A MNDO study of the structural and electronic properties of polysilane model compounds

The results of MNDO geometry optimizations on selected H? (SiH₂)_n? H polysilane model compounds are presented. Near energetic degeneracy is indicated for all-trans(T), alternating gauche–trans (GT), and all-gauche (G⁺G⁺) models (n = 10). The most stable (T) and least stable (G⁺G⁺) conformations are separated by only ca. 0.11 eV. The existence of low-energy barriers to moderate structural distortion is also suggested. Orbital localizations and charge density distributions along the “polymer” backbone are found to be sensitive functions of such distortion. The ground-state electronic distribution of the saturated all-trans silane chains are calculated to be considerably more polarizable than the fully conjugated H? (CH)_n? H π-electron framework of comparable length. The one-electron HOMO → LUMO excitation can be viewed essentially as an in-plane Si 3p → Si3s + H1s intramolecular charge transfer transition. The qualitatively different atomic orbital character of the HOMO and LUMO levels yields transition moment components for the separate repeat units which are relatively small. In the case of the rigidly trans conformation, the phase relationships of the transition moment terms are such as to constructively sum to a large net value reflecting strong optical absorption, as is observed experimentally.     

Theoretical study on a new kind of thienyl-functionalized polysilane

A series of theoretical calculations were performed on thienyl-functionalized polysilanes in order to understand their geometries, excited state spectra, energy band structures, and adsorption stability onto Au (111) surfaces. The thienyl functionalization causes a significant red-shift of the electronic spectra of the oligosilanes, which is due to the maximum absorption energy resulting from σ → π* transition in oligo(methylthienylsilane) much lower than that from σ → σ* transition, and a great decrease of the σ → σ* transition energy in thienyl end-functionalized oligo(dimethylsilane). The thienyl substitution exerts a small effect on the band gap of the polysilane, and yet disturbs slightly the σ electronic delocalization. There exists a stable adsorption between thienyl-functionalized polysilanes and Au (111) surfaces through non-bonding interaction of thiophenes with Au.     

10.5-eV photoionization mass spectrometry of aliphatic compounds

Vacuum ultraviolet photoionization by coherent 10.5-eV radiation is evaluated for soft ionization of organic molecules. Coherent 10.5-eV radiation is produced by frequency tripling the third harmonic of an Nd:YAG laser (355 nm) in a mixture of xenon and argon. A number of intermediate size, C₆ to C₈, aliphatic compounds are studied to determine the extent and characteristics of fragmentation. Compared with 12-eV electron impact ionization, all show higher molecular ion abundances, less fragmentation, and significant enhancement of low energy rearrangement ions. n-Alkanes, alkenes, ketones, carboxylic acids, and ethers all form predominant molecular ions. Aldehydes and amines show significant molecular ion abundances but also extensive fragmentation. Branched alkanes, dienes, alcohols, and esters show little or no molecular ion, but do have a single dominant fragment ion. Metastable broadening in these spectra gives qualitative information on the energetics of low energy rearrangements.     

Block interrupt polysilane derivatives

High molecular weight block interrupt polysilane derivatives have been prepared by the condensation of a number of α,ω-dichloropolysilanes with some aromatic bis silanols. In this manner, polymers containing polysilane blocks of 2, 4, or 6 catenated silicon atoms have been prepared. The materials are thermally stable and the UV spectroscopic properties mimic those of the corresponding monomers. Although these materials bleach upon irradiation in the deep UV, surprisingly, the block interrupt polysilanes behave as negative resists upon exposure to 254 nm radiation. This behavior contrasts with that of standard polysilane homopolymers which usually function in a positive lithographic mode.     

Block interrupt polysilane derivatives

High molecular weight block interrupt polysilane derivatives have been prepared by the condensation of a number of α,ω-dichloropolysilanes with some aromatic bis silanols. In this manner, polymers containing polysilane blocks of 2, 4, or 6 catenated silicon atoms have been prepared. The materials are thermally stable and the UV spectroscopic properties mimic those of the corresponding monomers. Although these materials bleach upon irradiation in the deep UV, surprisingly, the block interrupt polysilanes behave as negative resists upon exposure to 254 nm radiation. This behavior contrasts with that of standard polysilane homopolymers which usually function in a positive lithographic mode.     

