Chemical Physics Letters (v.357, #1-2)

29Si NMR chemical shifts of silane derivatives by Clémence Corminbœuf; Thomas Heine; Jacques Weber (1-7).
Geometries and 29Si NMR chemical shifts are calculated for silanes Si n H2n+2, n=1,…,5, methylsilanes SiH n Me4−n , methoxysilanes SiH n (OMe)4−n , and methylmethoxysilanes SiMe n (OMe)4−n , n=0,…,4. Geometries and 29Si NMR chemical shifts are in satisfying agreement with experiment within LCGTO-DFT at the DZVP/LDA level for geometries and IGLO-III/GGA (GGA=PW91,PBE) level for shielding constants, which is an improvement to B88PW86, P86PW86 and B3LYP results. If an auxiliary basis is applied to express the Coulomb potential, g-functions have to be included to reproduce SiOSi angles and 29Si NMR chemical shifts correctly.

We present a study on the vibrational dynamics of the HF/FHF stretch combination vibration of HF–pyridine complexes in diluted pyridine solution. The relaxation of the excited vibration is observed to occur in two steps. In the first step, energy is transferred from the excited combination vibration to the hydrogen bond F–H⋯F modes with a time constant of 0.51±0.09 ps. In a second relaxation process these hydrogen bond modes decay with a time constant of 2.6±0.3 ps.

Spectroscopic properties of Yb-doped GdCa4O(BO3)3 crystal by Huaidong Jiang; Jiyang Wang; Huaijin Zhang; Xiaobo Hu; Bing Teng; Chengqian Zhang; Pu Wang (15-19).
The doped nonlinear optical (NLO) crystal Yb:GdCa4O(BO3)3 (Yb:GdCOB) has been grown by using the Czochralski method, with large sizes and good optical quality. The polarized absorption and fluorescence spectra have been measured and analyzed according to the Judd–Ofelt theory. The ground-state energy-level splitting is 1002 cm−1. The spectral parameters of the crystal such as the emission lifetime, the absorption cross-section, and the peak emission cross-section have been calculated. In comparison with established laser crystals the results suggest that this crystal has potential as an efficient self-frequency-doubling (SFD) laser operation in the green.

Determination of the fictive temperature for a hyperquenched glass by Y.Z. Yue; J.deC. Christiansen; S.L. Jensen (20-24).
Based on heat capacity measurements, we propose a simple approach to determine the fictive temperature (T f) of a hyperquenched glass. The recovered enthalpy of heating is estimated, which equals the released amount of the excess inherent structure energy stored in the glass. The hyperquenching is reached by means of the fiber spinning. We analyze relationships between the released enthalpy, changes in the internal energy, and changes in the configurational entropy.

Molecular wheels on surfaces by Bidisa Das; K.L. Sebastian (25-31).
Can one have molecules, which are chemisorbed on a surface, but at the same time can exhibit fluxional–rotational motion on the surface and perhaps move on it? We present calculations to show that cyclopentadienyl, adsorbed to Si or Ge surfaces, can exhibit interesting spinning motion. The motion involves movement of the point of attachment around the ring, resulting in a wheel like motion of the molecule, though there is no net motion of the molecule in space. Our studies predict the activation energy for this to be rather low, and hence it should be possible to observe this experimentally.

Raman phonon spectra of pentacene polymorphs by A. Brillante; R.G. Della Valle; L. Farina; A. Girlando; M. Masino; E. Venuti (32-36).
We report for the first time lattice phonon Raman spectra of pentacene measured by means of a Raman microprobe technique. We experimentally prove the existence of two polymorphs, as expected from recent structural studies. A comparison with Quasi Harmonic Lattice Dynamics calculations, previously performed starting from the available X-ray data, help us in identifying the phase to which each crystal belongs.

Observation and decay of free and ligated metalloporphyrins in the gas phase by Niloufar Shafizadeh; Lahouari Krim; Sébastien Sorgues; Benoit Soep (37-44).
A soft desorption technique has been tested in order to vaporize metalloporphyrins in a supersonic jet and study their energetics and dynamics in the gas phase at low temperatures. Several metalloporphyrins: ZnTPP, NiTPP, CoTPP and ligated RuTPP–CO have been detected by resonant 2 photon ionization. ZnTPP was observed with high sensitivity owing to its long S1 lifetime, while the transition metal porphyrins displayed less intense ion yields due to short excited state lifetimes. The ligated ruthenium complex could be observed intact and femtosecond excitation at 400 nm revealed the ultrashort excited state dissociation of the ruthenium metalloporphyrin.

