Proton-Transfer Reaction Rates and Mechanisms - ACS Publications

3 Proton-Transfer Reaction Rates and Mechanisms EDWARD M. EYRING, DAVID B. MARSHALL, FRANK STROHBUSCH Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 24, 2015 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch003

University of Utah, Department of Chemistry, Salt Lake City, UT 84112 R. SÜTTINGER Institut für Physikalische Chemie der Universität Freiburg, D-78 Freiburg, Federal Republic of Germany Eight generalizations are given a r i s i n g from world-wide studies of proton transfer reactions i n aqueous media carried out over the past twenty– five years. Future directions of research on pro ton transfer kinetics are predicted, and recent kinetic studies by the authors on proton transfer in nonaqueous media (methanol, a c e t o n i t r i l e , and benzonitrile) are reviewed. Inorganic s o l u t i o n chemistry o f t e n i n v o l v e s p r o t o n t r a n s f e r s t o and from s o l v a t e d metal ions as w e l l as t o and from the a c i d s and bases t h a t complex metal i o n s . E i g h t g e n e r a l i z a t i o n s are presented below t h a t attempt t o summarize the i n s i g h t s r e garding proton t r a n s f e r r e a c t i o n s t h a t have emerged i n the p a s t quarter c e n t u r y . The m a s t e r f u l reviews by E i g e n (1_) and B e l l (2) p r o v i d e much more e x t e n s i v e a n a l y s i s o f most o f these points. Eight Generalizations 1. the type A

1

F o r thermodynamically favorable

+ B £

f

Β + A

2

α

2

Κ= k ^ , k

reactions

f

2 10

1 0

( K » l ) of

M ' V

+

1

(1)

i n v o l v i n g oxygen o r n i t r o g e n a c i d s w i t h BLO, HL O , or OH i n 10 -1 -1 aqueous s o l u t i o n a t room temperature, k^~10 M s and the r a t e constant i s s m a l l e r by a f a c t o r o f 10^^ i n the reverse (unfavorable) d i r e c t i o n ( 1 , 2 ) . 2. E i g e n s mechanism f o r p r o t o n t r a n s f e r r e a c t i o n s b e tween a c i d s (AH) and bases (B) proceeds through a n e u t r a l hydro1

0097-6156/82/0198-0063$06.00/0 © 1982 American Chemical Society In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

64

MECHANISTIC ASPECTS O F INORGANIC REACTIONS

gen bonded complex (AH...B) and an i o n p a i r (A ... HB) +

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 24, 2015 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch003

AH + Β J AH...B J A"... HB | A" + HB

+

(2)

w i t h these i n t e r m e d i a t e s undetectable i n aqueous s o l u t i o n b u t observable i n p o l a r o r g a n i c s o l v e n t s under s u i t a b l e c o n d i t i o n s (3). 3. Rate constants f o r the d i f f u s i o n - c o n t r o l l e d r e a c t i o n between a p r o t o n and a s p e c i e s A i n water decrease (4) by a f a c t o r o f 0.3 t o 0.5 f o r each p o s i t i v e charge added t o the r e a c t a n t A. Thus the r a t e constant f o r the r e a c t i o n o f a hydro2+ l y z e d metal i o n such as A£0H (aq) w i t h a s o l v e n t proton w i l l d e c l i n e w i t h the i n c r e a s i n g p o s i t i v e e l e c t r o s t a t i c charge o f the hydrolyzed metal i o n s p e c i e s . 4. I n t r a m o l e c u l a r hydrogen bonding, s t e r i c hindrance, and l o c a t i o n o f the mobile p r o t o n on a carbon atom ("carbon a c i d s " ) can a l l a c t t o decrease somewhat the r e a c t i o n r a t e s ( 5 ) . 5. Removal o f the p r o t o n from an i n t r a m o l e c u l a r hydrogen bond by a base occurs i n a two-step mechanism (a r a p i d e q u i l i b rium between Η-bonded and non-H-bonded forms f o l l o w e d by base c a t a l y z e d proton removal from the non-H-bonded form) r a t h e r than by d i r e c t a t t a c k o f the base on the i n t r a m o l e c u l a r l y hydrogen bonded s p e c i e s ( 6 ) . 6. Nuclear r e o r g a n i z a t i o n o r the r e h y b r i d i z a t i o n o f t h e carbon i s a main f a c t o r i n the r e t a r d a t i o n o f proton t r a n s f e r i n v o l v i n g carbon a c i d s , and s o l v a t i o n changes have much l e s s impact ( 7 ) . 7. Proton t r a n s f e r between e l e c t r o n e g a t i v e atoms i s f a s t e r the g r e a t e r the e l e c t r o n e g a t i v i t y o f t h e atoms between which the p r o t o n i s moving. Thus p r o t o n t r a n s f e r between n i t r o gen atoms i s slower and r a t e l i m i t i n g over a wider range o f ApK than f o r p r o t o n t r a n s f e r between oxygen atoms ( 8 ) . 8. Proton exchange r a t e s i n aqueous s o l u t i o n s a r e en hanced by s m a l l amounts (0.5% V/V) o f hydrophobic substances (e.g., methanol, dioxane) because o f a consequent i n c r e a s e i n Hbonded water s t r u c t u r e i n the h y d r a t i o n s h e l l s through which the p r o t o n t r a n s f e r i s mediated ( 9 ) . A m p l i f i c a t i o n o f G e n e r a l i z e d Conclusions In the f o l l o w i n g a m p l i f i c a t i o n o f these g e n e r a l i z a t i o n s , some a t t e n t i o n w i l l be g i v e n t o c o n t r o v e r s i a l aspects o f these statements. I t i s i n t e r e s t i n g t h a t an area o f s c i e n t i f i c study such as p r o t o n t r a n s f e r k i n e t i c s could be an a c t i v e one f o r over 25 y e a r s , p a r t i c u l a r l y because o f r e l a x a t i o n techniques, and s t i l l be one f o r which i t i s d i f f i c u l t t o make many g e n e r a l i z a t i o n s t h a t workers i n the f i e l d can endorse without major r e servations . The f i r s t g e n e r a l i z a t i o n simply a s s e r t s t h a t there a r e many

In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

3.

