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Vibrating molecular model study of halogenated alkane rotational isomers
Spectrochimica et CosmochimicaActa, 1964, Vol. 20, pp. 1843 to 1846. Pergamon Press Ltd. Printed in Northern Ireland
Vibrating molecular model study of halogenated alkane rotational isomers N. B. COLTHUP Central Research Division, American Cyanamid Stamford, Connecticut
Company,
(Received 28 April 1964) A b s t r a c t - - T h i s s t u d y r e v e a l s t h a t m o s t of t h e difference in C--C1 s t r e t c h i n g f r e q u e n c i e s in r o t a t i o n a l i s o m e r s of c h l o r o a l k a n e s is c a u s e d b y a c h a n g e in C - - C - - C b e n d i n g i n t e r a c t i o n .
CHLOROALKANES (and also bromo- and iodoalkanes) have long been k n o w n to exist as r o t a t i o n a l isomers which h a v e distinctly different infrared C--C1 stretching frequencies. These have been most r e c e n t l y studied b y SHIP~AN, FOLT and KR~MM [1] who m a d e close correlations for p r i m a r y , secondary, and t e r t i a r y chloroalkanes. T h e y f o u n d t h a t when a carbon a t o m was trans to the chlorine (Reference [l]), the f r e q u e n c y was r o u g h l y 10 per cent higher t h a n the f r e q u e n c y for the r o t a t i o n a l isomer where a h y d r o g e n was trans to the chlorine. I n this p a p e r the n - b u t y l chloride isomer where the non h y d r o g e n atoms zig-zag in one plane is called the trans isomer (Pc in Reference [1]). The gauche isomer results when the C--C1 b o n d is r o t a t e d 120 ° so t h a t in n - b u t y l chloride a h y d r o g e n is trans to the chlorine (PH) (see Figs. l(a) and l(b)). Vibrating ball and spring molecular models, first used b y K E T T E R I N G , SHULTS and ANDREWS [2], are essentially crude analog computers of simple valence force fields. Models are best used to d e m o n s t r a t e changes in vibrational interactions and f r e q u e n c y as a function of one variable, g e o m e t r y in this case. The present n - b u t y l chloride models are similar to those used in strained ring double bond studies [3] (see Fig. l(c)). E a c h " a t o m " is suspended b y a long r u b b e r thread. The model is connected to an eccentric on a variable speed m o t o r t h r o u g h a v e r y fine coupling wire. W h e n resonance occurs the model performs the a p p r o p r i a t e normal mode of v i b r a t i o n (see Fig. l(d)). Carbons were represented b y 1~" d i a m e t e r steel balls and chlorine b y a 5 rt d i a m e t e r ball. The mass ratio is 12" 35.1. The C - - C bonds were helical springs, 16 rail spring wire. 9 coils, ~ " diameter, ~1,, between atoms. The C--C1 bond was ~8 16 rail spring wire, 9 coils, ~la" diameter, -~- ~~" between atoms. A bond spring held at one end was a b o u t nine times easier to bend t h a n to stretch. The C - - C bond spring was 1.4 times as stiff as the C--C1 b o n d spring. The C--CI stretching frequencies obtained from the models are c o m p a r e d with the actual molecular frequencies in cm -~ [1] in Table 1. I t can be seen t h a t the frequency ratios are n e a r l y the same. T h e form of t h e v i b r a t i o n s and t h e f r e q u e n c y [1] J . J . SHIPMAN, V. L. FOLT, a n d S. KRIM~, Spectrochim. Acta 18, 1603 (1962). [2] C. F. KETTERIN(~, L. W . SHULTS, a n d D. H . ANDREWS, Phys. Bey. 36, 531 (1930). [3] N. B. COL~HUP, J. Chem. Ed. 38, 394 (1961). 6 1843
1844
N.B.
