1579
MAGNETOCHEMICAL STUDIESON DIPHENYLDIALKOXYSILANES
Magnetochemical Studies on Diphenyldialkoxysilanes by R. D. Goyal, R. R. Gupta, and R. L. Mital* Chemistry Department, Rajasthan University, Jaipur-4, India
(Received September 81, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
Diamagnetic susceptibilities of some diphenyldialkoxysilanes, (CeH&-Si [0(CHz),-H]2, are reported. xsi for these compounds was obtained from the intercept of X N us. n. n orbitals of the benzene ring are not conjugated with the silicon atom directly bound to the ring and no back-donation occurs in aromatic silicon systems. X;\I of these compounds also has been calculated theoretically from different wave-mechanical approximations. A poor agreement has been obtained between the observed xal values and the corresponding values calculated according to the method of Baudet, et al. This poor agreement has been analyzed in the light of different environmental conditions present in these molecules. Excellent agreement is obtained between experimental and calculated values using Hameka's method, which is attributed to the fact that in this approximation, the various structural factors, that contribute the molecular diamagnetism, have been taken into account.
Introduction It has been recently e~tablishedl-~that molecular diamagnetism is of much importance, usefulness, and interest in structural silicon chemistry. The results of such studies have been used to explain abnormal behavi0rl-j of silicon compounds containing Si-0 bonds in comparison to analogous carbon compounds containing C-0 bonds. Although much controversy6 regarding the nature of (C6H5)-Si bonds exists, no study appears to have been made on diphenyldialkoxysilanes with regard to molecular diamagnetism. The present investigation was undertaken with a view to extend experimental and theoretical studies of diamagnetism and to analyze the results in the light of structural factors. These studies provide information concerning bonding between the benzene ring and the silicon atom.
Experimental Section Six diphenyldialltoxysilanes (C~HS)~-S~-(OR)~ have been synthesized and ~haracterized.~Their susceptibilitiess have been measured with a sensitive Gouy balance calibrated with a number of standard substances and tried for more than 3 years in our laboratory. Reproducibility of the results was quite satisfactory. The Gouy force is of the order of k0.05 mg. The Gouy tube was hung in such a manner that its one end remained in the field, while the other end was outside the field, so that susceptibility could be calculated by a well-established method.',$ These susceptibility values are summarized in Table I.
Results and Discussion A plot of XM against n for diphenyldi-n-alkoxysilanes represented by (C6H&-Si- [O(CH2).-HIz has been made. A linear plot is obtained and the ordinate intercept of this plot is 135.2 which represents the mean susceptibility contribution of (CeH&-Si- [O(CHz),-H]z when n = 0. Taking X C ~ H = ~ 51.6,'O xo = 5.4,'
2.0," and using the additivity rule in the form = 2XCsHs 2x15 2x0 XSi = 135.2 the mean xsi in the series has been calculated to be 17.2 units. X I I = 168.4 (for n = 3//2) and XM = 212.8 (for n = 7 / 2 ) gives X C H ~= 44.4/(2 X 2 ) = 11.10 in this series. It has been reported that x ~ isi (20.6-21.0)j in silicon compounds containing four C-Si bonds. It has been further reported' that the value of xsi in silicon compounds depends on the number of Si-0 bonds present in a particular molecule. xsi = 17.0,', xsi = 18.42,' and XS, = 14.33' have been reported in compounds containing two, one, and four Si-0 bonds, respectively. These values refer to compounds which contain only one silicon atom. This variation in Si has been attributed t o the back-bonding (p,-d,) between oxygen and silicon atoms. xsi = 17.2 and x C H 2 = 11.10 in this series of diphenyl&-n-alkoxysilanes are comparable with xsi = 17.00 and XCH% = 11.17 in dimethyldi-nalkoxysilanes. This observation shows that xsi in both the series is almost the same. It can be noticed XH =
~(CsHa)z-Si--[0(CHz)o-H]~
+
+
+
(1) R. R . Gupta, Ph.D. Thesis, Rajasthan University, Jaipur, India, 1968. (2) R. L. Mital and R . R. Gupta, I m r g . Chim. Acta Reu., 4, 97 (1970). (3) R. L. Mital, J. Phys. Chem., 68, 1613 (1964). (4) R. L. Mital and R . R. Gupta, J . Amer. Chem. Soc., 91, 4664 (1969). (5) E. W. Abel, R . P. Rush, C. R . Jenkins, and T . Zobel, Trans. Faraday SOC.,60, 1214 (1964). (6) E. A. V. Ebsworth, "Volatile Silicon Compounds," Pergamon Press, Oxford, 1963, p 80. (7) B. C. Pant, Ph.D. Thesis, Rajasthan University, Jaipur, India, 1963. (8) All 'the susceptibility values are in - 10-6 cgs units throughout the paper. (9) B . N . Figgis and R. 5. Nyholm, J . Chem. SOC., 4190 (1958). (10) W. Haberdital, Sitzben., Deut. Akad. Wiss. Berlin, K1. Chem. Geol., Biol., No. 2 (1964); Angew. Chem., Int. Ed. Engl., 5, 288 (1966). (11) W. Haberditzl, 2. Chem., 1 , 255 (1961).
