J . Phys. Chem. 1991, 95, 7668-7673
7668
Ab Initio Molecular Orbital Study of the GaAs Hydrides Charles W. Bock,* Department of Chemistry, Philadelphia College of Textiles & Sciences, Philadelphia, Pennsylvania 19144, and American Research Institute, Marcus Hook, Pennsylvania 19061
Kerwin D. Dobbs, Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware I9716
Gilbert J. Mains,* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
and Mendel Trachtman Department of Chemistry, Philadelphia College of Textiles & Sciences, Philadelphia, Pennsylvania 191 44 (Received: January 7 , 1991)
The interaction of GaH, ( x = 0-3), electron deficient in the valence shell, and of ASH, ( x = 0-3), electron rich in the valence shell, has been studied by using ab initio molecular orbital methods, which include Mdler-Plesset correlation corrections to the fourth order, MP4SDTQ. In most instances, Hartree-Fock (HF) optimizations were used to determine local minima, and in a few cases, MP2=Full optimizations or MCSCF optimizations were also employed. This research corroborates the work reported by others for the electronic states of GaH, GaH2,GaH3, ASH, AsH2,AsH3,and GaAs. We report two structures for (GaA~)~(dimer); the lowest energy conformer appears to be a special case of p-bridged Ga-As2-Ga. Monovalent Ga is proposed to explain the lowest energy structure of GaAsH2. The lowest energy tetrahydride, H2Ga-AsH2, is a classical Lewis structure. A double Lewis acid/base adduct, (H2Ga-A~H2)2, which can be interpreted in terms of p-bridging, is the dimer structure. This research supports the formation of Lewis acid/base adducts in the thin-film deposition of GaAs by chemical vapor deposition.
Introduction GaAs and related compounds are of such intense interest that many international symposia have been dedicated to the study of these compounds.' Of course, the stimulus is the application of group Ill-V semiconductors in high-speed computers, lasers, light-emitting diodes, solar cells, and other solid-state devices. Most of the attention has focused on GaAs crystalline material. Recent studies of atoms adsorbed on various crystal faces using scanning tunneling microscopy2q3and photoelectron spectroscopf are especially noteworthy. However, thin GaAs films with desirable properties can be produced by metal-organic chemical vapor deposition (MOCVD), as well as by conventional molecular beam epitaxy (MBE), generating much interest in the chemistry of precursor molec~les.~ Studies of GaAs clustersb* and GaAs liquidg have also recently appeared. Our interest was stimulated by a recent calculation'0 of the structure and vibrational spectrum of Lewis acid/base adduct, H3Ga-AsH3, which could be involved as a precursor in the CVD ( I ) Christou. A., Rupprecht, H. S., Eds. Gallium Arsenide and Relared Compounds 1987. Fourteenth International Symposium, Heraklion, Cred, 28
Sept-Oct 1987; 1OP Conference Series No. 91; Institute of Physics: Philadelphia, 1988. ( 2 ) Griffith, J. E.; Kochanski, G . P. Ann. Rev. Mater. Sci. 1990, 20, 219. (3) Stroscio, J. A.; Feenstra, R. M.; Fein, A. P. Phys. Reu. B. 1987, 36,
7718. (4) Epp, J. M.;Dillard, J. G. Chem. Mater. 1990, 2, 449. ( 5 ) Omstead, T. R.; Jensen, K. F. Chem. Maier. 1990, 2, 39. (6) Balasubramanian, K. Chem. Phys. Leu. 1988, 150, 71. (7) Zhang, Q. L.; Liu. Y.;Curl, R. F.; Tittel, F. K.; Smalley, R. E. J. Chem. Phys. 1988,88, 1670. (8) Wang, L.; Chibante, L. P. F.; Tittel, F. K.; Curl, R. F.; Smalley, R. E. Chem. Phys. Leu. 1990, 172, 335. (9) Zhang. Q.M.; Chiarotti, G.; Selloni. A.; Car, R.; Parrinello, M. Phys. Rev. B. 1990, 42, 507 1. (IO) Dobbs, K. D.; Trachtman, M.; Bock, C. W.; Cowley, A. J. Phys. Chem. 1990, 94, 5210.
of GaAs. We have suggested" that p-H bridged dimers of BH3, AIH,, and GaH3 can be thought of as Lewis acid/base adducts in which one portion of the molecules, the center atoms with empty p orbitals, interacts with the bonding electron pairs of a companion molecule, more popularly termed three-center two-electron bonds. The role of group IIIA hydrides as atom traps has intrigued us since we found evidence for a H3B-Si adduct in a study of silicon boron hydridesI2 and for a H3B-AI adduct in a study of aluminum boron hydride^.'^ Although there have been theoretical studies of various group IIIA and group VA hydrides reported previously (vide infra), none have undertaken a comprehensive study of the mixed hydrides as reported here. This study of the Ga-As system represents an investigation with binary third period, group IIIA (IUPAC group 13) and VA (IUPAC group 15) compounds in which both electron-poor and electron-rich hydrogen-containing fragments occur and in which relativistic effects, specifically the possible 4s2 inert pair effect,I4 may be encountered.
Computational Methods Ab initio calculations were performed by using the GAUSSIAN 85 and GAUSSIAN 88 package^^^.'^ Of programs and GAMESS 90'' ( I I ) Bock, C. W.; Trachtman, M.; Murphy, C.; Muschert, B.; Mains, G . J. J. Phys. Chem. 1 9 9 1 , 95, 2339. (12) Mains, G. J.; Bock, C.; Trachtman, M. J. Phys. Chem. 1989, 93, 1745.
