Vacuum ultraviolet photolysis of CClF2CH2Cl

Photolysis of CClF₂CH₂Cl was studied by 147 nm vacuum ultraviolet irradiation. In the presence of NO; CF₂CH₂, CF₂CHCl, and CClF₂CN were produced. These products represent three different reaction paths; the molecular dechlorination, molecular dehydrochlorination, and chlorine radical elimination reactions. The reactant pressure and the addition gas (He or NO) pressure effects upon the product yield were studied. © 1994 John Wiley & Sons, Inc.     

UV angle-resolved photoelectron spectra of mixed methylene dihalides using synchrotron radiation

Photoelectron spectra for the dihalomethanes CH₂BrCl, CH₂ClI and CH₂Br₂ have been recorded with photons in the energy range 19–115 eV using Daresbury Laboratory Synchroton Radiation Source. Ionization energies have been measured for all valence falling within this energy including Br 3d and I 4d orbitals. Asymmetry parameters have been measured for all intense ionization processes and characteristic Cooper minima observed for halogen lone pair orbitals. For CH₂ClI lone pair orbital β spectra show strong evidence of mixed halogen character although a similar situation is not observed for CH₂BrCl. Partial photoionization cross sections are tabulated for the observed ionization processes. Detailed spectra are presented for the molecule CH₂ClI since this appears to be the first photoelectron study of this molecule.     

Quantum-chemical study of crystal formation of supramolecular silver compounds with trans-1,2-Bis(4-pyridyl)ethylene and their electronic absorption spectra

The energies of singlet-singlet transitions in different supramolecular compounds formed by interaction of trans-1,2-bis(4-pyridyl)ethylene (DPyEt) with AgNO₃ have been calculated by the TDDFT method. The calculations show that the absorption peaks at 305.7 and 318.3 nm can be assigned to a superposition of the spectra of small suprastructures, e.g., (Ag)(DPyEt)₂(NO₃), (Ag)₂(DPyEt)(NO₃)₂, and (Ag)(DPyEt)(NO₃). These transitions correspond to the excitation of an electron from the bonding orbital into the antibonding orbital with respect to the C=C bond. Strong absorption bands at 350–360 nm, typical of a solid phase, can be assigned to the transition from the MO of the lone pair of the nitrogen of DPyEt into the antibonding MO with respect to the C=C bond. Such bands are present in the calculated excitation spectra of the (Ag)₁(DPyEt)₃(NO₃)₁ and (Ag)₂(DPyEt)₄(NO₃)₂ suprastructures.     

Photodissociation of CH2I2 and Subsequent Electron Transfer in Solution

We studied photoinduced reactions of diiodomethane (CH₂I₂) upon excitation at 268 nm in acetonitrile and hexane by subpicosecond–nanosecond transient absorption spectroscopy. The transient spectra involve two absorption bands centered at around 400 (intense) and 540 nm (weak). The transients probed over the range 340–740 nm show common time profiles consisting of a fast rise (<200 fs), a fast decay (≈500 fs), and a slow rise. The two fast components were independent of solute concentration, whereas the slow rise became faster (7–50 ps) when the concentration in both solutions was increased. We assigned the fast components to the generation of a CH₂I radical by direct dissociation of the photoexcited CH₂I₂ and its disappearance by subsequent primary geminate recombination. The concentration‐dependent slow rise produced the absorption bands centered at 400 and 540 nm. The former consists of different time‐dependent bands at 385 and 430 nm. The band near 430 nm grew first and was assigned to a charge‐transfer (CT) complex, CH₂I₂^δ+???I^δ?, formed by a photofragment I atom and the solute CH₂I₂ molecule. The CT complex is followed by full electron transfer, which then develops the band of the ion pair CH₂I₂⁺???I^? at 385 nm on the picosecond timescale. On the nanosecond scale, I₃^? was generated after decay of the ion pair. The reaction scheme and kinetics were elucidated by the time‐resolved absorption spectra and the reaction rate equations. We ascribed concentration‐dependent dynamics to the CT‐complex formation in pre‐existing aggregates of CH₂I₂ and analyzed how solutes are aggregated at a given bulk concentration by evaluating a relative local concentration. Whereas the local concentration in hexane monotonically increased as a function of the bulk concentration, that in acetonitrile gradually became saturated. The number of CH₂I₂ molecules that can participate in CT‐complex formation has an upper limit that depends on the size of aggregation or spatial restriction in the neighboring region of the initially photoexcited CH₂I₂. Such conditions were achieved at lower concentrations in acetonitrile than in hexane.     

