169,815 research outputs found
High resolution IR-diode laser jet spectroscopy of malonaldehyde
No full textThe ro-vibrational infrared spectrum of malonaldehyde has been measured for the first time at high resolution. Employing diode laser spectroscopy in a pulsed slit seeded He jet expansion the rotational structure of the nu(6) (OH-bend/symmetric C=C,C=O stretch) fundamental could be completely resolved at a rotational temperature of about 7 K. The rotational structure of the (+) - tunnelling component was analyzed in terms of Watson's semi-rigid rotor Hamiltonian with a band centre at 1594.0898 cm(-1). Additional weaker signals were attributed to the (-) - tunnelling component. The estimated band centre around 1594.06 cm(-1) corresponds to a reduction of the tunnelling splitting upon excitation of nu(6) by only 0.03 cm(-1). (C) 2004 Elsevier B.V. All rights reserved
Two scales hydrodynamic limit for a model of malignant tumor cells
No full textWe consider a model introduced in [S. Luckhaus, L. Triolo, The continuum reaction-diffusion limit of a stochastic cellular growth model, Rend. Acc. Lincei (S.9) 15 (2004) 215-223] with two species (eta and xi) of particles, representing respectively malignant and normal cells. The basic motions of the eta particles are independent random walks, scaled diffusively. The xi particles move on a slower time scale and obey an exclusion rule among themselves and with the eta particles. The competition between the two species is ruled by a coupled birth and death process. We prove convergence in the hydrodynamic limit to a system of two reaction-diffusion equations with measure valued initial data. (c) 2006 Elsevier Masson SAS. All rights reserved
The vibrational spectrum of HONO: Fully coupled 6D direct dynamics
No full textThe vibrational spectrum of nitrous acid (HONO) in the electronic ground state is derived from fully coupled quantum-mechanical calculations. A global six-dimensional potential hypersurface is directly interpolated from density-functional calculations. A potential-optimized generalized coordinate discrete variable representation is combined with multidimensional successive truncation to yield the complete vibrational spectrum up to the first OH stretching overtone of both stable isomers. Higher OH stretching overtones up to 7nu(OH) are calculated selectively applying the adiabatic contraction technique. The ab initio prediction of the vibrational spectrum is in remarkably good agreement with available experimental data. The results indicate the onset of the intramolecular (1,3)-H-transfer between the first and second OH stretching overtone of cis-HONO. (C) 2003 American Institute of Physics
A monomers-in-dimers model for carboxylic acid dimers
No full textThe OH stretching fundamental band system of carboxylic acid dimers is studied using acetic acid and its isotopomers as a model system. Comparing experimental jet spectra with multidimensional quantum mechanical calculations the origin of the extremely broad vibrational band structure (Delta(ν) over tilde approximate to800 cm(-1)) is found in strong anharmonic resonances involving the OH stretching vibration. Within an adiabatic picture of hydrogen bonding a new monomers-in-dimers model allows to analyze the observed vibrational band structure in terms of the anharmonic quantum dynamics of the CO2H functional group. The results are discussed in terms of the time-dependent population dynamics and its implications for the mode-specificity of the vibrational predissociation of the hydrogen bonds. On a subpicosecond time scale the intramolecular vibrational energy redistribution of the dimer remains effectively localized within the six-dimensional manifold of the internal vibrations of the carboxyl group, conserving its local C-S symmetry. (C) 2003 American Institute of Physics
Rovibrational calculations for CH3+–Rg (Rg=He,Ne): The prototype disk-and-ball dimer
No full textRovibrational calculations in the intramolecular ground vibrational states of the CH3+-Rg dimers, Rg=He and Ne, are carried out on intermolecular ab initio potential energy surfaces (PESs) calculated at the MP2 level of theory using a basis set of aug-cc-pVTZ quality. The internal CH3+ coordinates in the dimer are kept frozen at the optimal monomer coordinates (D-3h symmetry, rigid monomer approximation). The three-dimensional (3D) intermolecular PESs of both dimers feature pronounced global minima at p-bound equilibrium structures: the Rg atom is attached to one side of the 2p(z) orbital of the central C atom along the C-3 symmetry axis (C-3v symmetry). The intermolecular C-He and C-Ne bonds are characterized by separations of R-e=1.93 and 2.21 Angstrom and dissociation energies of D-e=672 and 935 cm(-1), respectively. The PESs of these prototype disk-and-ball dimers reveal substantial angular-radial coupling in the region of the global minimum which leads to significant differences between the equilibrium and vibrationally averaged separations, R-e and R-0. The 3D rovibrational calculations on the rigid monomer PESs yield R-0=2.54 and 2.43 A and D-0=193 and 474 cm(-1) for CH3+-He and CH3+-Ne, respectively. In general, the spectroscopic constants derived for the ground vibrational states of both complexes are in good agreement with recent spectroscopic data obtained by infrared photodissociation spectroscopy. (C) 2002 American Institute of Physics
