Bent triatomic species

The best conditions for observing S3 are 440°C and lOmmHg when 10-20% of vapour species comprise this deep cherry-red bent triatomic species like ozone, p. 607, it has a singlet ground state. The best conditions for S4 are 450°C and 20 ramHg (concentration 20%) but the structure is still not definitely established and may, in fact be a strained ring, an unbranched diradical chain, or a branched-chain isostructural with S03(g) (p. 703). 

Ab initio study of the reorganization barrier for bent triatomic species "... 

Triatomic species can be linear, like CO2, or bent, like O3. The principles of orbital overlap do not depend on the identity of the atoms involved, so all second-row triatomic species with 16 valence electrons have the same bonding scheme as CO2 and are linear. For example, dinitrogen oxide (N2 O) has 16 valence electrons, so it has an orbital configuration identical to that of CO2. Each molecule is linear with an inner atom whose steric number is 2. As in CO2, the bonding framework of N2 O can be represented with sp hybrid orbitals. Both molecules have two perpendicular sets of three tt molecular orbitals. The resonance structures of N2 O, described... 

Bent Triatomic Molecules.—Calculations have been reported for many bent triatomic molecules (see Table 4). The general force field contains 2re + 4/ + 6/3 + 9A parameters, the relation to the primary spectroscopic constants being shown in Table 9. The fact that these are asymmetric top molecules, for which otj, a , and a can all be determined (generally from the microwave spectrum for the heavier molecules), means that 9 a values are available from each isotopic species to determine the 6 cubic force constants, so that the cubic force field is generally well determined. For the quartic force field the situation is much less satisfactory the experimental data on the anharmonic constants xrs are generally incomplete, and are in any case insufficient to fix all the quartic constants without good isotopic data. 

Stereochemical arrangement becomes an issue whenever an atom is bonded to two or more others. Thus triatomic species AB2 can be linear (e.g. C02, ) or bent (e.g. H20 1, ) it is observed... 

Linear triatomic anions The pioneering crystallographic studies of Odd Hassel140 on trihalides and related donor-acceptor species led to a far-reaching analysis of such Hassel compounds by Henry Bent.141 The triiodide ion (I3-, stable in aqueous solution) and other known linear trihalide XYZ- species also served as the prototype for Pimentel s incisive three-center MO analysis of hypervalency. We shall therefore begin with NBO/NRT investigation of a series of hypervalent and non-hypervalent triatomic anions in order to make contact with these classic studies. While ab initio studies add many quantitative details to the understanding of these species, the basic picture sketched by Bent and Pimentel is found to be essentially preserved. 

Table 2.2c lists the vibrational frequencies of triatomic halogeno compounds. The resonance Raman spectrum of the I3 ion gives a series of overtones of the Vi vibration [408,409]. The resonance Raman spectra of the l2Br and IBr2 ions and their complexes with amylose have been studied [410]. The same table also lists the vibrational frequencies of XHY-type (X,Y halogens) compounds. All these species are linear except the ClHCl ion, which was found to be bent by an inelastic neutron scattering (INS) and Raman spectral study [402]. 

The three fundamental modes of vibration are shown in Figure 4.12. In the case of a triatomic molecule, it is simple to deduce that the three modes of vibration are composed of two stretching modes (symmetric and asymmetric) and a bending mode. However, for larger molecules it is not so easy to visualize the modes of vibration. We return to this problem in the next section. The three normal modes of vibration of SO2 all give rise to a change in molecular dipole moment and are therefore IR active. A comparison of these results for CO2 and SO2 illustrates that vibrational spectroscopy can be used to determine whether an X3 or XY2 species is linear or bent. 

Diatomic Molecules. For triatomic molecules there are terms involving Coriolis interactions between vibrational modes in the correction from Bg to Bz, and the theory of the preceding sections is required. For linear or for bent symmetrical XY, molecules the rotational constants of a single isotopic species are sufficient to define the molecular structure completely, and thus for such molecules spectroscopic methods are superior in precision to the electron diffraction method. Electron diffraction studies serve as a test not only of the electron diffraction technique but also of the comparability between and r%. Some examples are given in Table 4. Besides the molecules CS., CIO, and CO, for which details are given, a... 

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