1066
NOTES
Relation of Ring Size to Ultraviolet Extinction Coefficient in Cyclosiloxanes Containing Phenyl Substituents on Silicon by C. R. Sporck and A. E. Coleman
spectrophotometer using- chloroform as solvent. The intensity is cmax measured at 265 mp and calculated in units of lo00 (moles of C6HS)-l. Each emax is the average of two independent determinations.
Table I : Ultraviolet Absorption Coefficients of Cyclosiloxanes
General Electric Compang, Silicone Products Department, Waterford, New York (Received September 81, 1964)
enax
In the course of investigations of the ultraviolet spectra of a large number of cyclosiloxanes containing alkyl and phenyl groups bonded to silicon, we have noted that t8heintensity a t the absorption maximum 'A transition appears to per mole of phenyl of the 'Lb be a function of the siloxane ring size. This result has been confirmed by the measurement of oscillator strength and published as a portion of a paper' dealing chiefly with monophenylsilanes and -siloxanes. We are making available here our data on a large number of cyclics. +
[lo' om.¶ M.p., "C. (CsHs)-*]
Compd.*
-
Cyclotrisiloxanes
I
m
PnSiOPzSiOPzSiO
190-191
417
87-88
412
111 PzSiOMezSiOMezSiO
64-64.5
412
IV
83.5-84
414
73.5-74.5
413
I1 PzSiOPzSiOMezSiO
r
I
PzSiOPzSiOMeVSiO I
1
V PzSiOPzSiOMeEtSiO r 1 VI PzSiOPaSiOMePrSiO
68-69 Average
s
Experimental
-
=
412 413 2.0
Cyclotetrasiloxanes
The methods for preparation of the cyclosiloxanes examined here have been published.2 A more detailed description of the synthetic methods will be provided in later publications of this laboratory. The structures, melting points, and intensities are tabulated in Table I. Typical spectra appear in Figure 1. All spectra were recorded with a Beckman DK-2
I
I
VI1 PzSiOPzSiOPzSiOPzSiO 1
VI11 PzSiOMezSiOPzSiOMezSiO IX
PzSiOPzSiOMezSiOMe2SiO I
199-200
366
130-131.5
364
I
71-73
360
87-89
368
Q2-94.5
370
1
X PzSiOPzSiOPzSiOMezSiO 1 PzSiOPzSiOPzSiOMeVSiO I
XI
I
I
XI1 PzSiOMe~SiOMezSiOMezSiOb
I
0.5
365
XI11
PzSiOPzSiOPzSiOMeEtSiO
90-91
XIV
Pd3iOPzSiOPzSiOMePrSiO
86-88 Average
36.8
S
-
=
366 366 3.0
Cyclopen tasiloxanes XV
PzSiOPrSiOPzSiOMezSiOMezSiO
87-89
355
67-68.5
346
179.5-182
355
PrSiOMezSiOMezSiOPzSiOMezSiOMe~SiO124-124.5
342
r XVI
I
PzSiOPzSiOMezSiOMezSiOMezSiO
Cy clohexasiloxanes 1
I
XVII
PnSiOPzSiOMezSiOP,SiOPzSiOMzSiO I
XVIII
1
I n this table P = phenyl, Me = methyl, Pr = n-propyl, E t = ethyl, and V = vinyl. This compound is a liquid a t room temperature; na'~ 1.4817.
'
WAVELENGTH (Mpl
Figure 1. Ultraviolet absorption spectra of 1,1,3,3-tetramethyl-5,5-diphenylcyclotrisiloxane (top) and 1,1,3,3,5,5-hexamethyl-7,7-diphenylcyclotetrasiloxane (bottom) in chloroform; 1.1 X mole of C8H5-/l.
The Journal of Physical Chemistrg
(1) J. F. Brown and P. I. Prescott, J . A m . Chem. SOC.,86, 1402
(1964). (2) C . R. Sporck, Belgian Patents 635.643-635.649 (1963).
