Bridging a Knowledge Gap from Siloxanes to Germoxanes and

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Bridging a Knowledge Gap from Siloxanes to Germoxanes and Stannoxanes. A Theoretical Natural Bond Orbital Study Ionut-Tudor Moraru, Petronela M. Petrar, and Gabriela Nemes J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b01208 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 11, 2017

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The Journal of Physical Chemistry

Bridging a Knowledge Gap from Siloxanes to Germoxanes and Stannoxanes. A Theoretical Natural Bond Orbital Study Ionuţ-Tudor Moraru, Petronela M. Petrar, Gabriela Nemeş* Babeș-Bolyai University, Faculty of Chemistry and Chemical Engineering, Arany János street no. 11, RO-400082, Cluj-Napoca, Romania

ABSTRACT: Explaining the nature of the E-O chemical bond (E = Si, Ge, Sn) has been a great challenge for theoretical chemists during the last decades. Among the large number of models used for this purpose, the one based on hyperconjugative interactions shed more light on the nature of chemical bonding in siloxanes. Starting from this concept, this study aimed to evaluate the impact of siloxane type hyperconjugative effects on the structural features of germoxanic and stannoxanic species and in addition to assess if p-d like back-bonding interactions can also play important roles in determining the particular structures of these heavier analogues of ethers. Natural Bond Orbital Deletion (NBO DEL) optimizations, carried out at the DFT level of theory, revealed that hyperconjugative effects dictate to a large extent the structural behavior of these species. Furthermore, this study points that p-d back-bonding interactions also influence the equilibrium geometry of these species, although acting as a secondary electronic effect within the E-O-E moieties (E =Si, Ge, Sn).

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Introduction The study of oxanic derivatives containing E-O-E moieties (E= heavy group 14 element) has been of great interest in the last period, from both a fundamental and an applicative point of view. These compounds are mostly used in the synthesis of new materials with controlled properties1,2, and a full understanding of the chemical bonding within the precursors should allow for a better design of the targeted materials. Siloxanes, germoxanes and stannoxanes have molecular structures exhibiting large E-O-E bond angles and short E-O bond lengths, unlike C-O-C fragments in analogue ethers. Experimental values for Si-O-Si angles, measured by electron diffraction in gaseous phase3-4, range between 144.1˚ in (H3Si)2O and 148.0˚ in the permethylated derivative, (Me3Si)2O. Large Ge-O-Ge bond angles have also been reported in (H3Ge)2O (126.5˚)5 and (Me3Ge)2O (around 141˚)6. The Sn-O-Sn angle in (H3Sn)2O has not been yet determined by experimental methods, but in (Me3Sn)2O, a Sn-O-Sn angle around 140˚ has been measured6. E-O bonds lengths range from 1.631 to 1.634 Å in siloxanes3,4 and from 1.766 to 1.770 Å in germoxanes5,6. The Sn-O distance in hexamethyldistannoxane6 is 1.940 Å. These bonds are noticeably shorter than the sum of the covalent radii7 (1.77 Å for Si-O, 1.86 Å for Ge-O, and 2.05 Å for Sn-O). A great number of literature studies aimed to explain the structural particularities of siloxanic units and most of them employed computational methods. A first theoretical model of the Si-O bonding was developed by Linus Pauling in the third decade of last century, who classifies the chemical bond as predominantly ionic8 based on differences in electronegativity. Several decades after, experiments carried out on low-quartz led to new insights concerning the bonding within siloxanic species9 and a covalent character was proposed for Si-O10,11. The unusual short bond length was rationalized as a consequence of pO→dSi back-bonding12-13.


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Subsequent computational studies questioned the accuracy of the back-bonding model14,15, the contribution of d orbitals being considered merely a polarization function This observation was generally accepted not only for siloxanic derivatives, but also for analogue derivatives of heavier main group elements16-19, though other studies20 emphasized the need of d orbitals in computing accurate geometries and also reasonable energies. However, for anionic species, d orbitals seem to play a more significant role in the chemical bonding21,22. Studies on this topic also suggested that the ionic behavior14,23-25 is more appropriate in explaining the structural features of Si-O-Si moieties. Electron densities26,27 or the enhanced steric effects of silyl groups28,29 were also evaluated. In addition, the strength of Si-O was compared to similar chemical bonds containing second row elements30,31, calculations succeeding to explain the geometrical parameters of some alumosilicates or their affinity towards some cations, by analyzing the Si-Al exchange energies within Si-O-Si linkages. The concept of vicinal hyperconjugative interactions is well documented for sylil groups and siloxanic derivatives32-36, but much less is known about their heavier analogues. The development of computational chemistry enabled a better awareness of these issues37-42, but the most important contribution regarding the clarification of the hyperconjugative phenomena within disiloxanic derivatives belongs to Weinhold and West33,44, whose work led to new perspectives in the field45. This

