@article{Hartmann2014,
  author    = {Hermann Hartmann},
  title     = {Zur Theorie der π-Elektronensysteme},
  journal   = {Zeitschrift f{\"u}r Naturforschung, A},
  volume    = {15},
  number    = {11},
  pages     = {993 -- 1003},
  doi       = {10.1515/zna-1960-1109},
  year      = {2014},
}