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Ring-puckering combination band spectra of an isotopic impurity: 1,3-Disilacyclobutane-1,1,3-d3
JOURNAL
OF MOLECULAR
SPECTROSCOPY
84,
139-145 (1980)
Ring-Puckering Combination Band Spectra of an Isotopic impurity: 1,3-Disilacyclobutane-1 ,l ,3-d3 P. W. JAGODZINSKI, R. M. IRWIN, J. M. COOKE, AND J. LAANE Department
of Chemistry.
Texas A&M University, College Station, Texas 77843
An isotope doping procedure has been used to produce a mixture containing 1,3-disilacyclobutane- 1,1,3,3-d, (predominantly) and 2% of the - 1,l ,3-d3 species. Investigation of the S-H stretching band at 2153 cm-’ of the d3 molecule shows fourteen sum and difference bands arising from the low-frequency ring-puckering motion. The analysis of this spectrum together with those for the d, and the -1,1,3,3-d4 molecules demonstrate that a uniform amount of SiH, rocking is coupled to the ring-puckering motion in all isotopic species and that the kinetic energy model previously proposed is valid. The technique for analyzing isotopic impurities demonstrated in this work should be valuable for future investigations.
INTRODUCTION
The 1,3-disilacyclobutane molecule, Hz-Hz, has proven itself to be spectroscopically,a most interesting entity. Its rich far-infrared and low-frequency Raman spectra arise from a ring-puckering vibration which is governed by a double-minimum potential energy function with an 87-cm-’ barrier height (1). In addition, its mid-infrared and mid-Raman spectra show numerous combination bands resulting from the puckering motion (2). For example, in the SiH, stretching region alone, seventy (51 IR and 19 Raman) different sum and difference bands were observed originating from three different band origins (3). Similar1 sum SEC and difference bands from the SiD, stretch were observed for Dz tCH&D, Hz. In the course of analyzing the complete spectra (4) of both the normal and deuterated molecules we discovered that the latter had a small isotopic impurity which gave rise to a spectrum near 2150 cm-’ in the Si-H stretching region. This arose from the estimated 1.0% hydrogen impurity in commercially available LiAlD, which was used as the reducing agent for the preparation of the d, molecule. Thus, the desired 1,3-disilacyclobutane- I,1 ,3,3-d4 product contained about 4% of the d, species, as can be seen from the following reaction, Cl&Si$SiCl, + LiAlD, -+ D,Si$iD, (99%D; l%H)
+ D,SiDiHD.
96%
4%
Since the d, molecule should show no fundamental bands in the region 20502250 cm-‘, the spectrum observed there was due to a species containing Si-H bonds. It will be demonstrated that the species was, in fact, the d, molecule. Such Si-H impurity bands are common in virtually all Si-D-containing molecules, and 139
0022-2852/80/l 10139-07$02.00/O Copyright 0 1980 by Academic All k&Is
of reproduction
Press, Inc.
in any form reserved.
140
JAGODZINSKI
2250
2200
2150 FREQUENCY
ET AL.
2100
50
(CM-‘)
FIG. 1. Ring-puckering combination band spectra of 1,3-disilacyclobutane-1, I ,3-d3 (29%) in an isotopic mixture. Total pressures of 5 and 125 Torr in a IO-cm cell were used.