Absorption spectra of SiCl4, Si2Cl6, SiF3CH3 and GeF4 in the VUV region

In the range 110–200 nm the absorption features of Si₂Cl₆ closely resemble those of SiCl₄ and the peaks observed are tentatively assigned to the Rydberg transitions of a Cl lone-pair electron. Two diffuse bands in the SiF₃CH₃ absorption are also assigned to Rydberg excitations. The spectrum of GeF₄ shows a broad band considered to be a valence excitation of the outermost orbital. The emission of the SiCl₂ (ùB₁→X̃¹A₁) transition was found in the photoexcitation of Si₂Cl₆.     

Angle-resolved photoelectron spectroscopy of the chloro-substituted methanes

The angular distribution parameter, β, was determined for the valence orbitals (IP ′ 21.2 eV) of CCl₄, CHCl₃, CH₂Cl₂, and CH₃Cl in the 10–30 eV photon energy range using dispersed polarized synchrotron radiation. The energy dependence of β in the photoelectron energy range of 2 to 10 eV for the non-bonding chlorine n(Cl) orbitals of these molecules was found to be similar for all n(Cl) orbitals investigated. The energy dependence of β for the σ orbitals in these molecules was similar to that observed previously for other σ orbitals. The experimental CCl₄ results were compared with theoretical CCl₄ results obtained using the Xα multiple scattering formalism. Theory predicts the existence of two strong shape resonances in each of the valence orbitals of CCl₄. The overall agreement between experiment and theory is evaluated along with the experimental evidence concerning the verification of the predicted shape resonances.     

Profile and size of lone pair orbitals in CH2−, NH3− and H2O-like systems

A simple method has been devised to define the profile and size of the lone pair orbitals in CH_2?, NH_3? and H₂O_? like systems. In addition, the results derived in the present work and those obtained within the framework of molecular orbital theory are compared.     

Special Features of the Geometry of the 2,2,2,4,4,4-Hexachloro-1,3-dimethyl-1,3-diaza-2,4-diphosphetane Molecule and Its Electron Distribution According to Data of ab initio Calculations

The results of a nonempirical calculation of the 2,2,2,4,4,4-hexachloro-1,3-dimethyl-1,3-diaza-2,4-diphosphetane (Cl₃PNCH₃)₂ molecule by the RHF 6-31G(d) method are in agreement with the data of X-ray structural analysis of this compound. Calculated ³⁵Cl NQR frequencies for axial and equatorial chlorine atoms are close to the experimental values. The population of the orbitals of the lone electron pairs and the p orbitals of the equatorial Cl atoms were significantly lower than those of the axial atoms. Among the MO there was no MO corresponding to a three-center bond involving a P atom and axial Cl and N atoms.     

Boron as an Electron‐Pair Donor for B⋅⋅⋅Cl Halogen Bonds

MP2/aug′‐cc‐pVTZ calculations were performed to investigate boron as an electron‐pair donor in halogen‐bonded complexes (CO)₂(HB):ClX and (N₂)₂(HB):ClX, for X=F, Cl, OH, NC, CN, CCH, CH₃, and H. Equilibrium halogen‐bonded complexes with boron as the electron‐pair donor are found on all of the potential surfaces, except for (CO)₂(HB):ClCH₃ and (N₂)₂(HB):ClF. The majority of these complexes are stabilized by traditional halogen bonds, except for (CO)₂(HB):ClF, (CO)₂(HB):ClCl, (N₂)₂(HB):ClCl, and (N₂)₂(HB):ClOH, which are stabilized by chlorine‐shared halogen bonds. These complexes have increased binding energies and shorter B?Cl distances. Charge transfer stabilizes all complexes and occurs from the B lone pair to the σ* Cl?A orbital of ClX, in which A is the atom of X directly bonded to Cl. A second reduced charge‐transfer interaction occurs in (CO)₂(HB):ClX complexes from the Cl lone pair to the π* C≡O orbitals. Equation‐of‐motion coupled cluster singles and doubles (EOM‐CCSD) spin–spin coupling constants, ^1xJ(B‐Cl), across the halogen bonds are also indicative of the changing nature of this bond. ^1xJ(B‐Cl) values for both series of complexes are positive at long distances, increase as the distance decreases, and then decrease as the halogen bonds change from traditional to chlorine‐shared bonds, and begin to approach the values for the covalent bonds in the corresponding ions [(CO)₂(HB)?Cl]⁺ and [(N₂)₂(HB)?Cl]⁺. Changes in ¹¹B chemical shieldings upon complexation correlate with changes in the charges on B.     

Bonding/antibonding character of “lone pair” molecular orbitals from their energy derivatives; consequences for experimental data

The derivative of molecular orbitals (MO) energies with respect to a bond length (dynamic orbital force [DOF]) is used to estimate the bonding/antibonding character of valence MOs along this bond, with a focus on lone pair MOs, in a series of small molecules: AH (A = F, Cl, Br), AH₂ (A = O, S, Se), AX₃ (A = N, P, As; X = H, F), and H₂CO. The HOMO DOF agrees with the calculated variation of bond length and force constant in the corresponding ground state cation, and of bond length variation by protonation. These results also agree with available experimental data. It is worthy to note that the p‐type HOMOs in AH and AH₂ are found bonding. The lone pair MO is bonding in NH₃, while it is antibonding in PH₃, AsH₃, and AF₃.     

