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Microwave Spectrum, r0 Structure, Dipole Moment, Barrier to Internal Rotation, and Ab Initio Calculations for Fluoromethylsilane James R. Durig,* Savitha S. Panikar, Peter Groner, and Hossein Nanaie† Department of Chemistry, UniVersity of Missouri - Kansas City, Kansas City, Missouri 64110-2499
Hans Bu¨rger and Peter Moritz Anorganische Chemie, Bergische UniVersita¨t, Wuppertal, 42097 Wuppertal, Germany ReceiVed: December 7, 2009; ReVised Manuscript ReceiVed: January 30, 2010
The microwave spectra of seven isotopomers of fluoromethylsilane, CH2FSiH3, in the ground vibrational state were measured and analyzed in the frequency range 18-40 GHz. The rotational and centrifugal distortion constants were evaluated by the least-squares treatment of the observed frequencies of a- and b-type R- and b-type Q-transitions. The values for the components of the dipole moment were obtained from the measurements of Stark effects from both a- and b-type transitions and the determined values are: |µa| ) 1.041(5), |µb| ) 1.311(6), and |µt| ) 1.674(4) D. Structural parameters have been determined and the heavy atom distances (r0) in Angstroms are: Si-C ) 1.8942(57) and C-F ) 1.4035(55) and the angle in degree, ∠SiCF ) 109.58(14). A semi-experimental re structure was also determined from experimental ground state rotational constants and vibration-rotation constants derived from ab initio force fields. The internal torsional fundamental, SiH3, was observed at 149.2 cm-1 with two accompanying hot bands at 138.8 and 127.5 cm-1. The barrier to internal rotation was obtained as 717.3(16) cm-1 (2.051(46) kcal mol-1) by combining the analysis of the microwave A and E splittings and the torsional fundamental and hot band frequencies. Ab initio calculations have been carried out with full electron correlation by the second-order perturbation method with several different basis sets up to MP2/6-311+G(d,p) to obtain geometrical parameters, barriers to internal rotation, and centrifugal distortion constants. Adjusted r0 structural parameters have been obtained by combining the ab initio MP2/6-311+G(d,p) predicted values with the determined rotational constants for the fluoride as well as with the previously reported microwave data for the chloro- and bromo- compounds. These experimental results are compared to the corresponding parameters for the carbon analogues. Introduction The microwave spectrum and the molecular structure of methylfluorosilane, CH3SiH2F, and other methylhalosilanes have been extensively studied.1–4 It has been shown that, on the basis of the rs structure, the methyl axis of the methylfluorosilane molecule is distorted from C3V symmetry by tilting toward the hydrogen atoms of the fluorosilyl group.3 However, the structures of halomethylsilanes, CH2XSiH3, have not been as extensively investigated. Schwendeman and Jacobs5 have studied the microwave spectrum of chloromethylsilane and reported a partial rs structure and the barrier to internal rotation of the SiH3 group of 2.55 kcal mol-1. However, the expected distortion of the SiH3 axis was not investigated. More recently, the microwave spectrum of bromomethylsilane was analyzed by Hayashi and Kuwada6 and a plausible structure, based on the rotational constants for four isotopic molecules, was provided. The synthesis of fluoromethylsilane by Bu¨rger and Moritz7 has made the structural analysis of this molecule by microwave and infrared spectroscopy possible. Therefore, we have undertaken this study to provide the structural parameters of this molecule and to compare them with those of chloro- and bromomethylsilane. Ab initio molecular orbital calculations are * To whom correspondence should be addressed. Phone: 01 816-2356038. Fax: 01 816-235-2290. E-mail:
[email protected]. † Present Address: Department of Physical Science, Claflin University, Orangeburg, SC. 29115.
