Nuclear Magnetic Resonance Study of the Hydrogen Bonding of Chloroform with Aliphatic Tertiary Amines and Ethers

BY KIM F. WONG AND SOON NG*

Department of Chemistry, University of Malaya, Kuala Lumpur, Malaysia

Received 3 1st May, 1974

The parameters for the hydrogen bonding of chloroform with 16 aliphatic tertiary ainines and 10 aliphatic ethers in cyclohexane medium have been determined by n.m.r. spectrometry. The hydrogen- bond chemical shift Am correlates extremely well with the change in enthalpy AH" in each series of the closely related bases. The ethers have a more uniform negative temperature coefficient of AM, than do the tertiary amines. In both the oxygen and the nitrogen bases, the alkyl substituents having the /3-methyl group, such as n-propyl and isobutyl, have greater steric hindrance effect in the hydrogen bonding than bulkier substituents having the a-methyl or the y-methyl group, such as t-butyl or n- butyl, respectively. There is a very good correlation between AAB and the sum of the polar sub- stituent constants (Taft u* values) in the ethers, while in the tertiary amines this correlation is fairly good except for t-butyldimethyl- and cyclohexyldimethyl-amines, in which the Am (and A H ) values are relatively large. It is suggested that a necessary condition for an uniform temperature coefficient of Am and for a good correlation between AAB and co* is that the hydrogen bonds should be formed in the minimum potential energy configuration. Severe steric hindrance causes the hydrogen bonds to be formed in higher energy configurations than the minimum.

The hydrogen bonding ability of a saturated nitrogen or oxygen base should be dependent on the inductive and steric effects of the substituents. In the hydrogen bonding of chloroform with aliphatic tertiary amines and aliphatic ethers, the strength of the hydrogen bond is expected to be dependent on the inductive effects of the substituents, but the steric hindrance effect of the substituents is expected to reduce the extent of the interaction and to prevent the hydrogen bond from attaining the minimum potential energy configuration.

The hydrogen bonding of chloroform with Lewis bases has been extensively studied by n.m.r. spectrometry.'* By this technique we have obtained the hydrogen bond- ing parameters for the interaction of chloroform with a series of aliphatic tertiary amines and a series of aliphatic ethers in order to determine (1) the hydrogen bond proton chemical shift, its temperature dependence, and its correlation with the change in enthalpy, and (2) the influence of structural factors of the substituents on the hydrogen bonding ability of the base.

EXPERIMENTAL

The aliphatic tertiary amines were either obtained commercially or synthesized according to procedures described in the l i t e r a t~ re ,~ and were purified and dried with KOH pellets. The aliphatic ethers were likewise ~ b t a i n e d , ~ and were dried with sodium. Chloroform was purified by a standard procedure. Cyclohexane was of spectroscopic grade and was dried with sodium.

The acid and base solutions in cyclohexane were accurately prepared in 10 cm3 volumetric flasks a t 20°C. The concentration of the chloroform was fixed at 0.050 mol dm-3 or less,

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K. F. WONG AND S . NG 623 while that of the base was varied in 9 solutions from 0.70 to 2.50 mol dm-3, such that the base concentration was at least 17 times, but less than 55 times, that of the chloroform concentration. The sample tubes, including one containing chloroform at the same concen- tration in neat cyclohexane, were sealed. The concentrations were corrected for volume changes with temperature.l

The chloroform proton signal was measured from the cyclohexane signal with a frequency counter on a Hitachi Perkin-Elmer R-20B high resolution 60 MHz n.m.r. spectrometer, using 60 Hz sweep-width and sweep-rate 0.15 Hz s-'. The permanent magnet is thermo- statically maintained at 34°C. Measurements were made at 10°C intervals in the range 10-54°C. Temperatures were measured using standard ethylene glycol and methanol samples.

