On the mechanism of intermode vibration-to-vibration energy transfer in CH3F excited by a tea CO2 laser

The v₃ mode of CH₃F was excited by irradiation with a TEA CO₂ laser pulse, and the time-resolved emission spectra of the v₃ overtone and the 3 μ;m region were observed. The results indicate that the population of the v₄ level behaves kinetically in the same manner as that of 2v₃ or 3v₃. This suggests an efficient energy transfer between these levels.     

密度泛函理论研究银上吸附对巯基吡啶的SERS化学增强效应

基于密度泛函理论计算和拉曼光谱理论分析,我们研究了对巯基吡啶(4MPY)分子的拉曼光谱和其在银上的表面增强拉曼光谱(SERS),并进一步探讨了SERS与界面吸附结构、异构化、质子化和氢键作用以及低能激发态的关系。首先,我们对两种分子异构体的相对稳定性和拉曼光谱进行了理论分析。在此基础上,进而研究了该分子与不同银簇作用时的拉曼光谱,结果表明,4MPY以巯基硫与银簇作用形成强的Ag―S键,导致拉曼光谱的线型不依赖于所选银簇的大小。接着我们考虑了吡啶氮端作用的两种情况。(1)当4MPY-银簇复合物同时以吡啶氮与水簇或水合质子簇形成氢键时,结果表明吡啶环的部分振动频率随氢键和质子化发生蓝移。(2)当考虑吡啶氮与银簇作用时,吡啶环三角畸变振动发生蓝移。上述情况不仅解释了实验观测的振动频率变化,而且表明了化学环境改变对相对拉曼强度的影响。最后,我们计算了当对巯基吡啶分子以单端或双端与银簇作用,在考虑激发光与低能激发态的能量匹配时,拉曼光谱强度与低能激发态的关系。计算结果表明,在双端吸附构型下,与吡啶氮成键的银簇受激发产生电荷转移态,不仅导致吡啶环v_(12)、v_1和v_(8a)振动的拉曼信号增强,而且选择性地增强吡啶环C―H面内对称弯曲振动v9a的拉曼信号。     

Big macrocyclic assemblies of carboranes (big MACs): synthesis and crystal structure of a macrocyclic assembly of four carboranes containing alternate ortho- and meta-carborane icosahedra linked by para-phenylene units

The macrocyclic compound, [1,2-C₂B₁₀H₁₀-1,4-C₆H₄-1,7-C₂B₁₀H₁₀-1,4-C₆H₄]₂ (5)—a novel cyclooctaphane, was prepared by condensation of the C,C′-dicopper(I) derivative of meta-carborane with 1,2-bis(4-iodophenyl)-ortho-carborane. The X-ray crystal structure of 5·C₆H₆·6C₆H₁₂ was determined at 150 K, revealing an extremely loose packing mode. Molecule 5 has a crystallographic C_s and local C_2v symmetry; the macrocycle adopts a butterfly (dihedral angle 143°) conformation with the ortho-carborane units at the wingtips and the phenylene ring planes roughly perpendicular to the wing planes. Multinuclear NMR spectra suggest that molecule 5 in solution inverts rapidly via the planar D_2h geometry, which (from ab initio HF/6-31G* calculations) is only 1 kcal mol⁻¹ higher in energy than the C_2v one. An attempt to prepare an even larger macrocycle, comprising three para-carborane and three ortho-carborane units linked by six para-phenylene units, was unsuccessful.     

Multidimensional anharmonic calculation of the vibrational frequencies and intensities for the trans and cis isomers of HONO with the use of normal coordinates

The equilibrium geometry and the potential energy and dipole moment surfaces have been determined for the cis and trans isomers of the HONO molecule by an ab initio Moller–Plesset (MP2) calculation with a wide set of atomic orbitals. The multidimensional anharmonic vibrational Schrodinger equations are solved using the variational method with the Hamiltonian and wave functions written in the normal coordinates of cis and trans isomers. All one- and two-dimensional and a number of three-dimensional vibrational problems are solved to obtain the energy levels and vibrational eigenfunctions. The frequencies and intensities for the fundamental, overtone and some combination bands are determined in good agreement with the available experimental results. The calculation shows the strength of coupling between different vibrational modes and reveals the presence of strong resonances between the (v₁, v₃, v₆) and (v₁, v₃−1, v₆+2) states of cis-HONO. This fact may be important for understanding the energy redistribution between the intermolecular degrees of freedom. The magnitude and direction of vibrationally averaged ground-state dipole moment of both isomers, as well as the direction of transition dipole moments, are in good agreement with the experimental findings. The changes in the values of dipole moment and some geometrical parameters of cis- and trans-HONO on vibrational excitation are also computed.     

