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   

Experimental detection and theoretical characterization of the H2-NHX van der Waals complex

The H2-NH(X) van der Waals complex has been examined using ab initio theory and detected via fluorescence excitation spectroscopy of the A(3)Pi-X(3)Sigma(-) transition. Electronic structure calculations show that the minimum energy geometry corresponds to collinear H2-NH(X), with a well depth of D(e)=116 cm(-1). The potential-energy surface supports a secondary minimum for a T-shaped geometry, where the H atom of NH points towards the middle of the H2 bond (C(2v) point group). For this geometry the well depth is 73 cm(-1). The laser excitation spectra for the complex show transitions to the H2+NH(A) dissociative continuum. The onset of the continuum establishes a binding energy of D(0)=32+/-2 cm(-1) for H2-NH(X). The fluorescence from bound levels of H2-NH(A) was not detected, most probably due to the rapid reactive decay [H2-NH(A)-->H+NH2]. The complex appears to be a promising candidate for studies of the photoinitiated H2+NH abstraction reaction under conditions were the reactants are prealigned by the van der Waals forces.     

Spectroscopic study of the Ne-Xe-NH3 van der Waals trimer

Rotational spectra of the Ne-Xe-NH3 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Both a- and b-type transitions of eight isotopologues, namely 20Ne-132Xe-14NH3, 20Ne-129Xe-14NH3, 20Ne-132Xe-15NH3, 20Ne-129Xe-15NH3, 20Ne-131Xe-15NH3, 22Ne-132Xe-15NH3, 22Ne-129Xe-15NH3, and 22Ne-131Xe-15NH3 were measured and assigned. Nuclear quadrupole hyperfine structures arising from the 14N (nuclear spin quantum number I = 1) and 131Xe (I = 3/2) nuclei were detected and analyzed. The determined rotational constants were used to fit structural parameters. A harmonic force field analysis was performed based on centrifugal distortion constants to extract information about vibrational motions of the complex. A comparison of van der Waals bond lengths and stretching force constants between the Ne-Xe-NH3 trimer and the corresponding dimers indicates that non-additive three-body effects are present in the trimer system. Analyses of the 14N and 131Xe nuclear quadrupole coupling constants suggest that the NH3 unit undergoes nearly free internal rotation within the complex and that the presence of Ne has little effect on the orientation of NH3 with respect to the Xe atom.     

Rotational spectra of the Xe-(H2O)2 van der Waals trimer: xenon as a probe of electronic structure and dynamics

Rotational spectra of three isotopomers of the Xe-(H2O)2 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Nine [nine, four] a-type and twelve [eleven, seven] b-type transitions were measured for the 132Xe-(H2O)2 [129Xe-(H2O)2, 131Xe-(H2O)2] isotopomer. The determined rotational and centrifugal distortion constants were used to extract information about the structure and vibrational motions of the complex. The nuclear quadrupole hyperfine structures due to the 131Xe (nuclear spin quantum number I=3/2) nucleus were also detected. The large value of the off-diagonal nuclear quadrupole coupling constant chiab in particular provides detailed insight into the electronic environment of the xenon atom and the orientations of the water molecules within the complex. An effective structure that best reproduces the experimental 131Xe nuclear quadrupole coupling constants is rationalized by ab initio calculations. An overall goal of this line of work is to determine how the successive solvation of a xenon atom with water molecules affects the xenon electron distribution and its intermolecular interactions. The results may provide molecular level interpretations of 129Xe NMR data from, for example, imaging experiments.     

Potential-energy surface and van der Waals motions of p-difluorobenzene-argon cation

The structure and dynamics of the van der Waals complex of argon with the p-difluorobenzene cation are investigated using the ab initio theory. The restricted open-shell M?ller-Plesset second-order perturbation method combined with the augmented correlation-consistent polarized valence double-zeta basis set is employed to determine the electronic ground-state potential-energy surface of the cationic complex. This surface is extremely flat in a wide region of the configuration space of the Ar atom which moves almost freely over the monomer ring. However, it is bound to the monomer stronger in the cationic than in the neutral complex. Its binding energy is calculated to be 621 cm(-1) at a distance of 3.445 A from the monomer center. The calculated dissociation energy of 572 cm(-1) agrees perfectly with the experimental value of 572+/-6 cm(-1) [S. M. Belm, R. J. Moulds, and D. Lawrence, J. Chem. Phys. 115, 10709 (2001)]. The effect of a strong coupling of large-amplitude intermolecular motions on the character of van der Waals vibrational states is investigated. The vibrational structure of the spectrum of the complex is explained and its earlier assignment is partly corrected.     

