Explicitly correlated benchmark calculations on C8H8 isomer energy separations: how accurate are DFT,double-hybrid,and composite ab initio procedures?

Accurate isomerization energies are obtained for a set of 45 C₈H₈ isomers by means of the high-level, ab initio W1-F12 thermochemical protocol. The 45 isomers involve a range of hydrocarbon functional groups, including (linear and cyclic) polyacetylene, polyyne, and cumulene moieties, as well as aromatic, anti-aromatic, and highly-strained rings. Performance of a variety of DFT functionals for the isomerization energies is evaluated. This proves to be a challenging test: only six of the 56 tested functionals attain root mean square deviations (RMSDs) below 3?kcal?mol^?1 (the performance of MP2), namely: 2.9 (B972-D), 2.8 (PW6B95), 2.7 (B3PW91-D), 2.2 (PWPB95-D3), 2.1 (ωB97X-D), and 1.2 (DSD-PBEP86) kcal?mol^?1. Isomers involving highly-strained fused rings or long cumulenic chains provide a ‘torture test’ for most functionals. Finally, we evaluate the performance of composite procedures (e.g. G4, G4(MP2), CBS-QB3, and CBS-APNO), as well as that of standard ab initio procedures (e.g. MP2, SCS-MP2, MP4, CCSD, and SCS-CCSD). Both connected triples and post-MP4 singles and doubles are important for accurate results. SCS-MP2 actually outperforms MP4(SDQ) for this problem, while SCS-MP3 yields similar performance as CCSD and slightly bests MP4. All the tested empirical composite procedures show excellent performance with RMSDs below 1?kcal?mol^?1.     

A computational study of photochemical isomerization reactions of thiophenes

The mechanisms of the photochemical isomerization reactions were investigated theoretically using three model systems; 2‐methylthiophene, 2‐cyanothiophene, and 2‐phenylthiophene. The CASSCF (10‐electron/eight‐orbital active space) and MP2‐CAS methods were employed with the 6‐311(d) basis set. Three mechanisms, i.e., the internal cyclization‐isomerization route (path A), the zwitterion‐tricyclic route (path B), and the direct route (path C), have been used to explore the real photochemical reaction mechanism of these three model molecules. The structures of the conical intersections, which play a key role in such phototranspositions, were obtained. The intermediates and transition structures of the ground states were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route is as follows: reactant → Franck‐Condon region → conical intersection → photoproduct. In particular, the conical intersection mechanism described in this work gives a better explanation than either the previously proposed internal cyclization‐isomerization (path A) or the zwitterion‐tricyclic pathway (path B) mechanisms, and is supported by the experimental observations. The results obtained allow a number of predictions to be made. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

水合氧化锆的晶化方式对Pt/WO3-ZrO2异构化活性的影响

 采用不同方法制备了一系列具有一定晶相结构的水合 ZrO2, 考察了制备方法对水合 ZrO2 晶化方式及 Pt/WO3-ZrO2 催化剂催化正己烷异构化活性的影响. 采用 X射线衍射, Raman 光谱和 NH3-程序升温脱附对催化剂进行了表征. 结果表明, 水合 ZrO2 的晶化方式与制备时氧气的存在与否有关, 也大大影响了催化剂的异构化活性. 以无氧条件下制备的水合 ZrO2 为载体时, Pt/WO3-ZrO2 催化剂具有较高的异构化活性, 而以在空气气氛中制得的水合 ZrO2 为载体时, Pt/WO3-ZrO2 几乎无异构化活性.     

Structure and dissociation of cyanogen halides BrCN and ICN

Structural properties and dissociation of cyanogen bromide BrCN, cyanogen iodide ICN, and their isomers have been studied in detail using ab initio MP2, CCSD, and CCSD(T) methods. After dissociation of BrCN and ICN, the fragmented atoms have been considered to be either in their ground state or in their valence excited states. The recent experimental value for the strength of the CN bond of CN radical has been compared with our value calculated from the dissociation of BrCN. Interesting results have been obtained when BrCN and ICN dissociated to atoms that are in their valence excited states. The structural properties of BrCN and ICN and dissociation energy of BrCN agree very well with the theoretical and experimental values wherever available. The dissociation pathways of ICN are first predicted. The isomerization energy of BrCN is compared with the existing theoretical result. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Heteropolynuclear palladium complexes with pyrazolate and its 3-tert-butyl derivatives: the effect of heterometal ions on the rate of isomerization

