Entropy effects in the fragmentation of 1,1-dimethylhydrazine ions

The 1,1-dimethylhydrazine ion ((CH3)2NNH2+*) has two low-energy dissociation channels, the loss of a hydrogen atom to form the fragment ion m/z 59, (CH3)(CH2)NNH2+, and the loss of a methyl radical to form the fragment ion m/z 45, the methylhydrazyl cation, CH3NNH2+. The dissociation of the 1,1-dimethylhydrazine ion has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy, in the photon energy range 8.25-31 eV, and tandem mass spectrometry. Theoretical breakdown curves have been obtained from a variational transition state theory (VTST) modeling of the two reaction channels and compared to those obtained from experiment. Seven transition states have been found at the B3-LYP/6-31+G(d) level of theory for the methyl radical loss channel in the internal energy range of 2.32-3.56 eV. The methyl loss channel transition states are found at R(N-C) = 4.265, 4.065, 3.965, 3.165, 2.765, 2.665, and 2.565 A over this internal energy range. Three transition states have been found for the hydrogen atom loss channel: R(H-C) = 2.298, 2.198, and 2.098 A. The DeltaS++(45) value, at an internal energy of 2.32 eV and a bond distance of R(N-C) = 4.265 A, is 65 J K-1 mol-1. As the internal energy increases to 3.56 eV the variational transition state moves to lower R value so that at R(N-C) = 2.565 A, the DeltaS++ decreases to 29 J K-1 mol-1. For the hydrogen atom loss channel the variation in DeltaS++ is less than that for the methyl loss channel. To obtain agreement with the experimental breakdown curves, DeltaS++(59) = 26-16 J K-1 mol-1 over the studied internal energy range. The 0 K enthalpies of formation (DeltafH0) for the two fragment ions m/z 45 and m/z 59 have been calculated from the 0 K activation energies (E0) obtained from the fitting procedure: DeltafH0[CH3NNH2+] = 906 +/- 6 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 822 +/- 7 kJ mol-1. The calculated G3 values are DeltafH0[CH3NNH2+] = 911 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 825 kJ mol-1. In addition to the two low-energy dissociation products, 21 other fragment ions have been observed in the dissociation of the 1,1-dimethylhydrazine ion as the photon energy was increased. Their appearance energies are reported.     

1,1-Carboboration

The use of very electrophilic boranes RB(C(6)F(5))(2) widens the scope of the 1,1-carboboration reaction substantially. Simple terminal alkynes HC≡CR undergo this reaction with the RB(C(6)F(5))(2) reagents rapidly under mild conditions to give high yields of very useful new alkenylborane products. Even internal alkynes RC≡CR undergo 1,1-carboboration with the RB(C(6)F(5))(2) reagents to provide a novel way of carbon-carbon σ-bond activation. Variants of these reactions involving phosphorus substituted alkynes and more complex bisalkynyl main group and transition metal substrates give rise to the formation of very interesting functionalized metallacyclic products upon treatment with RB(C(6)F(5))(2) reagents by means of reaction sequences involving selective 1,1-carboboration steps.     

UV near-resonance Raman spectroscopic study of 1,1'-bi-2-naphthol solutions

The normal and UV near-resonance Raman (UVRR) spectra of 1,1'-bi-2-naphthol (BN) in basic solution were measured and analyzed. Density functional theory (DFT) calculations were carried out to study the ground state geometry structure, vibrational frequencies nu, off-resonance Raman intensities I, and depolarization ratios rho of 1,1'-bi-2-naphtholate dianion (BN(2-)). On the basis of the calculated and experimental results of nu, I, and rho, the observed Raman bands were assigned in detail. The 1612 cm(-1) Raman band of BN in basic solution was found dramatically enhanced in the UV resonance Raman spectrum in comparison with the normal Raman spectrum. Analyzing the depolarization ratios of the 1366 and 1612 cm(-1) bands in the RR spectra manifests that both the symmetric and antisymmetric parts of transition polarizabilities contribute to the 1366 cm(-1) band, but that only the symmetric part contributes to the 1612 cm(-1) band.     

