Thermochemical threshold

Thermochemical threshold, bond strength, bond dissociation energy (BDE) are equivalent terms giving the enthalpy difference (at temperature 7) between parent ion and fragments for the dissociation. 

Table 1. TRPD Results Interpreted as Simple Bond Cleavages Yielding Direct Thermochemical Thresholds...

As discussed in Section VI, the benzene ion is the polyatomic ion that has received the most careful attention regarding the extrapolation of rate-energy data to find the true thermochemical threshold [Equation (11)]. The TRPD results over... 

Lin and DeMore (637) have irradiated mixtures of 03 and isobutane with monochromatic light of wavelengths from 2750 to 3340 A at —40 C. The bandwidth was 16 A. The relative quantum yields of OCD) production were obtained from the yield of isobutyl alcohol, a product of the reaction 0( D) + isobutane. The results are shown in Fig. VI -13. The quantum yields are constant below 3000 A and show a sharp cutoffat 3080 A, the thermochemical threshold wavelength for the production of O( D) + 02( A). 

Fig. VI-16. (a) The absorption spectrum of 0.3 torr of SOj. Path length, 6.95 cm resolution, 3 A. (A) Fluorescence intensity (undispersed) of 0.3 torr SOj as a function of incident wavelength. Resolution, 3 A the arrow shows the thermochemical threshold of dissociation corresponding to 2192 A. Reprinted with permission from H. Okabe J. Am. Ghent. Soc. 93,7095 (1971). Copyright by the American Chemical Society.

The quantum yield of 0( P) has been determined by resonance absorption at 130 nm (76), resonance fluorescence (73,75), and time-of-flight (TOF) mass spectrometer (72,74). The quantum yield of 0(1d) decreases from 0.9 to 0.1 in the 300 to 320 nm region (73). The production of 0(ls) is 0.1% or less in the entire region from 170 to 240 nm (78). Above 310 nm both 03 absorption and the 0(1d) quantum yield increase with vibrational excitation of O3 (79). For example at 314 nm, which is about 500 cm-l below the thermochemical threshold for process (32), the absorption cross section increases more than 8 times and the quantum yield increases 6 times. 

The latter process is not competitive with the former above the thermodynamic threshold. Furthermore, as long as the available energy was kept below the thermochemical threshold for the production of vibrationally excited CN radicals, it was possible to fit the observed rotational distributions with phase space theory. The upper electronic state that is involved in the two-photon dissociation was shown to originate below 22,000 cm l and is thought to be repulsive. It could be the same state that has its absorption maximum at 270 nm. 

A break was observed in the translational energy probability curve for CS radicals at the thermochemical threshold for the production of S( D). It was then assumed that only atoms were produced below this threshold, while only 3p atoms were produced above this threshold. With this assumption they were able to derive a lower limit of 0.25 for the S(3p)/S(lD) ratio. They were also able to calculate a vibrational distribution from the TOF curve that agrees with the LIF measurements. The overall conclusions of this paper are in direct disagreement with the work of Addison et al. 

The entire cycle of odd oxygen chemistry in the upper atmosphere is initiated by the photolysis of O2 below 242 nm, the thermochemical threshold (Table 1) for dissociation into ground state atomic oxygen formation in... 

The thermochemical threshold tor production of O2 ( 2) in a spin-allowed process is 266 nm so one cannot a priori exclude the possibility of its formation in ozone photolysis. However I know of no evidence which would confirm the production of these higher excited states at high energies in the Hartley continuum. Looking at the characteristic red emission at 763 nm would make detection of 02( S) possible a very real experimental problem arises because deactivation of O ( D) by the trace of O2 invariably present in any O3 sample yields O2CI). This issue certainly deserves closer scrutiny. 

The first few energetically accessible dissociation channels for (CN)2 are listed below, with their thermochemical thresholds ... 

Fig. 6.1. S(ate-to-state rate constants for the dissociation of N2O and v 2 are the quantum numbers for the N—O and N—N vibrations respectively, in the reactant molecule, and v,2 is the quantum number for the vibration in the product N2 molecule. The thermochemical threshold is at 13488 cm , but the microscopic rate constants are almost negligible below about 20000 cm. The symbols have the following meanings crosses, r 2=v.2=0 circles, Pj2=0, f,2 = l triangles, j2=4, d 2=2 asterisks, Dt2=4, ti,2=3-...

Reactions can have a finite energy threshold that is higher than the thermochemical threshold, meaning that ctr is effectively zero below some threshold energy even though the reaction is thermodynamically allowed. One then speaks of an activation barrier that needs to be surmormted for reaction to... 

As shown by Lifshitz, Schwarz and co-workers, the methane elimination from the acetone cation proceeds via two successive intermediates, a hydro-gen-bridged complex and a ketene-methane ion-neutral complex. The barrier separating these two entities lies 14.2 kj mol above the CH2CO" -1- CH4 asymptote. Tunnelling through this barrier is required to account for the appearance of the CH2CO" fragment at its thermochemical threshold. 

Concerted retro-Diels-Alder decomposition does occur in cyclohexene derivatives that bear substituents in the allylic positions in such compounds the energies of open-ring intermediates are higher than that of the transition state for the concerted process, which occurs near the thermochemical threshold (Turefiek and Havlas, 1986). The critical effect of substituents on the stereochemistry of the retro-Diels-Alder reaction was discussed in Section 8.3 (Turecek and HanuS 1984, Mandelbaum 1983). 

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