Atom Junctivity

Chlorine atom 1 (in l+2- [3] ) and the carbenic carbon in 18a (18a+18b- [19] ) are atomic junctive simplexes (type 26 and 27 (Figure 8.3), respectively) for each of them, aj=l. In the latter case, it is solely the central carbon that constitutes the junctive atom the rest of the molecule is not part of the simplex. Bond junctive simplexes are exemplified by the O-H bond (type 28) in 24a for the transformation 24a+24b- 25), and the C-C bond of 5 (type 29) in transformation 5+6- 7. In these cases, the junctive sites are the terminal atoms of the a-bond thus, a =2. Lastly, n-junctive simplexes are typified by ketone 9 (type 30, Figure 8.3) during the course of its reactions with an alkyllithium reagent (9+10- ll), and 12 (type 31, Figure 8.3 3 =2), in its Diels-Alder reactions (12+13- 14) in the latter, the junctive sites are the terminal carbons of the 7i-system. Hence, for each of 30 and 31, a,=2. 

X. Net Atom Junctivity (j ). Molecular Junctivity (j ), Process Junctivity (JforJrev)... 

Net atom junctivity, ja, of atom X, is the net number of new incipient directed bonds between X and atoms it becomes bonded to. It is the number of incipient directed bonds minus the number of severed directed bonds, and is given by Equation 8.1 ... 

The dimerization of cyclobutanol has a Jfor value of 2 the termolecular C-G-G association has a Jfor value of +5. The coordination of LiBr with macrocycle 245 has a Jfor value of +1 (assuming that Li is still in the bonding vicinity of Li in contradistinction, the complexation of LiBr with 15-crown-5 has a Jfor value of +4 (here, the severance of LiBr "bond" is assumed to be complete). For the complexation of Ag with macrocyclic tetraene 252, Jfor = +7 here too, the formal ionic bond with fluoroborate is considered completely severed, because of the necessary physical separation between Ag and BF4. The t-complexation 256+257- 258 is characterized by Jfor =+6 the atomic junctivities are shown in Figure 8.19. Finally, the formation of organometallic complexes 261, 265,270 and 274 have Jfo, values of +4, +8, +12 and +12 respectively. 

Figure 8.21. Valency, Coordination Number, and Atom Junctivity...

These examples show that atom junctivity/disjunctivity differs from and complements the concepts of valency and coordination number. 

Atom junctivity (jj) is not to be confused with (junctive) atomicity (ag). (Junctive) atomicity, ( s), identifies the number of bonding atoms in a simplex - fundamental or topological it determines the notation for the jimctive process (1,1) vs. (1,2). Atom junctivity, (jj), on the other hand, denotes the number of directed bonds being formed at a given junctive atom. 

Fundamental simplex junctivity, jg f is the sum of atom junctivities (ja. ) of all reacting atomic sites in a fundamental simplex s, and is given by Equation 8.2 ... 

The formation of a directed bond between two (or more) reactive atomic sites (belonging to one, two or more reactants/reactant moieties) en route to fransifion state, intermediate or product(s) (Figure 8.2) is, ipso facto, a junctive process. Such a process manifests itself in the formation of (a) incipient a bonds (l+2- [3] , 5+6- [7]), (b) full-fledged a bonds (l+2- 4, 5+6- 8, 9+l0- ll, 12+13- 14), (c) partial bonds due to dipole-dipole, dipole-ion (in forming host-guest complexes ... 

Note that following the definitions given above, the transformation [3] - 4, [19] - 20 are not considered junctive because they are not accompanied by ab origine (ab initio) bonding between relevant nonbonded pairs of atoms, despite the fact they are accompanied by increased bonding between the already-bonded pairs of atoms. That is to say, in these transformations no unlinked atoms become newly linked (therefore, they are not junctive) there is, however, enhanced bonding between already-linked atoms. Similarly, the 4- [3] and 20- [19] transformations are not disjunctive because there is only diminished bonding there is no severance (total disconnection) between pairs of bonded atoms. 

A junctive site is a reacting atom at which a change in directed bonding occurs. In Figure 8.2 (p. 3), all junctive sites are marked with arrows. A junctive simplex is the smallest portion of a molecule/molecular species that encompasses the reacting atomic sites involved in a junctive process. Jimctive simplexes are of two types - fundamental and topological. Processes are derived from fimdamental simplexes, or topological ones, or both (vide infra). 

In carbogenic systems, a hmdamental junctive simplex is, generally, an atom (type 26,27), a a-bond (type 28,29), or, a Jt-system (type 30,31), as represented in Figure 8.3 (p. 4). The atomicity of a simplex (ag) is the number of junctive atoms of the simplex taking part in a given junctive process. 

Simple binary and ternary junctive processes are denoted as (m,n)j, (m,n,p)j, respectively (m, n, p are the atomicities as of the participating simplexes i.e. rtvi,p=l,2. ). The corresponding reverse processes are denoted as (mnld, (nvi,p)d and they constitute simple disjunctive processes (subscripted suffixes j and d, mean junctive and disjunctive, respectively). Figure 8.4 depicts cartoon representations and notational designations of simple binary and ternary junctive processes (the square bracketed entity, in each case, represents a conjunctive slate). ... 

The simplest binary junctive process (l,l)j results from atom-atom combination e.g. 32- [33], 34- [35], 36- [37]. The (l,2)j atom-bond, atom-face junctive processes are exemplified by transformations 38- [39] and 40- [41], respectively. The (2,2)j junctive processes resulting from bond-bond, bond-face, and face-face associations are represented by 42- [43], 44- [45] and 46- [47], respectively. 

The ternary (1,1,l)j junctive process resulting from atom-atom-atom associations is given by 48- [49]. Similarly, (l,l,2)j processes arise from atom-atom-bond, atom-atom-face interactions (e.g. 50- [51], 52- [53]), whereas (l,2,2)j processes stem from atom-bond-bond, atom-bond-face and atom-face-face interactions (e.g. 54- [55], 56- [57], 58- [59]. Finally, the ternary bond-bond-bond, bond-bond-face, bond-face-face, and face-face-face associations are designated (2,2,2)j as in 60- [61], 62- [63], 64- [65], and 66- [67], respectively. 

The various types of typical monoatomic jimctive moieties are depicted, along with the characteristic vectors Vy in their relative orientations, in Figure 13.3 below. In case 1, vector Vy is parallel to the only bond in the case of 2-4 it is parallel to one of the three bonds at the jimctive atom. In cases 5-6, Vy is parallel to the axis of the K bond. For cases 7-10, vector Vy is based on the two substituents of the junctive atom. In the case of 11-14, the effective vector Vy is parallel to the main axis of the n system. In each of cases 1-14, the absolute sense (i.e. v vs. v, or vj vs. Vg, or Vy vs. Va) of each vector is determined on the basis of the actual substituents (Figure 13.4, vide infra). 

Figure 143. Angular Joins in (l,l)-Junctive Processes Between Atomic Orbital Prototypes 1-13...

In [m,n,p]j, [m,n]j and [m]j, m is the number of junctive atoms in molecule 1, n is the number of junctive atoms in molecule 2, and p is the number of jimctive atoms in molecule 3 the total number of numerals between the square brackets reflects the number of molecules/molecular entities taking in the junctive process. Thus, [nvn,p]j involves three molecules, [nwi]j involves two, and [m]j involves only one. The numerical value of each numeral indicates the number of junctive atoms in a molecule taking part in the junctive process. For example, in junctive process... 

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