also reported for all three of the halomethylsilanes, and the optimized structure and the dipole moment components are compared with the experimental results. Experimental Section Fluoromethylsilane and its isotopomers were synthesized by the method previously described.7 The sample of CH2FSiH3 enriched to 34% in 13C was synthesized from 13CBr4, which was diluted 1:2 with natural CBr4. This compound was converted to CFBr3 with SbF3, which was then transformed into CH2FSiH3 as previously reported.8 The partly deuterated isotopologues of CH2FSiH3 were obtained by unselective reduction of the precursor CFBr2SiCl38 with a mixture of (n-C4H9)3SnH and (n-C4H9)3SnD. The samples were stored in a sealed sample tube immersed in liquid nitrogen. The microwave spectra for the seven isotopomers of fluoromethylsilane were recorded in the frequency range 18.0-39.5 GHz on a Hewlett-Packard model 8460A MRR spectrometer with a Stark modulation of 33.33 kHz. The Stark cells were packed in dry ice during the spectral measurements. The frequency for each transition was obtained by averaging the forward and the reverse scans over the band peak. The precision for the measurements was estimated to be better than 0.05 MHz. The far infrared spectrum of gaseous fluoromethylsilane was recorded in the region from 80 to 350 cm-1 with a Bruker 120 HR vacuum interferometer equipped with a Si bolometer and a 6 µm Mylar beam splitter. The resolution was adjusted to 0.05
10.1021/jp911614d 2010 American Chemical Society Published on Web 03/03/2010
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cm-1. A cell measuring 28 cm and fitted with polyethylene windows was employed, and a pressure path length of 0.107 m bar was used. Microwave Spectra The microwave spectra of CH2FSiH3 and its isotopomers were predicted with the rotational constants of the structure optimized by MP2/6-31G(d) ab initio molecular orbital calculations (see Ab Initio Calculations section). According to the results of this theoretical analysis, CH2FSiH3 is a nearly symmetric prolate rotor (κ ) -0.96), and the microwave spectrum was expected to consist of a- and b-type transitions. These predictions led subsequently to straightforward assignments of the rotational transitions of the normal isotopomer. The b-type Q-branch series J1,J-1 r J0J contained the most prominent bands with fully developed Stark modulation at 2000 V/cm. This series exhibited internal rotation splittings into two equally intense bands with small separations. Numerous a- and b-type R-transitions with intermediate intensities were also identified and assigned. Somewhat larger internal rotation splittings were observed for a number of b-type R-transitions. The spectrum of the sample with the partially deuterated silyl group was used to identify transitions belonging to the Si-d0, Si-d1 and Si-d2 species. The Si-d3 species was either absent or present in such a low abundance that the measurement of its spectrum was not practical. The spectral assignments of the Si-d1 and Si-d2 isotopomers were guided by the strong J1,J-1 r J0J transitions and their well developed Stark effects. These assignments were followed by the identification and assignment of a-type transitions. Eventually, some b-type R-transitions of the d1 species were identified on the basis of the previous assignments. No internal rotation splittings were observed for any of the partially deuterated species. Neither were the expected splittings of the lines of 13CH2FSiH3 resolved presumably because of the low concentration of the 13C species. A complete listing of the measured and assigned frequencies of the normal isotopomer of CH2FSiH3, including the resolved internal rotation splittings, is presented in Table 1. A rotationinternal rotation Hamiltonian9 similar to the IAM method implemented by Woods10 was fitted simultaneously to the observed frequencies in Table 1 and to far infrared torsional transition frequencies (see Internal Rotation section). Since splittings generally were so small, frequencies of transitions with unresolved internal rotation splitting were used in the fit for the E components only. The residuals of the least-squares fit are also shown in Table 1. The standard Hamiltonian for asymmetric rotors with centrifugal distortion (A-reduction, Ir representation11) was used to fit the rotational spectra of the other isotopomers for which observed frequencies, assignments and residuals are listed in Table 2. The resulting rotational and centrifugal distortion constants for all isotopomers are listed in Table 3. The additional parameters of the rotation-internal rotation Hamiltonian for CH2FSiH3 are presented in the next section. Because the number of measured transitions of most isotopomers was insufficient to allow reliable determination of all quartic centrifugal distortion constants, the parameters ∆K (for two isotopomers) and δK (for all isotopomers) were kept constant at the values predicted by a vibrational force field derived from ab initio calculations (see Ab Initio Calculations section). Internal Rotation. The barrier to internal rotation of the silyl group was determined from a global fit to all observed rotational transition frequencies and to the torsional transition frequencies observed in the far infrared spectrum of gaseous CH2FSiH3
Durig et al. TABLE 1: Rotational Transition Frequencies (MHz) for CH2FSiH3 transition 212 r 111 202 r 101 132,11 r 123,10 211 r 110 142,13 r 133,10 183,15 r 174,14 110 r 101 211 r 202 836 r 927 312 r 303 413 r 404 514 r 505 615 r 606 505 r 414 313 r 212 152,14 r 143,11 303 r 202 322 r 221 321 r 202 716 r 707 918 r 827 312 r 211 817 r 808 193,16 r 184,15 111 r 000 735 r 826 918 r 909 161,15 r 153,12 1019 r 100,10 414 r 313 606 r 515 404 r 303 432 r 331 431 r 330 422 r 321 413 r 312 a
νA
∆νa
19 713.96
0.01
19 958.97 20 996.62
-0.15 -0.05
23 319.00
-0.01
27 334.16
-0.05
28 489.78
-0.06
29 148.99 29 319.64
-0.02 0.02
31 310.77 31 713.13
-0.02 0.01
33 525.99
-0.05
36 753.85 36 789.28
0.07 -0.09
νE
∆νa
18 850.20 19 284.67 19 714.37 19 732.45 19 959.37 20 996.92 22 568.16 23 015.97 23 318.60 23 699.84 24 634.18 25 837.97 27 334.09 27 872.22 28 271.04 28 490.15 28 910.41 28 937.43 28 963.49 29 148.88 29 319.95 29 594.24 31 310.66 31 713.44 31 772.67 33 525.64 33 848.36 36 754.20 36 789.18 37 686.80 38 419.48 38 516.31 38 597.25 38 597.87 38 642.74 39 450.67
-0.02 0.00 0.05 -0.01 -0.10 -0.02 0.02 0.04 -0.03 0.06 0.05 0.06 0.01 -0.04 0.00 -0.02 0.00 0.05 0.03 0.00 0.04 -0.02 0.00 0.05 0.02 -0.01 0.05 0.10 -0.06 0.00 -0.02 0.00 0.22b 0.72b -0.01 0.01
∆ν ) νobs - νcalc. b Not included in the least-squares fit.
