Linear GeC3Ge - ACS Publications - American Chemical Society

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Gas-Phase Spectroscopic Detection and Structural Elucidation of Carbon-Rich Group 14 Binary Clusters: Linear GeC3Ge Sven Thorwirth,*,† Volker Lutter,†,‡ Alireza Javadi Javed,† Jürgen Gauss,§ and Thomas F. Giesen‡ †

I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany Institut für Physik, Universität Kassel, Heinrich-Plett-Straße 40, 34132 Kassel, Germany § Institut für Physikalische Chemie, Universität Mainz, Duesbergweg 10-14, 55128 Mainz, Germany ‡

S Supporting Information *

ABSTRACT: Guided by high-level quantum-chemical calculations at the CCSD(T) level of theory, the first polyatomic germanium−carbon cluster, linear Ge2C3, has been observed at high spectral resolution in the gas phase through its remarkably complex fundamental antisymmetric C−C stretching mode ν3 located at 1932 cm−1. The observation of a total of six isotopic species permits the derivation of a highly accurate value for the equilibrium Ge−C bond length. The present study suggests that many more Ge−C species might be detectable in the future using a combination of laser-ablation techniques for production and high-resolution infrared and/or microwave techniques for spectroscopic detection.

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some more spectroscopic detail in the gas phase via its (2)3Π− X3Π electronic spectrum.25 Here, we present the first high-resolution spectroscopic study of a polyatomic GenCm cluster (linear Ge2C3, GeCC CGe) using infrared free-jet spectroscopy performed at a wavelength of 5.2 μm.

INTRODUCTION Carbon-rich materials, such as the group 14 binary C−Si and C−Ge systems, are of great importance or potential interest in diverse areas of research such as material science and (nano)electronics (e.g., refs 1−5 and references therein), spectroscopy,6 astrochemistry,7 etc. From the viewpoint of structural chemistry it is highly interesting to study structural changes in polyatomic clusters upon increasing the number of heteroatoms and/or the metallicity in these systems. Tricarbon, C3, for example, is a linear molecule,8 whereas its silicon derivatives SiC2, Si2C, and Si3 are all found nonlinear, either Tshaped (SiC2)9 or strongly bent.10,11 No corresponding germanium-bearing clusters have been studied experimentally so far, but high-level quantum-chemical calculations suggest an L- rather than a T-shaped geometry as the global minimium of GeC2,12,13 highlighting the structural diversity to be expected even for these very simple binary clusters of group 14 elements. Likewise, larger carbon-rich clusters also share structural similarities such as linear cumulenic carbon backbones terminated by heteroatoms on either one or both ends. For example, the pentaatomic binary group 14 clusters C5, Si2C3, and Ge2C3 are all calculated to be linear in their most stable geometrical arrangements.6,13−15 In the gas phase and at high spectral resolution, pure carbon clusters have been studied up to 13 carbon atoms,16 and a sizable number of silicon−carbon molecules have been studied using both infrared and microwave techniques,10,17−20 but there is only very little known for germanium−carbon clusters. Though several have been studied spectroscopically in solid argon, Ge2C3,15,21 Ge2C5, GeC3, GeC7, and GeC9 (refs 22−24), up to now it is only diatomic GeC that has been studied to © XXXX American Chemical Society

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EXPERIMENTAL METHODS Ge2C3 has been characterized in the Cologne laboratory using high-resolution infrared spectroscopy, employing a spectrometer described in detail elsewhere.26,27 Briefly, in the present study the spectrometer comprised a laser-ablation source (Nd:YAG laser frequency tripled to operate at a wavelength of 355 nm, a repetition rate of 20 Hz, and a pulse energy of 5− 8 mJ) for production of carbon-rich clusters, a tunable quantum cascade laser (QCL, Daylight Solutions) covering the wavenumber range from 1875 to 1945 cm−1 as the radiation source and a Herriott-type multireflection cell aligned such to allow for 48 passes of the QCL beam. Upon irradiation of appropriate target material (sample rods made from 3:2 stoichiometric mixture of graphite and germanium powder, Sigma-Aldrich) with high-energy laser pulses, the ablation products are carried through a reaction channel (8 mm) of a slit nozzle (cross section 1 mm × 12 mm; mounted on a Series 9 pulse valve) using He buffer gas from a high-pressure reservoir (20 bar). At the nozzle exit, the cluster-seeded He-pulse expands adiabatically into a vacuum chamber kept at a background pressure around 10−2 mbar. Using this method, in the free-jet expansion Received: November 20, 2015 Revised: December 14, 2015

