J. Phys. Chem. 1991, 95, 6494-6499
6494
Vibrational Spectra, Ring-Puckering Potential Energy Function, and Conformation of 1,3-Disilacyclopent-4-ene Lloyd F. Colegrove and Jaan Laane* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: April 2, 1991)
1,3-Disilacyclopent-4-enehas been synthesized and its far-infrared, mid-infrared, and Raman spectra have been analyzed. From the far-infrared data of the molecule in the vapor phase, which shows a series of 10 bands between 48 and 85 cm-I, the ring-puckering potential energy surface was determined to be V (cm-I) = 1.48 X lOSp+ 0.30 X l e x 2 ,where x is the ring-puckering d i n a t e in angstroms. This shows the molecule to be planar but to be not nearly as rigid as silacyclopent-2ene. Other features in the infrared, Raman, and NMR spectra confirm that the interactions between the silicon atoms and the carbon-carbon double bond, while present, are reduced relative to silacyclopent-2-ene.
Over the past two decades we and other workers have used far-infrared spectroscopy to determine the ring-puckering potential energy functions and conformations of ring molecules'*2such as c y ~ l o p e n t e n eand ~ ~ related compounds. We have labeled such molecules as "pseudo-four-membered rings"' since their out-ofplane ring-puckering vibrations resemble those of cyclobutane and other four-membered rings. Our previous results- have shown cyclopentene to be puckered with a dihedral angle of 26' and with a barrier to planarity of 232 cm-' (2.8 kJ/mol). Here, the two CH2-CH2torsional interactions overcome angle strain effects to produce the nonplanar structure. The two monosilanes, silacyclopent-3-ene'J (1) and silacyclopent-2-ene9Jo(2), however, are
H2
1
2
3
both planar. In the case of 1, the smaller CH2-SiH2 torsional interaction as compared to a CH2-CH2interaction allows the angle strain to overcome the torsional forces and to yield a planar ring conformation. The planarity of the -2-ene (2), however, is surprising in view of the fact that 2,3-dihydrofuran"J2 and 2,3-dihydrothiophene" are both puckered as a result of their CH2-CH2 interactions. Not only is the silacylopent-2-ene planar, but it is also extremely rigid as demonstrated by the steep walls of its ring-puckering potential energy function. This rigidity can be accounted forlo by assuming that the vibrational force constants associated with the - S i H 2 - C = C - linkage are substantially increased due to an interaction between the carbon-carbon double bond and the silicon atom. These force constants contribute to both the quartic and quadratic potential energy terms in the one-dimensional ring-puckering potential energy function V = ux4 + bx2 (1) where x is the ring-puckering coordinate and where a and b are the potential energy parameters. The constant a arises primarily from angle strain effects whereas b has contributions from torsional (1) Laane, J., Pure Appl. Chem. 1987, 59, 1307, and references therein. (2) Carreira, L. A.; Lord, R. C.; Malloy, T. B. In Topics in Currenr Chemistry; Dewar, M. J. S.,et al., Eds.; Springer-Verlag: Berlin, 1979; Vol. 82, references therein. (3) Laane, J.; Lord, R. C. J. Chem. Phys. 1967, 47, 4941.
(4) Villarreal, J. R.;Bauman, L. E.; Laane, J.; Harris, W.C.; Bush, S.F. J . Chcm. Phys. 1975,63. 3727. (5) Villarreal, J. R.;Bauman, L. E.: Laane, J. J. Chcm. Phys. 1976.80. 1172. (6) Bauman, L. E.; Killough, P. M.; Cooke, J. M.; Villarrea.1, J. R.;Laane, J. J . Chem. Phys. 1982,86,2000. (7) Laane, J. 1. Chem. Phys. 1969, 50, 776. (8) Killough, P. M.;Laane, J. J. Chcm. Phys. 1984, 80, 5475. (9) Laane, J. J. Chem. Phys. 1970.52, 358. (10) Kelly, M. B.; Laane, J. J. Chem. Phys. 1988, 92, 4056. ( 1 I ) Ueda, T.; Shimanouchi, T. J. Chem. Phys. 1967, 47, 5018. (12) Green, W.H. J. Chem. Phys. 1969, 50, 1619.
SCHEME I 4
....
6
10
.
.
3
forces (typically negative) and initial angle strain (typically positive). For 2 the latter forces greatly exceed the former and b is positive. In order to further investigate silicon atom interactions with carbon-carbon double bonds, we have synthesized 1.3-disilacyclopent-4-ene (3) for the first time and analyzed its far-infrared, mid-infrared, and Raman spectra. These results will be reported here.
