Characterization of Electronic Spectra of Rhodium Monohydride and Monodeuteride

Journal of Molecular Spectroscopy 201, 244 –248 (2000) doi:10.1006/jmsp.2000.8082, available online at http://www.idealibrary.com on

Characterization of Electronic Spectra of Rhodium Monohydride and Monodeuteride Walter J. Balfour,* Jianying Cao,† ,1 and Charles X. W. Qian* *Department of Chemistry, and †Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8W 3P6, Canada Received November 5, 1999; in revised form February 1, 2000

The electronic spectra of rhodium monohydride and monodeuteride, obtained using a molecular beam laser vaporization source, have been investigated from 400 to 500 nm, by laser excitation and dispersed fluorescence spectroscopies. The ground level of RhH is 3⌬ 3 with ␻ e ⬇ 2040 cm ⫺1 and ␻ e x e ⬇ 46 cm ⫺1. The corresponding RhD data are ␻ e ⬇ 1422 cm ⫺1 and ␻ e x e ⬇ 18 cm ⫺1. The estimated ground state bond length is r 0 ⫽ 0.159 nm. Excited states with ⍀ ⫽ 2, 3, and 4 have been observed. © 2000 Academic Press

INTRODUCTION

The importance of the transition metal– hydrogen bond in catalysis, organometallic chemistry, and, in some instances, astrophysics has given impetus to a large number of spectroscopic and theoretical studies of the diatomic hydrides. Rhodium monohydride is a species for which several detailed theoretical calculations have been performed (1– 4), but where no experimental molecular structural data are known. The calculations all predict a ground state of symmetry 3⌬, in contrast to observations for the 3d family member, CoH, where theory and experiment agree upon 3⌽ for the ground state (5, 6). This difference between CoH and RhH may be attributed (7) to a difference in ground state electron configuration in the metal atom: for Co this is d 7 s 2 , for Rh, d 8 s 1 . Since it is the metal s orbital that forms the sigma bond to hydrogen, d 7 s 2 should lead to a high-spin d 7 ␴ 2 ␴ * 1 configuration, whereas d 8 s 1 gives the low-spin d 8 ␴ 2 one. A weaker bond is anticipated from d 7 ␴ 2 ␴ * 1 due to the antibonding nature of the ␴* orbital. In accord with such considerations the ground state electron configuration of CoH is found to be d ␦ 3 d ␲ 3 ␴ 2 ( 3 ⌽), whereas the predicted configuration in RhH is d ␦ 3 d ␲ 4 ␴ 1 ( 3 ⌬). In this paper we wish to report the observation, under jet-cooled conditions, of fluorescence excitation spectra of RhH and RhD. The spectroscopic evidence confirms the 3 ⌬ i ground state symmetry designation.

NH 3/He experiments to generate spectra of RhN (9), two spectral features were noted, showing the open-structure characteristic of a diatomic hydride. Their identification with RhH was readily confirmed through experiments with H 2 doped in He. The corresponding RhD spectrum was also obtained using D 2. A gas mixture containing 2–5% H 2 or D 2 in He proved suitable. The experimental conditions and the equipment used to record laser-induced fluorescence (LIF) and dispersed fluorescence (DF) and to measure excited state lifetimes have been described in detail elsewhere (8, 9). The second harmonic (532 nm) output of a Nd:YAG laser (Continuum NY60) was used for the vaporization and a Nd:YAG pumped tunable dye laser (Lumonics HY600 and HD300) was used to excite the jetcooled RhH/RhD molecules. A 0.2 cm ⫺1 resolution was typical in the region of study where several dyes (Coumarin 440, 460, 480, and 500) were used. The spectra for RhH occur between 437 and 442 nm and between 462 and 468 nm. The RhD spectrum contains these and additional features in the region 400 – 470 nm. OBSERVATIONS

EXPERIMENTAL PROCEDURE

A total of 10 bands, 2 attributable to RhH and 8 to RhD, have been found between 400 and 470 nm. These bands are centered at 464.7 and 438.4 nm (RhH) and 463.4, 445.2, 443.0, 428.1, 427.8, 424.4, 422.6, and 409.2 nm (RhD). A recording of the strongest RhD band is shown in Fig. 1. A similar structure of open R, Q, and P branches is found in all the observed bands. The rotational temperature in the source was ⬇35 K with the result that the intensities within a branch decrease rapidly with increasing rotation such that, in the present experiments, we have been able to follow the branch structure in most instances only as far as J ⫽ 7 in RhH and J ⫽ 10 in RhD. Toward higher J, members of the branches gradually broaden and then split into two components, the splitting

