ABSTRACT
Possible reaction mechanisms of 1,3-silyl and 1,3-hydrogen thermal rearrangements of trimethylsilyl-1-pyrazoline and its model systems were theoretically explored using B3LYP, MP2, CR-CCSD(T), CASSCF(6,5), and MRMP2(6,5) theories. Nitrogen substitution at the center position of allylic moiety turned out to have a special stabilizing effect on diradical intermediates, allowing a stepwise pathway. This substitutional effect was attributed to the nitrogen lone pair electrons, which form strong pi-conjugations with diradicals. The second nitrogen substitution at the terminal allylic position selectively reduces the reaction barrier of antarafacial retention pathway, creating a competition between concerted and stepwise channels. The introduction of a five-membered ring imposes ring strain on the allylic moiety and increases steric hindrance, allowing no antarafacial channels. The combined effect of the nitrogen substitution and the five-membered ring further removes the possibility of concerted reaction pathways. Therefore 1,3-silyl migrations of 3-trimethylsilyl-1-pyrazoline were found to occur only through stepwise mechanisms, implying that the Woodward-Hoffmann rule is not operative. The 1,3-hydrogen migration also occur via a stepwise mechanism; however, it would not occur easily because its reaction barrier is much higher than that of 1,3-silyl migrations. Current study shows that a stepwise mechanism can be the dominant reaction pathway of some particular [1,3]-sigmatropic rearrangements.
Subject(s)
Models, Theoretical , Organosilicon Compounds/chemistry , Pyrazoles/chemistry , Silanes/chemistry , Models, ChemicalABSTRACT
Thephotochemical reaction of [3(3)](1,3,5)cyclophane 2, which is a photoprecursor for the formation of propella[3(3)]prismane 18, was studied using a sterilizing lamp (254 nm). Upon photolysis in dry and wet CH2Cl2 or MeOH in the presence of 2 mol/L aqueous HCl solution, the cyclophane 2 afforded novel cage compounds comprised of new skeletons, tetracyclo[6.3.1.0.(2,7)0(4,11)]dodeca-5,9-diene 43, hexacyclo[6.4.0.0.(2,6)0.(4,11)0.(5,10)0(9,12)]dodecane 44, and pentacyclo[6.4.0.0.(2,6)0.(4,11)0(5,10)]dodecane 45. All of these products were confirmed by the X-ray structural analyses. A possible mechanism for the formation of these photoproducts via the hexaprismane derivative 18 is proposed. The photophysical properties in the excited state of the [3n]cyclophanes ([3n]CP, n = 2-6) were investigated by measuring the emission spectra and determining the quantum yields and lifetimes of the fluorescence. All [3n]CPs show excimeric fluorescence without a monomeric one. The lifetime of the excimer fluorescence becomes gradually longer with the increasing number of the trimethylene bridges. The [3n]CPs also shows excimeric phosphorescence spectra without vibrational structures for n = 2, 4, and 5, while phosphorescence is absent for n = 3 and 6. With an increase in symmetry of the benzene skeleton in the [3(3)]- and [3(6)]CPs, the probability of the radiation (phosphorescence) process from the lowest triplet state may drastically decrease.
ABSTRACT
Multireference as well as density functional theories in combination with the surface integrated molecular orbital molecular mechanics were adopted to study the surface reactions of cyanogens on Si(100)-2x1 surface. Three different products were identified as minima in the initial surface reaction. Among these, the [2+2] product is both kinetically easily accessible and thermodynamically the most stable. Therefore, it can be considered as the experimentally found strongly bound surface species. Unlike other conjugated systems, the [4+2] product is less stable than the [2+2] product. Subsequent surface isomerization studies revealed that kinetically favorable channels exist between the initially formed low-temperature species and the high-temperature species, indicating that surface morphology changes gradually as a function of surface temperature. Theses two channels eventually lead to the same final surface products, which is consistent with experiment. Current study shows that the subsequent surface isomerizations are the key reactions to better understand the complex surface structures and their properties.
ABSTRACT
Multireference as well as single-reference wave functions were adopted to study the surface reaction mechanisms of hydrazine. The initial surface mechanisms resemble those of ammonia and its methyl derivatives. MRMP2 values indicate that the lifetime of initial molecularly adsorbed species should be longer than previously suggested. High energy path as well as low energy path of subsequent surface reactions were found. The theoretical initial surface product of low energy path is consistent with the experimentally suggested structure. Both paths eventually lead to very stable surface products, which are also consistent with the experimentally suggested structures. The reaction channels of the experimentally observed NH3 desorptions were also revealed. It was shown that the high reactivity of hydrazine as compared to ammonia and its methyl derivatives is due to the high nucleophilic ability of the additional nitrogen atom of hydrazine.
