2-Hydroxybutadiene: Preparation,ionization energy and heat of formation

2-hydroxybutadiene was generated in the gas phase and its heat of formation determined as ?77 ± 5 kJ/mol.     

The rate of the gas phase iodination of cyclobutane. The heat of formation of the cyclobutyl radical

The gas phase iodination of cyclobutane was studied spectrophotometrically in a static system over the temperature range 589° to 662°K. The early stage of the reaction was found to correspond to the general mechanism where the Arrenius parameters describing k₁ are given by log k₁/M^?1 sec^?1 = 11.66 ± 0.11 – 26.83 ± .31/θ, θ = 2.303RT in kcal/mole. The measured value of E₁, together with the fact that E_?1 = 1 ± 1 kcal/mole, provides ΔH(c-C₄H_7.) = 51.14 ± 1.0 kcal/mole, and the corresponding bond dissociation energy, D(c-C₄H₇? H) = 96.8 ± 1.0 kcal/mole. A bond dissociation energy of 1.8 kcal/mole higher than that for a normal secondary C? H bond corresponds to one half of the extra strain energy in cyclobutene compared to cyclobutane and is in excellent agreement with the recent value of Whittle, determined in a completely different system. Estimates of ΔH and entropy of cyclobutyl iodide are in very good agreement with the equilibrium constant K₁₂ deduced from the kinetic data. Also in good agreement with estimates of Arrhenius parameters is the rate of HI elimination from cyclobutyl iodide.     

On the fragmentation pathway of the ionized enol of glycine in the gas phase

Density functional and second-order many body perturbation approaches were used to compute the potential energy surface for the fragmentation of the ionized enol of glycine [H2NCH = C(OH)2]+* into water and aminoketene radical cation [H2N-HC = CO]+*. Two possible pathways were considered. The potential energy surfaces obtained are very similar and both predict the existence of a molecular complex in which the water is coordinated to the aminoketene moiety in two different fashions with a noticeable binding energy. The fragmentation is kinetically controlled by the step in which the molecular complex is formed from the most stable cation enol of glycine. Our quantum-mechanical data confirm the hypothesis that the ylide ion [H3NCHCOOH]+* is an intermediate in the water loss.     

Pyramidane 2. Further computational studies: potential energy surface, basicity and acidity, electron-withdrawing and electron-donating power, ionization energy and electron affinity, heat of formation and strain energy, and NMR chemical shifts

The novel cycloalkane pyramidane (tetracyclo[2.1.0.0^1,30^2,5]pentane, [3.3.3.3]fenestrane), C₅H₄, with a pyramidal carbon atom, was investigated further. Calculations at the B3LYP/6-31G^* and G2(MP2) levels supported earlier conclusions from QCISD(T)/6-31G^*//MP2(FC)/6-31G^* energies that pyramidane lies in a deep well (ca. 100 kJ mol⁻¹) on the potential energy surface. The pyramidal carbon is predicted to have a lone electron pair, and calculations (CBS-4) indicate that pyramidane is remarkably basic for a saturated hydrocarbon (proton affinity 976, cf. 922 and 915 kJ mol⁻¹ for pyridine and aniline, respectively). The calculated (CBS-4) acidity is similar to that of tetrahedrane and toluene; the pyramidyl group (C₅H₃) attached to an atom bearing a lone electron pair appears to be much more strongly electron-withdrawing than the phenyl group. The infrared CO stretching frequency and C–CHO rotational barriers of pyramCHO, PhCHO and cyclopropylCHO indicate that the pyramidyl group is comparable to phenyl and cyclopropyl in its ability to donate electrons to an electron-deficient carbon. The adiabatic ionization energy of pyramidane is ca. 9.0 eV (MP2/6-31G^*, energy differences and Koopmans’ theorem), similar to that of typical cycloalkanes. The heat of formation of pyramidane was calculated by the G2(MP2) method and isodesmic reactions to be to be 585 kJ mol⁻¹ and the strain energy was estimated to be 622 kJ mol⁻¹; pyramidane is 122 kJ mol⁻¹ more strained than its isomer spiropentadiene. Application of the NMR NICS method, varying the position of the probe nucleus, gave no evidence for benzenoid-type aromaticity in the potentially cyclobutadiene cation-like base of pyramidane.     

