Fourier transform microwave spectroscopy of ScS (X2Σ+) and YS (X2Σ+)

Journal of Molecular Spectroscopy 278 (2012) 35–40

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Fourier transform microwave spectroscopy of ScS (X2R+) and YS (X2R+) G.R. Adande, D.T. Halfen, L.M. Ziurys ⇑ Departments of Chemistry and Astronomy, Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, United States

a r t i c l e

i n f o

Article history: Received 24 May 2012 In revised form 5 July 2012 Available online 22 July 2012 Keywords: FTMW spectroscopy Scandium sulfide (ScS) Yttrium sulfide (YS) Hyperfine structure Laser ablation

a b s t r a c t The pure rotational spectra of the transition metal sulfide radicals ScS and YS in their 2R+ ground states have been measured in the range 8–48 GHz using Fourier transform microwave (FTMW) spectroscopy. The radicals were synthesized from the reaction of metal vapor, produced by laser ablation, and H2S gas, heavily diluted in argon. A DC discharge was needed in the case of ScS. Four rotational transitions were recorded for each molecule, in which multiple fine and hyperfine components were resolved. The spectra were analyzed with a case (b) Hamiltonian, and rotational, fine, and hyperfine constants were determined for both molecules, improving the precision of previous parameters established from optical and double resonance data. The quadrupole coupling constant eQq has been accurately established for ScS for the first time, as well. From the rotational constants, the bond lengths were determined to be 2.1288 Å for ScS and 2.2614 Å for YS. The hyperfine parameters suggest that, although ScS and YS are principally ionic molecules, they are more covalent than their oxygen analogs. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Transition metal sulfides play an important role in many areas of scientific research. For example, such sulfides have numerous applications in catalysis [1], as well as in the semiconductor industry [2]. They also are relevant in biology, being linked to the primitive development of autotrophic life [3]. Because of their chemical importance, numerous theoretical investigations have been conducted in order to understand the structural, electronic, and thermodynamic properties of transition metal sulfides [4–6]. Such studies, however, can be problematic because of the presence of low-lying electronic states. High resolution spectroscopic data are therefore necessary to benchmark and complement such calculations. Two interesting transition metal sulfides are yttrium monosulfide (YS) and scandium monosulfide (ScS). Their relatively simple electronic structure provides a good starting point for computational models investigating transition metal bonding. Scandium and yttrium both belong to the group III transition metals, which have the simplest open d shell configuration (s2d1). Both molecules have been studied spectroscopically at optical wavelengths, as well as by Fourier transform infrared and optical double resonance methods [7–13]. These studies have shown that both species have 2 + R ground states, with the unpaired electron likely situated in a r hybridized molecular orbital, predominantly located on the metal atom [4]. From the experimental determination of magnetic hyperfine parameters, it had been suggested that the unpaired electron in ScS occupies an orbital centered on the scandium atom with ⇑ Corresponding author. Fax: +1 520 621 5554. E-mail address: [email protected] (L.M. Ziurys). 0022-2852/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jms.2012.07.009

57% s character [10], while the equivalent orbital in YS is 53% s in composition [9]. Here we present the first measurements of the pure rotational spectra of YS and ScS in their 2R+ ground states, recorded using Fourier transform microwave (FTMW) techniques. These radicals were produced using a laser ablation source to generate metal vapor. For both molecules, the fine and hyperfine structures were resolved in multiple rotational transitions, allowing for improved determination of the spectroscopic parameters, as well as an accurate measurement of the quadrupole coupling constant of ScS. In this paper we describe these results and their analysis, and give an interpretation of the fine and hyperfine constants for the two radicals. 2. Experimental The pure rotational spectra of YS and ScS were measured using the Balle–Flygare type Fourier transform microwave (FTMW) spectrometer of the Ziurys group, described in detail elsewhere [14]. Briefly, the instrument consists of a vacuum chamber containing a Fabry–Perot cavity with two spherical aluminum mirrors in a near confocal arrangement. The system is maintained at an unloaded pressure of 108 Torr by a cryopump. Microwave radiation is launched into the cavity either through an antenna (4–40 GHz) or waveguide (40–60 GHz: see [15]) embedded in one mirror. Molecules of interest are introduced into the chamber via a pulsed supersonic nozzle. Molecular emission is collected by an antenna or waveguide embedded in the opposite mirror and detected as a function of time with a low noise amplifier, the so-called Free Induction Decay (FID). The time domain signal is digitized and

