1199
Spectra and Structure of Small-Ring Molecules
Spectra and Structure of Small-Ring Molecules. 33.’ Microwave Spectrum of Silacyclopentane J. R. Durig,**l3 Swiss Federal Institute of Technology, Laboratory for Physical Chemistry, Zurich, Switzerland
W. J. Lafferty,* National Bureau of Standards, Washington, D.C.
and V. F. Kalaslnsky Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received December 18, 1975) Publication costs assisted by the University of South Carolina
The microwave spectrum of silacyclopentane, 1-silacyclopentane-l,1-dz, and silacyclopentane-29Sihas been investigated in the spectral range of 8-40 GHz. The rotational lines of five vibrational excited states of the ring-puckering mode have also been assigned and are consistent with a high barrier to pseudorotation. Both the dipole moment measurements and the isotopic data indicate that the skeletal ring of this molecule is in the “twisted” CZconformation for the ground state. The a component of the dipole moment is 0.726 f 0.005 D and the c component has been determined to be less than 0.01 D with a l l 4 = 0.726 f 0.006 D. The isotopic data are sufficient to determine the following parameters: LHSiH = 108.76 f 0.26, r(Si-H) = 1.478 f 0.004. Other important structural features have been estimated.
Introduction
The phenomenon of pseudorotation has been studied spectroscopically for a number of years and has recently been reviewed by LaaneS4The general theory for free and hindered pseudorotation has been developed by Harris et al.5 and successfully employed to interpret experimental data.6 More recently, Ikeda et al.7 have investigated the implications of a barrier to pseudorotation using a strict two-dimensional model. The barriers to pseudorotation for a number of monosubstituted cyclopentanes have been determined from transitions in their far-infrareds-l7 and Ramanl*Jg spectra; however, it is usually not possible to determine the structure of the most stable ring conformation from the vibrational data alone. Silacyclopentane is one of the molecules which has been studied previously and the value of the barrier to pseudorotation was J~ there was some unfound to be 3.9 k c a l / m 0 1 . ~ ~However, certainty as to the most stable configuration, Cz or C,, although the evidence was strongly in favor of the twisted or C2 conformation. Rotational spectra have been shown to be useful in answering these questions.20,21In order to provide additional evidence for the relatively high barrier and to determine the symmetry of the heavy-atom skeleton in the ground vibrational state, the microwave spectra of silacyclopentane and silacyclopentane-l,l-dz have been recorded and analyzed in detail. The geometry of the ring has been estimated from a rigid model of the molecule in the ground state consistent with the observed rotational constants and the dipole moment has been determined from two different transitions. Experimental Section
The samples of silacyclopentane and silacyclopentane-dz were prepared and kindly provided by Dr. J. N. Willis, Jr.16,22 No further purification was necessary and each sample was quite stable in thz bronze waveguide. The microwave spectra
were recorded with the samples held a t dry ice temperatures on a conventional spectrometer with 80-kHz square wave modulation. Frequency measurements were reproducible within 0.05 MHz with an absolute accuracy estimated to be 0.1 MHz for the ground state lines and for higher vibrational states and isotopic species to f 0 . 5 MHz. The dipole moment measurements were made with a Stark field applied from a precision high-voltage dc power supply with sufficient square wave superimposed to modulate the transition. The waveguide spacing was calibrated relative to the dipole moment of 0CS.23 Spectrum and Assignment
