Joel L. Wolk

The performance of density function theory in describing gas-phase SN2 reactions at saturated nitrogen

Abstract The prototype gas-phase identity S N 2 reactions at neutral nitrogen X − +NH 2 X→NH 2 X+X − (where X=F, Cl, Br, I) were studied using the three hybrid Hartree–Fock DFT methods B3LYP, MPW1PW and MPW1K in conjunction with the 6-31+G(d,p) basis sets. Comparison of the results with the high-level G2(+) theory indicated that all of the three hybrid DFT methods can give reasonable values of the complexation energies (Δ H comp ) for the ion–molecule complex formed in the reaction X − +NH 2 X (X=Cl, Br, I). The overall barriers (Δ H ovr ≠ ) and central barriers (Δ H cent ≠ ) for all of the reactions calculated using B3LYP functional are significantly underestimated. The best transition structures were obtained using MPW1K/6-31+G(d,p) level, which appears to exhibit the best performance in describing the potential energy surface for S N 2 reactions at neutral nitrogen. Some correlations of central barriers found at the G2(+) level of theory are reproduced by the results of three hybrid DFT methods for the reactions X − +NH 2 X→NH 2 X+X − (X=Cl, Br, I).

A G2(+) level investigation of the gas-phase identity nucleophilic substitution at neutral oxygen

Abstract G2(+) level molecular orbital calculations have been carried out for the identity nucleophilic substitution at saturated oxygen, X−+HOX→HOX+X− (X=F, Cl, Br, I). A comparison with data for the analogous reactions at saturated nitrogen, X−+NH2X→NH2X+X−, and at saturated carbon, X−+CH3X→CH3X+X−, indicate that the substitution reaction at saturated oxygen proceeds via a classic SN2 pathway. The calculated intrinsic barriers ΔHcent≠ for substitution at oxygen are found to be much higher than the corresponding barriers for substitution at carbon and nitrogen, decreasing in the order F (106.3 kJ / mol )> Cl (92.5 kJ / mol )> Br (70.3 kJ / mol )> I (58.6 kJ / mol ) . Stabilization energies of the ion–molecule complexes decrease in the order F (187.9 kJ/mol )> Cl (97.5 kJ/mol )> Br (81.2 kJ / mol )> I (66.5 kJ / mol ) , that are also significantly higher than the corresponding values at carbon and nitrogen, and correlate well with the halogen electronegativities. The overall barriers relative to the reactants (ΔHovr≠) are negative for all halogens F (−81.7 kJ / mol ) , Cl (−5.1 kJ / mol ) , Br (−10.7 kJ / mol ) , I (−8.1 kJ / mol ) . These trend is similar to that for the analogous reaction at nitrogen, but contrasts to that for the reactions at carbon where the ΔHovr≠ are negative only for X=F.

Synthesis and field bioassay of some analogs of sex pheromone of citrus mealybug,Planococcus citri (Risso).

A series of structural analogs of (s+)-cis-(1R)-3-isopropenyl-2,2-dimethylcyclobutanemethanol acetate, sex pheromone of the citrus mealybug,Planococcus citri (Risso), was synthesized. The analogs were tested in a field bioassay in order to determine the structure-activity relationships of the pheromone. All changes in structure reduced the activity of the test compounds, to various degrees. The most active analog tested was the homolog (+)-cis-(1R)-3-isopropenyl-2,2-dimethylcyclobutaneethanol acetate (IV), whose activity, at a higher dosage, was comparable to that of the pheromone. The alcohol (+)-cis-(1R)-3-isopropenyl-2,2-dimethylcyclobutanemethanol was tested in mixtures with the pheromone and found to be neither an inhibitor nor a Synergist. The results show that all functional groups of the pheromone molecule are essential for optimal biological activity.

Preparation of Tyrian Purple (6,6'-Dibromoindigo): Past and Present

Over the past century, various synthetic approaches have been suggested to the most famous dye of antiquity, Tyrian purple (6,6′-dibromoindigo). These synthetic routes have been exhaustively surveyed and critically evaluated from the perspective of convenience, cost, safety and yield.

A Simple, Safe and Efficient Synthesis of Tyrian Purple (6,6'-Dibromoindigo)

6,6’-Dibromoindigo is a major component of the historic pigment Tyrian purple, arguably the most famous dye of antiquity. Over the last century, chemists have been interested in developing practical syntheses of the compound We describe herein a new, reasonably simple and efficient synthesis of Tyrian purple which opens the way to the production of large quantities of the dye with minimal hazards and at low cost.

Stabilization of carbenium ions by an α-azido group

Abstract The ability of the azido group to stabilize an α carbenium ion was calculated to be intermediate between that OH and NH 2 . The barrier for the decomposition of the title compound into N 2 and H 2 CN + is 4.6kcal/mol.

Relative reactivity of three and four membered rings ? the absence of charge effectElectronic supplementary information (ESI) available: Gaussian archive file. See http://www.rsc.org/suppdata/ob/b3/b314869f/

Radical (neutral) and electrophilic (cationic) ring opening reactions were studied computationally in order to probe the difference in reactivity between three and four membered rings. Using the Marcus equation we have shown that the activation energy for the four membered ring opening is close to the Marcus predicted barrier whereas three membered rings display much higher reactivity than that predicted by the Marcus equation. Thus, the reactivity of the three membered rings is enhanced, in addition to the strain release, by another factor which is not operative in the four membered rings. It is clear also that this factor is not charge dependent. The possible origin of this effect is discussed.