Final Report Summary - MA:CCC (Möbius aromaticity: a new Challenge for Computational Chemistry)
Project context
Möbius aromaticity has become a fascinating topic in chemistry from an experimental and theoretical point of view. Heilbronner, based on Hückel molecular orbital theory, predicts that singlet annulenes with 4n p electrons would be aromatic systems in twisted conformations (with odd number of half-twists) where the p orbitals lie on the surface of a Möbius strip, whereas singlet [4n+2] Möbius molecules would be antiaromatic (the opposite of the Hückel rule). The synthesis of a viable aromatic Möbius system has been a challenge for 40 years, as its difficulty mainly resides in the curvature of the system, resulting in a destabilisation due to the ring strain, which is larger than the stabilisation due to the Möbius aromaticity. In the last five years, expanded porphyrins have emerged as a new promising class of molecules for the creation of Möbius aromatic systems. The conformational flexibility, the number and the nature of substituents on the pyrrolic and meso positions, and the metallation of the porphyrins allow them to achieve different topologies with distinct aromaticities, magnetic, and electric properties. Specifically, the expanded porphyrins present a strong relationship between aromaticity, molecular geometry, and nonlinear optical properties (NLOP). One of the most appealing applications is the possibility to switch between Möbius and Hückel topologies only applying small changes in the external conditions or in the structure of the ring.
Project objectives
The main goals of this project are: a) to determine the potential of the Hückel-to-Möbius switches as optical switches and to predict/design new switches with the optimum properties, e.g. aromatic, magnetic, and electric; b) report the reaction mechanism of these topological switches and to provide new insight into the main factors (aromaticity, intramolecular hydrogen bonds, ring strain, and steric effects of the meso-substituents) that play a role in this interconversion process.
The key factors, which will determine their potential use as optical switches, are the high values of the NLOP and the large differences between the NLOP of the Möbius and Hückel conformations.
Project work
In a very recent work, we have carried out the first exhaustive evaluation of the electronic and vibrational contributions to static and infinite optical frequency (IOF) dynamic NLOP for a system (the bianthraquinodimethane modified [16] annulene) synthesised by Herges and co-workers with Hückel and Möbius topologies. In this work, we have employed different theoretical methods in order to evaluate, which one is capable to provide semiquantitative accuracy of the NLOP with a p-conjugated Hückel-Möbius switch. Our results concluded that BHandHLYP, M052X, and CAM-B3LYP methods correctly reproduce the X-ray crystal structure and provide similar nonlinear optical properties, which can be considered of a semiquantitative quality. Moreover, we found that the NLOP values for Hückel and Möbius conformations of the bianthraquinodimethane modified [16] annulene are very similar.
In another article, we studied the static and IOF dynamic NLOP of the Hückel and Möbius conformers of A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) synthesised by Latos-Grazynski and coworkers. The sum of the electronic and vibrational contributions second hyperpolarizability shows very high values (lying in the 4x105 and 5x106 a.u. range) and large differences of second hyperpolarizability between the Hückel and Möbius conformers are obtained, with the maximum difference at 1x106 a.u. The obtained results indicate that the expanded porphyrins are promising systems to manufacture Hückel-to-Möbius topological switches.
Moreover, we have another article submitted about the evaluation of the NLOP for the meso-hexakis(2,6-F2-phenyl)[28]hexaphyrin(1.1.1.1.1.1) synthesised by Sankar et al. With this system we obtained larger differences of second hyperpolarizability between the Hückel and Möbius conformers, around 2x106 a.u. than with the A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1). For this reason, we studied several meso-substituents with different electron withdrawing and electron releasing natures for the [28]hexaphyrin(1.1.1.1.1.1). We have also considered different positions for the meso-substituents in order to check the optimum conditions to have high values of the NLOP and large differences between the NLOP of the Möbius and Hückel conformations.
