The CO2 Molecule

There are 16 valence electrons—6 electrons (2s) (2p) from each oxygen atom and 4 electrons (2s) (2p) from the carbon atom. The molecule is linear and has a center of symmetry. Thus, we characterize the orbitals according to the symmetry operations which are found in We have for the carbon orbitals, 

For the oxygen orbitals it is practical to form first the possible linear combinations, and then find the transformation properties of these combinations  

The 16 valence electrons occupy the lowest orbitals in the ground state, giving the configuration (log (la f (2ag (2a f (1 ) (1 ).  

The filled shell configuration for the ground state transforms as 

Low-energy excited states occur by promoting an electron from the lir orbital to the 2 orbital. The electronic configuration 

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In Figure 6.35 the region labelled B is a deep pocket, the bottom of which represents the CO2 molecule at equilibrium with rj = r2 = r. Regions Ai and A2 are valleys which are... 

The value of n in the polymeric adsorbed species (CO) is larger on the 3c sites than on die 4c and 5c sites. The CO2 molecule is much more suongly adsorbed, indicating ion formation on the oxide surface, and the evidence suggests... 

Thermal desorption spectra of CO2 from a titania surface are shown in figure 2. It revealed two desorption peaks at temperature ca. 175 and 200 K. As reported, surface of titania have two structures which is similar to the results fomd by Tracy et al. [7]. Based on their study, it was confirmed that one peak at ca. 170 K was attributed to CO2 molecules bound to regular five-coordinate Ti site considered as the perfected titania structure. The second peak at ca. 200 K considered as the CO2 molecules bound to Ti referred to the... 

A detailed study of the C02- species on MgO has been carried out by Lunsford and Jayne 26). Electrons trapped at surface defects during UV irradiation of the sample are transferred to the CO2 molecule upon adsorption. By using 13C02 the hyperfine structure was obtained. The coupling constants are axx - 184, am = 184, and a = 230 G. An analysis of the data, similar to that carried out in Section II.B.2 for N02, indicates that the unpaired electron has 18% 2s character and 47% 2p character on the carbon atom. An OCO bond angle of 125° may be compared with an angle of 128° for CO2- in sodium formate. 

Any periodic distortion that causes polarization of a molecule can also cause interaction with the electric field component of radiation. An example is the asymmetric stretching vibration of the CO2 molecule, that creates a fluctuating dipole moment as shown below. 

Figure 6 illustrates the different forms of chemisorption for a C02 molecule. In the weak form of chemisorption the CO2 molecule is attached to the surface by two valence bonds, as depicted in Fig. 6a. This is an example of adsorption on a virtual Mott exciton, that is, not on a pre-prepared Mott exciton which, as already observed, represents a pair of free valencies of opposite sign (electron -j- hole) wandering through the crystal as an entity, but on an exciton produced in the very act of adsorption. As appears from Fig. 6a, this is the valence-saturated and electrically neutral...

Neglecting adsorption of the CO2 molecules and assuming that the different forms of chemisorption of CO2 are in equilibrium (case when electron equilibrium is established), we have... 

If the vibration does not produce a modulation of the dipole moment (e.g., as with the symmetric stretch vibration of the CO2 molecule), its infrared intensity vanishes because (3 l/3R i) = 0. One says that such transitions are infrared "inactive". 

The formation of an M—C hond is the one common feature. In these structures, /i indicates that the CO2 molecule bridges two or more metals with the subscript indicating the number of metal ions bridged. 

The result (a) is a measure of the number of CO2 molecules (CO2 is normally a gas in which the CO2 molecules are separated from each other and have individual physical identities). SiC>2 on the other hand is a complicated crystalline solid (quartz), in which each silicon is surrounded by more than two oxygens and each oxygen by more than one silicon. Because of these factors, there is no physically distinct cluster of one silicon with two oxygens. The result of (b) represents a count of the number of SiC>2 formula units. The Ca(N03)2 discussed in (c) is an ionic crystal of no specific size, the given sample containing 0.0915 moles of calcium ions and twice that number of moles of nitrate ions. 

After 100 seconds, the average migration is only 53 mm. So the beaker will remain filled with CO2 for an extended period. A common lecture demonstration involves pouring the gas from such a beaker over a candle flame, which extinguishes the candle. The carbon dioxide gas can be poured through air because it is heavier than air, yet the low rate of diffusion guarantees that the CO2 molecules will mainly remain together. 

In order to visualize normal vibrations, let us consider a mechanical model of the CO2 molecule shown in Fig. 1-10. Flere, the C and O atoms are represented by three balls, weighing in proportion to their atomic weights, that are connected by springs of a proper strength in proportion to their force constants. Suppose that the C—O bonds are stretched and released... 

The IR activity of small molecules can be determined by inspection of the mode of a normal vibration (normal mode). Obviously, the vibration of a homopolar diatomic molecule is not IR-active, whereas that of a heteropolar diatomic molecule is IR-active. As shown in Fig. 1-13, the dipole moment of the H2O molecule is changed during each normal vibration. Thus, all these vibrations are IR-active. From inspection of Fig. 1-11, one can readily see that V2 and v3 of the CO2 molecule are IR-active, whereas v is not IR-active. 

Compounds having double bonds (other than C==C double bonds) may also act as Lewis acids. In the reaction below, the OH" ion donates a pair of electrons to the CO2 molecule and becomes attached to the more electropositive atom of the double bond one of the two double bonds is converted to a single bond ... 

When there are two or more bonds—that is, more than two atoms in the molecule— polar bonds might cancel out each other s effects, resulting in a nonpolar molecule. For example, in carbon dioxide, two polar bonds connect the carbon and oxygen atoms. However, these bonds lie exactly opposite each other (along a straight line), and the effect of one polar bond is canceled by the effect of the other, so the CO2 molecule has no dipole it is a nonpolar molecule. 

Fermi resonance is observed if an overtone or a combination band has nearly the same energy as a fundamental. Coupling leads to a blue shift of the higher frequency and to a red shift of the lower one. Simultaneously, the intensity of the overtone increases while that of the fundamental decreases. In the CO2 molecule there is a resonance between the symmetrical stretching vibration (1343 cm, calculated without Fermi resonance) and the overtone of the deformation vibration (1334 cm ). Two other examples of Fermi resonance are ... 

As in the CO2 molecule, there is strong vibrational coupling between the two NO groups in the isoelectronic NO species. The observed difference between both stretching vibrations is therefore large Vas- 2360 cm 1/ 1396 cm (Hisatsune et al., 1962). 

CO2 is a poor donor but a good electron acceptor. Owing to its acidic character, it is frequently used to probe the basic properties of solid surfaces. IR evidence concerning the formation of carbonate-like species of different configurations has been reported for metal oxides [31], which accounts for the heterogeneity of the surface revealed by micro-calorimetric measurements. The possibility that CO2 could behave as a base and interact with Lewis acid sites should also be considered. However, these sites would have to be very strong Lewis acid sites and this particular adsorption mode of the CO2 molecule should be very weak and can usually be neglected [32]. 

The energetic inhomogeneity of the surface along the x and y directions is not taken into account, but this is not expected to affect the results significantly at 308 and 333 K [39]. The cross interaction potential parameters between different sites were calculated according to the Lorentz-Berthelot rules Oap = aa + and eafi= ( The potential energy t/ due to the walls inside the slit pore model for each atom of the CO2 molecule is given by the expression C/ = + Uw(H-r where H is the distance between the carbon centers across... 

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