The Journal of Physical Chemistry, Vol. 76, No. 11, 197.8
1580
R. D. GOYAL,R. R. GUPTA,AND R. L. MITAL 2401
Table I : Diamagnetic Susceptibility Data on Diphenyldialkoxysilanesof the General Formula [(CaHb)rSi-(OR)2]
I
I
I
I
i
I
J
(calcd)-From Btludet’s waveFrom mechanical Hameka’s methodb equationc 7-XM
R
x 11 (exptl)
CHs C2H.5 CHa-(CHz)z (CHs)zCH CHdCH2)g (CHs)-CH
156.80 179.25 201.58 202.46 223.62 224.46
I
XSi
17.2a 17.28 17.2a
... 17.2a
...
163.90 186.60 209.30 209.30 232.00 232.00
156.82 179.02 201.22
... 223.42
CzH6 Average value of xsi determined from the plot of xv against n in the series (CBHB)Z-S~-[O(CHI),-H]Z. References 13 and 14. Reference 16. a
0
that the replacement of two CaH6 groups by two CH, groups does not affect x s i or in other words the behavior of a CHa group is similar to that of a C6H6 group towards x s i . It can be concluded that the R orbitals of a benzene ring cannot become conjugated with the d orbitals of a silicon atom directly bound to the ring otherwise x s i should be affected by replacement of a methyl group by a phenyl group. These studies support ultraviolet studies6by showing that no back-donation occurs in aromatic silicon systems. The is0 isomers have larger diamagnetic susceptibilities than the normal isomers and are analogous to related series of carbon The larger diamagnetism of the is0 isomer relative to that of the normal isomer is explicable in terms of the argument of Angus.13 Calculation of Diamagnetic Susceptibility XM values of these compounds have been calculated theoretically by Baudet’s wave-mechanical method.14-16 I n this method for calculation of x M , the contributions to the molecular susceptibility from the following sources are taken into account,: (a) from the inner shells of electrons of each atom present (ISE), (b) from the bonding electrons in each bond (BE), (c) from the nonbonding lone-pair electrons in the outer shells (NBE), and (d) from the T electrons present in each bond. XM is expressed in the form XM(theoret) = ZXISE4- EXBE ZXNBE ZR electrons. Using the values of XISE, XBE, XNBE, R electrons, and X C ~ H= ~ 51.6, the XM values of these silicon compounds have been calculated and summarized in the Table I. A critical comparison of the calculated and experimental values shows a poor agreement between them (the percentage deviation is 3.55%). Although all the above contributions to the molecular susceptibility have been considered, structural factors (such as pT-d, bonding betxeen silicon and oxygen
+
The Journal of Physical Chemistry, Vol. 76, No. 11, 1978
1201
0
I
I
I
I
I
2
3
4
n-
5
6
Figure 1. A pIot of X M against n for the series [ ( C~H&-S~-(OC,HZ,+I)21.
atoms), which cannot be neglected, have not been taken into consideration in calculating wave-mechanically the magnetic susceptibility data, and this may be a probable reason for the poor agreement between calculated and observed XM values. Moreover, this method fails to give different XM values for positional isomers and contradicts experimental results. An attempt has been also made to calculate X M of these compounds from Hameka’s wave-mechanical equation” in which the molecular susceptibility of a compound is considered to be made up from (1) each atom, (2) each bond, and (3) each interaction between any two bonds starting from the same atom. Mathematically this equation can be represented by XRI: = The Sizes of the Xatoms Xbonds Xbondinteractions. terms in this equation depend on the atoms involved. Hameka’s equation for a compound (Cd&),-Si(OC,H2,+&, can be expressed as
+
M
=
+
aA
+ bB + abC
(where a
+ b = 4)
(12) P. W. Selwood, “Magneto-chemistry,” Interscience, New York, N. Y.,2nd ed, 1955, p 95. (13) W. R. Angus and W. K. Hill, Trans. Faraday Soc., 39, 197 (1943). (14) J. Baudet, J . C h i n . Phys., 58, 288 (1961). (15) A.Pacault, J. Hoarau, and A. Marchand, “Advances in Chemical Physics,” Vol. 3, I. Prigogine, Ed., Interscience, New York, N. Y., 1961,p 171. (16) J. Baudet, J. Tillieu, and J. Guy, C. R.Acad. Xci., Paris, 244, 2920 (1957). (17) H.F.Hameka, J . Chem. Phys., 34, 1996 (1961).
