AM1 study of conformations of oxocane and 1,3-dioxocane

Summary AM1 semi-empirical SCF MO calculations are reported for important conformations of oxocane (1) and 1,3-dioxocane (2). The boat-chair conformation of1 (BC-1) is found to be the most stable form, whereas the crown family conformation is calculated to be 3.7 kJ·mol^–1 less stable. The boat-boat form of1 is 15.9 kJ·mol^–1 less stable thanBC-1. The boat-chair conformation of2 (BC-1,3) is calculated to be the most stable form of 1,3-Dioxocane. The crown-family conformation and the boat-boat geometry of this compound are 4.2 and 8.3 kJ mol^–1 less stable thanBC-1,3.

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AM1-Rechnungen zu den Konformationen von Oxocane und 1,3-Dioxocan

Zusammenfassung Die wesentlichen Konformationen von Oxocan (1) und 1,3-Dioxocan (2) wurden mittels semiempirischer AM1-Rechnungen (SCF MO) untersucht. Die Wanne-Sessel-Konformation von1 (BC-1) ist am stabilsten; Konformationen aus der Klasse der Kronen sind um 3.7 kJ·mol^–1 energiereicher. Die Wanne-Wanne-Konformation von1 ist um 15.9 kJ·mol^–1 instabiler alsBC-1. Die Wanne-Sessel-Konformation von2 (BC-1,3) ist das stabilste Konformer von 1,3-Dioxocan. Kronenkonformationen und Wanne-Wanne-Geometrien sind um 4.2 und 8.3 kJ·mol^–1 energiereicher alsBC-1,3.

     

AM1 calculations of bond dissociation energies. Allylic and benzylic C-H bonds

AM1 method and correlation dependence between electronic relaxation energy and valence change on the C atom of the breaking bond were used to calculate the bond dissociation energies in 50 compounds with allylic or benzylic C-H bonds. The average calculation error is 0.8 kcal/mol.     

AM1 treatment of cyclacene tubes

Semiempirical molecular orbital calculations at the level of AM1 type were performed on the nanotubes generated from the Hückel type cyclacenes. It has been found that the cryptoannulenic effect exhibited by the parent cyclacenes tends to disappear as the tube length increases.     

单重态二碘代乙叉重排反应的AM1方法研究

采用量子化学中的ＡＭ１方法研究了单重态二碘代乙叉重排反应的机理。结果发现,该重排反应经过一个三元环过渡态,反应的能势为２５．８ｋＪ／ｍｏｌ。根据计算结果,详细研究了该反应的热力学及动力学函数,结果表明该反应是放热反应,放出的热量约为２５０ｋＪ／ｍｏｌ;该反应的Ａ因子稍小于１０１３ｓ－１,是典型的单分子反应。由于反应的平衡常数和速度常数都很大,故Ｉ２Ｃ＝Ｃ：一旦生成就立即转变为ＩＣ≡ＣＩ。     

1-Methylthio-1-phenyl-1-silacyclohexane: Synthesis,conformational preferences in gas and solution by GED,NMR and theoretical calculations

1-Methylthio-1-phenyl-1-silacyclohexane 1, the first silacyclohexane with the sulfur atom at silicon, was synthesized and its molecular structure and conformational preferences studied by gas-phase electron diffraction (GED) and low temperature ¹³C and ²⁹Si NMR spectroscopy (LT NMR). Quantum-chemical calculations were carried out both for the isolated species and solvate complexes in gas and in polar medium. The predominance of the 1-MeS_axPh_eq conformer in gas phase (1-Ph_eq:1-Ph_ax = 55:45, ΔG° = 0.13 kcal/mol) determined from GED is consistent with that measured in the freon solution by LT NMR (1-Ph_eq:1-Ph_ax = 65:35, ΔG° = 0.12 kcal/mol), the experimentally measured ratios being close to that estimated by quantum chemical calculations at both the DFT and MP2 levels of theory.     

