The Fourier transform microwave spectrum of YC2 (X˜2A1 ) and its 13C isotopologues: Chemical insight into metal dicarbides

Chemical Physics Letters 555 (2013) 31–37

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~ 2 A1 ) and its The Fourier transform microwave spectrum of YC2 (X isotopologues: Chemical insight into metal dicarbides

13

C

D.T. Halfen ⇑, J. Min, L.M. Ziurys Department of Chemistry and Department of Astronomy, Arizona Radio Observatory, and Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA

a r t i c l e

i n f o

Article history: Received 12 August 2012 In final form 22 October 2012 Available online 29 October 2012

a b s t r a c t ~ 2 A1 ) and its 13C isotopologues has been measured in The Fourier transform microwave spectrum of YC2 (X the 10–57 GHz range, the first FTMW study of a metal dicarbide species. The molecule was created from yttrium vapor and CH4 in argon in a supersonic jet with a discharge-assisted laser ablation source (DALAS). Rotational, fine structure, and Y and 13C hyperfine constants were determined for each isotopoð1Þ logue. The calculated r m structure is r(Y–C) = 2.187(4) Å, r(C–C) = 1.270(4) Å, and h(C–Y–C) = 33.74(7)°. The hyperfine parameters indicate that the unpaired electron resides principally in an sd hybridized orbital on the yttrium nucleus. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Transition metal dicarbide species (MC2) play a prominent role in chemistry. Since the discovery of metallo-carbohedrenes (metcars) in 1992, large metal–carbon clusters have been the subject of intense study [1–3]. These clusters are thought to be produced when MC2 units assemble in a cage structure [2]. Transition-metal dicarbides have also been shown to be the starting point for singlewalled carbon nanotubes (SWNT) and carbon nanocapsules [4,5], and may also be involved in the formation of endohedral metallofullerenes [6,7]. In addition, MC2 species are generated by H2 elimination in the reaction of metal atoms with acetylene; the metal acetylide MCCH is the other main product when insertion occurs [8–10]. Understanding such simple C–H activation reactions is fundamental in evaluating more complex organometallic processes [8]. Depending on the relative degrees of ionic vs. covalent bonding, monomeric metal dicarbide species in principle can have three structures: linear MCC, bent MCC, and cyclic T-shaped MC2. Spectroscopic studies of nonmetal and metalloid CCX compounds, such as CCN, CCS, CCP, and CCAs [11–14], have shown that these species are linear, while SiC2, in contrast, is cyclic [15–17]. Theoretical investigations of the metal dicarbides, both main group and transitions metals, suggest cyclic or T-shaped geometries [18,19]. Little experimental work has been done on these molecules, however, to verify such predictions, and current spectroscopy has been limited to YC2, AlC2, and most recently, ScC2 [20–24]. YC2 is of particular interest with regard to transition metal dicarbides [25]. Graphitic cages, as well as single-walled nanotubes,

⇑ Corresponding author. Fax: +1 520 621 5554. E-mail address: [email protected] (D.T. Halfen). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.062

have been known to form around YC2 particles [4,5]. Yttrium also forms some of the strongest bonds with carbon [25], and has a simple electron configuration, (core) 5s24d1, making YC2 an attractive model system. Furthermore, it has been demonstrated that the lowest energy pathway for the reaction of yttrium with acetylene leads to yttrium dicarbide, YC2, while a higher energy pathway produces YCCH [8–10]. There have been numerous theoretical studies of YC2, including those of Roszak & Balasubramanian, Jackson et al. and Glendening, using CASSCF/CI and DFT methods [6,9,26], as well as Puzzarini & Peterson [27], employing MRCI and CCSD(T) techniques. These cal~ 2 A1 culations concur that the molecule is a radical species with a X ground state and a cyclic T-shaped structure with a C–Y–C bond angle near 33°. Spectroscopically, YC2 was first investigated by Steimle et al. in 1997, who conducted medium-resolution laser-induced fluorescence (LIF) and dispersed LIF spectroscopy of the ~ 2 A1 –X ~ 2 A1 , 31 band [21]. This study was followed by that of BousA 0 quet & Steimle in 2001, who recorded optical Stark spectra and determined the ground state dipole moment of YC2 to be 6.38(3) D [22]. Shortly thereafter, Steimle et al. in 2002 measured high-resolution LIF spectra of the 000–000 and 310 bands of the ~ 2 A1 –X ~ 2 A1 transition of YC2 [23]. These authors also recorded rotaA tional spectra of the NKa,Kc = 101 000 through 404 303 transitions of this radical with pump/probe microwave optical double resonance (PPMODR) techniques, resolving the yttrium hyperfine structure. Here we present the first Fourier transform microwave (FTMW) study of the pure rotational spectrum of YC2. This Letter extends the past work of Steimle et al. in recording additional transitions of the main isotopologue, as well as spectra of singly- and doubly-substituted carbon-13 species Y13CC and Y13C2, which have not been observed previously. A new U-band (40–60 GHz) source was employed for some of these measurements. Spectroscopic

