trans-1,2-Dicyano-cyclopropane and other cyano-cyclopropane derivatives: A theoretical and experimental VA, VCD, Raman and ROA spectroscopic study

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

The structures of cyclopropane rings which carry σ-acceptor or σ-donor substituents have been studied using density-functional theory (DFT), and mean bond lengths and conformational parameters retrieved from the Cambridge Structural Database. It is confirmed that σ-acceptor substituents, e.g. halogens, generate positive ring bond-length asymmetry in which there is lengthening of the distal bond (opposite to the point of substitution), and shortening of the two vicinal bonds. This is due to withdrawal of electron density from the cyclopropane 1e′′ orbitals, which are bonding for the distal bond and antibonding for the vicinal bonds. For σ-donor substituents such as SiH3 or Si(CH3)3, the DFT and crystal structure data show negative ring bond-length asymmetry (distal bond shortened, vicinal bonds lengthened), owing to electron donation into the 4e′ ring orbital, which are also bonding for the distal bond and antibonding for the vicinal bonds. The results also show that —OH substituents induce weak positive asymmetry, but that the effects of methyl or amino substituents are either non-existent or extremely small, certainly too small to measure using crystal structure information.

Conformation and geometry of cyclopropane rings having π-acceptor substituents: a theoretical and database study

The 3e′ orbitals of cyclopropane have the correct symmetry to interact with low-lying unoccupied orbitals of π-acceptor substituents and maximum overlap occurs when the two orbital systems are parallel, i.e. when the π-acceptor bisects the ring in projection down the substituent bond. Since the cyclopropyl group is a common component of active pharmaceutical and agrochemical ingredients, it is important that these strong conjugative interactions are well modelled by computational techniques, and clearly represented in experimental crystal structures. Here we show that torsion angle distributions derived from crystal structure data in the Cambridge Structural Database are in excellent correspondence with torsional energy profiles computed using density functional theory (DFT) for a range of substituents: —COOR, —CONR 2, —NO2, vinyl and phenyl. We also show that crystal structure information is invaluable in modelling conformations of compounds that contain multiply substituted rings, where steric interactions require some substituents to adopt energetically disfavoured conformations. Further, conjugative interactions with π-acceptors lead to significant asymmetry in the cyclopropane ring bond lengths and again the experimental and computational results are in excellent agreement. Such asymmetry effects are additive, and this explains bond-length variations in cyclopropane rings bearing two or more π-acceptor substituents.

Theoretical approaches to the calculation of Raman optical activity spectra

In this article, we will give a brief account of the different approaches that have been presented in the literature for calculating Raman optical activity (ROA) spectra by ab initio methods. We will also outline the general structure of a self-consistent-field-based approach for analytic calculations of ROA spectra, including also contributions from London orbitals. The use of London orbitals ensures that the relevant ROA parameters are gauge origin independent. We will also give an outlook on the future of ab initio calculations of Raman optical activity spectra.