the observation would not have been particularly interesting except that sum and difference bands arising from the ring-puckering vibration in combination with the Si-H stretching in the impurity molecules was observed. These bands were quite weak in the sample containing only 4% of the d, molecule. Thus, it was decided to enrich the amount of d, in the isotopic mixture by intentionally adding more LiAlH4 to LiAlD, in the reaction mixture. The ring-puckering combination band spectra of such an enriched sample will be discussed. EXPERIMENTAL
DETAILS
1,3-Disilacyclobutane-1, 1,3,3-d4 was synthesized as previously described (2) except that the reducing agent used contained 10% LiAlH, in addition to the LiA1D4. Based on a statistical analysis, this yielded a mixture containing 66% d4, 29% d3, and 5% of various d, species. This material had a b.p. of 61”. The infrared spectrum of the vapor of this isotopic mixture in a lo-cm cell was recorded using a Digilab FTS-20 high-resolution vacuum spectrophotometer. RESULTS AND DISCUSSION
Figure 1 shows the infrared spectrum of the Si-H stretching band and the associated ring-puckering combination bands of 1,3-disilacyclobutane-l , 1,3-d, in the isotopic mixture at total pressures of 5 and 125 Torr (1.5 and 36 Torr partial pressures of d3). The fundamental frequency observed at 2153 cm-’ (and derived to be 2153.9 cm-l from the combination bands) is almost exactly the average of the four Si-H stretching frequencies (2157.0, 2156.5, 2154.3, and 2147 cm-‘) of the undeuterated molecule (3), as expected for an uncoupled motion. The frequencies of the sum and difference bands are presented in Table I and the transition diagram is shown in Fig. 2. The frequency differences of the combination bands relative to the derived band origin at 2153.9 cm-’ are also listed in Table I. These all agree within about one-half cm-’ with the values predicted from a linear extrapolation between corresponding d, and d, values and, thus, the spectra can be assigned with
RING-PUCKERING
COMBINATION
BAND SPECTRA
141
(1,101 t (l,9))
(I,81
+
I I
I I
(l,5) (1,4) (1,3) (I.21 (l,l) (l,OY
(0.10) (0.9) (0,8) (0.71 (0,6) (0.5)
I I
I
I I
I I I I
igkG+d& co,&
SUM
4
DIFFERENCE
FIG. 2. Combination band transitions observed for 1,3-disilacyclobutane1,1,3-d,.
confidence to the d3 species. For example, the frequencies for the 3 + 4 transition are 49.5 and 41.9 cm-’ for the d, and d, molecules respectively; extrapolation gives (3 X (41.9) + 49.5)/4 = 43.8 cm-’ for the d, species, and this may be compared to the observed value of 43.3 cm-l. TABLE I Infrared Combination Frequencies Transition
“SUM -
(cm-‘) of 1,3-Disilacyclobutane-1,1,3-d,
ysuII1-yo
vdiff
"o-"diff
1-2
2206.1a
52.8
2101.3a
52.6
3-4
2197.2
43.3
2110.7
43.2
4-5
2200.6
46.9
2106.9
47.0
5-6
2206.7a
52.8
2101.3a
52.6
6-7
2211.1
57.2
2096.7
57.2 61.3
7-a
2215.4
61.5
2092.6
a-9
2219.0
65.1
2088.8
65.1
9-10
-
-
2085.8
68.1
10-11
-
-
2082.5
71.4
a)
Frequency
%=2153.9
used twice
cm-'
142
JAGODZINSKI
ET AL.
TABLE II Potential and Kinetic Energy Functions for Isotopic Species of 1,3-Disilacyclobutane Potential Energy Functions Reduced'? V =A(z4-Bz')
Kinetic Energy Functions
Dimensioned: V =ax4-bx2
g44 = Caixi
Reduced
x 10-3
Molecule
-LB
C2H4Si2H4
15.20
4.79
2.38.105
9.11 x103
138.8
7.2055
C2H4Si2HD3
13.33
5.11
2.38x105
9.11 .1D3
169.0
5.9187 -0.0141 -4.5811
C2H4Si2D4
12.80
5.22
2.38~10~
9.11 x103
179.4
5.5740
a)
a(cm-'A-') b(cm-'A-')
Mass (a.u.) ~-__
10
a1 -
-
a3
aZ_-7.7293
-3.8879
0.1838 -
a4 -7.3983 -8.8289 -8.8062
see Ref. 5.