1581
RATESOF HYDROGEN ABSTRACTION BY CHd3B1) where A , B , and C are the parameters related to Hameka's various terms as explained below.
A
=
0.25XSi
XCoHa
XSi-CsHs
-I- 1*5X8i-(C61~6)2
where xsi-(c611s)z is the term due to the interactions of two Si-CsHh bonds to the same silicon atom. Similarly for other terms
B
=
0.25xsi
+
+
XOC,H~,+~
XSi-OC,Hz,+l
+ 1.5XSi-(OC,Hz,+i)?
As regards the difference between methyl and ethyl compounds, the same treatment leads to the conclusion that there will be a certain increment in susceptibility on replacing a methyl group by an ethyl group, but no
other parameter will be changed. X M for Si-(C6HS)4 = 212.201 gives A = 53.05 units and XM for Si(OCH3)*= 89.98' gives B = 22.5 units. X M for (c6Hs)z-Si-(OCHs)z = 156.80l gives C = 1.43. Using these values of A , B , and C and XCH, = 11.10 in the present homologous series, X M values of these compounds have been calculated and summarized in Table I. An excellent agreement is obtained between observed and calculated XM values. I n calculating X M by Hameka's equation all the structural factors (including p,-d, bonding between siIicon and oxygen) , which do affect diamagnetism, have been considered, and this no doubt accounts for the excellent agreement between the calculated and experimental values.
Acknowledgment. Thanks are due to Professor R. C. Mehrotra, Head of the Chemistry Department, for providing facilities in the department.
Predictions of the Rates of Hydrogen Abstraction by CH2(3B1) by the Bond-Energy Bond-Order Method1 by Robert W. Carr, Jr. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota (Received January 6, 1978)
66466
Publication costs borne completely by The Journal of Physical Chemistru
Activation energies for 25 hydrogen atom transfer reactions to CH2(3B1)were celculated by the bond-energy bond-order method. The computed activation energies cover the range 7.9 kea1 mol-' to 44.2 kea1 mol-'. Rate constants were calculated by using the empirical activation energies in conjunction with transition state theory over the temperature range 300 to 650'K. The computational results indicate that the low reactivity of CH2(3B1)toward CH bonds is due to high activation energies.
Introduction The presence of free radicals in reactions of methylene with hydrocarbons is well established.2 Their source has been attributed to abstraction of hydrogen from C-H bonds by methylene with the formation of methyl radicals and the corresponding radical cofragments. Recent work indicates that the free-radical precursor is the ubiquitous triplet ground state of methylene, CH2(3B1).3 Furthermore, the work with alkane substrates clearly indicates the importance of a hydrogen abstraction reaction of triplet methylene14 although other reactions can contribute to the free radical component of the reaction.5 The rate of hydrogen abstraction by triplet methylene is only qualitatively known. Triplet methylene is
sufficiently unreactive at 300'K that Braun, Bass, and Pilling5 were unable to observe any reaction with either hydrogen or methane in their flash photolysis (1) Supported by the U. S. Atomic Energy Commission under contract No. AT(11-1) 2026. This is AEC document COO-2026-7. (2) H. M. Frey and G. B. Kistiakowsky, J . Amer. Chem. Soc., 79, 6373 (1957); H. M. Frey, Proc. Chem. Soc., 318 (1959); W. E. Doering and H. Prinzbach, Tetrahedron, 6, 24 (1959).
(3) J. W. Simons and B. S. Rabinovitch, J . Phys. Chem., 68, 1322 (1964); R. W. Carr, Jr., and G. B. Kistiakowsky, ibid., 70, 119 (1966); S. Y. H o and W. A. Noyes, Jr., J . Amer. Chem. Soc., 89, 5091 (1967). (4) G. Z. Whitten and B. S. Rabinovitch, J . Phys. Chem., 69, 4348 (1965); R. W. Carr, Jr., ibid., 70, 1970 (1966); R. W. Cam, Jr., and B. M. Herzog, ibid., 71, 2688 (1967); M. L. Halberstadt and J. R. McNesby, J . Amer. Chem. Soc., 89, 3417 (1967). (5) W. Braun, A. M. Bass, and M. Paling, J . Chem. Phys., 5 2 , 5131 (1970).
The Journal of Physical Chemistru, Vol. 76,No. 11, 1972
