The Importance of Symmetry

This chapter discusses the application of symmetry to orbital-based computational chemistry problems. A number of textbooks on symmetry are listed in the bibliography at the end of this chapter. 

In order to obtain this savings in the computational cost, orbitals are symmetry-adapted. As various positive and negative combinations of orbitals are used, there are a number of ways to break down the total wave function. These various orbital functions will obey different sets of symmetry constraints, such as having positive or negative values across a mirror plane of the molecule. These various symmetry sets are called irreducible representations. 

Molecular orbitals are not unique. The same exact wave function could be expressed an infinite number of ways with different, but equivalent orbitals. Two commonly used sets of orbitals are localized orbitals and symmetry-adapted orbitals (also called canonical orbitals). Localized orbitals are sometimes used because they look very much like a chemist s qualitative models of molecular bonds, lone-pair electrons, core electrons, and the like. Symmetry-adapted orbitals are more commonly used because they allow the calculation to be executed much more quickly for high-symmetry molecules. Localized orbitals can give the fastest calculations for very large molecules without symmetry due to many long-distance interactions becoming negligible. 

Our discussions of the particle in a box, the harmonic oscillator, the hydrogen atom, and homonuclear diatomic molecules have all included emphasis on the role that symmetry plays in determining the qualitative nature of the eigenfunctions. When we encounter larger systems, detailed and accurate solutions become much more difficult to perform and interpret, but symmetry continues to exert strong control over the solutions. 

The simple Hiickel method was devised to treat electrons in unsaturated molecules like ethylene and benzene. By 1930 it was recognized that unsaturated hydrocarbons are chemically more reactive than are alkanes, and that their spectroscopic and thermodynamic properties are different too. The available evidence suggested the existence of loosely held electrons in unsaturated molecules. 

Hiickel found that, by treating only the n electrons explicitly, it is possible to reproduce theoretically many of the observed properties of unsaturated molecules such as the uniform C-C bond lengths of benzene, the high-energy barrier to internal rotation about double bonds, and the unusual chemical stability of benzene. Subsequent work by a large number of investigators has revealed many other useful correlations between experiment and this simple HMO method for n electrons. 

Treating only the n electrons explicitly and ignoring the a electrons is clearly an approximation, yet it appears to work surprisingly well. Physically, HiickeTs approximation may be viewed as one that has the tt electrons moving in a potential field due to the nuclei and a a core, which is assumed to be frozen as the n electrons move about. Mathematically, the a—tt separability approximation is 

Let us consider the implications of Eq. (8-1). We have already seen (Chapter 5), that a sum of energies is consistent with a sum of hamiltonians and a product-type wavefunction. This means that, if Eq. (8-1) is true, the wavefunction of our planar 

---

The importance of symmetry in the study of the electronic structure of atoms and molecules depends on the fact that wave functions must transform according to one of the symmetry species of the symmetry group of the molecule. Stated precisely, the eigenfunctions of a Hamiltonian form bases for irreducible representations of the symmetry group of the Hamiltoirian. This principle allows wave functions to be classified according to symmetry species it assists... 

These simple molecular orbital pictures provide useful descriptions of the structures and spectroscopic properties of planar conjugated molecules such as benzene and naphthalene, and heterocychc species such as pyridine. Heats of combustion or hydrogenation reflect the resonance stabilization of the ground states of these systems. Spectroscopic properties in the visible and near-ultraviolet depend on the nature and distribution of low-lying excited electronic states. The success of the simple molecular orbital description in rationalizing these experimental data speaks for the importance of symmetry in determining the basic characteristics of the molecular energy levels. 

The importance of symmetry aspects of the matrix element M defined in Eq. (56) has been variously emphasized, for instance, by Grimmeiss et al. (1974), Morgan (1975), Jaros (1977), Monemar and Samuelson (1978)—see their Appendix A, Banks et al. (1980a,b), Blow and Inkson (1980a,b, 1982), and Inkson(1981).To proceed, we first expand the impurity wave function in terms of the band functions, i.e.,... 

Symmetry is a common phenomenon in tlie world around us. IT Nature abhors a vacuum, it certainly seems to love symmetry It is difficult to overestimate the importance of symmetry in many aspects of science, not only chemistry. Just as the principle known as Occam s razor suggests that the simplest explanation for an observation is scientifically the best, so it is true that other tilings being equal, frequently the most symmetrical molecular structure is the preferable one. More important, die methods of analysis of symmetry allow simplified treatment of complex problems related to molecular structure. 

No equivalent data seem available for oxacyclobutane itself but a number of 3,3-substituted derivatives have been documented (17,30) and show clearly the importance of symmetry for crystallinity and the marked increase in the melting point produced by polar substituents. The melting points of the simpler ether polymers are somewhat lower than those of the corresponding polyolefins, presumably because of the greater flexibility of the polyether chain coupled with low van der Waal s forces. They must be determined primarily by the ability of the chains to fit into a lattice, i. e. by the shape of the chains, so there seems no reason... 

The existence of low symmetry reaction pathways obviates the importance of symmetry rules. The consequence of this is especially apparent in the reactions of enes and dienes... 

When we stress the importance of symmetry, it is not equivalent with declaring that everything must be symmetrical. In particular, when the importance of left-and-right symmetry is stressed, it is their relationship, rather than their equivalence, that has outstanding significance. 

