Unimolecular gas phase

These expressions are modified in the case of non-unimolecular gas phase reactions to... 

POLYRATE can be used for computing reaction rates from either the output of electronic structure calculations or using an analytic potential energy surface. If an analytic potential energy surface is used, the user must create subroutines to evaluate the potential energy and its derivatives then relink the program. POLYRATE can be used for unimolecular gas-phase reactions, bimolecular gas-phase reactions, or the reaction of a gas-phase molecule or adsorbed molecule on a solid surface. 

This review will first concentrate on the unimolecular gas-phase chemistry of diene and polyene ions, mainly cationic but also anionic species, including some of their alicyclic and triply unsaturated isomers, where appropriate. Well-established methodology, such as electron ionization (El) and chemical ionization (Cl), combined with MS/MS techniques in particular cases will be discussed, but also some special techniques which offer further potential to distinguish isomers will be mentioned. On this basis, selected examples on the bimolecular gas-phase ion chemistry of dienes and polyenes will be presented in order to illustrate the great potential of this field for further fundamental and applied research. A special section of this chapter will be devoted to shed some light on the present knowledge concerning the gas-phase derivatization of dienes and polyenes. A further section compiles some selected aspects of mass spectrometry of terpenoids and carotenoids. 

On the other hand, if the non-classical ion is a stable intermediate, the transition state for the 3,2 hydride shift requires a subst mtial reorganization, including the cleavage of the cyclopropyl ring, and, by analogy with unimolecular gas phase processes, a much higher pre-exponential factor might be expected. [In the cyclopropane-propylene reaction log A is 1ST 7 (Chambers and Kistiakowsky, 1954).] Contrary to expectation, the observed pre-exponential for the 3,2-shift is actually a little lower than for the 1,2,6-equilibration process. 

In the first place, we shall take a look at the recent advances in fast reaction photochemical kinetics and spectroscopy, in particular at picosecond laser flash photolysis and femtosecond observations. Next, photophysics and photochemistry in molecular beams will be considered. Here observations are made under single molecule-single photon conditions, and these experiments provide insight into the most fundamental unimolecular gas phase reactions. 

F. M. Bickelhaupt, R. H. Fokkens, L. J. de Koning, N. M. M. Nibbering, E. J. Baerends, S. J. Goede, andF. Bickelhaupt, Int. J. Mass Spectrom. Ion Processes, 103, 157 (1991).Isolated Excited Electronic States in the Unimolecular Gas-Phase Ion Dissociation Processes of the Radical Cations of Isocyanogen and Cyanogen. 

Prior to 1953, few kinetic works on the homogeneous, unimolecular gas-phase pyrolysis or elimination of simple alkyl halides were reported. According to these experimental data the commonly accepted mechanism consisted of a concerted four-membered cyclic transition state yielding the corresponding olefin and hydrogen halide as shown in equation 1. For molecular dehydrohalogenation, the presence of a /i-hydrogen adjacent to the C—X bond is necessary. 

The first case of neighboring group participation in homogeneous unimolecular gas-phase pyrolysis was deduced from the significant rate increase in the thermal decomposi-... 

The homogeneous unimolecular gas-phase pyrolysis of 3-methoxy-l-chloropropane and 4-methoxy-l-chlorobutane175 was described as in equations 81 and 82. 

Isotopic substitution affects rates of ionic decompositions and isomerisations in essentially the same ways as isotopic substitution affects rates of thermal reactions [360, 608, 654, 764, 905, 925]. Mass spectrometry does, however, own a few idiosyncracies in this area and it is important to distinguish clearly the different sorts of isotope effects involved. The term kinetic isotope effects in this review will be restricted to effects of isotopic substitution on the values of rate coefficients, k(E). Kinetic isotope effects on unimolecular gas-phase... 

This F test can also be used when the number of degrees of freedom is different for the two models. The most frequently encountered circumstance of this kind results from least-squares fitting with two models, one having n parameters and the other having the same n parameters plus p additional parameters associated with an elaboration of the first model. An example is j = a + versus y=a+bx- - cx. As a further simple example, consider a unimolecular gas-phase kinetics experiment such as Exp. 24. Strict first-order kinetics predicts that the partial pressure of decaying molecular species A will vary as... 

Table XI.4. Unimolecular Gas-phase Decompositions That Give Stable Molecules (Continued)...

Table XI.5. Unimolecular Gas-phase Reactions That Produce Radicals...