Photoelectron spectra,penning ionization electron spectra,and character of canonical molecular orbitals

When canonical molecular orbitals are expanded in terms of a set of localized molecular orbital building blocks, called bond orbitals, the character of the canonical molecular orbitals can be characterized according to the component bond orbitals resembling the core, lone pair, and localized bond building blocks in an intuitive Lewis structure. Weinhold's natural bond orbital method can produce a unique Lewis structure with total occupancy of its occupied bond orbitals exceeding 99.9% of the total electron density for simple molecules. Two useful indices, Lewis bond order and weight of lone pair orbitals, can be defined according to the weights of the bonding and lone pair components of this unique Lewis structure. Calculation results for molecules N₂, CO, CS, NO, HCN, C₂H₂, H₂O, and H₂S show that the former index can account for the vibrational structures of photoelectron spectroscopy, whereas the latter index can account for the band intensity enhancement of Penning ionization electron spectroscopy. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 882–892, 1998     

Synthesis,crystal structure,absorption properties,photoelectric behavior of organic–inorganic hybrid (CH3NH3)2CoCl4

Lead‐free and organic–inorganic hybrid (CH₃NH₃)₂CoCl₄ [(MA)₂CoCl₄] single crystal and thin film are prepared. The single crystal diffraction data of (MA)₂CoCl₄ are assigned to monoclinic, P2(1)/C space group (7.6590 × 12.6908 × 10.89350 Å, 90.0 × 96.5320 × 90.000). The absorption edge of (MA)₂CoCl₄ reaches 730 nm. The band gap for (MA)₂CoCl₄ is determined to be approximately 1.63 eV. To the best of our knowledge, this is the first study on (MA)₂CoCl₄ for optoelectronic applications. A low‐cost photodetector based on (MA)₂CoCl₄ thin film is efficient under different monochromatic light from 330 nm to 400 nm with different chopping frequencies (1.33–60 Hz). The strongest photoresponse (I_on ? I_off) is under 330 nm monochromatic light with 1.33 Hz according to our optimal condition. The calculational results by density functional theory show that the narrow valence bands and conduction bands are derived from the 3p orbitals of Cl and 3d orbitals of Co.     

A HeI photoelectron spectroscopy and theoretical study of 2,6-dichloropyrazine, 2,3-dichloropyrazine, 4,6-dichloropyrimidine and 3,6-dichloropyridazine

The electronic structures of C₄H₂Cl₂N₂ isomers have been studied by HeI photoelectron spectroscopy (PES) combined with the outer valence Green’s function (OVGF) calculations at 6-311++G(d,p) basis sets. The vertical first ionization potentials for 2,6-dichloropyrazine, 2,3-dichloropyrazine, 4,6-dichloropyrimidine and 3,6-dichloropyridazine are determined to be 9.93, 9.89, 10.45 and 10.07 eV, respectively, and the PE spectra were assigned based on molecular orbital analysis and by comparison with related compounds. The effects of chlorine substituent and different positions of nitrogen atoms in these C₄H₂Cl₂N₂ isomers have been carefully analyzed by comparing the C₄H₂Cl₂N₂ isomers with their matrixes (pyrazine, pyridazine, pyrimidine). The inductive effect of chlorine substituent leads to the stabilization of the nitrogen lone pair orbitals. The resonance effect predominates over the inductive effect, and the consequence is the destabilization of the π orbitals. Owing to the different positions, the effects of nitrogen atoms are different and lead to the different sequence of π orbitals.     

An ab initio evaluation of the role of p,π interaction: II. Molecules of the CH3COX series

The RHF/6-311G(d) and MP2/6-311G(d) calculations with full geometry optimization were performed for CH₃COX molecules (X = F, Cl, Br, CH₃). Variations in the populations of the p _y orbitals of their halogen and carbon atoms (orbitals whose symmetry axes are perpendicular to the molecular plane) from X = F to X = Cl, Br, and CH₃ are not associated with variations in the extent of the p,π conjugation between the lone electron pair of the halogen atom and the π-electron system of the carbonyl group. The bonding molecular orbitals formed by these atomic p _y orbitals are not determined by this interaction. The RHF/6-311G(d) and MP2/6-311G(d) calculations give similar results.     