(Figure 1). The three pronounced bands at 149.2, 138.8, and 127.5 cm-1 were assigned to the V ) 1 r 0, 2 r 1, and 3 r 2 torsional transitions. The first two were included in the global fit and the results are shown in Table 4. The third torsional transition could not be fit because the fitting program used calculates torsional transitions and splittings only for the lowest three torsional states, e.g., V ) 0, 1, and 2. The resulting internal rotation parameters are: barrier V3 ) 717.3(16) cm-1, moment of inertia of top about its axis Iτ ) 6.13(23) uÅ2, angle between principal inertial axis a and internal rotation axis β ) 21.8(49)°. The relative standard error for β is quite large (22%) and only a little smaller for Iτ and the dependent parameter F (3.8% and 7.2%, respectively) because the resolved internal rotation splittings are all very small. The relative standard error for V3 (0.2%) is quite small because the addition of two torsional transition frequencies to the least-squares fit of internal rotation splittings leads to much lower standard errors for the barrier and Iτ and significantly reduced correlation between the fitted parameters. The residual reported in Table 4 for the 3 r 2 transition was calculated with a different program. Molecular Structure. The experimental rotational constants listed in Table 3 were used to determine the molecular structure. The r0 structure was obtained by a fit of the structural parameters directly to the observed ground state rotational constants.12 Each rotational constant was given a weight equal to the inverse square of its standard error. An exception was made for CH2FSiH2D(s), where the larger standard errors of
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TABLE 2: Rotational Transition Frequencies (MHz) of the Ground Vibrational State of CH2FSiH3 12
transition
ν
212 r111 18 360.21 202 r101 18 757.42 211 r110 313 r212 27 536.11 303 r202 28 120.11 322 r221 321 r220 312 r211 28 746.63 414 r313 404 r303 37 463.87 423 r322 37 522.34b 432 r331 422 r321 37 584.62 413 r312 38 320.75 111 r000 28 709.99 19 731.80 110 r101 211 r202 20 141.69 20 768.20 312 r303 413 r404 21 625.20 514 r505 22 730.71 615 r606 24 106.61 716 r707 25 777.82 817 r808 27 771.09 918 r909 30 113.37 1019 r100,10 32 829.63 111,10 r110,11 35 940.85 121,11 r120,12 404 r313 19 116.93 505 r414 29 192.23 606 r515 131,13 r122,10 826 r735 142,13 r133,10 30 486.64 142,12 r133,11 152,14 r143,11 38 795.47 918 r827 34 568.59 a
12
CH2FSiH2D(a) ∆ν
a
12
CH2FSiH2D(s) ν
a
∆ν
12
CHDFSiH3
ν
a
∆ν
CH2FSiHD2(a) ν
∆ν
12
CH2FSiHD2(s)
13
a
ν
∆νa
18 741.50 19 190.73 19 654.57 28 107.56 28 767.90 28 797.40 28 826.16 29 477.11 37 468.11 38 323.24 38 390.58 38 411.81
0.01 0.01 -0.02 -0.02 0.02 -0.04 0.00 0.04 -0.06 -0.06 0.04 0.13
31 129.11 21 986.69 22 450.59 23 159.77
-0.06 -0.04 -0.01 -0.02
a
ν
∆ν
0.08 0.05 0.05 -0.01
27 758.32 28 442.32
-0.05 -0.05
-0.04
29 185.93 37 000.66 37 881.61
0.00 0.07 -0.06
38 051.01 38 903.36
0.03 -0.01
0.03 0.16 0.00 -0.06 -0.04 0.04 0.00 -0.03 -0.01 0.01 0.00 -0.01 0.00 0.04 0.02 -0.03
36 753.96 36 817.18
0.00 0.00
21 480.22 21 926.77 22 609.53 23 543.13 24 747.75 26 246.99 28 068.24 30 240.58 32 793.48 35 754.30 39 145.99
-0.01 -0.06 0.08 -0.03 0.04 -0.02 0.01 -0.01 0.00 -0.02 0.01
36 840.97
0.00
-0.05 0.00
-0.01
20 044.03 20 787.54 21 809.27 23 134.31 24 792.85 26 818.50 29 246.28 32 109.33 35 435.98 39 245.73
0.04 -0.01 0.02 0.02 0.01 -0.02 0.01 -0.03 0.00 0.01
30 234.75
0.04
34 518.97
0.00
31 753.68
0.00
35 029.66
18 794.42 19 209.93 19 845.77 20 716.79 21 842.44 23 246.24 24 954.67 26 995.93 29 397.91 32 185.99
29 084.28
0.00
0.00 -0.01 0.01 0.01 -0.02 0.00 0.01 0.01 -0.01 0.00
35 845.43 36 486.94
0.09 -0.05
36 584.66
-0.04
CH2FSiH3
18 286.29 19 009.08
-0.01 0.01
21 097.83 22 502.00 24 174.68 26 138.28 28 413.97 31 019.95 33 969.48
-0.03 0.01 0.00 -0.01 0.00 0.03 -0.02
26 942.11 28 837.22 31 098.53 33 756.69 36 840.07
0.11 0.11 0.08 -0.01 -0.20
29 972.95
0.00
28 284.48 38 803.97
0.05 -0.07
30 853.83
-0.01
34 109.64 30 275.83 30 978.25
0.05 -0.03 0.00
0.00
0.00 0.02
∆ν ) νobs- νcalc. b Not included in the least-squares fit.
