Ground state diatomic molecules

Atomic charges, moments, and volumes of ground-state diatomic molecules ... 

Alkali dimers are attractive reactants for molecular beam studies since they provide an unusual example of highly labile ground state diatomic molecules. The dimer bond is very weak ( >0(Li2) = 26kcalmol 1,D0(Cs2) =... 

TABLE 3. Average Electric Dipole Polarizabilities for Ground State Diatomic Molecules (in Units of 10 cm )... 

This presents a problem when we discuss the dipole moment of a polar heteronuclear diatomic molecule, AX, where X will be the more electronegative. In the chemical picmre, it is quite common to say that in the ground state the molecule lies along some axis and that it has a definite dipole moment. In the physical picmre, we say that the molecule has no measurable dipole moment in the ground state. 

As noted in the text, the average bond length in a diatomic molecule can be determined to high precision by microwave measurements. However, even in the ground state the molecule still has total energy E = Tico/2, so it is still vibrating. Use the force constant for CO from Table 3.2 to predict the spread Ax in the probability distribution. 

We can combine the LCAO method for with an aufbau principle, analogous to that developed for atoms, to describe the electron configuration of more complex molecules. Electrons available from the two atoms are fed into the MOs, starting with the MO of lowest energy. At most, two electrons can occupy each MO. The ground-state H2 molecule, therefore, accommodates two electrons with opposite spins in a a-gu bonding MO (Fig. 6.10). The diatomic molecule is more stable than the isolated atoms by the energy difference — 2AE. 

For the case of an atom-diatom Van der Waals molecule, the two possible types of resonance phenomena are schematically illustrated in Figure 1. There, Vj(R) represents the potential energy between the atom and the ground state diatom, while V (R) is that between an atom and a diatom in excited vibrational-rotational state (v, j ). 

These results do not agree with experimental results. At room temperature, while the translational motion of diatomic molecules may be treated classically, the rotation and vibration have quantum attributes. In addition, quantum mechanically one should also consider the electronic degrees of freedom. However, typical electronic excitation energies are very large compared to k T (they are of the order of a few electronvolts, and 1 eV corresponds to 10 000 K). Such internal degrees of freedom are considered frozen, and an electronic cloud in a diatomic molecule is assumed to be in its ground state f with degeneracy g. The two nuclei A and... 

To compare the relative populations of vibrational levels, the intensities of vibrational transitions out of these levels are compared. Figure B2.3.10 displays typical potential energy curves of the ground and an excited electronic state of a diatomic molecule. The intensity of a (v, v ) vibrational transition can be written as... 

As was shown in the preceding discussion (see also Sections Vin and IX), the rovibronic wave functions for a homonuclear diatomic molecule under the permutation of identical nuclei are symmetric for even J rotational quantum numbers in and E electronic states antisymmeUic for odd J values in and E elecbonic states symmetric for odd J values in E and E electronic states and antisymmeteic for even J values in Ej and E+ electeonic states. Note that the vibrational ground state is symmetric under pemrutation of the two nuclei. The most restrictive result arises therefore when the nuclear spin quantum number of the individual nuclei is 0. In this case, the nuclear spin function is always symmetric with respect to interchange of the identical nuclei, and hence only totally symmeUic rovibronic states are allowed since the total wave function must be symmetric for bosonic systems. For example, the nucleus has zero nuclear spin, and hence the rotational levels with odd values of J do not exist for the ground electronic state f EJ") of Cr. 

Table 7.5 States from ground configurations in diatomic molecules...

It is important to realize that electronic spectroscopy provides the fifth method, for heteronuclear diatomic molecules, of obtaining the intemuclear distance in the ground electronic state. The other four arise through the techniques of rotational spectroscopy (microwave, millimetre wave or far-infrared, and Raman) and vibration-rotation spectroscopy (infrared and Raman). In homonuclear diatomics, only the Raman techniques may be used. However, if the molecule is short-lived, as is the case, for example, with CuH and C2, electronic spectroscopy, because of its high sensitivity, is often the only means of determining the ground state intemuclear distance. 

To derive the states arising from a molecular orbital configuration in a diatomic molecule see Sections 7.2.2 and 7.2.4. There are two states, and 11 /2, arising from this configuration and is the lower in energy and, therefore, the ground state. 

An Xc2 excimer laser has been made to operate in this way, but of much greater importance are the noble gas halide lasers. These halides also have repulsive ground states and bound excited states they are examples of exciplexes. An exciplex is a complex consisting, in a diatomic molecule, of two different atoms, which is stable in an excited electronic state but dissociates readily in the ground state. In spite of this clear distinction between an excimer and an exciplex it is now common for all such lasers to be called excimer lasers. 

We have seen in Section 6.1.3.2 that, for diatomic molecules, vibrational energy levels, other than those with v = 1, in the ground electronic state are very often obtained not from... 

All heteronuclear diatomic molecules, in their ground electronic state, dissociate into neutral atoms, however strongly polar they may be. The simple explanation for this is that dissociation into a positive and a negative ion is much less likely because of the attractive force between the ions even at a relatively large separation. The highly polar Nal molecule is no exception. The lowest energy dissociation process is... 

To take a typical diatomic molecule such in its electronic ground state,... 

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