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DOI: 10.1021/acs.jpca.5b11374 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

prominent feature observed in solid argon at 1920.7 cm−1.15 Consequently, it is the major target for initial gas-phase studies in the infrared and should be located in the range from 1926 to 1941 cm−1, provided a typical red shift of 5−20 cm−1 as observed for similar vibrational stretching modes of other carbon chain molecules in argon vs the gas phase also holds here (see refs 49 and 50 and references therein). To further guide the high-resolution spectroscopic search for ν3 in the gas phase, corresponding high-level quantum-chemical calculations were performed at the coupled-cluster level of theory.51 From a harmonic force field calculated at the fcCCSD(T)/cc-pVQZ level and fc-CCSD(T)/cc-pVTZ anharmonic corrections obtained using second-order vibrational perturbation theory (VPT2),39,40 the ν3 mode was predicted at 1942 cm−1 (Table 1), in good agreement with the range estimated from the matrix investigation above. Ge2C3 is a heavy linear molecule and hence its rotational constant is small. Using the CCSD(T)/cc-pwCVQZ equilibrium structure and corresponding zero-point vibrational corrections from the fcCCSD(T)/cc-pVTZ force fields, the calculated rotational constant B0 only amounts to 356 MHz (Table 2). As a consequence, the vibration−rotation spectrum of Ge2C3 is predicted to be very dense. However, another degree of spectral complexity is introduced by taking into account the three isotopes of germanium with a natural abundance greater than 20% (74Ge, 36.7%; 72Ge, 27.3%; 70Ge, 20.4%), resulting in no less than six abundant isotopologs (Table 2). This argument is significant in such a way that ν3 is an almost exclusive C−Cstretching mode, so isotopic substitution of germanium will hardly shift the centers of individual isotopic vibrational bands causing all spectra of isotopic species to (at least partly) overlap. This is in accord with the initial matrix study, in which neither isotopic shifts nor broadening of the ν3 band were observed.15 Spin-statistical effects will also shape the overall appearance of the vibrational band: Like in the closely related C5 and Si2C3 species20,52,53 for identical pairs of I = 0 bosons (resulting in the molecular point group D∞h for the 74 Ge12C12C12C74Ge, 72Ge12C12C12C72Ge, 70Ge12C12C12C70Ge species), Bose−Einstein statistics restricts the rotational quantum numbers to even and odd values of J for the ground vibrational and first vibrationally excited state, respectively, and hence two adjacent rotation−vibration transitions will be separated by 4B rather than 2B, the latter holding for isotopic species of C∞v symmetry in which the inversion symmetry is broken (74Ge12C12C12C72Ge, 74Ge12C12C12C70Ge, 72 Ge12C12C12C70Ge). As a consequence, in a simplified picture of rotationally resolved yet isotopically overlapping bands, one would expect lines separated by 2B showing an alternating intensity pattern due to the different contributions from isotopic species of D∞h and C∞v symmetry. However, owing to nonzero frequency shifts between the isotopic vibrational bands centers and differences in the rotational constants, this picture will not be strictly valid.

rotational temperatures of the clusters typically reach values of 20−40 K. A few mm behind the nozzle exit, the QCL beam intersects the cluster pulse perpendicularly to the direction of the traveling free jet, and the transmitted intensity is recorded as a function of wavenumber using mercury cadmium telluride detectors that are cooled to the temperature of liquid nitrogen. After calibration using a Fabry−Perot etalon and standard calibration gases, the typical accuracy is on the order of a few times 10−4 cm−1. Absorption lines exhibit typical widths of some 100 MHz (fwhm), limited by the jet velocity components parallel to the line of sight (i.e., the direction of the QCL beam).

