Spectral hole-burning and magnetic circular dichroism of matrix-isolated copper phthalocyanine

Persistent spectral hole-burning in the Q-band region is reported for a concentrated (≈ 2 × 10⁻² mol l⁻¹) matrix of copper phthalocyanine in solid Ar at 1.6 K. Hole-burning occurs with a quantum efficiency of ≈ 10⁻⁶ and can be reversed by annealing at ≈ 28 K. Vibrational side holes allow determination of excited-state frequencies. A hole-burning mechanism involving triplet-state charge separation followed by rapid electron back-transfer is proposed. Temperature dependence of the MCD and the difference between MCD spectra taken before and after irradiation indicate that the excited state is split by crystal-field and spin-orbit interactions.     

Semiclassical nonadiabatic dynamics using a mixed wave-function representation

Nonadiabatic effects in quantum dynamics are described using a mixed polar/coordinate space representation of the wave function. The polar part evolves on dynamically determined potential surfaces that have diabatic and adiabatic potentials as limiting cases of weak localized and strong extended diabatic couplings. The coordinate space part, generalized to a matrix form, describes transitions between the surfaces. Choice of the effective potentials for the polar part and partitioning of the wave function enables one to represent the total wave function in terms of smooth components that can be accurately propagated semiclassically using the approximate quantum potential and small basis sets. Examples are given for two-state one-dimensional problems that model chemical reactions that demonstrate the capabilities of the method for various regimes of nonadiabatic dynamics.     

Quantum and classical studies of the O(3P) + H2(v = 0-3,j = 0) --> OH + H reaction using benchmark potential surfaces

We present results of time-dependent quantum mechanics (TDQM) and quasiclassical trajectory (QCT) studies of the excitation function for O(3P) + H2(v = 0-3,j = 0) --> OH + H from threshold to 30 kcal/mol collision energy using benchmark potential energy surfaces [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)]. For H2(v = 0) there is excellent agreement between quantum and classical results. The TDQM results show that the reactive threshold drops from 10 kcal/mol for v = 0 to 6 for v = 1, 5 for v = 2 and 4 for v = 3, suggesting a much slower increase in rate constant with vibrational excitation above v = 1 than below. For H2(v > 0), the classical results are larger than the quantum results by a factor approximately 2 near threshold, but the agreement monotonically improves until they are within approximately 10% near 30 kcal/mol collision energy. We believe these differences arise from stronger vibrational adiabaticity in the quantum dynamics, an effect examined before for this system at lower energies. We have also computed QCT OH(v',j') state-resolved cross sections and angular distributions. The QCT state-resolved OH(v') cross sections peak at the same vibrational quantum number as the H2 reagent. The OH rotational distributions are also quite hot and tend to cluster around high rotational quantum numbers. However, the dynamics seem to dictate a cutoff in the energy going into OH rotation indicating an angular momentum constraint. The state-resolved OH distributions were fit to probability functions based on conventional information theory extended to include an energy gap law for product vibrations.     

Violation of microscopic reversibility and the use of reverse quais-classical trajectories for calculating reaction cross sections

In order to calculate the transition probabilities (or cross sections) for reactive collisions, such as A + BC(ν, j)→ AB(ν′, j) + C, using the quasi-classical trajectory method, one quantizes the internal energy of the reagents and in addition adopts some algorithm for calculating the internal quantum numbers of the products. A serious consequence of this procedure is that the quasi-classical results do not obey microscopic reversibility. It is shown that for the collinear F + H₂(ν = 0) → FH(ν = 2, 3)+ H reaction (and its D₂ counterpart), the quasi-classical trajectory probabilities for the reverse reaction not only differ substantially from the forward ones but in general are in much better agreement with accurate quantum calculations. A similar situation was found for the collinear H + H₂(0) → H₂(1) + H reaction. We suggest that in doing quasi-classical calculations, the reverse of the process of interest should also be considered. Comparison of forward and reverse quasi-classical collinear calculations with accurate collinear quantum results could give an indication of whether forward or reverse calculations should be used for the three-dimensional case.     

Mischkristallbildung von As2O3 und Sb2O3

Zusammenfassung Durch Wegkochen von NH₃ aus ammoniakalischen Tartratlösungen des As₂O₃ und Sb₂O₃ in verschiedenem Verhältnis wurden einerseits monokline Mischkristalle der Oxyde mit einem Gehalt zwischen 100 und 70 Molprozent Arsen, andererseits kubische Mischkristalle mit 30 bis 100 Molprozent Antimon gewonnen. Die analytischen Ergebnisse werden durch Röntgen- und Infrarot-Untersuchungen bestätigt, aus den letzteren ergeben sich weitere Hinweise auf die Struktur.Mit 1 Abbildung1. Mitt.:E. Hayek, M. Hohenlohe-Profanter, B. Marcic undE. Beetz, Angew. Chem.70 307 (1958).     

Thermoanalytical investigation of some alkaline earth hydroxide hydrates on application as latent heat storage materials

Owing to their high specific melting enthalpy and the range of the melting temperatures the alkaline-earth hydroxide hydrates Ba(OH)₂·8H₂O and Sr(OH)₂·8H₂O are promising latent heat storage materials. The investigations of the melting and solidification behaviour of Sr(OH)₂·8H₂O and its mixtures with Ba(OH)₂·8H₂O, which had been performed by means of DTA and DSC methods in the closed system with a constant gross composition lead to statements on the melting temperature and specific melting enthalpyvs. concentration. Theoretical storage densities of 532 MJ/m³ are obtained for the mixture of Ba(OH)₂·8H₂O and Sr(OH)₂·8H₂O (80/20) and a value of 655 MJ/m³ can be achieved for Sr(OH)₂·8H₂O. The kinetics of rehydration to the octahydrates has a great influence on the storage temperature and storage density.     

Probing the structure of single-molecule surface-enhanced Raman scattering hot spots

We present here a detailed study of the specific nanoparticle structures that give rise to single-molecule surface-enhanced Raman scattering (SMSERS). A variety of structures are observed, but the simplest are dimers of Ag nanocrystals. We chose one of these structures for detailed study using electrodynamics calculations and found that the electromagnetic SERS enhancement factors of 10(9) are easily obtained and are consistent with single-molecule SERS activity.     

Effect of loop distortion on the stability and structural dynamics of DNA hairpin and dumbbell conjugates

The thermal stability and conformational dynamics of DNA hairpin and dumbbell conjugates having short A-tract base pair domains connected by tri- or hexa(ethylene glycol) linkers is reported. The formation of stable base-paired A-tract hairpins having oligo(ethylene glycol) linkers requires a minimum of four or five A-T base pairs. The formation of base-paired dumbbells having oligo(ethylene glycol) linkers by means of chemical ligation of nicked dumbbells requires a minimum of two A-T base pairs on either side of the nick. Molecular modeling indicates that the hexa(ethylene glycol) linker is sufficiently long to permit formation of strain-free loop regions and B-DNA base pair domains. In contrast, the tri(ethylene glycol) is too short to permit Watson-Crick base pairing between the bases attached to the linker. The shorter linker distorts the duplex, resulting in fluxional behavior in which the base pairs adjacent to the linker and at the open end of the hairpin dissociate on the nanosecond time scale. The loss of interstrand binding energy caused by these fluctuations leads to a difference of approximately 5 degrees C in melting temperature between EG3 and EG6 hairpins. An analysis of the fluxional behavior of the EG3 adjacent base-pair has been used to study the pathways for base flipping and base stacking, including the identification of rotated base (partially flipped) intermediates that have not been described previously for A-T base pairs.