1
CHAPTER I INTRODUCTION
I t is w e ll known th a t th e degree o f in t e r a c t io n betw een s o lv e n t and s o lu te is re s p o n s ib le f o r v a rio u s s o lv e n t e ffe c ts * U s u a lly , more p o la r th e s o lv e n t, th e g re a te r th e in te r a c t io n e .g . s o lv e n t s h i f t s . L it e r a t u r e in o rg a n ic compounds is e x te n s iv e , b u t n o t so in in o rg a n ic com pounds.
A ls o , th e oxygen atom s o f a c e ty la c e to n a te com plexes as w e ll as th e c h e la te r in g s th e m s e lv e s , can fu n c tio n as s it e s f o r H-bond fo rm a tio n . L it e r a t u r e on H-bond fo rm a tio n o f some
MCAcAc)^ 2 has o n ly been re p o rte d by X - r a y \ s o lv a tio n ^ , uv and v is ib le s p e c tr a l s tu d ie s .if
T h e re fo re , we c o n s id e r i t d e s ira b le to s tu d y s o lv e n t e ffe c ts o f some b iv a le n t and t r iv a le n t m e ta l a c e ty la c e to n a te s in a
v a r ie t y o f s o lv e n ts in th e uv r e g io n .
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1.1 T h e _F ra n c k-c o n d o n P r in c ip le and H ydrogen B o n d in g "
B o th M c C o n n e ll
f
B a y lis s 'a n d McRae^have d is c u s s e d th e e ffe c t o f s o lv e n t on an e le c tr o n ic t r a n s it io n . The e q u ilib r iu md is ta n c e s and o r ie n ta tio n s o f s o lv e n t m o le c u le s aro u n d a s o lu te m o le c u le a re d ic ta te d by th e e n e rg ie s o f s o lv a tio n . I f th e s o lu te m o le c u le makes an e le c tr o n ic t r a n s it io n , th e s o lv a tio n a rra n g e m e n t o f th e i n i t i a l s ta te is n o t n e c e s s a rily th e most s ta b le f o r th e new e le c tr o n ic d is t r ib u t io n and, in d e e d , c o u ld be e n e r g e tic a lly r e p u ls iv e . S in c e th e s o lv e n t m o le c u le s c a n n o t r e - o r ie n t fa s t enough d u rin g th e t r a n s it io n , (F ra n c k - Condon p r in c ip le ) such a change in s o lv a tio n e n e rg ie s
w o uld r e s u lt in a change in th e e n e rg y s e p a ra tio n o f th e two s ta te s and th e re w ould be a s o lv e n t s h i f t .
In an H -b o n d in g s y s te m , th e in te r a c tio n is lo c a liz e d a t fu n c tio n a l g roup s w ith e it h e r p ro to n d onor o r a c c e n to r p ro p e r
t i e s . A s o lv e n t s h i f t th e n in d ic a te s th a t th e H -b o n d in g p ro p e rtie s o f th e s e fu n c tio n a l g roup s in th e i n i t i a l s ta te d i f f e r fro m th e H -b o n d in g p r o p e r tie s o f th e s e same g roup s in th e f i n a l s ta te . The m ag nitud e o f th e s h i f t can be e v a lu a te d in te rm s o f th e F ra n c k-C o n d o n p r in c ip le . The fo llo w in g th e o ry , h o w e ve r, m ust n o t be re g a rd e d as d e f in it iv e .
F ig u re 1 p o rtra y s p o te n tia l fu n c tio n s f o r fo rm a tio n o f an H-bond A -ÏÏ. . . B w here R is th e H-bond le n g th ,
A-H is th e s o lv e n t m o le c u le and B is th e s o lu te m o le c u le .
3
F ig .la ( พ0>wt
'.i£Ne P»6Y
F i g „ 1 H y p o t h e t i c a l p o t o n t i a l c u r v e s : e n e r g y V S , H - b o n d l e n g t h . R « ' H y d r o g e n b o n d l e n g t h
k
H -b ond . In each case th e e n e rg y s e p a ra tio n o f th e non-H -b ond ed s p e c ie s (R = นo) is assumed to be id e n t ic a l to h ^ th e e n e rg y s e p a ra tio n in a n o n -H -b o n d in g s o lv e n t o f th e same r e f r a c t iv e in d e x . The H-bond en e rg y is d e s ig n a te d พ 'ไ'/ ะ: - & H ) and th e e x c it a t io n energ y im p lie d by th e Franck-C ond on p r in c ip le is la b e le d 0J-. (B y th e s e d e f in it io n s , พ and CJ a re a lw a y s p o s itiv e num b ers. O f couse e ith e r Uq o r พ^ c o u ld be z e r o .)