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QUANTUM-CHEMICAL CALCULATIONS Quantum-chemical calculations at the CCSD(T) level of theory 28 were performed using the CFOUR suite of programs29,30 in combination with Dunning’s hierarchies of correlation-consistent polarized valence31,32 and polarized core−valence basis sets.33−36 Structure calculations considering all electrons in the CCSD(T) treatment were performed with basis sets as large as cc-pwCVQZ. Harmonic, cubic, and semiquartic force fields were calculated using the cc-pVTZ basis in the frozen-core approximation and analytic secondderivatives procedures37 together with numerical differentiation for the third and fourth derivatives.38,39 Anharmonic corrections to vibrational frequencies, zero-point vibrational corrections to rotational constants as well as vibration−rotation interaction constants were then obtained using second-order vibrational perturbation theory (VPT2), for the relevant expressions, see ref 40. To explore scalar-relativistic effetcs, additional calculations were performed using the spin-free exact two-component scheme in its one-electron variant (SFX2c-1e).41−45 These calculations were performed with uncontracted versions of the ANO-RCC basis sets from ref 46.

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THEORETICAL SPECTRUM As a linear pentaatomic molecule, Ge2C3 (Figure 1) features seven fundamental vibrational modes, three of which are

Figure 1. Structural parameters of Ge2C3 (in Å). Calculations were performed at the CCSD(T) level using different basis sets as well as with and without inclusion of scalar-relativistic effects. The semiexperimental (reemp) Ge−C bond length was derived while the C−C distance was kept fixed at the CCSD(T)/cc-pwCVQZ value. For further details, see text.

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RESULTS AND DISCUSSION On the basis of the reasoning given above, the ν3 band of Ge2C3 was searched for in the gas phase in the 1926−1941 cm−1 wavenumber range. With the laser-ablation production technique and appropriate target material comprising elemental carbon (graphite) and germanium (see Experimental Methods section for details of the experiment) followed by supersonic jet expansion of the ablation products seeded in a large excess of

(doubly degenerate) bending and four are stretching modes (Table 1). Owing to D∞h point group symmetry of Ge2C3 (inherent to centrosymmetric isotopologs, see below) four of its fundamentals are infrared active and three of those (ν3, ν4, and ν6) were observed in solid argon previously.15,21 In analogy to the isovalent linear C5 and Si2C3 clusters,47,48 the antisymmetric C−C-stretching mode ν3 is calculated to be the most intense vibrational fundamental15,21 and the most B


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Table 1. Vibrational Wavenumbers and Rotation−Vibration Interaction Constants of 74Ge212C3 (Wavenumbers in cm−1, αi and qi in MHz) calculated a

vib mode

harm.

ν1(σg) ν2(σg) ν3(σu) ν4(σu) ν5(πg) ν6(πu) ν7(πu)

1491 271 1985 730 169 527 62

anharm.a

harm.b

anharm.c

1472 253 1945 743 166 516 55

1498 273 1982 734 165 523 60

1479 255 1942 747 162 512 53

experiment

1932.08997(9)d 735.3e 580.1e

parameter

calculateda

α1 α2 α3 α4 α5/q5 α6/q6 α7/q7

1.196 0.188 1.532 0.787 −1.152/0.051 −0.393/0.044 −0.998/0.135

experiment

1.5930(13)

a fc-CCSD(T)/cc-pVTZ calculations. bfc-CCSD(T)/cc-pVQZ calculations. cCalculated from the fc-CCSD(T)/cc-pVQZ harmonic force field and anharmonic corrections calculated using VPT2 at the fc-CCSD(T)/cc-pVTZ level. dGas-phase value (this work); observed at 1920.7 cm−1 in solid Ar15. eAr matrix value.21

