16842
J. Phys. Chem. 1995,99, 16842-16848
ESR Investigation of the HBBH(X3X) Radical in Neon and Argon Matrices at 4 K. Comparison with ab Initio SCF and CI Calculations Lon B. Knight, Jr.,* Kelly Kerr, P. K. Miller, and C. A. Arrington Department of Chemistry, Furman University, Greenuille, South Carolina 29613 Received: June 12, 1 9 9 p
The first definitive experimental characterization of the unusual HBBH molecule is reported. It has been generated by several different methods and trapped in neon and argon matrices for a detailed electron spin resonance (ESR) investigation. A complete resolution of the IlB and ‘H nuclear hyperfine structure into isotropic and dipolar components was possible. Ab initio CI calculations, conducted as part of this experimental study, yielded Aisoand Adip parameters in good agreement with the observed values. These ESR results offer the first confirmation that HBBH has a 3Xg- electronic ground state as predicted by earlier theoretical calculations. The HBBH molecule resembles acetylene with one electron removed from each of the x-type molecular orbitals. Molecules that contain boron-boron double bonds are extremely rare.
Introduction The first definitive experimental characterization of the most unusual diborene molecule (HBBH) is reported. It has been studied in detail by electron spin resonance (ESR) in neon and argon matrices at 4 K. Complete resolution of both the hydrogen and boron A tensors (nuclear hyperfine interaction) into dipolar and isotropic components allows a detailed electronic structure analysis to be conducted for the unpaired electrons in this 12-electron species-making it the “simplest” tetratomic radical studied to date. The measured Aiso and Adipolx nuclear hyperfine parameters show close agreement with ab initio theoretical calculations which were conducted as part of this experimental study. These ESR results are the first experimental measurements that confirm earlier theoretical predictions of an X3Xg- electronic ground state for HBBH.’g2 The only previous experimental observations reported for HBBH include a brief reference to its formation when we f i s t observed its ESR ~pectrum,~ a tentative vibrational assignment of its B-H stretch in an argon matrix: and mass spectrometric (MS) detection of its molecular cation5 These photoionization MS experiments show the formation of HBBH+ from B2H6 and report an estimated ionization potential of 8.25 eV for HBBH from its ground electronic, state. The valence molecular orbitals of HBBH are 202 2uu23 u 2 1xu2where the first two 0 type orbitals involve bonding and antibonding boron 2s electrons. The two unpaired electrons are found to occupy degenerate boron 2px and 2p, bonding orbitals. The HBBH molecule could be described as acetylene with one electron removed from each of the n type orbitals. The isotropic hyperfine interaction with both the boron and hydrogen nuclei arises primarily from spin polarization of the 0 electrons by the unpaired JL electrons. The HBBH radical is isoelectronic with C2 whose 3Xg- and 31-Iu states lie 6434 and 716 cm-I, respectively, above its X’Zg+ground state.6 A recent theoretical calculation for HBBH also finds the linear triplet state to be the global minimum with the ]Ag and IXg+ states lying 14.7 and 23.4 kcal/mol higher, respectively, at the CI level of theory.2 For the heavier group 13 atoms (Al, Ga, and In) the linear geometry for X2H2 molecules is only a transition state with a singlet state trans-bent isomer calculated to have the lowest energy. The HBBH molecule may be considered as the model of an unsaturated bond between two boron atoms2 ‘Abstract published in Aduance ACS Abstracts, October 15, 1995.
0022-365419512099-16842$09.0010
Except for the B2 radical which also has a 32g-ground state, there are apparently no other small molecules containing a boron-boron double bond.’ The electron-deficient nature of group 13 elements hinders the formation of such bonding. Earlier theoretical treatments called attention to the nonexistence of HBBH and the experimental challenge of building an ethylene-like bond between two boron atoms.8 However, a recent synthetic advance has produced an acyclic species that does contain a boron-boron double bond.9 Several theoretical calculations suggesting likely boron-boron double-bonded candidates have been reported; these include a diborane dianion analog of a substituted ethylene and the replacement of H atoms in HBBH with amino groups.8.’0 The stabilizing effect of ~ t . back-bonding was explored in detail. The calculated shorter B-B bond length in diborene relative to B2H4 has been suggested as evidence for some double-bond character in HBBH, although there is no experimental evidence of thk2S8 A bond dissociation energy of 153 kcal/mol from CI calculations for the B-B bond in ground state HBBH has been reported as well as an SCF B-B bond length of 1.509 A and a B-H length of 1.179 A. The bond energy is based upon dissociation into two BH(311) fragmenk2 These calculated properties show good agreement with our theoretical treatment which was primarily focused on calculating the nuclear hyperfine interactions, namely, the A,,, and A+ magnetic parameters for boron and hydrogen. The heat of formation and vibrational frequencies for HBBH have been calculated,” as well as its band structure and polymerization energy. Only nonstoichiometric polymeric ( B H x ) compounds ~ are known with x close to one.I2 A thorough review of the heavier metal hydrides has been presented which also contains numerous references to boron hydride ~hemistry.’