Formation in the Gas Phase

The enthalpy of combustion of isoxazole was only determined several years ago (78MI41615). For isoxazole, AH°c (298.15 K) =-(1649.85 0.50) kJ mol , from which the entropy of formation in the gas phase was derived as AH tig) = 78.50 0.54 kJ moF. The enthalpies of combustion of 3-amino-5-methylisoxazole and 5-amino-3,4-dimethyl-isoxazole have also been determined (73MI41606). 

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

Fig. 4. Plot of enthalpies of complex formation in the gas phase of CpNi complexes (Cp = cyclopentadienyl) vs those for the corresponding Mn complexes. The ligands are segregated into correlations for soft (O) (N and S donors) and hard (oxygen donors) ( ). Energies are in kcal mol-1. Redrawn after Ref. (14). 

The standard enthalpy of solvation of any substance A, therefore, is the difference between its standard enthalpy of formation in solution and its standard enthalpy of formation in the gas phase ... 

Such equilibria are known to consist of a number of consecutive complex equilibria. The formation constant / represents the free enthalpy of complex formation in the gas phase. This quantity can not be determined by experiment. 

Atomic layer deposition is a high vacuum process where small amounts of the precursors are leaked into the system sequentially with intermittent evacuations. The ALD enables the conformal deposition of atomically thin layers with precise thickness control at low temperatures without the typical aggregate formation in the gas-phase. 

Assuming that the rate of gas-to-particle conversion of any condensable species is greater than its rate of formation in the gas phase (which is the case for heten eneous nucleation predominant in the atmosphere, but may not be valid for homogeneous nucleation in clean-air smog-chamber studies) ... 

In 1965, the following double-scale equation was proposed (39) to correlate enthalpies of adduct formation in the gas phase and poor solvating solvents ... 

The enthalpy of formation in the gas phase was calculated by summing the enthalpy of formation of the solid species from Reference 50 with the sublimation enthalpy in Reference 1. 

Marston, G., C. D. McGill, and A. R. Rickard, Hydroxyl-Radical Formation in the Gas-Phase Ozonolysis of 2-Methylbut-2-ene, Geophys. Res. Lett, 25, 2177-2180 (1998). 

When we consider the metals of nanoscopic size, fine metal particles from micrometer to nanometer size can be synthesized by both physical and chemical methods. The former method provides the fine metal particles by decreasing the size by addition of energy to the bulk metal, while in the latter methods, fine particles can be produced by increasing the size from metal atoms obtained by reduction of metal ions in solution. Since chemical reactions usually take place in homogeneous solution in any case, this chapter includes most of the cases of synthesis and growth of fine metal particles. However, the polyol process, reaction in microemulsions, and formation in the gas phase are omitted, since they are described in later chapters by specialists in those fields. 

DFT was employed to study the mechanism of ammonolysis of phenyl formate in the gas phase, and the effect of various solvents on the title reaction was assessed by the polarizable continuum model (PCM). The calculated results show that the neutral concerted pathway is the most favourable one in the gas phase and in solution.24 The structure and stability of putative zwitterionic complexes in the ammonolysis of phenyl acetate were examined using DFT and ab initio methods by applying the explicit, up to 7H20, and implicit PCM solvation models. The stability of the zwitterionic tetrahedral intermediate required an explicit solvation by at least five water molecules with stabilization energy of approximately 35 kcalmol-1 25... 

A short review of the OVPO and the MCVD method is given. A more detailed discussion of the PCVD method is presented. Emphasis has been laid on the description of experiments in which pure and Ge02 -oxide doped silica have been deposited. It turns out that the PCVD method has some unique properties such as deposition without soot formation in the gas phase, moderate substrate temperature, high deposition efficiency and the possibility of rapid reactor movement. Besides germanium, other dopant elements such as boron and fluorine have also been successfully deposited simultaneously with silica. 

To examine the processes of cluster formation in the gas phase a Knudsen Cell has been used. A mixture of ZnO and graphite powders was positioned inside the effusion cell. The carbothermal reaction between ZnO and graphite was used to create zinc vapor. The effusion cell was heated up to a temperature above 1000°C. 

Recently there have been several observations of exciplex formation in the gas phase. Exciplexes between 9,10-dicyanoanthracene and 1,5-dimethyl-naphthalene (Itoh et ai, 1981) and 1,2-dimethoxybenzene, hexamethyl-benzene and 2,5-dimethylhexa-2,4-diene (Hirayama and Phillips, 1981) have been studied. The work of Hirayama and Phillips showed that the wavelength of exciplex emission in the gas phase is very similar to that obtained in cyclohexane solution. Measurement of wavelengths of exciplex emission in the gas phase enable one to probe solvent polarity properties using eqn (20) more accurately since Vq is now known. Here is the frequency of emission... 

It has been observed that a series of 2,4-alkanedionato adducts of cobalt(III)(salen), salen = bis(salicylideneaminato) dianion, undergo a thermally induced, intramolecular one-electron transfer reaction to cobalt(II)bis(salicylideneaminato) . The concomitant formation in the gas phase of a mixture of the /9-diketone (not more than 50%), methanol, ethanol and acetone has been explained as follows the thermally induced, homolytic fission of the Co—Odik bond gives a /3-diketonato radical which abstracts a hydrogen atom from a second /3-diketonate to form the corresponding diketone, whereas the dehydrogenated /3-diketonato radical decomposes into compounds of lower molecular weight. 

When the necessary data for substitution in these empirical relations are not available, the heat of vaporization must be estimated by analogy, or frankly guessed. For example, Pauling and Sherman give the heats of formation in the gas phase of a number of compounds such as uric acid, for which the estimate of the heats of vaporization can only be guesses of unknown accuracy. This may introduce errors of as much as 10 kcal, in the most unfavourable case, and Pauling and Sherman are careful not to draw conclusions from differences less than this. 

Nearly all fragments in the mass spectra for the complexes investigated showed sets of isotopic peaks, which indicate the presence of boron-containing fragments in these species. As seen from the composition of the ionic fragments, their formation in the gas phase largely occurs with detachment of dioximate fragments from the "parent" molecule or ion ... 

Several possible explanations have been offered. One is that the ground state of the nucleophile is destabilized by repulsion between the adjacent pairs of electrons another is that the transition state is stabilized by the extra pair of electrons a third is that the adjacent electron pair reduces solvation of the nucleophile. Evidence supporting the third explanation is that there was no alpha effect in the reaction of HOj with methyl formate in the gas phase, although HOj shows a strong alpha effect in solution. The a-efifect has been demonstrated to be remarkably dependent on the nature of the solvent. The a-effect is substantial for substitution at a carbonyl or other unsaturated carbon, at some inorganic atoms, and for reactions of a nucleophile with a carbocation, but is generally smaller or absent entirely for substitution at a saturated carbon. ... 

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