(13) Mains, G. J.; Bock, C.; Trachtman, M.; Finley, J.; McNamara, K.; Fisher, M.; Wociki, L. J. Phys. Chem. 1990, 94, 6996. (14) Pyykko, P. Chem. Reu. 1988, 88, 563. ( 1 5 ) G A U S S I A N 8s calculations were carried out by using the program as described in: Hehre, W. J.; Radon, L.; Schleyer, P. v. R.; Pople, J. A. Ab Inirio Molecular Orbital Theory; John Wiley & Sons: New York, 1986. (16) Frisch, M. J.; Head-Gordon, M.;Schlegel, H. B.; Raghavarchari, K.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whitesides, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J . P.; Fluder. E. M.; Topiol, s.;Pople, J. A . G A U S S I A N 88; Gaussian, Inc.: Pittsburgh, PA.
0022-3654/91/2095-7668%02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1669
Orbital Study of the GaAs Hydrides
TABLE I: Total Molecular Energies (IumeeS) and Zero-Point Energiea (ZPE, kcrl/mol) for Gallium Hydrides
GaH level HF/3-21 G(*)//HF/3-21G(*)' ZPE//HF/3-2 1G( *) MP2/HF/3-21G(*)//HF/3-21G(*Y HF/HuZSP*//HF)HUZSP* ZPE//HF/HUZSP* HF/HUZSP**//HF/HUZSP* MPZ/HUZSP**//HFHUZSP* MP3/HUZSP*//HF/HUZSP*
MP4(SDQ)/HUZSP**//HF/HUZSP* MP4(SDTQ)/HUZSP**//HF/HUZSP* MP2=FULL/HUZSP**//MP2=FULL/HUZSP** figure 'GAUSSIAN
8s.
GaH/t
GaH,
GaH, -1 91 5.705 21
-1921.91 3 97 2.5 -1 921.91 5 86 -1921.96558 -1921.97380 -1921.97745 -1921.979 13 -1 921.973 73
-1 91 5.106 49 6.5 -1915.16861 -1922.52205 6.6 -1922.525 65 -1922.59644 -1922.607 90 -1922.612 16 -1922.614 15 -1922.605 25
11.9 -1915.781 91 -1 923.1 20 02 12.1 -1923.12553 -1923.21545 -1 923.229 67 -1923.23440 -1923.236 55 -1923.224 22
Ib
IC
Id
e-,,
C-,, -1914.55027 2.4 -1914.612 78 -1921.966 37 2.3 -1921.967 82 -1922.03300 -1922.045 40 -1 922.050 09 -1 922.052 13 -1922.041 42 la
c2n
nc nc nc
D?"
nc = not calculated.
program on the CRAY Y-MP/832 at the Pittsburgh Supercomputing Center, the CRAY X-MP/48 at NCSA, an IBM 3090 a t Oklahoma State University, and on several different VAX computers located in Philadelphia and Stillwater. A few potential energy surfaces were computed initially by using the 3-21G(*) basis set which resides in GAUSSIAN 85. For the majority of the calculations, we employed Huzinaga et al.'sl* (433321/4321/4*) basis sets, which are designated as HUZSP*, where a diffuse d function was added to Ga and As and the outer s and p shells were split. In cases where p functions were added to hydrogen,1° the basis set is designated as HUZSP**. Electron correlation was included by performing single-point calculations at the MP2, MP3, MP4SDQ, and MP#DTQ/HUZSP** levels using HF/HUZSP* geometries. Vibrational frequencies were obtained from analytical second derivatives calculated at the RHF/HUZSP*//RHF/ HUZSP* levels of computation. MP2(FULL)HUZSP** optimizations were performed in many cases, and for GaAs, CASSCF calculations were also implemented by using the HUZSP** basis set.
Results and Discussion GaH. Balasubramanian and Kim20 have published a detailed study of GaH using CASSCF and effective core potentials, and we report the results of our all-electron calculations for comparison only. The interaction of a Ga(2P) atom with a hydrogen atom produces the l2+ state shown in Figure la. The MP2=FULL/HUZSP**/MP2=FULL/HUZSP**bond length is computed to be 1.672 A compared with 1.663 A reported ex~erimenta1ly.I~Balasubramanian and Kim" find a bond length of 1.662 A. We calculate De to be 59.7 kcal/mol at the MP2=FULL/HUZSP**/MP2=FULL/HUZSP** level compared with -64.5 kcal/mol (e~perimental'~) and Balasubramanian and Kim's 64.7 kcal/mol. The 311state of GaH, shown in Figure I b, is 50.7 kcal/mol higher in energy a t the MP4SDTQ/HUZSP**// RHF/HUZSP* level, in good agreement with the experimental value of 49.3 kcal/mol and 48.1 kcal/mol reported by Balasubramanian and Kim. The total molecular energies and zero point energies are reported in Table I. GaH? Balasubramanian2' also recently reported a study of GaH2 using CASSCF and a core potential. He finds a Ga-H bond length of 1.61 A and a bond angle of 120.4' by using this method. We show our lowest energy structure in Figure IC. We find GaH2 to exhibit a bond angle of 1 19.5' in the ground state and a bond length of 1.600 A, both results suggesting considerable sp2 hybridization in the bond formation. We calculate De(17) GAM= 90. Schmidt, M.w.; Baldridge, K. K.; Boatz,J. A.; Jensen, J. H.; Kosecki, S.; Gordon, M.S.:Nguyen, K. A.; Winduss, T. L.; Elbert, S. T. QCPE Bull. 1990. 10, 52-54. (18) Huzinaga, S.; Andzelm, J.; Klobukowski, M.;Radzio-Andzelm, E.; Sakai, Y.;Tatewaki, H. Gaussian Basis Sets f o r Molecular Calculations;
Elsevier: Amsterdam, 1984. (19) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IK Constants of Diatomic Molecules: Van Nostrand Reinhold Co.: New York, 1979. (20) Kim, G.; Balasubramanian, K. J . MOL Speclrosc. 1989. 134, 412. (21) Balasubramanian, K. Chem. Phys. Lett. 1989, 164, 231.