paper

presents

a

comprehensive

computational

study

investigating

vicinal

hyperconjugative interactions of the type LPO→σ*E-R (E = Si, Ge, Sn; R = H, C) in heavier analogues of ethers. This study also investigated the role of p→d-like interactions in determining the structural features of siloxanic, germoxanic and stannoxanic derivatives. Natural Bond


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Orbital (NBO)46-48 techniques in conjunction with DFT calculations were employed to extensively investigate all these interactions. Computational details All calculations were carried out using the Gaussian 09 package49 and involved constrained and unconstrained geometry optimizations, frequency analysis and NBO46-48 techniques, including NBO Deletion (NBO DEL) optimizations. Calculations were performed within the DFT method using B3LYP50,51 and PBE052 hybrid functionals and the valence triple-ζ basis set and Stuttgart effective core potentials, Def2-TZVPD53-55. Effective core potentials (ECPs) were computed for the relativistic core electrons of the Sn atom. In addition, a double-ζ basis set, Def2-SV(P)53-55, and a larger quadruple-ζ one, Def2-QZVPPD53-55 were used in order to determine whether basis set brings a major influence in the computed data. The integration grid used was of 99 radial shells and 950 angular points for each shell (99,950). The optimization criteria were set to tight and a vibrational analysis was performed to confirm that the optimized structures correspond to global minima. The Gaussian 09 implemented version of the NBO Program56 was used. NBO DEL techniques allowed geometry optimization in the absence of specific donor-acceptor interactions.

Results and discussion Structural features Molecular geometries of model compounds (Scheme 1) of the type (R3E)2O (E = Si, Ge, Sn; R= H, Me) were optimized through DFT calculations, using B3LYP and PBE0 hybrid functionals, and Def2-TZVPD basis set. Recent benchmark studies57 carried out on disiloxanic


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derivatives showed that both B3LYP and PBE0 functionals lead to good results, when used in conjunction with triple-ζ basis sets.

Scheme 1. Model structure of the investigated compounds containing the E-O-E moiety. The E-O-E bond angles and E-O bond lengths, along with reported experimental values are presented in Table 1. Similar geometrical parameters were obtained with both functionals and a good agreement with previously reported experimental data in gaseous phase was observed. However, B3LYP seemed to slightly overestimate the Si-O-Si and Ge-O-Ge angles in the siloxanic and germoxanic species, in comparison with PBE0 and experimental data. A better agreement was observed between B3LYP and experimental values for Sn-O-Sn, while PBE0 tended to underestimate these angles.

Table 1. Selected geometrical parameters for the global minima of (R3E)2O species (E = C, Si, Ge, Sn; R = H, Me).

E C Si Ge Sn

R H

E-O-E (°)

E-O (Å)

B3LYP

PBE0

expt.*

B3LYP

PBE0

expt.*

112.9

112.2

111.858

1.411

1.400

1.41558

59

1.444

1.430

1.42059

CH3

128.1

127.1

130.8

H

152.9

147.7

144.13,4

1.637

1.634

1.6343,4

CH3

159.5

150.5

148.03

1.643

1.641

1.6313

H

129.7

127.8

126.55

1.784

1.774

1.7665

6

1.790

1.780

1.7706

CH3

136.9

133.9

141.0

H

134.5

132.5

–

1.969

1.956

–

CH3

138.9

134.4

140.86

1.976

1.963

1.9406


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* Experimental values were measured by gaseous phase electron diffraction.

