Van Der Waals Dimers Research Articles

Formation of Short Zn-Zn Bonds Stabilized by Simple Cyanide and Isocyanide Ligands.

Cyanogen diluted in argon was reacted with laser ablated Zn atoms to produce the NCZnCN and NCZnZnCN cyanides and higher energy isocyanides ZnNC, CNZnNC, and CNZnZnNC, which were isolated in excess argon at 4 K. These reaction products, identified from the matrix infrared spectra of their -CN and -NC chromophore ligand stretching modes, were confirmed by 13 C and 15 N isotopic substitution and comparison with frequencies calculated by the B3LYP and CCSD(T) methods using the all electron aug-cc-pVTZ basis sets. The cyanide and isocyanide products were increased markedly by mercury arc UV photolysis, which covers the zinc atomic absorption. The above electronic structure calculations that produce appropriate ligand frequencies for these dizinc products also provide their Zn-Zn bond lengths: CCSD(T) calculations find a short 2.367 Å Zn-Zn bond in the NCZnZnCN cyanide, a shorter 2.347 Å Zn-Zn bond in the 37.4 kJ mol-1 higher energy isocyanide CNZnZnNC, and a longer 4.024 Å bond in the dizinc van der Waals dimer. Thus, the diatomic cyanide (-CN) and isocyanide (-NC) ligands are as capable of stabilizing the Zn-Zn bond as many much larger ligands based on their measured and our calculated Zn-Zn bond lengths. This is the first example of dizinc complexes stabilized by different ligand isomers. Additional weaker bands in this region can be assigned to the analogous trizinc molecules NCZnZnZnCN and CNZnZnZnNC.

Formation of Short Zn−Zn Bonds Stabilized by Simple Cyanide and Isocyanide Ligands

AbstractCyanogen diluted in argon was reacted with laser ablated Zn atoms to produce the NCZnCN and NCZnZnCN cyanides and higher energy isocyanides ZnNC, CNZnNC, and CNZnZnNC, which were isolated in excess argon at 4 K. These reaction products, identified from the matrix infrared spectra of their ‐CN and ‐NC chromophore ligand stretching modes, were confirmed by 13C and 15N isotopic substitution and comparison with frequencies calculated by the B3LYP and CCSD(T) methods using the all electron aug‐cc‐pVTZ basis sets. The cyanide and isocyanide products were increased markedly by mercury arc UV photolysis, which covers the zinc atomic absorption. The above electronic structure calculations that produce appropriate ligand frequencies for these dizinc products also provide their Zn−Zn bond lengths: CCSD(T) calculations find a short 2.367 Å Zn−Zn bond in the NCZnZnCN cyanide, a shorter 2.347 Å Zn−Zn bond in the 37.4 kJ mol−1 higher energy isocyanide CNZnZnNC, and a longer 4.024 Å bond in the dizinc van der Waals dimer. Thus, the diatomic cyanide (‐CN) and isocyanide (‐NC) ligands are as capable of stabilizing the Zn−Zn bond as many much larger ligands based on their measured and our calculated Zn−Zn bond lengths. This is the first example of dizinc complexes stabilized by different ligand isomers. Additional weaker bands in this region can be assigned to the analogous trizinc molecules NCZnZnZnCN and CNZnZnZnNC.

Computational study of the ro-vibrational spectrum of CO-CO2.

An accurate ab initio ground-state intermolecular potential energy surface (PES) was determined for the CO-CO2 van der Waals dimer. The Lanczos algorithm was used to compute rovibrational energies on this PES. For both the C-in and O-in T-shaped isomers, the fundamental transition frequencies agree well with previous experimental results. We confirm that the in-plane states previously observed are geared states. In addition, we have computed and assigned many other vibrational states. The rotational constants we determine from J = 1 energy levels agree well with their experimental counterparts. Planar and out-of-plane cuts of some of the wavefunctions we compute are quite different, indicating strong coupling between the bend and torsional modes. Because the stable isomers are T-shaped, vibration along the out-of-plane coordinates is very floppy. In CO-CO2, when the molecule is out-of-plane, interconversion of the isomers is possible, but the barrier height is higher than the in-plane geared barrier height.

