J. Phys. Chem. 1996, 100, 9339-9347
9339
Conformational Analysis of 1,4-Disilabutane and 1,5-Disilapentane by Combined Application of Gas-Phase Electron Diffraction and ab Initio Calculations and the Crystal Structure of 1,5-Disilapentane at Low Temperatures Norbert W. Mitzel, Bruce A. Smart, Alexander J. Blake, Heather E. Robertson, and David W. H. Rankin* Department of Chemistry, The UniVersity of Edinburgh, The Kings Buildings, West Mains Road, Edinburgh, EH9 3JJ, U.K. ReceiVed: January 2, 1996; In Final Form: February 19, 1996X
The gas-phase structures of the conformers of 1,4-disilabutane and 1,5-disilapentane have been analyzed from electron-diffraction data augmented by flexible restraints derived from ab initio calculations. This allowed the simultaneous refinement of 22 and 29 parameters for 1,4-disilabutane and 1,5-disilapentane, respectively. 1,4-Disilabutane has been found to be present in the vapour predominantly in the anti (A) form (76(2)% from the experiment, 83% predicted by theory). Consistency in the geometries is found between theoretical predictions and experimental findings, except for the torsion angle ∠(SiCCSi) of the gauche (G) conformer [exptl 78.5(21)°, theor 68.0°]. The AA conformer of 1,5-disilapentane was always found to be the lowest energy structure, while some doubt still remains about the ordering of the AG and G(+)G(-) conformers. The AA conformer is found to be the sole form present in the crystal [C2/c, a ) 15.585(8), b ) 4.704(3), c ) 9.895(6) Å, β ) 95.77(4)°, Z ) 4]. Good agreement is found for geometrical parameters determined experimentally in the gas phase and solid state and calculated by ab initio methods. The following values represent the most important distances (rg/Å) and angles (∠g/deg) found for the gas phase and crystal structures. 1,4-Disilabutane GED (A/G, esd’s correspond to 1σ): r(CSi) 1.882(1)/1.885(1), r(CC) 1.563(5)/1.563(5), r(SiH) both 1.499(3), ∠(CCSi) 110.7(2)/114.4(5), ∠(SiCCSi) 180.0/78.5(21). 1,5-Disilapentane GED [AA/G(+)G(-)]: r(CSi) 1.886(1)/1.888(1), r(CC) 1.537(2)/1.539(2), r(SiH) both 1.487(4), ∠(CCC)114.8(7)/118.8(7), ∠(CCSi) 114.1(4)/116.8(7), ∠(SiCCC) 180.0/60.9(10); X [%, AA/AG/G(+)G(-)] 28(4)/40(5)/26(6). 1,5-Disilapentane XRD: r(CSi) 1.868(2), r(CC) 1.527(2), ∠(CCC) 113.8(2), ∠(CCSi) 115.2(1), ∠(SiCCC) 180.0(1).
Introduction The conformations of simple alkanes such as butane and pentane are of fundamental importance to chemistry, as they are the basic models for conformations of hydrocarbon backbones in organic molecules. A large area of modern chemistry including pharmaceutical and biochemistry is based on the principles of conformational theory.1 In this context experimentally based analyses are of particular importance, as these are used as reference information for the implementation of modern molecular modeling programs. Many studies dealing with these model systems including butane, pentane,2-4 and the higher alkanes have been carried out to get as much information as possible about these systems. The influence of substituents on the conformations of C2 and C3 units has mainly been investigated with their halogen derivatives, but also compounds such as 2-fluoroethanol,5 2-chloroethanol,6 ethylene-1,2-dithiol,7 and, most recently, ethylenediamine8 have been studied. For the derivatives of ethane with the most electron withdrawing substituents (F, Cl, Br, NH2) the “gauche effect”9 has been found to be operative, i.e., the stabilization of the gauche vs the anti conformer. However, no information is available for derivatives of the small hydrocarbons bearing more electropositive substituents such as the heavier congeners in group 14. Therefore we considered the two disilaalkanes 1,4-disilabutane and 1,5disilapentane to be ideal candidates for such studies. Besides their conformational behavior, they are of interest because these compounds10 and their R,ω-dibromo derivatives X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00046-9 CCC: $12.00
in particular11 are promising single-source precursors for the deposition of thin films of β-silicon carbide. In chemical vapor deposition (CVD) experiments, both deposit silicon layers rather than those of β-SiC in the low-temperature regime and a loss of the bridging C2/3 unit as ethene and propene is observed, respectively. By contrast, β-SiC films are formed at high temperatures. Structural parameters for such compounds, preferably from the gas phase, are therefore highly desirable in order to get more detailed information about the complex chemistry in such CVD processes. Experimental Section The samples of 1,4-disilabutane and 1,5-disilapentane were prepared by modified12 literature procedures13 and purified by repeated distillations. Electron Diffraction. Electron-scattering intensity data for 1,4-disilabutane and 1,5-disilapentane were recorded on Kodak Electron Image plates using the Edinburgh gas diffraction apparatus.14 The samples were maintained at 230 and 273 K, respectively. The inlet nozzle was held at ambient temperature (293 K) during the experiments. Scattering data for benzene were recorded concurrently and used to calibrate the camera distance and electron wavelength. Data were obtained in digital form using the Joyce Loebl MDM6 microdensitometer15 at the EPSRC Daresbury Laboratory. The data analysis followed standard procedures, using established data reduction15 and leastsquares refinement programs16 and the scattering factors established by Fink and co-workers.17 Table 1 lists experimental data, including the weighting points needed to set up the offdiagonal weight matrix used in the least-squares refinement. © 1996 American Chemical Society
9340 J. Phys. Chem., Vol. 100, No. 22, 1996
Mitzel et al.
