Chemical Physics Letters (v.648, #C)

Contents (iii-xii).

The systematic study of the growth of water clusters is of interest. The potential energy profiles for the reactions F + (H2O) n , (n  = 1–4) have been investigated using the CCSD(T) method. Final energetics have been evaluated with the CCSD(T)/cc-pVQZ method. All the stationary points have been located. Structurally, the stationary points on the F + (H2O) n potential energy surfaces are related. Energetically, the water tetramer reaction F + (H2O)4, water trimer reaction F + (H2O)3, and water dimer reaction F + (H2O)2 are barrierless, while the water monomer reaction F + H2O has a ∼2 kcal/mol barrier.

Solid state synthesis and tunable luminescence of Li2SrSiO4:Eu2+/Ce3+ phosphors by Zhen Wei; Yongli Wang; Xiaobo Zhu; Jinyu Guan; Weixi Mao; Juanjuan Song (8-12).
Li2SrSiO4:Eu2+/Ce3+ phosphors have been synthesized by solid state reaction. Eu2+ and/or Ce3+ doped Li2SrSiO4 have the hexagonal crystal structure, indicating the doped Eu2+ and/or Ce3+ ions have no influence on the crystal structure of Li2SrSiO4. Under the excitation at 365 nm, Eu2+ or Ce3+ doped Li2SrSiO4 phosphors give characteristic emission bands of Eu2+ or Ce3+, respectively. For Li2SrSiO4:Eu2+/Ce3+ phosphors with the fixed Ce3+ concentration, two emission bands originating from Eu2+ and Ce3+ can be observed. On the basis of luminescent properties of Li2SrSiO4:Eu2+/Ce3+ phosphors, we can conclude that energy transfer from Ce3+ to Eu2+ occurs in Li2SrSiO4.

We have studied the pressure-induced phase transitions of I2O5 in diamond anvil cells using optical spectroscopy and X-ray diffraction. X-ray and Raman data indicate that the linear structure of phase I isostructurally transforms into a stacking layer structure of phase II at 5 GPa and, then, into an amorphous network of phase III at 15 GPa. Phase III further transforms into amorphous solid at 23 GPa, which can be recovered at ambient condition. Anisotropic van der Waals interaction between lone pairs of iodines and neighboring molecules drives to the observed phase change with increasing of iodine coordination number.

Fluorescence spectra of all-trans-1,6-diphenyl-1,3,5-hexatriene were measured in n-hexadecane at 99 °C by varying λ exc in the 294–404 nm range. Resolution of this spectral matrix into s-trans,s-trans and s-cis,s-trans conformer fluorescence spectra yields the λ exc dependence of fractional contributions which are converted to conformer specific fluorescence excitation spectra. Conformer absorption spectra obtained from the fluorescence excitation spectra are remarkably similar, but differ significantly from absorption spectra derived from a spectrothermal absorption spectral matrix measured in n-alkanes under isopolarizability conditions. The results reveal substantial conformer equilibration in the excited state. Theory is consistent with adiabatic conformer equilibration in the 21Ag state.

Interaction of atomic hydrogen with anthracene and polyacene from density functional theory by Ricardo M. Ferullo; Norberto J. Castellani; Patricia G. Belelli (25-30).
The interaction of atomic hydrogen with two linear polycyclic aromatic hydrocarbons (PAHs), anthracene and polyacene (the polymer of benzene), was studied within the density functional theory (DFT). Using a proper dispersion-corrected method (DFT-D) the preferential physisorption sites were explored. The activation barrier for the bond formation between a peripheral C and the incoming H was calculated to be 58.5 and 34.1 meV with pure DFT on anthracene and polyacene at its antiferromagnetic ground state, respectively. DFT-D, although improves the description of the physisorbed state, tends to underestimate the chemisorption barriers due an artifact arising from the dispersion correction.