Revealing the origin of excimer emission in anthracene crystals: The role of excitation wavelength and pressure

Understanding the formation mechanism of excimer emission in aromatic molecules is the fundamental to several areas of photochemistry, including photo-induced dimerization. In this study, we observed the formation process of excimer emission in anthracene crystals at normal pressure by varying excitation wavelength. It was found that the excimer emission in anthracene crystals was originated from photo-induced van der Waals (vdW) dimers of anthracene molecules. The formation and annihilation processes of the excimer emission were characterized during the pressure loading and unloading processes, which were assigned to the pressure-induced vdW dimers formation and annihilation. Note that the fluorescence spectrum of anthracene crystals was dominated by excimer emission at 5.88 GPa, the excitation wavelength dependence of excimer emission can be obtained. The observed frequency shift confirmed the role of excitation wavelength in fluorescence process. These findings have significant implications for understanding the origin of the excimer emission and open novel perspectives in the fluorescence research of aromatic molecules.

Explicitly correlated potential energy surface of the CO2-CO van der Waals dimer and applications.

The four-dimensional-potential energy surface (4D-PES) of the CO2-CO van der Waals complex is generated using the explicitly correlated coupled cluster with single, double, and perturbative triple excitation (CCSD(T)-F12) method in conjunction with the augmented correlation-consistent triple zeta (aug-cc-pVTZ) basis set. This 4D-PES is developed over the set of inter-molecular coordinates and where the CO2 and CO monomers are treated as rigid rotors. Afterwards, analytic fits of this 4D-PES are carried out. In addition to the already known C-bound and O-bound stable structures of CO2-CO, we characterise a new isomer: it has a T-shaped structure where the O atom of the CO2 moiety points into the centre of mass of CO. We also find the saddle points connecting these minimal structures. This new isomer may play a role during the intramolecular isomerization processes at low energies. Then, the 4D-PES expansion is incorporated into bound vibrational state computations of C-bound and O-bound complexes. We also computed the temperature dependence of the second virial coefficient for CO2-CO. A good agreement with experiments is found.

Formation of [(CnC1imTe)4Hg]2+ (n = 6, 8) upon in‐situ Generation of Dialkylimidazole‐2‐Tellurones in Ionic Liquids at Room Temperature

The synthesis of a homoleptic HgII complexes with in situ‐formed N‐heterocyclic carbene telluride ligands is reported. Tetrakis(1‐hexyl‐3‐methylimidazole‐2‐tellurone) mercury(II), [(C6C1imTe)4Hg]2+ (1), and tetrakis(1‐octyl‐3‐methylimidazole‐2‐tellurone) mercury(II), [(C8C1imTe)4Hg]2+ (2), emerge from reaction mixtures containing telluridomercurate anions and potassium cations in imidazolium‐based ionic liquids (CnC1Im)[BF4] (n = 6, 8), if these are exposed to ambient conditions, that is, air, moisture and room temperature. The compounds crystallize with two of the ionic liquid cations each, and form van der Waals dimers via Te···Te interactions in the crystal. Although being coordinated in a homoleptic manner, the orientation of the imidazolium ligands is such that two of them undergo π‐stacking interactions, while the others interact with neighboring units via their alkyl chains.

- stacking vs. C-H/ interaction: Excimer formation and charge resonance stabilization in van der Waals clusters of 9,9'-dimethylfluorene.

