Molecular solids tetrahedral

Pure NI3 has not been isolated, but the structure of its well-known extremely shock-sensitive adduct with NH3 has been elucidated — a feat of considerable technical virtuosity.Unlike the volatile, soluble, molecular solid NCI3, the involatile, insoluble compound [Nl3.NH3] has a polymeric structure in which tetrahedral NI4 units are comer-linked into infinite chains of -N-I-N-I- (215 and 230 pm) which in turn are linked into sheets by I-I interactions (336 pm) in the c-direction in addition, one I of each NI4 unit is also loosely attached to an NH3 (253 pm) that projects into the space between the sheets of tetra-hedra. The stmcture resembles that of the linked Si04 units in chain metasilicates (p. 349). A further interesting feature is the presence of linear or almost linear N-I-N groupings which suggest the presence of 3-centre, 4-electron bonds (pp. 63, 64) characteristic of polyhalides and xenon halides (pp. 835-8, 897). 

An instructive illustration of the effect of molecular motion in solids is the proton resonance from solid cyclohexane, studied by Andrew and Eades 101). Figure 10 illustrates their results on the variation of the second moment of the resonance with temperature. The second moment below 150°K is consistent with a Dsi molecular symmetry, tetrahedral bond angles, a C—C bond distance of 1.54 A and C—H bond distance of 1.10 A. This is ascertained by application of Van Vleck s formula, Equation (17), to calculate the inter- and intramolecular contribution to the second moment. Calculation of the intermolecular contribution was made on the basis of the x-ray determined structure of the solid. 

The phosphorus vapor condenses as white phosphorus, a soft, white, poisonous molecular solid consisting of tetrahedral P4 molecules (3). This allotrope is highly reactive, in part because of the strain associated with... 

A novel topological strategy has been examined for designing amorphous molecular solids suitable for optoelectronic applications. In this approach, chromophores were attached to a tetrahedral point of convergence. For instance, stilbenoid units were covalently linked to a tetraphenylmethane core by means of a palladium-catalyzed Suzuki coupling reaction [143]. The optical properties of these compounds were examined. 

The free spaces where Ps can form and o-Ps can have a reasonably long lifetime may be extrinsic defects, as just illustrated, or intrinsic defects, such as created when heating a pure solid compound. More generally, they may correspond to the natural voids present in any solid matrix (e.g., "free volume" in polymers, treated elsewhere in this book). Ps can be formed not only in molecular solids, including frozen liquids, but also in a number of ionic solids, even when the open spaces are rather small. For example, Ps is formed in such a highly packed lattice as KC1 [44, 45] where the largest space available corresponds to the tetrahedral sites circumscribed by 4 Cf anions, with a radius of only 0.0845 nm, resulting in an o-Ps lifetime of about 0.65 ns. 

Carbon occurs in the allotropes (different forms) diamond, graphite, and the fullerenes. The fullerenes are molecular solids (see Section 16.6), but diamond and graphite are typically network solids. In diamond, the hardest naturally occurring substance, each carbon atom is surrounded by a tetrahedral arrangement of other carbon atoms, as shown in Fig. 16.26(a). This structure is stabilized by covalent bonds, which, in terms of the localized electron model, are formed by the overlap of sp3 hybridized atomic orbitals on each carbon atom. 

A number of solids are composed only of atoms interconnected by a network of covalent bonds. These solids are often called covalent network solids. Quartz is a network solid, as is diamond. See Figure 9-20. In contrast to molecular solids, network solids are typically brittle, nonconductors of heat or electricity, and extremely hard. In a diamond, four other carbon atoms surround each carbon atom. This tetrahedral arrangement forms a strongly bonded crystal system that is extremely hard and has a very high melting point. 

In a molecular solid the fundamental particle is a molecule. Examples of molecular solids include ice (contains H2O molecules), dry ice (contains CO2 molecules), sulfur (contains Sg molecules), and white phosphorus (contains tetrahedral molecules). 

SECTION 12.7 Covalent-network solids consist of atoms held together in large networks by cot ent bonds. These solids are much harder and have higher melting points than molecular solids. Important examples include diamond, where the carbons are tetrahedrally coordinated to each other, and graphite where the carbon atoms form hexagonal layers through sp bonds. 

We note here that the ordinary condensed solid phase of CO2 is a molecular solid. At variance with silica and germania, a nonmolecular CO2 crystalline form of carbon dioxide in which carbon atoms are tetrahedrally coordinated to oxygen atoms only exists at high pressure [16,17], In addition, an amorphous phase of CO2, formed by a disordered arrangement of CO4 tetrahedra, has recently been obtained at very high pressure [18]. 

From the resnlts presented above, it is possible to establish a connection between free clusters and crystalline solids. The largest promotion energy from to the deformed tetrahedral and near-tetrahedral structures occurs for CO4 rather than for Si04 or Ge04 [19], a feature that correlates with the observations that CO2 is a molecular solid at ordinary pressure and that crystalline and amorphous forms of CO2 in which the C atom is tetrahedrally coordinated to four atoms are only produced at very high pressure. 

Tetraphosphorus Clusters. As mentioned at the beginning of this Chapter, phosphorus is a unique element displaying a modification formed by discrete molecular units. White phosphorus is a molecular solid made up of P4 tetrahedral clusters which are stable in solution as well as in the gas phase. 

An interesting feature of phosphorus pentachloride is that it is an ionic solid of tetrahedral PC14+ cations and octahedral PC16 anions, but it vaporizes to a gas of trigonal bipyramidal PC15 molecules (see Section 2.10). Phosphorus pentabromide is also molecular in the vapor and ionic as the solid but, in the solid, the anions are simply Br ions, presumably because six bulky Br atoms simply do not fit around a central P atom. 

The bands due to Fe(CO)4 are shown in Fig. 8. This spectrum (68) was particularly important because it showed that in the gas phase Fe(CO)4 had at least two vq—o vibrations. Although metal carbonyls have broad vC—o absorptions in the gas phase, much more overlapped than in solution or in a matrix, the presence of the two Vc—o bands of Fe(CO)4 was clear. These two bands show that in the gas phase Fe(CO)4 has a distorted non-tetrahedral structure. The frequencies of these bands were close to those of Fe(CO)4 isolated in a Ne matrix at 4 K (86). Previous matrix, isolation experiments (15) (see Section I,A) has shown that Fe(CO)4 in the matrix had a distorted C2v structure (Scheme 1) and a paramagnetic ground state. This conclusion has since been supported by both approximate (17,18) and ab initio (19) molecular orbital calculations for Fe(CO)4 with a 3B2 ground state. The observation of a distorted structure for Fe(CO)4 in the gas phase proved that the distortion of matrix-isolated Fe(CO)4 was not an artifact introduced by the solid state. 

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