Rate constants and isomeric branching of the Cl–isoprene reaction by Wenfeng Lei; Dan Zhang; Renyi Zhang; Luisa T. Molina; Mario J. Molina (45-50).
The formation rates and isomeric branching ratios of the four possible adducts arising from the Cl–isoprene reaction have been calculated using canonical variational transition state theory (CVTST). In addition, RRKM/master equation (ME) formalism is employed to investigate isomerization of the Cl–isoprene adducts. We found that there is no evidence for an energetic barrier for Cl addition to isoprene and the Morse potential well represents the energetics along the reaction coordinate. The results reveal the importance of terminal Cl addition to isoprene in partitioning of the final reaction products and in determining the reaction pathways.

B3LYP and CCSD(T) calculations with various basis sets show that the Sc+CO2→ScO+CO reaction can proceed by two distinct pathways, Sc+CO2→(η1-O)ScOCO→(η2-O,O)cyc-ScCO2→ScO+CO→ end-/side-OScCO with the largest barrier of 3.2 kcal/mol and barrier-less Sc+CO2→(η 2-C,O)(ScOC)O→end-OScCO→ScO+CO. The `end-on' OScCO complex bound by 7.2 kcal/mol relative to ScO+CO is predicted to be the dominant reaction product. The reaction is more than 30 kcal/mol exothermic and the Sc atom is expected to be efficient in reforming CO2 to carbon monoxide.

On the basis of density functional theory, a scheme for efficient calculating Fukui function, local softness, and hardness within atom-bond electronegativity equalization method plus σπ model is delivered. By the local softness obtained through this model, we investigated the regioselectivity of Diels–Alder reaction in light of local HSAB principle, and the result is in accordance with experimental and theoretical results. Moreover, we demonstrate for the first time that the maximum hardness principle is obeyed in the stereospecific Diels–Alder reactions, i.e., compared with exo isomer, the more stable endo isomer usually has larger hardness value, consequently, composes the main product.

Cl2 molecular elimination reaction from 1,2-dichloroethane by Li Zhu; Joseph W. Bozzelli (65-72).
The transition state energy for Cl2 molecular elimination from 1,2-dichloroethane is computed at compostite CBS-Q (based on B3LYP/6-31++G(d) optimized structure) and G3MP2 (based on MP2/6-31++G(d) optimized structure) levels of theory. Forward rate constants are calculated as 4.25E11  T0.826·exp(−93.4  kcal mol −1/RT) and 7.62E10  T0.892·exp(−98.5  kcal mol −1/RT)  s −1 , respectively for these two methods. Intrinsic reaction coordinate (IRC) analysis is performed to verify the transition state structures. While the overall, elimination of Cl2 plus olefin formation has a relative low ΔH rxn; the reaction with these barriers is evaluated to be not important in thermal reactions of chlorocarbons.

We studied the structures of UO2(CO3)3 4−, Ca2UO2(CO3)3 0, and Ba2UO2(CO3)3 0 using HF, MP2, and B3LYP methods. We obtained an unusual long uranyl bond for the UO2(CO3)3 4− complex by ECP calculations. This study shows that it is crucial to explicitly include the counterion(s) and the hydration water molecules in the calculation of the aqueous uranyl tricarbonate complex. The calculated structure of the hydrated Ca2UO2(CO3)3 0 has good agreement with the experiment.

Estimation of the anharmonic quantum density of states in large systems by Pascal Parneix; Nguyen-Thi Van-Oanh; Philippe Bréchignac (78-84).
We present a mixed quantum/classical model which allows an estimation of the quantum density of vibrational states in large anharmonic systems. The construction of the model is based on the modification of the harmonic quantum density of states by using the evolution of the classical anharmonic mean energy as a function of temperature, computed from a classical canonical Nosé simulation. The validity of the assumptions proposed in this model has been tested for two 1-D and 2-D model systems for which exact quantum calculations are feasible.

