Sov., 1960
EIWIRO?;MER.TdL INFLUENCE ON
BEHAVIOR O F LONGCH.4IRI .hIOLECULES
1643
THE ENVIROXMENTAL INFLUENCE ON THE BEHAVIOR OF LONG CHAIN MOLECULES1 BY
R. H.
ARANOW AND
L. WITTEN
i?I.-lS, 7818 Bellona Aue., Baltimore 12, Md. Received A p r i l 13?1960
Transition of a molecule upon a change of environment from the state of internal torsional oscillation to the state of hindered internal rotation can account quantitatively for the entropy of fusion per CH, group of long chain hydrocarbons, Traube’s rule of surface tension, the distribution ratios of long chain hydrocarbons between water and organic solvents immiscible with water, the solubility in water of liquid long chain hydrocarbons, and the effect of chain length and alcohol on critical micelle concentration. The solutions of crystalline long chain molecules in water are predicted to be a special class of non-ideal solution.
I. Introduction The phenomenon of hindered internal rotation has been the subject of much study.2 Systems in the temperature region where the hindered internal rotation can still be regarded as a torsional oscillation but where the torsional oscillation can occur around more than one position of potential minimum have interesting properties from the point of view of statistical mechanics. We call such systems pseudo-degenerate torsional oscillators for reasons which shall be described in Section I1 of this paper. In section I1 we shall review the relationship between the thermodynamic behavior and the statistical mechanical description of the pseudo-degenerate torsional oscillator. We shall discuss the partition function for two phase systems in order to demonstrate the effect of the pseudo-degenerate oscillat’or. In section I11 the results of the analysis made in section I1 will be utilized to explain the distribution ratios of long chain molecules between immiscible solvents, solubility of liquid long chain molecules in water, and effect of chain length on critical micelle concentration. In addition the theory of fusion of long chain hydrocarbons3”and of Traube’s R ~ l e 3will ~ be reviewed and restat’ed in order to demonstrate the common physical basis of all the phenomena mentioned. In section IV an example will be given of the biological implications of the pseudo-degenerate oscillator behavior. In addition we will state some predictions with regard to the solubility of crystalline state long chain hydrocarbons. 11. Theory I n the solid stat’e,in condensed surface films, and in aqueous solution the potential energy diagram for torsional oscillation about a single carbon-tocarbon bond is postulated to take the general form shown in Fig. la. The effect of interact’ion with neighboring molecules is to restrict the torsional oscillation to one preferred region of potential minimum. The postulate has been confirmed experimentally for the solid state and condensed filrn~.~JIn aqueous solution the water molecules (1) This research mas partially supported by t h e United States Air Force through the Air Force Office of Scientific Research of t h e Air Research and Development Command, under Contract Number AF 49(638)-735. Reproduction in whole or in part is permitted for any purpose of t h e United States Government. (2) See, for example S. Miaushima, “Structure of Molecules a n d Internal Rotation,” Academic Press, Inc., New York, N. Y . , 1954. (3)(a) R . H. Aranow. L. Witten and H. Andrews, THISJOURKAL. 62, 812 (1958); (b) R. H. A r a n o a and L. Witten, J. Chem. Phys., 28, 405 (1958).
