Article pubs.acs.org/JPCA
Electronic Transition of Palladium Monoboride Y. W. Ng, H. F. Pang, Yue Qian, and A. S.-C. Cheung* Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S Supporting Information *
ABSTRACT: The laser-induced fluorescence spectrum of palladium monoboride (PdB) in the visible region between 465 and 520 nm has been observed and analyzed. Gas-phase PdB molecules were produced by the reaction of diborane (B2H6) seeded in argon with laser ablated palladium atoms. Thirteen vibrational bands have been recorded, which included transitions of both Pd10B and Pd11B isotopic species. These bands belong to the [19.7]2Σ+−X2Σ+ system, with ground X2Σ+ state bond length, ro, determined to be 1.7278 Å. A molecular orbital energy level diagram was used to understand the observed ground and excited electronic states. This work represents the first experimental investigation of the electronic spectroscopy of the PdB molecule. addition, Kharat et al.9 using DFT calculations obtained a doublet ground state with bond length of 1.856 Å and vibrational frequency of 725.6 cm−1. We are engaged in the studied of electronic transitions of gas-phase TMB molecules to investigate their molecular and electronic structure. Two TMB molecules: IrB16 and CoB17 were previously reported. In this paper, we report the analysis of the [19.7]2Σ+−X2Σ+ system of the PdB molecule in the spectral region between 465 and 520 nm recorded using the technique of laser vaporization/reaction with free jet expansion and laser induced fluorescence (LIF) spectroscopy. Spectra of both Pd10B and Pd11B isotopes were observed and analyzed. Molecular constants for the [19.7]2Σ+ and X2Σ+ states are reported. A molecular orbital energy level diagram has been used to understand the observed electronic states.
I. INTRODUCTION Transition metal borides (TMB) attract our attention because of their special properties, which distinguish them from other borides.1 Most of the TMB are refractory materials with melting points above 2000 K, high hardness, and excellent conductivity.2 There have been recent reports on superhardness properties of the rhenium boride (ReB2) and ruthenium boride (RuB2) bulk3,4 and rhodium (RhB) and iridium (IrB) films.5 The electrical conductivity of titanium diboride (TiB2) is known to be 5 times better than that of the titanium metal itself.6 In addition, cobalt boride (CoB) thin film is known to be an excellent catalyst for the production of hydrogen in the hydrolysis of sodium borohydride solution.7 Futhermore, systematic theoretical studies have recently been performed on 3d, 4d, and 5d transition metal monoborides.8−10 Despite the important properties that transition metal borides have been found to possess and recent advances in theoretical calculations, experimental studies of TMB molecules are still very limited. Palladium (Pd) is an interesting metal with high catalytic activity. Palladium-catalyzed coupling reactions for connecting organic compounds are extremely useful in organic synthesis.11 Many of the complex reactions involving Pd are well-known, but simple diatomic molecules formed from Pd and main group elements are among the least studied. For instance, among the first row main group elements only preliminary work has been reported for PdC12,13 and PdO14 molecules, and for the PdN and PdF molecules, nothing is experimentally known. For the PdB molecule, Knight et al.15 used matrix isolation electron spin resonance (ESR) spectroscopy of species produced by laser vaporization of a Pd/B pressed powder mixture to show that PdB has an X2Σ+ ground state and reported magnetic hyperfine parameters for this ground state. This showed that the unpaired electron resides in a σ orbital that has approximately 56% Pd 5s character. They also performed ab initio calculations using the unrestricted Hartree−Fock method and limited STO-3G basis set, and obtained the bond length of the ground state to be 1.608 Å. In © 2012 American Chemical Society
II. EXPERIMENT The apparatus used in this study has been described in previous publications.17,18 A brief description of the relevant experimental conditions for obtaining the PdB spectrum is provided here. Pulses from a Nd:YAG laser with wavelength 532 nm and power 5−6 mJ, were focused onto the surface of a palladium rod to generate palladium atoms. PdB molecules were produced by reacting the ablated palladium atoms with 0.5% diborane (B2H6) seeded in argon. Tunable pulsed dye and optical parametric oscillator (OPO) lasers were used in this work. They were individually pumped by different Nd:YAG lasers with wavelength set to 355 nm, which gave tunable output wavelength in the ultraviolet and visible regions. The energy output from the tunable pulsed lasers was typically about 10 mJ/pulse, their wavelengths were measured individually by a wavelength meter with accuracy around ±0.02 cm−1, and their line widths were close to 0.07 cm−1. The tunable wavelength was used to excite the jet cooled PdB Received: September 13, 2012 Revised: November 1, 2012 Published: November 2, 2012 11568
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molecule and the obtained fluorescence signal was directed into a 0.25 m monochromator. The LIF signal was subsequently detected by a photomultiplier tube (PMT) installed behind the exit slit of the monochromator. The monochromator was used for two purposes: (i) obtaining wavelength resolved fluorescence spectrum, and (ii) as an optical filter in recording the LIF spectrum. The PMT output was fed into a fast oscilloscope for averaging and storage.