Results and Discussion Syntbesis and Characterization of 1,3-Disilacyclopent-4-ene. Scheme I shows the preparative procedure for 1,3-disiIacyclopent-4-ene (3). The saturated 1,1,3,3-tetrachloro-1,3-disilacyclopentane (8) is first prepared, utilizing two Grignard reactions and a hydrosilation step in a manner similar to that described by Fritz and Sch0ber.l' The ring is then chlorinated by using sulfuryl chloride, and the olefin (10) is produced by the elimination of HCl using 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU). The reduced disilane ring (3) is prepared by using lithium aluminum hydride. In a typical p r d u r e we can prepare about 3 g of 3 over a 6-week period starting from 220 g of chloromethyltrichlorosilane,which is a relatively expensive starting material. Our final product generally contains about 10% of the saturated 1,3-disilacyclopentane due to the difficulty of both purifying and conserving sample at the same time, particularly after the formation of 1,1,3,3,4-pentachloro-1,3-disilacyclopentane(9). This does not cause a major problem, however, since we have detailed spectrat4 of the 1,3-disilacyclopentane and can account for its presence in our sample of 3. Table I lists a number of the characteristic infrared and Raman frequencies for 3 and the tetrachloride (10) while Table I1 lists the principal mass spectral peaks for 3. Figure 1 shows the proton NMR chemical shifts for 1,2, and 3, their chlorinated derivatives, and several other related compounds. The infrared frequencies and NMR chemical shifts may be compared to those of various five-membered rings containing Six2(X = H, F, Cl) groups which are reported e1se~here.l~The various SiHz stretching and bending (13) Fritz, V. G.; Schober, P. Z . Anorg. Chcm. 1972, 372. (14) Colegrove, L. F. Ph.D. Dissertation, Texas A&M University, 1989.
0022-3654/91/2095-6494$02.50/00 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6495
Spectra of 1,3-Disilacyclopent-4-ene
o'0:*ou .*om2 emu 0:%cJ*
0: a&== ( :
b
J=(:
w
= (=J
m
19
t)
I vT
"2
Ll
1y1
"2
LI
w
c4
C'Z
m
la
Figure 1. Proton chemical shifts for several saturated and unsaturated rings containing silicon. TABLE I: Cbancteristic Infrared and Raman Frequencies of 1,1Msilacyclopent-4-eneand Its 1,1,3,3-Tetrachloro Derhrtivd
,
CH3CHSiH2CH2SiH2 infrared (vapor) Raman (liquid) cm-' intensity cm-I re1 int 3018 W 3017 P 100 2991 m 2979 256 2905 W 2922 P 215 2155 ws 2146 ws 2150 P lo00 1593 W 1600 P 43 1409 (B) mw 1410 22 1368 W 1368 9 1257 mw 1251 11 1093 ms 1094 P 33 9 999 S 1000 34 vs 952 948 (B) 3 vs 861 P 868 (B) 15 780 S 791 750 S 740 17 668 mw 672 P 190 654 P 50 550 W 565 40 370 P 20 263 13 48 W Os
CH=CHSiCI2CH2SiCl2 Raman (liquid) cm-' re1 int 3028 100 2965 301 2925 550 520 120 299 425 1600 67 1409 73 1358 25 1270 80 1089 47 140 185 795 679 580 530 478 335 248
assignment =C-H sym stretch CH2 antisym stretch CH2 sym stretch Six2 antisym stretch Sixz sym stretch C = C stretch CH2 deformation CH wag (0.p.) CH2 wag CH wag (i.p.) Six2deformation (0.p.) Six2 deformation (i.p.) Six2 wag (i.p.) CH2 rock ring stretch ring stretch ring stretch ring stretch ring deformation C=C twist ring bending
240 53 7 6 52 165 110 34 49
SYm AI B2
AI B2 A1
AI AI BI
BI AI Bl AI AI B2
BI AI A1
BI AI A2
B2
= strong, m = medium, w = weak, v = very; B = type B infrared band contour; P = polarized; i.p. = in-phase, 0.p. = out-of-phase.
