NMR Spectroscopy - American Chemical Society

10 N M R Spectroscopy at High Pressure

Downloaded by CORNELL UNIV on May 30, 2017 | http://pubs.acs.org Publication Date: July 9, 1982 | doi: 10.1021/bk-1982-0191.ch010

JIRI JONAS University of Illinois, Department of Chemistry, Urbana, IL 61801

The field of NMR spectroscopy at high pressure has been discussed in several recent review articles.(1-4) It has been well documented that for providing rigorous experimental tests of current theoretical models of liquids, one has to use pressure as an experimental parameter and separate the effects of density and temperature on molecular motions and interactions. This article will discuss our recent developments in the field of NMR spectroscopy at high pressure and, in particular, will focus on multinuclear high resolution NMR FT spectroscopy at high pressure. Two illustrative examples of this specific technique are given. First, the C NMR experiments of rhodium carbonyl clusters under high pressures of CO/H illustrate the promising future of the high resolution, high pressure FT NMR in studies of homogeneous catalytic processes.(4) Second example deals with the investigation of the pressure effects on conformational inversion in cyclohexane. (5) This represents the first experiment which has shown conclusively the importance of the pressure dependent collisional contribution to isomerization reactions as predicted by recent stochastic models of isomerization dynamics in liquids. In order to illustrate the scope and range of high pressure NMR experiments, the next part of the article discusses our recent results on relaxation and transport behavior of compressed supercritical water. The article finishes with a brief account of application of high pressure NMR to studies of the dynamic structure of various disordered systems. 13

2

Multinuclear

High

R e s o l u t i o n F T NMR a t 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 s p e c t r o s c o p y a t h i g h p r e s s u r e s r e p r e s e n t s one o f t h e p r o m i s i n g new a r e a s o f r e s e a r c h a t h i g h p r e s s u r e . Recent advances i n magnet t e c h n o l o g y have r e s u l t e d i n t h e development o f magnets c a p a b l e of a t t a i n i n g a h i g h homogeneity o f t h e magnetic f i e l d over t h e sample volume so t h a t even w i t h o u t sample s p i n n i n g , one c a n achieve v e r y h i g h r e s o l u t i o n . At the same t i m e , t h e F o u r i e r t r a n s f o r m t e c h n i q u e s make a l l t h e s e h i g h

0097-6156/82/0191-0199$06.00/0 ©

1982 A m e r i c a n Chemical Society

Levy; NMR Spectroscopy: New Methods and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

NMR

200

SPECTROSCOPY

r e s o l u t i o n e x p e r i m e n t s much e a s i e r t o be p e r f o r m e d a t h i g h p r e s s u r e s t h a n i t w a s t h e c a s e w i t h c l a s s i c a l CW t e c h n i q u e s . have

recently

described

(6)

a high resolution,

We

h i g h p r e s s u r e NMR

probe w h i c h i s s u i t a b l e f o r s u p e r c o n d u c t i n g magnets and w h i c h has a number o f f e a t u r e s w h i c h a r e i m p o r t a n t f o r f u t u r e a p p l i c a t i o n s o f t h i s s p e c i f i c NMR t e c h n i q u e : r a n g e ; i i ) h i g h r e s o l u t i o n (5 χ contamination

free

sample

i ) wide 10""^ f o r

pressure and temperature 6 mm s a m p l e s ) ; i i i )

cell.


I n a c o l l a b o r a t i v e e f f o r t w i t h D r . Β. T . H e a t o n from t h e U n i v e r s i t y o f K e n t a t C a n t e r b u r y , E n g l a n d , we h a v e r e c e n t l y a p p l i e d t h i s t e c h n i q u e (4) t o measure spectra of rhodium c a r b o n y l c l u s t e r s u n d e r h i g h p r e s s u r e o f CO a n d H , , . In con n e c t i o n w i t h the e f f o r t s by the p e t r o c h e m i c a l i n d u s t r y to f i n d c a t a l y t i c s y n t h e s e s w h i c h u s e CO a n d Η there i s increasing evidence that t r a n s i t i o n metal carbonyl c l u s t e r s are involved i n the c a t a l y t i c s y n t h e s i s of e t h y l e n e g l y c o l and i n the F i s c h e r Tropsch and r e l a t e d r e a c t i o n s . These c a t a l y t i c r e a c t i o n s r e q u i r e reasonably h i g h p r e s s u r e s and t e m p e r a t u r e s a n d , so f a r , i n f r a r e d s p e c t r o s c o p y has been t h e o n l y s p e c t r o s c o p i c technique used to monitor them. Although u s e f u l i n f o r m a t i o n has been o b t a i n e d i n t h i s way, i t i s g e n e r a l l y i m p o s s i b l e to o b t a i n unambiguous s t r u c t u r a l i d e n t i f i c a t i o n and mechanistic inter-/intra-exchange data which are of importance i n e s t a b l i s h i n g r e a c t i v i t y patterns of intermediates that are present under these c o n d i t i o n s . These are a p r e r e q u i s i t e f o r the basic

