July 5, 1954
SPECTRA OF TRIMER AND TETRAMER OF PHOSPHONITRIDIC DICHLORIDE [CONTRIBUTION FROM
TEE
3403
NAVAL RESEARCH LABORATORY]
Infrared and Raman Spectra of the Trimer and Tetramer of Phosphonitridic Dichloride' BY L. W. DAASCH RECEIVED FEBRUARY 6, 1954 The infrared spectra of (PNC12)s and (PNC12)d are given for the range from 260 to 5000 wave numbers for solutions in carbon tetrachloride or carbon disulfide arid for the solid state. Raman spectra are also given for solutions in carbon tetrahas D a h chloride and for the liquid state. An interpretation of the data strengthens the belief that the molecule (PNCIZ)~ symmetry. An assignment of fundamental frequencies by class is made for fourteen of the seventeen active fundamentals and certain frequencies are further designated as modes of vibration originating in the skeleton or PClz groups. The symmetry of ( P N c l z ) ~is shown to be either D q h or Dzd. The data are more in accord with the selection rules for Dlh than DZd, but do not unequivocally exclude the Dzd symmetry. Only a few class and group assignments are made for this compound.
5000 to 260 cm.-' (Figs. 1 and 2) are a composite of spectra obtained from saturated solutions of I11 and IV in carbon tetrachloride and carbon disulfide. The spectra in the solid state are the same as in soiutior~. TIw m numbers of the observed maxima in the infrared and Raman spectra are listed in Tables I and I1 and a partial vibrational assignment is given for each molecule. The Raman spectra of the liquids are the same as the solutions except where noted in the tables. Interpretation of Trimer Data. Symmetry, Selection Rules and Assignment.-An attempt will first be made to interpret the spectroscopic data on the basis of a molecule with D3h symmetry. For this symmetry the fundamental modes of vibration divide as follows: 4A1' 2A2' 1A1" 3AzK 6E' 433'' where classes A1' and E"are Raman active only, Az" is infrared active only, E' is Infrared spectra from 5000 to 450 cm.-l were obtained active in both infrared and Raman effects, and Az' on the prism instrument previously described.' The range and A,'' are inactive in both effects. Since Class from 550 t o 260 cm.-' was studied with a Perkin-Elmer AI' frequencies will be polarized, i t is evident that Model 12C equipped with a 30" CsBr prism. The Raman spectrograph used was an Applied Research Laboratory three of the four AI' frequencies are a t 365, 670 and commercial instrument which has three 60' flint glass prisms 785 cm.-'. Another weak, polarized band was obas the dispersing element and both photographic and photoelectric recording. The resolving power of this instrument served a t 100 cm.-l on one set of polarization recis listed %s 3 cm.? and the dispersion as 87 cm.-l per mm. ords but the band is certainly not as strong as reported previously,6 if, indeed, it is present a t all. a t 4358 A. The samples were prepared by the procedure of Schenck It will be noted also in Table I that there is disagreeand RomeIB and were vacuum distilled directly into the ment with the previous work on the intensity and Raman tube for that phase of the work. The samples were maintained in the liquid state while obtaining the Raman polarization of the 575 cm.-' Raman line. It is a t spectrum by surrounding the sample tube with another most only one-fourth as strong as 670 ern.-' and has glass tube around which was wound a loose spiral of nichrome a semi-quantitative depolarization factor of 0.8. wire. There was no mass polymerization from the heat6 The strong infrared bands a t 1218, 885, 612, 522 but the background was fairly large indicating slight decomand 336 cm. -I, with coincident weak depolarized poiition or polymerization. Polarization measurements were semi-quantitative since Raman lines, are undoubtedly E' vibrations. The no special effort was made to correct for convergence error. strict strong-weak intensity relationship in the two Also, some of the bands were too weak to measure their intensities accurately and only visual estimates of their polari- effects indicates that the other possible E' vibration zation were made. The observed depolarization factors should likewise exhibit strong infrared absorption. for the depolarized lines of CCI, were 0.88, 0.90 and 0.77 Now there must be an overlapping of bands of difso that one could be reasonably certain that other observed ferent species a t about 670 cm.