Symmetry-Constrained Properties Behave Differently for 2D or 3D Structures under the Same Point Group.

In chemistry and physics, two molecules belonging to the same point group are expected to behave similarly regarding various properties, following their character tables. Here, we show that the derivative of the dipole moment with respect to the normal coordinate of vibration might show different symmetry constraints if the molecule is planar, even if these molecules belong to the same point group. Examples of pairs of molecules featuring these conditions are presented. These findings open a new path toward a much deeper understanding of how 2D materials behave so differently compared to 3D materials featuring the very same atoms and arrangements (graphene and graphite, for example); chemists and physicists dealing with 2D materials could benefit from looking more deeply into pure mathematical relations that might be governing 2D systems in a different way when compared to 3D systems. The aid from mathematicians is welcomed.

On the quest for a molecular vibration whose absolute intensity is described solely by fluctuations of atomic dipoles: Can we find it?

The conditions for a molecular vibration to be active in the infrared spectrum while its absolute intensity is described solely by fluctuations of atomic dipoles are presented. A quick recipe of features of such a system is presented, to guide the seek for it. If such a system can be found, it will then be proved that population analyses based solely on a point-charge approximation are unrealistic as they cannot properly describe this intensity, a real, measurable, unambiguous property.

Energetics and electronics of polar Diels-Alder reactions at the atomic level: QTAIM and IQA analyses of complete IRC paths.

The mechanism of Diels-Alder reactions between cyclopentadiene and several cyanoethylenes was studied by means of Density Functional Theory calculations using QTAIM and IQA (Interacting Quantum Atoms) analyses along complete IRC paths. Each geometry from the IRC had its wavefunction computed and the topology of the electronic density for it was then evaluated. By means of IQA, the global energetic profile was partitioned among the various atoms in the molecule, providing insight into what atoms are the main ones responsible for the magnitude of the energy barriers. The (a)synchronicity of the reaction mechanisms featuring non-symmetrically substituted dienophiles was characterized, from QTAIM, by the electron densities and Laplacians over the LCP's as well as by the different atomic energy barriers obtained from IQA. The magnitude of the atomic barrier nicely explains the (a)synchronicity of the reaction mechanisms, and the degree of (a)synchronicity is nicely revealed by the difference between the earlier and later bond breaking and bond formations. The main conclusion is that important energetic and electronic changes are occurring before and after the position of the transition state structure, mainly for those asynchronous mechanisms, and although these provide essential insight into the reaction mechanism, most studies cannot assess this kind of information because they are focusing solely on reactants, transition states, and products. We advocate that the additional computational effort required for such analyses is more than compensated by the great amount of useful information it provides.