The volatilities of some adducts of trimethylindium

Journal of Crystal Growth 92 (1988) 37—45 North-Holland, Amsterdam

37

THE VOLATILITIES OF SOME ADDUCTS OF TRIMETHYLINDIUM Donald C. BRADLEY, Marc M. FAKTOR

“,

Dano M. FRIGO and Lesley M. SMITH

Department of Chemistry, Queen Mary College, Mile End Road, London El 4NS, UK Received 16 May 1988

The volatilities of some adducts of trimethylindium with amine and phosphine ligands were measured by the modified entrainment method (MEM). The results indicated that for some adducts both dissociative and undissociative vaporization takes place in the temperature ranges covered.

1. Introduction The chemistry of organo-indium compounds has recently received renewed attention because of their use as precursors in the growth of single crystal films of indium pnictides by organometallic vapour phase epitaxy (OMVPE) [1]. These materials and their solid solutions with other Group III pnictides, are of particular interest in the opto-electronic industry; the band gap and the mean interatomic distance, can be varied independently, thus facilitating the selection of emission wavelength whilst maintaining epitaxial lattice match [2].The growth of these solid solution films of precisely defined stoichiometry requires reliable and accurate knowledge of the partial pressures of each component precursor. Furthermore, the precise nature of the gas phase precursors is required for calculating their hydrodynamic fluxes to the advancing crystal interface [3]. An important class of precursor for OMVPE of Ill—V materials are adducts between indium trialkyls and Group V Lewis bases [4]. Therefore, it was decided to measure the volatilities of a number of adducts containing trimethylindium as potential indium precursors, and to investigate the

*

Deceased

stability of the adduct bond in the vapour for different Lewis bases. The modified entrainment method (MEM) is a dynamic technique for evaluating equilibrium constants for a variety of heterogeneous systems, including vapour pressures and dissociative vaporization [5]. For volatility studies, the sample is enclosed in a bottle, such that the condensed and vapour phases are in virtual equilibrium: the equilibrium is perturbed only imperceptibly by a small amount of vapour continuously issuing from the bottle via a cylindrical channel of known dimensions. Thus, problems of hindered vaporization are adequately overcome since only a fraction of the surface of the condensed phase needs to be evaporating in order to maintain the equilibrium partial pressures (the mathematical evaluation of the problems of hindered vaporization has been presented elsewhere [6]). The technique is therefore, particularly suitable for measuring the volatifities of air sensitive organometallics in which some degree of surface contamination is unavoidable. Decomposition may give rise to a semi-permeable skin which obstructs volatilization and/or lead to products which are either much more volatile than the test compound or even permanent gases. In the latter case, provided the decomposition products do not give rise to further decomposition [7], the dynamic system allows their escape before measurements are taken.

0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

38

D.C. Bradley er al.

/

Volatilities of some adducts of trimethylindium

2. Theory of modified entraimnent

the MEM channel. The total measured weight loss will then be given by

A full description of the theory of MEM is provided elsewhere [5], but in brief, it relates the saturated vapour pressure to the rate of mass transport across a diffusive resistance. The sample (A) is contained in a small bottle fitted with a cylindrical channel of length 1 and cross-sectional area C, through which the vapour can escape. The

This is analogous to the equation for Knudsen effusion under conditions of dissociative and non-dissociative vaporization [9,10]. With each P1 will be associated an exponential

vapour inside the bottle is assumed to be at the saturated vapour pressure of A, PA°, and outside the bottle the partial pressure is maintained effectively at zero by a stream of nitrogen carrier gas (N). It has been shown that ~A° is given by [5]:

of the type found in eq. (2). Therefore the plot of R ln ~1”versus 1/T will be a logarithm of a sum of exponentials: a mathematically ill-conditioned problem which normally requires extended ternperature ranges and great experimental accuracy to detect the inherent curvature in the plot and

(1)

WRTI/DANMAC,

where W is the measured rate of weight loss, DAN is the binary diffusion coefficient of A in nitrogen, and MA is the molecular weight of A. Since ~A (in atm) is given by ~ ~ ln PA° + (2) =

where

— -~-

i~H,,

-~—~

and

i~S.,,

are the standard enthalpy

and entropy changes upon vaporization, then from eqs. (1) and (2), if T/DAN is assumed to be a constant and the effects specific heat changes are neglected (i.e. ~ and z~S,,,are temperature-independent), then zlH~and a plot ofintercept R ln J~’versus will have slope at 1/T 1/T 0 of ~XS~ R ln( RT1/1O1325DANMAC). —

