Least-Squares Fitting of Time-Domain Signals for Fourier Transform Mass Spectrometry

To advance Fourier transform mass spectrometry (FTMS)-based molecular structure analysis, corresponding development of the FTMS signal processing methods and instrumentation is required. Here, we demonstrate utility of a least-squares fitting (LSF) method for analysis of FTMS time-domain (transient) signals. We evaluate the LSF method in the analysis of single- and multiple-component experimental and simulated ion cyclotron resonance (ICR) and Orbitrap FTMS transient signals. Overall, the LSF method allows one to estimate the analytical limits of the conventional instrumentation and signal processing methods in FTMS. Particularly, LSF provides accurate information on initial phases of sinusoidal components in a given transient. For instance, the phase distribution obtained for a statistical set of experimental transients reveals the effect of the first data-point problem in FT-ICR MS. Additionally, LSF might be useful to improve the implementation of the absorption-mode FT spectral representation for FTMS applications. Finally, LSF can find utility in characterization and development of filter-diagonalization method (FDM) MS.

A line-shape function in terms of changes in both molecular structure and force constants: a Gaussian approximation

We propose a new expression of a line-shape function (LSF) including the effects of changes in both force constants and the molecular structure within the harmonic-oscillator approximation. This expression enables us to calculate the LSF using only the data on molecular structures, force constants, and electronic energies in the initial and final electronic states without solving the eigenvalue equation for the normal vibration of a molecule. To derive the LSF expression, we consider one-photon emission from a polyatomic molecule thermalized in an electronic excited state, and derive the intensity distribution function for one-photon emission using not Lax and Kubo and Toyozawa's [J. Chem. Phys. 20, 1752 (1952); Prog. Theor. Phys. 13, 160 (1955)] generating function method but rather the density-matrix method. As a simple application, a Gaussian approximate LSF is applied to SO(2). As a result, it is found that the effect of change in force constants between the initial and final electronic states cannot be ignored, nor can the effect of change in the molecular structure between these two states. The LSF expression obtained is applicable to studies of not only radiative transition but also of electron-transfer and energy-transfer processes where both changes in molecular structure and force constants between the initial and final electronic states cannot be disregarded.     

A new technique for the early detection of stiffness in coupled differential equations and application to standard Runge-Kutta algorithms

Higher-order Runge-Kutta (RK) algorithms employing local truncation error (LTE) estimates have had very limited success in solving stiff differential equations. These LTEs do not recognize stiffness until the region of instability has been crossed after which no correction is possible. A new technique has been designed, using the local stiffness function (LSF), which can detect stiffness very early before instability occurs. The LSF is a normalized dimensionless ratio which is essentially based on the product of the step size and the geometric mean of all the slopes. It is exceedingly sensitive to the onset of stiffness. Together, the LSF and the LTE form a complementary pair which can cooperate to help solve some mildly stiff equations which were previously intractable to RK algorithms alone. Examples are given of implementation and LSF performance. Received: 18 September 1997 / Accepted: 13 February 1998 / Published online: 17 June 1998     

Dediazoniations of Arene Diazonium Ions in Homogenous Solution. Part IV: Change-over from a heterolytic to a homolytic mechanism in 2,2,2-trifluoroethanol pyridine mixtures

There are only two dediazoniation products of benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol (TFE), namely phenyl 2,2,2-trifluoroethyl ether ( 1 ) and fluorobenzene ( 2 ). The reaction kinetics are strictly first-order with respect to the diazonium salt. The addition of increasing amounts of pyridine to the system results in a gradual decrease in the yields of 1 and 2 and an increase in the yields of the homolytically formed products, benzene ( 3 ), biphenyl ( 4 ), isomeric phenylpyridines ( 5 ) and diazo tar ( 6 ). The reaction kinetics show that the rate of dediazoniation of the benzene diazonium salt increases with increasing amounts of pyridine. The reaction with added pyridine is no longer first-order with respect to the diazonium ion. The product analyses and the kinetic data are consistent with the view that in pure TFE this diazonium salt decomposes completely by a heterolytic mechanism. The addition of pyridine brings about a competitive homolytic mechanism which becomes increasingly dominant as the concentration of pyridine increases.     

Dehydrogenation of aromatic molecules under a scanning tunneling microscope: pathways and inelastic spectroscopy simulations

We have performed a theoretical study on the dehydrogenation of benzene and pyridine molecules on Cu(100) induced by a scanning tunneling microscope (STM). Density functional theory calculations have been used to characterize benzene, pyridine, and different dehydrogenation products. The adiabatic pathways for single and double dehydrogenation have been evaluated with the nudge elastic band method. After identification of the transition states, the analysis of the electronic structure along the reaction pathway yields interesting information on the electronic process that leads to H-scission. The adiabatic barriers show that the formation of double dehydrogenated fragments is difficult and probably beyond reach under the actual experimental conditions. However, nonadiabatic processes cannot be ruled out. Hence, in order to identify the final dehydrogenation products, the inelastic spectra are simulated and compared with the experimental ones. We can then assign phenyl (C6H5) and alpha-pyridil (alpha-C5H4N) as the STM-induced dehydrogenation products of benzene and pyridine, respectively. Our simulations permit us to understand why phenyl, pyridine, and alpha-pyridil present tunneling-active C-H stretch modes in opposition to benzene.     