The heteropolynuclear complexes [Pd(2)M'(2)(mu-pz)(6)] (M'=Ag (1), Au (2); pzH=pyrazole), HT-[Pd(2)M'(2)(mu-3-tBupz)(6)] (M'=Ag (3 a), Au (4 a); 3-tBupzH=3-tert-butylpyrazole), and HH-[Pd(2)Au(2)(mu-3-tBupz)(6)] (4 b) have been prepared and some of them were structurally characterized. When 3-tert-butylpyrazolate was employed as a bridging ligand, two linkage isomers (head-to-tail (HT) and head-to-head (HH)) arise from the difference in orientation of the substituent groups on the pyrazolate bridges between the two Pd atoms. (1)H NMR spectroscopy has been used to identify and to follow the reversible stereochemical rearrangement of the HH isomer of [Pd(2)Ag(2)(mu-3-tBupz)(6)] (3 b) to form the HT isomer 3 a in CDCl(3) and the HT isomer of [Pd(2)Au(2)(mu-3-tBupz)(6)] (4 a) to form the HH isomer 4 b in C(6)D(6). Kinetic studies of the reaction have established the rate law to be -d(HH)/dt=d(HT)/dt=k(2)[HH]-k(1)[HT] for 3 b and -d(HT)/dt=d(HH)/dt=k(1)[HT]-k(2)[HH] for 4 a, where k(1) and k(2) denote the rate of isomerization from the HT to the HH isomer and that from the HH to the HT isomer, respectively. For typical runs at 50 degrees C in C(6)D(6), k(1)=13.8x10(-5) s(-1), k(2)=18.6x10(-5) s(-1), and K(eq)=k(2)/k(1)=1.24 for 3 b, and k(1)=1.26x10(-5) s(-1), k(2)=3.52x10(-5) s(-1), and K(eq)=k(1)/k(2)=0.36 for 4 a. Temperature-dependent rate measurements reveal DeltaH(not equal) and DeltaS(not equal) to be 100(1) kJ mol(-1) and 0(3) J mol(-1) K(-1) for 3 b and 112(5) kJ mol(-1) and 20(17) J mol(-1) K(-1) for 4 a, respectively. The rate of isomerization is essentially unaffected by the concentration of the complex or by the presence of neutral bridging ligands. These data and observations imply that the isomerization involves an intramolecular exchange process.     

Optical processing with photochromic switches

The absorbance, fluorescence, and refractive index of a photochromic material can be modulated under the influence of optical stimulations. The reversible modification of these macroscopic properties is a result of photoinduced transformations at the molecular level. These processes can be exploited to mediate the interplay of optical signals and offer the opportunity to design and implement photonic devices for optical processing based on molecular components.     

Silylenes,Silenes, and Disilenes: Novel Silicon‐Based Reagents for Organic Synthesis?

Silylenes, silenes, and disilenes are silicon analogues of carbenes and alkenes. Since the first detection and isolation of these species a few decades ago, focus has been given to their fundamental structure and reactivity properties. Recent developments show that the time is set to exploit their unique chemistry in applied areas. Emerging applications in catalysis and stereoselective synthesis point to a new field within synthetic organosilicon chemistry.     

Conjecture: imines as unidirectional photodriven molecular motors-motional and constitutional dynamic devices

Compounds containing the C==N group, such as imines and their derivatives, may undergo syn-anti isomerization by two different routes: 1) photochemically, by out-of-plane rotation around the carbon-nitrogen double bond through a "perpendicular" form, and 2) thermally, by in-plane nitrogen inversion through a "linear" transition state. When the two interconversions occur in sequence, a full, closed process is accomplished, restoring the initial state of the system along two different steps. In a chiral imine-type compound, for example, with an asymmetric center next to the C==N function, photoinduced rotation may be expected to occur in one sense in preference to the opposite one. Thus, photoisomerization followed by thermal isomerization in a chiral imine compound generates unidirectional molecular motion. Generally, imine-type compounds represent unidirectional molecular photomotors converting light energy into mechanical motion. As they are also able to undergo exchange of the carbonyl and amine partners, they present constitutional dynamics. Thus, imine-type compounds are double dynamic, motional, and constitutional devices.     

The synthesis and characterization of all diastereomers of a linear symmetrically fused Tris-Tröger's base analogue: new chiral cleft compounds

The synthesis and characterization of all diastereomers of a linear symmetrically fused tris-Tr?ger's base analogue are described. The diastereomers are unambiguously assigned as syn-anti 1 a, anti-anti 1 b, and syn-syn 1 c isomers, by using X-ray diffraction analysis and NMR spectroscopy. For the first time, the anti-anti and the syn-syn diastereomers of a linear symmetrically fused tris-Tr?ger's base analogue have been synthesized. Molecules 1 a and 1 c are new cleft compounds and analysis of compound 1 a in the solid state shows inclusion of one molecule of CH(2)Cl(2) in the larger aromatic cleft, whereas in isomer 1 c disordered solvent molecules are trapped in the extended aromatic cleft. Furthermore, in the solid state, isomer 1 c forms infinite open channels along one of the crystallographic axes and perpendicular to this axis there are infinitely extending "wedged-ravines". Importantly, each of the diastereomers 1 a-c is resistant to inversion at the stereogenic nitrogen atoms under strongly and weakly acidic conditions in the range from room temperature (RT) to 95 degrees C. This observed configurational stability at the stereogenic nitrogens of 1 a-c is unique for analogues of Tr?ger's base in general to date. Finally, the ratio of cleft compounds 1 a and 1 c significantly increased relative to cavity compound 1 b when ammonium chloride was used as an additive in the Tr?ger's base condensation to 1 a-c suggesting a templating effect of the ammonium ion.