=

—

It has been shown that for an adduct AB which vaporizes exclusively to A plus B and for which A and B have not very dissimilar diffusion coeffi-

W

-~

(4)

~PIDINMi.

thence to obtain the individual thermodynamic parameters [11]. In most cases values of DAN are not known experimentally but from eq. (1) are required for the measurement of vapour pressure by the MEM. Furthermore, in general 75 T/DAN is power law [12]. Eq. (1)but thenusually becomes not a constant follows a T’ J~”R1T~75/DoMACT°75,

PA°=

where D 0 is the value of DAN at a reference temperature T0. A plot of R ln( J’~’/T 0.75) versus 1/T will then have slope —L~H.,.and intercept at 1/T 0 of 175/1O132SDOMAc). ~‘v R ln(RIT0 Theoretical and semi-empirical calculations [13,14] have suggested that for organometallic compounds of this general type Graham law [15] works well, and DAN may be calculated from =

—

DAN

=

KMK 1’12(T/373 15)i7S

cients, then the fluxes due to each component i can be related by [8] WcRT1/D~NM~C, (3)

(la)

(5)

where K is an empirical constant givenforbya 2 s~ kg1”2 mol~”2 3.36(±0.15)x 106 m variety of metallo-organic and branched organic molecules diffusing in nitrogen. The diffusion

where D 1N is the quasi-binary diffusion coefficient of i in nitrogen (which is the principal gaseous species since the i components are dilute). This equation may be extended to the case of partial dissociation in the vapour provided it is assumed that no re-equilibration of components occurs in

coefficients of the compounds in this paper may therefore be estimated to approximately ±5%by this very simple approach, and since this is much the largest error in the estimation of quantities in eq. (1), then the estimation of PA°may be assumed to have similar uncertainty.

D.C. Bradley et at

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Volatilities of some adducts of trimethylindium

3. Results and discussion Rates of weight loss were measured at several temperatures for the following adducts of trimethylindium in the liquid phase: Me3 InL, where L is PEt

(1)

mental plots are linear within experimental error and the values of m and s (table 2) may be related to the thermodynamic parameters or complex summations thereof. The estimation of the enthalpies of vaporization z~H~ ab initio is very difficult and is not pursued here. Estimation of standard entropies of vaporization L1S~is easier since statistical thermo-

P(NMe2 )~, (2) NH(C6H11)2,

39

(3)

NHCHMe(,CH2)3CHMe, ~4) NHCMe2 (CH2)3CMe2, (5)

dynamics may be used. For many simple systems since there is free rotation in the liquidus at the boiling point, the value of ~ can be approximately equated with the change in translational

N(CH2CH2)3CH,

entropy alone. By the Sackur—Tetrode equation

/

..

‘CH ~ k 2NE t)3,

~‘

(6)

1

[16], thedependent gas phaseupon translational weakly molecularentropy weight issoonly the

(8)

values of ~ S.,, for many compounds are comparable; a fact noted by Trouton [17] and Hildebrand [18]. Using eq. (5) to estimate the diffusion coeffi-

and also for (Me3 In)2tmen,

where tmem Me2N(CH2 ) 2NMe2. Displayed graphic formulae are presented in fig. 1. The results are presented in table 1. Then R ln( W/ T 0.75) was plotted versus 1 / T and the values of the slope m and intercepts s were obtamed; these plots are shown in fig. 2. The experi=

cients, the apparent ~1S,,, may be calculated from the intercepts s given in table 2. If dissociation congruent with vaporization is occurring, then as two gas phase molecules are produced per mole of condensed compound the apparent z.~S,,, will be substantially above the expected value of 85—90 J for a molecules variety of [19], metallomol K for small,1 simple and 105—125 J moF’ K organics, e.g. M(OBut)4 (M Ti, Zr) [20], Me3 InEMe3 (E N, P) [21]. Table 2 shows the ap~

/Et

Me

31nf—P~-Et

/2 MesJn+—P.ç-NMe2

Et (2)

—

—

/ Me31n~~_N..~\

=

NMe2

(1)

=

M

e>......~ Me31n&—N.,~

,>

Me Me

Me In~—N

(6)

