- Email: [email protected]
Steric inhibition of hydrogen-bonding in a secondary amide
Spectrochimica Acta, 1964, Vol. 20, pp. 1879 to 1882. Pergamon Press Ltd. Printed in Northern Ireland
RESEARCH NOTE
Steric inhibition of hydrogen-bonding in a secondary amide (Received 3 April 1964) A b s t r a c t - - S t e r i c i n h i b i t i o n of h y d r o g e n - b o n d i n g in alcohols [1-3] a n d p h e n o l s [ 4 - 6 ] h a s b e e n d e s c r i b e d in t h e l i t e r a t u r e . T h i s n o t e i l l u s t r a t e s a similar o c c u r r e n c e in a n N - m o n o s u b s t i t u t e d amide. THE i n f r a - r e d s p e c t r a of t h r e e c o m p o u n d s w i t h t h e g e n e r a l s t r u c t u r a l f o r m u l a R . C O N H . R were r e c o r d e d u s i n g a P e r k i n - E l m e r 12C s p e c t r o m e t e r c o n v e r t e d t o d o u b l e - p a s s o p e r a t i o n , a n d f i t t e d w i t h a l i t h i u m fluoride prism. T h e p o s s i b i l i t y of self-association o f t h e s e molecules t h r o u g h t h e p e p t i d e ( - - C O - - N I t - - ) l i n k was successively d e c r e a s e d b y a l i p h a t i e g r o u p s (R) of i n c r e a s i n g m o l e c u l a r v o l u m e a t t a c h e d t o b o t h ends. T h e c o m p o u n d s were N - m e t h y l a c e t a m i d e [R = - - ( C H a ) j, N-(isopropyl) i s o b u t y r a m i d e [R = - - C H ( C H a ) 2 ] , a n d N - ( t - b u t y l ) p i v a l a m i d e [R = --C(CHa)a]. Solutions of t h e s e m a t e r i a l s a t a p p r o x i m a t e m o l a r c o n c e n t r a t i o n s (c) of 0-5, 0.05 a n d 0.005 in c a r b o n t e t r a c h l o r i d e were m a d e u p a t r o o m t e m p e r a t u r e (22°C), a n d t h e i r i n f r a - r e d s p e c t r a r e c o r d e d b e t w e e n 3600 c m -1 a n d 2800 c m -1. T h e c o r r e s p o n d i n g cell l e n g t h s (1) of 0-03, 0.3 a n d 3.0 c m were c h o s e n so as t o give a b s o r p t i o n b a n d s of s u i t a b l e i n t e n s i t y , a n d so t h a t t h e p r o d u c t (e × 1) was c o n s t a n t . T h e s p e c t r a , c o n v e r t e d t o l i n e a r a b s o r b a n c e a n d r e c i p r o c a l c e n t i m e t r e scales are s h o w n in Fig. 1. T h e a b s o r p t i o n p e a k p o s i t i o n s for t h e s o l u t i o n s are listed in T a b l e 1, t o g e t h e r w i t h t h o s e for t h e solid a n d gaseous states, a n d t h e p r o p o s e d assignments. T h e m o n o m e r i c N H s t r e t c h i n g a b s o r p t i o n s a r e identified as t h e h i g h e s t f r e q u e n c y b a n d s i n t h e v a p o u r , a n d in s o l u t i o n b y t h e e n h a n c e m e n t of t h e i r r e l a t i v e i n t e n s i t y o n d i l u t i o n . O n t h e o t h e r h a n d , t h e a s s o c i a t e d N H s t r e t c h i n g b a n d s a t ~ 3 3 0 0 c m -1 ( ~ 3 4 0 0 c m -1 i n N - ( t - b u t y l ) p i v a l a m i d e ) b e c o n m p r o g r e s s i v e l y weaker, a n d also m o v e t o h i g h e r frequencies, o n d i l u t i o n . T h e a s s o c i a t i o n b a n d s a t ~ 3 1 0 0 c m -1 (not o b s e r v e d in N - ( t - b u t y l ) p i v a l a m i d e ) d i s a p p e a r comp l e t e l y a t c o n c e n t r a t i o n s n e a r 0.005 M. I n t h e solid s t a t e , t h e c o r r e s p o n d i n g a s s o c i a t e d N H s t r e t c h i n g b a n d s c ' m b e r e a d i l y identified b y c o m p a r i s o n w i t h t h e c o n c e n t r a t e d s o l u t i o n s p e c t r a . S o m e of t h e w e a k (in some cases, b r o a d ) a