Reversible carboxylation of N-heterocyclic carbenes

Spectroscopic analysis, thermogravimetric analysis, and crossover experiments performed on a series of imidazolium carboxylates revealed carboxylation was reversible with N-aryl substituted adducts.     

Fluorescence-detected two-dimensional electronic coherence spectroscopy by acousto-optic phase modulation

Two-dimensional electronic coherence spectroscopy (ECS) is an important method to study the coupling between distinct optical modes of a material system. Such studies often involve excitation using a sequence of phased ultrashort laser pulses. In conventional approaches, the delays between pulse temporal envelopes must be precisely monitored or maintained. Here, we introduce a new experimental scheme for phase-selective nonlinear ECS, which combines acousto-optic phase modulation with ultrashort laser excitation to produce intensity modulated nonlinear fluorescence signals. We isolate specific nonlinear signal contributions by synchronous detection, with respect to appropriately constructed references. Our method effectively decouples the relative temporal phases from the pulse envelopes of a collinear train of four sequential pulses. We thus achieve a robust and high signal-to-noise scheme for phase-selective ECS to investigate the resonant nonlinear optical response of photoluminescent systems. We demonstrate the validity of our method using a model quantum three-level system-atomic Rb vapor. Moreover, we show how our measurements determine the resonant complex-valued third-order susceptibility.     

17O NMR spectroscopic characterization and the mechanism of formation of alkyl hydrotrioxides (ROOOH) and hydrogen trioxide (HOOOH) in the low-temperature ozonation of isopropyl alcohol and isopropyl methyl ether: water-assisted decomposition

Low-temperature ozonation of isopropyl alcohol (1a) and isopropyl methyl ether (1b) in [D6]acetone, methyl acetate, and tert-butyl methyl ether at -78 degrees C produced the corresponding hydrotrioxides, Me2C(OH)(OOOH) (2a) and Me2C(OMe)(OOOH) (2b), along with hydrogen trioxide (HOOOH). All the polyoxides investigated were characterized for the first time by 17O NMR spectroscopy of highly 17O-enriched species. The assignment was confirmed by GIAO/MP2/6-31++G* calculations of 17O NMR chemical shifts, which were in excellent agreement with the experimental values. Ab initio density functional (DFT) calculations at the B3LYP/ 6-31G*+ZPE level have clarified the transition structure (TS1, deltaE = 7.4 and 10.6 kcalmol(-1), relative to isolated reactants and the complex 1a-ozone, respectively) for the ozonation of 1a: this, together with the formation of HOOOH and some other products, indicates the involvement of radical intermediates (R*, *OOOH) in the reaction. The activation parameters for the decomposition of the hydrotrioxides 2a and 2b (Ea, = 23.5+/-1.5 kcalmol(-1), logA = 16+/-1.8) were typical for a homolytic process in which cleavage of the ROOOH molecule occurs to yield a radical pair [RO* *OOH] and represents the lowest available energy pathway. Significantly the lower activation parameters for the decomposition of HOOOH (Ea = 16.5+/-2.2 kcalmol(-1), logA = 9.5+/-2.0) relative to those expected for the homolytic scission of the HO-OOH bond [bond dissociation energy (BDE) = 29.8 kcalmol(-1), CCSD(T)/6-311++G**] are in accord with the proposal that water behaves as a bifunctional catalyst and therefore participates in a "polar" (non-radical) decomposition process of this polyoxide. A relatively large acceleration of the decomposition of the hydrotrioxide 2a in [D6]acetone, accompanied by a significant lowering of the activation energies, was observed in the presence of a large excess of water. Thus intramolecular 1,3-proton transfer probably also involves the participation of water and is similar to the mechanism proposed for the decomposition of HOOOH. This hypothesis was further substantiated by the B3LYP/6-31++ G*+ZPE calculations for the participation of water in the decomposition of CH3OOOH, which revealed two stationary points on the potential energy surface corresponding to a CH3OOOH-HOH complex and a six-membered cyclic transition state TS2. The energy barriers were comparable with those calculated for HOOOH, that is, deltaE = 15.0 and 21.5 kcalmol(-1) relative to isolated reactants and the CH3OOOH-HOH complex, respectively.     

Wave packet interferometry and quantum state reconstruction by acousto-optic phase modulation

Studies of wave packet dynamics often involve phase-selective measurements of coherent optical signals generated from sequences of ultrashort laser pulses. In wave packet interferometry (WPI), the separation between the temporal envelopes of the pulses must be precisely monitored or maintained. Here we introduce a new (and easy to implement) experimental scheme for phase-selective measurements that combines acousto-optic phase modulation with ultrashort laser excitation to produce an intensity-modulated fluorescence signal. Synchronous detection, with respect to an appropriately constructed reference, allows the signal to be simultaneously measured at two phases differing by 90 degrees. Our method effectively decouples the relative temporal phase from the pulse envelopes of a collinear train of optical pulse pairs. We thus achieve a robust and high signal-to-noise scheme for WPI applications, such as quantum state reconstruction and electronic spectroscopy. The validity of the method is demonstrated, and state reconstruction is performed, on a model quantum system--atomic Rb vapor. Moreover, we show that our measurements recover the correct separation between the absorptive and dispersive contributions to the system susceptibility.     