Vibrational spectra of hexafluorodisiloxane and internal rotation around the SiO bond. An ab initio study

The structure of hexafluorodisiloxane was optimized at different levels of theory up to MP2/6-31G^*, and the vibrational spectrum was calculated and compared with the experimental one. It was shown that at the Hartree-Fock level the simple SCF/3-21 (O^*) basis set gives satisfactory results. The geometry was also optimized at two C_2v structures which appear to be saddle points on the SiF₃ rotor potential curve. The significant change of the SiOSi angle equilibrium value in the course of the SiF₃ group rotation was explained by assuming repulsion between lone pair electrons of fluorines and oxygen. This effect is also revealed in the predicted change in the v_asSiOSi frequency on going from the C₂ to the C_2v structure. This provides the explanation of some unusual features in the vibrational spectra of hexahalodisiloxanes, namely large gas-solid shifts of v_as SiOSi and splitting of this mode in the matrices.     

Observation Of Mode Selective Vibrational Excitation Of SF6 By Collisions With 4–10 eV Li And H Ions

Structured time of flight spectra of both Li⁺ and H⁺ ions scattered from ground state SF₆, have been measured at angles (5.0° v_c.m. 16.6°) less than the rainbow angle at E_c.m., = 4.3 eV and 9.7 eV, respectively. The structure can be arttributed to vibrational excitation of the v₃ mode by H⁺ and excitation of the v₄ mode by Li⁺. The relative transition probabilities obey a Poisson distribution and can be explained by a simple forced oscillator model.     

Ab initio molecular orbital study of Fe2Cl6 and FeAlCl6

Ab initio molecular orbital theory was used to determine the equilibrium structure and vibrational frequencies of Fe₂Cl₆ and FeAlCl₆. The equilibrium structure the Fe₂Cl₆ dimer has D_2h symmetry with a planar arrangement of the four membered {FeCl_brFeCl_br} ring, similar to the Al₂Cl₆ dimer. The calculated bond distances and vibrational frequencies are in good agreement with experiment. The potential energy surface for the puckering of the {FeCl_brFeCl_br} ring is extremely flat. This prevents an unambiguous assignment of either D_2h or C_2v symmetry to the Fe₂Cl₆ structure in electron diffraction measurements. The FeAlCl₆ molecule is found to have a C_2v structure similar to Fe₂Cl₆ with vibrational frequencies in good agreement with experiment.     

RDX分子内振动能量重分配的动力学研究

基于从头算分子动力学(Born-oppenheimer molecular dynamics, BOMD)模拟, 构建了环硝胺六氢-1,3,5-三硝基-1,3,5-三嗪(RDX)单分子不同振动模式之间的耦合矩阵, 并计算了在不同加载能量下从低频振动模式到高频振动模式的最优能量传输路径. 结果表明, RDX单分子中—NNO₂基团更有利于能量局域化, 振动模式v₃和v₄在从低频振动模式到高频振动模式的能量传输过程中扮演着重要角色. 通过对v₃和v₄两个振动模式的进一步分析发现, 加载能量的不同会导致RDX单分子能量传输路径的不同. 当加载能量较低时, RDX单分子倾向于从低频振动模式到中频振动模式再到高频振动模式的能量传输路径; 当加载能量较高时, 能量更倾向于从低频振动模式直接传输到高频振动模式上. 揭示了RDX分子内振动耦合能量转移的微观机制, 为进一步探索RDX将“机械能”转化为“化学能”的微观过程提供了理论基础.     