b s o r p t i o n s o b s e r v e d i n t h e solid s t a t e , a n d w h i c h p e r s i s t w i t h a p p r o x i m a t e l y e q u a l i n t e n s i t y o n d i l u t i o n in c a r b o n t e t r a c h l o r i d e c a n b e a c c o u n t e d for in t e r m s o f o v e r t o n e s or c o m b i n a t i o n t o n e s of f u n d a m e n t a l s a t lower frequencies. T h e w e a k b a n d a t ~ 3 2 2 0 c m -1 in t h e s o l u t i o n s p e c t r u m o f N - m e t h y l a e e t a m i d e r e m a i n s a l m o s t c o n s t a n t in i n t e n s i t y o n successive d i l u t i o n in c a r b o n t e t r a c h l o r i d e , b u t decreases in i n t e n s i t y o n p a r t i a l d e u t e r a t i o n , a n d c a n b e a c c o u n t e d for as a c o m b i n a t i o n t o n e of t h e i n t e n s e A m i d e I a n d A m i d e I I bands. O n t h e o t h e r h a n d , t h e b a n d s n e a r 3200 c m -1 in t h e s o l u t i o n s p e c t r a of N-(isopropyl) isob u t y r a m i d e a n d N - ( t - b u t y l ) p i v a l a m i d e do n o t d i m i n i s h a p p r e c i a b l y in i n t e n s i t y e i t h e r o n d i l u t i o n or p a r t i a l d e u t e r a t i o n , a n d p e r s i s t as weak, b r o a d b a n d s in t h e v a p o u r s t a t e . N o a s s i g n m e n t s have been made to these absorptions. T h e weak, b u t r e l a t i v e l y s h a r p b a n d a t 3275 c m -1 in t h e s p e c t r u m of N - ( t - b u t y l ) p i v a l a m i d e is a s s i g n e d t o t h e o v e r t o n e 2 × A m i d e I. T h i s a b s o r p t i o n does n o t d i m i n i s h in i n t e n s i t y e i t h e r o n d i l u t i o n or d e u t e r a t i o n . T h e c o r r e s p o n d i n g o v e r t o n e b a n d s in N - m e t h y l a c e t a m i d e a n d N-(isopropyl) i s o b u t y r a m i d e are o b s e r v e d in t h e v a p o u r p h a s e only, a t 3435 c m -1 a n d 3395 c m -1 respectively. 1879
1880
Research note
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1881
The broad intense absorption at ~ 3 3 0 0 cm -1 in the concentrated solution spectra of ~W-methylacetamide and N-(isopropyl) isobutyramide has been observed previously for a number of N-monosubstituted amides, and interpreted (assuming all the molecules to be in the t r a n s O
II ( - - C - - N - - ) configuration) as being due to a~sociation through hydrogen bonding between CO I H and N H links of neighbouring molecules [7] thus forming long polymeric chains. With increasing dilution these chains are broken, so that lower polymers are formed and the absorption peak Waves
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Fig. 1. The infra-red spectra of three N-monosubstituted amides (R.CONtt. R) in solution, under the conditions indicated in each diagram. moves to a higher frequency value. At, 0.5 M concentration in carbon tetraehloride, both N-methylaeetamide and N-(isopropyl) isobutyramide show this band with similar absorbanqe values and peak positions. The approximate bandwidth at half-height, measured without band separation, is considerably greater in N-methylaeetamide ( ~ 1 1 0 em -1) than in N-(isopropyl) isobutyramide (~,70 em -1) and the possible relation between the width of this band and sterie factors affecting hydrogen bonding deserves further study. The corresponding association band
1882
l~esearch note
in the spectrum of N-(t-butyl) pivalamide, where steric hindra.llee is at a maximum, is very weak (see Fig. 1) and its width at half-height difficult to assess. I t is of the order of 50 cm -1.