For our second objective we recently reported the reaction mechanism of the conformational switch between the Hückel planar and the singly twisted Möbius structures for two different meso-substituted [28]hexaphyrins(1.1.1.1.1.1) i.e. the meso-substituents are the hydrogen atoms and pentaflurophenyls. First, we have shown that the selection of the computational method is crucial for the correct energetic description of these topological switches. Secondly, we have pointed out that the nature of the meso-substituent can have an important effect in the thermochemistry and kinetics of these topological switches. These results provide new insight into the main factors (aromaticity, intramolecular hydrogen bonds, ring strain, and steric effects of the meso-substituent) that play a role in this interconversion process and allow the rationalising of factors controlling these switches.
In a further study (which has been just submitted) we have employed a more diverse set of substituents. Our results indicate that the ring strain controls the energy barrier of the rotational process; while the steric effect of the substituents results crucial for the relative stability of the Hückel and Möbius structures. On the other hand, the aromaticity shows a less relevant role in the thermochemistry and kinetics.
It is worth noting that we have two articles currently being written. The first study focuses on the steric effect and its implications to the relative stability between planar Hückel, singled-twisted Möbius, and figure-eight Hückel conformations. The second work studies the effect of one, two, three, and six trifluoromethyls in the thermochemistry and kinetics of the conformational switch between the Hückel planar, singled-twisted Möbius, and figure-eight structures for the meso-substituted [28]hexaphyrin(1.1.1.1.1.1) system.
Project impact
These results are very important to find new ways to control the thermochemistry and kinetics between these Möbius and Hückel structures and allow rationalizing the factors controlling these topological switches. Thus, for instance, it is possible predicting how to freeze a switch in a desired Möbius or Hückel conformation and propose new switches.
Last but not least, Dr Josep Maria Anglada and Dr. Miquel Torrent-Sucarrat have had a long collaboration of theoretical studies of the Chemistry of Earth atmosphere, especially about the reactivity of hydroxyl and hydroperoxyl radicals. During this time, we have finished some jobs already started, which led to additional publications, namely: a) The anharmonicity and the Eigen-Zundel Dilemma in the IR Spectrum of the Protonated 21 Water Cluster; b) The HO2 adsorption and acid dissociation process on the surface of the (H2O)20 and (H2O)21 clusters; c) The effects of a Single Water Molecule on the OH + H2O2 Reaction.12.
Möbius aromaticity has become a fascinating topic in chemistry from an experimental and theoretical point of view. Heilbronner, based on Hückel molecular orbital theory, predicts that singlet annulenes with 4n p electrons would be aromatic systems in twisted conformations (with odd number of half-twists) where the p orbitals lie on the surface of a Möbius strip, whereas singlet [4n+2] Möbius molecules would be antiaromatic (the opposite of the Hückel rule). The synthesis of a viable aromatic Möbius system has been a challenge for 40 years, as its difficulty mainly resides in the curvature of the system, resulting in a destabilisation due to the ring strain, which is larger than the stabilisation due to the Möbius aromaticity. In the last five years, expanded porphyrins have emerged as a new promising class of molecules for the creation of Möbius aromatic systems. The conformational flexibility, the number and the nature of substituents on the pyrrolic and meso positions, and the metallation of the porphyrins allow them to achieve different topologies with distinct aromaticities, magnetic, and electric properties. Specifically, the expanded porphyrins present a strong relationship between aromaticity, molecular geometry, and nonlinear optical properties (NLOP). One of the most appealing applications is the possibility to switch between Möbius and Hückel topologies only applying small changes in the external conditions or in the structure of the ring.
Project objectives
The main goals of this project are: a) to determine the potential of the Hückel-to-Möbius switches as optical switches and to predict/design new switches with the optimum properties, e.g. aromatic, magnetic, and electric; b) report the reaction mechanism of these topological switches and to provide new insight into the main factors (aromaticity, intramolecular hydrogen bonds, ring strain, and steric effects of the meso-substituents) that play a role in this interconversion process.
The key factors, which will determine their potential use as optical switches, are the high values of the NLOP and the large differences between the NLOP of the Möbius and Hückel conformations.
