Small para-hydrogen clusters doped with carbon monoxide: quantum Monte Carlo simulations and observed infrared spectra

The structures and rotational dynamics of clusters of a single carbon monoxide molecule solvated in para-hydrogen, (paraH(2))(N)-CO, have been simulated for sizes up to N=17 using the reptation Monte Carlo technique. The calculations indicate the presence of two series of R(0) rotational transitions with J=1<--0 for cold clusters, similar to those predicted and observed in the case of He(N)-CO. Infrared spectra of these clusters have been observed in the region of the C-O stretch ( approximately 2143 cm(-1)) in a pulsed supersonic jet expansion using a tunable diode laser probe. With the help of the calculations, the observed R(0) rotational transitions have been assigned up to N=9 for the b-type series and N=14 for the a-type series. Theory and experiment agree rather well, except that theory tends to overestimate the b-type energies. The (paraH(2))(12)-CO cluster is calculated to be particularly stable and (relatively) rigid, corresponding to completion of the first solvation shell, and it is observed to have the strongest a-type transition.     

Nuclear hyperfine interaction of rotating hydrogen: a spectroscopic investigation of hydrogen-OCS van der Waals complexes

The rotational spectra of five weakly bonded hydrogen-OCS complexes (paraH(2), orthoH(2), HD, orthoD(2), and paraD(2)) are measured. Hyperfine structure is resolved and analyzed in all except the complex with paraH(2), where I=0. For the two j=1 species, orthoH(2)-OCS and paraD(2)-OCS, nuclear hyperfine coupling constants are found to be d(a)=21.2(2) and 8.4(2) kHz, respectively, indicative of nearly free uniaxial rotation of the hydrogen around the b-inertial axis. Similar analyses for HD-OCS and orthoD(2)-OCS yield the quadrupole coupling constants eqQ(a)=16(2) and 30(2) kHz, respectively, showing that the internal rotational motions of HD and orthoD(2) in the complex are slightly hindered producing a small nonspherical distribution. For orthoD(2)-OCS, the observed hyperfine structure indicates that the nuclear spin states I=0 and 2 are strongly coupled in the rotation of the complex.     

Five-dimensional ab initio potential energy surface and predicted infrared spectra of H2-CO2 van der Waals complexes

The authors present a new five-dimensional potential energy surface for H2-CO2 including the Q3 normal mode for the nu3 antisymmetric stretching vibration of the CO2 molecule. The potential energies were calculated using the supermolecular approach with the full counterpoise correction at the CCSD(T) level with an aug-cc-pVTZ basis set supplemented with bond functions. The global minimum is at two equivalent T-shaped coplanar configurations with a well depth of 219.68 cm-1. The rovibrational energy levels for four species of H2-CO2 (paraH2-, orthoH2-, paraD2-, and orthoD2-CO2) were calculated employing the discrete variable representation (DVR) for radial variables and finite basis representation (FBR) for angular variables and the Lanczos algorithm. Our calculations showed that the off-diagonal intra- and intermolecular vibrational coupling could be neglected, and separation of the intramolecular vibration by averaging the total Hamiltonian with the wave function of a specific vibrational state of CO2 should be a good approximation with high accuracy. The calculated band origin shift in the infrared spectra in the nu3 region of CO2 is -0.113 cm-1 for paraH2-CO2 and -0.099 cm-1 for orthoH2-CO2, which agrees well with the observed values of -0.198 and -0.096 cm-1. The calculated rovibrational spectra for H2-CO2 are consistent with the available experimental spectra. For D2-CO2, it is predicted that only a-type transitions occur for paraD2-CO2, while both a-type and b-type transitions are significant for orthoD2-CO2.     

