Grace Shiahuy Chen

Push–pull substitution versus intrinsic or packing related N–N gauche preferences in azines. Synthesis, crystal structures and packing of asymmetrical acetophenone azines

The asymmetrical E,E-configured para-disubstituted 4-methoxyacetophenone azines with 4′-bromo-(1), 4′cyano-(2) and 4′-nitro-(3) substituents have been synthesized and their crystal structures have been determined. The synergetic reinforcement of ‘push’ and ‘pull’ effects does not suffice to overcome the intrinsic or packing induced N–N gauche preference, and azines 1–3 all assume distinctly gauche N–N conformations (dihedral angle of 115–140°). The crystal packing is characterized by offset T-shaped and parallel displaced face-to-face arene-arene contacts between pairs of azines with parallel or anti-parallel D→A orientations. This crystal architecture results in a net dipole of crystals of 1. The structural data are analysed in comparison with the symmetrical E,E-configured para-disubstituted acetophenone azines 4–7. With regard to this best possible reference data set, the structural parameters of 1–3 do not show any significant manifestation of special electronic interactions over the N-perturbed extended π-system to be associated with the asymmetry of the azines. While there exists no structural evidence of conjugation in these solid state structures, our results do not rule out asymmetry effects on the electronic structure of the ground or the excited states.

Asymmetrization Effects on Structures and Populations of the Ground State of Dipolar Donor)Acceptor-Substituted Molecular Organic NLO Materials

Asymmetric donor–acceptor‐substituted π‐conjugated systems with low dipole moments and structural components that favor parallel alignment of neighboring molecules are potential molecular organic materials with nonlinear optical properties for which near prefect dipole parallel alignment is possible. The asymmetrical 4‐methoxysubstituted acetophenone azines with the substituents fluorine (1), chlorine (2), bromine (3), cyano (4), and nitro (5) in the 4′‐position have been studied in this context, and for 2 and 3 the dipole parallel alignment has indeed been accomplished in crystals of the pure material. In the present study, the effects of asymmetrization on the structure and the electronic structures of the ground states of 1–5 have been explored at the RHF/6‐31G* level. The properties of the optimized structures of the azines 1–5, a comparative analysis of asymmetrical and symmetrical azines, and natural population analyses, all show no significant evidence for conjugation over the azine bridge. The concept of azine spacers as “conjugation stoppers” is useful and the dipole moments of asymmetrical azines are relatively small. The analysis suggests that the charge transfer between the donor and acceptor contributes to the dipole moment much less than is generally assumed. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1130–1140, 1998

A theoretical analysis of 2,3 sigmatropic shifts in allylic sulfilimines and sulfoximines

Abstract Ab initio calculations at the level MP4(SDTQ,full)/6-31G*//MP2(full)/6-31G* + AVZPE(MP2(full)/6-31G*) support the experimental observation that allylic sulfoximines do not undergo 2,3 sigmatropic shifts. This process is hindered by a high kinetic barrier although the reaction is strongly exothermic.

Comparative analysis of crystal structures of E,E-configured para-substituted acetophenone azines with halogen, oxygen, nitrogen and carbon functional groups

A comparative analysis is presented of the solid state structures of fifteen E,E-configured para-substituted acetophenone azines with halogen [–F (1a), –Cl (1b), –Br (1c)], oxygen [–OMe (2, 5 and 6), –OH (3), –OCOEt (3)], nitrogen [–NMe2(7), –NH2(8), –NHCOMe (9), –NO2(10)] and carbon [–Me (11), –CO2Et (12), –CN (13)] functional groups. The X-ray crystal structures of 2, 7–10 and 12 were determined and are reported. The data allow us systematically to examine the structural effects of the nature of the para-substituent in a series of closely related azines and to assess and distinguish between intrinsic electronic and steric effects and consequences of crystal packing. Stereoelectronic effects of the para-substituents are discussed in terms of contributions of various resonance forms and structural parameters are identified that may serve as indicators of their importance. Analyses are presented of conformational properties and of crucial bond lengths of the azines 1–13. The molecules also are analysed as para-disubstituted benzenes X–C6H4-Az and compared with X–C6H4-Z systems qualitatively to rank the electron-withdrawing ability of the Az group.

