J . Phys. Chem. 1993, 97, 1304-1 3 1 1
1304
Electron Spin Resonance Investigations of AlH2, AlHD, AlD2, AI(OH)2, and Al(OD)* in Neon Matrices at 4 K: Comparisons with ab Initio Theoretical Calculations Lon B. Knight, Jr.,’ James R. Woodward, Thomas J. Kirk, and C. A. Arrington Chemistry Department, Furman University, Greenville, South Carolina 2961 3 Received: August 3, 1992
The first electron spin resonance (ESR) results are reported for the divalent aluminum radicals AlH2, AlHD, AID2, Al(OH)2, and Al(OD)2. The radicals were generated by 254-nm photolysis of neon matrices at 4 K containing aluminum atoms codeposited with H2O or D2O. For low water concentrations in the approximate range 0.1-0.2% (H2O relative to neon) the AlH2 radical was produced, while Al(OH)2 was observed for higher water amounts in the 1-2% range. Configuration interaction (CI) theoretical calculations of the nuclear hyperfine interactions ( A tensors) were conducted a t the optimized geometry for AlH2 and the three conformers of Al(OH)2. The calculated isotropic and dipolar coupling constants showed reasonably close agreement with the observed values. On the basis of these magnetic parameters, the electronic structures of nine small aluminum radicals are compared. A correlation between the charge on aluminum and the magnitude of its Aisoparameter is discussed.
Introduction The first electron spin resonance (ESR) investigations of the divalent aluminum radicals AlH2 and Al(OH)2 are presented. Earlier gas-phase electronic spectroscopic studies of AlH2 established its Cz, ground state as XZAwith a bond angle of 120’ and a metal-hydrogen bond length of =1.59 A;I infrared bands for AlH2 in krypton matrices have also been reported; however, attempts to observe its ESR spectrum in these samples were unsuccessful.2 The experimental conditions under which AlH2 is observed by ESR in neon matrices suggest the occurrence of a rather unusual and interesting photochemical reaction pathway. The simultaneous deposition of thermally generated aluminum atoms and H20(g) (or D2O) produced intense ESR signals of aluminum atoms and HAlOH radical^.^ The UV photolysis of these matrices at =4 K yielded highly resolved ESR spectra of AlH2 (or AlHD/AlD2) provided the water content of the neon matrix was kept below approximately 0.1%. Matrix samples prepared under similar conditions but with higher water concentrations yielded another new aluminum radical, which is assigned to Al(OH)2. An excellent theoretical analysis of several small aluminum radicals has been reported recently, which includes thecalculation of the isotropic nuclear hyperfine parameters for AlH2 and Al(OH)2.4 These calculations employed unrestricted MP2 spin density matrix derived Fermi contact integrals. As part of this experimental study, we have calculated the isotropic and dipolar components of the A tensors for various geometries of these radicals to help facilitate the spectroscopic assignment and to more fully understand the magnetic interactions. The results of our CI type calculations, which included all singly and selected doubly excited configurations, are compared with the alternate theoretical approach described above. Both methods yielded results in reasonably close agreement with these neon matrix experimental measurements. The small number of valence electrons makes AlH2 an excellent candidate for such an experimental-theoretical comparison. The reasonably close agreement with theory, the generation conditions, and theexpected ESR spectral changes that occurred with deuterium substitutions in AlHD and AID2 confirm the spectroscopic assignment. In addition, the large AI (I = 5/2) hyperfine interaction that was observed is consistent with the trend that has been established for other small aluminum-centered radicals studied by matrix isolation ESR spectroscopy: A1160;5AlH+ and A1D+;3 YA1+;6 AlF+;’ HAIOH;’ and HAICH3.8 Other small aluminum radicals 0022-3654/93/2091- 1 304f04.00/0
studied by ESR in neon/argon matrices include A ~ ( C O ) ZAls,lo ,~ AlC (X4Z),I1and the intermetallic species PdA1.lZ The codeposition of aluminum atoms with NH3 and small organic reactants such as buta-1,3-diene in adamantane at 77 K has also been studied by ESR.13 A detailed ESR study of aluminum atom codeposition reactions with ethylene and acetylene in rare gas matrices has been reported.14 The ethylene complex has a *-coordinated structure, while a a-bonded adduct of vinyl structure was reported for the aluminum-acetylene product. While numerous diatomic metal hydrides (Le., BeH, HgH) having X2Z ground states have been studied by rare gas matrix ESR,I5only a few metal dihydrides have been