Rotational excitations of N2O in small helium clusters and the role of Bose permutation symmetry

We present a detailed study of the energetics, structures, and Bose properties of small clusters of (4)He containing a single nitrous oxide (N(2)O) molecule, from N=1 (4)He up to sizes corresponding to completion of the first solvation shell around N(2)O (N=16 (4)He). Ground state properties are calculated using the importance-sampled rigid-body diffusion Monte Carlo method, rotational excited state calculations are made with the projection operator imaginary time spectral evolution method, and Bose permutation exchange and associated superfluid properties are calculated with the finite temperature path integral method. For N< or =5 the helium atoms are seen to form an equatorial ring around the molecular axis, at N=6 helium density starts to occupy the second (local) minimum of the N(2)O-He interaction at the oxygen side of the molecule, and N=9 is the critical size at which there is onset of helium solvation all along the molecular axis. For N> or =8 six (4)He atoms are distributed in a symmetric, quasirigid ring around N(2)O. Path integral calculations show essentially complete superfluid response to rotation about the molecular axis for N> or =5, and a rise of the perpendicular superfluid response from zero to appreciable values for N> or =8. Rotational excited states are computed for three values of the total angular momentum, J=1-3, and the energy levels fitted to obtain effective spectroscopic constants that show excellent agreement with the experimentally observed N dependence of the effective rotational constant B(eff). The non-monotonic behavior of the rotational constant is seen to be due to the onset of long (4)He permutation exchanges and associated perpendicular superfluid response of the clusters for N> or =8. We provide a detailed analysis of the role of the helium solvation structure and superfluid properties in determining the effective rotational constants.     

Laser induced fluorescence spectroscopy of tetracene with large Ar, Ne, and H2 clusters in superfluid He nanodroplets

Clusters of tetracene molecules with different numbers of attached (Ar)(N), (Ne)(N) and (H(2))(N) particles (N = 1-2000) are assembled inside superfluid He nanodroplets and studied via laser-induced fluorescence. The frequency shift of the fluorescence spectrum of the tetracene molecules is studied as a function of cluster size and pickup order of tetracene and cluster species. For (Ar)(N) and (Ne)(N) clusters, our results indicate that the tetracene molecules reside inside the clusters when tetracene is captured by the He nanodroplet before the cluster species; conversely, the tetracene molecules stay on the surface of the clusters when tetracene is captured after the cluster species. In the case of (H(2))(N) clusters, however, tetracene molecules reside inside the (H(2))(N) clusters irrespective of the pickup order. We conclude that (Ar)(N) and (Ne)(N) clusters are rigid at T = 0.38 K, while (H(2))(N) clusters of up to N = 2000 remain fluxional at the same temperature. The results may also indicate the occurrence of heterogeneous nucleation of the (H(2))(N) clusters, which is induced by the interaction with tetracene chromophore molecules.     

High resolution infrared spectroscopy of carbon dioxide clusters up to (CO2)13

Thirteen specific infrared bands in the 2350 cm(-1) region are assigned to carbon dioxide clusters, (CO(2))(N), with N = 6, 7, 9, 10, 11, 12 and 13. The spectra are observed in direct absorption using a tuneable infrared laser to probe a pulsed supersonic jet expansion of a dilute mixture of CO(2) in He carrier gas. Assignments are aided by cluster structure calculations made using two reliable CO(2) intermolecular potential functions. For (CO(2))(6), two highly symmetric isomers are observed, one with S(6) symmetry (probably the more stable form), and the other with S(4) symmetry. (CO(2))(13) is also symmetric (S(6)), but the remaining clusters are asymmetric tops with no symmetry elements. The observed rotational constants tend to be slightly (≈2%) smaller than those from the predicted structures. The bands have increasing vibrational blueshifts with increasing cluster size, similar to those predicted by the resonant dipole-dipole interaction model but significantly larger in magnitude.     

Energetics and structures of the initial stages of nucleation of (SiO(2))(N) species: possible routes to highly symmetrical tetrahedral clusters

A detailed survey of the low energy isomer spectrum of (SiO(2))(N), N= 6-10, 13, 16 has been performed using interatomic potential based global optimisations refined via high-level density functional calculations. Within these spectra, including many isomers reported for the first time, structurally and energetically viable pathways for the initial stages of silica cluster growth through SiO(2) nucleation are identified. The role of the exceptionally stable (SiO(2))(8) ground state "magic" cluster is highlighted in the possible formation of highly symmetric fully tetrahedral clusters of size (SiO(2))(10) and (SiO(2))(16). These clusters are found to form a part of a natural (SiO(2))(N)N= 7, 10, 13, 16 sequence together with the C(3v) ground states for (SiO(2))(7) and (SiO(2))(13). The fully tetrahedral clusters are argued to be likely relatively long-lived metastable species in the process of gas phase SiO(2) nucleation due to the manner of their termination. It is speculated that larger tetrahedral (SiO(2))(40) clusters may exhibit porous structures.     