Utilization of the energy diagram in Fig. 2 permits a precise determination of the energy separations for many of the individual levels. For example, 2200.8 - 2101.3 = 99.5 cm-’ gives the energy spacing between the a = 4 and v = 6 puckering levels in the Si-H stretching ground state. This may be compared to values of 99.7 and 99.6 cm-’ obtained from the sum and difference band separations, respectively, in Table I. Comparison of other energy spacings also shows that the frequency differences in Table I may be considered reliable for both ground and excited states of the Si-H stretching mode. Thus, as expected and as was found for 1,3-disilacyclobutane-d, (3), the puckering levels are insignificantly perturbed by interactions with Si-H stretching motions. In our previous analysis (I) of the ring-puckering potential energy function for the d, and d., molecules it was necessary to assume a vibrational model in which the SiH, rocking coupled with the puckering motion. Only in this way could the observed isotopic shifts in the transitions be satisfactorily calculated using the same potential energy function (V = ax4 + bx2) for both isotopic species. A rocking parameter R was defined and this was determined to be 0.44 for both molecules. However, as was pointed out, it is not necessary for both isotopic forms to have the same amount of rocking-puckering interaction, and R may actually be different for the d,, and d4 systems. The data obtained for the d3 molecule in this work are therefore invaluable for establishing whether the same rocking parameter can adequately represent the molecular motion for all isotopic species. That is, we wish to determine whether a uniform degree of vibrational coupling between the ring-puckering and SiH, rocking modes is present in the do, d3, and d4 molecules. Tables II and III demonstrate that this is indeed the case. Using exactly the same vibrational model (with R = 0.44) to calculate the kinetic energy expansions and using the same potential energy function, all of the calculated frequency values were in excellent agreement with the observed values. Table II lists the kinetic energy (inverse reduced mass) expressions and potential energy functions used for the calculations. Table III presents the observed and calculated values for all three isotopic forms of 1,3-disilacyclobutane. The computational procedures used have previously been described (1,5). Because of the excellent agreement between the theoretical calculation and the observed data for all the molecules, we may
RING-PUCKERING
COMBINATION
143
BAND SPECTRA
TABLE III Ring-Puckering Transition Frequencies
for Isotopic Species of 1,3-Disilacyclobutane
IQ-H2
---_m~l D2SiCHpSlD2CH2
HDS~Cti2SiD2tH2
-
D-l
-
'1-2
56.0
56.1
2-3
31.6
31.4
0.2
-
24.2
-
3-4
49.5
50.1
-0.6
43.3
43.5
-0.2
3.0
-
52.8
-0.1
1.8
-
53.2
-0.4
-
1.5
-
52.0
52.5
-0.5
-
22.2
-
41.8
41.7
0.1
4-5
54.5
55.1
-0.6
46.9
47.1
-0.2
45.1
44.8
0.3
5-6
61.0
61.3
-0.3
52.8
52.9
-0.1
50.7
50.6
0.1
6-7
66.0
66.3
-0.3
57.2
57.4
-0.2
55.0
54.9
0.1
7-a
JO.6
70.8
-0.2
61.5
61.4
0.1
59.1
58.8
0.3
a-9
74.7
74.8
-0.1
65.1
65.1
0.0
62.5
62.3
0.2
9-10
78.5
78.4
0.1
68.1
68.4
-0.3
65.8
65.5
0.3
10-11
82.0
al.8
0.2
71.4
71.4
0.0
68.7
68.4
0.3
11-12
85.3
85.0
0.3
-
74.1
-
71.4
71.2
0.2
12-13
88.2
87.9
0.3
75.2
73.8
1.4
13-14
91.2
90.7
0.5
-77
76.2
-.a
14-15
93.9
93.4
0.5
79.0
78.4
0.6
15-16
-
95.9
-
81.2
80.6
0.6
69.6
90.5
-0.9
76.0
76.2
-0.2
o-3
a) Ref 1. b) Frm
V=2.3a.105x4-
9.11.103x2
cm-'.
reiterate our previous results with greater conviction and smaller uncertainty. Namely, the vibrational model with R = 0.44 (which means a 3.5” angle of rocking when the dihedral angle of puckering is at its equilibrium value of 23.5 ‘_ 0.5”) is highly satisfactory. The potential function is V(cm-*) = (2.38 t 0.1) x 105x4 - (9.11 2 0.1) x 103x” and this has a barrier to inversion of 87.2 -+ 1.O cm-‘. Figure 3 shows this potential energy function and energy levels for the d3 molecule. CONCLUSIONS
Three significant observations have resulted from this investigation. First, it was shown that considerable structural detail can be obtained for an isotopic species present in low concentrations. Even in the sample containing only 4% of the d, molecule, the scale expansion on the Fourier transform infrared instrument permitted the combination band structure to be examined. Since the deuterated product from many reactions with LiAlD, or other deuterating reagents will often
144
JAGODZINSKI
ET AL.
