Unimolecular dissociation of gaseous

It is the purpose of this review11 to show that a fairly substantial number of quite different unimolecular dissociations of gaseous cation radicals can be described conveniently by this concept. Among the reactions discussed are some unusual cleavage processes, the occurrence of which were difficult to explain without the involvement of hidden hydrogen migrations. 

Wahihaftig, A.L. Unimolecular Dissociations of Gaseous Ions, in Gaseous ion Chemistry and Mass Spectrometry, Futrell, J.H., editor John Wiley and Sons New York, 1986 7-24. 

EXPERIMENTAL APPROACHES TO THE UNIMOLECULAR DISSOCIATION OF GASEOUS CLUSTER IONS... 

However, central to any truly accurate determination of the radiative rate are the integrated absorption (emission) intensities, A", which for gaseous ions are almost completely unknown as are, usually, the vibrational frequencies. Fortunately, however, ab initio and density functional methods have recently been shown to be quite accurate in their predictions of vibrational spectra for a wide variety of systems, and there is no reason to suspect that this accuracy would not carry over to comparable data for gaseous ions. The one caveat must be that the low-frequency modes that are common in cluster ions will be decidedly anharmonic, and prediction of both these frequencies and their intensities may be suspect. However, these modes are not generally expected to be dominant contributors to the overall radiative rate. In addition, standard RRKM procedures can be applied to the unimolecular dissociation of the same adduct ions and, in principle therefore, the overall kinetics of formation of stabilized association complexes can be accurately modeled. 

The relatively long timescales of the ionization, isolation, thermalization, reaction, and detection sequences associated with low-pressure FTICR experiments are generally thought to preclude the use of this technique as a means of examining the unimolecular dissociation of conventional metastable ions occurring on the microsecond to millisecond timescale. Nonetheless, as just demonstrated (Section IIIC), intermediates with this order of magnitude of lifetime are routinely formed in the bimolecular reactions of gaseous ions with neutral molecules at low pressures in the FTICR cell, as in Equation (13). 

The unimolecular decomposition of gaseous HN3 at low pressures can be expected to start with a scission of either the H-NNN or HN-NN bond. Experiments show that the initial formation of NH and N2 is favored. The reaction enthalpies are 60 to 75 kJ/mol for the dissociation into N2(X) and NH(X) and about 385 kJ/mol for the dissociation into N3 and H see pp. 107/8. HF calculations of the potential energy surfaces employing a 6-31G basis set predicted that the reaction yielding N2 and NH prevails because of its low activation... 

One example of unimolecular dissociation is the breakup of gaseous sulfur hexafluoride [2551-62-4] 6 according to reaction 21 (106) ... 

At the present time a number of gaseous unimolecular reactions are known. The view that none exist, although it appeared plausible for a time, has now been definitely abandoned. Nevertheless, unimolecular reactions are rather exceptional and appear to be confined to molecules of rather complex structure. It is possible that the decomposition of diatomic molecules into atoms at high temperatures is unimolecular but more probable that it is bimolecular, the reverse reaction of recombination being termolecular. Thus the rate of dissociation of chlorine would be k1 [Cl2]2 while the rate of recombination of the atoms would be 2 [Cl]2 [Clg], according to the Herzfeld theory (p. 111). 

Time-resolved BIRD measurements provide an opportunity to precisely measure the kinetics of dissociation for isolated gaseous ions. The BIRD technique can, in principle, be used to study the dissociation kinetics of gaseous ions of any size. However, interpretation of the kinetic data depends on the size of the ions (or more accurately, the relative rates of absorption, emission, and dissociation). The general kinetic scheme for unimolecular reactions initiated by photon absorption at zero pressure is analogous to that described for IRMPD [see Eq. (16.3) in Section 16.3.1.1). Application of the steady-state approximation (i.e., the assumption that apparent rate constant ( mi) for unimolecular dissociation ... 

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