EYRING E T A L .

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65

r e a c t i o n s i n v o l v i n g a c i d s , such as h y d r o f l u o r i c a c i d and water, f o r which the r a t e s are d i f f u s i o n c o n t r o l l e d w i t h r a t e constants 1

of the order of 1 0 ^ M *s i n aqueous s o l u t i o n f o r the com b i n a t i o n of the i o n s . In f a c t , i t i s g e n e r a l l y found t h a t when the r e a c t i n g p a r t n e r s i n r e a c t i o n 1 have ΔρΚ > 0 ( i . e . , Κ » 1 ) , the value of i s independent of ΔρΚ ( d i f f u s i o n c o n t r o l l e d ) as


i n d i c a t e d by Bronsted p l o t s of l o g k^ v s . ΔρΚ having zero i n t h i s region.

Conversely, when ΔρΚ

of l o g k^ vs. ΔρΚ w i l l have u n i t slope.

< 0 (i.e., Κ «1),

slope a plot

Such Bronsted p l o t s are

discussed e x t e n s i v e l y by E i g e n (_1) and are a l s o considered by Margerum (10). The second statement has to do w i t h the n o t i o n t h a t i n the Eigen mechanism f o r proton t r a n s f e r there must be intermediate ion p a i r s . The reference t o the unpublished work of Kreevoy and Liang (3) r e f l e c t s the impact of t h e i r s t u d i e s on some of our own recent work surveyed below. In f a c t , there i s an extensive p u b l i s h e d l i t e r a t u r e concerning phenol-amine complexes i n which the e x i s t e n c e of the intermediates i n equation 2 has been estab l i s h e d i n d i f f e r e n t organic s o l v e n t s . One of the o l d e s t such papers i s t h a t of B e l l and Barrow (11) going back t o 1959. Others i n c l u d e Hudson and co-workers (12) i n 1972, and Baba and co-workers (13) i n 1969. The next g e n e r a l i z a t i o n , number 3 above, has t o do w i t h the n o t i o n t h a t two simple c a t i o n s w i l l r e a c t w i t h one another l e s s r a p i d l y than a c a t i o n and an anion of corresponding s i z e would. Table I presents examples from the l i t e r a t u r e , where, i n every case, a proton r e a c t s w i t h species of d i f f e r e n t charge types, and there i s a steady decrease i n the r a t e of r e a c t i o n as one proceeds from top t o bottom i n t h a t t a b l e . Table I I summarizes a temperature jump study (14) of the r e a c t i o n of hydroxide i o n w i t h v a r i o u s i n t r a m o l e c u l a r l y hydrogen bonded malonic a c i d monoanions and p o i n t s up the f a c t t h a t , as the s t e r i c hindrance i n c r e a s e s , a c o n s i d e r a b l e strengthening i n the hydrogen bond occurs w i t h a concomitant slowing down of the r a t e a t which the r e a c t i o n proceeds ( g e n e r a l i z a t i o n number 4 ) . At the time the authors d i d not foresee t h a t i t would be pos s i b l e t o d i s t i n g u i s h between whether the hydrogen bond was bro ken d i r e c t l y by the a t t a c k i n g base or whether, i n f a c t , there f i r s t had t o be a c o l l a p s e of the hydrogen bond i n t o an open form o f the anion t h a t would subsequently r e a c t w i t h the base. Thus, they simply p o s t u l a t e d the former mechanism ( d i r e c t a t tack) . G e n e r a l i z a t i o n number 5 r e f l e c t s the work of H i b b e r t and Awwal (6) who have concluded t h a t i t i s the l a t t e r k i n d of mech anism ( i n v o l v i n g the open form of the anion) t h a t p r e v a i l s i n i n t r a m o l e c u l a r hyrogen bond breaking r e a c t i o n s . This i s a p o i n t , however, on which there i s s t i l l room f o r e q u i v o c a t i o n .




66

Table I Experimental Rate Constants f o r Base P r o t o n a t i o n I l l u s t r a t i n g the I n f l u e n c e of I o n i c Charge on P r o t o n a t i o n Reactions i n Aqueous S o l u t i o n (25 C, μ = 0 M) H +

+

A

(n )-l +

Reactants

H

+

H

+

H

+

H

+

H

+

k

$

n

M

+

k , M s

Method

f

+ HS"

7..5 X 1 0

1 0

E-disp

+ N(CH,)„ ? 3 + CuOH + (NH,) CoOH

2, .5 X

10

1 0

NMR

X 10

1 0

Sound

~ 1 2+

+ Pt(en) (en )

Ref:

2

J

+

1,.4 X 10

9

T-jump

2,.6 X 1 0

8

T-jump

E i g e n , M.; Kruse, W.; Maass, G.; DeMaeyer, L. Progr. React. K i n . 1964, 2, 287.

Note t h a t the r a t e constant f o r d i f f u s i o n c o n t r o l l e d r e a c t i o n s between a proton and.a base decreases by a f a c t o r o f 0.3 t o 0.5 f o r each p o s i t i v e charge added to the base.


3.

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Reaction Rates and

67

Mechanisms.