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D.T. Halfen et al. / Chemical Physics Letters 555 (2013) 31–37

constants were established from these data, including both 13C and Y hyperfine parameters, and an improved molecular structure was determined. Here we present these measurements, their analysis, and the implications for the bonding in YC2.

2. Experimental The microwave spectrum of YC2 and its carbon-13 isotopologues was measured using the Balle-Flygare-type FTMW spectrometer of the Ziurys group [28,29]. This machine is composed of a large vacuum chamber with a background pressure near 108 Torr, evacuated with a cryopump. Inside the cell are two spherical mirrors in a near confocal arrangement, forming a Fabry–Perot cavity. Two sets of mirrors are used to cover the 4–60 GHz range. Molecules are created in a supersonic jet expansion, using a pulsed valve/laser ablation nozzle that lies at 40° relative to the optical axis. Microwaves are pulsed into the cavity through either an antenna (4–40 GHz) or waveguide (40–60 GHz) imbedded in one mirror. After interaction with the molecular beam, emission signals are collected by an antenna or waveguide imbedded in the second mirror and detected with a low noise amplifier (LNA). Time domain signals are acquired at a rate of 10 Hz and are processed with an FFT to produce spectra with 4 kHz resolution. The detected spectra appear as Doppler doublets with a line width of 10 kHz per feature, and the center frequency is taken as the average of the two Doppler components. For more details, see Ref. [29,30]. Yttrium dicarbide was created from the reaction of Y vapor and methane in the presence of a DC discharge using DALAS (Discharge-Assisted Laser Ablation Source) [31]. A Nd/YAG laser operating at 532 nm was used to ablate a yttrium rod, housed in a steel ablation adapter that is attached to a Teflon discharge nozzle, i.e. DALAS. Yttrium vapor was entrained in a mixture of 0.2–0.5% CH4 and/or 13CH4 in Ar (200 psi), which was pulsed through the nozzle with a backing pressure of 36 psi (248 kPa) and a flow of 50 sccm (standard cubic centimeters per minute). As the gas exited the source and expanded into the chamber, a 800–1200 V DC dis-

charge was applied. More details on DALAS can be found in Ref. [31]. The YC2 signals could also be produced with a 0.1% acetylene (C2H2) in argon mixture, but the spectra were 5 times weaker in intensity. Typically, 250–2000 pulsed averages were need for the main isotopologue and 500–2000 for the 13C-substituted species. Despite the presence of a l-metal shield, Zeeman doublets with a separation of about 20–40 kHz were seen in several of the DJ = DF = 0 components of the low N transitions. Transition frequencies were taken as the average of the observed components.

3. Results YC2 has C2v symmetry with a large dipole moment of

la = 6.38(3) D [22], and follows the a-type selection rules DKa = 0, DKc = ±1. The isotopologues Y12C2 and Y13C2 have equivalent nuclei, but this symmetry is broken in Y13CC, which is in the Cs point group. In Y12C2, the two 12C nuclei are bosons, such that the total wavefunction must be symmetric on exchange of these two nuclei. For Y13C2, the total wavefunction must be antisymmetric with respect to particle exchange because carbon-13 is a fermion. A 2A1 ground state term in the v = 0 level has symmetric electronic and vibrational wavefunctions with respect to particle exchange, such that the overall symmetry is determined by pairing the rotational wavefunction with the appropriate nuclear spin function. The spin of 12C is I = 0, such that Itot = 0 and the spin function is always symmetric. In the case of Y13C2, the spin of 13C is I = 1/2 and Itot = 0, 1; Itot = 0 is antisymmetric and Itot = 1 is symmetric. The symmetric rotational wavefunctions for a-type transitions are those with Ka,Kc = ee and eo, where e is even and o is odd, while those with Ka,Kc = oo and oe are antisymmetric [32]. Pairing the appropriate nuclear spin and rotational wavefunctions has important consequences, as summarized in Figure 1, which shows the energy level diagrams for YC2 and its 13C isotopologues for the N = 0 and 1 rotational levels. The spin-rotation fine structure levels are indicated by quantum number J, and those for hyperfine interactions by F. For Y12C2 (Itot = symm), only sym-