We report gas-phase enthalpies of formation for the set of energetic molecules NTO, DADE, LLM-105, TNT, RDX, TATB, HMX, and PETN using the G2, G3, G4, and ccCA-PS3 quantum composite methods. Calculations for HMX and PETN hitherto represent the largest molecules attempted with these methods. G3 and G4 calculations are typically close to one another, with a larger difference found between these methods and ccCA-PS3. Although there is significant uncertainty in experimental values, the mean absolute deviation between the average experimental value and calculations are 12, 6, 7, and 3 kcal/mol for G2, G3, G4, and ccCA-PS3, respectively.

Display OmittedWe have carried out a numerical study of the structural, thermodynamic and optical properties of the partially fluorinated 5-n-alkyl-2-(4-n-(perfluoroalkyl-metheleneoxy)phenyl) free-standing smectic film in air under the action of the external electric field E. Calculations, based upon the extended McMillan′s mean-field theory with anisotropic forces, show a stepwise reduction of the value of the Helmholtz free energy and the reflectivity of the partially fluorinated smectic film in air, as the temperature is raised above the bulk smectic-A-isotropic transition value. It has been shown, by solving the self-consistent nonlinear equations for the orientational and translational order parameters, that the electric field E may not only affect the layer-thinning transition sequences, but also change the first multilayer jump in the film thickness, whereas practically does not affect the reflectivity of the partially fluorinated smectic film in air. The partially fluorinated LC compounds reveal a compression of a smectic layer spacing during the layer-thinning transitions which is supported by the negative values of the thermal expansion coefficient.

Transition metal (TM) doped boron clusters have gained increasing interests and become a new research direction because the diameter of borospherene makes B cage a suitable candidate to accommodate a range of transitional metal atoms or small molecules inside to form endohedral M@B n borospherenes. Moreover, the formed M@B n clusters not only exhibit size and shape dependent properties, but also provide a rich source to discover novel molecular structures and nanostructures with novel physical and chemical properties.Geometries associated with relative stabilities and energy gaps of W@B n (n  = 20, 24, 28, 32) are systematically investigated by density functional theory. The calculated averaged atomic binding energies reveal that the W@B20 has enhanced stability over other clusters. Interestingly, the irregular W@B24 fullerene with bigger HOMO–LUMO gap is supposed to have stronger chemical activity. Moreover, the interactions between W and B24 cage is strongest one based upon the calculated binding energy between W and B cage, the doped W changes the properties of pure cages. The calculated dipoles of W@B n reveal that the irregular W@B24 cage is a nonpolar molecule.

Computational studies on linear, second and third-order nonlinear optical properties of novel styrylquinolinium dyes by A. Karakas; M. Karakaya; M. Taser; Y. Ceylan; A. Gozutok; A.K. Arof; Y. El Kouari; B. Sahraoui (47-52).
The electric dipole moments (μ), static dipole polarizabilities (α) and first hyperpolarizabilities (β) of styrylquinolinium dyes, D8 and D21, have been computed by density functional theory (DFT). The one-photon absorption (OPA) characterizations have been investigated using UV–vis spectroscopy and further interpreted using computational chemistry. The time-dependent Hartree–Fock (TDHF) method has been used to describe the dynamic dipole polarizabilities, dynamic second-order and also static and dynamic third-order nonlinear optical (NLO) properties. D8D21 have rather high β and second hyperpolarizabilities (γ). The highest occupied molecular orbitals (HOMO), the lowest unoccupied molecular orbitals (LUMO) and the HOMO–LUMO band gaps for D8D21 have been evaluated by DFT.

The dwell time of a molecule in a voltage dependent NMDA receptor channel is an important factor in defining its activity as channel blocker. A model has been designed, based on quantum chemical descriptors like geometrical parameters, charge distribution, electron density topography and global reactivity descriptors, to shed lights on the dwell time of a channel blocker. Structure and charge distribution studies indicate polarization of molecules with the electron density located at the core of the molecule. Electron density topography reveals ring critical point (ρ rcp), emerging as a signature parameter to understand the dwell time of a channel blocker molecule.