Table I I


Experimental Rate Constant Data I l l u s t r a t i n g the Role o f S t e r i c E f f e c t s i n Strengthening the I n t r a m o l e c u l a r Hydrogen Bond i n the Monoanion o f 2 , 2 - D i s u b s t i t u t e d Malonic A c i d s i n Aqueous S o l u t i o n (25 C, 0.1 M NaClO^) 0 It

r Substituents on Malonic A c i d

k , f

Diethyl 28 Ethyl-n-butyl 16 Ethylisoamyl 16 Ethylphenyl 14 Di-n-butyl 14 Di-n-heptyl 14 Di-n-propyl 13 E t h y l i s o p r o p y 15.5 Diisopropyl 4.5

M"

•1 -1 s

X 7 X 7 X 7 X 7 X 7 X 7 X 7 X 7 X 1θ' 1 0

1 0

1 0

1 0

1 0

1 0

1 0 1 0

+ OH

f

Ε * a

AG*/

5 6 6 6 6 6 6.5 7.5 8

2

A" + H0 2

6 6.5 6.5 6.5 6.5 6.5 6.5 7 7

AH*/

4.5 5.5 5.5 5 5 5 6 7 7.5

A

s

t

f"

-5 -3 -3 -4 -4 -4 -1 0 +2

+3 +6 +6 +2 +6 +6 +6 +12 +16

k Units: kcal/mol — U n i t s : e.u. Ref: M i l e s , M. H.; E y r i n g , Ε. M.; E p s t e i n , W. W.; Ostlund, R. E. J . Phys. Chem. 1965, 69, 467. Note t h a t the primary e f f e c t o f the a l k y l s u b s t i t u e n t s i s s t e r i c , r a t h e r than e l e c t r o n i c , w i t h o n l y branching on the c a r bon attached t o the parent malonic a c i d e f f e c t i v e i n c l o s i n g t h e jaws t o strengthen the i n t r a m o l e c u l a r hydrogen bond. Taking the " m e l t i n g " o f one water molecule from the d i a n i o n t o c o n t r i b u t e 5 e.u. t o A S ^ , °ne may estimate the number o f s o l v e n t molecules t h a t must be removed i n the reverse r e a c t i o n t o form the a c t i vated complex.




68

Perlmutter-Hayman and Shinar (15, 16) have s t u d i e d by tempera ture-jump the r e a c t i o n s o f bases w i t h d i f f e r e n t acid-base i n d i c a t o r s having i n t r a m o l e c u l a r hydrogen bonds. With T r o p a e o l i n 0, d i r e c t a t t a c k o f the base on the hydrogen b r i d g e predominates according t o t h e i r i n t e r p r e t a t i o n , whereas, f o r A l i z a r i n Yellow G, the observed r e l a x a t i o n i s a s c r i b e d c h i e f l y t o d i f f u s i o n c o n t r o l l e d r e a c t i o n between the base and t h a t p a r t o f the i n d i c a t o r present i n the open form. Thus, data e x i s t t h a t l e a d one t o doubt the g e n e r a l i t y o f statement number 5. Statement number 6 has t o do w i t h carbon a c i d s and i s sup ported by reference ( 7 ) . There a r e , i n f a c t , other references t h a t suggest s o l v e n t p l a y s a much more d i r e c t r o l e i n the k i n e t i c s o f p r o t o n a t i n g carbanions than statement number 6 would im p l y . F o r example, there i s evidence t h a t n u c l e a r r e o r g a n i z a t i o n and r e h y b r i d i z a t i o n o f the carbon atom are too r a p i d t o have much k i n e t i c importance when compared w i t h s o l v e n t r e o r i e n t a tion. The s t r o n g dependence o f carbanion p r o t o n a t i o n r a t e s on the s o l v e n t supports t h i s view. These r a t e s are t y p i c a l l y much f a s t e r i n organic s o l v e n t s , such as DMSO, than i n water. A par t i c u l a r r e a c t i o n t h a t was s t u d i e d i n d i f f e r e n t s o l v e n t s (17) i s C(N0 ) " + H 2

3

+

£ HC(N0 ) 2

3

(3)

In cyclohexanol and i n i s o b u t a n o l the r a t e s a r e d i f f u s i o n con t r o l l e d and 10 times f a s t e r than they are i n water, even though i n a l l s o l v e n t s the same r e h y b r i d i z a t i o n occurs. A recent com p a r i s o n (18) o f r a t e s o f p r o t o n a t i o n and methyl mercuration o f d e l o c a l i z e d carbanions i n aqueous s o l u t i o n by Raycheba and Geier a l s o addresses g e n e r a l i z a t i o n number 6. Only the methyl' merc u r a t i o n s are d i f f u s i o n c o n t r o l l e d and three t o four orders o f magnitude f a s t e r than the p r o t o n a t i o n . Thus, the wrong hydrogen bond s t r u c t u r e around the carbanion i n water s t r o n g l y i n h i b i t s proton t r a n s f e r , whereas a t t a c k o f the methyl mercury i o n i s not i n f l u e n c e d because t h i s i o n does not i n t e r a c t s i g n i f i c a n t l y w i t h the hydrogen bonded network. I n reference t o statement number 7, Kresge's k i n e t i c s t u d i e s (8) i n d i c a t e t h a t a proton t r a n s f e r from one oxygen t o another would be f a s t e r than t h a t from a n i t r o g e n t o another nitrogen. Some o f Kresge's recent unpublished work (19) sug gests t h a t the t r a n s f e r o f a proton from phosphorus t o oxygen i s somewhat slower than the corresponding t r a n s f e r between n i trogen and oxygen. While there i s nothing p a r t i c u l a r l y the matter w i t h statement number 7 as w r i t t e n , i t i s one t h a t c l e a r l y i s going t o undergo more e l a b o r a t i o n . The l a s t o f these e i g h t statements has t o do w i t h the idea t h a t , i f a very s m a l l amount o f an organic s o l v e n t such as meth a n o l i s introduced i n t o an aqueous s o l u t i o n , the r a t e o f reac t i o n ( i n v o l v i n g proton t r a n s f e r ) may speed up because o f t h e i n creased hydrogen-bonded water s t r u c t u r e .


3.