Energy Level Diagrams of YC2 Isotopologues

~ Y13CC (X2A )

~ YC2 (X2A1)

~ Y13C2 (X2A1)

110

110

110

111

111

111

3/2 101 1/2

1/2 000 NKa,Kc

J

F

2 1 1 0

101

1 0

000 NKa,Kc

5/2 3/2 3/2 1/2 3/2 1/2 1/2 3/2 1/2 1/2 J

F1

F

2 1 1 0

101

1 0

000 NKa,Kc

J

F1

F

Figure 1. Energy level diagrams of YC2 and its 13C isotopologues, illustrating the differences in level structure as a function of nuclear spin in 2A1 and 2A0 states. Rotational, fine structure, and hyperfine levels are indicated by N, J, and F, as well as F1, when appropriate. The transitions observed in this Letter are marked by arrows. In YC2, Ka = odd levels are not allowed by boson spin statistics of the 12C nuclei (I = 0), including the NKa,Kc = 110 and 111 levels, shaded in gray. YC2 has hyperfine splittings only from the 89Y spin (I = 1/2). Y13CC does not have C2v symmetry and all Ka levels are allowed, and hyperfine interactions from both the Y and 13C (I = 1/2) are present. For Y13C2, fermion statistics of the 13C nuclear spins pairs the Ka even levels with Itot = 0, and the Ka odd levels with Itot = 1. Therefore, the Ka = 0 levels do not have 13C hyperfine splitting, while those with Ka = 1 do.

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D.T. Halfen et al. / Chemical Physics Letters 555 (2013) 31–37 Table 1 ~ 2 A1 ).a Transition frequencies of YC2 and Y13C2 (X J0 ? J00

F0 ? F00

YC2

mobs

mo–mc

mobs

mo–mc

101 ? 000

0.5 ? 0.5 1.5 ? 0.5 1.5 ? 0.5 1.5 ? 1.5 1.5 ? 0.5 1.5 ? 0.5 1.5 ? 0.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 2.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 1.5 3.5 ? 2.5 3.5 ? 2.5 3.5 ? 2.5 3.5 ? 2.5 4.5 ? 3.5 4.5 ? 3.5 4.5 ? 3.5 4.5 ? 3.5 5.5 ? 4.5 5.5 ? 4.5

1?1 1?0 2?1 2?2 1?1 1?0 2?1 2?1 3?2 3?3 2?2 2?1 3?2 3?2 4?3 3?2 4?3 4?3 5?4 4?3 5?4 5?4 6?5

11401.079 11502.587 11557.241 22788.819 22865.377 22896.346 22945.008 22997.137 23039.059 34153.235 34290.711 34370.313 34403.431 34476.306 34508.363 45818.826 45843.808 45933.423 45958.082 57239.820 57259.121 57362.305 57381.517

0.008 0.003 0.001 0.001 0.010 0.015 0.017 0.005 0.001

10701.292 10795.578 10847.636

0.000 0.007 0.001

21583.493 21624.051

0.006 0.002

32258.367 32290.941 32356.608 32388.115

0.000 0.000 0.002 0.001

202 ? 101

303 ? 202

404 ? 303

505 ? 404

a b

Y13C2

NKa,Kc0 ? NKa,Kc00

b

0.007 0.005 0.003 0.004 0.001 0.000 0.004 0.000 0.001 0.002 0.002 0.001 0.001

In MHz. Unresolved Zeeman splitting, not included in fit.

