Properties of the halogen-hydride interaction: an ab initio and "atoms in molecules" analysis

X-Cl...H-Y interactions are analyzed by applying ab initio methods as well as the Bader theory. All calculations were performed using Pople's basis sets (6-311++G(2df,2pd) and 6-311++G(3df,3pd)) as well as the Dunning-type bases (aug-cc-pVDZ and aug-cc-pVTZ) within the MP2 method. For the complexes analyzed here, X-Cl and H-Y may be treated as a Lewis acid and a Lewis base, respectively. The Cl...H interactions are rather weak or at most moderate since, for the strongest interaction of the F3...HLi complex, the binding energy calculated at the MP2/6-311++G(3df,3pd) level of approximation amounts to -3.4 kcal/mol, and the H...Cl distance is equal to 2.65 A, less than the corresponding sum of van der Waals radii. These interactions may be classified as halogen-hydride interactions. However, some of the complexes analyzed, especially F3SiCl...HBeF and F3SiCl...HBeF, are very weakly bound, probably by typical van der Waals interactions.     

Time-dependent density functional theory calculation of van der Waals coefficient of sodium clusters

In this paper we employ all-electron ab initio time-dependent density functional theory based method to calculate the long range dipole-dipole dispersion coefficient (van der Waals coefficient) C(6) of sodium atom clusters containing even number of atoms ranging from 2 to 20 atoms. The dispersion coefficients are obtained via Casimir-Polder relation [Phys. Rev. 3, 360 (1948)]. The calculations are carried out with two different exchange-correlation potentials: (i) the asymptotically correct statistical average of orbital potential (SAOP) and (ii) Vosko-Wilk-Nusair representation [Can. J. Phys. 58, 1200 (1980)] of exchange-correlation potential within local density approximation. A comparison with the other theoretical results has been performed. We also present the results for the static polarizabilities of sodium clusters and also compare them with other theoretical and experimental results. These comparisons reveal that the SAOP results for C(6) and static polarizability are quite accurate and very close to the experimental results. We examine the relationship between volume of the cluster and van der Waals coefficient, and find that to a very high degree of correlation C(6) scales as the square of the volume. We also present the results for van der Waals coefficient corresponding to cluster-Ar atom and cluster-N(2) molecule interactions.     

Computational study on the ring distortions of 1,4-dicyanobenzene in the gas phase and in the crystal

A molecular mechanics software enhanced to perform empirical energy calculations on crystals (KESSHOU) was further developed to handle intermolecular electrostatic interactions as well. The packing of the molecules of 1,4-dicyanobenzene and 1,4-diisocyanobenzene in the crystal was studied. The role of the van der Waals and the electrostatic interactions in the balance of nonbonded atom-atom interactions is analyzed. The packing forces are dominated by van der Waals forces. The electrostatic interactions have higher stabilizing contribution for the dicyano isomer than for the diisocyano form. The dependence of the results on the size of the crystal, the molecular mechanics force field (MM2 vs MM3), and the dielectric constant are also assessed. Ab initio MP2/6–311G** geometries of the isolated molecules are in accordance with the observed benzene ring distortions determined by electron diffraction.     

Accurate description of van der Waals complexes by density functional theory including empirical corrections