TABLE 1: Experimental Conditions of Electron-Diffraction Experiments Including Weighting Functionsa and Correlation Parameters15 compound camera height/mm electron wavelength/Å ∆s/Å smin/Å sw1/Å sw2/Å smax/Å correlation parameter scale factor k
1,4-disilabutane 285.98 0.05668 0.2 2.0 4.0 12.2 14.4 0.0780 0.858(5)
128.12 0.05668 0.4 10.0 12.0 29.2 34.0 0.0518 0.859(18)
1,5-disilapentane 285.90 0.05876 0.2 2.0 4.0 12.2 14.2 0.4088 0.785(11)
128.24 0.05669 0.4 7.0 9.2 27.2 32.0 -0.3311 0.809(23)
a The trapezoidal weighting function gives 0% weight to the data points at smin and smax and 100% weight to data points between sw1 and sw2.
Crystallography. Growth of a single crystal of 1,5-disilapentane was performed on a Stoe Stadi-4 four-circle diffractometer fitted with an Oxford Cryosystems low-temperature device18 by slowly cooling the liquid sample sealed in a capillary from 163 K by 3 K and a rate of 5 K h-1. C3H12Si2, M ) 104.31, space group C2/c, a ) 15.585(8), b ) 4.704(3), c ) 9.895(6) Å; V ) 721.8 Å3, Z ) 4, Dcalc ) 0.960 g cm-3; T ) 113 K; µ(Mo KR) ) 0.367 mm-1; The structure was solved by direct methods19 and refined20 with 48 parameters (all non-H atoms anisotropic) to give R1 ) 2.15% and S ) 1.069 [based on 435 observations with F g 4σ(F)] and wR2 ) 5.64% for all 471 data (472 data collected). Final residual electron density: max ) 0.21, min ) -0.20 e Å-3. Many attempts to grow a crystal of 1,4-disilabutane failed. Each time a seed crystal was produced, the whole sample crystallized spontaneously on cooling. Theoretical Methods. All ab initio molecular orbital calculations were performed on a DEC Alpha APX 1000 computer using the Gaussian 92 program.21 Geometry optimizations on 1,4-disilabutane were performed at the SCF level of theory using the standard 3-21G*,22-24 6-31G*,25-27 and 6-311G**28,29 basis sets, while the larger two basis sets were used for optimizations at MP2 level of theory. Since improving the basis set beyond 6-31G* was found to have little effect on either the molecular geometry or the relative energies of the two forms of 1,4-disilabutane, no calculations on 1,5-disilapentane were performed using any higher basis set. Estimates of the effects of a more sophisticated treatment of electron correlation on the relative energies of conformers were obtained by performing single-point energy calculations at the CCSD(T) level of theory. Vibrational frequency calculations were calculated from analytic second derivatives at the SCF level using the 3-21G* and 6-31G* basis sets to determine the nature of all located stationary points. Frequency calculations on 1,5-disilapentane were restricted to the SCF level of theory, while additional frequency calculations on 1,4-disilabutane were performed at the 6-31G*/MP2 level. Cartesian force constants were derived from the frequency calculations at the highest level available and converted to internal coordinates and scaled by common factors using the program ASYM40 (which is similar to ASYM2030). In the case of 1,4-disilabutane, the symmetry coordinates of Murphy and co-workers for n-butane31 were used. 1,4-Disilabutane. Theoretical Calculations. Two forms, anti (A, C2h symmetry) and gauche (G, C2 symmetry), based on a fully staggered structure, can be envisaged for 1,4-disilabutane. Vibrational frequency calculations and geometry optimizations were performed on both structures, which were found to be local minima on the potential energy surface at all levels of theory.
The geometry of the A conformer proved to be rather insensitive to changes in the adopted theoretical method with improvements in neither the basis set nor level of theory resulting in significant changes in geometrical parameters. The Si-C bond length was found to vary across a range of less than 0.01 Å, while the C-C bond distance fell over an even narrower range if the result using the smallest of the basis sets is excluded. The other bond lengths were calculated to fall within similarly small ranges, while the bond angles and dihedral angles remained essentially unaffected and generally agreed within 1° of one another. Calculations on the G conformer yielded similar trends, with bond distances and angles again varying over narrow ranges. In general, values of molecular parameters were shown to differ rather little between the two isomers with one notable exception: the Si-C-C bond angle was found to be approximately 3° wider in the case of the G conformer. Presumably this difference arises because of the increase in steric crowding in the G conformer as compared to the anti form. The A conformer is the lowest energy form of 1,4-disilabutane. The energy difference between the two structures was investigated using basis sets up to 6-311G** and levels of theory up to CCSD(T). Calculations using the 3-21G* basis set at the SCF level of theory predicted an energy difference of 5.7 kJ mol-1, while larger basis sets yielded only modest increases in this value. The introduction of correlation at the 6-31G*/MP2 level of theory led to a decrease in the energy separation to 5.2 kJ mol-1, or 5.5 kJ mol-1 when corrected for zero-point vibrational energy. Further improvements in the level of theory (up to CCSD(T)) led to a slight but steady decrease in the relative stability of the A conformer. Since the effects of increasing basis set size and and improvement of the treatment of electron correlation are small and act in an opposing fashion, it seems that modestly sophisticated calculations are capable of providing reliable estimates of the relative energies of the two isomers, with the best value probably lying in the range 5.0-5.5 kJ mol-1, corresponding to a range of the percentage of the trans conformer