This laboratory has recently been engaged in a systematic spectroscopic investigation of rhodium-containing diatomic species produced by chemical reaction of laser-ablated rhodium metal with various substrates. In the course of Rh ⫹ CH 4/He experiments to generate spectra of RhC (8) and Rh ⫹ 1

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ELECTRONIC SPECTRUM OF RhH AND RhD

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TABLE 2 Observed RhH and RhD Bands and Their Molecular Constants a

FIG. 1. The 463.4-nm band of RhD. The features marked with an asterisk near 463.69, 464.75, and 464.82 nm, respectively, are the R(3), Q(3), and Q(4) lines of RhH contaminant.

being the same in magnitude for R( J), Q( J ⫹ 1), and P( J ⫹ 2). This upper state doubling, which is different in different bands, increases rapidly with increasing J and is assumed due to ⌳-type doubling. A list of the observed splittings is given in Table 1. It may be noted that these splittings are quite erratic for the 464.7-nm RhH band. Because of these irregularities and the rather limited range of observations, we have not attempted to fit the rotational line frequencies using model Hamiltonians. Instead, estimated B values have been obtained from ⌬ 2 F( J) second combination differences assuming ␯ ⫽ ␯ 0 ⫹ B⬘ J⬘( J⬘ ⫹ 1) ⫺ B⬙ J⬙( J⬙ ⫹ 1) and band origins ␯ 0 have been determined graphically using plots of 21[R( J ⫺ 1) ⫹ P( J)] versus J 2 and Q( J) versus J( J ⫹ 1). Table 2 contains a summary of the molecular constants and lifetimes for the excited states observed. The bands share a common lower level and first lines indiTABLE 1 Observed Splittings (cm ⴚ1) in Rotational Levels in the Excited States of RhH and RhD

cate its ⍀ value to be 3, as expected for the lowest spin– orbit substate of an inverted 3⌬ state. All but two of the observed bands have ⌬⍀ ⫽ 0; exceptions are the 424.4- and 409.2-nm bands of RhD which arise from ⍀⬘ ⫽ 2 4 ⍀⬙ ⫽ 3 and ⍀⬘ ⫽ 4 4 ⍀⬙ ⫽ 3 transitions, respectively. The relative positioning and characteristics of the 464.7- and 438.4-nm bands of RhH and the 463.4- and 443.0-nm bands of RhD suggest that they are corresponding (0, 0) and (1, 0) bands. Observations in dispersed fluorescence (DF) [Fig. 2] support these assignments. The relative (Franck–Condon) intensities in the DF spectra are a reflection of the nodal character of the upper state vibrational wavefunction. It can be seen that with the excitation wavelength chosen near 443.2 nm (RhD) emission to the v ⫽ 1 level of the ground electronic state is very weak. The same is true for RhH excitation near 437.5 nm. We have searched unsuccessfully for transitions to higher vibrational levels. No bands have been found to the red of 465 nm for either isotopomer nor have we had any success in a search for RhH analogues of the other RhD bands. As may been seen from the upper state data, summarized in Table 2, the higher energy RhD bands show no systematic behavior with regard to position, ⍀⬘, B⬘, or ⌳-splitting to suggest a ready classification. Line positions with assignments are listed for the [21.5]–X bands of RhH and RhD in Table 3. The strongest features in the DF spectra of Figs. 2a and 2b arise from emission to successive vibrational levels of the ground state. Such experiments have located levels v⬙ ⫽ 0 through 4 in RhH and v⬙ ⫽ 0 through 5 in RhD. These data give the following approximate values for the vibrational constants in the X 3 ⌬ 3 state: RhH:

␻ e⬙ ⫽ 2040 ⫾ 10 cm ⫺1

␻ e⬙ x e⬙ ⫽ 46 ⫾ 4 cm ⫺1

RhD:

␻ e⬙ ⫽ 1422 ⫾ 10 cm ⫺1

␻ e⬙ x e⬙ ⫽ 18 ⫾ 2 cm ⫺1 .

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TABLE 3 Rotational Assignments and Vacuum Wavenumbers (in cm ⴚ1) for RhH and RhD

FIG. 2. RhD-dispersed fluorescence spectra at various excitation wavelengths. Zero-wavenumber displacement marks the probe frequency: (a) 443.161 nm; (b) 463.564 nm; (c) 422.688 nm; (d) 424.815 nm; (e) 427.785 nm.