NOTES
Discussion The data of Table I clearly show that the average absorption int,ensity differences between the cyclot risiloxanes, cyclotetrasiloxanes, and higher cyclics are much larger than the standard deviations for each group of cyclics. The differences between the cyclopentasiloxanes and the cyclohexasiloxanes do not appear to be significant. If we consider the geometric configurations of these c.yclics in order of increasing ring size, we find that the average angle between the substituents on successive silicon atonis and conseqiiently the average distance between these substituents will decrease as the ring size increases. These changes in intergroup distance as the ring size changes from cyclotetrasiloxane to cyclopentasiloxane to cyclohexasiloxane could be less important as opportunity for ring puckering increases in the larger rings. Cases of rigidly enforced proximity and orientation of phenylene groups are found in the paracyclophanes a,nd lJ4-polymethylenebenzene cyclics studied by Cram3 and in the siloxane analog of the paracyclophanes studied by M a ~ K a y . MacKay's ~ compound is
1067
phenyl groups has nothing to do with the virtual identity of emnx for, say, compounds I and 111 of Table I among the trisiloxanes and for compounds VI1 and XI1 among the tetrasiloxanes. (3) D. J. Cram, N. L. Allinger, and H. Steinberg, J . A m . Chem. Soc., 76, 6132 (1954). (4) F.P.iMacKay, Thesis, The Pennsylvania State University Graduate School, 1956. (6) J. Petruska, J . Chem. Phye., 34, 1111, 1120 (1961). (6) I. Tinoco, J . A m . Chem. Sot., 82, 4785 (1960).
Elect rooxidation of the Tetraphenylborate Ion in Aqueous Solution at the Platinum
Disk Electrode by W. Richard Turner and Philip J. Elving University of Michigan, Ann Arbor, Michigan (Receiried October 6 , 1964)
The tetraphenylborate ion will undergo electrochemical oxidation a t the wax-impregnated graphite electrode in aqueous so1ution,lE2 although Geske3 has reported being unable to obtain voltammetric waves for this ion in aqueous solution at, the rotating platinum wire electrode. He was able to obtain a single wave in acetonitrile which was ascribed to the reactions
T n these compounds the benzene rings of the smaller cyclics are distorted from planarity and bathochromic and hypochromic effects are observed in the ultraviolet. In our compounds the distortion would be smaller, and we do not detect a bathochroniic shift but do observe B(CGHB).I- B(C6H6)2+ (C6H5)2 2e (1) a hypochromic effect, weaker than Cram's, upon going from the cyclotrisiloxane to the cyclotetrasiloxane. B(CsH6)2+ HzO B(Ci")2OH H+ (2) A still weaker hypochroniic effect is observed upon During the course of an investigation of the electrogoing from cyclotetrasiloxane to groups of cyclopentaoxidation of tetraphenylborate in aqueous solution at siloxanes and cyclohexasiloxanes, which are undifthe pyrolytic graphite electrode,4 the oxidation a t a ferentiated, It would appear that methyl, ethyl, platinum disk electrode was attempted. Waves were vinyl, and propyl groups on adjacent silicons have as obtained a t both the stationary and rotated platinum large an effect on the transition probability as does the disk electrodes which were comparable to those obphenyl group. tained a t the pyrolytic graphite electrode (Figure 1A). Brown and I'rescott' stress calculation of oscillator In addition to the wave observed by Geske, a second st rcngths and analyze them into vibrational and subwave has been characterized at the pyrolytic graphite stituent components following the method of P e t r ~ s k a . ~ electrode* However, in the case of our compounds, the substituents on silicon are similar so that intensity maxima B(CsHJ2OH 2H2O 4 are sufficient. The spectra of all the compounds are B(OH)3 (CsH5)2 2H+ 2e- (3) superposable with vertical shifts. Lastly, the development of Tinoco6 used by Brown (1) P.J. Elving and D. L. Smith, Anal. Chem., 32, 1849 (1960). and Prescott' to estimate hypochromism in mono(2) D.L. Smith, D. R. Jamieson, and P. J. Elving, ibid.. 32, 1253 phenylsiloxane structures cannot explain the results (1960). presented here as these authors have themselves ob(3) D.H.Geske, J . P h p . Chem. 63, 1062 (1959). served. Induced dipole-dipole interaction between (4) W.R.Turner and.'I J. Elving, Anal. Chem., 37, 207 (1965).
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+
+
+
+ +
+
+
Volume 69, Number S March 1965
