Hydrogen Triatomic

This section briefly considers the proton H+, the hydride ion H, the hydrogen molecule ion H2, the triatomic 2-electron species H3+ and the recently established cluster species +... 

The triatomic hydrogen molecule ion H3+ was first detected by J. J. Thomson in gas discharges and later fully characterized by mass spectrometry its relative atomic mass, 3.0235, clearly distinguishes it from HD (3.0219) and from tritium... 

Because FHF- epitomizes the limit of strong hydrogen bonding in a particularly simple geometrical form, let us examine some further aspects of its potential-energy surface. The triatomic species can generally be described in terms of three variables,... 

Carbon dioxide is a symmetrical, linear triatomic molecule (0 = C=0) with a zero dipole moment. The carbon-to-hydrogen bond distances are about 1.16A, which is about 0.06A shorter than typical carbonyl double bonds. This shorter bond length was interpreted by Pauling to indicate that greater resonance stabilization occurs with CO2 than with aldehydes, ketones, or amides. When combined with water, carbonic acid (H2CO3) forms, and depending on the pH of the solution, carbonic acid loses one or two protons to form bicarbonate and carbonate, respectively. The various thermodynamic parameters of these reactions are shown in Table I. 

The triatomic molecule allotrope of oxygen was discovered in 1839 by Schonbein who detected it from its smell and thus named it ozone (o eiv ozein emitting a smell). The nature of the molecule was established in 1865 by Soret. Thenard discovered hydrogen peroxide in 1818. Water was known as an Element, from Thales of Milet ( 600 B. C.) to the time of Cavendish... 

Water is an interesting and important liquid. As the combination of multidimensional vibrational spectroscopy and molecular dynamics helps us understand water better, more and more complex dynamics have been revealed [8,9]. We can briefly explain why a liquid of triatomic molecules turns out to be so immensely complicated Water s three atoms bestow all the complexity of multiple intramolecular vibrations, and in addition in water there are more hydrogen bonds (—3.57) than atoms ... 

The linear molecule BeH, will serve as our first example of a triatomic species. The molecular orbitals for this molecule are constructed from the Is orbitals on the hydrogen atoms (labeled H and H ) and the 2j and one of the 2p orbitals of beryllium (the one directed along the H—Be—H bond axis). The remaining two Ip orbitals of beryllium cannot enter into the bonding because they are perpendicular to the molecular axis and thus have zero net overlap with the hydrogen orbitals. 

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

The non-complementarity between the ellipsoidal 33-6H+ and the spherical halides results in much weaker binding and appreciable distortions of the ligand, as seen in the crystal structures of the cryptates 35 where the bound ion is F , Cl-, or Br-. In these complexes, F- is bound by a tetrahedral array of hydrogen bonds whereas Cl- and Br- display octahedral coordination (Fig. 4). Thus, 33-6H+ is a molecular receptor for the recognition of linear triatomic species of a size compatible with the size of the molecular cavity [3.11]. 

PROBLEM 19.5 Hydrogen cyanide, HCN, is a linear triatomic molecule. Draw its electron-dot structure, and indicate which hybrid orbitals are used by the carbon atom. 

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