TABLE 11: Principal Mass Spectral Peaks of
1,3-Disilrcyclopent-4-ene mass intensity 74 100 99 46 25 85 19 75 100 74 28 73 68 72 34 69 19 45 32 43
assignment M M-l M - CH, M - CHC M - CHCH (SiH2CH2SiH2)
M M M M M
- CHzCH - CH23CH2 - SiHl - SiHCHCH (SiH3CH2) - SiHICHCH (SiH,CH)
frequencies of 3 are typical for organosilanes. The C=C stretch at 1600 cm-' is intermediate between the values of 1620 and 1560 cm-I observed for 1 and 2, respectively, suggesting that there is less interaction between the silicon atom and the carbon-carbon double bond in 3 than there is in 2. As we shall see, this is also clearly demonstrated by the far-infrared spectrum. The ring vibrations of 3 tend to be a t lower frequencies than those of 1 or 2, and this is primarily due to the greater mass and lower force constants introduced by the addition of a second silicon atom to the ring. The effect on chemical shifts of introducing a SiHz or SiCl, group into a cyclopentene or cyclopentane ring can clearly be Seen in Figure 1. When the former is added to a ring, the CH2 protons adjacent to the SiH2are shifted by -0.6 f 0.4 ppm whereas CH2 protons B to the silicon are shifted by +0.3 0.1 ppm. The olefinic hydrogens adjacent or 0 to a SiH2 have their chemical shifts increased by 0.2 f 0.1 ppm relative to cyclopentene. However,
*
(IS) Chao, T. H.;Moore, S.J.; Lane, J. J . Organornet. Chcm. 1971.33, 157.
A
*u* 45
60
I
75 WAVENUMBER ( C M ' )
Figure 2. Far-infrared spectrum of the ring-puckeringvibration of 1,3-
disilacyclopent-4-ene. Recorded at 40 Torr pressure and 0.75-mpath length. The band at 56.0 cm-l is from the 1,3-disilacyclopentaneimpurity. in 2 and 3, where the silicon atom is adjacent to the double bond, the olefinic hydrogen /?to the silicon has an increase of 1.3 0.1 ppm, reflecting a significant interaction between the silicon atom and the r system. No similar effect is seen in 1where the silicon atom is not adjacent to the double bond. All of the chlorosilanes in Figure 1 show similar effects except that all of the chemical shifts for these molecules tend to be 0.1-0.3ppm higher due to the high electronegativity of chlorine atoms. Far-Infrared and Combination Band Spectra of 1,3-Disilacyclopent-Cene. The far-infrared ring-puckering spectrum of
*
64%
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991
Colegrove and Laane
TABLE 111: observed and Calculated Ring-Puckering Transitions for 1,3-Disilrcyelopent-4-ene relative frequency, cm-l absorbance transition obsd calcd' A obsd calcd' 49.5 0.44 0-1 48.8 -0.7 0.23 57.4 0.2 1-2 57.6 0.60 0.78 0.96 63.1 0.3 2-3 63.4 0.88 3-4 68.2 67.6 0.6 (1.00) (1.00) 71.5 0.6 4-5 72.1 0.91 0.94 74.9 0.3 5-6 75.2 0.85 0.83 0.3 0.77 0.70 6-7 78.3 78.0 7-8 80.8 80.8 0.0 0.68 0.56 83.3 -0.3 8-9 83.0 0.70 0.44 85.7 0.33 -0.9 0.50 9-10 84.8 "Using V (cm-I) = 1.48 X 10s9 + 0.30 X 1 ~ x 2 .
I
,
,
,
,
I
"
"
WAVENUMBER (CM-l) Figure 4. Sum bands from the Si-H stretch in combination with the ring-puckering mode of 1,3-disilacyclopent-4-ene.Bands indicated by solid lines originate from the infrared inactive (A2) Si-H2 stretching mode at 2155.2cm-'; those indicated by the dotted lines originate from the 2146.4cm-l Si-H2 stretching vibration (A,).
WAVENUMBER (CM.')
Figure 3. Si-H stretching region of 1,3-disilacyclopent-4-eneshowing the combination bands from the Si-H stretching and ring-puckering modes. Top spectrum: 1 Torr vapor pressure of sample in a IO-cmcell; botton spectrum: 45 Torr of sample.