Two-dimensional laser induced fluorescence spectroscopy of van der Waals complexes: fluorobenzene-Ar(n) (n = 1,2)

The technique of two-dimensional laser induced fluorescence (2D-LIF) spectroscopy has been used to observe the van der Waals complexes fluorobenzene-Ar and fluorobenzene-Ar(2) in the region of their S(1)-S(0) electronic origins. The 2D-LIF spectral images reveal a number of features assigned to the van der Waals vibrations in S(0) and S(1). An advantage of 2D-LIF spectroscopy is that the LIF spectrum associated with a particular species may be extracted from an image. This is illustrated for fluorobenzene-Ar. The S(1) van der Waals modes observed in this spectrum are consistent with previous observations using mass resolved resonance enhanced multiphoton ionisation techniques. For S(0), the two bending modes previously observed using a Raman technique were observed along with three new levels. These agree exceptionally well with ab initio calculations. The Fermi resonance between the stretch and bend overtone has been analysed in both the S(0) and S(1) states, revealing that the coupling is stronger in S(0) than in S(1). For fluorobenzene-Ar(2) the 2D-LIF spectral image reveals the S(0) symmetric stretch van der Waals vibration to be 35.0 cm(-1), closely matching the value predicted based on the fluorobenzene-Ar van der Waals stretch frequency. Rotational band contour analysis has been performed on the fluorobenzene-Ar 0(0)(0) transition to yield a set of S(1) rotational constants A' = 0.05871 ± 0.00014 cm(-1), B' = 0.03803 ± 0.00010 cm(-1), and C' = 0.03103 ± 0.00003 cm(-1). The rotational constants imply that in the S(1) 0(0) level the Ar is on average 3.488 ? from the fluorobenzene centre of mass and displaced from it towards the centre of the ring at an angle of ~6° to the normal. The rotational contour for fluorobenzene-Ar(2) was predicted using rotational constants calculated on the basis of the fluorobenzene-Ar geometry and compared with the experimental contour. The comparison is poor which, while due in part to expected saturation effects, suggests the presence of another band lying beneath the contour.     

Electronic spectroscopy of toluene-rare-gas clusters: the external heavy atom effect and vibrational predissociation

Toluene-X van der Waals clusters (where X = Ne, Ne2, Ar, Ar2, Kr, Xe) have been investigated by fluorescence excitation spectroscopy in the region of the S1-S0 transition. With the exception of Xe, for each rare-gas studied, we have assigned cluster transitions in the region of all the strong monomer vibrational bands up to 1000 cm(-1) above the origin band. We have further investigated the S1 relaxation dynamics for each vibrational level of each complex, via their fluorescence decay profiles. Clustering with neon has little appreciable effect on the vibrationless S1 lifetime. By contrast, the clusters with argon and krypton exhibit markedly shorter fluorescence lifetimes compared with the monomer. The effect is so severe in the case of toluene-Xe clusters that no fluorescence signals were observed. We interpret these results in terms of an external heavy atom effect in which the rate of intersystem crossing in toluene is influenced by the cluster partner. For clusters built upon excited S1 vibrational levels, the situation is potentially complicated by intramolecular vibrational redistribution and vibrational predissociation (VP). The majority of the fluorescence decay profiles were satisfactorily modeled using single exponential decays. The emission following pumping of the 37(1) level in the toluene-Kr cluster, however, is an exception. We have modeled the decay of this level with a simple kinetic scheme including VP and determined a predissociation rate of (1.04 +/- 0.54) x 10(7) s(-1).     

High resolution electronic spectra of 7-azaindole and its Ar atom van der Waals complex

Rotationally resolved fluorescence excitation spectra of the S(1)<--S(0) origin band of 7-azaindole [1H-pyrrolo(2,3-b)pyridine] and its argon atom van der Waals complex have been recorded and assigned. The derived rotational constants give information about the geometries of the two molecules in both electronic states. The equilibrium position of the argon atom in the azaindole complex is considerably different from its position in the corresponding indole complex. Furthermore, the argon atom moves when the UV photon is absorbed. There are significant differences in the intermolecular potential energy surfaces in the two electronic states. A large, vibration-state-dependent rotation of the S(1)<--S(0) electronic transition moment vector of 7-azaindole relative to that of indole suggests that these differences have their origin in S(1)/S(2) electronic state mixing in the isolated molecule, a mixing that is enhanced by nitrogen substitution in the six-membered ring.     

