J. Phys. Chem. 1996, 100, 16551-16554
16551
Structural Properties of [(Trichlorosilyl)amino]dichloroborane Max Mu1 hlha1 user, Marcus Gastreich, and Christel M. Marian* Institute of Physical and Theoretical Chemistry and Sonderforschungsbereich 408, UniVersity of Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany
Hardy Ju1 ngermann and Martin Jansen Institute of Inorganic Chemistry and Sonderforschungsbereich 408, UniVersity of Bonn, Gerhard-Domagk-Strasse 1, D-53121 Bonn, Germany ReceiVed: June 12, 1996; In Final Form: August 5, 1996X
Density functional calculations have been performed on [(trichlorosilyl)amino]dichloroborane (TADB, Cl3SiNHBCl2), a molecular precursor of the recently synthesized ceramic Si3B3N7. An infrared spectrum of the substance was determined both experimentally using a Fourier transform infrared spectrometer and by means of quantum chemical calculations. The computed infrared spectrum is in good agreement with experiment and allows the assignment of the more intense peaks to the various vibrational modes. Electronic structure calculations show that the nitrogen atom in TADB is not pyramidally coordinated; the molecule rather contains a planar Si-NH-BCl2 unit with a partial N-B double bond.
Introduction Recently, a new ceramic material of composition Si3B3N7 was synthesized from [(trichlorosilyl)amino]dichloroborane (Cl3SiNHBCl2, TADB) as a molecular precursor by ammonolysis and pyrolysis.1,2 From MAS NMR studies3 a few topological properties are known about Si3B3N7, e.g., it has been established that Si is tetragonally coordinated to four nitrogen atoms, while N and B show a 3-fold coordination. In addition there are indications that the Si-N-B structural unit of TADB is not broken up during during the ammonolysis and pyrolysis leading to the formation of Si3B3N7. Further experimental information on structural properties of the latter is presently not available, e.g., whether N exhibits a pyramidal or planar nearest-neighbor coordination. To shine some light on the properties of molecules containing the Si-N-B unit, we set out for a quantum chemical investigation of the structural parameters of TADB. To our knowledge experimental values of bond lengths and angles in TADB, e.g., by means of X-ray diffraction, have not yet been determined. The reliability of our theoretical predictions can be checked though by comparison of calculated harmonic frequencies and infrared intensities with a measured IR spectrum. For this purpose we synthesized the substance and recorded a spectrum on an FT-IR spectrometer. We have tried to keep the level of theoretical treatment modest since we aim at being able to study larger subunits of Si3B3N7 with two or three coordination spheres at the same level of approximation. To lend confidence to the structural data obtained we have, therefore, in addition to a check against experimental data, tested the consistency of our results on TADB also with more sophisticated methods. Technical Details Computational Details. Geometry optimizations were performed using a local density functional (LDF) approach. Herein we employ the Vosko-Wilk-Nusair4 correlation functional and the Slater local exchange interaction5 (SVWN). * To whom all correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01732-7 CCC: $12.00
The choice of an LDF was motivated by two reasons: first, the aim to study larger sections of covalently bound Si/B/N containing material by the same methods; with increasing cluster size the computing time required by more sophisticated methods like HF/MPn (Hartree-Fock/Møller-Plesset perturbation theory of nth order) or CCSD(T) (coupled-cluster theory including single, double, and, approximately, also triple excitations) would soon grow out of proportion. Second, experience on similar systems like trisilylamine and triaminoborane6 has shown that, apart from the hydrogen bonds which come out slightly too long, all other bonding parameters are comparable with the results of MP2 optimizations and much better than HF optimized values. We cross-checked our theoretical predictions for the most stable conformer of TADB also by use of the recently