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Future Trends Table I I I suggests some o f the proton t r a n s f e r k i n e t i c s t u d i e s one i s l i k e l y t o hear most about i n t h e near f u t u r e . The v e r y f i r s t e n t r y , c o l l o i d a l suspensions, i s one t h a t Pro f e s s o r Langford mentioned e a r l i e r i n these proceedings. I n the r e l a x a t i o n f i e l d , one o f the comparatively new developments has been the measurement o f k i n e t i c s o f i o n t r a n s f e r t o and from c o l l o i d a l suspensions. Yasunaga a t Hiroshima U n i v e r s i t y i s a pioneer i n t h i s type o f study (20, 21, 2 2 ) . H i s students take m a t e r i a l s such as i r o n oxides t h a t form c o l l o i d a l suspensions t h a t do not p r e c i p i t a t e r a p i d l y and measure the k i n e t i c s o f pro ton t r a n s f e r t o the c o l l o i d a l p a r t i c l e s u s i n g r e l a x a t i o n t e c h niques such as the pressure-jump method. Such s t u d i e s engender i n t e r e s t i n quarters t h a t one would not a n t i c i p a t e . F o r example, a c i v i l engineer a t S t a n f o r d U n i v e r s i t y r e c e n t l y sought i n f o r m a t i o n about the e l e c t r i c f i e l d jump (Ε-jump) r e l a x a t i o n technique. I t i s quite surprising that t h i s l e a s t w i d e l y used o f the r e l a x a t i o n methods would appeal t o engineers as a means o f measuring the k i n e t i c s o f t r a n s f e r o f heavy metals t o and from c o l l o i d a l suspensions as i s done i n c l e a r i n g water. T h i s , o f course, i s a very p r a c t i c a l problem f o r which engineers can deduce i n t e r e s t i n g f e a t u r e s from t h i s type o f fundamental k i n e t i c measurement. As f o r s t u d i e s o f i c e , a search o f the recent proton t r a n s f e r l i t e r a t u r e d i s c l o s e s t h a t i c e i s one o f the substances t h a t s t i l l generates i n t e r e s t (23, 24) p a r t i c u l a r l y as i t r e l a t e s t o membrane p r o t o n - t r a n s f e r problems. S o l i d b a t t e r y e l e c t r o l y t e s can a l s o i n v o l v e (25, 26, 27) proton t r a n s f e r s , although these are o b v i o u s l y v e r y slow compared t o t h e kinds o f r a t e s t h a t we are used t o c o n s i d e r i n g i n aqueous i n o r g a n i c s o l u t i o n s . Picosecond time regime k i n e t i c s t u d i e s o f proton t r a n s f e r are coming i n t o vogue (28, 29, 3 0 ) , p a r t i c u l a r l y f o r i n t r a molecular processes t h a t can be v e r y f a s t . Bound t o p l a y an i n c r e a s i n g l y important r o l e i n the e l u c i d a t i o n o f proton t r a n s f e r s are the gas phase i o n - s o l v e n t c l u s t e r techniques that r e v e a l d r a m a t i c a l l y the r o l e played by s o l v e n t molecules i n these r e a c t i o n s (31, 3 2 ) . Dr. Swaddle's d i s c u s s i o n o f volume measurements (33) i s an i n t e r e s t i n g one from our p o i n t o f view, because we l a t e l y have b u i l t an e l e c t r i c - f i e l d - j u m p c e l l t h a t would work a t f a i r l y high pressures. Our reason f o r doing so may be amusing. We thought t h a t perhaps i t would be p o s s i b l e t o s o l v a t e i o n s , o r a t l e a s t i o n p a i r s , u s i n g xenon as a s o l v e n t . This p o s s i b i l i t y had been suggested t o us by the work o f P e t e r Rentzepis ( 3 4 ) . We d i s c o v e r e d , t o our c h a g r i n , t h a t w h i l e one can indeed d i s solve r a t h e r l a r g e molecules, such as lysozyme, i n l i q u i d xenon, none o f the i o n p a i r s t h a t we t r i e d , i n c l u d i n g some v e r y l a r g e ions i n which the charge was spread over a f a i r l y l a r g e s i z e d



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MECHANISTIC ASPECTS OF INORGANIC REACTIONS

Table I I I E i g h t Areas f o r Future Research i n the Study of Proton T r a n s f e r K i n e t i c s

1. 2. 3. 4. 5. 6. 7. 8.

C o l l o i d a l suspensions Ice S o l i d battery electrodes Picosecond time regime Gas phase i o n - s o l v e n t i n t e r a c t i o n s Volumes of a c t i v a t i o n Solvent e f f e c t s Laser induced s o l v e n t i o n i z a t i o n



3.

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Proton-Transfer

Reaction Rates and

71

Mechanisms

molecule, a c t u a l l y d i s s o l v e i n the l i q u i d xenon (35). So we a r e apparently never going t o be doing Ε-jump s t u d i e s a t compara t i v e l y h i g h pressures i n l i q u i d xenon. But t h e r e i s no reason, i n p r i n c i p l e , why one c o u l d not perform e l e c t r i c f i e l d jump k i n e t i c s t u d i e s over an extended range o f pressures on many o f the aqueous systems s t u d i e d p r e v i o u s l y o n l y a t atmospheric pressure and thus deduce volumes o f a c t i v a t i o n f o r p r o t o n t r a n s f e r and other r a p i d r e a c t i o n s i n v o l v i n g charge n e u t r a l i z a t i o n . Before t r e a t i n g the l a s t two t o p i c s i n Table I I I l e t us consider a r h e t o r i c a l q u e s t i o n : I f indeed many o f the problems r e l a t i n g t o p r o t o n t r a n s f e r are reasonably w e l l s o l v e d from the p o i n t o f view o f someone who, l i k e Dale Margerum, goes ahead and measures k i n e t i c s o f l i g a n d exchange, a r e the k i n e t i c s o f p r o t o n t r a n s f e r s i n homogeneous aqueous and non-aqueous s o l u t i o n s s t i l l going t o be s t u d i e d f o r other reasons? The obvious answer i s "Yes" s i n c e one w i l l use proton t r a n s f e r k i n e t i c s s t u d i e s as a t o o l f o r i n v e s t i g a t i n g other p r o p e r t i e s o f chemical systems. In p a r t i c u l a r , we have been v e r y much i n t e r e s t e d i n u s i n g p r o t o n t r a n s f e r k i n e t i c s as a means o f measuring how i o n s o l v a t i o n changes as a s o l u t e e q u i l i b r i u m i s t r a n s f e r r e d from one organic s o l v e n t t o some other o r g a n i c s o l v e n t . The t o o l t h a t we have used i n most o f these s t u d i e s has been the e l e c t r i c f i e l d - j u m p technique. The square, h i g h v o l t a g e wave instrumenta t i o n w i t h spectrophotometric d e t e c t i o n (36,37) i s v e r y d i f f e r e n t from the d i s p e r s i v e Ε-jump apparatus w i t h a Wheatstone b r i d g e d e t e c t o r (38) t h a t Ken K u s t i n taught me how t o use i n E i g e n s l a b o r a t o r y over 20 years ago. I n the present day instrument the e x p o n e n t i a l decay i n the p h o t o m u l t i p l i e r v o l t a g e , a f t e r t h e h i g h v o l t a g e has been taken o f f the sample c e l l , y i e l d s t h e chemical r e l a x a t i o n ( o r r e l a x a t i o n s ) o f the chemical e q u i l i b r i u m (or e q u i l i b r i a ) . The time constant o f the l a s t e x p o n e n t i a l decay i s the chemical i n f o r m a t i o n o f i n t e r e s t i n such p r o t o n transfer kinetic studies. 1