Deisenhofer s description is a beautiful illustration for some of the ideas about the importance of symmetries occurring in an approximate way as discussed in the Introduction (Chapter 1). The near-C2 symmetry of the photosynthetic reaction center [16] and its elucidation [17] have been discussed in the literature. 

The importance of symmetry in structure does not mean that the highest symmetry is the most advantageous. This can be illustrated beautifully in molecular crystals. Lucretius proclaimed two millennia ago in his De rerum natura [65] ... 

As in the ABC case, the basis functions divide into four sets according to fz with 1,3, 3, and 1 functions in each set. However, of the three functions in the set with fz = % or — V2 two are symmetric and one antisymmetric. Hence each of the two 3X3 blocks of the secular equation factors into a 2 X 2 block and 1X1 block. Algebraic solutions are thus possible. Furthermore, the presence of symmetry reduces the number of allowed transitions from 15 to 9, because no transitions are allowed between states of different symmetry. (One of the nine is of extremely low intensity and is not observed.) Thus the A2B system provides a good example of the importance of symmetry in determining the structure of NMR spectra. 

Today, it is also unclear as to what is the scenario for the bifurcations of NHIMs as energy increases. In other words, the change from threshold regime to above-threshold regimes remains very unclear. The same is true for the importance of symmetries in the dynamics, especially so for the breaking of some symmetries through bifurcations. 

Dunlap performed A a calculations using reduced symmetry constraints to allow for the antiferromagnetic configuration. He found results reasonably consistent with the GVB-vdW picture that is, a long weak bond. In retrospect, this shows the importance of symmetry breaking, as well as the inadequacy of the A a potential. 

The following figure shows an isomerisation equilibrium by MS between two secondary ions. Two methyls can shift in the direction A—>B,8 but only one in the direction B—>A. The example of Fig. 169 illustrates the importance of symmetries in the reaction rates. 

Electronic structure calculations carried out at a variety of levels on these octahedral metal-halide clusters vary in detail (90-104). Our view of the metal-based orbitals responsible for metal-metal bonding in the M6Y8 4+ unit has evolved very little since Cotton and Haas semiempirical treatment in 1964 (90). Undoubtedly the importance of symmetry in the calculations has led to the consistent description of the bonding scheme. Only a brief summary emphasizing the relation between the proposed models to the physical and chemical properties of the clusters is presented here. For a more detailed discussion of the bonding numerous reviews are available (105-109). 

The qualitative ideas of valence bond (VB) theory provide a basis for understanding the relationships between structure and reactivity. Molecular orbital (MO) theory offers insight into the origin of the stability associated with delocalization and also the importance of symmetry. As a central premise of density functional theory (DFT) is that the electron density distribution determines molecular properties, there has be an effort to apply DFT to numerical evaluation of the qualitative concepts such as electronegativity, polarizability, hardness, and softness. The sections that follow explore the relationship of these concepts to the description of electron density provided by DFT. 

In some implementations, direct methods place a greater premium on the use of symmetry [28] to identify integrals with permuted indices that are identical to each other and rearranging the algorithm to take advantage of that. (In other implementations the direct method diminishes rather than increases the importance of symmetry.)... 

Now let us consider the effect of crystal environment on the magnetic moment of the lanthanides. In Table 10, we show the results of calculations of the magnetic moment of neodymium on several common crystal lattices. A trivalent Nd ion yields a spin moment of 3/lb and an orbital moment of 6/ib- In the final two columns of Table 10, we see that the SIC-LSD theory yields values slightly less than, but very close to, these numbers. This is independent of the crystal structure. The valence electron polarization varies markedly between different crystal structures from 0.34/ib on the fee structure to 0.90/Zb on the simple cubic structure. It is not at all surprising that the valence electron moments can differ so strongly between different crystal structures. The importance of symmetry in electronic structure calculations cannot be overestimated. Eor example, the hep lattice does not have a centre of inversion symmetry and this allows states with different parity to hybridize, so direct f-d hybridization is allowed. However, symmetry considerations forbid f-d hybridization in the cubic structures. Such differences in the way the valence electrons interact with the f-states will undoubtedly lead to strong variations in the valence band moments. 

All this, together with the lecognitioii of the importance of symmetry for the laws of physics makes the explosion of our knowledge during the second half of this century even more impressive. 

Now that the importance of symmetry in wavefunctions has been established, some of the utility of symmetry can be introduced. A water molecule, H2O, is in the position indicated by Figure 13.17. H2O has all of the symmetry elements described by the C2V point group, so it has E, C2, and two which we will designate a(xz) and (r(yz). Remembering from above that each symmetry operation can be defined as a matrix, we can construct matrices to define the symmetry operations for H2O. However, each atom in the molecule has x, y, and z coordinates, so there are a total... 

This synthesis of calcimycin was one of the first total syntheses of an ionophore. It was preceeded by syntheses of nonactin (Gerlach and Schmidt), followed shortly by syntheses of lasalocid A (X537A) (Ireland), and then by the landmark syntheses of monensin (2) developed by Kishi and Still. The Evans synthesis illustrates the importance of symmetry in synthesis design, and documented observations that were clearly useful to those that followed. The synthesis suffered somewhat from stereocontrol, particularly... 

The most important quantum-mechanical operator is the Hamiltonian operator, which corresponds to the energy. The importance of symmetry operators in the study of electronic wave functions arises from the fact that two commuting operators can have a set of common eigenfunctions (eigenfunctions of both operators). An eigenfunction of the Hamiltonian operator can also be an... 

---