Hydrogen bonding and dissociation effects on the gas phase proton transfer reactions of ozone

Recently, the proton affinity (PA) of ozone was experimentally determined by Cacace and Speranza [Science (1994) 265: 208] using a bracketing technique that involved the proton transfer (PT) reactions: O₃H⁺+B⇒O₃+BH⁺; for different Br?nsted bases B. These authors showed that the simple collision model is not adequate to describe PT. We now present a theoretical model reflecting this bracketing procedure by explicitly introducing H-bonding complexing, dissociation and PT contributions, to discuss the kinetic model that assumes that PT occurs through one elementary step. The methods used include semiempirical density functional theory and ab initio Hartree-Fock methods. The procedure is gauged by using estimated PA of ozone obtained from deprotonation reactions including the Br?nsted bases BNH₃, H₂O, HOCl, SO₂, CH₃F and Kr. The PA-obtained range was from 145.3 to 160.3 kcal/mol, in fair agreement with the experimental value of 148.0±3 kcal/mol. The model seems to be fairly independent of the reference bases used to evaluate the PA. H-bonding effects appear to be a determining factor to explain collision efficiencies. Received: 5 August 1997 / Accepted: 25 September 1997     

Kinetics and mechanism of 2-pyridylacetic acid pyrolysis in the gas phase: A joint experimental and theoretical study

A combination of the experimental and theoretical study was carried out on the reaction mechanism associated with the pyrolysis of 2-pyridylacetic acid in the gas phase. Methylpyridine and carbon dioxide were analyzed as the products, using a static system over the pressure range of 18–55 torr and the temperature of 541.2–583.4 K. The experimental kinetic data show that the pyrolysis process is homogeneous, unimolecular and proceeds through a concerted mechanism. Theoretical studies at the B3LYP level using the 6-31G^* basis set confirmed an asynchronous concerted mechanism for the reaction. Computed kinetic and activation parameters are in good agreement with the experimental one.     

荧光钙敏试剂Indo—1 AM的合成

Indo-1AM 是一种新的测定细胞内自由钙离子浓度的强荧光试剂。本文详细报导了从邻硝基酚及α-溴甲基苯甲酸酯出发经十—步反应的合成方法及其荧光性能,并提出了由 Indo-1酸o‖引入CH_3—C—OCH_2—(AM)基团的简易方法。     

AM1 treatment of some dinitro derivatives of [22](1,4)-cyclophane

Semiempirical molecular orbital treatment at the level of AM1(RHF) approach has been performed on [2₂](1,4)-cyclophanes (para-cyclophane derivatives) having two nitro substituents at different positions (the bridge positions and phane-deck(s)). All the isomeric structures presently studied are found to be stable but endothermic.     

Evaporation of the liquid stationary phase in gas chromatography

Summary The theory of the evaporation of the liquid stationary phase is elaborated and experimentally verified. On the basis of this theory the role played by the losses in the amount of liquid phase present is quantitatively determined. General techniques are examined which minimize the losses; these techniques are based on saturating the incoming carrier gas with liquid phase vapours and raising the pressure of the carrier gas in the column, e.g. by connecting a capillary to the column outlet in order to offer resistance to the gas flow. The application of these techniques ensures stable performance of the gas chromatographic columns using a volatile liquid phase.     

Synthesis,conformational preferences in gas and solution,and molecular gear rotation in 1-(dimethylamino)-1-phenyl-1-silacyclohexane by gas phase electron diffraction (GED), LT NMR and theoretical calculations

1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED), low temperature ¹³C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1-Ph_ax conformer (1-Ph_eq:1-Ph_ax ratio of 20:80%, ΔG°(317?K)?=??0.87?kcal/mol) in the gas phase is close to the theoretically estimated conformational equilibrium. In solution, low temperature NMR spectroscopy showed analyzable decoalescence of C_ipso and C(1,5) carbon signals in ¹³C NMR spectra at 103?K. Opposite to the gas state in the freon solution employed (CD₂Cl₂/CHFCl₂/CHFCl₂?=?1:1:3), which is still liquid at 100?K, the 1-Ph_eq conformer was found to be the preferred one [(1-Ph_eq: 1-Ph_ax?=?77%: 23%, K?=?77/23?=?2.8; ?ΔG°?=??RT ln K (at 103?K)?=?0.44?±?0.1?kcal/mol]. When comparing 1 with 1-phenyl-1-(X)silacylohexanes (X?=?H, Me, OMe, F, Cl), studied so far, the trend of predominance of the Ph_ax conformer in the gas phase and of the Ph_eq conformer in solution is confirmed.     

