Overlap dispersion and overlap induction
Hydrogen molecu le (H2) has two valence e l e c t r o n s i n a bonding lo M O . Sodium dimer has a l s o two va lence e l e c t r o n s i n a bonding 2u Nonetheless? t h e two MO's which have i d e n t i c a l symmetry+desri be two e n t i r e l y d y f f e r e n t bonding mechanisms+as evidenced by t h e f a c t t h a t H has a s t ronger bond than Na o r b i t a l symmetry imp1 i c a t i o n s f o r chemical bonding (Hucke l ' s Rule, Woodward-Hoffmann r u l e s ) we now r e a l i z e t h a t t h e r e i s more t o chemis t ry than o r b i t a l symmetry c o n t r o l : Under i d e n t i c a l symmetry c o n s t r a i n t s , two spec ies can e x h i b i t comple te ly d i f f e r e n t chemical behav io r . We h o l d t h i s t o be t h e key t h e o r e t i c a l problem o f t h e n e x t genera t i on o f a p p l i e d quantum chemists and here we o f f e r our own i n t e r p r e t a t i o n o f t h e phenomenon by us ing t h e q u a l i t a t i v e t h e o r e t i c a l t o o l which has been developed i n t h e l a s t t e n yea rs : Mo lecu la r O r b i t a l Valence Bond Theory ( r e f . 2 , 3 , 4 ) . We w i l l argue t h a t t h e chemical and phys i ca l p r o p e r t i e s o f a system depend j o i n t l y upon symmetry and a p r o p e r t y c a l l e d " c o l o r " , t h a t t h e mechanism o f e l e c t r o n d e l o c a l i z a t i o n i s color-dependent, and t h a t t h i s aspect o f chemical bonding can be made t ranspardn t o n l y by a C o n f i g u r a t i o n I n t e r a c t i o n ( C I ) t y p e fo rma l i sm such as t h e one employed i n MOVE t h e o r y . I n t h i s note, we argue t h a t t h e r e e x i s t f o u r t ypes o f chemical b i n d i n g mechanisms: The f a m i l i a r ove r lap ( "cova len t " ) and c l a s s i c a l Coulomb ( " i on i c " ) mechanisms and two more: two a r e respons ib le f o r t h e b i n d i n g o f t h e vas t m a j o r i t y o f chemical compounds o t h e r than those made up o f f i r s t row "nonmeta l l i c " atoms. MO. r a d i c a l c a t i o n has a weaker bond than H2 b u t Nag ( re? . 1). A f t e r nea r l y h a l f a cen tu ry o f p reoccupat ion w i t h 2 Over lap D ispe rs ion and I o n i c Over lap I n d u c t i o n . The l a t t e r I. THEORETICAL BACKGROUND The r e c i p e f o r f o r m a l l y implement ing MOVE theo ry i s as f o l l o w s : (a) A mo lecu le i n a g i v e n geometry i s d i s s e c t e d i n t o two fragments. v e n i e n t d i s s e c t i o n c h o i c e i s t h e one f o r which t h e l oca l symmetry o f t h e f ragments i s h i g h e s t . f i r s t p r i n c i p l e s o r computed by u s i n g an e f f e c t i v e o n e e l e c t r o n Hami l t on ian o f t h e Extended HMO (EHMO) v a r i e t y . (c) The e l e c t r o n s a r e d i s t r i b u t e d so as t o genera te t h e MOVE C o n f i g u r a t i o n Wavefunct ions (CW's) , O i l s . i n t o se ts , Tm's, and each s e t i s d iagona l i zed separa te l y t o genera te t h e d i a b a t i c s t a t e s , Bmi. a d i a b a t i c s t a t e s , 9 ; . The p a r t i t i o n i n g o f CW's i n t o s e t s i s c a r r i e d o u t i n such a manner so t h a t each Bml, t h e ground s t a t e r e s u l t i n g f rom d iagona l i z i n g t h e @ . I s o f T , rep resen ts j u s t one way o f making i n t e r f r a g m e n t a l bonds o r an t ibonds . W i t h ' t h i s coFvent ion, t h e approximate fo rm o f t h e ground t o t a l wavefunc t ion becomes: The most con(b) The symmetry-adapted fragment o r b i t a l s (MO's o r A0 s) a r e w r i t t e n f rom (d) The MOVB CW's a r e p a r t i t i o n e d (e) The d i a b a t i c s t a t e s are , f i n a l l y , d iagona l i zed t o produce t h e f i n a l O1 = E Am Rml Tha t i s t o say, h ighe r d i a b a t i c s t a t e s o f a p p r o p r i a t e symmetry (e.g., Bmi's w i t h i > 1) a r e deemed t o be r e l a t i v e l y un impor tan t . We now g i v e an i l l u s t r a t i o n o f our approach by showing t h e bond diagrammatic r e p r e s e n t a t i o n o f A=B. Accord ing t o t h e "d rawing convent ion" o f MOVB theory , dashed