The methyl isocyanide isomerization: A CNDO/2 study with partitioning of energy

Ab initio LCAO MO SCF calculations have been carried out to predict core electron binding energies and shifts in fluoro- and chloro-methanes. The quality of the calculations ranges from a better than double zeta basis set to minimal STO (3 G) basis set. Predictions of binding energies and shifts are made using Koopmans' theorem, hole state calculations and equivalent cores calculations. Using a flexible basis set there is very little difference in the prediction of shifts by these three methods but for minimal basis set calculations the equivalent cores calculations give the best results.     

Calculation of isotope enrichment using a unimolecular decomposition scheme for polyatomic molecules following infrared laser excition

A mechanism is proposed to account for laser-induced isotope enrichment reactions which proceed by unimolecular dissociation following selective vibrational excitation. Assuming that the internal vibrational energy relaxation is not complete during the molecular dissociative lifetime, both selective single and multiphoton absorption mechanisms were investigated.     

Ion-molecule reactions involving methyl isocyanide and methyl cyanide

Ion-molecule reactions involving methyl isocyanide and methyl cyanide have been performed in a new rf-only hexapole collision cell inserted in a large-scale tandem mass spectrometer. Beside protonation processes, N-methyl cyanogen ions (CH(3)N(+)CCN) and 1-methyleneiminium-1- ethylenium ions (CH(2)CN(+)CH(2)) have been produced in high yield and fully characterized by high-energy collisional activation. The unimolecular chemistry of the molecular ions of caffeine (1,3,7-trimethyl xanthine) has been revisited on the basis of these new results.     

The infrared spectra and conformation of methyl and ethyl propenyl ethers

The infrared spectra of the cis and trans geometric isomers of methyl and ethyl propenyl ethers have been recorded in the liquid and vapour phase, in the region 4000-200 cm^?1. Solid phase spectra have also been recorded for all but trans methyl propenyl ether. Evidence is given for the existence of two rotameric forms for both the trans compounds, in each case the more stable conformer being planar s-cis; but a single non-planar conformation is suggested for the two cis isomers. The enthalpy difference between the two rotameric forms in trans ethyl propenyl ether was found to be 4.6 ± 1.0 kJ mol^?1 (1100 ± 250 cal mol^?1).     

The infrared spectrum of CHD2NC and the ground state structure of methyl isocyanide

Analysis of the rovibration spectrum of CHD₂NC has enabled the rotation parameter (A₀B₀) to be determined. Knowledge of the microwave B₀ and C₀ constants enables A₀(CHD₂NC) to be determined, which, when used in conjunction with all other B₀ and C₀ constants for isotopic methyl isocyanides, allows the ground state structure to be defined within fairly narrow limits. The CH bond length is much shorter than that determined by the substitution technique (r_s structure). It is now in almost perfect agreement with the value predicted by the CH vibration frequency in CHD₂NC, and falls in the range of CH bond lengths exhibited by all other symmetric top methyl group molecules.     

The ground-state average structure of methyl isocyanide

The use of recently determined highly precise inertial data for various isotopic modifications of methyl isocyanide has enabled the ground-state average, or r_z, structure to be determined to within very narrow limits. Harmonic corrections to ground-state rotational constants have been calculated using a high-quality, experimentally determined harmonic force field. The derived zero-point inertial constants are sufficiently accurate to enable changes in the CH bond length and NCH bond angle on deuteration to be determined. The present r_z structure determination is believed to be a physically realistic estimate of the ground-state average geometry of methyl isocyanide.     

Molecular isomerization induced by ultrashort infrared pulses. II. Pump-dump isomerization using pairs of time-delayed half-cycle pulses

We investigate population transfer across the barrier in a double-well potential, induced by a pair of time-delayed single-lobe half-cycle pulses. We apply this setup both to a one-dimensional (1D) quartic model potential and to a three-dimensional potential representing HCN-->HNC isomerization. Overall the results for the two systems are similar, although in the 3D system some additional features appear not seen in the 1D case. The generic mechanism of population transfer is the preparation by the pump pulse of a wave packet involving delocalized states above the barrier, followed by the essentially 1D motion of the delocalized part of wave packet across the barrier, and the eventual de-excitation by the dump pulse to localized states in the other well. The correct timing is given by the well-to-well passage time of the wave packet and its recurrence properties, and by the signs of the field lobes which determine the direction and acceleration or deceleration of the wave packet motion. In the 3D system an additional pump-pump-dump mechanism linked to wave packet motion in the reagent well can mediate isomerization. Since the transfer time and the pulse durations are of the same order of magnitude, there is also a marked dependence of the dynamics and the transfer yield on the pulse duration. Our analysis also sheds light on the pronounced carrier envelope phase dependence previously observed for isomerization and molecular dissociation with one-cycle and sub-one-cycle pulses.     

