Taking snapshots of photoexcited molecules in disordered media by using pulsed synchrotron X-rays

Photoexcited molecules are quintessential reactants in photochemistry. Structures of these photoexcited molecules in disordered media in which a majority of photochemical reactions take place remained elusive for decades owing to a lack of suitable X-ray sources, despite their importance in understanding fundamental aspects in photochemistry. As new pulsed X-ray sources become available, short-lived excited-state molecular structures in disordered media can now be captured by using laser-pulse pump, X-ray pulse-probe techniques of third-generation synchrotron sources with time resolutions of 30-100 ps, as demonstrated by examples in this review. These studies provide unprecedented information on structural origins of molecular properties in the excited states. By using other ultrafast X-ray facilities that will be completed in the near future, time-resolution for the excited-state structure measurements should reach the femtosecond time scales, which will make "molecular movies" of bond breaking or formation, and vibrational relaxation, a reality.     

Synthesis and solid-state structures of new cyclophane host molecules

Five new cyclophane host molecules (corrals) are prepared by linking together two α,α′-di(4-hydroxyphenyl)-1,4-diisopropylbenzene or α,α′-di(3,5-dimethyl-4-hydroxyphenyl)-1,4-diisopropylbenzene units with two permethylene spacers. Three small cyclophane hosts (boxes) are synthesized by cyclization of α,α′-di(4-hydroxyphenyl)-1,4-diisopropylbenzene with di(bromomethyl)benzene compounds. Solid-state structures of one corral and one box are reported.     

Homocoenzyme B12 and Bishomocoenzyme B12: Covalent Structural Mimics for Homolyzed,Enzyme‐Bound Coenzyme B12

Efficient electrochemical syntheses of “homocoenzyme B₁₂” ( 2 , Co_β‐(5′‐deoxy‐5′‐adenosyl‐methyl)‐cob(III )alamin) and “bishomocoenzyme B₁₂” ( 3 , Co_β‐[2‐(5′‐deoxy‐5′‐adenosyl)‐ethyl]‐cob(III )alamin) are reported here. These syntheses have provided crystalline samples of 2 and 3 in 94 and 77 % yield, respectively. In addition, in‐depth investigations of the structures of 2 and 3 in solution were carried out and a high‐resolution crystal structure of 2 was obtained. The two homologues of coenzyme B₁₂ ( 2 and 3 ) are suggested to function as covalent structural mimics of the hypothetical enzyme‐bound “activated” (that is, “stretched” or even homolytically cleaved) states of the B₁₂ cofactor. From crude molecular models, the crucial distances from the corrin‐bound cobalt center to the C5′ atom of the (homo)adenosine moieties in 2 and 3 were estimated to be about 3.0 and 4.4 Å, respectively. These values are roughly the same as those found in the two “activated” forms of coenzyme B₁₂ in the crystal structure of glutamate mutase. Indeed, in the crystal structure of 2 , the cobalt center was observed to be at a distance of 2.99 Å from the C5′ atom of the homoadenosine moiety and the latter was found to be present in the unusual syn conformation. In solution, the organometallic moieties of 2 and 3 were shown to be rather flexible and to be considerably more dynamic than the equivalent group in coenzyme B₁₂. The homoadenosine moiety of 2 was indicated to occur in both the syn and the anti conformations.     

Note on the application of the reduced graph model in conjunction with search trees to the enumeration ofKekulé structures

The reduced graph model, when used in conjunction with the search trees method, provides a novel combinatorial procedure for the enumeration and generation ofKekulé structures. The procedure is suited for large benzenoid hydrocarbons consisting of cata- and thin peri-condensed parts.

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Zur Anwendung des Reduced Graph Model im Zusammenhang mit Search Trees zur Ermittlung der Anzahl möglicherKekulé-Strukturen

Zusammenfassung Das Modell erlaubt mit der im Titel genannten Kombination eine neuartige Methode zur Ermittlung und Generierung vonKekulé-Strukturen. Das Verfahren ist für große benzoide Kohlenwasserstoffe geeignet, die aus cata- und (dünnen) peri-kondensierten Teilstrukturen bestehen.

     

Pentaazadienido-Komplexe von Zink,Cadmium und Quecksilber Die Kristallstruktur von [Cd(EtOC6H4-N5-C6H4OEt)2(py)2] und [Hg(tol-N5-tol)2(py)]

Pentaazadienido Complexes of Zinc, Cadmium, and Mercury. The Crystal Structure of [Cd(EtOC₆H₄-N₅-C₆H₄OEt)₂(py)₂] and [Hg(tol-N₅-tol)₂(py)] The pentaazadienido complexes [M(EtOC₆H₄N₅C₆H₄OEt)₂] (M = Zn ( 1 ), Cd ( 2 )) are formed by the reaction of [M(NH₃)₄]²⁺ with [EtOC₆H₄N₅C₆H₄OEt]^? in aqueous ammonia. 2 crystallizes from pyridine as [Cd(EtOC₆H₄N₅C₆H₄OEt)₂py₂] ( 3 ) with the triclinic space group P1 and a = 937.2(2); b = 1422.7(2); c = 2085.5(2) pm; α = 75.28(1)°; β = 94.74(1)°; γ = 99.75(1)°; Z = 2. The central Cd²⁺ ion of 3 exhibits an octahedral coordination by two pyridine ligands in cis arrangement and two (N1, N3)-²⁺ chelating pentaazadienide ions. The reaction of [HgI₄]² with the 1,5-di(tolyl)pentaazadienide anion in aqueous ammonia affords [Hg(p-tol-N₅-tol)₂] ( 4 ), which crystallizes from pyridine in form of [Hg(tol-N₅-tol)₂py] ( 5 ) with the space group P1 and a = 1176.2(4); b = 1203.1(3); c = 1295.6(5) pm; α = 100.77(3)°; β = 110.08(3)°; γ = 94.29(2)°; Z = 2. In 5 the Hg²⁺ cation is threefold coordinated by two monodentate (N3)-η¹ pentaazadienid anions and one pyridine ligand. Within the N₅ chains of the pentaazadienid anions of 3 and 5 localized N? N double bonds are found in the positions N1? N2 and N4? N5 with distances between 125 and 129 pm.     