NMR field-cycling study of proton and deuteron spin relaxation in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl

NMR field-cycling measurements of the deuteron spin relaxation dispersion T₁(v) for the fully deuteriated nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB-d₁₉) over a broader Larmor frequency range (v≈10 kHz to 30 MHz) than reported so far in the literature basically confirm the magnetic relaxation mechanisms previously observed by frequency dependent proton spin studies of various nematogenic molecules, namely collective nematic modes of the director field in the kilohertz regime, and anisotropic reorientations of individual molecules (mainly self-diffusion for the protons and mainly rotations about the long axis for the deuterons) in the megahertz range. Within the experimental error limits such a model allows a self-consistent interpretation of the available deuteron and proton T₁(v) results for deuteriated or protonated 5CB, respectively. In particular, the magnitudes of the measured order fluctuation contributions are in approximate accordance, i.e. within a factor of less than two, with theoretical estimates from NMR line splittings and the relevant material parameters. More exact and more extensive deuteron studies are needed to locate the origin of the observed minor inconsistency.     

Vibrational dependence of the NH3 (v2)+NO and NO(v)+NH3 charge transfer cross sections

A tandem quadrupole mass spectrometer is used to study the charge transfer reactions NH₃⁺ + NO and NO⁺ + NH₃ over a collision energy range 1.5–13 eV. The vibrational state of the reagent ions is selected by resonance-enhanced multiphoton ionization. For the 0.9 eV exothermic process NH₃⁺ + NO → NH₃ + NO⁺ excitation of the v₂ umbrella bending mode (v₂ = 0–12) causes no marked change in the charge transfer cross section, while in the reverse process NO⁺ + NH₃ → NO + NH₃⁺ excitation of the NO⁺ vibration (v = 0–6) strongly enhanced the charge transfer cross section.     

Two-colour bound—free—bound spectroscopy of the [E1/2]c6S Rydberg states of CH3I and CD3I

In this resonantly enhanced multiphoton ionization (REMPI) experiment, an extended vibrational progression in the CI stretching mode (v₃) of methyl iodide (-h₃ and -d₃) is observed in the 1 + 1′ excitation of the [1/2] 6s; 0 Rydberg state when the pump photon wavelength lies in the bound → free absorption continuum. This is in contrast with one-colour coherent (non-resonant) two-photon excitation, where the v₃ mode is not excited. By working at several different fixed probe wavelengths and scanning the pump frequency, the relative contributions from the three intermediate repulsive states can be explored through changes in the relative strengths of the Ω = 0 and 1 components of the final Rydberg states. Extensive predissociation in the Rydberg states curtails the vibrational progression.     

Theoretical study on the aromaticity of the bimetallic clusters X2M2 (X=Si, Ge, M=Al, Ga)

Four neutral bimetallic clusters X₂M₂ (X=Si, Ge, M=Al, Ga) are investigated using density functional theory (DFT) and post-HF methods. The calculated results show that each of four X₂M₂ species has two energetically close stable isomers with rhombic structure (D_2h symmetry) and trapezoidal structure (C_2v symmetry) respectively. For the Ge₂Al₂ species the rhombic (D_2h) isomer is the ground state, whereas for other three species Ge₂Ga₂, Si₂Al₂, and Si₂Ga₂, the trapezoidal (C_2v) isomers are the ground states. The calculated magnetic susceptibility anisotropy (χ_anis) and nucleus-independent chemical shift (NICS) indicate that a strong diatropic ring current exists in the two heterocyclic planar isomers, suggesting they are highly aromatic. A detailed molecular orbital analysis further reveals that both heterocyclic isomers possess multiple aromaticity derived from one delocalized π MOs and two delocalized σ MOs.     

Thermodynamic characterization of ternary solutions of uncharged polymers

The thermodynamic interactions in aqueous solutions of uncharged polymers were studied. Using a gel-deswelling method, the water activities (chemical potentials) in binary and ternary (two polymers in one solvent) solutions of methylcellulose (MC), polyvinyl alcohol (PVA) and polyvinyl pirrolidone (PVP), respectively were determined at various polymer volume fractions (1.0 × 10⁻² < v₂ < 1.0 × 10⁻¹). On the theoretical basis of the Flory–Huggins approximations, the relevant solvent–segment (χ₁₂ or χ₁₃) and segment–segment pair interaction parameters (χ₂₃) have been calculated.