from 80 to 83% (at 293 K). Gas-Phase Structure Analysis. The large number of geometrical parameters necessary for the definition of this system required the adoption of some assumptions to simplify the problem. The model for the conformational mixture of 1,4disilabutane consists of both A (C2h) and G (C2) conformers. Calculated vibrational amplitudes (see above) were used throughout unless refinement was possible. The amplitudes obtained in this way were used to generate a structure in the rg space. All C-H and Si-H bond lengths for both conformers were assumed to be identical, and local C3V symmetry was assumed for the CSiH3 groups. The CH2 units were placed in idealized positions, i.e., lying in a plane perpendicular to the CCSi plane. Initially, the differences ∆(C-C), ∆(C-Si) and ∆(CCSi) between the bond lengths and bond angels in the two conformers, the angle ∠(HCH) and an angle ∆∠[SiC(H)2] representing the deviation of the HCH plane from the bisector of the angle C-C-Si (toward the C atom) were fixed at the calculated values. These parameters have been chosen for convenience in the specification of the model, and since many have little structural interest otherwise, more precise descriptions were not considered. Figure 1 shows the molecular structures of both conformers according to the list of geometrical parameter values given in Table 2. Internuclear distances of the two conformers of 1,4disilabutane are given in Table 4. The molecular scattering intensity curve is shown in Figure 2. As is seen in Figure 3, the radial distribution curve of 1,4-disilabutane consists of five well-resolved peaks representing the C-H, Si-H/C-C, Si-
1,4-Disilabutane and 1,5-Disilapentane
J. Phys. Chem., Vol. 100, No. 22, 1996 9341 TABLE 3: Theoretical and Experimental Energy Differences between A and G Conformers of 1,4-Disilabutane (in kJ mol-1) and Composition of the Gas Phase at 293 K (Esd’s (2σ) for the Mole Fractions Are Derived from Figure 4 and Correspond to the 95% Level of Confidence method 3-21G* 6-31G*
SCF SCF MP2 MP3 MP4DQ MP4SDQ CCSD CCSD(T) SCF MP2
6-311G*
∆E
∆E0
%G
5.72 6.27 5.20 4.94 4.85 4.76 4.82 4.70 6.54 5.50
5.99 6.60 5.51 5.52a 5.16a 5.07a 5.13a 5.01a 6.87a 5.81a 4.6(2)b
14.6 11.8 17.2 17.2 19.4 20.0 19.6 20.4 10.6 15.5 24(2)
GED a
Estimation of ∆E0 based on zero point energies calculated at the 6-31G*/MP2 level. b 4.4(2) if corrected for entropy effects (see text).
TABLE 4: Selected Distances (ra/Å) and Vibrational Amplitudes (ua/Å) for 1,4-Disilabutanea A conformer Figure 1. Molecular geometries of the A and G conformers for 1,4disilabutane as determined by gas-phase electron diffraction.
TABLE 2: Selected Structural Results for 1,4-Disilabutane: Ab Initio Calculations at Different Levels of Theory (re/Å; ∠e/deg)), GED (rg/Å; ∠g/deg)a 3-21G* SCF
6-31G* SCF MP2
6-311G* SCF MP2
C-C C-Si C-H Si-H ∠CCSi ∠HCH ∠CSiH ∠SiCH
1.570 1.887 1.088a 1.478a 112.4 106.6 110.5 109.3
A conformer: 1.547 1.543 1.894 1.889 1.089a 1.097a 1.479a 1.488a 113.6 112.3 105.8 106.0 110.5 110.3 108.5 109.0
1.546 1.890 1.089a 1.481a 113.9 105.9 108.9 108.2
1.546 1.885 1.097a 1.479a 112.7 106.1 110.1 108.8
1.563(5) 1.882(1) 1.141(2) 1.499(3) 110.7(2) 108.1(16) 112.1(7) 108.2(5)
C-C C-Si C-H Si-H ∠CCSi ∠HCH ∠CSiH ∠SiCH ∠Si-C-C-Si ∠twSiH3
1.568 1.890 1.089a 1.478a 115.6 106.0 110.6 108.7 68.9 168.8
G conformer: 1.547 1.543 1.897 1.893 1.089a 1.097a 1.478a 1.488a 116.8 115.5 105.4 105.7 110.8 110.4 107.8 108.8 70.4 67.2 171.0 169.2
1.546 1.892 1.089a 1.480a 116.9 105.5 110.8 107.6 71.9 171.8
1.546 1.888 1.097a 1.478a 115.7 105.8 110.2 108.0 68.0 169.7
1.563(5)df 1.885(1)df 1.141(2)s 1.499(3)s 114.4(5)drf 108.1(5)s 112.4(6)s 108.2(4)s 78.5(21) 153.9rf
GED
Symbols: aaverage, ffixed, rfrefined then fixed, dffixed difference to A conformer, drfdifference to conformer A refined then fixed, ssingle value refined for both conformers. a
C, Si‚‚‚C and Si‚‚‚Si(A) distances. The parameters r(C-H) and r(Si-C) are well refinable, whereas r(C-C), r(Si-H), and u(Si-H) are highly correlated, as indicated by the correlation matrix (Table 5), giving rise to some problems during the refinement. The distance r(Si-C) for the dominant A conformer refined to 1.882(1) Å, a value which is in excellent agreement with the predictions of the ab initio calculations (1.885 Å). The distance r(C-C) refined to 1.563(5) Å. Refinement of the differences ∆r(C-C) and ∆r(Si-C) between A and G conformers was not possible, and so these differences were fixed at the values theoretically predicted. However, in the case of the angle ∠(SiCC) [112.6(2)° for the A conformer] it was possible to refine the parameter itself and the difference ∆(CCSi) [3.7(4)°] by restraining this difference flexibly to the predicted value
G conformer
ra
ua
[T]
ra
ua
[T]
C-Si C-C Si-H C-H C‚‚‚Si Si‚‚‚Si C‚‚‚H Si‚‚‚H Si‚‚‚H
1.880(1) 1.561(5) 1.493(3) 1.136(2) 2.836(2) 4.555(2) 2.803(11) 2.481(6) 3.042(7)
0.059(2) 0.057(4) 0.097(4) 0.078(3) 0.097(3) 0.106(2) 0.166rf 0.112(7) 0.174(15)
1 2 3 4 5 6 8 9 10
Si‚‚‚H
5.060(14) 5.643(10)
0.356(53) 0.182
12 t12
C‚‚‚H C‚‚‚H
2.237(7) 3.294(13)
0.100 0.296rf
t9 15
H‚‚‚H
4.135(7) 2.398(12)
0.120f 0.138
17 t9
1.883(1) 1.561(5) 1.494(3) 1.136(2) 2.901(7) 3.788(2) 2.807(11) 2.468(6) 3.868(9) 2.969(25) 5.219(14) 4.062(43) 3.776(33) 2.224(7) 3.627(11) 3.184(15) 4.130(8) 2.398(12)
0.059 0.057 0.094 0.077 0.090 0.228(20) 0.161 0.131 0.200f 0.111 0.177 0.467f 0.328f 0.106 0.231 0.236f 0.122f 0.146
t1 t2 t3 t4 t5 7 t8 t9 11 t10 t12 13 14 t9 t15 16 18 t9
a Symbols: [T] ) tying scheme indicating which amplitudes are independently refining or tied together, ffixed, rfrefined then fixed. Values given in parentheses are estimated standard deviations, obtained in the least-squares refinement.