The spectrum in general was weak, but lines were sharp. The pattern of R-branch lines was typical of an a-type rotor with a 14 value close to zero in that lines associated with a given J-level transition were separated by as much as a few GHz. Initial line assignments were made on the basis of a spectrum predicted from a structure in which the Si-H and Si-C distances were assumed to be the same as in d i m e t h y l ~ i l a n e , ~ ~ and the C-H and C-C bond lengths the same as in propane.25 These assignments were verified by observing the Stark effect in various lines. The R-branch assignments were helpful in assigning some of the a-type Q branches in the spectrum. A search for signals attributable to molecules with isotopic substitution in natural abundance was carried out. The 29Si isotopic species was identified, with intensities consistent with its 4.7% abundance. Other weaker lines that might be due to 30Si,or a- or (3-13C were observed but the data were somewhat questionable and are not included. Observed transition frequencies for the -do, -l,l-d2, and 29Si species are listed in Table I. The rotational constants shown have been calculated using an iterative least-squares program. All the transitions shown were used in the program, except in the case of the -do compound where only the lower J levels were used. We feel that these eight R branches and one Q branch give rotational constants unaffected by centrifugal The Journal of Physical Chemistry, Voi. 80, No. 11, 1976
1200
J. R. Durig, W. J. Lafferty, and V. F. Kalaslnsky
TABLE I: Microwave Transition Frequencies (MHz), Rotational Constants (MHz), K Values, Moments of Inertia (u A2),a and Inertial Defects [A = I, - (Ia+ a)]for Some Isotopic Species of Silacyclopentane 28Si Transition
Obsd 12 121.76 1 2 873.09 15 072.57 22 617.57 17 801.01 18 206.58 20 395.60 21 971.40 22 584.77 23 270.54 23 404.97 26 596.19 27 939.48 28 166.74 29 400.00 30 311.15 28 639.64 28 674.62 32 418.33
295i
1,l-d2
Obsd
- calcdb
-0.02 0.09 -0.01 -0.01
-0.02 -0.01
-0.09 0.11
-0.02 -0.32 -0.16 -0.16 0.05 0.01 0.01
0.04 -0.24 -0.17 -0.34
34 902.20 -0.32 37 222.30 -0.09 37 760.10 -0.54 33 973.91 -0.47 33 981.96 -0.45 37 962.48 -0.48 39 299.01 -0.62 39 300.72 -0.63 18 932.62 -0.28 22 538.30 -0.69 5 473.39 f 0.04 4 137.01 f 0.02 2 661.55 f 0.02 0.049 92.334 f 0.002 122.160 f 0.001 189.882 f 0.001 -24.612
Obsd
Obsd
- calcd
11401.50 12 133.06 13 864.87
0.07 0.06 0.02
16 833.37 17 327.53 18 949.81 20 388.35 20 571.90 22 089.92 22 300.96 24 860.99
0.03 0.04 0.09 -0.03 -0.04 -0.08 -0.02 -0.04
25 979.22 26 710.50 27 794.15
0.01
0.06
Obsd
Obsd
- calcd
23 078.60 23 227.29 26 336.35 27 718.86 27 833.03
0.04 0.16 -0.01 -0.02 0.05
28 408.92 28 449.28 32 131.73
0.19
-0.02
5 308.25 f 0.16 3 774.14 f 0.03 2 542.43 f 0.03 -0.109 95.206 f 0.005 133.906 f 0.002 198.778 f 0.002 -30.334
0.11
-0.06
33 701.79 33 711.61
-0.08
38 984.47 38 986.57
-0.06 -0.11
0.01
5 473.42 f 0.20 4 085.26 f 0.04 2 639.99 f 0.02 0.020 92.333 f 0.004 123.708 f 0.002 191.432 f 0.002 -24.609
- -
Moments of inertia were calculated using the conversion factor 505 379 u A2 MHz. Errors are those implied from the errors in the rotational constants. Only the J = 2 1 and 3 2 transitions were used in the least-squares determination of the rotational constants.
distortion. In the other cases, since no Q branches were identified, we have used all the data available. In addition, a number of relatively strong excited state lines were apparent on the high-frequency side of the ground state lines. We have identified these in the -do compound as excited states of the ring-puckering motion and were able to measure five higher levels of this mode. These data are found in Table I1 along with the derived rotational constants and moments of inertia. Dipole Moment Stark coefficients were calculated by the method of Golden and Wilson26from a computer program written by B e a u d e P and modified by W. Kirchoff. Comparison of these rigid-rotor Stark coefficients to those obtained from a least-squares fit of the observed Stark transitions at various field values yield the dipole moment components. The two transitions chosen for these measurements were those which have very large type-c dependence but the c component was found to be less The Journal of Physical Chemistry, Vol. 80, No. 1 I , 1976
