Experimental and theoretical study of the pyrolysis of trichlorosilane

Pyrolysis of trichlorosilane (TCS) and copyrolysis of TCS with 1,3-butadiene were studied. The enthalpies and activation energies for the reactions of the products of TCS pyrolysis were found by quantum-chemical calculations. A direct study of the pyrolysis of TCS by mass spectrometry was carried out. Based on the thermochemical parameters found by quantum-chemical calculations and on the results of GLC and mass spectrometry concerning the composition of the pyrolysis products, it was concluded that the pyrolysis of TCS follows a scheme that includes formation of radicals and silylenes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1659–1662, September, 1997.     

Experimental and theoretical study of the 2-alkoxyethylidene rearrangement

The rearrangement of 2-ethoxyethylidene, generated photochemically from a nonnitrogenous precursor, leads to ethyl vinyl ether. Although this product could result, in principle, from a 1,2-hydrogen shift and/or a 1,2-ethoxy shift in the carbene, a deuterium labeling study indicates an essentially exclusive preference for hydrogen migration. The experimental results are in agreement with CCSD and W1BD calculations for the simpler 2-methoxyethylidene system that show a prohibitively large barrier for the methoxy shift and a negligible barrier for the hydride shift.     

Experimental and theoretical study on the photoionization of styrene

This study employed a vacuum ultraviolet synchrotron radiation source and reflectron time-of-flight mass spectrometry (TOF-MS) to investigate the photoionization and dissociation of styrene. By analyzing the photoionization mass spectrum and efficiency curve alongside G3B3 theoretical calculations, we determined the ionization energy of the molecular ion, appearance energy of fragment ions, and relevant dissociation pathways. The major ion peaks observed in the photoionization mass spectra of styrene correspond to C₈H₈⁺, C₈H₇⁺ and C₆H₆⁺. The ionization energy of styrene is measured as 8.46 ± 0.03 eV, whereas the appearance energies of C₈H₇⁺ and C₆H₆⁺ are found to be 12.42 ± 0.03 and 12.22 ± 0.03 eV, respectively, in agreement with theoretical values. The main channel for the photodissociation of styrene molecular ions is the formation of benzene ions, whereas the dissociation channel that loses hydrogen atoms is the secondary channel. Based on the experimental results and empirical formulas, the required dissociation energies (E_d) of C₈H₇⁺, C₈H₆⁺ and C₆H₆⁺ are calculated to be (3.96 ± 0.06), (4.00 ± 0.06) and (3.76 ± 0.06) eV, respectively. Combined with related thermochemical parameters, the standard enthalpies of formations of C₈H₈⁺, C₈H₇⁺, C₈H₆⁺ and C₆H₆⁺ are determined to be 964.2, 1346.3, 1350.2 and 1327.0 kJ/mol, respectively. Based on the theoretical study, the kinetic factors controlling the styrene dissociation reaction process are determined by using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. This provides a reference for further research on the atmospheric photooxidation reaction mechanism of styrene in atmospheric and interstellar environments.     

Experimental and theoretical study of the conformational analysis of hydrochlorothiazide

Hydrochlorothiazide was characterized in order to determine the possible structural modifications at different temperatures due to its importance as a drug to control heart diseases and a diuretic. This compound could present conformers due to the rotation of the NH₂ group, which was studied by using different techniques such as Thermal Analysis, IR spectroscopy, X-Ray Diffraction and complemented by theoretical calculations. The theoretical and experimental results point to the conclusion that no polymorphic forms are present in the compound under study. The calculations confirm the apparent difference in values between theory and experiment for the vibration frequencies explained by the hydrogen bonds between near molecules.     