H'Si/\Si’D 600
-
300
-
200
-
V (cm-‘)
I -0
I 3
I -0
2
I
I
-0
I, 0.0
I
I x
I, 0.1
I 02
I
I 0.3
(A,
FIG. 3. Ring-puckering potential energy function for 1.3-disilacyclobutane-1,1,3-d,.
contain some H impurity, small concentrations of mixed isotopic species will generally result. Thus, isotopic impurities possessing -MHD or -MHD* groups (M = Si, Ge, Sn, etc.) may often be studied, especially in the generally uncluttered M-H stretching regions. The reaction mixtures may also intentionally be enriched with additional H1 impurity to increase the production of the desired isotopic molecule. Similarly, small amounts of LiA1D4 may be added to LiAlH, to produce some d1 species. However, such studies may prove more difficult because the M-D stretching regions are generally not so free from interference as M-H regions. In certain cases molecular symmetry will be affected by mixed H-D substitution and new spectroscopic features may appear in the spectra. This was demonstrated in our laboratory for cyclopentene-l-d,, and -1,2,3,3-d4, which show previously inactive ring-twisting motions in their infrared spectra (6). The second important contribution from this study is that the coupled puckeringrocking model for 1,3-disilacyclobutane with uniform interaction between the two motions for all isotopic molecules was shown to be valid. 1,3-Disilacyclobutane is thus a rare example of a molecule for which both the dynamical motion of the ringpuckering and the potential energy function have been well characterized. Although
RING-PUCKERING
COMBINATION
145
BAND SPECTRA
several dozen potential energy functions have been generated for ring molecules over the past 15 years (7,8), many of the dimensioned potential energy parameters (which depend on the vibrational model through the kinetic energy terms) have not been well determined due to insufficient analyses using isotopic substitution. Consequently, a wealth of information on the dihedral angles and the potential constants (a and b in V = ax4 + 6x2) must be considered as dubious. The third observation from this study concerns reaction mechanisms, but it is of interest to spectroscopists utilizing deuterated species. In our reaction with a mixture of LiAlD, and LiAlH, we obtained a statistical distribution of products (at least no d, species were detected). This implies that the reducing species scrambled in solution or that reduction took place randomly, one bond at a time. If each unscrambled LiAlH, had reacted with both Si-Cl bonds in an SiCl, group all at one time, almost 20% of the molecules should have contained one SiHp group and very little d3 species should have been produced. ACKNOWLEDGMENT This work was supported
RECEIVED:
December
by the National
Science
Foundation.
10, 1979 REFERENCES
I. R. M. IRWIN. J. M. COOKE, AND J. LAANE, 1. Amer. Chem. Sot. 99, 3273-3278 (1977). 2. R. M. IRWIN AND J. LAANE, to be published; R. M. IRWIN, Ph.D. thesis, Texas A&M University, 1978. 3. R. M. IRWIN AND J. LAANE, J. Mol. Spectrosc. 70, 307-313 (1978). 4. R. M. IRWIN AND J. LAANE, J. Phys. Chem. 82, 28452850 (1978). 5. J. LAANE, Appl. Specrrosc. 24, 73-80 (1970). 6. J. R. VILLARREAL, L. E. BAUMAN, AND J. LAANE. .I. Phys. Chem. 80, 1172-1177 (1976). 7. T. B. MALLOY, JR., L. E. BAUMAN, AND L. A. CARREIRA, in “Topics in Stereochemistry,” Vol. 14 (E. L. Eliel and N. L. Allinger, Eds.). Wiley-Interscience, New York, 1979. 8. L. A. CARREIRA, R. C. LORD, AND T. B. MALLOY, JR., in “Topics in Current (M. J. S. Dewar et al., Eds.), Springer-Verlag, Berlin/New York, 1979.
