C₅H₄:  Pyramidane and Its Low-Lying Isomers

Abstract

The chemistry of the C 5 H 4 singlet potential energy surface was investigated at sophisticated levels of theory with a focus on the stability of pyramidane (tetracyclo[2.1.0.0 1,3 0 2,5 ]pentane or [3.3.3.3]fenestrane), a structure featuring a carbon atom at the apex of a square pyramid. Zero-point corrected relative energetics were predicted with both coupled cluster and density functional methodologies. Computations with both methodologies agree qualitatively with previous theoretical results, demonstrating that the pyramidane structure is a true minimum with substantial barriers to isomerization. At the CCSD(T)/TZ2P level a relative energy of 24 kcal/mol was predicted for the transition state to tricyclo[2.1.0.0 2,5 ]pent-3-ylidene, the lowest barrier to isomerization of pyramidane. The transition state to bicyclo[2.1.0]pent-2-ene-5-ylidene, the other transition state known to lead directly to the pyramidane structure, was found to lie 33 kcal/mol above pyramidane. Relative energies are also provided for several lower-lying C 5 H 4 isomers, including isomers incorporating linear carbon chains.

Butterfly Methanes: Designing a Novel Class of anti‐van′t Hoff Carbons

“…Except 2 b , all the structures show axial X−C−X angle (α) closer to 180° and for 3 b , the equatorial angle Y−C−Y (β) shows a significant deviation from classical geometry. The bond lengths of all the compounds are within the acceptable limits and also in the range of earlier theoretical studies . The shorter C−B bond lengths than the typical C−B single bond (1.554 Å) in CH 3 −BH 2 indicate the π‐back donation from Cp z to the empty p‐orbitals of boron …”

“…The bond lengths of all the compounds are within the acceptable limits and also in the range of earlier theoretical studies. [26][27][28][29][30][31][32][33][34][35][36][37][38] The shorter CÀ B bond lengths than the typical CÀ B single bond (1.554 Å) in CH 3 À BH 2 indicate the π-back donation from Cp z to the empty p-orbitals of boron. [39] We computed vibrational modes of different structures at the B3LYP/6-311 + + G** level of theory ( Table 2).…”

Chemistry of Energetically Activated Cumulenes—From Allene (H2CCCH2) to Hexapentaene (H2CCCCCCH2)

“…[9][10][11][12][13][14][15][16] Whereas lower members of the cumulene carbenes have C 2v symmetry and 1 A 1 electronic ground states, [17][18][19] the situation for the closed-shell cumulenes is more complex. [20][21][22] Here, molecules having an even number of carbon atoms such as butatriene (H 2 C=C = C=CH 2 ) and hexapentaene (H 2 C=C = C= C = C=CH 2 ) belong to the D 2h point group (all four hydrogen atoms are in one symmetry plane); [23][24][25][26] on the other hand, cumulenes with an odd number of carbon atoms as found in propadiene/allene (H 2 C=C = CH 2 ) and pentatetraene (H 2 C=C = C=C = CH 2 ) dictate an inherent D 2d symmetry [27] and hence a pair-wise, staggered arrangement of the hydrogen atoms ( Figure 1, Table 1). This correlates with the 1 A g and 1 A 1 electronic ground states for the even-and odd-numbered cumulenes, respectively.…”

Rotational spectrum of CH3C13CCCH: Determination of the equilibrium structure of methyldiacetylene from microwave spectroscopy and ab initio calculations

“…Finally, since in Table 3 theoretical equilibrium structures available in the literature [43,44] are also reported, a brief comment on how they compare with the present results is deserved. An overall agreement is observed, even if the level of accuracy is definitely lower than that presented in this investigation.…”