P.A. Cox - Inorganic chemistry (793955), страница 13
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Each element gives a characteristicseries of lines, the intensities of which can be calibrated against samples of known composition to determine theamounts present in the unknown sample. A long-established technique is atomic absorption spectroscopy usingsamples sprayed into a hot flame. A more recent development which offers greater sensitivity and reliability isinductively coupled plasma atomic emission spectroscopy (ICPAES). The sample is injected into a plasma (hotionized gas) at a temperature around 10000°C which ensures more complete atomization than in the flame technique.In the technique of X-ray fluorescence (XRF) characteristic X-ray wavelengths are produced from a solid sample,and may be used to identify elements present (see Topic A4).
The method is less accurate than those based on theatomic spectra of gases, but is useful for solid samples, especially minerals that may contain many elements. X-rays maybe excited by the electron beam in an electron microscope, and the resulting energy dispersive X-ray analysis(EDAX) can be used to give approximate atomic analyses of individual grains of a powdered solid and to estimate thechemical homogeneity of a sample.Mass spectrometryFor a molecular compound, the full molecular formula can be established from the empirical formula and the molecularmass (RMM). Various physical properties, including the vapour density of a gas, and so-called colligative propertiesB7—METHODS OF CHARACTERIZATION51(such as freezing point depression) in solution, can be used to determine the RMM. However the most importanttechnique in modern research is mass spectrometry (MS) where molecular ions are accelerated in an electric field,and then pass through a magnetic field where their paths are bent to an extent that depends on the mass/charge ratio.The traditional MS method requires a volatile sample, ionized by electron bombardment, but methods are now availablethat overcome the limitations of that method.
Direct desorption from solids by a laser beam or by fast atombombardment (FAB) allow measurement of involatile compounds. Solutions may also be sprayed directly into thespectrometer inlet and the spectrum measured after the solvent has evaporated.In the case of the compound (X) above, a major peak is found at mass number 214 as expected for CrC9H6O3, butnone at multiples of this value, showing that the molecular formula in this case is the same as the empirical formuladetermined from elemental analysis. Much more information can be obtained however. Individual isotopes are seen byMS, and the pattern of isotopic distribution should confirm the identification. For example the most abundant isotopeof Cr has mass 52, but there are others at 50, 53 and 54.
Carbon has 1.1% of 13C along with the major isotope 12C. Thusthe appearance of the spectrum can be checked against the detailed isotopic distribution expected from the formula.Compounds also show fragmentation patterns resulting from decomposition of the ions in the spectrometer. Inaddition to the molecular peak at 214 mass units from X, the appearance of others at 186, 158 and 130 shows the loss ofone, two and three fragments each of mass 28, which are most likely to be CO units. Although fragmentation reactionscan sometimes be much more complicated, the spectrum of X strongly suggests the existence of three separate COgroups, easily lost from the molecule, and in this case probably bonded to the metal. The characteristic nature of thefragmentation process thus enables some structural information to be obtained, as well as making MS a powerfulfingerprinting technique for known compounds.Spectroscopic methodsTogether with MS, IR and NMR spectroscopies are the most valuable fingerprinting techniques for molecularcompounds.
Features of the spectra also enable structural information to be obtained about a new compound, especiallythe presence of known functional groups and some aspects of its symmetry.IR measures the frequencies of molecular vibrations which depend on the masses of atoms and the force constants(i.e. the ‘stiffness’) of chemical bonds (see Topic C8). Spectra can be measured for pure gaseous and liquid samples, butsolids are usually measured by grinding them to make a mull with a heavy hydrocarbon liquid (‘nujol’) which hasrelatively few, and well known, IR bands.
Many types of chemical bond, such as C-H and C=O, give bands withcharacteristic IR frequencies and can thus be identified. In the case of compound X discussed above, bands appear whichare characteristic of aromatic C-H bonds (suggesting a C6H6 benzene ring) and of C=O groups bound to a metal atom(see Topic H9).The number of bands appearing in an IR spectrum can often give information about the symmetry of a molecule (seeTopic C3). The technique is especially useful in conjunction with Raman spectroscopy, another way of measuringvibrational frequencies.
Raman spectroscopy can also be used in media such as aqueous solution, where IRmeasurements are difficult or impossible because of the strong absorption by water.NMR has very different principles, depending on the properties of nuclear spin (analogous to electron spin,Topic A3). Not all nuclei possess spin, and of those which do some are much easier to obtain spectra from than others.The most familiar NMR nucleus is 1H; 13C is also useful for organic and organometallic compounds, and 19F and 31P areparticularly easy inorganic nuclei to study, although many others can be used. Two features of NMR are important.Spectra show chemical shifts, with frequencies for a given nucleus varying with the chemical environment.
Spinspin coupling arises from the interaction of active nuclei separated by one or more chemical bonds, and givescharacteristic patterns which allow aspects of the connectivity of atoms to be determined. In compound X, the 1Hspectrum shows that all six hydrogen atoms in the molecule are in an identical chemical environment, with a chemical52SECTION B—INTRODUCTION TO INORGANIC SUBSTANCESshift consistent with a benzene ring attached symmetrically to the chromium atom as shown in 1. 13C NMR confirmstwo different carbon environments with 6 and 3 atoms respectively, with chemical shifts appropriate to the structureshown.One feature of NMR can sometimes be misleading.
Compared with most other techniques it samples molecules over arelatively long time-scale (typically 0.01 to 0.1 seconds). Some molecules are fluxional with atoms exchanging rapidlybetween different positions, and when this happens NMR may ‘see’ these positions as equivalent. For example IR anddiffraction methods show clearly that the PF5 molecule has a trigonal bipyramidal structure with two different Fpositions (equatorial and axial, see Topic C2).
However, fluorine atoms exchange so quickly between these twopositions that in 19F NMR all five atoms appear equivalent.Other spectroscopic methods can be useful in some circumstances. Visible/UV absorption spectra depend on theexcitation of electrons from filled into empty orbitals.
The technique has some limited use in fingerprinting but isespecially suited to investigations of electronic structure, in particular the energy difference between molecular orbitals(see Topics C4–C6 and D7). Topic H8 discusses applications to transition metal complexes, as well as the use ofmagnetic measurements to determine the number of unpaired electrons.Diffraction methodsDiffraction is an interference phenomenon occurring when waves are scattered by objects in different positions.Electron diffraction depends on the wave-like properties of electrons and can be used in various ways. Oneapplication in inorganic chemistry is the determination of bond lengths and angles of molecules in the gas phase.
Itsscope is limited as only volatile substances may be studied, and a full interpretation is only possible for moleculescontaining rather few atoms.Of much greater general use is X-ray diffraction, which is by far the most important structural technique inchemistry. It depends on the fact that X-ray wavelengths are comparable to the spacing between atoms in crystals.Interference thus occurs between radiation scattered by different atoms, and scattered X-rays emerge only at certainangles from a crystal, depending on the wavelength of radiation and the inter-atomic spacings. Two different techniquesmay be employed. X-ray powder diffraction (XRPD) is performed on finely divided powdered (thuspolycrystalline) samples. It enables the dimensions of the crystal unit cell to be determined (see Topic D1).
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