Browse ORBi by ORBi project

- Background
- Content
- Benefits and challenges
- Legal aspects
- Functions and services
- Team
- Help and tutorials

Unimolecular reaction dynamics from kinetic energy release distributions. 8. Protonated fluorobenzene and structure of the phenyl ion Lorquet, Jean-Claude ; in Journal of Physical Chemistry A (2001), 105(15), 3719-3724 A previous interpretation of the kinetic energy release distribution (KERD) observed in the fragmentation of protonated fluorobenzene is confirmed by a maximum entropy analysis. The KERD is bimodal, with ... [more ▼] A previous interpretation of the kinetic energy release distribution (KERD) observed in the fragmentation of protonated fluorobenzene is confirmed by a maximum entropy analysis. The KERD is bimodal, with an intense and broad component due to the production of the cyclic phenyl ion. Ab initio calculations indicate the existence of several open-chain isomers, with an energy about 1.0-1.2 eV higher than that of the phenyl ring. A weaker component of the KERD corresponds to the formation of one or several of these acyclic structures. Its abundance represents about 6% of the main component (but drops down to 3% for the perdeuterated isomer). More translational energy than the statistical estimate is released during the dissociation process because of the presence of a barrier along the reaction path leading to the generation of the cyclic phenyl ion. About one-half of the energy of the barrier is released as translation, thereby indicating the operation of strong exit-channel interactions between separating fragments. © 2001 American Chemical Society. [less ▲] Detailed reference viewed: 16 (0 ULg)Landmarks in the theory of mass spectra Lorquet, Jean-Claude in International Journal of Mass Spectrometry (2000), 200(1-3), 43-56 Statistical theories of mass spectra are based on two assumptions. The first one, which postulates efficient phase space sampling, is substantiated by various experimentation and is now theoretically much ... [more ▼] Statistical theories of mass spectra are based on two assumptions. The first one, which postulates efficient phase space sampling, is substantiated by various experimentation and is now theoretically much better understood. The efficiency of phase space sampling can be estimated and is found to be quite good. Much effort remains to be done concerning the second assumption. A new impetus should be given to the concept of transition state. A better understanding of the role played by the conservation of angular momentum, the exact significance of transition state switching, and the incorporation of quantum effects are set as goals for the future. © 2000 Elsevier Science B.V. [less ▲] Detailed reference viewed: 6 (0 ULg)Unimolecular reaction dynamics from kinetic energy release distributions. VI. Energy-selected ions Lorquet, Jean-Claude in International Journal of Mass Spectrometry (2000), 201(1-3), 59-67 The kinetic energy distribution (KERD) determined by Brand et al. [Chem. Phys. 76 (1983) 111] for the iodine loss from energy-selected iodopropane ions has been analyzed by the maximum entropy method. The ... [more ▼] The kinetic energy distribution (KERD) determined by Brand et al. [Chem. Phys. 76 (1983) 111] for the iodine loss from energy-selected iodopropane ions has been analyzed by the maximum entropy method. The entropy deficiency DS has been determined and the value of e(-DS), which measures the efficiency of phase-space sampling, has been found equal to about 96%, thus corresponding to a nearly fully statistical situation. The phase space theory of Klots describes a slightly more constrained situation with e(-DS) ≃ 77%. A rapid determination of the sampled fraction of phase space is possible from energy-selected KERDs, based on the fact that there exists a direct relationship between the efficiency e(-DS) and the reduced maximum of the KERD ε(red)/(M) = ε(M) (E)/<ε>(E). The product ε(M)(E) P(ε(M)/E) or ε(red)/(M) P(ε(red)/(M)) holds the same property. (C) 2000 Elsevier Science B.V. [less ▲] Detailed reference viewed: 15 (1 ULg)Dissociation of Difluoroetylene Cations: an ion-neutral complex along the fluoroethylidene+ + HF reaction path. Leyh, Bernard ; Lorquet, Andrée ; Lorquet, Jean-Claude et al in Gelpi, E. (Ed.) 15th International Mass Spectrometry Conference: Book of Abstracts (2000) The collision activated dissociation and dissociative photoionization of 1,1- and 1,2-C2H2F2 in the HF-loss channel are investigated. The measured KER ditributions are interpreted by the opening of two ... [more ▼] The collision activated dissociation and