Isomerization threshold

It has been shown recently that the vibrational spectra of HCP [33-36], HOCl [36-39], and HOBr [40,41] obtained from quantum mechanical calculations on global ab initio surfaces can be reproduced accurately in the low to intermediate energy regime (75% of the isomerization threshold for HCP, 95% of the dissociation threshold for HOCl and HOBr) with an integrable Fermi resonance Hamiltonian. Based on the analysis of this Hamiltonian, this section proposes an interpretation of the most salient feature of the dynamics of these molecules, namely the first saddle-node bifurcation, which takes place in the intermediate energy regime. 

To this end, the specialized techniques described below have been developed to determine the structure of gas-phase ions [36]. In most of these methods, the measurements are compared against a reference compound, with the premise that if the unknown ion and the ion of a known structure give identical mass spectral data, the two ions possess the same structure. The reader is, however, reminded that ions sampled by these methods have different lifetimes and energy content. Therefore, the conclusions drawn from different methods may not be identical, but rather, depend on the type of ions sampled, their energy content, and the isomerization threshold. 

In a reaction, the threshold energy that must be exceeded for molecules to become activated toward chemical reaction. 2. In conformational isomerizations, the energy needed to interconvert stable conformers. 3. More generally, the energy needed to transform, activate, or interconvert a particular molecular entity(ies). 

Olefin Isomerization. One other type of hydrocarbon isomerization is on the threshold of commercialization—namely, that of olefins. Processes for olefin isomerization were first developed some 15 years ago (11, 14, 20) after it was recognized that highly branched olefins have higher octane numbers than do their straight-chain isomers, and that the octane numbers of olefins increase as the double bond moves toward the middle of the molecule. 

The questions that have arisen in regard to isomerizations in solution also apply to isomerizations in clusters. One of these questions, which has now been addressed, is the threshold energy and its variation with size of the cluster. Data on the threshold energies were obtained in the microcanonical study by Heikal et al. [1], New questions, however, also arise for clusters How rapidly is the energy transferred between the solute and the solvent molecules in the cluster Outside the threshold region a reduction in krale(E) with... 

Activation parameters for the high pressure gas-phase approach of 1,2-d2-cyclopropanes to cis, trans equilibrium (equation 1) have been reported as log A, a(kcal mol"1) of 16.0,64.2 and 16.4,65.176,77. From pressure-dependent measurements of rate constants and calculations based on RRKM theory, the threshold energy E for the cis, trans isomerization has been estimated to be 61.1 kcal mol"1 and 61.3 kcal mol"11 16 1, s. 

When a molecule is supplied with an amount of energy that exceeds some threshold energy, a unimolecular reaction can take place, that is, a dissociation or an isomerization. We distinguish between a true unimolecular reaction that can be initiated by absorption of electromagnetic radiation (photo-activation) and an apparent unimolecular reaction initiated by bimolecular collisions (thermal activation). For the apparent unimolecular reaction, the time scales for the activation and the subsequent reaction are well separated. When such a separation is possible, for true or apparent unimolecular reactions, the reaction is also referred to as an indirect reaction. We will discuss the following. 

Catalytic isomerization of 3,4-dichlorobutene catalyzed by Pd nanoparticles of Pd-PPX film was studied at 100°C [91], The ratio of trans- to cis-1, 4-dichlorobutene for the reaction in this system with low concentration of Pd nanoparticles is 10, and coincides with the ratio obtained for the reaction with the usual palladium catalyst. But the selectivity of the reaction decreases with increasing of Pd concentration the yield of trans-l, 4-dichlorobutene decreases while the yield of cA-1,4-dichlorobutene remains constant. This result shows that the change in the catalytic properties of the composite is determined by interactions between nanoparticles rather than by the size effects. At catalytic reaction catalyzed by Pd-PPX films, where the volume content of Pd nanoparticles is close to percolation threshold, the trans-to-cis ratio for produced isomers of 1,4-dichlorobutene is 2.9 that is close to equilibrium value of this ratio. 

An earlier off-odor problem that surfaced was the presence of 4-phenyl-cyclohexene in styrene-butadiene coated paper (Koszinowski et al. 1980). This compound was created by a Diels-Alder condensation reaction involving a molecule of styrene and butadiene and is differentiated by its odor from the isomeric 3 and 1-phenyl-cyclohex-ene compounds which cannot be formed by such a condensation reaction. The recognition threshold of this compound in the headspace over an aqueous solution lies around a concentration of 10 pg/kg (10 ppb). The typical odor of this compound at concentrations of 4-phenyl-cyclohexene in paper over 4 mg/kg (4 ppm) is easily identified. AGC determination in this concentration range is also possible without difficulty and its identification with MS using the relative molecular mass of 158 and one of the retro Diels-Alder decomposition product fragments at m/e = 104 (styrene) and mJe = 54 (butadiene) is definitely possible. 

Flavor threshold values for carbonyl compounds are influenced by factors such as the number of carbon atoms, degree and location of unsaturated double bonds, isomerism, additive and/or antagonistic effects of mixtures of carbonyls and the medium in which the carbonyls exist (Day et al., 1963 Meijboom, 1964). Many carbonyls have up to 100 times greater flavor potency in an aqueous medium (e.g., liquid milk) than in a fat or oil (e.g., butteroil). Hence, off-flavor tends to be noticeable at lower concentrations of carbonyl compounds in liquid milk than in butter. 

Dian BC, Clarkson JR, Zwier TS (2004) Direct measurement of energy thresholds to conformational isomerization in tryptamine. Science 303 1169-1173. 

For example, the lowest-energy dissociation of 61 to form H2N-CH-OH and CO requires 129 kj mol-1 threshold energy, but must overcome a 310 kj mol-1 barrier for the rate-determining isomerization to H2N-CH(OH)-CO [260]. a-Radicals derived from methyl glycinate (62) and glycine AT-methylamide (63) are somewhat less stable, but still show fractions of undissociated survivor ions in the +NR+ mass spectra. This increased reactivity is supported by theoretical calculations that show additional dissociation channels, e.g., elimination of CH3OH from 62 and elimination of CO from 63 [260]. 

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