Ring expansions in substituted phenylnitrenes: an AM1 SCF-MO study

The semi-empirical AM1 SCF-MO method is used to study the ring expansion of ortho-substituted phenylnitrenes in the singlet state. This two-step rearrangement involves nitrene atom insertion into the phenyl ring to give a bicyclic azepine intermediate, which undergoes electrocyclic ring opening to yield the monocyclic keteneimine product. The first step of nitrene insertion in ortho-substituted phenylnitrenes may occur either towards the substituent side or away from it, while the second step is simply sequential to the first. Nine ortho-substituted phenylnitrene systems X–C₆H₄–N (X=H, CH₃, CN, NH₂, NO₂, OH, F, SH and Cl) are considered for study here.

Comparison of activation energy barriers for both steps predicts that the first step of azepine formation would be the rate-determining step. Electron-withdrawing substituents promote this step, and vice versa for electron-donating ones. The azepine intermediate is unstable compared to the keteneimine product due to strain. The transition state for the second step is predicted to have aromatic character, unlike the azepine and ketenimine. In general, the ring expansion is predicted to be favoured towards the unsubstituted side of the ring rather than towards the substituted side, where steric factors play the major role. The two successive steps of the ring expansion proceed via transition states which are more or less similar to each other, the second one having aromatic character. Competition between ring expansion and decay of the singlet phenylnitrene to the triplet state, as estimated by calculated enthalpy terms, is predicted to favour decay to the triplet state.     

A simple detection system for gas chromatography based on molecular absorption spectrometry in the gas phase

Summary A simple and inexpensive detection system for gas chromatography, based on gas-phase, molecular absorption measurements, is presented in which the chromatographic column is directly joined to the spectrophotometer flow cell, without heated transfer lines. A mixture of eight benzene compounds (including methyl, halogen and nitrogen derivatives) were separated and analyzed. Parameters affecting separation (temperature program and carrier gas flow) and measurement quality (wavelength and integration time) were studied and a measurement program designed to modify the wavelength during chromatography. The analytical characteristics of each compound were calculated, obtaining detection limits ranging from 0.5 to 9 g mL^–1. Finally, the method was applied to several synthetic mixtures, with good results.     

Sugars in the gas phase: The conformational properties of erythrose, threose, and erythrulose characterized by quantum chemistry calculations

The structural properties of the three open chain C₄H₈O₄ sugars, i.e. two aldoses (erythrose and threose) and one ketose (erythrulose), have been investigated by DFT and ab initio calculations to get accurate structures and relative energies. The structure of all the conformers predicted within 10 kJ/mol has been optimized at the B3LYP/6-311++G(d,p) level of the theory. Two types of intramolecular hydrogen bonds have been clearly identified. They are related to the hydroxyl and to the carbonyl oxygen atoms and are of weak and middle strength, respectively. The most stable structures have been optimized at the B3LYP/6-311++G(2df,p) and at the MP2/6-311++G(2df,p) levels of the theory in order to calculate accurate rotational parameters and dipole moment for their future detection in the microwave range in the gas phase. Their corresponding harmonic IR spectra have also been calculated and their fingerprint signature is discussed in the region of the OH stretching vibrations, of the torsion of the C–O bonds and of the deformation of the C–C skeleton.     

Quantum-chemical investigation of the mechanism of cyclocondensation of 4-hydroxy-4-methylpentan-2-one with cyanoacetamide using the AM1 method

The mechanism of formation of 4,6,6-trimethyl-2-oxo-1,2,5,6-tetrahydropyridine-3-carbonitrile from 4-hydroxy-4-methylpentan-2-one and cyanoacetamide in the presence of ammonium acetate has been studied by the AM1 method. It was found that, under the reaction conditions, the amide is readily converted to an iminol tautomeric form which takes part in subsequent reaction. It was shown that the reaction is a cascade process forming two intermediates. The final product 4,6,6-trimethyl-2-oxo-1,2,5,6-tetrahydropyridine-3-carbonitrile is formed from the product of a Knoevenagel condensation via an intramolecular nucleophilic substitution mechanism. On the basis of the activation energies obtained it can be deduced that the limiting stage is the deprotonation process of the cyanoacetiminol. Dedicated to Professor L. I. Belen’kii with gratitude and deep respect. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 209–216, February, 2006.     