dissociative photoionization of 1,1- and 1,2-C2H2F2 in the HF-loss channel are investigated. The measured KER ditributions are interpreted by the opening of two competitive reaction channels leading to two C2HF+ isomers. This hypothesis is checked by ab initio calculations. [less ▲] Detailed reference viewed: 24 (5 ULg)Unimolecular Reaction Dynamics from Kinetic Energy Release Distributions: extent of Phase Space Sampling. ; Locht, Robert ; Lorquet, Andrée et al in Gelpi, Ed. (Ed.) 15th International Mass Spectrometry Conference: Book of Abstracts. (2000) The dissociative photoionization of C2H3Br+ in the Br and HBr loss channels is reported. The KER distributions are analyzed by the Maximum Entropy Method (MEM). It can be concluded that at low and high ... [more ▼] The dissociative photoionization of C2H3Br+ in the Br and HBr loss channels is reported. The KER distributions are analyzed by the Maximum Entropy Method (MEM). It can be concluded that at low and high internal energy the dissociations are statistical. At intermediate internal energies the excess energy is not completely randomized. [less ▲] Detailed reference viewed: 52 (8 ULg)Unimolecular reaction dynamics from kinetic energy release distributions. III. A comparative study of the halogenobenzene cations ; Leyh, Bernard ; Remacle, Françoise et al in Journal of Chemical Physics (1999), 110(2-12), 2911-2921 The translational kinetic energy release distribution (KERD) in the halogen loss reaction of the chloro-, bromo-, and iodobenzene cations has been experimentally determined in the microsecond time scale ... [more ▼] The translational kinetic energy release distribution (KERD) in the halogen loss reaction of the chloro-, bromo-, and iodobenzene cations has been experimentally determined in the microsecond time scale and theoretically analyzed by the maximum entropy method. The KERD is constrained by the square root of the translational energy, i.e., by the momentum gap law. This can be understood in terms of quantum-mechanical resonances controlled by a matrix element involving a localized bound state and a rapidly oscillating continuum wave function, as in the case of a vibrational predissociation process. The energy partitioning between the reaction coordinate and the set of the remaining coordinates is nearly statistical, but not quite: less translational energy is channeled into the reaction coordinate than the statistical estimate. The measured entropy deficiency leads to values of the order of 80% for the fraction of phase space sampled by the pair of fragments with respect to the statistical value. In the case of the dissociation of the chlorobenzene ion, it is necessary to take into account a second process which corresponds to the formation of the chlorine atom in the excited electronic state 2P 1/2 in addition to the ground state 2/ 1/2. The observations are compatible with the presence of a small barrier (of the order of 0.12 eV) along the reaction path connecting the D̃ 2A 1 state of C 6H 5Cl + to the Cl( 2P 1/2) + C 6H 5 +(X̃ 1A 1) asymptote. © 1999 American Institute of Physics. [less ▲] Detailed reference viewed: 10 (0 ULg)Unimolecular Dynamics from Kinetic Energy Release Distributions. V. How does the Efficiency of Phase Space Sampling vary with Internal Energy? ; Locht, Robert ; Lorquet, Andrée et al in Journal of Chemical Physics (1999), 111(20), 9259-9266 A retarding field technique coupled with a quadrupole mass analyzer has been used to obtain the kinetic energy release distributions (KERDs) for C2H3Br+->[C2H3]++Br dissociation as a function of the ... [more ▼] A retarding field technique coupled with a quadrupole mass analyzer has been used to obtain the kinetic energy release distributions (KERDs) for C2H3Br+->[C2H3]++Br dissociation as a function of the internal energy. The KERDs obtained by dissociative photoionization using the He(I), Ne(I) and Ar(II) resonance lines were analyzed by the maximum entropy method and were found to be well described by introducing a single dynamical constraint, namely the relative translational momentum of the fragments. Ab initio calculations reveal the highly fluxional character of the C2H3+ ion. As the energy increases, several vibrational modes are converted in turn into large-amplitude motions. Our main result is that, upon increasing internal energy, the fraction of phase space sampled by the pair of dissociating fragments is shown to first decrease, pass through a shallow minimum around 75%, and then increase again, reaching almost 100% at high internal energies (8 eV). This behaviour at high internal energies is interpreted as resulting