-' since the Raman values for strong lines were fairly accurate. Nevertheless, line a t 670 cm.-I is definitely polarized and therethe lines are only indicated as polarized (p) or depolarized fore Class AI', while there is a strong band a t 672 (dp) with questionable weak lines so indicated. cm.-' in the infrared spectrum where AI' is inacResults and Discussion tive. Keeping in mind the strong-weak relationThe infrared spectra of both compounds from ship this infrared band is assigned to the remaining E' frequency. (1) Nomenclature follows that outlined in Ckcm. Eng. News, 80, 4515 (1952). Any other depolarized Raman lines must be class (2) L. Brockway and W. Bright, THIS J O W R N A L , 65, 1551 (1943). E" so that 162, 173 and 210 cm.-' are surely of this (3) J. Ketelaar and T.de Vries, Rec. trav. chim., 58, 1081 (1939). class and the last E" frequency is probably a t 575 (4) Hereafter (PNC1z)r referred to as I11 and (PNC1r)r as IV. cm.-', the only remaining depolarized line of any (5) L. Daasch and D. Smith, Anal. Chcm., 23, 853 (1951). (6) A. Ficquelmont, M . Magat and L. Ochs, Compl. t e n d . , 208, 1900 strength. (1939). An assignment for the three vibrations of class (7) J. Rud Nielsen, F. Crawford and D. Smith, I.O p f .SOC.Am.,87, A*'' is not apparent although one should only have 296 (1947). to look for infrared active-Raman inactive bands. (8) R. Schenk and G.Romer. Bcr., 67B,1343 (1924). Introduction From electron diffraction studies2 the structure proposed for (PNC12)3 is a cyclic configuration similar t o benzene, but consisting of alternating phosphorus and nitrogen atoms. I n addition, each phosphorus is bonded externally to two chlorine atoms. A similar eight-membered ring is proposed3 for (PNC12)4.4 The infrared spectra for I11 and IV were previously reported6 without discussion of the molecular structure. The Raman spectrum of I11 was published some years ago.6 The present report increases the range of the infrared spectrum, gives the Raman spectra of the pure liquids as well as carbon tetrachloride solutions, and discusses the data in regard to the probable molecular structure. Experimental
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W o v e Numbers in cm;l.
5
9
7
6
15
14
II
m
17
Wavelength i n Microns.
-e
W a v e Numbers in cm;‘,
I-
COMPOUND
I
IlM~TRAlUIIl
I ___--
40
C l l i LINGYH
20
NAVAL RESEARCH
1 0
20
_ _ _ _ _ _ _ 26
22
21
za
27
ze
30
32
31
33
Wavelength in Microns.
Fig. 1.-Infrared 25oo;roo
lJ00
I
F
1100
spectrum of (PNCL),.
W a v e Numbers in cm-I 1000 300 800
700
600
100
I
80
60
~
-
~
-I
I 40
-
I
20
I
5 0
f
O4
5
6
7
9
c
+e
.=
500
400
IO
II
12
13
14
15
16
I7
Wavelength i n Microns. W a v e Numbcrs In cm;‘. 3 SO
100
!! p.
Wovelength in Microns.
Fig. 2.-Itifrared
spectrum of (PNC1I)d.
Another accidental overlapping class AI’ Raman band with 311 infrared Imnd of niedium intensity occurs a t ‘785 cm.-I. The infrared band is assigned to an Aq”vibration. There are other bands in class A2” at 900 e ~ i i . -and ~ above, which are be-
lieved to be overtones or combinations. The approximate “group,;’ frequencies indicate that the two remaining Az fundamentals should be below the one just assigned, perhaps beyond the range of the infrared spectrum.
July 5, 1954
SPECTRA OF TRIMER AND TETRAMER OF PHOSPHONITRIDIC DICHLORIDE
3405
TABLE I
INFRARED Present work
100 (vw, P?) 162 (4,dp) 173 (3, dp) 210 (3, dp) 340 (1, dp) 365 (IO, P) 523 (0.5, -) 575 (1, dp) 610 (0.5, -) 670 (4, P)
Raman
Ref. (6)
AND RAMAN DATAON ( PNC12)1
Infrared
A’ ( ? ) E”
100 (5.3, P) 162 (4.6, dp) 177 (4.5, dp) 206 (4.0, -) 312 (VW,-) 339 (2.7, dp) 366 (7.5, P) 533 (1.2, dp) 585 (5.5, P) 614 (1.0, -) 672 (6, P) 707 (VW,-)
T85 ( 2 , P ? ) 885 ( V W , -) 940 ( V W , -)
1230 vw 1368
Remarks”
Assign.
