Chemical Physics Letters (v.342, #3-4)

Based on an ab initio method, the optimization of possible geometry structures and the calculation of the adsorbed energies, charges and populations have been carried out for the adsorbed-decomposition of NO, CO and CH2O on a TiO 2(1 1 0) defect surface. The orbital interaction particulars and a promising mechanism of the adsorbed-decomposition of NO on TiO 2(1 1 0) (1×1) defect surface are proposed.

Carbon monoxide-assisted growth of carbon nanotubes by Y.H Tang; Y.F Zheng; C.S Lee; N Wang; S.T Lee; T.K Sham (259-264).
Carbon monoxide was used to synthesize carbon nanotubes (CNTs) in a hot-filament chemical vapor deposition (HFCVD) system. Transmission electron microscopic investigation showed that the CNTs were multi-walled and had an average diameter of 30 nm and 20 layers. Nickel was observed in the tips of the nanotubes and played a crucial role in the formation of multi-walled carbon nanotubes (MWNT)s. The CNTs synthesized from carbon monoxide validate the oxide-assisted growth method.

Reversible water-solubilization of single-walled carbon nanotubes by polymer wrapping by Michael J. O'Connell; Peter Boul; Lars M. Ericson; Chad Huffman; Yuhuang Wang; Erik Haroz; Cynthia Kuper; Jim Tour; Kevin D. Ausman; Richard E. Smalley (265-271).
Single-walled carbon nanotubes (SWNTs) have been solubilized in water by non-covalently associating them with linear polymers, most successfully with polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS). This association is characterized by tight, uniform association of the polymers with the sides of the nanotubes. A general thermodynamic drive for this wrapping is discussed, wherein the polymer disrupts both the hydrophobic interface with water and the smooth tube–tube interactions in aggregates. The nanotubes can be unwrapped by changing the solvent system. This solubilization process opens the door to solution chemistry on pristine nanotubes, as well as their introduction into biologically relevant systems.

Radical-induced core destruction of monolayer-protected metal clusters by N. Sandhyarani; T. Pradeep; Joseph.S. Francisco (272-276).
Chlorine radicals formed by the UV irradiation of solvents such as CCl4 react with alkanethiolate-protected gold and silver clusters forming the corresponding metal salts and variously halogenated disulfides. Reaction rates depend on the chain length of the monolayer; whereas the reaction is over within tens of seconds for clusters protected with shorter chain monolayers, those with longer chains need several minutes for complete conversion.

Electron photodetachment from [Fe(CN)6]4−: photoelectron relaxation and geminate recombination by Victor Lenchenkov; Jeremiah Kloepfer; Victor Vilchiz; Stephen E. Bradforth (277-286).
The relaxation of photoelectrons detached from aqueous [Fe(CN)6]4− is captured via 50 fs pump-multicolor probe spectroscopy. The recovered relaxation (570 fs, 0.53 eV exponential spectral shifting due to solvation) is very similar to the electron dynamics subsequent to ionization of water, however the trapping time is slower (∼310  fs ). The observed geminate recombination varies strongly with the effective charge on the photodetached species. The influence of the Coulomb potential is successfully unraveled from the recombination dynamics while extracting the average electron photoejection distance. In contrast to threshold detachment of monovalent anions, electrons are ejected to ∼15  A ̊ .

A novel oscillating system with no dark oscillations, possessing a single photochemical autocatalytic feedback loop triggered by VUV photolysis of ozone, is proposed. This gas phase photochemical oscillator is extremely simple, involving only ozone, triplet oxygen radical, molecular oxygen, and one additional species, which provides the necessary destabilizing exit reaction. The phase diagram for the suggested system is constructed, and experimentally realistic values of rate constants and light intensity corresponding to the oscillatory regime are found.

By a direct fit to 431 experimental wave numbers of pure rotational and vibration-rotational transitions of ArH+X1 Σ + in six isotopic variants, 16 coefficients of radial functions defining the Born–Oppenheimer potential energy, adiabatic and nonadiabatic effects have been determined at σ ̂ =0.885 and F=4.07×1015. Using a relation between nonadiabatic rotational effects and electric and magnetic properties of the molecule, a permanent dipole moment of 40 Ar 1 H + is estimated as μ 0=2.12(55) D which agrees both with the ab initio value 2.2(1) D and the experimental result μ 0=3.0(6) D within quoted error limits.