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GAUCHE
C-C-C PLAN E
1
c,
B
I
c,
S c, sc# ""
./CI-C,-Cz
c,
PLANE
j
UC
c~'~
C-C-C PLANE
- ,
/ @ C~ ( A B O V E
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Fig. l. Models of n - b u t y l chloride performing t h e C--C1 stretching vibration. (a) and (b) are sketches of t h e skeletal atoms, the equilibrium positions of which lie in t h e C - - C - - C plane, e x c e p t for t h e gauche chlorine. (e) and (d) are sketches of t h e s t a t i o n a r y and v i b r a t i n g trans m o d e l t a k e n f r o m photographs. The elongated highlight (d) on each a t o m defines t h e v i b r a t i o n a l a m p l i t u d e in d i a g r a m s below. I n (e) and (f) t h e p h o t o g r a p h i c film is parallel to the plane containing C1, C 1 and C 2 b o t h the trans and gauche models. F o r these three atoms, b o n d stretching (S) and bending (B) interactions are similar. I n (g) a n d (h) the film is parallel to t h e C C - - C plane. N o t e t h e m a r k e d difference in C1---C2--C a bending i n t e r a c t i o n (B) in the trans (g) and gauche (h) models.
Table 1. C--C1 stretching frequencies in n - b u t y l chloride Models
trans (Pc) gauche (PH) f r e q u e n c y ratio (PH:Pc)
280 cycles per min. 250 1 : 1.119
Molecules 730 cm -1 649 1 : 1.123
Vibrating molecular model study of halogenated alkane rotational isomers
1845
differences between trans and gauche isomers were not very sensitive to small changes in the C---X force constant or to an increase to infinity of the mass of the terminal X-atom. This implies t h a t the same effect should occur in bromo- and iodoalkanes as indeed it does [4]. The frequencies were not particularly sensitive to changes in the orientation of the fourth carbon. The suspension system of long rubber threads had little effect on the frequencies. Pendulum motions which stretched the coupling wire and vertical translation motions which stretched the rubber threads had ver y low frequencies relative to the C--C1 frequency. We varied the orientation of the whole model so t h a t the large amplitude C 1 motion was sometimes horizontal and sometimes at an angle which stretched the rubber suspension thread. The model frequency was practically independent of orientation of the model in the suspension system which we felt was valid for non planar vibrations. We assumed t h a t because of the above results we had a satisfactory representation of the interactions involved which in this case were purely skeletal. I n the C--C1 vibration the carbons move essentially at rigi~t angles to the direction t he y move in the CH 2 rocking vibrations so little or no interaction is expected with these. The CH 2 wagging vibrations are too high in frequency to interact. The vibrating models were photographed and the vibrational amplitude components parallel to the plane of the film were measured from the photographs. As seen in Figs. l(e) and l(f), the "C--C1 stretching" vibration consists of the chlorine and the second carbon (C2) both moving away from C1 with small amplitude while C 1 moves away from the chlorine with large amplitude in the C1--C1--C 2 plane and nearly at right angle to the C~--C 2 bond. The "C--CI" vibration thus consists of a large stretching (S) of the C--C1 bond and a small in-phase stretching (8) of the C1--C 2bond and a bending (B) of the C1--C1--C 2angle in both rotational isomers. There is only one large change in interaction between the rotational isomers. When Ca is trans to the chlorine the C1--Ce--C a angle is sharply bent when the C1 atom moves away from the chlorine. When Ca is not trans to the chlorine the C1--C~--C a angle is not bent appreciably (see Figs. l(g) and l(h)). Thus as C 1 moves away from the chlorine it encounters additional resistance to motion in the trans isomer not present in the gauche which accounts for the 10 per cent higher frequency. In the nomenclature used in Reference [1], P is primary chloride and S is secondary chloride. The subscript H or C is the atom or atoms trans to the chlorine. The secondary chlorides can be pictured as follows. Starting with isopropyl chloride we add a fourth carbon on one methyl in one of three orientations. I t can be trans to the chlorine (ScH), trans to the other methyl (SHH)SO t h a t the carbon chain is planar or trans to the hydrogen on the branched carbon (SHH') SO t h a t the carbon chain is non-planar. I f another carbon is added on the other methyl (3-chloropentane) in addition to the above isomers it is possible to have two carbons trans to the chlorine (Scc), or two carbons trans to the hydrogen on the branched carbon (SHH") [1]. A model of 2-chloropentane was made by clipping another " c a r b o n " on the carbon with the chlorine in the butyl chloride model. In all the rotational isomers the branched carbon (C2) moved with large amplitude away from the chlorine in a direction approximately perpendicular to the C~--C~--C~ carbon chain plane:while the [4] F. F. BENTLY,:N. T. 1VfcDEvIT~:and A. L. ROZEK,Spectrochim. Acta 2{), 105 (1964).
