Ab initio Infrared Spectra and Inversion Barriers of Dibenzo [a,c]cycloocta-5,8-diones

Abstract

Ab initio calculations are carried out for stereo-specific

inversions of 6XX´-7YY´-dibenzo[a,c]cyclooctadiene-5,8-

dione, 1: X=X´=Y=Y´=H; 2: X=X´=Y=Y´=CH₃; 3: X=Y=

H, X´=Y´=CH₃; 4_cis: Y´=H, X´=Y=CH₃; 4_trans : X´=CH₃,

Y´=Y=H; 5_cis: X=Y´=H, X´=Y= i-pr and 5_trans: X=X´= i-pr,

Y´=Y=H. Inversion energy barriers of 1–5, to their corresponding

conformers 1'–5', are determined via ab initio calculations.

These are preformed at HF/6-31G* and MP2/6-31+G*

levels of theory, using Gaussian 98 system of programs. The

order of energy barriers/(kcal/mol) found for the inversions

is: 3⇌3' (34.00) > 2⇌2' (28.05) > 5_cis⇌5'_cis (27.21) >

5_trans⇌5'_trans (24.75) > 4_cis⇌4'_cis (22.35) > 4_trans⇌ 4'_trans

(21.62) > 1⇌1' (19.47). Enantiomeric conformers 4_cis and

4´_cis have the same energy. Similarly, 5_cis and 5´_cis, which are

mirror images, have the same energy. Diastereomeric conformers

4_trans and 4´_trans are at different energies. So are 5trans

and 5´_trans. Important factor that cause to observing two absorbance

for each of cis and an absorbance for each of 4_trans

and 4´_trans; 5_trans and 5´_trans is relative position of carbonyl group

and closer benzene ring.