reported, namely, ScHz, YH2, PdH2+,CoHz, RhH2, and IrH2.16 The ESR information obtained in such an inert environment provides direct information concerning the electronic structure of the valence region. The observed magnetic parameters for AlH2 will also be compared with those previously reported for the isovalent BH2 radical,” so that an electronic structure comparison can be made between “simple” first- and second-row radicals. The matrix codeposition of aluminum and other metals with water was thoroughly investigated in earlier vibrational and electronic spectroscopic studies. Aluminum atoms were observed to react spontaneously with water to form the insertion product HAlOH, while the heavier group-IIIA metals only formed water adducts before photolysis.I8 The importance of understanding fundamental water/metal atom reactions has been described previously. Catalytic abilities of such reactions may involve assisting H atom transfer.I8J9 The presence of Al(0H)Z in aluminum combustion reactions has been proposed, but no spectroscopic or other direct detection of this species has been reported previously.20 Knowledge concerning the reactions of transient aluminum compounds under high-energy conditions is important for numerous applications, given the widespread use of such materials. Hydrogen or other molecular solidscontaining metal atoms are thought to be among the best prospects for highenergy density materials.21 For example, calculationsconducted on A12H6 indicate such potential.22 Efforts are currently under way in our laboratory to generate the AIH3+ and A12Ha+ cation radicals for matrix ESR study. The rare gas matrix isolation technique has been used to study a wide variety of small neutral radicals including high-spin carbon clusters23and metal cluster^.^^^^^ Generation methods have also been developed for matrix ESR investigations of isolated radical cations and anions, including photoionization, electron bom0 1993 American Chemical Society
Electron Spin Resonance Investigations
I-J I \
Figure 1. Rare gas matrix ESR apparatus used in these studies for the photochemical generation of AlH2 and Al(OH)2 is shown in schematic form. A resistively heated high-temperature effusion oven was used to produce Al(g). The three ports labeled 'W" are windows; the 9-in. electromagnet with a IO-cm gap is mounted on tracks and is rolled into position when needed for recording ESR spectra. The hydraulic lifter assembly moves the matrix target between the deposition position and the X-band microwave cavity, which contains the 100-kHz modulation coils.
bardment, and X-irradiation.25-z7 Electron transfer from sodium atoms to ethylene oxide molecules in argon matrices has been shown to produce the ring-opened anion radical.28 Recent examples of matrix ESR studies in our laboratory include CH30H+,27CH3F+,29BNB,30BNH,31,GaAs+, GaP+,32C2+,33 CH2-,17 Si2+, and Ge2+.34
Experimental Section The rare gas matrix isolation apparatus and associated ESR equipment have been describedp r e v i o ~ s l y A . ~liquid ~ ~ ~helium ~~~~ Heli-Tran (APD) cryostat was used to cool thecopper flat matrix deposition target ( 5 cm, 6 mm width, and 1.30 mm thick) to approximately 4 K. In another similar cryostat a closed-cycle helium refrigerator capable of 4 K operation was employed (APD Heli-Plex HS-4). For both cryostats, a hydraulic system was used to move the attached matrix target between the deposition position and the X-band microwave cavity, which was located inside the high-vacuum apparatus. See Figure 1. Severaldifferent arrangementswere tried for achieving a steady and reproducible rate of aluminumvaporization sothat the amount of water and other matrix deposition conditions could be varied in a systematic and reproducible manner in a series of different experiments. The direct vaporization of A1 from resistively heated tantalum or molybdenum effusion ovens is not suitable since molten aluminum alloy "wets" the tantalum tube, which causes abrupt changes in the vaporization rate and inhibits the ability to read the surface temperature in a consistent manner with an optical pyrometer. A satisfactory arrangement that yielded reproducible vaporization conditions was achieved by using an aluminum oxide ceramic liner inside a 9-mm4.d. tantalum tube (0.020-in. wall thickness). The Ta tube was 5 cm in length and was supported by Ta end pieces and straps mounted on watercooled copper feed-through electrodes, which supplied the high amperage heating current.33 The effusion orfice in the center of the cell assembly had a diameter of 2 mm. Prior to loading, the oven assembly was outgassed to approximately 2200 K for several hours until the background pressure in the matrix apparatus dropped to 2 X Torr. After loading the liner with spectroscopicgrade aluminum (Alfa), the cell was outgassed overnight at approximately 1000 K before the start of a given series of matrix experiments. A quartz microbalance (QM 301 Veeco) mounted in the matrix deposition vicinity was also used to monitor the vaporization process. An optical pyrometer was used to measure the outer Ta surface temperature, which was typically in the range 1050-1 150 OC for most of the aluminum deposition experiments. On some experiments the neon matrix gas and H20(g) were introducedthrough separate inlet tubes. The neon flow wasvaried
