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The Raman spectra of the hydrochloride salts of N-methylacetamide
Spectrochimicakcta, 1966,Vol. 22, pp. 1125 to 1129. PergamonPress Ltd. Printed in NorthernIreland
The Raman spectra of the hydrochloride salts of N-methylacetamide O. D. :BONNER, K . W. BUI~ZL a n d G. B. WOOLSEY Department of Chemistry, University of South Carolina, Columbia, South Carolina (Received 15 July 1965) A b s t r a c t - - T h e Raman spectra of the 1 : 1 and the 2 : 1 salt of N-methylacetamide.HC1 reveal that the lone pair of nitrogen electrons is involved in bonding in both cases. However, while these electrons in the 1 : 1 salt are in a bond with a proton to give CHsCON+H~CHS, they are in the
C - - N bond in the case of the 2 : 1 salt, where the proton goes to the oxygen atom of the N-methylacetamide molecule. A possible structure for the 2 : 1 salt is proposed. l. INTRODUCTION •-METHYLACETAMLDE ( ~ ) iS k n o w n t o f o r m t h e following t w o t y p e s o f salts w i t h h y d r o c h l o r i d e : t h e 1:1 salt C H s C O N H C H a ' H C 1 a n d t h e 2:1 salt [CHsCONHCHs]2.HC1 [1, 2]. A p r e p a r a t i o n o f these t w o salts reveals t h a t t h e 2: 1 salt is f o r m e d p r e f e r e n t i a l l y c o m p a r e d to t h e 1 : 1 salt which releases HC1 e v e n a t r o o m t e m p e r a t u r e to some e x t e n t . T h e g r e a t e r s t a b i l i t y o f t h e 2 : 1 salt is also i n d i c a t e d b y its higher m e l t i n g p o i n t (87-89°C) c o m p a r e d to t h e m e l t i n g p o i n t o f t h e 1 : 1 salt a t 69.4-76.2°C. Since n o t h i n g a b o u t t h e s t r u c t u r e of these salts is k n o w n , t h e i r R a m a n s p e c t r a were m e a s u r e d a n d c o m p a r e d w i t h t h e s p e c t r u m o f p u r e N M A a t t h e s a m e t e m perature (~32°C). 2. EXPERIMENTAL D'ALELIO [1] r e p o r t e d t h a t t h e 1:1 salt is f o r m e d b y b u b b l i n g d r y H C I t h r o u g h a solution o f N M A in a n o n p o l a r solvent while in absence o f a n y solvent t h e 2:1 salt results. W e f o u n d t h a t t h e 2 : 1 salt can be o b t a i n e d easily b y this m e t h o d while t h e 1 : 1 salt results o n l y b y b u b b l i n g t h e d r y HC1 gas v e r y v i g o r o u s l y t h r o u g h a v e r y dilute solution of N M A in benzene. H o w e v e r , if t h e solution is n o t dilute enough, or if t h e stirring is n o t efficient, t h e 2: 1 salt will result despite t h e use of a n o n p o l a r solvent. T h e salts t h u s o b t a i n e d were w a s h e d w i t h a c e t o n e to r e m o v e excess N M A a n d t h e n dried carefully. N M A was purified b y zone refining a n d h a d a m e l t i n g p o i n t of 30.5°C. T h e R a m a n s p e c t r a were o b t a i n e d w i t h a C a r y Model-81 R a m a n S p e c t r e p h o t o m e t e r . F o r liquid N M A t h e s t a n d a r d C a r y 7 m m cell w i t h a v o l u m e of 5 m l was used, while for t h e solid s a m p l e s a cell similar to t h e one described b y BUSE¥ a n d KELLER [3] was e m p l o y e d . [1] G. D'A~ELIO, J. Am. Chem. Soc. 59, 109 (1937). [2] F. F. BLICKE and J. H. BURCEmCLTER,J. Am. Chem. Soc. 64, 451 (1942). [3] 1%. H. BUSEY and O. L. :KELLER, JR. J. Chem. Phys. 41, 215 (1954). 1125
1126
0. D.
BONNER,K .