Proton T r a n s f e r s i n Methanol, A c e t o n i t r i l e , and B e n z o n i t r i l e Methanol i s one o f the easy s o l v e n t s t o work w i t h u s i n g the e l e c t r i c - f i e l d - j u m p technique. The p r e p a r a t i o n o f the s o l v e n t i s not n e a r l y as arduous as i s t h a t o f some other s o l v e n t s such as a c e t o n i t r i l e . I n methanol we observed t h a t p i c r i c a c i d anion protonates a t the d i f f u s i o n c o n t r o l l e d r a t e whereas d i p i c r y l amine s t e r i c a l l y hinders the p r o t o n from recombining w i t h i t . (39).



72


In another study (40) we found t h a t p r o t o n a t i o n o f p y r i d i n e i s d i f f u s i o n - c o n t r o l l e d w i t h a one-to-one solute-methanol com p l e x as the r e a c t i v e s p e c i e s . Thus, w h i l e methanol p l a y s essen t i a l l y no r o l e i n the p r o t o n t r a n s f e r t o d i p i c r y l a m i n e i n t h e f i r s t study, i t i s indeed i n t i m a t e l y i n v o l v e d i n the p r o t o n transfer to pyridine. Now l e t us c o n s i d e r the r e s u l t s o f an Ε-jump study (14) o f p r o t o n t r a n s f e r between p i c r i c a c i d (A) and methyl r e d (B) k

AH + Β

l

2

A

in acetonitrile.

y

+ +

HB

(4)

The formula f o r methyl red i s

r-COOH N(CH ) 3

2

Rates a r e found t o be a f a c t o r o f t e n slower than d i f f u s i o n con t r o l suggesting a requirement f o r s o l v e n t r e o r g a n i z a t i o n around the s t r o n g l y s o l v a t e d c a t i o n s . Another p r o t o n t r a n s f e r s t u d i e d by the Ε-jump technique i n a c e t o n i t r i l e (42) i s t h a t between p - n i t r o p h e n o l (AH) and t r i ethylamine ( B ) . The e x t i n c t i o n c o e f f i c i e n t s f o r each o f t h e species i n t h e f o l l o w i n g e q u i l i b r i u m have been measured by Kreevoy and L i a n g ( 3 ) : +

AH + Β £ AH...B £ A"... HB J A" + BH λ = 306 nm 320 400 427

+

(5)

One can be e x c i t e d about a s p e c t r o p h o t o m e t r y Ε-jump study o f t h i s system because, i n p r i n c i p l e , i t should be p o s s i b l e t o mea sure t h e r e l a x a t i o n times a s s o c i a t e d w i t h each o f the s u c c e s s i v e equilibria. The i n t e r m e d i a t e s are s t a b l e , b u t t h e r e l a x a t i o n data a r e c o n s i s t e n t w i t h the s i n g l e e q u i l i b r i u m


3.

EYRING E T A L .


AH + B j> A

Proton-Transfer

Reaction Rates and

Mechanisms

+ BH

73 (6)

r e g a r d l e s s of the m o n i t o r i n g wavelength. The i o n recombination 9 -1 -1 r a t e constant, ~9 χ 10 M s , i s 5 times slower than a d i f fusion controlled rate. A requirement f o r s o l v e n t r e o r g a n i z a t i o n around the c a t i o n i s again p o s t u l a t e d . Thus, i n both cases i n v o l v i n g a c e t o n i t r i l e , s o l v e n t movement i s i n t i m a t e l y involved i n proton t r a n s f e r . Whereas i n a c e t o n i t r i l e the r a t e l i m i t i n g step i s an open ing of the s o l v e n t s h e l l of a r e a c t a n t , i n b e n z o n i t r i l e the back r e a c t i o n of (5) between the protonated a c r i d i n e orange c a t i o n (BH ) and the 3-methyl-4-nitrophenolate i o n (A ) t o form the i o n p a i r i s d i f f u s i o n c o n t r o l l e d (although the o v e r a l l r e a c t i o n t o the n e u t r a l molecules i s an endothermic p r o c e s s ) . Because of i t s lower d i e l e c t r i c constant than a c e t o n i t r i l e , the e l e c t r o s t a t i c i n t e r a c t i o n s between r e a c t a n t s i n b e n z o n i t r i l e outweigh s p e c i f i c solvent effects. I n other words, i n b e n z o n i t r i l e a r a t e l i m i t i n g c o u p l i n g of p r o t o n t r a n s f e r t o the r e o r i e n t a t i o n of s o l v e n t d i p o l e s does not occur and the measured r a t e s are v e r y f a s t . The i o n recombination ( I ) + ( I I ) ·+ i n b e n z o n i t r i l e has a d i f f u s i o n c o n t r o l l e d s p e c i f i c r a t e ( t h e o r e t i c a l ) k = 9 -1 -1 (4.3 - 5.6) χ 10 M s and a measured (T-jump) s p e c i f i c r a t e k = (3.5 ± 0.8) χ 10 M" S " a t 0.1 M i o n i c s t r e n g t h . Table IV i s an attempt t o summarize the r e s u l t s of these p r o t o n t r a n s f e r s t u d i e s i n nonaqueous s o l v e n t s . There i s no systematic t r e n d i n what seems t o be the r a t e l i m i t i n g step i n c o n t r a s t t o the a t t r a c t i v e E i g e n - W i l k i n s g e n e r a l i z a t i o n f o r the mechanism of metal i o n complexation. O b v i o u s l y , many more pro t o n t r a n s f e r k i n e t i c s t u d i e s i n nonaqueous s o l u t i o n s are needed f o r b e a u t i f u l g e n e r a l i z a t i o n s t o emerge. Whether i n v e s t i g a t o r s w i l l have the p a t i e n c e t o c a r r y them out or not i s the o n l y uncertainty. 9

