215-216 HH W10-IR-rev

215-216 HH W10-IR-rev - 215-216 HH W10 Notes - Dr. Masato...

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Unformatted text preview: 215-216 HH W10 Notes - Dr. Masato Koreeda Topic: _IR Spectroscopy_ page 1 of 3. Harwood/Claridge) • Uses the light with ν = 1013 ~ 1014 cycles/s (i.e., ≈ 1 ~ 10 kcal/mol) I. IR Spectrum – Fourier Transform (FT) System 2.5 µ 7µ 15 µ Date: January 6, 2010 Infra-red (IR) Spectroscopy (Ege’s book: Section 12.2; Chapter 3 of the textbook – 25 µ −−−−−wavelength ( = 10 cm) 100 ↑ % transparency -4 ________________________________________________________________________ ________________________________________________ 0 -1 4,000 3,000 1,430 666 500 400 cm -----wave numbers fingerprint region A. Δ = hv = hc/λ 4 v = 1 x 10 = wavenumbers (cm-1) λ ≡ # of waves per cm w a v e le n g th in m ic r o n s ( µ ) [ 1 µ = 1 0 - 4 c m = 1 0 - 6 m ] h = P la n c k ’ s c o n s ta n t; c = v e lo c ity o f th e lig h t λ = w a v e le n g th -1 1 cm corresponds to 2.86 cal/mol ∴ 4,000 cm-1 ≡ 11.44 kcal/mol 400 cm-1 ≡ 1.14 kcal/mol “ w a v e n u m b e r s a r e p r o p o r t io n a l t o t h e e n e r g y ” B. The 1,430 ~ 500 cm-1 (7 ~ 20 µ) region: Fingerprint region • U s e d f o r c o n f ir m in g th e id e n tity o f c o m p o u n d s . • N o t u s e f u l f o r th e id e n tif ic a tio n o f f u n c tio n a l g r o u p s . C. 4,000 ~ 1,430 cm-1 (2.5 ~ 7 µ) region: • M o s t o f th e c h a r a c te r is tic s tr e tc h in g v ib r a tio n p e a k s a p p e a r in th is r e g io n . • U s e f u l f o r th e id e n tif ic a tio n o f f u n c tio n a l g r o u p s s u c h a s O - H , N - H ( o r N H 2 ) , C = O , C = N , C ≡ C , C ≡ N , e tc . D . I R s p e c tr a a r e r e c o r d e d a s : ( 1 ) n e a t /l iq u id ( l iq u id s a m p l e s ; D ia m o n d A T R F T I R * ) ; g r o u n d s o l id ( D ia m o n d A T R F T IR ) ( 2 ) K B r d is c ( s o lid s a m p le s ) [ f o r b e s t r e s u lts , th e d is c h a s to b e tr a n s p a r e n t] ( 3 ) s o m e tim e s in s o lu tio n ( in C H C l3 , C H C l3 o r d io x a n e ) ( 4 ) e v e n a s a g a s * A T R F T I R = A tte n u a te d T o ta l- R e f le c tio n F o u r ie r T r a n s f o r m I R 215-216 HH W10 Notes - Dr. Masato Koreeda Topic: _IR Spectroscopy_ page 2 of 3. Date: January 6, 2010 I I . I R a b s o r p t io n p e a k s . • M o le c u la r v ib r a tio n s a r e d e te c te d . T h e tw o ty p e s o f m o le c u la r v ib r a tio n s a r e : ( 1 ) S tr e tc h in g v ib r a tio n s ( o f te n d e s ig n a te d a s “ v ” ) e.g., CO v C=O T h e s e a r e g e n e r a lly s tr o n g e r th a n b e n d in g v ib r a tio n s a n d m o r e r e lia b le in id e n tif y in g f u n c tio n a l g r o u p s . ( 2 ) B e n d i n g o r d e f o r m a t i o n v i b r a t i o n s ( o f t e n d e s i g n a t e d a s “ δ” ) . e.g., H C H H C H H C H H C H scissoring wagging twisting rocking M