Radiation-chemical aspects of chemical evolution and radiation chemistry of simple cyano compounds

Radiat. Phys. Chem. Vol. 15, pp. 195-201 Pergamon Press Ltd., 1980. Printed in Great Britain

RADIATION-CHEMICAL ASPECTS OF CHEMICAL EVOLUTION A N D RADIATION CHEMISTRY OF SIMPLE C Y A N O C O M P O U N D S IVAN G. DRAGANI(~ and ZORICA D. DRAGANI(~t Boris Kidri~ Institute of Nuclear Sciences, Vin6a, Yugoslavia

(Received 20 February 1979) Abstract--The role of ionizing radiation as one of energy sources for chemical evolution processes is examined. The discovery of fossile nuclear reactors in old uranium deposits of Oklo (Africa) suggests that natural nuclear reactors could have have been the sources of abundant, localized, radiation in the past of our planet. A brief survey of radiation chemistry of aqueous solutions of cyanides and simple nitriles is given. Some results obtained at BKI (Vin~a) suggest that simple cyano compounds could have been an important raw material for primordial organic synthesis in aqueous medium and ionizing radiation as the source of energy. It has been concluded that the studies of chemical evolution can be a rewarding domain for radiation chemistry to transfer some of its achievements. CHEMICAL E V O L U T I O N A N D R A D I A T I O N CHEMISTRY The studies of chemical evolution concern the sequence of events on the primitive Earth that led from simple inorganic compounds to complex organic molecules and eventually to living matter. We know nothing precise about the time of its beginning and its duration. It could start only after the planet's surface became hospitable for organic synthesis and it had to end when the abundance of life made it hostile for primodial organic synthesis. ") This occurred somewhere between 4.1 and 3.2 billion years ago, although it is not unreasonable to assume that chemical evolution proceeded up to the beginning of Middle Precambrian, simultaneously with development of scarce, primitive, forms of life. '2) It is generally admitted that ionizing radiation was one of the energy sources for chemical evolution processes. The estimates of its abundance on the early Earth depend on the model used in calculation. The conclusions generally reached show ") that its amount could have been of the same order of magnitude as that of electric discharges, shock waves or UV radiations with wavelengths shorter than 1500 ,~,. These estimates neglect a probable contribution of natural nuclear reactors and radioactive elements produced by them. t3) They take into account only radioactive *Present address: Dr. Zorica Dragani6, Boris Kidri~ Institute of Nuclear Sciences, P.O. Box 522, 11001 Beograd, Yugoslavia. 195

substances dispersed in the rocks and this might be the reason why ionizing radiation is considered by some chemists as a minor partner in the group of energy sources. Radiation chemistry has presently still a modest place in the interdiscliplinary approach to prebiological evolution. Radiation chemists very rarely direct their experiments towards the problems of prebiotic chemistry. The reason is that phenomena of interest occur mainly at larger radiation doses and usually with low radiation-chemical yields. And just the contrary is preferred: large radiation yields that can be reliably determined, and low doses where the accumulation of other radiolytic products is small and can be neglected in a kinetic treatment of reaction schemes. Chemists who occasionally use ionizing radiation as a tool to produce a chemical change profit little, if at all, from considerable theoretical achievements in radiation chemistry that could be helpful in gaining a deeper insight into the examined prebiotic phenomena. There are many reasons to believe that radiation-induced chemical reactions could be of interest to aqueous prebiotic chemistry, ta'4) The energy of ionizing radiation is deposited in radiation paths in water and chemical changes occur mainly along radiation tracks. Once formed, the products of these reactions diffuse away and can escape the degradation due to the continued action of radiation. Other advantages of ionizing radiation over other types of energy are: (i) In dilute solutions, such as those of interest to prebiotic chemistry,

196

I . G . DRAGANI¢and Z. D. DRAGANI~

the energy is deposited in water only. (ii) Oxygen, if initially present in water, is consumed already at lower radiation doses and the radiolysis proceeds in anoxic conditions. (iii) The free-radicals produced by radiation induce chemical changes reacting with solutes or with themselves. Their reactions are known to be quite independent in wider ranges of temperature, solute concentration and solution's pH. Also, they are known to induce oligomerizations and polymerizations without the need for a catalyst or specific conditions.