Figure 1. GaH, and ASH, (x = 0-3). Approximate atomic localization of lone pair electrons is shown by vertical heavy arrows. Arrow pointing up is the a spin; arrow pointing down is the B spin. Numbers in parentheses refer to MP2=FULL//HUZSP**//MPZ=FLJLL/HUZSP'* values. (a) GaH IZ,RHF/HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): Ga-H = 1.6893 (1.6725). (b) GaH ,II UHF/ HUZSP*//UHF/HUZSP* structure. Bond lengths (angstroms): Ga-H = 1.6016 (1.5885). (c) GaH, In UHF/HUZSP*//UHF/ HUZSP* structure. Bond lengths (angstroms): Ga-H = 1.6002 (1.5854). Bond angles (degrees): HGa-H = 119.4 (120.35). (d) GaH, RHF/HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): Ga-H = 1.5821 (1.5686). Bond angles (degrees): H-Ga-H = 120.0 (120.0). (e) ASH IZ, RHF/HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): Ga-H = 1.5283 (1.5224). (0 ASH 'Z UHF/HUZSP*//UHF/HUZSP* structure. Bond lengths (angstroms): AS-H = 1 S290 ( 1 3248). (9) AsHz UHF/HUZSP*//UHF/HUZSP* structure. Bond lengths (angstroms): As-H = 1.5228 (1.5162). Bond angles (degrees): H-Ga-H = 92.3 (91.4). ( h ) ASH' RHF/ HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): As-H = 1.5169 (1.5106). Bond angles (degrees): H-As-H = 92.3 (91.4).
(HGa-H) to be only 41.2 kcal/mol, in good agreement with 40.6 kcal/mol computed by Balasubramanian. GaH, is not stable with respect to Ga(,P) and H,, suggesting that the energy required to break the lone pair on the Ga-H and H-H bonds is essentially that obtained by the formation of two Ga-H bonds. Total molecular energies and zero point energies are reported in Table I. CaHe Gallane and digallane have been the subject of numerous recent publications."~2'~2ZThe molecule, shown in Figure Id, exhibits D3h symmetry in the isolated state, typical for sp2 hybridization. We find a Ga-H bond length of 1.582 A, which compares well with 1.58 A reported by Balasubramanian2' before second-order configuration interaction. We calculate De(H2Ga-H) to be 75.8 kcal/mol compared with 80.9 kcal/mol computed by Balasubramanian. Thus, as noted previously,21GaH, is much less stable than might be expected because of the closed electron shells of both GaH and GaH,. Total molecular energies and zero point energies are reported in Table 1. (22) Lammertsma,
K.; Leszczynski, J. J. Phys. Chem. 1990. 94, 2806.
7670 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Bock et al.
TABLE II: Total Mokculrr Energha (hrtrees) and Zero-PointEaergies (ZPE, kcd/mol) for Arsenic Hydrides AsH/t ASH ASH,
level
HF/3-2 1 G(*)//HF3-21 G(*)' ZPE//HF/3-2 1G( *) MP2/HF/3-21 G( *)//HF/3-21G( *)' HF/HUZSP*//HF/HUZSP* ZPE//HF/HUZSP* HF/HUZSP**//HF/HUZSP* MP2/HUZSP**//HFHUZSP* MP3/HUZSP**//HF/HUZSP*
MP4(SDTQ)/HUZSP**//HF/HUZSP* MP4(SDTQ)/ HUZSP* *//H F/HUZSP*
MP2=FULL/HUZSPS*//MP2=FULL/HUZSP** ,. figure 'GAUSSIAN 85.
C."
c.0
nc nc nc -2232.719 3.2 -2232.721 -2232.804 -2232.819 -2232.822 -2232.823 -2232.812
le
-2224.11295 3.3
33
63 79 17 49 97 68
-2224.206 72 -2232.640 92 3.3 -2232.643 50 -2232.737 95 -2232.75479 -2232.759 13 -2232.76079 -2232.753 14 If
ASH?
c,
Ck
-2224.760 77 8.2 -2224.86002 -2233.285 54 8.2 -2233.290 08 -2233.396 88 -2233.413 91 3 -2233.417 644 -2233.41973 -2233.41251 Ig
-2225.347 09 14.7 -2225.465 12 -2233.871 16 14.7 -2233.877 81 -2234.009 73 -2234.029 1 I -2234.033 04 -2234.035 76 -2234.018 82 Ih
nc = not calculated.