Me Me Me

~

Et~Et

Me Me Me 1n~—N

/\ Me Me

parent z~S,,,for all the compounds; the apparent value of ~ H.,, in J mol 1 is numerically equal to the slope m but with the sign reversed. Inspection of table 2 shows that the values of s and consequently those of the apparent L~S~fall into two distinct classes: the lower values, which we believe to be indicative of simple vaporization; and the higher values which are indicative either of dissociative vaporization or of a very large rotational entropy Srot in the gas phase which is suppressed in the liquid. The value of Srot depends

Me 1n~—N

Me Me (5)

—

\/

N—~InMe3

Et

Fig. 1. Displayed graphic formulae of the compounds.

upon the moments of inertia of the molecule and for most of the compounds considered here, these are not very large; they can be estimated for some of these compounds from the crystal structures [22]. However, a large Srot is not sufficient to account for a non-negligible contribution of L~Srot to the overall ~S~: there must be some intermolecular forces which hinder rotation in the liquidus even at the boiling point, e.g. hydrogen .

40

D.C. Bradley et al.

/

Volatilities of some adducts of trimethylindium

Table 1 MEM results for the compounds Compound

T (K)

X lO~ (kg s~)

75)

— R ln(J~i”/T° (J mol~ K 1)

(Torr)

(1) Me 3InPEt3

a,b)

(2) Me,InP(NMe2)3

cd)

(3) Me3InNH(C6H11),

a.e)

353.9 362.4 369.7 377.2 386.4 394.8 406.2

1.27 2.21 3.27 4.61 8.00 12.7 22.7

245.2 240.7 237.6 234.9 230.4 266.7 222.1

1.34 2.29 3.35 4.65 7.93 12.3 21.6

393.2 402.2 410.2 414.7 420.4 425.2 431.4

9.11 14.6 20.8 26.8 35.0 44.9 58.9

229.5 225.7 222.9 220.8 218.7 216.7 214.6

9.47 15.0 20.9 26.7 34.5 43.9 57.0

367.2 375.4 381.7 388.2 396.4

2.72 5.10 8.27 14.1 24.0

239.1 234.0 230.1 225.8 221.5

2.52 ~ 4.65 0 7.46 0 12.5 ~ 21.0 0

9.58 15.4 20.1 27.0 37.6 53.6 67.3

228.4 224.7 222.5 220.2 217.5 214.7 212.9

11.6 18.4 23.7 31.4 43.2 60.7 75.5

226.0 222.4 218.1 214.6 210.8 208.1

12.9 0 20.9 0 33.3 0 51.3 0 112 80.7f)

(4)

Me3InNHCHMe(CH2),~HMe c.g)

354.9 364.7 370.9 377.4 384.3 391.75 396.9

(5)

Me3InI’IHCMe2(CH2)3CMe2

360.1 367.4 374.9 382.1 388.9 395.0

(6)

Me,InN(CH,CH,) 3CH

(7)

Me,In(CH2NE6)3

(8)

(Me3In)2tmen~”~

jJ)

j~

Notes to table 1 on next page.

c,h)

12.9 21.2 34.3 53.6 120 85

377.8 389.9 401.9 415.2 432.7

2.71 5.10 9.21 17.2 37.1

239.3 234.2 229.5 224.5 218.4

2.31 4.24 7.50 13.6 28.6

342.6 362.1 378.7 389.2 396.7

4.16 12.8 29.1 48.4 66.4

23.1 226.1 219.6 215.5 213.0

3.44 10.1 22.4 36.4 49.3

370.2 378.4 389.2 398.2 407.2 ~ 412.7 m)

1.88 3.58 7.53 13.8 21.0 24.4

242.2 237.0 231.0 226.1 222.7 221.6

1.280 2.40 0 4.95 0 8.92 0 13.3 0 15.3 ~

D.C. Bradley er a!.

/

Volatilities of some adducts of trimethylindium

41

(~) S

-210

—

\

(~)

\•%%~\I~\\~

‘~ 0

-220

\\

(3)

-

-~

L1~

N

0 .~ ~~1

—230

-

-240

-

*

\

(6)

2.3

2.4

I

I

2.5

2.6

2.7

\

2.8

2.9

3.0

1)

1o~,T(iC

Fig. 2. R ln( L~’/T°75) versus 1/T for the compounds. The numbering scheme is as in fig. 1.

Notes to table I a) 1 = 2.185 X 102 m, C = 2.835 X 10—6 m2. c) ~ = 2.506 x 102 m, c = 2.835 x 10—6 m2. ~ MA = 0.341 kg mol~. g) MA = 0.273 kg mol~. I) l=1.830X10’ m, C=3.801X106 m2. k) MA = 0.331 kg mo11. m) Decomposition.

b) d)

MA MA

= =

0.278 kg mol’. 0.323 kg mo11

0 Apparent PA° (see text). h) MA = 0.301 kg mol~. ~ MA=O.27l kg mol~. ‘~ MA = 0.436 kg mo11.

42

D.C. Bradley et al.