1,3-disilacyclopent-4-ene(3) is shown in Figure 2. The spectrum of the olefinic ring shows a regular series originating with the relatively weak band a t 48.8 cm-' and then continuing as shown in Table 111. These bands are type C for this B2 vibrational mode of the C, molecule. A band present at 56.0 cm-' is due to a small amount of 1,3-disilacyclopentane impurity in the ~amp1e.I~ The spectrum of 3 is characteristic of a planar ring system with a potential energy function dominated by a quartic term. It is similar to that observed for silacyclopent-3-ene (I).'** Mid-infrared combination band spectra in the SiH2stretching region provide further confirmation of the ring-puckering transition frequencies. Figure 3 shows this region for "low" (1 Torr) and "high" (45 Torr) vapor pressures of the sample, and Figures 4 and 5 show in expanded form the sum and difference bands, respectively. Table IV lists the observed combination bands and compares the sums and differences to the far-infrared values. Two sets of sum and difference bands were seen, one originating from the A2 (infrared inactive) out-of-phase antisymmetric SiHz the other from the A, in-phase stretching vibration at 2155.2 an-,, symmetric stretching mode at 2146.4 cm-I. The combination bands off the A2 mode are more intense, especially on the sum side, than those from the A, vibration. These transitions have BI symmetry (A2 X B2 = B,) and thus have type A band contours. The combinations from the A , stretching mode have type C contours (A, X B2 = B2). The Bl and B2 SiH2 stretching modes do not give rise to observed combination bands since these would either be inactive (BI X B2 = A2) or produce type B bands (B, X B, = A,) with no sharp Q branches, respectively. What is demonstrated by Table IV is that the sum and difference values agree very closely with the far-infrared frequencies, not only confirming the far-infrared values but also demonstrating that
I
2105
2090
1
"
2075
"
I
20t I
WAVENUMBER (CM") Figure 5. Difference bands from the Si-H stretch in combination with the ring-puckering mode of 1,3-disilacyclopent-4-ene.Bands indicated by solid lines originate from 2155.2 cm-I; those indicated by the dotted lines originate from 2146.4cm-I.
the ring-puckering levels are changed very little in the excited vibrational states of the SiH2 stretching modes. Calculation of the Ring-Puckering Potential Furdon. In order to determine the vibrational potential energy function (eq 1) for 1,3-disilacyclopent-rl-ene, we must utilize the one-dimensional Hamiltonian H(x) = (-h2/2) d/dx gu(X) d/dx
+ V(X)
(2)
where gu(x) is the kinetic energy (reciprocal reduced mass) expansion. Subscripts 1-3 for the gN terms are reserved for the molecular rotations. The g&) expansion in terms of x has been
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6491
Spectra of 1,3-Disilacyclopent-4-ene
TABLE IV: Combisrtioo Band Frequencies (em-') of 1,3-Disilacyclopent-4-ene YO = 2146.4 transition vprtar sum A diff 2097.6 2194.8 48.4 0-1 48.8 2088.8 2203.7 57.3 1-2 57.6 63.6 2083.1 63.4 22 10.0 2-3 68.2 2078.8 68.2 2214.6 3-4 72.1 2074.2 72.1 2218.5 4-5 75.5 2070.6 75.2 2221.9 5-6 78.6 2068.0 78.3 2225.0 6-7 81.0 2064.9 7-8 80.8 2227.4 8-9 83.0 2229.8 83.4 84.8 2231.6 85.2 9-10
CH=-CHSiH2CH&H2
138.7
'The form of the kinetic energy expansion is g&)
=
0.007209
VO
A 49.0 57.6 63.3 67.6 72.2 75.8 78.4 81.5
A 49.5 57.6 63.9 68.4 72.6 75.4 78.1
sum 2204.7 2212.8 2219.1 2223.6 2227.8 2230.6 2233.3
-0.009428
5
2155.2 diff 2106.2 2097.6 2091.8 2087.2 2083.1 2080.1 2077.0 2075.0 2071.7 2068.9
A 49.0 57.6 63.4 68.0 72.1 75.2 78.2 80.2 83.5 86.3
-0.06640
0.1762
xf.oA).".
TABLE VI: Calculated Structural Parameters for
1,3-Disilacyclopent-4-ene" length, A 1.341 1.869 1.102 1.932 1.527 1.113
atoms C(4)=C(5) C(4)-Si C(4)-H
Si-C(2) Si-H C(2)-H
C(4)4(5)-Si(l) C(4)=C(5)-H
C-Si-C H-Si-H Si-C(Z)--Si
H-C(2)-H
angle, deg 117.1 122.8 100.8 111.3 104.0 109.9
I
-/
2001
CHICHCHzCHzCHz 7 CH2CHICHSiHzCH2 CH2CHPCHCHzSiH2 , CHICHSiHzCHzSiH2
.
117.9 129.6 128.6 138.7
7.06 18.28 2.13 1.48
-2.56 1.84 -0.05
0.30
24.4 31.5 15.4 13.0
1 I
0
B -6.17 2.40 -0.28 2.16
,I
63.4
9.2 -0.1 0.0 0.1 PUCKERINGCOORDINATE
9.3
0.3
0.2
X(A)
Figure 6. Ring-puckering potential energy function and far-infrared transitions for 1,3-disilacyclopent-4-ene.