proposed Hartree-Fock/density functional hybrid method B3LYP7,8 which has proven to yield highly reliable geometrical and spectroscopic parameters. Relative stabilities of various conformers are more sensitive though. Therefore, we have performed single-point HF calculations taking into account electron correlation in Møller-Plesset perturbation theory up to fourth order. The atomic orbital (AO) basis was a split-valence 6-31G* basis taken from the GAUSSIAN92/DFT9 basis set library. Four different arrangements of the nuclei I-IV (Figure 1) were chosen as starting points for a geometry optimization. In I and II the BCl2 and the NH groups are coplanar; among the Cl atoms in the SiCl3 group in I the Cl atom in the cisoid position with respect to the hydrogen atom is also located in this plane, while in II the SiCl3 group is rotated through 60° such that here the transoid standing Cl lies in the BNH plane. III and IV can be thought of as being generated from I and II, respectively, by rotating the BCl2 group through 90° out of plane. The search for minima on the potential hypersurface was in a first step confined to conserve the symmetry of the molecule at the respective starting point. At points of zero gradient, second derivatives of the energy were computed in order to check for imaginary frequencies; at a given saddle point the molecule was distorted along the corresponding normal coordinate, and a minimum was searched without symmetry constraints. Infrared intensities were computed from the derivative of the dipole moment at the optimized geometry. © 1996 American Chemical Society
16552 J. Phys. Chem., Vol. 100, No. 41, 1996
Mu¨hlha¨user et al. TABLE 1: Bonding Parameters and Shared Electron Numbers (SEN) of TADB at Its Equilibrium Structure I and the Two Saddle Points II and IV molecular conformation B3LYP
SVWN I
II
R [Å]
R [Å]
SEN
R [Å]
SEN
R [Å]
SEN
1.02 1.74 1.42 2.06 2.05 1.77
1.03 1.73 1.41 2.04 2.03 1.75
1.24 1.24 1.71 1.11 1.10 1.34
1.03 1.73 1.41 2.02 2.04 1.75
1.24 1.24 1.70 1.10 1.14 1.36
1.03 1.70 1.44 2.04 2.04 1.75
1.24 1.32 1.54 1.11 1.11 1.38
N-H N-Si N-B Si-Cl(5) Si-Cl(6,7) B-Cl(8,9)
B3LYP
SVWN I
Figure 1. Starting points for geometry optimizations on TADB.
Most of the calculations were performed using the GAUSSIAN92/DFT program package.9 The characterization of the bonding properties is based on a Roby-Davidson-Ahlrichs (RDA) population analysis.10-13 In this approach the charge not assigned to a particular center is minimized by the use of modified AOs; the overlap populations obtained in this waysthe two-center contributions being denoted shared electron numbers (SEN)sare much less sensitive with respect to the choice of AO basis sets as in the usual Mulliken population analysis and can be taken as a measure of bond strength.13 The RDA population analysis was applied to Roothaan-SCF molecular orbitals employing the TURBOMOLE program package.14 Experimental Setup. The IR spectroscopic measurements were conducted on a Fourier transform infrared (FT-IR) spectrometer (Type IFS 113v, Bruker Co., Karlsruhe, Germany). The appliance has a modified interferometer by Genzel. Compared to a machine with Michelson arrangement and with the same dimension of the optical unit, it reaches a resolution enhancement of 100%. A globar was used as the light source in the MIR range, and a mercury cadmium telluride photodetector registered the transmission. No baseline corrections and smoothing operators were used. A fluid probe of 10% TADB in hexane was deposited in a cuvette with KBr windows and measured in the absence of wet and air at a temperature of 20 °C. Results and Discussion Potential Energy Surface and Bonding Parameters. Among the conformations I-IV (Figure 1) only structure I represents a minimum. II is a saddle point exhibiting one vibrational mode with an imaginary frequency. Distortion along this direction leads directly to structure I. Both exhibit very similar bonding parameters: a planar Si-NH-BCl2 unit with a partial N-B double bond corresponding to an N-B bond length of 1.41 Å and a shared electron number of 1.71 electrons (Table 1). Inspection of Table 1 also shows that bond