1

1

Proton T r a n s f e r i n IR Laser E x c i t e d Solvents Another s i t u a t i o n i n which an a l r e a d y w e l l - s t u d i e d p r o t o n t r a n s f e r r e a c t i o n serves as a probe o f a p h y s i c a l phenomenon has been suggested by K n i g h t , Goodall and Greenhow (43, 44). They i o n i z e d water w i t h s i n g l e photons of Nd:glass l a s e r i n f r a r e d r a d i a t i o n and measured an i o n recombination r a t e constant f o r the r e a c t i o n +

H ( a q ) + 0H~(aq)

H 0(£) 2

(7)

i n e x c e l l e n t agreement w i t h t h a t r e p o r t e d by E i g e n and De Maeyer (45). One might a t f i r s t wonder why, more than two decades a f t e r the c l a s s i c Eigen-De Maeyer experiments, someone would remeasure the k i n e t i c s of i o n recombination i n water. The


74



Table IV Summary o f the R e l a t i o n s h i p s between Nonaqueous Solvent P r o p e r t i e s and Rate L i m i t i n g Steps f o r Proton T r a n s f e r

Solvent Type

Solvents

Rate L i m i t i n g Step

Polar, Protic

Water, Methanol

Polar, Aprotic Moderately P o l a r , Aprotic Low P o l a r , A p r o t i c

Acetonitrile Benzonitrile

D i f f u s i o n together of r e a c t a n t s Solvent r e o r g a n i z a t i o n D i f f u s i o n together of reactants R o t a t i o n o f encounter complex

Chlorobenzene

100

Wavelength, microns Figure 1. The near IR absorption spectrum of anhydrous liquid hydrofluoric acid with a few of the many possible IR wavelengths obtainable from a neodymiumdoped glass laser superimposed.



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Reaction Rates and

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75

i n t r i g u i n g aspect o f such an experiment i n any neat s o l v e n t i s t h a t the i o n - c r e a t i n g mechanism competes s u c c e s s f u l l y i n time w i t h the t h e r m a l i z a t i o n o f the near i n f r a r e d l a s e r energy de p o s i t e d i n the s o l v e n t v i b r a t i o n s . C l e a r l y , one ought to study other u l t r a p u r e , a u t o i o n i z i n g s o l v e n t s to see whether there i s something i n t r i n s i c a l l y p e c u l i a r about water, such as i t s hydro gen bonded s t r u c t u r e , t h a t makes p o s s i b l e a long l i f e t i m e f o r the d e r e a l i z a t i o n o f the thermal energy deposited by the l a s e r . As F i g u r e 1 suggests, i t i s now easy t o generate i n f r a r e d p u l s e s from a l a s e r at a v a r i e t y o f wavelengths and thus e x c i t e a p a r t i c u l a r s o l v e n t at many d i f f e r e n t i n f r a r e d wavelengths t o see what impact t h a t v a r i a b l e has on t h i s process o f c r e a t i n g i o n s i n the pure s o l v e n t . Hydrogen f l u o r i d e i s the most obvious o f s e v e r a l a u t o i o n i z i n g s o l v e n t s t h a t an i n o r g a n i c chemist could probe i n t h i s f a s h i o n . Acknowledgments We thank Professors M. M. Kreevoy and A . J . Kresge f o r en l i g h t e n i n g comments and the donors o f the Petroleum Research Fund, administered by the American Chemical S o c i e t y , f o r p a r t i a l support o f t h i s r e s e a r c h .

Literature Cited 1. Eigen, M. Angew. Chem., Int. Ed., Engl. 1964, 3, 1. 2. Bell, R. P. "The Proton in Chemistry;" Chapman and Hall, London, 1973, pp.130, 194. 3. Kreevoy, M. M.; Liang, Τ. Μ., unpublished work. 4. De Maeyer, L.; Kustin, K. Ann. Rev. Phys. Chem. 1963, 14, 5. 5. Bell, R. P. op. cit., pp. 130, 131. 6. Hibbert, F.; Awwal, A. J. Chem. Soc., Perkin II 1978, 939. 7. Okuyama, T.; Ikenouchi, Y.; Fueno, T. J. Am. Chem. Soc. 1978, 100, 6162. 8. Kresge, A. J. Pure Appl. Chem. 1981, 53, 189. 9. Nicola, C. U.; Labhardt, Α.; Schwarz, G. Ber. Bunsenges. Phys. Chem. 1979, 83, 43; for an alternative view see Symons, M. C. R. Acc. Chem. Res. 1981, 14, 179. 10. Margerum, D. W. This Volume, American Chemical Society: Washington, D. C., 1982. 11. Bell, C. L.; Barrow, G. M. J. Chem. Phys. 1959, 31 1158. 12. Hudson, R. Α.; Scott, R. M.; Vinogradov, S. N. J. Phys. Chem. 1972, 76, 1989. 13. Baba, H.; Matsuyama, Α.; Kokubun, H. Spectrochim. Acta 1969, 25A, 1709. 14. Miles, M. H.; Eyring, Ε. M.; Epstein, W. W.; Ostlund, R. E. J. Phys. Chem. 1965, 69, 467. 15. Perlmutter-Hayman, B.; Shinar, R. Int. J. Chem. Kinet. 1977, 9, 1.



