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Tetrel Bonds in Infinite Molecular Chains by Electronic Structure Theory and Their Role for Crystal Stabilization Janine George† and Richard Dronskowski*,†,‡ †

Institute of Inorganic Chemistry, Chair of Solid-State and Quantum Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany ‡ Jülich-Aachen Research Alliance (JARA-HPC), RWTH Aachen University, 52056 Aachen, Germany S Supporting Information *

ABSTRACT: Intermolecular bonds play a crucial role in the rational design of crystal structures, dubbed crystal engineering. The relatively new term tetrel bonds (TBs) describes a long-known type of such interactions presently in the focus of quantum chemical cluster calculations. Here, we energetically explore the strengths and cooperativity of these interactions in infinite chains, a possible arrangement of such tetrel bonds in extended crystals, by periodic density functional theory. In the chains, the TBs are amplified due to cooperativity by up to 60%. Moreover, we computationally take apart crystals stabilized by infinite tetrelbonded chains and assess the importance of the TBs for the crystal stabilization. Tetrel bonds can amount to 70% of the overall interaction energy within some crystals, and they can also be energetically decisive for the taken crystal structure; their individual strengths also compete with the collective packing within the crystal structures.

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INTRODUCTION Intermolecular interactions beyond hydrogen bonds such as tetrel bonds play a crucial role in understanding and, hopefully more frequently in the future, designing crystal structures.1−4 A tetrel bond is a weak bonding interaction between a covalently bonded tetrel atom Tr with a filled noble gas shell on the one side and an acceptor atom X on the other, as depicted in Scheme 1.5

cluster approaches. Moreover, there have been various investigations dealing with the importance of directed intermolecular bonds in comparison to nonlocalized bonding in crystal structures.19−23 Recently, we have already bridged the gap between such gas-phase and solid-state calculations targeted at the cooperativity of pnictogen, chalcogen, and halogen bonds of interconnected X(CN)y (X = pnictogen, chalcogen, or halogen, y = 1−3) molecules, namely by using an infinite chain model.24 At about the same time, the chemistry of X(CN)3 species (X = pnictogen) and their anions defined the focus of insightful experimental investigations.25,26 Such an infinite chain model has also been used by others to understand the cooperativity in hydrogen-bond-induced supramolecular polymerization.27 Moreover, we have directly investigated chalcogen, halogen, and hydrogen bonds in terms of their cooperativity in crystals, and we have assessed their importance for crystal stabilization by cutting the crystal into monomers, dimers, chains, and layers.28−30 This method further allowed discrimination between different structural models for αchitin.31 Also, this method was successfully used to identify new substrates for molecular beam epitaxy.32 In this work, we now calculate the interaction energies and cooperativities of tetrel bonds using an infinite chain model and assess their importance for crystal stabilization in terms of interaction energies. For reasons of brevity and consistency

Scheme 1. Cartoon of a Tetrel Bond between a Tetrel Cyanide (Tr = C, Si, Ge, Sn) Molecule and an Acceptor Atom X

Tetrel bonds have been known for a long time, both experimentally6,7 as well as theoretically.8,9 In recent years, there have been various investigations on molecules with σholes capable of building such tetrel bonds,10,11 their energetics,12,13 organic chemistry,2,14 their interplay with each other (e.g., cooperativity),15,16 and their role in other σ-bond interactions.17,18 There are also studies dealing with their importance in understanding crystal structures.1,2,4 So far, such calculations have been carried out for the gas phase using © 2017 American Chemical Society

Received: December 19, 2016 Revised: January 20, 2017 Published: January 20, 2017 1381

DOI: 10.1021/acs.jpca.6b12732 J. Phys. Chem. A 2017, 121, 1381−1387

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The Journal of Physical Chemistry A

bond or molecule. To perform the evaluation of ΔEint, we did not optimize the structure of the isolated monomer.

with our previous study on other σ-hole interactions,24 we concentrate on tetrel bonds between a tetrel atom (Tr) and nitrogen (N). The chosen systems for the infinite chain model are depicted in Scheme 2, and our choice is due to the fact that

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RESULTS AND DISCUSSION Infinite Molecular Chains. To start with, let us assess the structurally optimized infinite one-dimensional chains. As a first approximation to calculate the cooperativity, we use one molecule in the repetition unit (see again Scheme 2). Regrettably, the chains of Si(CN)4 and H3SiCN could not be included in these cooperativity investigations of Tr···N type because they could not be optimized in such a way that the intermolecular Si···N bond was shorter than the Si−C bond. As mentioned before, the cooperativity of tetrel bonds in f inite chains of H3SiCN molecules has already been investigated.16 Furthermore, their cooperativity was compared to the cooperativity and energetic stabilization within chains of H3SiNC molecules in which the CN species is flipped. In agreement with our results, the cooperativity and overall stabilization of H3SiNC chains is indeed larger than those of H3SiCN. Coming back to all of the other systems, their interaction energies of the dimers, trimers, and chains are shown in Figure 1a.

