Gases diffraction methods

This final section of this chapter is devoted to the interplay between theory and the model-driven experimental techniques, such as diffraction methods. Gas-phase electron diffraction and powder diffraction methods are more heavily reliant on direct input from simulation, for example in providing starting structures for least-squares refinement analyses (Section 2.11.3), whereas in the spectroscopic techniques discussed above the need is more for validation and assignment. 

In this chapter, we have chosen from the scientific literature accounts of symposia published at intervals during the period 1920 1990. They are personal choices illustrating what we believe reflect significant developments in experimental techniques and concepts during this time. Initially there was a dependence on gas-phase pressure measurements and the construction of adsorption isotherms, followed by the development of mass spectrometry for gas analysis, surface spectroscopies with infrared spectroscopy dominant, but soon to be followed by Auger and photoelectron spectroscopy, field emission, field ionisation and diffraction methods. 

The methods available for structure determination are surveyed. Those that are applicable to the gas phase, i.e. electron diffraction and rotational spectroscopy, are suitable mainly for small molecules. Data for the crystalline phase are usually relatively straightforward to obtain, but acquiring reliable structural data for silicon compounds as liquids or in solution by diffraction methods or liquid crystal NMR spectroscopy remains a challenge. 

It is finally assumed that with all force constants and potential functions correctly specified in terms of the electronic configuration of the molecule, the nuclear arrangement that minimizes the steric strain corresponds to the observed structure of the isolated (gas phase) molecule. In practice however, the adjustable parameters, in virtually all cases, are chosen to reproduce molecular structures observed by solid-state diffraction methods. The parameters are therefore conditioned by the crystal environment and the minimized structure corresponds to neither gas phase nor isolated molecule [109],... 

The synthesis of the trismethylenemethane iron tricarbonyl complex [(CH2)3C]-Fe(CO)3 was reported by Emerson et al. in 1966 (27). The geometry of this compound in the gas phase was investigated by Almenningen et al. (28) using electron diffraction methods. These authors pointed out some structural peculiarities which were not amenable to a simple explanation, in particular, why the hypothetical planar (CH2)3C radical is distorted when bound to the Fe(CO)3 conical fragment in such a way that the carbon atoms of the CH2 groups are displaced toward — the iron atom (Fig. 9). 

With respect to the naked metal atoms, this is the largest metalloid duster that has ever been structurally determined by diffraction methods. The Ga2 unit in the center of the 64 naked Ga atoms is remarkable and unique in this entire field of chemistry [Figure 2.3-28(c)]. The Ga2 unit, which contains a bond that is almost as short (2.35 A) as the above-mentioned Ga-Ga triple bond (2.32 A) and resembles the Ga2 unit of a-Ga (2.45 A), is surrounded by a Ga32 shell in the form of a football with icosahedral caps [see d-Ga (Figure 2.3-17)]. The apex and base atoms of this Ga32 unit are naked and are oriented towards each other in the crystal in an unusual fashion (see below). The Ga2Ga32 unit is surrounded by a belt of 30 Ga atoms that are also naked . Finally the entire Ga framework is protected by 20 GaR groups. 

D had a large experimental uncertainty, but is nevertheless close to the later result of 4.16 0.4 D (Kulakowska et al. 1974), obtained from capacitance measurements of a solution in dioxane. The diffraction method has the advantage that it gives not only the magnitude but also the direction of the dipole moment. Gas-phase microwave measurements are also capable of providing all three components of the dipole moment, but only the magnitude is obtained from dielectric solution measurements. 

Mpst of the structural information about complex gas molecules has been obtained by the electron-diffraction method. Values of interatomic distances and bond angles determined by this method before 1950 are summarized in a review article by P. W. Allen and L. E. Sutton, Acta Cryst. 3, 46 (1950). ValueB of interatomic distances and bond angles for organic molecules determined by both x-ray diffraction of crystals and electron diffraction of gas molecules are summarized in a 90-page table in G. W. Wheland s book Resonance in Organic Chemistry, John Wiley and Sons, New York, 1955, and by L. E. Sutton in Tables of Interatomic Distances and Configurations in Molecules and Ions, Chemical Society, London, 1958. (Later references to the latter book will be to Sutton Interatomic Distances.)... 

As a result of the development of the x-ray method of studying the structure of crystals and the band-spectroscopic method and especially the electron-diffraction method of studying gas molecules, a large amount of information about interatomic distances in molecules and crystals has been collected. It has been found that the values of interatomic distances corresponding to covalent bonds can be correlated in a simple way in terms of a set of values of covalent bond radii of atoms, as described below.1... 

