- https://science-univ.com/quantum-chem/sigma-pi/
- https://www1.meijo-u.ac.jp/~tnagata/education/ochem1/2021/ochem1_03.pdf
- https://chemblogno1.blogspot.com/2019/10/sp-sp3-sp2-sp3.html
- https://www1.doshisha.ac.jp/~bukka/lecture/general/resume_g/GC-13-06.pdf
- https://www2.ph.sci.toho-u.ac.jp/tamura/Hakone96/HKNchem.htm
- https://www.kagakudojin.co.jp/appendices/kaito/c18152/c18152-01kaito.pdf
