H I G H - P R E S S U R E HIGH-RESOLUTION N U C L E A R M A G N E T I C R E S O N A N C E : P R E S S U R E DEPENDENCE OF THE HINDERED ROTATION OF THE A M I D E -
G R O U P IN HIGHER A M I D E S
H. Hauer, G. Volkel, E. Lang and H.-D. Ludemann
Institut fur Biophysik und physikalische Biochemie, Universitat Regensburg, Postfach 397, D-8400 Regensburg, Federal Republic of Germany
The s t u d y of the temperature dependence of the exchange broadened NMR-spectra has become the most prominent method f o r the d e t e r m i n a - t i o n o f the a c t i v a t i o n parameters AG*, AH*, AS*, i n t r a - and i n t e r - m o l e c u l a r rearrangements.
R e c e n t l y two groups (1,2) developed h i g h - p r e s s u r e h i g h - r e s o l u t i o n NMR c e l l s capable of o b t a i n i n g s p e c t r a i n wide temperature range a t p r e s s u r e s up t o 300 MPa w i t h a r e s o l u t i o n < 1 Hz, and a p p l i e d t h e s e c e l l s f o r the d e t e r m i n a t i o n of the a c t i v a t i o n volumes AV* o f i n o r g a - n i c r e a c t i o n s (2) and o r g a n i c i n t r a m o l e c u l a r rearrangements ( 3 ) . The f i r s t c l a s s of compounds s t u d i e d e x t e n s i v e l y i n our group, were the N,N-dimethylamides. The a c t i v a t i o n volume f o r the i n v e r s i o n o f
3 the dimethylaminogroup (R1 = CH^) i n most s o l v e n t s i s around +10 cm •
-1
mol ( 3 ) .
Two d i f f e r e n t e x p l a n a t i o n s have been g i v e n f o r t h i s magnitude and the p o s i t i v e s i g n of AV*. Le Noble (4) proposed, t h a t the r e d u c t i o n o f the d i p o l e - d i p o l e - a t t r a c t i o n i n the t r a n s i t i o n s t a t e i s r e s p o n s i b l e f o r the volume i n c r e a s e found. Our group proposed t h a t the e f f e c t i s b e s t d e s c r i b e d by a s i m p l e s t e r i c a l model (3) . I n o r d e r t o r o t a t e around the N-C(=0)-bond, the dimethylaminogroup r e q u i r e s t h a t a t o r o i d a l volume must be k e p t f r e e of o t h e r m o l e c u l e s . The d i s l o d g e - ment o f the s o l v e n t m o l e c u l e s d u r i n g t h i s r o t a t i o n i s the main cause
f o r the volume e f f e c t s o b s e r v e d .
N u c l e a r M a g n e t i c Resonance 795 The s t u d y o f the p r e s s u r e dependence o f u n s u b s t i t u t e d amides (R^=H) where the e l e c t r o s t r i c t i o n e f f e c t s are maximal and g e o m e t r i c a l e f - f e c t s m i n i m a l , and the study o f s y m m e t r i c a l N - a l k y l a m i d e s w i t h e t h y l - (R^=C2H5-) o r i s o p r o p y l g r o u p s ( Rq= 1 i C ^ H ^ , where e l e c t r o - s t r i c t i o n e f f e c t s are reduced and s t e r i c a l e f f e c t s enhanced, s h o u l d r e v e a l which o f the mechanisms i s r e s p o n s i b l e f o r the e f f e c t s ob- s e r v e d .
We have t h e r e f o r e s t u d i e d the p r e s s u r e and temperature dependence o f the p r o t o n h i g h r e s o l u t i o n NMR s p e c t r a o f N , N - d i i s o p r o p y l b e n z a m i d e
( D i P r B ) , N,N-diethylbenzamide (DEB), acetamide (AA) and formamide (FoA). Computer s i m u l a t i o n o f t h e complex exchange broadened A^B, A^B2 and A B Cn s p e c t r a , by a p p l i c a t i o n o f t h e DNMR 5 program, y i e l d s the i n v e r s i o n r a t e f o r t h e r o t a t i o n o f the amide group as f u n c t i o n o f p r e s s u r e and temperature. I n f i g . 1 t y p i c a l s p e c t r a f o r D i P r B a t
150 MPa a r e g i v e n . Table 1 compares t h e a c t i v a t i o n volumes d e r i v e d from t h e a n a l y s i s o f t h e s p e c t r a o f a l l amides w i t h some r e s u l t s ob- t a i n e d p r e v i o u s l y f o r two d i m e t h y l a m i d e s . The r e s u l t s a v a i l a b l e t o date c o r r o b o r a t e q u a l i t a t i v e l y t h e g e o m e t r i c a l model proposed f o r the d i m e t h y l a m i d e s : DEB and D i P r B do show r e l a t i v e l y l a r g e a c t i -
v a t i o n volumes w h i l e the e f f e c t s However, the r e s u l t s do r e v e a l a l s o the q u a n t i t a t i v e l i m i t a t i o n s o f the s i m p l e s t e r i c a l model proposed ( 3 ) . The a c t i v a t i o n volume o f t h e p r i m a r y found i n AA and FoA a r e much s m a l l e r .
amides can s t i l l be d e s c r i b e d w i t h s u f f i c i e n t a c c u r a c y by t h i s model.
The s u b s t i t u t i o n o f the amide p r o - tons by two e t h y l - r e s p . i s o p r o p y l - groups i n t r o d u c e s two c o n f o r m a t i o n a l l y f l e x i b l e groups i n t o t h e r o t a t i n g m o i e t y . The p r e f e r r e d t o r s i o n a n g l e s
100 Hz H0
o f these groups around t h e N-C bond are unknown, and i t i s thus impos- s i b l e t o extend t h e model c a l c u l a - F i g . 1: T-dependence o f t h e
DiPrB p r o t o n s p e c t r a a t 150 MPa
t i o n s t o these groups. T h e o r e t i c a l l y one would e x p e c t at l e a s t a s l i g h t
796 H. Hauer et al.
i n c r e a s e o f AV* when t h e dimethylaminogroup i s r e p l a c e d by a d i e t h y l - aminogroup. E x p e r i m e n t a l l y a s l i g h t decrease i s observed when DMB and DEB a r e compared.
TABLE 1
AG and AV of the Amide I n v e r s i o n i n Symmetrical Amides
R1 R2 S o l v e n t Cone.
(%w/w)
AG* (5MPa) k J - m o l "1 L i t .
AV*
cm^ -mol-1
CH3 CH3 Acetone 20 75 75 10.0
C6H5 CH3
n 62.5 62 8.6
_ II _
C2H5
_ ii _ H
61 - 7.4
_ I I _ C H ( C H3)2
_ ti _ II
57 - 5.4
CH3 H Ace tone/DMF
(1:1)
it 71 70 2.2
H H Acetone 10 69 74. 5 2.3
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