Infrared spectra of seeded hydrogen clusters: (paraH2)N - OCS, (orthoH2)N - OCS, and (HD)N - OCS, N = 2 - 7

Infrared spectra of hydrogen-carbonyl sulfide clusters containing paraH2, orthoH2, or HD have been studied in the 2060 cm(-1) region of the C-O stretching vibration. The clusters were formed in pulsed supersonic jet expansions and probed using a tunable infrared diode laser spectrometer. Simple symmetric rotor type spectra were observed and assigned for clusters containing up to N = 7 hydrogen molecules. There was no resolved K structure, and Q-branch features were present for orthoH2 and HD but absent for paraH2. These characteristics can be rationalized in terms of near symmetric rotor structures, very low effective rotational temperatures (0.15 to 0.6 K), and nuclear spin statistics. The observed vibrational shifts were compared with those from recent observations on the same clusters embedded in helium nanodroplets. The observed rotational constants for the paraH2 clusters are in good agreement with a recent quantum Monte Carlo simulation. Some mixed clusters were also observed, such as HD-HD-He-OCS and paraH2 - orthoH2 - OCS.     

Spectroscopy and potential energy surface of the H2-CO2 van der Waals complex: experimental and theoretical studies

A 4-D ab initio potential energy surface is calculated for the intermolecular interaction of hydrogen and carbon dioxide, using the CCSD(T) method with a large basis set. The surface has a global minimum with a well depth of 212 cm(-1) and an intermolecular distance of 2.98 A for a planar configuration with both the O-C-O and H-H axes perpendicular to the intermolecular axis. Bound state calculations are performed for the H(2)-CO(2) van der Waals complex with H(2) in both the para and ortho spin states, and the binding energy of paraH(2)-CO(2)(50.4 cm(-1)) is found to be significantly less than that of orthoH(2)-CO(2)(71.7 cm(-1)). The surface supports 7 bound intermolecular vibrational states for paraH(2)-CO(2) and 19 for orthoH(2)-CO(2), and the lower rotational levels with J< or = 4 follow an asymmetric rotor pattern. The calculated infrared spectrum of paraH(2)-CO(2) agrees well with experiment. For orthoH(2)-CO(2), the ground state rotational levels allowed by symmetry are found to have (K(a), K(c))=(even, odd) or (odd, even). This somewhat unexpected fact enables the previously observed experimental spectrum to be assigned for the first time, in good agreement with theory, and indicates that the orientation of hydrogen is perpendicular to the intermolecular axis in the ground state of the orthoH(2)-CO(2) complex.     

Structure of Ag clusters Grown on Fs-Defect Sites of an MgO(1 0 0) surface

The structure of AgN clusters (N=1-4, 6, 8, 10), both in the gas phase and grown on the MgO(1 0 0) surface containing Fs-defects, has been investigated by a density functional basin-hopping (DF-BH) approach. In analogy with what observed in the case of gold clusters, it is found that the presence of the defect implies a double frustration and a cylindrical invariance of the metal-surface interaction, causing small Ag clusters growing around the Fs defect to be highly fluxional. Nevertheless, two different structural crossovers are found to be induced by the metal-defect interaction for the adsorbed clusters such that: 1) planar structures prevail for Nor=7), prevail for N=6 and N=8; 3) distorted face-centered cubic (fcc) structures grown pseudomorphically on the defected surface prevail for N=10. The transition from fivefold to fcc motifs is rationalized in terms of the double-frustration effect, which increases the bond strain of the noncrystalline structures. Detrapping energies from the defect were also calculated; the lowest energy pathway corresponds to the detachment of a dimer.     