Found on only one polarization record
E” E“ 315 w 336 s
E‘ AI ’ E’ E” E’ AI’ and E‘
522 vs 612 s 672 s 690 vw 740 w 782 m 885 s 990 m 1040 vw 1060 vw 1092 m 1121 m 1200 m 1218 vs 1310 1368 1660 1750 1880 1975 2025 2075 2095
Ai’
” See Table I11 for meaning of V I to V Z O . Group Frequencies.-It is fortunate that certain molecular vibrations “concentrate” in a portion or group of the molecule. These group frequencies are effective tools in spectroscopy when the atomic masses participating in the vibration are much different from one another. One is not so fortunate as this in I11 and IV where the chlorine and phosphorus have comparable atomic masses and nitrogen has about one-third the atomic weight of either of these. In this case not much could be expected from group frequency correlations were it not for the well known fact that, in general, skeletal frequencies arising from cyclic structures have strong group characteristics. Indeed, a comparison of the spectra of I11 with its esters6 (,PN(QR)& clearly indicates that t h e E’ vibration a t 1218 ern.-' is a ring vibration, preserved regardless of the mass attached to the phosphorus atom, and that those a t 885 and 522 ern.-' are likely to be ring vibrations also. Furthermore, the approximate motion for vibrations of this class suggests that a P=N stretching vibration (Table 111) would be the only possibility for a frequency as high as 1218 ~ r n . - ’ . ~The only other ring vibration that could come above 750 ern.-' is the elongation mode which entails both stretching and de(9) It might be pointed out that since the P=N bonds are llkely resonating around the ring of 111, the absorption in the region for organophosphorus compounds containing a -0 bond direct evidence that the P=O bond is likewise a resonating or bond
very same is inpolar
A,”
Ring dcfoririatioil; ~g Ring vibration; VI4 PCl? stretch; V I ? V I 4 ?1 PC12 vibrations; vi and
E’
V I and u8 Ring clongatioii;
E’
P=N valence;
iiild
VIS(?)
PIZ
VI,
formation of the P=N bonds. Therefore, the 885 cm.-l band is thought to arise from this mode. Except for the fact that symmetric PCl2 stretch frequencies have in the past been below 800 cm.-‘ (the 672 or 612 ern.-' band probably accounts for this vibration) , there is no basis on which the other three group frequencies given in Table I11 can be divided among the experimentally observed frequencies of this class. I n the class E” vibrations, the ring deformation mode is perpendicular to the plane of the ring (Table 111)and experience has shown that these “out of plane” modes are usually found a t very low frequencies. Likewise, the PClz deformation and torsional modes will probably be a t low frequencies. Therefore the three lowest depolarized Raman bands are assigned to these modes. The fourth vibration is described as an unsymmetrical, out of phase, PClz valence vibration and consequently couHcome a t a higher frequency (575 cm.-l). The PClz unsymmetrical, in phase, valence vibration is probably the only class Az” vibrational mode that might have a frequency as high as 782 ern.-'. Thus, as mentioned previously, the bands a t 990 ern.-' and above 1092 ern.-' which follow the A2”selection rules must be interpreted as combinations or overtones. Class AI’ has a “breathing” ring vibration (corresponding to P-N=P stretch) which might be expected above 1000 cm.-’ since the P=N stretch
L. W. DAASCH
3406
Vol. 76
TABLE I1 INFRARED AND RAMAN DATAO N (PhTCla), Assignment Raman 137 158 169 178 192 259 300
Infrared
Class
Activity
Ai'
R
3Eg
339 355 370 393
169 and 178 believed t o be doublet although not well resolved
vw vw w w
PI A2'
A2"
I.A. I.R.
Raman line considerably broadened and peakintensityweakened in liquid spectrum
(5, dp) (0.5, -)" (0.5, -)
YIO
E'
730 ms (0.5,--)* 790 s 895 ( 1 . 0 , d p ? ) 895 s 1315 vs 1700 vw L860 vw 2020 v w 2060 vw 2200 vw
PI YS
vg
R and 1.R
512 (shoulder) 520 vs 579 w 590 w 615 vs
V.5 VO
Ai " Alg
I.A.