Experimental and theoretical study of the electronic spectrum of BeAl

The electronic structure of BeAl was investigated by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. BeAl was formed by pulsed laser ablation of a Be/Al alloy in the presence of helium carrier gas, followed by a free jet expansion into vacuum. In agreement with recent ab initio studies, the molecule was found to have a (2)Pi(1/2) ground state. Transitions to two low lying electronic states, (2)(2)Pi(1/2)(v') <-- X (2)Pi(1/2) (v' = 0) and (1)(2)Delta(v') <-- X (2)Pi(1/2) (v' = 0,1), were observed and rotationally analyzed. An additional band system, identified as (4)(2)Sigma(+)(v') <-- X (2)Pi(1/2), was found in the 28 000-30 100 cm(-1) energy range. This transition exhibited an unusual pattern of vibrational levels resulting from an avoided crossing with the (5)(2)Sigma(+) electronic state. New multi-reference configuration interaction calculations were carried out to facilitate the interpretation of the UV bands.An ionization energy of 48 124(80) cm(-1) was determined for BeAl from photoionization efficiency (PIE) measurements. Fine structure in the PIE curve was attributed to resonances with Rydberg series correlating with vibrationally excited states of the BeAl(+) ion. Analysis of this structure yielded a vibrational frequency of 240(20) cm(-1) for the cation.     

Experimental and theoretical electron density study of estrone

The electron density and the electrostatic potential (ESP) distributions of estrone have been determined using X-ray diffraction analysis and compared with theoretical calculations in the solid and gas phases. X-ray diffraction measurements are performed with a Rigaku Rapid rotating anode diffractometer at 20 K. The electron density in the estrone crystal has been described with the multipole model, which allowed extensive topological analysis and calculation of the ESP. From DFT calculations in the solid state a theoretical X-ray diffraction data set has been produced and treated in the same way as the experimental data. Two sets of single molecule DFT calculations were performed: (a) An electron density distribution was obtained via a single-point calculation with a large basis set at the experimental geometry and subsequently analyzed according to the quantum theory of atoms in molecules (AIM) to obtain the bond and most atomic properties, and (b) another electron density distribution was obtained with a smaller basis set, but at a geometry optimized using the same basis set for the analysis of atomic energies. An interesting locally stabilizing hydrogen-hydrogen bond path linking H(1) and H(11B) is found which represents the first characterization of such bonding in a steroid molecule. AIM delocalization indices were shown to be well correlated to the experimental electron density at the bond critical points through an exponential relationship. The aromaticity of ring A, chemical bonding, the O(1)...O(2) distance necessary for estrogenic activity, and the electrostatic potential features are also discussed.     

Experimental and theoretical charge density study of the neurotransmitter taurine

The experimental electron density distribution in taurine, 2-aminoethane sulfonic acid, 1, has been determined from high-resolution X-ray diffraction data collected at a temperature of 100 K. Taurine crystallizes as a zwitterion in the monoclinic space group P2(1)/c. Topological analysis of the experimental electron density and a comparison with high-level theoretical gas-phase calculations show that the crystal environment has a significant influence on the electronic configuration of the sulfonate moiety in 1, which in the crystal is more delocalized than in the gas phase. This crystal effect is mainly due to hydrogen bonding.     

Experimental and theoretical study of substituent effects of iodonitrobenzenes

The electronic structures and substituent effects of o-, m-, and p-iodonitrobenzene have been studied by ultraviolet photoelectron spectroscopy (UPS). The observed bands were interpreted on the basis of empirical arguments and theoretical calculations. The analysis of electronic effects of the donor/acceptor substituent groups is essential for the reliable assignment of the observed photoelectron spectra. The investigation of pi- and n-orbital ionization potentials enabled us to describe the correlation between substituent effects and the relative reactivities of the iodonitrobenzenes. It was found that the energy order of the pi(2) and n(II) parallel orbitals is reversed as a result of the combined influence of the electron-withdrawing nitro group and the electron-donating iodine atom. Distinct changes of the pi and n bands occur in o-iodonitrobenzene. This characteristic depends on the conjugation between the pi orbitals of the benzene ring and the nitro group and the interaction of in-plane lone pairs of iodine and one of the oxygen atoms of the nitro group in the adjacent position. This might contribute to the high reactivity of o-iodonitrobenzene in a number of reactions.     

Experimental and theoretical study of the photodissociation of bromo-3-fluorobenzene

The UV photodissociation of bromo-3-fluorobenzene under collisionless conditions has been studied as a function of the excitation wavelength between 255 and 265 nm. The experiments were performed using ultrafast pump-probe laser spectroscopy. To aid in the interpretation of the results, it was necessary to extend the theoretical framework substantially compared to previous studies, to also include quantum dynamical simulations employing a two-dimensional nuclear Hamiltonian. The nonadiabatic potential energy surfaces (PES) were parameterized against high-level MS-CASTP2 quantum chemical calculations, using both the C-Br distance and the out-of-plane bending of the bromine as nuclear parameters. We show that the wavelength dependence of the photodissociation via the S0-->1pipi*-->1pisigma* channel, accessible with a approximately 260 nm pulse, is captured in this model. We thereby present the first correlation between experiments and theory within the quantitative regime.     