'The kinetic energy models assume no rocking of CHI or SiHz group (R = 0). The reduced parameters A and B in Y = A(9 + 89)are independent of ,u and arc defined in ref 19.
calculated for the 'bisector model" according to methods previously des~ribed.'~.'' This is presented in Table V. The structural parameters used were calculated by utilizing Allinger's'* molecular mechanics program (MMZSI), and these are presented in Table VI. The gU(x) expansion is used in eq 2 so that the potential energy parameters a and 6 in eq 1 can be determined to fit the observed frequencies. The matrix diagonalization techniques for calculating the energies of the vibrational quantum states are well established.' The resulting potential function is V (cm-I) = 1.48 X 105x4+ 0.30 X 104x2 (3)
60.8
\
"C(2) is between the silicon atoms; C(4) and C(5) are olefinic. TABLE VII: Ring-Puckering Potential Energy Plvrmetem" for FircMwkred Riag Oleflm Containing Silicon a, IO5 b, lo4 molecule p,au cm-I/A4 cm-I/A4 A,cm-l
I
600 h
i1
0
r 400. W
5 200.
and this gives rise to the calculated transition frequencies shown in Table 111. The agreement between observed and calculated frequencies is excellent, demonstrating that the determination of the potential function is unambiguous. Figure 6 shows the ring-puckering potential energy function, the vibrational quantum states, and the observed far-infrared transitions for this planar ring system. Transitions up to the 10th excited state, which lies 712 cm-I above the ground state, can be seen. Table VI1 compares the dimensioned potential energy parameters a and 6 and the reduced (dimensionless) parameted9 A and
Figure 7. Comparison of the ring-puckering potential energy functions of 1,3-disilacyclopent-4-ene(3). silacyclopent-3-ene (I), and silacyclopent-2-ene (2).
(16) Malloy, T. B. J. Mol. Specrrosc. 1972, 41. 504. (17) Laane, J.; Harthcock, M. A.; Killough, P.M.; Bauman. L. E.;Cooke, J. M. J. Mol. Spectrosc. 1982. 91. 286. ( 18) Burkert, U.; Allinger. N.L. Molecular Mechanics; ACS Monograph; American Chemical Society: Washington, DC, 1982. (19) Laane, J. Appl. Specrrosc. 1970, 24(l), 73.
B of 1,3-disilacyclopent-4-eneto those of cyclopentene and the two silacyclopentenes. Figure 7 shows the potential functions for the three silicon compounds. What is evident is that the function for 1,3-disilacyclopent-4-ene(3) is much more similar to that of silacyclopent-3-ene (1) than to that of the 2-ene (2). It does not
\>,, 0 03
02
0.1
;y
, ,//'
0.0
ai
02
oa
PUCKERING COORDINATE X(H)
6498 The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 have a steep potential like that of 2. Molecule 1 has an almost perfect balance between the angle strain effects and the weak SiH2-CH2 torsions, resulting in a nearly perfect quartic (Le., the constant b in eq 1 is very close to zero). It also has no possibility for interaction between the silicon atom and the carbon-rbon double bond and thus has a very broad potential energy function characteristic of a very floppy molecule with a large-amplitude ring-puckering vibration ( f 3 0 ° in the dihedral angle). As discussed above, even though it has increased torsional forces resulting from the CH2-CH2 internal rotation, molecule 2 is extremely rigid because of the A interaction with the silicon atom. Molecule 3,studied here, shows a much smaller degree of such interactions. If it had no Si-.rr interactions, 3 would be expected to be even less rigid than 1. In five-membered rings of this type we have found'.'' that the angle bending during a ring-puckering occurs almost exclusively at the atoms adjacent to the double bond, which in 3 are silicons and in 1 are carbons. Since the angle bending force constants for CSiC angles are considerably lower than for CCC angles, the angle strain effects will be less in 3 than in 1. The torsional interactions for both molecules should be similar since each has two SiH2-CH2 linkages. The fact that 3 has a steeper potential energy curve than 1 (Figure 7) must then be a ramification of the fact that other forces are responsible for making the ring more rigid. These forces presumably come from the interactions (d,-p,) of the silicon atom with the carbon-carbon double bond, similar to what was observed for 2. However, the effect for 3 is much less than for 2. The reasons for this are not obvious, but may well be due to the different geometrical relationships in the two molecules.
