Gas-phase investigations

Experimental information on the complexes between dihalogens and methylated amines is still comparatively scarce. Gas-phase investigations are available only for the complexes of trimethylamine with F2 [41 ] and with C1F [42], So far only a few theoretical investigations on XY- amine complexes have been presented [16,17,22,24,28,32,34,43,44]. On the basis of rotational spectroscopic analysis, the N(CH3)3 C1F complex was described as being dominated by a significant contribution of an ionic [(CH3)NC1]+- -F valence bond structure [41]. For the (CH3)3N F2 complex an even stronger tendency toward an ionic [(CH3)NF]+- F structure was reported [40]. 

It should be noted that, due to space limitations, we restrict ourselves to the species that have been characterized in the condensed phase. Experimental or quantum chemical gas-phase investigations will be cited when appropriate however, their literature coverage is not complete. 

The next step in the direction of a deeper understanding of nanostructured materials depends on being able to isolate the individual structurally determined cluster units from the crystal lattice and then determine the physical properties of the single clusters in question. This long-term objective has been partially achieved in the gas phase investigation of a structurally determined Gai9R6 cluster [R = C(SiMe3)3] in an FT mass spectrometer (cf. Section 2.3.4.2.5, Ga clusters)... 

According to the complexity of the phenomenon of solvation, experimental results which might have bearings for its understanding are spread throughout the literature. Gas-phase investigations of intermolecular interactions are of a basic interest 23 ) and a large amount work has been done on the thermodynamics and spectroscopic properties of liquid mixtures 29,237-239)... 

Whereas theoretical studies of dihydrogen bonding where dihydrogen-bonded complexes are isolated, approximating the gas phase, are very numerons, gas-phase experiments with these complexes are relatively rare. This circumstance is probably connected with the nontrivial experimental techniqnes used for gas-phase investigations. 

Mg(OH2)6] with a simple glycosidic linkage (glucose-PPi) in the gas phase (investigation of the complete manifold of conformational possibilities was not possible). Even so, eight representative conformations—all with comparable stabilities—were reported. 

A comprehensive picture of alkyl iodides pyrolyses has been presented by Benson124. These substrates are sensitive and difficult to handle in homogeneous molecular HI elimination studies and this is the reason for the comparatively few gas-phase investigations. Concurrent radical and unimolecular mechanisms are frequently observed in organic iodides decomposition. 

In order to get more insight into the factors that determine thermal stability, which is crucial for the safe manipulation of (fluoromethyl)silanes, we have investigated the conventional thermal [24] and laser-induced decomposition [25, 26] of the parent silanes F H3 CSiH3. These compounds are stable gases at ambient temperature, and they may be conveniently studied by IR and multinuclear NMR spectroscopy. Therefore these appear to be particularly suited for gas phase investigations. 

Previous theoretical and experimental gas phase investigations showed that SiH3 is 53.7 kcal mol more stable than CH3 [31]. However, substitution, particularly by Jt-donors, reduces this difference Table 2 updates prior results, [33] at the higher MP2(FC)/6-31G level of theory, with a set of second and third period groups. 

Besides the above-mentioned application for volatile, stable compounds, the technique of matrix isolation is superior to gas-phase investigation of compounds with a low vapor pressure. The vapor pressure of molybdenum hexacarbonyl, Mo(CO)6, for instance, is only 10 mbar at room temperature, which is insufficient for ordinary gas-phase infrared measurements. However, if Mo(CO)6 is flushed with a stream of matrix gas and if the final mixture is deposited as a matrix, then the density of the Mo(CO)6 molecules increases considerably. This integration effect (the deposition time varies from 5 minutes to several hours) is very important for high-temperature species and has interesting analytical applications. 

Adsorbed moleeules and intermediates at high pressures can be detected by vibrational speetroseopies provided they can differentiate between gas phase and surfaee signals. For example, Fig. 4 shows a (conventional) IRAS spectrum of CO at 50mbar on Pd(l 11) at 300 K (113,114). The signal of adsorbed on-top CO at approximately 2060 cm is nearly obscured by the rovibrational absorption spee-trum of the CO gas phase. In contrast, as shown below, SFG and PM-IRAS selectively probe the adsorbed surface species and thus provide surface-sensitive information, even in the presence of a gas phase. Investigations of the catalyst structure and composition under working conditions can be earried out by high-pressure (HP-) STM and (HP-) XPS, provided that the instruments are properly modified (9,115). 

The counter-current moving bed reactor was first mentioned by Takeuchi et al. (1978). However, so far only gas-phase investigations are known. The lack of applications of the TMBR are for the same reasons as with the TMB process, namely difficulties with particle movement, back mixing of the solid, and abrasion of the particles. 

The solubility coefficient is the ratio of the equilibrium concentration of the dissolved penetrant to its partial pressure in the the gas phase. Investigations of sorption isotherms for gases in glassy polymers have lead to the hypothesis that the equilibrium concentration is the result... 

Consequently, under- or supersaturation can be adjusted precisely and varied rapidly under electrochemical conditions, in contrast to gas-phase investigations. 

While newer enzymatic procedures facilitate protein cleavages into characteristic peptide fragments, the overall purpose of the gas-phase investigations of such peptides remains the determination of the amino acid sequences and their homolo-... 

Almost all of the calculations on polyads were performed for isolated base complexes thus neglecting effects of nucleic backbone, solvent and also of entropic contributions. Erqreriments most directly related to quantum-chemical results are gas phase investigations on base pairs.Nevertheless, it makes also sense to compare the results of quantum-chemical studies on base pairs or polyads to complete three-dimensional nucleic acid structures. One should realize, however, that there is a long way to go from results of quantum-chemical studies on building blocks to nucleic acid structure and function. The power of this approach is that it can help to separate the effects of different parts of the nucleic acid, such as backbone and bases, on the overall structure. In addition, it provides useful information on charge distribution and electrostatic potentials relevant for the determination of interaction sites with cations such as K, the most abundant cation in cells. Finally, the interaction energies can be studied in great detail. For example non-additive contributions can be determined. 

Carbeck, J. D., J. C. Severs, J. Gao, Q. Wu, R. D. Smith, and G. M. Whitesides, Correlation between the charge of proteins in solution and in the gas phase investigated by protein charge ladders, capillary electrophoresis, and electrospray ionization mass spectrometry, J. Phys. Chem. B, 102, 10596-10601, 1998. 

Several cyclometalation studies have been conducted in the gas phase. In the reactions of Cr", Mn, and Fe with octynes/ " Fe with isonitriles, " and Cu with nitriles/cyclometalation follows initial coordination via tt or lone-pair electrons. Such reactions have demonstrated both C—H and C—C activation. Another gas-phase investigation involved the reorganization of FeCH fragments (x = In contrast with the results of matrix and surface studies, only FeCH... 

Production of unligated (bare) transition metal ions for gas-phase investigations has been accomplished primarily by well-established methods electron impact on volatile precursors such as metal carbonyls, laser desorption from pure... 

A gas-phase investigation of the system F+HNO3 and the determination of absolute rate constants for the reaction of the NO3 radical,... 

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