Kinetics and mechanisms of the unimolecular elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane in the gas phase: experimental and theoretical study

The gas-phase thermal elimination of 2,2-diethoxypropane was found to give ethanol, acetone, and ethylene, while 1,1-diethoxycyclohexane yielded 1-ethoxycyclohexene and ethanol. The kinetics determinations were carried out, with the reaction vessels deactivated with allyl bromide, and the presence of the free radical suppressor cyclohexene and toluene. Temperature and pressure ranges were 240.1-358.3 °C and 38-102 Torr. The elimination reactions are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 2,2-diethoxypropane, log k(1) (s(-1)) = (13.04 ± 0.07) - (186.6 ± 0.8) kJ mol(-1) (2.303RT)(-1); for the intermediate 2-ethoxypropene, log k(1) (s(-1)) = (13.36 ± 0.33) - (188.8 ± 3.4) kJ mol(-1) (2.303RT)(-1); and for 1,1-diethoxycyclohexane, log k = (14.02 ± 0.11) - (176.6 ± 1.1) kJ mol(-1) (2.303RT)(-1). Theoretical calculations of these reactions using DFT methods B3LYP, MPW1PW91, and PBEPBE, with 6-31G(d,p) and 6-31++G(d,p) basis set, demonstrated that the elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane proceeds through a concerted nonsynchronous four-membered cyclic transition state type of mechanism. The rate-determining factor in these reactions is the elongation of the C-O bond. The intermediate product of 2,2-diethoxypropane elimination, that is, 2-ethoxypropene, further decomposes through a concerted cyclic six-membered cyclic transition state mechanism.     

1,1-二甲基-2-芳基环丙烷的光化学

报道了用二环己烷胺锂盐作为碱,由苄氯和异丁烯合成1,1-二甲基-2-(1-萘基)环丙烷(1)和1,1-二甲基-2-(9-菲基)环丙烷(2)的方法,(1)或(2)的直接光解得到2-甲基-4-芳基-1-丁烯(主要产物),芳基乙烯和2,3-二甲基-2-丁烯。二苯甲醇和二苯甲酮存在下的敏化试验表明,(1)和(2)都淬灭苯频哪醇化反应,并证明确实发生了三线态向(1)或(2)的能量转移,因此是单线态的反应,不存在由芳基所造成的垂直激发态的稳定化现象,这就揭示了芳环的P轨道不参与反应过渡态。因此,重排反应系经过Hückel式四电子周环过渡态进行,该过渡态遵循“大K值对小K值“规则。     

Ground and excited state dissociation dynamics of ionized 1,1-difluoroethene

The kinetic energy release distributions (KERDs) for the fluorine atom loss from the 1,1-difluoroethene cation have been recorded with two spectrometers in two different energy ranges. A first experiment uses dissociative photoionization with the He(I) and Ne(I) resonance lines, providing the ions with a broad internal energy range, up to 7 eV above the dissociation threshold. The second experiment samples the metastable range, and the average ion internal energy is limited to about 0.2 eV above the threshold. In both energy domains, KERDs are found to be bimodal. Each component has been analyzed by the maximum entropy method. The narrow, low kinetic energy components display for both experiments the characteristics of a statistical, simple bond cleavage reaction: constraint equal to the square root of the fragment kinetic energy and ergodicity index higher than 90%. Furthermore, this component is satisfactorily accounted for in the metastable time scale by the orbiting transition state theory. Potential energy surfaces corresponding to the five lowest electronic states of the dissociating 1,1-C2H2F2+ ion have been investigated by ab initio calculations at various levels. The equilibrium geometry of these states, their dissociation energies, and their vibrational wavenumbers have been calculated, and a few conical intersections between these surfaces have been identified. It comes out that the ionic ground state X2B1 is adiabatically correlated with the lowest dissociation asymptote. Its potential energy curve increases in a monotonic way along the reaction coordinate, giving rise to the narrow KERD component. Two states embedded in the third photoelectron band (B2A1 at 15.95 eV and C2B2 at 16.17 eV) also correlate with the lowest asymptote at 14.24 eV. We suggest that their repulsive behavior along the reaction coordinate be responsible for the KERD high kinetic energy contribution.     