mines in Gabon (Africa). Its basic aspects seem to be well understood after two international meetings. t8'9~ The conditions of criticallity were met about 2 billion years ago, when natural uranium contained 3% of 235U. The chain fission processes were maintained during several hundred thousand years in six distinct zones, because of an efficient neutron moderation (convenient H20/uranium ratio) and reasonable amounts of nuclear impurities. A generally accepted view "°~ is that the conditions of precambrian Earth were such that the Oklo phenomenon was not a singular occurrence, and the search for other fossile nuclear reactors is g o i n g o n . tg) Nevertheless, their early occurrence and possible role as energy sources for evolutionary processes has not been considered, and we have examined the requirements for criticallity on the early Earth, when the abundance of 23~U was between 20 and 10%. "') In this calculation the chemical composition of uranium deposit is assumed to be the same as in the Oklo case, as none of the chemical characteristics of the Oklo deposits appear to be particularly unique. Water was taken as moderator, and H20/UO2 mass ratio was varied between 0.05 and 1.5. The "naked" sphere model was used in computer calculations of the radius of critical sphere (Rcr, in meters) for various UO2 contents. Figure 1 shows that 3--6% of uranium could have been enough for triggering a chain fission process. The radius of critical sphere depends strongly on the amount of water present and, in above conditions, could be about 1 meter only provided that H20/UO2 mass ratio is adequate (see the insert in Fig. 1). The geochemistry of uranium-ore forming processes on the early Earth is far from being under-

NATURAL NUCLEAR REACTORS AND O T H E R SOURCES O F I O N I Z I N G R A D I A T I O N S ON T H E E A R L Y EARTH The sources of ionizing radiations of interest to chemical evolution processes are given in Table I. Natural nuclear reactors merit a particular attention both because of energy liberated during chain fission processes and ionizing radiations emitted by fission products. These reactors were suitable, natural, arrangements of uranium and water which enabled the chain fission of uranium 235. Fissionable uranium 235 decays with a half-life of 0.7 billion years and its concentrations on the early Earth were up to thirty times larger than 0.7% presently. Because of its larger concentrations the criticallity was easier to achieve in the past of our planet, as noted already 20 yr ago by Kuroda. <5~He suggested that fossile nuclear reactors should exist in deposits having an age of about 2 billion years and that the absence of large uranium deposits older than this period of time could result from their destructive disintegration during criticallity. The first evidence of a natural (fossile) nuclear reactor was reported in 1972 as the Oklo phenomenon/~'7~ named after the Oklo uranium

TABLE |. THE SOURCES OF IONIZING RADIATION ON PRIMITIVE EARTH Source Remark C

e

r

t

a

40 K

i

n

Half-life 1.31x109 years

23~h

Half-life 1.41x10 I0 years

235u

Half-life

0.7x109

years

238 U

Half-life 4.5xi09

years

244pu

Half-life

years

P

r

o

b

Natural nuclear reactors, and various radioactive nuclei which were produced by chain fission processes. P

o

s

s

Superheavy elements in the island of stability, of atomic ntm%bers around 114.

a

b

1

7.6x107

e

Tb~ sate radiations and radioelements as in man-made nuclear reactors.

i

b

1

e

Half-lives estimated on theoretical gounds between 108 and 109 years.

197

Radiation-chemical aspects of chemical evolution

I 3f "

,

(4.1 109yr ago) 5%UOZx

15

0 I0

II.o

11.5 H20/UO2

3.2 x 109 yr ogo

to i .c_ ct 5

oL.5

I

xOXe,.

9 4.1xlO yrago

I

I

0..5 LO H20/UO 2 (mass ratio)

I

1.5

FIG. 1. The requirements for criticality in a sphere with constant radius, Re, in meters: 3 m (O, A) or 2 m (0, A). Insert: Dependence of Rcr on H20/UO2 at constant UO2 content. According to Ref. 11. stood, but it seems certain that in anoxic atmosphere many sediments were laid down which are presently of economic significance for their content of gold and/or uranium. The origin of these gold-uranium reefs is discussed elsewhere <2~>and points out to the possibility of early formation of uranium deposits by exogenic processes, composed from a cycle of weathering-erosion-transportation-sedimentation steps. It should be noted that plutonium 244 is given in the group of certain sources. The evidence for its presence on the early Earth is obtained from the isotopic anomaly of the stable isotope into which it decays. "2~ Superheavy elements are recently reported in the mineral monazite from Madagascar. "3~ Search for further evidence of their presence in the Earth's crust is still going on and they are given in the group of possible sources. More work is needed before a conclusion is reached on the significance of these radioactive elements as energy sources for chemical evolution. S I M P L E C Y A N O C O M P O U N D S AS A R A W M A T E R I A L FOR P R I M O R D I A L ORGANIC SYNTHESIS It is almost certain that hydrocyanic acid and some of its alkyl derivatives were present both in the atmosphere and in waters covering the surface of primitive Earth. This is suggested by their abundance in the interstellar space as well as the easiness by which they form in a variety of prebiotic experiments. They have been frequently and