Multi-arrangement quantum dynamics in 6D: cis-trans isomerization and 1,3-hydrogen transfer in HONO
No full textThe overtone spectrum and wave packet dynamics of nitrous acid (HONO) are studied with a global six-dimensional potential energy function interpolated directly from density functional calculations together with the corresponding dipole hypersurfaces. The quantum dynamics for the cis-trans isomerization and the symmetric 1,3-hydrogen transfer are treated in full dimensionality in terms of the generalized Z-matrix discrete variable representation. For the quantum mechanical description of complicated rearrangements a new approach to multi-arrangement quantum dynamics is introduced and applied to the symmetric hydrogen exchange tunneling in cis-HONO. The cis-trans isomerization is found to be dominated by adiabatic barrier crossing with only minor tunneling contributions, but with pronounced mode selectivity. The OH-stretching overtones of trans-HONO are adiabatically almost completely separated from the OH torsional dynamics with extremely slow intramolecular energy redistribution. The 1,3-hydrogen transfer, by contrast, proceeds largely via coherent tunneling even significantly below the barrier. The process is clearly non-adiabatic (at least in terms of valence coordinates) but remains highly state specific. While the absorption spectrum of trans-HONO remains largely unaffected, OH-stretching overtones of cis-HONO (above the barrier between 2v(OH) and 3v(OH)) decompose into highly fragmented absorption patterns with corresponding tunneling periods on the picosecond time scale. (C) 2004 Elsevier B.V. All rights reserved
Fermi resonance and conformation in glycolaldehyde particles
No full textThe infrared spectra of gas-phase glycolaldehyde and small glycolaldehyde particles both exhibit a split carbonyl band around 1730 cm(-1). Ab initio calculations show that this splitting can be traced back to a Fermi resonance involving the C=O-stretching band and the first overtone of the C-C-stretching band. The analysis of this resonance leads to detailed information about the conformation of glycolaldehyde in the particles. The results can be summarized as follows. (i) In the particles, monomers with OCCO dihedral angles of 0degrees and CCOH dihedral angles around 50degrees are strongly preferred. (ii) The intramolecular hydrogen bond observed in the gas phase is partly replaced by intermolecular hydrogen bonds. (iii) The calculated infrared spectrum and the results from the Fermi resonance analysis are in good agreement with the formation of long chains of monomeric glycolaldehyde. This is in contrast to the solid bulk, which consists of chemically bound dimers
Observation and quantification of the hydrogen bond effect on O-H overtone intensities in an alcohol dimer
No full textFirst overtone (2 nu(OH)) transitions of the two OH-stretching modes in isolated 2,2,2-trifluoroethanol dimers are observed for the first time. Their relative intensities provide quantitative evidence for the pronounced IR intensity suppression of two-quantum hydrogen-bonded OH-stretching excitations relative to fundamental transitions by up to three orders of magnitude. The OH anharmonicity is shown to increase by 15% upon hydrogen bonding to another oxygen, whereas it remains constant upon hydrogen bonding to a fluorine atom. The experimental results are supported by reduced-dimensionality anharmonic calculations in up to three dimensions. (C) 2008 Elsevier B.V. All rights reserved
Tunneling dynamics of the NH chromophore in NHD2 during and after coherent infrared excitation
No full textInternational audienceThe time dependent quantum dynamics of the large amplitude motion of the NH stretching chromophore in NHD2 is investigated during and after coherent multiphoton excitation by calculation of the wave packet evolution using global analytical potential energy and electric dipole hypersurfaces of ammonia derived from ab initio calculations. Intramolecular vibrational redistribution between the NH stretching and bending motion and coupling to the radiation field induces a diffusion of probability density into the NH chromophore space, which includes the inversion coordinate. However, inversion remains essentially dominated by a tunneling process, even at average energies well above the inversion barrier. (C) 2003 American Institute of Physics
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