All-trans- and t,T,t,C,t,T,t-deca-1,3,5,7,9-pentaenes: Ab initio structures, vibrational analyses, and some regularities in the series of related molecules

Total geometry optimization and calculation of the force constants for all- trans- and t,T,t,C,t,T,t-deca-1,3,5,7,9-pentaene were carried out at the ab initio, HF/6-31G level. The HF/6-31G//HF/6-31G force fields were modified using empirical scale factors transferred from trans-buta-1,3-diene augment ed by an additional scale factor for the central formal carbon-carbon doubl e bond coordinates (determined previously for all-trans-hexa-1,3,5-triene). The total number of scale factors was seven. The vibrational problems for both decapentaenes were solved using the respective scaled HF/6-31G//HF/6-3 1G force field. Infrared intensities and Raman activities were calculated f rom the unsealed HF/6-31G//HF/6-31G force fields. Complete assignment of al l the fundamental vibrational frequencies is given. Geometrical parameters, vibrational frequencies and force constants are compared with the correspo nding values of buta-1,3-diene, hexa-1,3,5-triene and octa-1,3,5,7-tetraene . Regularities in the properties of this molecular series are discussed. Sp ecial attention is given to the possibility of using the vibrational spectr a for detection of distortions from the regular tuans structure of these ol igoenes.