~ A recent review of binary boron species lists approximately 100 discrete molecule^.^^ The ease of thermal decomposition of such hydrides is potentially important in the creation of thin metallic films involved in the manufacture of semiconductor devices. The hydrides may be active intermediates in the thermolysis reactions associated with important CVD processes.I3 One example would be the formation of p-silicon layers by boron ion implantation from species such as B~HN+ produced by diborane discharges. The pulsed laser vaporization of boron has been employed in a series of vibrational spectroscopic matrix studies involving boron atom reactions with small reactant molecules including CH4,I5-l60 2 , ” H20,I6.l8CO,I9 N and N2,20.21COZ, 22 C2H2,23 0 1995 American Chemical Society
HBBH(X3Z) in Ne and Ar Matrices
J. Phys. Chem., Vol. 99, No. 46, I995 16843
H2,4 and CH3Br.I6 Rare gas matrix isolation ESR studies of gas neon at a flow rate of 5 sccm and B2Hs(g) were simultasmall boron-containing radicals include B atoms,24B2,7 BO,25 neously deposited from separate inlet tubes. The ratio of B2I&f BF+,26BS,27BC?8 BH2,29BF2,30BC0,3'BNH?' B3,32BNB,33 neon was varied on different depositions from approximately H2B0,34 and B z H ~ - .Several ~~ of these radical investigations 1/100 to 1/10 000. Matrix depositions conducted under these have also included ab initio CI calculations of the nuclear conditions for 60 min produced intense ESR absorptions that hyperfine interactions. ESR, theoretical, and structural studies are assigned to HBBH. Similar experiments employing argon have also been reported for the diboranyl (B2H5) r a d i ~ a l . ~ ~ - ~discharges ~ and argon matrices also produced the HBBH spectrum, but its intensity was nearly 10-fold weaker than that Experimental Section observed in neon. Using previously described procedures and equipment, it was Several high-energy generation methods and the associated found that the same ESR spectrum assigned to HBBH in the equipment used in our laboratory for ESR studies of both neutral experiments could be produced by electron and charged radicals have been described p r e v i o ~ s l yThe . ~ ~ ~ ~open ~ ~ tube ~ ~ discharge ~ bombardment?' Electrons from a tungsten filament were techniques include photoionization from an open tube neon accelerated to 50 eV and directed toward the copper matrix resonance lamp (16.8 eV), electron bombardment (50-60 eV), target simultaneous with neon (or argon) and B2&(g) deposition fast atom bombardment with neutral neon beams (3-10 keV), under flow conditions similar to those described above. Electron X-irradiation of deposited matrix samples (20- 100 keV), currents in the range 0.01 -0.05 mA were used, as measured at conventional high-temperature (> 1000 K) vaporizatiodreacthe deposition target. In addition to observing the ESR spectrum tions, ion-neutral codeposition reactions, and various pulsed of HBBH, the B2(X3Z)signals were also detected under these laser vaporization schemes. Five independent production electron bombardment condition^.^ methods were employed in these HBBH matrix ESR studies in Frequency-tripled output at 355 nm from a Nd:YAG pulsed order to make a more definitive spectral assignment. A brief laser was focused just above the matrix deposition target. The account of each technique is presented with more details of the laser was operated at 10 Hz and 15 mJ of energy per pulse. methods described in the literature cited above. Even though The deposition of neon matrix gas and B2H6 under these "gas HBBH is a neutral radical, it was generated under conditions phase" laser bombardment conditions produced a weak ESR developed primarily for trapping radical ions. Often these highly spectrum of HBBH. In another arrangement, the diborane was energetic conditions produce unexpected neutral radical ESR first deposited onto a 77 K copper surface located about 5 cm signals which are considerably more intense than those of the from the 4 K matrix deposition target. Focused pulsed laser intended ion species. The five different generation methods irradiation of the condensed diborane produced more intense that proved successful in producing matrix samples of HBBH ESR signals of HBBH. This condensed p h a s e h e r vaporization from B&(g) were photolysis at 16.8 eV, X-irradiation, electron arrangement is similar to that previously employed in our bombardment at 50 eV, laser irradiation directly in the gas phase, laboratory for the insertion of copper atoms into CH3F for and laser output focused onto a copper surface that was coated generating the CH3CuF radical.42 In these B2H6 experiments, with condensed B2H6. we were attempting to generate copper-boron-hydrogen For most experiments, it