The solvent activity curves (ln a₁ versus polymer volume fraction) can be well described by a polynomial of third-degree in both the binary and the ternary solutions of the polymers. The solvent–segment interaction parameters exhibit a slight dependence on the polymer concentration. For each binary solution, the χ₁₂−v₂ function can be fitted by a straight line wich has a small positive slope. In the mixtures of two polymers, the values of the segment–segment (χ₂₃) interaction parameters were close to zero or sligthly negative (χ₂₃ 0 ± 0.03), indicating that under the studied conditions, the polymers in the ternary solutions are compatible.     

Multiply Charged Anions,Maximum Charge Acceptance,and Higher Electron Affinities of Molecules,Superatoms,and ClustersSCI北大核心CSCD

The addition of electrons to form gas-phase multiply charged anions (MCAs) normally requires sophisticated experiments or calculations.In this work, the factors stabilizing the MCAs, the maximum electron uptake of gas-phase molecules, X, and the electronic stability of MCAs X^Q-, are discussed. The drawbacks encountered when applying computational and/or conceptual density functional theory (DFT) to MCAs are highlighted. We develop and test a different model based on the valence-state concept. As in DFT, the electronic energy, E(N, v_ex), is a continuous function of the average electron number, N, and the external potential, v_ex, of the nuclei. The valence-state-parabola is a second-order polynomial that allows extending E(N, v_ex) to dianions and higher MCAs. The model expresses the maximum electron acceptance, Q_max, and the higher electron affinities, A_Q, as simple functions of the first electron affinity, A₁, and the ionization energy, I, of the "ancestor" system. Thus, the maximum electron acceptance is Q_{max, calc} = 1 + 12A₁/7(I -A₁). The ground-state parabola model of the conceptual DFT yields approximately half of this value, and it is termed Q_{max, GS} = ${}^{1}\!\!\diagup\!\!{}_{2}\; $ + A₁/(I -A₁). A large variety of molecules are evaluated including fullerenes, metal clusters, super-pnictogens, super-halogens (OF₃), super-alkali species (OLi₃), and neutral or charged transition-metal complexes, AB_mL_n^0/+/-. The calculated second electron affinity A_{2, calc} = A₁-(7/12)(I -A₁) is linearly correlated to the literature references A_{2, lit} with a correlation coefficient R = 0.998. A₂ or A₃ values are predicted for further 24 species. The appearance sizes, n_ap^3-, of triply charged anionic clusters and fullerenes are calculated in agreement with the literature.     

The rotational spectrum of propynyl isocyanide

Propynyl isocyanide, CH₃C₂NC, has been prepared by vacuum pyrolysis of pentacarbonyl-(1,2-dichloropropenyl isocyanide) chromium, (CO)₅Cr–CN–C(Cl)=C(Cl)CH₃, and its ground state millimeter and microwave spectrum has been observed for the first time. r_s structural parameters of this molecule with a C_3v symmetry could be obtained from the rotational constants of several isotopomers: r_(C1–C2)=1.456(2) Å, r_(C2–C3)=1.206(2) Å, r_(C3–N)= 1.316(2) Å, r_(N–C4)= 1.175(2) Å, r_(H–C1)= 1.090(1) Å, >HCC=110.7(4)°. The nitrogen quadrupole coupling constant has been determined to be 878(2) kHz and measurements of the Stark effect allowed to obtain an electric dipole moment of 4.19(3) Debye. The results fit well into a series of related compounds and are in good agreement with data from ab initio calculations.     

Model of silicooxygen ring vibrations

In order to assign the bands in the IR spectra of silicates to the appropriate normal vibrations, a vibrational model has been proposed. A complex silicooxygen ring is considered as a ‘unit cell' composed of the appropriate number of [SiO₄]⁴⁻ tetrahedra. According to this model, in the ring silicates spectra we have to observe bands due to internal vibrations of individual tetrahedra and bands corresponding exclusively to the ring structure. Change in the tetrahedra symmetry from T_d (ideal tetrahedron) to C_2v (tetrahedron in a ring) and then to the ring symmetry: D_3h, D_4h and D_6h (ideal rings) with respect to reducible representations makes it possible to differentiate between the bands due to ring structure (pseudo-lattice vibrations) and internal modes of tetrahedra. It has been established that in the case of all ideal rings there is only one IR active vibrational mode, namely the one symmetric with respect to the axis of the highest fold, i.e. A₂″ in the case of 3-membered rings and A_2u in the case of 4- and 6-membered rings. The model proposed has been verified for different membered ring silicates.     