An empirical method to account for van der Waals interactions in practical calculations with the density functional theory (termed DFT-D) is tested for a wide variety of molecular complexes. As in previous schemes, the dispersive energy is described by damped interatomic potentials of the form C6R(-6). The use of pure, gradient-corrected density functionals (BLYP and PBE), together with the resolution-of-the-identity (RI) approximation for the Coulomb operator, allows very efficient computations for large systems. Opposed to previous work, extended AO basis sets of polarized TZV or QZV quality are employed, which reduces the basis set superposition error to a negligible extend. By using a global scaling factor for the atomic C6 coefficients, the functional dependence of the results could be strongly reduced. The "double counting" of correlation effects for strongly bound complexes is found to be insignificant if steep damping functions are employed. The method is applied to a total of 29 complexes of atoms and small molecules (Ne, CH4, NH3, H2O, CH3F, N2, F2, formic acid, ethene, and ethine) with each other and with benzene, to benzene, naphthalene, pyrene, and coronene dimers, the naphthalene trimer, coronene. H2O and four H-bonded and stacked DNA base pairs (AT and GC). In almost all cases, very good agreement with reliable theoretical or experimental results for binding energies and intermolecular distances is obtained. For stacked aromatic systems and the important base pairs, the DFT-D-BLYP model seems to be even superior to standard MP2 treatments that systematically overbind. The good results obtained suggest the approach as a practical tool to describe the properties of many important van der Waals systems in chemistry. Furthermore, the DFT-D data may either be used to calibrate much simpler (e.g., force-field) potentials or the optimized structures can be used as input for more accurate ab initio calculations of the interaction energies.     

Two-step evaluation of binding energy and potential energy surface of van der Waals complexes

Evaluation of intermolecular distance and binding energy (BE) of van der Waals complex/cluster at ab initio level of theory is computationally demanding when many monomers are involved. Starting from MP2 energy, we reached a two-step evaluation method of BE of van der Waals complex/cluster through reasonable approximations; BE = BE(HF) + sum Mi> Mj{BE (Mi- Mj)(MP2 or MP2.5) - BE(Mi-Mj)(HF)} where HF represents the Hartree-Fock calculation, Mi, Mj, etc. are interacting monomers, and MP2.5 represents the arithmetic mean of MP2 and MP3. The first term is the usual BE of the complex/cluster evaluated at the HF level. The second term is the sum of the difference in two-body BE between the correlated and HF levels of theory. This equation was applied to various van der Waals complexes consisting of up-to-four monomers at MP2 and MP2.5 levels of theory. We found that this method is capable of providing precise estimate of the BE and reproducing well the potential energy surface of van der Waals complexes/clusters; the maximum error of the BE is less than 1 kcal/mol and 1% in most cases except for several limited cases. The origins of error in these cases are discussed in detail.     

A post-Hartree-Fock model of intermolecular interactions: inclusion of higher-order corrections

We have previously demonstrated that the dipole moment of the exchange hole can be used to derive intermolecular C(6) dispersion coefficients [J. Chem. Phys. 122, 154104 (2005)]. This was subsequently the basis for a novel post-Hartree-Fock model of intermolecular interactions [J. Chem. Phys. 123, 024101 (2005)]. In the present work, the model is extended to include higher-order dispersion coefficients C(8) and C(10). The extended model performs very well for prediction of intermonomer separations and binding energies of 45 van der Waals complexes. In particular, it performs twice as well as basis-set extrapolated MP2 theory for dispersion-bound complexes, with minimal computational cost.     

Evaluation of the individual hydrogen bonding energies in N-methylacetamide chains

The individual hydrogen bonding energies in N-methylacetamide chains were evaluated at the MP2/6-31+G** level including BSSE correction and at the B3LYP/6-311++G(3df,2pd) level including BSSE and van der Waals correction. The calculation results indicate that compared with MP2 results, B3LYP calculations without van der Waals correction underestimate the individual hydrogen bonding energies about 5.4 kJ mol^?1 for both the terminal and central hydrogen bonds, whereas B3LYP calculations with van der Waals correction produce almost the same individual hydrogen bonding energies as MP2 does for those terminal hydrogen bonds, but still underestimate the individual hydrogen bonding energies about 2.5 kJ mol^?1 for the hydrogen bonds near the center. Our calculation results show that the individual hydrogen bonding energy becomes more negative (more attractive) as the chain becomes longer and that the hydrogen bonds close to the interior of the chain are stronger than those near the ends. The weakest individual hydrogen bonding energy is about ?29.0 kJ mol^?1 found in the dimer, whereas with the growth of the N-methylacetamide chain the individual hydrogen bonding energy was estimated to be as large as ?62.5 kJ mol^?1 found in the N-methylacetamide decamer, showing that there is a significant hydrogen bond cooperative effect in N-methylacetamide chains. The natural bond orbital analysis indicates that a stronger hydrogen bond corresponds to a larger positive charge for the H atom and a larger negative charge for the O atom in the N-H?O=C bond, corresponds to a stronger second-order stabilization energy between the oxygen lone pair and the N-H antibonding orbital, and corresponds to more charge transfer between the hydrogen bonded donor and acceptor molecules.     