ASH. Binning and C ~ r t i s have s ~ ~discussed ASH, ( x = 1-3), and the details of their analysis need not be repeated. As expected, the ground state is the 'Gstate shown in Figure le, obtained from interacting a hydrogen atom with a ground-state A s ( ~ S )atom. The As-H bond length found was 1.522 A at the MP2(FULL)HUZSP* */ / MP2( FULL)HUZSP** level compared with 1.520 A reported by Binning and Curtiss at the Hartree-Fock (HF) level and 1.534 A found e~perimental1y.l~Pairing the electrons on the As atom in As-H, vide Figure If, raised the energy by 40.6 kcal/mol at the MP4SDTQ/HUZSP**//HF/HUZSP* level, compared with the experimentally observed 30.3 kcal/mol for the isolated atom,24 suggesting an interaction between the unpaired electrons and the bonding electrons, perhaps signaling a change in hybridization. Total molecular energies and zero point energies are reported in Table 11. AsH2. We find an H-As-H bond angle of 92.3O at the UHF/HUZSP*//UHF/HUZSP* level and 91.4' a t the UMP2=FULL/HUZSP**//UMP2=FULL/HUZSP** level for the structure shown in Figure Ig, compared with 90.7O reported by Binning and Curtiss' using a modified Dunning basis set and 90.7O reported experimentally. The As-H bond lengths are 1.523 A at the U H F level and 1.516 A at the MP2=FULL level compared with 1.516 A by Binning and Curtiss and an experimental value of 1.5 18 A. The bond angles suggest little hybridization of the s2 lone pair electrons on the As atom. Total molecular energies and zero point energies are reported in Table 11. ASH,. See Figure 1h. The H-As-H bond angle in arsine was found to be 93.7O at the RHF/HUZSP*//RHF/HUZSP* level, compared with 94.1' found by Binning and Curtiss and 92.1' experimentally. Similarly, we find the As-H bond length to be 1.5 17 A, compared with the Binning and Curtiss value of 1.5 1 1 A, the latter being identical with the experimental value. The bond angles indicate little or no sp3 hybridization. Total molecular energies and zero point energies are reported in Table 11. CaAs. The lowest energy structure found, given in Figure 2a, corresponds to the 3Z- state as might be expected from interacting a Ga(2P) atom with the As('S) atom, both atoms preserving the 4s2 lone pairs in the combination. The primary orbital configuration is 4 s ~ ~ u * ~ 4 p u ~ a , ' The u , , ~u. orbitals are localized on the As atom and do not contribute to the bonding. Hence, a single bond distance is found and shown in Figure 2a. Promoting the 4su* /3 electron to the 4 p , orbital produces the 4sdu*'4pd?r,2?ry' configuration and results in the formation of a ?r bond. This leaves a single a spin electron on each of the Ga and As atoms, which gives a 3rI state. The bond distance is consistent with a double bond. The structure is given in Figure 2b. Spin pairing the 4su* electron and the 4p7r produces a I l l state, also with a double bond, shown in Figure 2c. Promoting both 4su* into the ?r system produces a 4 s ~ ~ u * ~ 4 p a ~ a~ I ,Z2 state ? r ~ ,which is triply bonded and is shown in Figure 2d. In this sequence of states, the CASSCF equilibrium bond distances decrease from 2.6033 A (single bond) to 2.3697 A and 2.3369 A (double bonds) to 2.2297 A (triple (23) Binning, R. C.; Curtiss, L. A. J . Chem. Phys. 1990, 92, 1860. (24) Moore. C . Atomic Energy Levels. NBS Circ. 1952. 467.
w tt (f)
(9)
Figure 2. GaAs and (GaA&. Approximate atomic localization of lone pair electrons is shown by vertical heavy arrows. Arrow pointing up is the a spin; arrow pointing down is the (3 spin. Numbers in parentheses
values. refer to MP2=FULL/HUZSPt*//MP2=FULL/HUZSP** Numbers in square brackets are CASSCF/HUZSP**//HUZSP** values with eight electrons distributed in eight orbitals. (a) GaAs )2UHF/HUZSP*//UHF/HUZSP* structure. Bond lengths (angstroms): Ga-As = 2.5978 (2.5627) [2.6033]. (b) GaAs 'II UHF/HUZSP*// UHF/HUZSP* structure. Bond lengths (angstroms): Cia-As = 2.4185 (2.3374) [2.3697]. (c) GaAs 'II UHF/HUZSP*//UHF/HUZSP* structure. Bond lengths (angstroms): Cia-As = 2.4294 (2.3718) [2.3369]. (d) GaAs 'ZtRHF/HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): Ga-As = 2.2457 (2.1688) [2.2297]. (e) GaAs 'A UHF/HUZSP*//UHF/HUZSP* structure. Bond lengths (angstroms): Ga-As = 2.5779 (2.5474) [2.6004]. (f) (GaAs), RHF/ HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): GaAs = 2.7016, As-As = 2.2728. Bond angles (degrees): Ga-As-As = 65.1, planar molecule. (g) (GaAs)z RHF/HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): Ga-As = 2.4191, As-As = 4.1527. Bond angles (degrees): Ga-As-As = 59.1, planar molecule. bond). Finally, the orbital configuration 4~u~u*~4p~??r~l?r~*' gives a ' A state, shown in Figure 2e, which is singly bonded again, with a CASSCF bond distance of 2.6004 A. B a l a s ~ b r a m a n i a n ~ ~ ~ reports a double minimum for the 'Z state, triply bonded at short distances. BalasubramanianZSbhas also found that the ground state of As, is triply bonded with a bond length of 2.164 A (CASSCF), not very different from the As2 experimental value25b of 2.103 A, nor very different from the triple bond distance for GaAs observed here. It is interesting to observe that the pentuplet state, Le., the state with four unpaired electrons, was unbound at the HF/HUZSP* level, as expected. The total molecular (25) (a) Balasubramanian, K. J . Chem. Phys. 1987.86, 3410. (b) Balasubramanian, K.J. Mol. Specrrosc. 1987, / 2 / , 465. (c) Balasubramanian. K . J. Phys. Chem. 1986, 90,6186.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7671