For the interaction between a proton donor (A) and a base (B), represented by the equilibrium

A+B + AB where AB is the 1 : 1 hydrogen-bonded dimer, the equilibrium constant K, assuining rapid exchange between A and AB, is given by

Here [Ale and [Bl0 are the initial molar concentrations of the acid and the base, respectively, in cyclohexane solvent ; Aobs is the difference between the observed proton chemical shift in the solution and that for the free acid at the same temperature; and AAB, the hydrogen- bond chemical shift, is the difference between the chemical shift of the proton in AB and that in A at the same temperature. For [Bl0 4 [Ale, eqn (1) reduces to the modified Benesi- Hildebrand equation '9

I/Aobs = ~ / A A B +~/AABKCBIO. (2 ) K and AAB are obtained simultaneously from eqn (1) by means of Rose-Drago plots of K-' against AAB. Initially an approximate value of AAB is obtained from the double- reciprocal plot according to eqn (2), and then values of K-l were calculated around this value from eqn (1) for the observed shift Aobs of a solution so that a plot of K-l against AAB is obtained for this solution. However, the concentrations of chloroform and base chosen experimentally permit eqn (2) to be valid and the K and A m values obtained from either eqn (1) or eqn (2) agree well within experimental error. The calculations were performed on a Hewlett-Packard programmable calculator, model 9100 A.

The " best fit " K values at different temperatures were used in the van't Hoff equation to obtain the enthalpy and the entropy changes :

In K = AS"/R - AH"/RT. (3) The hydrogen bonding parameters for the compounds that were reported in a preliminary

report The values shown in the table for these compounds are therefore more reliable than the values in the preliminary report.

have been re-determined and evaluated by means of eqn (1).

RESULTS AND DISCUSSION

The parameters for the hydrogen bonding between chloroform and a series of 16 aliphatic tertiary amines and a series of 10 aliphatic ethers in cyclohexane medium are shown in the table. For those bases in which the chloroform proton shift accompanying the hydrogen bond formation is small (< 0.15 p.p.m. a t 1 .O mol dm-3 in the base), measurements were made at only the probe temperature, 34"C, and from these data the values of K and AAB were evaluated by means of eqn (2). Because the boiling point of diethyl ether is low (34.5"C), measurements were made at only the probe temperature.

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624 N.M.R. STUDY OF HYDROGEN BONDING

1. THE HYDROGEN BOND CHEMICAL SHIFT, AAB, ITS TEMPERATURE DEPENDENCE A N D CORRELATION WITH THE CHANGE IN ENTHALPY, A H "

As shown in the table the magnitude of AAB decreases with increase in temperature. The temperature dependence of AAB has been explained in terms of a model of 1 : 1 complexes given by Muller and Reiter which suggests that, as the hydrogen bond length increases with the temperature, the magnetic environment of the proton approaches that of the uncomplexed chloroform so that AAB decreases, as observed. The hydrogen bond formed by chloroform with an oxygen or a nitrogen base may be represented by a shallow anharmonic potential energy curve,l as shown in fig. 1. The temperature dependence of the AAB of chloroform hydrogen bonded to aliphatic tertiary amines and ethers appears to be linear over the temperature range, 10-54"C,

TABLE 1 .-PARAMETERS OBTAINED FOR THE HYDROGEN BONDING OF CHLOROFORM WITH VARIOUS NITROGEN AND OXYGEN BASES IN CYCLOHEXANE MEDIUM*

34OC AN3Ip.p.m. = f(r)t - A H o / -ASo/ (AAB/~AF/"I)/

base K/dm3 mol- 1 13mlp.p.m. ( r in "C) kJ mol-1 J K-1 mol-1 p.p.m. (kJ)-l mol

1. Me2NPrn 2. Me2NBun 3. Me2NBui 4. Me2NBuS 5 . MezNBut 6. Me2N-

7. MeNEt, 8. MeNPr2 9. MeNPri

10. MeNEtBd 11. N-methyl-

cyclohexyl

pyrrolidine 12. EtSN

14. EtzNBun 13. Et2NPrn

15. PrzN 16. PriO 17. BunOMe 18. BuiOMe 19. BuSOMe 20. Bu'OMe 21. tetrahydro-

furan 22. tetrahydro-

PYran 23. EtZO 24. neopentyl-

OMe 25. t-pentylOMe 26. MeNBu;

0.46 0.47 0.24 0.36 0.55

0.58 0.49 0.26 0.28 0.30

0.69 0.38 0.25 0.26 0.13 0.30 0.30 0.26 0.30 0.41

0.51

0.45 0.37

0.15 0.080 0.17

1.42 1.44 1.40 1.44 1.70

1.55 1.52 1.46 1 S O 1.57

1.45 1.56 1.53 1.57 1.52 1.04 0.78 0.73 0.88 0.91

0.85

0.80 0.83

0.72 0.42 0.61

-0.0048t+ 1.57 14.6 -00.0046t+ 1.60 14.5 -0.0048t+ 1.56 14.3 - 0.0029t+ 1.53 14.6 -0.0019t+ 1.77 18.2

