Isomerism unimolecular reactions

From stochastic molecnlar dynamics calcnlations on the same system, in the viscosity regime covered by the experiment, it appears that intra- and intennolecnlar energy flow occur on comparable time scales, which leads to the conclnsion that cyclohexane isomerization in liquid CS2 is an activated process [99]. Classical molecnlar dynamics calcnlations [104] also reprodnce the observed non-monotonic viscosity dependence of ic. Furthennore, they also yield a solvent contribntion to the free energy of activation for tlie isomerization reaction which in liquid CS, increases by abont 0.4 kJ moC when the solvent density is increased from 1.3 to 1.5 g cm T Tims the molecnlar dynamics calcnlations support the conclnsion that the high-pressure limit of this unimolecular reaction is not attained in liquid solntion at ambient pressure. It has to be remembered, though, that the analysis of the measnred isomerization rates depends critically on the estimated valne of... 

Khundkar L R, Marcus R A and Zewail A H 1983 Unimolecular reactions at low energies and RRKM-behaviour isomerization and dissociation J. Phys. Chem. 87 2473-6... 

A) During the luultiphoton excitation of molecular vibrations witli IR lasers, many (typically 10-50) photons are absorbed in a quasi-resonant stepwise process until the absorbed energy is suflFicient to initiate a unimolecular reaction, dissociation, or isomerization, usually in the electronic ground state. 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules ia a gas at low pressure can be taken to be isolated for the short time between coUisions. Unimolecular reactions such as photodissociation or isomerization iaduced by photon absorption can sometimes be studied between coUisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity ia one direction and almost zero velocity ia perpendicular directions. Not only does this reduce coUisions, it also aUows bimolecular iateractions to be studied ia intersecting beams and iacreases the detail with which unimolecular processes that can be studied, because beams facUitate dozens of refined measurement techniques. (J) Means have been found to trap molecules, isolate them, and keep them motionless at a predetermined position ia space (11). Thus far, effort has been directed toward just manipulating the molecules, but the future is bright for exploiting the isolated molecules for kinetic and dynamic studies. 

Unimolecular reactions that take place by way of cyclic transition states typically have negative entropies of activation because of the loss of rotational degrees of freedom associated with the highly ordered transition state. For example, thermal isomerization of allyl vinyl ether to 4-pentenal has AS = —8eu. ... 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

Free valence also persists in unimolecular reactions of radicals, such as decomposition and isomerization. 

P3.03.23. EFFECT OF PRESSURE ON UNIMOLECULAR REACTIONS. CYCLOPROPANE ISOMERIZATION. 

Our interest in thermally activated unimolecular reactions is in the change of kuni with pressure from the high to the zero pressure limit, and in the pressure dependence of the isotope effect over that range. One particularly interesting study carried out by Rabinovitch and Schneider (reading list) focused on the isomerization of methyl isocyanide, CH3NC, to methyl cyanide, CH3CN... 

The thermal isomerization of cyclopropane to propylene is perhaps the most important single example of a unimolecular reaction. This system has been studied by numerous workers. Following the work of Trautz and Winkler (1922), who showed that the reaction was first order and had an energy of activation of about 63,900 cal mole measured in the temperature range 550-650° C, Chambers and Kistiakowsky (1934) studied the reaction in greater detail and with higher precision from 469-519° C. They confirmed that it was first order and, for the reaction at its high-pressure limit, obtained the Arrhenius equation... 

Much of the recent work on the cyclopropane-propylene isomerization has had one of two objectives, either to try and determine which of the two reaction paths suggested by the early workers is involved, or to test the various theories of unimolecular reactions. Comer and Pease (1945), using catalytic hydrogenation to analyse their reaction product, but otherwise working under similar conditions to Chambers and Kistiakow-sky, suggested that all the results obtained could be represented just as well by the reaction scheme... 