(3.44° with an uncertainty estimated of 0.4°). This means thatthe theoretically derived value for ∆(CCSi) was treated as an extra observation and assigned a best estimate of an uncertainty.32 Similar restraints were applied for ∠(CSiH) [restraint: 110.9 (10)°] and ∠(HCH) [restraint: 106.1(20)°] to allow more parameters to refine and so to obtain more realistic standard deviations. The uncertainties of these restraints are necessarily subjective but are based on our estimates of the likely reliability of the computed values in the highest level calculations.32 The torsion angle τ(SiCCSi), which defines the skeletal geometry of the G conformer, refined to 78.5(21)°. It is markedly dependent on ∆(CCSi) and to a smaller extent on the twist angle of the SiH3 group, τ(SiH3). The magnitude of τ(SiH3) has been explored and the parameter fixed at the value with the best fit to the data, which also gave the lowest value for τ(SiCCSi). A free refinement of τ(SiH3) was not possible. Following this procedure, the refinement converged to a final weighted RG value of 4.23% (2.99 and 6.96% for long and short camera distance data sets, respectively). Figure 4 shows the dependence of the fit, expressed as the RG value, on the conformational composition. The mole fraction of the A conformer was determined to be 76 ( 2% at 293 K
9342 J. Phys. Chem., Vol. 100, No. 22, 1996
Mitzel et al. TABLE 5: Correlation Martrix (×100) for Parameters of 1,4-Disilabutane (Only Elements with Absolute Values Greater than 50 Are Shown) r(Si-H) ∠CCSi ∠HCH ∠SiCCSia u(Si-H) u(Si‚‚‚H) k2b r(C-C) r(Si-H) ∠CCSi ∠CSiH ∆∠SiCCc ∠SiCCSia u(C-Si) a
-80
-77 56
-80 62 58
-60 -55
-59 58 82
b
For G conformer. Difference between SiCC angles for A and G conformers. c Scale factor for short camera data set.
Figure 2. Electron diffraction molecular scattering intensity and difference (experiment - theory) curves of 1,4-disilabutane. Theoretical data are shown in the regions 0-2 and 34-36 Å-1, for which no experimental data are available.
Figure 4. Dependence of the RG value on the conformational composition of 1,4-disilabutane. The dotted line indicates the threshold of the 95% confidence level according to the Hamilton test.
Figure 3. Radial distribution and difference curves for 1,4-disilabutane. The experimental curve was calculated from the combined molecular intensities with use of theoretical data for the region 0 e s/Å e 2.0 and B ) 0.002 Å2. The vertical lines indicate the positions of distances (some assigned), and their lengths are proportional to the relative areas. The upper two curves show the contributions of the individual conformers.
(corresponding to a Gibbs energy difference of 4.6 ( 0.2 kJ mol-1), according to a Hamilton test at the 95% confidence level.33 1,5-Disilapentane. Theoretical Calculations. Calculations on 1,5-disilapentane were performed with a smaller number of basis sets than was the case for 1,4-disilabutane because of size considerations. Four conformers based on fully staggered arrangements of 1,5-disilapentane can be envisaged and calculations were performed on all four of these structures, which are listed below in order of increasing energy: (1) anti-anti conformer, C2V symmetry (AA) (2) anti-gauche conformer, C1 symmetry (AG)
(3) gauche(+)-gauche(-) conformer, C2 symmetry [G(+)G(-)] (4) gauche(+)-gauche(+) conformer, Cs symmetry [G(+)G(+)] Of the four proposed structures, the first three represent local minima on the potential energy surface, while the G(+)G(+) isomer was shown to be the transition state between the AG and G(+)G(-) conformers [imaginary frequency (a′′ symmetry) of 72i cm-1 (3-21G*/SCF) or 95i cm-1 (6-31G*/SCF)]. The instability of this conformation is attributed to repulsions between the two H3Si groups, which would come quite close in this conformer. The molecular geometries of the three located local minima are discussed in detail below. The results are listed in Table 6. AA conformer, C2V: As was found for both isomers of 1,4disilabutane, the Si-C bond length of the AA conformer proved to be rather insensitive to the adopted theoretical method with the three estimates falling within a range of 0.01 Å. Similarly small variations were also observed for other structural parameters: the C-C bond distance was calculated to be slightly longer at the 3-21G*/SCF level than at more sophisticated levels, while electron correlation serves to lengthen bonds to hydrogen by approximately 0.01 Å. Bond angles and dihedral angles generally fell within ranges of 1°, with the C-C-Si bond angle of 113.2° (6-31G*/MP2) being almost identical with that observed in the A conformer of 1,4-disilabutane at the same level of theory. AG conformer, C1: Calculations on the AG conformer yielded similar trends to those observed for other systems studied: the Si-C bond distance fell over a narrow range, while estimates of the C-C bond distance at the 3-21G*/SCF level appear to be too long. The variation in bond angles with the level of theory was once again found to be small. The values of the two Si-C-C bond angles were found to be dependent upon whether the H3Si group belongs to a G or A type part of the
1,4-Disilabutane and 1,5-Disilapentane