understanding of

the mechanism of

these

catalytic

processes.

M u l t i n u c l e a r NMR s t u d i e s a t a t m o s p h e r i c p r e s s u r e , i . e . , under l e s s extreme c o n d i t i o n s , have y i e l d e d v a l u a b l e s t r u c t u r a l and m e c h a n i s t i c i n f o r m a t i o n on t r a n s i t i o n m e t a l c a r b o n y l c l u s t e r s and b o t h the above l i m i t a t i o n s s u f f e r e d by i n f r a r e d s p e c t r o s c o p y have n o w b e e n o v e r c o m e b y m e a s u r i n g h i g h r e s o l u t i o n NMR s p e c t r a a t high pressure. Well-resolved C NMR s p e c t r a , w i t h e x c e l l e n t s i g n a l / n o i s e , have been o b t a i n e d on t h e c a t a l y t i c system i n v o l v e d i n the formation of ethylene g l y c o l . The p e n t a n u c l e a r c l u s t e r , i s o l a t e d from the low pressure

[ R h ^ ( C O ) ] " , has r e c e n t l y ( c a . 10 b a r ) r e a c t i o n o f

[Rh-^CCO)™] ~ w i t h carbon monoxide. Because of t h i s c l u s t e r i n the c a t a l y t i c synthesis of

been

of the involvement ethylene glycol

f r o m s y n t h e s i s g a s , i t was o f i n t e r e s t t o m o n i t o r t h i s r e a c t i o n by C n . m . r . spectroscopy at h i g h e r p r e s s u r e s of carbon monoxide and i n t h e p r e s e n c e o f h y d r o g e n to see whether o t h e r s p e c i e s a r e formed. The 4 5 . 2

MHz

C n.m.r.

spectrum of

(NMe Bz) [Rh 3

2

1 2

(C0)

]

in

t h i s h i g h p r e s s u r e c e l l , ( F i g u r e l a , b ) , has good s i g n a l / n o i s e ( 5 0 : 1 ) a n d r e s o l u t i o n ( c a . 25 H z ) a n d i s s i m i l a r t o t h a t o b t a i n e d p r e v i o u s l y on a c o n v e n t i o n a l h i g h r e s o l u t i o n spectrometer. Upon p r e s s u r i z i n g w i t h u p t o 5 7 5 b a r o f CO ( 1 3 . 7 % C0) at 52°C, there i s a progressive transformation of [ R h , ( C 0 ) « ] into [Rh (CO)- ]" b u t , e v e n a f t e r c a . 2 4 h u n d e r 7 7 b a r CO a n d 3 h u n d e r 575 b a r CO, t h e r e i s s t i l l e v i d e n c e f o r u n r e a c t e d 1 3

2

5

2 =

0

5



10.

NMR

JONAS

Spectroscopy

at High

Pressure

201

Figure 1. 45.2-MHz C NMR spectra in a high pressure probe a, (NMe Bz) [Rh (CO) ] at —40°C in acetone-a ; and b, after pressurizing (850 bar) with CO/ H (2.1:1; 13.7% CO) at -32.8°C. S = solvent; * = impurity, [Rh (CO) ] ^ 13