Scheme 2. Infinite Molecular Chain Interconnected by Tetrel Bonds

several of the molecules also form corresponding crystal structures possibly stabilized by tetrel bonds (C(CN)4,33 (CH3)2Tr(CN)2, with Tr = Si, Ge, Sn34). Two molecules and their tetrel-bonded chains have already been investigated for cooperativity in a cluster approach (H3CCN,15 H3SiCN16); therefore, we also include higher and lower homologues of such molecules. Finally, the importance of the tetrel bonds for the aforementioned crystal structures will be discussed afterward.

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THEORY Electronic structure calculations based on density functional theory (DFT) were performed with the Vienna ab initio simulation package (VASP 5.4.1).35−38 The meta GGA functional M06L39 and the projector-augmented wave40,41 method were used. The M06L functional has already been successfully used for quantifying interaction energies of tetrel bonds.42 Unless mentioned otherwise, we refer to this level of theory. For test calculations, we also used the PBE functional43 corrected by the D3 dispersion correction.44 The kinetic energy cutoff of the plane-wave expansion was 500 eV. As done before,24 the chain models were structurally optimized under the constraint of constant volume; the lattice parameters and spatial coordinates were allowed to relax. At least 20 Å of vacuum was inserted in the directions without translational symmetry. The convergence criterion for all structural optimizations, on which the energetic evaluations are based, was 5 × 10−3 eV/Å, and the convergence of the electronic structure was at least 1 × 10−5eV. All phonon calculations were performed with PHONOPY45,46 and the finite-displacement method;47 the finite displacement was 0.03 Å for the infinite chains and 0.01 Å for the crystals. The convergence criterion for the previous structure optimization was at least 1 × 10−5 eV, and the convergence of the electronic structure was at least 1 × 10−7eV. All projected Crystal Orbital Hamilton Population (COHP) calculations and their energy integration (ICOHP) for the infinite chains were performed with Lobster 2.0.48−51 We used a 3 × 1 × 1 supercell (multiplication along the chain) for the initial VASP calculations. The interaction energies ΔEint were calculated using the following formula, as also done in previous studies:24,28−31

Figure 1. (a) Interaction energies of tetrel-bonded dimers, trimers, and chains. The interaction energy is normalized per tetrel bond. (b) Shortening of the Tr···N bond lengths (in percent) upon going from the dimer to the chain.

Interaction energy and cooperativity clearly grow from C to Sn for all chain types. This tendency has already been found for pnictogen, chalcogen, and halogen bonds of X(CN)y molecules and for other tetrel bonds in neutral complexes.1,12 As expected, the tetrel bonds built by Tr(CN)4 are the strongest due to the largest number of strongly electronegative end groups. Also, the Tr···N bond lengths shrink more strongly for the systems including Sn than for the ones including other tetrels (see Figure 1b). Note that the dimer structures were only optimized in terms of bond length but not in terms of energetic evaluation. Astonishingly, the Sn···N interaction differs heavily from all other tetrel bonds with respect to interaction energy and intermolecular bond shrinking. This tendency was also reproduced for the Tr(CN)4 molecular chains at the PBE +D3/PAW level of theory to make sure that this anomaly is not only an effect of one specific functional. More numerical details may be found in the SI. We will come back to this special feature below. To assess the quality of the here-employed infinite chain model, the phonon densities of states (PDOS) for the investigated Tr(CN)4 chains were calculated. Because all

ΔE int = Ecrystal/chain/dimer − n × Emonomer

Ecrystal/chain/dimer is the energy of the crystal, chain, or dimer, Emonomer is the energy of the monomer cut from this crystal, chain, or dimer, and n is the number of monomers in the crystal, chain, or dimer. Afterward, ΔEint was normalized per 1382


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Figure 2. PDOS for Sn(CN)4. The one imaginary mode indicating dynamical instability is visualized in a 2 × 1 × 1 supercell and a Newman-like projection on the right. In the projection, the molecular chain of Sn(CN)4 molecules is sketched along the Sn···N bond.