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converted by Fast Fourier Transform (FFT) to generate a spectrum, which appears as Doppler doublets, resulting from the jet expansion of the mixture relative to the electric field in the cavity. The transition frequency is taken to be the average of the doublets. The resolution of the FTMW spectrometer is 4 kHz. Both ScS and YS were created by the reaction of H2S with metal vapor produced by laser ablation. A gas mixture of 0.1% H2S in 200 psi of argon was introduced into the cavity by a pulsed valve (General Valve, 0.8 mm nozzle orifice), to which the ablation source was attached such that the gas mixture and the metal vapor were injected approximately at the same time into the cavity. A pulsed Nd:YAG laser beam (200 mJ/pulse) was used to ablate the metal, contained in the form of a rotating, translating rod. In the case of ScS, a DC discharge (0.6 kV, 20 mA) was also necessary for molecule production, applied to the metal/gas mixture immediately following the ablation source. (Yttrium reacted with H2S spontaneously without the need of a discharge.) The details of the discharge assisted laser ablation source, called DALAS, can be found in Sun et al. [16].

3. Results The rotational measurements were based on the constants obtained by Stringat et al. [13] and Azuma and Childs [8] for YS and Steimle et al. [10] for ScS. Because the magnetic moment of yttrium is relatively small, the hyperfine pattern in YS follows a classic bbJ coupling scheme, such that J = N + S and F = J + I. The nuclear spin of yttrium is I = 1/2. Four rotational transitions of YS (N = 1 ? 0 to N = 4 ? 3) were recorded over the range 8–34 GHz; see Table 1. Frequency predictions using the previous spectroscopic parameters for YS were typically reliable to ±15 MHz. Fifteen hyperfine components of this radical were recorded in total. Representative spectra of YS are shown in Fig. 1. In the upper panel, two hyperfine components in the N = 2 ? 1 rotational transition near 16.6 GHz are displayed, labeled by quantum number F, with one from each spin–rotation doublet, indicated by J. Similarly, the lower panel shows two hyperfine components of the N = 3 ? 2 transition neat 24.9 GHz, one from each spin–rotation component. All lines exhibit Doppler doublets, indicated by brackets. In contrast to yttrium, scandium has a large magnetic moment and a nuclear spin of I = 7/2. As a consequence, the Fermi contact term in ScS is very large relative to the spin–rotation interaction. The hyperfine structure is therefore of the same order of magnitude as the fine structure, following a bbs coupling scheme as opposed to bbJ, as for YS. Four rotational transitions were recorded for ScS in the range 11–50 GHz, each consisting of numerous

Fig. 1. Representative FTMW spectra recorded for YS (X2R+). In the upper panel, two hyperfine components of the N = 2 ? 1 rotational transition near 16.6 GHz are displayed, indicated by quantum number F, each arising from a different spin doublet, labeled by J. In the lower panel, two hyperfine lines from the N = 3 ? 2 rotational transition near 24.9 GHz are shown, also originating in separate spin– rotation components. There is a frequency beak in each spectrum in order to show the two spectral features. Doppler doublets are indicated by brackets. Each spectral feature shown was measured in one 600 kHz wide scan, with 1000 pulses per scan.

hyperfine components: see Table 2. For comparison with YS, the bbJ notation was used for labeling the transitions. The transition frequencies were typically ±3 MHz away from predictions, based on the previous constants.