was found that a convenient source radicals, but none were detected. of B2H6(g) was a 1% diborane-argon mixture (Matheson electronic grade). Just prior to an experiment, pure diborane A detailed description of X-irradiation for the production of was separated from the argon carrier by collection in a stainless matrix-isolated ion and neutral radicals has recently been steel cold trap maintained at 77 K. The sample purity was This method was found to produce moderately intense ESR confirmed by quadrupole mass spectrometric analysis as it signals of HBBH by X-irradiating neon matrices at 60 keV. entered the matrix isolation apparatus. Other B2&(g) samples Several experiments conducted over a wide range of concentrawere generated by adding NaB& to phosphoric acid. A tions found that the most dilute matrices yielded the most intense diborane sample isotropically enriched to 90% in 'OB was kindly HBBH signals. The ratio of to rare gas was varied from provided by Professor Jerry Odom at the University of South approximately 1/100 to 1/10 OOO. Apparently more aggregation Carolina. occurs in the more concentrated matrices. The distinguishing characteristics of X-irradiation generation compared to the other Both liquid helium and 4 K closed cycle refrigerators (APD methods employed is that it is conducted after the matrix HS4) were employed for these experiments. By means of a small hydraulic system, the cryostat was moved up and down deposition process is completed. A typical matrix sample was under high-vacuum conditions over a distance of 8 cm in order prepared by depositing the B2&(g)/neon mixture at 5 sccm for to position the attached matrix target either in the deposition a period of 30 min, followed by 40 min of X-irradiation using position or in the X-band microwave cavity which was mounted a previously described experimental a1~angement.4~ in the vacuum chamber. Background pressures in the system were typically 5 x Torr prior to deposition; it increased ESR Analysis and Results to approximately 5 x Torr during neon deposition as measured by an ionization vacuum gauge calibrated for nitrogen. The low-resolution overall ESR spectra assigned to HBBH in a 3Z ground state are shown in Figures 1 and 2 for neon and The initial series of experiments was intended to generate argon matrix samples at 4 K. Since S = 1, there are two fine the BH' radical by photoionizing B&(g) during neon depositions. The ESR spectrum of BH+(X2Z) has not been observed structure transitions (AMs = f l ) for both the parallel (8 = 0') and perpendicular (8 = 90") directions, where 8 is the angle despite numerous attempts with a variety of methods. The between the molecular symmetry axis and the applied magnetic microwave-powered open-ended neon discharge lamp (9 mm field. Using conventional labels, these four features are 0.d.; fused silica) was operated with 40 W forward and 2W reflected power and a neon flow of 2.0 sccm. The output from designated Zl, Z2, XYI, and XY2 on the spectra where the this lamp, which includes 16.8 eV photons as well as metastable subscripts simply give the increasing order of the applied magnetic field.24 Unusually narrow line width absorptions are neon atoms, was directed toward the 4 K matrix deposition surface; both copper and single crystal sapphire matrix targets observed for this randomly oriented powder sample of HBBH. were used over the course of these HBBH experiments. Matrix In addition to these four absorption features, the AMs = 2
Knight et al.
16844 J. Phys. Chem., Vol. 99, No. 46, 1995
AMS=2
XYI
1
HBBH:
X3X:NEON 4 K
TABLE 1: Observed vs Calculated ESR Line Positions (gauss) and Magnetic Parameters for HBBH(X3Z,-) in Neon and Argon Matrices at 4 K neon" A M s = 2' Zld XYId XY2d Z2d
argonb
obsd
calcd
obsd
calcd
1568 2236 2766 3976 4612
1568 2235 2767 3975 4613
1577 2268 2786 3956 4576
1576 2269 2787 3957 4575
Magnetic Parameters g l = 2.0010(5) (neon and argon) D = 3330(6) MHz (neon) and 3228(8) MHz (argon) "B: \All = 4.6(3) MHz; IAlll = 24.9(6) MHz (neon) IH: l A ~ l= 45.9(4) MHz; ~AIII = 24.9(6) MHz (neon)
gll=
i400 2200 3000 3800 4600 Figure 1. The top ESR spectrum, produced by photoionizing B2Hs at
16.8 eV during deposition, was observed in a neon matrix and assigned to HBBH in its X32:,- ground electronic state. The perpendicular (0 = 90') fine structure transitions ( A M s = + l ) are labeled XYI and XY2 and show a triplet hyperfine pattern arising from two equivalent H atoms; the corresponding parallel (e = 0') absorptions are labeled Z2 and ZI, respectively. Higher resolution spectra of these various features and the AMs = 2 transition are shown in Figures 3-7. The inserts above and below the Z lines were recorded at "X8" amplifications. The features labeled "H'are the hyperfine doublet components of H atoms trapped in neon; other background radical signals in the geregion are not shown; these include CH3, BH2, B atoms, HzO+, HCO, and trace amounts of N2'. The lower spectrum was simulated from an exact diagonalization program utilizing the magnetic parameters listed in Table 1; g, designates the magnetic field position corresponding to the free spin g value (2.0023).