J. Phys. Chem., Vol. 100, No. 22, 1996 9343
TABLE 6: Selected Structural Results for 1,5-Disilapentane: Ab Initio Calculations at Different Levels of theory (re/Å; ∠e/deg)), GED (rg/Å; ∠g/deg), XRD (r/Å; ∠/deg)a 3-21G*/SCF
6-31G*/SCF
6-31G*/MP2
C-C C-Si C-H Si-H ∠CCC ∠CCSi ∠CSiH ∠SiCH ∠HCH ∠SiCCC ∠τ(SiH3)
1.553 1.888 1.088 1.478 111.9 113.8 110.6 109.2 106.5 180 180
1.538 1.844 1.089 1.479 113.1 114.4 110.5 108.5 105.9 180 180
AA conformer 1.535 1.890 1.097 1.488 112.0 113.2 110.4 109.1 106.2 180 180
1.537(2) 1.886(1) 1.140(2)s 1.487(4)s 114.8(7) 114.1(4) 110.5(12)s 107.4(6) 107.2(14)s 180f 180f
C-C C-Si C-H Si-H ∠CCC ∠CCSi ∠CSiH ∠SiCH ∠SiCCC ∠τ(SiH3)
1.553; 1.555 1.890; 1.887 1.085a 1.478a 113.0 113.5; 115.3 110.6a 108.7a 175.6; 64.9 179.6;45.1
1.538; 1.540 1.896; 1.894 1.087a 1.479a 114.1 114.1; 116.7 110.6a 109.2a 176.2; 66.1 179.6;48.1
AG conformer 1.534;1.537 1.892; 1.889 1.097a 1.488a 113.9 112.8; 115.0 110.5a 108.8 175.6; 63.6 179.3;45.7
1.536(2)df; 1.537(2)df 1.889(1)df; 1.886(1)df 1.140(2)s 1.487(4)s 116.2(9)c 112.7(7)c; 115.2(6)c 110.5(12)s 107.7(6);107.1(6) 176.3(17)c; 63.7(22)c 180.0f; 25.3(40)df
C-C C-Si C-H Si-H ∠CCC ∠CCSi ∠CSiH ∠SiCH ∠SiCCC ∠τ(SiH3)
1.556 1.890 1.09 1.478 114.3 115.3 110.7a 108.4a 63.4 49.3
1.540 1.897 1.097 1.479 115.6 116.6 109.4a 107.5a 64.1 51.8
G(+)G(-) conformer 1.537 1.892 1.097 1.488 115.0 115.0 109.7a 108.2a 61.0 49.9
GED
XRD 1.527(2) 1.868(2) 0.96(2) 1.35(2) 113.8(2) 115.2(1) 111.1a 107.8a 106(2) 180.0(1)
1.539(2)df 1.888(1)df 1.140(2)s 1.487(4)s 118.8(7)c 116.8(7)c 110.5(12)s 106.6(6) 60.9(10) 29.5(40)
a Symbols: average, cdifference flexibly constrained, dfdifference fixed, ffixed, ssingle value refined for all conformers. Values given in parantheses are estimated standard deviations, obtained in the least-squares refinement.
TABLE 7: 1,5-Disilapentane: Relative Theoretical and Experimental Conformational Energies and Predicted Gas-Phase Compositions at 293 K (in kJ mol-1) (Energies Given Relative to the Ground-State Conformer (AA)) method 3-21G* 6-31G* 6-31G* GED
SCF SCF MP2
∆E AG
G(+)G(-)
G(+)G(+)
∆E0 AG
G(+)G(-)
G(+)G(+)
% AA
AG
G(+)G(-)
3.33 4.31 3.30
6.60 8.53 6.00
19.23 21.65 19.94
3.78 4.67 3.66a 2.5(3)b
7.73 9.61 7.08a 1.4(5)b
19.85 21.99 20.28a
51.8 61.8 50.0 28(4)
43.9 36.4 44.6 40(5)
4.3 1.8 5.4 32(6)
a Estimation of ∆E based on zero-point energies calculated at the 6-31G*/SCF level. b 3.0(5) (AG) and 1.8(7) [G(+)G(-)] if corrected for 0 entropy effects (see text). The esd’s (2σ) for the mole fractions (and energy differences) are derived from Figure 8 and correspond to the 95% level of confidence.
molecule. The A type Si-C-C-C unit was found to have aC-C-Si angle of 112.8° (6-31G*/MP2), compared to 112.3° for A 1,4-disilabutane at the same level of theory, while a value of 115.0° (6-31G*/MP2) was predicted for the C-C-Si angle in a G type Si-C-C-C skeleton (compared to 115.5° for G 1,4-disilabutane). G(+)G(-), C2: Optimized geometrical parameters for the G(+)G(-) form of 1,5-disilapentane are reported in Table 6. The main structural features, which are not sensitive to the details of the basis set and are not greatly influenced by correlation effects, are very similar to those already described for the other conformers. The C-C-Si bond angle was found to fall in the range 115.0-116.6°. The relative energies of the four conformers were calculated and are reported in Table 7 both with and without corrections for zero-point energy. Changes in theoretical treatment did not change the energy ordering of the four isomers and led to only modest changes in the relative energies. The AA conformer was always found to be lowest in energy while the AG structure was predicted to lie between 3.3 and 4.7 kJ mol-1 higher in
energy. Our best estimate of the energy separation is 3.66 kJ mol-1 (6-31G*/MP2 + ZPE). We do not anticipate that improvements in either the basis set or theoretical treatment will lead to substantial changes in the relative energy of conformers since such improvements were found to have almost no effect on the relative energies of the A and G forms of 1,4disilabutane. The remaining two structures were found to lie 7.1 kJ mol-1 [G(+)G(-)] and 20.3 kJ mol-1 [G(+)G(+)] above the AA conformer at the 6-31G*/MP2 level when the effects of zero-point energy were taken into consideration. Gas-Phase Structure Analysis. The model of 1,5-disilapentane consisted of a mixture of a C2h symmetric AA conformer, a C2 symmetric G(+)G(-) conformer, and an AG conformer of C1 symmetry. The fourth possible conformer, G(+)G(+), was not considered since the calculations predicted that this structure does not correspond to a local minimum on the energy hyperface and is far too high in energy to play a detectable role. As compared with 1,4-disilabutane, 1,5-disilapentane has even more degrees of freedom, and consequently there is even
9344 J. Phys. Chem., Vol. 100, No. 22, 1996
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Figure 6. Electron-diffraction molecular scattering intensity and difference (experiment - theory) curves for 1,5-disilapentane. Theoretical data are shown in the regions 0-2 and 32-36 Å-1, for which no experimental data are available.