from the conjugated effect of intramolecular vibrational redistribution (IVR) and radiationless transitions among potential energy surfaces. Our findings are consistent with the coincidence data of Miller and Bear, reanalyzed here, and with the KERD of the metastable dissociation. [less ▲] Detailed reference viewed: 38 (10 ULg)Intramolecular vibrational relaxation seen as expansion in phase space. III. The long-time limit ; Lorquet, Jean-Claude in Journal of Chemical Physics (1997), 107(17), 6677-6680 Asymptotic formulas that describe the behavior of the function N(T) measuring the phase space volume sampled by a nonstationary wave packet during its time evolution are derived. It is shown that, in the ... [more ▼] Asymptotic formulas that describe the behavior of the function N(T) measuring the phase space volume sampled by a nonstationary wave packet during its time evolution are derived. It is shown that, in the long-time limit, N(T)∼T-1 when the dynamics is regular, whereas N(T)∼T-2 In T for the chaotic case. © 1997 American Institute of Physics. [less ▲] Detailed reference viewed: 16 (0 ULg)Intramolecular vibrational relaxation seen as expansion in phase space. II. Reference ergodic systems ; Lorquet, Jean-Claude in Journal of Chemical Physics (1996), 104(4), 1362-1369 The aim of the paper is to estimate the volume of phase space that is, in principle, available to a nonstationary wave packet during its intramolecular vibrational relaxation. For that purpose, use is ... [more ▼] The aim of the paper is to estimate the volume of phase space that is, in principle, available to a nonstationary wave packet during its intramolecular vibrational relaxation. For that purpose, use is made of the maximum entropy method, together with the concept of constrained ergodicity to construct two so-called reference ergodic systems. The first one concerns thermal excitation processes. In that case, the only two constraints that are imposed on the intramolecular dynamics arise from the normalization of the wave function and from the conservation of. energy. These constraints affect the zeroth and first moments of the spectrum. The second reference system concerns a situation where, as an additional constraint, use is made of the information that the system has been prepared spectroscopically, i.e., by a specific .excitation process, consisting in the coherent excitation of an initial pure state. Then, the second moment of the spectrum, denoted cr, is shown to provide the appropriate additional constraint. Translated into the time domain, the prior knowledge of the dynamics used as a constraint is limited to an infinitesimally brief period of time [0,dt] with the remaining evolution determined by the maximum entropy method. The spectroscopic reference system constructed in that way can be understood as the one that samples the maximal volume of phase space available to a wave packet having a specified average energy and being put in motion by a specified initial force. Closed-form expressions are obtained for the phase space volumes occupied by these two reference systems for various simple parametrizations of the function D(E) that expresses the density of states as a function of the internal energy (power laws or exponential increase). Thermal reference systems are found to sample a larger volume of phase space than their spectroscopic counterparts. The difference between these two cases depends critically on the value of σ, and also on the symmetry characteristics of the excitation process. In general, the volumes occupied by the reference systems, thermal as well as spectroscopic, can be expressed as ηE avD(E av), where E av is the (conserved) average energy of the wave packet and η is a correcting factor that depends on the functional form of D(E) and on the nature of the imposed constraints. In all cases studied, the value of η was found not to greatly differ from 1. The method has been applied to the analysis of three experimental photoelectron spectra presenting different spectral characteristics (X̃ 2A 1 state of NH 3 +, X̃ 2B 3 state of C 2H 4 +, and the X̃ 2A″ state of C 2H 3F +). The fractional occupancy index F defined by Heller as the fraction of the available phase space eventually explored up to the break time T B could be determined. After a time of the order of 100 fs, F was found to be of the order of a few percent for thermal excitation. When the molecule presents some symmetry, the expansion of the wave packet is restricted to that part of phase space spanned by the totally symmetric wave functions. The use of this additional a priori knowledge increases the fractional index F. © 1996 American Institute of Physics. [less ▲] Detailed reference viewed: 25 (2 ULg)Unimolecular Reaction Dynamics from Kinetic Energy Release Distributions. 