1
HBBH: ARGON 4K
'
'
/I
I
1600 2400 3200 4000 4800 Figure 2. The overall argon matrix ESR spectrum of HBBH (X3Zg-) is shown. The absorption features have significantly greater line widths in argon, making resolution of the nuclear hyperfine structure impossible. Compare this argon spectrum to the analogous neon spectrum of Figure 1. The H atom doublet lines are labeled "H'.
transition, which is highly characteristic of a 3C state with a small D value, was observed at low magnetic fields in both matrices. Given the axial symmetry of this radical, there are only three magnetic parameters which determine the resonance magnetic field positions of these five observed absorption features, namely, gll, gL, and the D value (zfs). A unique determination of these magnetic parameters can be made by fitting the observed lines to those calculated using exact diagonalization solutions to the conventional spin Hamiltonian:
H = pGgS
+ SDS + SAT
where standard symbols are employed.24 A description of our computer programs for such exact diagonalization line fitting procedures has been given p r e v i o ~ s l y . ~ A - ~comparison ~~~* of
Microwave frequency was 9590.0(3) MHz. Individual hyperfine lines were resolved for neon but are not listed. See text. Calculated line positions were from an exact diagonalization analysis using the magnetic parameters listed in this table. Microwave frequency was 9584.0(3) MHz. Hyperfine structure was not resolved in argon. The AMs = 2 transition had its maximum intensity at 8 = 52" where 0 is the angle between the applied magnetic field and the molecular symmetry axis of HBBH. Z I and Z2 designate the parallel (e = 0') fine structure ( A M s = +1) line positions; XY2 and XYI designate the corresponding perpendicular (0 = 90') line positions, respectively.
the observed and calculated line positions is given in Table 1 for the neon and argon matrix samples. The magnetic parameters are gil = g l = 2.0010(5) for both matrices; D = 3330(6) MHz for neon and D = 3228( 10) MHz for argon. The decrease in the D value of 3.1% going from neon to argon is consistent with the trend of such matrix shifts previously reported for other triplet radicals. The sensitivity of D to environmental effects is well-known and has been discussed p r e v i ~ u s l y . ~The ~ observed neon to argon decrease in D for B2(X3X) was 5.4%. A simulated ESR spectrum using the neon magnetic parameters determined for HBBH is shown in the lower half of Figure 1. Note that the relative intensities and phases of the various features on the simulated spectrum show excellent agreement with the observed spectra shown in Figures 1 and 2. It is interesting that both the simulated and observed spectra show comparable intensities for the "forbidden" Ahis = 2 and the Perpendicular Ahis = f l absorption features. Other ESR radical signals detected during the course of these experiments include intense H atom lines, BH2,29BO,25 and B2.' Definitive evidence that the fine structure transitions analyzed above are correctly assigned to HBBH was obtained from higher resolution ESR spectra which clearly exhibited the required nuclear hyperfine substructure. Literally dozens of different depositions had to be conducted under slightly different conditions of formation, concentration, and matrix annealing in order to obtain sufficiently small line widths to resolve the nuclear hyperfine structure, especially that of boron. The expanded scale ESR spectra presented in Figure 3 show the resolution of the XYl and XY2 perpendicular fine structure transitions into triplets which have the expected 1:2:1 intensity ratio for two equivalent hydrogen atoms ( I = I/*). The hydrogen IAll parameter of 16.4(2) G can be obtained from each of these XY transitions. Each hydrogen component is further resolved into a septet hyperfine pattern for two equivalent "B(Z = 3/2) atoms with an lAll value of 1.6(1) G as shown in Figure 4 for the XY, transition. Exactly the same nuclear hyperfine pattern was observed on the higher field XY2 perpendicular absorption. The less intense and more closely spaced IoB(I= 3; 20% abundance) hyperfine lines expected for the loBloB and I0B1lB isotopic combinations were not resolved since all components of these patterns would be contained within the "B"B septet. The
HBBH(X3Z) in Ne and Ar Matrices
XY,
J. Phys. Chem., Vol. 99, No. 46, 1995 16845
MI-
Kpi
NEON 4K
+-'
STRUCTURE&
H HYPERFINE STRUCTURE
Figure 3. ESR spectra of the hydrogen triplet hyperfine structure on each of the XY perpendicular (8 = 90") fine structure transitions of
the HBBH radical are shown. The boron hyperfine splitting, not resolved on these spectra, is shown in Figure 4 for the XYI transition. HBBH:NEON 4 K
Figure 5. High-resolution ESR spectra of the ZI and Z2 parallel (8 = 0") fine structure transitions for HBBH are shown. See overall spectrum in Figure 1. The equally spaced nine-line hyperfine pattem results from the circumstance that All for ''B(I=3/2)is fortuitously equivalent to the All for hydrogen, which is 8.9 G. Under this condition the resulting
septet of triplets should have a nine-line overlapping pattem as shown in this experimental spectrum of HBBH isolated in a neon matrix at 4 K.
HBBH: NEON 4 K AMS= 2 90;
I
I
2762 2782 Figure 4. The highly resolved ESR spectrum of the low-field XYI perpendicular (8 = 90") fine structure transition of HBBH is shown. It consists of a hydrogen 1:2:1 triplet with each component further resolved into septets which arise from two equivalent 1'B(I=3/2)atoms. The triplet splitting is 16.4 G, and the septet splitting is 1.6 G. A similar hyperfine pattem was observed on the high-field XY2 transition.