successfully used in prebiotic simulation experiments.(", ,5~ Simple cyano compounds represent also a challenging starting material in radiation-chemical approaches to the problems of primordial organic synthesis. With an aqueous solution of a cyanide or nitrile we have a well defined model system: all main atom constituents of biomonomers (C, N, H and O) are present in water to which we can supply a well defined amount of energy. The freeradical model of water radiolysis offers a useful tool for a quantitative correlation of the energy input and the amounts of the products formed. It helps also to get an insight into the reaction kinetics and considerations of probable reaction schemes. Fast experimental techniques, which are currently used in radiation chemistry, enable us to get the information on free-radicals, the shortlived species that play important roles in observed chemical changes. An investigation of radiationinduced chemical changes in aqueous simple cyano compounds is in progress and Table 2 gives some information on the published work. RADIOLYSIS OF AQUEOUS SOLUTIONS OF SIMPLE CYANO COMPOUNDS Several nanoseconds after the passage of ionizing radiation through an oxygen-free, dilute, aqueous solution, the cyano molecules are attacked by the products of water radiolysis, H, OH and e~q. The main point of attack is the carbon-nitrogen triple bond in the cyano group (1)

H+RCN---}RC(H)==lq

(2) OH + RCN ~ RC(OH)==lq

or or

Rt~--NH Rt~=NOH

(3) e~-q+ R C N ~ (RCN)-. Amino and imino groups are also involved when the solute is cyanamide or dicyandiamide. Table 3 summarizes the rate constant values of reactions (1)-(3). It has been noted that the values of k(e~q + RCN) satisfy Taft's empirical relation <37~and support the assumption that e~q reacts only by addition to the cyano group. The inductive effect of the substituent R determines the rates constants of reactions e S , + R C N , except for NHzC(=NH)NHCN where also other functional groups are involved. Free-radicals Table 4 lists the data on free-radicals observed by fast kinetics spectrophotometry in pulsed electron beam experiments. Computer calculated concentrations had to be used when registered absorbance was composed of contributions of

198

I. G. DRAGANI(~and Z. D. DRAGANI(~ T A B L E 2. RADIOLYSIS OF AQUEOUS SOLUTIONS OF CYANIDES AND NITRILES

Workin~ oonditions RCN

HCN w CN-

Reference

R:H, CH3, C2H5, CH2C~, (CH2)2CN y-radiolysis in Krad dose ranget pH 2 and 6

16

Pulse radiolysis Pulse radiolysis, pH 3,7-14

17 18

Pulse radiolysis, pH 1,9-15

19

KCN

y-radiolysis, pH ii

20

(C~) 2

Pulse radiolysis, pH 6

21

7-radiolysis in Mrad dose-range

22

Amino acids in hydrolysates of radiolyhic products, pH 6

23

Pulse radiolysis y-radiolysis in Krad and Mrad dose-range¢

24 25

C2H5CN

~2C~

pH 2,4 and 5. h~2C(=NH)~CN

Pulse radiolysis; y-radiolysis in the Krad and Mrad dose-range, pH 2,4 and 6.

26

y-radiolysis in the Mrad dose-rangep

27, 38

i/4 2,4 and 5,8 y-radiolysis, l~i i-i0 Amino acids in hydrolysates of radlolytic

28,29,30 31

products, pn 5,8

~4CN

Amino acids in hydrolysates of radiolytic products

32

y-radiolysis in the Mrad dose-range, pH 9

33, 38

Amino acids in hydrolysates of radiolytic

31

products Na(~ o r I~CS]

y-radiolysis in the Mrad dose-range, pH 11,3 Amino acids in h ~ l y s a t e s of radiolytic

33, 38 31

p~duc~

several species that absorb light at wavelength on which the observation was made. For this purpose a computer simulation of the reaction mechanism was made by taking into account all the reactions and their rate constants/~'~> Besides the reactions of the studied cyano compound, we had to take into account also the reactions in pure irradiated water and the kinetic system consisted of about twenty nonlinear differential equations. In some cases the computation was used to derive the rate constants of reactions that could not be experimentally obtained. When these fitted rate constants had to be derived the system was selected so that only one parameter had a dominant effect on experimentally observed changes.