Driving matrix liquid crystal displays

Liquid crystal displays had a humble beginning with wrist watches in the seventies. Continued research and development in this multi-disciplinary field have resulted in displays with increased size and complexity. After three decades of growth in performance, LCDs now offer a formidable challenge to the cathode ray tubes (CRT). A major contribution to the growth of LCD technology has come from the developments in addressing techniques used for driving matrix LCDs. There are several approaches like passive matrix addressing, active matrix addressing and plasma addressing to drive a matrix display. Passive matrix LCD has a simple construction and uses the intrinsic non-linear characteristic of the LCD for driving. Departure from conventional line by line addressing of a passive matrix has resulted in improved performance of the display. Orthogonal functions have played a crucial role in the development of passive matrix addressing. Simple orthogonal functions that are useful for driving a matrix LCD are introduced. The basics of driving several rows simultaneously (multi-line addressing) are discussed by drawing analogies from multiplexing in communication. The impact of multi-line addressing techniques on the performance of the passive matrix LCDs in comparison with the conventional technique will be discussed.     

On the governing equations for the compressing process and its coupling with other processes

As a continuation of a recent linear analysis by Mao et al.(Acta Mech Sin,2010,26:355),in this paper we propose a general theoretical formulation for the compressing process in complex Newtonian fluid flows,which covers gas dynamics,aeroacoustics,nonlinear thermoviscous acoustics,viscous shock layer,etc.,as its special branches.The principle on which our formulation is based is the maximally natural and dynamic Helmholtz decomposition of the Navier-Stokes equation,along with the kinematic Helmholtz decompos...     

Nickel-catalyzed cycloaddition of unsaturated hydrocarbons and carbonyl compounds

A mild and general route for preparing pyrans and dienones from carbonyls and diynes is described. Ni imidazolylidene complexes were used to mediate cyclizations between diynes and aldehydes. The reaction of an enyne and an aldehyde afforded a mixture of cyclized products. In addition, a spiropyran was prepared from the cycloaddition of a diyne and cyclohexanone. [reaction: see text]     

An in situ approach for nickel-catalyzed cycloaddition

A convenient method for preparing pyridines from air-stable, commercially available catalyst precursors is described. The addition of n-BuLi to Ni(acac)2 and an NHC salt (such as IPr.HCl or SIPr.HCl) rapidly generates an active Ni0/NHC catalyst for the cycloaddition of diynes and nitriles that affords pyridines without a decrease in observed yields. The in situ method also converts diynes and carbon dioxide to the corresponding pyrones.     

Nickel-catalyzed cycloadditions of unsaturated hydrocarbons, aldehydes, and ketones

The nickel-catalyzed cycloaddition of unsaturated hydrocarbons and carbonyls is reported. Diynes and enynes were used as coupling partners. Carbonyl substrates include both aldehdyes and ketones. Reactions of diynes and aldehydes afforded the [3,3] electrocyclic ring-opened tautomers, rather than pyrans, in high yields. The cycloaddition reaction of enynes and aldehydes afforded two distinct products. A new carbon-carbon bond is formed, prior to a competitive beta-hydrogen elimination of a nickel alkoxide, between the carbonyl carbon and either one of the carbons of the olefin or the alkyne. The steric hindrance of the enyne greatly affected the chemoselectivity of the cycloaddition of enynes and aldehydes. In some cases, dihydropyran was also formed. The scope of the cycloaddition reaction was expanded to include the coupling of enynes and ketones. No beta-hydrogen elimination was observed in cycloaddition reaction of enynes and ketones. Instead, C-O bond-forming reductive elimination occurred exclusively to afford dihydropyrans in excellent yields. In all cases, complete chemoselectivity was observed; only dihydropyrans where the carbonyl carbon forms a carbon-carbon bond with a carbon of the olefin, rather than of the alkyne, were observed. All cycloaddition reactions occur at room temperature and employ nickel catalysts bearing the hindered 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or its saturated analogue, 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolin-2-ylidene (SIPr).     

Nickel-Catalyzed Cycloisomerization of Enynes: Catalyst Generation via C-H Activation of Carbene Ligands

The combination of Ni(0) and an N-heterocyclic carbene acts as a precatalyst for the cycloisomerization of enynes to afford 1,3-dienes. During the course of the reaction, a nickel hydride is formed from oxidative addition of the ortho C-H on the carbene ligand. Deuterium labeling studies are presented.