (X = F, Cl, Br) systems studied by means of the molecular orbital and Quantum Chemical Topology methods

Ab initio studies carried out at the MP2(full)/6-311+G(2df) and MP2(full)/aug-cc-pVTZ-PP computational levels reveals that dinitrogen (N(2)) and cuprous halides (CuX, X = F, Cl, Br) form three types of systems with the side-on and end-on coordination of N(2): N[triple bond]N-CuX (C(infinity v)), N(2)-CuX (C(2v)) stabilized by the donor-acceptor bonds and weak van der Waals complexes N(2)...XCu (C(2v)) with dominant dispersive forces. An electron density transfer between the N(2) and CuX depends on type of the N(2) coordination and a comparison of the NPA charges yields the [N[triple bond]N](delta+)-[CuX](delta-) and [N(2)](delta-)-[CuX](delta+) formula. According to the NBO analysis, the Cu-N coordinate bonds are governed by predominant LP(N2)-->sigma*(Cu-X) "2e-delocalization" in the most stable N[triple bond]N-CuX systems, meanwhile back donation LP(Cu)-->pi*(N-N) prevails in less stable N(2)-CuX molecules. A topological analysis of the electron density (AIM) presents single BCP between the Cu and N nuclei in the N[triple bond]N-CuX, two BCPs corresponding to two donor-acceptor Cu-N bonds in the N(2)-CuX and single BCP between electron density maximum of the N[triple bond]N bond and halogen nucleus in the van der Waals complexes N(2)...XCu. In all systems values of the Laplacian nabla(2)rho(r)(r(BCP)) are positive and they decrease following a trend of the complex stability i.e. N[triple bond]N-CuX (C(infinity v)) > N(2)-CuX (C(2v)) > N(2)...XCu (C(2v)). A topological analysis of the electron localization function (ELF) reveals strongly ionic bond in isolated CuF and a contribution of covalent character in the Cu-Cl and Cu-Br bonds. The donor-acceptor bonds Cu-N are characterized by bonding disynaptic basins V(Cu,N) with attractors localized at positions corresponding to slightly distorted lone pairs V(N) in isolated N(2). In the N[triple bond]N-CuX systems, there were no creation of any new bonding attractors in regions where classically the donor-acceptor bonds are expected and there is no sign of typical covalent bond Cu-N with the bonding pair. Calculations carried out for the N[triple bond]N-CuX reveal small polarization of the electron density in the N[triple bond]N bond, which is reflected by the bond polarity index being in range of 0.14 (F) to 0.11 (Cl).

Density functional and ab initio studies on structures and energies of the ground state of CrCO

AbstractCrCO was studied using density functional theory (DFT) and ab initio methods. We obtained the two‐dimensional potential energy surface (PES), geometry, and vibrational frequencies for CrCO in a septet state. Two minimum structures were found in the CCSD(T) calculation, including a local minimum that is a weak van der Waals (vdW) complex. All DFT methods yield only one minimum structure. We demonstrate that the bond dissociation energy (0.50 kcal/mol) and vibrational frequency (1981.1 cm−1) of CrCO calculated using CCSD(T) are in better agreement with experimental values (<1.5 kcal/mol and 1977 cm−1) than any of the reported theoretical studies. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Quantum-chemical study of CHCl3–SO2 association

CHCl(3)-SO(2) association is studied by high-level quantum-chemical calculations of stationary points of the dimer electronic potential-energy hypersurface, including correlated second-order Moller-Plesset and CCSD(T) calculations with basis sets up to 6-311++G(d,p). During geometry optimization, frequency, and energy calculations, a self-written computer code embedding the GAMESS ab initio program suite applies counterpoise correction of the basis set superposition error. A CH...O hydrogen-bonded complex (DeltaE(0)=-8.73 kJmol) with a 2.4 A intermolecular H...O distance and two very weak van der Waals complexes (DeltaE(0)=-3.78 and -2.94 kJmol) are located on the counterpoise-corrected potential-energy surface. The intermolecular interactions are characterized by Kitaura-Morokuma interaction energy decompositions and Mulliken electron population analyses. The unusual hydrogen bond is distinguished by a CH-bond contraction, a pronounced enhancement of the IR intensity and a shift to higher frequency ("blueshift") of the CH-stretching vibration compared to the CHCl(3) monomer. Spectroscopy and association in liquid solution is also discussed; our results provide an alternative explanation for features in the CH-stretching vibration spectrum of chloroform dissolved in liquid sulfur dioxide which have been attributed previously to an intermolecular Fermi resonance.

Paradigm Pre‐Reactive Van Der Waals Complexes: X—HX and X—H2 (X: F, Cl, Br)

AbstractFor Abstract see ChemInform Abstract in Full Text.