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16. Perlmutter-Hayman, B.; Shinar, R. Int. J. Chem. Kinet. 1978, 10, 407. 17 Chaudri, S. Α.; Asmus, K.-D. J. Chem. Soc., Faraday I 1972, 68, 385. 18. Raycheba, J. M. T.; Geier, G. Inorg. Chem. 1979, 18, 2486. 19. Kresge, A. J. Private communication. 20. Ashida, M.; Sasaki, M.; Kan, H.; Yasunaga, T.; Hachiya, K.; Inoue, T. J. Colloid Interface Sci. 1978, 67, 219. 21. Hachiya, K.; Ashida, M.; Sasaki, M.; Kan, H.; Inoue, T.; Yasunaga, T. J. Phys. Chem. 1979, 83, 1866. 22. Astumian, R. D.; Sasaki, M.; Yasunaga, T.; Schelly, Z. A. J. Phys. Chem., in press. 23. Knapp, E. W.; Schulten, K.; Schulten, Z. Chem. Phys. 1980, 46 215. 24. Kunst, M.; Warman, J. M. Nature 1980, 288, 465. 25. Farrington, G. C.; Briant, J. L. Mat. Res. Bull. 1978, 13, 763. 26. Farrington, G. C.; Briant, J. L. Science 1979, 204, 1371. 27. Roth, W. L.; Anne, M.; Tranqui, D. Revue de Chimie Minerale 1980, 17, 379. 28. Smith, Κ. K.; Kaufmann, Κ. J.; Huppert, D.; Gutman, M. Chem. Phys. Lett. 1979, 64, 522. 29. Hetherington, W. Μ., III; Miukeels, R. H.; Eisenthal, Κ. B. Chem. Phys. Lett. 1979, 66, 230. 30. Barbara, P. F.; Brus, L. E.; Rentzepis, P. M. J. Am. Chem. Soc. 1980, 102, 5631. 31. Farneth, W. E.; Brauman, J. I. J. Am. Chem. Soc. 1976, 98, 7891. 32. McDonald, R. N.; Chowdhury, A. K.; Setser, D. W. J. Am. Chem. Soc. 1980, 102, 4836. 33. Swaddle, T. W. This Volume, American Chemical Society: Washington, D. C., 1982. 34. Rentzepis, P. M.; Douglass, D. C. Biophys. J. 1981, 33, 271a. 35. Marshall, D. B.; Strohbusch, F.; Eyring, E. M. J. Chem. Eng. Data 1981, 26, 333. 36. Olsen, S. L.; Silver, R. L.; Holmes, L. P.; Auborn, J. J.; Warrick, P., Jr.; Eyring, Ε. M. Rev. Sci. Instrum. 1971, 42, 1247. 37. Olsen, S. L.; Holmes, L. P.; Eyring, Ε. M. Rev. Sci. In strum. 1971, 45, 859. 38. Eigen, M.; De Maeyer, L. "Investigation of Rates and Mechanisms of Reactions," Technique of Organic Chemistry, Vol. 8, Part 2, Friess, S. L.; Lewis, E. S.; Weissberger, A. Eds.; Interscience Publishers: New York, 1963; p. 988. 39. Strohbusch, F.; Marshall, D. B.; Vazquez, F. Α.; Cummings, A. L.; Eyring, E. M. J. Chem. Soc., Faraday I 1979, 75, 2137.



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ET

AL.

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Reaction Rates and

Mechanisms

77

40. Marshall, D. B.; Eyring, Ε. M.; Strohbusch, F.; White, R. D. J. Am. Chem. Soc. 1980, 102, 7065. 41. Strohbusch, F.; Marshall, D. B.; Eyring, E. M. J. Phys. Chem. 1978, 82, 2447. 42. Marshall, D. B.; Strohbusch, F.; Eyring, E. M. J. Phys. Chem. 1981, 85, 2270. 43. Goodall, D. M.; Greenhow, R. C. Chem. Phys. Lett. 1971, 9, 583. 44. Knight, B.; Goodall, D. M.; Greenhow, R. C. J. Chem. Soc., Faraday II 1979, 75, 841. 45. Eigen, M.; De Maeyer, L. Z. Elektrochem. 1955, 59, 986. RECEIVED April 5,1982.


General Discussion—Proton-Transfer Reaction Rates and Mechanisms


Leader:

Ramesh Patel

DR. RAMESH PATEL (Clarkson C o l l e g e ) : I t appears t h a t s t u d i e s on c o l l o i d a l systems may represent an extremely impor t a n t area f o r the f u t u r e . We have a l s o been doing some c o l l o i d a l work, p a r t i c u l a r l y d e a l i n g w i t h the s o l u t i o n chemistry t h a t precedes the formation of very h i g h l y monodispersed c o l l o i d a l p a r t i c l e s . One such system w i t h i r o n phosphate has been i n c l u d e d i n the p o s t e r p r e s e n t a t i o n . Yasunaga has s t u d i e d c o l l o i d a l systems i n v o l v i n g t i t a n i u m d i o x i d e . What I f i n d very curious i f t h a t he r e p o r t s t h a t the recombination r a t e of hydroxyl ions r e a c t i n g w i t h the t i t a n i u m i s orders of magnitude s m a l l e r than what one f i n d s i n other sys tems, i n c l u d i n g i c e , of course, and other s o l u t i o n s . I was won d e r i n g whether you have any comment. DR. EYRING: I have no f a c t , what you say i s t r u e .