2. A Study of the Reaction C6H5Br+->C6H5++Br by the Maximum Entropy Method. ; Remacle, Françoise ; Leyh, Bernard et al in Journal of Physical Chemistry (1996), 100(19), 8003-8007 The kinetic translational energy released in the unimolecular fragmentation reaction C6H5Br+->C6H5++Br has been experimentally studied in the microsecond time scale and theoretically analyzed by the ... [more ▼] The kinetic translational energy released in the unimolecular fragmentation reaction C6H5Br+->C6H5++Br has been experimentally studied in the microsecond time scale and theoretically analyzed by the maximum entropy formalism. The appropriate functional form relating the actual distribution to its prior distribution (eq.2.3) involves the square root of the kinetic energy (i.e. the momentum associated with the relative translational energy). A value of 0.26+/-0.02 eV is obtained by the entropy deficiency distribution at an internal energy of 0.85 eV above the reaction threshold. From this value, it can be concluded that 77% of the transition state phase space is efficiently sampled. [less ▲] Detailed reference viewed: 7 (0 ULg)Unimolecular dissociation of selectively excited polyatomic molecules Remacle, Françoise ; Lorquet, Jean-Claude ; in Chemical Physics Letters (1993), 209(4), 315-324 Selective activation processes (e.g., induced by a Franck-Condon transition) lead to an initially restricted sampling of the available phase space. The rate constant calculated by RRKM theory need then ... [more ▼] Selective activation processes (e.g., induced by a Franck-Condon transition) lead to an initially restricted sampling of the available phase space. The rate constant calculated by RRKM theory need then not be an upper bound to the exact value. A faster reaction rate is possible if the sampled phase space is preferentially coupled to the reaction coordinate. Such will be the case if the limited region of phase space accessed during the earlier stages of the energy redistribution is significantly spanned by "local mode" states. A computational example is provided as an illustration. It also shows how recrossings of the transition state result in an enhanced sampling of the bound phase space. © 1993. [less ▲] Detailed reference viewed: 11 (0 ULg)Unimolecular reaction dynamics from Kinetic Energy Release Distributions. 1 - Dissociation of Bromobenzene Ions. Lorquet, Jean-Claude ; Leyh, Bernard in Organic Mass Spectrometry (1993), 28 Preliminary data on a new method for extracting data from the kinetic energy release distribution of metastable fragment ions are presented. This method can give information concerning the energy ... [more ▼] Preliminary data on a new method for extracting data from the kinetic energy release distribution of metastable fragment ions are presented. This method can give information concerning the energy threshold of the reaction and the energy dependence of the rate constant (i.e. its logarithmic derivative). It was applied to bromine loss from bromobenzene cations. [less ▲] Detailed reference viewed: 23 (1 ULg)Quantum effects in competitive unimolecular reactions Remacle, Françoise ; Desouter-Lecomte, Michèle ; Lorquet, Jean-Claude in Chemical Physics (1991), 153(1-2), 201-217 The decay law P(t) of a microcanonical ensemble of metastable states prepared in an energy interval δE is usually nonexponential because the widths Γ of each individual resonance fluctuate around an ... [more ▼] The decay law P(t) of a microcanonical ensemble of metastable states prepared in an energy interval δE is usually nonexponential because the widths Γ of each individual resonance fluctuate around an energy-dependent average K(E) which is satisfactorily approximated by RRKM theory. The kinetic equations used at the macroscopic level are therefore not necessarily transferable to elementary processes when the interval δE contains more than a single resonance. For example, two competitive unimolecular reactions need not be characterized by the same effective evolution in time. Particular attention is focused on the quantum expressions of the yields of fragmentation Yk(E, t) when they differ from those Fk(E, t) derived from the macroscopic kinetic scheme. The discussion is based on the value of the parameter νeff = 2K(E)2/σ2 which is a measure of the dispersion of the individual rate constants. Two applications are presented. First, we consider a competition between an electronic and a