Ultrasonic degradation of polysilane polymers

The degradation of four polysilanes in solution irradiated by ultrasonic waves has been studied by measurements of reduced viscosity and dynamic mechanical behavior. In all cases ultrasonic degradation occurs. The ultrasonic waves cause the average molecular weight to decrease and the molecular-weight distribution to become narrower in a relatively short period. The degree and rate of degradation are related to the intensity of the ultrasonic waves. In addition, the degree of degradation decreases with increasing polymer concentration and the rate increases with increasing initial molecular weight of the polysilane. After irradiation, the molecular weights of polysilanes tended to the same value from various initial values.     

Threshold-photoelectron-spectroscopy-coincidence study of the double photoionization of HBr

A threshold-photoelectron-coincidence spectrum of HBr has been recorded in the 32.2-35.8 eV photon energy range, with a resolution of approximately 0.01 eV, using a synchrotron radiation source. The X (3)Sigma(-) and a (1)Delta(2) states of the HBr(2+) dication are clearly observed in the spectrum, while there is no clear evidence for the formation of the b (1)Sigma(+) electronic state. For the first two states, the vibrational states v=0-3 have been resolved, while for the ground X (3)Sigma(-) state also spin-orbit splitting has been detected. The results appear in good agreement with previous experimental observations. A comparison with theoretical predictions indicates the role of "noncovalent" contributions to the interaction between the two atomic partners for the formation of metastable states.     

Vacuum ultraviolet photoionization mass spectrometric study of ethylenediamine

The photoionization and dissociative photoionizations of ethylenediamine have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions NH(2)CHCH(3)(+), NH(2)CH=CH(2)(+), CH(2)NH(2)(+), NH(3)(+), NH(2)CH(2)CHNH(2)(+) and NH(2)CH(2)CH(2)NH(2)(+) have been obtained. In addition, the energetics of the dissociative photoionization is investigated with ab initio Gaussian-3 (G3) calculations. The computational results are useful in analyzing the dissociation channels near the ionization thresholds. With the help of the G3 results, the dissociation channels for the formation of the aforementioned fragment ions have been established.     

Vacuum ultraviolet photoionization mass spectrometric study of cyclohexene

In this work, photoionization and dissociation of cyclohexene have been studied by means of coupling a reflectron time‐of‐flight mass spectrometer with the tunable vacuum ultraviolet (VUV) synchrotron radiation. The adiabatic ionization energy of cyclohexene as well as the appearance energies of its fragment ions C₆H₉⁺, C₆H₇⁺, C₅H₇⁺, C₅H₅⁺, C₄H₆⁺, C₄H₅⁺, C₃H₅⁺ and C₃H₃⁺ were derived from the onset of the photoionization efficiency (PIE) curves. The optimized structures for the transition states and intermediates on the ground state potential energy surfaces related to photodissociation of cyclohexene were characterized at the ωB97X‐D/6‐31+g(d,p) level. The coupled cluster method, CCSD(T)/cc‐pVTZ, was employed to calculate the corresponding energies with the zero‐point energy corrections by the ωB97X‐D/6‐31+g(d,p) approach. Combining experimental and theoretical results, possible formation pathways of the fragment ions were proposed and discussed in detail. The retro‐Cope rearrangement was found to play a crucial role in the formation of C₄H₆⁺, C₄H₅⁺ and C₃H₅⁺. Intramolecular hydrogen migrations were observed as dominant processes in most of the fragmentation pathways of cyclohexene. The present research provides a clear picture of the photoionization and dissociation processes of cyclohexene in the 8‐ to 15.5‐eV photon energy region. Copyright © 2016 John Wiley & Sons, Ltd.     