1,1-Dimethyl-1-silaethylene : Heat of formation,ionization potential and the energy of the siliconcarbon π-bond

The thermodynamic cycle consisting of thermal decomposition and dissociative ionization processes for 1,1-dimethyl-1-silacyclobutane is calculated. The heat of formation and the ionization potential (IP) for 1,1-dimethyl-1-sila-ethylene (DMSE) have been obtained: ΔH^o_f(DMSE) = 15.5 ± 5 kcal/mol; IP(DMSE) = 7.5 ± 0.3 eV. The siliconcarbon π-bond energy in DMSE is estimated: D_π(SiC)  28 ± 8 kcal/mol.     

吡啶气相分子的共振多光子电离谱

本工作观测了双光子能量在35180-36002cm^-^1范围内吡啶分子的共振多光子电离谱。对电子跃迁S~1(^1B~1)→S~0(^1A~l)的振动带做了归属, 首次在^1B~1态观察到7个新的振动模。讨论了N原子取代对分子结构及振动模的影响。     

Electron affinities,gas phase acidities,and potential energy curves: Benzene

The experimental electron affinities of benzene, E_a(Bz), 0.4 to ?4.8 eV, are evaluated. Multiple negative ion states are proposed to account for different electron affinities. The semi‐empirical procedure known as “configuration interaction or unrestricted orbitals to relate experimental quantities to self‐consistent field values by estimating electron correlation” (CURES‐EC) has several advantages: (i) supports multiple E_a(Bz), (ii) supports the E_a(phenyl) and the D(C? H,Bz), (iii) supports the gas phase acidity of benzene from the latter, (iv) predicts the singlet–triplet split for the phenyl anion of 1.2(2) eV, and (v) predicts the existence of an excited quartet state of the benzene anion with an E_a(Bz), ?2.5(2) eV. Nine ionic Morse curves are calculated from CURES‐EC properties and experimental data. These are compared with quantum mechanical crossing “c” potentials obtained using a subroutine in commercial software and ab initio and density functional theory (DFT) procedures. Curves are calculated for the proposed quartet state of the benzene anion. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Electronic spectroscopy and ionization potential of UO2 in the gas phase

The electronic spectroscopy of UO(2) has been examined using multiphoton ionization with mass-selected detection of the UO(2) (+) ions. Supersonic jet cooling was used to reduce the spectral congestion. Twenty-two vibronic bands of neutral UO(2) were observed in the range from 17,400 to 32,000 cm(-1). These bands originated from the U(5fphi(u)7ssigma(g))O(2) X (3)Phi(2u) and (3)Phi(3u) states. The stronger band systems are attributed to metal-centered 7p<--7s transitions. Threshold ionization measurements were used to determine the ionization potentials of UO and UO(2). These were found to be higher than the values obtained previously from electron impact measurements but in agreement with the results of recent theoretical calculations.     

The heat of formation and lattice energy of copper hydride

The heat of decomposition of copper hydride was measured, and this employed in a Born-Haber cycle to yield a lattice energy of 288·6 kcal/mole. Computation of the lattice energy of a hypothetical ionic Cu⁺H⁻ by a Born-Landé equation resulted in 216 kcal/mole. These quantities were used to estimate the heat of transformation of hypothetical Cu⁺H⁻ into real CuH; other thermochemical properties were calculated as well. These results, along with considerations of electronegativity, bond energy of CuH(g), and structure, all point to the covalent nature of copper hydride.     