Electron detachment dissociation of peptide di-anions: an electron–hole recombination phenomenon by Bogdan A. Budnik; Kim F. Haselmann; Roman A. Zubarev (299-302).
A novel electron–ion reaction mimicking positron capture is reported for gas-phase polypeptide di-anions. Bombardment of the latter by >10 eV electrons produced electron detachment followed by backbone dissociation, noticeably cleavage of the N–C α bond. On the other hand, reaction with hydrogen cations resulted in losses of neutral groups but not in backbone cleavage. It is proposed that the N–C α dissociation is due to recombination along the peptide chain of a positive radical charge (hole) with a negative charge. The new reaction is likely to find application in mass spectrometry for primary structure determination of acidic polypeptides.

The relative retardation of intramolecular charge transfer (ICT) rate and solvation dynamics of coumarin 490 (C-490) in the water pool of the Aerosol OT (AOT)-heptane reverse micelle compared to normal water have been investigated using picosecond time resolved emission spectroscopy. We have observed a maximum 3.5 times change in the ICT rate of C-490 in the water pool of the reverse micelle compared to normal water. The retardation of ICT rate is much smaller compared to the several thousands fold decrease in the solvation dynamics in the water pool of the reverse micelle compared to pure water.

A 2D 15 N–15 N chemical shift correlation method is proposed to assign resonances in non-selectively or uniformly 15N labeled biological solids. A reverse cross-polarization sequence is used to transfer 15N magnetization to the directly bonded amide-protons through the 1 H–15 N dipolar coupling and a longitudinal proton magnetization transfer via 1 H–1 H dipolar couplings is used to mix amide-1H resonances. It is shown that this procedure allows the observation of cross-peaks between 15 N chemical shift resonances of adjacent amino acid residues with a proton mixing time as short as 10 μs in a polycrystalline sample of N-acetyl-L–15N-valyl-L–15N-leucine.

The effect of reversible binding of the autocatalyst on the lateral instability of reaction fronts by Éva Jakab; Dezső Horváth; Ágota Tóth; John H. Merkin; Stephen K. Scott (317-322).
Numerical studies of lateral instability have been carried out on reaction fronts arising in a cubic autocatalytic system with a reversible removal of the autocatalyst introduced as a fast equilibrium producing an immobile complex. We have shown that even a small amount of complexing agent substantially decreases the critical ratio of diffusion coefficients; and since no abrupt extinction of front property exists, lateral instability may even occur in systems where the autocatalyst diffuses faster than the reactant.

Optical limiting response in a unsymmetrical dithiolene metal complex (Me4N)2[Zn(dmit)(Sph)2] by Zhenrong Sun; Minghong Tong; Heping Zeng; Liangen Ding; Zugeng Wang; Zhizhan Xu; Jie Dai; Guoqing Bian (323-327).
We report the measurements of reverse saturable absorption (RSA) and optical limiting responses in a unsymmetric dithiolene metal complex (Me4N)2[Zn(dmit)(Sph)2] with nanosecond and picosecond laser pulses at 532 nm. The experimental results show that the triplet–triplet absorption is responsible for the measured RSA, resulting in optical limiting responses. The measured data can be well simulated by a rate equation model to obtain the absorption cross-section of the excited state.

Triplet state characteristics of 2,2- and 4,4-biphenyldiols have been investigated in different organic solvents using 248 nm nanosecond laser flash photolysis technique. While for 2,2-biphenyldiol, the triplet state of the molecule is produced as the only transient, for 4,4-biphenyldiol, some phenoxyl radicals are also formed along with the triplet state following the laser excitation. Triplet quantum yields of 2,2-biphenyldiol in different solvents are seen to be much lower than those of 4,4-biphenyldiol. The differences in the laser flash photolysis results of the two diols have been explained on the basis of the presence and the absence of intramolecular hydrogen bonding in the two molecules.

