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Aromaticity and Conformational Flexibility of Five-Membered Struct Chem (2016) 27:101–109 DOI 10.1007/s11224-015-0707-4 ORIGINAL RESEARCH Aromaticity and conformational flexibility of five-membered monoheterocycles: pyrrole-like and thiophene-like structures 1 1,2 3 4,5 Irina V. Omelchenko • Oleg V. Shishkin • Leonid Gorb • Jerzy Leszczynski Received: 13 November 2015 / Accepted: 18 November 2015 / Published online: 7 December 2015 Ó Springer Science+Business Media New York 2015 Abstract Aromaticity and conformational flexibility of Keywords Aromaticity Á Five-membered heterocycles Á the series of five-membered monoheterocycles with group Aromaticity indices Á Aromatic ring flexibility 14–16 heteroatoms, having one or two lone pairs, were studied with ab initio methods using NICS, ASE and I5 indices. For non-planar molecules like phosphole, aro- Introduction maticity of their planar transition states was also studied, and a special modification of ASE index was proposed to Five-membered aromatic heterocycles have been widely that end. It was found that the presence of two lone pairs is investigated because of their importance for biochemistry, generally preferable for aromaticity of all heterocycles medicine, technology and other aspects of life and industry except CPD and silolyl dianions. Heterocycles with group [1]. Aromaticity is a central concept in the chemistry of 16 heteroatoms have consistently lower aromaticity com- heterocycles; it is the main ground for classification and for pared to other groups. A lot of structures should be clas- rationalization of their properties and reactivity [2, 3]. Most sified as moderate aromatic and non-aromatic. Energies of of the popular quantitative indices of aromaticity were out-of-plane deformation do not correlate with other indi- developed and tested on the basis of both six- and five- ces studied, but reveal the same qualitative trends. Gener- membered heterocycles [4–7]. Degree of aromaticity of a ally, aromaticity of five-membered monoheterocycles great number of different five-membered heterocycles was depends strongly on both heteroatom type and number of carefully calculated and recalculated within the bounds of lone pairs on it. discussion about multidimensionality of aromaticity con- cept by Katritzky et al. [8–14]. It was concluded finally that for large number of different molecules under study there is Oleg V. Shishkin: Deceased. no statistically significant correlation between different aromaticity indices; however, any of them can be used for a & Irina V. Omelchenko rough classification of molecules into aromatic, non-aro- [email protected] matic and antiaromatic species [12]. 1 Effect of heteroatom on aromaticity could be examined SSI ‘Institute for Single Crystals’, National Academy of Sciences of Ukraine, 60 Lenin Ave., Kharkiv 61001, Ukraine by replacement of one (CH) group in carbocycle by iso- electronic heteroatomic group. It should considerably 2 V.N.Karazin Kharkiv National University, 4 Svobody Sq., Kharkiv 61077, Ukraine change p-electron delocalization in the ring, primarily due to different electronegativity and atomic radii of carbon 3 Badger Technical Services, LLC, Vicksburg, MS, USA 4 atom and heteroatom [15]. For six-membered monohete- US Army ERDC, 3532 Manor Dr, Vicksburg, MS 39180, rocycles, it was found that aromaticity diminishes with the USA increasing heteroatom number, but all monoheterocyclic 5 Department of Chemistry and Biochemistry, Interdisciplinary analogues of benzene are highly aromatic [16]. Center for Nanotoxicity, Jackson State University, P.O. Box 17910, 1325 Lynch Street, Jackson, MS 39217, Cyclopentadienyl (CPD) anion is fully aromatic five- USA membered analogue of benzene with 6 p-electron system. 123 102 Struct Chem (2016) 27:101–109 However, aromaticity of known five-membered mono- analogues with 3rd row heteroatoms, with similar but a bit heterocycles varies in a much more wide range than of six- smaller degree of aromaticity [29, 30]. This is in consistent membered monoheterocycles. Even for well-studied with theoretical data on six-membered monoheterocycles molecules that have aromatic system formally analogous to [16]. Recent review about selenaheterocycles [31] shows benzene, it is very different. Pyrrole and thiophene are its similarity to thiophene. Arsole molecule was found to known to be highly aromatic, but furan is considered rather be non-planar and non-aromatic as well as phosphole, with as a non-aromatic molecule [15]. aromatic planar transition state, but inversion barrier of As A lower stability of other monoheterocyclic analogues atom is higher [21]. Germolyl anion is less stable and less of CPD anion hinders the experimental studies of their aromatic than silolyl anion, while its dianion is aromatic aromaticity [15]. Here the computational chemistry tech- again [32, 33]. niques come into play. Among such rare heterocycles, the One can note that listed molecules considered being phosphole is possibly the most studied one. Being formally monoheterocyclic analogues of CPD anion are not exactly similar to the highly aromatic planar pyrrole, the unsub- of the same type