Energy Barriers For Internal Rotation Research Articles

Bulky Phosphinines: From a Molecular Design to an Application in Homogeneous Catalysis

The design and preparation of an asymmetrically substituted and bulky phosphinine was achieved by introducing sterically demanding substituents into specific positions of a rigid phosphorus-heterocyclic framework. Compound 5 shows, at the same time, axial chirality and a sufficiently high energy barrier for internal rotation to prevent enantiomerization. Both enantiomers of 5 were isolated by means of chiral analytical HPLC, and their absolute configurations could be assigned by combining experimental data and DFT calculations. Despite its substitution pattern, 5 can still coordinate to transition-metal centers through the lone pair of electrons on the phosphorus atom. Rapid C-H activation on the adjacent aryl substituent at the 2-position of the phosphorus heterocycle was achieved by using [{Cp*IrCl2}2] (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) as a metal precursor. A racemic mixture of 5 was applied as a π-accepting low-coordinate phosphorus ligand in the Rh-catalyzed hydroformylation of trans-2-octene, which showed a clear preference for the formation of 2-methyloctanal.

Dendron‐Containing Tetraphenylethylene Compounds: Dependence of Fluorescence and Photocyclization Reactivity on the Dendron Generation

The synthesis and characterization of four dendron-containing tetraphenylethylenes (TPEs), 1(1)-1(4), were synthesized, along with a TPE compound that contained four OCH(2) Ph groups (referred to as 1(0)) for comparison. Photophysical studies revealed that the TPE core became emissive after linking dendrons onto its periphery. Moreover, the fluorescence intensity was significantly enhanced when high-generation dendrons were linked onto the TPE core; the fluorescence intensity increased in the following order: 1(1)<1(2)<1(3)<1(4). This phenomenon was tentatively attributed to an enhancement in the energy barrier for internal rotation and torsion of the TPE core to which four dendrons were connected. In addition, the photocyclization of the TPE core into the respective 9,10-diphenylphenanthrene was facilitated when high-generation dendrons were linked to the TPE core. Again, the photocycliztion reactivity increased in the following order: 1(1)<1(2)<1(3)<1(4). We found that the fluorescence and photocyclization reactivity of TPE could be modulated by covalent interactions with dendrons, and such modulation was strongly dependent on the dendron-generation.

Ab initio and DFT investigation of fluorinated methyl hydroperoxides: Structures, rotational barriers, and thermochemical properties

Ab initio and density functional theory (DFT) calculations have been performed on CH 2FOOH, CHF 2OOH, CF 3OOH, CF 2ClOOH, and CFCl 2OOH. Geometries of stable conformers are optimized at the MP2(FULL)/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. The enthalpies of formation ( Δ H f , 298 ° ) and the ROO H, RO OH, and R OOH bond dissociation enthalpies (BDEs) are estimated for each of the studied hydroperoxides using the calculated reaction enthalpies ( Δ H rxn , 298 ° ) of the adopted isodesmic reactions. The results show that the progressive fluorine substitution of hydrogen atoms in methyl group results in an increase in each of BDE(O H), BDE(O O), and BDE(C O). This has been explained in terms of the stabilizing influence of fluorine-substituted methyl groups. However, the replacement of F by Cl when going from CF 3OOH to CFCl 2OOH leads to a decrease in both BDE(O O) and BDE(C O). Potential energy barriers for internal rotation about C O and O O are calculated at the B3LYP/6-31G(d,p) level for each of the studied hydroperoxides.

Dielectric relaxation and molecular conformational energy of some arylazo benzothiazine derivatives

We carry out semi-empirical calculations on some arylazo benzothiazine derivatives to obtain the energy barriers for internal rotation and dipole moments, using the PM3 Hamiltonian. By means of the correlation function method and some probabilistic procedures we describe then the dielectric relaxation of these molecules and we try to explain what the contribution of different relaxation modes is in the process of relaxation. Moreover, we account for the consequences of the nonrigidity of the molecules in dielectric response.

Dielectric relaxation and molecular conformational energy of 2,4′-DPE, 3,4′-DPE, 2,3′-DPE and 3,3′-DPE (dipyridyl ethylenes)

Semi-empirical calculations were carried out on four isomers: 2,4′-, 3,4′-, 2,3′-, and 3,3′-dipyridyl ethylenes (DPE). The energy barriers for internal rotation were obtained using the PM3 Hamiltonian. The Goulon-Rivail and Williams theory were used to describe the dielectric relaxation of these molecules. It was found that steric hindrance should be taken into account for complete analyses of the dielectric responses of these molecules.

Conformational preference in methylphenyl sulphoxide and in ortho substituted fluorine derivatives: a theoretical approach

Ab initio MO calculations (3-21G *//3-1G * basis set) have been performed for methylphenyl sulphoxide ( 1), 2-fluoro- ( 2) and 2,6-difluoro- ( 3) derivatives in order to examine the structural properties of their conformational ground state (s). The conformational energy profile for rotation around the C ar. -S bond has been constructed in the STO-3G * basis set and the minima have been located at the 3-21G * level allowing relaxation of the geometrical parameters of the methylsulphinyl group. For methylphenyl sulphoxide one energy minimum is found and corresponds to the SO bond nearly eclipsed with the phenyl ring (twist angle 7.31°). Two energy minima are found for compound 2 and in the more stable conformation the SO bond is coplanar with the ring and anti with respect to the ortho fluorine substituent. In the less stable ground state the lone pair of the sulphur atom settles in the nodal plane of the π-electron cloud of the phenyl ring. This conformation is structurally close to the conformational ground state of compound 3. For compound 1 the properties of the transition state have also been determined at the 3-21G * level. The energy barrier for internal rotation corresponds to the S-C bond of the methyl sulphinyl group eclipsed with one ortho C-H bond. The origin of the conformational ground- and transition-states in these molecules is discussed on the basis of their electronic structure.

Ab initio internal rotation barriers of bifurans

The energy barriers for internal rotation about the interannular single CC bond in bifurans were calculated by means of ab initio MO calculations. The STO-3G and 4-31G basis sets were used. Geometrical optimization of the most significant interannular internal parameters was performed with the STO-3G basis set. The calculated barrier heights, relative to cis conformer energies, follow the sequence 2,3′- > 2,2′- > 3,3′- and have the values of 4.2, 3.7 and 3.3 kcal mol −1, respectively. The same sequence holds for 4-31G calculations but very significant differences are obtained for the 3,3′-bifuran barrier height and for the 2,2′-bifuran relative cis— trans stability. The 4-31G barrier height values were 4.5, 3.4 and 0.8 kcal mol −1, respectively. In all calculations, trans conformers appeared to be more stable than cis except for 2,3′-bifuran, where the opposite is true. Finally, the MO structures of the highest occupied MOs of bifurans are discussed on the basis of the electronic structure of furan.