12824
J. Phys. Chem. 1994, 98, 12824-12827
Observed and Calculated Raman Spectra of the GazHs and Ga& Molecules Philip F. Souter, Lester Andrews,* Anthony J. Downs,* and Tim M. Greene Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, U.K. OX1 3QR
Buyong Ma and Henry F. Schaefer, III* Center for Computational Quantum Chemistry, The Universip of Georgia, Athens, Georgia 30602 Received: August 17, 1994@
The Raman spectra of both Ga& and Ga2D6 isolated in solid matrices at low temperature reveal the five strongest Raman-active fundamentals in excellent agreement with the newly calculated Raman intensities and frequencies.
Introduction After a checkered history going back nearly half a century, the elusive binary hydride of gallium, [GaH31n, was first authenticated only recently1V2as a major product of the reaction between monochlorogallane and lithium tetrahydridogallate at subambient temperatures. That the compound vaporizes at low pressures predominantly as the digallane molecule, H2GabH)2GaH2, with a D z structure ~ akin to that of B2H.5, has been deduced partly on the evidence of the electron diffraction pattern of the vapor but primarily on the basis of the infrared spectrum of the compound in the gaseous and matrix-isolated For such a centrosymmetric molecule the Raman spectrum provides additional, complementary information and, importantly, confirmation of the structure and bonding model. Ab initio molecular orbital theory has demonstrated clearly its success in predicting the harmonic vibrational frequencies of polyatomic molecules, a capability which should apply equally to symmetric fundamentals and Raman intensities as well as to antisymmetric modes and infrared inten~ities.~-~ We report here a comparison of the observed and calculated Raman spectra of digallane, which are in excellent agreement.
Experimental and Calculations Digallane and [2H.&ligallane were synthesized by the reaction between monochlorogallane and lithium tetrahydridogallate and between their deuterio derivatives, respectively, in accordance with the method described previously.1,2 Owing to the fragile nature of the product, the reaction was carried out in purposebuilt, all-glass apparatus, which had been "flamed out" rigorously under vacuum so as to remove any traces of moisture or other volatile impurities. The thermal instability of digallane meant that great care had to be taken to ensure that the temperature of the walls of the reaction vessel and attached traps never exceeded ca. -20 "C. The purity of the product was checked by reference to its infrared and 'H NMR In order to measure the Raman spectrum of a solid matrix incorporating digallane, an all-glass inlet system was constructed consisting of a jacketed, single-sleeved tube with a 3 mm i.d. and a 12 mm o.d., as illustrated in Figure 1. The jacket and sleeve combination facilitated uniform cooling of the inner nozzle through the turbulence created in the flow of the coolant gas, thereby ensuring that the inlet system was held at ca. -30 "C. The all-glass inlet system was connected to the matrix apparatus via a bored-through l12 in. Ultratorr fitting which @
Abstract published in Advance ACS Abstracts, November 15, 1994.
0022-365419412098-12824$04.5010
allowed the nozzle to be pulled in and out while maintaining a vacuum seal; this permitted deposition of the sample, followed by withdrawal of the nozzle, rotation of the inner window, and then excitation of the Raman spectrum of the deposit. The sample of digallane or [2H6]digallanewas contained in a glass ampule attached to the inlet assembly via a break-seal. Once this had been opened, the contents of the ampule were transferred to a new, as yet unsealed, ampule which was in turn joined to the spray-on unit and to a supply of the matrix gas, care being taken to ensure that all the relevant surfaces were cooled below -20 "C. The sample was held at ca. -50 "C and its vapor mixed with the matrix gas, which had been precooled, before deposition via the spray-on unit on a CsI window held at ca. 20 K and at an angle of 45" to the nozzle (see Figure l).' Close approach of the nozzle to the substrate (8-10 mm) allowed the formation of a domed matrix deposit. In early trials the mixture of compound and matrix gas was sprayed on for the whole experiment, but later experiments revealed that equally intense spectra could be obtained by spraying on a bed of matrix gas alone prior to "doming off' this deposit with the mixture. Illumination of the sample by the output from an Arf laser caused the whole dome to scatter, thereby imparting significantly higher intensity to the scattering than could be achieved from a flat condensate. The Raman spectra of both digallane and [2H6]digallanewere thus recorded on a Spex Ramalog 4 spectrophotometer at a resolution of 2 cm-'. For the calculations, Dunning's 14sllp5d basis set for gallium8 was contracted to 10s8p2d, the orbital exponents for the polarization functions being %(Ga) = 0.16. Two different basis sets were used for hydrogen, namely, (i) DZP, a standard Huzinaga-D~nning~~~ contracted Gaussian basis H(4sl2s) augmented with a set of p functions on the hydrogen atoms [ap(H) = 0.751, and (ii) TZP, a Hunzinaga-Dunning H(5d3s) contracted Gaussian basis set augmented with a set of p functions on the hydrogen atoms [ap(H) = 0.751. Harmonic vibrational frequencies and IR and Raman intensities were determined using analytical third-derivative methods at the SCF level with the aid of the program PSI developed by the Schaefer research group.l0 Estimates of the frequencies and infrared intensities have been reported previ~usly;~ Raman intensities for comparison with the new experimental observations are reported for the first time.
