Gas-phase data

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

There is an excellent correlation between these data and the gas-phase data, in terms both of the stability order and the energy differences between carbocations. A plot of the gas-phase hydride affinity versus the ionization enthalpy gives a line of slope 1.63 with a correlation coefficient of 0.973. This result is in agreement with the expectation that the gas-phase stability would be somewhat more sensitive to structure than the solution-phase stability. The energy gap between tertiary and secondary ions is about 17kcal/mol in the gas phase and about 9.5 kcal/mole in the SO2CIF solution. 

Carbanion-stabilizing effects have been calculated at several levels of theory. Table 7.6 gives some gas-phase data. The AMI and PM3 semiempirical calculations have also been done in water. The order NO2 > CH=0 > CN > Ph > CH2=CH is in accord with the experimental trends and reflects charge delocalization. The electronegative substituents F, OH, and NH2 are stabilizing by virtue of polar effects. The small stabilization provided by CH3 is presumabfy a polarization effect. 

Be sure to remind students that these frequencies are gas phase data and arc thus not the same as the more-faniiliar solution spectra (we will treat solvated systems in Chapter 9). Even so, such gas phase calculations make excellent discovery-based exercises, For example, students may be asked to explain the substituent effects observed tising basic chemistry knowledge. 

Some gas phase data suggest that a certain fraction of the transition states for some reactions are reflected back to products. One can multiply the right side of Eq. (7-55) by k, the transmission coefficient, to account for this, in which case k < 1. We shall ignore this factor k, taking it as unity. Indeed, we shall ignore a large body of experimental research on gas phase reactions and the theoretical calculations on them. 

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Ti2. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

So far we have not touched on the fact that the important topic of solvation energy is not yet taken into account. The extent to which solvation influences gas-phase energy values can be considerable. As an example, gas-phase data for fundamental enolisation reactions are included in Table 1. Related aqueous solution phase data can be derived from equilibrium constants 31). The gas-phase heats of enolisation for acetone and propionaldehyde are 19.5 and 13 keal/mol, respectively. The corresponding free energies of enolisation in solution are 9.9 and 5.4 kcal/mol. (Whether the difference between gas and solution derives from enthalpy or entropy effects is irrelevant at this stage.) Despite this, our experience with gas-phase enthalpies calculated by the methods described in this chapter leads us to believe that even the current approach is most valuable for evaluation of reactivity. 

Mononuclear ER4 and simple four-coordinate compounds of E(IV) states are the baseline for viewing the other coordination numbers, the effect of bulky ligands, bonds to other E or metals, E(II) compounds, multiple bonds and other phenomena discussed in later sections. Basic parameters for some simple compounds are presented in Table 1, taken from the gas-phase data summarized by Molloy and Zuckerman5 and Haaland6. These data show the unperturbed molecules in the gas phase and provide the base for... 

Gas-phase data, 298°K. Energy units kcal mol. (Data from Skinner and Pilcher, 1963)... 

Gas-phase data, 25°C. Entropy changes are corrected for symmetry and optical isomers. (Data from Allinger and Zalkow, 1960) b For closure to trans form c For closure to cis form... 

Infrared Measurements. Elementary-step models can be fit to transient gas-phase data (5) which are obtained by the methods described above. The models will then predict the surface intermediate concentration during transients and at steady state. It is clear that it is also important to observe these surface species experimentally, during transients as well as at steady state. Infrared spectroscopy can be used during catalysis in the presence of the gas phase, so it plays an important role in transient studies. 

The chemistry of uranium interacting with atmospheric components, like carbon, nitrogen, and oxygen, poses a formidable challenge to both experimentalists and theoreticians. Few spectroscopic observations for actinide compounds are suitable for direct comparison with properties calculated for isolated molecules (ideally, gas phase data are required for such comparisons). It has been found that even data for molecules isolated in cryogenic rare gas matrixes, a medium that is usually considered to be minimally perturbing, can... 

For many species and solvents these assumptions are thought to be sensible, whereas in some cases they may lead to significant errors (see following discussion). Nevertheless, this was the approach used by Bordwell and co-workers [342], They combined most of the terms of equation 16.32 in a single constant, C, which was empirically adjusted to give better agreement with gas-phase data [343-345], Equation 16.33 illustrates this procedure C = 306.7 kJ mol-1 is valid for S = dimethylsulfoxide (DMSO) and when the oxidation potential of R- is anchored on the ferrocenium/ferrocene Fc+/Fc) couple instead of the SHE in water. 