Molecular products The reactions of intermediates, produced by reactions (1)-(3), with one another or with cyanomolecules lead to the decomposition of cyanides and nitriles. This is followed by formation of a variety of products. Their radiationchemical yields were measured under different experimental conditions. The essential findings are the following: (i) The radiation--chemical yields of decomposition vary from 3.3 (CH3CN, pH 6) to 13.8 (HCN, pH 6). (ii) Small molecules produced are: H2, CO2, NH3, RCHO, RNH2, urea and biuret. Their radiation-chemical yields strongly depend on the cyano compound and pH of solution. (iii) The amounts of nitrogen from decomposed molecules,

3.5xi06

CH3CN

1.3x107

6.7x106

2.7xi06

3.6xi07 3x107 3.6x107

(~C~)2

h~H2CN

h~2C(=NH)h~CN

HCN

(]q-

1.6x107

1.06x107

Ixl07

f~2 ((IN)2

C2H5(IN

< 107

(CN)2

2.7x106 1.5x106

H

Cc~

6.6x108 2x108

l.lxl09

1.5x109

1.7x109

7.1x109

Ixl08 1.5x108

2.5xi07 3x107

2. ixl0I0

e-aq

7.1x109

6x107

< 5x107

5.6xi06

8.5x106

3x107

7.3x107

5.5xi06

107

OH

18, 19

16 29 19

26

24

16

16

16 34 35

16 34 35 36

21

Reference

TABLE 3. RATE CONSTANTS OF REACTIONS OF NITRILES AND CYANIDES WITH H , e~q AND O H , k IN d m 3 tool -I s -I

450 355

OH, adduct of dicyand/a~de :':~;C (=O) NHCN

£230 1200 e230 1800

1150

1700

1900

1800

£250 390

1250

150

300 400 and 200

400 450

E255 1 1 5 0

~240 1500

a R is the sut~tituent in nltriles.

< 220 < 220

325

~ 2 C (=0)~

~3~ 2

370

< 240

RC (OH)=N or ~'=NC~ a

~K~

< 240 < 220

350

HC(OH)~ or H~NOH OHCH=N

H adduct of dicyandi~nide

300

h~2C(H)~ or h~2~qqH

275 275

< 255

< 240

280 290

or ~=NH

£1' M-Icon-!

440 and 290 360 and 2100

lmax, nm

or ~

RC(H)~

HC(H)~ H2CN

I~2C(=NH)NHCN !-

(h~2CN)-

(CN)2-

Intermediate

CNoq-

~E[2C (=~'I ) ~'K3~

h~2C (=NH)N~K~q

h~2oq

h~q2f3q

CH3C~ , C2H5C~ , (CH2)2CN' (CH2Oq)2

HCN H(~

h~H2C(=NH)NH(~

NH2CN

CH3CN , C2H5CN CH2(CN)2 , (CH2aq)2

HCN HCN

NH2C(--NH)NHCN

NH2CN

(CN)2

Cc~0ound

19

18

26

26

24

24

16

16 19

26

24

16 16

16 19

26

24

21

Ref.

TABLE 4. FREE-RADICAL INTERMEDIATES IN IRRADIATED AQUEOUS SOLUTIONS OF NITRILES AND CYANIDES

O

o

B

I. G. DRAGANI¢and Z. D. DRAGANI¢

200

which are incorporated into nonvolatile radiolytic products, vary from 60% (propionitrile, pH 6) to 76% (acetonitrile, pH 6). (iv) Up to 20% of nitrogen from nonvolatile products is in amide bonds.

Oligomers with peptidic properties acids released on hydrolysis

and amino

The examination of radiolytically produced compounds in irradiated aqueous cyano-systems shows that peptide back-bone appears in irradiated cyanides and in acetonitrile and propionitrile samples. The supporting evidence was: positive biuret reaction (a routine test in peptide measurements), t22'27"33~ the IR spectra with bands appearing in regions characteristic for peptides t38) and the release of several protein and nonprotein amino acids on acid hydrolysis. ~23'3~> A detailed examination of these phenomena has shown that they are all correlated with absorbed doses of radiation as one would expect from genuine radiolytic products. Table 5 summarizes the data on amino acids. It can be seen that, with some exception only, the same protein and non-protein amino acids appear in all irradiated cyano compounds. Quantitative measurements show that radiation-chemical yields vary from 0.6 for glycine to 0.003 for glutaminc acid. Glycine is the most abundant amino acid in hydrolysates of cyanides and acetonitrile while alanine is in propionitrile. TABLE 5.