Figure 5. Molecular geometries of the three stable conformers of 1,5disilapentane as determined by gas-phase electron diffraction.
greater necessity for adoption of reasonable constraints in the analysis of the diffraction data. All C-H and Si-H bond lengths (and corresponding amplitudes of vibration) within one molecule and for the three conformers were assumed to be constant. Local C3V symmetry was assumed for the CSiH3 units. The CH2 units were placed in idealized positions, i.e., lying on the planes perpendicular to the CCSi and CCC planes. The deviation of the HCH planes from the bisector of the angle ∠(CCSi) defined a parameter, which was initially fixed to the weighted ab initio mean value, but freely refined in the late stages of refinement. A flexible restraint of 106.0, corresponding to the weighted average of calculated values, with an estimated uncertainty of 2°, was applied to the angle ∠(HCH), which subsequently refined to 107.2(14)°. Differences in the bond lengths C-C and C-Si of the three conformers have been fixed to the calculated differences in values. Flexible restraints were also applied to the differences between the angles ∠(CCC) and ∠(CCSi) for the three conformers, with the values of these restraints being the calculated differences and the uncertainties estimated as 1°. The torsion angle ∠(SiCCC) for the AA conformer was fixed at 180°. A flexible restraint for the deviation of the dihedral angle ∠(SiCCC) in the A part of the AG conformer from that in the AA conformer (from 180°, i.e., perfect A conformation) was introduced [deviation: -4.4(20)°]: this parameter then refined to 3.7(17)° [corresponding to an angle ∠(SiCCC) of 176.3(17)°]. The angle ∠(SiCCC) for the G(+)G(-) conformer refined unrestrained to 60.9(10)°. The corresponding difference to the angle ∠(SiCCC) in the AG conformer refined to -2.8(23)° after introducing it to be subject to a further flexible restraint [2.6(30)°], which gives an ∠(SiCCC) angle of 63.7(22)° in the AG conformer. Using these eight flexible restraints and including finally 29 parameters in the refinement allowed
Figure 7. Radial distribution curves for 1,5-disilapentane. The experimental curve was calculated from the combined molecular intensities with use of theoretical data for the region 0 e s/Å e 2.0 and B ) 0.002 Å2. The vertical lines indicate the positions of distances (some assigned), and their lengths are proportional to the relative areas. The upper three curves show the contribution of the individual conformers.
better estimates of uncertainties to be obtained, compared to those refinements in which all differences were set at fixed values. The experimental molecular scattering intensity curve is shown in Figure 6. The radial distribution curve of 1,5disilapentane (Figure 7) shows seven well-distinguished features, corresponding to the C-H, Si-H/C-C, Si-C, Si‚‚‚C, and the Si‚‚‚Si distances of the G(+)G(-), AG, and AA conformers, respectively. The distances r(CH) and r(SiC) are well determined, whereas the unambiguous refinement of the correlated distances r(SiH) and r(CC) was more difficult (see correlation
1,4-Disilabutane and 1,5-Disilapentane
J. Phys. Chem., Vol. 100, No. 22, 1996 9345
TABLE 8: Selected Distances (ra/Å) and Vibrational Amplitudes (ua/Å) for 1,5-Disilapentanea AA conformer
[T]
C-Si
1.884(1)
0.061(2)
1
C-C
1.535(2)
0.051(2)
2
Si-H C-H Si‚‚‚C
1.477(4) 1.136(2) 2.875(6)
0.124(3) 0.068(2) 0.090(3)
3 4 5
C‚‚‚C Si‚‚‚C
2.587(10) 4.290(7)
0.074 0.081(5)
t5 6
Si‚‚‚Si Si‚‚‚H
5.737(13) 2.473(9)
0.126(8) 0.104(21) 10
8 2.481(9)
AG conformer 1.887(1) 1.881(1) 1.534(2) 1.537(2) s s 2.854(11) 2.893(9) 2.608(12) 4.289(9) 3.430(22) 5.186(17) 0.105 2.466(9)
0.053 0.062 0.051 0.0501 s s 0.089 0.087 0.075 0.081 0.181(f) 0.176 t10 0.107
[T] t1 t1 t2 t2 t5 t5 t5 t6 7 t8 2.464(9) t10
G(+)G(-) conformer 1.886(1)
0.060
t1
1.537(2)
0.0501
t2
s s 2.921(11)
s s 0.087
t5
2.647(10)
0.076
t5
3.492(12) 4.398(35) 0.107 2.464(9)
0.158 0.196(24) 9 t10 0.107
t7 t10
[T] ) tying scheme indicating which amplitudes are independently refining or tied together, f ) fixed, s ) single value refined for all conformers. Values given in parentheses are estimated standard deviations, obtained in the least-squares refinement. a
Figure 8. Dependence of the RG value on the conformational composition of 1,5-disilapentane. The contour plot is based on 143 refinements, which were performed with variation of the mole fractions in steps of 2%. The local minimum at X(AA) ) 40 and X(AG) ) 36% does not correspond to physically meaningful structures of the three conformers. The bold line at RG ) 6.85% corresponds to the 95% level of confidence according to the Hamilton test.