W . BUNZL a n d G. B . WOOLSEY
3. RESULTS AND DISCUSSIO~ The R a m a n spectrum of pure NMA has been measured b y MIZUS~IMA [4] and M~AZAWA [5], who have assigned the observed bands. Since they agree well with our values we used their assignments in Table 1 which gives the R a m a n spectra of NMA and the salts. Table
1 : 1 Salt 90 170 -
-
-
-
628(11)
1. R a m a n
2: 1 Salt 120 --
-
-610(3)
820(7}
665(6) 825(3)
878(15)
910(8)
-
-
9 0 5 ( 4 )
--
-
-
spectra of the 1 : 1 and 2 : 1 salt of N-methylacetamide hydrochloride and of l~-methylacetamide NMA ---
A s s i g n m e n t for NMA "( J
291(1) 435(3) 630(6)
Lattice vibrations 7 0 % CI-Is--N--C bending; 30% O = G - - N b e n d i n g 70% C H s - - C in p l a n e rocking Amide IV
--
-
881(8) -
4 0 % CHn--C stretching; 2 0 % O~----C--N bending
-
--
988(1)
C~[s(--C ) r e e k i n g *
1160(4)
1160(3)
1290(7)
--
1305(6)
C H s ( - - N ) rocking* Amide III
1350(3)
1348(9)
1378(3)
1395(3)
1395(8)
1412(4)
-
-
CHs(--C) sym. b e n d i n g CHs(---N) sym. b e n d i n g
--
1450(5)
1450(3)
CHs(--C), C H a ( - - N ) asym. b e n d i n g
1690(6b)
1595(4)
1655(5)
Amide I
--
2756(4)
2725(1)
2790(7)
2788(5)
2810(2)
-
-
2815(3)
--
--
2920(12)
2940(25)
2952(25)
2935(25)
2985(5)
2995(9)
2994(9)
3020(6)
--
3042(7) ---
3050(5) -3200(2b)
2900(3) CH s sym. s t r e t c h i n g CH s a s y m . s t r e t c h i n g
--3080(2b) "~ 3300(3bb) )
F e r m i resonance doublet i n v o l v i n g A m i d e I I a n d N H • • • O stretching.
* C H s ( - - N ) a n d CHs(--C ) denote m e t h y l groups a t t a c h e d to nitrogen a n d carbon atoms, respectively'.
We will discuss the spectra from the following three points of view: (a) The change of the vibrations of the CH3(--N) group in the two forms of the salts compared to NMA. BRAV~HOLTZ [6] showed that the vibrations of this Methyl group depend strongly on modifications which involve the lone pair of electrons of the nitrogen atom and that they can be used as a test to decide whether or not this lone pair of electrons is involved in bonding. (b) The change of the C ~ O stretching frequency in the salts compared to the C = O stretching frequency in NMA. (c) The change of the N H stretching frequency in the salts compared to that in NMA.
The R a m a n spectra of the hydrochloride salts of N-methylacetamide
1127
3.1 The N-Methylacetamide spectrum A comparison of the spectra from MIZUSHIMA [4] and MIYAZAWA [5] shows agreement for most of the bands. However, while MLZUSHI~L~[4] reports two weak bands at 2808 and 2900 cm -1, MIYAZAWA [5] does not report them. Since we also observe these two bands at 2810 and 2900 cm -1 quite clearly, there should be no doubt about their reality. We can explain them if we keep in mind t h a t the vibrations of the two Methyl groups CH3(--C ) and CH3(---N ) in NMA need not be identical. This has been shown by BRAUN~OLTZ [6] who discusses this fact in detail for compounds having CHa(--N ) groups and where the nitrogen atom retains its lone pair of electrons. KOHLRAVSC~[7] has suggested t h a t in all these cases the observed bands at 2815 and 2880 cm -1 constitute a Fermi resonance doublet which arises from resonance of the C - - H stretching fundamental at 2820 em -1 and the overtone of the symmetric methyl deformation mode of 1430 cm -1. There is no reason why this should not happen as well with the CHa(--N) group in NMA. The two resonance structures of NMA CHaC--~ONHCH3 CH30-C-N+HCH3 (I)
(II)
show us t h a t the lone pair of the nitrogen electrons is only partially delocalized and t h a t it still belongs to some extent to the nitrogen atom. We therefore believe t h a t our observed bands at 2810 and 2900 cm -~ can be explained in the same way; i.e. as a Fermi resonance doublet involving the overtone of the sym. Methyl deformation mode (2 × 1412 see Table 1) and the C - - H stretch fundamental which is about at 2820 cm -~. MZYAZAWA[5] assigns the two bands at 2994 and 2935 to the asymmetric and the symmetric stretching vibration of the CH a group; but this Methyl group is now obviously the CHa(--C) group, since these two bands are usually observed for all aliphatic CH a groups. 3.2 The 1 : 1 salt CHaCONHCHa'HCI I n the range of the stretching vibrations of the Methyl groups we observe here the bands at 2790, 2940 and 2985 cm -1. Since we do not expect the CHa(--C ) group to change during the salt formation we assign here again the two bands at 2985 and 2940 cm -~ to the asymmetric and symmetric stretching vibration of this methyl group. We already mentioned t h a t the reason for the two bands at 2900 and 2810 cm -1 in ~q~CIAis the presence of the lone pair of the nitrogen electrons. From the absence of these bands in the 1 : 1 salt we conclude then t h a t this lone pair of electrons is now involved in a bond. This behavior has been observed by WXLDRON [8] who reports data for the methylammonium ion and its N-deutero-derivative. The involvement of the lone electron pair in bond formation always causes a shift of the C - - H stretching [4] [5] [6J [7] [8]
S. 1Vlizusnz~ et al. J . A m . Chem. Soc. 72, 3490 (1950). T. MI~'AZAWAet al. J . Chem. P h y s . 29, 611 (1958). J. T. BRAUNHOLTZ et al. J . Chem. Soc. 2780 (1958). K. W. F. KO~LRAUSCH, R a m a n Speetren, p. 216, Becker and Erler, Leipzig (1943). R. I). WALDRON, J . Chem. P h y s . 21, 734 (1953).