/

Volatilities of some adducts of trimethylindium

unaffected: the amine would behave as inert carrier gas and merely alter the diffusion coefficient

Table 2 Volatilization parameters from graphical analyses Compound (1) (2) (3) (4)

m a) 3 x10

a)

—

63(1) c) 67(1) c) 89(2) 52(1) ~

a)

A b)

B b)

126(2) 133(3) 194(4) 112(3)

3290 3500 4650 2720

9.44 9.81 13.0 a) 8.71

(J moF1 K~)

66(1) 60(2) + 2(3) —82(2) —

d)

e)

(5) 74(1) —20(1) 172(2) d) 3860 ~ (6) 63(2) ‘~ —71(3) 118(4) 3290 (7) 56(1) c) — 71(1) 117(2) 2920 (8) 85(1) — 12(1) 175(2) d) ~ e) a) Uncertainty limits in parentheses. b) Parameters in log10(P,~/Torr) — A/T + B. c) — m ~ H,, (J mol 1) ,1)

a)

11.9 a) 9.03 9.01 12.0 a)

A

bonding in water [19], or some transitory oligomeric species involving 5-coordinate indium such as

(Me2N)3P

evaporating component, then

famine

=

~Me

3In, i.e. the flux of Me3In would also be suppressed proportionately by shifting the equilibrium towards associated adduct by the principle of mass action. A series of MEM experiments were performed on the adduct, alternating between pure nitrogen and nitrogen plus amine, in the gas stream; the results

are presented in table 3. The effect is dramatic and reversible, therefore the partial pressure of amine in the indicating carrier gas that is comparable

Apparent ~,,. Parameters for apparent ~o

Me3,1n E

by a very then smallthe amount. If the adduct dissociated however, concentration gradient across the MEM channel would be reduced, thus reducing the amine flux from the bottle. Furthermore, since there can be no unlimited build-up of any

~Me2 (Me2N)2P—~~InMe,

possible accounting for the slightly higher ~ S~.for this compound. To distinguish dissociation from high rotational entropy a technique was required which was sensitive to the species present in the vapour phase. An obvious technique is vapour density [23], or at lower pressures torsion effusion [9].However, since the MEM was originally designed as a technique for measuring general heterogeneous equilibria, we adapted the apparatus to show conclusively that one of the compounds whose intercept was in the higher range, viz. _________________

Me3InNHCMe2(CH2)3CMe2 dissociates partially upon vaporization. This involved entraimng a small amount of one of the adduct components, i.e.

NHCMe2(CH2)3CMe2 in the carrier gas. If the adduct evaporated nondissociatively then the J.~”would be essentially

with that due to dissociative evaporation of the adduct. Furthermore, the reversibility of this suppression of W shows that it is not due to some absorption process. Coates and Whitcombe [21] showed that the amount of dissociation of some similar adducts at similar temperatures was vanishingly small. It was noted in the X-ray crystal structure of Me3InNHCMe2(CH2)3CMe2 that the In—N interaction was long and, therefore, weak due to Van der Waals repulsion between the methyl substituents on the metal and the amine

Table 3 ______________ MEM results for Me3InH1~Me2(CH2)3CMe2 with NHCMe2(CH2)3~iMe2in the carrier gas stream a) T (°C) 70.75 71.75 72.4

Carrier gas contents N2 N2 + = 0.3% amine N2 + = 1.5% amine

72.7

N2

78.85 78.85 78.4 89.9 89.9 89.9 90.0

N2 N2 + N2 N2 N2 + N2 N2 +

W x 1011 (kg s~) 5.30 2.79 2.21 5.65

—

0.3% amine

=

0.3% amine

—

1.5% amine

13.8 8.58 12.6 20.6 16.4 19.0 14.2

90.25 N2 2 m; C=2.835x10618.7 m2. a) l=1.791x10

D.C. Bradley et at

/

Vo!ati!ities of some adducts of trimethylindium

[22]; we have provided further evidence that the adduct bond is relatively weak. It is noteworthy that, as was predicted in “The ory of modified entrainment” (vide supra), although the semi-logarithmic plot for this adduct should in principle be curved, no curvature was detectable within the experimental error and limited temperature range.