Table 1 Observed transition frequencies of YS (X2R+).

a

mobsa

mobs  mcalca

N0 ? N00

J0 ? J00

F0 ? F00

1?0

1/2 ? 1/2 3/2 ? 1/2 3/2 ? 1/2

1?1 1?0 2?1

8296.712 8327.470 8348.735

0.003 0.003 0.001

2?1

3/2 ? 1/2 3/2 ? 1/2 5/2 ? 3/2 5/2 ? 3/2

1?0 2?1 2?1 3?2

16624.146 16649.810 16654.901 16674.080

0.003 0.002 0.001 0.006

3?2

5/2 ? 3/2 5/2 ? 3/2 7/2 ? 5/2 7/2 ? 5/2

2?1 3?2 3?2 4?3

24955.916 24974.668 24982.239 24999.912

0.006 0.002 0.003 0.002

4?3

7/2 ? 5/2 7/2 ? 5/2 9/2 ? 7/2 9/2 ? 7/2

3?2 4?3 4?3 5?4

33282.711 33299.468 33309.440 33325.797

0.009 0.001 0.002 0.004

In MHz.

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G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40 Table 2 Observed transition frequencies of ScS (X2R+).

a

N0 ? N00

J0 ? J00

F0 ? F00

mobsa

mobs  mcalca

1?0

3/2 ? 1/2 3/2 ? 1/2 3/2 ? 1/2

3?3 5?4 4?4

11805.146 11849.830 11898.629

0.001 0.005 0.006

2?1

3/2 ? 3/2 3/2 ? 1/2 3/2 ? 3/2 3/2 ? 1/2 5/2 ? 1/2 5/2 ? 3/2 5/2 ? 3/2 3/2 ? 1/2 5/2 ? 3/2 5/2 ? 1/2 5/2 ? 3/2 3/2 ? 1/2

3?4 2?3 4?4 5?4 4?4 2?2 5?4 3?3 6?5 3?4 5?5 4?3

23504.279 23576.110 23607.576 23629.356 23646.380 23664.364 23669.405 23686.733 23696.680 23699.674 23718.195 23790.044

0.005 0.009 0.002 0.001 0.002 0.001 0.000 0.009 0.001 0.005 0.002 0.002

3?2

5/2 ? 3/2 5/2 ? 3/2 5/2 ? 3/2 5/2 ? 3/2 7/2 ? 5/2 7/2 ? 5/2 5/2 ? 5/2 7/2 ? 5/2 5/2 ? 3/2 7/2 ? 5/2 7/2 ? 3/2 7/2 ? 5/2 7/2 ? 5/2 7/2 ? 5/2 7/2 ? 5/2 7/2 ? 5/2 5/2 ? 3/2

1?2 4?4 3?3 6?5 5?5 3?2 5?4 4?3 2?2 6?5 5?4 4?4 2?2 7?6 6?6 3?3 4?3

35416.208 35442.239 35454.586 35457.035 35461.236 35470.531 35471.275 35472.333 35480.732 35512.429 35523.071 35525.635 35530.564 35532.415 35533.948 35535.677 35545.540

0.001 0.001 0.003 0.001 0.004 0.004 0.000 0.002 0.008 0.000 0.003 0.003 0.008 0.002 0.005 0.002 0.001

4?3

7/2 ? 5/2 7/2 ? 5/2 7/2 ? 5/2 9/2 ? 7/2 9/2 ? 7/2 9/2 ? 7/2 7/2 ? 5/2 9/2 ? 7/2 7/2 ? 5/2 9/2 ? 7/2

4?4 7?6 6?5 5?4 4?3 6?5 5?4 7?6 4?3 8?7

47259.004 47285.741 47298.714 47305.885 47311.688 47345.299 47347.847 47348.429 47349.957 47364.951

0.001 0.013 0.004 0.002 0.003 0.002 0.004 0.007 0.001 0.005

In MHz.