Studies of exciton and hole stabilization in multichromophoric systems underpin our understanding of electron transfer and transport in materials and biomolecules. The simplest model systems are dimeric, and recently we compared the gas-phase spectroscopy and dynamics of van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9'-dimethylfluorene (F1) to assess how sterically controlled facial encumbrance modulates the dynamics of excimer formation and charge resonance stabilization (CRS). Dimers of fluorene and MF show only excimer emission upon electronic excitation, and significant CRS as evidenced in a reduced ionization potential for the dimer relative the monomer. By contrast, the dimer of F1 shows no excimeric emission, rather structured emission from the locally excited state of a tilted (non π-stacked) dimer, evidencing the importance of C-H/π interactions and increased steric constraints that restrict a cofacial approach. In this work, we report our full results on van der Waals clusters of F1, using a combination of theory and experiments that include laser-induced fluorescence, mass-selected two-color resonant two-photon ionization spectroscopy, and two-color appearance potential measurements. We use the latter to derive the binding energies of the F1 dimer in ground, excited, and cation radical states. Our results are compared with van der Waals and covalently linked clusters of fluorene to assess both the relative strength of π-stacking and C-H/π interactions in polyaromatic assemblies and the role of π-stacking in excimer formation and CRS.

Coulomb over-the-barrier Monte Carlo simulation to probe ion-dimer collision dynamics

We present a combined theoretical and experimental study of primary and post-collision mechanisms involved when colliding low energy multiply charged ions with van der Waals dimers. The collision dynamics is investigated using a classical calculation based on the Coulombic Over-the- Barrier Model adapted to rare-gas dimer targets. Despite its simplicity, the model predictions are found in very good agreement with experimental results obtained using COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS), both for the relative yields of the different relaxation processes and for the associated transverse momentum exchange distributions between the projectile and the target. This agreement shows to which extent van der Waals dimers can be assimilated to independent atoms.

Electronic structure and spectra of the RbHe van der Waals system including spin orbit interaction

The potential energy interaction, the spectroscopic properties and dipole functions of the RbHe van der Waals dimer have been investigated. We used a one-electron pseudopotential approach and large Gaussian basis sets to represent the two atoms Rb and He. The Rb+ core and the electron-He interactions were replaced by semi-local pseudopotentials and a core-core interaction is included. Therefore, the number of active electrons of RbHe is reduced to only one electron. Consequently, the potential energy curves and dipole moments for many electronic states dissociating into Rb(5s,5p,4d,6s,6p,5d,7s)+He are performed at the SCF level. In addition, the spin-orbit coupling is included in the calculation. The Rb+He interaction, in its ground state, is taken from accurate CCSD (T) calculations and fitted to an analytical expression for a better description of the potential in all internuclear ranges. The spectroscopic properties of the RbHe electronic states are extracted. The comparison of these constants has shown a very good agreement for the ground state as well as for the lower excited states when compared with existing theoretical and experimental studies.

Effect of Facial Encumbrance on Excimer Formation and Charge Resonance Stabilization in Model Bichromophoric Assemblies

Excimer formation and charge resonance stabilization in covalently linked bichromophoric systems with flexible spacers are important processes relevant to biochemistry and functional materials. Requiring a π-stacked cofacial arrangement of a pair of aromatic molecules at a van der Waals contact, the underlying geometrical reorganization that accompanies these events continues to be debated. Here we use a variety of methods including two-color resonant two-photon ionization spectroscopy (2CR2PI), ion yield measurements, hole-burning spectroscopy (HB), and laser-induced fluorescence (LIF) excitation and emission spectroscopy to compare the gas-phase spectroscopy and dynamics of the van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9′-dimethylfluorene (F1). The goal of this work is to probe the influence of methyl substitution on the dynamics of excimer formation and charge resonance (CR) stabilization. The fluorene dimer, (F)2, displays lifetime broadened electronic spectra and the dominance of...

Interatomic potentials of van der Waals dimers Hg2 and Cd2: Probing discrepancies between theory and experiment

Results of new all-electron ab initio calculations and revisit of experimental studies of the interatomic potentials of lower-lying ungerade excited and ground electronic energy states of the Hg2 and Cd2 van der Waals complexes are used as probes of discrepancies between theory and experiment. From simulations of the previously and presently measured LIF excitation and dispersed emission spectra new analytical representations of the excited- and the ground-state interatomic potentials are proposed. An inverted perturbation approach was also used to improve the studied interatomic potentials. The comparison of the new ab-initio calculated potentials with the results of the analyses illustrates an improve theory-to-experiment agreement for such a demanding system like Hg2 or Cd2.