Orbital Study of the GaAs Hydrides
TABLE 111: Total Molecular Energies (hrtrcca) and Zero-Point Energies (ZPE, kcal/mol) for CaAs and CaAs Dimers GaAs('Z-) GaAs('II) GaAs(lII) GaAs(lZt) GaAs('A) level c-0 c-0 c-0 c-0 c-0 Ga2As2 nc nc nc nc nc -4137.55702 HF/3-21 G(*)//HF/3-21 G( *)" nc nc nc nc nc 0.4 ZPE//HF/3-2 IG(*) HF/HUZSP*//HF/HUZSP* -4153.58092 -4153.562 11 -4153.54771' -4153.49327 -4153.51854 -8307.21491 nc 1.4 nc nc nc 0.4 ZPE//HF/HUZSP* HF/HUZSP**//HF/HUZSP* -4153.58092 -4153.562 1 1 -4153.54771' -4153.49327 -4153.51854 -8307.21491 MP2/HUZSP**//HFHUZSP* -4153.715 18 -4153.70641 -4153.68527' -4153.67855 -4153.66575 -8307.59502 -4153.73574 -4153.72489 -4153.70486' -4153.690 10 -4153.688 15 -8307.60906 MP3/HUZSPD*//HF/HUZSP* MP4(SDQ)/HUZSP**//HF/ -4153.741 64 -4153.73036 -4153.71205' -4153.69794 -4153.695 1 1 -8307.61759 HUZSP* MP4(SDTQ)/HUZSP**//HF/ -4153.74608 -4153.73677 -4153.71995' -4153.71 171 -4153.70038 -8307.64390 HUZSP* MP2=FULL/HUZSPS*//MP2-4153.73250 -4153.72522 -4153.70337' -4153.69906 -4153.68320 nc FULL/HUZSP** CASSCF/HUZSP*//CASSCF/ -4153.61902 -4153.61072 -4153.591 Off -4153.58825 -4153.57754 nc HUZSPSb 2a 2b 2c 2d 2e 2f figure "GAUSSIAN 85.
GazAsz nc nc -8307.147 19 1.8 -8307.147 19 -8307.531 13 -8307.54461 -8307.551 86 -8307.58032 nc nc 2g
bEight electrons distributed in eight orbitals. 'Guess=Mixed used to generate this singlet state. nc = not calculated.
energies and zero point energies are reported in Table 111. Finally, we find a good correspondence between C A S S C F and HF geometries as well as structures for all t h e states considered here. Ga2Asz. Following Balasubramanian? we calculated the two rhombus structures shown in parts f and g of Figure 2. A s expected on simple bond strength considerations,6 the rhombus structure with t h e shortest As-As bond length, Figure 2f, is the more stable by 39.9 kcal/mol a t t h e M P 4 S D T Q / H U Z S P * / / HF/HUZSP* level. Since these are singlet states, it is difficult t o rationalize the structure in Figure 2f as t h e double adduct of two Ga atoms with Asz as Balasubramanian suggested. A study of the overlap population finds 0.280 electron between the terminal G a atoms and the As atoms. T h e As-As bond length, 2.273 A, is closer to that found experimentally6 for double-bonded As2, 3Z-u, suggesting the tetramer is not a triple A-As bond but a double As-As bond. T h e Ga atoms could then singly bond to each of t h e terminal A s atoms. However, t h e Ga-As bond in Figure 2f is 2.701 A, considerably longer than t h e single bond shown in Figure 2a. If a single bond were formed, it would leave the low lying p orbitals of G a in search of electron density, which would explain t h e orientation toward the other A s atom. Thus, the orientation of the G a atoms can be rationalized in terms of Lewis acid/base interaction. T h e Ga atoms a r e p-bridged across the two As atom and form symmetric three-center tweelectron bonds. I t is interesting to note that Balasubramanian% recently reported the ground state of GaAs, to be an isosceles triangle with a Ga-As bond length of 2.85 A and a As-As bond length of 2.2 A. These results a r e not inconsistent with t h e idea that the Ga(zP) atoms a r e p-bridged across the triplet state of As,. Turning briefly t o Figure 2g, we find t h e Ga-Ga SCF bond distance to be 2.482 A, a little longer than the Ga-Ga single bond found by Balasubramanianzk for 32- Ga2, and the Ga-As SCF bonds to be 2.419 A long, almost exactfy the bond lengths in Figure 2a. T h e molecule appears to be a double Lewis acid/base complex between two 'I: Ga-As molecules; the dissociation energy is 67.0 kcal/mol a t t h e MP4SDTQ/HUZSP**//HF/HUZSP* level. W e have not examined the linear structures, but like Balasubramanian, we suspect that Ga-As-As-Ga is the most stable and probably represents Lewis double adducts between two Ga atoms a n d the lone pair electrons of triple-bonded As,. In view of the many electronic states available t o GaAs, vide Table 111, it would be imprudent to speculate further on either of these two structures found thusfar. The total molecular energies and zero point energies are reported in Table 111. W e have not examined all the possible isomers of the dimer. CaAsHz. Simple Lewis theory would predict t h e structure shown in Figure 3 b t o be the most stable dihydride. However, the structure that places both hydrogens on the stronger bonding arsenic atom and leaves Ga in a monovalent condition, Le., Figure ~~
~~
(26) Balasubramanian, K.J . Chern. Phys. 1987, 87, 3518.