The Journal of Physical Chemistry, Vol. 97, No. 7. 1993 1305 over a wide range but was typically maintained at 5.0 std cm3 min-' as measured by a thermal conductivity flow meter (Tylan FM 360). The relative amount of H*O(g) introduced was monitored by a thermocouple gauge mounted in the water vapor supply line. While this method of gas introduction proved to be reproducible, it could not provide an accurate ratio of neon to H2O(g) actually deposited. Accurate ratios were determined by preparing neon/H20 mixtures in a 5-Lheated glass reservoir by using standard vacuum techniques and two capacitance-type pressure gauges-one calibrated up to 1000 Torr for neon and the other in the 0-100-Torr range for H2O(g) vapor pressure measurements. Conversion from a series of experimentsinvolving H20 to D2O required extensive passivation before deuteriumsubstituted radicals were observed. A 100-W low-pressure mercurylamp (GEHlOOA4/T) wasused toirradiate thematrix samples at 254 nm following deposition. The outer Pyrex shroud of this lamp was cut away. Unfocused output from the lamp, located approximately 10 cm from the matrix sample, passed through a 2-in. diameter fused silica vacuum window. The matrix sample exhibited no temperature rise during the UV irradiation process, which was usually conducted for 30 min.
Results Neon and argon blank deposition experiments conducted in the manner described above with an aluminum vaporization temperature of approximately 1100 OC and no intentional introduction of water vapor yielded aluminum atom ESR signals as previously de~cribed.~JS The only other radicals detected were low-level background amounts of H, CH3, and HCO, which are commonly observed when high energies are present during such deposition^.^^ Irradiation at 254 nm of these blank neon and argon aluminum depositsyielded no new radicals in the magnetic field range 0-9000 G. Aluminum deposition in neon and argon matrices with water amounts in the 0.1-2.0% range yielded similar results to those described above plus the generation of the HAlOH insertion radical product. A new study of this radical, whose ESRspectrum was previously assigned on a preliminary basis,j has been made that involves deuterium substitution and the first report of I7O hyperfine interaction (hfi).35 Irradiation of these argon matrices produced increases in the H atom signals and small decreases in the amount of HAlOH present, however, neither AlH2 nor Al(OH)2 radicals were observed in argon matrices in any of these experiments. For neon samples, the irradiation-induced changes were dramatically different. For neon matrices containing water amounts in the low range of 0.054.2%, irradiation caused the appearanceof an intense sextet-of-triplets hyperfine pattern, which is assigned to AlH2 as discussed below. The HAlOH radicals initially present before irradiation were nearly eliminated and a 100-fold increase in the H atom signals over background levels was also observed for these 'low water" neon matrices. For high water amounts (1-2%), irradiated neon matrices yielded no detectable amounts of AlH2 but exhibited a new and intense sextet hyperfine pattern, which is assigned to Al(OH)2. By contrast, the irradiation-induced growth in the H atom signals for these 'high water" matrices was small. The different spectral changescaused by irradiating neon matrices prepared with similar A1 atom amounts but with different water contents are summarized in Figure 2. It should also be reported that irradiation of blank neon and argon samples containing only water (no AI) showed no detectable radical production. Experimental attempts to generate and matrix isolate A1H2 by the pulsed laser vaporizationof aluminum while passing hydrogen gas over its surface have been unsuccessful. This high-energy method has been used to produce neon samples of isolated AlH+, AlF+, AlC, and several other radicals.3~~-~Laser-vaporized aluminum codeposited with a neon/Hz mixture was also unsuccessful in producing AIH2.