vibrational predissociation in a triatomic van der Waals-like model with a sparse density of states and a maximum number of selection rules. The shorter the lifetime, the larger the dispersion of the widths of the resonances lying within δE, and the more severe the discrepancy between Yk(E, t) and Fk(E, t). In the quantum treatment, fragmentation proceeds with unequal effective rate constants in the two decay channels. The second application deals with the other extreme where the density of states is so large that statistical methods are useful. Explicit quantum mechanical expressions of the yields in a competitive fragmentation process are given for some distributions of the widths, χν 2 and convolutions of χν 2. Here again, a discrepancy can be observed between Yk(E, t) and Fk(E, t): competitive unimolecular reactions need not be characterized by the same rate. It is demonstrated that, when the density of states is so large that the resonances are quasi-degenerate, the mean and the variance of the widths can be obtained from the invariant Ω matrix. When the resonances overlap, the variance of the widths is always greater than the variance of any kind of zero-orders widths. Except in unrealistic cases (either because a single resonance is excited or because all the widths are the same) it is dubious to disprove a mechanism on the basis of arguments borrowed from conventional macroscopic kinetics in an energy-resolved experiment as has been done many times in the past. © 1991. [less ▲] Detailed reference viewed: 6 (0 ULg)Quantum effects in competitive decay Remacle, Françoise ; Desouter, Michèle ; Lorquet, Jean-Claude in Chemical Physics (1991), 153 Detailed reference viewed: 5 (3 ULg)Intramolecular vibrational energy relaxation seen as expansion in phase space. I. Some experimental results for H2O+(X̃ 2B1), C2H4 +(X̃ 2B3), and HCN+(B̃2Σ +) Lorquet, Jean-Claude ; in Journal of Chemical Physics (1990), 93 It has been shown by Heller that a nonstationary wave packet resulting from a Franck-Condon transition evolves on the potential energy surface of the final electronic state and propagates through phase ... [more ▼] It has been shown by Heller that a nonstationary wave packet resulting from a Franck-Condon transition evolves on the potential energy surface of the final electronic state and propagates through phase space at a rate which can be determined from the autocorrelation function \C(t)\2 = | 〈∅(0) |∅(t) ) |2. Since C(t) can be obtained by Fourier transformation of an optical spectrum S(E), i.e., from an observable quantity, it is possible to derive from an experimental measurement information concerning the density operator of a so-called dynamical statistical ensemble (DSE). This density operator, denoted ρav, represents a statistical mixture of the eigenstates of the system with weights determined by the dynamics of the system. It becomes diagonal after a so-called break time Script T sign;B. Its measure, according to a definition due to Stechel, can be interpreted as an effective number of states (denoted script N) that significantly contribute to the dynamics. The break time Script T sign;B represents the finite period of time allowed to expand in the phase space and after which no further progress can be made. Therefore, the number script N∞ of phase space cells which are accessed after a very long interval of time (or in practice after the break time) remains limited. Information on the validity of statistical theories of unimolecular reactions is contained in the fraction ℱ of the available phase space which is eventually explored. In order to assess the representativity of the sampling, it is necessary to account for the selection rule which requires all the states counted in script N ∞ to belong to the totally symmetric representation. It is also appropriate to estimate the role played by Fermi resonances and similar vibrational interactions which bring about energy flow into zero-order antisymmetric modes. A method to carry out the necessary partitionings is suggested. The functions script N T and ℛ T, and the quantities Script T signB, script N ∞, t script N, and ℱ have been determined from experimental data in three cases. In each case, the rate ℛT = d script N r/dT starts from an initial value of zero, increases up to a maximum which is reached after a time of the order of 10-14 s, and then exhibits an overall decrease upon which oscillations are superimposed. For state X̃ 2B1 of H2O+, Script T signB ≃a 2.4×10-14 s and Script T sign ≃ 0.3. The wave packet never accesses that part of the phase space that corresponds to the excitation of