Femtosecond laser photoionization time-of-flight mass spectrometry of nitro-aromatic explosives and explosives related compounds

The ultrafast laser-induced photoionization and photodissociation processes of the nitroaromatic containing explosive and explosive related compounds (ERCs) nitrobenzene (NB), 1,3-dinitrobenzene (DNB), m-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) have been investigated at three laser wavelengths and power densities using a time-of-flight mass spectrometer. Examination of the mass spectra of these compounds reveals the enhanced formation of the molecular ion [M⁺] when ultraviolet (332 nm) and visible (495 nm) light is used relative to infrared (795 nm) radiation. In addition, at 795 nm and a power density of 3. 5 × 10¹⁴ W/cm², the presence of a competition between multiphoton ionization (MPI) and Coulomb explosion (CE) channels is revealed by peak shape analysis, and is thought to be operative under these conditions for all of the molecules investigated.     

A ladder polysilane wrapped with amylose

A mixture of tricyclic ladder polysilane 1 and amylose, upon pH adjustment of aqueous media from very basic to neutral, affords a supramolecular complex in which a preferential twisted (helical) conformation is induced on the ladder polysilane to display optical activity. Under the experimental conditions used, 1 assumes a left-handed twisted structure in the left-handed helical channel of amylose.     

Theoretical study of the X-ray photoionization spectra of polycycloalkanes

The dependence of X-ray photoionization spectra on structural characteristics is assessed by one-particle Green's function calculations on the nearly isomeric adamantane, 1,4-dimethyl bicyclo [2,2,2] octane and 1,1,4,4-tetramethyl cyclohexane compounds. These calculations are carried out using the quasi-particle approximation and the diagonal 2ph-TDA renormalization scheme for the self-energy. The simulated spectra fingerprint remarkably the atomic connectivity from their inner valence part, through the interplay of energy degeneracies and the influence of torsional strains on electron binding energies. The boundary region between the inner and outer valence bands provides also specific signatures for molecular characteristics pertaining to conformational questions, namely the enforced conversion from the chair to the boat form of the cyclohexane units generating these compounds. © 1996 John Wiley & Sons, Inc.     

A vacuum ultraviolet photoionization mass spectrometric study of acetone

The photoionization and dissociative photoionization of acetone have been studied at the photon energy range of 8-20 eV. Photoionization efficiency spectra for ions CH3COCH3+, CH3+, C2H3+, C3H3+, C3H5+, CH(2-)CO+, CH3CO+, C3H4O+, and CH3COCH2+ have been measured. In addition, the energetics of the dissociative photoionization has been examined by ab initio Gaussian-3 (G3) calculations. The computational results are useful in establishing the dissociation channels near the ionization thresholds. With the help of G3 results, the dissociation channels for the formation of the fragment ions CH3CO+, CH2CO+, CH3+, C3H3+, and CH3COCH2+ have been established. The G3 results are in fair to excellent agreement with the experimental data.     

Chirality control in optically active polysilane aggregates

A novel strategy for controlling the higher order chirality of aggregates prepared from enantiopure polysilanes is experimentally probed and discussed. Structurally similar poly[n-alkyl(aryl)]silanes were synthesized in which one side chain comprised the chiral (S)-2-methylbutyl group and the other an achiral m- or p-alkyl-substituted phenyl ring. In solution the polymers adopt helical conformations with the same induced preferential screw sense chirality, as evidenced by circular dichroism (CD) spectroscopy. Aggregates, however, formed by addition of a nonsolvent to a polymer solution, show oppositely signed CD spectra. Consistent results were obtained for another series of poly[p-n-alkyl(aryl)]silanes where alkyl is butyl, propyl, and ethyl. The sense of the aggregate higher order chirality is dependent on the chemical composition and environment and is coarse-tunable by adjusting the length of the achiral side chain and fine-tunable by adjusting the good/poor solvent ratio. The origin of these effects is discussed with reference to a simple model.     

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.     