Molecular dissociation by mid-IR femtosecond pulses by L. Windhorn; T. Witte; J.S. Yeston; D. Proch; M. Motzkus; K.L. Kompa; W. Fuß (85-90).
By focusing a MIR femtosecond laser in a cell containing gas-phase metal carbonyls, the resonant infrared multiphoton dissociation of molecules was observed. Cr(CO)6,Mo(CO)6,W(CO)6, and Fe(CO)5 could easily be dissociated, which requires an excitation to at least v=7 or 8 of the CO stretch vibration. After irradiation with ∼150 fs pulses at 5  μm the metal carbonyl practically disappears in favor of free CO, as detected by the IR spectrum. By comparing the power dependence of the total conversion with a model, we can infer that only few vibrational degrees of freedom are involved in the excitation process.

Finite single-wall carbon nanotubes (p,q), characterized by p=n+l+1, q=nl, 0⩽ln, and n=1,2,…, consisting of even nanotube (NT) sections may be viewed as a chain of aromatic `molecules' with each `molecular-unit' consisting of two NT sections of 4n+2 sp2-bonded carbon atoms. Such (group II) NTs, have large energy gaps for short tubes, whereas for tubes of intermediate lengths, the energy gap decreases monotonically as a function of NT length. The energy gap behaviors are totally different from the previously studied group I NTs with p=n+l and q=nl, which exhibited oscillatory characteristics.

Structures of hydrogen molecules in single-walled carbon nanotubes by Yuchen Ma; Yueyuan Xia; Mingwen Zhao; Minju Ying (97-102).
Molecular-dynamics simulations were used to investigate the structures of hydrogen molecules formed within (5,5), (9,0) and (10,10) single-walled carbon nanotubes (SWNT). In the (5,5) and (9,0) SWNTs, H2 molecules preferably form linear lines and helical curves, showing the strong confinement of small-diameter SWNTs. The structure of H2 molecules in the (5,5) and (9,0) SWNTs is different, showing the dependence on the helicities of the SWNTs. The accumulation process of H2 molecules in the (10,10) SWNT is also presented, showing the formation of H2 molecules cylindrical shells.

A medium size and rigid molecule (2,3-naphto-1,3-dioxole) has been selected for this study because full anisotropic reorientation is expected and because its symmetry elements dictate the orientation of the rotation–diffusion tensor. NMR measurements include direct cross-relaxation rates (which yield the three rotation–diffusion coefficients by assuming the length of CH bonds) and remote cross-relaxation rates (which, by using these rotation–diffusion coefficients, yield distances between a given carbon and remote protons). Two different solvents have been used: carbon disulfide and dimethyl sulfoxide. In both solvents, the same type of reorientation anisotropy is observed although with different ratios of rotation–diffusion coefficient values, presumably due to specific intermolecular interactions undergone by the dioxole ring. This would also explain geometrical variations at the level of this moiety.

Symmetrically substituted perhaloporphyrins, 2,3,7,8,12,13,17,18-octahalo (bromo and chloro)-5,10,15,20-tetraphenylporphyrins (H2OBTPP and H2OCTPP) have been examined as receptors for binding to Lewis bases. The kinetics of base binding to haloporphyrins showed an enhanced binding constants for H2OBTPP relative to H2OCTPP and follows a linear trend with increase in pK a values of the bases. The binding of Lewis-bases to perhaloporphyrin core is largely influenced by the extent of nonplanarity of the porphyrin core than the electron-withdrawing ability of the substituents. The higher binding constants of H2OBTPP relative to H2OCTPP have been interpreted in terms of greater nonplarity of the former than the latter.

Raman spectroscopic study on photochromic reaction of a diarylethene derivative by Chie Okabe; Nobuyuki Tanaka; Tuyoshi Fukaminato; Tsuyoshi Kawai; Masahiro Irie; Yoshinori Nibu; Hiroko Shimada; Alexander Goldberg; Shinichiro Nakamura; Hiroshi Sekiya (113-118).
Photochromic reaction of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene has been studied by FT-Raman spectroscopy. The Raman bands in the 1300–1700  cm −1 region of the open-ring isomer of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene are clearly distinguished from those of the closed-ring isomer. The vibrational assignment has been made by measuring the polarized Raman spectrum and density functional theory calculations at the B3LYP/6-31G** level.