Results and Discussion The spectra of digallane, Ga&, and [2H6]digallane,Ga2D6, have been measured with the molecules isolated in either a 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 49, 1994 12825
Letters Thermocouple
Bored-through ID" Ulaatorr fitting
Sample in
\
/ Refrigerator
/
Nl
r/
/
Coolant out
/Heater
\
lmal
Dome of deposit Laser
Caesium iodide window
.
Figure 1 Schematic cross-sectional view of the matrix Raman apparatus.
TABLE 1: Observed and Calculated Vibrational Fundamentals of the GazHs MoleculPb predicted activity predicted infrared Raman depol observed mode frequency (in intensity' intensityd ratio, e frequency (in cm-l) 0.093 2000,g 1986," 1979' 0 556 V I (a,) 2082 v2 v3
(ag)
(a,) v4 (ag) v5 (au) v6 ( b g )
~1 ( b g ) YE ( b d
~9
(b2J
(b7.u)
VIO v11 (big)
v12 h g ) VI3
(blJ
v14 (blu) VI5 ( h g ) VI6
(b3u)
VI9 (b3u) v18 (b3u)
1571 792 238 473 1321 374 2075 872 239 2068 498 1249 693 82 1 2074 1400 729
0 0 0 0 0 0 488 220 10 0 0 406 20 1 0 144 1365 785
100 20 5 0 6 7 0 0 0 163 5
0 0 34
0 0 0
0.213 0.7 0.16 0 0.75 0.75 0 0 0 0.75 0.75 0 0 0.75 0 0 0
1474,g 1471," 1461' 736,g 733,h 733'
not obs not obs not obs not obs 1999 76W
not obs 2011,g 1997," 1987'
not obs 1202) 652' 763,g 761," 752' 1976' 1279 671'
observed activity infrared Raman intensity' intensity/ 0 0 0 0
0 0 0 30 30 0 0 0 50 30 0 20 50 100
100 10 20
0 0 0 0 0 0 0 15 0 0 0 20 0 0 0
a Obstrved spectra relate to the vapor (infrared) and the matrix-isolated molecule (infrared and Raman). Calculated at the TZP SCF level of theory. Calculated infrared intensity (km mol-'). Calculated Raman intensity (A4 amu-I). Observed infrared intensities relative to the strongest band set to 100 arbitrary units. /Observed Raman intensities relative to the strongest band set to 100 arbitrary units. 8 Observed Raman shift for an argon matrix. * Observed Raman shift for a dinitrogen matrix. Observed Raman shift for a methane matrix. j Observed gas-phase infrared frequency (ref 2). Observed infrared frequency for a dinitrogen matrix.