Acknowledgements--The author thanks Professor 1=$. C. LoRD for helpful discussions and the Massachusetts Institute of Technology for the use of facilities in the Spectroscopy Laboratory. This investigation has been supported by grants to the Chester B e a t t y Research Institute (Institute of Cancer Research: Royal Cancer Hospital) from the Medical Research Council, the British Empire Cancer Campaign and the National Cancer Institute of the National Institutes of Health, U.S. Public Health Service. The work was completed during the tenure of an Eleanor l~oosevelt International Cancer Fellowship, for which the author expresses his thanks to the International Union Against Cancer. Chester Beatty Research Institute Fulham Road London S. W.3, England [1] [2] [3] [4] [5] [6] [7]
R. LTJMLEY JONI~S
F . A . SMITH and E. C. CREITZ, J. Research N . B . S . 46, 145 (1951). .N.D. COGGESgALLand E. L. SAIER,J. Am. Chem. Soc. 78, 5414 (1951). L. P. KuH~, J. A ~ . Chem. Soc. 74, 2492 (1952), 86, 5950 (1958). 1~. D. COGC~ESHALL,J. Am. Chem. See. 69, 1620 (1947). W. C. SEAlCSand L. J. KITCH~,br, J. Am. Chem. Soc. 71, 4110 (1949). L. J. BELLAMY and l~. L. WILLIAMS, Prec. Roy. Soc. (London) A254, 119 (1960). L. J. B~LI~A~Y, The Infrared Spectra of Complex Molecules, pp. 206-209 (2nd Edition), London, Methuen: New York, J o h n Wiley (1958).
RESEARCH NOTE
Steric inhibition of hydrogen-bonding in a secondary amide (Received 3 April 1964) A b s t r a c t - - S t e r i c i n h i b i t i o n of h y d r o g e n - b o n d i n g in alcohols [1-3] a n d p h e n o l s [ 4 - 6 ] h a s b e e n d e s c r i b e d in t h e l i t e r a t u r e . T h i s n o t e i l l u s t r a t e s a similar o c c u r r e n c e in a n N - m o n o s u b s t i t u t e d amide. THE i n f r a - r e d s p e c t r a of t h r e e c o m p o u n d s w i t h t h e g e n e r a l s t r u c t u r a l f o r m u l a R . C O N H . R were r e c o r d e d u s i n g a P e r k i n - E l m e r 12C s p e c t r o m e t e r c o n v e r t e d t o d o u b l e - p a s s o p e r a t i o n , a n d f i t t e d w i t h a l i t h i u m fluoride prism. T h e p o s s i b i l i t y of self-association o f t h e s e molecules t h r o u g h t h e p e p t i d e ( - - C O - - N I t - - ) l i n k was successively d e c r e a s e d b y a l i p h a t i e g r o u p s (R) of i n c r e a s i n g m o l e c u l a r v o l u m e a t t a c h e d t o b o t h ends. T h e c o m p o u n d s were N - m e t h y l a c e t a m i d e [R = - - ( C H a ) j, N-(isopropyl) i s o b u t y r a m i d e [R = - - C H ( C H a ) 2 ] , a n d N - ( t - b u t y l ) p i v a l a m i d e [R = --C(CHa)a]. Solutions of t h e s e m a t e r i a l s a t a p p r o x i m a t e m o l a r c o n c e n t r a t i o n s (c) of 0-5, 0.05 a n d 0.005 in c a r b o n t e t r a c h l o r i d e were m a d e u p a t r o o m t e m p e r a t u r e (22°C), a n d t h e i r i n f r a - r e d s p e c t r a r e c o r d e d b e t w e e n 3600 c m -1 a n d 2800 c m -1. T h e c o r r e s p o n d i n g cell l e n g t h s (1) of 0-03, 0.3 a n d 3.0 c m were c h o s e n so as t o give a b s o r p t i o n b a n d s of s u i t a b l e i n t e n s i t y , a n d so t h a t t h e p r o d u c t (e × 1) was c o n s t a n t . T h e s p e c