Metal-metal interactions in heterobimetallic d8-d10 complexes. Structures and spectroscopic investigation of [M'M' '(mu-dcpm)2(CN)2]+ (M' = Pt,Pd; M' ' = Cu,Ag, Au) and related complexes by UV-vis absorption and resonance Raman spectroscopy and ab initio calculations

X-ray structural and spectroscopic properties of a series of heterodinuclear d(8)-d(10) metal complexes [M'M' '(mu-dcpm)(2)(CN)(2)](+) containing d(8) Pt(II), Pd(II), or Ni(II) and d(10) Au(I), Ag(I), or Cu(I) ions with a dcpm bridging ligand have been studied (dcpm = bis(dicyclohexylphosphino)methane; M' = Pt, M' ' = Au 4, Ag 5, Cu, 6; M' ' = Au, M' = Pd 7, Ni 8). X-ray crystal analyses showed that the metal...metal distances in these heteronuclear metal complexes are shorter than the sum of van der Waals radii of the M' and M' ' atoms. The UV-vis absorption spectra of 4-6 display red-shifted intense absorption bands from the absorption spectra of the mononuclear trans-[Pt(phosphine)(2)(CN)(2)] and [M' '(phosphine)(2)](+) counterparts, attributable to metal-metal interactions. The resonance Raman spectra confirmed assignments of (1)[nd(sigma)-->(n + 1)p(sigma)] electronic transitions to the absorption bands at 317 and 331 nm in 4 and 6, respectively. The results of theoretical calculations at the MP2 level reveal an attractive interaction energy curve for the skewed [trans-Pt(PH(3))(2)(CN)(2)-Au(PH(3))(2)(+)] dimer. The interaction energy of Pt(II)-Au(I) was calculated to be ca. 0.45 ev.     

The Nature of the Binding of Au, Ag, and Pd to Benzene, Coronene, and Graphene: From Benchmark CCSD(T) Calculations to Plane-Wave DFT Calculations

The adsorption of Ag, Au, and Pd atoms on benzene, coronene, and graphene has been studied using post Hartree-Fock wave function theory (CCSD(T), MP2) and density functional theory (M06-2X, DFT-D3, PBE, vdW-DF) methods. The CCSD(T) benchmark binding energies for benzene-M (M = Pd, Au, Ag) complexes are 19.7, 4.2, and 2.3 kcal/mol, respectively. We found that the nature of binding of the three metals is different: While silver binds predominantly through dispersion interactions, the binding of palladium has a covalent character, and the binding of gold involves a subtle combination of charge transfer and dispersion interactions as well as relativistic effects. We demonstrate that the CCSD(T) benchmark binding energies for benzene-M complexes can be reproduced in plane-wave density functional theory calculations by including a fraction of the exact exchange and a nonempirical van der Waals correction (EE+vdW). Applying the EE+vdW method, we obtained binding energies for the graphene-M (M = Pd, Au, Ag) complexes of 17.4, 5.6, and 4.3 kcal/mol, respectively. The trends in binding energies found for the benzene-M complexes correspond to those in coronene and graphene complexes. DFT methods that use empirical corrections to account for the effects of vdW interactions significantly overestimate binding energies in some of the studied systems.     

