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Steric Effects Vs. Electron Delocalization RSC Advances PAPER View Article Online View Journal | View Issue Steric effects vs. electron delocalization: a new look into the stability of diastereomers, conformers Cite this: RSC Adv.,2021,11, 20691 and constitutional isomers† Sopanant Datta and Taweetham Limpanuparb * A quantum chemical investigation of the stability of compounds with identical formulas was carried out on 23 classes of compounds made of C, N, P, O and S atoms as core structures and halogens H, F, Cl, Br and I as Received 13th April 2021 substituents. All possible structures were generated and investigated by quantum mechanical methods. The Accepted 24th May 2021 prevalence of a formula in which its Z configuration, gauche conformation or meta isomer is the most stable DOI: 10.1039/d1ra02877d form is calculated and discussed. Quantitative and qualitative models to explain the stability of the 23 classes rsc.li/rsc-advances of halogenated compounds were also proposed. hyperconjugation in a similar vein to the ethane case,17,18 but Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 1. Introduction the reasoning underlying the preference for meta isomer is still Steric effects, non-bonded interactions leading to avoidance of lacking. spatial congestion of atoms or groups, are oen the central In addition to carbon-backbone compounds in Table 1, there theme in the discussion of stability of diastereomers, are many experimental and theoretical studies for other back- 21 ] 22,23 ] 24 conformers and constitutional isomers. Reasoning based on bones discussed in this paper, namely, C3, C N, C P, 25–31 32 33,34 35 36,37 38–40 41 steric effects is relatively intuitive and gives rise to a generally N]N, N]P, P]P, C–N, C–P, C–O, C–S, N– 38,42–44 38,43 38,43,45,46 38,42,47 47 48,49 50 accepted rule of thumb that an E conguration, anti conformer N, N–P, P–P, N–O, N–S, P–O, P–S, O– 38,42,51,52 53,54 38,55 and para isomer in diastereomers, conformational and consti- O, O–S, and S–S. 21,56 This article is licensed under a tutional isomers, respectively, should be the most stable forms. Inspired by Bent's rule, which states how orbital hybrid- Many ndings in contrast to steric predictions exist in the izations can explain trends of bond lengths and bond angles in literature. Table 1 shows experimental and theoretical investi- a series of compounds correctly while the steric argument fails, Open Access Article. Published on 10 June 2021. Downloaded 10/1/2021 4:09:56 AM. gations of the Z con guration, gauche conformer and meta in this paper, we want to advance the understanding of energy isomer being the most stable forms in carbon-backbone prediction of chemical structures that are derived from the compounds. The experiments include heat of combustion or same molecular formula. hydrogenation and spectroscopic measurement while the Non-superimposable structures of the same molecular theoretical studies are mainly quantum mechanical methods. formula can be enantiomers, diastereomers, conformers and Even when steric effect reasoning correctly predicts the constitutional isomers (structural isomers). As energies of result, controversy ensues. For example, a number of organic enantiomers are identical, they are excluded from our investi- chemistry textbooks attributed the relative stability of the gation. For the other three types of isomerism, E and Z cong- 0 staggered conformation of ethane to steric factor alone. This urations in A]A compounds and halocyclopropanes represent 0 has led to controversy discussed at length across the scientic diastereomerism, gauche and anti conformers in A–A community for over eight years.1–8 compounds represent conformational isomerism and ortho, Electron delocalization effects, on the other hand, are rela- meta and para structures in halobenzenes represent constitu- tively more complicated. The reasoning for energy prediction tional isomerism. oen involves resonance structures9–15 (formerly called meso- meric effect) or hyperconjugation16–20 (delocalization) of 2. Materials and methods orbitals. Specic reasonings for each case of exception to steric 2.1 Quantum chemical calculations and datasets prediction are shown in Table 1. The preference for Z congu- ration and gauche conformation are primarily due to All possible structures made of two atoms A]A0 and A–A0 where 0 A, A ˛ {C, N, P, O, S} or C3 or C6 as the core structure and combinations of halogen (H, F, Cl, Br, I) as substituents were Science Division, Mahidol University International College, Mahidol University, 57–59 Salaya, Nakhon Pathom 73170, Thailand. E-mail: [email protected] previously generated by our group. Up to four levels of † The data presented in this study are openly available in Open Science theory (HF, B3LYP, MP2 