Conclusions The far-infrared spectrum of 1,3-disilacyclopent-4-ene(3)and the ring-puckering potential energy function derived therefrom clearly show this molecule to be planar and relatively nonrigid. It is "stiffer" than silacyclopent-3-ene (1) but much more floppy than silacyclopent-2-ene (2). Mid-infrared combination bands involving the ring-puckering vibration and the mid-infrared and Raman spectra of the other vibrational modes confirm the planarity of the molecule. The C=C stretching frequency of 1600 cm-I (vs 1620 cm-' for 1 and 1560 cm-' for 2) indicates a modest interaction with the silicon atoms. The proton NMR spectra show that in 2 and 3 the olefinic hydrogens /3 to the silicon atoms do feel their influence through the double bond. Thus, there is considerable evidence that in 3 there is a moderate amount of interaction between the silicon atoms and the carbon-carbon A system. However, this phenomenon is much smaller than it is in
*A.
Experimental Section Synthetic Metbods. 1,3-Disilacyclopent-4ene(3) was prepared according to the scheme shown above (Scheme I). The procedure described by Fritz and SchoberI3 was modified due to the availability of different starting products and a better reagent for eliminating HCl. All solvents were dried over lithium aluminum hydride and freshly distilled for each reaction. All glassware was carefully dried before each reaction. The standard synthetic setup included a three-neck round botton flask, an overhead paddle stirrer (or magnetic stirrer), a Friedrich condenser, a side-arm addition funnel, and a thermometer. Dried nitrogen was used to purge each reaction vessel. (Cblorometbyl)vinyldicblorosilane (6). Magnesium turnings (29.2 g, 1.2 mol) were placed into 500 mL of tetrahydrofuran (THF), and 1 mL each of 1,Zdibromoethane and vinyl bromide were introduced into the reaction vessel with gentle heating and constant stirring to initiate the reaction. Vinyl bromide (128.3 g, 1.2 mol) in 50 mL of THF was added dropwise and the temperature of the reaction mixture was maintained at 45 O C . The reaction vessel was then heated to reflux for an additional 3 h. The resulting solution was decanted into a dropping funnel which was gently warmed with heating tape to keep the Grignard in solution. This was added dropwise, using vigorous stirring, to 439 g (2.4 mol) of (chloromethy1)trichlorosilane in 1000 mL of ethyl
Colegrove and Laane ether. (Chloromethy1)vinyldichlorosilane(6) was produced along with small amounts of the bi- and trisubstituted silanes. The magnesium salts were separated from the reaction mixture by using vacuum filtration, and the ether and THF solvents and the unreacted starting material (C1CH2SiCl3) were removed by distillation. Periodic filtration of the mixture was required because the MgBrCl salt tended to fall out of solution as the solvents were removed. The distilled crude product (45.0 g, 0.25 mol), which was determined to be 90% pure based on its NMR spectrum, was taken off in the 121-130 OC temperature range. This represents a 55% yield for the monovinyl compound (6) based on the expected statistical distribution of products (58% of the (chloromethy1)trichlorosilane starting product, 35% of 6, and 7% of the divinyl species). The reaction was also carried out using a 1:l ratio (rather than 1:2) of Grignard to the chloromethyltrichlorosilane. In this case, 47.1 g (0.26 mol) of crude product 6 was produced from 1.2 mol (220 g) of the (chloromethy1)trichlorosilane (48% yield based on 90% purity and an expected statistical yield of 44% of the monsubstituted material 6 ) . The 1:l reaction produces more of the di- and trivinyl derivatives, which make the separation more difficult, but it does allow a smaller amount of the chloromethyltrichlorosilane to be used in the reaction. 1,1,1,4,4,5-Hexachloro-1,4-disilapentane(7). Into a reaction vessel, which was vented through a dry-iceacetone trap and oil bubbler, was placed 87.5 g (0.5mol) of 6 and 10 mg of hexachloroplatinic acid. The mixture was heated to a gentle reflux and 81 g (0.6 mol) of trichlorosilane was added dropwise over a 48-h period. The reaction was allowed to reflux for an additional 12 h. Excess trichlorosilane was removed by simple distillation. The product 7 (1 16 g, 0.37 mol, yield 74%) was obtained by vacuum distillation (bp 81-90 OC at 1 mmHg). 