Spectroscopy of DABCO-rare-gas and DABCO-DABCO van der Waals complexes

The excited electronic origin bands of several DABCO containing van der Waals complexes have been observed via (1+1) resonance enhanced multi-photon ionization. Sharp resonances with widths of about 3 cm^–1 are seen for DABCO-Rg _n=1,2,3 (Rg is Ar, Kr or Xe), for the DABCO-DABCO dimer and for DABCO-DABCO-Ar. The origins of the rare-gas complexes are blue shifted with respect to the monomer origin. Broad features originating from DABCO-Rg _n complexes with highn, appear to higher energies than the complex origins, with widths of 120 cm^–1.     

IR predissociation of halogenated methane van der Waals complexes

The infrared dissociation spectra of several halogenated methanes (CHCl₂F, CH₃F, CF₃I and CF₃Br) have been studied. Bonded and free C–F stretch excitations have been observed.H-bridges forming the bonding in CHCl₂F or CH₃F dimers lead to very large red-shifts in the dissociation spectrum with respect to the gas-phase absorptions. Double resonance techniques have been employed to study the homogeneity of the spectral features.     

Microwave and ab initio studies of rare gas-methane van der Waals complexes

Rotational spectra of the weakly bound Kr-methane van der Waals complex were recorded using a pulsed molecular beam Fourier transform microwave spectrometer in the range from 3.5 to 18 GHz. Spectra of 25 isotopomers of Kr-methane were assigned and analyzed. For isotopomers containing CH4, 13CH4, and CD4, two sets of transitions with K = 0 and one with K = 1 were recorded, correlating to the j = 0, 1, and 2 rotational levels of free methane, respectively (j is the rotational angular momentum quantum number of the methane monomer). For isotopomers containing CH3D and CHD3, two K = 0 components were recorded, correlating to the j(k) = 0(0) and 1(1) rotational levels of free methane (k corresponds to the projection of j onto the C3 axis of CH3D and CHD3). The obtained spectroscopic results were used to derive van der Waals bond distance R, van der Waals stretching frequency nu(s), and the corresponding stretching force constant k(s). Nuclear spin statistical weights of individual states were obtained from molecular symmetry group analyses and were compared with the observed relative transition intensities. The tentatively assigned j = 2 transitions were more intense than predicted from symmetry considerations. This is attributed to a relatively large effective dipole moment of this state, supported by ab initio dipole moment calculations. Ab initio potential energy calculations of Kr-CH4 and Ar-CH4 were done at the coupled cluster level of theory, with single and double excitations and perturbative inclusion of triple excitations, using the aug-cc-pVTZ basis set supplemented with bond functions. The theoretical results show that the angular dynamics of the dimer does not change significantly when the binding partner of methane changes from Ar to Kr. The dipole moment of Ar-CH4 was calculated at various configurations, providing a qualitative explanation for the unsuccessful spectral searches for rotational transitions of Ar-CH4.     

Characteristics and relaxation dynamics of van der Waals complexes between p-difluorobenzene and Ne

Characteristics of the single and double Ne van der Waals complexes of p-difluorobenzene (pDFB) have been explored with ultraviolet fluorescence excitation and dispersed fluorescence spectroscopy. Eight S(1)-S(0) fluorescence excitation bands involving six ring modes of pDFB-Ne and two bands of pDFB-Ne(2) have been identified. Band assignments are confirmed by dispersed fluorescence from the pumped band. Shifts of the complex bands from the analogous monomer bands are generally 4 cm(-1) to the red for pDFB-Ne and 8 cm(-1) for pDFB-Ne(2). None of the observed ring modes is significantly perturbed by complexation in either the S(1) or S(0) states. The pDFB-Ne S(1) van der Waals binding energy D(0')相似文献   

The electronic spectroscopy and photophysics of tropolone and its van der Waals complexes

Studies of the electronic spectroscopy of tropolone in a variety of media are reviewed. Attempts to understand the effects of the surrounding medium on tropolone in its ground and first excited singlet states by studying the spectra and dynamics of its van der Waals complexes are described. The van der Waals complexes studied to date fall into two groups. Those which are primarily dispersively bound exhibit red microscopic solvent shifts, have observable tunneling doublet splittings and have structures in which the solvent species are bound above and below the plane of the chromophore in the 1∶1 and 1∶2 clusters. Those which are primarily hydrogen-bonded exhibit blue microscopic solvent shifts and exhibit no observable tunneling doublets.     

Rotational spectra of the van der Waals complexes of molecular hydrogen and OCS

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2-OCS, ortho-H2-OCS, HD-OCS, para-D2-OCS, and ortho-D2-OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2-OCS relative to ortho-H2-OCS increases exponentially with decreasing normal H2 component in H2He gas mixtures, making the observation of para-H2-OCS in the presence of the more strongly bound ortho-H2-OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2-OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2-OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD-OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2-OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2-OCS.     

Augmented van der Waals equations of state: SAFT-VR versus Yukawa based van der Waals equation

Performance of the SAFT-VR equation of state developed for the hard sphere based simple fluids, namely the square-well, Sutherland and Yukawa fluids, is examined by comparing its results with simulation data and an augmented van der Waals (vdW) equation based on a Yukawa (Y) reference. Its shown that both for the equilibrium vapor-liquid data and data along selected isotherms in the liquid and supercritical fluid phases the vdW(Y) equation provides better results, particularly when going to lower temperatures.     