2742
natural abundances of the three isotopic HBBH radicals divided by the number of hyperfine lines would be in the ratio of 291 411 for IIBIIB, IlBlOB, and loBtOB,respectively. Hence, the 'OB hfs could not be completely resolved even on those experiments utilizing the B2H6 sample enriched to 90% in log. The H hfs of H"B"BH was identical to that observed for the H'OB'OBH radical. Resolution of the inherently weaker and usually broader parallel absorptions (Zl and Z2) was especially difficult. Prior to obtaining resolved spectra of these features, we had estimated a hydrogen lAlll value of lO(3) G and IlB IAlll value of 1l(4) G based solely upon total line width changes that were observed when B2H6 samples enriched to 90% in 1°B(Z=3) were employed. The nuclear g factor for 1°B is approximately onethird as large as that for IlB. In more recent experiments, resolved spectra of the Z lines were finally obtained which revealed a nine-line pattern with an equal spacing of 8.9(2) G. This is the expected observation for the circumstance where AII(H)and AII("B) happen to be equivalent. Highly amplified and expanded scale spectra of the Zl and Z2 absorptions exhibiting these properties are shown in Figure 5. It is a rare occurrence that a full resolution of the A tensors can be made for a high-spin powder sample. Such detailed hyperfine measurements will allow a thorough comparison with the theoretical results discussed below. In the earlier matrix study
15'38 1568 1598 1628 I658 I688 17'18 Figure 6. The observed ESR spectrum for the AMs = 2 transition of HBBH is shown in the top trace. The complex hyperfine structure results from the overlap of "B and 'H splitting at various 8 values, where 8 is the angle between the applied magnetic field and the
molecular symmetry axis. Using the neon magnetic parameters for HBBH listed in Table 1, the HRE vs 8 plot and the simulated ESR spectrum (shown in the lower portion of the figure) were generated from our exact diagonalization programs. Note that the spectral intensity is greatest at the 8MlN position of 52", which corresponds to the phase up peak of the simulated and observed spectra. The simulated spectrum appears simpler and narrower since nuclear hyperfine splitting was not included. of B2 it was not possible to resolve the nuclear hyperfie splitting of the parallel lines. The observed Ah& = 2 fine structure absorption feature for the neon sample is shown in the top section of Figure 6, where a direct comparison is made with an H ~ vss 0 plot which is presented on the same magnetic field scale. The simulated ESR spectrum associated with this case is also presented, but no nuclear hyperfine structure is included on the simulated spectrum. Note that the most intense feature of the simulated spectrum occurs at the turning point of the angular plot which is labeled "&IN"; in this case, &IN = 52". This phase-up position of the spectrum is usually referred to as the HMIN position. As is evident on the simulated spectrum, there is a weak phase down absorption feature at the perpendicular (0=
16846 J. Phys. Chem., Vol. 99, No. 46, 1995
Knight et al.
TABLE 2: Calculated Geometries and Energies for BH and HBBH molecule HBBH(3X) HBBH('X) BH@) BH('X) a
B-H
(A)
1.177 1.178 1.193 1.239
B-B
(A)
1.516 1.538
HF energy"
MP2 energy
MP4SDTQ energy
-50.433 -50.382 -25.114 -25.127
-50.568 -50.532 -25.157 -25.191
-50.600 -50.568 -25.170 -25.215
All energies in hartrees.
""A NEON 4 K
J
7
96
Theoretical Results The geometry of HBBH was calculated using Gaussian 92 with the 6-3 llG** basis set and fourth-order Moller-Plesset evaluation of the energy.44 Both the triplet and singlet ground states were calculated; the energies and optimal bond distances are given in Table 2. When the singlet state calculation was started with a geometry having an H-B-B angle of 150", it optimized the energy at the linear geometry. The linear triplet state is 20 kcaUmo1 lower in energy than the linear singlet state and has a B-B bond length that is shorter by about 0.02 A. The B-B bond energy for triplet HBBH, taken as the difference in energy between the HBBH minimum and the energy of one BH('Z) and one BH(3Z), is 135 kcavmol. The energy of HBBH(3X) with respect to two ground state BH('E) molecules
TABLE 3: Comparison of Experimental and Calculated Nuclear Hyperfine Parameters (MHz) for HBBH in its X3&- Ground Electronic State
obsd" calcdb
-5.2(4) -19.1
-19.7(6) -20.0
-38.9(5) -36.5
14.0(7) 13.6
All experimental values are from neon matrix ESR results. The expenmental values of All and Al used to calculate these A,,, and Adlp parameters were All = -24.9(6), A l = -45.9(4) MHz for IH and AI,= -24.9(6), A 1 = 4.6(3) MHz for "B. The Adjp parameter listed here is equivalent to A= described in the text. The hyperfine parameters were calculated using the MELDF programs.45
