1 σ-Aromaticity about cyclopropene Dewar firstly deduced in 1979, that cyclopropene should have σ-Aromaticity with the aromaticity energy to cyclopropane (55.1kcal/mol) as in Table 1. σ-Conjugation and σ-Aromaticity M. J. Dewar, Bull. Soc. Chim. Belg. 1979, 88,

2 a Eclipsing strain energy (kcal/mol). b Angle strain energy (kcal/mol). c σ-Aromatic energy (kcal/mol).

3 Then, in 1996, Peeter Burk and his group calculated the 1 HNMR of cyclopropene, obtainted the chemical shift 7.9 (δ, in ppm), which is higher than 7, indicating aromatic character. He explained this phenomenon as a result of in-plane delocalization of electrons in σ-bonds (σ- aromaticity). J. Phys. Chem. 1996, 100, Aromaticity of Substituted Cyclopropenes: A Theoretical Study Table 2. Calculated Magnetic Susceptibilities, Components of Magnetic Susceptibility Tensor, Magnetic Susceptibility, and 1 H Chemical Shifts, respectively in (ppm cgs) and (in ppm).

4 However, in 2005, Schleyer denied the downfield proton chemical shift for a suitable aromaticity criteria. In the paper, he calculated various δ values of different compounds compared them with NICS values, they are not always consistent with each other. Figure1. Computed isotropic NMR δ 1 H values referenced to the benzene proton chemical shift (taken to be δ=7.3). The magnitude of the out-of-plane component of the shielding tensor, σ Ⅱ (π) is given in brackets. NICS(0) values are in ring centers. Downfield Proton Chemical Shifts Are Not Reliable Aromaticity Indicators Org. Lett., Vol. 7, No. 8, 2005

He also concluded that cyclopropane and cyclopropene (Table 3.) cannot be characterized as aromatic, either by the 1 HMR of the sp 3 hybridized CH groups (δ=0-0.7 ppm) or by the π-electron contributions to the out-of-plane component of the shielding tensor [ σ Ⅱ (π)]. 5 Table 3. Magnetic Properties of Monocyclic Hydrocarbons, The Protons of Neither the σ- Aromatic Cyclopropane nor the σ- Antiaromatic Cyclobutane Are Shifted Downfield

6 In 2010, Jemmis calculated the isomerization reaction 1, isomerization energy endothermic by 12.6 kcal/mol 1 (the corresponding energy from experimental enthalpy of formation is 10.3 kcal/mol) 2 ， indicating nonaromaticity.. ΔH =+12.6 kcal/mol 1. Are Metallocene-Acetylene (M=Ti, Zr, Hf) Complexes Aromatic Metallacyclopropenes? Organometallics 2010, 29, 76– NIST Chemistry WebBook (