Orbitals, molecular unlike atoms

The NO molecule illustrates what must be done with a single unpaired electron on the central atom [see predictive rule 2(c), p 119). It must occupy a molecular orbital but, unlike the usual situation, the orbital won t be filled. In... 

Molecular orbitals have many of the same characteristics as atomic orbitals. For example, an MO can hold a maximum of two electrons (with opposite spins), it has a definite energy, and we can visualize its electron-density distribution by using a contour representation, as we did with atomic orbitals. Unlike atomic orbitals, however, MOs are associated with an entire molecule, not with a single atom. 

Specifically, floating spherical Gaussian orbitals (FSGO), developed from an extension of Frost s simple electron pair model of molecular electronic structure, are used as basis functions in SCF-MO and Cl calculations. As will be described in the following section, these functions, unlike atomic basis orbitals, generally are not confined to atoms, but are allowed to occupy electron-rich regions corresponding... 

The excited electron in the a antibonding orbital is in a molecular orbital that has 2u symmetry, and the electron still in the tt orbital has Ilg symmetry. The direct product of these two symmetries is simply Hg. (Verify this.) The a and tt electrons can have either the same direction spin or different direction spin (rather, the z components of their spin), so multiplicities of 3 or 1 are possible. Therefore, the term symbols for this excited-state electron configuration are and II states are doubly degenerate. In this case, there are two possible, degenerate tt molecular orbitals for the unexcited electron. Complete term symbols would have values for ft included. For the triplet state, H can be 2,1, or 0. For the singlet state, Q, can only be 1. Unlike atomic term symbols, it is relatively uncommon to see the fi values listed explicitly in the term symbols of diatomic molecules. One would see Ilu to represent all three individual states, rather than Ilu 2> Ilu i, and Ilu o-... 

As discussed in Chapter 8 (see page 350), we imagine that an atom has available to it a set of orbitals (Is, 2s, 2p, etc.) and can be built up electron by electron by placing electrons into these orbitals in a specified order. In a similar manner, with molecular orbital theory, we imagine that each molecule has available to it a set of orbitals, called molecular orbitals (MOs) and can be built up electron by electron by placing electrons into these orbitals in a specified order. Unlike atomic orbitals, which are centered on a single nucleus, molecular orbitals are defined with respect to all the nuclei. 

Although P3 procedures perform well for a variety of atomic and molecular species, caution is necessary when applying this method to open-shell reference states. Systems with broken symmetry in unrestricted Hartree-Fock orbitals should be avoided. Systems with high multireference character are unlikely to be described well by the P3 or any other diagonal approximation. In such cases, a renormalized elec-... 

The bond strengths of MO + and resistances to reduction decrease in the order U > Np > Pu > Am. In the case of U02+ the force constants for U — 0 bonds are high and indicate a multiplicity in excess of two. In M02+ there is a combination of the appropriate d and/ orbitals which produce one a-bond plus one 7r bond in U02+ the molecular orbitals are filled while succeeding electrons pass into the non-bonding orbitals which results in an instability of MO + in the sequence U-Am. Unlike UO +, U02 is unstable and it is possible that this instability is due to the sensitivity of the energy of the 5/electrons to toal charge which will result in an overlap of electrons between the U and 0 atoms. 

The MO theory treats molecular bonds as a sharing of electrons between nuclei. Unlike the valence bond theory, which treats the electrons as locahzed balloons of electron density, the MO theory says that the electrons are delocalized. That means that they are spread out over the entire molecule. Now, when two atoms come together, their two atomic orbitals react to form two possible molecular orbitals. 

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