explanation

f o r i t e i t h e r , but

in

DR. PATEL: One reason f o r much of the i n t e r e s t which pre v a i l s i n t h i s area r i g h t now, e s p e c i a l l y w i t h i r o n ' I I ) , has to do w i t h the c o r r o s i o n of s t e e l i n i n d u s t r y and a l s o i n nuclear r e a c t o r s . Normally one t h i n k s of forming p r e c i p i t a t e s or par t i c l e s by adding base t o a s o l u t i o n and c o o l i n g i t down. I f i r o n ( I I I ) s o l u t i o n s are made more a c i d i c and i f you r a i s e the temperature, these c o n d i t i o n s l e a d t o the formation of v e r y , very w e l l - d e f i n e d p a r t i c l e s . A very important event i n t h i s i s the p r o t o n t r a n s f e r k i n e t i c s t h a t l e a d to the formation of the h y d r o l y s i s of many of these t r i v a l e n t i o n s . DR. THOMAS MEYER ( U n i v e r s i t y of North C a r o l i n a ) : F i r s t , do you have any comments to make about chemical r e a c t i o n s i n which proton t r a n s f e r accompanies e l e c t r o n t r a n s f e r ? Second, do you have any comments to make on s i t u a t i o n s where p r o t o n t r a n s f e r takes p l a c e between i n t e r f a c e s , e.g., from one s o l v e n t to anoth er or perhaps from a s o l v e n t i n t o a membrane? DR. EYRING: C e r t a i n l y the l a t t e r of the two subjects you are t a l k i n g about i s one t h a t i s of p a r t i c u l a r i n t e r e s t t o us. We have i n v e s t e d a great d e a l of our recent e f f o r t on a t e c h nique c a l l e d photoacoustic spectroscopy, which we initially thought we could use f o r l o o k i n g a t r e a c t i o n s o c c u r r i n g a t a surface. Up to t h i s p o i n t , however, we have been disappointed because, i n f a c t , the s i g n a l - t o - n o i s e r a t i o i s so bad t h a t i t would take a very long time to o b t a i n s a t i s f a c t o r y data. Thus, the r e a c t i o n would have to be extremely slow before we would be able t o say anything about i t from photoacoustic spectroscopic measurements.


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Reaction Rates and

Mechanisms

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DR. EPHRAIM BUHKS ( U n i v e r s i t y of Delaware): I would l i k e to ask your o p i n i o n about a p o s s i b l e i n t e r p r e t a t i o n of proton t r a n s f e r i n terms of n u c l e a r t u n n e l i n g e f f e c t s . Might i t be p o s s i b l e t h a t as the energy of the v i b r a t i o n a l modes becomes very l a r g e , the c l a s s i c a l r a t e theory might not work? DR. EYRING: W e l l , I t h i n k we are a l l conscious of the f a c t t h a t B e l l w r i t e s e x t e n s i v e l y on the s u b j e c t of t u n n e l i n g i n connection w i t h proton t r a n s f e r . I n f a c t , there i s a recent book t h a t was p u b l i s h e d w i t h i n the l a s t year t h a t i s on t h a t p a r t i c u l a r t o p i c [ B e l l , R. P. "The Tunnel E f f e c t i n Chemistry"; Chapman and H a l l : London, 1980]. DR. NORTON (Colorado State U n i v e r s i t y ) : As a novice i n t h i s f i e l d , something t h a t i s s t a r t i n g t o worry me about metal l i c systems i s the d i f f i c u l t y of d i s t i n g u i s h i n g between an orthodox proton t r a n s f e r as opposed to an e l e c t r o n t r a n s f e r f o l l o w e d by hydrogen atom t r a n s f e r i n the reverse d i r e c t i o n . I s t h i s ever a problem i n the kinds of more c l a s s i c a l systems you have been d e s c r i b i n g ? DR. EYRING: I f i t i s , I am not aware of i t . One of the a t t r a c t i v e f e a t u r e s about the e a r l y f a s t r e a c t i o n s t u d i e s of proton t r a n s f e r systems i s t h a t they were comparatively simple. Some of the problems which have been mentioned today, such as p e r c h l o r a t e ions causing c o m p l i c a t i o n s , are not present i n e x t r a pure water t o which Dr. Ken K u s t i n has added j u s t a t r a c e of HF. I f you take a membrane or some other k i n d of a s u r f a c e , the proton t r a n s f e r i s c e r t a i n l y a great d e a l more complex. My research group has not t r i e d t o do i t . We have gone o f f on a tangent, where we went l o o k i n g f o r a probe t h a t would be u s e f u l f o r l o o k i n g a t surfaces k i n e t i c a l l y , and d i s c o v e r e d , to our c h a g r i n , t h a t photoacoustic spectroscopy i s much more s u i t able f o r i n v e s t i g a t i n g the i n f r a r e d spectrum of p o l y a c e t y l e n e . That happens t o be an e x c i t i n g t o p i c , and there are indeed p u b l i c a t i o n s a r i s i n g from our work on photoacoustic s p e c t r o scopy. But I must confess, i t i s a l i t t l e f r u s t r a t i n g t o have become i n v o l v e d i n a f i e l d l i k e t h a t , f u l l y i n t e n d i n g t o use i t as a k i n e t i c t o o l , and then d i s c o v e r i n g t h a t i t j u s t i s n ' t very suitable. I t may be s u i t a b l e f o r making k i n e t i c s t u d i e s a t electrode surfaces. Bard a t Texas has done some i n t e r e s t i n g experiments which suggest t h a t i f one i s l o o k i n g a t a s m a l l d i f f e r e n c e between two b i g f e a t u r e s , as may be the case a t the s u r f a c e of an e l e c t r o d e , perhaps photoacoustic spectroscopy may be used f o r k i n e t i c measurements. But f o r the kinds of sys tems t h a t we thought would be i n t e r e s t i n g , such as proton t r a n s f e r t o and from a membrane, t h i s technique does not appear t o be a promising k i n e t i c t o o l . DR. anything

J . KERRY THOMAS ( U n i v e r s i t y of Notre Dame): I s there wrong w i t h p u t t i n g a f l u o r e s c e n t probe t h a t i s pH



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dependent i n these systems? Fluorescence i s a very s e n s i t i v e method. I t i s used i n many membrane s t u d i e s where m o n i t o r i n g o f a s p e c i f i c process i s r e q u i r e d . One could l o c a t e such a probe i n a s e l e c t e d p o s i t i o n and e s s e n t i a l l y use t h i s as a method f o r checking d i f f u s i o n i n t h a t r e g i o n . We do t h i s i n m i c e l l e sys tems, and i t i s a r e l a t i v e l y easy method. That i s d i s t i n c t l y p o s s i b l e .


DR. EYRING:


Proton-Transfer Reaction Rates and Mechanisms - ACS Publications

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