-0.0031t+ 1.66 16.7 -0.0020t+ 1.58 16.5 -0.0027tf 1.55 15.6 -0.0029tf 1.59 16.6 - 0.0031 t+ 1.66 16.9

- O.O24t+ 1.53 15.2 -00.0034t+ 1.67 17.0 -00.0030t+ 1.63 16.4 - 0.0025t+ 1.65 17.0 -O.Wlt+ 1.66 15.1 - O.W98t+ 1.07 15.2 -0.00082t+0.81 11.7 -0.00097tf 0.75 10.9 - O.W70t+ 0.90 12.3 -0.00104t+0.95 13.4

-00.00098t+0.88 12.3

-00.W86t+ 0.83 11.7

54 53 59 56 64

59 60 62 65 64

53 64 65 66 67 60 48 47 50 51

44

45

0.0966 0.0994 0.0976 0.0985 0.0936

0.0929 0.0918 0.0938 0.0907 0.0925

0.0952 0.091 8 0.0934 0.0924 0.101 0.0683 0.0668 0.0669 0.0712 0.0681

0.0690

0.0683

* Uncertainties : K f 0.01 dm3 mol-', AAB k 0.02 p.p.m., AH" k 0.6 kJ mol-', ASo z 2 J 1C-I mol-'. For compounds 15,24,25 and 26, the uncertainties are somewhat larger than for the other compounds.

-t In Am/(p.p.m.) = f ( t ) , the standard deviation u < 0.02 p.p.m., but for compounds 3, 8, 9, 13, 14 and 15, u > 0.02 p.p.m.

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K. F . W O N G A N D S . N G 625

as given in the table. The temperature coefficient A A B in the aliphatic ethers is quite uniform and has the value -0.009~0.001 p.p.m. per 10°C rise in cyclohexane medium. In the tertiary amines the temperature coefficient of A A B varies considerably and is two to five times larger than in the case of the ethers. It is suggested (see next section) that a necessary condition for an uniform temperature coefficient of A A B in a series of closely related bases is that the hydrogen bonds formed must be in the minimum potential energy configuration, as represented by point M in fig. 1. The larger AAB and the more varied temperature coefficient in the tertiary amines may be attributed, respectively, to the higher poIarizability of the nitrogen lone-pair and to the hydrogen bonds being formed in higher potential energy configurations, as represented by point H in fig. 1, as a result of the steric hindrance effect of the alkyl substituents. The hydrogen bond represented by point H is longer and weaker than that represented by point M.

The ratio of A A B (34°C) to the change in enthalpy AH" is remarkably constant for the two series of aliphatic ethers and tertiary amines, showing the good linear correla- tion between A A B and AH". For the aliphatic ethers the ratio has an average value

I FIG. 1 .-Schematic diagram of the potential energy of hydrogen bonding versus the hydrogen-base

distance.

15 -

3

CI I

13- c, Y

Q - a I

0.7 0.8 0.9 I .O AAB/P.P.~.

FIG. ?,.-Correlation between AHo and AAB (34°C) for the hydrogen bonding of chloroform with various aliphatic ethers. The numbers refer to the ethers in table 1. The least squares line through the points is given by : -AH"/(kJ mol-') = 13.7 Am/(p.p.m) + 0.8 with linear correlation co-

efficient p = 0.9821, and standard deviation (J = 0.2 kJ mol-l.

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626 N . M . R . STUDY O F H Y D R O G E N B O N D I N G

of 0.0684 p.p.m. (kJ)-l mol with standard deviation 0 = 0.0015 p.p.m. (kJ)-' mol. For the tertiary amines the ratio has an average value of 0.0944 p.p.m. (kJ)-l mol with o = 0.0027 p.p.m. (kJ)-l mol. Fig. 2 shows the plot of AH" against AAB (34°C) for the ethers. For the tertiary amines, as the temperature coefficient of Am is relatively large and varies considerably, the correlation of Am (34°C) with AH" is expected to have somewhat poorer linear correlation than in the case of the aliphatic ethers, as shown in fig. 3.

