The semiempirical PM3 method, calibrated against ab initio HF͞6-31؉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 AcO ؊ with DCE; (ii) the structures of the transition states in the gasphase 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 kcal͞mol. 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.

A single-crystal of ammonia, NH3, was grown in a thin-walled capillary at 178 K, and high-resolution X-ray diffraction data were obtained for this compound at 160 K in order to obtain information about electron density distribution. Conventional and multipole refinements and deformation electron density maps indicated small but significant N−H bond bending inside the NH3 tetrahedron that is in agreement with ab initio quantum-chemical calculations and the VSEPR model. Topological analysis of the experimental charge density distribution in the ammonia molecule has been performed, and the data are compared with high-level quantum-chemical calculations. Some features of the intermolecular hydrogen bonds in the crystal are discussed.

Stable imidazol-2-ylidene-borane adducts 8 and 9 are obtained from 1,3,4,5-tetraalkylimidazol-2-ylidenes 7 and MezS . BH3 or EtzO . BF3. An X-ray structure analysis of 8 b has beenperformed. An a b initio calculation [MP2(fu)/6-31G**] reveals a low energy difference between two rotamers. The bond orders of the cyclic bonds imply only a minor TC interaction between the N-C -N and the C-C fragment. Addukte des Borans mit elektroneutralen Kohlenstoff-Basen sind selten. Den schon langer bekannten Verbindungen 1-3[2-41 konnten wir kurzlich den Komplex eines ,,ylidischen" Olefins 4 zur Seite stellenL5]. Carbene weisen als Liganden in Komplexverbindungen fur gewohnlich Akzeptoreigenschaften aufL6] und sind deshalb meist schlechte Koordinationspartner fur elektronenarme Zentren; dies macht den bislang fehlenden Nachweis der Existenz von Carben-Boranen begreiflich. Die wegweisenden Arbeiten von Wanzlick['] uber nucleophile Carbene haben mit der Isolierung eines stabilen Irnidaz~l-Zylidens[~~ durch Arduengo eine ungeahnte Aktualisierung erfahrenL9]. Die mittlerweile bequem und mit betrachtlicher Variationsbreite der Substituenten['""] zugangliche Substanzklasse hat den seit langem bekannten Metallkomplexen['21 in kiirzester Zeit eine Reihe von Addukten 5 und 6 beigefugt (X = Se['], Te[13], CH2['], CS2[14], IC,F5['51). Der kurzlich bekannt gewordenen Koordination von 1,3-Dimesitylimidazol-2-yliden an das A1H3-FragmentL'61 wollen wir die Boran-Addukte 8 gegeniiberstellen. Synthese und Eigenschaften der Carben-Borane 8a-c und 9Der nucleophile Charakter der Imidazol-Zylidene 7 eriibrigt fur diese Carbene im Falle der Addukt-Bildung die in den Metall-Komplexen['21 bekannte ,,klassische" Ruckbindung und ermoglicht somit die Anbindung auch elektronenarmer Fragmente. Tatsachlich sind durch Umsetzung

~ ~~ ~~The molecular and solid-state structure of borazine has been determined at 115 and 160 K by single-crystal X-ray crystallography. The Borazine is generally introduced in textbooks as "inorganic benzene" under aspects of the isoelectronic relationship. This holds only for some physical and chemical properties. The prominent discrepancies in the reactivity are attributed to the significant difference between the electronegativities of boron and nitrogen atoms. Among the physical properties the density and boiling point are quite similar. However, the melting points differ by approximately 60°C. This suggests that the solid state of borazine must be quite different from that of benzene.The crystal structures under various conditions of benzene are known for a long time"], all of them exhibit the well-known herringbone pattern in the space groups Pnma and P 2 , k A crystal structure determination of borazine has never been performed, obviously due to crystallization problems.Moreover, even the molecular structures of both compounds are still debated. The vast amount of experimental data of benzene does not allow an unambiguous assignment of the symmetry of the equilibrium However, the results from quantum-mechanical calculations favor DhhThe molecular structure of borazine has been determined by gas-phase electron diffractionL4I. In the latest electron diffraction study by Harshbarger et al.[4c] a planar D3h and two nonplanar (C2 and C3,) models were discussed. The C3, model was excluded because of inconsistency with a microwave investigation. A decision between a D3h model with large vibrational amplitudes perpendicular to the ring plane and a nonplanar C, model was not possible. Thus, distinct assignments of the molecular geometry and symmetry by diffraction methods have not yet been made.After many attempts we succeeded in growing a cylindrical single crystal by means of a miniature zone-melting procedure in a capillary directly on the diffractometer applying an infrared heatThe structure refinements in the tetragonal space group P43212 from the diffraction data at 160 and 115 K give no evidence for solid-state phase transitions; the molecules behave like rigid bodies in the given temperature interval. Results and DiscussionBorazine has crystallographic C2 symmetry, the molecule deviates slightly from the expected D3h symmetry ( The bond angles in the six-membered ring are significantly different, 117.1(1)' at the boron atoms and 122.9(1)" at the nitrogen atoms (mean values). Similar findings have been mentioned for borazine derivati~es[6"~~1, but possible substituent effects or the accuracy of the results did not allow an unambiguous statement.The phenomenon of the alternating angles in the sixmembered ring can be understood by Bent's rule considering a rehybridization of the ring The more electronegative nitrogen atoms prefer hybrid orbitals with lower s character. Therefore, the increased p character at the boron atoms results in decreasing bond angles, whereas the opposite behavior is observed at the ...

Noticeable deformations of the benzene framework caused by annelation with strained rings are observed in the crystallographic studies on 1–4. These are the most strained benzene derivatives yet known. The angles at the sp2 carbon atoms in 1 and 3 indicated by an arrowhead are the largest known: 176.9 and 174.9°, respectively. Molecules 2 and 4 display a slight bond length alternation in the arene moiety (0.010–0.023 Å).