Predicting the Open-Shell Character of Polycyclic Hydrocarbons in Terms of Clar Sextets

Abstract

Rather unexpected spin-symmetry breakings of mean-field single determinants occur in singlet ground states of many families of alternating conjugated hydrocarbons which accept a full on-bond electron pairing. These symmetry breakings may be seen as an indication of the existence of unpaired electrons. Although qualitative, the concept of disjoint electronic sextets proposed by Clar (hereafter called CS) is at least a heuristic tool for predicting various features of fused polybenzenic hydrocarbons. The present work shows that identifying the preferred CS distribution enables one to rationalize the existence of one or several spin-symmetry breakings, i.e., the existence and the number of unpaired electrons in alternant fused polycyclic hydrocarbons via a simple recipe for the prediction of these features. This recipe is based on comparison between various distributions of CS on the molecular frame, subject to a restriction concerning the fragments of the graph that do not belong to the CS. This rule is successfully confronted to UDFT calculations and to a recently proposed criterion predicting the possible occurrence of spin-symmetry breaking from the topological Hückel Hamiltonian. The confrontation runs on series of rhombuses, periacenes, anthenes, and other graphene flakes or nanoribbons. The CS distribution definitely offers a qualitative guide to look at the possible occurrence of (multiple) symmetry breakings in polycyclic architectures which are commonly seen as closed-shell singlets.

Exchange Rules for Diradical π-Conjugated Hydrocarbons

“…Naively, one could expect that diradicals always had a ground state with S = 1, due to the similitude with the case of open-shell atoms. Whereas this is often the case in a variety of systems 3,[11][12][13][14][15] , in others there is a violation of the Hund's rule 12,[16][17][18][19][20][21] . Therefore, the sign of the exchange interaction in this class of diradicals is not always the same 5 .…”

Revitalizing Spin Natural Orbital Analysis: Electronic Structures of Mixed‐Valence Compounds, Singlet Biradicals, and Antiferromagnetically Coupled Systems

“…SNO analysis of a teranthene derivative reported in reference . a) Lewis structure, commonly used to describe the teranthene derivative, note that once the number of Clar sextet is maximized, there will be two sites with unpaired electrons. b) Spin density plot.…”

New [4]helicene derivatives: Synthesis, characterization and photophysical properties

“…Polycyclic aromatic hydrocarbons (PAHs) represent an important class of organic compounds which have two or more fused benzene rings. These compounds have received considerable attention due to their fascinating chemistry [1] and unique physical properties [2][3].…”

“…H NMR (300 MHz, CDCl3): δ (ppm): 7,15 (t, J = 8.4 Hz, 2H); 7.53-7.56 (m, 2H); 7.63 (s, 1H); 7.75 (dd, J1 = 8.7 Hz, J2 = 1.5 Hz); 7.85-7.97 (m, 5H); 8.16 (s, 1H, Hvinyl); 13 C NMR (75 MHz, CDCl3): δ (ppm): 110.51 (C); 111.08 (C); 115.56 (CH); 115.85 (CH); 117.48 (CN); 121.90 (CH); 125.78 (CH); 126.53 (CH); 126.62 (CH); 127.21 (CH); 127.99 (C); 128.47 (C); 129.57 (CH); 129.64 (CH); 130.87 (CH); 130.98 (CH); 132.78 (C); 132.92 (C); 140.26 (CH); 143.37 (C).4.4.3. (Z)-1-Methyl-p-[2-cyano-2-(naphthalen-2-yl)]vinylbenzoate (9c)Yellow solid, 91%, m.p = 144-146 °C;1 H NMR (300 MHz, CDCl3): δ (ppm): 3.96 (s, 3H, OCH3); 7.54-7.57 (m, 2H); 7.70 (s, 1H); 7.75 (d, J = 8.7 Hz, 1H); 7.85-7.93 (m, 3H); 7.97 (d, J= 8.1 Hz , 2H); 8.13 (d, J = 8.1 Hz, 2H); 8.20 (s, 1H, Hvinyl); 13 C NMR (75 MHz, CDCl3): δ (ppm): 51.87 (CH3); 113.56 (C); 117.10 (CN); 121.84 (CH); 126.27 (CH); 126.63 (CH); 126.87 (CH); 127.24 (CH); 128.11 (CH); 128.58 (CH); 128.68 (2CH); 129.64 (2CH); 130.67 (C); 130.94 (C); 132.73 (C); 133.12 (C); 137.37 (C); 140.02 (CH); 165.82 (C=O).4.4.4. (Z)-3-(p-Methoxyphenyl)-2-(naphthalen-2-yl)acrylonitrile (9d)A C C E P T E D M A N U S C R I P TYellow solid, 92%, m.p = 129-131 °C;1 H NMR (300 MHz, CDCl3): δ (ppm): 3.91 (s, 3H, OCH3); 7.01 (d, J = 8.4 Hz, 2H); 7.52-7.55 (m, 2H ), 7.62 (s, 1H); 7.75 (d, J = 7,8 Hz, 1H); 7.86-7.97 (m, 5H); 8.16 (s, 1H , Hvinyl); 13 C NMR (75 MHz, CDCl3): δ (ppm): 54.90 (OCH3); 108.43 (C); 113.97 (2CH and CN); 122.08 (CH); 125.26 (CH); 126.19 (C); 126.26 (CH); 126.33 (CH); 127.13 (CH); 127.86 (CH); 128.27 (CH); 130.70 (2CH); 131.66 (C); 132.76 (C); 132.91 (C); 141.32 (CH); 161.03 (C).…”

“…(Z)-3-(p-Cyanophenyl)-2-(naphthalen-2-yl)acrylonitrile (9e) Yellow solid, 84%, m.p = 205-207 °C; 1 H NMR (300 MHz, CDCl3): δ (ppm): 7.57-7.60 (m, 2H); 7.67 (s, 1H); 7.76-7.79 (m, 3H); 7.88-7.89 (m ,1H); 7.94 (d, J = 9 Hz, 2H); 8.01 (d, J = 6 Hz, 2H); 8.22 (s ,1H, Hvinyl); 13 C NMR (75 MHz, CDCl3): δ (ppm): 113.11 (C); 115.06 (C); 116.62 (CN); 117.60 (CN); 121.73 (CH); 126.5 (CH); 126.71 (CH); 127.40 (CH); 127.24 (CH); 128.12 (CH); 128.68 (CH); 129.08 (2CH); 130.32 (CH); 132.09 (2CH); 132.74 (C); 133.33 (C); 137.45 (C); 138.64 (C).4.4.6. (Z)-3-(p-(Methylthio)phenyl)-2-(naphthalen-2-yl)acrylonitrile (9f)Yellow solid, 87%, m.p = 142-144 °C;1 H NMR (300 MHz, CDCl3): δ (ppm): 2.54 (s, 3H, CH3); 7.32 (d, J = 8.4 Hz, 2H); 7.51-7.59 (m, 2H ); 7.60 (s, 1H); 7.73 (d, J = 8.4 Hz, 1H); 7.83-7.92 (m, 4H); 8.15 (s, 1H, Hvinyl); 13 C NMR (75 MHz, CDCl3): δ (ppm): 14.52 (CH3); 109.81 (C); 117.81 (CN); 121.96 (CH); 125.31 (2CH); 125.57 (CH); 126.46 (CH); 127.20 (CH); 127.96 (CH); 128.38 (CH); 129.18 (2CH); 129.69 (C); 131.29 (C); 132.82 (2C); 140.98 (2CH); 142.16 (C). 4.4.7.…”