T h e re a re th re e q u a n titie s o f in t e r e s t w h ic h m ig h t be e x p e rim e n ta lly a c c e s s ib le ,
ๆ/^, ๆรQ
andๆ/0
0 , th e fre q u e n c ie s a p p ro p ria te to maximum a b s o rp tio n , maximum e m is s io n , and th e 0 -0 t r a n s it io n , r e s p e c tiv e ly . These fre q u e n c ie s a re r e la te d to th e q u a n tity V 0 by th e r e la t io n sF i g . l a p o r t r a y s t h e s i t u a t i o n w h e n t h e e x c i t e d s t a t e f o r m s a w e a k e r H - b o n d a n d i n F i g . 1 b t h e e x c i t e d s t a t e f o r m s t h e s t r o n g e r
H w ร \ - ฬ 0 - พ 1 4 น* 1 L .1 }
K ( v e - v 0 ) = ะ w o พ , - ^ 0 ( O
พ . - พ 1
( 5 )
The m ost in t e r e s t in g q u a n tity is Wq - พ
^ ,
w h ic h g iv e sd ir e c t ly th e d iffe r e n c e in H-bond e n e rg ie s o f ground and e x c ite d s ta te s . In F ig . 1a (Wq} พ^) we see th a t th e u s u a l e x p e c ta tio n s o f th e p o te n tia l fo rm p re d ic t th a t Uq'Jt พ0 and Ui ,1 > Cc*0 .
Mo u p p e r l i m i t c a n b e s e y o n พ ^ , s i n c e t h e s t e e p s i d e o f t h e p o t e n t i a l f u n c t i o n c o u l d r i s e s u f f i c i e n t l y f a s t t o m a k e น ( F ig . 1 a ).
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In any e v e n t, i t w i l l o n ly be a c o in c id e n c e i f CO, = พ and we see th a t hA i/^can be e xp e c te d , a p r i o r i , to m easure Wq . S in c e พ , is p o s itiv e , eq. 1 p re d ic ts th a t i f Wq > พ1 th e n ใ£ > 0 , i . e . , a b lu e - s h if t w h ic h exceeds th e d iffe r e n c e V/q-ViL by น) 1 w i l l be ob served in a b s o rp tio n . S im ila r ly , in e m is s io n , a s h i f t w i l l be ob served w h ic h is le s s th a n พ - V/^ by aic ( i . e . , e ith e r a re d s h i f t o r b lu e s h i f t may be o b se rve d ; in e it h e r c a se , th e s h i f t w i l l be s m a ll compared to VJq ) . I f พ1 = 0 , no s h i f t w i l l be ob se rve d in e m is s io n s in c e th e n v/q = พ0 .
In F ig . lb (Uq <. './1 ) th e a n h a rm o n ic ity e x p e c ta tio n s
p re d ic t th a t พ1 "> <^1 and น 1.1>«1 . Now th e a b s o rp tio n sp ectrum w i l l show a s h i f t w h ic h i s le s s by น)1 th a n th e re d s h i f t im p lie d by th e n e g a tiv e v a lu e o f Wq " พ1 . The e m is s io n sp ectrum w i l l show a re d s h i f t e xc e e d in g j i.r - พ1 I by top .
I f a b lu e s h i f t a b s o rp tio n is o b se rve d F ig . 1a i s a p p lic a b le and th e b lu e s h if t s exceeds th e d iffe r e n c e by พ - พ1 .
I f a re d s h i f t is ob served in a b s o rp tio n th e n F ig . 1๖ is
a p p lic a b le (พ0 <.พ 1 ) and th e re d s h i f t is le s s th a n th e d iffe r e n c e พ - Wq. I f o n ly a s m a ll b lu e s h i f t is ob se rve d in a b s o rp tio n , e it h e r o f F ig s . 1a o r 1b c o u ld be a p p lic a b le . S im ila r ly , in e m is s io n , a b lu e s h i f t im p lie s F ig . 1a, a la rg e re d s h i f t im p lie s F ig . 1b, and a s m a ll re d s h i f t i s am b ig uous. Under any
c irc u m s ta n c e s ,^ A a n d c o n ta in th e q u a n tity
o
= ’'•'0 - พ 1 .6
* *
V» —» Tf . In th e V1-*IY t r a n s it io n , one o f th e e le c tro n s i s p rom oted fro m th e n o n -b o n d in g o r b it a l to an em pty a n ti-b o n d in g T f- o r b ita l. The one n o n -b o n d in g e le c tro n re m a in in g on th e oxygen
atom ( f o r exam p le, as in th e c a rb o n y l g ro u p ) i s n o t adeq uate
to s u s ta in th e hyd rog en bond and th e e x c ite d s ta te is d e - s ta b iliz e d . Hence th e hyd rog en bond is weakened in th e p ro c e s s o f p ro m o tio n and th e energ y o f t r a n s it io n is in c re a s e d , ( i . e . b lu e s h i f t ) , *•
f t - *'ll. A g a in u s in g th e c a rb o n y l group as an exam p le.*•
T h is t r a n s it io n f u r t h e r s ta b iliz e â th e e x c ite d H-bonded s ta te due to d e lo c a liz a tio n o f th e Tf e le c tio n s . Hence th e t r a n s it io n en e rg y i s d ecreased i . e . a re d s h i f t i s re c o rd e d .
T h e b l u e s h i f t p h e n o m e n o n h a s b e e n u s e d t o c h a r a c t e r i z e
«า -*TÎ t r a n s i t i o n s a n d t o d i s t i n g u i s h t h e m f r o m t r a n s i t i o n s ( r e d s h i f t s ) , 8 » 9 ’ 1 0