metric rotational levels exist (i.e. Ka = 0, 2, 4, etc. components); odd Ka levels are not allowed, as Figure 1 shows. Because the spin function can be symmetric or antisymmetric for Y13C2, all Ka levels are allowed, but the Ka = 0, 2, 4, etc. components are paired with Itot = 0, and thus have no 13C hyperfine interactions. In contrast, the Ka = 1, 3, 5, etc. levels have Itot = 1 and thus hyperfine structure is possible. For Y13CC, there are no equivalent nuclei and each rotational level is split into Y and 13C hyperfine states, see Figure 1. The coupling scheme for this species is best described as F1 = J + I1(Y), F = F1 + I2(13C). The measurements for YC2 were based on the work of Steimle et al. First, spectra were recorded for the lines observed by these authors in the NKa,Kc = 202 ? 101 transition near 22 GHz, and chemical conditions optimized. Weaker hyperfine lines were then located, and the NKa,Kc = 101 ? 000, 303 ? 202, and 404 ? 303 transitions were measured accordingly. Frequency predictions were then made for the NKa,Kc = 505 ? 404 transition, based on a preliminary fit of the data, and additional lines recorded. The FTMW measurements were within ±50 kHz of the 11 lines recorded by Steimle et al. The transition frequencies of the 13C isotopologues were predicted by scaling the YC2 constants by the appropriate reduced mass. Locating spectra of these two species required more extensive searches, covering 10–12 MHz in frequency. The rotational frequencies measured for YC2 and Y13C2 in their ~ 2 A1 ground states are listed in Table 1. As is evident in the table, X only data for the Ka = 0 components were recorded. The Ka = 1 levels for Y13CC and Y13C2 and the Ka = 2 states for YC2 exist. The Ka = 1 components for the two isotopic species were searched for but could not be identified in the data. The values of B and C given by Steimle et al. for YC2 were 99% correlated [23]. Thus, the value of (B–C), which is directly related to the frequency separation of the Ka = 1 lines, has very high uncertainty, and our search may not have covered a sufficient range to locate these components. The Ka = 2 levels lie over 14 K in energy, and would not likely be very populated in the jet expansion, where the rotational temperature is near 5 K. In the Ka = 0 components, each rotational transition N + 1 ? N is split into spin-rotation doublets, and additional splittings are generated by the yttrium nuclear spin of I(89Y) = 1/

2. Twenty-three hyperfine lines were recorded for the main isotopologue over five rotational transitions, and nine for Y13C2 in three transitions. One line for YC2 had unresolved Zeeman splitting and was omitted from the fit, as indicated in the table. From this line, the residual magnetic field in the chamber is estimated to be 30 mG [32], about 8% of the Earth’s magnetic field of 350 mG, as measured in Tucson, Arizona. In Table 2, the data recorded for Y13CC are presented. In this case, 13C hyperfine structure was resolved, as expected, in addition to that from yttrium, which creates doublets of doublets, as both spins have I = 1/2. (Additional, weaker hyperfine components are present where DF = 0). Quantum number F1 labels the splitting due to the Y nucleus, as discussed, and those arising from 13C by F. In total, 19 hyperfine lines from three rotational transitions were observed for Y13CC (see Table 2). Only a-type transitions were measured; no b-type lines were observed because the b-type Table 2 ~ 2A0 ).a Transition frequencies of Y13CC (X NKa,Kc0 ? NKa,Kc00

J0 ? J00

F 01 ? F 001

F0 ? F00

mobs

mo–mc

101 ? 000

0.5 ? 0.5 1.5 ? 0.5 1.5 ? 0.5 1.5 ? 0.5 1.5 ? 0.5 1.5 ? 0.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 1.5 2.5 ? 1.5 3.5 ? 2.5 3.5 ? 2.5 3.5 ? 2.5 3.5 ? 2.5

1?1 1?0 2?1 2?1 1?0 2?1 2?1 2?1 3?2 3?2 2?1 2?1 3?2 3?2 3?2 3?2 3?2 4?3 4?3

1.5 ? 1.5 1.5 ? 0.5 2.5 ? 1.5 1.5 ? 0.5 1.5 ? 0.5 2.5 ? 1.5 1.5 ? 0.5 2.5 ? 1.5 3.5 ? 2.5 2.5 ? 1.5 2.5 ? 1.5 1.5 ? 0.5 3.5 ? 2.5 2.5 ? 1.5 2.5 ? 2.5 2.5 ? 1.5 3.5 ? 2.5 4.5 ? 3.5 3.5 ? 2.5

11030.529 11135.697 11187.673 11191.572 22161.884 22209.210 22259.516 22261.951 22301.962 22302.991 33269.433 33270.039 33301.593 33304.640 33307.392 33371.227 33373.127 33403.947 33404.448

0.003 0.004 0.001 0.006 0.003 0.003 0.006 0.000 0.002 0.000 0.005 0.008 0.008 0.013 0.002 0.000 0.003 0.002 0.001

202 ? 101

303 ? 202

a

In MHz; the coupling scheme is F1 = J + I1(Y) and F = F1 + I2(13C).