I I I I I 1

18 3 - I

8 % a 2 16

I

14

L i I .4 1.5 I .6 I .?

AAB / P . P . ~ . FIG. 3.-Correlation between AH" and Am (34°C) for the hydrogen bonding of chloroform with various aliphatic tertiary amines. The numbers refer to the amines in table 1. The least squares line through the points (with points 5 and 15 omitted) is given by : -AH"/(kJ mol-') = 16.9A AB/

(p.p.m.) - 9.4 with p = 0.9636 and Q = 0.3 kJ mol-'.

The results show that in hydrogen bonding there is good correlation between the hydrogen bond chemical shift in n.m.r. spectra and the change in enthalpy for a series of closely related bases and a fixed proton donor. Therefore to estimate the AH" for another member in the series, it is only necessary to determine the AAB at the temperature at which the correlation is known. For example, for the interaction of chloroform with diethyl ether the AH" value is predicted to be - 12.2 5 0.4 k J mol-l, as the value of A A B (34°C) is measured to be 0.83 p.p.m.

2. I N F L U E N C E OF SUBSTITUENT S T R U C T U R A L FACTORS O N THE H Y D R O G E N BONDING ABILITY OF THE BASE

The hydrogen bonding ability of a Lewis base should be dependent on the polar and steric effects of the substituents. The Taft polar substituent constant o* are appropriate measures of the inductive effects of the substituent groups.1o For alkyl substituents, the o* values are: Me 0.000, Et -0.100, Pr" -0.115, Pri -0.190, Bun - 0.130, Bu' - 0.125, Bus - 0.210, But - 0.300, cyclohexyl - 0.150, and neopentyl -0.165. The Taft steric substituent constants E, l o may not be relevant in the hydrogen bonding of chloroform to the nitrogen or oxygen lone pair, as the E, values are based on the steric strain arising on conversion of an sp2 initial state to an sp3 activated complex in acid-catalysed ester hydrolysis, which is a very different situation.

The curves in fig. 4 for the variation of the observed chloroform proton shifts with the concentration of the series of aliphatic ethers show that the substituents with

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K. F. WONG AND S . NG 627

the fl-methyl group, such as isobutyl, neopentyl and t-pentyl, result in considerably smaller observed shifts than the n-butyl or the t-butyl substituent. The data in the table show the smaller extent and strength of the hydrogen bonding in the ethers having substituents with the /3-methyl group. Examination of models show that the segmental rotation in the substituent about the C(a)-C(p) bond would bring the /?-methyl group close to the region of the lone pairs in the position to block the approach of the proton donor. This condition becomes more prevalent when the /?-methyl group occurs in a bulky substituent, such as t-pentyl or neopentyl, and when the size of the other substituent increases, requiring the P-methyl group to be oriented to the same side of the oxygen as the lone pairs, in order to reduce the steric interaction with this other substituent. The s-butyl substituent also has a B-methyl group but it appears that in this case the steric effect is more than compensated for by the relatively large cr* value of this substituent.

30

x" 2o 1 a 2

10

1.0 2.0 3;O

[ether] FIG. 4.-The variation of the observed chloroform proton shifts at 60 MHz and 34°C with the molar concentration of various aliphatic ethers : t, ButOMe ; s, BusOMe ; n, BunOMe ; i, BuiOMe ; nP, neopentyl-OMe ; tA, t-pentyl-OMe. Concentration of chloroform fixed at 0.050 mol dm-3. Aobs =

G(CHC13 in ether-cyclohexane solution) - G(CHC1, in neat cyclohexane).

The contribution of the alkyl substituents to the hydrogen bonding ability of the nitrogen or oxygen bases may be measured by the sum of the polar substituent constants, ED*. Fig. 5 shows that for the aliphatic ethers a fairly good straight line is obtained in the plot of A A B (34°C) against -Zh*. From this correlation it can be predicted that the alicyclic chain in tetrahydrofuran has o* = -0.210+0.025 and that in tetrahydropyran has o* = -0.165f0.025. It is to be noted that the point (24) for neopentyl methyl ether deviates substantially from the correlation, and the deviation is worse in the case of t-pentyl methyl ether (not plotted), indicating that the correlation does not hold for substituents which severely limit the access of the proton donor to the oxygen lone pair. In this case severe steric hindrance prevents the hydrogen bond of the minimum energy configuration to be formed. The hydrogen bond that is formed may be represented by point H on the potential energy curve in fig. 1, at which point there is an attractive force acting on the hydrogen, whereas at the minimum energy configuration, represented by point M, the attractive force is balanced by the repulsive force.ll It appears that a necessary condition for the A A B (or AH") correlation with Co* to hold is that the hydrogen bond formed must be in the minimum energy configuration. In the aliphatic ethers it appears that this