The photochemistry of 3,4,5-trideuteriopyridine

Irradiation of 3,4,5-trideuteriopyridine at 254 nm in the vapor phase results in the formation of a mixture of 2,3,4-trideuteriopyridine and 2,3,6-trideuteriopyridine. The formation of these products is consistent with a photoisomerization mechanism involving equilibrating azaprefulvene intermediates. This is the first direct evidence that pyridine vapor undergoes photoisomerization resulting in transposition of the pyridine ring atoms.     

Reactivity of pyridine-2,4,6-tricarboxylic acid toward Zn(II) salts under different reaction conditions

Pyridine-2,4,6-tricarboxylic acid (ptcH(3)) readily reacts with a Zn(II) salt at room temperature to form different products depending upon the presence or absence of pyridine in the reaction mixture. In the presence of pyridine, the ligand breaks to form infinitely zigzag coordination polymers with the empirical formula [Zn(Ox)(py)(2)]n(Ox = oxalate, py = pyridine). The backbone is formed from Zn(II)-oxalate where two pyridine molecules are coordinated to each Zn(II) ion giving it hexacoordination. The orientation of the bound pyridines is slightly different when Zn(II)-nitrate is used compared to that when Zn(II)-sulfate (or acetate) salt is used. In absence of pyridine, the ligand remains intact and forms a mixture of a carboxylate-bridged coordination polymer and a discrete carboxylate-bridged 12-membered metallomacrocycle.     

Synthesis and study of chiral NADH models in the thieno[2,3-b]pyridine series

Thieno[2,3-b]pyridine derivatives, functionalized on the thiophene ring have been synthesized in two different ways: electrophilic substitution of thieno[2,3-b]pyridine derivatives or construction of a pyridine ring starting from disubstituted thiophene compounds. The products obtained were used in the synthesis of chiral NADH models.     

Synthesis and Utility of Dihydropyridine Boronic Esters

When activated by an acylating agent, pyridine boronic esters react with organometallic reagents to form a dihydropyridine boronic ester. This intermediate allows access to a number of valuable substituted pyridine, dihydropyridine, and piperidine products.     

Preparation of ethynylpyridine macrocycles by oxidative coupling of an ethynylpyridine trimer with terminal acetylenes

Macrocycles consisted of pyridine rings and acetylene bonds were prepared by Eglinton coupling from a tandem precursor bearing two terminal alkynyl groups. The composition of molecular size in the cyclized products changed by the reaction solvent. In pyridine, 9-meric and bigger macrocycles were obtained, while that of 6-mer was not. On the other hand, in pyridine/THF mixed solvent, the 6-mer was obtained as a major product.     

Competitive free radical 4-nitrophenylation of pyridine and thiophene

Competitive free radical 4-nitrophenylation of pyridine and thiophene shows a relative reactivity of thiophene:pyridine of 1.3:1 as based on product isolation. A three-center reaction is proposed to account for the predominance of alpha substitution in each system. Column chromatography and ¹H nmr are used for separation and identification of products.     

Remarkable Chichibabin-type cyclotrimerisation of 3-nitrotyrosine, tyrosine and phenylalanine to 3,5-diphenylpyridine derivatives induced by hypochlorous acid

Reaction of 3-nitrotyrosine with HOCl in aqueous phosphate buffer (pH 7.4) leads to a mixture of extractable products, including 3,5-di(4-hydroxy-3-nitrophenyl)pyridine (15% isolated yield) and 3,5-di(4-hydroxy-3-nitrophenyl)-2-(4-hydroxy-3-nitrophenylmethyl)pyridine (3%) arising by a Chichibabin-like pyridine synthesis via N-chloroimine intermediates. Under the same conditions, phenylalanine gives 3,5-diphenylpyridine in 9% isolated yield, while tyrosine leads to 3,5-di(4-hydroxyphenyl)pyridine (3%) and 3-(3-chloro-4-hydroxyphenyl)-5-(4-hydroxyphenyl)pyridine (3%).     