The gas phase pjnrolysis of alkyl hahdes has been extensively reviewed 58>, and in general the unimolecular gas phase reactions of alkyl halides parallel their reactivity in a mass spectrometer. For example, ethylchloride yields ethylene and HCl on thermolysis 5 >, and the ethylene ion in the mass spectrum of ethyl chloride is significantly more intense than the molecule ion. 1,2-dichloroethane also eliminated HCl thermolytically and the corresponding ion is the base peak in its mass spectrum. Elimination of HCl is also common to the mass spectra and thermochemistry of chloroprene dimers.Although in this case the major ion at mje 91 had no definite analog in the thermochemistry. This is probably due to the fact that mje 91 was a tropylium ion which would not be stabihzed as a neutral. 

MP2 study of substituent effects of 2-substituted alkyl ethyl methylcarbamates in homogeneous, unimolecular gas phase elimination reaction ... 

Unimolecular gas phase studies try to isolate reacting molecules from their environment. Insofar as this is successful, gas phase studies provide the most unambiguous data on the intramolecular forces which control reaction rates and pathways. The energetic and conformational requirements of transition state species are of paramount interest, and with the stringent limitations placed on the data by modern reaction rate theories, the results may be critically examined and meaningfully evaluated. A critical survey of the data leading to the rejection of some and a selection of the best parameters in others, has been one of our primary concerns. Transition state theory has been assumed, and the methods and criteria employed in the calculations are based on this theory. They are outlined very briefly for each... 

A number of authors have pointed out that spectra acquired using various desorption techniques have many features in common (11-14). Generally the ions detected are even electro ionj molecular ions formed by protonation or addition of NH-, Na, etc., and fragment ions formed by elimination of neutral molecules. It is not clear to what extent pyrolysis and solvolysis reactions augment the contributions of unimolecular gas phase decompositions to spectra obtained using the various desorption techniques. Glucuronic Acid Conjugate Desorption... 

Intramolecular Hydrogen Tranter in Unimolecular Gas-phase Reactions 8S7... 

Intramolecular hydrogen transfer is another important class of chemical reactions that has been widely studied using transition state theory. Unimolecular gas-phase reactions are most often treated using RRKM theory [60], which combines a microcanonical transition state theory treatment of the unimolecular reaction step with models for energy redistribution within the molecule. In this presentation we will focus on the unimolecular reaction step and assume that energy redistribution is rapid, which is equivalent to the high-pressure limit of RRKM theory. 

The induction time for the initial endothermic bond breaking reaction can be calculated using the high pressure, high temperature transition state theory. Experimental unimolecular gas phase reaction rates under low temperature (<1000K) shock conditions obey the usual Arrhenius law ... 

Within the Eyring transition state theory for a unimolecular gas-phase reaction SiN [SiN] —> SiN with the activated complex [S1n] the rate constant k is then related to the activation free enthalpy of the... 

The unimolecular gas-phase thermolysis of RR CHC(S)OSi(CH3)3 at 380-760 °C gives the corresponding ketene as the major product [40]. The 0-trimethylsilyl... 

For reactions in a condensed phase (solid or liquid solution) it is often convenient to. .consider the reactants in the initial and final states at a fixed finite separation, as making small vibrations around the positions of minimum potential energy due to the interactions with the medium. Therefore, the quasiclassical approximation for this motion, assumed to correspond to the reaction coordinate, may be used for the same reason as for a unimolecular gas phase reaction which occurs at high presure. 

The kinetics and mechanism of the unimolecular gas-phase elimination of 2-(dimethylamino)ethyl chloride have been examined by using DFT methods to explain the enhanced reactivity in gas-phase elimination compared to the parent compound ethyl chloride. The TS located on the minimum energy path had a four-centred cyclic configuration comprising chlorine, hydrogen, and two carbon atoms and benefited from electron delocalization involving the dimethylamino substituent. 

Rate constants for the chemical exchange processes that occur in alkyl nitrites, cyclohexane, substituted cyclohexanes, sulfur tetrafluoride and formamide are pressure dependent. The mechanism for these thermally initiated, unimolecular gas-phase processes, reported by Lindemann in 1922, involves competition between the reaction and collisional deactivation of the critically energized molecule. A (Fqn [3]). 

The unimolecular gas-phase elimination kinetics of 2-methoxy-l-chloroethane, 3-methoxy-l-chloropropane, and 4-methoxy-l-chlorobutane has been studied using density functional theory (DFT) methods. Results calculated for 2-methoxy-l-chloroethane and 3-methoxy-l-chloropropane suggest that the corresponding olefin forms by dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state. In the case of 4-methoxy-l-chlorobutane, in addition to the 1,2-elimination mechanism, anchimeric assistance by the methoxy group, through a polar five-centered cyclic transition state, provides 4-methoxybutene, tetrahydrofuran, and chloromethane. Polarization of the C-Cl bond is rate limiting in these elimination reactions. 

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