TABLE 3: Experimental Rotational and Centrifugal Distortion Constants in the Ground State of CH2FSiH3 Isotopomersa A (MHz) B (MHz) C (MHz) ∆J (kHz) ∆JK (kHz) ∆K (kHz) δJ (kHz) δK (kHz) sb (kHz) nc
CH2FSiH3
CH2FSiH2D(s)
CH2FSiH2D(a)
CH2FSiHD2(s)
CH2FSiHD2(a)
CHDFSiH3
27 170.625(14) 5043.4148(24) 4602.2605(24) 4.670(25) -35.06(41) 248.2(11) 0.6998(37) 2.342d 45 47
25 863.319(28) 4822.700(19) 4382.937(12) 1.93(33) -13.5(19) 173.058d 0.5970(83) 3.221d 42 14
24 221.083(19) 4892.7484(24) 4489.1422(25) 5.013(36) -23.92(64) 228.9(49) 0.6922(51) 0.159d 39 25
21 807.745(41) 4737.922(31) 4396.723(24) 3.92(73) -15.2(21) 110.26d 0.6041(73) -13.386d 44 13
23 072.865(67) 4687.097(86) 4278.345(34) 6.2(11) -12(14) 117.36d 0.564(39) 1.770d 12 12
24 068.034(23) 4984.3284(33) 4508.3760(25) 5.613(23) -22.80(99) 500(97) 0.7137(55) 3.697d 38 20
13
CH2FSiH3
26 558.056(32) 5027.7982(48) 4571.2189(38) 4.124(49) -34.85(72) 169.5(23) 0.6561(69) 2.728d 74 27
a Uncertainties in parentheses are standard errors in units of last digit. b Standard deviation. c Number of frequencies fitted. d Constant, from ab initio calculations (MP2/6-31G(d)).
CH2FSiHD2(s) were used because the standard errors seemed to be too low. The initial diagnostic least-squares method13 turned into a regular nonlinear least-squares procedure after a few iterations. For an approximate semi-experimental re structure, equilibrium rotational constants were derived from the ground state constants by applying corrections for the vibration-rotation interactions.14 For that purpose, vibration-rotation constants were derived from quadratic and cubic force fields obtained by MP2/6-31G(d) ab initio calculations, applying a single scaling factor of 0.95 to all quadratic force constants. The parameters of the re structure were fit to the resulting equilibrium rotational
constants by the same nonlinear least-squares procedure. The parameters of the r0 and semi-experimental re structure are shown in Table 5 in comparison with ab initio structures. The equilibrium rotational constants and their residuals in the leastsquares fit are listed in Table 6. If the axis of the internal rotor is defined as the direction that forms equal angles with each of the Si-H bond directions, this internal rotor axis intersects the C-Si direction at an angle of 1.28(9)° (re) or 1.36(21)° (r0), tilting the SiH3 group away from the F atom. According to the structure analysis, the C-Si bond direction is at an angle of 24.1° with respect to the a principal inertial axis. Therefore, the internal rotation axis forms an angle
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Durig et al. sets 6-31G(d), 6-311G(d), and 6-311+G(d,p). The results are shown in Table 5 in comparison with the experimental r0 and semi-experimental re structures. The centrifugal distortion constants and the vibration-rotation constants were derived from quadratic and cubic force fields determined in MP2(full)/ 6-31G(d) calculations for the structure optimized by the same method and with the same basis set. Discussion
Figure 1. Far infrared spectrum of CH2FSiH3, 0.107 m bar, resolution 0.05 cm-1. Asterisks denote rotational lines of HCl present in trace.