1846
N . B . CoLT~uP
chlorine and C 1 and C 3 m o v e d with small a m p l i t u d e a w a y from C 2. W h e n C 4 was trans to the chlorine (Sen) the C~--C3--C 4 angle was b e n t as before in the P c isomer whereas when C a was not trans to the chlorine (t.~HH, SHtt' ) the C~--C3--C 4 angle was not b e n t appreciably. The SHH:SCH f r e q u e n c y ratio was a b o u t 1.00:1.09 for the models and for the 2-chloropentane molecule (612 and 670 cm -1) [1]. We were unable to d e t e c t a significant f r e q u e n c y difference between SHH and SHH'. The SH~ : SHH' f r e q u e n c y ratio for the 2-ehlorobutane molecule is 1.00:1.03 (609:628 cm -1) b u t the SHH and SHH t bands are possibly closer in f r e q u e n c y in the 2-chloropentane molecule since two bands were not resolved in this region [1]. I t is suggested t h a t this one change in C1--C~--C 3 bending interaction is responsible for most of the differences in the C - - X stretching frequencies of the halogenated alkane r o t a t i o n a l isomers. This includes p r i m a r y , secondary, and t e r t i a r y chloroalkane rotational isomers [1], axial and equatorial chlorocyclohexanes [5] and bromoand iodoalkane isomers [4]. [5] D. H. R. BARTO.~,J. E. PAGE and C. W. SIIOPPEE,J. Chem. Soc. 331 (1956).
Vibrating molecular model study of halogenated alkane rotational isomers N. B. COLTHUP Central Research Division, American Cyanamid Stamford, Connecticut
Company,
(Received 28 April 1964) A b s t r a c t - - T h i s s t u d y r e v e a l s t h a t m o s t of t h e difference in C--C1 s t r e t c h i n g f r e q u e n c i e s in r o t a t i o n a l i s o m e r s of c h l o r o a l k a n e s is c a u s e d b y a c h a n g e in C - - C - - C b e n d i n g i n t e r a c t i o n .
CHLOROALKANES (and also bromo- and iodoalkanes) have long been k n o w n to exist as r o t a t i o n a l isomers which h a v e distinctly different infrared C--C1 stretching frequencies. These have been most r e c e n t l y studied b y SHIP~AN, FOLT and KR~MM [1] who m a d e close correlations for p r i m a r y , secondary, and t e r t i a r y chloroalkanes. T h e y f o u n d t h a t when a carbon a t o m was trans to the chlorine (Reference [l]), the f r e q u e n c y was r o u g h l y 10 per cent higher t h a n the f r e q u e n c y for the r o t a t i o n a l isomer where a h y d r o g e n was trans to the chlorine. I n this p a p e r the n - b u t y l chloride isomer where the non h y d r o g e n atoms zig-zag in one plane is called the trans isomer (Pc in Reference [1]). The gauche isomer results when the C--C1 b o n d is r o t a t e d 120 ° so t h a t in n - b u t y l chloride a h y d r o g e n is trans to the chlorine (PH) (see Figs. l(a) and l(b)). Vibrating ball and spring molecular models, first