The evolution of the principle of circumscribing

Circumscribing benzenoid polycyclic aromatic hydrocarbons was first introduced in 1982 under the topic of a periodic table for benzenoid hydrocarbons and isomer enumeration. This circumscribing process was rendered on benzene to successively generate coronene, circumcoronene, dicircumcornene, and so on as the first constant-isomer series. Subsequently, numerous benzenoid constant-isomer series were uncovered because of the coupling of the benzenoid periodic table (Table PAH6) and circumscribing. Circumscribing even carbon monocyclic polyenes produced [n]cyclacenes (n = number of hexagonal rings) which are monotier nanotubes. Starting with [n]cyclacenes and successive circumscribing generates (n,0) Zigzag nanotubes of increasing length. Successive circumscribing cycloparaphenylenes leads to (n,n) Armchair nanotubes of increasing length. Successive supercircumscribing cycloparaphenylenes leads to total-resonant-sextet Armchair nanotubes of maximum stability. Circumscribing polycyclic conjugated systems was subsequently introduced for other ring sizes. Circumscribing polycyclic hydrocarbons with a combination of pentagonal and hexagonal rings leads to fullerenes when 12 pentagonal ring is reached. Generation of altans is a specialised type of circumscribing in which the encircling annulene is attached via its alternate degree-2 vertices to the degree-2 vertices of the core structure. We show that monocapped nanotubes are circumscribed altans.

环绕合成（circumscribing）苯型多环芳烃（benzenoid polycyclic aromatic hydrocarbons）的方法最早于1982年被提出，彼时该研究隶属于苯型烃周期表及异构体计数的研究主题。该环绕合成过程以苯为起始底物，依次生成蔻（coronene）、环绕蔻（circumcoronene）、双环绕蔻（dicircumcornene）等化合物，进而构建出首个恒定异构体系列。后续得益于苯型烃周期表（PAH6表）与环绕合成法的结合应用，大量苯型恒定异构体系列被发掘。对偶数碳单环多烯烃实施环绕合成，可得到[n]环省（[n]cyclacenes，其中n为六元环数目），这类化合物属于单层级纳米管。以[n]环省为起始底物，通过连续环绕合成可得到长度持续递增的(n,0)型锯齿型纳米管（zigzag nanotubes）。对环对苯撑（cycloparaphenylenes）实施连续环绕合成，可得到长度持续递增的(n,n)型扶手椅型纳米管（armchair nanotubes）。对环对苯撑实施连续超环绕合成，可得到稳定性最优的全共振六隅体扶手椅型纳米管（total-resonant-sextet armchair nanotubes）。后续环绕合成法被推广应用于其他环尺寸的多环共轭体系。对同时包含五元和六元环的多环烃实施环绕合成，当体系中存在12个五边形环时，即可得到富勒烯（fullerenes）。阿尔坦结构（altans）的构建是一类特殊的环绕合成过程：环绕形成的轮烯（annulene）通过其交替二度顶点（degree-2 vertices），与核心结构的二度顶点相连。本研究证实，单封端纳米管（monocapped nanotubes）即属于环绕型阿尔坦结构。