TABLE 4: Observed Torsional Transitions, Barrier to Internal Rotation, V3, Moment of Inertia of the Internal Rotor, Iτ, Angle between Internal Rotation Axis and a Principal Axis, β, and Internal Rotation Constant, Fa transition
νobs (cm-1)
νobs - νcalc (cm-1)
1r0 2r1 3 r 2b V3 (cm-1) Iτ (uÅ2) β (°) F (cm-1)
149.2 138.8 127.5 717.3(16) 6.13(23) 21.8(49) 3.89(28)
0.07 -0.06 0.3
a Uncertainties in parentheses are standard errors in units of last digit. b Not used in fit because of limitations of the fitting program; νobs - νcalc was obtained with a different program that calculates torsional transition frequencies from F and the potential constants.
of 22.8° with the a axis. This angle agrees well with the angle of the internal rotation axis obtained during the internal rotation analysis of 21.8(49)°, which unfortunately has quite large error limits, rendering the agreement somewhat fortuitous. Electric Dipole Moment. The electric dipole moment and its components along the principal axes were determined from the quadratic Stark effects of the transitions 111 r 000, 313 r 212, 413 r 312, 505 r 414, and 817 r 808. The squares of the dipole moment components were calculated by a linear least-squares fit of the frequencies of the observed Stark components as a function of the square of the applied electric field. The Stark coefficients were obtained by second-order perturbation theory from the observed rotational constants. The electric field was calibrated by the Stark effect of the 3 r 2 transition of OCS, which has a dipole moment of 0.71619 D.15 The Stark coefficients ∆ν/E2 of a number of arbitrarily selected Stark components and the resulting dipole moment are listed in Table 7. Ab Initio Calculations The LCAO-MO calculations were performed with the Gaussian-98 program.16 The gradient method of Pulay17 was used to determine the energy minima by simultaneous relaxation of all structural parameters compatible with the point group Cs of the molecular structure. Initially, the structure of CH2FSiH3 was defined by using the parameters of methylfluorosilane and fluoroethane.4 It was subsequently optimized in restricted Hartree-Fock calculations with the 6-31G(d) basis set and in calculations with electron correlation by the perturbation method to second order according to Møller-Plesset18 with the basis
The r0 structure of fluoromethylsilane has been determined from 21 rotational constants without utilizing any constraints. The structural values are reasonable and agree within the error limits with the structural parameters of other molecules which either contain a silyl group or a fluoromethyl group. However, comparing the rs value of 1.387 Å for the C-F bond distance in fluoroethane4 with that obtained for the CH2FSiH3 molecule in this study (1.403 Å, r0), we find a relatively large difference of 0.016 Å. However, this discrepancy almost disappears when the r0 distance for this bond in fluoroethane is evaluated with our program from the reported4 rotational constants for 20 isotopic species. The resulting distance of 1.400(4) Å agrees well with the value of the C-F distance for fluoromethylsilane. The r0 value for the C-H bond distance is shorter for the C-H bonds in CH2FSiH3 by 0.003 Å than the corresponding bonds in fluoroethane. The HCF angle of 106.82(13)° is smaller in fluoroethane4 than the corresponding angle of 108.21(39)° in fluoromethylsilane, but the difference is only twice as large as the combined standard errors. A similar difference is found for the HCH angle (108.88(8)° in FCH2CH3, compared to 107.33(23)° in CH2FSiH3). Also of interest is the effect of the fluorine atom on the C-Si bond distance in comparison with other halogen substituents. In the microwave study5 of CH2ClSiH3, the reported structural parameters had three different values where the most probable value for the C-Si distance (rs) was 1.889 Å but with an uncertainty of 0.010 Å. Similar uncertainties were also reported for the C-Cl and C-H distances where the uncertainties for the two heavy atom distances arose from the problem of small principal coordinates because the “a” coordinate of C consistently gave imaginary results. The reported structure I was obtained by simply setting the a-coordinate to zero. Structure II was obtained by invoking the center of mass condition by moving the CH2 triangle. These two methods lead to Si-C distances of 1.894 Å for structure I and 1.884 Å for structure II. The opposite effect was observed for the C-Cl distance with values of 1.783 Å for structure I and 1.793 Å for structure II. The “best” structure was obtained by taking the averages of the two values and assuming that the difference of 0.010 Å was the uncertainty (Table 8). The reported structural parameters for CH2BrSiH36 were concluded to be the same as those reported for CH2ClSiH3, except for the C-Br distance which was assumed to be the same as in ethylbromide (Table 8). However, by combining the values of the reported six rotational constants for the 79Br and 81Br isotopomers of CH2BrSiH3 with the predicted parameters from ab initio MP2(full)/6-311+G(d,p) calculations, it is possible to obtain all structural parameters of this molecule with relatively small uncertainties. We have shown19 that ab initio MP2(full)/6-311+G(d,p) calculations predict the r0 structural parameters for more than 50 carbonhydrogen distances to better than 0.002 Å compared to the experimentally determined values from isolated C-H stretching frequencies,20 which agree with the values from earlier microwave studies. Therefore, the carbon-hydrogen distances can be taken from the MP2/6-311+G(d,p) predicted values for the halomethylsilanes.
