AM1 calculations of rotation around essential single bonds and preferred conformations in conjugated molecules

Abstract

Equilibrium twist angles, rotational barriers around essential single bonds, and preferred conformations for over 60 conjugated organic molecules were calculated using the semiempirical AM1 (Austin model 1) method. Comparison with ab initio and experimental data shows that AM1 can be applied quite successfully to conformational problems of this type. Relatively large errors are, however, found for compounds in which lone pair-hydrogen and especially lone pair-lone pair interactions are decisive for their conformational behavior. AM1-calculated rotational barriers in conjugated molecules, however, are found to be much too low. Moreover, AM1 does not seem to even correctly reproduce the trend of rotational barriers within a series of structurally related compounds. E / Z energy differences obtained by AM1 are also frequently considerably too low. In contrast to rotational barriers, their trends and thus conformational preferences, however, are quite satisfactorily calculated by this method. Ionization potentials obtained by AM1 are too high by about 0.5 eV. However, trends are quite well predicted.

Molecular Design of Novel Conjugated Polymers for Blue-Light-Emitting Devices

“…The AM1 method has been successfully used to investigate geometrical structures and conformations of large molecules in particular, although the method yields low rotational barriers. 25 It also has produced excellent agreement with the experimental observation in the case of polymers. 16 For the geometrical optimizations, we chose 6 wave vectors with a regular interval from 0 to π /a (where a is a translation vector), and imposed the C2v constraint on the monomer units.…”

Electronic effects of heterocyclic substituents. Spectroscopical and theoretical (AM1) study in a series of heterocyclic carboxaldehydes

“…AM 1 gives the S-0 trans conformation as 4.8 k.J mol-' more stable than the corresponding S-0 cis conformation, whereas ab initio 3-2 1G//3-21G calculations (13) mol-'. This result, which can be logically extended to the corresponding benzo[b] derivative 3, can be accounted for by the overestimation of lone pair -lone pair repulsions that have already been noticed with the AM1 method (15).…”

“…On the other hand, six-membered heterocycles show downfield shifts (positive values of AS) in the case of 2-and 4-substitution (13,15,16,18), whereas 3-substituted pyridine and quinoline carboxaLdehydes (14,17) show negative values of AS. These results are also in accordance with the r-deficient character of pyridines, which leads to an electron-withdrawing character at positions 2 and 4.…”

“…Furans and pyrroles (5)(6)(7)(8)(9)(10)(11)(12) show the most marked differences in shielding when comparing positions 2 and 3. Thiophenes and pyridines (1)(2)(3)(4)(13)(14)(15)(16)(17)(18) show comparatively fewer important differences in this respect.…”

Internal Rotation of Ester Linkage in Phenyl Benzoate and Hydroxybenzoic Acid Dimer as Models of Aromatic Polyesters Using Density Functional Theory

“…Fabian pointed out that the rotational barrier of various conjugated molecules calculated by the AM1 method were about 50% lower than the ab initio results and experimental data. [9] S bond The internal rotation energy profile for S bond (see Figure 5) is the most important one because it determines the linearity of aromatic polyesters. HF/6-31G*, HF/STO-3G [11] , and MP2/6-31G* (d 0 ) [10] calculations failed to give a stable cis-conformer, while B3LYP/6-31G* gave a cis-conformer (cis-form) as a local minimum structure.…”