surrounding a given long chain hydrocarbon form a cage where hydrogen bonds between water molecules may be likened to bars. The complete rotation around a carbon-to-carbon bond thus involves not only the relative motion of the parts of the long chain hydrocarbon molecule but also the simultaneous breaking or distortion of many hydrogen bonds of the cage surrounding an individual molecule. Since the total motion requires high energy, the cage of water molecules has the effect of confining a particular bond to the configuration it is in initially. I n addition if the shape of the hydrocarbon molecule has a significant effect upon the number of hydrogen bonds in the surrounding cage it is reasonable that those shapes which permit maximum hydrogen bonding would correspond to minimum potential energy and hence be preferred. It is a postulate of the theory that there is a t room temperature in aqueous solution one or at most a very small number of such over-all molecular shapes. In the pure liquid state, in organic solutions and in dilute liquid surface films (“gaseous film state”), the potential diagram is postulated to take the form shown in Fig. lb. I n the vapor phase it has been confirmed experimentally for relatively short chain molecules that all configurations around a given carbon-to-carbon bond are not equally preferred.2 Figure l b is probably not a good representation for the potential energy of rotation about a carboncarbon bond for long chain molecules in the vapor phase because the relative depths of the three potential minima are considerably different and vary markedly with time due t o the interaction with distant atoms and to the many possible configurations of other bonds. I n the organic liquid state and in dilute films, however, interaction of the molecule as a whole with neighboring molecules may tend to average out the effect of the other parts of the molecule associated with a given carbon-to-carbon bond. The contribution to the potential energy of rotation about a given carbon-carbon bond of the more distant CH, groups is strongly affected by the interaction of those groups with their environments, i e . , neighboring molecules. When this interaction is strong, the effective moment of inertia of the two groups around a given carbon-carbon bond should be relatively insensitive to the particular configuration A. BIiiller, Proc. R o y . SOC.(London). 188, 514 (1932). ( 5 ) N. K. Adam, “ T h e Physics a n d Chemistry of Surfaces,” Oxford
(4)
University Press, 1941, Chap. 11.
R. H. ARANOW AND L. WITTEN
1644
Vol. 64
In summary, for the system in Fig la, Q = i
(- ei/kT) ; for the system in Fig. l b Q =
exp
3 exp 3
0
0
8 Figure- Io
(- +j)/kT). We should point out that these approximate forms can be obtained in many different ways for the problem a t hand. The important physical requirement is the impossibility of the molecules performing hindered rotations for the one case and the corresponding possibility for the second case. Let us look now at the canonical single particle partition function, 2, for a molecule having n carbon to carbon bonds. In an environment such as that of Fig. la, the partition function can be represented by
9 F i g u r e - Ib
Fig. 1.-Potential energy curve and energy levels for rotation around a carbon-to-carbon bond: (a) represents conditions in the liquid state, on surfaces, and in organic solutions; (b) represents conditions in the solid state, and in aqueous solution.
of the more distant atoms of the chain. Hence the simplified diagram shown in Fig. l b may be a valid representation, a t least as an average. That such an averaging process is physically reasonable may be inferred from experiments on 1,&dichloroethane which indicate that the difference in the energy of the succesive minima of the potential curve is of the order of 1 kcal. in the gaseous state while there is no energy difference in the liquid state.2 The physical concepts presented here are offered as postulates and we will check some consequences of the postulates. The general characteristics of the energy level diagram for the torsional oscillator (Fig. la) are similar to those of the linear harmonic oscillator. The main characteristics of the energy level diagram for the system in Fig. lb6 are that the nondegenerate levels of the torsional oscillator (Fig. la) have been shifted down and that there are three times as many energy levels available as there would be without the two secondary minima. In the region above the potential barrier the levels become like those of the free internal rotator. But this region is not of interest in the present paper, for the molecules are rarely excited t o these levels a t room temperature. These general features are preserved even with alteration in the detailed shape of the potential energy curve. In particular, the general features remain even if the depths of the potential minima and the heights of the maxima are varied by different amounts. Because of the threefold increase in number of energy levels, we can separate the levels into groups of three and call (ej) the average energy level of the j t h group. The partition function Q for the oscillator can be approximated by Q = C 3 exp j
(- (€j)/kT);k is Boltzmann’s constant and T ia the
temperature. This form is also obtained for a triply degenerate oscillator; we can call the system a pseudodegenerate torsional oscillator. (6) See G. Herzberg, “Molecular Spectra and Molecular Structure,” Vol. 11, D. Van Nostrand Co., Inc., New York, N. Y.,1945, D. 225.
