12215
J. Phys. Chem. 1994, 98, 12215-12222
Microwave Study of the CVD Precursor Trimethylamine- Alane, (CH3)3NAlH3: Rotational Spectra, 27Al and 14N Nuclear Quadrupole Coupling Constants, and Molecular Structure H. E. Warner, Y. Wang, C. Ward, C. W. Gillies,* and L. Interrante Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180 Received: September 7, 1994@
The rotational spectra of six isotopomers of trimethylamine-alane have been observed by pulsed beam Fourier transform microwave spectroscopy. Hyperfine analyses of the symmetric top isotopomers, (CH&NAlH3, (CH3)3NAlD3, and (CH3)3l5NA1H3, determined the spectroscopic constants, BO, DJ, DJK, eQq(27A1), and eQq(14N). For (CH3)3NAlH3, BO = 2745.113(1) MHz, DJ = 643(24) Hz, DJK= 1372(250) Hz, eQq(27A1)= 25.032(6) MHz, and eQq(14N) = -3.777(5) MHz. Spectral assignments have also been made for the asymmetric top isotopomers, [(CH3)2l3CH3]NAlH3,(CH3)3NAlH2D,and (CH3)3NAlHD2. Substitution structural parameters of rNc = 1.487(2) and 6°C = 109.9(2)' are obtained from the microwave moments of inertia. With the assumption of rAlN = 2.063(7) A from an electron diffraction structure reported previously (Almenningen, A.; Gundersen, G.; Haugen, T.; Haaland, A. Acta Chem. Scand. 1972, 26, 3928) microwave substitution coordinates for the aluminum hydrogens give THAI = 1.585(2) A and OHAN = 98.1(6)'. The structure of (CH&NAlH3 is compared to the related complexes, H3A1NH3, H3NBH3, and (CH&NBH3. In the same series, bond orders calculated from measured values of eQq(27A1) and eQq("B) are compared to A1-N and B-N dative bond dissociation energies.
I. Introduction The Lewis acid-base adduct of trimethylaluminum and ammonia, (CH&AlNH3, is known to undergo thermal decomposition, through loss of methane, to produce the trimeric amide, [(CH&AlNH2]3. The latter has been used as a single-source precursor in the thermal chemical vapor deposition (CVD) of A1N thin fi1ms.l A structural isomer of this adduct, (CH3)3NAlH3, on the other hand, decomposes thermally by eliminating (CH3)3N to give A1 and HZ gas.2 This isomer has also been used as a CVD precursor, but it gives A1 films. Pursuant to our interest in the gas phase and surface phase chemistry which takes place in the overall CVD processes involving these precursors, microwave spectroscopy and mass spectrometry are being used to characterize the gas phase species present upon vaporization of (CH&AlNH3, [(CH3)2AlNH2]3, and (CH3)3NAlH3. Recent mass spectral studies of [(CH3)2AlNH213 have provided evidence for a gas phase equilibrium of the dimer and trimer of the monomeric amide, ( C H ~ ) Z A ~ N There H ~ . ~has been no gas phase work reported for (CH&AlNH3; however, the solid state infrared and Raman spectra are known4 and a recent study employed matrix isolation infrared techniques in an unsuccessful attempt to trap chemical intermediates in the (CH3)3Al -t- NH3 r e a ~ t i o n . The ~ thermolysis of (CH3)3AlNH3 was investigated in solution by calorimetry and 'H NMR spectroscopy.6 This work established that the trimer, [(CH3)2AlNH2]3,is produced from the adduct, (CH&AlNH3, accompanied by the evolution of methane, and a proposed mechanism suggests that the monomer, (CH3)2AlNH2, catalyzes trimer formation. The kinetics and mechanisms of the thermal decomposition of (CH3)3NAIH3 on silicon and aluminum surfaces have been studied by Dubois, Zegarshi, Kao, and Nuzzo.' Despite the fact that (CH3)3NAlH3 was first prepared in 1942,8 there has been little experimental work reported on this substance in the gas phase. In 1963 Fraser et al. observed the infrared spectrum in the vapor phase and demonstrated that the compound was monomeric with C3" ~ymmetry.~ Nine years later Almenningen @
Abstract published in Advance ACS Abstracts, November 1, 1994.