Five intermolecular vibrations of the CO2 dimer observed via infrared combination bands.

The weakly bound van der Waals dimer (CO2)2 has long been of considerable theoretical and experimental interest. Here, we study its low frequency intermolecular vibrations by means of combination bands in the region of the CO2 monomer ν3 fundamental (≈2350 cm-1), which are observed using a tunable infrared laser to probe a pulsed supersonic slit jet expansion. With the help of a recent high level ab initio calculation by Wang, Carrington, and Dawes, four intermolecular frequencies are assigned: the in-plane disrotatory bend (22.26 cm-1); the out-of-plane torsion (23.24 cm-1); twice the disrotatory bend (31.51 cm-1); and the in-plane conrotatory bend (92.25 cm-1). The disrotatory bend and torsion, separated by only 0.98 cm-1, are strongly mixed by Coriolis interactions. The disrotatory bend overtone is well behaved, but the conrotatory bend is highly perturbed and could not be well fitted. The latter perturbations could be due to tunneling effects, which have not previously been observed experimentally for CO2 dimer. A fifth combination band, located 1.3 cm-1 below the conrotatory bend, remains unassigned.

Radical Cations of the Monomer and van der Waals Dimer of a Methionine Residue as Prototypes of (2 Center-3 Electron) SN and SS Bonds. Molecular Simulations of Their Absorption Spectra in Water.

Oxidation of peptides or proteins by the OH(•) radicals produced by pulse radiolysis yields species identified by their absorption spectra in the UV-visible domain. However, the case of methionine (Met) in peptides is complex because its oxidation can lead to various free radicals with 2 center-3 electron (2c-3e) bonds. We have performed Monte Carlo/density functional theory molecular simulations of the radical cation of the methylated methionine aminoacid, Met(•+), taken as a model of the methonine residue of peptides, and of the radical cation of its van der Waals dimer, Met2(•+). The cation of the methionine residue displays a 2c-3e SN bond. The cation of dimer Met2(•+) displays three quasidegenerate conformers, one stabilized by a 2c-3e SS bond and the other two stabilized by ion-molecule interactions and made up of a neutral and a cationic unit. These conformers are characterized by their charge and spin density localization and their UV-visible absorption spectra. These spectra enable a discussion of the absorption spectra of the literature; in particular, we emphasize the role of dimers before and after the oxidation process.

First Experimental Evidence for the Diverse Requirements of Excimer vs Hole Stabilization in π-Stacked Assemblies

Exciton formation and charge separation and transport are key dynamical events in a variety of functional polymeric materials and biological systems, including DNA. Beyond the necessary cofacial approach of a pair of aromatic molecules at van der Waals contact, the extent of overlap and necessary geometrical reorganization for optimal stabilization of an excimer vs dimer cation radical remain unresolved. Here, we compare experimentally the dynamics of excimer formation (via emission) and charge stabilization (via threshold ionization) of a novel covalently linked, cofacially stacked fluorene dimer (F2) with the unlinked van der Waals dimer of fluorene, that is, (F)2. Although the measured ionization potentials are identical, the excimeric state is stabilized by up to ∼30 kJ/mol in covalently linked F2. Supported by theory, this work demonstrates for the first time experimentally that optimal stabilization of an excimer requires a perfect sandwich-like geometry with maximal overlap, whereas hole stabilization in π-stacked aggregates is less geometrically restrictive.