C-H...N and C-H...S hydrogen bonds--influence of hybridization on their strength

Simple complexes connected through C-H...S and C-H...N interactions are investigated: CH4...NH3, C2H4...NH3, C2H2...NH3, CH4...SH2, C2H4...SH2, and C2H2...SH2. Ab initio and DFT calculations are performed (SCF, MP2, B3LYP) using different basis sets up to the MP2/aug-cc-pVQZ//MP2/aug-cc-pVDZ level of approximation. The Bader theory is applied since MP2/6-311++G(d,p) wave functions are used to find and to characterize bond critical points in terms of electron densities and their Laplacians. The influence of hybridization on the properties of C-H...S and C-H...N systems is also studied showing that the strength of such interactions increases in the following order: C(sp3)-H...Y, C(sp2)-H...Y, C(sp)-H...Y, where Y = S, N--it is in line with the previous findings on C-H...O hydrogen bonds. The results also show that CH4...SH2 and C2H4...SH2 complexes should be rather classified as van der Waals interactions and not as hydrogen bonds. The frequency associated with the C-H stretch of C(sp3)-H...S is blue-shifted.     

A polarizable dipole–dipole interaction model for evaluation of the interaction energies for NH···OC and CH···OC hydrogen‐bonded complexes

In this article, a polarizable dipole–dipole interaction model is established to estimate the equilibrium hydrogen bond distances and the interaction energies for hydrogen‐bonded complexes containing peptide amides and nucleic acid bases. We regard the chemical bonds N? H, C?O, and C? H as bond dipoles. The magnitude of the bond dipole moment varies according to its environment. We apply this polarizable dipole–dipole interaction model to a series of hydrogen‐bonded complexes containing the N? H···O?C and C? H···O?C hydrogen bonds, such as simple amide‐amide dimers, base‐base dimers, peptide‐base dimers, and β‐sheet models. We find that a simple two‐term function, only containing the permanent dipole–dipole interactions and the van der Waals interactions, can produce the equilibrium hydrogen bond distances compared favorably with those produced by the MP2/6‐31G(d) method, whereas the high‐quality counterpoise‐corrected (CP‐corrected) MP2/aug‐cc‐pVTZ interaction energies for the hydrogen‐bonded complexes can be well‐reproduced by a four‐term function which involves the permanent dipole–dipole interactions, the van der Waals interactions, the polarization contributions, and a corrected term. Based on the calculation results obtained from this polarizable dipole–dipole interaction model, the natures of the hydrogen bonding interactions in these hydrogen‐bonded complexes are further discussed. © 2013 Wiley Periodicals, Inc.     

Metallophilic attractions between d8-d10 heterometallic compounds trans-[Pt(PH3)2(CN)2] and M(PH3)2+ (M=Ag or Cu): Ab initio study

The weak metal-metal interactions of Pt(II)-Ag(I)/Cu(I) have been investigated by ab initio method at MP2 level through the model complexes [trans-Pt(PH3)2(CN)2-M(PH3)2+] (M=Ag,Cu). The calculated interaction energy of 12.9 and 11.5 kcal mol(-1) for [trans-Pt(PH3)2(CN)2-Ag(PH3)2+] and [trans-Pt(PH3)2(CN)2-Cu(PH3)2+] respectively, are in the middle of the van der Waals force and the strong hydrogen bond. The estimated equilibrium separations between Pt and M, r(eq)(Pt-M) (3.32 A for M=Ag and 3.23 A for M=Cu), lie within the region expected for weak metal-metal interaction. The electronic dispersive contributions dominate the weak interaction.     