Pritchard et al. (1953), using an improved analjdiical technique, were able to study the reaction down to a pressure of less than 0-1 mm (where the rate constant is only 10 % of the high-pressure value). Their results indicated quite clearly that the reaction was unimolecular, and in the pressure region, where the rate constant had decreased appreciably addition of inert gases did lead to an increase in rate. At the same time. Slater (1953) applied his theory of unimolecular reactions to this isomerization. 

The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

Bowen, R.D. Stapleton, B.J. Williams, D.H. Nonconcerted Unimolecular Reactions of Ions in the Gas Phase Isomerization of Weakly Coordinated Carbonium Ions. J. Chem. Soc., Chem. Commun. 1978, 24-26. 

Isomerizations are important unimolecular reactions that result in the intramolecular rearrangement of atoms, and their rate parameters are of the same order of magnitude as other unimolecular reactions. Consequently, they can have significant impact on product distributions in high-temperature processes. A large number of different types of isomerization reactions seem to be possible, in which stable as well as radical species serve as reactants (Benson, 1976). Unfortunately, with the exception of cis-trans isomerizations, accurate kinetic information is scarce for many of these reactions. This is, in part, caused by experimental difficulties associated with the detection of isomers and with the presence of parallel reactions. However, with computational quantum mechanics theoretical estimations of barrier heights in isomerizations are now possible. 

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

Both unimolecular and bimolecular reactions are common throughout chemistry and biochemistry. Binding of a hormone to a reactor is a bimolecular process as is a substrate binding to an enzyme. Radioactive decay is often used as an example of a unimolecular reaction. However, this is a nuclear reaction rather than a chemical reaction. Examples of chemical unimolecular reactions would include isomerizations, decompositions, and dis-associations. See also Chemical Kinetics Elementary Reaction Unimolecular Bimolecular Transition-State Theory Elementary Reaction... 

Formaldonitrone, CH2=N(H)—O (3), the elusive simplest organic nitrone, has been prepared transiently in the gas phase by femtosecond collisional neutralization of its cation radical, CH2—N(H)—0+". The latter was generated by dissociative ionization of 1,2-oxazolidine. Nitrone 3 showed negligible dissociation upon collisional neutralization and was distinguished from its tautomers formaldoxime 2 and nitrosomethane 1 that gave different NR mass spectra. The enthalpy of formation was calculated from enthalpies of atomization and two isodesmic reactions as Af//29s(3) = 58 1 kJmol . The calculated, large activation barriers for isomerization of 3 (179 and 212 kJmoH for 3 anti-2 and 3 1, respectivelyindicate that once 3 is formed and diluted in the gas phase it should not isomerize unimolecularly to either 1 or (syn/anti) 2. 

For pentyl radical, internal H-atom transfers can occur regardless of whether further oxidation occurs. These unimolecular reactions can directly compete with oxidation steps and so have implications for low-temperature combustion. For instance, n-pentyl radical can quickly isomerize to iso-pentyl radical via 1,4-H atom transfer each of these radicals can undergo p-scission reactions to yield a new alkyl radical + alkene ... 

In contrast to cyclization and rearrangement as the unimolecular reaction, the EZ isomerization of olefins is difficult due to a drastic and unenviable change in the size and shape of the occupied space by substituents on the double bond during isomerization in the crystalline state. Some (Z,Z)-muconic derivatives provide a geometrical isomer as the photoproduct in a high yield, but not a polymer, under UV irradiation in the crystalline state, as is described in the Introduction (Scheme 1 and Table 1). This isomerization is a crystal-to-crystal reaction with an excellent selectivity, which is completely different from ordinary photoisomerizations. 

The formation of oxidation products a-c in a range of G values (0.7-3.8) during the 7-R of S in 02-saturated DCE suggests that a-c would be produced from complicated reactions of peroxy radicals with S (Table 5). On the other hand, the regioselective formation of 3d with large G values (2.6-3.0) in oxidation of 3 with O2 is explained by spin localization on the p-olefinic carbon because of the contribution of (B) in 3. The results of products analyses are essentially identical with prediction based on k and ko for S measured with PR. It should be emphasized that the reactivities of c-t unimolecular isomerization and reaction of S with O2 can be understood in terms of charge-spin separation induced by p-MeO. 