Ring breathing Symmetrical PClz stretch, in phase Trigonal ring deformation PClz deformation, in phase P=r\' stretch (trigonal) PClz wag, in phase (motion in plane of PCh group) PC12 torsion, in phase? UnsymmetricalPCl~stretch,inphase Ring deformation (out of plane of ring j PC12 rock, in phase (motion perpendicular to plane of PCI2j
P=K stretch in plane Ring elongation Y I ~Symmetrical PC12 stretch, out phase Ring deformation, in plane Y1.5 PC12 wag, out of phase (motion plane of PC12j vi6 PCl2 deformation, out of phase ~ 1 7PClz unsymmetrical stretch, out phase V I S PClz rock, out of phase? Y I O Ring deformation (out of plane ring) Y Z O PCl2 torsion, out of phase? ~ 1 1 ~ 1 2
538 (5, P) 590 620 668 725
Approximate motion
Y$
404 (10, P )
434 (4, P)" 512 ( 0 . 5 , P?)
Remarks
vz
(6, dp) (3, dp)" (8, dp) (8. dp) (5. dp) (4, dp) ( 0 . 5 , -)b
350 ( 1 . 5 , P ) b
(Ddh
symmetry)
TABLE 111 GROUPFREQUENCIES IN ( PNClZ)3
EU
PCh vibration
PCls vibration Blg and E"
Pu Ring vibration P=N stretch
a Not observed on liquid spectrum, perhaps due to background or unresolved from broadened neighboring lines. b Raman line observed on liquid spectrum only.
vibration of ciass E' is a t 1218 cm.-'. However, in benzene the Alg breathing frequency (990 cm.-l) is much lower than the comparable E, and E, class C-C=C stretching frequencies which occur10 a t -1500 cm.-l. Likewise for 111the polarized band a t 785 cm.-l which is assigned to the Ai' breathing mode is lower than the E' infrared active ring frequencies which come a t 1218 and 885 cm.-l. Of the three remaining polarized bands the one a t 670 cm.-' is assigned to a PClz symmetric stretch vibration and the one a t 365 cm.-l to the trigonal ring deformation. The doubtful band a t 100 cm.-l may be the remaining PClz deformation vibration. The group of bands on the low and high frequency sides of the very strong infrared band a t 1218 cm.-l and the group of bands in the 1650 to 2100 cm.-l region have been purposely ignored in the discussion of the assignment. These high frequency bands are undoubtedly combinations and overtones. The list of such bands which could be active in the infrared for Dsh symmetry is quite lengthy so that suitable combinations are easily found. However, in view of the number of possible combinations i t would add little support to the assignment of the fundamentals, especially when it is realized that some of the possible combinations include bands from the inactive Az' and AI" classes which are completely unknown for this molecule. Nevertheless, a peculiarity for each of these groups of observed bands should be noted. Most of (10) G.Herzberg, "Molecular Spectra and Molecular Structure, XI," D Van Nostrand Co , Inc., New York, N. Y , 1945, pp 362-369.
E"
R
of
in
of
of
the peaks about the 1218 crn.-l band are symnietrically placed. That is, there are bands a t .to5 j t 3 j cm.-l and k l 5 4 ( h 4 ) . In addition, on the iow frequency side there are absorptions a t 1263 and 178 cm.-l from the 1218 cm.-' maximum. What may be of significance is that the 95, 154 and 178 cm.-l values are close to the three lowest Raman frequencies and the bands may be the conlbinations 1218 f X,where X takes on the above values. The other group of frequencies, 1G50 to 2100 crn.-l, can also be explained as Combinations but the peculiarity is the concentration of the bands in these regions to the exclusion of the 1400 to 1750 cm.-l and 2100 to 5000 cm.-l regions. The three singlets and two doublets are definitely reminiscent of a similar group of bands in substituted benzenes which occur in about the same part of the spectrum. Interpretation of the Tetramer Data. Symmetry, Selection Rules and Structure.-The structure of I V has been studied previously using X-ray diffraction methods3 These studies show that the structure is probably an eight-membered ring similar to I11 but with the ring now puckered into a sort of skewed ''tub'' structure with Sd symmetry. However, as shown in Table IV the tetramer with this symmetry should display 11 polarized and 21 depolarized Raman lines and in addition there should be 10 coincidences betm-een the Ranian and infrared spectra. The spectroscopic data in Table 11, summarized in Table IV, shows that the symmetry is higher than S1 in the liquid or solution media because the total number of observed bands and coincidences is so much less than calculated for Sq symmetry. It is possible, of course, that in the
July 5, 1954
S P E C T R A OF
3407
TRIMER AND TETRAMER OF P H O S P H O N I T R I D I C DICHLORIDE
solid state crystal forces distort the molecule into the lower symmetry. TABLE IV SELECTION RULESFOR POSSIBLETETRAMER STRUCTURES Symmetry
Raman fundamentals DepolPolarized arized
D 4H Infrared fundamentals
21 16 14 11 8
10 10 15 15 6 16 10 Dzd 9 0 Dah 4 5 10wtos+2vw 6 Obsd." below 1350 cm -l a Four additional very weak Raman lines were observed but their polarization was not measured.