PDOSs look similar, we only show the PDOS of the Sn(CN)4 chain in Figure 2 and provide all others in the SI. There is only one imaginary mode indicating dynamical instability, and this mode is schematically illustrated in Figure 2 also: neighboring molecules rotate in opposite directions around the axis defined by the tetrel bond. This imaginary mode is not surprising because the end atoms of the different molecules building the chain tend to avoid each other, the source of the famous “eclipsed” and “staggered” conformations known from alkane stereochemistry. Thus, the chains with only one molecule in the repetition unit are well optimized. The strengthened interaction energy for Sn and the short Sn···N bond leads us to assume that the type of interaction may change from C to Sn. Therefore, we will quantify the covalent bonds in the chains of C(CN)4, Ge(CN)4, and Sn(CN)4 by ICOHP values as regards the Tr···N and Tr−C bonds; because the ICOHP designates the individual bond’s contribution to the sum of the Kohn−Sham eigenvalues, it may serve as a simple DFT measure for bond covalency. The calculated values are shown in Figure 3a. Indeed, the ICOHP of the supposedly weak Sn···N tetrel bond reaches about 3/4 of the regular Sn−C single bond, indicative of a significant interaction. Likewise, the ICOHP value of Sn···N is in the range of very strong hydrogen bonds.52 The same tendency can be illustrated by looking at the interaction energies per bond between the parts of the chains (Figure 3b). To do so, the different monomers are computationally “cut apart” from the chain, as shown in Scheme 3, without changing their geometry otherwise, impossible in practice but easily possible in theory. Indeed, the course of ICOHP values and interaction energies are different in magnitude but quite similar in tendency; the covalent Tr−C single bond and the Tr···N tetrel bond are in the same energetic range for Tr = Sn but not for Tr = C. As expected, the weakest interaction, C···N, yields an ICOHP value of zero because only weak dispersion but no covalency exists between C and N. We finally note that the numerical values of ICOHP and interaction energies differ in size, for simple reasons. ICOHPs represent an arbitrary partitioning of the band structure energy of a stable system and indicate what one particular bond contributes to the latter energy.48 The interactions energies, however, are differences between total energies of specially prepared structural cuts that are impossible

Figure 3. (a) ICOHP values for the relevant bonds of the C(CN)4, Ge(CN)4, and Sn(CN)4 chains and (b) interaction energies in the same chains assessed by cutting different monomers from the chains.

to realize in nature. Hence, both represent two variants of a similar “gedankenexperiment”. Summing up, both cooperativity and covalency of the Tr···N bonds grow from C to Sn while going down the main group. This is also reflected in the bond becoming shorter from the dimer to the chain for these compounds. We proceed with investigating the tetrel-bonded chains directly in the crystal structures. Crystal Structure of C(CN)4. The crystal structure of C(CN)4 exhibits two different types of tetrel-bonded chains,33 as shown in Figure 4, left. Britton,33 who determined the C(CN)4 crystal structure in 1974 already, compares the latter with the alternative SiF4 structure type53 (see Figure 4, right). He further reasons that the tighter packing of the chains in the 1383


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The Journal of Physical Chemistry A Scheme 3. Monomers Cut from the Tetrel-Bonded Chain to Evaluate the Interaction Energies of the Intermolecular Tr··· N (monomer 1, blue) and the Intramolecular Tr−C Bond (monomer 2, green)

Figure 5. Interaction energies per molecule for two possible structures of C(CN)4.

basis of the bond lengths alone, the chains in the [C(CN)4] type are only slightly less strong than those in [SiF4]. That is to say that the tetrel bonds of C(CN)4 in the [SiF4] type are almost optimized when comparing them with an ideal chain, the energy difference being just 2 kJ/mol. The energetic gain of C(CN)4 adopting its own [C(CN)4] type, however, can be mainly attributed to interactions beyond direct interactions; therefore, the favorable chain packing is the main reason for stabilization. Naturally, the tighter packing is also reflected in a smaller volume of the optimized experimental C(CN)4 structure (120 instead of 142 Å3/molecule). For molecular solids, packing energies can be multiple times larger than the ones resulting from direct contacts and turn out as very important for the chosen crystal structure.19,20 Hence, cutting the crystal allows one to energetically reason why the experimental and not the alternative structure of SiF4 prevails simply by quantifying the packing in terms of an interaction energy. Also, the importance of the tetrel bonds may be quantified for specific crystal structures. Although the tetrel bonds are not decisive for the crystal structure taken here, their interaction energy amounts to 55% of the overall stabilization energy of the C(CN)4 crystal, and thus, they are nonetheless important for the energetic stabilization of the crystal. We go on to another comparison of two possible polymorphs for (CH3)2Tr(CN)2 structures and assess the importance of the tetrel-bonded chains for these structures.