12

2

30

s

2

6

13

6


15

2

N M R SPECTROSCOPY

202

[ R h ( C O ) ] - ; t h e r e i s no change i n t h e spectrum a t t h i s temperature on further p r e s s u r i z i n g with (Ρ = 850 b a r ; C0:H = 2.1:1). Warming t o room t e m p e r a t u r e , f o l l o w e d b y c o o l i n g to - 3 2 . 8 ° C , r e s u l t s i n complete f o r m a t i o n o f [Rh_(C0) (Figures l a , b ) ; t h i s spectrum i s s i m i l a r to that obtained p r e v i o u s l y , b u t , i n a d d i t i o n t o t h e r e s o n a n c e d u e t o CO ^ - ^ a t 1 8 4 . 3 p . p . m . , t h e r e i s a r e s o n a n c e d u e t o CO a t l§2.0 p?p.m. A t room t e m p e r a t u r e , t h e s p e c t r u m under 1000 b a ? 6 o / H ( 2 . 1 : 1 ) i s s i m i l a r t o t h a t o b t a i n e d u n d e r 1 0 b a r o f CO a n d c o n s i s t s o f a resonance a t 247.5 p.p.m. due t o t h e e q u a t o r i a l e d g e - b r i d g i n g carbonyls and a broad resonance a t c a . 200 p.p.m. due to the remaining carbonyls undergoing i n t r a - m o l e c u l a r exchange. 1 2

3 0

2

1

0


a

u t

o n

2

I t i s s i g n i f i c a n t t h a t , even under such h i g h pressures of C O , e x c h a n g e o f c a r b o n y l s w i t h CO i s n o t f a s t o n t h e NMR t i m e scale at low temperatures. Thus, h i g h pressure n . m . r . studies offer the p o s s i b i l i t y of structural i d e n t i f i c a t i o n of i n t e r mediates formed under extreme c o n d i t i o n s o f r e a c t i o n s o f i n d u s t r i a l i n t e r e s t and further studies of p o t e n t i a l c a t a l y t i c systems a r e underway. The s t u d y o f t h e p r e s s u r e e f f e c t s o n t h e c o n f o r m a t i o n a l inversion of cyclohexane i n d i f f e r e n t solvents represents the second i l l u s t r a t i v e example o f t h e a p p l i c a t i o n o f h i g h r e s o l u t i o n , h i g h p r e s s u r e F T NMR. A l a r g e n u m b e r o f s t u d i e s e m p l o y i n g d i f f e r e n t NMR t e c h n i q u e s have been devoted to t h e i n v e s t i g a t i o n of t h e temperature dependence o f t h e r i n g i n v e r s i o n of cyclohexane. This i s not s u r p r i s i n g i n view of the fact that the problem of cyclohexane inversion represents the seminal problem i n conformational analysis. What i s s u r p r i s i n g , however, t h a t a l l p r e v i o u s s t u d i e s have used a s i n g l e s o l v e n t - carbon d i s u l f i d e , a n d , that only a l i m i t e d p r e s s u r e study (up t o 2 kbar) o f c y c l o h e x a n e i n q u a t e r n a r y m i x t u r e h a s been p e r f o r m e d by Ludemann e t a l . (7) There were two main m o t i v a t i o n s f o r o u r s t u d y . F i r s t , we wanted to f o l l o w t h e e f f e c t s of pressure on t h e c o n f o r m a t i o n a l inversion of cyclohexane i n different solvents: acetone-d^, carbon d i s u l f i d e and perdeuterated methylcyclohexane. Secondly, in view of recent theoretical research i n the area of stochastic models f o r i s o m e r i z a t i o n r e a c t i o n s as proposed by Montgomery, C h a n d l e r a n d B e r n e ( 8 ) a n d b y S k i n n e r a n d W o l y n e s ( 9 ) we a t t e m p t e d to provide the f i r s t experimental proof of t h e t h e o r e t i c a l p r e d i c t i o n of l a r g e pressure e f f e c t s on t h e t r a n s m i s s i o n c o e f f i c i e n t , or otherwise stated, a large c o l l i s i o n a l contribution to the a c t i v a t i o n volume f o r an i s o m e r i z a t i o n r e a c t i o n . Our r e s u l t s show t h a t we w e r e s u c c e s s f u l i n b o t h a s p e c t s . I t i s accepted t h a t t h e lowest energy path f o r cyclohexane r i n g inversion proceeds v i a a h a l f - c h a i r t r a n s i t i o n state with t h e t w i s t b o a t a n d i t s s l i g h t l y ( F i g u r e 2) h i g h e r e n e r g y p s e u d o rotation partner, the boat, as intermediates. When we c o m p l e t e d t h e h i g h p r e s s u r e NMR m e a s u r e m e n t s , w e f o u n d t h a t t h e r i n g