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D.T. Halfen et al. / Chemical Physics Letters 555 (2013) 31–37

~2 YC2 (X A1): NK ,K = 303 a

J = 2.5

J = 3.5

1.5

F=3 F=3 F=2

34370.1

~2 13 Y C2 (X A1): NK ,K = 303

202

c

a

2.5

F=4

J = 2.5

3

2

F=2

F=3

F=3

2

2.5 F=4

3

2

2

34403.5

34476.4

34508.4

32258.1

~2 YC2 (X A1): NK ,K = 505 a

J = 4.5

404

J = 5.5

3.5 F=5

c

F=5

4

4.5 F=6

5

4

3

57240.0

57259.2

57362.3

57381.5

Frequency (MHz) Figure 2. FTMW spectra of the NKa,Kc = 303 ? 202 (upper panel) and 505 ? 404 ~ 2 A1 ) near 34 and 57 GHz. Each (lower panel) rotational transitions of YC2 (X rotational transition consists of fine structure doublets, marked by J, each of which is further split into two hyperfine components, labeled by F, due to the Y nuclear spin. There are three frequency breaks in the data in order to show all four features. Doppler doublets, indicated by brackets, are apparent in each line profile. The NKa,Kc = 303 ? 202 and 505 ? 404 spectra were each created from four, 500 kHz-wide scans, with 270–2500 and 1800–2000 pulse averages per scan, respectively.

dipole moment is estimated to be 0.1 D, based on that of 13CC [33]. Therefore, the b-type transitions would be a factor of 64 times weaker than the a-type lines that were measured, and thus not detectable given the noise levels. Spectra of the NKa,Kc = 303 ? 202 (upper panel) and 505 ? 404 (lower panel) transitions of YC2 near 34 and 57 GHz are presented in Figure 2. Each transition consists of a spin-rotation doublet, labeled by J, which is further split into two hyperfine lines due to the spin of the 89Y nucleus, and indicated by F. There are three frequency breaks in each spectrum in order to show all four lines.

32258.5

32291.0

32356.6

32388.0

Frequency (MHz)

Frequency (MHz)

57239.6

J = 3.5

1.5

1

34370.5

F=4

1

202

c

~ 2 A1 ) near Figure 3. FTMW spectrum of the NKa,Kc = 303 ? 202 transition of Y13C2 (X 32 GHz. The spectrum is very similar to that of YC2, with spin-rotation doublets, indicated by J, which are further split into two yttrium hyperfine components, labeled by F. Carbon-13 hyperfine structure is not present in the Ka = 0 component of Y13C2 because of fermion statistics. There are three frequency breaks in the spectrum to show the four components. Doppler doublets for each feature are indicated by brackets. The spectrum was created from four, 500 kHz-wide scans with 2000 shots each.

Each feature is also split into Doppler doublets, indicated by brackets, due to the orientation of the molecular jet relative to the electric field of the cavity. In Figure 3, a spectrum of the NKa,Kc = 303 ? 202 transition of Y13C2 is displayed near 32 GHz. The pattern is virtually identical to that of the main isotopologue, with a ‘doublet of doublets’ resulting from spin-rotation and Y hyperfine interactions, labeled by J and F. Again, there are three frequency breaks in the spectrum in order to show all four lines, and the Doppler doublets are indicated by brackets. Figure 4 shows lines arising from the NKa,Kc = 303 ? 202 transition for Y13CC near 33 GHz. There are multiple frequency breaks in the spectrum in order to display six of the nine components measured for this transition. The Doppler doublets present in each feature are indicated by brackets. For this species, hyperfine coupling from the single 13C nucleus now splits each yttrium F1 hyperfine component into additional lines, labeled with F. The relative contributions of the 89Y and 13C hyperfine interactions is illustrated by the J = 3.5 ? 2.5 spin component, shown in the figure. Yttrium generates the F1 = 4 ? 3 and 3 ? 2 splitting of about 30 MHz, while that due to 13C is much smaller, less than 0.5 MHz for the F = 4.5 ? 3.5 and 3.5 ? 2.5 lines, for example. 4. Analysis The spectra of YC2 and its 13C isotopologues were individually fit with an S-reduced asymmetric top Watson Hamiltonian that included rotation, centrifugal distortion, spin-rotation,and magnetic hyperfine interactions [34]:

^ eff ¼ H ^ rot þ H ^ cd þ H ^ sr þ H ^ mhf : H

ð1Þ

The spectroscopic constants were determined using the nonlinear least squares fitting program SPFIT [35], and are given in Table 3. In each fit, A and eaa, as well as the ratio of B to C, were fixed to the values from Steimle et al. [23]. In addition, DNK was