Chemistry,
Vol. XX
OF MOLECULAR
SPECTROSCOPY
84,
139-145 (1980)
Ring-Puckering Combination Band Spectra of an Isotopic impurity: 1,3-Disilacyclobutane-1 ,l ,3-d3 P. W. JAGODZINSKI, R. M. IRWIN, J. M. COOKE, AND J. LAANE Department
of Chemistry.
Texas A&M University, College Station, Texas 77843
An isotope doping procedure has been used to produce a mixture containing 1,3-disilacyclobutane- 1,1,3,3-d, (predominantly) and 2% of the - 1,l ,3-d3 species. Investigation of the S-H stretching band at 2153 cm-’ of the d3 molecule shows fourteen sum and difference bands arising from the low-frequency ring-puckering motion. The analysis of this spectrum together with those for the d, and the -1,1,3,3-d4 molecules demonstrate that a uniform amount of SiH, rocking is coupled to the ring-puckering motion in all isotopic species and that the kinetic energy model previously proposed is valid. The technique for analyzing isotopic impurities demonstrated in this work should be valuable for future investigations.
INTRODUCTION
The 1,3-disilacyclobutane molecule, Hz-Hz, has proven itself to be spectroscopically,a most interesting entity. Its rich far-infrared and low-frequency Raman spectra arise from a ring-puckering vibration which is governed by a double-minimum potential energy function with an 87-cm-’ barrier height (1). In addition, its mid-infrared and mid-Raman spectra show numerous combination bands resulting from the puckering motion (2). For example, in the SiH, stretching region alone, seventy (51 IR and 19 Raman) different sum and difference bands were observed originating from three different band origins (3). Similar1 sum SEC and difference bands from the SiD, stretch were observed for Dz tCH&D, Hz. In the course of analyzing the complete spectra (4) of both the normal and deuterated molecules we discovered that the latter had a small isotopic impurity which gave rise to a spectrum near 2150 cm-’ in the Si-H stretching region. This arose from the estimated 1.0% hydrogen impurity in commercially available LiAlD, which was used as the reducing agent for the preparation of the d, molecule. Thus, the desired 1,3-disilacyclobutane- I,1 ,3,3-d4 product contained about 4% of the d, species, as can be seen from the following reaction, Cl&Si$SiCl, + LiAlD, -+ D,Si$iD, (99%D; l%H)
+ D,SiDiHD.
96%
4%
Since the d, molecule should show no fundamental bands in the region 20502250 cm-‘, the spectrum observed there was due to a species containing Si-H bonds. It will be demonstrated that the species was, in fact, the d, molecule. Such Si-H impurity bands are common in virtually all Si-D-containing molecules, and 139
0022-2852/80/l 10139-07$02.00/O Copyright 0 1980 by Academic All k&Is
of reproduction
Press, Inc.
in any form reserved.
140
JAGODZINSKI
2250
2200
2150 FREQUENCY
ET AL.
2100
50
(CM-‘)
FIG. 1. Ring-puckering combination band spectra of 1,3-disilacyclobutane-1, I ,3-d3 (29%) in an isotopic mixture. Total pressures of 5 and 125 Torr in a IO-cm cell were used.