We have measured the change of the equilibrium constant with pressure for four foldamers of m-phenylene ethynylene. From the change in ln  K over short pressure intervals, we have extracted the change in partial molar volume vs pressure in polymethylmethacrylate (PMMA) and polytertbutylmethacrylate (PtBMA). For a given foldamer the ratio of ΔV in the two media is larger for shorter oligomers. The effect of replacing hydrogen by CH3 in the foldamer is greater in PMMA than in PtBMA. In a given medium, the ΔV per monomer unit is essentially independent of the chain length.

Anions of the hydrogen-bonded guanine–cytosine dimer – theoretical study by Johan Smets; Abraham F. Jalbout; Ludwik Adamowicz (342-346).
The quantum mechanical calculations performed in this work revealed an interesting nature of the stationary states of an excess electron in the guanine–cytosine (G–C) anion. The electron can form either a dipole-bound (DB) state or a covalent state. The coexistence of the two states may enhance the ability of the G–C dimer to capture an excess electron.

The scaling study for the hydrogen bonding networks by Hai-jun Wang; Xin-wu Ba; Min Zhao; Ze-Sheng Li (347-352).
The theory of reversible gelation is shown to be applicable to the hydrogen bonding (HB) system by analyzing their similarities in statistical viewpoint. The size distribution of HB cluster, the gelation condition and the relationship between the equilibrium conversions and Gibbs free energy for the one-component HB system are given. Based on these results, the sol–gel phase transition for some HB systems is predicted to take place by obtaining the generalized scaling law satisfied by the radius of gyration of HB clusters. Furthermore, taking water clusters as an example, several possible experimental plans to test the sol–gel phase transition are proposed.

We apply symmetry theorems to the problem of heteronuclear dipolar recoupling in the presence of magic-angle spinning in solid-state NMR. Examples are shown in which the 13C NMR signal is acquired while rotor-synchronized pulse sequences with the symmetry R181 7 or R182 5 are applied to the 1H spins. This allows recoupling of heteronuclear dipolar interactions combined with homonuclear decoupling of the irradiated 1H spins. The structure of the 13C NMR spectrum is sensitive to bond lengths and bond angles. A two-dimensional procedure is described for applications to multiply isotopically labelled systems.

Three-frequency nuclear quadrupole resonance of spin-1 nuclei by K.L. Sauer; B.H. Suits; A.N. Garroway; J.B. Miller (362-368).
We introduce a new nuclear quadrupole resonance (NQR) method for the detection of spin-1 nuclei, where the transition excited and directly detected is not irradiated at all. It is demonstrated, theoretically and experimentally, that the irradiation of a powder sample containing spin-1 nuclei by two of the three characteristic NQR frequencies can result in free induction decay (FID) and echo signals at the third NQR frequency. We present the optimal conditions for such three-frequency NQR experiments and compare theory with experiment using 14N (I=1) in a powder sample of sodium nitrite.

Topological coordinates for toroidal structures by István László; André Rassat; P.W. Fowler; Ante Graovac (369-374).
Physically realistic 3D geometries for toroidal trivalent networks can be produced from graph theoretical information alone, using the eigenvectors resulting from diagonalisation of the vertex adjacency matrix. Arguments from the problem of a quantum particle constrained to move on a surface show that three vectors suffice for zero-genus spherical cages, whereas four are needed for decorations of surfaces with genus 1 (in contrast to previous suggestions). Solutions for the problems arising from the systematic high degeneracies in the spectra of polyhex tori are proposed.

A new formulation of the short-range anisotropic orientating potential experienced by a biaxial apolar particle dissolved in a uniaxial mesophase is given, where the interactions between the solute and the single molecules constituting the solvent are explicitly taken into account. To test the model, the order parameters of naphthalene have been calculated by using the Monte Carlo-Metropolis method and the results compared with 1 H-NMR experimental data: the excellent agreement with the order parameters of the same molecule in the `magic' nematic mixture 55 wt% ZLI1132 + EBBA seems to confirm once more that long-range orientational effects can be neglected for solutes dissolved in that phase.