The rotational contribution has been assumed separable from all other contributions, $; also the rotational contribution has been assumed t o be representable as a product of factors each making the contribution Q. Similarly the partition function in an environment such as that of Fig. l b can be represented by
The main dependence on the chain length, n, will be in the exponent as explicity shown in these expressions. Actually the factor $ has a dependence on n as do the energy levels 6. These dependences can be expected to be small compared with the exponential dependence and will be ignored in the approximation with which we are dealing. We are more interested in demonstrating the physical features and gross predictions of our picture rather than the details. There are two convenient ways of evaluating the statistical mechanical behavior of two phase systems in order to show the effect of hindered internal rotation. The more rigorous approach for the problem at hand involves the evaluation of p >the chemical potential, which must have the same value in both phases a t equilibrium. Another approach consists of regarding the two phases (which we shall designate as CY and p) as being two regions of a single larger system where the energy levels (including the interfacial values) of a molecule depend on the location of the molecule within the systemwe can call E ( C Ythe ) ~ energy levels when the molecule is in the phase CY, E (p)i the energy levels in the phase p, and E CY,^), the energy levels at the interface.
i
1
(3)
where na is the number of molecules in phase CY, NT is the total number of molecules, and k is Boltzmann’s constant. We use this second approach to describe the effects of internal rotation because, for the problem at hand, it is more direct than the first approach. For the case of 51 solute distributed
Kov. , 1960
ENVIRONMENTAL INFLUENCE ON BEHAVIOR OF LONGCHAINMOLECULES
between two phases the effect of the solvent on the energy levels can be included by considering the potential of average forces’ of the solvent on the solute. The form of equation 3 is then retained but the influence of the solvent is implicit in the evaluation of energy levels. If the contribution of translational motion t o the partition function of the single molecule can be regarded as separable and having the form (4)
+ +
+
where V = Va V , Vas, then Z t r = Z t r ( a ) Ztr!fi) Ztr(a,p). This form corresponds mathematically to the situation where €translation = Etr.(a) or Etr(P) or etr(a,P) but not all simultaneously. Hence by partitioning the partition function into three classes, a,p and (alp), one can arrive at the form
+
Va 1L.a
i\‘T
-
-
e-Ei(a)/kT j
T’,
e-Ei(a)/kT
+ Vg
e-Ei(S)/kT
+
(5)
i
j
V(a,,q
e - E (a.B)/kT 1
where Ej(a) refers to thejth energy level of all nontranslational motions for a molecule whose translational energy lies anywhere in the set (Etr(P)) e-Ei(a)/kT
z
3
V(a,p)
e-EI(a,B)/kT 1
and e-Ei(d/kT
%a-
v. nsvi3
=
-2
e-Ei(P)/hT I.
1% Assuming now that the motions of internal rotaexp(-E,IkT)
exp(-E,lkT)
At room temperatures; if kT is much smaller than the spacing between (eo) and (€1) or between eo, and el, then pn E
If $JI)B n and
-
fi 3n (e-(ao)-m/kT)n $s
(9)
is at most only a slowly varying function of eo we get PnlPn-1 s 3 This approximation states that the difference in all other external internal behavior in the two media is relatively insensitive to chain length although the external and internal rotational behavior in a given medium may exhibit considerable dependence on chain length. The results of the experimental determination of p are shown in Table I. The approximate rule of three is followed fairly well for those systems where the two solvents are very immiscible. The above data has been computed using the data of R. C. Archibald.8 The assumptions made in the calculations are : 1. Volume of organic solution = sum of volumes of individual components; 2. Moles of water per liter of solution = 55.5. For those cases where the organic solvent is present to a large extent in the aqueous phase the solute molecule is surrounded by both water and other organic solvent. Hence the solute molecule is expected to have greater freedom for internal rotations than in water alone but less than in organic material alone. Its environment can be represented to be something between Figs. l a and l b or to alternate between the two. Hence the contribution to the partition function of a rotational state of such a molecule will be g exp - ei/kT)” where .Q is a (eo)
( i
(7)
111. Applications to Physical Chemistry A. The Distribution Ratios of Long Chain Hydrocarbons between Two Immiscible Solvents. -Consider an idealized system of two truly immiscible solvents (water and an organic solvent which is insoluble in water) with a long chain hydrocarbon in dilute solution in the aqueous phase at concentrations much less than the critical micelle concentration. In fact we assume that in both phases the solutions are infinitely dilute. The definition of the distribution ratio p is the ratio of the concentration of solute molecules in the organic phase (a)to that in the water phase (p). From equation 7 P =
1645
(8)
j
tion are separable from all other motions and recalling that the internal rotational contribution has been assumed representable as a product of n equivalent factors, and using (l), (2) and (8)
(7) W. G. McMillanand J. E. hlayer, J . Chem. Phys., 13, 276 (1945).