AMINOACIDSIN HYDROLYSATES

~-C~ 3-aspartic acid

~

REMARKS It is quite certain that radioactivity on the early Earth was more abundant and came more frequently in localized form than it is usually admitted. Natural nuclear reactors, such as the recently discovered one in Africa, could be of particular importance. Because of its specific way of energy deposition, ionizing radiation could have been a more effective energy source for chemical evolution than is presently accepted. This is especially true for processes in aqueous

OF COMPOUNDS FROM IRRADIATED CYANIDES

H~ a ~ Aspartic acid

Arginine appears as a free-amino acid in a fairly large yield (0.2) in irradiated NH2CN only, where radiolytic products do not show peptidic properties. It should be noted that the above findings concern the examinations of bulks of nonvolatile products in irradiated systems. The isolation and characterization of individual oligomers with peptidic characteristics is presently in progress. Results with irradiated ammonium cyanide show that fractionated material consists of neutral, basic and acid oligomers. Infrared spectra suggest that peptide-bonded fragments are in oligomers of probably the urea-aldehyde type. They represent up to about 30% of the oligomer, as estimated from the total amount of amioo acid in hydrolysate, t39)

~4~

~

Na~ a C~3~c c2nsc~ ~2c~d

~

++

ANDNITRILES

4+

+++

++

+

++

+++ Serine Sacrosine

+

Glutamic acid Glycine

s-amino-n-butyric acid

+

+

+

++

+++

+

+4+

+

+

+

+++

+++

+++

+4+

4+

4++

+++

+4-+

+++

4++

+++

++

+++

+++

++

+++

+4+ +++

+++

+

4-+

Arginine

++4-: ident/fied by amino acid analyzer and GC-M~. ~-~: identified by amino acid analyzer and GC. +: identified by amino acid analyzer or GC. +x: identified by amino acid analyzer and infrared spectra after separaticn by paper chromatography. a: Ref. 31. b: Nef. 32. c: Ref. 23. d: Ref, 25.

+

4++

Valine ~-alanine

+

++

+x

Radiation-chemical aspects of chemical evolution media where the free-radicals and radical-ions play an important role. The examination of radiation chemistry of simple cyanides and nitriles is at its beginnining and this brief review is justified more by indicating its potential possibilities for primordial organic synthesis than by summarizing what has been achieved. The abiotic formation of peptidic skeleton in aqueous medium without intervening formation of amino acids might be of particular interest. The fragments with peptidic characteristics do not appear in N - C N type of nitriles (cyanamide and its dimer), in contrast to C - C N nitriles and cyanides. The present results on aqueous cyano compounds encourage other radiation chemical investigations of interest to primordial organic synthesis. The studies of chemical evolution can be a rewarding domaine for radiation chemistry to transfer some of its achievements. REFERENCES 1. S. L. MILLERand L. E. ORGEL, The Origins of Life on the Earth, North Holland, Amsterdam 1974, Chap. 5. 2. M. G. RUTTEN, The Origins of Life by Natural Causes. Elsevier, Amsterdam 1971, pp. 398-403. (a) Chapter 13 and the references given here. 3. I. G. DRAGANICand Z. D. DRAGANI(~,XXIX International Astronautics Congress. Dubrovnik, Yugoslavia, 1978, To appear in Acta Astronautica. 4. C. PONNAMPERUMAand M. SWEENEY, The role of ionizing radiation in primordial organic synthesis. In A. W. Schwartz (editor), Theory and Experiment in Exobiology, 1971, Vol. I, Wolters-Noordhoff, The Netherlands. 5. P. K. KURODA,J. chem. Phys. 1956, 25, 781. 6. R. BODU, H. BOUZIGUES,N. MORIN and J. P. PFIFFELMANN,C. R. Acad. Sc. 1972, 275 D, 1731. 7. M. NEUILLY, J. BUSSAC,C. FREJACQUES,G. NIEF, G. VENDRYESand J. IVON, C. R. Acad. Sc. 1972, 275 D, 1847. 8. The Oklo Phenomenon. IAEA, Vienna, 1975. 9. Natural Fission Reactors. IAEA, Vienna, 1978. 10. M. MAURETTE,Ann. RefJ. Nucl. Sci. 1976, 26, 319. l 1. D. ALTIPARMAKOVand I. DRAGANI~,To be published. 12. E. C. ALEXANDER,JR., R. S. LEWIS, J. H. REYNOLDS and M. C. MICHEL, Science 1971, 172, 837.