2,6-二氯吡啶的定向合成及结构测定

研究了以吡啶为原料,通过光氯化反应定向合成2,6-二氯吡啶。并对光氯化反应的工艺进行了研究。当吡啶与氯气配比为1:2.3,反应温度为200℃,反应5h时,光氯化反应的转化率可达95.7%。反应的进程和产品的纯度通过GC-MS进行控制。并用元素分析、红外光谱、质谱对产物的结构进行了表征。     

Selective preparation of pyridines, pyridones, and iminopyridines from two different alkynes via azazirconacycles

Selective preparation of pyridine derivatives from two different alkynes and a nitrile was achieved by a novel procedure in which an alkyne and a nitrile couple first to give an azazirconacyclopentadiene followed by reaction with the second alkyne in the presence of 1 equiv of NiCl(2)(PPh(3))(2). This procedure gives only single products of pyridine derivatives from two different symmetrical alkynes and a nitrile. Our novel procedure can be used even with two similar alkyl-substituted alkynes such as 3-hexyne and 4-octyne. Two possible pyridine isomers from 3-hexyne, 4-octyne, and acetonitrile could be completely and independently prepared as single products by this method. The origin of the selectivity comes from the addition order of two different alkynes. This method was applied for the formation of pyridones and iminopyridines using isocyanate and carbodiimide derivatives instead of nitriles, respectively. Reaction of an alkyne with Cp(2)ZrEt(2) and an isocyanate or a carbodiimide gives an azazirconacycle. Treatment of the azazirconacycle with the second alkyne in the presence of 1 equiv of NiCl(2)(PPh(3))(2) gave a pyridone or an iminopyridine derivative. The use of two different unsymmetrical alkynes afforded the pyridine with five different substituents when the first alkyne has a trialkylsilyl group and the second alkyne has a phenyl group as functional groups. On the other hand, azazirconacyclopentadienes reacted with propargyl bromide in the presence of CuCl with excellent regioselectivity to give tetrasubstituted pyridine derivatives as single products. With the assistance of the trialkylsilyl groups, pyridines with all different substituents including H were also prepared.     

A New Approach to the Synthesis of Dithienylethanediones and Dithienylacetylenes

1,2-Dithienylethanediones were synthesized by modified acylation of thiophene and its homologs in the presence of aluminum chloride and pyridine. The bishydrazones of the products were converted into dithienylacetylenes by oxidation with atmospheric oxygen in pyridine in the presence of CuCl.     

Electrochemical reduction of NO2 studied by the use of cone-shaped electrodes

The electrochemical reduction, in an all solid state cell, of NO₂ and O₂ on LSF (strontium substituted lanthanum ferrite) based electrodes has been studied using the cone-shaped electrode technique. It was shown that the reduction of NO₂ occurs much faster on LSF based electrodes than the reduction of O₂. The electrochemical reduction of NO₂ on LSF compounds is probably catalyzed by Fe(III) and oxide ion vacancies. The same seems to be true for the electrochemical reduction of O₂. The largest difference in current densities for the electrochemical reduction of NO₂ and O₂ was found for the compound LSF70 (La_0.3Sr_0.7FeO_3−δ).     

吡啶团簇的飞秒光电离和从头计算研究

用飞秒激光电离飞行时间质谱研究了吡啶分子团簇在400 nm波长下的多光子光电离,实验观测到一系列的质子化和非质子化团簇离子.结果表明,质子转移也能发生在弱氢键结合的分子间.通过分析离子峰宽和离子信号强度随气源压力的变化,得到质子化团簇离子来源于大团簇离子的碎裂,而非质子化团簇离子是中性团簇直接电离的结果.从头计算结果表明,吡啶团簇是通过弱氢键C-H…N 结合在一起的,并且团簇离子离解倾向于生成质子化产物.     

Indole related compounds. Reaction of 1-protected-indoles with pyridine substrates

The products resulting from the reactions of 1-benzenesulfonyl-2-lithioindole and 2-lithio-1-methylindole with different pyridine substrates (pyridine, 1-methoxy-pyridinium salt and 2-bromopyridine) have been studied. The use of lithiocyclohexylisopyropylamide as a lithiating agent of indoles has been investigated. The preparation of four of the possible benzenesulfonyl derivatives of 2-(2′ -pyridyl)indole is described.     

Enantio- and Regioselective Copper-Catalyzed 1,2-Dearomatization of Pyridines

A copper-catalyzed dearomative alkynylation of pyridines is reported with excellent regio- and enantioselectivities. The synthetically valuable enantioenriched 2-alkynyl-1,2-dihydropyridine products afforded are generated from the readily available feedstock, pyridine, and commercially available terminal alkynes. The three-component reaction between a pyridine, a terminal alkyne, and methyl chloroformate employs copper chloride and StackPhos, a chiral biaryl P,N- ligand, as the catalytic system. Under mild reaction conditions, the desired 1,2-addition products are delivered in up to 99 % yield with regioselectivity ratios up to 25 : 1 and enantioselectivities values of up to 99 % ee. Activated and non-activated terminal alkynes containing a wide range of functional groups are well tolerated. Even acetylene gas delivered mono-alkynylated products in high yield and ee. Application of the methodology in an efficient enantioselective synthesis of the chiral piperidine indolizidine, coniceine, is reported.     

α-Aminoxylation of Ketones and β-Chloro-α-aminoxylation of Enones with TEMPO and Chlorocatecholborane

Oxidation of various cyclic and acyclic ketones under mild conditions with chlorocatecholborane, a bulky pyridine base, and TEMPO to the corresponding α-aminoxylated products in good to excellent yields (52-99%) is described. For enones as substrates, products of a β-chloro-α-aminoxylation are obtained (70-89%).