is 107 kcdmol. We calculate a barrier of 55 kcal/mol on the linear geometry energy surface for formation of HBBH from two ground state BH molecules. The valence shell molecular orbitals of HBBH are just those expected for this eight-electron system. It is acetylene with one electron removed from each of the n orbitals. The unpaired electrons are in orbitals of pure p character. The isotropic hyperfine interaction with both the boron and hydrogen nuclei arises from spin polarization of the CJ electrons by the unpaired n electrons. The hyperfine parameters were calculated with the MELDF suite of programs using the geometry for HBBH determined in the Gaussian 92 c a l ~ u l a t i o n . The ~ ~ Dunning double-zeta basis set was and CI calculations were carried out using four reference configurations with all "single excitations. An energy threshold of 1 x hartrees was used for selection of excited configurations. The energy calculated by MELDF is -50.5967 hartrees with -50.6100 hartrees as the extrapolated full CI limit. A comparison of the observed and calculated hyperfine parameters are given in Table 3. These theoretical values show excellent agreement with the experimental results except for the Aiso parameter for boron. As previously discussed, this quantity is especially difficult to calculate with a high degree of accuracy since the corehalence balancing requirements involve the combination of two large numbers of opposite sign.' When the isotropic interaction is inherently small, the deviation on a percent basis is usually found to be large. A similar circumstance has been discussed in detail for the case of boron atoms and the BZ m ~ l e c u l e .For ~ the boron atom a high-level calculation yielded A,,, = - 1.8 compared to an argon matrix value of 19 MHz. The agreement between theory and experiment for the dipolar interaction is almost always much closer; in the case of the boron atom, this same calculation yielded an Adlp value of 114 MHz compared to the experimental value of 108 MHz. For BZthe calculated A,,, parameter was found to vary from -13 to 0, depending upon the theoretical method employed; A,,, determined from argon matrix ESR results for BZ was 15 MHz. Discussion The utilization of several different generation methods, the highly resolved nuclear hyperfine patterns, and the close experimental-theoretical agreement for the A values make possible a definitive ESR assignment for the presence of HBBH in a 3Zground state in.these neon and argon matrix samples. The observation that the ESR absorptions were not weakened or eliminated by visible light photolysis supports the assignment of a neutral radical species. It is well-established that a positive response to such photolysis is strong evidence in support of an isolated cation or anion radicaLm These observations constitute the first experimental evidence confirming the earlier theoretical
HBBH(X32) in Ne and
Ar Matrices
J. Phys. Chem., Vol. 99, No. 46, 1995 16847
predictions of a 32ground state for the HBBH molecule. All experiments involved the exposure of B2H6 to high-energy conditions. Unfortunately, it was not possible to detect nonradical species in these various experiments. The lack of such overall information makes it difficult to describe the HBBH formation mechanism. Experiments designed to generate HBBH from boron and hydrogen atoms by passing H2 over laser-vaporized boron proved unsuccessful. The failure to detect BH+ remains a mystery, but new attempts are planned that involve the thermal cracking of B2H6 under ionizing deposition conditions. The loss of H and/or H2 from B2H6 in a series of steps seems to be the most likely formation process for HBBH under these matrix deposition conditions. The direct combination of two ground state BH molecules in the matrix seems unlikely given the high-energy barrier that was calculated for this reaction. However, the failure to detect the BH+(X2Z) radical might indicate the rapid reaction of BH BH+ to yield HBBH+, which is neutralized in the matrix environment. Since the diborene cation would most probably have a 211 electronic ground state, its presence in a rare gas matrix would go undetected by ESR. The inability to detect 211 states results from the extreme anisotropy of the g tensor expected for such radicals. The commonly applied free atom comparison method (FACM) can be used to show that the observed IlB dipolar hyperfine interaction is consistent with that expected for the electronic structure of HBBH which places the two unpaired electrons in px and py orbitals where 2 is the molecular symmetry axis. As observed, the isotropic interaction should be quite small for a p-type radical since no direct admixture of boron 2s character occurs. By comparison, in the BO radical (XzZ) where 2s admixture is significant Aiso for IlB is quite large with a value of 1033 M H Z . ~The ~ axial symmetry of HBBH and the requirement that the trace of the dipolar tensor be zero produces only one independent dipolar parameter, namely AZ = -2Am = - 2 A y y . For a given magnetic nucleus, the following relationships apply:
+
+
A,,= Aiso A, A,
+
and A, = Aiso Axx = Aiso- A d 2
= s P s ( 3 z 2- r2)/r5dz and Aiso= 8/3ng,g,,/l~,(S(r))