Solvent effects on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. A laser flash photolysis study

[reaction: see text] A laser flash photolysis study has been carried out to assess solvent effects on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. The rearrangement rate constant k decreases by increasing solvent polarity and an excellent correlation with negative slope is obtained between log k and the solvent polarity parameter E(T)N. These evidences are in full agreement with the previous indication that the extent of internal charge separation decreases on going from the starting 1,1-diarylalkoxyl radical to the transition state.     

Potential constants and Coriolis coupling constants of perhalogenated ethylenes Part 1. 1,1-Cl2C=CF2, 1,1-Cl2C=CBr2 and 1,1-F2C=CBr2

Coriolis coupling constants have been calculated from force field computations and used to evaluate the inertial defect of 1,1-dichlorodifluoroethylene, 1,1-dichlorodibromoethylene and 1,1-difluorodibromoethylene. The inertial defect values for the ground vibrational state of 1,1-C1₂C=CF₂ = 0.2450, 1,1-Cl₂C=CBr₂ = 0.3740 and 1,1-F₂C=CBr₂ = 0.4190 amu Ų show corrrespondence with the observed values of similar ethylene-type molecules.     

The thermal unimolecular isomerization of 1,1-divinylcyclopropane

The thermal isomerization of 1,1-divinylcyclopropane has been investigated in the gas phase in the temperature range of 238 to 288°C and for pressures in the range of 4 to 12 torr. The isomerization to 1-vinylcyclopentene is homogeneous and kinetically first order and almost certainly unimolecular. The rate constants yield the Arrhenius equation or The assumption of a similar transition state for this isomerization with that of l-methyl-l-vinylcyclopropane leads to a value for an alkylpentadienyl radical stabilisation energy of 19.2 ± 1.6 kcal/mole (80.4 ± 6.7 kJ/mole), and this value is compared with other estimates.     

High-energy kinetic study of chemically activated 1,1-dichlorocyclopropane

1,1-Dichlorocyclopropane has been produced by addition of CH₂(¹A₁) to 1,1-dichloroethylene. CH₂(¹A₁) was generated by the photolysis of ketene at 277–334 nm. The 1,1-dichlorocyclopropane was formed in a chemically activated state, had an energy content between 386 and 400 kJ/mol, and reacted in two parallel channels to 2,3-dichloropropene and 1,1-dichloropropene. 1,1-Dichloropropene was also formed directly by insertion of CH₂(¹A₁) into the CH bond of 1,1-dichloroethylene. As secondary reactions elimination of HCl from chemically activated 2,3-dichloropropene occurred with 3-chloropropyne and chloroallene as products. In some of the experiments perfluoropropane was added as an inert gas. The apparent rate constants for the isomerization and elimination reactions are reported. The results of RRKM calculations including distribution functions for the activated 1,1-dichlorocyclopropane and a step-ladder model for the deactivation verify the proposed reaction scheme.     

Excited state properties of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles: A quantum chemical characterization

Structure, electronic states, photoluminescence, and carries transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles for light-emitting diodes were investigated experimentally [G. Yu, et al, J. Am. Chem. Soc. 2005, 127, 6335], and the excellent electroluminescent (EL) properties of them have been found. In this paper, excited state properties of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles are studied with quantum chemistry method as well as the 3D real space analysis methods. The transition densities of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles show that the orientations and strengths of dipole moments the neutral, anionic and cationic ones are significantly different, where the neutral 1,1-disubstituted 2,3,4,5-tetraphenylsiloles show the Frenkel character; while the anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles show the plasmon character. The charge difference densities of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles reveal the important diversity of the orientations and the results of intramolecular charge transfer (ICT). The theoretical results also reveal the contribution of the substituents at the 1,1-position to the neutral and charged excited state properties of 2,3,4,5-tetraphenylsiloles. The calculated results are consistent with the experimental results, and further provide the insight of understanding to the excited state properties of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles.     

Asymmetric epoxidation of 1,1-disubstituted terminal olefins by chiral dioxirane via a planar-like transition state

Various 1,1-disubstituted terminal olefins have been investigated for asymmetric epoxidation using chiral ketone catalysts. Up to 88% ee has been achieved with a lactam ketone, and a planar transition state is likely to be a major reaction pathway.     