parameter greater than one but less than three. Carrying through the analysis in this way, one will get for the ratio of distribution functions
_-P n t l
3 -
Pn
9
Thus for p n + l / p n = 2, g = 1.5. These considerations may explain why ratios in the table may be less than 3. However, a quantitative evaluation of this effect and of other causes of deviation from the “Rule of Three” are beyond the aims of our general discussion. The solubility of liquid long chain hydrocarbons in water may be regarded as a special case of distribution ratio where p = d/s, d is the moles per liter of pure liquid solute, and s in the solubility in water expressed as moles per liter of water. (Note: we have assumed that the volume of solution is approximately the volume of solvent.) The fact that the solubility decreases by a factor of three for every CH, group added may thus be expressed as Pn+l - &+1)/8/3 E Pn
4n)lS
Shinodagaccounted for the effect of chain length on critical micelle concentration and the effect of alcohol on critical micelle concentration by equating the chemical potential in the aqueous phase to that in the liquid micelle interior (liquid phase of the sol(8) R. C. Archibald, J . Am. Chem. Soc., 64,3178 (1932). (9) K. Shinoda, Bull. Chem. SOC.Japan, 26, 101 (1953).
R. H. ARANOW ASD L. WITTEN
1646
Vol. 64
TABLE I TYPICAL EXPERIMENTS RESULTS FOR DISTRIBUTION RATIOSTUDIES Solvent
n-Amyl alcohol
Solubility of solvent in water, g./100 ml. HzO Solute
2.7 P
Acetic acid
n-Butyl alcohol
Pn P
~
Pn-
1
Butyric acid
67.0
Recalling the definition of entropy S
=
-a
~(-M’TnZ)
s,,
Pn-
2.07 12.3 2.14 26.3
2.65 167.3
3.03
1.95 51.3
2.99
498
2.24 115.0
The first term is assumed to be a constant independent of n. If each bond of two molecules neighboring in the homologous series contributes approximately the same amount to that part of the partition function which is independent of chain length, then A(AS) = AS, - AS,-1 becomes
. /
The second term which is temperature dependent is comparatively small. It has a value which can be explained in terms of excitation energy levels above the lowest rotational level which can occur a t the melting temperatures of these long chain compounds. This theory is in good agreement with observation for even numbered n.3a If the entropy of the solid state transition of odd n molecules is included in the analysis these molecules also give good agreement. C. Review of the Application to the Theoretical Derivation of Traube’s Rule.-Traube’s Rule is often stated in the following way: The concentration of the members of the homologous series required to achieve equal surface tension lowering of the aqueous solution diminishes about threefold for each CH2 group added to the chain. Langmuirll showed that the rule could be expressed as kT In 3
(10) M. L. Corrin and W. (1947).
n.Harkins, J. Am.