1128
O.D. BONNER, K. ~ . BU~ZL and G. B. WOOLSEY
f u n d a m e n t a l at 2820 cm -1 to a v a l u e of a b o u t 100 w a v e n u m b e r s greater. I n t h e case of the 1 : 1 salt it would be shifted so close to t h e strong b a n d of t h e s y m m e t r i c s t r e t c h i n g v i b r a t i o n of t h e C H a ( - - C ) g r o u p t h a t it could n o t be detected. H o w e v e r , while in N M A t h e 2820 b a n d i n t e r a c t s b y F e r m i r e s o n a n c e w i t h t h e o v e r t o n e of t h e C H a ( - - N ) d e f o r m a t i o n m o d e (2 x 1412 cm-1), this should n o t be possible in t h e 1 : 1 salt, due to t h e u p w a r d shift of t h e 2820 c m -1 b a n d . W e therefore e x p e c t to o b s e r v e in t h e 1 : 1 salt this o v e r t o n e w i t h o u t a n y p e r t u r b a t i o n . T h e C H a ( - - N ) d e f o r m a t i o n m o d e in t h e salt is a t 1395 c m -1 a n d t h e corresponding overt o n e is t h e n a t 2790 c m -x. Since we do indeed o b s e r v e a b a n d a t this w a v e n u m b e r , we assign it to this o v e r t o n e . N e x t we consider t h e C---~O s t r e t c h i n g m o d e which a b s o r b s a t 1655 c m -1 for ~ M A . I n t h e 1 : 1 salt it is shifted to 1690 c m -x, indicating t h a t t h e force c o n s t a n t a n d w i t h it t h e s t r e n g t h of t h e C = O b o n d has increased. F r o m the r e s o n a n c e s t r u c t u r e s of N M A we see t h a t a n y c o n t r i b u t i o n of S t r u c t u r e I I w e a k e n s t h e C---~O b o n d a n d we t h e r e f o r e a t t r i b u t e t h e o b s e r v e d increase of t h e C = O b o n d s t r e n g t h to a decrease of t h e c o n t r i b u t i o n of resonance S t r u c t u r e I I . This, t o g e t h e r w i t h the f a c t t h a t t h e lone pair of t h e n i t r o g e n electrons is i n v o l v e d in b o n d i n g leads us to t h e result t h a t t h e p r o t o n of the h y d r o c h l o r i d e is b o n d e d to t h e n i t r o g e n a t o m of the N M A molecule, for t h e n r e s o n a n c e S t r u c t u r e I I is no longer possible a n d t h e o b s e r v e d stiffening of t h e C ~ O b o n d should occur. F u r t h e r evidence for t h e presence of t h e NH2+ g r o u p can be o b t a i n e d f r o m t h e a b s o r p t i o n b a n d s a t w a v e n u m b e r s g r e a t e r t h a n 3000 cm -1. MIYAZAWX [9] h a s s h o w n t h a t the t w o b r o a d b a n d s a t 3080 a n d 3300 c m -~ in N M A are a F e r m i reson a n c e d o u b l e t which involve t h e i n t e r a c t i o n o f t h e o v e r t o n e of t h e A m i d e I I b a n d a n d the s t r e t c h i n g v i b r a t i o n of !qH • • • 0 in a h y d r o g e n b o n d . I n t h e case of t h e 1 : 1 salt we find t w o b a n d s a t w a v e n u m b e r s g r e a t e r t h a n 3000 cm -x. A v e r y s h a r p b a n d a t 3042 a n d a m o d e r a t e l y b r o a d b a n d a t 3020 c m -~. Since t h e NH2+ g r o u p is k n o w n to a b s o r b in a b r o a d b a n d a t a r o u n d 3000 e m -1 (e.g. d i - n - b u t y l a m i n e h y d r o c h l o r i d e [10] a t 2986 c m -1) we a t t r i b u t e o u r b a n d a t 3020 c m -1 to t h e NH2+ s t r e t c h i n g mode. T h e b a n d a t 3042 c m -1 is p r o b a b l y a c o m b i n a t i o n b a n d or a n o v e r t o n e . T h e corresponding NH2 + d e f o r m a t i o n m o d e which should b e b e t w e e n 1600 a n d 1700 cm -1 could n o t be d e t e c t e d as a s e p a r a t e b a n d . H o w e v e r , t h e f a c t t h a t t h e b a n d a t 1690 c m -1 (resulting f r o m t h e C ~ O s t r e t c h i n g f r e q u e n c y ) is so u n u s u a l l y b r o a d indicates s t r o n g l y t h a t t h e N H 2 + d e f o r m a t i o n m o d e is also i n v o l v e d in this b a n d . 