the observation would not have been particularly interesting except that sum and difference bands arising from the ring-puckering vibration in combination with the Si-H stretching in the impurity molecules was observed. These bands were quite weak in the sample containing only 4% of the d, molecule. Thus, it was decided to enrich the amount of d, in the isotopic mixture by intentionally adding more LiAlH4 to LiAlD, in the reaction mixture. The ring-puckering combination band spectra of such an enriched sample will be discussed. EXPERIMENTAL
DETAILS
1,3-Disilacyclobutane-1, 1,3,3-d4 was synthesized as previously described (2) except that the reducing agent used contained 10% LiAlH, in addition to the LiA1D4. Based on a statistical analysis, this yielded a mixture containing 66% d4, 29% d3, and 5% of various d, species. This material had a b.p. of 61”. The infrared spectrum of the vapor of this isotopic mixture in a lo-cm cell was recorded using a Digilab FTS-20 high-resolution vacuum spectrophotometer. RESULTS AND DISCUSSION
Figure 1 shows the infrared spectrum of the Si-H stretching band and the associated ring-puckering combination bands of 1,3-disilacyclobutane-l , 1,3-d, in the isotopic mixture at total pressures of 5 and 125 Torr (1.5 and 36 Torr partial pressures of d3). The fundamental frequency observed at 2153 cm-’ (and derived to be 2153.9 cm-l from the combination bands) is almost exactly the average of the four Si-H stretching frequencies (2157.0, 2156.5, 2154.3, and 2147 cm-‘) of the undeuterated molecule (3), as expected for an uncoupled motion. The frequencies of the sum and difference bands are presented in Table I and the transition diagram is shown in Fig. 2. The frequency differences of the combination bands relative to the derived band origin at 2153.9 cm-’ are also listed in Table I. These all agree within about one-half cm-’ with the values predicted from a linear extrapolation between corresponding d, and d, values and, thus, the spectra can be assigned with
RING-PUCKERING
COMBINATION
BAND SPECTRA
141
(1,101 t (l,9))
(I,81
+
I I
I I
(l,5) (1,4) (1,3) (I.21 (l,l) (l,OY
(0.10) (0.9) (0,8) (0.71 (0,6) (0.5)
I I
I
I I
I I I I
igkG+d& co,&
SUM
4
DIFFERENCE
FIG. 2. Combination band transitions observed for 1,3-disilacyclobutane1,1,3-d,.
confidence to the d3 species. For example, the frequencies for the 3 + 4 transition are 49.5 and 41.9 cm-’ for the d, and d, molecules respectively; extrapolation gives (3 X (41.9) + 49.5)/4 = 43.8 cm-’ for the d, species, and this may be compared to the observed value of 43.3 cm-l. TABLE I Infrared Combination Frequencies Transition
“SUM -
(cm-‘) of 1,3-Disilacyclobutane-1,1,3-d,
ysuII1-yo
vdiff
"o-"diff
1-2
2206.1a
52.8
2101.3a
52.6
3-4
2197.2
43.3
2110.7
43.2
4-5
2200.6
46.9
2106.9
47.0
5-6
2206.7a
52.8
2101.3a
52.6
6-7
2211.1
57.2
2096.7
57.2 61.3
7-a
2215.4
61.5
2092.6
a-9
2219.0
65.1
2088.8
65.1
9-10
-
-
2085.8
68.1
10-11
-
-
2082.5
71.4
a)
Frequency
%=2153.9
used twice
cm-'
142
JAGODZINSKI
ET AL.
TABLE II Potential and Kinetic Energy Functions for Isotopic Species of 1,3-Disilacyclobutane Potential Energy Functions Reduced'? V =A(z4-Bz')
Kinetic Energy Functions
Dimensioned: V =ax4-bx2
g44 = Caixi
Reduced
x 10-3
Molecule
-LB
C2H4Si2H4
15.20
4.79
2.38.105
9.11 x103
138.8
7.2055
C2H4Si2HD3
13.33
5.11
2.38x105
9.11 .1D3
169.0
5.9187 -0.0141 -4.5811
C2H4Si2D4
12.80
5.22
2.38~10~
9.11 x103
179.4
5.5740
a)
a(cm-'A-') b(cm-'A-')
Mass (a.u.) ~-__
10
a1 -
-
a3
aZ_-7.7293
-3.8879
0.1838 -
a4 -7.3983 -8.8289 -8.8062
see Ref. 5.