Electronic excitations of C(60) aggregates

Excitation properties of the isolated C(60) and (C(60))(N) model clusters (N = 2, 3, 4, 6 and 13) are studied using an a priori parameterized and self-consistent Hamiltonian, the Complete Neglect of Differential Overlap considering the l azimuthal quantum number method. This method properly describes electron excitations of the isolated C(60) after the configuration interaction of singles (CIS) procedure, when those are compared with experimental data in n-hexane solution and in a molecular beam. Geometry models of (C(60))(N) clusters to model the effect of aggregation were obtained from the fullerene fcc crystal. Some peaks in the low energy edge of the absorption spectrum appear corresponding to clustering effects, as well as small increases of bandwidths in the strong bands at the UV region. An analysis of the theoretical absorption spectrum for dimer models has been carried out, taking into account the influence of the distance between fullerene centers. The density of states of CIS for fullerene clusters in the range from 2.0 to 6.5 eV shows the possibility of electron transitions as functions of the size of the clusters.     

HF in clusters of molecular hydrogen. I. Size evolution of quantum solvation by parahydrogen molecules

We present a theoretical study of the quantum solvation of the HF molecule by a small number of parahydrogen molecules, having n = 1-13 solvent particles. The minimum-energy cluster structures determined for n = 1-12 have all of the H(2) molecules in the first solvent shell. The first solvent shell closes at n = 12 and its geometry is icosahedral, with the HF molecule at the center. The quantum-mechanical ground-state properties of the clusters are calculated exactly using the diffusion Monte Carlo method. The zero-point energy of (p-H(2))(n)HF clusters is unusually large, amounting to 86% of the potential well depth for n > 7. The radial probability distribution functions (PDFs) confirm that the first solvent shell is complete for n = 12, and that the 13th p-H(2) molecule begins to fill the second solvent shell. The p-H(2) molecules execute large-amplitude motions and are highly mobile, making the solvent cage exceptionally fluxional. The anisotropy of the solvent, very pronounced for small clusters, decreases rapidly with increasing n, so that for n approximately 8-9 the solvent environment is practically isotropic. The analysis of the pair angular PDF reveals that for a given n, the parahydrogen solvent density around the HF is modulated in a pattern which clearly reflects the lowest-energy cluster configuration. The rigidity of the solvent clusters displays an interesting size dependence, increasing from n = 6 to 9, becoming floppier for n = 10, and increasing again up to n = 12, as the solvent shell is filled. The rigidity of the solvent cage appears to reach its maximum for n = 12, the point at which the first solvent shell is closed.     

Correlation of Solution and Gas Phase Complexation Assessed by Electrospray Ionization Mass Spectrometry: Application to One-, Two-, and Three-Ring Macrocycles

Electrospray ionization mass spectrometry (ESI-MS) was used to probe multiple cation complexation by C(12)H(25)(CH(2))(12)(CH(2))(12)C(12)H(25), 2, and <18N>CH(2)C(6)H(4)CH(2), 3. Complexation of two cations (2Na(+), 2 K(+), or Na(+) and K(+)) by 3 and three cations by 2 (3 Na(+), 3 K(+), and mixtures) as well as mixed proton-metallic cation complexes of both were observed. The K(+)/Na(+) cation-binding selectivity of 18-crown-6 was studied by ESI-MS of a methanol solution, and the selectivity profile was favorably compared with data obtained previously by ion-selective electrode techniques in the same solvent.     

Rotational fluctuation of molecules in quantum clusters. II. Molecular rotation and superfluidity in OCS-doped helium-4 clusters

In this paper, quantum fluctuations of a carbonyl sulfide molecule in helium-4 clusters are studied as a function of cluster size N in a small-to-large size regime (2or=20, which is larger than the experimental nanodroplet value. Superfluid response of the doped cluster is found to show remarkable anisotropy especially for N相似文献   

Effect of bending and torsion rigidity on self-diffusion in polymer melts: a molecular-dynamics study

Extensive molecular-dynamics simulations have been performed to study the effect of chain conformational rigidity, controlled by bending and torsion potentials, on self-diffusion in polymer melts. The polymer model employs a novel torsion potential that avoids computational singularities without the need to impose rigid constraints on the bending angles. Two power laws are traditionally used to characterize the dependence of the self-diffusion coefficient on polymer length: D proportional to N(-nu) with nu=1 for NNe (reptation regime), Ne being the entanglement length. Our simulations, at constant temperature and density, up to N=250 reveal that, as the chain rigidity increases, the exponent nu gradually increases towards nu=2.0 for NNe. The value of Ne is slightly increased from 70 for flexible chains, up to the point where the crossover becomes undefined. This behavior is confirmed also by an analysis of the bead mean-square displacement. Subsequent investigations of the Rouse modes, dynamical structure factor, and chain trajectories indicate that the pre-reptation regime, for short stiff chains, is a modified Rouse regime rather than reptation.     