Anchoring the potential energy surface of an important atmospheric van der Waals dimer, the H2O⋯O2 complex

The focus of this paper is the weakly-bound H2O⋯O2 van der Waals dimer. The global minimum has an electronic dissociation energy of 0.70kcalmol−1 at the spin unrestricted CCSD(T) level of theory (UCCSD(T)) in conjunction with a correlation consistent quadruple-zeta basis set augmented with diffuse functions on all heavy atoms, denoted haQZ (i.e., cc-pVQZ for H and aug-cc-pVQZ for O). Eleven other stationary points are also characterized at the UCCSD(T)/haQZ level of theory, all of which lie within 0.5kcalmol−1 of the global minimum and have at least one imaginary frequency (ni⩾1). The spin unrestricted MP2 method (UMP2) yields very similar structures and energies for the twelve stationary points. However, the UMP2 vibrational frequencies provide a qualitatively different description of the potential energy surface (more than a single minimum), of the frequency shifts upon complexation and of the resulting infrared intensities of the complex. The UCCSD(T)/haQZ computations indicate only one of the twelve H2O⋯O2 structures is a minimum and that it exhibits appreciable infrared intensity for one of the lowest energy intermolecular vib rational modes, unlike the related H2O⋯N2 system.

Small and efficient basis sets for the evaluation of accurate interaction energies: aromatic molecule-argon ground-state intermolecular potentials and rovibrational states.

By evaluating a representative set of CCSD(T) ground state interaction energies for van der Waals dimers formed by aromatic molecules and the argon atom, we test the performance of the polarized basis sets of Sadlej et al. (J. Comput. Chem. 2005, 26, 145; Collect. Czech. Chem. Commun. 1988, 53, 1995) and the augmented polarization-consistent bases of Jensen (J. Chem. Phys. 2002, 117, 9234) in providing accurate intermolecular potentials for the benzene-, naphthalene-, and anthracene-argon complexes. The basis sets are extended by addition of midbond functions. As reference we consider CCSD(T) results obtained with Dunning's bases. For the benzene complex a systematic basis set study resulted in the selection of the (Z)Pol-33211 and the aug-pc-1-33321 bases to obtain the intermolecular potential energy surface. The interaction energy values and the shape of the CCSD(T)/(Z)Pol-33211 calculated potential are very close to the best available CCSD(T)/aug-cc-pVTZ-33211 potential with the former basis set being considerably smaller. The corresponding differences for the CCSD(T)/aug-pc-1-33321 potential are larger. In the case of the naphthalene-argon complex, following a similar study, we selected the (Z)Pol-3322 and aug-pc-1-333221 bases. The potentials show four symmetric absolute minima with energies of -483.2 cm(-1) for the (Z)Pol-3322 and -486.7 cm(-1) for the aug-pc-1-333221 basis set. To further check the performance of the selected basis sets, we evaluate intermolecular bound states of the complexes. The differences between calculated vibrational levels using the CCSD(T)/(Z)Pol-33211 and CCSD(T)/aug-cc-pVTZ-33211 benzene-argon potentials are small and for the lowest energy levels do not exceed 0.70 cm(-1). Such differences are substantially larger for the CCSD(T)/aug-pc-1-33321 calculated potential. For naphthalene-argon, bound state calculations demonstrate that the (Z)Pol-3322 and aug-pc-1-333221 potentials are of similar quality. The results show that these surfaces differ substantially from the available MP2/aug-cc-pVDZ potential. For the anthracene-argon complex it proved advantageous to calculate interaction energies by using the (Z)Pol and the aug-pc-1 basis sets, and we expect it to be increasingly so for complexes containing larger aromatic molecules.

A Parametrized Coupled-Pair Functional for Molecular Interactions: PCPF-MI.

We present a parametrized coupled-pair functional, optimized to describe molecular interactions (PCPF-MI). The method has the same computational cost as singles and doubles configuration interaction, is size extensive, and yields energies that are stationary with respect to variations in all of its excitation coefficients. This last property facilitates the construction of density matrices and the evaluation of one- and two-electron properties. For the S22, HSG, S66, and A24 databases of van der Waals dimers, PCPF-MI computations yield interaction energies with mean unsigned errors of 0.326, 0.149, 0.214, and 0.044 kcal mol(-1), respectively, relative to benchmark computations. PCPF-MI interaction energies consistently improve upon those obtained from several other coupled-pair methods, including the averaged coupled-pair functional (ACPF), the coupled electron-pair approximation [CEPA(n), n = 0,1,3], and averaged quadratic coupled cluster (AQCC). Optimal parameters for spin-component-scaled (SCS) variants of each of these methods are also presented. SCS-CEPA(0), SCS-CEPA(1), SCS-ACPF, and SCS-PCPF-MI all perform similarly, with average errors for the four databases of roughly 0.1 kcal mol(-1). The PCPF-MI method, without additional SCS parametrization, has an average error of 0.192 kcal mol(-1) over the four databases and, when compared to the SCS-parametrized coupled-pair methods, has the added benefit that the energy is stationary with respect to variations in all its excitation coefficients.