antisymmetric vibrations. For state X̃ 2B3u of C2 H4 +, Script T signB ≃1.6 × 10-13 s and Script T sign ≃ * 5×10-4. This fraction raises to 6 × 10 -3 if measured with respect to the effectively available phase space. When the spectrum consists of a discrete part followed by a dissociation continuum, the method can be extended to study the behavior of the bound part of the wave packet only. This has been applied to state B̃ 2Σ+ of HCN+ which is characterized by a very irregular spectrum. This case offers an example of complete occupation of phase space after a break time which is of the order of 2 ×10 -13 s. © 1990 American Institute of Physics. [less ▲] Detailed reference viewed: 16 (0 ULg)Extracting laws of decay in the femto-picosecond range from autocorrelation functions Remacle, Françoise ; Desouter, Michèle ; Lorquet, Jean-Claude in Journal of Chemical Physics (1989), 91 Detailed reference viewed: 6 (2 ULg)Couplings between Normal Modes studied by the Correlation Function. Duschinsky Effect and Fermi Resonance. ; Leyh, Bernard ; Lorquet, Jean-Claude in Chemical Physics (1989), 132 The dynamical information contained in a correlation function obtained by the Fourier transform of an electronic spectrum can be used to study strong intermode couplings, such as the Duschinsky effect (DE ... [more ▼] The dynamical information contained in a correlation function obtained by the Fourier transform of an electronic spectrum can be used to study strong intermode couplings, such as the Duschinsky effect (DE) and the Fermi resonance (FR). Both of them complicate the calculation of the correlation function by destroying its factorisability. In some particular cases, the DE can greatly simplify the form of the correlation function by concealing one of its inherent frequencies. The DE never leads to a beat or to a systematic decrease of the correlation function. A simple classical approximation for the correlation function which takes into account the Lissajous motion of the center of the wave packet, but does not allow for its deformation or spreading is found to be useful in a harmonic model. The FR leads to a beat in the correlation function which results from a periodic energy transfer from the active to the inactive mode. A practical method is given to extract the perturbed and unperturbed energies as well as the coupling matrix element of a FR from a low-resolution spectrum by Fourier transformation of just that part of the spectrum which corresponds to the quasidegenerate interacting states. The case of the B2Sigma+u state of CS2+ is treated as an example. [less ▲] Detailed reference viewed: 47 (0 ULg)Nonadiabatic unimolecular reactions. III. Dissociation mechanisms of methylnitrite and deuterated methylnitrite ions Leyh-Nihant, Brigitte ; Lorquet, Jean-Claude in Journal of Chemical Physics (1988), 88(9), 5606-5619 At low energies, methylnitrite ions dissociate via two channels giving rise to CH3O + NO+ or to [CH3O+] + NO fragments. Peculiar characteristics have been detected in the dissociation of energy-selected ... [more ▼] At low energies, methylnitrite ions dissociate via two channels giving rise to CH3O + NO+ or to [CH3O+] + NO fragments. Peculiar characteristics have been detected in the dissociation of energy-selected parent ions, viz., remarkably low rate constants, one of which is found to remain insensitive to an increase of the internal energy, and large isotope effect. These peculiarities are accounted for by a statistical, nonadiabatic model. Ab initio calculations, confirmed by multipolar expansions reveal that the potential energy curves which correlate to these two dissociation asymptotes cross. The crossing takes place at a large value along the reaction coordinate R, indicating a long-range interaction. Production of the CH3O + NO+ fragments results from a simple bond cleavage taking place on a single diabatic surface. On the other hand, production of [CH3O+] + NO fragments is brought about by a transition from one diabatic surface to the other. It leads to deformed methoxy ions which immediately rearrange to the much more stable H2COH + structure. The nonadiabatic rate constant has been calculated by a statistical method. The contribution of each channel is weighted by a transmission coefficient which is equal to the nonadiabatic transition probability. Implementation of this statistical treatment requires partitioning the set of degrees of freedom as follows: {R, y, d, v}. It is necessary to withdraw the isomerization mode y from the statistical treatment, because the equilibrium positions along this coordinate are very different in each