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628 N . M . R . STUDY OF H Y D R O G E N B O N D I N G

configuration is normally attained, except in the case of a very bulky substituent having one or more /I-methyl groups such as the neopentyl or the t-pentyl substituent. It is reasonable that this condition is also necessary for the AAB correlation with the AH", as the aliphatic ethers exhibit excellent linear correlation between A A B and AH".

i 0.7

L 1 I 1 1

-XU+

FIG. 5.--Correlation between A- (34°C) and the sum of the polar substituent constants, Xu*, in the aliphatic tertiary amines (top), and the aliphatic ethers (bottom). The numbers refer to the compounds in table 1. Top : the least squares line through the points for the tertiary amines (with points 5 and 6 omitted) has p = 0.8330 and is given by : Am/(p.p.m.) = 0.49 IXcr*l + 1.37. Below : the least squares line through the points for the ethers (with point 24 omitted) has p = 0.9712 and is

OJ 0.2 0.3 0.4

given by : AAB/(p.p.m.) = 1.06 lAu*l 4- 0.62.

In the case of the aliphatic tertiary amines the steric factors of the substituents, especially those having the P-methyl group, would be even more important in deter- mining the hydrogen bonding ability of the nitrogen. Fig. 6 shows the variation of the observed chloroform proton shifts with the concentration of the series of butyl- dimethylamines, and clearly demonstrates that the substituents having the P-methyl group, s-butyl and isobutyl, cause considerably smaller shifts than the n-butyl or the t-butyl substituents, resulting in considerably smaller values of K for the s-butyl- and isobutyl-substituted amines. The table also shows that the n-propyl-substituted amines consistently have slightly smaller K values than the corresponding n-butyl- and isopropyl-substituted amines, indicating that the n-propyl group (p-methyl substitution) causes more steric hindrance than the n-butyl or the isopropyl group in hydrogen bonding to these tertiary amines. In the segmental rotation of the substituent about the C(a)-C(p) bond the B-methyl group can get closer to the nitrogen lone pair than the a-methyl group of an isopropyl or a t-butyl substituent, and the tendency of the p-methyl group to be oriented to the region of the lone pair, and hence the steric hindrance effect, would increase as one or both of the other substituents become bulkier. Examination of models show that the segmental rotation in the n-butyl substituent about the C(a)-C(B) bond would be restricted if

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K. F. WONG A N D S. NG 629 the y-methyl group is not oriented farthest away from the region of the oxygen or nitrogen lone pair, and this might explain why the steric effect of the n-butyl substituent in the hydrogen bonding is not greater than that of the n-propyl substituent.

1 I I I

[tert-amine] FIG. 6.-The variation of the observed chloroform proton shifts at 60 MHz and 34°C with the molar concentration of the series of butyldimethylamines : t, ButNMez ; n, BunNMez ; s, BusNMe2 ; i, BuiNMe2. Concentration of chloroform fixed at 0.050 mol dm-3. Aobs defined as in the caption

of fig. 4.