o s t o f th e s e p e a k s s h o w u p in th e 1 ,4 0 0 ~ 4 0 0 c m - 1 . N o t s o u s e f u l; u n r e lia b le f o r th e id e n tif ic a tio n o f f u n c tio n a l g r o u p s . (1) Stretching vibrations H a r m o n ic o s c illa to r m o d e l ( b a s e d o n H o o k e ’ s la w ) m1 m2 m a s s e s o f a to m s 1 a n d 2 F = kx = m • d2x/dt2 (force) v= 1 2πc f (m1 + m2) m1 • m2 f : f o r c e c o n s ta n t ( in d ic a tio n o f h o w s tr o n g th e b o n d is ) € (a) v for absoption rel. force const. 1,600 960 425 (b) Because of v = v C ≡ C 2200 cm-1 v C = C ~1600 cm-1 v C C ~700 cm-1 1 f (m1 + m2) 2πc m1 • m2 v for vO-H is larger than that of vC-H. i.e., in addition to the force constant, masses of the two atoms (m1 and m2) have € to be taken into consideration. A better use of this formula is the comparison of isotopically labeled compounds, e.g., vC-H: 2900 cm-1 vs vC-D: 2050 cm-1 T h e f o rc e c o n s ta n ts f o r C - H a n d C - D a re v irtu a lly id e n tic a l; th is d if f e re n c e in w a v e n u m e rs c o m e s f ro m th a t in th e m a s s e s ( H v s D ). 215-216 HH W10 Notes - Dr. Masato Koreeda Topic: _IR Spectroscopy_ page 3 of 3. Date: January 6, 2010 III. A Few Key Issues Concerning IR Stretching Vibrations (1) Stronger intensity expected for “polar” bonds B o n d s th a t c a u s e la r g e d ip o le m o m e n t c h a n g e s u p o n v ib r a tio n ⇒ s tr o n g s tr e tc h in g v ib r a tio n a b s o r p tio n – th e d e te c tio n o f I R p e a k s is b a s e d o n th e c h a n g e in th e d ip o le m o m e n ts u p o n v ib r a tio n in e a c h b o n d . ( 2 ) H ig h e r v ⇒ h i g h e r e n e r g y r e q u i r e d f o r s t r e t c h i n g o f t h e b o n d . n o te : M a k e s u r e to c o m p a r e w ith in th e s a m e c o m b in a tio n o f a to m s . Examples: (a) νC=O O 1715 cm-1 1685 cm-1 more singlebonded C=O O O O 1655 cm-1 even more singlebonded C=O O O O O O O O O ( b ) v a r io u s C = O f u n c tio n a l g r o u p s ( i) T h e r e s o n a n c e e f f e c t m a k e s C = O m o r e s in g le b o n d e d ⇒ lo w e r c m - 1 f o r ν C = O O R Z R' R O Z R' R O Z R' ( ii) I f Z is a h ig h ly e le c tr o n e g a tiv e a to m , the inductive effect through the C-Z sigma bond O ⇒ makes the C=O bond shorter, i.e., stronger C=O bond R' R Z ⇒ higher cm-1 for νC=O I t is th e f in e b a la n c e o f th e s e tw o o p p o s in g e f f e c ts th a t d e te r m in e s th e ν C = O o f e a c h o f R - C ( = O ) - Z - R .’ acid halides O R O R F 1800 cm-1 Cl esters/lactones νC=O ~1735 cm-1 O R O R' ketones νC=O ~1715 cm-1 O R R' amides νC=O ~1660 cm-1 O R N R" R' R O N R' R" 1830 cm-1 inductive effect more significant The inductive effect by the OR' oxygen atom is a slightly more significant contributor than the resonance effect by the OR' lone pair An extremely significant contributor... ...
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