A Theoretical Study of Monosubstituted Cyclopropenyl System

Abstract

MONDO-Forces calculations have been performed, with complete optimization of geometry on X-cyclopropenyl system (cations, radicals and anions), where X is H, O", OH, CH 3 , CN, N0 2 , F and CF 3 . All substituents prefer planar structure when substituted on both cations and radicals, while they prefer pyramidal structure in the case of anions except CF 3 . The substituents O", OH and F act as electron releasing, while CHO, N0 2 and CF 3 act as electron withdrawing when substituted on cyclopropenyl system. CH 3 and CN show amphielectronic behaviour. They act as electron releasing on the cations and withdrawing on both radicals and anions depending on electron demand. In the case of cations and radicals, all substituents were found to increase the vicinal bonds and decrease the distal bonds and bond angles to which the substituent is attached. For anions the substitutents show no such regularity because the substituents are out of the three-membered ring plane. All substituents increase the stability of the cyclopropenyl system except CF 3 in the case of the cation.The cyclopropenyl cation is the smallest aromatic molecule which has two ^-electrons and satisfies the Hückel 4 n + 2 rule. The electronic structure is of great interset since the molecule is highly strained but despite of this the ion is stable in the form of salts, e. g. C 3 H 3 • SbCl^, and also in polar solvents [1]. D 3h configuration of the ion is indicated by its IR and NMR spectra [1], Allen has reported a structural analysis for two cyclopropenyl cation derivatives based upon Xray crystallographic data [2]. Clark carried out ab initio LCAO SCF MO Calculations on both C 3 H 3 + and C3H3 and discussed the aromaticitiy and antiaromaticity [3]. Ha et al. reported an ab initio LCAO SCF MO investigation on C 3 H 3 + , C 3 Hj and C 3 H 3~ and discussed the geometry and stability of these species [4j. Random et al. aiso carried out ab initio calculations on C3H3 using STO-3G and 6-31G and obtained the equilibrium structure of the ion [5]. Takada and Ohno also published an ab initio CI calculation on the electronic structure of C3H3 [6]. The most recent ab initio calculations on the molecular structure and vibrational spectrum of C3H3 were performed by Xie et al. [7], and Lee et al. [8], Cyclopropenyl free radical, the simplest member of the series of fully conjugated cyclic radicals, has been the subject of experimental [9-13] and theoretical studies [4,[14][15][16][17][18][19][20],The theoretical work by Chipman and Miller [20] predicts an ethylenic structure of C s symmetry as the lowest energy form. The hydrogen atom at the apex of the isoceles triangle is bent substantially out of the ring plane. The allylic structure, which satisfies the Jahn-Teller theorem, is predicted to be 5 kcal/mole higher in energy. Experimental work by Closs and Redwine [13] support the C s structure and rules out the allylic structure. There is no experimental information on the properties of the neutral radical, so it is necessary to resort to theoretical studies.The cyc...

Stationary Points on Potential Energy Surface of Cyclic C3H3 with Coupled-Cluster Approaches and Density Functional Theory

“…Our results are consistent with previous results ,,,,,,− , in that the C s ( 2 A′) structure is the global minimum. The 2 A″( C s ) state is a transition state along the a″ mode connecting the 2 A′( C s ) states according to our results.…”

“…As will be discussed below, the C s ( 2 A″) structure and the C 2 ( 2 A) structure can be obtained from a deformation of the C 2 v ( 2 A 2 ) structure along the b 1 and a 2 vibrational modes, respectively. Minimum-energy structures for each symmetry obtained in this work are generally in good agreement with previous results. ,− ,,− ,− , The C1–C2 bond in the C 2 v ( 2 B 1 ) structure by Tachibana is about 0.023 Å shorter than that of the present work. The C2–C3 bond of the C s ( 2 A′) structure with MCSCF by Kass and with CASSCF by Bersuker is about 0.02 Å longer than the present result.…”

“…Minimum-energy structures for each symmetry obtained in this work are generally in good agreement with previous results. 5,[20][21][22]24,[27][28][29][30][32][33][34]36 The C1−C2 bond in the C 2v ( 2 B 1 ) structure by Tachibana 28 Harmonic frequencies of all the structures using various methods are listed in Tables S1−S7 of supporting information together with previous results. 36 Two sets of frequencies for the two degenerate states of the D 3h and C 3v structures are presented.…”

“…C 3 H 3 is an indispensable radical in reactions involving cyclic aromatic hydrocarbons, soot formation processes, and some combustion reactions. − For example, it is an important intermediate in forming the aromatic ring through recombination of two propargyl radicals. − , In addition, C 3 H 3 has been observed as one of the products in the C­( 3 P) + C 3 H 6 reaction in the atmospheres of Titan and giant planets as well as in the interstellar medium . Numerous experimental − and theoretical studies ,− have reported on this radical because of its significance. Previous studies ,,, show that several isomers of C 3 H 3 exist, including propargyl radical, cyclopropenyl radical (c-C 3 H 3 ), and others.…”

“…Experimental studies on the electron spin resonance (ESR) spectrum , and ionization potentials (IPs) of C 3 H 3 have been reported. On the other hand, the electronic structure of C 3 H 3 has also been investigated ,,− ,− ,− using various methods such as Hartree–Fock theory (HF), density functional theory (DFT), , coupled-cluster theory (CC), , configuration interaction (CI), multiconfiguration self-consistent field (MCSCF), and complete active space SCF (CASSCF). , Most of these studies focus on the aromaticity or antiaromaticity of C 3 H 3 , since aromaticity is closely related to its stability. In addition, adiabatic IPs and electron affinities (EAs) of C 3 H 3 have also been reported, , and its IP with G2 theory agrees well with the experimental value…”