Microwave Spectroscopic Investigation and Structural Determination of the Ar–Difluoropyridine van der Waals Complexes

The ground state rotational spectra of the van der Waals dimers of 2,3-, 2,4-, 2,5-, 2,6-, and 3,5-difluoropyridine with argon were investigated in the range of 4 to 26 GHz using both Fourier transform microwave (FTMW) and chirped pulse Fourier transform microwave (cp-FTMW) spectroscopy. The spectroscopic parameters derived from fitting the observed transitions were used to estimate the van der Waals coordinates of the five weakly bound complexes and to quantitatively describe their internal dynamics using a pseudodiatomic model. The experimental structures are in fair agreement with MP2 calculations (6-311++G(2d, 2p)) and have the centers of mass of the two moieties separated by 3.486 to 3.545 Å with the Ar atom lying 2-10° from the c' axis of the ring monomer.

Chemical Reactions Triggered Using Electrons Photodetached from "Clean" Distributions of Anions Deposited in Cryogenic Matrices via Counterion Codeposition.

Application of matrix isolation spectroscopy to ionic species is typically complicated by the presence of neutral contaminants during matrix deposition. Herein we demonstrate that simultaneous deposition of balanced currents of counterions with mass-selected ions of interest generates "clean" distributions of matrix-isolated metal carbonyl anions, where the only bands appearing in the CO-stretching region of the vibrational spectrum arise from ions. (Neutrals are initially absent.) Photodetachment by mild irradiation with visible light leads to complete conversion of the anions into their corresponding neutral species. The photodetached electrons, in turn, initiate covalent chemistry, inducing C-C bond formation following electron-capture by CO van der Waals dimers to produce trans-OCCO(-). The initial clean distribution of ions enables clear connections to be drawn between the spectral changes occurring at each experimental step, thus demonstrating the potential of the counterion codeposition technique to facilitate detailed studies of chemistry involving ions and electron transfer in cryogenic matrices.

Dispersion-correcting potentials can significantly improve the bond dissociation enthalpies and noncovalent binding energies predicted by density-functional theory.

Dispersion-correcting potentials (DCPs) are atom-centered Gaussian functions that are applied in a manner that is similar to effective core potentials. Previous work on DCPs has focussed on their use as a simple means of improving the ability of conventional density-functional theory methods to predict the binding energies of noncovalently bonded molecular dimers. We show in this work that DCPs developed for use with the LC-ωPBE functional along with 6-31+G(2d,2p) basis sets are capable of simultaneously improving predicted noncovalent binding energies of van der Waals dimer complexes and covalent bond dissociation enthalpies in molecules. Specifically, the DCPs developed herein for the C, H, N, and O atoms provide binding energies for a set of 66 noncovalently bonded molecular dimers (the "S66" set) with a mean absolute error (MAE) of 0.21 kcal/mol, which represents an improvement of more than a factor of 10 over unadorned LC-ωPBE/6-31+G(2d,2p) and almost a factor of two improvement over LC-ωPBE/6-31+G(2d,2p) used in conjunction with the "D3" pairwise dispersion energy corrections. In addition, the DCPs reduce the MAE of calculated X-H and X-Y (X,Y = C, H, N, O) bond dissociation enthalpies for a set of 40 species from 3.2 kcal/mol obtained with unadorned LC-ωPBE/6-31+G(2d,2p) to 1.6 kcal/mol. Our findings demonstrate that broad improvements to the performance of DFT methods may be achievable through the use of DCPs.