201

13. R. V. GENTRY,T. A. CAHILL, N. R. FLETCHER,H. C. KAUFMANN,L. R. MEDSKER,J. W. NELSON and R. G. FLOCCHINI,Phys. Rev. Lett. 1976, 37, 11. 14. S. W. Fox and K. DOSE,Molecular Evolution and the Origins of Life, Freeman, San Francisko, 1972. 15. D. H. KENYONand G. STEINMAN, Biochemical Predestination, McGraw-Hill, New York, 1969. 16. I. DRAGANIt~, Z. DRAGANI~, LJ. PETKOVI(~ and A. NIKOLI(~, J. Am. Chem. Soc. 1973, 95, 7193. 17. I. G. DRAGANIt~,Z. D. DRAGANIt~and V. M. MARKOVI(~, Int. J. Radiat. Phys. Chem. 1976, 8, 339. 18. D. BEHAR,J. phys. Chem. 1974, 78, 2660. 19. H. BOCHLER,R. E. BOHLERand R. COOPER,J. phys. Chem. 1976, 80, 1549. 20. B. H. J. BIELSKI and A. O. ALLEN, J. Am. Chem. Soc. 1977, 99, 5931. 21. I. G. DRAGANI(~,Z. D. DRAGANI(~and R. A. HOEROYD,J. phys. Chem. 1971, 75, 608. 22. I. G. DRAGANI(~,Z. D. DRAGANI(:and M. J. SHUSHTARIAN,Radiat. Res. 1976, 66, 54. 23. I. DRAGANI(~,Z. DRAGANIC,A. SHIMOYAMAand C. PONNAMPERUMA,Origins of Life 1977, 8, 377. 24. I. G. DRAGANII~,Z. D. DRAGANI(~and K. SEHESTED,J. phys. Chem. 1978, 82, 757. 25. Z. D. DRAGANI(~,I. G. DRAGANI(2and S. V. JOVANOVIC, Radiat. Res. 1978, 73, 508. 26. Z. D. DRAGANI(2,I. G. DRAGANI(2and K. SEHESTED,J. phys. Chem. 1979, 83, 220. 27. Z. D. DRAGANI(;, I. G. DRAGANI(~and M. BOROVl~ANIN, Radiat. Res. 1976, 66, 42. 28. H. OGURA,J. Radiat. Res. (Japan) 1967, 8, 93. 29. H. OGURA,Bull. Chem. Soc; (Japan) 1968, 41, 2871. 30. H. OGURA,T. FUJIMURA,S. MUROZONO,K. HIRANO and M. KONDO, J. Nucl. Sci. Technol. 1972, 9, 339. 31. Z. DRAGANI(~, I. DRAGANI(~, A. SHIMOYAMAand C. PONNAMPERUMA,Origins of Life 1977, 8, 371. 32. M. A. SWEENEY, A. P. TOSTE and C. PONNAMPERUMA, Origins of Life 1976, 7, 187. 33. Z. D. DRAOANI(~, |. G. DRAGANI( and V. NIKETIC, Radiat. Res. 1977, 69,223. 34. E. J. HARTand M. ANBAR,The Hydrated Electron, p. 238. Wiley-Interscience, New York, 1970. 35. P. NETA, G. R. HOLDREN and R. H. SCHULER, J. phys. Chem. 1971, 75, 449. 36. P. NETA, R. W. FESSENDENand R. H. SCHULER,J. phys. Chem. 1971, 75, 1654. 37. R. W. TAFT, J., Steric Effects in Organic Chemistry (Edited by M. S. Newman), p. 556. Wiley, New York, 1956. 38. I. G. DRAGANI(~,Z. D. DRAGANIt~,S. JOVANOVICand S. V. RIBNIKAR,J. Mol. Evol. 1977, 10, 103. 39. Z. DRAGANI(2, V. NIKETI(~, S. JOVANOVI(7 and I. DRAGANI(2,3". Mol. Evol. To be published.