Table 2. Relative Band Intensities, Selected Molecular Parameters (Equilibrium Rotational Constants (Be), Zero-Point Vibrational Corrections (ΔB0), (Scaled) Ground-State Rotational Constants (B0,sc), Rotational Constants of the Excited Vibrational State (B3), Rotation−Vibration Interaction Constants (α3), in MHz), and ν3 Band Centers (in cm−1) of Ge2C3 Isotopologs calculated

experiment

speciesa

rel intb

Bec

ΔB0d

B0e

74−74 74−72 74−70 72−72 72−70 70−70

1.00 0.75 0.56 0.56 0.42 0.32

355.385 360.170 365.194 364.978 370.026 375.097

−0.691 −0.699 −0.708 −0.708 −0.717 −0.726

356.076 360.870 365.903 365.686 370.742 375.823

B0,scf

B0

B3

α3

ν3

360.6 365.6 365.4 370.5 375.5

355.810(26) 360.584(20) 365.496(29) 365.496(31) 370.479(141) 375.544(250)

354.217(26) 358.972(20) 363.870(28) 363.871(30) 368.831(139) 373.881(247)

1.5930(13) 1.6120(12) 1.6251(18) 1.6249(19) 1.6478(89) 1.6628(151)

1932.089973(91) 1932.106657(76) 1932.123932(87) 1932.12392(10) 1932.14179(46) 1932.16050(82)

a

Mass number of germanium nuclei in given isotopolog. bRelative intensity; from isotopic abundances and statistical weights, see text. cCCSD(T)/ cc-pwCVQZ calculations. dfc-CCSD(T)/cc-pVTZ calculations. eB0 = Be − ΔB0. fB0,sc(iso) = (B0,exp/B0)74GeCCC74Ge × B0(iso)

Figure 2. ν3 vibrational band of Ge2C3 at 1932 cm−1 observed in the gas phase vs a simulation based on the best-fit parameters (Table 2) and a temperature of 30 K.

helium, a vibrational band was finally detected around 1932 cm−1 (Figure 2) whose characteristics are fully compatible with an assignment to the ν3 band of linear Ge2C3: (i) The band is not observed using pure graphite rods but only if target material made of both graphite and germanium is used and (ii) exhibits tight line spacing as expected for a molecule with a very small rotational constant. Most importantly, (iii) in selected regions within the band, the spectral pattern clearly shows alternating line intensities as expected from the spin-statistical considerations discussed earlier.

Spectroscopic assignment was based on the calculated B0 and B3 rotational constants (Tables 1 and 2; see also the Supporting Information for further details on the spectroscopic assignment procedure). Starting with the isotopic species with the strongest individual vibrational band, 74GeCCC74Ge, the spectroscopic constants of a standard linear molecule Hamiltonian were fitted to individual lines in the spectrum. With only three parameters released in the fit (the ground vibrational-state constant B0, the rotational constant belonging to the ν3 mode, B3 (alternatively, the rotation−vibration interaction constant α3), and the vibrational band center ν3), all 67 lines attributed to this C


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Figure 3. Close-up of the ν3 vibrational band of Ge2C3 near 1931 cm−1 showing simulations (at 30 K) of the individual isotopic ν3 bands (red traces), their superposition (green), and the experimental spectrum (black).