t r a , c o n v e r t e d t o l i n e a r a b s o r b a n c e a n d r e c i p r o c a l c e n t i m e t r e scales are s h o w n in Fig. 1. T h e a b s o r p t i o n p e a k p o s i t i o n s for t h e s o l u t i o n s are listed in T a b l e 1, t o g e t h e r w i t h t h o s e for t h e solid a n d gaseous states, a n d t h e p r o p o s e d assignments. T h e m o n o m e r i c N H s t r e t c h i n g a b s o r p t i o n s a r e identified as t h e h i g h e s t f r e q u e n c y b a n d s i n t h e v a p o u r , a n d in s o l u t i o n b y t h e e n h a n c e m e n t of t h e i r r e l a t i v e i n t e n s i t y o n d i l u t i o n . O n t h e o t h e r h a n d , t h e a s s o c i a t e d N H s t r e t c h i n g b a n d s a t ~ 3 3 0 0 c m -1 ( ~ 3 4 0 0 c m -1 i n N - ( t - b u t y l ) p i v a l a m i d e ) b e c o n m p r o g r e s s i v e l y weaker, a n d also m o v e t o h i g h e r frequencies, o n d i l u t i o n . T h e a s s o c i a t i o n b a n d s a t ~ 3 1 0 0 c m -1 (not o b s e r v e d in N - ( t - b u t y l ) p i v a l a m i d e ) d i s a p p e a r comp l e t e l y a t c o n c e n t r a t i o n s n e a r 0.005 M. I n t h e solid s t a t e , t h e c o r r e s p o n d i n g a s s o c i a t e d N H s t r e t c h i n g b a n d s c ' m b e r e a d i l y identified b y c o m p a r i s o n w i t h t h e c o n c e n t r a t e d s o l u t i o n s p e c t r a . S o m e of t h e w e a k (in some cases, b r o a d ) a b s o r p t i o n s o b s e r v e d i n t h e solid s t a t e , a n d w h i c h p e r s i s t w i t h a p p r o x i m a t e l y e q u a l i n t e n s i t y o n d i l u t i o n in c a r b o n t e t r a c h l o r i d e c a n b e a c c o u n t e d for in t e r m s o f o v e r t o n e s or c o m b i n a t i o n t o n e s of f u n d a m e n t a l s a t lower frequencies. T h e w e a k b a n d a t ~ 3 2 2 0 c m -1 in t h e s o l u t i o n s p e c t r u m o f N - m e t h y l a e e t a m i d e r e m a i n s a l m o s t c o n s t a n t in i n t e n s i t y o n successive d i l u t i o n in c a r b o n t e t r a c h l o r i d e , b u t decreases in i n t e n s i t y o n p a r t i a l d e u t e r a t i o n , a n d c a n b e a c c o u n t e d for as a c o m b i n a t i o n t o n e of t h e i n t e n s e A m i d e I a n d A m i d e I I bands. O n t h e o t h e r h a n d , t h e b a n d s n e a r 3200 c m -1 in t h e s o l u t i o n s p e c t r a of N-(isopropyl) isob u t y r a m i d e a n d N - ( t - b u t y l ) p i v a l a m i d e do n o t d i m i n i s h a p p r e c i a b l y in i n t e n s i t y e i t h e r o n d i l u t i o n or p a r t i a l d e u t e r a t i o n , a n d p e r s i s t as weak, b r o a d b a n d s in t h e v a p o u r s t a t e . N o a s s i g n m e n t s have been made to these absorptions. T h e weak, b u t r e l a t i v e l y s h a r p b a n d a t 3275 c m -1 in t h e s p e c t r u m of N - ( t - b u t y l ) p i v a l a m i d e is a s s i g n e d t o t h e o v e r t o n e 2 × A m i d e I. T h i s a b s o r p t i o n does n o t d i m i n i s h in i n t e n s i t y e i t h e r o n d i l u t i o n or d e u t e r a t i o n . T h e c o r r e s p o n d i n g o v e r t o n e b a n d s in N - m e t h y l a c e t a m i d e a n d N-(isopropyl) i s o b u t y r a m i d e are o b s e r v e d in t h e v a p o u r p h a s e only, a t 3435 c m -1 a n d 3395 c m -1 respectively. 1879
