Bond diagram

It will be convenient to describe first the binary. sulfur nitrides SjN,. and then the related cationic and anionic species, S,Nv. The sulfur imides and other cyclic S-N compounds will then be discus.sed and this will be followed by sections on S-N-halogen and S-N-O compounds. Several compounds which feature i.solated S<—N, S-N, S = N and S=N bonds have already been mentioned in the. section on SF4 e.g. F4S NC,H, F5S-NF2. F2S = NCF3, and FiS=N (p. 687). Flowever. many SN compounds do not lend themselves to simple bond diagrams, - and formal oxidation states are often unhelpful or even misleading. 

It is not possible to write down a single, satisfactory, classical bonding diagram for S4N4 and, in valence-bond theory, numerous resonance hybrids must be considered of which the following are typical ... 

FIGURE 2.7. (a) Three active pz orbitals that are used in the quantum treatment of the X + CH3-Y— X-CH3 + Y Sw2 reaction, (b) Valence-bond diagrams for the six possible valence-bond states for four electrons in three active orbitals, (c) Relative approximate energy levels of the valence-bond states in the gas phase (see Table 2.4 for the estimation of these energies). 

Valence bond diagrams, for SN2 reactions, 60 Valence bond (VB) model for diatomic molecules, 15-22 empirical (EVB), 58-59 EVB mapping potential, 87, 88... 

Fig. 1. The vector-bond diagrams for three structures of the canonical set of fourteen for n — 4, and some of their superposition patterns.

To find the coefficient of a given exchange integral in a matrix element, (I/F/PII), draw the vector-bond diagram for structure II, change it as indicated by the permutation diagram for P, and form the superposition pattern of I and PI I. The coefficient is then given, except for the factor (—l)p, by the above rules for the Coulomb coefficient that is, it is (— 1)F(— V)r2 in t>. 

The bond diagrams provide an obvious simple method of determining the allowed spectral terms for equivalent electrons with Russell-Saunders coupling, which may be convenient for the reason that it separates states of different multiplicity at the start. 

If Rumer tableaux are used for ft, these may in many cases be put in a one-to-one relation with classical bonding diagrams used by chemists. 

Connectivity indexes in particular may be used easily because each index can be calculated from valence bond diagrams. These indexes can also be correlated... 

Excitation of the protonated Schiff s base causes a movement of positive charge from the N toward the ring. The C-ll-C-12 bond loses much of its double-bond character. The valence bond diagrams shown here should not be taken too literally, but they give a good qualitative picture of the redistribution of electrons that occurs when the molecule is excited. 

Isomerization of the retinal Schiff s base can occur when the molecule is excited with light, because the C-l 1-C-12 bond loses much of its double-bond character in the excited state. The valence bond diagrams of figure S2.7 illustrate this point. In the ground state of rhodopsin, the potential energy barrier to rotation about the C-l 1-C-l2 bond is on the order of 30 kcal/mol. This barrier essentially vanishes in the excited state. In fact, the energy of the excited molecule probably is minimal when the C-11 -C-l2 bond is twisted by about 90° (fig. S2.8). The excited molecule oscillates briefly about this intermediate conformation, and when it decays back to a ground state it usually settles into the ail-trans isomer, bathorhodopsin. 

Anion lattice The key to the simple description of ionic compounds. Often close-packed. Bond diagram The key to the simple description of covalent compounds. Often a fragment of a close-packed array 7>. 

S. Shaik, A. Shurki, Angew. Chem. Int. Ed. Engl. 38, 586 (1999). Valence Bond Diagrams and Chemical Reactivity. 

Pictorial Representation of Valence Bond Wave Functions by Bond Diagrams... 

Since we agreed that a bond needs not necessarily involve only two AOs on two centers, we must agree on some pictorial representation of a bond. This bond diagram is in Fig. 3.2, and shows two spin-paired electrons in general orbitals 9 and cp2, by a line connecting these orbitals. This bond diagram represents the wave function in Equation 3.12... 

FIGURE 3.2 A generic bond diagram representation of two spin-paired electrons in orbitals cpi and bond pair is indicated by a line connecting the orbitals. 

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