## 5 Current Coherence in Donor-Acceptor Systems  
Electron flows in molecules are determined by the _energy_ conditions. The molecular energetics is reflected by SE of QM. Indeed, the entropy/information flows in molecular systems are carried by the state electronic flows. The QIT description thus offers only a supplementary, alternative framework for exploring and ultimately better understanding the rules and structures of chemistry. The energetics of a nondegenerate ground-state of the whole molecular system is devoid of any phase aspect, while subtle preferences involving reactants may already involve local phases of electronic states in the substrate subsystems, reflecting their entanglement in the whole reactive complex [63; 64; 65; 66; 67; 68; 69; 70; 71]. As an illustrative example let us now recall the energy-preferred _complementary_ reactive complex A--B [67; 70; 94; 95] shown in Figure 1, consisting of the basic subsystem  
\[\mathrm{B}=(a_{\mathrm{B}}\mid\ldots\mid b_{\mathrm{B}})\equiv(a_{\mathrm{B}} \mid b_{\mathrm{B}})\]  
and its reaction companion--the acidic substrate  
\[\mathrm{A}=(a_{\mathrm{A}}\mid\ldots\mid b_{\mathrm{A}})\equiv(a_{\mathrm{A} }\mid b_{\mathrm{A}}),\]  
where \(a_{\mathrm{X}}\) and \(b_{\mathrm{X}}\) denote the active acidic and basic sites of X, respectively. The four molecular fragments, \(\lambda\in\){(\(a_{\mathrm{A}}\), \(b_{\mathrm{A}}\)), (\(a_{\mathrm{B}}\), \(b_{\mathrm{B}}\))}, define the active parts in the system charge reconstruction. The acidic (electron acceptor) part is relatively harder, i.e., less responsive to external perturbations, thus exhibiting lower values of the fragment Fukui function or chemical softness descriptor, while the basic (electron donor) fragment is relatively more polarizable, as indeed reflected by higher response descriptors of its electron density or site populations. The acidic part \(a_{\mathrm{X}}\) exerts an electron-_accepting_ (stabilizing) influence on the neighboring part of another reactant Y, while the basic fragment \(b_{\mathrm{X}}\) produces an electron-_donor_ (destabilizing) effect on the fragment Y in its vicinity. Figure 1: The Charge-Transfer (CT) \(\{b_{\mathrm{X}}\)\(\rightarrow\)\(a_{\mathrm{Y}}\}\) and Polarizational (P\({}_{\mathrm{X}}\)) \(\{a_{\mathrm{X}}\)\(\rightarrow\)\(b_{\mathrm{X}}\}\) electron flows involving acidic A = (\(a_{\mathrm{A}}\mid b_{\mathrm{A}}\)) and basic B = (\(a_{\mathrm{B}}\mid b_{\mathrm{B}}\)) reactants in the complementary arrangement R\({}_{c}\) of their acidic (\(a\)) and basic (\(b\)) sites in reactive complex R = (A\(|\)B). These electronic flows are seen to produce an effective concerted (circular) flux of electrons in the equilibrium reactive system R\({}^{*}\) = (A\({}^{*}\)! B ) = (\(a_{\mathrm{A}}\)! _b_A\({}^{*}\)! _a_\({}^{*}\)! _b_B\({}^{*}\)) as a whole, with all fragments exhibiting the “flow-through” current pattern, which precludes an exaggerated depletion or accumulation of electrons on any site in reactive system. In the most stable complementary (\(c\)) complex of Figure 1[94; 95], a geometrically accessible \(a\)-fragment of one reactant faces the geometrically accessible \(b\)-fragment of the other substrate  
\[R_{c}\equiv\left[ \begin{array}{c}a_{\rm A}-b_{\rm B}\\ b_{\rm A}-a_{\rm B}\end{array} \right], \tag{46}\]  
while in a less favorable regional HSAB-type coordination the acidic (basic) fragment of one reactant faces the like-fragment of the other substrate:  
\[R_{\rm HSAB}\equiv\left[ \begin{array}{c}a_{\rm A}-a_{\rm B}\\ b_{\rm A}-b_{\rm B}\end{array} \right]. \tag{47}\]  
A relative stability of \(R_{c}\) reflects an electrostatic preference: an electron-rich (repulsive, basic) fragment of one reactant indeed prefers to face an electron-deficient (attractive, acidic) part of the reaction partner.