3.3 T h e 2:1 salt [CHaCOHNCHa]2-HC1 While we f o u n d in t h e 1 : 1 salt t h a t t h e C ~ 0 s t r e t c h i n g f r e q u e n c y was shifted t o higher w a v e n u m b e r s in c o m p a r i s o n to NMA, we find n o w t h a t it is shifted d o w n t o 1595 c m -1. This obviously indicates a decrease in t h e s t r e n g t h of t h e C = O b o n d c o m p a r e d to t h e one in NMA. This can be e x p l a i n e d if we a s s u m e t h a t t h e p r o t o n in t h e 2: 1 salt is b o n d e d to t h e o x y g e n a t o m o f each second N M A molecule. Since for these N M A molecules t h e r e s o n a n c e S t r u c t u r e I is no longer possible, we e x p e c t t h e m to h a v e o n l y r e s o n a n c e S t r u c t u r e H w i t h a N ~ C double b o n d involving t h e [9] T. MIYAZAWA,J. ~Iol. Spectroscopy 4, 168 (1960). [10] C. BRISE~TE and C. SANDORFY,Can. J. Chem. 38, 34 (1960).
The tCaman spectra of the hydrochloride salts of N-methylacetamide
1129
lone pair of the nitrogen electrons. F u r t h e r m o r e , from the observation of the b r o a d b a n d at 3200 cm -1 we conclude t h a t we h a v e O - - H • • • N and N - - H • • • 0 h y d r o g e n bonds present. This is indicated in Fig. 1 which shows a possible s t r u c t u r e for t h e 2: 1 salt :
--"\ ~
CH3
/
N--C~.
cLe\
CH3
0
c. 14
/
3
".. "l',y.c~ ,""N j 0/3
CH3
~/c~®
"'O----H--N~ ,~C--O--H--CH~
Fig. 1. Proposed structure for the 2 : 1 salt [CHaCONHCHa]s.HC1.
W e indicated t h a t t h e vibrations of the C H 3 ( - - N ) group are v e r y sensitive to t h e presence o f the lone pair of the nitrogen electrons. I f our assumed s t r u c t u r e is correct we e x p e c t t h e u n p e r t u r b a t e d C - - H stretching f u n d a m e n t a l of the CH3(--N) group a t 2820 cm -1 to shift to a higher wave n u m b e r b u t not as far as in the 1 : 1 salt since 50 p e r c e n t of the molecules in the 2: 1 salt h a v e their lone pair of nitrogen electrons o n l y p a r t l y i n v o l v e d in bonding via h y d r o g e n bonding. The observed b a n d a t 2920 cm -z fulfills these requirements. I t is also too far r e m o v e d f r o m the o v e r t o n e o f t h e C H 3 ( - - N ) d e f o r m a t i o n m o d e (2 × 1395 = 2790 cm -1) to i n t e r a c t w i t h it b y F e r m i resonance as in NMA. Therefore the observed b a n d at 2788 cm -1 can be readily identified as this o v e r t o n e o f the C H 3 ( - - N ) d e f o r m a t i o n mode. T h e two bands at 2952 a n d 2995 cm -1 are here again t h e s y m m e t r i c a n d the a s y m metric stretching f r e q u e n c y of the CHa(--C) group. The b a n d a t 3050 cm -1 m i g h t here again be the o v e r t o n e of t h e Amide I I band. W e m e n t i o n e d before the difficulty in the p r e p a r a t i o n o f the 1 : 1 salt from a benzene solution o f NMA if its c o n c e n t r a t i o n is n o t dilute enough, even t h o u g h HC1 is used in excess. F r o m the fact t h a t the 2 : 1 salt is usually formed, we can conclude t h a t the a c t i v a t i o n e n e r g y for the transition from the 2:1 salt to t h e 1:1 salt is appreciable. This is n o t unreasonable since we k n o w t h a t the s t r u c t u r e of the two salt forms, characterized b y the position of the proton, is entirely different. This m e a n s t h a t the transition from t h e 2: 1 salt to the 1 : 1 salt involves not only f u r t h e r addition of protons b u t also a r e a r r a n g e m e n t of t h e protons already present.