JONAS

NMR

Spectroscopy

HALF-CHAIR TRANSITION STATE

at High

Pressure

203

HALF-BOAT TRANSITION STATE

Figure 2. Representation of the chair-to-chair interconversion process in cyclohexane showing the possible intermediates (boat and twist-boat) and transition states (half-chair and half-boat). The magnetically nonequivalent protons, H and H , undergo mutual exchange during ring inversion. A

B


NMR


204

SPECTROSCOPY

i n v e r s i o n r a t e i s a c c e l e r a t e d with i n c r e a s i n g pressure as can be seen i n Figure 3. I t i s c l e a r from Figure 3 that the i n v e r s i o n r a t e e x h i b i t s the pressure dependence i n the order: acetone-d^, CS^ and deuterated methylcyclohexane. I t i s important to emphasize that the slope of the logarithm of the i n v e r s i o n r a t e i s s t r o n g l y n o n l i n e a r with pressure. In order to understand the r e s u l t s l e t us b r i e f l y o u t l i n e the main f e a t u r e s or b e t t e r s t a t e d , the main r e s u l t s of the s t o c h a s t i c models (8, 9) f o r the i s o m e r i z a t i o n dynamics. The d e t a i l s can be found i n the o r i g i n a l papers. (8, 9) In t h i s model i t i s proposed that there i s a c o n t r i b u t i o n to the a c t i v a t i o n volume from dynamical e f f e c t s , because the r e a c t i o n coordinate i s coupled to the surrounding medium. This leads to the r e s u l t t h a t the transmission c o e f f i c i e n t κ ( u s u a l l y assumed to be u n i t y i n the conventional t r a n s i t i o n s t a t e theory), i s a f u n c t i o n of the " c o l l i s i o n frequency" which r e f l e c t s the a c t u a l coupling to the surrounding medium. In our case of cyclohexane, which has no d i p o l e moment, we may take the v i s c o s i t y of the solvent as the measure of the reduced c o l l i s i o n frequency. The theoretical calculations (8, 9) have shown that the transmission c o e f f i c i e n t i s s t r o n g l y dependent on the c o l l i s i o n frequency. This formalism allows us to c a l c u l a t e the c o l l i s i o n a l (dynamical) c o n t r i b u t i o n to the a c t i v a t i o n volume. One may w r i t e k(t) = κ k ^

(1)

where k ( t ) i s the observed i s o m e r i z a t i o n r a t e , κ i s the transmis s i o n c o e f f i c i e n t and k represents the c l a s s i c a l r a t e i n the t r a n s i t i o n s t a t e theory. The r e s u l t of the c a l c u l a t i o n i s that T S T

Δν^ OBS

= Δν^ C0LL V

+ Δν^ TST V

V

J

(2)

ί + where A V g i s the observed a c t i v a t i o n volume, ^ Q L L f c o l l i s i o n a l c o n t r i b u t i o n to the a c t i v a t i o n volume, and AV£ has i t s usual meaning. For cyclohexane one may estimate AV^g to be - 1.5 cm /mole. According to the s t o c h a s t i c model the transmission coef ficient i s dependent on the c o l l i s i o n frequency, i . e . , on the s t r e n g t h of the coupling to the surrounding medium. The theory p r e d i c t s that a t weak coupling ( i n e r t i a l regime) the i n c r e a s e of pressure leads to the increase i n c o l l i s i o n a l frequency with the subsequent increase i n the i n v e r s i o n r a t e (or κ ) . On the other hand, i n the strong coupling l i m i t ( d i f f u s i v e regime) the increase i n pressure ( c o l l i s i o n a l frequency) slows down the isomerization rate. A f t e r a n a l y z i n g our experimental data we were pleased to f i n d r e s u l t s given i n F i g u r e 4. The observed a c t i v a t i o n volume l)BS 8 l y pressure dependent as i s the c o l l i s i o n a l V

QB

i s

h

0

T

i s

s t r o n


e


Figure 3.

P=4,4 kbar

6

C D„CD P = 2.3 kbark=l28sec-'

ACET0NE-D 6

Experimental NMR spectra of cyclohexane at 218 Κ illustrating the effect of solvent and pressure on the ring-inversion process.

3


NMR

206

ο

ι

1 Δ


Δ

SPECTROSCOPY

1.5 Δ

Δ

-0.5

1.0

-1.0

—10.5

•Δνί

-1.5

5Τ

Ο

ΙΟ

Ji

-2.0

1-0-5

-2.5

-1.0

_, •il ο

-1.5

NMR Spectroscopy - American Chemical Society

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