Carbon-13 NMR spectroscopy of heterocyclic compounds—III : A 20 MHz study of chemical shifts and carbon-proton coupling constants for the methyl coumarins

Complete assignments of chemical shifts and extensive assignments of carbon-proton coupling constants are presented for the 3-, 4-, 6-, 7- and 8-methylcoumarins, together with similar parameters for 5,8- and 6,8-dimethylcoumarin. Shifts in 6,8-dimethylcoumarin can be predicted with high precision from the substituent effects observed in the 6- and 8-methyl derivatives, and shifts are predicted for the 5-methyl derivative, which was not studied.     

Carbon-13 NMR spectroscopy of heterocyclic compounds—II : A 20 MHz study of chemical shifts and carbon-proton coupling constants for coumarin and some bromocoumarins

Complete assignments of chemical shifts and extensive assignments of carbon-proton coupling constants are presented for coumarin through a study also involving detailed measurements on 3-bromo-, 6-bromo-, 7-bromo-, 3,6-dibromo- and 3,6,8-tribromo-coumarin.     

The fluorination of 2-methylpropane over cobalt trifluoride

2-Methylpropane has been fluorinated over cobalt trifluoride at 140-200°C to give a mixture of at least 30 components, of which 22 have been identified with some certainty and five, more tentatively; this represents well over 95% of the reaction mixture. About 95% of the identified products were polyfluoro(2-methylpropanes); the rest were polyfluorobutanes. The fluorination was not completely random; there was a clear preference (by some 10 times greater than chance) for compounds in which the tertiary hydrogen had been replaced by fluorine. A scheme for the fluorination pathway is proposed. Attempts to dehydrofluorinate 1 ,3 -pentafluoro-2-fluoromethylpropane failed.     

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.     

Reactivities of monomers toward the 2-cyano-2-propyl radical by NMR study of end groups in copolymers

Copolymerizations of styrene (S) with methyl methacrylate (MMA) and methacrylonitrile (MAN) at 60°C were performed with azoisobutyronitrile as initiator, enriched in its methyl groups with ¹³C. From the NMR spectrum of a copolymer it is possible to compare the numbers of initiator fragments attached to the two types of monomer unit; this information can be used to study the competition between the monomers for capture of the radicals formed from the initiator. The velocity constants for addition of the 2-cyano-2-propyl radical to S, MMA, and MAN are in the proportions 1:0.56:0.34; it is concluded that polar effects are of some importance in the reactions of the radical.     

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...     

Endgroups in Copolymers of Acenaphthylene with Methyl Methacrylate Prepared with Azoisobutyronitrile as Initiator

Copolymers of acenaphthylene (ACN) with methyl methacrylate (MMA) have been prepared with azobis(isobutyronitrile-β, β-¹³C₂) as initiator, The endgroups derived from the initiator have been examined by ¹³C-NMR spectroscopy; those attached to ACN units have been distinguished from those attached to MMA units and quantitative comparisons of their numbers have been made. It has been deduced that at 60°C ACN is four times as reactive as MMA toward the (CH₃)₂ C(CN) radical. The marked preference for initiation involving ACN means that, for all copolymers, the ratio of ACN to MMA is appreciably greater for the sites adjacent to the (CH₃)₂ C(CN)– endgroups than for the whole copolymer.     

End-groups in polymers and copolymers of ethyl acrylate prepared using azoisobutyronitrile as initiator

Samples of azoisobutyronitrile enriched with carbon-13 or deuterium have been used for NMR studies of the initiator fragments incorporated in polymers of ethyl acrylate and copolymers of that monomer with styrene. It has been deduced that at 60° styrene is 3.3 times as reactive as ethyl acrylate towards the 2-cyano-2-propyl radical.     

Reactions of the 2-cyano-2-propyl radical with vinyl acetate and other monomers: A study of head-addition

Polymers have been prepared at 60 and 100°C using initiators giving the 2-cyano-2-propyl radical enriched with carbon-13 in the nitrile group. Examination by NMR of the (CH₃)₂C(CN)? end groups in the resulting polymers of vinyl acetate (VAC) and of appropriate model compounds has indicated that up to 20% of these end groups correspond to the product of head-addition of the primary radical to VAC at 60°C. Similar end groups have been found in poly(vinyl formate) but not in poly(methyl methacrylate) and polystyrene. In polyVAC prepared at 100°C, the proportion of end groups attached to the head of a monomeric unit is higher than for polymers made at 60°C.