Chem. S o r . , 69, 683
(13)
where An - An-l = work required to bring one CH2 group from the interior t o the surface. Calling FI(n) the free energy of an n-chain molecule in the solution, Fs(n) the free energy on the surface AF(n) = F.(n) - F l ( n ) and A ( A F ) = AF(n) A(AF) i Z -kT In 3 (per molecule)
-__
1
5.95
2.79
2.68
352
Pn -
P
1
2.56
An - A,1
we get for the entropy of fusion of an n-chain molecule A S n = s, per mole
Pn-
63.1
116.3
Ute itself). Using the solubility data just discussed to evaluate the contribution of the bond to the change in chemical potential, Shinoda got quantitative agreement between theory and experiment. lo We refer the reader to the original papers for details of the calculation. The behavior of the alcohol is computed assuming that the alcohol distributes itself between the micelle and the water. For the alcohol with n carbon-to-carbon bonds pn 0: exp (n In 3) is the value Shinoda used. He again got quantitative agreement between theory and experiment. I n summary, the phenomena of solubility, critical micelle concentration as a function of chain length and added alcohol, and distribution ratio studies all exhibit the same approximate “rule of three” which can be explained theoretically by a model which allows pseudo-degenerate torsional oscillation in any liquid organic phase but which only permits torsional oscillation in a single configuration in the aqueous phase. B. Review of the Application to the Entropy of Fusion of Long Chain Hydrocarbon Compounds. -The carbon-carbon bond in the solid state behaves as a torsional oscillator while in the liquid state it behaves as a pseudo-degenerate torsional oscillator. Thus, using s subscripts for solid and 1 for liquid
Pn
_ .
22.6 2.74
3.08
35.3
8.84 2.63
3.28
675
P
1
45.2
219
Caproic acid
Pn-
16.46 3.22
Valeric acid
Pn -
6.28 3.75
20.8
Ethyl methyl ketone
12.5
7.Y
5.56
Propionic acid
&Amyl alcohol
- A F ( n - 1)
This rule may be statistically derived3b (11) I. Langmuir. ibid., 38, 1948 (1917).
(14)
Nov., 1960
ENVIRONMENTAL IXFLUENCE ON
BEHAWOR O F LONGCHAIN ~ ~ O L E C U L E S A ueous J u t ion
-Assolid
Hence per molecule 4- 6 (16) 6 is a small correction term which arises from the $# and $1 contributions as well as excitation to higher energy levels. The contribution to A(AF) from these terms is expected to be much smaller than kT In 3. Traube’s rule may also be considered as a special case of the distribution ratio rule A ( A F ) E -kT In 3
P n =
Csurfaoe (n) -~ Caolution
(n)
For equal lowering of surace tension, Csurface (n- 1)Then P” -- Caoln (n - 1) _ = 3 (experimentally) ph--l
Csoln (n)
Csurface(n)
=
=
3(by the ratio rule) (17)
Positive deviations from Traube’s rule have been attributed to excitation to higher energy levels. Traube’s Rule also applies to perfluoro and whydroperfluorocarboxylic acids in which CH2groups are replaced by CF2 groups. IV. Application to Biology An example of the application of our model to biology is a phenomena sometimes known as Ferguson’s rule for the effect of narcotics as a function of chain length. An example of this rule is the effect of alcohols on tadpoles.12 The concentration of alcohol in the aqueous region which just stops tadpole motion is known as, the “limiting concentration.” Ferguson’s rule states that the limiting concentration decreases threefold with each additional CH2 group added. We may now interpret this rule as an example of the distribution ratio rule. The given limiting concentrations in the aqueous region all correspond according to the rule to just one concentration of the hydroxyl group within an organic region of the tadpole. Since the narcotic effect depends on the hydroxyl group our interpretation is reasonable and consistent. The argument that the effect is one of equal lowering of surface tension is countered when it is demonstrated that straight chain amines have no narcotic action (no inhibitory action dependent on chain length). Since the surface effects of amines follow the same behavior as alcohols the narcotic action cannot be a surface effect. The application of the rule of three resulting from our theory leads to the conclusion that the narcotic action must be taking place in an organic liquid region. We feel that other examples of the importance of carbon-carbon bond behavior in biological processes will undoubtedly be demonstrated in the future. (12) Otto Warburg, ”Heavy Metal Prosthetic Groups and Enzyme Action,” Tr. b y Alexander Lawson, Oxford, Clarendon Press 1949, p. 7, F. H. Johnson, H. Eyring and M. J. PoliEsar, “The Kinetic Basis of Molecular Biology,” John Wiley and Sons, New York, N. Y.. 1954. 11. 429-432.