Effects of electron correlation and spin projection on rotational barriers of trithiocarbenium ion [C(SH)3]+ and Radical Dication [C(SH)3]?,2+

Theoretical level dependencies are discussed of relative isomer stabilities and rotational barriers of trithiomethyl cation [C(SH)3]+ (a) and of radical dication [C(SH3)]⋅,2+ (b). Spin polarization and dynamic electron correlation are very important for the radical dictation. Removal of an electron from one of the degenerate π‐HOMOs of C3h symmetric [C(SH)3]+ stabilizes the remaining π electron to such an extent that the unpaired electron is not in the HOMO of the dictation. The radial π MOs “diving below the Fermi level” facilitates strong spin polarization because of its energetic proximity to σ MOs. Projection of the first three higher spin states eliminates spin contaminations, and the terms E(PUHF(s+3))‐E(UHF) and E(PMP4(s+3))‐E(MP4) are discussed. The combination of annihilation of spin contamination and electron correlation is essential for the determination of relative energies and rotational barriers of the radical dication. The results obtained at this level match the results of high level QCISD(T) calculations in a near‐quantitative fashion. Perturbation theory alone does not correct for spin contamination even if it is carried to full fourth order and includes triple excitations; the E(PMP4(s+3))‐E(MP4) values are all negative and can exceed 5 kcal/mol in magnitude. Previous studies showed that annihilation of spin contaminations is important in regions of potential energy surfaces where σ bonds are broken (homolytic dissociation), formed (radical addition), or both (H abstraction by radical). Our findings stress that the annihilation of spin contaminations can be just as important for any process that greatly alters spin polarization and even if that process proceeds without breaking or forming of σ bonds. For comparison, density functional theory also was employed in the potential energy surface analyses. The results obtained with the B3LYP formalism were found to be less susceptible to spin contamination and resulted in rather good agreement with the best pertubation and configuration interaction results.

DIPOLE MOMENTS OF THE NONLINEAR OPTICAL MATERIALS NPO AND POM

Solution-phase measurements and ab initio quantum-mechanical calculations (MP2/6-311G**//MP2/6-31G*) of the dipole moments of 4-nitropyridine N-oxide (µexptl= 0.83 ± 0.04 D, µcalc= 0.97 D) and 3-methyl-4-nitropyridine N-oxide (µexptl= 0.69 ± 0.05 D, µcalc= 0.89 D) show that the electronic effects of methyl substitution are localized and much smaller (Δµ < 0.2 D) than had previously been thought.

β,β-Dichlorovinyldiazonium or dichloro(diazomethyl)carbenium ion? Crystal structure and electron density distribution of β,β-dichlorovinyldiazonium hexachloroantimonate

The crystal structure of β,β-dichlorovinyldiazonium hexachloroantimonate, 2·SbCl6, and the topological electron density analysis of 2 provide compelling evidence for the importance of chlorine-stabilized carbenium ion resonance forms for the stabilization of vinyldiazonium ions by β-substitution.

Studies on Quinazolines. 12.^1 Design of 4-Amino-8-arylquinazoline Derivatives as Potential Non-Peptide Corticotropin-Releasing Hormone Receptor I (CRHR1) Antagonists

Four 8-aryl-4-(N-cyclopropylmethyl-N-propyl)amino-2-methylquinazolines were synthesized, and their binding affinity for corticotropin-releasing hormone type 1 receptor (CRHR1) was investigated. Compounds 22 and 23 possessed high rCRHR1 affinities of K(subscript i)=13 and 50 nM, respectively. The quinazoline derivatives showed parallel SAR to the other known bicyclic system; the ortho-substituent on the 8-aryl ring is indispensable.

Seleno- and Tellurocarbenium Ions

Abstract: α-Chalcogeno substituted carbenium salts [(RX)3C]+ PF6-(X = S, Se, Te; R = 2,4,6-iPr3C6H2) obtained from reaction of the copper complexes [(bipy)CuXR] with CBr4 are stabilized to the same extent; but the electronic mechanism is very different.