Acknowledgement--The authors are pleased to acknowledge the support of the N'ational Science Foundation under grant GP-3475.
The Raman spectra of the hydrochloride salts of N-methylacetamide O. D. :BONNER, K . W. BUI~ZL a n d G. B. WOOLSEY Department of Chemistry, University of South Carolina, Columbia, South Carolina (Received 15 July 1965) A b s t r a c t - - T h e Raman spectra of the 1 : 1 and the 2 : 1 salt of N-methylacetamide.HC1 reveal that the lone pair of nitrogen electrons is involved in bonding in both cases. However, while these electrons in the 1 : 1 salt are in a bond with a proton to give CHsCON+H~CHS, they are in the
C - - N bond in the case of the 2 : 1 salt, where the proton goes to the oxygen atom of the N-methylacetamide molecule. A possible structure for the 2 : 1 salt is proposed. l. INTRODUCTION •-METHYLACETAMLDE ( ~ ) iS k n o w n t o f o r m t h e following t w o t y p e s o f salts w i t h h y d r o c h l o r i d e : t h e 1:1 salt C H s C O N H C H a ' H C 1 a n d t h e 2:1 salt [CHsCONHCHs]2.HC1 [1, 2]. A p r e p a r a t i o n o f these t w o salts reveals t h a t t h e 2: 1 salt is f o r m e d p r e f e r e n t i a l l y c o m p a r e d to t h e 1 : 1 salt which releases HC1 e v e n a t r o o m t e m p e r a t u r e to some e x t e n t . T h e g r e a t e r s t a b i l i t y o f t h e 2 : 1 salt is also i n d i c a t e d b y its higher m e l t i n g p o i n t (87-89°C) c o m p a r e d to t h e m e l t i n g p o i n t o f t h e 1 : 1 salt a t 69.4-76.2°C. Since n o t h i n g a b o u t t h e s t r u c t u r e of these salts is k n o w n , t h e i r R a m a n s p e c t r a were m e a s u r e d a n d c o m p a r e d w i t h t h e s p e c t r u m o f p u r e N M A a t t h e s a m e t e m perature (~32°C). 2. EXPERIMENTAL D'ALELIO [1] r e p o r t e d t h a t t h e 1:1 salt is f o r m e d b y b u b b l i n g d r y H C I t h r o u g h a solution o f N M A in a n o n p o l a r solvent while in absence o f a n y solvent t h e 2:1 salt results. W e f o u n d t h a t t h e 2 : 1 salt can be o b t a i n e d easily b y this m e t h o d while t h e 1 : 1 salt results o n l y b y b u b b l i n g t h e d r y HC1 gas v e r y v i g o r o u s l y t h r o u g h a v e r y dilute solution of N M A in benzene. H o w e v e r , if t h e solution is n o t dilute enough, or if t h e stirring is n o t efficient, t h e 2: 1 salt will result despite t h e use of a n o n p o l a r solvent. T h e salts t h u s o b t a i n e d were w a s h e d w i t h a c e t o n e to r e m o v e excess N M A a n d t h e n dried carefully. N M A was purified b y zone refining a n d h a d a m e l t i n g p o i n t of 30.5°C. T h e R a m a n s p e c t r a were o b t a i n e d w i t h a C a r y Model-81 R a m a n S p e c t r e p h o t o m e t e r . F o r liquid N M A t h e s t a n d a r d C a r y 7 m m cell w i t h a v o l u m e of 5 m l was used, while for t h e solid s a m p l e s a cell similar to t h e one described b y BUSE¥ a n d KELLER [3] was e m p l o y e d . [1] G. D'A~ELIO, J. Am. Chem. Soc. 59, 109 (1937). [2] F. F. BLICKE and J. H. BURCEmCLTER,J. Am. Chem. Soc. 64, 451 (1942). [3] 1%. H. BUSEY and O. L. :KELLER, JR. J. Chem. Phys. 41, 215 (1954). 1125
1126
0. D.
BONNER,K .