43

tohn microbalance

~1r’ -__________

stainless steel fibre

heating ______

meter

performed L~SV The values teston infor the thegas other higher phase compounds range, dissociation viz.with Me

apparent was not 3InNH(C6 H,1)2 and (Me, In)2tmen. Therefore, the assignment of values for z1H~ and AS.,, (table 2) for these compounds remains tentative; for Me3In~HCMe2(CH2)3~Me2

=

—A/T+ B.

bottle

flow

nitrogen Inlet

________________

(6)

thermocouple

heating

fluid inlet

they are clearly not true thermodynamic parameters, whereas for the remaining adducts they probably are. However, even though the MEM (as with many other techniques for determining vapour pressures) does not give true vapour pressures if dissociative vaporization is occurring, the apparent pressures behave similarly. Therefore they may still be used as precursors for OMVPE since their dosimetry is as well behaved as those which do not dissociate. Table 2 gives parameters for the vapour pressure (real or apparent) equation for each compound, viz. log,0(P~/Torr)

Jacket ssmple

to VOCuum—~~ pump

column ~

-

to bubbler and fume cupboard —thermocouple leads

Fig. 3. Apparatus for MEM.

The bottle was suspended half way down a cylindrical colunm fitted with a heating jacket through which silicone fluid (heated by a Cobra Ultra thermostat) was passed. The temperature was measured using a chromel—alumel thermocouple, the hot end of which was placed in the thermocouple column (a hollow Pyrex tube, the sealed end of which was situated <2 mm below the base of the MEM bottle). The nitrogen carrier gas was purified

4. Experimental The apparatus used is shown in fig. 3. The MEM bottle consisted of a Pyrex round-bottomed flask fitted with a Quickfit B7 socket. The stopper was constructed of precision bore tubing and was shaped and ground to fit the socket. The resulting channel, which ran longitudinally through the stopper, was typically 2 cm long by 2 mm diameter. The bottle was suspended from one arm of a Cahn R-100 microbalance via a cradling glass hook on the MEM bottle and a stainless steel fibre. The output from the balance was relayed to a strip chart recorder thus obtaining plots of weight versus time,

by

successive passage through columns containing manganese(II) oxide, 4A molecular sieves, and a special form of ignited silica containing a highly reduced chromium dopant, and introduced into the system at the balance casing via a needle valve and a flow meter. After passage down the column containing the MEM bottle, the waste gases passed through a silicone fluid bubbler and into a fume cupboard. A constriction in this colunm just above the MEM bottle accelerated the gas flow over the opening of the channel on the bottle, ensuring that the partial pressure of sample there remained effectively at zero. The compounds were prepared as outlined in the literature [22,24,25]. As they were all very air/moisture sensitive, the samples were intro-

44

D.C. Bradley et a!.

/

Volatilities of some adducts of trimethylindium

duced into the MEM bottle using normal Schienk-style techniques. Before loading the bottle into the apparatus the latter was evacuated for several hours to ~ ~o 2 Torr using a rotary oil pump. The tap to vacuum was then closed and the system was filled with nitrogen from the inlet, This alternate evacuation and filling was repeated three times. Then, with nitrogen flowing, the thermocouple mounting was completely detached from the bottom of the apparatus and the MEM bottle was rapidly introduced into the system and suspended from the balance. The thermocouple mounting was re-attached and the system was alternately evacuated and filled with nitrogen three times. The vacuum pump was then disconnected and the nitrogen inlet and outlet were opened in quick succession. The temperature was raised to the required value and plots of weight versus time obtained. Except for the smallest rates of weight loss, these plots were continued until at least 1 mg in W had been lost. The traces suffered from some noise, presumably due to fluctuations in the nitrogen pressure, but this was small (±1—2% of full scale deflection) and regular (— 0.1 Hz). Consequently, the noise caused no problems in obtaining J~Vconsistent to <2% for separate, isothermal traces. Plots of R ln( J~”/T°75) versus 1 / T were linear within experimental error, and slopes and intercepts were analysed by linear least squares

this vapour pressure was extrapolated approximately to higher temperatures using a typical value for ~ H~.The ratio of the flow rates in the two inlets could be varied and hence, assuming 100% bubbler efficiency, the final partial pressures of amine in the carrier gas were calculated, as are shown in table 3.