The Gas Electron Diffraction Method , P. Andersen and O. Hassell, in Physical Methods in Heterocyclic Chemistry , ed. A. R. Katritzky, Academic Press, New York, 1971, vol. 3, pp. 27-51. 

It follows from what has been said above that free-volume is a value that is determined by both hole volume and empty volume, the latter being connected with the packing mode. In this case an empty volume ve = vr — vw, where vr is the real (observed) volume at temperature T and vw the volume of the substances as calculated from the Van der Waals dimensions obtained by an X-ray diffraction method or from the gas-kinetic collision cross-section3. Then the expansion volume vex = v - v0, where v0 is the volume occupied by the molecules at 0 K in a close-packed crystalline state. 

Problems of this nature, however, can be somewhat alleviated by the availability of complementary gas phase electron diffraction or micro-wave data, from which values for the crucial structural parameters uninfluenced by solid-state bonding and packing effects can be evaluated. Changes in these reference values coupled with a suggestive stereochemistry between the donor-acceptor components of the coordinate bond are often sufficient to confirm bonding. The utility of combined microwave spectroscopic and X-ray diffraction methods is amply illustrated by the cyano derivatives of di- and trimethylgermane. 

It should be mentioned that in some cases it may happen that a questioned ink can be more positively identified through presence of fluorescent or other unique components in the formulation. When sufficient questioned ink is available and the proprietory formula composition has been furnished, further analysis can lead to the identification of a component which may provide additional proof of the identity of the ink. For example, there are a variety of fatty acids, resins, and viscosity adjusters added to inks which can be readily identified by TLC or gas liquid chromatography (GLC), when sufficient ink is available. As further examples, amorphous carbon and graphite, which are common dispersion ingredients in ballpoint inks, can be distinguished using electron diffraction methods. 

Diffraction methods, which are beyond doubt the most informative approach, are at the same time the most cumbersome. It should also be stressed that, although this approach may allow the complete structure of a molecule to be given in the gas or solid phase, in practice nonvolatility, instability, and difficulties inherent in crystal growth, of the samples may interfere significantly. Where, however, these difficulties are overcome the results obtained are profitable whatever the effort invested. 

In addition to routinely used methods, such as elemental analysis, IR and UV-vis-NIR spectra, thermogravimetry-differential thermal analysis (TG-DTA), single-crystal X-ray diffraction, and gas adsorption, there are some important characterization methods for coordination polymers. 

Table 5.1. Bond lengths in monomers and hydrogen-bonded dimers of some simple carboxylic acids from gas diffraction methods [366, 367]...

Many of these conclusions from vibrational spectroscopy have been confirmed by other techniques, such as diffraction methods. This is also true for some alkaline-earth dihalides, which were investigated as matrix-isolated species by IR and Raman spectroscopy and studied in the gas phase with the help of electron diffraction e.g., MgC electron diffraction in the gas phase d Mxcn = 219 pm, a = 180° (Gershikov and Spiridonov, 1981) matrix-isolated MgCl2 V] = 327 cm", vi = 93 cm, v, = 601 cm (Lesieki and Nibler, 1976). The bonding and the structures of all alkaline-earth metal dihalides have been discussed in detail (Spoliti et al., 1980). 

Persistent Re(CO)3(PR3)2 radicals have been spectroscopically and even crystallographically characterized, for example, Re(CO)3(PCy3)2 (5), suggesting the square-pyramidal structure to be the most favored one see Diffraction Methods in Inorganic Chemistry). Subcarbonyls hke Re(CO)2 have been studied in rare gas matrices. 

The well-known NLO molecular crystal POM (3-methyl-4-nitropyridine-1-oxide) is simulated through cluster calculations by Guillaume et al.216 Semi-empirical and MP2 ab initio results are considered and comparisons of the NLO response with those obtained from the usual oriented gas model are made. POM is also selected by Hamzaoui et al 11 as an example of an NLO molecular crystal on which to test their procedure for relating the polarizabilities to the multipolar components of the ground state charge distribution determined by X-ray diffraction methods. 

The structure of this molecule has been studied in the crystalline and vapour states by spectroscopic and diffraction methods, most recently by i.r. in a solid Kr matrix. All these studies give a bond angle close to 119-5° and the bond length 1 -43 A. The dipole moment in the gas phase is noted above. 

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