Sampling efficiency of molecular dynamics and Monte Carlo method in protein simulation by Hiroshi Yamashita; Shigeru Endo; Hiroshi Wako; Akinori Kidera (382-386).
Molecular dynamics (MD) and Monte Carlo (MC) method were compared in terms of the sampling efficiency in protein simulations. In the comparison, both methods use torsion angles as the degrees of freedom and the same force field, ECEPP/2. The MC method used here is the force-bias scaled-collective-variable Monte Carlo (SCV MC) [A. Kidera, Int. J. Quant. Chem. 75 (1999) 207], which corresponds to a finite step size extension to Brownian dynamics. It is shown that MD has about 1.5 times larger sampling efficiency. This difference is attributed to the inertia force term in MD, which does not exist in MC.

Molecular docking of α-cyclodextrin inclusion complexes by genetic algorithm and empirical binding free energy function by Wensheng Cai; Baoyun Xia; Xueguang Shao; Qingxiang Guo; B. Maigret; Zhongxiao Pan (387-396).
A molecular docking method that predicts the lowest energy geometries of inclusion complexes between host and guests was developed and tested, in combination with a new simple empirical function that estimates the free energy of binding. The total interaction energies of the host–guest inclusion complexes were optimized using a genetic algorithm (GA). The docking method was applied to 43 complexes of α-cyclodextrin (α-CD) and mono- or 1,4-disubstituted benzenes with known binding constants. The new simple empirical free energy function was calibrated by the 43 docked complex structures and gave a good relationship between the predicted binding constants and the observed values.

On the prediction of band gaps from hybrid functional theory by J. Muscat; A. Wander; N.M. Harrison (397-401).
Details of the band gap and band widths within materials are of fundamental importance to a wide range of applications. A hybrid scheme is used to predict the band gaps of a variety of materials. The electronic structure of silicon is examined in some detail and comparisons with alternative theories are made. Agreement with experimentally derived band gaps is at least as good as that obtained with sophisticated correlated calculations or perturbation theories. The functional is straightforward to implement, computationally efficient and produces ground state energy surfaces which are significantly more accurate than those computed using the best gradient corrected density functionals currently in use.

Gaussian-theory calculations, G1, G2 and G3, are in good agreement with experiment for the proton transfer reaction free energy from phenol and 3-chlorophenol to water. This is in contrast to earlier calculations at the G1 and G2 levels that had indicated errors of 600–750  kJ mol −1 . There are no apparent artifacts of MP4-level basis set corrections in these calculations. There is no evidence that these theories perform any less well when applied to aromatic compounds than systems for which they have been successfully applied previously.

A B3LYP/6-31G** study on the chlorination of ammonia by hypochlorous acid by J. Andrés; M. Canle L.; M.V. Garcı́a; L.F. Rodrı́guez Vázquez; J.A. Santaballa (405-410).
B3LYP/6-31G** calculations were performed on the chlorination of NH3 by HOCl, considering explicit participation of zero, one, two, three and four water molecules. Detailed analysis of the free energy profiles shows the mechanism is water-assisted. Cl transfer from the O of HOCl to the N of NH3 is accompanied by proton transfers along a chain of hydrogen-bonded water molecules. Preferential stabilisation of the transition structure arises from cooperative fluctuations involving the hydrogen-bonded chain of water molecules. Calculations show solvent reorganization associated with proton translocations is coupled with Cl transfer in the vicinity of the transition structure.

On the semiclassical dissociation yields of the doubly excited states of H2 by Itamar Borges; Carlos E. Bielschowsky (411-416).
We have calculated semiclassical dissociation yields for the dipole allowed states Q1 1 Π u(1), Q1 1 Σ + u(1), Q1 1 Σ + u(2), and Q2 1 Π u(1) and for the dipole forbidden states Q1 1 Σ + g(1) and Q2 1 Σ + g(1) of H2 employing two different sets of accurate resonant widths and our computed potential energy curves. The semiclassical dissociation yields thus obtained have been compared with experimental estimations and employed to discuss photon and electron impact dissociation measurements through these purely repulsive states. A set of recommended dissociation yields values for each state has been suggested for future use.