In case bbs, S couples with I instead of N to give the intermediate quantum number G, taking the place of J, i.e. I + S = G. G then couples with N to create F. In this notation, all the transitions reported in Table 2 would either be assigned to G = 3 or G = 4. Adding N0 or N00 to the respective G generates the F0 or F00 , as given in the table. Representative spectra of ScS are shown in Fig. 2. The top panel displays two hyperfine components of the N = 2 ? 1 rotational transition near 23.6 GHz, both arising in the J = 5/2 ? 3/2 fine structure doublet (or G = 3, F = 3 ? 2 and G = 4, F = 6 ? 5). Three hyperfine lines of the N = 3 ? 2 transition near 35.5 GHz are shown in the lower panel, one arising from the J = 5/2 ? 3/2 doublet and the other two from the 7/2 ? 5/2 doublet (all G = 4). Hyperfine transitions are labeled by F, and the Doppler doublets are indicated by brackets. 4. Analysis Both molecules were fit with the non-linear least-squares analysis program SPFIT [17]. The following bbJ effective Hamiltonian was used [18]:

Heff ¼ Hrot þ Hsr þ Hmhf þ HeQq þ Hnsr

ð1Þ

Rotational, spin–rotation, magnetic hyperfine, electric quadrupole and nuclear spin–rotation interactions were considered in the analyses.

The fitted spectroscopic parameters for YS and ScS are presented in Table 3. Also given in the table are the constants previously obtained from the optical and double resonance data. In the analysis of YS, only five parameters were necessary to obtain an rms of 4 kHz, the experimental precision. In contrast, in the double resonance study of YS [8], both CI and cD were additionally used in the spectral fitting. Both parameters did not significantly improve the fit in this work and therefore were not included. As Table 3 shows, the FTMW study has improved the precision of the rotational constants, and the fine and hyperfine parameters are consistent with the past double resonance work. In the case of ScS, the FTMW data has increased the accuracy of the spectroscopic constants by factors of 10–100, including the first reliable determination of the quadrupole coupling constant eQq. In this analysis, CI was necessary to achieve an rms comparable to the experimental precision (4 kHz). 5. Discussion 5.1. Hyperfine and fine structure interactions The electronic configuration of ScS is postulated to be: (core) 10r2 4p4 11r1 [4,10,19]. The unpaired electron in ScS thus lies in the 11r orbital, which is thought to be centered on the scandium

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be used to evaluate the amount of s character in a given orbital by comparing it to the atomic value of scandium, 2823 MHz [20]. The ratio is then [bF(ScS)/bF(Sc)] = |c1|2  0.58, such that the unpaired electron in ScS is in an orbital with 58% s character. This value is smaller than that found for ScO, where the analogous electron has 69% s character [20]. The decrease in s character upon replacement of O by S suggests that the atomic 3d orbital is better stabilized by the less electronegative sulfur atom. Similarly, the YS electronic configuration is thought to be (core) 13r2 6p4 14r1 [11]. As with scandium, the 5p orbitals in yttrium lie more than 16 000 cm1 above the 5s level [21], and are unlikely to contribute appreciably to the 14r orbital. This orbital is thus likely to be sd hybridized, with a small sulfur 3p contribution. The atomic value of the Fermi contact term for the yttrium atom is 1250 MHz [8]. Therefore, the unpaired electron in YS is 53% s in character, as opposed to 62% for YO [22]. Again, the amount of s character decreases with replacement of the oxygen atom with sulfur. The dipolar hyperfine constant c is defined as [23]:

c¼

  3 1 X ð3 cos2 hi  1Þ g s lB g N lN 2 n r 3i s

ð2Þ

Assuming a negligible role for the sulfur 3p orbital, contributions to c must come principally from the metal d orbital that is hybridized with the metal s orbital, as the angular expectation value for an s electron is zero. Assuming pure sd hybridization, the dipolar hyperfine parameter reduces to