species can be fit to within the experimental uncertainty (5 × 10−4 cm−1). On the basis of this initial spectroscopic assignment and empirically scaled molecular parameters, the less abundant isotopic species were assigned one by one in the following. The final fit values in Table 2 are in very good agreement with those calculated initially. Empirically scaled values of the ground-state rotational constants B0,sc are in even better agreement with experiment. As expected, vibrational shifts between the individual isotopic band centers are very small as all are found within an interval of less than 0.1 cm−1. On the basis of the experimental molecular parameters in Table 2, the ν3 spectrum can now be simulated to near perfect agreement for both the overall band shape (Figure 2) and the finer details. Figure 3 displays a close-up of the interference of individual isotopic rotation-vibration lines and their constructive superposition culminating in the P(48) transition at 1930.8 cm−1, the dominant feature within the entire band. Owing to D∞h symmetry and the related simple molecular structure of Ge2C3 featuring only two unique bond distances, rGe−C and rC−C, the six isotopic species investigated here in principle permit the derivation of accurate structural information: Because the moment of inertia I in a molecule equals Σimiri2, in Ge2C3 it almost exclusively depends on the germanium atoms (due to their much greater mass compared to carbon and their outermost position relative to the center of mass). As a consequence and due to the fact that no 13C isotopic species were observed here, it turns out that no information on the rC−C distance can be derived from the present data set. However, the best-estimate rC−C bond length may be kept fixed in a structural refinement procedure to obtain a highly accurate bond length rGe−C. In this approach, the six experimental ground-state rotational constants of Ge2C3 were corrected for the zero-point vibrational effects calculated at the fc-CCSD(T)/cc-pVTZ level of theory (Table 2) using the relation Be = B0 + ΔB0. rGe−C was then determined in a leastsquares adjustment to the six semiexperimental equilibrium moments of inertia while keeping rC−C fixed at the value determined at the CCSD(T)/cc-pwCVQZ level (1.2893 Å, Figure 1). This procedure results in an equilibrium structural bond length for rGe−C of 1.7695(1) Å. Taking into account an uncertainty of 10−3 Å for rC−C (see, e.g., ref 54) this translates into a more conservative empirical value of rGe−C of 1.770(1) Å. This bond length is somewhat shorter than the equilibrium value obtained for the GeC diatomic radical (1.805 Å).25 The same trend is observed in the isovalent SiC and Si2C3 species, for which bond lengths of 1.7182(2)55 and 1.6859 Å56 have

been derived, respectively. The semiexperimental Ge−C bond distance is within 0.002 Å of the value calculated at the CCSD(T)/cc-pwCVQZ level of theory (Figure 1) and hence close to the agreement found in molecules containing secondrow elements.57 When dealing with third-row elements, however, an interesting issue is the significance of relativistic effects. Our calculations using the SFX2c-1e scheme (see Figure 1 and the Quantum-Chemical Calculations section for further details) show that scalar-relativistic effects are only on the order of 0.003 Å for the Ge−C distance, whereas for the C−C distance they are more or less negligible.

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CONCLUSIONS Given the relative ease with which carbon-rich clusters can be produced in laser-ablation experiments such as the one used here (e.g., refs 58 and 59), it would be surprising if other metal−carbon and germanium−carbon clusters in particular were not detectable in the infrared and/or using microwave (pure rotational) spectroscopy (the latter method being restricted to polar molecules). Plausible candidates include GeC, GeC2, Ge2C, and GeC3, the silicon−carbon variants of which have all been detected at high spectral resolution in the gas phase (and are also known astronomcial molecules, see refs 10 and 60−63 and references therein). Although the minimum energy structures of Ge2C and GeC3 are predicted to be similar to the ones of Si2C and SiC3 (ref 13) the putative L-shaped minimum of GeC2 seems particularly appealing. Preliminary CCSD(T) calculations performed in the course of the present investigation predict substantial dipole moment components of μa = 3.7 D and μb = 0.6 D, highlighting the intrinsic characteristics of a promising target for future Fourier transform microwave (FTMW) studies. Ultimately, more complex (three-dimensional) carbon-rich clusters harboring germanium or silicon (see, e.g., ref 64 and references therein) may be subjected to high-resolution spectroscopic techniques, to study more comprehensively the evolution of cluster formation from the molecular phase to bulk matter.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b11374. Spectroscopic data for all isotopologs and related fit files (PDF) D


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AUTHOR INFORMATION

Corresponding Author

*S. Thorwirth. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support by the Deutsche Forschungsgemeinschaft (DFG grants TH 1301/3-1, TH 1301/32, GA 370/6-1, and GA 370/6-2) and additional funding through DFG grant SFB 956. We also thank the Regional Computing Center of the Universität zu Köln (RRZK) for providing computing time on the DFG-funded High Performance Computing (HPC) system CHEOPS.

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Linear GeC3Ge - ACS Publications - American Chemical Society

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