than 0.01 D. The a component of the dipole moment was constant within the experimental error in the vibrational states, u = 0, 1,2, and 3; I ( uI wd u )I = 0.726 f 0.005 D. The total 4 0.726 f 0.006 D. dipole moment was determined to be 1 ~ = The dipole moment, transition moments, and the transitions used to determine them are given in Table 111. This is comparable to the value of 0.75 D found in dirnethyl~ilane.~~ Molecular Conformation Qualitatively it is possible to make use of the data to determine the molecular conformation of silacyclopentane. The least likely conformation is the planar, Czur form. More probable are the C, (envelope) or Cz (twisted) configurations. Of the three possibilities only the C, form is expected to have dipole-moment components in more than one direction, and in particular these would be a and c components. We have found that within the experimental error, wc is zero for silacyclopentane. Additionally, the isotopic substitution of the silicon atom
Spectra and Structure of Small-Ring Molecules
1201
TABLE 11: Microwave Transition Frequencies (MHz), Rotational Constants (MHz), K Values, Moments of Inertia (u
A2),nand Inertial Defects [A = IC- (Ia+ a)]for the v39 Vibrational States of Silacyclopentane-28Si ~~
u=2
u = l
Transition
Obsd - calcd
Obsd
18 220.64 0.23 20 416.30 0.15 21 988.76 -0.08 22 611.86 -0.03 -0.76 23 292.35 23 425.69 0.34 0.11 26 620.59 27 955.31 -0.20 0.08 28 197.37 0.04 29 440.27 -0.14 30 341.02 0.30 32 445.90 0.00 34 936.69 5 467.92 f 0.48 4 140.86 f 0.07 2 664.52 f 0.08 0.053 92.426 f 0.008 122.047 f 0.002 189.670 f 0.006 -24.803
Obsd
Obsd calcd
22 003.57 0.26 22 634.55 0.15 23 313.86 0.12 23 444.32 0.12 26 641.48 0.01 27 969.05 0.03 28 223.26 0.04 29 474.07 0.00 30 365.84 0.04 32 469.13 -0.20 34 965.46 -0.28 5 462.45 f 0.27 4 143.89 f 0.04 2 667.36 f 0.05 0.056 92.519 f 0.005 121.958 f 0.002 189.468 f 0.004 -25.009
u=3 Obsd
22 015.48 22 653.36 23 331.99 23 460.95 26 659.34 27 980.41 28 245.16
~
u=4
Obsd calcd
0.21 0.23 0.11 0.18 0.02 0.16 0.10
Obsd
22 025.34 22 668.38 23 348.15 23 475.88 26 674.58 27 989.33 28 263.17
Obsd calcd
0.41 0.01 -0.02 0.13 0.06 -0.18
u=5
Obsd
Obsd calcd
22 033.29
0.11
23 362.63 -0.02 23 489.38 0.14 26 687.62 -0.02 27 998.14 0.00
-0.05
30 386.08 -0.07 30 402.50 0.03 32 507.20 -0.02 32 522.57 -0.13 32 489.33 -0.33 35 027.63 0.01 34 989.91 -0.28 35 010.41 -0.16 5 449.23 & 0.24 5 457.43 f 0.42 5 452.68 f 0.34 4 149.69 f 0.04 4 148.22 f 0.05 4 146.34 f 0.06 2 674.26 f 0.03 2 669.89 f 0.06 2 672.21 f 0.05 0.062 0.063 0.059 92.743 f 0.005 92.685 f 0.006 92.604 f 0.007 121.787 f 0.002 121.886 f 0.002 121.830 f 0.002 188.979 f 0.002 189.288 f 0.005 189.124 & LO04 -25.202 -25.391 -25.551
See Table I. TABLE III: Stark Coefficients [(MHz/cm2)/V2] and Dipole Moment of Silacyclopentane
Transitions 404
+ -
303
414 +- 313
In/il
Obsd X IO6
3 2 3 2
0.8131 0.2817 1.246 0.4927 pa = 0.726 & 0.005 D fiLc = 0.00 f pt = 0.726 f
Calcd X lo6 0.8135 0.2823 1.243 0.4931
0.01
0.006 D
gives no change (within experimental error) in the moment of inertia about the a axis (see Table I). This implies that the silacyclopentane molecule has an axis of symmetry and that it passes through the silicon atom. Finally, we rule out the planar, C2", form on the basis of the values of the inertial defect, A, shown in Table I. These values are too large for any reasonable out-of-plane hydrogen coordinates. We must conclude, then, that silacyclopentane exists in a permanently twisted, C2, configuration in the ground state. Structure With the isotopic data available, certain features of the molecular structural can be determined, while others may be inferred. By taking advantage of the symmetry of silacyclopentane in the formulation of Kraitchman's equationszs i t is possible to calculate substitution coordinates for the silicon atom and its protons. We find r(Si-H) = 1.478 f 0.004 8,and L(H-Si-H) = 108.76 f 0.26". These are similar to the corresponding quantities in d i m e t h y l ~ i l a n e . ~ ~ While it is not possible to determine the exact structure of silacyclopentane we may estimate some of the other salient features. By assuming the remaining bond lengths and exte-
TABLE IV: Assumed and Calculated Structural Parameters for Silacyclopentane
Substitution parameters Assumed parameters
Estimated parameters
r(Si-H) = 1.478 f 0.004 8, LHSiH = 108.76 f 0.26' r(Si-C) = 1.87 8, r(C-C) = 1.548, r(C-H) = 1.098, L(HC,Si) = 109.5' L(HC&!6) = 109.5' L(CSiC) = 96.7' L(SiCC) = 105.1" L(CCC) = 113.7" 7 = 21'