Achieving control over microscopic dynamics of molecules with external fields has long been a major goal in chemical reaction dynamics. This goal stimulated the development of quantum control schemes, which have been applied with spectacular results to unimolecular reactions, such as photodissociation or isomerization reactions. Attaining control over bimolecular reactions in a gas has proven to be a much bigger challenge. 

G.M. Wieder and R.A. Marcus. Dissociation and Isomerization of Vibrationally Excited Species. II. Unimolecular Reaction Rate Theory and Its Application. J. Chem. Phys., 37 1835-1852,1962. 

Since A, B, and C are regions in the phase space of single closed system, the transitions between A and represent a unimolecular reaction or isomerization, rather than a general reaction in the sense of chemical kinetics. Unlike some unimolecular reactions, (e.g the decomposition of diatomic molecules) the molecular dynamics system of eq. 1 will be assumed to have sufficiently many well-coupled degrees of freedom that transitions between reactant and product regions occur spontaneously, without outside interference. 

In the case of closed-shell organic molecules M can be an excited singlet or triplet state. M can react on its own in unimolecular reactions (dissociations, isomerizations) or it can react with another (ground state) molecule N in bimolecular processes (e.g. additions, substitutions, etc.). 

A summary of the major chemical reactions of free radicals is given in Table 4.3. Broadly speaking these can be classified as unimolecular reactions of dissociations and isomerizations, and bimolecular reactions of additions, disproportionations, substitutions, etc. The complexity of many photochemical reactions stems in fact from these free radical reactions, for a single species formed in a simple primary process can lead to a variety of final products. 

Photoinduced unimolecular reactions often have kinetics of the order of ps. One example of isomerization in ps times is shown in Figure 8.9. This is the photochromic reaction of a spiropyran. The photoinduced process takes place... 

These are essentially unimolecular reactions of dissociation and of isomerization, studied mostly in the gas phase. We shall consider here a few examples of such reactions. The dissociation of IGN can be written as... 

As a typical unimolecular reaction of disilenes, the /i,Z-isomcrization is discussed first. In contrast to the isomerization of an alkene that occurs via the rotation around the C = C double bond with an activation energy of ca. 60kcalmol-1 the E,Z-isomerization of disilenes is known to occur more easily. As shown in review OW, the E,Z-isomerization in aryl-substituted disilenes 3,4,20,26, and 27 proceeds under mild conditions to allow the kinetic studies at 40-80 °C by NMR spectroscopy. Recently, the T,Z-isomerization between tetrakis(trialkylsilyl)disilenes ( )- and (Z)-33 was found to occur more rapidly with the rates of the NMR time scale at 30 °C 63... 

One can again consider two general categories direct reactions and complex mode reactions. Saddle points are found for some, but not all unimolecular reactions. Thus, for the unimolecular dissociation of HoO in its electronic ground state no saddle point is found (see Figs 3.1.5 and 3.1.6). For an isomerization like HCN —> HNC, a saddle... 

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. 

In principle, one can induce and control unimolecular reactions directly in the electronic ground state via intense IR fields. Note that this resembles traditional thermal unimolecular reactions, in the sense that the dynamics is confined to the electronic ground state. High intensities are typically required in order to climb up the vibrational ladder and induce bond breaking (or isomerization). The dissociation probability is substantially enhanced when the frequency of the field is time dependent, i.e., the frequency must decrease as a function of time in order to accommodate the anharmonicity of the potential. Selective bond breaking in polyatomic molecules is, in addition, complicated by the fact that the dynamics in various bond-stretching coordinates is coupled due to anharmonic terms in the potential. 

Example 11.2), and unimolecular reactions like photodissociation and isomerization. 

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