W . BUNZL a n d G. B . WOOLSEY
3. RESULTS AND DISCUSSIO~ The R a m a n spectrum of pure NMA has been measured b y MIZUS~IMA [4] and M~AZAWA [5], who have assigned the observed bands. Since they agree well with our values we used their assignments in Table 1 which gives the R a m a n spectra of NMA and the salts. Table
1 : 1 Salt 90 170 -
-
-
-
628(11)
1. R a m a n
2: 1 Salt 120 --
-
-610(3)
820(7}
665(6) 825(3)
878(15)
910(8)
-
-
9 0 5 ( 4 )
--
-
-
spectra of the 1 : 1 and 2 : 1 salt of N-methylacetamide hydrochloride and of l~-methylacetamide NMA ---
A s s i g n m e n t for NMA "( J
291(1) 435(3) 630(6)
Lattice vibrations 7 0 % CI-Is--N--C bending; 30% O = G - - N b e n d i n g 70% C H s - - C in p l a n e rocking Amide IV
--
-
881(8) -
4 0 % CHn--C stretching; 2 0 % O~----C--N bending
-
--
988(1)
C~[s(--C ) r e e k i n g *
1160(4)
1160(3)
1290(7)
--
1305(6)
C H s ( - - N ) rocking* Amide III
1350(3)
1348(9)
1378(3)
1395(3)
1395(8)
1412(4)
-
-
CHs(--C) sym. b e n d i n g CHs(---N) sym. b e n d i n g
--
1450(5)
1450(3)
CHs(--C), C H a ( - - N ) asym. b e n d i n g
1690(6b)
1595(4)
1655(5)
Amide I
--
2756(4)
2725(1)
2790(7)
2788(5)
2810(2)
-
-
2815(3)
--
--
2920(12)
2940(25)
2952(25)
2935(25)
2985(5)
2995(9)
2994(9)
3020(6)
--
3042(7) ---
3050(5) -3200(2b)
2900(3) CH s sym. s t r e t c h i n g CH s a s y m . s t r e t c h i n g
--3080(2b) "~ 3300(3bb) )
F e r m i resonance doublet i n v o l v i n g A m i d e I I a n d N H • • • O stretching.
* C H s ( - - N ) a n d CHs(--C ) denote m e t h y l groups a t t a c h e d to nitrogen a n d carbon atoms, respectively'.
We will discuss the spectra from the following three points of view: (a) The change of the vibrations of the CH3(--N) group in the two forms of the salts compared to NMA. BRAV~HOLTZ [6] showed that the vibrations of this Methyl group depend strongly on modifications which involve the lone pair of electrons of the nitrogen atom and that they can be used as a test to decide whether or not this lone pair of electrons is involved in bonding. (b) The change of the C ~ O stretching frequency in the salts compared to the C = O stretching frequency in NMA. (c) The change of the N H stretching frequency in the salts compared to that in NMA.
The R a m a n spectra of the hydrochloride salts of N-methylacetamide
1127
3.1 The N-Methylacetamide spectrum A comparison of the spectra from MIZUSHIMA [4] and MIYAZAWA [5] shows agreement for most of the bands. However, while MLZUSHI~L~[4] reports two weak bands at 2808 and 2900 cm -1, MIYAZAWA [5] does not report them. Since we also observe these two bands at 2810 and 2900 cm -1 quite clearly, there should be no doubt about their reality. We can explain them if we keep in mind t h a t the vibrations of the two Methyl groups CH3(--C ) and CH3(---N ) in NMA need not be identical. This has been shown by BRAUN~OLTZ [6] who discusses this fact in detail for compounds having CHa(--N ) groups and where the nitrogen atom retains its lone pair of electrons. KOHLRAVSC~[7] has suggested t h a t in all these cases the observed bands at 2815 and 2880 cm -1 constitute a Fermi resonance doublet which arises from resonance of the C - - H stretching fundamental at 2820 em -1 and the overtone of the symmetric methyl deformation mode of 1430 cm -1. There is no reason why this should not happen as well with the CHa(--N) group in NMA. The two resonance structures of NMA CHaC--~ONHCH3 CH30-C-N+HCH3 (I)
(II)
show us t h a t the lone pair of the nitrogen electrons is only partially delocalized and t h a t it still belongs to some extent to the nitrogen atom. We therefore believe t h a t our observed bands at 2810 and 2900 cm -~ can be explained in the same way; i.e. as a Fermi resonance doublet involving the overtone of the sym. Methyl deformation mode (2 × 1412 see Table 1) and the C - - H stretch fundamental which is about at 2820 cm -~. MZYAZAWA[5] assigns the two bands at 2994 and 2935 to the asymmetric and the symmetric stretching vibration of the CH a group; but this Methyl group is now obviously the CHa(--C) group, since these two bands are usually observed for all aliphatic CH a groups. 3.2 The 1 : 1 salt CHaCONHCHa'HCI I n the range of the stretching vibrations of the Methyl groups we observe here the bands at 2790, 2940 and 2985 cm -1. Since we do not expect the CHa(--C ) group to change during the salt formation we assign here again the two bands at 2985 and 2940 cm -~ to the asymmetric and symmetric stretching vibration of this methyl group. We already mentioned t h a t the reason for the two bands at 2900 and 2810 cm -1 in ~q~CIAis the presence of the lone pair of the nitrogen electrons. From the absence of these bands in the 1 : 1 salt we conclude then t h a t this lone pair of electrons is now involved in a bond. This behavior has been observed by WXLDRON [8] who reports data for the methylammonium ion and its N-deutero-derivative. The involvement of the lone electron pair in bond formation always causes a shift of the C - - H stretching [4] [5] [6J [7] [8]
S. 1Vlizusnz~ et al. J . A m . Chem. Soc. 72, 3490 (1950). T. MI~'AZAWAet al. J . Chem. P h y s . 29, 611 (1958). J. T. BRAUNHOLTZ et al. J . Chem. Soc. 2780 (1958). K. W. F. KO~LRAUSCH, R a m a n Speetren, p. 216, Becker and Erler, Leipzig (1943). R. I). WALDRON, J . Chem. P h y s . 21, 734 (1953).
