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1 Synthesis of Bridging Indium Hydride on a Semiconductor H Synthesis of Bridging Indium Hydride on a H H Semiconductor Surface H H H In Qiang Fu, Lian Li, Connie H. Li and Robert F. Hicks* +10H P H H [110] Department of Chemical Engineering H H University of California, Los Angeles H Los Angeles, CA 90095-1592 H [110] (a) (b) Electron-deficient bridging hydrides of the group IIIA elements have been studied for many years.1 A series of Figure 1. Ball-and-stick models of InP (001) (2x4) borane compounds exist with 3-center-2-electron bridging surface (a) before and (b) after hydrogen adsorption. 2 hydrogen bonds, ranging from B2H6 to B10H12. By contrast, An atomically smooth and homogeneous surface was structures formed by the heavier elements are fewer in prepared by depositing a thin film of indium phosphide onto number. Alane occurs only as a polymer network of (AlH3)n 12 3 an InP (001) crystal by metalorganic vapor-phase epitaxy. chains. Digallane has been synthesized in the gas-phase and Then the crystal was removed from the reactor and transferred in an inert-gas matrix at cryogenic temperatures by Downs directly into an ultrahigh vacuum chamber, where it was and coworkers.4 These authors also observed the 5 annealed at 773 K for 30 min to produce an indium-rich (2x4) oligomerization of this molecule in the condensed state. reconstruction. The unit cell of this structure consisted of an Pullumbi et al. obtained indane, InH3 (1), in low- 6 In-P hetero-dimer straddling across four In-In dimers as temperature matrix isolation experiments. Other compounds shown in Figure 1(a).11 Scanning tunneling micrographs of with terminal In-H bonds have been prepared by the surface reveal rows of triangular gray spots that are incorporating bulky ligands into the organometallic characteristic of the reconstruction. This was the only surface molecules, such as in [Li(thf)2]-[{(Me3Si)3C}2In2H5] (2), phase detected by the STM, and defects, such as missing K3[K(Me2SiO)7]-[InH(CH2CMe3)3]4 (3), InH{2- 7 dimers, occupied less than 1% of the available area. Atomic Me2NCH2(C6H4)}2 (4), and Me2InB3H8 (5). The only known hydrogen was produced by dissociating hydrogen molecules compounds containing bridging In-H bonds are leaked into the vacuum chamber over a white-hot tungsten K[H{In(CH2CMe3)3}2] (6) and [Li(tmeda)2]-[Me3In-H- 7 filament. The atoms adsorbed onto the (2x4) surface to InMe3] (7), both of which contain negatively charged indium saturation coverage, and their vibrational properties were clusters. Attempts to synthesis diindane, In2H6 (8) have not recorded by internal reflection of the infrared light through been successful, presumably because this molecule is 13 8 the indium phosphide crystal. thermodynamically unstable. Consequently, our knowledge Shown in Figure 2 are polarized reflectance spectra of of bridge-bonded hydrides of indium and thallium is limited 6,8-10 hydrogen adsorbed on the indium-rich InP (001) surface. to theoretical calculations of their structure. Note that the low-frequency cut-off at 1000 cm-1 is due to InP We have overcome the thermodynamic constraints faced in lattice vibrations, which absorb all the infrared light. Perusal (InH3)n molecules by using the indium phosphide (001) of the spectra reveals that there are three groups of bands surface as a rigid backbone upon which to stabilize the appearing in the frequency ranges 2350-2200, 1750-1600 and electron-deficient bridging hydride. Indium dimers present on the metal-rich InP surface provide the ideal sites for formation 20 of this structure.11 Hydrogen atoms insert into the dimer bonds (bond length ∼2.84 Å), and relieve some of the strain P built into the reconstructed surface. The hydrogen bonds are 15 [110] ) S further strengthened by electron transfer from phosphorous to -5 [110] indium through the In-P back-bonds. In this paper, we report 10 on the vibrational properties of bridging indium hydride and compare the experimental results to ab initio cluster R/R (x10 ∆ p-polarized calculations of In2H6 and In3H9. 5 (1) Taylor, M. J.; Brothers, P J. Chemistry of Aluminium, s-polarized Gallium, Indium and Thallium; Downs, A. J., Ed.; Blackie: 0 Glasgow, 1993; P111. 3000 2500 2000 1500 1000 (2) Greenwood, N. N. The Chemistry of Boron; Pergamon Wavenumber (cm-1) Press: Oxford, 1975. Figure 2. Polarized reflectance spectra of adsorbed (3) Almenningen, A.; Anderson, G. A.; Forgaard, F. R.; hydrogen on the InP (001) (2x4) at 300K. Haaland, A. Acta. Chem. Scand. 