distances and bond angles computed by either employing the SVWN density functional or the more advanced B3LYP are very similar. Due to electrostatic repulsion between the in-plane chlorine atom of the SiCl3 group and the innermost chlorine (Cl(9)) of BCl2 the Si-N-B angle is somewhat larger in structure II compared to I. As shown in Table 2 rotation of the SiCl3 group requires only little energy. For all methods employed in the present work consistently a value of approximately 12 kJ/mol is found indicating an almost free rotation around the Si-N bond. The electronic structure is retained in this case. The highest molecular orbital (HOMO) in conformation (I) is a mostly
IV
Si-N-B Si-N-H H-N-B N-B-Cl(8) N-B-Cl(9) Cl(8)-B-Cl(9) Cl(5)-Si-N Cl(6)-Si-N Cl(6)-Si-Cl(7) Cl(5)-Si-Cl(6)
II
III
∠ [deg]
∠ [deg]
∠ [deg]
∠ [deg]
131.6 113.6 114.8 119.2 121.4 119.5 105.1 112.0 109.0 109.3
128.0 115.4 116.5 119.7 120.1 120.2 105.0 111.8 109.3 109.4
136.4 111.2 112.4 117.6 123.4 119.0 108.2 112.7 107.7 109.9
123.9 117.7 118.4 121.0 121.0 117.9 112.0 105.5 110.1 107.6
TABLE 2: Potential Energy Content of TADB [kJ/mol] at Distinct Points of the Potential Surface Relative to Structure Ia structure method
II
IV
SVWN SCF MP2 MP3 MP4SDQ
+10.9 +11.3 +12.6 +12.1 +12.1
+67.8 +67.8 +68.6 +68.6 +68.2
a Optimization of the geometrical parameters has been performed at the SVWN level in all cases.
nonbonding a′′ orbital. It has major contributions from the nitrogen pz orbital with a slight bonding overlap to boron and considerable charge density in Cl lone-pair orbitals. The lowest unoccupied MO (LUMO) has again a′′ symmetry and is slightly Si-N bonding while showing a nodal plane between N and B. Upon rotation of the SiCl3 group the HOMO swaps places with the second highest occupied MO, a linear combination of chlorine lone-pair orbitals, but no exchange between occupied and unoccupied orbitals occurs. By contrast, rotating the BCl2 group by 90° has a major effect on the electronic structure. In C1 symmetry structure III distorts toward IV, which represents another saddle point on the potential energy surface located 68 kJ/mol above the minimum I. The HOMO has again large contributions from N pz and Cl lonepair orbitals, but now there is a slight Si-N bonding and N-B antibonding overlap. Accordingly, the B-N shared electron number is smaller and the B-N bond longer than in I but still some double bonding character can be made out. The Si-N and B-N bond strengths are coupled, so in this case in which the BCl2 group is perpendicular to the Si-NH-B plane the Si-N bond gains some double-bonding character and becomes shorter. Infrared Spectrum. The experimentally observed IR spectrum is shown in Figure 2. In addition, the locations of the most intense peaks are displayed in Table 3 together with the quantum chemically determined harmonic vibrational frequencies and corresponding infrared intensities. In this collection
Structure of [(Trichlorosilyl)amino]dichloroborane
J. Phys. Chem., Vol. 100, No. 41, 1996 16553
Figure 2. Experimentally recorded IR spectrum of 10% TADB in hexane. The peaks at 2962, 2933, and 2874 cm-1 belong to C-H stretching modes of the solvent.
TABLE 3: Experimentally Observed IR Spectrum and Calculated Harmonic Frequencies ν [cm-1] and Relative Infrared Intensities I [%] of TADB theory global minimum (I) experimenta mode 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
ν
I
485
11
565
15
626 60 856 14 926/956 53 1222 22 1346/1369 100 3375 19
B3LYP ν
Ib
SVWN ν
Ic
saddle point (II) SVWN ν
Id
36 0 42 0 26i 0 59 0 63 0 48 0 78 0 79 0 109 0 146 0 143 0 133 0 163 0 161 0 151 0 176 1 175 1 174 1 205 1 198 1 204 0 273 1 267 1 256 0 274 1 269 1 268 2 353 1 349 1 355 0 478 10 485 8 480 6 492 0 478 0 469 0 561 12 572 8 558 9 590 2 574 3 593 2 613 15 624 17 643 16 640 41 645 36 643 36 850 8 861 10 854 6 941 56 950 40 944 43 1242 21 1187 11 1220 9 1385 100 1369 100 1381 100 3538 9 3466 9 3413 7
sym A′′ A′′ A′ A′ A′′ A′ A′ A′′ A′ A′ A′ A′′ A′ A′′ A′ A′′ A′ A′ A′ A′ A′
a Only the most intense transitions are given. For a complete overview see Figure 2. b 100% ≡ 705 km/mol. c 100% ≡ 729 km/mol. d 100% ≡ 788 km/mol.