We study spin-forbidden transitions of Os polypyridyl sensitizers by two-component relativistic time-dependent density functional theory with the spin–orbit interaction based on Tamm–Dancoff approximation. The absorption spectra, including spin-forbidden-transition peaks, for the Os complexes are reasonably reproduced in comparison with the experimental ones. The extension of the conjugated lengths in the Os complexes is investigated and found to be effective to enhance photo absorption for spin-allowed transitions as well as spin-forbidden ones. This study provides fruitful information for a design of new dyes in terms of conjugation lengths.

Iodination of poly(vinyl chloride) (PVC) was prepared by addition of I2 or I through physical adsorption and chemical modification techniques forming (PVC–I2) (composite I) and (PVC–I) (composite II), respectively.Investigations were performed using TGA, DSC, FTIR, UV–vis absorbance analyses and ac conductivity measurements. The activation energy of decomposition, ΔE d , was found to be 215.15 kJ/mole and 43.1 kJ/mole for PVC and composite II, respectively. Both direct and indirect optical transitions near the absorption edge are observed. Addition of I2 or I decreases the optical energy gap for PVC. The chemical replacement of Cl ion in PVC matrix with I ion in composite II increases the electrical conductivity by two orders. A possible conduction mechanisms are suggested.

Understanding effect of structure and stability on transformation of CH4 hydrate to CO2 hydrate by Jinxiang Liu; Yujie Yan; Haiying Liu; Jiafang Xu; Jun Zhang; Gang Chen (75-80).
Understanding the transformation process of CH4 hydrate to CO2 hydrate is crucial to develop the CH4 ―CO2 replacement technique for CH4 production and CO2 sequestration. Ab initio calculations show that the transformation will slightly distort the host lattice and decrease the binding strength of guest molecules, but it is a thermodynamically spontaneous process dominated by the entropic contribution. Moreover, ab initio molecular dynamics simulations suggest that the dynamics of the host lattice is independent on the guest molecules, while CO2 in hydrate exhibits slower translational and rotational motion than CH4 in hydrate.

Display OmittedIn this study, we report the quantum-mechanical characterization of two novel α- and β-type 2D pure boron sheets, i.e., α6- and β14-sheets, constructed from the experimentally available B36 and B35 building blocks. Ten isomeric configurations were located. Using the calculated binding energies, the thermodynamic stability of these structures was considered in detail. Additionally, we calculated the work functions of α6- and β14-sheets. The results clearly demonstrate that their work functions (approximately 4.6 eV) are the highest among all of the reported mixed triangular-hexagonal type 2D boron sheets and are very similar to that of graphene.

Electric behavior of interlayer water in graphene oxide films by V.A. Smirnov; V.P. Vasil’ev; N.N. Denisov; Yu.V. Baskakova; V.A. Dubovitskii (87-90).
Current–voltage characteristics of graphene oxide (GO) films were explored in water vapor as a function of film water content. It has been found that the films under study may contain free and bound water that can be completely eliminated in vacuum. A new phenomenon – a decrease in current with time at a constant voltage – was revealed. This was associated with elimination of water in the absence of electrochemical processes. The theoretical model was suggested to rationalize the behavior of water diffusivity in the process of electroconductivity.

Second- and third-order nonlinear optical properties of unsubstituted and mono-substituted chalcones by Luis M.G. Abegão; Ruben D. Fonseca; Francisco A. Santos; Gabriela B. Souza; André Luis B.S. Barreiros; Marizeth L. Barreiros; M.A.R.C. Alencar; Cleber R. Mendonça; Daniel L. Silva; Leonardo De Boni; J.J. Rodrigues (91-96).
This work describes the second and third orders of nonlinear optics properties of unsubstituted chalcone (C15H12O) and mono-substituted chalcone (C16H14O2) in solution, using hyper-Rayleigh scattering and Z-Scan techniques to determine the first molecular hyperpolarizability (β) and the two-photon absorption (2PA) cross section respectively. β Values of 25.4 × 10−30  esu and 31.6 × 10−30  esu, for unsubstituted and mono-substituted chalcone, respectively, dissolved in methanol have been obtained. The highest values of 2PA cross-sections obtained were 9 GM and 14 GM for unsubstituted and mono-substituted chalcone, respectively. The experimental 2PA cross sections obtained for each chalcone are in good agreement with theoretical results.