c¼ 2

+

Fig. 2. Representative FTMW spectra recorded for ScS (X R ). In the upper panel, two hyperfine components of the N = 2 ? 1 rotational transition near 23.6 GHz are presented, both arising in the J = 5/2 ? 3/2 fine structure doublet and labeled by the F quantum number. In the lower panel, three hyperfine lines of the N = 3 ? 2 transition near 35.5 GHz are displayed, one arising from the J = 5/2 ? 3/2 doublet and the others from the 7/2 ? 5/2 component. There are frequency breaks in each spectrum in order to display multiple hyperfine transitions. Doppler doublets are indicated by brackets. Each spectral feature shown was measured in one 600 kHz wide scan, with 1500–2000 pulses per scan.

nucleus [10]. The orbital composition is primarily a mixture of metal 4s and 3d character, because the 4p orbitals of scandium lie much higher in energy (20 000 cm1 above the 4s level) [19]. In analogy to the metal oxides, the 11r orbital is thought to be mostly non-bonding, with slight bonding character due to a small admixture of the sulfur 3p orbital [19]. Because the Fermi contact term only arises from the contribution of electrons in s orbitals, it can

  3 3 cos2 h  1 g s lB g N lN 2 r3 dr

ð3Þ

For dr electrons, h3 cos2 hi  1i = 4/7. Using this factor, h1/r3i can be calculated for the unpaired electron in both scandium and yttrium sulfide. For ScS, h1/r3i  1.013 a.u3 for the unpaired electron. This value can be compared to the atomic value for the scandium d electron of h1/r3i  0.911 a.u3 [10] and for the ion Sc+: h1/r3i  1.851 a.u3 [24]. The magnitude of h1/r3i of the unpaired electron is significantly closer to the neutral value, suggesting a considerable degree of covalency in the Sc–S bond. For YS, the unpaired electron has h1/r3i  1.887 a.u3. The analogous atomic values are h1/r3i  2.373 for the Y+ ion [24] and h1/r3i  1.711 a.u3 for the neutral atom [25]. The trend found in scandium for h1/r3i is repeated in yttrium, indicating some fraction of covalent character also in this molecule. In this work, the quadrupole coupling constant for ScS has been accurately determined for the first time. In the context of a Townes–Dailey analysis, the quadrupole coupling constant eQq0

Table 3 Spectroscopic constants for ScS (X2R+) and YS (X2R+).a Parameter

ScS

ScS (optical)b

YS

YS (optical)

B D

bF c CI eQq

5915.2294(12) 0.002873(50) 96.3356(73) – 1671.2(2.4) 112.558(12) 0.01665(98) 55.709(54)

5914.72(39) 0.002893(23) 92.8(2.2) – 1673.9(6.2) 116(41) – 63(159) 56(27)e

4163.0992(21) 0.001331(79) 42.252(15) – 667.8(1.2) 42.470(94)

4160.2(2.7)c 0.0011(6)c 42.2382(6)d 1.8243(21)  104d 667.479(60)d 42.684(54)d 0.0046(6)d

rms of fit r0 (Å)

0.004 2.128824(2)

c cD

a b c d e

Constants in MHz unlesss specified. Errors quoted are 3r. From [12], unless otherwise specified. From [13]. From [8]. From [10].

2.13750(6)

0.004 2.261416(1)

G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40

ScS and YS, respectively. The values of the spin–rotation parameters in the corresponding oxides are c = 3.2175 MHz (ScO) and 9.2254 MHz (YO). Very small or negative values of the spin–rotation constant in the oxides have been attributed to second-order spin–orbit coupling from unobserved low-lying electronic 2P states, arising from the promotion of one electron from the HOMO p shell [22]. In the metal sulfides, other electronic states must be contributing to the second-order spin–orbit interaction, generating net positive spin–rotation constants.