dinitrogen or an argon matrix, with the results shown in Figures 2 and 3. The depicted Raman shifts are from the 514.5 nm laser line, but identical spectra were obtained using the 488.0 and 476.5 nm lines. Both sets of spectra reveal five Ramanactive fundamentals, namely, V I , v2, v3, ~ 1 1 ,and Y15. The dinitrogen matrices displayed one band for each fundamental, whereas the argon matrices revealed two such bands. This could be due either to matrix site effects (cf the infrared spectra of digallane in these two matrices2) or to a degree of aggregation in the argon matrices resulting from less efficient isolation. The second of these two explanations seems the more likely in view of experiments carried out with different concentrations of digallane; at lower concentrations the bands at higher wavenumber gained in intensity relative to their lower energy neighbours which may thus originate in an oligomer. Methane matrices also revealed only one band per fundamental. Al-
though the dinitrogen matrices were clearer and gave better isolation of the digallane, the Raman shifts of the Ga2H6 and GazDa molecules trapped in argon are almost certainly nearer to the true gas-phase values. The five observed fundamentals are indeed predicted by the TZP SCF calculations to be the most intense bands in Raman scattering, and the predicted relative intensities match remarkably well the measured intensities, although ~3 and ~ 1 scatter 5 somewhat more strongly than predicted. Depolarization measurements showed that only V I and v2 exhibited 4 values significantly less than 0.75 (ca. 0.5 in each case). Comparisons with the calculated 4 values suggest that the scattering of the matrix prevents the measurement of true depolarization ratios, but find pleasing agreement in that the two bands with the smallest predicted depolarizationratios were measurably polarized in practice.
12826 J. Phys. Chem., Vol. 98, No. 49, 1994
Letters
TABLE 2: Observed and Calculated Vibrational Fundamentals of the Ga$( Moleculdb predicted activity observed activity predicted infrared Ra" depol observed infrared Ra" mode frequency (in cm-l)b intensityc intensityd ratio, e frequency (in cm-l) intensityC intensity 0 100 275 0.095 1433,g 1424," 1420' v1 (ag) 1476 0 0 30 51 0.108 1060: 1059." 1050' 1112 0 vz (ag) 9 0.72 527,g 525." 523' 0 30 v3 (a,) 565 0 0 0 5 0.18 not obs 234 0 v4 (ag) 0 0 not obs 0 0 v5 (au) 335 0 0 0 3 0.75 not obs v6 (be) 935 0 3 0.75 not obs 0 0 v.i (bg) 274 0 0 0 1439 40 0 v8 (b2u) 1483 257 0 0 55Y 20 0 ~9 (b2u) 623 111 0 0 not obs 0 0 VIO (bzu) 169 5 81 0.75 1451,g 1442." 1434' 0 30 V I I (big) 1478 0 3 0.75 not obs 0 0 V I Z (big) 363 0 86@ 100 0 892 215 0 0 ~ 1 (blu) 3 0 0 439k 25 0 VI4 (blu) 495 100 VI5 (b3g) 581 0 17 0.75 543,g 543," 536' 0 30 0 0 1416, 20 0 VI6 (b3u) 1469 61 25 0 0 0 92Y VI7 (b3u) 995 711 0 0 4841 100 0 V I E (b3J 523 396 a Observed spectra relate to the vapor (infrared) and the matrix-isolated molecule (infrared and Raman). Calculated at the TZP SCF level of theory. Calculated infrared intensity (km mol-[). Calculated Raman intensity (A4am&). e Observed infrared intensities relative to the strongest band set to 100 arbitrary units. fobserved Raman intensities relative to the strongest band set to 100 arbitrary units. 8 Observed Raman shift for an argon matrix. Observed Raman shift for a dinitrogen matrix. ' Observed Raman shift for a methane matrix. I Observed gas-phase infrared frequency (ref 2). Observed infrared frequency for a dinitrogen matrix.
(b)
I 603
I
too
1600
I \I
1
I
I
2100
500
Rarmn r h l f t l c m - I
1000
I500
Ramon shlft I c m - l
Figure 2. Raman spectrum of the Ga& molecule isolated in (a) argon and (b) dinitrogen matrices at 20 K.