Utilization of the energy diagram in Fig. 2 permits a precise determination of the energy separations for many of the individual levels. For example, 2200.8 - 2101.3 = 99.5 cm-’ gives the energy spacing between the a = 4 and v = 6 puckering levels in the Si-H stretching ground state. This may be compared to values of 99.7 and 99.6 cm-’ obtained from the sum and difference band separations, respectively, in Table I. Comparison of other energy spacings also shows that the frequency differences in Table I may be considered reliable for both ground and excited states of the Si-H stretching mode. Thus, as expected and as was found for 1,3-disilacyclobutane-d, (3), the puckering levels are insignificantly perturbed by interactions with Si-H stretching motions. In our previous analysis (I) of the ring-puckering potential energy function for the d, and d., molecules it was necessary to assume a vibrational model in which the SiH, rocking coupled with the puckering motion. Only in this way could the observed isotopic shifts in the transitions be satisfactorily calculated using the same potential energy function (V = ax4 + bx2) for both isotopic species. A rocking parameter R was defined and this was determined to be 0.44 for both molecules. However, as was pointed out, it is not necessary for both isotopic forms to have the same amount of rocking-puckering interaction, and R may actually be different for the d,, and d4 systems. The data obtained for the d3 molecule in this work are therefore invaluable for establishing whether the same rocking parameter can adequately represent the molecular motion for all isotopic species. That is, we wish to determine whether a uniform degree of vibrational coupling between the ring-puckering and SiH, rocking modes is present in the do, d3, and d4 molecules. Tables II and III demonstrate that this is indeed the case. Using exactly the same vibrational model (with R = 0.44) to calculate the kinetic energy expansions and using the same potential energy function, all of the calculated frequency values were in excellent agreement with the observed values. Table II lists the kinetic energy (inverse reduced mass) expressions and potential energy functions used for the calculations. Table III presents the observed and calculated values for all three isotopic forms of 1,3-disilacyclobutane. The computational procedures used have previously been described (1,5). Because of the excellent agreement between the theoretical calculation and the observed data for all the molecules, we may
RING-PUCKERING
COMBINATION
143
BAND SPECTRA
TABLE III Ring-Puckering Transition Frequencies
for Isotopic Species of 1,3-Disilacyclobutane
IQ-H2
---_m~l D2SiCHpSlD2CH2
HDS~Cti2SiD2tH2
-
D-l
-
'1-2
56.0
56.1
2-3
31.6
31.4
0.2
-
24.2
-
3-4
49.5
50.1
-0.6
43.3
43.5
-0.2
3.0
-
52.8
-0.1
1.8
-
53.2
-0.4
-
1.5
-
52.0
52.5
-0.5
-
22.2
-
41.8
41.7
0.1
4-5
54.5
55.1
-0.6
46.9
47.1
-0.2
45.1
44.8
0.3
5-6
61.0
61.3
-0.3
52.8
52.9
-0.1
50.7
50.6
0.1
6-7
66.0
66.3
-0.3
57.2
57.4
-0.2
55.0
54.9
0.1
7-a
JO.6
70.8
-0.2
61.5
61.4
0.1
59.1
58.8
0.3
a-9
74.7
74.8
-0.1
65.1
65.1
0.0
62.5
62.3
0.2
9-10
78.5
78.4
0.1
68.1
68.4
-0.3
65.8
65.5
0.3
10-11
82.0
al.8
0.2
71.4
71.4
0.0
68.7
68.4
0.3
11-12
85.3
85.0
0.3
-
74.1
-
71.4
71.2
0.2
12-13
88.2
87.9
0.3
75.2
73.8
1.4
13-14
91.2
90.7
0.5
-77
76.2
-.a
14-15
93.9
93.4
0.5
79.0
78.4
0.6
15-16
-
95.9
-
81.2
80.6
0.6
69.6
90.5
-0.9
76.0
76.2
-0.2
o-3
a) Ref 1. b) Frm
V=2.3a.105x4-
9.11.103x2
cm-'.
reiterate our previous results with greater conviction and smaller uncertainty. Namely, the vibrational model with R = 0.44 (which means a 3.5” angle of rocking when the dihedral angle of puckering is at its equilibrium value of 23.5 ‘_ 0.5”) is highly satisfactory. The potential function is V(cm-*) = (2.38 t 0.1) x 105x4 - (9.11 2 0.1) x 103x” and this has a barrier to inversion of 87.2 -+ 1.O cm-‘. Figure 3 shows this potential energy function and energy levels for the d3 molecule. CONCLUSIONS
Three significant observations have resulted from this investigation. First, it was shown that considerable structural detail can be obtained for an isotopic species present in low concentrations. Even in the sample containing only 4% of the d, molecule, the scale expansion on the Fourier transform infrared instrument permitted the combination band structure to be examined. Since the deuterated product from many reactions with LiAlD, or other deuterating reagents will often
144
JAGODZINSKI
ET AL.