Sa
cay
11 5
D 2D
Coincidences
The possible structures of higher symmetry are drawn in Fig. 3, along with an S4 structure. The symmetry C4,. need not be considered for the structure of an isolated, stationary molecule in the crystal since the site group must be a subgroup of both the molecular symmetryll and the space group symmetry12 and ClV does not have S4 as a subgroup. Although the molecule could attain this symmetry in solution and in the liquid state the number of observed coincidences is much less than required for ClV symmetry (Table IV). This leaves symmetries D 2 d and D 4 h as possibilities. In D 2 d and D 4 h symmetries any polarized Raman lines should have no coinciding infrared bands because Class A1 is inactive in the infrared spectrum for both these symmetries. Thus, eliminating as accidental those coincidences between polarized Raman lines and infrared bands, there are four other coincidences. On this basis alone one might be inclined to eliminate the symmetry D 2 d which requires ten coincidences. However, the fact that the infrared spectrum extends to only 260 cm.-' makes it impossible to be certain that there are not infrared bands coincident with some of the six depolarized Raman lines below 260 crn.-'. On the other hand, if D 2 d symmetry were assumed, some of these depolarized bands would, without doubt, prove to be in class B1 since there are four depolarized Raman active bands in this class which would have to be included in the assignment for the Raman spectrum. The observed coincidences, therefore, could not possibly number ten. In any case, with the data available the D 2 d symmetry requires six more coincidences to be displayed whereas D a h requires four observed coincidences to b e accidental. The D 4 h structure is a t least less in conflict with this portion of the data than D 2 d . Considering the Raman data only, the total number of depoiarized bands seems to indicate a D 4 h structure. D 2 h requires 14, D 4 h requires 11; and there are only eight definitely depolarized bands with a possibility of four others, if all the very weak lines prove to be depolarized. The five polarized bands are intermediate in number between the required four for D 4 h and six for D z d . (11) Molecular symmetry is used here t o mean the symmetry of an
isolated molecule. not the "apparent" molecular symmetry deduced from X-ray intensity measurements The X-ray measurements may indicate molecular symmetries higher than the site group symmetry in such cases as tunneling or random occupation of alternate orientations. (12) R Halford, J Chcm. p h y s . , 14, g(1948).
4H
Fig. 3.-Possible
s4
structure for (PNClZ),.
The infrared absorption spectrum between 260 and 1350 ern.-' (about the upper limit for possible fundamentals) has six strong bands, four weak, and two in the very weak category. Here again ignorance of the 0-260 cm.-' region is a handicap but D d h structure with nine infrared fundamentals fits the available infrared data much better than D 2 d with sixteen infrared active fundamentals. I n summation, the available spectroscopic data favor the structure with D 4 h symmetry although they do not eliminate D 2 d symmetry. One other bit of evidence would favor the planar D4h structure even in the crystal. The X-ray studies3 which indicated a molecular structure with- S 4 symmetry also contained the information that the P=N distances around the ring were equal. Resonance in the ring of IV was postulated to account for this equality of P=N distances and for the remarkable chemical stability of the ring in IV. However, a common requirement for what is apparently resonance of a high order is that the structure involved in the resonance must be planar,13 and for this molecule the planar structure has D 4 h symmetry. Group Frequencies and Class Assignments.The infrared spectra of I11 and IV are strikingly similar, there being only two strong bands a t 336 and 672 cm.-' in I11 which have no counterparts in IV. Likewise the strong depolarized Raman lines below 250 cm.-' in I11 are almost matched by lines in IV. Using this similarity and the selection rules for D 4 h symmetry14 a partial assignment by class is given for IV in Table 11. There are two class E, ring vibrations, one of which corresponds to P=N. stretch and the other to. a ring distortion(Fig, 4). In cyclooctatetraene1° the corresponding vibrations have the frequencies 1635 and 1399 cm.-'. The infrared bands a t 1315 and 895 cm.-l in IV would be logical choices.l6 The PClz . s t r e w vihratbn~of class E, comes at 615 cm,-1 and one of the remaining ring vibrations a t 520 cm.-'. Lippincott16lists another frequency in cyclooctatetraene as being concentrated in the C=C stretch(13) E. Alexander, "Principles of Ionic Organic Reactions," John Wiley and Sons, Inc., New York. N. Y..1950, p. 20. 4Blg (14) Fundamental modes divide as follows: AI, f 2Aeu 4Bz 3Eg 7EU where classes Ai,, Big, Beg and E, are Raman active only and Ai, and Eu are infrared active only. (15) 895 cm.-' Raman line is considered accidentally coincident as are 725, 820 and 590 cm. -1. ( 1 6 ) E. Lippincott, R. Lord and R . McDonald, THISJOURNAL, 73, ETU(19sl).