matrix in Table 9) but finally achieved. The refinement converged at RG ) 6.68% (7.85 and 5.97% for the long and short camera distance data, respectively). The optimum molecular geometries of the three conformers of 1,5-disilapentane are shown in Figure 5, geometrical parameters (rg) are given in Table 6, and intermolecular distances and vibrational amplitudes are listed in Table 8. Figure 7 also contains information about the theoretical radial distribution curves of the three conformers of 1,5-disilapentane. The characteristic peaks for each conformer are those related to the Si‚‚‚Si (over four bonds) and the Si‚‚‚C (over three bonds) distances. These distances all appear in the experimental curve, indicating that all three conformers are present in comparable amounts. The composition was determined by exploring the variations in RG over a matrix of different mole fractions (X) of AA and AG conformers [X of the G(+)G(-) conformer is the difference to 100%]. The result is shown in Figure 8, with the range of the 95% confidence level (Hamilton test) being at RG ) 6.85%. The obtained ratio is 28(3):40(4):32(5)% for AA, AG, and G(+)G(-) conformers, respectively. Crystal Structure. Numerous attempts were made to obtain a single crystal of 1,4-disilabutane, but all were unsuccessful. However, a single crystal of 1,5-disilapentane could be grown in situ in a capillary. The material showed a melting point of 163(2) K. 1,5-Disilapentane crystallizes in its AA form, which
Figure 9. Packing of molecules of 1,5-disilapentane in the crystal. View along the b axis.
is also an indication that this is probably the ground-state conformation. The crystal belongs to the monoclinic system, space group C2/c, with a 2-fold symmetry axis passing through the central carbon atom. The torsion angles ∠(SiCCC) are -180.0(1)°. The molecules are all aligned parallel, as can be seen in the packing plot given in Figure 9. This is similar to the packing for the higher paraffins,34 but unlike that for the analogous n-pentane (space group Pbcn),35 which crystallizes in layers of parallel molecules but with an angle between the chain vectors of different layers. The closest intermolecular non-H atom distance is 3.97 Å for the Si‚‚‚Si contact, which is almost exactly twice the van der Waals radius of silicon (4.0 Å). Other structural parameters are listed in Table 6 with the theoretical and gas-phase data for comparison. They are discussed in context in the following section. Discussion 1,4-Disilabutane. The results from the GED data are based on the measurement of 20 refining parameters, which is more than would ordinarily be possible. This was achieved by making extensive use of constraints derived from high level ab initio calculations,43 with four of them used as flexible constraints to support the refinement where a lack of information would otherwise make it necessary to fix the parameter to values of unknown reliability. This leads to much more realistic estimated standard deviations, particularly for strongly correlated parameters, but nevertheless care is suggested in the interpretation of the listed parameters. However, there is an encouraging consistency between theory and experiment in most of the structural parameters for 1,4-
9346 J. Phys. Chem., Vol. 100, No. 22, 1996
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TABLE 9: Correlation Martrix (×100) for Parameters of 1,5-Disilapentane (Only Elements with Absolute Values Greater Than 50 Are Shown) r(C-C)
∠CCC
∠(CSiH)
∠(CCSiH)a
u(C-H)
u(Si-C)
56 56
70
65
90
u(C-C)
-59
r(Si-H) ∠(HCH) ∆∠(SiCCC)b ∠(SiCCC)c ∆∠(CCC)d ∆[CC(H2)]e u(Si-H) u(Si-C) u(C-C) u(Si‚‚‚C) k2f
-66 -67 -59 -58 68
-58
53 79
a
Torsion angle CCSiH for G(+)G(-) conformer. b Deviation of the torsion angle SiCCC in the A part of the AG conformer from 180°. c Torsion angle SiCCC for the G(+)G(-) conformer. d Difference between the ∠(CCC) angles in conformers AA and G(+)G(-). e Deviation of the CHH plane from the bisector of the ∠(CCSi) angle. f Scale factor for short camera distance.
TABLE 10: Structural and Composition Parameters of Conformational Mixtures for Selected Compounds X-CH2-CH2-X X SiH3 CH3 I Br Cl F
rg rg ra ra ra ra
TABLE 11: Structural and Composition Parameters of Conformational Mixtures for Selected Compounds X-CH2-CH2-CH2-X
r(CC)
τ(XCCX)c
X(G)/%
ref
1.563(5)a 1.531(2)b 1.479(33)b 1.506(12)b 1.510(6)b 1.504(3)b
78.5(21) 72.4(48) 79(16) 73.0(46) 73.4(31) 71.5(2)
22(2) 36(7) 12(12) 4.9(18) 18.6(38) 94.4(38)
this work 2 37 38 39 40
X/%
a A conformer. b Average value for both conformers. c For the G conformer.
disilabutane. Deviations occur for the C-C bond length, which is determined to be 1.563(5) Å by GED but calculated to be 1.546 Å. This is long for a C-C bond but might be explained by two electropositive SiH3 groups bound at each end. Table 10 shows a marked correlation between the C-C distance and electronegativity of substituents for a series of compounds X-CH2-CH2-X (with X ) F, Cl, Br, CH3, SiH3). The difference in C-C bond lengths between 1,4-disilabutane (GED) and 1,4dibromo-1,4-disilabutane (1.541(8) Å solid-state XRD35) is just within 3 esd’s, but the bromo derivative has a markedly shorter Si-C bond [1.853(4) Å] than the parent compound [1.882(1) Å]. Another parameter which deserves some comment is the torsion angle ∠(Si-C-C-Si) for the G conformer, which has been predicted by theory to be 68.0° but determined by GED to be 78.5(21)°. Comparison with other experimental values for 1,2-derivatives of ethane (Table 10) leads to the conclusion that values greater than the predicted 68.0° are not unexpected. The energy difference between the conformers in 1,4disilabutane predicted by ab initio calculations is about 20% (5.5 kJ mol-1 at the MP2/6-31G* level) higher than estimated from the GED data [4.6(2) kJ mol-1]. Variation in the level of theory shows the value calculated at the CCSD(T)/6-31G* level to be closest to the experiment (Table 3). These values are based on eq 1, with the vibrational partition functions Qvib,A
nG/nA ) (MG/MA)(Qvib,G/Qvib,A)exp(-∆E/RT)
(1)
and Qvib,G assumed to be equal and so neglecting entropy effects. nA and nG are the mole fractions of A and G conformer, Ma and Mb are the multiplicities of each conformer, ∆E is the difference in energies between the two conformers, R is the gas constant and T the temperature. The effect of neglecting the partition functions has been estimated by deriving estimates of them from the harmonic frequencies calculated at the MP2/6-31G* level of theory. Under these conditions, the term (Qvib,G/Qvib,A) has been calculated to be 0.944 and if that is taken into account in the
ra(CC)
X SiH3 CH3 Br Cl F a
AA a
AG
1.535(2) 32(5) 42(5) 1.531(2)b 1.527(5)b 3(2) 30(2) 1.529(4)b 3(3) 24(2) 1.515(3)b 0 27(2)
G(+)G(-) G(+)G(+) 26(5) 67(2) 73(2) 63(4)
0 0 0 0 10(5)
ref this work 3 41 42 43
For AA conformer. b Single value refined for all conformers.
