Search

Several spectroscopic methods were used in studying the conformations of tetrakis(trifluoromethyl)diphosphane and -diarsane to determine the gauche effect. The first complex exists as trans conformer in the solid and as a mixture of about 70% trans and 30% gauche in the liquid and gaseous states based on its vibrational spectra. It has a trans-to-gauche ratio of 90:10 while the trans conformer of tetrakis(trifluoromethyl)diarsane has been detected in all phases.

The X-ray crystallographic structures of crystalline fluorenylidenecycloproparenes 7 and 8 and of dibenzocycloheptatrienylidene 9 were investigated. Theoretical studies were used to analyze the structure, dipole moment, charge distribution and thermodynamic stability of the unidentified methylidenecycloopropabenzene 3, the derived parent tria-, penta- and heptafulvalene derivatives 4-6 and the crystalline derivatives 7,8 and 9. Results showed that hydrocarbons are polar and that the cycloproparenylidene moiety functions as electron donor.

1H-Cyclopropa[b]naphthalene is converted into novel polar olefins 15-19 via reaction of the 1,1-disilyl with anthrone-like ketones. The alkenes so formed are crystalline, polar compounds that range in color from yellow to magenta. Solutions of the 10,10-dimethylanthrylidene 15, the xanthenylidene 16, and N-methylacrydinylidene 18 fluoresce; parent anthrone 19 likely phosphoresces. X-ray crystal structure determinations are reported for 15, 18, and the thioxanthenylidene 17. Theoretical studies using ab initio calculations at the HF/6-31G level have been performed to provide assessments of the structures, charge distributions, and dipole moments of 15-19.

The semiempirical PM3 method, calibrated against ab initio HF/6-31+G(d) theory, has been used to elucidate the reaction of 1,2-dichloroethane (DCE) with the carboxylate of Asp-124 at the active site of haloalkane dehalogenase of Xanthobacter autothropicus. Asp-124 and 13 other amino acid side chains that make up the active site cavity (Glu-56, Trp-125, Phe-128, Phe-172, Trp-175, Leu-179, Val-219, Phe-222, Pro-223, Val-226, Leu-262, Leu-263, and His-289) were included in the calculations. The three most significant observations of the present study are that: (i) the DCE substrate and Asp-124 carboxylate, in the reactive ES complex, are present as an ion-molecule complex with a structure similar to that seen in the gas-phase reaction of Ac[O.sup.-] with DCE; (ii) the structures of the transition states in the gas-phase and enzymatic reaction are much the same where the structure formed at the active site is somewhat exploded; and (iii) the enthalpies in going from ground states to transition states in the enzymatic and gas-phase reactions differ by only a couple kcal/mol. The dehalogenase derives its catalytic power from: (i) bringing the electrophile and nucleophile together in a low-dielectric environment in an orientation that allows the reaction to occur without much structural reorganization; (ii) desolvation; and (iii) stabilizing the leaving chloride anion by Trp-125 and Trp-175 through hydrogen bonding.

The [S.sub.N]2 displacements of chloride ion from C[H.sub.3]Cl, [C.sub.2][H.sub.5]Cl, and [C.sub.2][H.sub.4][Cl.sub.2] by acetate and hydroxide ions have been investigated, using ab initio molecular orbital theory at the HF/6-31+G(d), MP2/6-31+G(d), and MP4/6-31+G(d) levels of theory. The central barriers (calculated from the initial ion-molecule complex) of the reactions, the differences of the overall reaction energies, and the geometries of the transition states are compared. Essential stereochemical changes before and after the displacement reactions are described for selected cases. The gas phase reactions of hydroxide with C[H.sub.3]Cl, [C.sub.2][H.sub.5]Cl, and [C.sub.2][H.sub.4][Cl.sub.2] have no overall barrier, but there is a small overall barrier for the reactions of acetate with C[H.sub.3]Cl, [C.sub.2][H.sub.5]Cl, and [C.sub.2][H.sub.4][Cl.sub.2]. A self-consistent reaction field solvation model was used to examine the [S.sub.N]2 reactions between methyl chloride and hydroxide ion and between 1,2-dichloroethane and acetate in solution. As expected, the reactions in polar solvent have a large barrier. However, the transition state structures determined by ab initio calculations change only slightly in the presence of a highly polar solvent as compared with the gas phase. We also calibrated the PM3 method for future study of an enzymatic [S.sub.N]2 displacement of halogen.

The dependence of hydrogen-bond strength on the hydrogen bonding distance and hydrogen bond angles of intermolecular hydrogen bonds were analyzed by utilizing ab initio computational methods. The hydrogen bonding distance of the amide hydrogen bonds were 2.1 amstrong with minimum steric overlap due to low interactions between the two formamides. The hydrogen bond between two N-methylacetamide exhibits decreasing constant interaction energy compared to the hydrogen bond of the intermollecular formamide.

X-ray structure determinations of tetrakis(trifluoromethyl)diphosphane (2c, mp -82 degrees C, triclinic, P&onemacr;; Z = 1, a = 529.7(3) pm, b = 681.6(2) pm, c = 802.8(3) pm, alpha = 108.58(1) degrees, beta = 99.66(1) degrees, gamma = 103.29(1) degrees, wR2 = 0.204) and -diarsane (3c, mp -52 degrees C, monoclinic, P2(1)/c; Z = 2, a = 769.5(3) pm, b = 750.0(3) pm, c = 960.7(2) pm, beta = 105.26(1) degrees, wR2 = 0.115), both at -100(3) degrees C, reveal the molecules to adopt the trans conformation in the solid. Compared with the tetramethyl derivatives, the E-E (224.6(2)/246.3(1) pm, E = P, As) and E-C (188.3(4)/201.3(7) pm) bonds are elongated by 4.5/4.8 pm and 3.4/3.4 pm, respectively. From gas electron diffraction studies of diphosphane 2c a mixture of 85(10)% trans and 15(10)% gauche conformers can be deduced; diarsane 3c shows the trans form exclusively. The molecular parameters (E-E, 224.8(11)/245.2(6); E-C, 189.6(4)/ 201.2(4) pm) agree excellently with those determined for the crystalline state. As a result of quantum chemical calculations at Hartree-Fock and hybrid density functional levels of theory using 6-311+G basis sets, the gauche conformer of hydrazine derivative 1c and the trans conformer of diarsane 3c are clearly lowest in energy. However, for diphosphane 2c the gauche and not the trans form is found to be slightly more stable. Variations of calculated E-E and E-C bond lengths are analyzed and compared with corresponding values of the parent compounds E(2)H(4) (1a to 3a) as well as the tetramethyl derivatives 1b to 3b.