5. ConcLusions The volatilities of a number of OVMPE precursors have been measured. These values are currently required by crystal growers who need to obtain accurate and reproducible dosimetry into the growth equipment. The method used here gave the simple relationships between vapour pressures (real or apparent) and temperature for each compound thus fulfilling the requirements for the crystal growers. The experimental data showed that in some cases these precursors volatilised with partial dissociation into their adduct components, and in such cases the acquisition of accurate thermodynamic parameters for the vaporisation processes is difficult. An independent method of studying the complex vaporization equilibria was performed on one of the compounds and will be applied to others in the future.

analysis. =

One of the compounds whose intercept at 1/T 0 indicated that it might be dissociating, viz.

Acknowledgements

Me 3 InNHCMe2 (CH2 )3~Me2, was tested for dissociation by performing an MEM experiment with a small amount of NHCMe2(CH2)3CMe, entrained in the carrier gas stream. The apparatus shown in fig. 3 was modified by having two nitrogen gas inlets: one through the balance casing, as usual, and one which passed through a bubbler containing the amine (in a thermostatically controlled bath) and was introduced at the top of the MEM column containing the sample bottle. The vapour pressure of the amine was measured by manometry to be 3.9 Torr at 22.5°C;

We thank the Director of Research, British Telecom Research Laboratories, Martlesham Heath, Ipswich (DMF), and the Procurement Executive, Ministry of Defence, of the Radar and Signals Research Establishment, Malvern (LMS), for financial support.

References [1] H.M. Manasevit and WI. Simpson, J. Electrochem. Soc. 116 (1969) 1725. [2] GB. Stringfellow, Ed., Proc. 3rd Intern. Conf. on Metalorganic Vapor Phase Epitaxy (North-Holland, Amsterdam, 1986) [J. Crystal Growth 77 (1986)].

D.C. Bradley et at

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Vo!atilities of some adducts of trimethylindium

[3] MM. Faktor and I. Garrett, Growth of Crystals from the Vapour (Chapman and Hall, London, 1974). [4] R.H. Moss, J. Crystal Growth 68 (1984) 78. [5] D. Battat, M.M. Faktor, I. Garrett and R.H. Moss, J. Chem. Soc., Faraday Trans. 70 (1974) 2267. [6] M.H. Lyons, PhD Thesis, University of London (1982); B. de Largy, A. Finch, P.J. Gardner and N. Kell, J. Chem. Soc., Faraday Trans. I, 79 (1983) 383. [7] D.C. Bradley and MM. Faktor, J. Appl. Chem. 9 (1959) 5425; Trans. Faraday Soc. 55 (1959) 2117. [8] D.M. Frigo, PhD Thesis, University of London (1985). [9] C.G. de Kruif, J. Chem. Phys. 77 (1982) 6247. [10] D.C. Bradley, M.M. Faktor and D.M. Frigo, j. crystal Growth 89 (1988) 227. [11] W.J. Wiscombe, J. Computat. Phys. 24 (1977) 416. [12] E.N. Fuller, PD. Schettler and J.C. Giddings, Ind. Eng. Chem. 58 (5) (1966) 18. [13] D.C. Bradley, M.M. Faktor, D.M. Frigo and L.M. Smith, Chemtronics 2 (1987) 17. [14] D.C. Bradley, MM. Faktor, D.M. Frigo and Ky. Young, Chemtronics 3 (1988) 50.

45

[15] JR. Partington, An Advanced Treatise on Physical Chemistry, Vol. 1 (Longmans—Green, London, 1949) p. 903. [16] B.J. McClelland, Statistical Thermodynamics (Chapman and Hall, London, 1973). [17] F. Trouton, Phil. Mag. 18 (1884) 54. [18] J.H. Hildebrand, J. Am. Chem. Soc. 37 (1915) 970. [19] P.W. Atkins, Physical Chemistry, 3rd ed. (Oxford University Press, Oxford, 1986) p. 105. [20] D.C. Bradley and J.D. Swanwick, J. Chem. Soc. (1958) 3207; (1959) 748. [21] G.E. Coates and R.A. Whitcombe, J. Chem. Soc. (1956) 3351. [22] D.C. Bradley, H. Dawes, D.M. Frigo, M.B. Hursthouse and B. Hussain, J. Organomet. Chem. 325 (1987) 55. [23] G.E. Coates, F. Glockling and ND. Huck, J. Chem. Soc. (1952) 4496. [24] A. Storr and B.S. Thomas, Can. J. Chem. 23 (1970) 3667. [25] K.A. Aitchison, PhD Thesis, University of London (1983).