One of the two enthalpies of formation for the tertiary alkyl nitrites is most probably incorrect. As seen in Table 1, not only is the slope for the liquid-phase data less negative than that for the gas-phase data, but it is much less steep than all other liquid-phase data of which we know. Part of the discrepancy is due to the inverted order of the archival enthalpies of vaporization t-butyl nitrite = 34.4 kJmol-1 and t-pentyl nitrite = 33.5 kJ mol 1. 

The solution basicities in water at 25 °C of ortho-, meta- and para-substituted primary, secondary and tertiary anilines have been widely discussed by Smith1. Preliminary gas-phase data were reported by Bohme2. Subsequently, the gas-phase basicities (proton... 

Although simple, a model system containing one solvent molecule together with one ion already provides valuable insight into the nature of the ion-solvent interaction. There is also convincing evidence that this two body potential dominates in much more complicated situations like in the liquid state 88,89,162). Molecular data for one to one complexes can be calculated with sufficient accuracy within reasonable time limits. Gas-phase data reported in Chapter III provide a direct basis for comparison of the calculated results. 

Finding DII FEK-8. While no agent was detected in the scrubbing solutions and scrubber filters, the ability of the GPCR process to destroy HD in mortars and neat GB could not be confirmed because sampling and analysis problems hampered the gathering of gas-phase data. 

The substrate in these studies was restricted to be rigid, and Morse functions were used for the hydrogen-surface and two-body interactions. The parameters in the Morse functions were determined for single hydrogen atoms adsorbed on the tungsten surface by fitting to extended Huckel molecular orbital (EHMO) results, and the H2 Morse parameters were fit to gas-phase data. The Sato parameter, which enters the many-body LEPS prescription, was varied to produce a potential barrier for the desorption of H2 from the surface which matched experimental results. 

You may assume that gas phase data may be used for reactions in toluene.)... 

The Criegee mechanism, widely accepted for the liquid-phase reaction, does not adequately explain the available gas-phase data. O Neal and Blumstein suggested a biradical structure for the first gas-phase intermediate and proposed three types of unimolecular hydrogen abstraction reactions (Figure 3-10). 

Some recent work from this laboratory describes some experiments which have the potential of extracting gas phase data from studies in polar solvents 8, 12,13). Only when the solvent is properly selected for the particular system to be studied can one hope to approach data relatively free of solvation or association energies. For many systems, the solubility limitations clearly make quantitative measurement impossible at our present level of understanding. 

Both the quantity and quality of gas-phase experimental structural data rapidly diminish with incorporation of elements beyond the second row of the Periodic Table. Solid-phase structures abound, but differences in detailed geometries from gas-phase structures due to crystal packing may be significant and preclude accurate comparisons with the calculations. There are, however, sufficient gas-phase data primarily on very small molecules to enable adequate assessment to be made. 

We will begin with the first group of species and then move to the second. To initially avoid a discussion of solvation effects on carbanion stability, we will begin with gas-phase data for simple representatives of model classes of compounds (Table 3.2). Later, the gas-phase data will be compared to data that have been obtained in solution (Section 4.3.3). 

The paucity of gas phase data for azoles (see Section V,A) has led us to use substituted pyridines as reference compounds. Care has been taken to select the appropriate substituted pyridines for comparison purposes. Thus, 2-X-imidazoles, 3(5)-X-1,2,4-triazoles, and 5-X-tetrazoles have been compared to 2-X-pyridines, while 3,5-di-X-1,2,4-triazoles have been compared to 2,6-di-X-pyridines. The choice of pyridines has been determined by the large amount of data available for these compounds (81JOC891 83MI2 86UP12). 

Earlier attempts to use IR spectroscopy to identify adsorbed dioxygen species by comparison with solid-state and gas-phase data for O2, O2, and 0 have led to a number of conflicting assignments in the literature. These arise for two principal reasons first, the IR bands for the dioxygen species adsorbed on the surface often lie at frequencies intermediate between those expected for the individual species on the basis of the solid-state and gas-phase data, and the data for different systems tend to cover a range of frequencies (Appendix C) second, there is considerable overlap between the... 

Solution studies (Albery and Kreevoy, 1978), gas phase data (Pellerite and Brauman, 1980, 1983) and theoretical calculations (Wolfe et al., 1981) all indicate an enormous variation in the barriers for identity exchange of CH3X (113). For example, in (113) the experimental gas-phase barrier for... 

In the February 2005 version of the CSD database, 349 strnctures containing one or more direct zinc-carbon interactions have been found (excluding strnctnres containing the Zn—CN structural motif). Together with the gas-phase data, this means that at that date a total of 354 molecular structures had been determined. Of these strnctnres a large majority (225) deals with compounds in which 4-coordinate zinc is present. 

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