ISSN:0005-9021    

DOI:10.1002/bbpc.19971010302

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractWe present the conceptual background for intramolecular vibrational (rotational) redistribution (IVR) in polyatomic molecules and its analysis from high resolution spectra. We illustrate some of the basic concepts relating to the use of effective and real molecular hamiltonians for IVR with examples drawn from our previous spectroscopic investigations of the chiral molecule CHFClBr. The importance of the initial state for the dynamics is shown, demonstrating that simple intuitive concepts of survival probabilities for some arbitrary chromophore (“zero order bright”) state are insufficient for a quantitative understanding of IVR. An approximate (C3‐like) symmetry prevents relaxation towards quasi‐equilibrium in highly excited C1‐asymmetrical CHFClBr. The coherent infrared multiphoton excitation of CHFClBr at the CH‐stretching frequency leads to coherent vibrational states with a very wide energy distribution corresponding to short time oscillatory behaviour of the CH‐stretching motion with a period of 11–12 fs. On longer time scales between 50 and 500 fs substantial vibrational motion is found in the CH‐bending coordinates. For modest excitation energies both the initial vibrational angular momentum quantum number (l= 0) and a' symmetry of the wavepacket are well conserved for 1–2 ps. At higher excitations and longer times (2–4 ps) these approximate symmetries are lost. These results are briefly discussed in relation to a general classification of IVR mechanisms and in relation to reactive

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010303

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractThe features and mechanisms of intramolecular energy redistribution (IVR) and state mixing of HOCl at chemically significant energies have been investigated using intra‐cavity laser absorption spectroscopy (ICLAS). These studies revealed that for the observed vibrational states up to about 75% of the dissociation energyE0both strong and weak interactions (Fermi‐type, Coriolis) exist but producing onlylocalizedperturbations in the spectra. The 5v1band, however, at about 85% of the dissociation thresholdE0(O‐Cl bond) has been found to begloballyand strongly mixed with the 4v1+ 2v2+v3zero‐order vibrational state, and both states appeared in turn coupled to other background states by additional (weaker) interactions. The transfer from frequency to time domain led to the conclusion that this spectral signature is characteristic for efficient but nevertheless restricted intramolecular vibrational energy redistribution in the overtone spectrum of HOCl. The intramolecular dynamics of HOCl is compared to the dynamics of HCN and discussed in terms of low‐order interactions between zero‐order states (ZOS) grouped in diff

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010304

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractDouble‐resonance vibrational overtone excitation spectra of small molecules, in combination with laser‐induced fluorescence product detection, provide information on both their energetics and intramolecular energy redistribution dynamics. Application of this approach to measure OH stretch overtone spectra of HOOH and NH2OH near their dissociation threshold provides estimates of the O‐O and N‐O bond energies of 17051.8±4 cm−1and 21620±20 cm−1respectively.In symmetrical molecules, such as HOOH, the double‐resonance excitation technique allows us to prepare molecules in either pure OH stretch overtone levels or combination levels in which the excitation energy is divided between two identical OH stretch oscillators. Comparison of the resulting vibrational overtone linewidths indicates no clear difference between the pure OH stretch and the local‐local combination levels. However, comparison of the energy transfer rates from pure OH stretch vibrations and those in which one quantum of energy is put in a low frequency mode reveals faster rates from the latter. This observation clearly indicates that coupling strengths depend markedly on the vibrational character of the coupling partner and not simply on the total energy.Comparison of the OH stretch overtone spectra of the structurally similar molecules HOOH, NH2OH, and CH3OH reveal similar rates of vibrational energy redistribution, despite the difference in the density of states in these molecules. This observation reinforces the notion that energy flow between the OH stretch and dark background states is not statistical but rather is controlled by coupling to a small subset of