Knight et al.
1306 The Journal of Physical Chemistry, Vol. 97, No. 7, 1993
h
,x
4K
AIH2 NEON 4K AI M I = - 3 / 2
I
SMULATED
3f20
1
3830'
39?0
Figure 2. Aluminum atoms deposited with H20 produce the insertion product radical HAlOH as shown in the center ESR spectrum B, which was recorded prior to 254-nm photolysis of the neon matrix sample. For high water concentrationsin the 0.5-246 range, UV photolysis of such matrices reduced slightly the HAlOH amount and produced intense Al(OH)? ESR signals as shown in the top spectrum A. For low water concentrations(approximately0.1%),UV photolysissignificantlyreduced the HAlOH amount and produced intense H atom and AlH2 ESR absorptions as shown in the bottom spectrum C. For all three aluminum radicals, the AI:M,= -3/? transition was monitored. Obviously,different initial matrices (B) were employed for these UV photolysis observations; however, care was taken to maintain the same aluminum vaporization conditionsin both cases. The UV photolysisof 'blank" matricescontaining only neon and water produced no ESR absorptions from 0 to 9000 G. 2700 ,
2900 ,
3)OO ,
3070
spectrum. Comparison with a computer-simulatedspectrum is shown in the lower trace. The indicated X , Y, 2 triplets ( 1:2:1) result from hyperfine interactionswith the twoequivalentH atoms. The AI MI = 3 / 2 transition of AIH2 is presented in Figure 5. &H2:NEON 4K
1
1
I
2500
3020
Figure 4. Observed ESR line shape for AlH2 showing three resolved g tensor components for the AI M I= -'/? transition is shown in the top
Ly"Ili--cI 3730
'2
y 3770
3300
2815
I
2865
AI M,=3/2
X.Y 2915
2465
Figure 5. This expanded scale presentationof the AI MI = '/?transition in AIH2 should be compared with its high-field counterpart (MI = -3/2) shown in Figure 4. The high-field transition shows clear separation between the X , Y,and 2 components, while X and Yare only partially
AIH2: 4K NEON
resolved for this low-field transition. The triplet structure results from hyperfine interaction with the two equivalent hydrogens. spectrum that shows nearly perfect agreement with the experimental spectrum in the upper half of this figure. The 54 observed and calculated ESR lines for AlH2 consisting of a sextet-of-triplets for each of the three g components are listed in Table I. The magnetic parameters for AIH2 and Al(OH)? were extracted from the observed line positions by using exact diagonalizationsolutionsto the followingspin hamiltonian: 3300
'
3500
'
3700
'
3900
4100
Figure 3. Overall ESR spectrum of AIH2 in a neon matrix at 4 K is shown. It consists of a widely spaced AI (I = s/2) sextet of smaller 1:2:1 triplets for the two equivalent H atoms. Two aluminum atom lines and the H atom lines are indicated. The samplewas prepared by the photolysis (254 nm) of a neon matrix containing aluminum atoms and a small amount (approximately 0.18) of H20. See Figures 4 and 5 for expanded scale presentations of the M I= *'/? transitions. See Figure 6 for a simulated spectrum of AIH2 over this same magnetic field range.