the infrared bands near 3000 cm-1 (Figure 2) which originate from C-H modes in the solvent hexane are not considered. The theoretical data are given not only at the global minimum but also at the geometry of conformation II. This might be helpful since we assume that the SiCl3 rotation (corresponding to mode 1) is considerably excited at thermal energies. For the 10 most intense infrared-active modes isotopic shifts also
were calculated for a substitution of single elements with their most frequent isotopes. These data are collected in Table 4 and can already give some hints to which nuclear displacements contribute to a specific vibrational mode. In detail, beginning with the highest frequency mode, the following assignments are made, and the results are compared with the experimentally recorded IR spectrum. Mode 21 is a pure N-H stretching vibration. Experimentally, the fundamental N-H stretching vibrational mode is observed at 3375 cm-1. Our theoretical values amount to 3538 and 3466 cm-1 for the B3LYP and SVWN functionals, respectively. Comparing these data with experiment, one has to keep in mind that the latter are harmonic frequencies. The anharmonicity parameter is estimated to be of the order of 80 cm-1, corresponding to its value in the free NH molecule.5 Adding this estimate of the anharmonicity correction to the computed harmonic frequency yields a calculated zero vibrational frequency of about 3460 or 3390 cm-1, respectively. For the other vibrational modes anharmonicity corrections are expected to be much smaller in absolute value and are therefore neglected. The most intense mode in our theoretical IR spectrum is mode 20, an almost pure B-N stretching mode, in accord with experiment where a strong band is observed at 1346 cm-1 with a 10B shoulder at 1369 cm-1. The next, quite intense peak in the experimental spectrum is localized at 1222 cm-1. Mode 19, which we calculate at 1242 or 1186 cm-1, respectively, is an H-N-B bending vibration with small coupling to the Si-N stretching and BCl2 antisymmetric stretching motions. In the spectrum of the deuterated species this vibrational mode should be easily recognized; we predict a red-shift of its transition frequency by almost 150 cm-1. Mode 18 is mostly the BCl2 antisymmetric stretching mode with small contributions from Si-N-B and Si-N-H bending. Experimentally peaks at 926 and 957 cm-1 can be assigned to this motion matching well our calculated value of 941 cm-1 (B3LYP) or 950 cm-1 (SVWN) and an isotopic blue-shift of 32 cm-1. The experimentally observed peak at 856 cm-1 corresponds to mode 17,
16554 J. Phys. Chem., Vol. 100, No. 41, 1996
Mu¨hlha¨user et al.
TABLE 4: Isotopic Shifts of Harmonic Frequencies [cm-1] in the 10 Most Intense Infrared-Active Modes of TADB (1) Calculated at the LDF/SVWN Level of Treatment mode 12 13 14 15 16 17 18 19 20 21
isotopic shifts
unshifted frequency 14 1 11 35 Cl28 3 Si N H B Cl2
Cl3SiNH BCl2
484.5 572.3 573.8 623.5 645.4 861.3 949.9 1186.9 1368.7 3466.0
8.9 2.2 3.2 0.9 0.3 3.6 31.8 17.9 31.5 0.0
35
10
15
Cl3Si NHBCl2
37Cl SiNHBCl 3 2
Cl3SiNHB37Cl2
Cl329SiNHBCl2
Cl3SiN2HBCl2
-3.2 -7.0 -2.6 -1.0 -0.7 -12.6 -0.4 -2.0 -20.7 -7.7
-6.3 -1.5 -0.7 -4.2 -4.8 -0.2 0.0 0.0 0.0 0.0
-0.6 -5.4 -0.1 -0.8 0.0 -0.8 -3.0 -0.3 -0.1 0.0
-2.0 -0.9 -0.7 -6.2 -6.8 -5.0 -0.2 -0.1 -0.1 0.0
-5.0 -7.7 -62.7 -10.8 -4.3 -93.9 -52.9 -146.4 -16.6 -930.7