H'Si/\Si’D 600
-
300
-
200
-
V (cm-‘)
I -0
I 3
I -0
2
I
I
-0
I, 0.0
I
I x
I, 0.1
I 02
I
I 0.3
(A,
FIG. 3. Ring-puckering potential energy function for 1.3-disilacyclobutane-1,1,3-d,.
contain some H impurity, small concentrations of mixed isotopic species will generally result. Thus, isotopic impurities possessing -MHD or -MHD* groups (M = Si, Ge, Sn, etc.) may often be studied, especially in the generally uncluttered M-H stretching regions. The reaction mixtures may also intentionally be enriched with additional H1 impurity to increase the production of the desired isotopic molecule. Similarly, small amounts of LiA1D4 may be added to LiAlH, to produce some d1 species. However, such studies may prove more difficult because the M-D stretching regions are generally not so free from interference as M-H regions. In certain cases molecular symmetry will be affected by mixed H-D substitution and new spectroscopic features may appear in the spectra. This was demonstrated in our laboratory for cyclopentene-l-d,, and -1,2,3,3-d4, which show previously inactive ring-twisting motions in their infrared spectra (6). The second important contribution from this study is that the coupled puckeringrocking model for 1,3-disilacyclobutane with uniform interaction between the two motions for all isotopic molecules was shown to be valid. 1,3-Disilacyclobutane is thus a rare example of a molecule for which both the dynamical motion of the ringpuckering and the potential energy function have been well characterized. Although
RING-PUCKERING
COMBINATION
145
BAND SPECTRA
several dozen potential energy functions have been generated for ring molecules over the past 15 years (7,8), many of the dimensioned potential energy parameters (which depend on the vibrational model through the kinetic energy terms) have not been well determined due to insufficient analyses using isotopic substitution. Consequently, a wealth of information on the dihedral angles and the potential constants (a and b in V = ax4 + 6x2) must be considered as dubious. The third observation from this study concerns reaction mechanisms, but it is of interest to spectroscopists utilizing deuterated species. In our reaction with a mixture of LiAlD, and LiAlH, we obtained a statistical distribution of products (at least no d, species were detected). This implies that the reducing species scrambled in solution or that reduction took place randomly, one bond at a time. If each unscrambled LiAlH, had reacted with both Si-Cl bonds in an SiCl, group all at one time, almost 20% of the molecules should have contained one SiHp group and very little d3 species should have been produced. ACKNOWLEDGMENT This work was supported
RECEIVED:
December
by the National
Science
Foundation.
10, 1979 REFERENCES
I. R. M. IRWIN. J. M. COOKE, AND J. LAANE, 1. Amer. Chem. Sot. 99, 3273-3278 (1977). 2. R. M. IRWIN AND J. LAANE, to be published; R. M. IRWIN, Ph.D. thesis, Texas A&M University, 1978. 3. R. M. IRWIN AND J. LAANE, J. Mol. Spectrosc. 70, 307-313 (1978). 4. R. M. IRWIN AND J. LAANE, J. Phys. Chem. 82, 28452850 (1978). 5. J. LAANE, Appl. Specrrosc. 24, 73-80 (1970). 6. J. R. VILLARREAL, L. E. BAUMAN, AND J. LAANE. .I. Phys. Chem. 80, 1172-1177 (1976). 7. T. B. MALLOY, JR., L. E. BAUMAN, AND L. A. CARREIRA, in “Topics in Stereochemistry,” Vol. 14 (E. L. Eliel and N. L. Allinger, Eds.). Wiley-Interscience, New York, 1979. 8. L. A. CARREIRA, R. C. LORD, AND T. B. MALLOY, JR., in “Topics in Current (M. J. S. Dewar et al., Eds.), Springer-Verlag, Berlin/New York, 1979.
Chemistry,
Vol. XX