1128
O.D. BONNER, K. ~ . BU~ZL and G. B. WOOLSEY
f u n d a m e n t a l at 2820 cm -1 to a v a l u e of a b o u t 100 w a v e n u m b e r s greater. I n t h e case of the 1 : 1 salt it would be shifted so close to t h e strong b a n d of t h e s y m m e t r i c s t r e t c h i n g v i b r a t i o n of t h e C H a ( - - C ) g r o u p t h a t it could n o t be detected. H o w e v e r , while in N M A t h e 2820 b a n d i n t e r a c t s b y F e r m i r e s o n a n c e w i t h t h e o v e r t o n e of t h e C H a ( - - N ) d e f o r m a t i o n m o d e (2 x 1412 cm-1), this should n o t be possible in t h e 1 : 1 salt, due to t h e u p w a r d shift of t h e 2820 c m -1 b a n d . W e therefore e x p e c t to o b s e r v e in t h e 1 : 1 salt this o v e r t o n e w i t h o u t a n y p e r t u r b a t i o n . T h e C H a ( - - N ) d e f o r m a t i o n m o d e in t h e salt is a t 1395 c m -1 a n d t h e corresponding overt o n e is t h e n a t 2790 c m -x. Since we do indeed o b s e r v e a b a n d a t this w a v e n u m b e r , we assign it to this o v e r t o n e . N e x t we consider t h e C---~O s t r e t c h i n g m o d e which a b s o r b s a t 1655 c m -1 for ~ M A . I n t h e 1 : 1 salt it is shifted to 1690 c m -x, indicating t h a t t h e force c o n s t a n t a n d w i t h it t h e s t r e n g t h of t h e C = O b o n d has increased. F r o m the r e s o n a n c e s t r u c t u r e s of N M A we see t h a t a n y c o n t r i b u t i o n of S t r u c t u r e I I w e a k e n s t h e C---~O b o n d a n d we t h e r e f o r e a t t r i b u t e t h e o b s e r v e d increase of t h e C = O b o n d s t r e n g t h to a decrease of t h e c o n t r i b u t i o n of resonance S t r u c t u r e I I . This, t o g e t h e r w i t h the f a c t t h a t t h e lone pair of t h e n i t r o g e n electrons is i n v o l v e d in b o n d i n g leads us to t h e result t h a t t h e p r o t o n of the h y d r o c h l o r i d e is b o n d e d to t h e n i t r o g e n a t o m of the N M A molecule, for t h e n r e s o n a n c e S t r u c t u r e I I is no longer possible a n d t h e o b s e r v e d stiffening of t h e C ~ O b o n d should occur. F u r t h e r evidence for t h e presence of t h e NH2+ g r o u p can be o b t a i n e d f r o m t h e a b s o r p t i o n b a n d s a t w a v e n u m b e r s g r e a t e r t h a n 3000 cm -1. MIYAZAWX [9] h a s s h o w n t h a t the t w o b r o a d b a n d s a t 3080 a n d 3300 c m -~ in N M A are a F e r m i reson a n c e d o u b l e t which involve t h e i n t e r a c t i o n o f t h e o v e r t o n e of t h e A m i d e I I b a n d a n d the s t r e t c h i n g v i b r a t i o n of !qH • • • 0 in a h y d r o g e n b o n d . I n t h e case of t h e 1 : 1 salt we find t w o b a n d s a t w a v e n u m b e r s g r e a t e r t h a n 3000 cm -x. A v e r y s h a r p b a n d a t 3042 a n d a m o d e r a t e l y b r o a d b a n d a t 3020 c m -~. Since t h e NH2+ g r o u p is k n o w n to a b s o r b in a b r o a d b a n d a t a r o u n d 3000 e m -1 (e.g. d i - n - b u t y l a m i n e h y d r o c h l o r i d e [10] a t 2986 c m -1) we a t t r i b u t e o u r b a n d a t 3020 c m -1 to t h e NH2+ s t r e t c h i n g mode. T h e b a n d a t 3042 c m -1 is p r o b a b l y a c o m b i n a t i o n b a n d or a n o v e r t o n e . T h e corresponding NH2 + d e f o r m a t i o n m o d e which should b e b e t w e e n 1600 a n d 1700 cm -1 could n o t be d e t e c t e d as a s e p a r a t e b a n d . H o w e v e r , t h e f a c t t h a t t h e b a n d a t 1690 c m -1 (resulting f r o m t h e C ~ O s t r e t c h i n g f r e q u e n c y ) is so u n u s u a l l y b r o a d indicates s t r o n g l y t h a t t h e N H 2 + d e f o r m a t i o n m o d e is also i n v o l v e d in this b a n d . 