MP2 study of substituent effects of 2-substituted alkyl ethyl methylcarbamates in homogeneous, unimolecular gas phase elimination reaction

Møller-Plesset MP2/6-31G method was used to examine the gas-phase elimination of 2-substituted alkyl ethyl N,N-dimethylcarbamates. The results of these calculations support a concerted non-synchronous six-membered cyclic transition state mechanism for carbamates containing a C_β–H bond at the alkyl side of the ester. These substrates produce the N,N-dimethylcarbamic acid and the corresponding olefin. The unstable intermediate, N,N-dimethylcarbamic acid, rapidly decomposes through a four-membered cyclic transition state to dimethylamine and CO₂ gas. Correlation of the logarithm of theoretical rate coefficients against original Taft's σ* values gave an approximate straight line (ρ*=−1.39, r=0.9558 at 360 °C). In addition to this fact, when log k_rel is plotted against the theoretical log k_rel for 2-substituted ethyl N,N-dimethylcarbamates a reasonable straight line (r=0.9919 at 360 °C) is obtained, suggesting similar mechanism.     

细菌光合反应中心Q~A和Q~B间电子转移反应的量子化学研究

用量子化学半经验的AM1和密度泛函DFT(BELYP/6-31G(d))方法分别优化了质体醌MQ1(Q~A)、泛醌UQ1(Q~B)及其阳离子自由基的结构。用Nelsen方法计算了电子转移反应MQ1-UQ1→MQ1UQ^-~1的内重组能λi。用线性反应坐标方法构造了该电子转移反应的双势阱,两透热势能面在反应坐标R≈0.30处相交。对该电子转移体系进行闭壳层的单点计算,并用Koopmans定理计算了体系的分裂能△,得到△随线性反应坐标R的变化关系。结果表明,在R=0.342处△有一极小值,从而得到该电子转移反应的电子转移矩阵元Vrp,并由此确定了反应的过渡态。在此基础上,用两球模型计算了反应的溶剂重组能λ0。本文还计算了该电子转移反应的活化自由能△G。最后,根据Marcus电子转移理论计算了该反应的速率常数ket为5.93×10^4s^-^1,由此得到该反应的半衰期与文献报道的结果一致。     

1,3,3-三硝基氮杂环丁烷及其衍生物的热解机理的AM1研究

用ＳＣＦ－ＭＯ－ＡＭ１方法计算研究了１，３，３－三硝基氮杂环丁烷和它的三个衍生物的分子几何构型，电子结构和热解机理，分子中连结偕二硝基的两个Ｃ－Ｎ键级较小，均裂该建所需活化能较低，可能是优先发生的主要热解引发反应，四个标题物均裂该键的活化能和该键的Ｗｉｂｅｒｇ键级之间存在良好的线性关系，线性相关系数为０．９９。     

7─羟基喹啉与甲醇、乙醇形成1：2桥式氢键化合物的AM1法研究

利用半经验ＡＭ１方法计算了基态与第一激发态７－羟基喹啉的两种异构体及其与甲醇等溶济分子形成１：２桥式氢键化合物的结构与稳定性。在基态，烯醇式结构比酮式结构稳定；而在第一激发态酮式比烯醇式稳定。１：２桥式氢键的形成使得酮式能量降低比烯醇式多。烯醇式１：２桥式氢键化合物呈交叉式结构，酮式１：２桥式氢键化合物呈折叠式结构，激发态的形成对氢键结构影响不大。在７－羟基喹啉羟基（或羰基）的邻位和间位引入取代基后，对喹啉环和桥式氢键结构的影响均不明显。