#
m
Figure 3. GaAsH,. Approximate atomic localization of lone pair electrons is shown by vertical heavy arrows. Arrow pointing up is the a spin; arrow pointing down is the spin. Numbers in parentheses refer to MP2=FULL//HUZSP**//MP2=FULL/HUZSP** values. (a) GaAsHz RHF/HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): As-H = 1.5251 (1.5212), Ga-As = 2.5807 (2.5428). Bond angles (degrees): H-As-H = 93.2, H-As-Ga = 90.2 (84.6). (b) HGa=AsH RHF/HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): Ga-As = 2.2462 (2.2385), Ga-H = 1.5685 (1.5588), As-H = 1.5340 (1.5367). Bond angles (degrees): H-Ga-As = 179.4 (180.4), H-As-Ga = 85.8 (79.6). (c) HGa-ASH IA" UHF/ HUZSP*//UHF/HUZSP* structure. Bond lengths (angstroms): HGa = 1.5815 (I.5588), Ga-As = 2.4431 (2.2383), As-H = 1.5167 (1.5367). Bond angles (degrees): H-Ga-As = 119.6, Ga-As-H = 94.8 (79.6). (d) HGa-ASH 'A" UHF/HUZSP*//UHF/HUZSP* structure. Bond lengths (angstroms): H-Ga = 1.61 12, Ga-As = 2.5044, As-H 3: 1.5224. Bond angles (degrees): H-Ga-As = 116.4, Ga-As-H = 94.8. Molecule nearly planar, T = 179.999. (e) HzGa-ASH RHF/ HUZSP*//RHF/HUZSP* structure. Bond lengths (angstroms): HGa = 1.5778 (1.5636), Ga-As = 2.3770 (2.3410). Bond angles (degrees): H-Ga-H = 123.9 (124.9), H-Ga-As = 118.1 (117.5).
3a, is more stable by 11.6 kcal/mol a t the MP4SDTQ/ H U Z S P * * / / H F / H U Z S P * level and 8.9 kcal/mol a t the MP2= FULL/ HUZSP* */HUZSP* *// M P 2 = FULL/HUZSP* * / H U Z S P * * level. There seems to be little doubt t h a t Figure 3a is the ground-state structure. T h e bond angles suggest t h a t A s has not hybridized to sp3; Le., the As bonds a r e pure p a bonds, and hence, by inference the lone pair on the As atom is composed of essentially pure s electrons. Gallium in the +1 oxidation state is not new;27 e.g., GazO, Ga2S, and G a X a r e often postulated in GaXz solids. This stability may be due to the 4s2 inert pair effect or possibly a simple manifestation of orbital contraction a s one proceeds across the 3d block. Regardless of the explanation, comparing the lower energy structure in Figure 3a to that in Figure (27) Cotton, F. A.; Wilkinson, G. Aduanced Inorganic Chemistry, 5th 4.; John Wiley & Sons: N e w York, 1988; p 229.
Bock et al.
7672 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 TABLE I V Total Molecular Energies (hartrees) and Zero-Point Energies GaAsH2 level c, -41 38.798 48 H F/ 3-2 1 G(*) / / H F/ 3-2 1 G( *)" 9.7 ZPE//HF/3-21G(*) -4138.97909 MP2/H F/3-2 1G( *)//H F/3-2 1G( *)' -4154.735 21 HF/HUZSP*//HF/HUZSP* ZPE/ / H F/ HUZSP* 9.6 -41 54.739 94 H F/ H UZSP88 / / H F/HUZSP* -41 54.924 73 MP2/HUZSP**//HFHUZSP* -4 154.948 96 M P3/H UZSP* * / / H F/ HUZSP* -41 54.955 25 MP4(SDQ) / HUZSP**//H F/ HUZSP* -41 54.961 28 M P4( SDTQ) / H UZSP* *//H F/ HUZSP* -4 1 54.943 69 M P2= FULL/ H UZSP* *//MP2= FULL/HUZSP* figure 3a "GAUSSIAN 8s.
(ZPE, kcal/mol) for Isomers of GaAsH2 HGa=AsH HGaAsH HGaAsH/t -41 38.775 39 8.7 -41 38.967 50 -41 54.709 54 8.7 -4 1 54.7 I 3 93 -41 54.909 68 -41 54.928 97 -4154.934 16 -41 54.942 77 -4 1 54.929 54 3b
H,GaAs
C.
c2B
nc
nc
nc -41 54.707 68 8.0 -41 54.723 81' -4154.891 776 -41 54.9 I3 4 9 nc -41 54.925 67' nc 5c
nc -41 54.706 37 8.0 -41 54.7 10 52 -41 54.873 84 -41 54.896 38 -41 54.902 64 -41 54.907 76 nc 3d
-41 38.737 00 8.4 -41 38.907 8 1 -41 54.674 16 8.5 -41 54.677 82 -4154.852 12 -41 54.876 64 -41 54.884 04 -41 54.889 61 -4154.871 48 3e
C,
'Spin projected. nc = not calculated.