1880
Research note
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Research note
1881
The broad intense absorption at ~ 3 3 0 0 cm -1 in the concentrated solution spectra of ~W-methylacetamide and N-(isopropyl) isobutyramide has been observed previously for a number of N-monosubstituted amides, and interpreted (assuming all the molecules to be in the t r a n s O
II ( - - C - - N - - ) configuration) as being due to a~sociation through hydrogen bonding between CO I H and N H links of neighbouring molecules [7] thus forming long polymeric chains. With increasing dilution these chains are broken, so that lower polymers are formed and the absorption peak Waves
3600
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cm
3FO0
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2900
~-80G
,
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Fig. 1. The infra-red spectra of three N-monosubstituted amides (R.CONtt. R) in solution, under the conditions indicated in each diagram. moves to a higher frequency value. At, 0.5 M concentration in carbon tetraehloride, both N-methylaeetamide and N-(isopropyl) isobutyramide show this band with similar absorbanqe values and peak positions. The approximate bandwidth at half-height, measured without band separation, is considerably greater in N-methylaeetamide ( ~ 1 1 0 em -1) than in N-(isopropyl) isobutyramide (~,70 em -1) and the possible relation between the width of this band and sterie factors affecting hydrogen bonding deserves further study. The corresponding association band
1882
l~esearch note
in the spectrum of N-(t-butyl) pivalamide, where steric hindra.llee is at a maximum, is very weak (see Fig. 1) and its width at half-height difficult to assess. I t is of the order of 50 cm -1.
Acknowledgements--The author thanks Professor 1=$. C. LoRD for helpful discussions and the Massachusetts Institute of Technology for the use of facilities in the Spectroscopy Laboratory. This investigation has been supported by grants to the Chester B e a t t y Research Institute (Institute of Cancer Research: Royal Cancer Hospital) from the Medical Research Council, the British Empire Cancer Campaign and the National Cancer Institute of the National Institutes of Health, U.S. Public Health Service. The work was completed during the tenure of an Eleanor l~oosevelt International Cancer Fellowship, for which the author expresses his thanks to the International Union Against Cancer. Chester Beatty Research Institute Fulham Road London S. W.3, England [1] [2] [3] [4] [5] [6] [7]
R. LTJMLEY JONI~S
F . A . SMITH and E. C. CREITZ, J. Research N . B . S . 46, 145 (1951). .N.D. COGGESgALLand E. L. SAIER,J. Am. Chem. Soc. 78, 5414 (1951). L. P. KuH~, J. A ~ . Chem. Soc. 74, 2492 (1952), 86, 5950 (1958). 1~. D. COGC~ESHALL,J. Am. Chem. See. 69, 1620 (1947). W. C. SEAlCSand L. J. KITCH~,br, J. Am. Chem. Soc. 71, 4110 (1949). L. J. BELLAMY and l~. L. WILLIAMS, Prec. Roy. Soc. (London) A254, 119 (1960). L. J. B~LI~A~Y, The Infrared Spectra of Complex Molecules, pp. 206-209 (2nd Edition), London, Methuen: New York, J o h n Wiley (1958).