F
In the case of GazD6, the region between 1050 and 1110 cm-' revealed, in addition to the v 2 fundamental near 1059 cm-l, two extra bands at 1093 and 1108 cm-l, presumably associated with the overtones 2vg and 2 ~ 1 5 ,the intensities of which are enhanced through Fermi resonance with V Z . Methane matrices also showed two extra bands at 1079 and 1098 cm-l with a similar pattem of intensities. Ratios of the observed and theoretical frequencies for digallane and d i b ~ r a n e ~make , ~ ~ an interesting comparison. Similar motions are found to give similar ratios: for example, for V I , vg, ~ 1 1 ,and V16, the terminal M-H stretching fundamentals, VobdVcdc varies between 0.952 and 0.972 for digallane (M = Ga) and between 0.933 and 0.936 for diborane (M = B). For v 2 , v6, ~ 1 3 and , v17, the M-H-M stretching fundamentals, Vob&calc varies between 0.909 and 0.962 for digallane and between 0.896 and 0.940 for diborane, and for v 3 , v g , ~ 1 5 and , v18, the terminal MH2 wagging, twisting, and scissoring motions, v&&c& varies between 0.872 and 0.929 for digallane and between 0.859 and 0.920 for diborane. The GaH2 wagging mode vg at 760 cm-I gives the largest discrepancy between theory and experiment (vobdvc& = 0.872), but this is also the motion in diborane with the largest error in prediction (YobJ vCdc= 0.859), tending to suggest that the difficulty in modeling the potential surfaces associated with these types of vibration is a universal one. That the ratios of vobS/v& are slightly higher
for the deuteriated molecule (e.g.,0.961 for the v1 fundamental of Ga& vs 0.970 for Ga&) reflects reduced anharmonicity for deuterium compared with hydrogen vibrations. Attempts to locate other Raman-active fundamentals proved fruitless, but the intensities of these features were thus shown to be wholly consistent with the scattering cross sections predicted by the ab initio calculations. One notable absence from the measured Raman spectrum is any clear sign of the lowest energy ag mode, v 4 , involving breathing of the central Ga@-H)zGa ring and approximating most nearly to what could be regarded as a Ga* G a stretching motion. However, the low intensity of this mode in Raman scattering is well anticipated by the calculations, thereby affording relatively clear evidence that, despite the closeness of approach, no bond path exists between the two gallium atoms of the molecule! As with diborane,12 so too with digallane, it may yet be possible to locate the remaining Raman-active vibrations, as well as the a, mode, v5 (silent in both infrared absorption and Raman scattering), and the low-frequency b?, mode, v10, by searching the IR spectrum for signs of combination bands implicating the relevant fundamentals.
i 3. Raman spectrum of the Gaz& molecule isolated in (a) argon and (b) dinitrogen matrices at 20 K.
Conclusion In summary, the Raman spectra of matrix-isolated digallane and [2€Lj]digallane have been measured and reveal the five most
Letters intense Raman-active fundamentals. The band positions, intensities, and polarization properties are in excellent agreement with the theoretical values based on a D2h diborane-like structure.
Acknowledgment. The authors thank EPSRC and NSF for financial support of this research and for the award of a studentship (to P.F.S.)and Y. Yamaguchi for helpful discussion. L.A. is a Sesquicentennial Associate of the University of Virginia and a Senior Fulbright Scholar. References and Notes (1) Downs, A. J.; Goode, M. J.; Pulham,C. R. J. Am. Chem. Soc. 1989, 1936.
111,
J. Phys. Chem., Vol. 98, No. 49, 1994 12827 (2) Pulham,C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.; Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149. (3) Liang, C.; Davy,R. D.; Schaefer,H. F., I l l Chem. Phys. Lett. 1989, 159, 393. (4) Lammertsma, K.; Leszczynski,J. J. Phys. Chem. 1990, 94, 2806. (5) Duke, B. J. J . Mol. Struct. (THEOCHEM) 1990, 208, 197. (6) Shen, M.; Schaefer, H. F., III J . Chem. Phys. 1992, 96, 2868. (7) Downs, A. J.; Hawkins,M. Adv. Infrared Raman Spectrosc. 1983, IO, 1. (8) Dunning, T. H., Jr. J. Chem. Phys. 1977, 66, 1382. (9) Huzinaga, S. J . Chem. Phys. 1965, 42, 1293. (10) Janssen, C. L.; Seidl, E. T.; Hamilton, T. P.; Yamaguchi, Y.; Remington, R.; Xie, Y.; Vacex, G.; Sherrill, C. D.; Crawford, T. D.; Fermann, J. T.; Allen, W. D.; Brooks, B. R.; Fitzgerald, G . B.; Schaefer, H. F., III PSITECH, Inc., Watkinsville, GA, 1994. (11) Duncan, J. L. J. Mol. Spectrosc. 1985, 113, 63. (12) Duncan, J. L.; McKean, D. C.; Torto, I.; Nivellini, G. D. J . Mol. Spectrosc. 1981, 85, 16.