ASI.=Kj+urRh3
1647
Pure
liguid
\/ASn=K,-nR&3
Solid
Fig. 2.-Cycle
to demonstrate effect of chain length on heat of solution.
V. Further Application to Physico-chemical Behavior The rgodel of the carbon-to-carbon bond in aqueous solution is roughly the same as that in the solid state. Hence we would expect the homologous series in water (dissolved from the solid state) to act as a special class of non-ideal solution. For ideal solutions (two components) where N 1 = mole fraction of the solute and AHfl molar heat of fusion of the pure solute. Since the long chain compounds gain internal freedom when they melt but do not gain this freedom when they go into aqueous solution we expect these compounds to follow the equation -d=In- N I
dT
AHa0ln- ASaoln
RTZ
RT
(19)
where AHsolncorresponds to the heat required t o bring a mole of the solute from the crystal into aqueous solution without “internal melting.” Our prediction based on our model is that AHsoln will be almost independent of chain length. h direct test of this prediction has not yet been made. But a special test can be applied. Consider the following system a t equilibrium at the melting point of the solute system: pure crystalline molecules, pure liquid molecules and aqueous solution of the molecules. For this system the entropy of fusion study shows that the entropy per bond gained upon bringing a mole from the crystal to the pure liquid is approximately R In 3. From the distribution ratio rule this entropy per bond is lost when the liquid molecule goes into aqueous solution. Hence when a mole of material is brought from the aqueous region into the crystalline state the entropy change should be approximately independent of chain length. The cycle is illustrated in Fig. 2. Since the different molecules are compared at different temperatures a small temperature dependence for AS111 should be observed. This temperature dependence will be indirectly a function of n through the relationship between the melting point and n. (Kote As111 = - A S s o l n as the process of solution is considered a reverse of the process in the diagram; i.e., A8111 = A S of crystallization from aqueous solution.) Careful freezing point studies should confirm our predictions and illustrate that the homologous series form a special class of nonideal solutions.
VI.
summary
The diverse phenomena of entropy of fusion, Traube’s Rule of surface tension, solubility of liquid hydrocarbons, distribution ratio of long chain hy-
1648
FRANKLIN J. WRIGHT
drocarbons and critical micelle concentration as a function of chain length and of added alcohol can all be unified by a model of CHz group behavior which permits pseudo-degenerate torsional oscillation in organic media and on aqueous surfaces but which requires one preferred configuration for torsional oscillation in the solid and in aqueous media. It may be argued that the factor 3 is a coincidence and could be caused primarily by changes in the behavior of water associated with the introduction of CH2 groups; however the phenomena of entropy of fusion is independent of the presence of water. Against the argument that the factor 3 is probably inherent in the energetic behavior (internal and external) of the CHZ group only and not a result of the effect of the geometry of the molecule on torsional oscillation is the fact that Traube’s rule is obeyed by the CF2 group. This group has a geometry similar to the CH2 group but has very different internal and external interaction energy. The Traube rule behavior indicates that the geometry is the determining factor. If future experiments demonstrate that the “rule of three” also holds for entropy of fusion of long chain fluorocarbons and that the solubility of liquid long chain fluorocarbons in water goes down by a factor of three for each CF2 added, we will have much stronger evidence for believing that the geometry is the deciding factor. We hope that this article will be useful in demonstrating the unity underlying
Vol. 64