1,1,3,3-Tetrachloro-1,3-disilacyclopentane(8). Magnesium turnings (9 g, 0.4 mol, carefully heatdried in a vacuum and purged with nitrogen) were placed into a reaction vessel containing 200 mL of anhydrous ether. 1,2-Dibromoethane (1 mL) and 7 (1 mL) were added to initiate the Grignard reaction, and then 100 g (0.32 mol) of 7 in 50 mL of ether was added dropwise. A mustardcolored paste containing MgC12 resulted, and to this was added an additional 200 mL of ether. The mixture was then heated to reflux for 6 h. The magnesium salts were vacuum filtered from the mixture and 8 (67 g, 0.28 mol, yield 88%) was obtained by vacuum distillation (36-40 OC at 1 mmHg). 1,1,3,3,4-Pentachloro-1,3-disilacyclopentane ( 9 ) . 1,1,3,3Tetrachloro-l,3-disilacyclopentane(8) (40 g, 0.1 7 mol) and sulfuryl chloride (21 g, 0.17 mol, freshly distilled) were mixed in a reaction vessel. Utilizing magnetic stirring, the mixture was heated to reflux (71 "C) and 100 mg of benzoyl peroxide was added. The evolution of gas (HCl and SO2)was vigorous for a 30-min period. The mixture was allowed to stir for 4 h, and then an additional 100 mg of benzoyl peroxide was added. After four more hours of stirring the reaction mixture was fractionally distilled under vacuum to obtain 9 (27 g, 0.10 mol, yield 598, bp 41-46 OC a t 1 mmHg). In this separation it was difficult to remove the last traces of 8 and ultimately this leads to a small amount of the saturated 1,3-disilacyclopentane in samples of 3. 1,1,3,3-Tetrachloro-1,3-disilacyclopent-4-ene(10). The pentachloride 9 (33 g, 0.12 mol) and 20 mL of dried carbon tetrachloride were placed into a reaction vessel. With magnetic stirring, (DBU) was 18 g (0.12 mol) of 1,8-diazabicyclo[5.4.0]undec-7-ene added dropwise. The moderately exothermic reaction immediately produced a clear, waxy precipitate. During the addition the precipitate turned green. After 2 h of stirring, the precipitate was removed from the solute by utilizing vacuum filtration in an anaerobic environment. The resulting solution was fractionally distilled under vacuum yielding 10 (20 g, 0.084 mol, yield 7056, bp 35-40 OC at 1 mmHg). NMR spectra of the product showed it to contain about 10% of 8. 1,3-Disilacyclopent-4-ene(3). The unsaturated tetrachloride 10 (20 g, 0.084 mol) was added dropwise to lithium aluminum hydride (34 g, 0.9 mol) in 50 mL of dried n-butyl ether and allowed to stir for 24 h. The solution was vacuum transferred
6499
J. Phys. Chem. 1991, 95, 6499-6502 from the reaction flask and distilled to obtain 3.5 g (0.035 mol, yield 42%) of 3 (pb 87-92 "C). This sample contained about 10% of 1,3-disilacyclopentane. Also distilled off a t slightly lower temperatures were ring cleavage side products. Characterization and Spectroscopic Measurements. All compounds were characterized by using nuclear magnetic resonance spectra (Varian XL-2000 and EM-390), mass spectra (VG Analytical 70S), and infrared and Raman spectra. Mid-infrared gas-phase spectra of 3 at vapor pressures of 5 and 45 Torr contained in a IO-cm gas cell with KBr windows were recorded on a Digilab FTS-60 interferometer. Liquid-phase Raman spectra were recorded with a Cary 82 monochromator equipped with a Coherent Radiation Innova 20 argon ion laser source. Detailed
Measurements of Line Strengths in the InGaAsP Laser
spectra are available e1~ewhere.I~Mid-infrared spectra of the combination bands in the SiHz stretching region were recorded on a Bomem DA3.002 interferometer using the IO-cm cell. The Bomem instrument was also used to acquire the far-infrared spectrum of 3. An adjustable path length (up to 21 m) Wilks multireflectance cell was used to contain the sample for these scans. A liquid helium cooled germanium bolometer was utilized as the detector, and 1000 scans were typically coadded. In the 40125-cm-' range, a 25 pm thick beamsplitter and a mercury lamp source were utilized.
Acknowledgment. We thank the National Science Foundation and the Robert A. Welch Foundation for financial support.