experimental C(CN)4 structure might be responsible for the fact that the SiF4 structure is not taken. More than 4 decades later, we can easily compare both structures energetically and calculate their PDOS. As expected, the experimentally reported structure is energetically more stable by −10 kJ/mol per molecule than the alternative SiF4 structure; therefore, the structural preference is clearly based on energetic reasons. In addition, the PDOS for C(CN)4 adopting the [SiF4] type shows imaginary modes (Figure 4, right), while the optimized experimental structure has none (Figure 4, left), indicative of dynamical stability. Here is another, more chemical way to consider the structural alternatives. In both structure types, [C(CN)4] and [SiF4], there are four tetrel bonds (8 × 1/2 bond) per C(CN)4 molecule. One may easily cut the crystal structures into these chains and compare their interaction energies without further chain optimization (see Figure 5). In both crystals, there are interactions beyond the tetrel-bonded chains (marked by “beyond chains”, ΔEint = ΔEint,crystal − 4 × ΔEint,chain). The interaction energy in the experimental [C(CN)4] type due to interactions beyond the tetrel bond, however, is clearly larger than that in the [SiF4]. As already reasoned by Britton on the

Figure 4. (Left) (a) Crystal structure of C(CN)4 built from two different chain types.33 For sketch simplicity, the nitrogen atoms are connected by a tetrahedron to make chain comparison easier. (b) PDOS of C(CN)4 without imaginary modes, indicating dynamical stability. (Right) Same as before but assuming C(CN)4 to crystallize in the [SiF4] type. All crystal structures were visualized by VESTA.54 1384


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Figure 6. (a) (CH3)2Ge(CN)2 structure and the tetrel-bonded chains within this structure. (b) As before but for (CH3)2Sn(CN)2. All crystal structures were visualized by VESTA.54

(CH3)2Ge(CN)2 vs (CH3)2Sn(CN)2 Structure. (CH3)2Ge(CN)2 and (CH3)2Sn(CN)2 crystallize in two different crystal structures, as shown in Figure 6.34 Konnert et al.34 suggest that the stronger Tr···N interactions of the Sn compound may be responsible for the fact that (CH3)2Sn(CN)2 does not adopt the structure of (CH3)2Ge(CN)2. The two crystal structures are both formed by chains of (CH3)2Tr(CN)2 molecules but differ in the number of chain types. [(CH3)2Sn(CN)2] is built up by chains including only one (CH3)2Sn(CN)2 molecule in its repetition unit. These chains are similar to those calculated in the first part of the article. [(CH3)2Ge(CN)2] shows the same chain type plus another one with two molecules in the repetition unit, as shown in Figure 6, left. Here, we aim to support Konnert’s reasoning by total energies. We take both structures, replace Ge with Sn and vice versa, and optimize all four structures. To do so, one needs to essentially guess the positions of the hydrogen atoms, but this should not play a crucial role in this investigation, in particular, because structural optimization, as witnessed previously, leads to a good fit of the hydrogen positions from DFT with those from experiment whenever available.55 The real lattice parameters and the theoretical ones are in good agreement given that we do not include the zero-point energy and temperature effects (see Table 1); only the b parameter of

Figure 7. PDOS for the optimized experimental structures of (CH3)2Tr(CN)2 (Tr = Ge, Sn). The C−N stretching vibrations are at around 2300 cm−1. This range is comparable to the experimental range (2100−2200 cm−1) found by Konnert et al.34

