Polyatomic ions, orientation

Polar molecule A molecule in which there is a separation of charge and hence positive and negative poles, 183-185 dipole force, 237 orientation, 183 Polarimeter, 600 Polarity, 184-185 Pollutants, 6 Polyamide, 615-616 Polyatomic ion, 36,39 Polyatomic molecules, 654 Polyester A large molecule made up of ester units, 614-615 Polyethylene, 611-612... 

Molecular rotation In a normal crystal every atom occupies a precise mean position, about which it vibrates to a degree depending on the temperature molecules or polyatomic ions have precisely defined orientations as well as precise mean positions. When such a crystal is heated, the amplitude of the thermal vibrations of the atoms increases with the temperature until a point is reached at which the regular structure breaks down, that is, the crystal melts. But in a few types of crystal it appears that notation of molecules or polyatomic... 

The mass determination of ionic species (atomic or polyatomic ions) in mass spectrometry is always a comparative measurement, which means the mass of an ionic species is determined with respect to reference masses of elements (or substances) used for mass calibration. The reference mass is thus acquired from the mass unit (m = In = 1/12) of the mass of the neutral carbon isotope (m = 1.66 X 10 kg). A mass calibration is easy to perform in solid-state mass spectrometry if the sample contains carbon (using carbon cluster ions with whole masses, as discussed above). The so-called doublet method was apphed formerly, e.g., ions and doubly charged Mg + forming a doublet at the same nominal mass number 12 were considered, where they are slightly displaced with respect to one another. The doublet method is no longer of relevance in modern inorganic mass spectrometry. Orientation in the mass spectra can be carried out via the matrix, minor and trace elements after mass calibration and by comparing the measured isotopic pattern of elements with theoretical values. 

FIGURE 1.24 The device for evaluation of orientationally averaged projections of polyatomic ions through exposure of wax models rotating around three perpendicular axes. (From Mack, Jr., E., J. Am. Chem. Soc., 47, 2468, 1925.)... 

Though at high E/N the ion transport depends in a complex way on collision integrals of various orders (2.2.1), all those quantities in classic transport theory are orientationally averaged. This implies free rotation of ions regardless of E, which is true only for atomic ions or at sufficiently low E. Polyatomic ions in gas can be oriented with respect to E by two unrelated mechanisms—collisional alignment considered here and dipole alignment (2.7). 

Stereoisomers Molecules or polyatomic ions with the same atoms and the same bonds but differing in the geometrical orientations of the atoms and bonds. 

We learned in Chapter 4 that an isolated atom has its electrons arranged in orbitals in the way that leads to the lowest total energy for the atom. Usually, however, these pure atomic orbitals do not have the correct energies or orientations to describe where the electrons are when an atom is bonded to other atoms. When other atoms are nearby as in a molecule or ion, an atom can combine its valence shell orbitals to form a new set of orbitals that is at a lower total energy in the presence of the other atoms than the pure atomic orbitals would be. This process is called hybridization, and the new orbitals that are formed are called hybrid orbitals. These hybrid orbitals can overlap with orbitals on other atoms to share electrons and form bonds. Such hybrid orbitals usually give an improved description of the experimentally observed geometry of the molecule or ion. In fact, the concept of hybrid orbitals was developed specifically to explain the geometries of polyatomic ions and molecules. 

TABLE 5.10 Rotational orientation Correlation Times x/ps of Several Polyatomic Ions in Various Solvents... 

Orientational disorder in ionic solids arises because a diatomic or polyatomic ion has available to it two or more distingirishable orientations in the crystal lattice. This kind of disorder is a fairly common occurrence, especially when the polyatomic ions are sufficiently symmetrical. If these ions are associated with monoatomic ions of opposite charge, the... 

Two theories go hand in hand in a discussion of covalent bonding. The valence shell electron pair repulsion (VSEPR) theory helps us to understand and predict the spatial arrangement of atoms in a polyatomic molecule or ion. It does not, however, explain hoav bonding occurs, ] ist where it occurs and where unshared pairs of valence shell electrons are directed. The valence bond (VB) theory describes how the bonding takes place, in terms of overlapping atomic orbitals. In this theory, the atomic orbitals discussed in Chapter 5 are often mixed, or hybridized, to form new orbitals with different spatial orientations. Used together, these two simple ideas enable us to understand the bonding, molecular shapes, and properties of a wide variety of polyatomic molecules and ions. 

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