Experimental and theoretical study of thermodynamic properties of levoglucosan

The heat capacity of levoglucosan was measured over the temperature range (5 to 370) K by adiabatic calorimetry. The temperatures and enthalpies of a solid-phase transition and fusion for the compound were found by DSC. The obtained results allowed us to calculate thermodynamic properties of crystalline levoglucosan in the temperature range (0 to 384) K. The enthalpy of sublimation for the low-temperature crystal phase was found from the temperature-dependent saturated vapor pressures determined by the Knudsen effusion method. The thermodynamic properties of gaseous levoglucosan were calculated by methods of statistical thermodynamics using the molecular parameters from quantum chemical calculations. The enthalpy of formation of the crystalline compound was found from the experiments in a combustion calorimeter. The gas-phase enthalpy of formation was also obtained at the G4 level of theory. The thermodynamic analysis of equilibria of levoglucosan formation from cellulose, starch, and glucose was conducted.     

Ring-chain isomerism in conjugated guanylhydrazones: Experimental and theoretical study

Treatment of 1,3-diaryl-propene-2-one with aminoguanidine under acidic conditions for a short reaction period (1?h) delivers - in accordance with a literature report - the corresponding guanylhydrazones. However, when the reaction time was increased to 12?h, formation of the ring annulated product 4,5-dihydro-1H-pyrazole-1-carboximidamide was observed. This is the first case of ring-chain isomerism in conjugated guanylhydrazones. The acyclic conjugated guanylhydrazone and the corresponding annulated product (4,5-dihydro-1H-pyrazole-1-carboximidamide) could be clearly distinguished by means of UV and NMR spectroscopy. The formation of the ring isomer was further confirmed by single crystal XRD analysis. The time-dependent ¹H NMR study indicated the gradual transformation of the open-chain compound into the cyclic one. The mechanistic insights into the formation of these two products were explored using quantum chemical methods which revealed that the ring isomer is thermodynamically more stable than the open-chain isomer by 6–11?kcal/mol and the barrier for cyclization was found to be 31.37?kcal/mol.     

Theoretical study of free-radical migrations

Free-radical migrations have been investigated by ab initio molecular orbital theory to account for the facts that 1,2-migrations are observed for Cl-atoms, aryl or vinyl groups, whereas for H atoms only 1,5- and 1,6-migrations have been seen in contrast to the easy 1,2-hydride shifts in cations. The optimum geometry and energy models of the transition states of 1,2- and 1,5-migrations, and of their corresponding initial states have been determined using the Gaussian 70 STO-3G RHF method. The calculated activation energies ΔE are in agreement with experimental observations. The main features of the 1,2-migration reactions are (i) ΔE is more important for elements of the first and second row of the periodic classification than for those belonging to the third row; (ii) protonation strongly reduces ΔE. For both 1,2- and 1,5-migrations a correlation exists between ΔE and the energetic change of the frontier orbital, Δ?_somo: the reactions are under frontier orbital control.     

Experimental and theoretical study of the basicity of azoles and their analogs

The thermodynamic ionization constants of 30 imidazole, pyrazole, triazole, and thiazole derivatives and their structural analogs were determined by potentiometric titration in acetonitrile. The effect of individual structural factors on the basicity of the azole molecules is discussed. It is demonstrated that the best theoretical basicity index is the magnitude of the electrostatic energy of interaction () of the unshared pair of the nitrogen atom being protonated with the system of charges. The values were calculated by means of the MO self-consistent-field (MO SCF) method with the Pariser-Parr-Pople approximation and by the simple Hückel method. The pK_a- correlation equations obtained make it possible to predict the pK_a values of azoles with an average accuracy of 0.5 pK_a units for acetonitrile solutions and 0.3 units for aqueous solutions.Communication XI of the series Basicity and Structure of Azomethines and Their Structural Analogs. See [1] for communication X.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 552–557, April, 1972.     