conversion of experimental composition into relative energies, an experimental energy difference of 4.4 kJ mol-1 results, being only slightly lower than the uncorrected one. 1,5-Disilapentane. The refinement on the GED data for 1,5disilapentane was performed using 29 parameters with 8 of them subject to flexible restraints. The Si-C bond length of the AA conformer of 1,5-disilapentane was determined to be slightly longer in the gas phase [1.886(1) Å] (ab initio calculated value 1.890 Å) than in the solid state [1.868(2) Å]. The corresponding values for the C-C distances are much closer [GED 1.537(2), ab initio 1.535, XRD 1.527(2) Å]. In contrast to 1,4disilabutane, the lengthening of the C-C bond is less pronounced in this compound, if compared with the corresponding R,ω-dichloroalkanes (Table 11), presumably because there is only one SiH3 or Cl group adjacent to each C-C bond. The angles ∠(CCC) and ∠(CCSi) in the solid state [113.8(2) and 115.2(1)°] are in reasonable agreement with those of the undistorted molecule in the gas phase [114.8(7) and 114.1(4)°]. This indicates the absence of any dominant packing forces and is consistent with the observation that all intermolecular contacts between the molecules in the crystal are larger than or comparable with the corresponding van der Waals radii. Both parameters are predicted by theory to have slightly lower values. The torsion angles ∠(SiCCC) of all three conformers refine to values which are in excellent agreement to the theoretical predictions. However, this discussion should focus on the mean values of A and G parts of the molecules only, as the refinements of the differences were supported by theoretical derived flexible constraints. The quality of agreement is unchanged by this restricted view. The greatest amount of uncertainty about the conformational mixture of 1,5-disilapentane arises from the conformational composition parameters. The complexity of the system does not allow these ratios to be determined to a high degree of accuracy, and subsequently all conclusions must be tentative. However, the composition at 293 K suggests that the order of stability of the conformers is AA, G(+)G(-), AG, but the esd’s are so large that G(+)G(-) and AG cannot be separated. This
1,4-Disilabutane and 1,5-Disilapentane is surprising at first glance, but may be understood in the context of the conformational vapour compositions of 1,3-dibromo, 1,3dichloro, and 1,3-difluoro derivatives of propane, which are strongly dominated by the G(+)G(-) conformers (Table 11). For 1,3-difluoropropane even the G(+)G(+) conformer is present to a significant extent in the mixture. This has been called the “gauche effect”9 and attributed to the high polarity of the C-X bonds. Since the polarity of the Si-C bonds is the inverse of X-C bonds (X ) F, Cl, Br), a different conformational behavior is expected. However, the data do not show the presence of an “inverse gauche effect” but indicate a more complicated picture. On the other hand, the gauche effect should be most pronounced in the case of the 1,2-dihalogenoethanes, which is clearly the case for 1,2-difluoroethane. For the 1,2-dichloro and 1,2-dibromo derivatives, however, the A conformer is the more stable. It is therefore surprising that G conformations are preferred in the case of the corresponding derivatives of propane, but these findings have been explained by empirical rules derived for halogen-substituted alkanes.36 Despite the close similarity of the geometries for 1,5disilapentane derived from GED analysis and ab initio computations, the experimental relative energies are not supported by the ab initio calculations. The order of relative stabilities has been calculated to be clearly AA, AG, G(+)G(-), confirming AA to be the energetic ground-state conformation, but leaving some doubt about the relative energies of AG and G(+)G(-) conformers (see Table 7). Correcting the experimentally derived energy differences for entropy effects by taking the terms (Qvib,AG/Qvib,AA) and (Qvib,G(+)G(-)/Qvib,AA) (estimated from the calculated harmonic frequencies) into account leads to an even bigger difference between theory and experiment, with the experimental energy differences being 2.8 (AG) and 1.0 kJ mol-1 [G(+)G(-)] vs the AA conformer. Further work, both theoretical and experimental will be necessary, to clarify this point. Acknowledgment. This work was supported by the EPSRC (Grant GR/K04194) and the European Union (Human Capital and Mobility fellowship for N.W.M.). The Daresbury Laboratory is thanked for tracing the electron-diffraction plates. Supporting Information Available: Crystal and refinement data table, list of atomic coordinates and anisotropic displacement parameters, full list of bond lengths and angles, for the crystal structure determination of 1,5-disilapentane and full correlation matrices for gas-phase electron diffraction refinements (4 pages). Ordering information is given on any current masthead page. References and Notes (1) Elliel, E. L.; Allinger, N. L.; Angyal, S. J.; Morrison, G. A. Conformational Analysis; Interscience Publishers: New York, 1967. (2) Heenan, R. K; Bartell, L. S. J. Chem. Phys. 1983, 78, 1270. (3) Bonham, R. A.; Bartell, L. S.; Kohl, D. A. J. Am. Chem. Soc. 1959, 81, 4765.
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