Molecular Mechanics (MM4) Studies on Unusually Long Carbon–Carbon Bond Distances in Hydrocarbons

Allinger, Norman L.; Lii, Jenn‐Huei; Schaefer, Henry F.

doi:10.1021/acs.jctc.5b00926

Cited by 12 publications

(9 citation statements)

References 22 publications

(33 reference statements)

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“…1 ). This realization sparked a race towards the longest covalent C–C bonds [ 5 – 6 ]: already impressive 1.67 Å in hexakis(3,5-di- tert -butylphenyl)ethane is not even a limit and stable diamondoid dimer with a central C–C bond as long as 1.71 Å has been achieved [ 7 – 8 ]. Bulky alkyl groups assist not only in achieving the longest C–C bonds, but also the shortest intermolecular H···H contacts [ 9 ], which are otherwise tackled by squeezing them inside the cages [ 10 ].…”

Section: Introductionmentioning

confidence: 99%

Steric “attraction”: not by dispersion alone

Gryn’ova¹,

Corminbœuf²

2018

Beilstein J. Org. Chem.

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Non-covalent interactions between neutral, sterically hindered organic molecules generally involve a strong stabilizing contribution from dispersion forces that in many systems turns the ‘steric repulsion’ into a ‘steric attraction’. In addition to London dispersion, such systems benefit from electrostatic stabilization, which arises from a short-range effect of charge penetration and gets bigger with increasing steric bulk. In the present work, we quantify this contribution for a diverse set of molecular cores, ranging from unsubstituted benzene and cyclohexane to their derivatives carrying tert-butyl, phenyl, cyclohexyl and adamantyl substituents. While the importance of electrostatic interactions in the dimers of sp2-rich (e.g., π-conjugated) cores is well appreciated, less polarizable assemblies of sp3-rich systems with multiple short-range CH···HC contacts between the bulky cyclohexyl and adamantyl moieties are also significantly influenced by electrostatics. Charge penetration is drastically larger in absolute terms for the sp2-rich cores, but still has a non-negligible effect on the sp3-rich dimers, investigated herein, both in terms of their energetics and equilibrium interaction distances. These results emphasize the importance of this electrostatic effect, which has so far been less recognized in aliphatic systems compared to London dispersion, and are therefore likely to have implications for the development of force fields and methods for crystal structure prediction.

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Section: Introductionmentioning

confidence: 99%

Steric “attraction”: not by dispersion alone

Gryn’ova¹,

Corminbœuf²

2018

Beilstein J. Org. Chem.

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“…Allinger tested his MM4(2015) molecular mechanics force field with overcrowded molecules including 11 2 . [113] While it is recognized "[…]that the forces that determine these bond lengths [are] i. e., stretching, bending, torsional, van der Waals, and electrostatic […]" it follows that "Long CÀ C bonds are […] mainly the result of steric e ects […]." It is surprising that the question was not asked why these molecules exist after all, not to speak of an explanation what stabilized them.…”

Section: Dissociation Of Hexaphenylethanesmentioning

confidence: 99%

Computational Chemistry as a Conceptual Game Changer: Understanding the Role of London Dispersion in Hexaphenylethane Derivatives (Gomberg Systems)

Rösel

Schreiner

2022

Israel Journal of Chemistry

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The present personal perspective sheds light on the checkered history of hexaphenylethane (HPE) and some of its key derivatives, including successes and failures in interpreting experimental as well as computational data. HPE has become a testing ground for chemical theory since the groundbreaking work of Gomberg in 1900. It sparked the growth of theoretical carbon chemistry beyond tetravalency, forced chemists to improve theory of color and chemical bonding by mesomeric resonance, and challenged the purely repulsive view on substituent effects. Understanding the origin of the instability of HPE on the one hand and the stability of the sterically much more crowded all-meta t Busubstituted HPE on the other hand may well be considered an important turning point in the appreciation of London dispersion (LD) and the analytical power of computational chemistry.

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“…Molecules 1-53 are categorized into seven groups Group I-Group VII and targeted CC bonds are indicated by red coloration. The C−C bonds of Group I molecules (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13) and Group II molecules (14-18) are exposed to increasing steric repulsion as the bulkiness of substituents increases. Group III molecules (19)(20)(21)(22)(23)(24) represent different diamondoid configurations.…”

Section: Introductionmentioning

confidence: 99%

“…However, it may exceed this standard value considerably, which has led to a competition of pushing the CC bonds to their limits by finding the best strategy for synthesizing the longest but still intact CC bond [ 2 , 3 , 4 , 5 , 6 , 7 ]. Whereas the question of the practical use of such ultra-long C−C bonds is still open, the study of these extreme cases including novel CC bonding situations such as e.g., the recently established CC tetrel bonding [ 8 , 9 , 10 ] strongly contributes to enriching our understanding of the CC bond and the concept of the chemical bond in general [ 11 , 12 , 13 ]. Besides long C−C bonds other elongated bonding situations are also of interest, notably long OO bonds and dative bonding.…”

Section: Introductionmentioning

confidence: 99%

Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study

et al. 2021

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For decades one has strived to synthesize a compound with the longest covalent C-C bond applying predominantly steric hindrance and/or strain to achieve this goal. On the other hand electronic effects have been added to the repertoire, such as realized in the electron deficient ethane radical cation in its D3d form. Recently, negative hyperconjugation effects occurring in diamino-o-carborane analogs such as di-N,N-dimethylamino-o-carborane have been held responsible for their long C-C bonds. In this work we systematically analyzed CC bonding in a diverse set of 53 molecules including clamped bonds, highly sterically strained complexes such as diamondoid dimers, electron deficient species, and di-N,N-dimethylamino-o-carborane to cover the whole spectrum of possibilities for elongating a covalent C-C bond to the limit. As a quantitative intrinsic bond strength measure, we utilized local vibrational CC stretching force constants ka(CC) and related bond strength orders BSO n(CC), computed at the ωB97X-D/aug-cc-pVTZ level of theory. Our systematic study quantifies for the first time that whereas steric hindrance and/or strain definitely elongate a C-C bond, electronic effects can lead to even longer and weaker C-C bonds. Within our set of molecules the electron deficient ethane radical cation, in D3d symmetry, acquires the longest C-C bond with a length of 1.935 Å followed by di-N,N-dimethylamino-o-carborane with a bond length of 1.930 Å. However, the C-C bond in di-N,N-dimethylamino-o-carborane is the weakest with a BSO n value of 0.209 compared to 0.286 for the ethane radical cation; another example that the longer bond is not always the weaker bond. Based on our findings we provide new guidelines for the general characterization of CC bonds based on local vibrational CC stretching force constants and for future design of compounds with long C-C bonds.

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Molecular Mechanics (MM4) Studies on Unusually Long Carbon–Carbon Bond Distances in Hydrocarbons

Cited by 12 publications

References 22 publications

Steric “attraction”: not by dispersion alone

Steric “attraction”: not by dispersion alone

Computational Chemistry as a Conceptual Game Changer: Understanding the Role of London Dispersion in Hexaphenylethane Derivatives (Gomberg Systems)

Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study

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