The kinetics and thermochemistry of the gas phase reaction of cyclopentadiene and iodine. Relevance to the heat of formation of cyclopentadienyl iodide and cyclopentadienyl radical

The rate of the reaction of cyclopentadiene with iodine has been followed spectrophotometrically over the temperature range 171.7° to 276.5°C. The reaction first proceeds almost to the point of equilibrium with cyclopentadienyl iodide and HI, although the final products are fulvalene and HI. Equilibrium constants obtained are those predicted by bond additivity. A third-law value of δH⁰_{f 298} (c-C₅H₅I,g) = 49 kcal/mole is obtained. Rate studies of the reaction up to the iodide equilibrium, yield values for the rate constant . Uncertainty in the Arrhenius parameters, as well as doubts as to the applicability of the usual assumption that E₃ = 1 ± 1 kcal/mole, make difficult an evaluation of total cyclopentadienyl stabilization energy (TSE) from these data. However, the value is probably 15 < TSE < 20.     

Reactivity in the gas phase. Behaviour of isoxazoles under negative ion chemical ionization conditions

[M ? H⁺]^? ions of isoxazole (la), 3-methylisoxazole (1b), 5-methylisoxazole (1c), 5-phenylisoxazole (1d) and benzoylacetonitrile (2a) are generated using NICI/OH^? or NICI/NH₂^? techniques. Their fragmentation pathways are rationalized on the basis of collision-induced dissociation and mass-analysed ion kinetic energy spectra and by deuterium labelling studies. 5-Substituted isoxazoles 1c and 1d, after selective deprotonation at position 3, mainly undergo N ? O bond cleavage to the stable α-cyanoenolate NC ? CH ? CR ? O^? (R = Me, Ph) that fragments by loss of R? CN, or R? H, or H₂O. The same α-cyanoenolate anion (R = Ph) is obtained from 2a with OH^?, or NH₂^?, confirming the structure assigned to the [M ? H⁺]^? ion of 1d, On the contrary, 1b is deprotonated mainly at position 5 leading, via N? O and C(3)? C(4) bond cleavages, to H? C ≡ C? O ^? and CH₃CN. Isoxazole (1a) undergoes deprotonation at either position and subsequent fragmentations. Deuterium labelling revealed an extensive exchange between the hydrogen atoms in the ortho position of the phenyl group and the deuterium atom in the α-cyanenolate NC ? CD = CPh ? O^?.     

Keto and enol forms of methyl acetate molecular ions,their stability and interconvertibility prior to fragmentation in the gas phase

The heats of formation of the keto and enol forms of the molecular ion of methyl acetate are 577 ± 4kJ mol^?1 and 477 ± 4KJ mol^?1 respectively. Fragmentation by loss of CH₃O˙ takes place at the thermochemical threshold for [CH₃CO]⁺ formation for both isomers, which may therefore freely interconvert at internal energies corresponding to this decomposition threshold.     

A theoretical investigation of atmospheric sulfur chemistry. 1. The HSO/HOS energy separation and the heat of formation of HSO, HOS, and HS2

The energy separation between the ground-state structures of HSO and HOS has been determined by using two independent ab initio methods. In the first method, the optimized geometry of all species was obtained at the HF/6-31G(d) level, as were harmonic vibrational frequencies for zero-point energy corrections. The energies were calculated by using fourth-order Moller-Plesset perturbation theory and a 6-31G(d,p) basis set. After corrections for extrapolation of the Moller-Plesset series to infinite order and extension of the basis set to include diffuse sp-, extra d-, and f-type Gaussian functions, the predicted energy separation, including zero-point vibrational effects, is 2.5 kcal/mol. HOS is the more stable isomer. The second method uses a double-zeta basis augmented with an extra set of p functions and two sets of d functions on the sulfur and oxygen atoms and a double-zeta + p basis on hydrogen. With this basis, equilibrium structures of HSO and HOS were obtained from MCSCF calculations; the energy separation between these structures was corrected by using large scale configuration interaction. In good agreement with the first method, HOS is the more stable isomer by 3.1 kcal/mol. Through calculation of the energy change in the reaction HO2 + XY --> O2 + HXY, the first method predicts the heats of formation of HXY = HSO, HOS, and HS2 to be -0.4, -2.9, and 26.7 kcal/mol, respectively.