a stretching vibration between Si and an (NH) unit with small contributions from the BCl2 symmetric stretching motion. Mode 16 is the out-of-plane vibration with highest frequency. Its major components are in-phase displacements of the two outof-plane chlorine atoms Cl(5) and Cl(7) accompanied by a pyramidalization at the nitrogen center in the opposite direction. It has a large intensity. Close to mode 16 we calculate an inplane vibrational mode also with considerable intensity. It is mainly a stretching motion employing silicon and the in-plane chlorine of the SiCl3 group coupling slightly to a symmetric stretching motion of the boron-bonded chlorine atoms against the (BNH) unit. In the experimental IR spectrum an intense peak with a maximum at 626 cm-1 and a shoulder toward lower wavenumbers is detected in this frequency region which presumably accommodates two fundamental transitions. Mode 14 represents an in-phase out-of-plane motion of B and H, 180° phase shifted with respect to the Si motion. Its calculated transition intensity is too small for the band to be detected in the IR spectrum. The normal coordinates of mode 13 are the other linear combination of the (BNH)-Cl2 symmetric stretching and smaller contributions from the Si-Cl(6) stretch. Its transition frequencies match the observed peak at 565 cm-1. Finally, the lowest frequency mode with nonnegligible infrared intensity is mode 11, a symmetric stretching vibration of the three Cl atoms against the (SiNH) unit combined with little BCl2 antisymmetric stretch. Its calculated transition frequency is again in good agreement with experiment. At room temperature, the internal rotation of the SiCl3 group about the Si-B bond might be considerably excited since the rotational barriers amounts to only 11-12 kJ/mol. However, as shown in Table 3, the harmonic frequencies at the saddlepoint structure (II) do not differ dramatically from those at the global minimum in the high-frequency regime. We do, therefore, not expect major changes in the spectrum due to the excitation of this torsional motion. Summary Our calculations on [(trichlorosilyl)amino]dichloroborane (TADB) show that this substance contains a planar Si-NH-
BCl2 unit with a partial N-B double bond. Theoretically and experimentally determined infrared spectra are in good agreement. This lends confidence to the computed structural data and allows an assignment of hitherto unidentified spectral signals. In addition, the current investigation shows that a Kohn-Sham scheme employing the Slater exchange and the Vosko-Wilk-Nusair correlation functional is capable of predicting reliably bonding parameters and relative energies for molecules in this class of silicon-, boron-, and nitrogencontaining compounds. References and Notes (1) Wagner, O. Molekulare Precursoren fu¨ r Bor-siliciumnitridMischkeramiken; Diploma Thesis, University of Bonn, Bonn, 1991. (2) Baldus, H.-P.; Wagner, O.; Jansen, M. Mater. Res. Soc. Symp. Proc. 1992, 271, 821. (3) Lo¨ffelholz, J. Neue pra¨ paratiVe Zuga¨ nge zu nitridischen Mischkeramiken; Dissertation, University of Bonn, Bonn, 1994. (4) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (5) Slater, J. C. Phys. ReV. 1951, 81, 385. (6) Gastreich, M. Quantenchemische Untersuchung Von Strukturelementen Si/B/N-haltiger Keramiken; Diploma thesis, University of Bonn, Bonn, 1996; available via http://www.thch.uni-bonn.de/tc/ghost.html. (7) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (8) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92/DFT, Revision f.4. Gaussian, Inc., Pittsburgh PA, 1993. (10) Davidson, E. R. J. Chem. Phys. 1967, 46, 3320. (11) Roby, K. R. Mol. Phys. 1974, 27, 81. (12) Heinzmann, R.; Ahlrichs, R. Theor. Chim. Acta 1976, 42, 33. (13) Erhardt, C.; Ahlrichs, R. Theor. Chim. Acta 1985, 68, 231. (14) Ha¨ser, M.; Ahlrichs, R. J. Comput. Chem. 1989, 10, 104. (15) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure; van Nostrand: New York, 1979; Vol. 4.
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