Polyatomic species

The bonding in molecules containing more than two atoms can also be described in terms of molecular orbitals. We will not attempt to do this the energy level structure is considerably more complex than the one we considered. However, one point is worth mentioning. In polyatomic species, a pi molecular orbital can be spread over die entire molecule rather than being concentrated between two atoms. 

HOWTO WRITE THE LEWIS STRUCTURE OF A POLYATOMIC SPECIES... 

The Lewis structure of a polyatomic species is obtained by using all the valence electrons to complete the octets (or duplets) of the atoms present by forming single or multiple bonds and leaving some electrons as lone pairs. 

A fourth success concerns the high degree of unsaturation found in the observed list of molecules. Very few highly saturated molecules are detected, and those that are saturated tend to be found in highly localized sources known as hot cores, where they are probably formed via H-atom hydrogenation on grain surfaces.54 The reason that ion-molecule reactions do not produce more saturated polyatomic species is, as discussed above, the small number of reactions between hydrocarbon ions and H2 that can occur rapidly. 

Bjorn E, Freeh W, Hoffmann E, Liidke C (1998) Investigation and quantification of spectroscopic interferences from polyatomic species in inductively coupled plasma mass spectrometry using electrothermal vaporization or pneumatic nebulization for sample introduction. Spectrochim Acta 53B 1766... 

In this chapter, the descriptions of molecular structure will be primarily in terms of a valence bond approach, but the molecular orbital method will be discussed in Chapter 5. As we shall see, construction of molecular orbital diagrams for polyatomic species is simplified by making use of symmetry, which will also be discussed in Chapter 5. 

Elemental phosphorus is only one of several elements whose structures are polyatomic species. Another is the structure of elemental sulfur, which consists of puckered S8 rings ... 

It should not be inferred that the crystal structures described so far apply to only binary compounds. Either the cation or anion may be a polyatomic species. For example, many ammonium compounds have crystal structures that are identical to those of the corresponding rubidium or potassium compounds because the radius NH4+ ion (148 pm) is similar to that of K+ (133 pm) or Rb+ (148 pm). Both NO j and CO, have ionic radii (189 and 185 pm, respectively) that are very close to that of Cl- (181 pm), so many nitrates and carbonates have structures identical to the corresponding chloride compounds. Keep in mind that the structures shown so far are general types that are not necessarily restricted to binary compounds or the compounds from which they are named. 

Polyatomic species containing atoms from group IVA are produced by reducing the elements in liquid ammonia that contains some dissolved sodium. In accord with the hard-soft interaction principle (see Chapter 9), isolation of species containing large anions is best accomplished when a large cation of... 

Identification of molecules in space, even small molecules, by IR astronomy requires a rotational progression in the spectrum to be measured and resolved by the telescopes. For the transitions in the simpler molecules such as CO the telescope must be capable of aresolution of 2150/1.93 1114, which is within the resolution limit of the UK Infrared Telescope (3000-5000). However, the rotational constant for CO is rather large and many molecules, especially polyatomic species, will have a rotation constant ten times smaller than this, placing the observation of a resolved rotational progression beyond the resolution of the telescopes. Confidence in the identification of the molecule is then severely dented. The problem is worse for visible astronomy. 

This fusion method produces an acid-soluble glass bead which is dissolved in nitric acid. It avoids the use of hydrofluoric and perchloric acids. The disadvantages are the cost of platinum labware, and the large quantities of LiB02 used to produce the glass bead increase the total dissolved solids and may contribute to polyatomic species. Dilution to counter these effects may reduce the elements of interest below the instrumental detection limits. 

For monoatomic ions, the RE has the same numerical value as the IE of the neutral for diatomic or polyatomic species differences due to storage of energy in internal modes or electronic excitation may occur. Ionization of the analyte via CE is effected if [49]... 

More recently, the advent of the collision/reaction cell technology has revolutionised commercial quadrupole ICP-MS systems. A gas, such as hydrogen, helium or ammonia, is introduced into the reaction cell (placed inside the mass spectrometer and preceding the analyser quadrupole), where it reacts and dissociates or neutralises the polyatomic species or precursors. Through collision and reaction with appropriate gases in a cell, interferences... 

The expressions in (3.72) and (3.73) are valid only for monatomic ideal gases such as He or Ar, and must be replaced by somewhat different expressions for diatomic or polyatomic molecules (Sidebar 3.8). However, the classical expressions for polyatomic heat capacity exhibit serious errors (except at high temperatures) due to the important effects of quantum mechanics. (The failure of classical mechanics to describe the heat capacities of polyatomic species motivated Einstein s pioneering application of Planck s quantum theory to molecular vibrational phenomena.) For present purposes, we may envision taking more accurate heat capacity data from experiment [e.g., in equations such as (3.84a)] if polyatomic species are to be considered. The term perfect gas is sometimes employed to distinguish the monatomic case [for which (3.72), (3.73) are satisfactory] from more general polyatomic ideal gases with Cv> nR. 

In inductively coupled plasma-mass spectrometry, isobaric interference occurs between species with the same mass and charge. Interference can be eliminated if the mass spectral resolution is sufficiently great or by dissociating an interfering polyatomic species with a collision cell. When laser ablation is used to sample a solid, matrix-matched standards are often necessary for quantitative analysis. 

Adsorption and Transport of Polyatomic Species in One-dimensional Systems Exact Forms of the Thermodynamic Functions and Chemical Diffusion Coefficient... 

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