Infrared spectrum of ArHD: Simulation through the accurate calculation of photodissociation cross sections

The absolute values of the photodissociation cross sections and lineshapes for 29 sets of predissociative transitions of the ArHD van der Waals complex have been computed. These lineshapes are summed with the correct statistical weighting and compared with the experimentally observed spectrum of McKellar.     

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.     

The structure of phosphacymantrene

Proton NMR spectra of phosphacymantrene (π-phospholyl manganese tricarbonyl) orientated in the nematic phases of liquid crystals have been investigated. The derived H-H and H-P direct dipolar coupling constants have been used to determine the relative proton-proton and proton-phosphorus distances. A comparison of the geometrical data of various 5-membered aromatic heterocycles shows that the relative distances between the protons closest to the heteroatom increase with the van der Waals radius of the heteroatom. The results suggest that NMR spectroscopy of orientated molecules can be used to determine van der Waals radii.     

Van der Waals surface graphs and molecular shape

A van der Waals surface graph is the graph defined on a van der Waals surface by the intersections of the atomic van der Waals spheres. A van der Waals shape graph has a vertex for each atom with a visible face on the van der Waals surface, and edges between vertices representing atoms with adjacent faces on the van der Waals surface. These are discrete invariants of three‐dimensional molecular shape. Some basic properties of van der Waals surface graphs are studied, including their relationship with the Voronoi diagram of the atom centres, and a class of molecular embeddings is identified for which the dual of the van der Waals surface graph coincides with the van der Waals shape graph. This revised version was published online in July 2006 with corrections to the Cover Date.     

An ab initio multireference doubles excitation configuration interaction study of low-lying electronic states of Cd2 using Slater-type orbitals

Potential-energy curves for the ground state and lower excited states of the Cd₂ dimer have been calculated. They are obtained using a multireference doubles excitation configuration interaction procedure and employing Slater basis sets, previously optimized at the self-consistent-field level for excited states of the Cd atom. The spectroscopic constants and excitation energies for the bound states of Cd₂ have been compared with experimental as well as other theoretical results. The ground state of Cd₂ is essentially repulsive and presents a shallow van der Waals minimum. The computed adiabatic electronic transitions are in good agreement with the experimental ones. Received: 16 September 1999 / Accepted: 3 February 2000 / Published online: 2 May 2000     

van der Waals interaction between internal aqueous droplets and the external aqueous phase in double emulsions

A mathematical model for analyzing the van der Waals interaction between the internal aqueous droplets (W(1)) and the external aqueous phase (W(2)) of double emulsions has been established. The effects of Hamaker constants of the materials forming the system, especially those of the two different adsorbed surfactant layers with uniform density (A(1) and A(2)), on the van der Waals interaction were investigated. The overall van der Waals interaction across the oil film is a combined result of four individual parts, that is, W(1)-W(2), A(1)-A(2), W(1)-A(1), and A(2)-W(2) van der Waals interaction, and it may be either attractive or repulsive depending on many factors. It was found that the overall van der Waals interaction is dominated by the W(1)-W(2) interaction at large separation distances between the W(1)/O and O/W(2) interfaces, while it is mostly determined by the A(1)-A(2) interaction when the two interfaces are extremely close. Specifically, in the cases when the value of the Hamaker constant of the oil phase is intermediate between those of W(1) and W(2) and there is a thick oil film separating the two interfaces, a weak repulsive overall van der Waals interaction will prevail. If the Hamaker constant of the oil phase is intermediate between those of A(1) and A(2) and the two interfaces are very close, the overall van der Waals interaction will be dominated by the strong repulsive A(1)-A(2) interaction. The repulsive van der Waals interaction at such cases helps stabilize the double emulsions.     

Van der Wals团簇ο-xylene N2的共振双光子电离光谱

The one-color resonant two -photon ionization technique is employed to study jetcooled van der Waals(vdW) complex o-xylene?N2 through the S0-S1 transition around the band. The spectra obtained exhibit rich information about the complex intermolecular vdW vibrational modes. We have tentatively assigned all the observed spectral features. The structure of the complex has been obtained by calculation of the minimum energy structure.     

