Theoretical vibrational spectra as a tool in the identification of unusual organic molecules

Abstract

-Reactive organic systems are often synthesized at low temperature in a frozen matrix and identified by their IR spectra.Typically these are low resolution spectra contamiflated by reactants and other products. In such cases theoretical vibrational spectra, computed initio, can be of help in the identification and analysis of these spectra.Thus the theoretical IR spectrum of cyclobutadiene is in accord with a rectangular rather than square structure. An experiment that was thought to show the contrary by proving the identity of cyclobutadiene-1,2--d2 and -1,4--d2 is reinterpreted as having obtained an equal mixture of these nonequivalent isomers. Similarly revisions are suggested in the interpretation of the observed spectrum of thiirene. In particular, the C-S bond is calculated to be abnormally long and weak, and the corresponding stretching vibration is predicted at unusually low frequency.

Tetra-radical and ionic S1/S0 conical intersections of cyclobutadiene

“…That is, the optimized structure of CBD optimized by CAS (8,8) is comparable to that obtained by CCSD. The structure of CBD optimized by CAS(8,8)/cc-pVDZ (C@C = 1.35 Å and C-C = 1.59 Å) are favorably compared with the previous theoretical calculations by CCSD(T)/cc-pVTZ (C@C = 1.34 Å, C-C = 1.56 Å) [24] and MR-AQCC/SA-4-CASSCF/cc-pVDZ (C@C = 1.36 Å, C-C = 1.57 Å) [15].…”

“…According to more sophisticated methods [12,24], CBD has two dipole-forbidden near-UV transitions that correspond to the 1 1 B 1g state (b 3g to b 2g single-electron excited state) as the S 1 state and the 2 1 A g state (b 3g to b 2g double-electron excited state) as the second singlet excited (S 2 ) state. At the CAS (8,8) level, however, the order of the 1 1 B 1g and 2 1 A g states is inverted. That is, the FC energies of double-and single-electron excited states were calculated as 4.07 and 4.13 eV, respectively.…”

“…To verify the existence of this CI, we determined the S 1 /S 0 energy difference at the MRMP2//CASSCF/cc-pVDZ level, which was estimated to be 10.1 kJ mol À1 (0.1 eV). Hence, even if the dynamical electron correlation is taken into account, the S 1 / S 0 CI certainly exists around the CI tetra by CAS (8,8)/cc-pVDZ. Its structure fits our qualitative consideration, i.e., the orbitals of each of the four carbon atoms show sp 3 -hybridized character.…”

“…For a long time, many theoretical and experimental chemists have scrutinized CBD in comparison with benzene to reveal the nature of aromaticity/ antiaromaticity and localization/delocalization [2,3]. Although the structure of CBD has not been established experimentally, many theoretical and experimental results suggest that it has the rectangular D 2h structure rather than the D 4h square structure in the singlet ground state (S 0 ) [4][5][6][7][8][9][10][11]. The distortion from D 4h to D 2h is due to the second-order Jahn-Teller effect [12].…”

Ground state potential energy surface between cyclobutadiene and tetrahedrane looked down from S1/S0 conical intersections

“…38 In order to reduce this disagreement, eight p electrons in eight p orbitals that correspond to the p system of two acethylenes, i.e., the four p electrons in four p orbitals and four s electrons in four s orbitals were used as the active space [CAS (8,8)]. The result of CASSCF appeared to be improved by adding the s orbital to the active space.…”