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D.T. Halfen et al. / Chemical Physics Letters 555 (2013) 31–37

~2 13 12 Y C C (X A'): NK ,K = 303 a

J = 2.5

c

J = 3.5

1.5

2.5

F = 3.5 2

33269.8

3 3.5

2

3.5

2

4

2.5

3

1.5

33371.2

33270.2 33301.5

2.5

2

2.5

0.5

3

3.5

2.5

1

1.5

4 4.5

3 F1 = 3

202

33373.2

33404.0

33404.5

Frequency (MHz) ~ 2 A0 ) near 33 GHz. The spectrum is more complex that those of YC2 and Y13C2 because of the Figure 4. FTMW spectrum of the NKa,Kc = 303 ? 202 transition of Y13CC (XX presence of both Y and 13C hyperfine structure. Each fine structure component, labeled by J, is first split into F1 components due to the yttrium nuclear spin, separated by about 30 MHz. Additional, smaller splittings of order a few MHz or less, arising from the 13C nuclear spin of ½ and labeled by F, produce the observed spectrum. Each feature exhibits Doppler doublets, indicated by brackets, and there are four frequency breaks in the data to display multiple lines. The spectrum is a composite of four, 500 kHz wide scans, and 1 MHz-wide scan-composite of two 500 kHz-wide scans; each 500 kHz scan is 2000 pulse averages.

Table 3 ~ 2A1), Y13CC (X ~ 2A0 ), and Y13C2 (X ~ 2A1). Spectroscopic constants of YC2 (X Parametera

MW 52 246c 6054.3651(25)d 5434.6584(23)d 5744.5117(17) 0.02 087(17) 1.537c 1.00(35)  106 eaa 2123c ebb 103.6879(67)d ecc 163.440(11)d (ebb + ecc)/2 133.5637(62) 0.04 205(25) DsN aF (Y) 566.56(17) Taa (Y) 44.340(36) TaaD (Y) 0.0327(57) aF (13C) aFD (13C) Taa (13C) TaaD (13C) rms 0.006 A B C (B + C)/2 DN DNK HN

a b c d

Y13CC

YC2

Y13C2

Opticalb

MW

MW

52 246(13) 6054.3(5.4) 5434.6(5.4) 5744.5(3.8) 0.0210(90)

52 246c 5860.7142(52)d 5260.8291(46)d 5560.7716(35) 0.03 447(26) 1.537c

52 246c 5682.651(12)d 5100.991(11)d 5391.8210(82) 0.04 920(58) 1.537c

2123(72) 265.6(2.7) 0c 132.8(1.3) 0.3127(45) 589.2(1.2) 45.0(6.3)

2123c 100.327(37)d 158.141(59)d 129.234(70) 0.0459(17) 565.25(25) 44.69(25) 0.032(11) 28.115(79) 0.0123(95) 3.40(12) 0.080(27) 0.005

2123c 97.334(79)d 153.42(12)d 125.379(74) 0.0455(34) 566.85(35) 43.70(49) 0.024(17)

0.003

In MHz; errors are 3r in the last quoted decimal places. Ref. [23]. Held fixed. Ratio held fixed, see text.

fixed to the value of DJK for SiC2 [16,17] and the ratio of ebb to ecc to that calculated for AlC2 [36]. More rotational transitions and asymmetry components would be needed to determine more global constants. Several higher-order distortion constants were also used in the analysis: HN for the main isotopologue and TaaD for both Y and 13C hyperfine interactions, as well as aFD for Y13CC (see Table 3). These terms reflect the rather asymmetric nature of the molecule. The asymmetry parameter for YC2 is j = 0.974 [32]. This value indicates more asymmetry than for the near-prolate asymmetric top SrSH (j = 0.999), for example, and is closer to the values found for T-shaped species NaCN and KCN (j = 0.957 and