TABLE V: Total Molecular Energies (hartrees) and Zero-Point Energies (ZPE, kcal/mol) for GaAsH, Isomers and ( H & ~ - A s H ~ ) ~ H2Ga-AsH2 HGa. .ASH, HAs..*GaH3 (H2Ga-AsH2)2 level C, c, C. C. AEdimer nc -41 39.949 54 -41 39.906 13 nc nc H F/3-2 1G( *)//H F/3-2 1G(*)" ZPE//HF/3-2 1G( *) 18.8 18.6 nc MP2/HF/3-2 1G(*)//HF/3-21 G(*)" -4140.14225 -4140.09300 nc nc -22.0 -4155.88439 -4155.837 16 -4155.78230 -831 1.803 83 H F/HUZSP*//HF/HUZSP* 18.9 18.2 16.9 nc ZPE//HF/HUZSP* -4155.89278 -4155.85572 -4155.79789 nc H F/HUZSP**// HF/HUZSP* -34.8 -4156.10231 -4156.05679 -4155.997 59 -8312.19346' MP2/HUZSP**//HFHUZSP* -32.6 -41 56.129 07 -41 56.088 01 -41 56.028 10 -83 12.240 616 M P3/ HUZSP* *//H F/ HUZSP* -32.3 MP4(SDQ)/HUZSP**//HF/HUZSP* -4156.135 72 -4156.09644 -4156.03694 -8312.255 23' nc M P4( SDTQ)/ HUZSP*/ / H F/ HUZSP* -4156.141 89 -4156.101 44 -4156.04229 nc nc MP2=FULL/HUZSP**//MP2=FULL/HUZSP** -4156.122 54 nc nc nc 4b 4c d figure 4a "GAUSSIAN 85.
MPX/HUZSP*//HF/HUZSP*. nc = not calculated.
Figure 4. GaAsH4. Approximate atomic localization of lone pair electrons is shown by vertical heavy arrows. Arrow pointing up is the a spin; arrow pointing down is the fl spin. Numbers in parentheses refer to MP2=FULL//HUZSP**//MP2=FULL/HUZSP** values. (a) H2Ga-as Hz R H F/ HUZSP* // RH F/ HUZSP* structure. Bond lengths (angstroms): H-Ga = 1.5815 (1.5677), Ga-As = 2.4431 (2.4125). Bond angles (degrees): H-Ga-H = 120.6 ( 1 21.3), H-Ga-As = I 19.6, Ga-As-H = 94.8 (93.2), H-As-H = 94.8 (94.2). (b) HGa-AsH3 R H F/ HU ZSP*/ / R H F/ HUZSP* structure. Bond lengths (angstroms): H-Ga = 1.6925, Ga-As = 3.4839. Bond angles (degrees): H4-Ga-As = 70.4, H-As-H = 94.9, Ga-As-H, = 139.0, Ga-As-H = 112.1.
3b suggests a localization of hydrogen impurities around As sites in the gallane/arsine CVD of GaAs. The double bond in the classical structure, Figure 3b, is fairly weak. Breaking the double bond to form the excited singlet state (Figure 3c) requires 10.7 kcal/mol at the MP4SDTQ/HUZSP**//HF/HUZSP* level; further breaking the Ga=As bond to form the triplet state, Figure 3d, requires only 1 1.2 kcal/mol at this level. The singlet dihydride with both H atoms on the Ga atom (Figure 3e) is, as expected, the least stable structure found, some 33.6 kcal/mol above the structure in Figure 3a. Total molecular energies and zero point energies are reported in Table 1V. CaAsH,. The lowest energy tetrahydride structure is the classical Lewis structure, shown in Figure 4a. Observe again that the bond angles made by As correspond to essentially pure p orbitals, suggesting that the lone pair has pure s characteristics,
Figure 5. (GaH2AsH2)2.Approximate atomic localization of lone pair electrons is shown by vertical heavy arrows. Arrow pointing up is the a spin; arrow pointing down is the fl spin. RHF/HUZSP*//RHF/ HUZSP* structure. Bond lengths (angstroms): H-Ga = 1.5760, H-As = I S O 1 2, Ga-As = 2.5822. Bond angles (degrees): H-Ga-H = 125.3, H-As-H = 100.1. The heavy atom system is planar.
as in Figure 3a. The H-Ga-H bond angle clearly supports sp2 hybridization for Ga. Rotating GaH, 90' produces the rotational transition state -3.0 kcal/mol higher in energy. Observe that the electron-poor Ga atom finds it expedient to hybridize to lower its energy, whereas the electron-rich As atom does not, suggesting a polarization of the s2 lone hybrid on As toward the empty Ga p orbital. Figure 4b is extremely interesting from the Lewis acid/base adduct viewpoint. The Ga-H moiety is orienting its empty p orbitals to share the lone pair on the As. This distorts the As-H bonds about 4 O from pure p bonding. The lone pair on Ga appears to distort the upper hydrogen atom, H3, significantly increasing the Ga-As-H3 bond angle by almost 20'. This may be the converse of the attraction of the Si-H bond by the empty p orbital found by Luke et a1.28in HSi-BH2, in essence an VSEPR effect. The geometry of this molecule is nearly the same as that of arsine and H-Ga; see parts a and h of Figure 1. The reason for the rotation of the Ga-bonded hydrogen, H4, toward arsine in this complex is not clear. We searched for an adduct between (28) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y.; Karni, M.; Chandrasekhar,J.; Schleyer, P.v. R. J . Am. Chem. Soc. 1986,108,270.
J. Phys. Chem. 1991, 95,7673-7679 gallane and As-H without success. We also searched for an adduct in which As was associated with four hydrogens tetrahedrally located about the gallium, as in tetrahydroboride compounds, but all starting structures dissociated or led to the structure in either Figure 4a or Figure 4b. Total molecular energies and zero point energies are reported in Table V. Ga2As2HB.The structure analogous to cyclobutane was found to be stable and is depicted in Figure 5 . The Ga-H and As-H SCF bond lengths in this dimer (1.576 and 1.501 A, respectively) differ only slightly from those in the corresponding monomer, H2Ga-AsH2, Figure 4a (1.58 1 and 1.517 A, respectively), at the RHF/HUZSP*//RHF/HUZSP* level. The change in the Ga-As bond lengths in going from the dimer (2.582 A) to the monomer (2.640 A) is somewhat larger than expected, but both of these bond lengths are much smaller than the Ga-As bond length (2.821 A) reported by Dobbs et al.'O at the same computational level. These results suggest a synergistic interaction between the dative bonds formed between the two GaHz-AsH2 monomers such that all four partners can achieve sd hybridization. The dimerization energy is almost exactly double the value reported at the same level for the H3Ga.-AsH3 adduct, 2 X 15.6 kcal/mol, and the dimerization is best viewed as a double Lewis acid/base interaction. While one might think of this as GaHz forming p bridges across H2As-AsH2, it should be emphasized that these bonds are genuine two-electron two-center bonds. Preliminary calculations suggest that this dimeric form of GalAs2H8is about 10 kcal/mol more stable at the R H F level than H(AsH2)Ga(p-H2)GaH(AsH2). The total molecular energies are reported in Table V.