While the calculated C-O-C angle in dimethylether was close to 109˚, the computed E-O-E angles in species of the type (H3E)2O (E = Si, Ge, Sn) were much wider (Table 1). The C-O-C angle increased with the bulkiness of the substituents; calculated values reached 128.1˚ (B3LYP) and 127.1˚ (PBE0) in di-tert-butyl ether. In contrast, the widening of the E-O-E angles (E = Si, Ge, Sn) from (H3E)2O to (Me3E)2O derivatives was less important. The calculated values of Si-O, Ge-O and Sn-O were significantly shorter than the sum of the covalent radii of the atoms, with values of 0.14-0.15 Å for the disiloxanic derivatives and 0.08-0.09 Å for the digermoxanic and distannoxanic ones (Table 1). The most stable conformations for the (H3E)2O derivatives were found to be those in which the hydrogen atoms are in eclipsed orientations (see Table S1 in Supporting Information). In (Me3E)2O derivatives, the calculated values of the C-E-E-C dihedrals ranged between 30˚ and 45˚ (Table S1). The molecular structures of (H3O)2E derivatives were also optimized using smaller (Def2-SV(P)) and larger (Def2-QZVPPD) basis sets. The calculated E-O-E angles and E-O bond lengths obtained using these basis sets were in good agreement with those delivered by the triple-ζ one (Tables S2 and S3). Constrained geometry optimizations, with the E-O-E angles fixed at 180˚, were performed to estimate the linearization potentials for the E-O-E units in all investigated compounds (E = C, Si, Ge, Sn), which were calculated as the difference between the energy of the constrained molecular geometry and the global minimum (Table S4). Larger potentials were calculated for the C-O-C units in ethers than for the E-O-E moieties in siloxanic, germoxanic and stannoxanic species. The calculated linearization potential for the


6

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C-O-C moiety was >33 kcal/mol in dimethylether, and >20 kcal/mol in the hexamethylated derivative. The lowest potentials were obtained for the Si-O-Si fragments in all siloxanic derivatives, with values 0.1 kcal/mol identified, several vicinal hyperconjugative interactions of the type LPO→σ*E-H and back-bonding LPO→RyE effects were detected for each of the (H3Si)2O, (H3Ge)2O and (H3Sn)2O species (Scheme 2).


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Scheme 2. Representation of hyperconjugative and back-bonding interactions occurring within the E-O-E (E = Si, Ge, Sn) derivatives.

The total hyperconjugation energy computed as the sum of all LPO→σ*E-R interactions (∑LPO→σ*E-R) for each of the investigated species is presented in Table 2. However, individual interactions contribute to the total energy in different ratios, depending on several factors, such as the spatial orientation or the nature of the atomic orbitals (OAs) involved.

Table 2. Computed hyperconjugation energies occurring at the E-O-E level for (R3E)2O derivatives (E = Si, Ge, Sn; R = H, Me). Si E

B3LYP Σ (LPO→σ*E-R) (kcal/mol)

Ge

Sn

R H

PBE0

B3LYP

PBE0

B3LYP

PBE0

37.4

38.2

27.2

28.4

18.3

19.2

CH3 45.7

46.5

29.8

31.6

20.5

21.5

Among the investigated structures, the total hyperconjugation energy from interactions occurring within the E-O-E moiety (E = Si, Ge, Sn) decreased with the increasing of the atomic number of E and the bulkiness of the substituent on the E atom (Table 2). The higher


8

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hyperconjugation energy within the permethylated species is due to a better orbital overlap between the LPs and σ*E-C when compared with the LPs - σ*E-H overlapping. Moreover, the NBO analysis led to similar results for both B3LYP and PBE0 functionals, with differences of less than 2 kcal/mol in each case. NB orbitals involved in the identified hyperconjugative interactions for one of the two bonds from the E-O-E unit in disiloxanes, digermoxanes and distannoxanes are presented in Figure 1; similar interactions were of course observed along the other bond. NBO calculations showed that the electron donor behavior of the two LPs on the oxygen atom within the hyperconjugative interaction is not identical. While one of the lone pairs (LPO1) (Figure 1 - a1-a3, b1-b3, c1-c3) presents a certain extent of s character, the other lone pair, LPO2 (Figure 1 - a4-a5, b4-b5 and c4-c5) behaves as a pure p atomic orbital (Table S5). The amount of s character in the LPO1 increases from disiloxane (8-11%) to distannoxane (40-41%). Similar trends were observed for the oxygen LPs in the permethylated species even though the s character of LPO1 is less pronounced than for each of the corresponding (H3E)2O (Table S5). The electron acceptor antibonding orbitals σ*E-R (R = H, C) have contributions from E (60-70%) and R (30-40%) AOs. Atomic orbitals from the E atom have in all of the cases around 25% s and 75% p character, while H atoms contribute to σ*E-H with pure s orbitals. For σ*E-C, the s/p ratio in E atomic orbitals was found to be also around 1/3 (Table S6 and Table S7), with no contribution from d AOs. Both σ*E-H and σ*E-C exhibit an elongated shape along E-H and E-C chemical bonds, the main difference between the two lying in the number of nodal planes, two for σ*E-H (Figure 1) and three for σ*E-C (Figure S1).