Hopv1 Overtone Band at
1.5 pm Using an
T. J. Johnson, F. G. Wienhold, J. P. Burrows, G. W. Harris,* Atmospheric Chemistry Department, Max Planck institute for Chemistry, P.O. Box 3060, W-6500 Mainz, Germany
and H. Burkhard DBP Telekom Research Institute, P.O. Box 10 00 03, W-6100 Darmstadt, Germany (Received: April 25, 1991; In Final Form: June 14, 1991)
We report the first observation of resolved rotational-vibrational overtone (2q) absorptions of the hydroperoxyl radical (HO,) in the 1.5-pm region, using two-tone frequency modulation spectroscopy (TTFMS) with an InGaAsP laser diode and White-cell optics. Photolysis of Cl2/H2/02 mixtures was used to produce the H02, and the concentration was determined by modulated-photolysis UV absorption spectroscopy. The line center absorption cross sections for the strongest lines ranged between and 10 X cm2 molecule-' under Doppler-limited conditions. For the strongest line this corresponds to an 1.3 X cm2molecule-' cm-I, a line strength of the same order of magnitude as lines previously integrated line strength S of 1.6 X observed in the relatively weak uI fundamental.
Introduction The hydroperoxyl radical, HOz, is a pivotal atmospheric species, being closely coupled to OH,the most important oxidant in both tropospheric and stratospheric chemistry.' Currently several techniques are under development to quantitatively measure atmospheric concentrations of H 0 2 and the associated ROz family of radicals, either spectroscopically or by chemical conversion methods.2 Recent improvements3 in the chemical conversion method have made it much more useful, but the technique suffers from the drawback that it cannot distinguish between H 0 2 and ROZ. The detection limits appear to be -2 pptv, slightly better than those obtained with the matrix isolation EPR techniques, one of the few spectroscopic techniques to have successfully measured tropospheric HO,.' The EPR technique is sensitive, but requires relatively long collection times (ca. 1 h) and off-line analysis in the laboratory. Optical measurements to monitor atmospheric HO, using its UV bands are prevented by interference from adjacent O3bands and by the relative weakness of the unstructured H 0 2 absorption. Atmospheric measurements using the mid-IR absorptions via tunable diode laser absorption spectroscopy (TDLAS) have not as yet been realized due to the small concentrations and line as well as sampling problems. However, HO, is known to have a few absorption bands in the near-IR region, a vibronic progression in the low-lying 2A' 2A" transition beginning a t 1.425 pm, as well as the first vibrational overtone in the OH stretch centered at 1.504 pm. Since Hunziker and Wendt' first identified
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* To whom correspondence should be addressed. 0022-3654/9 1/2095-6499$02.50/0
and assigned these bands under low resolution in 1974, there has been much interest2 as to the magnitude of the individual line strengths: Measuring H 0 2 in the near-IR region is potentially more attractive than measuring in the mid-IR region because the generally weaker line strengths can in part be compensated for by measuring a t higher pressures before pressure broadening becomes important. Using broader lines is also more practicable in the near-IR region because there are fewer interfering absorptions as compared to the mid-IR region. Most importantly, because two-tone frequency modulation spectroscopy (TTFMS) in the near-IR region offers the possibility of measuring at higher pressures without significant loss of signal, the use of open-path multiple reflection cells may be feasible. This would avoid the severe problems of sampling H02 at reduced pressures, which is necessary for optimum performance of a mid-IR TDL spectrometer aimed at tropospheric measurements. The 1.5-pm lasers are (1) (a) Warneck, P. Chemistry ofthe Natural Atmosphere, Academic Press: San Diego, CF, 1988. (b) Finlayson-Pitts, B. J.; Pitts, J. N. Atmospheric Chemistry, Wiley-Interscience: New York, 1986. (2) Cantrell, C. A.; Shetter, R. E.; McDaniel, A. H.; Calvert, J. G. Measurement Methods for Peroxy Radicals in the Atmosphere. In Measurement Challenges in Atmospheric Chemistry; Adv. Chem. Ser. No. 232; American Chemical Society: Washington, DC, in press. This article provides an excellent review of techniques aimed at measuring H 0 2 and R02. (3) Hastie, D. R.; Weipenmayer, M.; Burrows, J. P.; Harris, G. W. Anal. Chem.. acceDted for oublication. (4) 'Miheicic, D.; qolz-Thomas, A,; PBtz, H. W.; Kley, D.; Mihelcic, M. J . Atm. Chem. 1990. 11. 271. (5) Zahniser, M. S.; McCurdy, K. E.; Stanton, A. C. J . Phys. Chem. 1989, 93, 1066. ( 6 ) Zahniser, M. S.;Stanton, A. C. J . Chem. Phys. 1984, 80, 4951. (7) Hunziker, H. E.; Wendt, H. R. J . Chem. Phys. 1974, 60, 4622.
0 1991 American Chemical Society