Ccc2). We corroborate Konnert’s finding that the M−CH3 distances are shorter than the M−CN distances in all experimentally determined phases, (CH3)2Ge(CN)2 and (CH 3 ) 2 Sn(CN) 2 . Hence, the results of the structure optimization support the experimental structure determination and the assignment of N and C. Also, the suggested H positions were supported by the dynamic stability of the derived structures. After cutting these structures into the crystal-generating chains as shown in Figure 6 and evaluating the interaction energies, there are significant differences, which we depict in Figure 8. In both structures, each molecule takes part in two tetrel bonds (4 × 1/2 bond). Both experimental structures are energetically more stable than their fictive counterparts because whenever (CH3)2Tr(CN)2 adopts its native structure the total interaction energies are larger. In both cases, the stabilization correlates with significantly more stable tetrel-bonded chains. The interaction energies beyond the tetrel bonds, however, are almost unaffected by the taken structure or, alternatively, the nature of the tetrel. Nonetheless, the “beyond energies” are slightly larger for the [(CH3)2Sn(CN)2] type. Upon going from Ge to Sn, the energetic share of the tetrel bond in comparison to the overall interaction energy grows from around 50 to more than 70%. Note that this is in the range of the stabilization caused by chalcogen bonds in chalcogen cyanides (41−79%).30 And that is to say that the tetrel bonds appear to be decisive for

Table 1. Comparison of the Experimental34 and Computed Lattice Parameters at the M06L Level of Theory parameter (Å)

experimental (298 K)

(CH3)2Ge(CN)2 a 13.64 b 7.49 c 6.35 (CH3)2Sn(CN)2 a 9.00 b 9.74 c 7.95

DFT (0 K, without zero-point energy)

(2) (1) (1)

12.69 7.04 6.23

(2) (2) (1)

8.98 8.51 7.93

(CH3)2Sn(CN)2 is drastically underestimated by theory. Strangely, the experimental b value is also drastically larger than the experimental value of the isostructural (CH3)2Pb(CN)2 compound despite the latter including the larger Pb atom. The PDOS for the optimized experimental structures of (CH3)2Ge(CN)2 and (CH3)2Sn(CN)2 and also those of the virtual counterparts do not show any significant imaginary modes. The PDOS of the optimized experimental structures are shown in Figure 7. Including the H atoms leads to a slightly different space group for (CH3)2Sn(CN)2 than that given in the original publication34 (group−subgroup relation: Fmm2 → 1385


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Richard Dronskowski: 0000-0002-1925-9624 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge computational resources provided by JARA-HPC at RWTH Aachen University (Project jara0069). Moreover, J.G. thanks the Fonds der Chemischen Industrie for a Ph.D. scholarship.

