Search results for "Molecule"

High-spin → low-spin relaxation in the two-step spincrossover compound [Fe(pic)3]Cl2EtOH (pic = 2-picolylamine)

1998

Abstract The spin-crossover compound [Fe(pic) 3 ]Cl 2 EtOH (pic = 2-picolylamine) shows an unusual two-step spin transition. This is thought to be caused by specific nearest-neighbour interactions and short-range correlations and requires a theoretical treatment of the elastic interactions between the spin-changing molecules beyond the mean-field approximation. Such short-range correlations also influence the high-spin → low-spin relaxation following the light-induced population of the high-spin state at cryogenic temperatures, leading to characteristic deviations from the predictions of a mean-field treatment. These deviations are directly observable by comparison of the full and unperturb…

Spectroscopic signatures for planar equilibrium geometries in methyl-substituted oligothiophenes

2009

In recent studies it was demonstrated that temperature-dependent optical spectroscopy is a valuable tool for revealing the differences in the geometries of flexible molecules like oligothiophenes (OTs) in the ground (S0) and first excited (S1) electronic states, by examining the symmetry relations between the absorption and emission spectra: while at low temperature the spectra show mirror symmetry, pointing to planar geometries in S0 and S1, the symmetry relation breaks down at ambient temperature due to thermal population of torsional modes. In the present joint spectroscopic and theoretical study, we demonstrate that this behavior is also observed for di- and tetramethyl-substituted OTs,…

(R)-(−)-Quinuclidin-3-ol

2013

The structure of the title compound [alternatively called (R)-(−)-1-azabicyclo[2.2.2]octan-3-ol], C7H13NO, at 100 K has hexagonal (P61) symmetry. The structure shows a twist along the C—N pseudo-threefold axis. In the crystal, molecules are linkedviaO—H...N hydrogen bonds, forming infinite chains along thec-axis direction. The crystal studied was twinned by merohedry (twin law: 010, 100, 00-1; population: 0.925:0.075)

NMDA receptor antagonist treatment induces a long-lasting increase in the number of proliferating cells, PSA-NCAM-immunoreactive granule neurons and …

2001

During adulthood, neural precursors located in the subgranular zone of the dentate gyrus continue to proliferate, leading to the generation of new granule neurons. These recently generated cells transiently express the polysialylated form of the neural cell adhesion molecule, PSA-NCAM, and are supported by radial glia-like cells that are likely to play a role in neuronal migration and differentiation, or even act as their precursors. Previous reports indicate that treatment with NMDA receptor antagonists stimulates adult neurogenesis in the dentate gyrus, and because of the potential therapeutic value of this approach, we were interested in further characterizing the consequences of pharmac…

Origin of the Red Sites and Energy Transfer Rates in Single MEH-PPV Chains at Low Temperature

2011

Single poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) molecules dispersed in thin poly(methylmethacrylate) films have been investigated by fluorescence emission, excitation and time-resolved spectroscopy at 1.2 K. For the molecular weight studied (∼200 kDa) a bimodal distribution of emission maxima is observed. Based on a comparison of the spectroscopic properties of blue and red sites and on polarisation-resolved measurements, we argue in agreement with recent quantum-chemical calculations that the red subpopulation most probably does not arise from interchromophoric excitation delocalisation but is to be attributed to longer chromophoric units originating from orde…

2016

The photochemistry of benzophenone, a paradigmatic organic molecule for photosensitization, was investigated by means of surface-hopping ab initio molecular dynamics. Different mechanisms were found to be relevant within the first 600 fs after excitation; the long-debated direct (S1 → T1) and indirect (S1 → T2 → T1) mechanisms for population of the low-lying triplet state are both possible, with the latter being prevalent. Moreover, we established the existence of a kinetic equilibrium between the two triplet states, never observed before. This fact implies that a significant fraction of the overall population resides in T2, eventually allowing one to revisit the usual spectroscopic assignm…

Spectroscopic characterization and biological activity ofL-methionyl-L histidinato complexes of R2Sn(IV) ions (R?=?Me, nBu, Ph) and X-ray structure o…

2000

Complexes of L-methionyl-L-histidine (H2MetHis) with R2Sn(IV) ions (R = Me, nBu, Ph) have been synthesized. The crystal and molecular structures of Me2SnMetHis·0.5MeOH have been determined by X-ray diffraction. The title compound contains two crystallographically independent molecular units possessing the same trigonal-bipyramidal geometry at tin, each dimethyltin(IV) moiety being coordinated by the terminal amino nitrogen, deprotonated peptide nitrogen and terminal carboxylate group, neither the imidazole nor thioether groups being involved in bonding. IR spectroscopy was used to probe the structure of the complexes in the solid state, and the structure in solution (CD3OD) was assessed by …

Structural organic chemistry in ecotoxicology

1993

Abstract Role of the methods of structural chemistry in assessment of the ecotoxic effects of organic compounds is described as examples of fate modeling based on structural properties, screening analyses of the affected population, use of model substances and various structure-specific determination methods. Case examples from literature are given and, in addition, structure screening and verification for a contaminant in herring is described.

Vibrational population of H 2 + after electroionization of thermal H2

1993

In an ion trap experiment we have determined the vibrational population of the lowest 9 vibrational levels of H2+. We used photodissociation of the trapped molecules by 248 nm light from an excimer laser and the dependence of the photodissociation cross section from the vibrational state. Our results are in good agreement to calculations, which are based on the Franck-Condon principle, but include a variation of the internuclear distance in the transition matrix element.

Hartree-Fock calculations and photoelectron spectra of SSO and NSF

1974

Results from ab initio Hartree-Fock calculations on the ground states of SSO and NSF are reported. The calculations employ large basis sets of Gaussian functions of essentially double zeta quality. The photoelectron spectra of the bent triatomic molecules with 18 valence electrons, i.e. SSO, NSF, O3, SO2, NSCl are compared. Further experimental details of the photoelectron spectrum of SSO are presented and an assignment of the observed ionic states of SSO⊕ is attempted. The calculated dissociation energies, dipole moments and the population analyses are given. The correlation effect is qualitatively discussed in connection with the applicability of Koopmans' theorem for the bent tri-atomic …