Electronic and Geometrical Structures of Cyclopropanes: Linear Correlation of Ionization Energies with CC Bond Lengths in Monosubstituted Cyclopropanes

Abstract

The unique structure of the three-membered ring renders cyclopropane derivatives appropriate candidates for studies of substituent effects on molecular properties. The molecules are relatively small and can be treated adequately by suitable and comprehensible models.[1] It is well-known that substituents affect the geometry of the ring, [2±5] and qualitative models for the interplay between electronic and geometrical structures with different degrees of sophistication have been proposed (see below). However, quantitative models are still awaiting their elucidation. Recently, we have shown that in p-acceptor substituted cyclopropanes (eight compounds, R 0.962) [6] as well as in halogenocyclopropanes (12 compounds, R 0.928) [7] the difference Dr in the lengths of vicinal and distal bonds is linearly correlated with the energy difference DIP w of the Walsh orbitals w S and w A . The former parameter was calculated by using the density functional theory (DFT) [8] B3LYP method, [9] and the latter parameter was determined from ionization potentials (IPs) measured by photoelectron (PE) spectroscopy. For both series of compounds, the correlation coefficient R can be considered as highly satisfactory. Changes in the C À C bond lengths can thus be explained quantitatively by the energy split of two molecular orbitals (MOs). Herein, we will show that such a linear relationship is of almost general validity for cyclopropane derivatives and applies for most types of substituents.In Figure 1, some orbitals of the cyclopropyl group, which are relevant for an investigation of substituent effects on ring structure, [1, 10±13] are depicted. These are the three Walsh orbitals [16] J.

The Singular Gas-Phase Structure of 1-Aminocyclopropanecarboxylic Acid (Ac3c)

“…At this stage, one may wonder about the role played by the amino group in the conformational propensities observed for Ac 3 c apart from its participation in hydrogen bonding. The question arises as to whether any specific interaction between the amino group and the cyclopropane ring could be responsible, to a certain extent, for the conformational behavior exhibited by Ac 3 c. In contrast to the well-established capacity of the three-membered carbocycle to donate electron density to π-acceptors, ,, its interaction with electron-rich substituents like the amino group is more complex and not completely understood. ,, Cyclopropylamine has been shown to exist as a mixture of two conformers. , The lowest-energy form is characterized by a symmetrical structure, in which the two N–H bonds are staggered with respect to the cyclopropane bonds and the nitrogen lone-pair orbital points toward the cycle. , This arrangement corresponds to that encountered in the I and III conformers of Ac 3 c. The minor cyclopropylamine conformer is about 700 cm –1 higher in energy and presents the amino group rotated by ca. 130° with respect to the preferred orientation. ,, Although conjugative interactions were initially invoked to explain the structure of the dominant conformer, the finding that the energy barriers associated to torsion around the N–C angle are similar in cyclopropylamine and isopropylamine provided a solid argument against the existence of π-electron transfer from the amino to the cyclopropane bonds.…”

Geometry and conformation of cyclopropane derivatives having σ-acceptor and σ-donor substituents: a theoretical and crystal structure database study

“…This asymmetry ( = + 0.006 Å , Á = + 0.008 Å ) is typical for a weak -acceptor, being close to values exhibited by single Cl or Br substituents but much less than asymmetry parameters calculated for monofluorocyclopropane (Table 1). DFT results for cyclopropanol calculated by Rademacher (2003) using the B3LYP method yield a distal bond length of 1.521 Å and vicinal bonds of 1.502 Å , i.e. = + 0.009 Å , Á = + 0.013 Å .…”