and CCSD) were used in these studies Framework with the reference number 6ECP4. See DOI: 10.17605/OSF.IO/6ECP4. and B3LYP frequency calculations were completed for all © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2021,11,20691–20700 | 20691 View Article Online RSC Advances Paper Table 1 Summary of exceptions to steric prediction for carbon-backbone compounds in gas phase Case Exceptions to steric prediction and reasonings Z Early experimenters such as Demiel conjectured that more electronegative atoms are on the same side in the most stable – conguration18,31,60 72 isomers.11,13 Representative examples compiled by Eliel et al.73 include CHF]CHF, CHF]CHCl, CHF]CHBr, CHF]CHI, CHCl] CHCl, CHBr]CHBr. Halogens can be both donors and acceptors of electronic charges. When the donor and acceptor are on the same side, the structure is more stable due to the interaction of a and b substituents in the scheme.73 ff s / s* p / s* s / s* s / Yamamoto et al. proposed four delocalization e ects, -LP (ns CC), -LP (np CC), AP ( CX C0H0 ) and SP ( CX s* ¼ p ff ff C0X0 ), in C2H2X2 compounds where X F, Cl, Br and found that -LP e ect is the major origin of cis e ect in the three – compounds.18 20 73 Gauche For CH2F–CH2F, the gauche form is preferred due to hyperconjugative interactions. The dominant one is the 16,17,74–84 s s* 17 conformation antiperiplanar CH to CF delocalization described in the scheme. Potential energy surfaces of rotamers have been thoroughly investigated. For CH2F–CH2F, the twofold (V2) potential actually has an energy minimum when the F–C–C–F torsional angle is Æ90.79 Rotational barriers can be small such that the shi in equilibrium can be easily observed when polar solvents promote the interconversion of anti to gauche Creative Commons Attribution-NonCommercial 3.0 Unported Licence. conformers. “Bent bond” may offer an explanation for the destabilization of the anti conformer.17,80,81 – Meta isomer85 87 For diuorobenzene, heat of combustion results clearly showed that the meta isomer is the most stable.87 Computational studies showed that meta isomers are the most stable forms in most cases of dihalobenzenes. Taskinen This article is licensed under a attributed this to the absence of electronic interactions (shown below) between the two halogen substituents when they are at 1,3-positions.86 Open Access Article. Published on 10 June 2021. Downloaded 10/1/2021 4:09:56 AM. structures. As shown in Table 2, we improved upon existing imaginary frequency under certain conditions.88 Therefore, results by performing single-point electronic energy calculation sample frequency calculations at MP2/6-311++G(d,p) were per- at CCSD(T)/6-311++G(d,p) on diastereomer and conformer formed on all classes and only the benzene class was found to groups. For the constitutional isomer group, original MP2/6- have imaginary frequencies. In agreement with the previous 311++G(d,p) energies were used due to prohibitive computa- study,88 we found that imaginary frequencies disappear at MP2/ tional cost of CCSD(T) in benzene class of compounds. Our 6-311G(d,p) and the electronic energies are very close to MP2/6- À choice of methodology was inspired by observations in three 311++G(d,p). While mean absolute deviation is 12.4 kcal mol 1 dihaloethenes that HF is not good enough for geometries and from 1505 structures, R2 and slope for the energies from the two MP2 is better than DFT for energy calculation.18 HF and B3LYP basis sets are virtually unity. In other words, energies of all results are available in the supporting information† for structures are shied by similar magnitudes in the same comparison. We used them for diagnostic purpose in some direction. As a result, we use the basis 6-311++G(d,p) consis- difficult cases of rotamers. tently for all classes of compounds in this study. Pople's basis set of 6-311++G(d,p) was used due to the Optimized geometries of selected A]A0,A–A0 and hal- availability of iodine atom and its reasonable computational obenzene compounds were compared to gas-phase experi- cost. However, Pople's basis sets are well-known to produce mental data in previous studies.18,89,90,101 The current level of 20692 | RSC Adv.,2021,11, 20691–20700 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Paper RSC Advances Table 2 Class of compounds and the number of structures for the purpose of prevalence rate calculation Creative Commons Attribution-NonCommercial 3.0 Unported Licence. This article is licensed under a Open Access Article. Published on 10 June 2021. Downloaded 10/1/2021 4:09:56 AM. theory and/or basis set yields acceptable results. Additional difficult cases of rotamers, different convergence criteria for conrmation with solid-phase X-ray structures from the Cam- energies and forces were applied.
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