III. RESULTS AND DISCUSSION The LIF spectrum of PdB in the visible region between 465 and 520 nm has been recorded. Figure 1 is a broad-band scan of the
Figure 3. (0, 0) band of the [19.7]2Σ+−X2Σ+ transition of PdB.
width of 0.07 cm−1. Spectra of both Pd10B and Pd11B isotopes have been observed, and the presence of transition lines of the five Pd isotopes of PdB in some of the bands confirmed the carrier of the spectrum is the PdB molecule. Most of the observed vibrational bands, however, have overlapping rotational structure. As shown in Figure 3, the isotopic transition lines are crowded together, making the observed line width greater than the laser line width. The transition lines were identified relatively easily; other vibrational bands could also be analyzed rotationally. The molecular Hamiltonian and matrix elements for a 2Σ+ state in Hund’s case (b) coupling scheme have been discussed by Amiot.20 Energy levels of the 2Σ+ state used in this work were also calculated in case (b) using
Figure 1. Low-resolution LIF spectrum of PdB.
PdB spectrum; two vibrational progressions originating from v″ = 0 and v″ = 1 are easily identified. Figure 2 summaries the
Ĥ = BR2 − DR4 + γ R·S
(1)
where B and D are respectively the rotational and centrifugal distortion constants, and γ is the spin-rotation constant. R is the rotational angular momentum, and S is the spin angular momentum. Molecular constants for the two 2Σ states involved were retrieved using a least-squares fitting program. The transition lines of the observed vibrational bands were fitted in two steps. First, each band was fitted individually and, subsequently, all the bands of a particular isotope were merged together in a single fit. With our relatively low temperature molecular source, the highest J value observed was 13.5, so the centrifugal distortion constant D was set to zero in the fit. The obtained molecular constants for both the Pd10B and Pd11B isotopic species are listed in Table 1. The equilibrium molecular constants obtained are given in Table 2. For the ground X2Σ+ state, only two vibrational levels were observed and the vibrational separation, ΔG1/2, for the Pd11B isotope was determined to be 753.98 cm−1. The bond length, ro, determined for the [19.7]2Σ+ and the X2Σ+ states are respectively 1.8302 and 1.7278 Å. Because molecular parameters of isotopic molecules are approximately related by different powers of the mass dependence ρ = (μ/μi)1/2, where μ and μi are the reduced masses of Pd11B and one of the isotopes, respectively. The most abundant isotope is Pd11B and isotopic effects are calculated relative to it. Table 3 presents the observed molecular constants for both isotopic molecules; the values for the Pd10B isotope were calculated from those of Pd11B using the above-mentioned isotopic relationship.19 The agreement of
Figure 2. Observed vibrational transitions of PdB.
