Exploration of C₆H₆ Potential Energy Surface:  A Computational Effort to Unravel the Relative Stabilities and Synthetic Feasibility of New Benzene Isomers

Abstract

Ab initio (MP2, CCSD(T)) and hybrid density functional theory (B3LYP) calculations with up to triple-ζ basis set were done to locate all possible minima, where each carbon in the molecule is tetracoordinate, on the C6H6 potential energy surface. The search was initiated with a total of 218 structures, and in few cases, geometrical and stereoisomers were considered. The exhaustive study on all these topological structures resulted in a total of 263 stationary points on the C6H6 potential energy surface. The B3LYP level characterizes 209 as minima, 31 as transition states, 8 as second-order, 7 as third-order, and 1 as fourth-order saddle points. The remaining 7 structures could be located as stationary points only at the MP2 level. The molecules were classified into acyclic, monocyclic, bicyclic, tricyclic, and tetracyclic. The acyclic isomers fall within a range of 60−80 kcal/mol higher in energy compared to benzene. Among the cyclic structures, the range of relative stabilities of minima is larger, viz., monocyclic (31−146 kcal/mol), bicyclic (72−159 kcal/mol), tricyclic (72−300 kcal/mol), and tetracyclic (102−156 kcal/mol). Strain due to small three and four membered rings and constrained double and triple bonds control the relative stabilities of these isomers. The computed isomers exhibit various novel bonding modes for carbon, namely, planar tetracoordinate, hypervalent, pyramidal, bent/twisted double bonds, vicinal dicarbenes, nonlinear triple bonds, and so forth. Absolute chemical hardness values have no correlation with the relative stabilities, and about 45 molecules have higher hardness values than that of benzene.

Novel α-spirocyclic (alkyl)(amino)carbenes at the theoretical crossroad of flexibility and rigidity

“…The value of experimental and computational efforts in reaching the reactivity surfaces of carbenes is evident [1][2][3][4]. After successful preparation of stable (amino)(aryl)carbenes [5], Bertrand and co-workers [6] reported the synthesis, X-ray structure, and ligand properties of the first isolable acyclic (alkyl)(amino)carbene in 2004.…”

Let’s Play with Valence Isomers: The Influence of Different Main Group Elements on Their Relative Stability

“…Applying this definition to C 6 H 6 would lead to more than 330 isomers featuring tetravalent arrangement around all carbons. 1 Even, excluding the diastereomers and enantiomers, a computer-aided procedure generated 217 possible C 6 H 6 isomers. 2 In 1966, in order to better classify all these isomers, Romanian chemist Balaban defined an interesting new type of isomeric relationship that he named ''valence isomerism.…”

A heavy analogue of the smallest bridgehead alkene stabilized by a base

“…Accession codes: The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC 1007442 (1ÁDMAP), CCDC 1007443 (2Á(benzene) 0.5 ), CCDC 1007444 (2Á(2,3-dimethy-1,3-butadiene) 0.5 ), CCDC 1007445 (4Á(benzene)), CCDC 1007446 (7), CCDC 1007447 (8) and CCDC 1007448 (9). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.…”

“…1a) is a bridgehead alkene with the smallest number of bridge atoms and several possible canonical structures containing a strained double bond 2 , even though BBE has been considered to be an exceptional bridgehead alkene ('zero-bridge alkene') because of the lack of a twisted double-bond geometry observed in typical bridgehead alkenes. Although BBE and its derivatives have been proposed as reactive intermediates 6 and theoretically investigated as the isomers of C 4 H 4 molecules present in important fundamental organic molecules such as tetrahedrane and cyclobutadiene [7][8][9][10][11] , the synthesis and experimental observation of BBE and its derivatives have not been reported yet.…”