Hole-burning spectroscopy and ab initio calculations for the aniline dimer by Norifumi Yamamoto; Kazuyuki Hino; Koichi Mogi; Kazuhiko Ohashi; Yoshiko Sakai; Hiroshi Sekiya (417-424).
The mass-resolved spectrum indicates that only one conformation contributes to sharp peaks observed in the S1←S0 resonance two-photon ionization (R2PI) spectrum. However, geometry optimizations at the MP2/cc-pVDZ level suggest that two conformational isomers are stable: a head-to-head conformation with a single NH⋯N hydrogen bond and a head-to-tail conformation with double NH2⋯π hydrogen bonds. The calculations show that the head-to-tail conformation is more stable by 1.18  kcal mol −1 .

Resonance Raman and ab initio investigation of I2-olefin complexes by Xuming Zheng; Wei-Hai Fang; David Lee Phillips (425-433).
We report resonance Raman spectra of selected I2:olefin complexes obtained with excitation within the ∼270 nm absorption band of the complex. Most of the Raman intensity appears in the overtones and combination bands of the nominal I–I stretch and CC stretch modes. Ab initio calculations were done to examine the structure of the ground state of the complex. The Raman vibrational frequencies and the ab initio optimized geometry indicate both the I–I and CC bond order decrease moderately in the ground state complex.

Conformational dynamics of the dimethyl phosphate anion in solution by Eric Schwegler; Giulia Galli; François Gygi (434-440).
The conformational dynamics of the DNA phosphodiester linkage in solution is modeled by performing first principles molecular dynamics simulations of the dimethyl phosphate anion ((CH3O)2PO2 or DMP) in water, in the presence of a sodium cation. We observe conformational changes in DMP in solution, which are related to the approach of the sodium cation to the anionic oxygens of DMP. These simulations demonstrate that the presence of a counterion may play a significant role in driving the transition between different conformers of DMP.

On interpreting the photoelectron spectra of MgO by Charles W. Bauschlicher; Harry Partridge (441-446).
The 2Σ+ and 2Π states of MgO and the 1Σ+ , 1Π, and 3Π states of MgO are studied using the ACPF approach. The computed spectroscopic constants are in good agreement with the available experimental data. The computed Franck–Condon factors and photodetachment overlaps are compared with experiment.

The heat of formation of sulfine, CH2SO, revisited: a CBS-QB3 study by Paul J.A. Ruttink; Peter C. Burgers; Moschoula A. Trikoupis; Johan K. Terlouw (447-451).
The heat of formation (ΔH f) of sulfine, CH2SO, has been determined by the CBS-QB3 quantum chemical method, using 10 reactions, including the isodesmic reaction CH2SO+SO2→CH2S+SO3. The derived ΔH f of sulfine, −30±6  kJ/mol at 298 K, lies midway between two previously calculated values: −9±14 and −52±10 kJ/mol. The CBS-QB3 derived ΔH f (0 K) was very recently validated against the very accurate Weizmann-1 (W1) method [L.N. Heydorn, et al., Z. Phys. Chem. 215 (2001) 141] and there is excellent agreement between the two methods, within 3 kJ/mol. Our recommended value is evaluated against experimental observables, such as the measured proton affinity of CH2SO and the appearance energy of CH2S+OH from dimethyl sulfoxide ions, CH3S(O)CH3 •+.

High-level ab initio calculations of the torsional potential of glyoxal by J.C. Sancho-Garcı́a; A.J. Pérez-Jiménez; J.M. Pérez-Jordá; F. Moscardó (452-460).
The torsional barrier of glyoxal has been extensively studied using state-of-the-art ab initio methodologies ranging from Hartree–Fock (HF) and Möller–Plesset theories up to several developments of the coupled-cluster (CC) formalism, namely: CCD, CCSD, CCSD(T), BD and BD(T). We present a thorough discussion about the influence of the basis set size on the results, as well as a comparison between all the theories employed. The values obtained from the CCSD or CCSD(T) treatment achieve the so-called `calibration accuracy' (±1  kJ/mol) from which an extremely accurate torsional potential has been derived.

The lifetimes were measured for rovibronic levels lying slightly lower than the top of the potential barrier in the H 2   EF 1Σ g + double-minimum state, with a delayed coincidence method following electron impact excitation from the ground state. Dependence of lifetimes on the rotational levels in a pair of vibrational levels is explained in terms of perturbation caused by rotation–vibration interaction between these levels. Lifetimes of some D 1Π u and B 1Σ u + levels were also measured to estimate the cascade effects.