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W. T. GRUBB
340s
Vol. 76
Raman active class Big, it has been assigned to the 895 crn.-' Raman line. One further assignment might be made and that is the infrared active-Raman inactive band a t 790 crn.-l. In I11 there was believed to be an overlapping of AI' and Az" modes a t about this frequency. The Az" mode was assigned to a PCIZwagging, in phase vibration. This same type of motion in IV does come in class Az, which matches the selection rules of the observed barid a t 790 cm.-'. t Other than to say that the three vibrations of class E, .probably occur among the six observed '? Raman lines below 260 cm.-', nothing further in the way of assignment has been made. There are still 3B1g 4- 4B2, lAzu 3E, whose range cannot be given, much less proposals for assignment to observed frequencies. Conclusions The selection rules for D 3 h symmetry in the molecule (PNC12)3are shown to account for the observed Fig. 4.-Approximate motion of (PNCl9)t in certain of its infrared and Raman data. Selection rules for neither D 4 h or D z d fully account for the observed data on fundamental modes. (PNCIZ)~ and yet the moleculeis believed to have ing motion. This is a class B1 vibration for D4 one of these two symmetries. The data a t present symmetry and has been assigned in cyclooctatetra- favor the D l h structure. Vibrational assignments, ene to a frequency of 1601 cm.-'. The form of the as yet incomplete, are given for both molecules. Acknowledgment.-The author is happy to acvibration must be something like (c) in Fig. 4 which is actually more comparable to what has been called knowledge the assistance of R. B. Fox and E. J. a ring elongation vibration and which in I11 has Kohn of our High Polymer Branch in making available samples of the trimer and tetramer. been assigned to the infrared band a t 885 cm.-'. Therefore in IV, where this vibration is now in the WASHIVGTON, D. C.
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[CONTRIBUTION FROM
THE
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RESEARCH LABORATORY, GENERAL E L E C ~ R Co.] IC
A Rate Study of the Silanol Condensation Reaction at 25" in Alcoholic Solvents' BY W. T. GRCBB RECEIVED JANUARY 16, 1954
A method is described for measuring the rates of silanol condensation reactions. The method consists of a titration of sampies of the reacting soiution with Kari Fischer reagent. This ritrasion measures rhe mtai o n t e n t o f SiGH grmpsplus Hz0 in the solution. Complete condensation of a silanol solution results in a reduction of the apparent water concentration to one-half of its initial value. Using this method of analysis, a detailed study has been made of the condensation of trimethylsilanol in the solvent methanol with the following results. The condensation of trimethylsilanol is found to be inCHaOH $ (CH3)3complete in this system. Incomplete condensation is caused by the rapid reaction (CH3)aSiOH HzO, which removes silanol from the system, while the following condensation equilibrium is established slowly. SiOCHs CH30H. The formula for the condensation equilibrium constant (CH3)&XOH ( CH1)3SiOCH3e (CHa)aSiOSi(CH3)3 is K = [( CHg)3SiOCHa]2[Hz0]/[(CHa)3SiOSi(CH3)a]. This form for the equilibrium constant reveals that essentially no free trimethylsilanol is present a t equilibrium. The equilibrium constant has the same value when different concentrations of both acidic and basic catalysts are employed, and the same equilibrium is attained by starting from a mixture of hexamethyldisiioxane and water, thus demonstrating that this is a true thermodynamic equilibrium. It is found by studying the rate of attainment of the condensation equilibrium that the rate determining reaction in condensation is (CH3)3SiOCHa (CH&SiOH + (CH3)&3iOSi(CH3)3 C a O H . A rate law deduced from this reaction agrees with the experimental results. The rate law is found to be the same for both acidic and basic catalysts although HC1 is about 500 times as active as KOH in promoting condensation. Silanols containing bulkier substituents react much more slowly. Triethylsilanol and triphenylsilanol react, respectively, 1/600 and