rior bond angles, we can concentrate on the more important internal ring angles and, of course, T,the twist angle. The twist angle can be thought of as the dihedral angle between the plane defined by CUI-Si-C,, and one that contains the Si and Cp atoms. The assumed and subsequently derived parameters are given in Table IV. We found that the a moment of inertia was sensitive only to variations in the C-Si-C angle. This is to be expected for a molecule with silacyclopentane's symmetry. The other two valence angles and T were determined in each case from all three moments of inertia after &-Si-C was fixed a t a value consistent with I,. The structure given in Table IV reproduces the ground state rotational constants of the three isotopic species to within 1.0%. Error limits on the structural features have not been included since they would be very large and difficult to determine. We feel that this structure is a reasonable one, even though a number of assumptions were made. Vibrational Excited States We have measured five excited states of the ring-puckering motion in the normal isotopic species and the results are found The Journal of Physical Chemistry, Vol. 80, No. 11, 1976
J. R. Durig, W. J. Lafferty, and V. F. Kalasinsky
1202
Figure 1. Plot of the change in rotational constants (MHz) with increasing vibrational quantum number of the ring-puckering motion.
in Table 11. The increase in the values of the inertial defect, A, with increasing vibrational state are consistent with a motion of this sort. The value of the barrier to interconversion of the two equivalent twisted forms is expected, from the observed satellite pattern, to be fairly large. The variation in the rotational constants with increasing vibrational state is plotted in Figure 1. The linearity in the graphs is typical of molecules with appreciable barriers to i n t e r c o n v e r s i ~ nAlso . ~ ~ ~indicative ~~ of a high barrier in silacyclopentane is the lack of observed splitting in the first five excited states. This implies that the fifth vibrational level, which is about 500 cm-l above the ground state, is still well below the top of the potential well. This is consistent with the value of the barrier (3.9 kcaljmol) determined from far-infrared data.16J7 Discussion
The conformation of silacyclopentane has been established on the basis of dipole-moment components, isotopic substitution, and inertial defects to be the twisted, Cp, form. In the vibrational studies,16J7it had not been possible to distinguish between the C2 and C, forms. The C2 configuration had been assumed, as this is in qualitative agreement with the treatment of Pitzer and D ~ n a t h .The ~ l twisted form has also been found to be the stable one in germacyclopentane,2° cyclopentaThese molecules show none,3O and methylenecy~lopentane.~~ barriers to pseudorotation of 4.14,l5J9 2.15,32933and 1.829,34 kcal/mol, respectively. It has been implied16J7that the large difference between the observed barrier to pseudorotation and that predicted using the formulation of Pitzer and Donath is due to the nontransferability of torsional barriers from simple molecules to ring compounds. The structural features of ring compounds are probably an important factor causing this to be true. The difference between the C-Si-C angles in dimethylsilane (110.98°)24and silacyclopentane (96.7O) is certainly significant in this respect. In addition, this smaller angle introduces ring strain that is absent in unsubstituted cyclopentane. The twist angle, T , must be important since the nonbonded interactions between adjacent CH2 groups are affected by this parameter. It is not surprising that quantitative agreement with theory31 is generally poor.
The Journal of Physical Chemistry, Vol. 80, No. 1 I , 1976
The recent treatments of pseudorotation as a two-dimensional p r ~ b l e m linvolving ~ ? ~ ~ the puckering and radial modes have been interesting insofar as the barriers obtained have been lower than the previous simpler treatments. Additional vibrational data necessary for these studies were obtained from the far-infrared spectra of a number of isotopic species33 or the Raman spectra of gaseous ~amp1es.l~ The Raman spectra of silacyclopentane should prove to be very useful in determining a more complete potential surface governing the low-frequency ring vibrations.