1972, 26, 2315. (4) Downs, A. J.; Goode, M. J.; Pulham, C. R. J. Am. Chem. (9) Bennett, F. R.; Connelly, J. P. J. Phys. Chem. 1996, 100, Soc. 1989, 111, 1936. 9308. (5) Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. (10) Duke, B. J.; Liang, C.; Schaefer, H. F. J. Am. Chem. H.; Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149. Soc. 1991, 113, 2884 (6) Pullumbi, P.; Bouteiller, Y.; Manceron, L.; Mijoule, C. (11) Schmidt, W.G.; Bechstedt, F. Surf. Sci. 1998, 409, 474. Chem. Phys. 1994, 185, 25. (12) Li, L.; Han, B.-K.; Fu, Q.; Hicks, R. F. Phys. Rev. Lett. (7) Hibbs, D. E.; Hursthouse, M. B.; Jones, C.; Smithies, N. 1999, 82,1879. A. Organometallics 1998, 17, 3108 (and references therein). (13) Hicks, R.F.; Qi, H.; Fu, Q.; Han, B.-K.; Li, L. J. Chem. (8) Hunt, P.; Schwerdtfeger, P. Inorg. Chem. 1996, 35, 2085. Phys. 1999, 110, 10498. Qi, H.; Gee, P.E.; Hicks, R.F. Phys. Rev. Lett. 1994, 72, 250. 1 1600-1000 cm-1. The high frequency peaks between 2350 and latter vibration.8,9 The value obtained by us is in the same 2200 cm-1, which are more prominent in the s-polarized range as the broad, low-frequency band observed for spectrum, are attributed to phosphorous-hydrogen stretching hydrogen-terminated InP (001), and confirms our assignment vibrations.12 Conversely, the peak centered at 1610 cm-1 is of this feature to a bridging hydrogen bond. due to the vibration of a terminal indium hydride. The two intense, broad bands at 1350 and 1150 cm-1, which Table I. Vibrational Frequencies of Indanes. appear only in the p-polarized spectrum, are assigned to the asymmetric stretching modes of bridging indium hydride. InH3 In2H6 In3H9 This assignment is consistent with our previous work on Fa Ib Sc FISFIS 13 hydrogen adsorption on gallium arsenide. On gallium-rich 594 166 a2" 158 0 ag 29 0 a GaAs (001), a series of intense, broad peaks due to the 603 276 e' 188 4 b2u 114 7 b 1738 0 a1' 331 0 b2g 118 0 a asymmetric stretches of bridging gallium hydrides are 1746 578 e’ 334 0 au 154 1 a -1 observed at frequencies between 1750 and 1400 cm . The 369 0 b1g 165 3 b heavier indium atoms and weaker In-H bonds should produce 517 490 b3u 209 6 b 519 124 b 296 1 a similar features but shifted several hundred wavenumbers to 1u 571 0 ag 324 60 b lower frequencies. 615 0 b3g 387 9 a In order to verify the assignments of the vibrational bands, 639 116 b2u 408 243 b ab initio calculations using density functional theory (DFT) 1064 236 b1u 485 173 b 1137 0 b2g 529 466 b have been performed on indane (InH3), diindane (In2H6) and 1193 1090 b3u 546 92 a penta-coordinated triindane (In3H9). Perdew-Wang’s 1276 0 ag 582 0 a correlation and exchange functional were adopted for this 1779 137 b3u 585 27 b 14 1784 0 ag 634 114 b purpose. An (18s/14p/9d)/[5s/5p/3d] contracted basis set 1799 0 b 642 150 a 15 1g was used for indium, while Pople's triple-ζ plus polarization 1805 418 b2u 797 949 b (6-311G**) basis set was used for hydrogen.16 The 861 4 a calculations were performed with the Gaussian98 quantum 1051 4 a 17 1137 241 b chemistry suite. Shown below are the optimized structures 1281 1004 b of the indium hydride molecules, having D3h, D2h, and C2 1306 103 a symmetry, respectively. 1382 166 b 1389 210 a H H 1777 99 b H 1779 26 a H 133.0o 1.734 1.893 H 2.024 1.739 1798 122 b H In In In 1.757 1.964 H 97.4o 2.136 H1.880 1800 185 a H 98.9o H 1.738 In o 1808 210 a In H 100.9 H H a -1 b c H H frequency (cm ), intensity (km/mol), symmetry 1.739 H In o H 133.5 H For triindane, a penta-coordinated structure has been The vibrational properties are summarized in Table I. The obtained with three InH3 loosely connected as shown above. frequencies of the asymmetric In-H-In stretching modes are This is the same configuration used by Schaefer and coworkers in their study of borane, alane and gallane highlighted in bold. Since indane is the only experimentally 10 observed molecule, it is used to gauge the quality of the trimers. The optimization of In3H9 yielded a local minimum with no imaginary frequencies. As can be seen in Table I, the calculations. The infrared spectrum of InH3 exhibits three -1 6 trimer yields multiple vibrational bands for the bridging bands at 1754.5, 613.2 and 607.8 cm . Previous DFT -1 calculations by Bennett et al.9 yielded 1786, 630 and 615 hydride at 1137, 1281 and 1382/1389 cm . The additional cm-1, whereas Hunt et.
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