Spectroscopy of Ar-SH and Ar-SD. I. Observation of rotation-vibration transitions of a van der Waals mode by double-resonance spectroscopy

Rotation-vibration transitions of a van der Waals bending vibration, P = 1/2 <-- 3/2, of the Ar-SHSD (X 2pi) complexes in the electronic ground state have been observed by applying newly developed microwave-millimeter-wave double-resonance spectroscopy. The rotational energy-level structure for the two isotopomers, with hyperfine structure due to the hydrogen or deuterium nuclei and parity doublings in the P = 1/2 state, has now been clarified. Detailed explanation of the double-resonance technique is also given.     

p-Difluorobenzene-argon ground state intermolecular potential energy surface

The ground state intermolecular potential energy surface for the p-difluorobenzene-Ar van der Waals complex is evaluated using the coupled cluster singles and doubles including connected triple excitations [CCSD(T)] model and the augmented correlation consistent polarized valence double-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. The surface minima are characterized by the Ar atom located above and below the difluorobenzene center of mass at a distance of 3.5290 A. The corresponding binding energy is -398.856 cm(-1). The surface is used in the evaluation of the intermolecular level structure of the complex. The results clearly improve previously available data and show the importance of using a good correlation method and basis set when dealing with van der Waals complexes.     

Structural and electronic effects on one-bond spin-spin coupling constants 1J(B-N), 1J(B-H), and 1J(B-F) for complexes of nitrogen bases with BH3 and its fluoro-substituted derivatives

Ab initio equation-of-motion coupled cluster (EOM-CCSD) one-bond spin-spin coupling constants (1)J(B-N), (1)J(B-H), and (1)J(B-F) have been evaluated for complexes X:BH(n)F(3-n) with X = N(2), NCH, NCLi, H(2)CNH, NF(3), and NH(3), for n = 0-3. These complexes can be classified as either covalent or van der Waals complexes, on the basis of their binding energies and B-N distances. (1)J(B-N) for covalent complexes varies significantly from -19 to +9 Hz, whereas (1)J(B-N) is less than 2 Hz for van der Waals complexes. An absolute value of (1)J(B-N) of 3 Hz or greater indicates that the complex is covalently bonded, but a small value of this coupling constant does not necessarily mean that it is a van der Waals complex, in view of the variation among these complexes found for (1)J(B-N) as a function of the B-N distance. Deformation of the boron acid upon complex formation and electron donation by the nitrogen base has opposing effects on both (1)J(B-H) and (1)J(B-F). These effects are relatively small in van der Waals complexes. In covalent complexes, electron donation has the dominant effect on (1)J(B-H), and on (1)J(B-F) in complexes with BH(2)F and BHF(2), but acid deformation has the dominant effect on (1)J(B-F) in complexes with BF(3). Values of both (1)J(B-H) and (1)J(B-F) reflect the van der Waals or covalent nature of the B-N bond.     

Fluorescence of glyoxal in supersonic jets

Fluorescence excitation spectra have been measured in helium supersonic jets for S₁—S₀ O⁰₀ and 8¹₀ transitions of glyoxal. The polarization of the 8¹₀ transition was determined. The corresponding bands of the glyoxal—He van der Waals complex are absent.     

Rotational spectroscopic and ab initio studies of the Xe-H2O van der Waals dimer

An ab initio potential energy surface of the Xe-H(2)O van der Waals dimer was constructed at the coupled cluster level of theory with single, double, and pertubatively included triple excitations. For the Xe atom, the small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta (aug-cc-pVQZ-PP) basis set was used. Dunning's augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis set was chosen for O and H atoms. Midbond functions were used to supplement the atom-centered basis sets. Rotational spectra of the Xe-H(2)O van der Waals dimer were recorded with a pulsed-nozzle Fourier transform microwave spectrometer. Rotational transitions within two internal rotor states, namely, the 0(00) and 1(01) states, were measured and assigned. Nuclear quadrupole hyperfine structures due to the (131)Xe (I = (3)/(2)), D (I = 1) and (17)O (I = (5)/(2)) nuclei were also observed and analyzed. Information about the molecular structure and the H(2)O angular motions was extracted from the spectroscopic results with the assistance of the ab initio potential.