various seemingly unrelated examples of the physical behavior of hydrocarbons and that the mechanism proposed to explain this unity will be subjected to further analysis and experimental test. Basic in our interpretation of the various physical effects correlated in this paper is that the free energy changes involved have an origin in entropy considerations and do not arise from changes in potential energy. Many of our colleagues have argued that even if the effects are primarily entropy effects, the origin of the entropy effects lies not in the changing behavior of the hydrocarbon molecules but rather in the changed behavior of water molecules in the presence of a hydrocarbon molecule. Namely, it is argued, that immediately near a hydrocarbon chain water acquires an ordered icelike structure.13 When the hydrocarbon chain is removed water loses this ordered arrangement and becomes a randomly arranged liquid with a corresponding increase in entropy. Since all of the effects we mention except the entropy of fusion involve water, they can all be explained in this alternate fashion. The entropy of fusion, it is generally agreed, does have the origin we ascribe to it; it is according to this second view, a pure coincidence that the entropy increase per CH2 group which is approximately R In 3 for fusion is also approximately R In 3 for the aqueous cases. It seems to us crucial to resolve experimentally the issue regarding the two alternate explanations. (13) H. S. Frank and M. W. Evans, J . Chem. Phys., l S , 276 (1945).
FLASH PHOTOLYSIS OF CARBON DISULFIDE AND ITS PHOTOCHEMICALLY INITIATED OXIDATION1 BY FRANKLIN J. WRIGHT Central Basic Research Laboratory, Esso Research and Engineering Company, finden, New Jersey Received April 18, 1060
The absorption spectrum taken a few milliseconds after flash photolysis of gaseous CSZunder isothermal conditions at 20’ shows characteristic bands due to CS and Sa. The disappearance of SZis rapid and follows second-order kinetics. The rate constant of this reaction has been measured in terms of the unknown extinction coefficients of S2a t 2712 and 2662 A. It was found to be independent of inert gas pressure over a 350-fold range and to be unaffected by the presence of oxygen. The removal of CS has been shown to be a heterogeneous process occurrin at the walls of the reaction vessel. A slowly growing continuous absorption was also observed. The waxing and waning oathis continuum as well as its wave length dependence was studied as a function of time a t three inert gas pressures in the range 50 to 400 mm. The origin of this continuous absorption is to be found in the sulfur sol formed through the condensation of sulfur to solid particles. The usual coagulation equation for homogeneous aerosols has been shown to ap ly to this condensation process. Flash photolysis of CSZunder isothermal conditions in the presence of oxygen results ine! t appearance of the spectrum of SO2 and of that now ascribed to SzO. The addition of oxygen was found to decrease the amount of Sz formed but to have no appreciable effect on its rate of disappearance.
The absorption spectrum taken a few milli- obtaining sulfur radicals in a relatively simple seconds after illumination of gaseous carbon disul- environment, fide by a high intensity flash shows the characterThis paper describes a preliminary study of sulistic bands of 5 2 and CS.2 The lifetimes of these fur radicals both in the presence and in the abtwo species are greatly different; whereas Sz dis- sence of oxygen. The work was carried out several appears in a matter of a few milliseconds, the years ago but only recently has the identity of one spectrum of CS remains visible for several min- of the principal intermediate spectra-that now uteseZb Flash photolysis of CS2 under isothermal assigned to SzO-been established. conditions offers, therefore, a convenient way of Experimental (1) This work was carried out at the Physical Chemistry Department, University of Cambridge. (2) (a) R. G. W. Norriah and G. Porter, Proc. Roy. Soe. (London) A’AOO, 284 (1950); (b) G. Porter, Disc. Faraday Soc., 9, 60 (1950).
The apparatus used for this work waa essentially the same as that used in previous investigations.8-6 The absorption spectra were recorded photographically using a Hilger E 1 spectrograph.