AMr: ESR Spectral Assignment. The overall ESR spectrum assigned to AlHz is shown in Figure 3. It covers the magnetic field range from approximately 2500 to 4100 G with ge corresponding to 3420 G. The spectrum is straightforward to analyze as an anisotropic and widely spaced A1 ( I = s/2) sextet of smaller spaced 1:2:1 triplets resulting from two equivalent H atoms. This spectrum was observed only after 254-nm irradiation of neon matrices containing deposited aluminum atoms and small amounts of water as described in detail above. The AlH2 radical has not been previously detected by ESR under any experimental conditions. Theorthorhombicnatureof the AIH2spectrumis clearly evident in Figures 4 and 5 where expanded-scale presentations of the AkM1 = i3/? transitions show the resolution of three distinct g components, which exhibit the classical line shape for a powder sample. The lower trace of Figure 4 is a computer-simulated
where all symbols have their standard meaningsls and the sum was taken over all magnetic nuclei. It was not necessary to include A1quadrupoleeffectsin order to fit the observed lines to calculated positions well within the experimental uncertainty of i O . 5 G for all lines. Computer routines written in our laboratory were used in the analysis procedure and in the generation of the simulated ESR spectra. A description of this program and its general capabilities has been presented.7v29-33In the case of AlH2, the spin determinant had dimensions of 48 X 48. It had to be diagonalized for each magnetic field increment and each value of the 0 and 6 angles employed. Over 8 million diagonalizations were conduced in the analysis procedure. We have recently modified our ESR programs for compatabilitywith an IBM RISC 6000 machine. The observed overall AlHz ESR spectrum of Figure 3 shows excellent agreement with the simulated trace shown in Figure 6, using the magnetic parameters of Table I1 and the above diagonalization program. For this overall simulation an average of the X and Y magnetic parameters was employed, giving the spectrum an axially symmetric appearance to better match the low resolution wide scale experimental spectrum of Figure 3. The Lorentzian line-widthparameter was set at 1.5 G. The simulated trace clearly shows the phase inversion in the first derivative absorptionspectrum that is observed for the low-field lines relative
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1307
Electron Spin Resonance Investigations
TABLE I: Observed and Calculated ESR Line Positions for AM2in a Neon Matrix at 4 K' 1 0 -1 1 0 -1 'I2 1 0 -1 -112 1 0 -1 -312 1 0 -1 -512 1 0 -1
2640 2683 2728 2864 2907 295 1 3116 3159 3203 3396 3439 3483 3105 3749 3793 4042 4085 4130
512
2643 2689 2736 2868 2914 296 1 3121 3167 3214 3403 3449 3496 3714 3760 3806 4052 4098 4145
2513 2559 2606 2800 2846 2892 3110 3156 3202 3443 3489 3535 3800 3846 3892 4180 4226 4272
The microwave frequency was 9584.9 (3) MHz. The calculated line positions, from an exact diagonalization analysis, agreed with these observed lines within the experimental uncertainty of f0.7 G. The magnetic parameters for AIH2 obtained from these line positions are listed in Table I1 and compared with theoretical results in Table IV. 2500
2700
SIMULATED
2900
3100
3300
AIH2 1
4 .
1
A /I ll
V 5/2
3/2
MI =
1/21
- '(2
-5/2
-3/2
For longer passivation periods of the apparatus with D20, the AIH2 triplets were barely detectable, while the AlHD pattern was overlapped by the more intense components of the AID2 quintet. Al(0H)Z: ESR Assignment. For those neon matrices containing aluminum and water amounts in the high range of 1-2%, irradiation produced a new sextet of perpendicular and paralleltype lines with A values considerably larger than those observed for AlH2, A10, and HAlOH (see Table 11). The lowest and highest field transitions (A1:MI = &5/2)of this new aluminum radical species are shown in Figure 8. Its A1:MI = -3/2 transition is shown in Figure 2 and line positions for all 12 of its absorption features are listed in Table 111. These lines were easily fit to the above spin hamiltonian producing the g and A values listed in Table 11. Given the unusually large line widths (=14 G FWHM), these observed lines did not fully resolve into three g components as might beexpected for thenonlinear Al(OH)2 radical. However, the calculated amount of anisotropy would be contained in the observed absorption envelopes. As shown in Figure 8, the phases of the parallel and perpendicular lines are reversed at high field relative to the analogouslow-field absorptionfeatures as expected for a powder sample exhibiting this type of A tensor anisotropy. A spectral simulationof the line shapesusing the derived magnetic parameters shows near perfect agreement with the observed properties, even reflecting the correct relative intensity of the parallel and perpendicular components of each transition group. The observed aluminum A values and the lack of resolved H hyperfine structure is consistent with the theoretical calculations on AI(OH)2 discussed below. The deuterium-substitution experiments produced the same set of lines but their line widths of approximately 10G were noticeably smaller as would be expected for A1(OD)2. In summary, the assignment of this new species as the dihydroxyaluminum radical is supported by the observed hyperfine pattern, the sample preparation procedure and the ingredients known to be present, the comparison with similar aluminum radicals with large A values, the elimination of other obvious choices, and the detailed theoretical calculations of its expected magnetic parameters for three different conformers.
Discussion
,
,
,
I
!
