14 Resonance Raman Spectroscopy as a Complement to Other Techniques for Making Electronic Band Assignments for (Re X ) Ions 2-
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch014
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ROBIN J. H. CLARK and MARTIN J. STEAD University College London, Christopher Ingold Laboratories, 20 Gordon Street, London WC1H 0AJ England 2-
The resonance Raman (RR) spectra of the [ReX] ions, X = F, Cl, Br or I, obtained at resonance with the δ* t r a n s i t i o n . T h i s conclusion i s substantiated by measurements o f the d e p o l a r i z a t i o n r a t i o s o f the bands at _ resonance. These have been measured f o r the [Re2Fg] ~, [Re2Clg] and [Re2Brg]2- ions and are found t o be 1/3, a s i t u a t i o n which can only a r i s e i f the resonant e l e c t r o n i c t r a n s i t i o n i s zp o l a r i z e d , as the δ* «- δ t r a n s i t i o n ( i n the point group) must be. I r r a d i a t i o n w i t h i n the contour o f the second e l e c t r o n i c t r a n s i t i o n o f each ion produces e n t i r e l y d i f f e r e n t RR spectra i n each case. These spectra (Figure 3) are c h a r a c t e r i s e d by resonance enhancement to the v mode and i t s overtones, where v i s the t o t a l l y symmetric metal-halogen s t r e t c h i n g mode (624, 362, 211 and 152 cm" f o r X = F, CI, Br and I, r e s p e c t i v e l y ) . Thus the p r i n c i p a l s t r u c t u r a l change undergone by each ion on e x c i t a t i o n i s , i n t h i s case, along the metal-halogen coordinate, a r e s u l t i n keeping with that expected i n consequence o f a nonbonding-to-antibonding halogen-to-metal c h a r g e - t r a n s f e r t r a n s i t i o n . Thus, the assignment o f the second strong band i n the e l e c t r o n i c spectra o f the [Re2Xg] " lu( *) ( ) gM> U A , t r a n s i t i o n follows n a t u r a l l y . T h i s assignment i s confirmed 6y the f a c t that the measured d e p o l a r i z a t i o n r a t i o o f the V2 band o f the [ R e 2 C l g ] ~ and [Re2Brg] ~ ions at resonance i s 1/8, a s i t u a t i o n which can only a r i s e i f the resonant e l e c t r o n i c t r a n s i t i o n i s xy p o l a r i z e d . This i s p r e c i s e l y the p o l a r i z a t i o n r e q u i r e d ( i n D ^ ) f o r the *E *A transition. 1 a
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Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch014
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RR spectra obtained at resonance with the t h i r d e l e c t r o n i c band o f each i o n leads to resonance enhancement t o both the and V2 bands (and t h e i r overtones) t o comparable extents. Although no f i r m assignment o f the resonant e l e c t r o n i c band can be made i n t h i s case, the r e s u l t s are c o n s i s t e n t with the assignment e (TT*)