Charge transfer and van der Waals states of jet-cooled dialkylaniline complexes with anthracene

We report here a study of the influence of a physical parameter (i.e. the ionization energy of different donor aromatic molecules) on the spectroscopic and dynamic properties of a series of molecular complexes A-D (A acceptor, D donor) where A is the anthracene molecule and D is one of the following dialkylanilines: dimethyl, diethyl, dipropyl or dibutyl. All complexes exhibit the same spectroscopic behavior already observed for dimethylaniline and diethylaniline cases and tentatively explained by the existence of two isomeric forms for each complex. Decay times and the broad band maximum frequency shifts for the exciplex-like emission type are experiencing a continuous variation in agreement with the change of the ionization energy of the donors. This is confirmed by calculations done using a very simple model based on the interactions between the diabatic A*D and A⁻D⁺ states of the complexes. This agreement is in favor of the initial assumption, that most physical parameters (complexation geometry, coupling between the diabatic states) are only weakly perturbed when changing the donor molecule and this despite an expected increased steric hindrance.     

Effect of ionic van der Waals forces on the diffuse potential of model colloids

In this work, the role of ionic dispersion forces in specific ion effects is evaluated through Monte Carlo simulations in the primitive model. More specifically, we assess the effect of such forces on the diffuse potential, since this property is essential to understand the electrokinetic behavior of colloids. In this way, ion specificity arises naturally since ion dispersion forces depend on ionic polarizability, which differs for ions with the same valence. This property is included in the primitive model by means of the Lifshitz theory. The results for different ions are summarized with the help of a nondimensional parameter characterizing the relative weight of ionic van der Waals interactions. Our data reveal that for small ions with high polarizability the ionic dispersion forces can considerably contribute to the specificity of the diffuse potential. In any case, the specific ion effects due to ion polarizability are strongly influenced by ion size.     

Thermodynamic scaling of the viscosity of van der Waals, H-bonded, and ionic liquids

Viscosities eta and their temperature T and volume V dependences are reported for seven molecular liquids and polymers. In combination with literature viscosity data for five other liquids, we show that the superpositioning of relaxation times for various glass-forming materials when expressed as a function of TV(gamma), where the exponent gamma is a material constant, can be extended to the viscosity. The latter is usually measured to higher temperatures than the corresponding relaxation times, demonstrating the validity of the thermodynamic scaling throughout the supercooled and higher T regimes. The value of gamma for a given liquid principally reflects the magnitude of the intermolecular forces (e.g., steepness of the repulsive potential); thus, we find decreasing gamma in going from van der Waals fluids to ionic liquids. For some strongly H-bonded materials, such as low molecular weight polypropylene glycol and water, the superpositioning fails, due to the nontrivial change of chemical structure (degree of H bonding) with thermodynamic conditions.     

Spectroscopy and dynamics of 9,10-dichloroanthracene-Arn van der Waals complexes

In this paper we report on the measurements of the absorption spectra of large van der Waals complexes in planar supersonic jets. The absorption spectra and the fluorescence excitation spectra of the complexes of 9,10-dichloroanthracene (DCA) with Ar atoms are reported for the S₀ → S₁(0) vibrationless transition of DCA·Ar_n (n = 1?6), and from the S₀ → S₁ (1390 cm^?1) transition of DCA·Ar_n (n = 1?4). Information on the structure of these complexes was inferred from the additivity of the spectral shifts per added rare-gas atom (ASSRA) for DCA·AR_n (n = 1.2) and from deviations from the ASSRA for CDA-Ar_n (n = 3?6). The vibrational predissociation (VP) dynamics of DCA-Ar_n (n = 1?3) complexes was interrogated by fluorescence quantum yield, Y, measurements. The value of the S₁ (1390 cm^?1) state of DCA·Ar_n (n = 1?3) exhibits a dramatic enhancement relative to that of DCA. Utilizing the dependence of Y on the excess vibrational energy of bare DCA, we were able to estimate the internal energy of the fragments resulting from VP of DCA·Ar_n of DCA·Ar_n (n = 1?3). An upper limit of ? 100 ps was estimated for the VP (and/or vibrational energy redistribution) lifetime from the S₁ (1390 cm^?1) state of DCA·Ar₃.     

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.     

Vibrational predissociation of van der Waals complexes: Quasi-classical results with Gaussian-weighted trajectories

A detailed application of the Gaussian-weighted trajectory method to the photodissociation of the RgBr₂ (Rg = He, Ne, Ar) van der Waals triatomics is presented. In agreement with previous applications on molecular collisions, the approach significantly enhances the quasi-classical predictions of product state distributions with respect to those obtained with the Standard Binning procedure, especially near a vibrational channel closing. The different molecules studied shed light on the sort of improvement to expect for various densities of vibrational quantum-states involved in the fragmentation process. Extension to larger polyatomic molecules, its possible difficulties and solutions are briefly sketched.     

van der Waals interactions across stratified media

Working at the Lifshitz level, we investigate the van der Waals interactions across a series of layers with a periodic motif. We derive the complete form of the van der Waals interaction as an explicit function of the number of periodic layers. We then compare our result with an approximation based on an anisotropic-continuum representation of the stratified medium. Satisfactory agreement between discrete-layer and continuum models is reached only for thicknesses of ten or more layers.     

Superlubricity using repulsive van der Waals forces

Using colloid probe atomic force microscopy, we show that if repulsive van der Waals forces exist between two surfaces prior to their contact then friction is essentially precluded and supersliding is achieved. The friction measurements presented here are of the same order as the lowest ever recorded friction coefficients in liquid, though they are achieved by a completely different approach. A gold sphere attached to an AFM cantilever is forced to interact with a smooth Teflon surface (templated on mica). In cyclohexane, a repulsive van der Waals force is observed that diverges at short separations. The friction coefficient associated with this system is on the order of 0.0003. When the refractive index of the liquid is changed, the force can be tuned from repulsive to attractive and adhesive. The friction coefficient increases as the Hamaker constant becomes more positive and the divergent repulsive force, which prevents solid-solid contact, gets switched off.