Monte Carlo studies of isomers, structures, and properties in benzene-cyclohexane clusters: computation strategy and application to the dimer and trimer, (C6H6)(C6H12)n, n = 1-2

Low-temperature isomeric energies, structures, and properties of benzene-cyclohexane clusters are investigated via Monte Carlo simulations. The Monte Carlo strategy is first documented and then applied to (C(6)H(6))(C(6)H(12)) and (C(6)H(6))(C(6)H(12))(2) using four different potential energy surfaces. Results identify a single parallel-displaced dimer isomer. MP2 optimizations and frequency calculations support the Monte Carlo dimer structure and identify the van der Waals mode observed in vibronic spectra. Caloric simulations identify two temperatures where structural transitions occur and imply an experimental temperature below 10 K for dimers in cold supersonic expansions. The (C(6)H(6))(C(6)H(12))(2) studies identify eight independent trimer isomers: three form parallel-stacked (sandwich) arrangements with the two cyclohexane moieties related through a D(6)(h) transformation. The remaining five trimer isomers are trigonal, with no overall symmetry. Caloric studies indicate that the sandwich and trigonal isomeric classes coexist independently below 60 K, consistent with trimer vibronic spectra that contain two independent van der Waals progressions.     

Collisional energy-transfer processes involving van der waals bonds

Collisional energy transfer to van der Waals complexes is studied via trajectory calculations. Efficient build-up of energy in the van der Waals bond and its subsequent fragmentation is a result of the flow of energy from translation through the chemically bonded molecular unit. Despite such an efficient energy flow, migration of the vibrational energy initially present in the molecular unit into the van der Waals bond is not important. V-V energy transfer between the excited molecular unit and the incident molecule is very inefficient. O₂ is chosen for the model calculations.     

Experimental measurement of the van der Waals binding energy of X-O2 clusters (X=Xe, CH3I, C3H6, C6H12)

Van der Waals binding energies for the X-O(2) complexes (X=Xe, CH(3)I, C(3)H(6), C(6)H(12)) are determined by analysis of experimental velocity map imaging data for O((3)P(2)) atoms arising from UV-photodissociation of the complex [A. V. Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. Several dissociation pathways have been observed, we focus on the channel corresponding to prompt dissociation of X-O(2) into X+2O((3)P) fragments, which is present for complexes of O(2) with all partners X. Our method is based on analysis of the kinetic energy of all three photofragments, where the O atom kinetic energy was directly measured in the experiment and the kinetic energy of the X partner was calculated using momentum conservation, along with the measured angular anisotropy for O atom recoil. We exploit the fact that the clusters are all T-shaped or nearly T-shaped, which we also confirm by ab initio calculations, along with knowledge of the transition dipole governing radiative absorption by the complex. The effect of partitioning the kinetic energy between translation along the X-O(2) and O-O coordinates on the angular anisotropy of the O atom recoil direction is discussed. Van der Waals binding energies of 110±20 cm(-1), 280±20 cm(-1), 135±30 cm(-1), and 585±20 cm(-1) are determined for Xe-O(2), CH(3)I-O(2), C(3)H(6)-O(2), and C(6)H(12)-O(2) clusters, respectively.     

Higher order two- and three-body dispersion coefficients for alkali isoelectronic sequences by a variationally stable procedure

Using the variationally stable method of Gao and Starace, and the simple ground state wave function of the valence electron previously suggested by Patil and Tang, the multipolar polarizabilities of Li, Na, K, Rb, Cs, Be(+), Mg(+), Ca(+), Sr(+), Ba(+), the two-body dispersion coefficients of homonuclear and heteronuclear interactions from C(6) to C(40), as well as the three-body dispersion coefficients Z(L(1), L(2), L(3)) (up to L(i) = 5), are investigated. Higher order van der Waals dispersion coefficients C(n) (n > 24) and Z(L(1), L(2), L(3)) (L(i) > 3) are reported for the first time. Comparisons with previous calculations found in the literature show that this approach is capable of yielding precise and fast convergent values for higher order dispersion coefficients for alkali-metal atoms.     

Formation and dynamics of van der Waals molecules in buffer-gas traps

We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, (3)He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag(3)He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag(3)He, and report on our spectroscopic search for Ag(3)He, which produced a null result.