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a1.

b1.

LPO1→σ*Si1-H1 B3LYP: 5.2 kcal/mol PBE0: 5.1 kcal/mol

a2.

c1.

LPO1→σ*Ge1-H1 B3LYP: 2.4 kcal/mol PBE0: 2.6 kcal/mol

b2.


a3.

c2.

b3.

a4.

LPO1→σ*Ge1-H3 B3LYP:1.0 kcal/mol PBE0: 1.0 kcal/mol


LPO1→σ*Sn1-H3 B3LYP: 0.7 kcal/mol PBE0: 0.8 kcal/mol

c4.


b5.



c3.

b4.

a5.




Page 10 of 26


c5.



Figure 1. NB orbitals involved in the vicinal hyperconjugative interactions of the type LPO1→σ*E1-H and LPO2→σ*E1-H for (H3Si)2O (a1-a5), (H3Ge)2O (b1-b5) and (H3Sn)2O (c1-c5). Computed energies of the second order perturbation theory analysis, according to B3LYP and PBE0 hybrid functionals. The atom numbering used here is the same with the one in Scheme 2.


10

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The energy of the hyperconjugative interactions was strongly influenced by the relative orientations of σ*E-R orbitals and the oxygen’s LPs. Thus, the in plane hyperconjugations (the donor and the acceptor components lay in the same plane) were found to be energetically more favorable than out-of-plane ones (with the LPs and the antibonding orbitals situated in syn or perpendicular orientations). For instance, in plane interactions involving LPO1 in the (H3E)2O species were found to be more than two times greater in energy than the out of plane ones (Figure 1, a1-a3, b1-b3, c1-c3). For LPO2 only two hyperconjugative interactions were detected (Figure 1, a4-a5, b4-b5, c4-c5). Another possible interaction (LPO2→σ*E1-H1) had a calculated energy lower than 0.1 kcal/mol, due to a poor overlap between the LP and the antibonding orbital, situated in perpendicular planes. For two other hyperconjugations involving LPO2, the angle between LP and σ*E1-H2 and σ*E1-H3 was about 30˚. In most cases, the LPO2→σ*E1-H2 and LPO2→σ*E1-H3 interactions were energetically more favorable than the in plane LPO1→σ*E1-H1, fact which can be attributed to the atomic orbital consistency of the two lone pairs. Thus, the more pronounced s character of LPO1 led to a volume contraction of this lone pair by comparison to the pure p LPO2, which contributes to poorer orbital overlap within the adjacent hyperconjugations. This assumption can be better understood by comparing the computed hyperconjugation energies for disiloxane and distannoxane. In disiloxane, LPO1 had a less pronounced s character (around 10%) leading to hyperconjugative interactions having similar energies (or even higher according to B3LYP functional) with the ones involving LPO2. In contrast, in distannoxane, the more pronounced s character of the LPO1 (around 40%) caused a reduced overlap with respect to the pure p orbital and therefore noticeably smaller interaction energy by comparison with the ones involving the pure lone pair (Figure 1). Similar observations


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were made for the permethylated species and Figure S1 presents calculated values for hexamethyldisloxane. Similarly with data reported here, the calculated energies corresponding to the strongest in plane and out of plane interactions in disiloxane (LPO1→ σ*Si-H and LPO2→ σ*Si-H) were 5.4 kcal/mol and 5.6 kcal/mol43. NBO calculations using the Def2-SV(P) split-valence and the De2-QZVPPD quadruple-ζ basis sets were carried out on the (H3O)2E derivatives (Table S8). Similar contributions of AOs in the NBOs involved in these interactions were obtained for each of the basis sets used.

Natural Bond Orbital Deletion optimizations In order to elucidate the effect of LPO→σ*E-R hyperconjugations, we have performed NBO DEL optimizations in the absence of these interactions. The quantification of these interactions’ effects on the structural behavior of the E-O-E units was made by comparing the molecular geometries optimized using DEL calculations with the ones of the global minima. NBO DEL optimizations can be performed only in Z matrix input format, and the maximum number of internal coordinates which can be set as variables in the optimization procedure is limited to 50. For each (H3E)2O derivative, the Z-matrix contains 21 internal coordinates, while for (Me3E)2O species, the increased number of atoms leads to 75 coordinates. In order to meet the limits imposed by NBO DEL techniques, constrained optimizations have been performed for (Me3E)2O species, all geometrical parameters, with the exception of the E-O-E angle and E-O and E-C distances, being constrained. Table 3 presents the calculated values of the E-O-E angles and E-O bond lengths for the (H3E)2O species, using unconstrained NBO DEL optimizations, and calculated differences with respect to the global minima. Optimizations in the absence of LPO→σ*E-H interactions led to a


12

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consistent decrease of the E-O-E angles while the length of the E-O chemical bonds increased (Table 3). Furthermore, these NBO DEL unconstrained optimizations showed that the H-E-E-H dihedrals were strongly influenced by LPO→σ*E-H interactions. The removal of these effects (Table S9) led to a change in the orientation of H atoms from an eclipsed to an anti orientation.