The molecular structure of beryllocene, (C5H5)2Be. A reinvestigation by gas phase electron diffraction

The electron scattering pattern of gaseous dicyclopentadienylberyllium, Cp₂Be, has been recorded from s = 2.00 to 39.00 Å⁻¹ with a nozzle temperature of about 120°C. Molecular models of D_5d symmetry or models containing one π-bonded and one σ-bonded Cp ring are not compatible with the data. The possibility the gaseous Cp₂Be consists fo a mixture D_5d and π-Cp, σ-Cp conformers is considered and rejected. A model of C_5v symmetry can be brought into satisfactory agreement with the data. It is also found that a slip sandwich model obtained from the C_5v model by moving sideways the ring which is at the greatest distance from Be, while keeping the two rings essentially parallel is compatible with the electron diffraction data. The best fit between experimental and calculated intensity curves is obtained with a model with a sideways slip of 0.8(1) Å. This model is similar to that indicated by the X-ray diffraction investigations by Wong and coworkers [4,5]. It is suggested that the potential energy of the molecule does not change much as the magnitude of the slip changes and that the molecule thus undergoes large amplitude vibration.     

Vibrational frequencies and structural determination of 1,1-difluoro-1,2-propadiene

The normal mode frequencies and corresponding vibrational assignments of 1,1-difluoro-1,2-propadiene in C_2v symmetry are examined theoretically using the 98 set of quantum chemistry codes. All normal modes were successfully assigned to one of 11 types of motion (C=C stretch, C–H stretch, C–F stretch, CH₂ scissors, CF₂ scissors, CH₂ wag, CF₂ wag, CH₂ rock, CF₂ rock, CF₂ twist, and C=C=C bend) predicted by a group theoretical analysis. Comparing the vibrational frequencies with IR and Raman spectra available in the literature, a set of scaling factors is derived. Predicted infrared intensities and Raman activities are reported. Predicted normal mode frequencies of 1,1-difluoro-1,2-propadiene-d₂ are reported.     

On the 6Πu state of Na2

The 6¹Π_u state of sodium dimer has been observed up to v = 53 in excitation spectra of the system, recorded by polarisation labelling spectroscopy technique. The Dunham coefficients are derived and the potential energy curve constructed by the inverted perturbation approach method. Equilibrium constants for the 6¹Π_u state of Na₂ are: T_e = 35446.06 ± 0.04 cm⁻¹ (with respect to the minimum of the electronic ground state), Y₁₀ = 111.388 ± 0.019 cm⁻¹, Y₀₁ = 0.112122 ± 0.000017 cm⁻¹.     

Angular and frequency dependent spin relaxation study of liquid crystalline cyanobiphenyls

NMR field-cycling measurements of the Larmor frequency (v) and angular (Δ) dependences of the longitudinal proton spin relaxation time T₁ for the nematic liquid crystals 5CB and 8CB allow a more detailed analysis of the underlying molecular motions than data available previously. All T₁ (v, Δ) dispersion profiles essentially distinguish three frequency ranges where T₁ is governed by either local field effects, collective motions (director order fluctuations), or rotational and translational diffusion of individual molecules or molecular groups, respectively. The angular dependence supports and extends previous conclusions about the significance of the order fluctuation term at low (kHz) and high (MHz) Larmor frequencies; in addition it is the basis for the disentanglement of local field effects, which involve Jeener's dipolar relaxation, and of the sophisticated rotational relaxation models suggested in the literature by Dong, Nordio and Vold. It is found that Vold's third rate concept gives the best explanation of the measurements. The results on the rotational diffusion processes essentially agree with deuteron studies from the literature, but also reveal clear distinctions with regard to the anisotropy parameter σ, essentially due to the improved separation from the order fluctuation contribution.