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010305

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractThe combination of overtone spectroscopy and detailed quantum mechanical modelling of rovibrational dynamics provides an unique tool for investigating many aspects of intramolecular dynamics such as dynamical symmetries, coupling hierarchies, and the separation of time scales, which control the intramolecular energy flow at high excitations relevant for unimolecular reactions.Hydroxylamine NH2OH, which plays an important role in N/O/H chemistry in general, provides an intriguing example for the interplay between high frequency stretch‐bend motions and low frequency torsion/inversion tunneling. This paper discusses the dynamical analysis of its high resolution infrared absorption spectrum up to the visible range on the basis of up to five‐dimensional ab initio models using vibrational DVR grids of several million points.The torsion‐inversion process is found to follow a consecutive mechanism with very slow (≥ 1 μs) inversion tunneling below thetrans‐cisisomerisation barrier (2673 cm−1at MP2/TZ2P). The metastablecisisomer shows localised zero point and fundamental level eigenfunctions. From the overtone spectrum fast energy redistribution is deduced within the OH and NH2manifolds, respectively, on time scales below 100 fs. A direct coupling between the two moieties leads to a slow decay of OH stretching excitation within 3 ps. Further significant contributions to the effective homogeneous broadening observed in high OH stretch overtones are predicted from the 1,2‐H shift leading to ammonia oxide NH3O with a barrier around 18200 cm−1(CASPT2) and a NH3O well depth exceeding 1 eV.The spectroscopic characterisation of the metastable isomers of hydroxyl amine,cis‐NH2OH and NH3O, by stimulated emission pumping experiments is suggested on the basis of ab initio calculations for electronica

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010306

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractOvertone spectroscopy by vibrationally assisted dissociation and photofragment ionization (OSVADPI) has been coupled with the detection of fragments in a mass spectrometer allowing isotope selective overtone spectroscopy (ISOS) on naturally occurring isotopomer mixtures. The room temperature CH‐chromophore spectra of CF2HCl (polyad componentNj= 41near 11430 cm−1) and of chloroform (Nj= 21near 5942 cm−1andNj= 31near 8726 cm−1) have been investigated in this way and discussed in relation to intramolecular energy transfer. The features of the central Q‐branch regions are assigned with resolution of the Cl‐isotopomers, and Q‐branch maxima and Cl‐isotopomer shifts are determined. The effect of the strong Fermi resonance, mixing CH‐stretching and bending modes, on the position of vibrational hot band transitions is demonstrated. We describe in detail the extension of our technique into the near and mid infrared range with a high resolution optical parametric oscillator system which has a wide range of potential applications as a general infrared spectroscopy with isotope and mass s

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010307

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractAb initio, multi‐reference, configuration interaction calculations are reported for the reactions.Two equivalent, barrier‐less paths are found for the CH3+ H recombination, two inequivalent, barrier‐less paths are found for the CH2F+H and CHF2+ H recombinations (depending on which side of the radical the hydrogen atom approaches) and only one barrier‐less path is found for the CF3+ H recombination. The minimum energy path for H atom approaching CF3from the concave side is predicted to have a barrier of 27 kcal/mol. Both the minimum energy path energies and the transitional frequencies as a function of RCHfor all four reactions are predicted to be quite

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010308

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractThe rovibrational (J= 0–4) level structure of protonated nitrogen (HN+2) is investigated in detail for the CCSD(T) potential energy surface of Schmatz and Botschwina. The discrete variable representation (DVR) for the angular coordinate in conjunction with a distributed Gaussian basis set (DGB) for the radial degrees of freedom was used to perform the nuclear dynamics calculations. Both low lying and highly excited states are examined. The total number of bound states withJ= 0 up to the first dissociation threshold at 44409 cm−1is determined to be about 5000. We propose an empirical model for the level structure of HN+2which incorporates Roe's suggestion for stretch‐bend coupling. The empirical numbers and densities of states agree well with the corresponding numerically exact quantum mechanical values for the given su

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010309

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractStationary points of the potential surface for the SN2 reaction F−+ CH3Cl → FCH3+ Cl−have been investigated by large‐scale CCSD(T) calculations. The ion‐dipole complexes in the reactant and product channels have well depths of 15.8 and 9.6 kcal mol−1, respectively, and are separated by a small barrier of 3.3 ± 0.3 kcal mol−1. The molar reaction enthalpy at 298 K is obtained to be

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010310

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY

摘要:

AbstractThe variational canonical flexible transition state theory expressions for the thermal reaction rate constant are reviewed and then applied to the barrier‐less recombination reactions H + CCH, H + CF3, and CH + H2. The rate constant expressions can classically accommodate any description of the hindering potential governing relative orientation changes between the reactants at a given distance. The distance held fixed during the relative orientation changes can be fully optimized. The three applications display comparable and large variations between the harmonic oscillator and free rotor limits of the hindering potential. The actual hindering potentials are all derived from similarab initioelectronic structure calculations. With these potentials, the rate constant for H + CF3is somewhat free‐rotor like, the rate constant for CH + H2is mostly harmonic‐oscillator like, and the rate constant for H+CCH is largely in between these two l

ISSN:0005-9021    

DOI:10.1002/bbpc.19971010311

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1997

数据来源: WILEY