I
l
l
3500 3700 3900 4100 Figured This overall computer-simulated ESR spectrum of AIH2 should be compared with the experimental spectrum shown in Figure 3. The very weak parallel (11) features indicated in this axially symmetric approximation correspond to the 'z" components. See Figures 4 and 5 for the detailed line shape on more expanded magnetic field sweeps.Note the relatively large peak height intensity of the AI M I= ' 1 2 transition. Thearrow near 34206 indicates themagnetic field positioncorresponding to ge. 3300
to the high-field transitions. The third group (MI = of the AI sextet is significantlymore intense on a peak height basis since the various absorption components occur at nearly the same magnetic field for all molecular orientations. Inspection of the conventional B vs HRES plots for all the M Itransitions clearly shows this as the reason for the dominant intensity of the M I= l / 2 group, which is characteristic of aluminum radicals with large hyperfine splittings and small g tensor anisotropy. The H20/D20 substitution experimentscaused each of the six triplets in AlH2 to become a doublet-of-triplet pattern, consistent with the formation of AlHD. The doublet A value for H in AlHD agreed exactly with the triplet A value for H in AIH2. Furthermore, the D(I = l ) triplet A value of 6.7 (5) G was that expected on the basis of the nuclei gN ratio for H and D. With a limited degree of D20 passivation, the observed spectrum consisted of a mixture of the AlH2 triplets and the AlHD doubletof-triplets as shown in Figure 7 for the AI:Mr = I / z transition.
The formation mechanism of AlH2 and Al(OH)2 under the matrix photolysisconditionsdescribed above cannot be monitored solely by ESR spectroscopy, since nonradical species might be involved. The observationthat neither species could be observed in argon matrices suggests that diffusion-controlled processes might be involved, since neon is known to be a less rigid medium. Intense HAlOH ESR signals can be detected in both neon and argon matrices and there was poor quantitative correlation between the decrease of HAlOH and the formation of AlH2 as judged by ESR intensities. The photolyticgeneration of H atoms that can diffuse and combine with the non-radical AlH species might be occurring. The large production of H atoms when AlH2 is observed can be seen by comparing spectra B and C of Figure 2. The breaking of strong ionic A 1 4 bonds does not seem likely with the photolytic energy available. The formationof A1(OH)2at higher water concentrationsmight involve the trapping of H20 and HAlOH in or near the same lattice site. Photolysis could then produce AI(OH)2 + H2, accompanied by only small amounts of H atom generationcompare spectra B and A of Figure 2. The presence of water cluster molecules might also be important in Al(OH)2 formation. Despite the large number of separate matrix deposition experiments (approximately 65) conducted under a wide variety of formation conditions, it is interesting that AlH2 and AI(OH)2 were never detected in thesame matrixsample. Moreexperiments involving different photolytic energies and combined ESR/IR monitoring will be required to further probe the interesting formation mechanism of AIH2 and AI(OH)2 in neon matrices.
Knight et al.
1308 The Journal of Physical Chemistry, Vol. 97, No. 7, 1993
TABLE II: A Comprrison of the Observed Magnetic Parameters for Several Small Aluminum Radicals (MHz)'
AIH2b HAIOH' AI(OH)2' AIH+ AIF+ AlW HAICH3g PdAIh AI (CO) 2'
*
2.0012 (8) 2.0015 (4) 2.000 (1) 2.0018 (3) 2.0015 (5) 2.0015 (3) 2.000 (1) 2.010 (1) 2.0043 (3)
2.0026 (6) 1.9973 (6) 2.0015 (4) 1.9975 (4) 1.998 (1) 1.9996 (3) 2.oooo (5) 2.0004 (3) 2.002 (1) 2.002 (1) 2.0343 (5) 2.0021 (3) 1.9990 (3)
784 (2) 786 (2) 888 (1) 870(1) 1182 (3) 1537 (2) 2782 (6) 713 (1) 712 723 84 (1) 146(1) 39(1)
932 (2) 993 (1) 1295 (4) 1685 (2) 2893 (8) 872 (1) 880 182 (2) 39(1)
834 (2) 917 (2) 1220 (4) 1586 (2) 2819 (7) 766 (1) 772 117 (2) 75 (1)
123(I) 130(1) 286 (1) 290 (1)