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TABLE 5: Experimental and Ab Initio Structural Parameters of CH2FSiH3a r(C-Si) r(C-F) r(H4Si) r(H5Si) ) r(H6Si) r(H7C) ) r(H8C) R(FCSi) R(H4SiC) R(H5SiC) ) R(H6SiC) R(H7CSi) ) R(H8CSi) R(H5SiH6) R(H4SiH5) ) R(H4SiH6) R(H7CH8) R(H7CF) ) R(H8CF) τ(H5SiCH4) ) -τ(H6SiCH4) τ(H7CSiF)) -τ(H8CSiF) b
MP2/ 6-31G(d)
MP2/ 6-311G(d,p)
MP2/ 6-311+G(d,p)
r0
reb
1.8977 1.4107 1.4871 1.4820 1.0948 107.64 109.15 109.25 112.58 109.71 109.73 107.87 107.99 119.99 118.90
1.8948 1.4000 1.4769 1.4732 1.0940 109.08 108.46 109.42 111.84 109.85 109.83 107.71 108.12 119.80 119.55
1.8974 1.4061 1.4770 1.4728 1.0938 109.43 107.68 109.50 112.09 110.24 109.94 107.82 107.60 119.51 119.90
1.8942(57) 1.4035(55) 1.4779(63) 1.4738(7) 1.0911(12) 109.58(14) 107.28(28) 109.32(10) 111.69(35) 110.15(18) 110.35(16) 107.33(23) 108.21(39) 119.67(11) 119.90(21)
1.8856(25) 1.4007(24) 1.4784(28) 1.4725(3) 1.0905(5) 109.45(6) 108.05(12) 109.96(4) 111.80(15) 110.08(8) 109.38(7) 108.00(10) 107.81(17) 119.32(5) 119.38(9)
a Distances r in Å, bond angles R and dihedral angles τ in °; uncertainties in parentheses are standard errors in units of last digit. Semi-experimental structure (see text).
TABLE 6: Equilibrium (Be) and Ground State (B0) Rotational Constants (MHz) and Their Residuals from the Least-Squares Fit Structure CH2FSiH3
A B C A B C A B C A B C A B C A B C A B C
CH2FSiH2D(s) CH2FSiH2D(a) CH2FSiHD2(s) CH2FSiHD2(a) CHDFSiH3 13
CH2FSiH3
Be,obs
Be,obs - Be,calc
B0,obs
B0,obs - B0,calc
sqrt(weight)
27 315.9725 5076.5677 4634.6883 26 028.2442 4851.6644 4411.2777 24 350.1968 4922.8888 4518.8005 21 924.0213 4765.4796 4423.9341 23 215.8310 4713.8156 4304.5514 24 197.0036 5016.6176 4539.7339 26 691.8924 5060.5749 4603.1594
-0.3245 -0.0350 0.0042 1.0849 0.8389 1.0567 0.3028 0.0146 -0.0001 0.0112 0.2360 -0.0676 -3.1505 -2.9877 -2.8771 0.1302 0.0312 -0.0173 0.6308 -0.0209 0.0564
27 170.6247 5043.4148 4602.2605 25 863.3188 4822.6996 4382.9367 24 221.0826 4892.7484 4489.1422 21 807.7446 4737.9215 4396.7230 23 072.8652 4687.0969 4278.3448 24 068.0344 4984.3284 4508.3760 26 558.0561 5027.7982 4571.2189
-1.3645 0.0588 -0.2443 1.0710 0.2504 1.1926 0.3633 -0.0521 0.0878 -0.6048 -0.2272 0.4847 -1.2807 -3.9093 -2.6340 0.1115 -0.1468 0.1354 5.8524 0.3911 0.0216
71 417 417 24 32 42 53 417 400 24 32 42 15 12 29 43 303 400 32 208 263
TABLE 7: Dipole Moment Componentsa (D) and Selected Values for Stark Coefficients (MHz cm2 V-2) for CH2FSiH3 (∆ν/E2)106 transition
M
Obs.
Obs. - Calc.
111 r000 313 r212 413 r312 505 r414 817 r808 |µa| |µb| |µc| |µt|
0 1 3 4 7 1.041(5) 1.311(6) 0 (by symmetry) 1.674(4)
4.230 14.960 -10.970 3.380 18.010
0.034 -0.131 -0.063 0.009 0.102
a Uncertainties in parentheses are standard errors in units of last digit.
It has also been shown that Si-H distances can be obtained from the frequencies of the isolated stretching modes.21 The distances determined from the stretching frequencies are usually 0.004-0.006 Å longer than the values predicted from ab initio MP2(full)/6-311+G(d,p) calculations.22,23 Thus, with the Si-H and C-H distances determined by these methods within 0.002 Å and the corresponding bond angles to an expected uncertainty
of 0.5°, there are only three heavy atom distances to determine from the reported rotational constants for the CH2XSiH3 (X ) F, Cl, Br) molecules. The values for all structural parameters of CH2BrSiH3 are listed in Table 8, and the values of the rotational constants obtained from these parameters are shown in Table 9. The differences between the experimental rotational constants and the ones obtained from the structural parameters are 0.5-0.6 MHz. Considering the level of accuracy for the C-H and Si-H predicted distances as well as the small mass of the hydrogen atom, the effects on the rotational constants from the errors of the predicted values are orders of magnitude smaller than those from the heavy atom skeletal structure. Therefore, an adjustment of 0.002 Å for either heavy atom distance has a large effect on the values of the rotational constants. Thus, the uncertainty in the heavy atom distances is believed to be at most 0.003 Å. Similarly, adjusted r0 structural parameters for CH2ClSiH3 have been determined, and the results (Table 8) have significantly smaller uncertainties than the values reported earlier.5 These adjusted structural parameters for bromoand chloromethylsilane can be used for comparison with the corresponding parameters of fluoromethylsilane.