In fig. 5 is shown a reasonably good correlation between AAB (34°C) and Zo* for the tertiary amines, although the correlation is not as good as in the case of the aliphatic ethers. It is to be noted that (Bu')),NMe does not fit into the correlation. The small A A B value (0.61 p.p.m. at 34°C) is no doubt the result of the severe steric hindrance effect of the two s-butyl groups which would prevent the chloroform mole- cule from approaching close enough to the nitrogen to form a hydrogen bond of the minimum potential energy configuration, so that the hydrogen bond that is formed is represented by a point such as H in fig. 1. On the other hand, the points (5 and 6 ) for Bu'NMe, and cyclohexyl-NMe, deviate substantially from the correlation in that the A A B values are relatively large. A possible cause of this behaviour may be elucidated by comparing Bu'NMe, (point 5) and Bu'NEtMe (point 10). The K and AH" values are much larger for the former (0.55 dm3 mol-1 at 34"C, - 18.2 kJ mol-') than for the latter (0.30 dm3 mol-l at 34"C, - 16.9 kJ mol-l) ; these data emphasize the importance of the steric effect of the substituents in determining the nature of the hydrogen bond. Examination of a model shows that the methyl group of the ethyl substituent in Bu'NEtMe in order to reduce steric interaction with the other two substituents has to be oriented to the same side of the nitrogen as the lone pair. In view of this configuration, the approach of the chloroform molecule is more hindered than in the case of Bu'NMe,, and the hydrogen bond in Bu'NEtMe is there- fore ascribed to a higher potential energy configuration. The trend of the points in fig. 5 suggests that the hydrogen bonds in the other tertiary amines are in varying degrees of higher energy configurations than the hydrogen bonds in Bu'NMe, and cyclohexyl-NMe2. According to the Muller-Reiter model hydrogen bonds in

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630 N . M . R . STUDY OF H Y D R O G E N BONDING

configurations higher than that of the minimum energy can be expected to have greater temperature coefficient of A A B . As the higher energy levels are more closely spaced, the populations of these levels would increase more rapidly with the tempera- ture in the case of a hydrogen bond whose ground state is higher than the minimum energy configuration. The temperature coefficients of A A B shown in the table are generally higher for those tertiary amines in which steric hindrance to the hydrogen bond formation is considered substantial, and indeed the temperature coefficient of A A B for Bu'NMe, is the smallest. The varied temperature coefficient of A A B and the poorer correlation between A A B and Co* in the tertiary amines than in the ethers can therefore be attributed to the greater degree of steric hindrance to the hydrogen bond formation in the tertiary amines, so that the hydrogen bonds formed are of energy configurations in varying degrees higher than that of the minimum potential energy.

The question arises why in Bu'NMe, and cyclohexyl-NMe, the hydrogen bonds are in a configuration of lower potential energy than in the other tertiary amines. The salient feature of these two aliphatic tertiary amines is the large bulk of one substituent and the relatively small size of the other two substituents. The two methyl substituents make it easy for the proton donor to approach close to the nitrogen lone pair, while the bulky substituent excludes a large volume around the molecule.12 The hydrogen bond formed in this excluded volume would be " sheltered " from collisions with the solvent molecules, so that the interaction is facilitated, resulting in a hydrogen bond of the minimum energy configuration. This effect of a bulky substituent in the hydrogen bonding may be called steric assistance.

G. R. Wiley and S. I. Miller, J. Amer. Chem. Soc., 1972, 94, 3287. F. L. Slejko, R. S. Drago and D. G. Brown, J. Amer. Chem. SOC., 1972, 94,9210. L. Spialter and J. A. Pappalardo, The Acyclic Aliphatic Tertiary Amines (Macmillan, New York, 1965), and references therein. (a) J. F. Norris and G. W. Rigby, J. Amer. Clzem. SOC., 1932,54,2088 ; (b) A. I. Vogel, J. Chem. Soc., 1948, 616. I. D. Kuntz, Jr., F. P. Gasparro, M. D. Johnston, Jr. and R. P. Taylor, J. Amer. Chem. SOC., 1968,90,4778. J. Homer, M. H. Everdell, C. J. Jackson and P. M. Whitney, J.C.S. Faraduy iI,1972,68,874 ; J. Homer and P. M. Whitney, J.C.S. Faraday Z, 1973, 69, 1985. N. J. Rose and R. S. Drago, J. Amer. Chem. Soc., 1959, 81, 6138.

* K. F. Wong, T. S. Pang, and S. Ng, Chem. Coinm., 1974, 55. N. Muller and R. C. Reiter, J. Chem. Phys., 1965, 42, 3265.

l o R. W. Taft, in Steric Eflects in Organic Chemistry, ed. M. S . Newman (Wiley, New York, 1956), chap. 13. S. Leave11 and R. F. Curl, Jr., J. Mol. Spectr., 1973, 45, 428 ; H. Jones and R. F. Cull, Jr., J. Mol. Spectr., 1972, 42, 65. For the concept of excluded volume see, for example, E. H. Kennard, Kinetic Theory of Gases (McGraw-Hill, New York, 1938), chap. V,

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