Armchair biphenylene nanoribbons are investigated by using density functional theory. The nanoribbon that contains one biphenylene subunit in a unit cell is a semiconductor with a direct band gap larger than 1 eV, while that containing four biphenylene subunits is a metal. The semiconducting nanoribbon has high electron mobility of 57 174 cm2  V−1  s−1, superior to armchair graphene nanoribbons. Negative differential resistance behavior is observed in two electronic devices composed of the semiconducting and metallic nanoribbons. The on/off ratios are in the order of 103. All these indicate that armchair biphenylene nanoribbons are potential candidates for ultra-small logic devices.

Structural and dipolar fluctuations in liquid water: A Car–Parrinello molecular dynamics study by Ioannis Skarmoutsos; Marco Masia; Elvira Guardia (102-108).
A Car–Parrinello molecular dynamics simulation was performed to investigate the local tetrahedral order, molecular dipole fluctuations and their interrelation with hydrogen bonding in liquid water. Water molecules were classified in three types, exhibiting low, intermediate and high tetrahedral order. Transitions from low to high tetrahedrally ordered structures take place only through transitions to the intermediate state. The molecular dipole moments depend strongly on the tetrahedral order and hydrogen bonding. The average dipole moment of water molecules with a strong tetrahedral order around them comes in excellent agreement with previous estimations of the dipole moment of ice Ih molecules.

On the basis of ab initio quantum mechanics (QM) calculation, the obtained electron heat capacity is implemented into energy equation of electron subsystem in two temperature model (TTM). Upon laser irradiation on the copper film, energy transfer from the electron subsystem to the lattice subsystem is modeled by including the electron–phonon coupling factor in molecular dynamics (MD) and TTM coupled simulation. The results show temperature and thermal melting difference between the QM-MD-TTM integrated simulation and pure MD-TTM coupled simulation. The successful construction of the QM-MD-TTM integrated simulation provides a general way that is accessible to other metals in laser heating.

In this Letter, novel Y-type chromophores with dual electron donor groups, containing either styryl or azobenzene based π-conjugated bridge structures, were synthesized and their chemical structures, molecular configuration, microscopic optical properties as well as thermal properties were systematically characterized. The experimental results indicated that eight times increasing of second-order molecular hyperpolarizability as well as 50–100 nm blue shift of maximum absorption band for azobenzene based chromophore were observed by introducing Y-type dual electron donor groups, which was derived from the highly efficient ‘total charge transfer’ in this kind of chromophore as confirmed by the density functional theory calculation.

A screened automated structural search with semiempirical methods by Yukihiro Ota; Sergi Ruiz-Barragan; Masahiko Machida; Motoyuki Shiga (119-123).
Display OmittedWe developed an interface program between a program suite for an automated search of chemical reaction pathways, GRRM, and a program package of semiempirical methods, MOPAC. A two-step structural search is proposed as an application of this interface program. A screening test is first performed by semiempirical calculations. Subsequently, a reoptimization procedure is done by ab initio or density functional calculations. We apply this approach to ion adsorption on cellulose. The computational efficiency is also shown for a GRRM search. The interface program is suitable for the structural search of large molecular systems for which semiempirical methods are applicable.

Monitoring changes of paramagnetically-shifted 31P signals in phospholipid vesicles by Rebecca E. Joyce; Thomas L. Williams; Louise C. Serpell; Iain J. Day (124-129).
Phospholipid vesicles are commonly used as biomimetics in the investigation of the interaction of various species with cell membranes. In this letter we present a 31P NMR investigation of a simple vesicle system using a paramagnetic shift reagent to probe the inner and outer layers of the lipid bilayer. Time-dependent changes in the 31P NMR signal are observed, which differ whether the paramagnetic species is inside or outside the vesicle, and on the choice of buffer solution used. An interpretation of these results is given in terms of the interaction of the paramagnetic shift reagent with the lipids.