DISCUSSION

For comparison, the ⌬G( 21) values in the [21.5] state are considerably smaller, namely 1298.2 cm ⫺1 for RhH and 997.2 cm ⫺1 for RhD. We believe the [21.5] state, which has ⍀ ⫽ 3, to be a 3⌬ 3 substate also (vide infra). Close examination shows that there are additional weak features in the DF spectra shown in Figs. 2a and 2b. These arise through emission to a state lying approximately 820 cm ⫺1 above the ground state. Similar observations apply to excitation to other ⍀ ⫽ 3 levels in both RhH and RhD. The spectrum from excitation at 427.8 nm shows the emission to the T ⬇ 820 cm ⫺1 state and several of its vibrational levels more prominently (Fig. 2e). We believe the level at T ⬇ 820 cm ⫺1 to be the 3 ⌬ 2 spin– orbit component of the ground state. In support of this assignment we note that the DF spectrum using 424.8-nm radiation, i.e., excitation to an ⍀ ⫽ 2 state (see Fig. 2d), is somewhat different. Here emission at the probe frequency, ⍀ ⫽ 2 3 X 3 ⌬ 3 , v ⫽ 0, is relatively weak and the stronger features of the spectrum are those where ⌬⍀ ⫽ 0. We have not found the X 3 ⌬ 1 component, expected in the vicinity of T ⫽ 1650 cm ⫺1 . A summary of the intervals identified in the DF spectra and their assignments may be found in Table 4.

Spectra are known and have been characterized for all 3d transition metal monohydrides. For the 4d and 5d metals details are sparse: in the 4d series experimental spectroscopic data are available only for a few states of YH (10), PdH (11), and AgH (12), although all members of the family have been treated theoretically. In the case of RhH, calculations have been made incorporating relativistic effects and spin– orbit coupling. The modified coupled-pair functional method of

TABLE 4 Observed Low-Lying Levels of RhH and RhD as Measured by Dispersed Fluorescence Spectroscopy a

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ELECTRONIC SPECTRUM OF RhH AND RhD

Langhoff et al. (4) gives r e ⫽ 0.1575 nm and ␻ e ⫽ 2057 cm ⫺1. Balasubramanian (13), using complete active space multiconfigurational self-consistent field (CASSCF) with second-order configuration interaction (CI), found r e ⫽ 0.1576 nm and ␻ e ⫽ 1971 cm ⫺1. These predictions are close to the experimental values obtained in this work: r e ⫽ 0.159 nm, ␻ e ⬇ 2040 cm ⫺1. There are many low-lying electronic states of RhH predicted. The electron configuration of the ground state is predominantly ␴ 2␴ 1␲4␦3. Balasubramanian and Liao (3) have used CASSCF, followed by first-order CI, to calculate the energies of eight other electronic states arising from low-lying MO configurations. These calculations predict, in increasing energy order, states of symmetry 1⌺ ⫹ (␴ 2␲ 4␦ 4), 3⌽ (␴ 2␴ 2␲ 3␦ 3), 3⌸ (␴ 2␴ 2␲ 3␦ 3), 3⌺ ⫺ (␴ 2␴ 2␦ 4␲ 2), and 1⌸ (␴ 2␴ 2␲ 3␦ 3), all below 10 000 cm⫺1. We have searched in the energy range 0 –11 000 cm⫺1, but no evidence for any of these states has been found in our DF spectra. The various excited states found in the LIF experiments lie considerably beyond the limits of the energies calculated. The bonding interaction between the metal and hydrogen comes from Rh (5s)–H (1s) overlap. The 4d 8 electrons are essentially nonbonding and the molecule may be approximately described as Rh ⫹H ⫺. There are several factors that one needs to consider in order to understand the bonding interaction between the rhodium and hydrogen atoms that gives the 3⌬ ground state. First, since the atomic configuration of Rh is 4d 8 5s 1 , it is evident that the 5s atomic orbital is at a considerably higher energy than the 4d orbital. Indeed, the 4d 7 5s 2 configuration lies some 13 000 cm ⫺1 above the 4d 8 5s 1 (14). Second, the bonding/antibonding interactions involve mainly three AOs of the ␴ irreducible representation in the C ⬁v point group, i.e., 4d z 2 and 5s on Rh and 1s on H. Since the H atom has a higher ionization potential than the Rh atom (110 000 cm ⫺1 versus 60 000 cm ⫺1), it is reasonable to assume that the ␴-bonding orbital is largely hydrogen 1s in character. The ␴ nonbonding and antibonding orbitals, on the other hand, are dominated by the hybridized 5s and 4d z 2 orbitals of Rh. The formation of the RhH bond is then, to a large extent, the transfer of a Rh electron to the H atom. The electronic structure may therefore be understood approximately as a supermultiplet due to the Rh ⫹ ion. A consequence of the above consideration is an electron configuration with only one electron in the ␴ nonbonding orbital, since its energy may be significantly higher than the other nonbonding orbitals (i.e., 4d ␲ and 4d ␦ ). Another issue is the relative energies of the nonbonding ␦ and ␲ orbitals. It is likely that interactions/mixing with higher energy ␲-orbitals, e.g., Rh 5p ␲ and H 2p ␲ , will push the energy of the nonbonding 4d ␲ orbital lower than that of the 4d ␦ . As far as the Rh ⫹H ⫺ supermultiplet is concerned, the situation in RhH is not unlike that for NiH which has been discussed in depth by Gray et al. (15). We consider the Rh ⫹ 3F terms which arise from the ground electron configuration 4d 8 and the excited configuration 4d 7 ( 4 F)5s 1 . As the H ⫺ ligand is brought up to Rh ⫹, to form RhH, there is a splitting of each 3F ion term into 3⌽, 3⌬, 3⌸, and 3⌺ ⫺ molecular states. Where the