thirteen vibrational bands recorded for the Pd10B and Pd11B isotopic species. Figure 3 shows the band head region of the (0, 0) band of the [19.7]2Σ+−X2Σ+ transition of PdB, which consists of the two R heads. Two P branches were also identified; they correspond to the two R branches. For a typical 2 Σ(b)−2Σ(b) transition, there are only four main branches: R1, R2, P1, and P2,19 which matched well with our observation and were in agreement with the assignment of this transition. With our relatively low temperature source, the highest J value observed was 17.5. Due to the fact that palladium has six isotopes and five of them have relatively high abundance [104Pd (11.14%), 105Pd (22.33%), 106Pd (27.33%), 108Pd (26.46%), 110 Pd (11.72%)], these mass differences cause the rotational constants of each isotope to differ slightly; hence the observed spectrum shows broader features than expected for a laser line 11569
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Table 1. Molecular Constants for [19.7]2Σ+ and the X2Σ+ States of Pd11B and Pd10B (cm−1)a Pd11B state
v
T
B
γ
[19.7] Σ
4 3 2 1 0 1 0
21809.22(2) 21321.48(1) 20816.71(1) 20294.47(1) 19753.35(1) 753.98(1) 0
0.4403(6) 0.4485(4) 0.4582(3) 0.4661(2) 0.4741(3) 0.5317(2) 0.5353(2)
−0.402(7) −0.366(6) −0.321(6) −0.276(5) −0.218(6) −0.047(5) −0.044(5)
2 +
X2Σ+ a
Pd10B T
B
γ
21381.31(1) 20856.65(1) 20312.74(1)
0.4893(3) 0.4988(4) 0.5081(3)
−0.366(6) −0.326(6) −0.269(5)
786.86(1) 0
0.5794(4) 0.5837(2)
−0.011(6) −0.026(5)
Error in parentheses are one standard deviation in unit of the last significant figure quoted.
Table 2. Equilibrium Molecular Constants for Pd11B (cm−1) state
parameter
Pd11B
[19.7] Σ
Te ωe ωeχe Be 10−3αe ΔG1/2 Be 10−3αe
19476.6(6) 558.2(6) 8.9(1) 0.4787(5) 8.5(2) 753.98(9) 0.5372(4) 3.7
2 +
X2Σ+
Table 3. Observed and Calculated Isotopic Displacement and the B Value (cm−1) for the [19.7]2Σ+ and X2Σ+ States of Pd10B observed
calculated
state
v
To
B
To
B
[19.7]2Σ+
3 2 1 1 0
21381.31 20856.65 20312.74 786.86 0
0.4893 0.4987 0.5081 0.5794 0.5837
20381.22 20856.63 20312.97 787.23 0
0.4894 0.4987 0.5080 0.5796 0.5836
X2Σ+
Figure 4. (1, 1) band of the [19.7]2Σ+−X2Σ+ transition of PdB.
Table 4. Effective Molecular Constants and Origin Used To Analyze the (1, 1) Band of the [19.7]2Σ+−X2Σ+ Transition of Pd11B (cm−1) isotope
these molecular constants is excellent, which confirms again unambiguously the carrier of the spectrum is PdB. The rotational features of different Pd isotopes are rarely analyzed under the tunable pulsed laser resolution, which is usually due to the fact that either, for low vibrational levels, the isotopic displacements are so small that the rotational lines are not resolved or, for the high vibrational levels, these rotational features are so widely spread that heavy overlaps of rotational lines are common and further analysis becomes extremely difficult if not impossible. However, Figure 4 shows the (1, 1) band of this transition, which has rotational features between the situations described above, showing nicely resolved rotational lines of the five Pd isotopic species. For this particular band, the molecular spectrum simulation program, PGOPHER, has been employed for analyzing the isotopic transition lines. In this case, the 2Σ+−2Σ+ transition was taken as two independent 1Σ+−1Σ+ transitions with effective rotational constants and isotopic displacements, but no spinrotation constant was involved. Effective molecular constants were calculated using isotopic relationships and the P and R branches were simulated by varying the isotopic displacements. The obtained P1 and P2 branches are given in Figure 4 and the effective molecular constants used are listed in Table 4. The good agreement of the simulated and observed spectra indicated the analysis is correct and the rotational constants are indeed connected by the expected isotopic relationship.