Table 3. Calculated values for the E-O-E angles and E-O bond lengths according to NBO DEL unconstrained optimizations, following the removal of LPO→σ*E-R interactions. Differences between the NBO DEL optimization calculated values and the ones corresponding to equilibrium geometries.

E

R

NBO DEL opt Σ (LPO→σ*E-R) E-O-E (°)

B3LYP ∆ (NBO DEL – Equilibrium Geom.)

E-O (Å) ∆ E-O-E (°) ∆ E-O (Å)

PBE0 NBO DEL opt Σ (LPO→σ*E-R)

∆ (NBO DEL – Equilibrium Geom.)

E-O-E (°) E-O (Å) ∆ E-O-E (°)

∆ E-O (Å)

Si

H

124.2

1.706

-28.7

0.069

120.9

1.702

-26.8

0.068

Ge

H

117.4

1.841

-12.3

0.057

115.6

1.829

-12.2

0.055

Sn

H

121.9

2.015

-12.6

0.046

119.9

1.999

-12.6

0.043

The calculated values for E-O-E angles and E-O bond lengths in (Me3E)2O species, according to the constrained NBO DEL optimizations, are presented in Table S10. In addition, for a better comparison, the (H3E)2O species have also been optimized with the same constraints used for (Me3E)2O derivatives. Comparisons between constrained and unconstrained DEL optimizations for the (H3E)2O derivatives showed that the constrained calculations can be used as a relevant approach in estimating the manner in which the hyperconjugative effects impact the widening of the E-O-E angles and the length of the E-O bonds. By optimizing the molecular structures in the absence of LPO→σ*E-R interactions, the E-O-E angles were lowered by comparison to the equilibrium values while the E-O bonds were


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elongated. However, the calculated E-O-E angle for each derivative was still wider than the C-O-C ones contained in the analogue ethers while the lengths of the E-O bonds remained shorter than the sum of the covalent radii of the constituent atoms, even in the absence of these adjacent hyperconjugative interactions. Therefore, we have further assessed the cumulative effect of the vicinal LPO→σ*E-R interactions and LPO→RyE back-bonding ones. The total energy of the identified back-bonding interactions (ΣLPO→RyE) is presented in Tables S11 and S12 respectively, together with the amount of d character in RyE. However, the sum of the hyperconjugative interactions was higher than that of the back-bonding ones. The interaction energies decreased with the increasing of the atomic number of E and the bulkiness of the substituent on the E atom. The RyE orbitals had mainly a d character, which decreased in the group (from ~ 95% in siloxanes to ~ 60% in stannoxanes, see Tables S11 and S12). Considering that the LPs behave as almost pure p AOs, while RyE contain high amounts of d orbitals, the LPO→RyE interactions can be approximated as classical pO→dE back-bonding effects, especially for the siloxanic species. The back-bonding interaction energies and the contribution of d AOs in RyE orbitals, calculated using Def2-SV(P) and Def2-QZVPPD basis sets are presented in the Supporting Information (Tables S13 – S16). According to the results obtained, the choice of basis set influenced the energy of LPO→RyE interactions to some extent. Thus, the total back-bonding energy computed for the disiloxane molecule by using the double-ζ and quadruple-ζ basis sets were with around 3 kcal/mol smaller and respectively 3 kcal/mol higher compared to the ones delivered by the triple-ζ one; for digermoxane and distannoxane, the calculated energy differences between these basis sets were smaller. While d orbitals are considered rather polarization functions, more complete basis sets should lead to a decrease of the back-bonding


