The barrier to the methyl rotation inCis‐2‐butene and its isomerization energy toTrans‐2‐butene, revisited

Abstract

We respond to the two questions posed by Weinhold, Schleyer, and McKee (WSM) in their study of cis-2-butene (Weinhold et al., J Comput Chem 2014, 35, 1499), in which they solicit explanations for the relative conformational energies of this molecule in terms of the Quantum Theory of Atoms in Molecules (QTAIM). WSM requested answers to the questions: (1) why is cis-2-butene less stable than trans-2-butene despite the presence of a hydrogen-hydrogen (H⋯H) bond path in the former but not in the latter if the H⋯H bond path is stabilizing? (2) Why is the potential well of the conformational global minimum of cis-2-butene only 0.8 kcal/mol deep when the H⋯H bonding is stabilizing by 5 kcal/mol? Both questions raised by WSM are answered by considering the changes in the energies of all atoms as a function of the rotation of one of the two methyl groups from the minimum-energy structure, which exhibits the H⋯H bond path, to the transition state, which is devoid of this bond path. It is found that the stability gained by the H⋯H bonding interaction is cancelled by the destabilization of one of the ethylenic carbon atoms which, alone, destabilizes the system by as much as 5 kcal/mol in the global minimum conformation. Further, it is found that the 1.1 kcal/mol stability of trans-2-butene with respect to the cis-isomer is driven by the considerable destabilization of the ethylenic carbons by 11 kcal/mol, while the changes in the atomic energies of the other corresponding atoms in the two isomers account for the observed different stabilities. The error introduced into QTAIM atomic energies by neglecting the virials of the forces on the nuclei for partially optimized structures is discussed.

Energetic Analysis of Conjugated Hydrocarbons Using the Interacting Quantum Atoms Method

“…We also analyzed the hydrogen–hydrogen bonding observed in some of the aromatic molecules studied. This type of interactions has been extensively analyzed in the literature, in particular for the equilibrium geometries of some linear and branched alkanes and for the isomerization of 2‐butene using IQA and other approaches . In particular, byphenyl has been a molecule where the H⋯H interactions have called the attention because they appear in its planar transition state (TS) .…”

On the Stability of Cis‐ and Trans‐2‐Butene Isomers. An Insight Based on the FAMSEC, IQA, and ETS‐NOCV Schemes

“…A while later Matta, Sadjadi, Braden, and Frenking, abbreviated as MSBF, revisited answers to the above questions (1–2) based on in depth description of the electronic structure including the analyses of changes in atomic energies on rotations . It has been concluded, contrary to WSM, that: (i) CH•••HC interactions are stabilizing and the higher energy of cis ‐2‐butene originates largely from the destabilization of the ethylenic C5C7 fragment, (ii) to understand the barriers to methyl rotations one must take into account not only the changes in the atomic energies of the hydrogen atoms involved in the CH•••HC contact, but predominantly variations in the energies of all the remaining hydrogen and carbon atoms.…”

“…In the present study, we hope to settle the above dispute by further and in depth characterization of the stability of cis ‐ and trans ‐2‐butene isomers by means of various tools suitable for description of (non)bonding situations which were not used in the previous studies . We applied predominantly the fragment attributed molecular system energy change (FAMSEC) approach by Cukrowski [7e] which is rooted in the interacting quantum atoms (IQA) method by Pendás et al It has been proven that the FAMSEC scheme is well suited for an in depth analysis of the energy contribution made by interactions to relative energies of isomers regardless if they are of a (de)stabilizing nature and whether atoms involved are QTAIM or covalently (non)bonded.…”

Non-Covalent Interactions in Hydrogen Storage Materials LiN(CH3)2BH3 and KN(CH3)2BH3

“…It is noteworthy that these types of connections are intuitively considered as destabilizing due to the lack of electrostatic attraction between hydrogen atoms-homopolar dihydrogen interactions (especially the intramolecular ones) are still a matter of debate in the literature [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]. Very recently more and more evidence has been reported in the literature that highlight the stabilizing nature of homopolar dihydrogen interactions [17,21,29,[36][37][38][39][40][41][42][43][44][45][46]. Accordingly, in this work we provide complementary results which shed light on energetic, quantitative and qualitative characteristics of non-covalent interactions that contribute to the stability of LiN(CH3)2BH3 and KN(CH3)2BH3 crystals.…”