0.985) [37–39]. The rms values of the fits are between 3 and 6 kHz, in good agreement with the experimental uncertainty of ±4 kHz. Given the differences in the data sets, the parameters determined from the microwave data for YC2 agree well with those from the optical work of Steimle et al. [23], in particular (B + C)/2 and DN. For the spin-rotation constants, Steimle et al. could not independently determine ecc and therefore fixed it to 0. In our fit, both ebb and ecc were fitted but their ratio was held fixed; however, the average spin-rotation parameter (ebb + ecc)/2 for our work agrees with that of Steimle et al. to within the uncertainties. The dipolar term Taa is in excellent agreement with c, as given by the Steimle et al. paper, recognizing that Taa = 23 c [23]. On the other hand, the aF and DsN constants are smaller than those reported by Steimle et al., most likely a reflection of the low N microwave data set. When their PPMODR data is analyzed alone, aF and DsN reflect our values. Combining the LIF and PPMODR data apparently has a significant effect on these parameters. 5. Discussion Assuming the molecule has C2v symmetry and a T-shaped geometry, as established by Steimle et al. [23], the structure of YC2 was determined by a nonlinear least-squares analysis in the STRFIT code [40]. An r0 structure was first established using the sum of the rotational constants (B + C) of all three isotopologues. ð1Þ In addition, an r m geometry was calculated using the (B + C) values and the A constant from Steimle et al. [23]. This structure partially takes into account the effects of zero-point vibrations by modeling the mass dependence of the moment of inertia, bringing it closer to the equilibrium geometry [41]. The structural parameters for YC2, as well as the r0 values from Steimle et al. and the theoretical geometry from Puzzarini & Peterson, are listed in Table 4 [23,27]. ð1Þ From the r m analysis, the bond lengths and angle were established to be r(Y–C) = 2.187(4) Å, r(C–C) = 1.270(4) Å, and h(C–Y– C) = 33.74(7)°. These parameters agree with the theoretical values and with those from Steimle et al. to within 0.01 Å and 0.02° [23,27].

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D.T. Halfen et al. / Chemical Physics Letters 555 (2013) 31–37

Table 4 Structural parameters of YC2 and related molecules. Molecule

r(MC) (Å)

r(CC) (Å)

h(CMC) (deg.)

Method

Ref.

YC2

2.194(2)

1.264(2)

33.5(1)

r0a

2.187(4)

1.270(4)

33.74(7)

rm

2.1946 2.1998

1.2697 1.2690

33.63 33.53

1.83 232(58)

1.26 855(36) 1.2425 1.3391(13) 1.20 241(9)

40.505(25)

r0 re, CCSD(T)/ CBS + CV + SO rs re, Optical rz, MW re, Infrared, Raman

This Letter This Letter [23] [27]

SiC2 C2 CH2@CH2 HC„CH a b

ð1Þ b

[17] [45] [43] [42]

Fitted with (B + C) of isotopologues from this Letter. Additionally fitted with A of Y12C2 from Ref. [23]; cb = 0.116(68).

The C–C bond length of YC2 (1.270 Å) lies between that of acetylene (1.202 Å) and ethylene (1.339 Å) [42,43]. This result suggests that the C–C bond is a mixture of a double and triple bond, with a bond order of about 2.5. The Y–C bond length, in comparison with theoretical work [9], is indicative of a single bond. The perpendicular distance from the Y nucleus to the C–C moiety was calculated to be 2.093 Å, larger than the Y atomic radius of 1.80 Å [44]. Table 4 also lists geometries of related species. As the table show, the bond length of the C2 (X1 Rþ g ) molecule is 1.2425 Å [45], a triple bond, and thus slightly shorter than the C–C bond in YC2. The electron density contributed by the Y atom appears to increase the distance between the C atoms. In SiC2, the only other T-shaped dicarbide molecule with an accurate experimental structure, the geometry changes relative to YC2. The C–Si–C angle in this molecule is larger (40.5° vs 33.7°), and the heteroatom-carbon bond length is shorter (1.8323(6) Å as opposed to 2.187(4) Å). The C–C bond length in SiC2 (1.2685(4) Å) [17], in contrast, is almost identical to that of YC2. These findings suggest that the C–C bond in the triangular dicarbide species does not change significantly on substitution of Y for Si, and that the C–M–C angle is a function of the perpendicular distance between the heteroatom and the C2 group. A larger heteroatom lengthens the distance to the C2 moiety and decreases the angle. Further studies of dicarbide species are certainly needed to verify this simple picture. The electron configuration for YC2 is proposed to be (core) 12a1213a125b1214a11. The 12a1, 13a1, and 5b1 orbitals are principally associated with the C2 moiety; the 14a1 orbital is thought to be composed mainly of the 5s orbital of Y. The contribution of the 5s orbital to the 14a1 orbital can be evaluated by comparing the Y atomic Fermi contact term with the molecular constant aF, which is defined as [32]:

aF ¼

8p g l g l hjWð0Þj2 i: 3 s B N N

ð2Þ

Comparison of the yttrium atomic Fermi contact term (1189 MHz [46]) to that of YC2 shows that the 5s contribution to the 14a1 orbital is 48%. This value is considerably different from that calculated by Glendening, who suggested that the orbital had 87% 5s character [9]. The remaining 52% contribution to the 14a1 orbital is probably due to the 4dr orbital of Y. This result indicates that the 14a1 orbital of YC2 has substantial sd hybridization. Therefore, YC2 appears to be more covalent than previously thought. The dipolar constant Taa is defined as [23]:

T aa

  3 cos2 h  1 ¼ g s lB g N lN : r3

ð3Þ

Assuming that the 4dr orbital makes the major angular contribution to the 14a1 orbital, and thus to Taa, the expectation value of

hr13 i for YC2 can be calculated. The result is hr13 i = 2.0  1025 cm3 or 2.94 a03, using the angular expectation value of 4/7 for the dr orbital. The hr13 i parameter for the Y+ ion (5s14d1) is 1.6  1025 cm3 or 2.373 a03 [47], and that for the neutral atom (5s24d1) is 1.711 a03 [48]. The values of hr13 i in the neutral Y atom and Y+ reflects the size of the 4d orbital, which becomes more contracted for the ion. This calculation would infer that the unpaired electron in YC2 also lies in a more contracted orbital, perhaps resulting from sd hybridization and also from carrying a more electropositive charge. Glendening suggests a charge of +1.21 on the Y atom but with 87% 5s character of the 14a1 orbital, clearly too large. The effective charge on Y must be smaller, perhaps +0.6. The 13C hyperfine constants of YC2 have been measured here for the first time, providing additional insight into bonding. The value of aF(13C) for Y13CC is 28.115(79) MHz, much smaller (<1%) than the atomic parameter of 3777 MHz [47]. In addition, the dipolar term Taa(13C) is only 3.40(12) MHz. These numbers clearly show that little unpaired electron density exists on the carbon atoms in YC2. The unpaired electron must reside principally on the Y nucleus in an orbital that is sd hybridized. The bonding in YC2 has previously been described as highly ionic with a stoichiometry of Y2þ C2 2 [9]. In the ionic scheme, the two electrons from the Y nucleus are completely transferred to the C2 moiety, with the remaining unpaired electron residing in the 5s (now 14a1) orbital. The data presented here, however, suggest that this model is oversimplified, for various reasons. First, the C2 2 moiety has a triple carbon–carbon bond, where as YC2 does not. Secondly, the 14a1 orbital is not primarily 5s, but has a major contribution from the 4dr orbital. Hence, there is significant covalent character to the Y–C2 bond. This bonding is created when the 2pp a1 orbital of the C2 moiety, which is in the plane of the molecule, interacts with the Y 5s/4dr a1 hybrid orbital, which has electron density pointed perpendicular to the C–C axis. The two electrons, which in the ionic scheme would reside on the C2 moiety, are actually shared with the Y atom, reducing the C–C bond order from 3 (triple bond) to 2.5. The remaining unpaired electron resides in the corresponding ‘anti-bonding’ orbital with sd hybridization, which is localized near the Y nucleus. 6. Conclusion This work is the first FTMW study of a metal dicarbide, in this case YC2. Through measurements of the 13C isotopologues, which had not been previously studied, a more accurate structure has been determined for this interesting species. In addition, 13C hyperfine splittings in Y13CC were observed for the first time, providing further insight into the bonding in the molecule. The C–C bond in YC2 appears to be intermediate between a double and triple bond, with the C–M–C bond angle related to the size of the heteroatom. The unpaired electron resides in an sd hybridized orbital, with more covalent character than predicted theoretically. In addition, the sharing of electron density between the metal atom and the C–C moiety to some extent destabilizes the molecule relative to the C2 species, activating the C–C bond. This effect could explain the tendency for MC2 units to form clusters. In this case, the metal atom would take the role of the activating group. Additional spectroscopic studies of metal dicarbide species would be very useful in further elucidating their chemical and physical properties. Acknowledgment This research is supported by NSF Grant CHE-1057924. References [1] B.C. Guo, K.P. Kerns, A.W. Castleman Jr., Science 255 (1992) 1411. [2] S. Wei, B.C. Guo, J. Purnell, S. Buzza, A.W. Castleman Jr., J. Phys. Chem. 96 (1992) 4166.

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