Acknowledgment. The authors gratefully acknowledge the financial support of this study by the donors of the Petroleum Research Fund administered by the American Chemical Society. References and Notes (1) For part XXXII, see J. Chem. Phys., 63, 2015 (1975). (2) W. J. Lafferty received his Ph.D. under the direction of Professor Lord in 1961. J. R . Durig received his Ph.D. under the direction of Professor Lord in 1962. (3) Work done at the National Bureau of Standards and the Swiss Federal Institute of Technology while on sabbatical leave from the University of South Carolina. (4) J. Laane in "Vibrational Spectra and Structure", Vol. I, J. R. Durig, Ed., Marcel Dekker, New York, N.Y. 1972, Chapter 2. (5) D. 0. Harris, G. G. Engerholm, C. A. Toirnan, A. C. Luntz, R. A. Keller, H. Kim, and W. D. Gwinn, J. Chem. Phys., 50, 2438 (1969). (6) G. G. Engerholm, A. C. Luntz, W. D. Gwinn, and D. 0. Harris, J. Chem. Phys., 50, 2446 (1969). (7) T. Ikeda, R. C. Lord, T. B. Malloy, Jr., and T. Ueda, J. Chem. Phys., 56, 1434 (1972).
(8)W. J. Lafferty. D. W. Robinson, R. V. St. Louis, J. W. Russel, and H. L. Strauss, J. Chem. Phys., 42, 2915 (1965). (9) J. R. Durig, G. L. Coulter, and D. W. Wertz, J. Mol. Specfrosc., 27, 285 (1968). (10) J. R. Durig and D. W. Wertz, J. Chem. Pbys., 49, 675 (1968). (1 1) J. A. Greenhouse and H. L. Strauss, J. Chem. Phys., 50, 124 (1969). (12) D. W. Wertz, J. Chem. Phys., 51, 2133 (1969). (13) W. H. Green, A. B. Harvey, and J. A. Greenhouse, J. Cbem. Phys., 54,850 (1971). (14) J. R. Durig, J. M. Karriker, and D. W. Wertz, J. Mol. Spectrosc., 31, 237 (1969). (15) J. R. Durig and J. N. Willis, Jr., J. Chem. Phys., 52, 6108 (1970). (16) J. R. Durig and J. N. Willis, Jr., J. Mol. Spectrosc., 32, 320 (1969). (17) J. Laane, J. Cbem. Phys., 50, 1946 (1969). (18) L. A. Carreira, G. J. Jiang, W. B. Person, and J. N. Willis, Jr.. J. Chem. Phys., 56, 1440 (1972). (19) J. R. Durig, Y. S.Li, and L. A. Carreira, J. Chem. Phys., 58, 2393 (1973). (20) E.C. Thomas and V. W. Laurie, J. Cbem. Phys., 51, 4327 (1969). (21) J. A. Wells and T. B. Malloy. Jr., J. Chem. Phys., 60, 2132 (1974). (22) J. N. Willis, Jr., Ph.D. Thesis, University of South Carolina, 1970. (23) J. S. Muenter, J. Chem. Phys., 48, 4544 (1969). (24) L. Pierce, J. Chem. Phys., 34, 498 (1961). (25) D. R. Lide, Jr., J. Chem. Phys., 33, 1519 (1960). (26) S. Golden and E. B. Wilson, J. Chem. Pbys., 7, 669 (1948). (27) R. A. Beaudet, Ph.D. Thesis, Harvard University, 1961. (28) J. Kraitchrnan, Am. J. Pbys., 21, 17 (1953). (29) J. R. Durig, Y. S. Li, and L. A. Carreira, J. Chem. Phys., 57, 1896 (1972). (30) H. Kim and W. D. Gwinn, J. Chsm. Pbys., 51, 1815 (1969). (31) K. S. Pitzer and W. E. Donath, J. Am. Chem. SOC., 81,3213 (1959). (32) L. A. Carreira and R . C. Lord, J. Chem. Phys., 51, 3225 (1969). (33) T. lkeda and R. C. Lord, J. Chem. Phys., 56, 4450 (1972). (34) T. E. Malloy, Jr., F. Fisher, J. Laane, and R. M. Hedges, J. Mol. Spectrosc., 40, 239 (1971).