electronic state. Physically, this means that the reaction involves tunneling from one surface to the other along the displaced degree of freedom y. The large isotope effect has a double origin: part of it (a factor of ∼9) results from tunneling along γ; the remainder (an additional factor of 3) comes from the usual RRKM-like effect on the densities of states. The degrees d constitute a set of four low-energy bending modes which form a sink for the internal energy. The nonadiabatic transition probability is determined by the off-diagonal matrix element V12(Rc). Effective potential energy curves have been calculated by extending the Quack and Troe method to nonadiabatic reactions. It turns out that the excitation of the set d leads to a decrease of V12(Rc) and hence to a decrease of the nonadiabatic transmission coefficient. This accounts for the weak dependence of the rate constants kCH3O+ and kCD3O+ vs the energy (at least above a certain energy threshold). © 1988 American Institute of Physics. [less ▲] Detailed reference viewed: 15 (0 ULg)NONADIABATIC UNIMOLECULAR REACTIONS .2. ISOTOPE EFFECTS ON THE KINETIC-ENERGY RELEASE Remacle, Françoise ; Dehareng, Dominique ; Lorquet, Jean-Claude in Journal of Physical Chemistry (1988), 92(16), 4784-4787 This paper investigates the isotope effect that occurs when XOCO+ ions dissociate into XOC+ + 0 on a microsecond time scale (X = H or D). The reaction mechanism involves an electronic spin-forbidden ... [more ▼] This paper investigates the isotope effect that occurs when XOCO+ ions dissociate into XOC+ + 0 on a microsecond time scale (X = H or D). The reaction mechanism involves an electronic spin-forbidden predissociation between a stable singlet state and a repulsive triplet. Application of the statistical equations developed in the previous paper shows that, at a given energy, the predissociation rate constant is consistently smaller for DOCO’ than for HOCO+. Therefore, the internal energy necessary to bring about dissociation of the hydrogenated compound with a given rate constant is always lower than that of the deuteriated compound by a quantity AE* which is found to be equal to ca. 0.050 eV when k = lo6 s-l and to ca 0.020 eV for k = lo3 s-1. As a result, the excess energy which is released as kinetic energy carried by the fragments is substantially greater for the deuteriated than for the hydrogenated compound. This accounts for experiments which indicate that, in the microsecond time scale, DOCO’ gives rise to a dished metastable peak whereas the corresponding signal for HOCO’ is simply Gaussian. [less ▲] Detailed reference viewed: 26 (5 ULg)Constructing approximatively diabatic states from LCAO-SCF-CI calculations Desouter-Lecomte, Michèle ; Dehareng, Dominique ; Lorquet, Jean-Claude in Journal of Chemical Physics (1987), 86(3), 1429-1436 We consider here two approaches which have been proposed in the literature to obtain diabatic states from ab initio calculations. First by calculating explicitely the coupling vector g which describes the ... [more ▼] We consider here two approaches which have been proposed in the literature to obtain diabatic states from ab initio calculations. First by calculating explicitely the coupling vector g which describes the nonadiabatic interaction between the adiabatic states. Second, by some extrapolation process of the wavefunctions obtained at a particular reference point. The coupling vector is a sum of three contributions. The first two represent the the change in character of the adiabatic states in the region of nonadiabatic coupling due the variation of the CI and LCAO coefficients, whereas the third results from the translation of the atomic orbitals with the moving nuclear centers. Criteria have been given to recognize when it is possible to transform a set of CI wave functions into a pair of useful diabatic states. A particularly favorable situation is obtained when the interacting electronic states are doubly excited with respect to each other. Within the two-states approximation, the first term, depending on the CI coefficients, is strictly irrotational and never gives rise to problems. One can expect situations where it is also true for the second term depending on the LCAO coefficients. However, the third term of the coupling can never be described as a rotation of two diabatic functions. Nevertheless, the latter contribution can frequently be neglected, at least when the coupling is strong. The theory of the electron transfer factors (ETF's) provides further insight into the problem and confirm our conclusions. [less ▲] Detailed reference viewed: 54 (4 ULg) |
||