Interactions of transition metal atoms with He

The adiabatic interaction potentials were obtained for the paradigm transition metal-rare gas interactions: Sc(2D)-He and Ti(3F)-He and their di-cations. The ab initio approach included the coupled cluster and multireference configuration interaction methods. He atoms form very weak van der Waals complexes with Sc and Ti with well-depths of ca. 4-5 cm-1. The interactions are characterized by the nearly-degenerate manifolds of adiabatic states with splittings of the order of 0.1 cm-1 or less. The anisotropy of the Ti-He interaction is smaller than that for the Sc-He interaction. The origin of the weak anisotropy of these interactions was analyzed. The exchange repulsion was found to be nearly the same in the \({\rm\Sigma}\), \({\rm\Pi}\) and \({\rm\Delta}\) states due to the valence d-electrons being submerged under the doubly filled 4s electron sub-shell. The anisotropy of the total potential is controlled by the weakly-anisotropic dispersion interaction.

Paradigm pre-reactive van der Waals complexes: X–HX and X–H2 (X = F, Cl, Br)

This review describes recent progress in ab initio calculations and modeling of weak pre-reactive van der Waals complexes that appear in the entrance channels of benchmark atom–diatom reactions. Examples from recent work are used to demonstrate how relevant potential energy surfaces are obtained and modeled from first principles. The paradigm complexes include the X(2P)–HX and X(2P)–H2 (X = F, Cl, Br) systems, with O(3P)–HCl included for comparison. In these complexes an interaction with either the HX or the H2 molecule splits a degenerate P state of an open-shell atom into three potential energy surfaces, two of which are of the same symmetry. Application of state-of-the art highly correlated methods, CCSD(T) and MRCI, to the evaluation of adiabatic and diabatic states is discussed. Nonadiabatic coupling involving potential surfaces is compared for a number of complexes. Computational modeling of this term and its relationship to electrostatic interaction are also described. Spin–orbit coupling is shown to have dramatic effects on the structural and dynamic properties of these complexes.

Lone pair interactions with coinage metal atoms: Weak van der Waals complexes of the coinage metal atoms with water and ammonia

Interaction energies between the coinage metal atoms (Cu, Ag, and Au) and lone-pair donating molecules (H2O and NH3) are calculated by the spin adapted restricted open-shell Hartree–Fock coupled cluster method with the scalar relativistic effects accounted for by the Douglas–Kroll approximation. All ammonia complexes CuNH3, AgNH3, and AuNH3 are found to be of C3v symmetry with the counterpoise corrected interaction energies equal to −16.68, −6.87, and −14.64 mH for Cu, Ag, and Au, respectively. In the case of the water molecule the complexes are much weaker with the counterpoise corrected interaction energies equal to −3.78, −1.81, and −1.77 mH, for the three metal atoms, respectively. Moreover, all complexes with the water molecule are nonplanar. For both lone-pair donating molecules the structure and energetics of their complexes with the coinage metal atoms is mostly due to electron correlation effects. The relativistic effects are found to increase the bonding energies in the series of the ammonia complexes, whereas they reduce the bonding energy in the AgOH2 complex and are essentially negligible for CuOH2 and AuOH2. The calculated complex geometries and interaction energies are discussed in terms of different models. The pattern of interaction energies is discussed in terms of the balance between long-range induction and dispersion contributions and short-range forces. Also the possibility of some charge transfer from the lone-pair donor to the metal atom is considered and supported by analysis of the ionization potential and electron affinity data. The relativistic reduction of the size of the coinage metal atoms is found to be of importance as well. The calculated structural data are used to interpret the experimental observation concerning the existence of well resolved resonantly enhanced multiphoton ionization (REMPI) spectra of the ammonia–silver complexes and the absence of the corresponding spectra of the water–silver complex. This experimental difference between the ammonia and water complexes is explained in terms of the very flat interaction energy function for the wagging motion of the water molecule in AgOH2. This large amplitude vibration makes the structure of the complex undefined and is responsible for the nonexistence of the vibrationally resolved REMPI spectrum of the complex.

Characterization of dihydrogen-bonded D–H⋯H–A complexes on the basis of infrared and magnetic resonance spectroscopic parameters

The structural, energetic, and spectroscopic properties of the dihydrogen-bonded complexes LiH⋯H2, LiH⋯CH4, LiH⋯C2H6, and LiH⋯C2H2 are investigated. In particular, the interaction energy is decomposed into physically meaningful contributions, and the calculated vibrational frequencies, the magnetic resonance shielding constants, and inter- and intramolecular spin–spin coupling constants are analyzed in terms of their correlation with the interaction energy. Unlike the other three complexes, which can be classified as weak van der Waals complexes, the LiH⋯C2H2 complex resembles a conventional hydrogen-bonded system. The complexation-induced changes in the vibrational frequencies and in the magnetic resonance shielding constants correlate with the interaction energy, as does the reduced coupling JHX2h between the proton of LiH and hydrogen or carbon nucleus of the proton donor, while JHH1h do not correlate with the interaction energy. The calculations have been carried out using Møller–Plesset perturbation theory, coupled-cluster theory, and density-functional theory.