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Durig et al.
TABLE 8: Comparison of Structural Parameters (Å and Degree), Rotational Constants (MHz) and Dipole Moments (Debye) of Monohalomethylsilanes CH2BrSiH3 structural MP2/6parameter 311+G(d,p)
microwavea
CH2ClSiH3 r0 adjustedb
MP2/6311+G(d,p)
rs microwavec
CH2FSiH3 r0 adjustedb
MP2/6311+G(d,p)
r0 microwaveb
r0 adjustedb
r(Si-C) 1.887 1.889 1.886(3) 1.889 1.889(10) 1.886(3) 1.897 1.894(6) 1.892(3) r(Si-Hs) 1.478 1.477 1.484(2) 1.477 1.477(5) 1.484(2) 1.477 1.478(6) 1.483(2) r(Si-Ha) 1.473 1.477 1.479(2) 1.473 1.477(5) 1.478(2) 1.473 1.474(1) 1.479(2) r(C-H) 1.091 1.096 1.091(2) 1.091 1.096(10) 1.091(2) 1.094 1.091(1) 1.094(2) r(C-X) 1.947 1.950 1.946(3) 1.789 1.788(10) 1.791(3) 1.406 1.403(5) 1.403(3) ∠SiCX 110.5 109.3 109.1(5) 111.0 109.3(3) 109.3(5) 109.4 109.5(1) 109.5(5) ∠CSiHs 108.4 109.3 108.4(5) 108.2 109.3(5) 108.2(5) 107.7 107.3(3) 107.7(5) ∠CSiHa 109.3 109.3 109.3(5) 109.3 109.3(5) 109.3(5) 109.5 109.3(1) 109.8(5) ∠HaSiHs 109.8 110.6 109.8(5) 109.9 110.6(5) 109.9(5) 109.9 110.3(2) 109.8(5) ∠HaSiHa 110.3 110.6 110.3(5) 110.1 110.6(5) 110.2(5) 110.2 110.1(2) 109.8(5) ∠SiCH 112.1 109.3 112.1(5) 111.3 109.3(5) 111.3(5) 112.1 111.7(3) 113.0(5) ∠XCH 106.8 108.8 107.5(5) 107.6 108.5(5) 108.6(5) 107.6 108.2(4) 107.1(5) ∠HCH 108.3 107.5 108.2(5) 107.7 107.5(5) 107.7(5) 107.8 107.3(2) 106.7(5) |τHaSiCX| 60.3 60.3 60.3(5) 60.3 60.1(5) 60.3(5) 60.5 60.0(2) 60.4(5) |τHCSiX| 119.0 119.0 119.0(5) 119.9 119.4(5) 119.9(5) 119.3 119.9(2) 119.3(5) A 20 542.68 20 075.59(49) 20 075.00 22 340.17 21 759.21 21 758.89 27 126.88 27 170.61 27 174.03 B 2114.36 2151.50(2) 2150.93 3142.05 3204.08 3203.22 5029.17 5043.41 5044.69 C 1985.07 2012.49(3) 2013.05 2896.86 2938.09 2939.11 4590.81 4602.26 4603.99 |µa| 1.679 1.629 1.432 |µb| 1.198 1.307 1.589 |µc| 0.000 0.000 0.000 |µt| 2.062 2.088 2.139 b
a Reference 6. These estimated parameters reproduce the rotational constants within 120, 13, and 12 MHz for A, B and C, respectively. This work. c Reference 5.
TABLE 9: Comparison of Rotational Constants (MHz) Obtained from Modified Ab Initio Structural Parameters and Fit to the Microwave Determined Rotational Constants of Monohalomethylsilanes rotational constants A B C A B C A B C A B C A B C A B C A B C a
molecules 12
CH2 BrSiH3
12
CH281BrSiH3
79
observeda 20 075.6 2151.5 2012.5 20 064.5 2134.5 1997.5
calculated 20 075.0 2150.9 2013.1 20 064.0 2133.9 1998.0
|∆| 0.6 0.6 0.6 0.5 0.6 0.5
molecules 12
CH2 ClSiH3
13
CH235ClSiH3
12
CH235Cl29SiH3
12
CH237ClSiH3
35
observedb 21 759.2 3204.1 2938.1 21 080.4 3204.4 2925.5 21 726.8 3155.4 2896.5 21 692.9 3125.1 2870.4
calculated 21 759.8 3203.6 2938.7 21 077.6 3203.6 2926.0 21 726.8 3154.8 2897.1 21 693.0 3124.7 2871.0
|∆| 0.6 0.5 0.6 2.8 0.8 0.5 0.0 0.6 0.6 0.1 0.4 0.6
molecules 12
CH2FSiH3
13
CH2FSiH3
12
CH2FSiH2D(a)
12
CH2FSiH2D(s)
12
CH2FSiHD2(a)
12
CH2FSiHD2(s)
12
CHDFSiH3
observedc
calculated
|∆|
27 170.6 5043.4 4602.3 26 558.0 5027.8 4571.2 24 221.1 4892.8 4489.1 25 863.4 4822.7 4382.9 23 072.9 4687.0 4278.3 21 807.9 4738.0 4396.7 24 068.0 4984.3 4508.4
27 174.0 5044.7 4604.0 26 555.2 5028.8 4572.7 24 214.6 4892.2 4488.8 25 868.9 4821.7 4381.6 23 071.3 4688.7 4279.4 21 796.6 4735.7 4394.6 24 079.9 4983.8 4508.3
3.4 1.3 1.7 2.8 1.0 1.5 6.5 0.5 0.3 5.5 1.0 1.3 1.6 1.7 1.1 11.3 2.2 2.1 11.9 0.5 0.1
Reference 6. b Reference 5. c This study.