Quantum chemical treatment of the structure and absorption spectra of 1,1-diphenylpropyllithium and its complexes

The spatial and electronic structure and absorption spectra of butyllithium, 1-phenylpropyllithium, and 1,1-diphenylpropyllithium and their anionic and radical forms are calculated. Ionization potentials are low (about 6 eV) for all the organolithium compounds. The low-frequency electron transition in the visible range of the spectra of aromatic organolithium compounds is due to the electron density transfer from the α-carbon atom to the aromatic rings. Complexes of 1,1-diphenylpropyllithium with methylbenzoate and acetophenone, possessing low electron affinity (0.92 and 0.9 eV), are considered It is assumed that thermal electron transition occurs in the excited triplet state of the complex and is accompanied by a removal of the Li⁺ cation. A comparison of the experimental spectrum of the polystyryl-l,l-diphenylethyllithium+methylbenzoate reaction system with the calculated spectra of the MB^.-and AP^.-radical onions and MB.Li and AP.Li radicals showed that the best agreement is observed for AP.Li. Translated from Zhurnal Struktumoi Khimii, Vol. 38, No. 1, pp. 59–67, January–February, 1997.     

Preparation of 1,1-disubstituted silacyclopentadienes

Several new 1,1-disubstituted siloles containing substituents on the ring carbon atoms have been synthesized. The new siloles: 1,1-dihydrido-2,5-bis(trimethylsilyl)-3,4-diphenylsilole (5), 1,1-dihydrido-2,5-dimethyl-3,4-diphenylsilole (6), 1,1-dimethoxy-2,5-bis(trimethylsilyl)-3,4-diphenylsilole (7), 1,1-bis(4-methoxyphenyl)-2,5-bis(trimethylsilyl)-3,4-diphenylsilole (8), 1,1-dipropoxy-2,5-bis(trimethylsilyl)-3,4-diphenylsilole (9), and 1,1-dibromo-2,5-bis(trimethylsilyl)-3,4-diphenylsilole (13) were prepared from reactions originating from the previously reported, 1,1-bis(diethylamino)-2,5-bis(trimethylsilyl)-3,4-diphenylsilole (1) or 1,1-bis(diethylamino)-2,5-dimethyl-3,4-diphenylsilole (2). In addition, three other new organosilane byproducts were observed and isolated during the current study, bis(4-methoxyphenyl)bis(phenylethynyl)silane (11), bis(4-methoxyphenyl)di(propoxy)silane (12) and 1-bromo-4-bromodi(methoxy)silyl-1,4-bis(trimethylsilyl)-3,4-diphenyl-1,3-butadiene (14). Compounds 13 and 14 were characterized by X-ray crystallography and 14 is the first 1,1-dibromosilole whose solid state structure has been determined.     

Thermal [3,3]-rearrangement of 1,1-disubstituted allyl carboxylates: lone pair participation and the geminal bond participation

The diastereoselectivity of the [3,3]-rearrangement of 1,1-disubsstituted allyl carboxylates was studied. In this heteroatom-containing system, the transition state has a boat-like transition structure (TS) because of the participation of the lone pairs and the secondary orbital interaction. Although the TS for the [1,3]-rearrangement has a far higher barrier, it does not proceed in the usual antarafacial manner due to the cyclic orbital interaction among two lone pairs of the carboxylate and the allylic lumo. In conjunction with the geminal bond participation, delocalization to the σ-bond at the Z-position shows a bonding character in the transition state of the [3,3]-rearrangement. Therefore, we predicted that a more electron-withdrawing σ-bond prefers the Z-position in the product. We designed the 1,1-disubstituted substrates with trimethylsilyl and pentyl groups, and found that the trimethylsilyl group prefers the Z-position despite its steric bulkiness. We confirmed our prediction by experimentation. This Z-selectivity was improved when a trimethylgermyl group was used.     

Elektrophile Substitutionsreaktionen des 1,1-Difluoro-1H-cyclopropabenzols

Electrophilic Substitution Reactions of 1,1-;Difluoro-1H-cyclopropabenzene 1,1-Difluoro-1H-cyclopropabenzene ( 1 ) can be deprotonated with strong bases at C( 2 ). The resulting 1,1-di-fluoro-2-lithio-1H-cyclopropabenzene ( 2 ) reacts with electrophiles to form C( 2 )-substituted derivatives of 1 . The Diels-Alder reactions with electron-poor dienes, characteristic for 1H-cyclopropabenzene, do not occur with the 1,1-difluoro analogue 1.     

