Effect of Fluorine Substitution on the Energies of Small Ring Compounds

Abstract

The effect of fluorine substitution on the energies of small ring compounds has been examined via ab initio calculations at the MP2/6-31G*, MP2/6-311+G*, and B3LYP/6-311+G* theoretical levels, along with correction for differences in zero-point energies. The introduction of a fluorine into a cyclopropane ring leads to destabilization, which results from the higher s character in the orbitals forming the bonds to a substituent. On the other hand, this is not seen with cyclopropene. The effect of 3-substituents on cyclopropene was examined by studying cyclopropenone, cyclopropenethione, methylenecyclopropene, and 3,3-difluorocyclopropene. The stabilization was largest and about equal with the O and S substituents. Smaller effects were observed with CH2 and F2 substituents. The nature of the effects was studied making use of electron density difference maps. The structures and energies of the series of C4H4 and C4F4 derivatives were studied at the above theoretical levels. In most cases, fluorine substitution led to stabilization with respect to but-3-en-1-yne, but with tetrahedrane there was considerable destabilization. Fluorine substitution leads to destabilization of cyclobutyne with respect to cyclobutene.

Computational Study of Cyclobutane-1,3-diylidene Dicarbenes: Ground-State Spin Multiplicity and New Strategy toward the Synthesis of Bicyclo[1.1.0]but-1(3)-enes

“…Substituent Effects on the Structure of 2(S σπ ). As found in the BBE structures (Entries 5,8,13,16,21,and 24), the distorted double-bond length d in 2a2c(S σπ ) was significantly dependent on the substituent X. Thus, the bond length d was found to be in the order X = SiH 3 (143, 139 pm) < H (146, 140 pm) < F (153, 152 pm) in the corresponding (D 2h , C 2v ) Figure 4.…”

“…Thus, the energy gap was found to increase with increasing electronegativity of the substituent X, i.e., 13.94 eV (2b(S σπ ), X = F) > 12.03 eV (2a(S σπ ), X = H) > 9.46 (2c(S σπ ), X = SiH 3 ). The substituent effect may explain the smaller energy gap between T σ and S σπ for 2c (X = SiH 3 ) than those for 2b (X = H) and 2a (X = F), Entries 2,5,10,13,18,and 21 (Table 1) and Figure 4.…”

“…The substituent effect on the relative energetic stability (¦E Sσπ ) for the BBE structure 2(S σπ ) can be attributed to hyperconjugative aromaticity and antiaromaticity as exemplified in the cyclopropene system 3a3c (Figure 5a). 13,36 Thus, electron-donating substituents such as SiH 3 destabilize the cyclopropene structure 3c (X = SiH 3 ) compared to the parent 3a (X = H) because of the pseudo-4π electron (e) antiaromaticity. On the other hand, pseudo-2π e aromaticity is induced by strongly electronwithdrawing substituents such as fluorine atoms to stabilize the cyclopropene structure 3b (X = F) compared to 3a.…”

“…12 The smallest pyramidalized alkene, i.e., bicyclo-[1.1.0]but-1(3)-ene (BBE) (1), which has been determined to be an equilibrium structure, 13 has not yet been isolated. 14 The strain energy (SE) of compound 1 was computed to be 127 kcal mol ¹1 by Wiberg.…”

Are the Six-Memberedλ5-Phosphorins Aromatic or Ylidic?

“…Using the reaction in Scheme 3, Greenberg et al [47] evaluated the stabilization energy to be 9.6 kcal/mol at HF/4-31G. The higher-level calculations (MP2 and B3LYP), carried out by Borden [49] and by Wiberg and Marquez [50] gave very similar values. It is generally accepted that this enhanced stabilization is due to the delocalization of p-electrons into the appropriate CÀF s* orbital (i.e., negative hyperconjugation).…”