Project work
In a very recent work, we have carried out the first exhaustive evaluation of the electronic and vibrational contributions to static and infinite optical frequency (IOF) dynamic NLOP for a system (the bianthraquinodimethane modified [16] annulene) synthesised by Herges and co-workers with Hückel and Möbius topologies. In this work, we have employed different theoretical methods in order to evaluate, which one is capable to provide semiquantitative accuracy of the NLOP with a p-conjugated Hückel-Möbius switch. Our results concluded that BHandHLYP, M052X, and CAM-B3LYP methods correctly reproduce the X-ray crystal structure and provide similar nonlinear optical properties, which can be considered of a semiquantitative quality. Moreover, we found that the NLOP values for Hückel and Möbius conformations of the bianthraquinodimethane modified [16] annulene are very similar.
In another article, we studied the static and IOF dynamic NLOP of the Hückel and Möbius conformers of A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) synthesised by Latos-Grazynski and coworkers. The sum of the electronic and vibrational contributions second hyperpolarizability shows very high values (lying in the 4x105 and 5x106 a.u. range) and large differences of second hyperpolarizability between the Hückel and Möbius conformers are obtained, with the maximum difference at 1x106 a.u. The obtained results indicate that the expanded porphyrins are promising systems to manufacture Hückel-to-Möbius topological switches.
Moreover, we have another article submitted about the evaluation of the NLOP for the meso-hexakis(2,6-F2-phenyl)[28]hexaphyrin(1.1.1.1.1.1) synthesised by Sankar et al. With this system we obtained larger differences of second hyperpolarizability between the Hückel and Möbius conformers, around 2x106 a.u. than with the A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1). For this reason, we studied several meso-substituents with different electron withdrawing and electron releasing natures for the [28]hexaphyrin(1.1.1.1.1.1). We have also considered different positions for the meso-substituents in order to check the optimum conditions to have high values of the NLOP and large differences between the NLOP of the Möbius and Hückel conformations.
For our second objective we recently reported the reaction mechanism of the conformational switch between the Hückel planar and the singly twisted Möbius structures for two different meso-substituted [28]hexaphyrins(1.1.1.1.1.1) i.e. the meso-substituents are the hydrogen atoms and pentaflurophenyls. First, we have shown that the selection of the computational method is crucial for the correct energetic description of these topological switches. Secondly, we have pointed out that the nature of the meso-substituent can have an important effect in the thermochemistry and kinetics of these topological switches. These results provide new insight into the main factors (aromaticity, intramolecular hydrogen bonds, ring strain, and steric effects of the meso-substituent) that play a role in this interconversion process and allow the rationalising of factors controlling these switches.
In a further study (which has been just submitted) we have employed a more diverse set of substituents. Our results indicate that the ring strain controls the energy barrier of the rotational process; while the steric effect of the substituents results crucial for the relative stability of the Hückel and Möbius structures. On the other hand, the aromaticity shows a less relevant role in the thermochemistry and kinetics.
It is worth noting that we have two articles currently being written. The first study focuses on the steric effect and its implications to the relative stability between planar Hückel, singled-twisted Möbius, and figure-eight Hückel conformations. The second work studies the effect of one, two, three, and six trifluoromethyls in the thermochemistry and kinetics of the conformational switch between the Hückel planar, singled-twisted Möbius, and figure-eight structures for the meso-substituted [28]hexaphyrin(1.1.1.1.1.1) system.
Project impact
These results are very important to find new ways to control the thermochemistry and kinetics between these Möbius and Hückel structures and allow rationalizing the factors controlling these topological switches. Thus, for instance, it is possible predicting how to freeze a switch in a desired Möbius or Hückel conformation and propose new switches.
Last but not least, Dr Josep Maria Anglada and Dr. Miquel Torrent-Sucarrat have had a long collaboration of theoretical studies of the Chemistry of Earth atmosphere, especially about the reactivity of hydroxyl and hydroperoxyl radicals. During this time, we have finished some jobs already started, which led to additional publications, namely: a) The anharmonicity and the Eigen-Zundel Dilemma in the IR Spectrum of the Protonated 21 Water Cluster; b) The HO2 adsorption and acid dissociation process on the surface of the (H2O)20 and (H2O)21 clusters; c) The effects of a Single Water Molecule on the OH + H2O2 Reaction.12.