Determination ofsyn/anti Isomerism in DCNQI Derivatives by 2D Exchange Spectroscopy: Theoretical Underpinning

Abstract

Keywords: Ab initio calculations / NMR spectroscopy / 2D EXSY / CyanoiminesThe dynamic syn/anti isomerism resulting from the inversion of the cyano group at the C=N double bond in a series of substituted DCNQIs has been investigated in solution by two-dimensional exchange spectroscopy (2D EXSY). The isomers formed were characterised by 1 H NMR at 223 K to slow down the inversion process of the cyanoimine group as well as by H,H COSY spectra. Whereas compounds 6, 7, 9 and 10 show only one isomer, compounds 8 and 11 show two isomers with two inversion processes and compounds 12 and 13 show three isomers and four inversion processes. The energy barrier for the syn/anti isomerization of the NCN groups has been estimated from the rate constants determined from the 2D EXSY spectra, and very close values (13.16−13.93 kcal/ mol for a single inversion) were found for all compounds. Isomerizations involving two NCN groups (compounds 12 and 13) exhibited higher activation free energy values (13.41−14.40 kcal/mol). Theoretical calculations of these free energy barriers are in excellent agreement with the experi-

Charge-Transfer-Induced Isomerization of DCNQI on Cu(100)

“…This shows that the isomerization does not take place while the molecules are forming part of an island, but after detaching from it. With this data in mind, the following scenario can be depicted: In agreement with our and other experimental and theoretical results, the anti-DCNQI is the more stable isomer [20][21] and DCNQI adsorbs therefore on the copper surface from the gas phase (in the absence of condensation by Hbonding) in the anti form. Up to a substrate temperature of 245 K, the thermal energy is not large enough for the molecules to overcome the isomerization barrier between the anti and the syn form.…”

“…19 The stereochemistry of DNCQI and its derivatives has been thoroughly investigated. [20][21] For the parent compound, syn-anti configurations are interconverted in solution with an activation 4 energy E a ~ 0.5 eV, although the anti-isomer is slightly preferred (80:20), since the dipole moments are reduced in this configuration. 20 Theoretical calculations reveal that isomerization takes place via inversion of the cyano group around the imine nitrogen atom, with a gas-phase activation energy of 0.63 eV 21 (our own calculations give E a ~ 0.66 eV).…”

“…[20][21] For the parent compound, syn-anti configurations are interconverted in solution with an activation 4 energy E a ~ 0.5 eV, although the anti-isomer is slightly preferred (80:20), since the dipole moments are reduced in this configuration. 20 Theoretical calculations reveal that isomerization takes place via inversion of the cyano group around the imine nitrogen atom, with a gas-phase activation energy of 0.63 eV 21 (our own calculations give E a ~ 0.66 eV). Although submolecular features are hardly resolved due to the strong interaction with the copper surface (see below), a close look at the STM images seems to indicate that the molecular shape is spindle-like in the LT-phase (Figure 2c) with a center of symmetry that is absent in the RT phase, where the molecular shape is more croissant-like (Figure 2d), which suggests that in the LT phase the molecules are in the anti configuration, while in the RT-phase they are in the syn configuration.…”

Mechanistic Insights into Transfer Hydrogenation Catalysis by [Ir(cod)(NHC)2]+Complexes with Functionalized N-Heterocyclic Carbene Ligands

“…Experimental rotational rates for 5·Br were obtained from magnetization transfer experiments using 2D EXSY NMR spectroscopy. This method has been increasingly applied to the study of dynamic processes, the determination of rotational barriers, and conformational analysis . The basis of a quantitative 2D EXSY experiment is the relationship between the intensity of the exchange cross-peaks and the rate constants for chemical exchange.…”

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…Accurate 2D EXSY measurements of dynamics in N ‐acetylated proline and dehydroproline esters 8 (X=H, F) have shown a remarkable increase of their rotation activation energies by the presence of the double bond . An interesting case of flat rotor and stator (Figure ‐III) undergoing a nearly equal exchange is isomeric 2,3‐dimethylanthraquinone‐9‐cyanimides ( 9 ) reported by Günther and separately by Hoz and colleagues . Analysis of the 2D EXSY pattern helped to unambiguously assign the methyl groups.…”