酯交换法合成碳酸甲乙酯研究进展

碳酸甲乙酯(EMC)具有诸多优异的物理和化学性能,作为锂电池电解液溶剂已经被行业广泛认可,酯交换法是中国目前工业生产EMC的主要方法。本研究系统综述了碳酸酯交换反应热力学、动力学、均相及非均相催化剂、反应机理及反应工艺等方面的研究,重点评述了近五年酯交换法制备EMC的最新进展。均相催化剂中以pK_b值(碱度系数)为标准讨论了可溶碱类催化剂碱强度和催化效率之间的关系,探究了咪唑类离子液体阴、阳离子结构对反应效果的影响规律。针对工业上普遍采用的甲醇钠催化剂,描述了其失活现象并阐述了失活机理。详细比较和讨论了非均相催化剂的制备方法、表面酸碱性与催化效率之间的关系,综合评价了不同类别的催化剂催化酯交换反应的优缺点。着眼绿色、高纯、低成本EMC合成技术,高效固体碱催化剂和涉及气、液、固三相的催化精馏技术是今后开发的重点和发展方向。     

Particulate formation induced by infrared laser dielectric breakdown

Laser induced dielectric breakdown has been utilized to initiate, sustain and study a number of chemical reactions. A 1 joule per line TEA CO₂ laser has been used as the source to induce dielectric breakdown. Product analysis was carried out by mass spectrometry and infrared spectroscopy. The reaction and method are discussed in terms of efficiency, selectivity and scope.     

CO2 laser induced isomerization and photodissociation of 1,2-dichloroethylene

Infrared photoisomerization of both trans- and cis-1,2-dichloroethylene molecules sensitized by SF₆ has been observed by using a CO₂ laser. The direct infrared photoisomerization has also been observed for the trans molecule. The reaction rate of the sensitized photoisomerization increases by 6 times as compared with the direct process. The dielectric breakdown induced by an intense laser pulse enhances a dissociative reaction.     

Oxidation of methyl and ethyl butanoates

This paper describes an experimental and modeling study of the oxidation of methyl and ethyl butanoates in a shock tube. The ignition delays of these two esters mixed with oxygen and argon for equivalence ratios from 0.25 to 2 and ester concentrations of 0.5% and 1% were measured behind a reflected shock wave for temperatures from 1250 to 2000 K and pressures around 8 atm. To extend the range of studied temperatures in the case of methyl butanoate, two sets of measurements were also made in a jet‐stirred reactor at 800 and 850 K, at atmospheric pressure, at residence times varying between 1.5 and 9 s and for equivalence ratios of 0.5 and 1. Detailed mechanisms for the combustion of methyl and ethyl butanoates have been automatically generated using a version of EXGAS software improved to take into account these oxygenated reactants. These mechanisms have been validated through comparison of simulated and experimental results in both types of reactor. The main reaction pathways have been derived from reaction flux and sensitivity analyses performed at different temperatures. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 226–252, 2010     

Formation of aluminacyclobutenes via carbon monoxide and isocyanide insertion

Reaction of LAl[eta2-(CSiMe3)2] (L = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3) with carbon monoxide and tert-butyl isocyanide afforded unique AlC3 aluminacyclobutenes via insertion into one of the aluminium-carbon bonds.     

Competitive reactions of coordinated carbon monoxide and methyl isocyanide with amines

The relative reactivities of CO and CNR ligands with CH₃NH₂ were investigated in complexes which contained both ligands. Like (C₅H₅Fe(CO)₃⁺; the (C₅H₅)Fe(CO)₂(CNCH₃)⁺ complex reacts with CH₃NH₂ to give the carbamoyl complex (C₅)Fe(CO)(CNCH₃)(CONHCH₃); this is a readily reversible reaction. In contrast, (C₅H₅)Fe(CO)(CNCH₃)₂⁺ reacts with CH₃NH₂ to give the amidinium or carbene complex, (C₅H₅)Fe(CO)(CNCH₃)[C(NHCH₃)₂]⁺]. In a slow reaction, (C₅H₅)Fe(PPh₃)(CO)(CNCH₃)⁺ forms the amidinium complex, (C₅H₅)Fe(PPh₃)(CO)[C(NHCH₃)₂]⁺. Factors that affect the site of CH₃NH₂ reaction are discussed. The complexes have been characterized by IR and NMR spectroscopy; a variable temperature NMR study of (C₅H₅)Fe(CO)(CNCH₃)[C(NHCH₃)₂]⁺ indicates restricted rotation around the CN bonds of the amidinium ligand.     