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(1) Bauzá, A.; Mooibroek, T. J.; Frontera, A. Tetrel-Bonding Interaction: Rediscovered Supramolecular Force? Angew. Chem., Int. Ed. 2013, 52, 12317−12321. (2) Thomas, S. P.; Pavan, M. S.; Guru Row, T. N. Experimental evidence for ’carbon bonding’ in the solid state from charge density analysis. Chem. Commun. 2014, 50, 49−51. (3) Bishop, R. Organic crystal engineering beyond the Pauling hydrogen bond. CrystEngComm 2015, 17, 7448−7460. (4) Bauzá, A.; Mooibroek, T. J.; Frontera, A. Tetrel Bonding Interactions. Chem. Rec. 2016, 16, 473−487. (5) Murray, J. S.; Lane, P.; Politzer, P. Expansion of the σ-hole concept. J. Mol. Model. 2009, 15, 723−729. (6) Urban, R.-D.; Rouille, G.; Takami, M. Free-jet IR spectroscopy of SiF4−N2 and SiF4−CO complexes in the 10 μm region. J. Mol. Struct. 1997, 413-414, 511−519. (7) Ruoff, R.; Emilsson, T.; Jaman, A.; Germann, T.; Gutowsky, H. Rotational spectra, dipole moment, and structure of the SiF4−NH3 dimer. J. Chem. Phys. 1992, 96, 3441−3446. (8) Rossi, A.; Jasinski, J. Theoretical studies of neutral silaneammonia adducts. Chem. Phys. Lett. 1990, 169, 399−404. (9) Alkorta, I.; Rozas, I.; Elguero, J. Molecular Complexes between Silicon Derivatives and Electron-Rich Groups. J. Phys. Chem. A 2001, 105, 743−749. (10) Wittmaack, B. K.; Crigger, C.; Guarino, M.; Donald, K. J. Charge Saturation and Neutral Substitutions in Halomethanes and Their Group 14 Analogues. J. Phys. Chem. A 2011, 115, 8743−8753. (11) McDowell, S. A. C.; Joseph, J. A. Variation of sigma-hole magnitude with M valence electron population in MXnY4‑n molecules (n = 1−4; M = C, Si, Ge; X, Y = F, Cl, Br). Phys. Chem. Chem. Phys. 2014, 16, 669−671. (12) McDowell, S. A. C.; Joseph, J. A. The effect of atomic ions on model σ-hole bonded complexes of AH3Y (A = C, Si, Ge; Y = F, Cl, Br). Phys. Chem. Chem. Phys. 2014, 16, 10854−10860. (13) Del Bene, J. E.; Alkorta, I.; Elguero, J. Anionic complexes of F− and Cl− with substituted methanes: Hydrogen, halogen, and tetrel bonds. Chem. Phys. Lett. 2016, 655−656, 115−119. (14) Grabowski, S. J. Tetrel bond−σ-hole bond as a preliminary stage of the SN2 reaction. Phys. Chem. Chem. Phys. 2014, 16, 1824−1834. (15) Esrafili, M. D.; Mohammadirad, N.; Solimannejad, M. Tetrel bond cooperativity in open-chain (CH3CN)n and (CH3NC)n clusters (n = 2−7): An ab initio study. Chem. Phys. Lett. 2015, 628, 16−20. (16) Marín-Luna, M.; Alkorta, I.; Elguero, J. Cooperativity in Tetrel Bonds. J. Phys. Chem. A 2016, 120, 648−56. (17) Guo, X.; Liu, Y.-W.; Li, Q.-Z.; Li, W.-Z.; Cheng, J.-B. Competition and cooperativity between tetrel bond and chalcogen bond in complexes involving F2CX (X = Se and Te). Chem. Phys. Lett. 2015, 620, 7−12. (18) Scheiner, S. Comparison of CH···O, SH···O, Chalcogen, and Tetrel Bonds Formed by Neutral and Cationic Sulfur-Containing Compounds. J. Phys. Chem. A 2015, 119, 9189−9199. (19) Dunitz, J. D.; Gavezzotti, A. Molecular Recognition in Organic Crystals: Directed Intermolecular Bonds or Nonlocalized Bonding? Angew. Chem., Int. Ed. 2005, 44, 1766−1787. (20) Tiekink, E. R. T. Molecular crystals by design? Chem. Commun. 2014, 50, 11079−11082.

Figure 8. Interaction energy per molecule for (CH3)2Ge(CN)2 and (CH3)2Sn(CN)2 in the experimental structures of both (CH3)2Ge(CN)2 and (CH3)2Sn(CN)2. The stabilization of the tetrel-bonded chains correlates with the overall stability of the structures; therefore, the tetrel-bonded chains appear to be responsible for the different structures taken.

the change of structures from [(CH 3 ) 2 Ge(CN) 2 ] to [(CH3)2Sn(CN)2].

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CONCLUSIONS Similar to pnictogen, chalcogen, and halogen bonds, tetrel bonds of the type X···N show cooperativity in infinite chains in that the tetrel bonds can be amplified by about 60% in such chains. The tetrel bonds of Sn are significantly shorter and stronger than those built by Ge, Si, or C, and one finds a strong covalent contribution as mirrored by ICOHP values. Moreover, such tetrel bonds play a significant role in the crystal stabilization because the energetic stabilization caused by the tetrel bonds can amount up to 70% of the entire stabilization energy of the crystal, although this may be an exceptional example. To the best of our knowledge, this share of the crystal stabilization caused by the tetrel bonds has not been evaluated before, and the type of the present study could provide more insight about the importance of localized interactions such as tetrel bonds. Bond lengths alone cannot provide such information. Also, we have shown that a tetrel-bonded chain and its stabilization can be decisive for the structure taken by the molecules. In addition, the experimental structures of (CH3)2Ge(CN)2 and (CH3)2Sn(CN)2 have been corroborated as being dynamically stable, and the heretofore unknown H positions have been supplied. The experimental structure for C(CN)4 appears to be dynamically stable as well.

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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.6b12732. Interaction energies in kJ/mol and Tr···N in Å for the chain models and additional PDOS graphics and optimized structures in the POSCAR format (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49 (0)241 80-92642. Phone: +49 (0)241 80-93642. ORCID

Janine George: 0000-0001-8907-0336 1386


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