Diatomic molecules dihydrogen

This chapter consists of the application of the symmetry concepts of Chapter 2 to the construction of molecular orbitals for a range of diatomic molecules. The principles of molecular orbital theory are developed in the discussion of the bonding of the simplest molecular species, the one-electron dihydrogen molecule-ion, H2+, and the simplest molecule, the two-electron dihydrogen molecule. Valence bond theory is introduced and compared with molecular orbital theory. The photo-electron spectrum of the dihydrogen molecule is described and interpreted. 

The dihydrogen molecule (s1 + s1) is formed through the interaction of s-electrons only. There is no angular momentum to impart structure to the diatomic molecule, which hence cannot be described in classical geometrical terms other than a spherically symmetrical distribution of electron and proton densities. It has no shape, no bond and, unless it interacts with external fields, no geometrical features. Compounds such as LiH, Li2, etc. belong to the same class of amorphous molecules. 

Pure rotational transitions of symmetrical diatomic molecules like dihydrogen are forbidden in infrared spectroscopy by the dipole selection rule but are active in Raman spectroscopy because they are anisotropically polarisable. They are in principle observable in INS although the scattering is weak except for dihydrogen. These rotational transitions offer the prospect of probing the local environment of the dihydrogen molecule, as we shall see in this chapter. 

All homonuclear diatomic molecules having nuclides with non-zero spin are expected to show nuclear spin isomers. The effect was first detected in dihydrogen where it is particularly noticeable, and it has also been established for D2, T2, N2, N2, Oj, etc. When the two nuclear spins are parallel (ort/jo-hydrogen) the resultant nuclear spin quantum number is 1 (i.e. 5 -b j) and the state is threefold degenerate (2S -(-... 

We will first present the EDA results for the dihydrogen molecule which is the standard molecule in curricula and textbooks for discussing the nature of covalent bonding. Then we compare the results for H2 with heavier diatomic molecules N2 and isoelectronic CO... 

Dihydrogen is described as a resonance hybrid of these contributing resonance or canonical structures. In the case of H2, an example of a homonuclear diatomic molecule which is necessarily symmetrical, we simplify the picture to 1.11. Each of structures 1.11a, 1.11b and 1.11c is a resonance structure and the double-headed arrows indicate the resonance between them. The contributions made by 1.11b and 1.11c are equal. The term resonance hybrid is somewhat unfortunate but is too firmly estabhshed to be eradicated. 

Dihydrogen is a colourless, odourless gas, sparingly soluble in all solvents, and at 298 K and 1 bar pressure, it conforms closely to the ideal gas laws. The solid state structure of H2 can be described in terms of an hep lattice (see Section 6.3), but values of the melting point, enthalpy of fusion, boiling point and enthalpy of vaporization are all very low (Table 10.3), consistent with there being only weak van der Waals forces between the H2 molecules. The covalent bond in H2 is unusually strong for a single bond in a diatomic molecule. 

Elemental hydrogen exists at room temperature as a colorless, odorless, tasteless gas composed of diatomic molecules. We can call H2 dihydrogen, but it is more commonly referred to as either molecular hydrogen or simply hydrogen. Because H2 is nonpolar and has only two electrons, attractive forces between molecules are extremely weak. As a result, its melting point (—259 °C) and boihng point (—253 °C) are very low. 

Figure 5.34 The proposed template patterning mechanism in the formation of biomimetic microskeletal structures, where the circles with tails represent the cationic surfactant, the crosses represent the anionic dihydrogen phosphate counter-ions and the connected circles represent the tetraethylene glycol (TEG) molecules. Reprinted with permission from Nature Publishing Group, S. Oliver, A. Kuperman, N. Coombs, A. Lough and G. A. Ozin, Lamellar aluminophosphates with surface patterns that mimic diatom and radiolarian microskeletons. Nature, 378, 47-50 1 995.

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