used b y K E T T E R I N G , SHULTS and ANDREWS [2], are essentially crude analog computers of simple valence force fields. Models are best used to d e m o n s t r a t e changes in vibrational interactions and f r e q u e n c y as a function of one variable, g e o m e t r y in this case. The present n - b u t y l chloride models are similar to those used in strained ring double bond studies [3] (see Fig. l(c)). E a c h " a t o m " is suspended b y a long r u b b e r thread. The model is connected to an eccentric on a variable speed m o t o r t h r o u g h a v e r y fine coupling wire. W h e n resonance occurs the model performs the a p p r o p r i a t e normal mode of v i b r a t i o n (see Fig. l(d)). Carbons were represented b y 1~" d i a m e t e r steel balls and chlorine b y a 5 rt d i a m e t e r ball. The mass ratio is 12" 35.1. The C - - C bonds were helical springs, 16 rail spring wire. 9 coils, ~ " diameter, ~1,, between atoms. The C--C1 bond was ~8 16 rail spring wire, 9 coils, ~la" diameter, -~- ~~" between atoms. A bond spring held at one end was a b o u t nine times easier to bend t h a n to stretch. The C - - C bond spring was 1.4 times as stiff as the C--C1 b o n d spring. The C--CI stretching frequencies obtained from the models are c o m p a r e d with the actual molecular frequencies in cm -~ [1] in Table 1. I t can be seen t h a t the frequency ratios are n e a r l y the same. T h e form of t h e v i b r a t i o n s and t h e f r e q u e n c y [1] J . J . SHIPMAN, V. L. FOLT, a n d S. KRIM~, Spectrochim. Acta 18, 1603 (1962). [2] C. F. KETTERIN(~, L. W . SHULTS, a n d D. H . ANDREWS, Phys. Bey. 36, 531 (1930). [3] N. B. COL~HUP, J. Chem. Ed. 38, 394 (1961). 6 1843
1844
N.B.
,RA,,,s
®
CoLTnvP
GAUCHE
C-C-C PLAN E
1
c,
B
I
c,
S c, sc# ""
./CI-C,-Cz
c,
PLANE
j
UC
c~'~
C-C-C PLANE
- ,
/ @ C~ ( A B O V E
PLANE)
Fig. l. Models of n - b u t y l chloride performing t h e C--C1 stretching vibration. (a) and (b) are sketches of t h e skeletal atoms, the equilibrium positions of which lie in t h e C - - C - - C plane, e x c e p t for t h e gauche chlorine. (e) and (d) are sketches of t h e s t a t i o n a r y and v i b r a t i n g trans m o d e l t a k e n f r o m photographs. The elongated highlight (d) on each a t o m defines t h e v i b r a t i o n a l a m p l i t u d e in d i a g r a m s below. I n (e) and (f) t h e p h o t o g r a p h i c film is parallel to the plane containing C1, C 1 and C 2 b o t h the trans and gauche models. F o r these three atoms, b o n d stretching (S) and bending (B) interactions are similar. I n (g) a n d (h) the film is parallel to t h e C C - - C plane. N o t e t h e m a r k e d difference in C1---C2--C a bending i n t e r a c t i o n (B) in the trans (g) and gauche (h) models.