where evaluation is conducted over the total spin density ( P s ) and all symbols have their standard definition^.^^ The above equations give the link between the experimentally determined parameters (All and Al) and the dipolar interaction (Az) in the following expression: AZ = ~ ( A I-I A1)/3. The absolute signs of the A values cannot be determined from these ESR results alone, but the only choice that produces agreement with the high-level CI calculations discussed above is All = -24.9(6) MHz and A l = 4.6(3) MHz, which yields A, = - 19.7(6) MHz. An alternate expression for Az, namely ((3 cos2 0 - 1)/$), is useful in accounting for the negative sign of the dipolar interaction, where 0 gives the angle between the electron’s position relative to the nucleus and the applied magnetic field. The numerator of this dipolar expression becomes -1 at 0 = 90°, which is the dominant angle for px and py electrons when the applied magnetic field is along the Z axis direction. The FACM procedure can provide an approximate quantitative value of AZ based upon a free atom boron value for Various calculated values of (r-3)2pfor a free boron atom are available which vary from 159 MHz (47) to 134 MHz (48)to 142 MHz (7). Hence, FACM estimates of AZ for IlB in HBBH range from -32 to -27 MHz, which is reasonably close to the observed molecular value of - 19.7(6) MHz, especially con-
sidering that overlap effects and two center contributions have been neglected. The experimental A, value for IlB in the B2 molecular radical was found to be -26(1) MHz in an argon matrix. Given the location of the hydrogen nuclei in HBBH relative to the 2p, and 2pyboron orbitals which contain the spin density, it is expected that A, for hydrogen would be positive as observed (+14.0 MHz). This is because the effective value of 0 in the dipolar expression given above causes (3 cos2 0 - 1) to be positive. Spin polarization of the B-H 0 bond by the 2p electrons on boron would be expected to produce a negative Aiso value for hydrogen. The observed value of -39 MHz should be compared with values of -65 MHz for CH324and -76 MHz for H20+,49where the unpaired electron in each case occupies a 2p orbital on the central atom that is perpendicular to the 0 hydrogen bond. Acknowledgment. Project support from the National Science Foundation (CHE-93 19291) and student support from an NSF REU site grant are gratefully acknowledged. A Duke Endowment grant to Furman University, a DuPont College Science Grant, and support from the 3M Company provided valuable financial assistance for these studies. The authors express their sincere appreciation to Drs. William L. Luken and John C. Culberson for conducting initial theoretical calculations on HBBH which gave early estimates of the electronic ground state, the hyperfiie parameters, and the D tensor. We are also grateful to Professor Jeny Odom at the University of South Carolina for providing a diborane sample enriched to 90% in log. References and Notes (1) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J . Am. Chem. SOC.1975, 97, 3402. (2) Breboux, G.; Barthelat, J. C. J. Am. Chem. SOC.1993, 115, 4870. (3) Knight, L. B., Jr. In Chemistry and Physics of Matrix-Isolated Species; Andrews, L., Moskovits, M., Eds.; North-Holland: Amsterdam, 1989; Chapter 7, p 195. (4) Tague, T. J., Jr.; Andrews, L. J . Am. Chem. SOC.1994, 116, 4970. (5) Ruscic, B.; Mayhew, C. A.; Berkowitz, J. J . Chem. Phys. 1988, 88, 5580. (6) Haber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (7) Knight, L. B., Jr.; Gregory, B. W.; Cobranchi, S. T.; Feller, D.; Davidson, E. R. J . Am. Chem. SOC.1987, 109, 3521. ( 8 ) Jouany, C.; Barthelat, J. C.; Daudey, J. P. Chem. Phys. Lett. 1987, 136, 52. (9) Moezzi, A.; Olmstead, M. M.; Power, P. P. J . Am. Chem. SOC. 1992, 114, 2715. (10) Kaufmann, E.; Schleyer, P. v. R. Inorg. Chem. 1988, 27, 3987. (11) Sana. M.: Lerov. G.: Henriet. C. J . Mol. Struct. (THEOCHEW 1989, 187, 233. (12) Armstronn. D. R. Theor. Chim. Acta 1981. 60. 159. J.; Pulham, C. R. Chem. SOC.Rev. 1994, 23, 175. (13) Downs, (14) Greenwood, N. N. Chem. SOC.Rev. 1992, 21, 49. (15) Hassanzadeh, P.; Hannachi, Y.; Andrews, L. J . Phys. Chem. 1993, 97, 6418. (16) (a) Jeong, G. H.; Boucher, R.; Klabunde, K. J. J . Am. Chem. Soc. 1990, 112, 3332. (b) Klabunde, K. J.; Jeong, G. H. J . Am. Chem. SOC. 1986, 108, 7 103. (17) (a) Burkholder, T. R.; Andrews, L. J . Chem. Phys. 1991,95, 8697. (b) Boucher, R.; Wang, Yi; Klabunde, K. J. High Temp. Sci. 1991, 31, 87. (18) Andrews, L.; Burkholder, T. R. J . Phys. Chem. 1991, 95, 8554. (19) Burkholder, T. R.;Andrews, L. J . Phys. Chem. 1992,96, 10195. (20) Hassanzadeh, P.; Andrews, L. J . Phys. Chem. 1992, 96, 9177. (21) Andrews, L.; Hassanzadeh, P.; Burkholder, T. R.; Martin, J. M. L. J. Phys. Chem. 1993, 98, 922. (22) Burkholder, T. R.; Andrews, L.; Bartlett, R. J. J . Phys. Chem. 1993, 97, 3500. (23) Andrews, L.; Hassanzadeh, P.; Martin, J. M. L.; Taylor, P. R. J . Phys. Chem. 1993, 97, 5839. (24) Weltner, W., Jr. Magnetic Atoms and Molecules; Dover: Mineola, NY, 1989. (25) Knight, L. B., Jr.; Herlong, J. 0.;Kirk, T. J.; Arrington, C. A. J . Chem. Phys. 1992, 96, 5604.