Dynamik linearer Molekülanionen in den Hydrogensulfiden von Natrium,Kalium und Rubidium: Differential-Scanning-Kalorimetrie,Röntgen- und Neutronenbeugung

Dynamical Behaviour of Linear Molecular Anions in the Hydrogensulfides of Sodium, Potassium and Rubidium: Differential Scanning Calorimetry, X-ray and Neutron Diffraction Hydrogensulfides of the alkali metals M ? Na, K, Rb were prepared in autoclaves by the reaction of the corresponding metals with H₂S and D₂S, respectively, in the temperature range from 50°C to 150°C. Differential scanning calorimetry, X-ray and neutron diffraction methods reveal that both, the HS^?-and DS^?-compounds occur in three crystalline modifications with HT ? high-, MT ? medium- and LT ? low-temperature form: The temperatures and enthalpies for the changes of modifications of the H- and D-compounds are given and the atomic arrangements revealed mainly by neutron diffraction data are discussed, in relation to, for example, size of cations.     

Photochemical synthesis of a palladium dichloromethyl complex, {(hexyl)HC(N-methyl-imidazol-2-yl)2}Pd(CHCl2)Cl.: X-ray molecular structures of {(hexyl)HC(N-methylimidazolyl-2-yl)2}Pd(X)Cl, X=Cl, and CHCl2

The reaction of (hexyl)HC(mim)₂ (1, mim=N-methyl-imidazol-2-yl) with (cod)PdMeCl in C₆H₆ yields {(hexyl)HC(mim)₂}Pd(Me)Cl (3). The photochemical reaction of 3 with CH₂Cl₂ at 23 °C in ambient room light yields {(hexyl)HC(mim)₂}Pd(CHCl₂)Cl (4). It is proposed that this reaction proceeds by homolytic scission of the PdMe bond of 3.     

SinH/SinH-(n=3～8)的结构和电子亲合能的研究

选用四种不同的密度泛函理论方法(B3LYP，B3P86，BLYP，BP86)，在全电子的双ξ加极化加弥散函数基组(DZP )下，对SinH／SinH^-(n=3～8)体系进行研究，获得它们的基态结构和电子亲合能。预测Si3H／Si3H^-,Si4H／Si4H^-,Si5H／Si5H^-,Si6H／Si6H^-，Si7H／Si7H^-和Si8H／Si8H^-的基态结构分别为C2v(^2B2)／C2v(1^A1)氢桥结构，Cs(^2A’)／C(^1A’)，C2v(^2B2)／C2v(^1A1)，C2v(^2B2或^2B1)／C4v(^1A1)，C5v(^2A1)／C5v(^1A1)和C5(^2A‘‘)／C3v(^1A1)。在电子亲合能方面，B3LYP方法预测的电子亲合能是最可靠的，预测Si3H，Si4H，Si5H，Si6H，Si7H和Si8H的电子亲合能分别为2．56，2．59，2．84，2．86，3．19和3．14eV。     

Diborane (4) derivatives via coupling of amine monobromocyanoboranes: Study of the bromination of amine cyanoborane and molecular structures of the amine dibromocyanoboranes

The first examples of diborane (4) compounds derived from amine cyanoboranes are described. A series of monobromo derivatives of amine cyanoboranes (A:BHBrCN), and dibromo derivatives (A:BBr₂CN), 1-7, were prepared. Lithiation of the monobromo derivative of trimethylamine cyanoborane, using n-BuLi, did not produce the C-lithiated intermediate Li⁺ [CH₂NMe₂BHBrCN]⁻, but instead the B-lithiated intermediate Li⁺ [Me₃NBHCN]⁻, was obtained. This intermediate, when allowed to react for 16 h, coupled with the un-lithiated trimethylamine monobromocyanoborane (Me₃NBHBrCN) and resulted in diborane (4) derivative formation as the 2LiBr complex. The same result was obtained when one equiv of the trimethylamine monobromocyanoborane was added to the reaction mixture 1 h after lithiation. Following the same procedure, novel diborane (4) derivatives of amine cyanoboranes were successfully obtained, 8-11, as their 2LiBr complexes from the monobromo derivatives of the corresponding amine cyanoboranes. Molecular structures of the trimethylamine dibromocyanoborane, 6, and the triethylamine dibromocyanoborane, 7, were determined using X-ray crystallography.     

C_(59)F_(2n)HN(n=1,2)异构体的结构与稳定性的理论研究

分别用MNDO和AM1两种半经验方法,对C59F2nHN (n = 1, 2) 的异构体进行几何构型全优化,结合频率分析及HF/6-31G单点能计算,确定了各异构体的基态结构及其相对稳定性。计算结果表明,C59HN的F加成物的立体选择性规律与C60的不同,最稳定异构体不是1-2加成物。C59F2HN的最稳定异构体是1-4加成的6, 18-或12, 15-异构体; C59F4HN的最稳定异构体是1-4,1-4加成的6, 18; 12, 15-异构体,其能量远小于其它各异构体的能量。N原子取代碳笼骨架C原子后,改变了碳笼F加成物的立体选择性规律。