of electronic configuration of the five- stituted phosphole molecule was found to be non-aromatic membered ring. Pyrrole, phosphole and arsole, together and non-planar [17–20]. It was shown that a huge inversion with CPD, silolyl and germolyl anions, have only one lone barrier of tricoordinated phosphorous atom forces it to have pair (LP) on heteroatom that either involved or not into p- pyramidal geometry. That prevents conjugation of phos- system of the ring. Monoheterocycles with group 16 het- phorous lone pair with p-system of the ring [20]. Inversion eroatoms, pyrrolide and phospholide anions, as well as barriers grow significantly when moving down from N to silolyl and germolyl dianions, have two lone pairs, with Bi for group 15 tricoordinated atoms [21], while aromatic one of them involved (or not) into p-delocalization [15]. stabilization energy remains approximately the same [22]. The second type of heterocycles has notable electron Aromaticity of the planar transition state of phosphole was redundancy that can affect aromaticity. In the present work, predicted to be rather high [20]. we attempt to compare aromaticity of five-membered Another five-membered ring of great interest is silolyl monoheterocycles of both types (Scheme 1) within the anion, the CPD anion derivative with one carbon atom uniform approach and methods. That can help to locate substituted by Si atom. Chemistry of silaaromatic com- systematic changes depending on the heteroatom type. It pounds was considerably developed in the last decades, and also can be useful for comparison of the effect of het- the role of quantum chemical prediction for such unsta- eroatom on aromaticity of five- and six-membered mono- ble molecules is great [23]. For silolyl anion, the situation heterocycles with the same heteroatoms [16]. Since for six- with aromatic stabilization was found to be very similar to membered cycles the out-of-plane deformation energy was phosphole. Planar conformation of the molecule is a tran- found to be a good estimate of aromaticity or antiaro- sition state that has a significant degree of aromaticity [24]. maticity [34–36], applicability of this index for five- The equilibrium geometry of this anion is non-planar membered monoheterocycles was also comprehensively - because of the high inversion barrier of [SiC2H] moiety; tested. therefore, it is non-aromatic [23]. For both phosphole and silolyl anion, elimination of the ‘‘inversion problem’’ by removing of the proton from Methods of calculation heteroatom leads to significant increase in aromaticity. Silolyl dianions are known to be planar and highly aro- The structures of all molecules under consideration have matic [23–25]. Phopholide anion is highly aromatic too been optimized using the Møller-Plesset second-order [19, 20, 26]. Some anions of pyrrole derivatives are more perturbation theory [37] with Dunning’s correlation-con- aromatic than neutral forms, which indicates the presence sistent triple-zeta basis set [38] (MP2/cc-pVTZ method). of the same steric strain in pyrrole [27]. However, the Character of the stationary points on the potential energy inversion barrier of the nitrogen atom is much smaller than surface was checked by calculations of Hessian at the same aromatic stabilization energy of the planar ring [21]. It is interesting that the presence of electron-donating sub- 34 stituents increases aromaticity and decreases heteroatom X with one LP X with two LP's pyramidality in both phosphole [19, 28] and silolyl dianion CH-+NH OH C2-N - O [23], which forms intermediate structures between parent 2 5 non-aromatic CPD-like rings and their aromatic anionic SiH-+PH SH X Si2- P- S 1 states. GeH- AsH SeH+ Ge2- As- Se Heterocycles with heavier 4th row heteroatoms are less studied and generally considered to be very similar to their Scheme 1 Initial classification of the heterocycles under study 123 Struct Chem (2016) 27:101–109 103 level of theory; no imaginary eigenvalues for equilibrium order saddle point possibly due to the strong vibration geometries were found. For molecules that are non-planar coupling of lower modes; therefore, energy obtained during in the equilibrium state, also planar transition states were the partial geometry optimization with fixed planar con- TS located, with sole negative eigenvalues of Hessian matrix figuration of the [CC2H] moiety was taken for ASE for each, which corresponds to the X heteroatom inversion. estimation. The most stable conformations of rings were Aromaticity was estimated with several well-known taken throughout. All ASE calculations reported here indices [39] of structural (Bird’s I5 index [5]), magnetic include the zero-point energy (ZPE) correction. (NICS [6]) and energetic (ASE [40]) nature. All indices Conformational flexibility of rings was studied at the were calculated using the same level of theory. Bond same level of theory by scan of each of symmetry-inde- orders for Bird’s index calculation were obtained from pendent endocyclic torsion angles in the range ±30° with Wiberg indices [41] calculated in the frame of NBO 5° steps. All remaining geometrical parameters were opti- analysis [42].
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