Dissociative photoionization and thermochemistry of dihalomethane compounds studied by threshold photoelectron photoion coincidence spectroscopy

The dissociative photoionization studies have been performed for a set of dihalomethane CH(2)XY (X,Y = Cl, Br, and I) molecules employing the threshold photoelectron photoion coincidence (TPEPICO) technique. Accurate dissociation onsets for the first and second dissociation limits have been recorded in the 10-13 eV energy range, and ionization potentials have been measured for these compounds. By using our experimental dissociation onsets and the known heat of formation of CH(2)Cl(2) molecule, it has been possible to derive the 0 and 298 K heats of formation of all six neutral dihalomethanes as well as their ionic fragments, CH(2)Cl(+), CH(2)Br(+), and CH(2)I(+), to a precision better than 3 kJ/mol. These new measurements serve to fill the lack of reliable experimental thermochemical information on these molecules, correct the old literature values by up to 19 kJ/mol, and reduce their uncertainties. From our thermochemical results it has also been possible to derive a consistent set of bond dissociation energies for the dihalomethanes.     

Theoretical study of fluoromethane photoionization cross sections and angular distributions

Conclusions The valence-orbital photoionization cross sections show a nonmonotone behavior near threshold, which indicates that there are quasistationary states in the continuum. The features in the sections are more pronounced when there are more fluorine atoms [9].X systematically underestimates the cross sections. MSAA gives better agreement with experiment, since the method is parameterized by means of atomic cross sections. The overestimated cross sections near threshold in MSAA are due to the use of phases calculated for the ground-state potential. It is better to calculate the phases for the ionized state, which should shift the virtual level responsible for the overestimation in the continuum.The X asymmetry parameters for the upper valency orbitals agree well with measurements. Relaxation and correlation make it problematic to use X (essentially a one-electron method) for the inner valency orbitals.Deceased.Far East University. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 1, pp. 80–86, January–February, 1989.     

Investigation of substituted-benzene dopants for charge exchange ionization of nonpolar compounds by atmospheric pressure photoionization

Atmospheric pressure photoionization (APPI) using a dopant enables both polar and nonpolar compounds to be analyzed by LC/MS. To date, the charge exchange ionization pathway utilized for nonpolar compounds has only been efficient under restrictive conditions, mainly because the usual charge exchange reagent ions--the dopant photoions themselves--tend to be consumed in proton transfer reactions with solvent and/or dopant neutrals. This research aims to elucidate the factors affecting the reactivities of substituted-benzene dopant ions; another, overriding, objective is to discover new dopants for better implementing charge exchange ionization in reversed-phase LC/MS applications. The desirable properties for a charge exchange dopant include low reactivity of its photoions with solvent and dopant neutrals and high ionization energy (IE). Reactivity tests were performed for diverse substituted-benzene compounds, with substituents ranging from strongly electron withdrawing (EW) to strongly electron donating (ED). The results indicate that both the tendency of a dopant's photoions to be lost through proton transfer reactions and its IE depend on the electron donating/withdrawing properties of its substituent(s): ED groups decrease reactivity and IE, while EW groups increase reactivity and IE. Exceptions to the reactivity trend for dopants with ED groups occur when the substituent is itself acidic. All told, the desirable properties for a charge exchange dopant tend towards mutual exclusivity. Of the singly-substituted benzenes tested, chloro- and bromobenzene provide the best compromise between low reactivity and high IE. Several fluoroanisoles, with counteracting EW and ED groups, may also provide improved performance relative to the established dopants.     

The double photoionization of HCl: an ion-electron coincidence study

The double photoionization of HCl molecules by synchrotron radiation has been studied in the energy range between 30 and 50 eV. The HCl(2+) and Cl(2+) product ions have been detected by a photoelectron-photoion-coincidence technique, while the H(+)+Cl(+) formation, which follows the double ionization of HCl, has been studied by photoelectron-photoion-photoion coincidence. The photon energy threshold for the production of HCl(2+) ions has been found to be 35.4+/-0.6 eV, while for the dissociative channel leading to H(+)+Cl(+), it has been measured a threshold at 36.4+/-0.6 eV and a change in the slope of the cross-section energy dependence at 38.7+/-0.7 eV. The production of H+Cl(2+) occurs with a threshold photon energy of 42.8+/-1.1 eV. These results appear to be in a good agreement with previous data by different experimental techniques and recent theoretical calculations performed by our laboratory.