The r(C-Si) bond distance in CH2ClSiH3 is 1.889(10) Å compared to 1.894(6) Å in CH2FSiH3, but in methyl- and methylhalosilanes the r(C-Si) bond distances are much smaller. For example, in CH3SiH2F it is reported4 to be 1.845(4) Å and in the CH3SiH2Cl molecule4 it is 1.856(3) Å, compared to 1.864(l) Å in CH3SiH3.24 Thus, substitution of the hydrogen atom by halogen on the silyl group causes a shortening of the r(C-Si) bond, whereas substitution of hydrogen on the carbon atom causes elongation. The r0 structural parameters of fluoromethylsilane are in reasonable agreement with the optimized structural parameters calculated with the MP2(full)/6-311+G(d, p) basis set. It could be argued that r0 distances (CH2ClSiH3, CH2FSiH3) cannot be compared with rs distances (CH3SiH2F, CH3SiH2Cl, CH3SiH3) because they are defined differently.
However, except for pathological cases such as in CH3CH2F mentioned above, r0 - rs differences of distances between nonhydrogen atoms rarely exceed 0.01 Å. Since the r0 distances in question of about 1.89 Å are at least 0.025 Å larger than the rs distances, the comparison is sufficiently valid to establish that the C-Si bond is shortened by halogen substitution on Si but lengthened by substitution on C. The barrier to internal rotation of the silyl group in CH2FSiH3 is 717(2) cm-1 from our study. The value for this parameter in the CH2ClSiH3 molecule was reported by Schwendeman and Jacobs5 to be 892(17) cm-1 from the analysis of four rotational bands in the first torsional excited state exhibiting A and E splittings. They observed splittings as large as 1.3 MHz. However, one expects the barrier for CH2ClSiH3 to be higher
Study of Fluoromethylsilane than for CH2FSiH3. For example, the barrier for CH2ClCH3 is 1234(3) cm-1 (3.53 kcal mol-1),25 whereas the barrier for CH2FCH3 is 1171(1) cm-1 (3.35 kcal mol-1).26 The angle between the inertial axis a and the internal rotation axis derived from the internal rotation analysis is 21.8(49)°, whereas the internuclear vector between the carbon and the silicon atoms crosses the axis at an angle of 24.1°. The difference of 2.3° is only about half the standard error (4.9°) from the internal rotation analysis. Taking into account the small tilt angle of 1.1(2)°, the discrepancy becomes even less significant. The magnitude of the electric dipole moment of CH2FSiH3 and of its components along the a- and b-axes were 1.674(4), 1.041(5), and 1.311(6) D, respectively, as determined from the Stark effect. As is often observed, the MP2 ab initio calculations predicted these quantities significantly higher (Table 8). Acknowledgment. Taken in part from the dissertation of S. S. Panikar, which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree. J.R.D. acknowledges the University of Missouri-Kansas City for a Faculty Research Grant for partial financial support of this research. References and Notes (1) Pierce, L. J. Chem. Phys. 1958, 29, 383. (2) Zeil, W.; Christen, D. J. Phys. Chem. 1980, 84, 1790. (3) Shiki, Y.; Oyamada, M.; Hayashi, M. J. Mol. Spectrosc. 1982, 92, 375. (4) Hayashi, M.; Fujitake, M.; Inagusa, T.; Miyazaki, S. J. Mol. Struct. 1990, 216, 9. (5) Schwendeman, R. H.; Jacobs, G. D. J. Chem. Phys. 1962, 36, 1251. (6) Hayashi, M.; Kuwada, K. Bull. Chem. Soc. Jpn. 1973, 46, 2691. (7) Bu¨rger, H.; Moritz, P. J. Organomet. Chem. 1992, 427, 293. (8) Bu¨rger, H.; Moritz, P. Organometallics 1993, 12, 4930. (9) Groner, P.; Attia, G. M.; Mohamad, A. B.; Sullivan, J. F.; Li, Y. S.; Durig, J. R. J. Chem. Phys. 1989, 91, 1434.
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