The lowest E1(3Σ+) Rydberg state of the CdNe van der Waals complex was investigated by an optical–optical double resonance method. The A0 +(3Π) and B1(3Σ+) states were used as intermediates in the excitation from the X0 +(1Σ+) ground state. Bound–bound excitation spectra of the E1←A0+ transition were recorded. They constitute a first observation of CdNe in the E1 state. Spectroscopical parameters of the E1-state potential well were determined. In the excitation spectrum of the E1←B1 transition, a nodal structure of bound-free transitions was observed and elucidated by a projection of the B1-state vibrational wave-function onto the E1-state potential barrier according to the prediction of ab initio calculations.

Resonance Raman characterization of porphycene anions by Rabbani M. Gulam; Toshio Matsushita; Saburo Neya; Noriaki Funasaki; Junji Teraoka (126-130).
Resonance Raman spectra of reduced porphycenes were measured in an effort to characterize structural changes concomitant with chemical reduction. Frequency shifts were observed for reduced porphycenes in certain skeletal vibrational modes. The observed resonance Raman behavior was generally consistent with the results of vibrational analysis by quantum chemical calculations based on density functional methods.

A drug delivery system with cubic liquid-crystalline phase structure (cubic phase) containing the anti-cancer drug Carboplatin is studied. It is demonstrated that the combination of pulsed field gradient (PFG) NMR and MAS-NMR is a useful tool to study the biophysical properties of a cubic phase. The linewidth in 1H-NMR spectra is narrowed by MAS, which can be exploited to perform PFG diffusion NMR experiments under high-resolution conditions. Measurement of self-diffusion coefficients of all components of the cubic phase becomes possible. The influence of polyethylene glycol chains on the drug mobility is discussed. It is shown that polyethylene glycol chains interact with Carboplatin.

Investigations of triplet excimer of naphthalenophane by emission and transient absorption measurements in solution by Minoru Yamaji; Hidehiko Tsukada; Jun Nishimura; Haruo Shizuka; Seiji Tobita (137-142).
By using syn-[3.4](1,5)naphthalenophane (44NPP) whose naphthalene rings are in a parallel form, photophysical and photochemical properties of naphthalene triplet excimer are reported. 34NPP emits only excimer fluorescence at 295 and 77 K, and phosphorescence was absent even at 77 K. Based on the transient absorption spectra of 34NPP, it is shown that the excimeric triplet state of 34NPP is formed without the locally excited triplet at 295 and 77 K. The triplet excimer of 34NPP is produced via intersystem crossing from the singlet excimer state with a rate of 1.4×107   s −1 at 295 K. The triplet excimer in a parallel configuration is shown to be non-phosphorescent.

The complex (quinap)ReI(CO)3Cl, with quinap =1-(2-diphenylphosphino-1-naphthyl)isoquinoline, shows a luminescence at λ max =582  nm with φ=0.12 and τ=1.4  μs in solution (e.g., CH2Cl2) at r.t. It is suggested that this emission originates from a ReI to quinap metal-to-ligand charge transfer triplet.

The time-dependent anisotropy r g (t) – measured by exciting degenerate levels in an ensemble of stationary molecules of sufficiently high symmetry and probing a transition from the g-fold degenerate levels (g=2 or 3) to a non-degenerate level – falls (within a few hundred femtoseconds) to an asymptotic value significantly lower than r g (0). The loss of electronic coherence in such systems is examined afresh, and the anisotropy is shown to follow a double or single exponential decay depending on whether the random variables of interest (population and phase) are statistically independent or not; some errors in previous derivations are also corrected.

We demonstrate that the fluorescence spectrum from water droplets containing the dye acridine can be used to accurately determine the pH of droplets in the radius range 15–60  μm and in the pH range 3–10. Using a laser pump wavelength of 355 nm and probing the fluorescence with a 0.5 m spectrograph and CCD detector, the particle size parameters are sufficiently large in this radius range that the fluorescence spectrum conforms to that of the bulk phase, permitting a direct unambiguous determination of droplet pH. This could provide a sensitive method for studying directly the uptake of acidic or basic reagents on aerosol droplets.