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16848 J. Phys. Chem., Vol. 99, No. 46, 1995 (26) (a) Knight, L. B., Jr.; Earl, E.; Ligon, A. R.; Cobranchi, D. P.; Woodward, J. R.; Bostick, J. M.; Davidson, E. R.; Feller, D. J . Am. Chem. SOC.1986, 108, 5065. (b) Knight, L. B., Jr.; Ligon, A,; Cobranchi, S. T.; Cobranchi, D. P.; Earl, E.; Feller, D.; Davidson, E. R. J . Chem. Phys. 1986, 85, 5437. (27) Brom, J. M.; Weltner, W., Jr. J . Chem. Phys. 1972, 57, 3379. (28) Knight, L. B., Jr.; Cobranchi, S. T.; Petty, J. T.; Earl, E.; Feller, D.; Davidson, E. R. J . Chem. Phys. 1989, 90, 690. (29) Knight, L. B., Jr.; Winiski, M.; Miller, P.; Amngton, C. A,; Feller, D. J. Chem. Phys. 1989, 91, 4468. (30) Nelson, W. H.; Gordy, W. J. Chem. Phys. 1%9, 51, 4710. (31) Hamrick, Y. M.; Van Zee, R. J.; Godbout, J. T.; Weltner, W., Jr.; Lauderdale, W. J.; Stanton, J. F.; Bartlett, R. J. J . Phys. Chem. 1991, 95, 2840. (32) Hamrick, Y. M.; Van Zee, R. J.; Weltner, W., Jr. J . Chem. Phys. 1991, 95, 3009. (33) Knight. L. B.. Jr.: Hill. D. W.: Kirk. T. J.: Amneton. C. A. J . Phys. Chem.’ 1992: 96, 555. (341 Graham. W. M. R.: Weltner. W., Jr. J . Chem. Phys. . 1976.65, 1516. (35) Kasai, P. H. Acc. Chem. Res. 1971, 4, 329. (36) Hasegawa, A,; Sohma, J. Mol. Phys. 1974, 27, 389. (37) Curtiss, L. A,; Pople, J. A. J . Chem. Phys. 1989, 91, 4189. (38) Ruscic, B.; Schwarz, M.; Berkowitz, J. J . Chem. Phys. 1989, 91, 4183. (39) Knight, L. B., Jr.; Ken, K.; Villanueva, M.; McKinley, A. J.; Feller, D. J . Chem. Phys. 1992, 97, 5363. (40) (a) Knight, L. B., Jr. Acc. Chem. Res. 1986, 19, 313. (b) Knight, L. B., Jr. Radical Ionic Sysrems; Lund, A., Shiotani, M., Eds.; Kluwer Academic: Dordrecht, 1991; pp 73-97. I
Knight et al. (41) Knight, L. B., Jr.; Bostick, J. M.; Woodward, R.; Steadman, J. J . Chem. Phys. 1983, 78, 6415. (42) Knight, L. B., Jr.; Cobranchi, S. T.; Gregory, B. W.; Jones, G. C., Jr. J . Chem. Phys. 1988, 88, 524. (43) Knight, L. B., Jr.; Jones, G. C.; King, G. M.; Babb, R. M.; McKinley, A. J. J . Chem. Phys, in press. (44) GAUSSIAN 92, Revision: Frisch, C. M. J.; Trucks, G. W.; HeadGordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1992. (45) MELDF was originally written by L. McMurchie, S. Elbert, S. Langhoff, and E. R. Davidson. It has been substantially modified by D. Feller, R. Cave, D. Rawlings, R. Frey, R. Daasch, L. Mitzche, P. Phillips, K. Iberle, C. Jackels, and E. R. Davidson. (46) (a) Dunning, T. H., Jr. J . Chem. Phys. 1989.90, 1007. (b) Dunning, T. H., Jr.; Hay, P. J. In Methods of Elecrronic Structure Theory; Schaefer 111, H. F., Ed.; Plenum Press: New York, 1977; Vol. 2. (47) Morton, J. R.; Preston, K. P. J. Magn. Reson. 1978, 30, 577. (48) Koh, A. K.; Miller, D. J. At. Datu Nucl. Data Tables 1985, 33, 235. (49) (a) Strahan, S. E.; Mueller, R. P.; Saykally, R. J. J . Chem. Phys. 1986, 85, 1252. (b) Knight, L. B., Jr.; Steadman, J. J. Chem. Phys. 1983, 78, 5940.
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