Conclusions The acid character of GaH,, a consequence of its electron. deficient valence shell, and the base character of ASH,, a consequence of its electron-rich valence shell, manifest themselves in interesting ways. These Lewis acid/base characteristics,coupled with the fact that Ga-H bonds are inherently weaker than As-H
7673
bonds, explain the structures presented here and, the authors believe, all of the interesting chemistry reported thusfar. Just as ammonia preferentially chemisorbs on the 1 11A face of GaAs, containing only Ga atoms, one must expect that BH3 will preferentially chemisorb on the 1 11B face of GaAs, containing only As atoms. Indeed, since GaH, (x = 0-2) can both bond and Serve as an Lewis acid site, one might expect these moieties to serve as bridges between As atoms on the 1 1 1B face of GaAs. Leone et al.29report a potentially significant change in the kinetics of desorbing Ga from As-terminated Si( IOO), perhaps signaling a change in surface bonding at Ga merages below half a monolayer. The remarkable stability of monovalent Ga in Ga-AsH2 may be a manifestation of the fact that both Ga and As, as third period elements, try to preserve their s2 shell unhybridized. Whether this is an early indication of relativistic effects or simply the result of the 3d block contraction is not known at this time. The preservation of the s2 character has the ultimate effect of reducing the Lewis basicity of As, because the lone pair is spread all over the molecule, rather than localized as, for example, in ammonia, where sp3 hybridization effectively localizes the lone pair. Interestingly, gallium readily hybridizes to sp2 structures when substituents are present but As does not. In any case, one must expect to encounter monovalent Ga in GaH,-AsH,-type adducts as well as fragments of arsine in the MOCVD and CVD of GaAs. Acknowledgment. K.D.D. acknowledges a generous allotment of computer time on the CRAY X-MP/24 at the University of Texas. M.T. acknowledges support from the Pittsburgh Supercomputer Center, Grant No. CHRSSJP, for computer time on the CRAY Y-MP/832. G.J.M. is grateful for an NCSA Grant CHE890003N, which permitted a&ss to the CRAY-2 system, and to Oklahoma State University for access to the IBM 3090, which contributed to the understanding of these systems. (29) Smilgys, R. V.; Oostra, D. J.; Leone, S.R. J . Vac. Sci. Techno/. E 1990,8, 1102.
Anion Blndlng to Polar Molecules Harold Basch, Department of Chemistry, Bar- Ilan University, Ramat- Gan, Israel
M. Krauss,* and W . J. Stevens Biotechnology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received: February 11, 1991) The interaction of phosphoric acid and chlorinesubstitutedphosphoric acid with hydroxyl and chlorine anions yields a number of hydrogen-bonded and five-coordinate adducts. Stable hydrogen-bonded structures are found both for adducts with OHand C1- and for proton-transferred structures involving H 2 0 in the complexes H4POPO,Cl-, and H2P03Cl;. The hydrogen bond energy of the OH- and C1- adducts is calculated to be large due to the polar character of the P-O and 0-H bonds. Ligand-field analysis of the hydrogen bonding determines the electrostatic interaction as dominant for these adducts. Five-coordinate phosphorus structures corresponding to reaction intermediates are also found at a higher energy than the hydrogen-bonded structures. These adducts have very polar bonds. Axial C1 ligands are completely dissociative in all studied structures. Stability and binding energy trends upon chlorine substitution are related to the local polarity of the bonds.
Introduction The electronic characteristics of the bonding in the phosphates determines the mode of nucleophilic attack and the subsequent reaction. Recent ab initio calculations have found that the P-O bond is described as essentially a single, polar bond in molecules as varied as PH30, HP03, and Po3-.1-3 We have extended the ( I ) Kutrelnigg, W. Angcw. Chem., Inr. Ed. Engl. 1984, 23, 272. (2) (a) Rajca, A.; Rice, J. E.; Streitweiser, A. Jr.; Schaefer, H. F. Ill J . Am. Chem. Soc. 1987,109,4189. (b) Streitweiser, A., Jr.; McDowell, R. S.; Glaser, R. J . Compur. Chem. 1987. 8, 188.
0022-3654/91/2095-7673$02.50/0
analysis to the phosphates with the same conclusion for both the anion and neutral phosphoric acid.4 There is an appreciable d s population in the phosphate (approximately 0.3) on the P atom, which has been ascribed to back-bonding from the 0 to the P atom.s However, electronic properties such as the moments and (3) Cramer, C. J.; Dykstra. C. E.; Denmark, S.E. Chem. fhys. k r r . 1987, 136, 17. (4) Basch, H.; Krauss, M.; Stevens, W . Comparison of the Electronic Structure of the P-O and P-S bonds. Submitted to 1. Mol. Sfrucr. ( T h e e chem).
0 1991 American Chemical Society