Location dependent orientational structure and dynamics of ethane in ZSM5 by Siddharth Gautam; Tingting Liu; Sumant Patankar; David Tomasko; David Cole (130-136).
Orientational structure and dynamics of ethane confined in ZSM5 zeolite at four different loadings are reported. The effect of pore geometry on ethane is studied by isolating the contribution from ethane molecules in different locations, viz. straight channels, sinusoidal channels and their intersections. Orientational dynamics is found to be hindered in general and exhibits librational motion, with the extant of hindrance being the greatest in sinusoidal channels. Librational motion becomes faster with an increase in loading. This counterintuitive finding is consistent with experiments reported elsewhere and is explained on the basis of a decreased orientational anisotropy at higher loadings.

Influence of the strength of polarizing electric field on free relaxation of electric birefringence in poly(butyl-isocyanate) solutions by N.V. Tsvetkov; M.E. Mikhailova; E.V. Lebedeva; A.A. Lezov; V.B. Rogozhin; T.A. Rotinyan (137-142).
Free relaxation of electric birefringence in tetrachloromethane solution of high molecular weight poly(butyl-isocyanate) was studied. The effect of electric field strength on the average relaxation time was observed. The relaxation spectrum was analyzed using the Rouse and Zimm theories. With increase in the electric field strength, the contribution of fast (deformation) relaxation modes also increased significantly. It is assumed that certain changes in intramolecular mobility occur under the influence of electric field.

Synthesis of high saturation magnetization FeCo nanoparticles by polyol reduction method by F.J. Yang; J. Yao; J.J. Min; J.H. Li; X.Q. Chen (143-146).
FeCo nanoparticles with different compositions were prepared by a polyol reduction method and annealed in gas mixtures. All FeCo nanoparticles show large saturation magnetization (over 220 emu/g). The largest saturation magnetization of 273 emu/g was observed in the Fe55Co45 sample. As for Fe48Co52, the impurity phase of CoFe2O4 existed when nanoparticles were annealed at low temperature (200–400 °C). While annealed at above 450 °C, pure Fe48Co52 nanoparticles with large saturation magnetization of 230 emu/g were obtained. These FeCo nanoparticles with large saturation magnetization have great potential in some industry fields.

A high quality four-dimensional intermolecular PES for Ar–CS2 involving the Q 1 and Q 3 normal modes for the ν 1 symmetric stretching vibration and ν 3 antisymmetric stretching vibration of CS2 is presented. Two vibrationally averaged potentials with CS2 at the vibrational ground and ν 1  +  ν 3 excited states are generated from the four-dimensional potential. Each potential has a T-shaped global minimum and two equivalent linear minima. The radial DVR/angular FBR method and the Lanczos algorithm are applied to calculate the rovibrational energy levels and bound states. The predicted band origin shift of the complex is −0.0495 cm−1. The spectroscopic parameters are also predicted.

OLi3O anion: Designing the strongest base to date using OLi3 superalkali by Ambrish Kumar Srivastava; Neeraj Misra (152-155).
LiO (proton affinity = 1785 kJ/mol) is the strongest base reported in the literature, followed by CH3 (proton affinity = 1743 kJ/mol). Using ab initio and DFT calculations, we propose the design of a novel base OLi3O, by interacting OLi3 superalkali with O atom, which is even more basic than LiO. The proton affinity of OLi3O isomers lies in the range 1803–1928 kJ/mol. This strongest base is kinetically and thermodynamically stable, which can be synthesized by reacting Li2O with LiO at least in gas phase. This study may provide suitable path to the experimentalists to further explore this novel species.