Table 4 Quadrupole coupling constants of scandium species. Species

eQq (MHz)

ScS ScO ScCl ScF

55.709(54) 72.240(15) 68.2067(90) 74.09(15)

39

5.2. Periodic trends in 3D-transition metal monosulfides

Fig. 3. A comparison of experimentally-determined and theoretical [4] ground state bond lengths for the 3d transition metal sulfides and oxides. The experimental bond length values are r0 and the theoretical ones are re. The experimental and theory values are in relatively good agreement. The sulfides and oxide bond lengths show subtle variations across the periodic table, with notable differences at scandium and zinc, and from iron to nickel.

From the measured rotational constant of ScS in its ground state, a r0 bond length of 2.1288 Å has been determined. In Fig. 3, the experimentally-determined bond lengths for the 3d transition metal oxides and sulfides are plotted, as well as the values obtained from theory using DFT methods [4]. The agreement between theory and experiment is rather good for ScS and ScO. Nonetheless, there are two notable differences between the oxide and sulfide series. First, the bond length of ScS is greater than that of ZnS by almost 0.1 Å. In contrast, that of ScO is smaller than the bond distance of ZnO by about 0.03 Å. Secondly, while FeO, CoO and NiO have similar bond lengths, the bond lengths decrease steadily from FeS to NiS. This effect can be qualitatively explained by comparing the atomic orbitals. The energy separation between the 4s and 3d orbitals of the transition metals and the 2p orbital of oxygen is generally larger than the separation with the 3p orbital of sulfur by about 34,000 cm1 [27]. Consequently, there is more valence orbital overlap between the atoms in the monosulfides (i.e. increased bonding character), while in the oxides, these orbitals are predominantly non-bonding. Therefore, addition of electrons into the valence orbitals partly stabilizes the monosulfide molecules, shortening the bond lengths, as noted by Bridgeman and Rothery [4]; this stabilization is not as significant for the monoxide species.

can be expressed in terms of eQq320, the quadrupole coupling created by a 3d orbital of scandium [26]:

6. Conclusion

  1 eQq0 ¼ eQq320  ndr þ ndp  ndd 2

The pure rotational spectra of ScS and YS in their 2R+ ground states have been measured using FTMW spectroscopy, in combination with laser ablation. Spectroscopic constants have been improved for both radical species. Analysis of the hyperfine parameters indicates that both YS and ScS are somewhat more covalent than their oxygen analogs. In addition, YS is slightly less ionic than ScS. These data support the theoretical prediction that transition-metal bonds to sulfur are different than those to oxygen.

ð4Þ

Here ni are the orbital populations. Assuming the only contribution to eQq in ScS is the unpaired electron in the hybridized sd orbital, ndr = 1 and the other populations are zero. The term eQq320 can be evaluated using the formula [26]:

eQq320 ¼ 2:353

 3  2lðl þ 1Þ a0 Q ð2l þ 3Þð2l  1Þ r

ð5Þ

For a scandium nucleus, Q = 23.1 fm2 [26], l = 2 for a d electron, and h1/r3i can be obtained from c, as discussed. The coupling constant eQq is then calculated to be 31.46 MHz. This value is about 56% of the experimental constant of 55.709 MHz, which suggests that core electrons in scandium sulfide also contribute to eQq. Some insight into the bonding in ScS can be gleamed from a comparison of quadrupole parameters among scandium species, as listed in Table 4. For ScO, eQq = 72.24 MHz [20], while the respective values are 74.09 MHz and 68.21 MHz for ScF and ScCl. [26]. For ScS, the eQq = 55.709 MHz. Thus, while the quadrupole constants indicate that the electronic distribution is similar in these molecules, there are some differences. ScS is apparently the most covalent of the four molecules, while ScF is the more ionic. This result is perhaps expected as oxygen and the halogens are more electronegative than sulfur. Finally, the values of the spin–rotation parameters should be noted. These constants are c = 96.3356 MHz and 42.252 MHz for

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