14

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effect. However, the greatest interaction energies were obtained for the Def2-QZVPPD basis set, fact which is opposite to this general accepted statement and thus supports, even though in a smaller ratio, the participation of d orbitals in the E-O chemical bond. In order to evaluate the impact gained by the corroboration of all LPO→σ*E-R and LPO→RyE interactions, NBO DEL optimizations were also performed, removing in this case all these effects. Following the unconstrained DEL procedures for (H3E)2O species (Table 4), the Si-O-Si and Ge-O-Ge angles were lowered down to the C-O-C values of their analogue ethers. Furthermore, the Si-O and Ge-O distances were significantly elongated, to values very close to the sum of the covalent radii. The widening of Sn-O-Sn angle was less influenced by the removal of LPO→RySn interactions while the Sn-O bond lengths increased after the extra deletion of these back-bonding effects, getting closer to the sum of the their covalent radii (Table 4). In addition, the cumulative removal of LPO→σ*E-R and LPO→RyE effects led also to an anti orientation of the H-E-E-H dihedrals (Table S17).

Table 4. Calculated values for the E-O-E angles and E-O bond lengths according to unconstrained NBO DEL optimizations, following the removal of LPO→σ*E-R and LPO→RyE interactions. Differences between the values obtained using NBO DEL optimizations and the ones computed for the equilibrium geometries. B3LYP

E R

NBO DEL opt Σ (LPO→σ*E-R + LPO→RyE)

PBE0


NBO DEL opt Σ (LPO→σ*E-R + LPO→RyE)

E-O-E (°) E-O (Å) ∆ E-O-E (°) ∆ E-O (Å) E-O-E (°)


E-O (Å)

∆ E-O-E (°)

∆ E-O (Å)

Si H

114.5

1.769

-38.4

0.132

111.5

1.760

-36.2

0.126

Ge H

115.6

1.861

-14.3

0.077

113.9

1.847

-13.9

0.073

Sn H

121.2

2.039

-13.3

0.070

119.8

2.021

-12.7

0.065


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For the constrained DEL optimizations performed on the (Me3E)2O species (Table S18), the impact of LPO→RyE interactions on the E-O-E angles was not as pronounced than for the (H3E)2O derivatives, in good agreement with the decrease in energy of these effects from (H3E)2O to (Me3E)2O. However, the DEL calculated E-O-E angles within the hexamethyl derivatives dropped below the C-O-C value in di-tert-buthyl ether.

Conclusions This work showed that vicinal hyperconjugative interactions of the type LPO→σ*E-R play an important role in determining the structural features of germoxanes and stannoxanes, similarly to previous observations for siloxanic species. However, DFT calculations and NBO analysis suggested that back-bonding LPO→dE interactions also dictate, albeit to a lesser extent, the structural behavior of all these derivatives. Assessment of the basis set dependence on LPO→dE effects revealed the implication of these interactions in bonding, although in smaller ratios, as secondary electronic effects. Several geometrical parameters in these species are better explained when taking into account the cumulative effect of both hyperconjugative and back-bonding interactions, according to the NBO DEL optimizations. Thus, in the absence of all these effects, the E-O-E angles decreased to values similar to C-O-C angles in analogue ethers, or to even lower values. However, the Sn-OSn angle was less influenced by the deletion of LPO→RySn effects. The energy of these interactions and also the d-character of the Ry orbitals in the investigated E-O-E units decreased with the increase of the atomic number of E. Deletion of hyperconjugative and back-bonding effects also impacted the length of the E-O bonds; Si-O and Ge-O distances were elongated to


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the sum of covalent radii following the NBO DEL optimizations. In addition, the most stable eclipsed conformations of these species were strongly influenced by these interactions.

ASSOCIATED CONTENT Supporting Information. Tables containing computed data of several geometrical parameters, potentials to linearization, contribution of the AOs in the NBOs involved in hyperconjugative and p→d-like back-bonding interactions and their corresponding energies; image illustrating the NBOs involved in the hyperconjugative interactions for the permethylated derivatives; DFT coordinates of the analyzed compounds. This material is available free of charge.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT I. M. thanks Babes-Bolyai University for financial support from the Research Scholarship, being also grateful to the “World Federation of Scientist” Scholarship. Additional computational


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resources were provided by the high-performance computational facility MADECIP, POSCCE, COD SMIS 48801/1862 co-financed by the European Regional Development Fund of the European Union. The authors thank Professor Romuald Poteau (Université Paul Sabatier, Toulouse) for the helpful discussions. ABBREVIATIONS NBO, Natural Bond Orbital; AO. Atomic Orbital; NBO DEL, Natural Bond Orbital Deletion; LP, Lone Pair; Ry, Rydberg Orbital.

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TOC Graphic


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Bridging a Knowledge Gap from Siloxanes to Germoxanes and

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