A Thorough Systematic Conformational Study of an Experimentally Known Lantern‐Like Superphane

The results of a thorough, systematic conformational study of the experimentally known lantern-like superphane, SP, are presented. It is shown that in SP, the relative energies of the lowest-energy conformers within a given type increase with the number of imine nitrogen atoms pointing inward the SP cage ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {N}_{\mathrm{in}} $$</mml:annotation></mml:semantics> </mml:math> ). Thus, the conformer with all outward-pointing N atoms is the most energetically stable. The most energetically stable conformers of the types with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> <mml:mo>≤</mml:mo> <mml:mn>2</mml:mn></mml:mrow> <mml:annotation>$$ {N}_{\mathrm{in}}\le 2 $$</mml:annotation></mml:semantics> </mml:math> have a pinwheel structure, whereas those of types with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> <mml:mo>≥</mml:mo> <mml:mn>6</mml:mn></mml:mrow> <mml:annotation>$$ {N}_{\mathrm{in}}\ge 6 $$</mml:annotation></mml:semantics> </mml:math> , on the contrary, feature rosette-like structures. Both DFT-based and GFN2-xTB-based minima searches are performed. Both these methods locate the same minima, and the energies correlate fairly well. The latter method, however, gives smaller energetic differences between conformers, and dispersion effects are more pronounced. Molecular dynamics (MD) simulations show that periods of low potential energy are related mainly to conformers with low <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {N}_{\mathrm{in}} $$</mml:annotation></mml:semantics> </mml:math> , close to 4. Structures with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>3</mml:mn> <mml:mo>≤</mml:mo> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> <mml:mo>≤</mml:mo> <mml:mn>5</mml:mn></mml:mrow> <mml:annotation>$$ 3\le {N}_{\mathrm{in}}\le 5 $$</mml:annotation></mml:semantics> </mml:math> are more abundant than those with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> <mml:mo><</mml:mo> <mml:mn>2</mml:mn></mml:mrow> <mml:annotation>$$ {N}_{\mathrm{in}}<2 $$</mml:annotation></mml:semantics> </mml:math> . The influence of dispersion, electrostatic interactions, and angular stresses on the structural motifs of SP conformers and their energies is also investigated. The stabilizing effect due to dispersion decreases with increasing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {N}_{\mathrm{in}} $$</mml:annotation></mml:semantics> </mml:math> , and rosettes are more stabilized than pinwheels. The high relative energies of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>12</mml:mn></mml:mrow> <mml:annotation>$$ {N}_{\mathrm{in}}=12 $$</mml:annotation></mml:semantics> </mml:math> conformers result from the unfavorable barrel-shaped structure, which leads to high angular stresses. We propose several possibilities for the experimental (based on IR spectra) determination of the conformational type of the SP superphane (i.e., determination of the putative number <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow> <mml:mrow><mml:mtext>in</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {N}_{\mathrm{in}} $$</mml:annotation></mml:semantics> </mml:math> ).