Solvolysis of 1,1-dimethyl-4-alkenyl chlorides: evidence for pi-participation

Tertiary 1,1-dimethyl-4-alkenyl chloride (1) solvolyzes with significantly reduced secondary beta-deuterium kinetic isotope effect (substrate with two trideuteromethyl groups) and has a lower entropy and enthalpy of activation than the referent saturated analogue 4 (k(H)/k(D) = 1.30 +/- 0.03 vs k(H)/k(D) = 1.79 +/- 0.01; Delta Delta H(++) = -9 kJ mol(-1), Delta Delta S(++) = -36 J mol(-1) K(-1), in 80% v/v aqueous ethanol), indicating participation of the double bond in the rate-determining step. Transition structure 1-TS computed at the MP2(fc)/6-31G(d) level of theory revealed that the reaction proceeds through a late transition state with considerably pronounced double bond participation and a substantially cleaved C-Cl bond. The doubly unsaturated compound 3 (1,1-dimethyl-4,8-alkadienyl chloride) solvolyzes with further reduction of the isotope effect, and a drastically lower entropy of activation (k(H)/k(D) = 1.14 +/- 0.01; DeltaS(++) = -152 +/- 12 J mol(-1) K(-1), in 80% v/v aqueous ethanol), suggesting that the solvolysis of 3 proceeds by way of extended pi-participation, i.e., the assistance of both double bonds in the rate-determining step.     

Azomethyne derivatives of 1,3-benzothiazine 1,1-dioxide

The Schiff's bases 2-alkyl-4-oxo-3,4-dihydro-2H-1,2-benzothiazine 1,1-dioxides have been synthesized for the first time, their structures in solution and in the crystalline state and their ability to form complexes have been investigated. The unusual condensation reaction of 1,2-benzothiazine 1,1-dioxide with ethyl orthoformate and 4-aminoantipyrine has been observed.     

Kinetics of the hydrogen abstraction reactions of 1,1‐ and 1,2‐difluoroethane with hydroxyl radical: an ab initio study

The hydrogen abstraction reactions of 1,1‐ and 1,2‐difluoroethane with the OH radical have been investigated by the ab initio molecular orbital theory. The geometries of the reactants, products, and transition states have been optimized at the (U)MP2=full level of theory in conjunction with 6‐311G(d,p) basis functions. Single‐point (U)MP2=full with larger basis set, such as 6‐311G(3d,2p), and QCISD(T)=full/6‐311G(d,p) calculations have also been carried out to observe the effects of basis sets utilized and higher order electron correlation. Three and four reaction channels have been identified for 1,1‐ and 1,2‐difluoroethane, respectively. In the case of 1,1‐difluoroethane, hydrogen abstraction from the α‐carbon has been found to be easier than that from the β‐carbon. The barriers of the four reaction channels for 1,2‐difluoroethane are close to each other. Weak hydrogen bonding interactions have been observed between hydroxyl hydrogen and a fluorine atom in the transition states. Rate constants for the reactions of 1,1‐ and 1,2‐difluoroethane with the OH radical have been calculated using the standard transition state theory and found to be in good agreement with the experimental results. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1305–1318, 2000     

1,1-Diaminosubstituted 2-azaallenium salts: preparation and barriers to topomerization

Dialkylcarbodiimides, $\text{5}$, react with iminium salts, $\text{6}$, to afford the formal 1,1-diaminosubstituted 2-azaallenium salts $\text{3}$. According to dynamic NMR measurements compounds $\text{3}$ must be regarded as alkylidene-guanidinium salts $\text{A}$, undergoing a fast topomerization via a transition state $\text{B}$ with allene geometry (2G⁼₂₆₅ = 45.9 ± 1 kJmol^-1 for $\text{3f}$) and a slower rotation around the peripheral C-N bonds (transition state $\text{C}$; 2G⁼₂₉₀⁼ 58.0 ± 1 kJmol^-1 for $\text{3f}$).