Spectroscopic and theoretical study of the weakly bound H2-HCCCN dimer

Rotational spectra of the H(2)-HCCCN complex were studied using a pulsed-nozzle Fourier transform microwave spectrometer. Complexes containing the main and several minor isotopologues of cyanoacetylene (HCCC(15)N, DCCCN, and various (13)C containing isotopologues) and the two spin isomers of the H(2) molecule (paraH(2) and orthoH(2)) were investigated. Transitions of complexes with (14)N and D containing isotopologues have nuclear quadrupole hyperfine structures, which were measured and analyzed. Transitions of orthoH(2) molecule containing complexes show additional hyperfine structures due to nuclear magnetic proton spin-proton spin coupling of the hydrogen nuclei in the H(2) molecule. For orthoH(2)-HCCCN, both strong a- and weaker b-type transitions were measured and analyzed using a semirigid asymmetric rotor model. For the paraH(2)-HCCCN complex, only a-type transitions could be observed. The dimer complexes are floppy and have near T-shaped structures. Intermolecular interaction potential energy surfaces were calculated for H(2)-HCCCN using the coupled-cluster method with single and double excitations and noniterative inclusion of triple excitations [CCSD(T)]. Three orientations of the hydrogen molecule within the complex were considered. Equal weighting of the surfaces corresponding to the three hydrogen orientations provided an averaged potential energy surface. Bound-state rotational energy levels supported by the surfaces were determined for the different hydrogen orientations, as well as for the averaged surface. Simple scaling of the surfaces improved the agreement with the experimental results and produced surfaces with near spectroscopic accuracy.     

Finite temperature path integral Monte Carlo simulations of structural and dynamical properties of Ar(N)-CO(2) clusters

We report finite temperature quantum mechanical simulations of structural and dynamical properties of Ar(N)-CO(2) clusters using a path integral Monte Carlo algorithm. The simulations are based on a newly developed analytical Ar-CO(2) interaction potential obtained by fitting ab initio results to an anisotropic two-dimensional Morse∕Long-range function. The calculated distributions of argon atoms around the CO(2) molecule in Ar(N)-CO(2) clusters with different sizes are consistent to the previous studies of the configurations of the clusters. A first-order perturbation theory is used to quantitatively predict the CO(2) vibrational frequency shift in different clusters. The first-solvation shell is completed at N = 17. Interestingly, our simulations for larger Ar(N)-CO(2) clusters showed several different structures of the argon shell around the doped CO(2) molecule. The observed two distinct peaks (2338.8 and 2344.5 cm(-1)) in the υ(3) band of CO(2) may be due to the different arrangements of argon atoms around the dopant molecule.     

A three-dimensional ferromagnet based on linked copper-azido clusters

Two novel three-dimensional coordination polymers [Cu(6)(N(3))(12)(N-Eten)(2)](n) (1) (N-Eten=N-ethylethylenediamine) and {[Cu(9)(N(3))(18)(1,2-pn)(4)].H(2)O}(n) (2) (1,2-pn=1,2-diaminopropane) have been synthesized by the self-assembly reactions of Cu(NO(3))(2).3H(2)O, NaN(3) and small diamine ligands. Their molecular structures were determined by single-crystal X-ray diffraction. Complex 1 is composed of a neutral 3D coordination framework based on unprecedented hexanuclear copper(ii) clusters which features three types of bridging modes for azide (mu-1,1, mu-1,3 and mu-1,1,3). Complex 2 is a novel 3D coordination polymer featuring octanuclear copper-azido clusters and [Cu(diamine)(2)](2+) units which are linked by azido bridges. Magnetic studies for complex show ferromagnetic ordering at 3.5 K, where the azido bridges mediate ferromagnetic coupling between adjacent Cu(II) ions. The magnetic data for 1 were fitted to a uniform hexanuclear copper model which yielded g=2.21, J=6.26 cm(-1), zJ'=0.39 cm(-1). Complex 2 shows ferromagnetic coupling in the octanuclear unit and antiferromagnetic interaction between neighboring units.