Electric field induced modification of magnetism in platinum tripod on pt (111) surface by T.H. Rana; A. Kashyap; S. Biswas; R.F. Sabirianov (156-160).
Pt (111) hexagonal surface with tripod geometry constructed from four Pt atoms is investigated by first principle calculation. Interestingly, appreciable spin magnetic moment of 0.11 μ B per Pt tripod atom and 0.9 μ B per unit cell is observed without application of external electric field (EF). Further, the magnetic moment per Pt atom and/or unit cell can be tuned using EF of ±0.5 V/Å. Negative EF enhances the spin magnetic moment of tripod Pt atom and unit cell up to 0.137 μ B and 1.29 μ B , respectively. Positive EF reduces the spin magnetic moment of tripod Pt atom and unit cell down to 0.087 μ B and 0.45 μ B , respectively.

Mechanisms of monovacancy diffusion in graphene by Jack D. Wadey; Alexander Markevich; Alex Robertson; Jamie Warner; Angus Kirkland; Elena Besley (161-165).
Display OmittedA comprehensive investigation of monovacancy diffusion in graphene has been carried out with the use of density functional theory and the climbing image nudged elastic band method. An out-of-plane spiro structure is found for the first-order saddle point, which defines the transition state in the vacancy diffusion pathway. The obtained activation energy for diffusion is significantly lower than the reported values for the in-plane saddle point structures. The time between consecutive vacancy jumps in graphene is estimated to be in the range of 100–200 s at room temperature in a good agreement with experimental observations.

We present a theoretical analysis of the adsorption of NH3 molecule on (111) surfaces of PtM (Fe, Co, Ni) alloys in their L10 structure using the first principles density functional approach, through comparison with its adsorption on the parent M surfaces, i.e. BCC Fe (011), HCP Co (0001) and FCC Ni (111). While NH3–Pt interaction is stronger than that of NH3 with the elemental M-surfaces, while it is weaker than the strength of interaction of NH3 with M-site on the surface of PtM alloy. This remarkable reversal of the trend in relative strengths of the NH3-M binding is shown to originate from a combined effects of three mechanisms charge transfer, magnetism and strain which constitute descriptors for design new catalysts.Adsorption of NH3 molecule on HCP Co (0001) surface. (a) The density of states of NH3 molecule in gas phase, (b) the Co-d projected DOS and NH3 projected DOS for NH3 adsorbed Co (0001) surface and (c) d-projected DOS for the Co (0001). Display OmittedAdsorption of a molecule or group with an atom which is less electronegative than oxygen (O) and directly interacting with the surface is very relevant to development of PtM (M = 3d-transition metal) catalysts with high activity. Here, we present theoretical analysis of the adsorption of NH3 molecule (N being less electronegative than O) on (111) surfaces of PtM (Fe, Co, Ni) alloys using the first principles density functional approach. We find that, while NH3–Pt interaction is stronger than that of NH3 with the elemental M-surfaces, it is weaker than the strength of interaction of NH3 with M-site on the surface of PtM alloy.

Exploring host–guest complexation mechanisms by a molecular dynamics/quantum mechanics/continuum solvent model approach by Renlong Ye; Xuemei Nie; Yumei Zhou; Chung F. Wong; Xuedong Gong; Wei Jiang; Weihua Tang; Yan A. Wang; Thomas Heine; Baojing Zhou (170-177).
Display OmittedWe introduce a molecular dynamics/quantum mechanics/continuum solvent model (MD/QM/CSM) approach to investigate binding mechanisms of host–guest systems. The representative conformations of host, guest, and their complex generated from MD simulations at the molecular-mechanics level are used for binding free energy calculations based on a QM/CSM model. We use this approach to explore the binding mechanisms of β-cyclodextrin (β-CD) and 2, 6-di-methyl-β-CD (DM-β-CD) with various guest molecules. Our results suggest that solvent effects play a more important role in determining the relative binding affinities of DM-β-CD than those of β-CD mainly because the former is more flexible than the latter.