Experimental and theoretical study of the infrared spectra of BrHI- and BrDI-

Gas phase vibrational spectra of BrHI- and BrDI- have been measured from 6 to 17 microm (590-1666 cm(-1)) using tunable infrared radiation from the free electron laser for infrared experiments in order to characterize the strong hydrogen bond in these species. BrHI-.Ar and BrDI-.Ar complexes were produced and mass selected, and the depletion of their signal due to vibrational predissociation was monitored as a function of photon energy. Additionally, BrHI- and BrDI- were dissociated into HBr (DBr) and I- via resonant infrared multiphoton dissociation. The spectra show numerous transitions, which had not been observed by previous matrix studies. New ab initio calculations of the potential-energy surface and the dipole moment are presented and are used in variational ro-vibrational calculations to assign the spectral features. These calculations highlight the importance of basis set in the simulation of heavy atoms such as iodine. Further, they demonstrate extensive mode mixing between the bend and the H-atom stretch modes in BrHI- and BrDI- due to Fermi resonances. These interactions result in major deviations from simple harmonic estimates of the vibrational energies. As a result of this new analysis, previous matrix-isolation spectra assignments are reevaluated.     

Experimental and theoretical study of the reaction of OH radical with sabinene

The gas phase sabinene + OH reaction is studied both experimentally and theoretically. Product yields from the reaction of sabinene with OH radicals have been measured in the absence of NOx in the UCC chamber (Cork, Ireland) and in the presence of NOx in the LISA chamber. Three primary carbonyl compounds were observed and quantified: acetone in [(24 +/- 6)%], formaldehyde in [(25 +/- 6)%] and sabinaketone in [(20 +/- 6)%]. The simultaneous quantification of these compounds is one of the major results of this work. The mechanism of product formation for this reaction has been studied using the quantum chemical DFT-B3LYP (6-31G(d,p) method. According to these calculations, the H-atom abstraction channel from sabinene by OH in the initial oxidation step may be taken into account to explain the acetone production. Sabinaketone and formaldehyde are mainly products of the addition channels of OH on the -C=CH2 double bond of sabinene. This is the first theoretical work on the title reaction.     

Experimental and theoretical NMR study of selected oxocarboxylic acid oximes

1H and 13C NMR spectra of the oxocarboxylic acid oximes 2-hydroxyiminopropanoic acid (1), 2-(4-methylthiazol-2-yl)-2-(hydroxyimino)acetic acid (2) and 2-cyano-2-(hydroxyimino)acetic acid (3) were measured in DMSO-d6, D2O and acetone-d6 solutions. The data indicate the presence of hydrogen bonding in 1 and 2 and a strong electron-withdrawing effect due to the cyano group in 3. The effect of intra- and intermolecular hydrogen bonding on the hydrogen and carbon chemical shifts in these molecules was studied theoretically. Total energy calculations of the stability of various hydrogen-bonded species, in addition to equilibrium parameters and chemical shifts, were calculated using ab initio methods (RHF, MP2) and density functional theory (B3LYP), implemented in the Gaussian 98 software package. The gauge-including atomic orbital (GIAO) method was used to predict magnetic shielding constants. Chemical shift calculations for the most stable species agree fairly well with the observed data, especially for the hydroxyl protons. Substituents adjacent to the alpha-carbon show some influence of the oximic and carboxyl groups on the 13C chemical shifts, as expected for groups with different polar and anisotropic character.     

Protodesilylation of 2,6-disubstituted silyphosphinines. Experimental and theoretical study

2,6-Disilylphosphinines react with HCl in ethereal solution to cleanly yield the corresponding 2,6-unsubstituted derivatives. DFT calculations allowed rationalization of the mechanism of this protodesilylation.     

Benchmark thermodynamic properties of alkanediamines: Experimental and theoretical study

Vapor pressures of (dl)-1,2-propanediamine and 2-methyl-1,2-propanediamine were measured using the transpiration method. Molar enthalpies of vaporization were derived from the vapor pressure temperature dependence. Thermodynamic data on alkanediamines available in the literature were collected and treated uniformly. Consistency of the experimental data set for alkanediamines was evaluated with group-contribution and quantum-chemical methods.The standard molar entropy of formation and the standard molar Gibbs function of formation have been calculated. Vaporization and formation enthalpies of alkanediamines of benchmark quality are recommended for practical thermochemical calculations and validation of empirical and theoretical methods.