Table 1. C--C1 stretching frequencies in n - b u t y l chloride Models
trans (Pc) gauche (PH) f r e q u e n c y ratio (PH:Pc)
280 cycles per min. 250 1 : 1.119
Molecules 730 cm -1 649 1 : 1.123
Vibrating molecular model study of halogenated alkane rotational isomers
1845
differences between trans and gauche isomers were not very sensitive to small changes in the C---X force constant or to an increase to infinity of the mass of the terminal X-atom. This implies t h a t the same effect should occur in bromo- and iodoalkanes as indeed it does [4]. The frequencies were not particularly sensitive to changes in the orientation of the fourth carbon. The suspension system of long rubber threads had little effect on the frequencies. Pendulum motions which stretched the coupling wire and vertical translation motions which stretched the rubber threads had ver y low frequencies relative to the C--C1 frequency. We varied the orientation of the whole model so t h a t the large amplitude C 1 motion was sometimes horizontal and sometimes at an angle which stretched the rubber suspension thread. The model frequency was practically independent of orientation of the model in the suspension system which we felt was valid for non planar vibrations. We assumed t h a t because of the above results we had a satisfactory representation of the interactions involved which in this case were purely skeletal. I n the C--C1 vibration the carbons move essentially at rigi~t angles to the direction t he y move in the CH 2 rocking vibrations so little or no interaction is expected with these. The CH 2 wagging vibrations are too high in frequency to interact. The vibrating models were photographed and the vibrational amplitude components parallel to the plane of the film were measured from the photographs. As seen in Figs. l(e) and l(f), the "C--C1 stretching" vibration consists of the chlorine and the second carbon (C2) both moving away from C1 with small amplitude while C 1 moves away from the chlorine with large amplitude in the C1--C1--C 2 plane and nearly at right angle to the C~--C 2 bond. The "C--CI" vibration thus consists of a large stretching (S) of the C--C1 bond and a small in-phase stretching (8) of the C1--C 2bond and a bending (B) of the C1--C1--C 2angle in both rotational isomers. There is only one large change in interaction between the rotational isomers. When Ca is trans to the chlorine the C1--Ce--C a angle is sharply bent when the C1 atom moves away from the chlorine. When Ca is not trans to the chlorine the C1--C~--C a angle is not bent appreciably (see Figs. l(g) and l(h)). Thus as C 1 moves away from the chlorine it encounters additional resistance to motion in the trans isomer not present in the gauche which accounts for the 10 per cent higher frequency. In the nomenclature used in Reference [1], P is primary chloride and S is secondary chloride. The subscript H or C is the atom or atoms trans to the chlorine. The secondary chlorides can be pictured as follows. Starting with isopropyl chloride we add a fourth carbon on one methyl in one of three orientations. I t can be trans to the chlorine (ScH), trans to the other methyl (SHH)SO t h a t the carbon chain is planar or trans to the hydrogen on the branched carbon (SHH') SO t h a t the carbon chain is non-planar. I f another carbon is added on the other methyl (3-chloropentane) in addition to the above isomers it is possible to have two carbons trans to the chlorine (Scc), or two carbons trans to the hydrogen on the branched carbon (SHH") [1]. A model of 2-chloropentane was made by clipping another " c a r b o n " on the carbon with the chlorine in the butyl chloride model. In all the rotational isomers the branched carbon (C2) moved with large amplitude away from the chlorine in a direction approximately perpendicular to the C~--C~--C~ carbon chain plane:while the [4] F. F. BENTLY,:N. T. 1VfcDEvIT~:and A. L. ROZEK,Spectrochim. Acta 2{), 105 (1964).
1846
N . B . CoLT~uP
chlorine and C 1 and C 3 m o v e d with small a m p l i t u d e a w a y from C 2. W h e n C 4 was trans to the chlorine (Sen) the C~--C3--C 4 angle was b e n t as before in the P c isomer whereas when C a was not trans to the chlorine (t.~HH, SHtt' ) the C~--C3--C 4 angle was not b e n t appreciably. The SHH:SCH f r e q u e n c y ratio was a b o u t 1.00:1.09 for the models and for the 2-chloropentane molecule (612 and 670 cm -1) [1]. We were unable to d e t e c t a significant f r e q u e n c y difference between SHH and SHH'. The SH~ : SHH' f r e q u e n c y ratio for the 2-ehlorobutane molecule is 1.00:1.03 (609:628 cm -1) b u t the SHH and SHH t bands are possibly closer in f r e q u e n c y in the 2-chloropentane molecule since two bands were not resolved in this region [1]. I t is suggested t h a t this one change in C1--C~--C 3 bending interaction is responsible for most of the differences in the C - - X stretching frequencies of the halogenated alkane r o t a t i o n a l isomers. This includes p r i m a r y , secondary, and t e r t i a r y chloroalkane rotational isomers [1], axial and equatorial chlorocyclohexanes [5] and bromoand iodoalkane isomers [4]. [5] D. H. R. BARTO.~,J. E. PAGE and C. W. SIIOPPEE,J. Chem. Soc. 331 (1956).