Gate opening effect for carbon dioxide in ZIF-8 by molecular dynamics – Confirmed, but at high CO2 pressure by T. Chokbunpiam; S. Fritzsche; C. Chmelik; J. Caro; W. Janke; S. Hannongbua (178-181).
Since the simulation of the CO2 adsorption in ZIF-8 for the ZIF-8 HP (high-pressure phase) and ZIF-8 AP (conventional phase) structures – as defined in [1] – gave similar results [2], Molecular Dynamics (MD) has been applied to decide which of these two structures exists at which CO2 pressure. MD simulations with flexible lattice show that a transition from the ZIF-8 AP to the ZIF-8 HP structure takes place at extremely high CO2 pressure, i.e. at high loading.

Polyethylene glycol gold-nanoparticles: Facile nanostructuration of doxorubicin and its complex with DNA molecules for SERS detection by Jolanda Spadavecchia; Ramesh Perumal; Sandra Casale; Jean-Marc Krafft; Christophe Methivier; Claire-Marie Pradier (182-188).
Schematic representation of the biosensor elaboration strategy: (A) interaction of doxorubicin with DNA onto pegylated gold nanostructured surface. (B) Interaction of doxorubicin loaded to PEG AuNPs with DNA onto pegylated gold nanostructured surface.We report the synthesis of dicarboxylic acid-terminated polyethylene-glycol (PEG)–gold nanoparticles by a simple one-step method, and their further use to form nanostructured surfaces for biomolecule immobilization. The synthesized nano-scale particles were conjugated with probe/target oligonucleotides in order to evaluate intercalation phenomenon in the presence of doxorubicin drug via surface enhanced Raman spectroscopy (SERS) analysis.

Quantum chemical investigations on symmetrical polyradicals of meta-xylylene chains are done using unrestricted DFT – broken symmetry formalism, to determine whether the polymers are quasi-linear or helical, and their possible magnetic characteristics. As more radical centers are added, successively placed phenyl rings become twisted in the same direction to form a coiled chain. Each polyradical possesses a high-spin ground state with large adiabatic as well as vertical coupling constants. The adiabatic coupling constant exponentially decays with an increasing number of radical centers. Also, the Heisenberg–Dirac–Van Vleck coupling constants are generally large. The polymers are predicted to be very strong paramagnets.

Spectroscopic investigation of a brightly colored psittacofulvin pigment from parrot feathers by František Adamec; Jordan A. Greco; Amy M. LaFountain; Nikki M. Magdaong; Marcel Fuciman; Robert R. Birge; Tomáš Polívka; Harry A. Frank (195-199).
Octadecaoctaenal is a psittacofulvin pigment associated with the bright red coloration of parrots. It consists of a linear polyene chain terminated by an aldehyde group and therefore provides an opportunity to examine the fundamental factors controlling the excited state ordering and dynamics of polyenals. Steady-state and ultrafast time-resolved spectroscopy were performed on octadecaoctaenal and a derivative in which the aldehyde group was converted to a methylenehydroxyl group. It was found that for octadecaoctaenal, solvent proticity was more important than polarity in determining its excited state lifetime. Theoretical computations were carried out to reveal the origin of the effect.

Display OmittedCoupled spin and reaction dynamics of spin-correlated radical pairs is often described by a master equation due to Haberkorn. In a recent paper it was claimed that this equation is inconsistent with a description by a sum of individual quantum trajectories. The apparent inconsistency arises from an incorrect description of reaction attempts in the trajectory simulations. Results from quantum trajectory and Haberkorn master equation simulations coincide if these attempts are correctly described as quantum measurements of the spin state that happen on encounter of the two radicals.

Display OmittedWe recently unraveled a major inconsistency in the traditional description of radical-pair quantum dynamics by studying single-molecule quantum trajectories and comparing their prediction with Haberkorn's master equation. A comment by Jeschke claimed that the inconsistency arises because we did not properly include quantum state projections in the traditional approach. We here show that Jeschke stipulates quantum trajectories involving unphysical quantum states with negative populations. Moreover, the author's Monte Carlo simulation and its agreement with Haberkorn's master equation is a demonstration of an algebraic tautology, establishing the consistency of an unphysical master equation with circularly defined unphysical trajectories.