104
PdB PdB 106 PdB 108 PdB 110 PdB 105
104
PdB PdB 106 PdB 108 PdB 110 PdB 105
origin
B′eff
F1 Component 19539.96 0.4573 19540.11 0.4569 19540.26 0.4565 19540.55 0.4557 19540.83 0.4549 F2 Component 19540.55 0.4749 19540.71 0.4744 19540.86 0.4740 19541.15 0.4732 19541.43 0.4724
B″eff 0.5318 0.5313 0.5309 0.5299 0.5290 0.5323 0.5318 0.5313 0.5304 0.5295
IV. ELECTRONIC CONFIGURATION AND ELECTRONIC STATES Using the molecular orbital (MO) energy level diagram discussed by DaBell et al.13 for the PdC molecule, we examined the observed transitions in this work. Figure 5 represents qualitatively the relative energy order of the MOs formed from the Pd and B atoms. The MO energy levels derive from the 5s and 4d atomic orbitals (AOs) of the Pd atom and the 2p AO of the B atom. The lowest energy 11σ and 5π MOs and the higher energy 13σ and 6π MOs are formed from the main group 2p 11570
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It is instructive to compare the spectroscopic properties of the group VIIIA metal monoborides: PdB, NiB,22 and PtB,23 which are monoborides in the same group. In this connection, the analysis of the [19.7]2Σ+−X2Σ+ transition of NiB,22 and the [20.2]3/2−X2Σ+ and [21.2]1/2−X2Σ+ transitions of PtB23 have recently been reported. All three molecules, NiB, PdB, and PtB, have the same ground configuration and term, σ1, X2Σ+. The ground state bond lengths, ro, of NiB, PdB, and PtB were determined, respectively, to be 1.698, 1.7278, and 1.752 Å for which the PdB value fits very well the trend of the increase in bond length down the group.
■
S Supporting Information *
Figure 5. Molecular orbital energy level diagram of PdB.
Tables of assigned rotational lines. This information is available free of charge via the Internet at http://pubs.acs.org
AO of B atom and Pd 4dσ and 4dπ AOs. The 12σ MO is essentially the Pd 5s AO. The 2δ MO is the Pd 4dδ AO, because there are no other δ symmetry orbitals available. The PdC molecule has a X1Σ+ ground state, and the electronic configuration is (11σ)2 (5π)4 (2δ)4 (12σ)2. Because PdC has one more electron than the PdB molecule, if one electron is taken away from the outermost orbital (i.e., 12σ orbital), the ground of PdB would logically be (11σ )2 (5π )4 (2δ)4 (12σ )1 → X2Σ+
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*Tel: (852) 2859 2155. Fax: (852) 2857 1586. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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(2)
(11σ )2 (5π )4 (2δ)3 (12σ )2 → A2Δ
(3)
(11σ )2 (5π )4 (2δ)4 (6π )1 → B2Π
(4)
AUTHOR INFORMATION
Corresponding Author
ACKNOWLEDGMENTS The work described here was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKU 701008). We thank the reviewer for suggestions to improve the paper. We also thank Professor Colin Western (University of Bristol, U.K.) for permissions to use the PGOPHER software.
The matrix isolation ESR study confirms this ground state and shows that the unpaired σ electron has 56% Pd 5s character.15 Low-lying states that are expected are
(11σ )2 (5π )4 (2δ)4 (13σ )1 → C2Σ+
ASSOCIATED CONTENT
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(5)
From our observed spectrum, there is no doubt that the X Σ state is the ground state of PdB, with one unpaired electron in the 12σ orbital. To assign the upper state of our observed transitions, we assume single configurations can be used to describe the electronic states involved; we consider the promotion of an electron from 12σ nonbonding MO to a higher energy 13σ antibonding MO. The vibrational frequency and the bond length of the excited 2Σ+ state are respectively lower and longer, which match quite well the expectation that the 13σ antibonding orbital is occupied. Knight et al.15 using UHF calculations predicted the bond length of PdB to be 1.608 Å, but this is much shorter than our measured value of 1.7278 Å. In contrast, Kharat et al.9 obtained a doublet ground state with a bond length of 1.856 Å using DFT methods. This is much longer than our experimental value. Our result, 1.7278 Å, however, is very similar to the bond length (1.712 Å) found for the closely related molecules, PdC.12 These molecules only differ by the presence of an additional electron in the nonbonding 12σ orbital in PdC, so their bond lengths are expected to be quite similar, as is found here. It is very likely that the [19.7]2 Σ + state is the C 2 Σ + with electronic configuration as shown in (5). As far as the spin rotation constant, γ, is concerned, the ground state value is small and negative, which is normal and the negative size originates from the phase convention.21 However, the γ value of the excited state is an order of magnitude larger. This is very likely due to additional contributions from interactions between the 2Σ+ and other nearby states through second-order effects. 2 +
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