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FIG. 3. Energy level diagram comparing CoH and RhH. The metal ion energy levels have been taken from Moore’s tables (14). For the molecules ab initio values have been used. The upper group of RhH levels are shown schematically as dashed lines since they have not been treated theoretically.

energy splitting is not large, we may expect the groups of four states to have a separation approximately that of the parent Rh ⫹ 3 F ion states. The particular ordering of the molecular states depends upon specific interactions within the molecule. The cases of the 3d CoH and its 4d relative, RhH, are compared in Fig. 3 where the ordering of molecular states from the ground ion terms is as predicted by ab initio methods (3, 5). What is evident from this figure is that a 3 ⌽–X 3 ⌽ transition is expected in CoH near 10 000 cm ⫺1, while the analogous 3 ⌬–X 3 ⌬ transition in RhH is expected near 25 000 cm ⫺1. An A⬘ 3 ⌽ 4 –X 3 ⌽ 4 transition has indeed been located in CoH, whose (0, 0) band lies near 12 400 cm ⫺1 (6), and this RhH study identifies excited levels with ⍀ ⫽ 2, 3, and 4 in the 21 500 –24 500 cm ⫺1 region. It is probable that these RhH levels are derived from 3⌸, 3⌬, and 3⌽ states, respectively. Balasubramanian and Liao also estimated the ground state 3 ⌬ 1– 3⌬ 3 spin– orbit splitting at ⬃1400 cm ⫺1, which is consistent with our ⬃820 cm ⫺1 determination of the energy of the middle 3⌬ 2 component. For an unperturbed 3⌬ state derived from a ␴␦ 3 configuration, A so ⫽ ⫺ 21 a ␦ . With a ␦ approximated by the Rh atomic ␨ 4d , the 3⌬ 2 and 3⌬ 1 components are expected at T ⫽ 625 and 1250 cm ⫺1, respectively. While the ground state vibrational constants that we have measured are approximate and the anharmonicity of the upper [21.5] state remains undetermined, a rough estimate can be made of the expected RhH–RhD isotopic shift for the [21.5]– X (0, 0) band. The predicted value of ⬃ ⫺140 cm ⫺1 is sufficiently different from the observed one (⫺59 cm ⫺1) to suggest that one or both of the v ⫽ 0 [21.5] states is perturbed. Given the density of low-lying states predicted theoretically this would not be too surprising. Two observations suggest that the rhodium monohydride upper states suffer from predissociation: (i) The RhD spectrum is much richer than the RhH spectrum. It is only for the RhD excited state of lowest energy among those observed that the RhH analogue has been found; and (ii) in some branches there

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is a marked intensity drop off beyond a certain J⬘ value. For example, in the 427.8-nm band of RhD, lines with J⬘ ⫽ 4 and 5 are quite strong and lines with J⬘ ⱖ 6 are completely absent. This is not unusual behavior. It has been pointed out by McCarthy and Field (11) that “most transition metal M–H molecules can be expected to have universally predissociated electronic states at E evr ⬎ D 00 ,” and for good reason. The current best estimate for the dissociation energy of RhH is 20 600 ⫾ 1750 cm ⫺1 from gas-phase thermochemistry (16). ACKNOWLEDGMENTS The authors thank Scott Fouge`re and Chi Zhou for assistance in obtaining some of the spectra. Financial support was received from the University of Victoria and the Natural Sciences and Engineering Research Council of Canada.

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