0022-365419412098-12215$04.50/0
et al. determined the molecular geometry by employing gas phase electron diffraction.'O Very few microwave spectroscopic studies of aluminumcontaining compounds have appeared in the literature. These include only aluminum diatomic series, AlF,I1-l4 AlCl,11915s16 A1Br,15,16AlI,l5,l7A10,'* and AlS.19 In order to pursue highresolution microwave studies of the AlN and A1 CVD processes, the present investigation of (CH3)3NAlH3 was undertaken using pulsed nozzle Fourier transform microwave spectroscopy. In 1976 Kirby, Smith, and KrotoZ0observed the J = 5 4, 6 5, and 7 6 rotational transitions of the normal isotopomer in the 26.5-40.0 GHz region, determining the rotational constant, Bo, to be consistent with the electron diffraction structural data. This microwave work employed a conventional Stark-modulated absorption spectrometer. It was not possible to resolve the 27Aland 14N nuclear hyperfine structure, and the results were not published in the open literature. In the present microwave study, a hyperfine analysis of the J = 1 0, 2 1, and 3 2 transitions of the normal isotopomer has determined the 27Aland 14Nnuclear quadrupole coupling constants. This is the first gas phase measurement of an 27Alnuclear quadrupole coupling constant in a polyatomic molecule. High-resolutionmicrowave spectra are also reported for (CH3)3l5NA1H3, (CH&NAlD3, (CHhNAlD2, (CH3)3NAlHzD, and [(CH3)2l3CH3]NA1H3. The rotational constants obtained from these spectra are used in conjunction with the reported value of the Al-N bond distance from an electron diffraction study lo to obtain an improved molecular structure of (CH3)3NAlH3. These structural results are related to ab initio studies of H3AlNH321 and (CH3)3NA1H322in the Discussion section. They are also compared to the related compounds, H3NBH3 and (CH3)3NBH3, which have well-determined gas phase molecular structure^.^^^^^ The Al-N bond order, the number of electrons donated from the nitrogen to aluminum, is calculated from the measured value of eQq(27A1). An analogous B-N bond order is determined from the published value of eQq("B) for H3NBH3.25 These Al-N and B-N bond orders and the B-N bond order reported for (CH3)3NBH326are compared to +
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0 1994 American Chemical Society
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12216 J. Phys. Chem., Vol. 98, No. 47, 1994
Warner et al.
theoretical and experimental determinations of the dative bond The spectrometer electronics follow closely the 30 MHz dissociation energies in the Al-N2lxZ2 and B-N a d d ~ c t s . ~ ~ , single-sideband ~~ modulation described in ref 31. A pulse generator provides the timing sequence required for operation 11. Experimental Section of the s p e c t r ~ m e t e r .Each ~ ~ cycle begins with a microwave pulse of frequency Y 30 MHz where Y is the carrier frequency A. Preparation of Trimethylamine- Alane. (CH3)3NAIH3 from the microwave synthesizer by opening a pin diode switch was prepared from lithium aluminum hydride and trimethylfor 1 ps. After a short delay of 2 ps, a second pin diode switch amine hydrochloride, employing procedures described in the is opened, allowing the background noise from the cavity to be l i t e r a t ~ r e . ~Typically, .~~ 1 g of dry trimethylamine hydrochloride downconverted to an IF (intermediate frequency) of 30 MHz. was added slowly to a stirred mixture containing 100 mL of A second mixer downconverts the IF to zero frequency, dry ether and 0.5 g of dry, finely ground lithium aluminum whereupon the signal is amplified, phase inverted, and then hydride at room temperature under nitrogen. The reaction was digitized using a digital oscilloscope. complete after hydrogen gas stopped bubbling from the solution. A gray-colored powder appeared upon evaporation of the ether Once the noise data acquisition is complete, a gas beam pulse solvent. Caution: trimethylamine-alane is a fire and explosion is released into the cavity. Then the pulse sequence described hazard in air and should be handled in a glovebox or in a for the noise is repeated without phase inversion of the gasvacuum. Sublimation of the crude product gave trimethylpulse-on signal. In order to obtain background subtraction, this amine-alane in 79% yield as a white crystalline solid (melting signal is added to the phase-inverted background signal. point of 73-74 "C). (CH3)315NA1H3was prepared similarly Typically, the signals are digitized at a rate of 0.4 pslpoint, by using 99% 15N-enrichedtrimethylamine hydrochloride. The employing 512 record lengths. In searches for strong transitions, synthesis of (CH&NAID3 was carried out as described above 64 cycles are averaged to obtain free induction decays (FID) using LiAlD4. Partial deuteration at the alane hydrogen having reasonably good signal to noise. Weak transitions may positions was obtained by employing 50% by weight mixtures require averaging several thousand cycles. The digital oscilof L i A l b and LiAlD4. loscope performs a fast Fourier transform (FIT)on the averaged B. Microwave Spectroscopy. The rotational spectrum of data. High spectral sensitivity is obtained by using low-noise (CH3)3NAIH3 was observed with a recently constructed pulsed microwave amplifiers (