3.3 T h e 2:1 salt [CHaCOHNCHa]2-HC1 While we f o u n d in t h e 1 : 1 salt t h a t t h e C ~ 0 s t r e t c h i n g f r e q u e n c y was shifted t o higher w a v e n u m b e r s in c o m p a r i s o n to NMA, we find n o w t h a t it is shifted d o w n t o 1595 c m -1. This obviously indicates a decrease in t h e s t r e n g t h of t h e C = O b o n d c o m p a r e d to t h e one in NMA. This can be e x p l a i n e d if we a s s u m e t h a t t h e p r o t o n in t h e 2: 1 salt is b o n d e d to t h e o x y g e n a t o m o f each second N M A molecule. Since for these N M A molecules t h e r e s o n a n c e S t r u c t u r e I is no longer possible, we e x p e c t t h e m to h a v e o n l y r e s o n a n c e S t r u c t u r e H w i t h a N ~ C double b o n d involving t h e [9] T. MIYAZAWA,J. ~Iol. Spectroscopy 4, 168 (1960). [10] C. BRISE~TE and C. SANDORFY,Can. J. Chem. 38, 34 (1960).
The tCaman spectra of the hydrochloride salts of N-methylacetamide
1129
lone pair of the nitrogen electrons. F u r t h e r m o r e , from the observation of the b r o a d b a n d at 3200 cm -1 we conclude t h a t we h a v e O - - H • • • N and N - - H • • • 0 h y d r o g e n bonds present. This is indicated in Fig. 1 which shows a possible s t r u c t u r e for t h e 2: 1 salt :
--"\ ~
CH3
/
N--C~.
cLe\
CH3
0
c. 14
/
3
".. "l',y.c~ ,""N j 0/3
CH3
~/c~®
"'O----H--N~ ,~C--O--H--CH~
Fig. 1. Proposed structure for the 2 : 1 salt [CHaCONHCHa]s.HC1.
W e indicated t h a t t h e vibrations of the C H 3 ( - - N ) group are v e r y sensitive to t h e presence o f the lone pair of the nitrogen electrons. I f our assumed s t r u c t u r e is correct we e x p e c t t h e u n p e r t u r b a t e d C - - H stretching f u n d a m e n t a l of the CH3(--N) group a t 2820 cm -1 to shift to a higher wave n u m b e r b u t not as far as in the 1 : 1 salt since 50 p e r c e n t of the molecules in the 2: 1 salt h a v e their lone pair of nitrogen electrons o n l y p a r t l y i n v o l v e d in bonding via h y d r o g e n bonding. The observed b a n d a t 2920 cm -z fulfills these requirements. I t is also too far r e m o v e d f r o m the o v e r t o n e o f t h e C H 3 ( - - N ) d e f o r m a t i o n m o d e (2 × 1395 = 2790 cm -1) to i n t e r a c t w i t h it b y F e r m i resonance as in NMA. Therefore the observed b a n d at 2788 cm -1 can be readily identified as this o v e r t o n e o f the C H 3 ( - - N ) d e f o r m a t i o n mode. T h e two bands at 2952 a n d 2995 cm -1 are here again t h e s y m m e t r i c a n d the a s y m metric stretching f r e q u e n c y of the CHa(--C) group. The b a n d a t 3050 cm -1 m i g h t here again be the o v e r t o n e of t h e Amide I I band. W e m e n t i o n e d before the difficulty in the p r e p a r a t i o n o f the 1 : 1 salt from a benzene solution o f NMA if its c o n c e n t r a t i o n is n o t dilute enough, even t h o u g h HC1 is used in excess. F r o m the fact t h a t the 2 : 1 salt is usually formed, we can conclude t h a t the a c t i v a t i o n e n e r g y for the transition from the 2:1 salt to t h e 1:1 salt is appreciable. This is n o t unreasonable since we k n o w t h a t the s t r u c t u r e of the two salt forms, characterized b y the position of the proton, is entirely different. This m e a n s t h a t the transition from t h e 2: 1 salt to the 1 : 1 salt involves not only f u r t h e r addition of protons b u t also a r e a r r a n g e m e n t of t h e protons already present.
Acknowledgement--The authors are pleased to acknowledge the support of the N'ational Science Foundation under grant GP-3475.