Ab initio studies of hydrogen bond formation in methyl cyanide or methyl isocyanide and methanol systems

CH₃CN ? HOCH₃ and CH₃NC ? HOCH₃ hydrogen-bonded systems have been studied theoretically by ab initio MO methods using a 4–31 G basis set at their equilibrium geometries. The stabilization energies of these hydrogen bonds are 5.4 and 5.9 kcal/mol, respectively. The nature of these hydrogen bonds is discussed in the light of frontier orbital theory and the topological properties of the charge density of the chemical bond.     

Acid-catalyzed isomerization of pivalaldehyde to methyl isopropyl ketone via a reactive protosolvated carboxonium ion intermediate.

Quantitative rearrangement of pivalaldehyde to methyl isopropyl ketone is observed in acids such as trifluoromethanesulfonic acid, anhydrous HF, and trifluoroethyl alcohol-BF3 but not in trifluoroacetic acid. Studies in a mixture of trifluoroacetic acid and trifluoromethanesulfonic acid show that acids with H(o) < or = -11 are able to carry out complete isomerization. These results and density functional theory calculations at the B3LYP/6-31G level suggest that protonated pivalaldehyde undergoes further protosolvation at higher acidities to a reactive superelectrophilic species resulting in rearrangement. A mechanism for the pivalaldehyde rearrangement to methyl isopropyl ketone in strong protic acids involving a reactive protosolvated superelectrophilic intermediate is suggested. Aspects of the related mechanism of the reaction with isobutane with CO in HF/BF3 medium leading to methyl isopropyl ketone are also discussed.     

High intensity infrared laser irradiation calorimetry: direct-determination of heat input to chlorodifluoromethane and ethyl acetate

The heat absorbed when chlorodifluoromethane (CF₂HCl) and ethyl acetate were irradiated with a pulsed infrared CO₂-TEA laser was measured using a laser calorimeter. The measurements show that CF₂HCl at high pressure (> 10 torr) was completely thermally equilibrated during the laser pulse, while at low pressure (<2 torr) this system deviated considerably from thermal equilibrium. Ethyl acetate, however, was found to be non-thermal over the entire pressure range studied (0.5 to 20 torr). Implications of these observations to infrared induced unimolecular decompositions and molecular energy transfer are discussed.     

Studies of the reactions of lithium atoms with methyl cyanide and methyl isocyanide in inert gas matrices

Linear LiNC is known to be the more stable isomer when isolated in inert gas matrices at low temperatures (4–15K).¹ In an attempt to form the other geometrical isomer, LiCN, studies of the reaction of lithium atoms with CH₃CN and CH₃NC were undertaken. A new absorption in the CN stretching region was observed when lithium atoms were co-condensed with CH₃NC in solid argon and xenon matrices. Photolysis of the matrices with a medium pressure Hg lamp during deposition of Li/CH₃NC seemed to favour the formation of the more stable isomer, LiNC. Reactions of lithium atoms with CH₃CN only led to the formation of LiNC, with or without photolysis.     

Time-resolved infrared spectroscopy of thiopeptide isomerization and hydrogen-bond breaking

The photoisomerization of the protected tetrathioxopeptide Boc-Ala-Gly(=S)-Ala-Aib-OMe was followed using time-resolved infrared spectroscopy in the amide I region in combination with isotope labeling. In acetonitrile at room temperature, approximately half of the molecules are found in a loop conformation, restrained by an intramolecular hydrogen bond, while the other half adopts more extended conformations. UV-excitation of the thioxopeptide unit immediately weakens the intramolecular hydrogen bond. After the molecules have relaxed to the electronic ground state with a 130 ps time-constant, a delayed re-formation of the intramolecular hydrogen bond is observed for molecules returning to the initial trans conformation of the thioamide bond, while the loop structure is permanently broken when the molecules isomerize to the cis conformation.