Ab Initio SCF MO and CI Studies of the Electronic States of Butadiene

Ab initio SCF MO calculations carried out for the ground electronic states of cis‐ and trans‐butadiene show the trans configuration to be more stable by approximately 5 kcal, total energy − 154.7103 hartrees. The set of SCF MO's resulting from these calculations is subsequently employed as a basis for a series of configuration–interaction (CI) treatments investigating the electronic spectrum of butadiene. Results of CI treatments which consider only $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}$ single excitation configurations bear a definite resemblance to previous semiempirical studies and give rise to the excited state ordering ${}^3{B}_u{,}^3{A}_g{,}^1{B}_u,\mathrm{and}{}^1{A}_g$ , in agreement with the current experimental assignment although the singlets are calculated relatively high in energy. Extension of the CI treatment to include multiple excitation configurations and $\mathrm{\sigma \hspace{0.17em}\to \hspace{0.17em}\pi *}$ excitation decidely alters the spectrum by lowering the ${}^1{A}_g$ to 2.5 eV below the ${}^1{B}_u$ , and also by introducing ${}^3{B}_u\mathrm{and}{}^3{A}_u$ states lower than the ${}^1{B}_u:$ a ${\mathrm{\sigma \hspace{0.17em}\to \hspace{0.17em}\pi *}}^1{A}_u$ state is also calculated to lie close to the ${}^1{B}_u$ . The relevance of these ab initio results to basic assumptions of the semiempirical model and to the current experimental interpretation of the butadiene spectrum is discussed.