Infrared spectra of B(OMe)3, ClB(OMe)2 and Cl2BOMe species,isolated CH stretching frequencies and bond strengths

Infrared spectra in the gas phase are reported over the range 3100-500 cm⁻¹ for species of B(OMe)₃, ClB(OMe)₂ and Cl₂BOMe, with CH₃, CD₃ and CHD₂ substitution. A detailed analysis of νCH and νCD data in all three species of Cl₂BOMe yields strong evidence for the presence of three kinds of CH bond, two of them weak and one of them strong. The methyl group is then twisted, probably through 10–20°, out of the eclipsed or staggered conformation. The CHD₂ spectra of the di and trimethoxy compounds are less susceptible to analysis, but suggest also the presence of two weak and strong bonds, the former increasing in weakness as the number of methoxy groups increases. This is as expected from the increased competition likely between the lone pair electrons for the empty boron orbital. The spectra of the CD₃ species permit a clear assignment of νBO, δ_sCH₃, δ_sCD₃ and δ_asCD₃ modes. In Cl(COCH₃)₂, ν_sBO lies at 1278 cm⁻¹.     

Photoelectron Spectra of Azabenzenes and Azanaphthalenes: I. Pyridine,diazines, s-triazine and s-tetrazine

The experimental and theoretical basis of a recently proposed reassignment of the bands in the PE. spectra of pyridine, pyridazine, pyrimidine and pyrazine is discussed in detail. A characteristic feature of the derived orbital sequence is that it takes the ‘through-space’ and ‘through-bond’ interaction between the ‘lone pair’ basis orbitals explicitly into account. A simple parametrization of the orbital energies, based on HMO-type models for the π-orbitals and for the ‘lone pair’ linear combinations, yields excellent agreement with the observed band positions in the PE. spectra of s-triazine and s-tetrazine. Our new assignment is compared to those proposed previously.     

Transient absorption spectra of 2-hydroxybenzophenone photostabilizers

Transient absorption spectra (400–600 nm) of 2-hydroxybenzophenone and the methyl methacrylate copolymer of 2-hydroxy, 3-allyl, 4,4'-dimethoxybenzophenone following 355 nm excitation (7–480 ps delay) are reported. A short-lived, 435 nm transient (τ ≈ 10 ps in CH₂Cl₂) for both molecules is assigned to the lowest excited singlet before internal proton transfer. Weaker, broad T-T absorption is observed after 480 ps.     

Sulfur and chlorine K-shell x-ray absorption spectra of SCl2, S2Cl2, SOCl2, and SO2Cl2

The X-ray absorption spectra of dichlorosulfide (SCl₂), dichlorodisulfide (S₂Cl₂), thionyl chloride (SOCl₂) and sulfuryl chloride (SO₂Cl₂) have been recorded with synchrotron radiation in the regions of S 1s and Cl 1s excitation and ionization. Ionization current spectra of SOCl₂ and SO₂Cl₂ in the Cl 1s regions are also presented. The main spectral features are assigned to K-shell excitations to σ^* orbitals associated with the SCl, SS and SO bonds in these molecules.     

Valence bond studies of AH2 molecules

Minimal basis set (STO) molecular orbital and valence-bond calculations are reported for the³ B ₁ and¹ A ₁ states of CH₂. The open-shell molecular orbital calculations used the Roothaan formulation. The valence-bond calculations used the Prosser-Hagstrom biorthogonalisation technique to evaluate the cofactors required in using Löwdin's formulae. Optimisation of geometry and orbital exponents in the molecular orbital calculation on the³ B ₁ state gave a geometry of R_C-H=2.11 a.u. and H-C-H=123.2 °. The energy obtained was ?38.8355 a.u. The molecular orbital and valencebond calculations are compared. In the valence-bond calculations the variation with bond-length and bond-angle of the configuration energies was studied. Valence bond “build-up” studies are also reported. Valence-bond calculations using hybrid orbitals instead of natural atomic orbitals showed that the perfect-pairing approximation is not as good for CH₂ as BeH₂. The nature of the lone-pair and bonding orbitals is found to be significantly different between the³ B ₁ and¹ A ₁ states. In the³ B ₁ state the 2s and 2p orbitals are fairly equally mixed between both types of orbital. However in the¹ A ₁ state the bonding orbitals have mainly 2p character and the lone pair orbitals have mainly 2s character. As was found for H₂O, the bonding hybrid orbitals do not follow the hydrogen nuclei as the bond angle varies but continue to point approximately in their equilibrium directions.     

Coupling between Substituents as a Function of Cage Structure: Synthesis and Valence Ionized States of Bridgehead Disubstituted Parent and Hexafluorinated Bicyclo[1.1.1]pentane Derivatives C5X6Y2

He(I) photoelectron spectroscopy was used to examine the valence‐shell electronic structure of three new and seven previously known bicyclo[1.1.1]pentane derivatives, 1,3‐Y₂‐C₅X₆ (for X=H, Y=H, Cl, Br, I, CN; for X=F, Y=H, Br, I, CN). A larger series (X=H or F, Y=H, F, Cl, Br, I, At, CN) has been studied computationally with the SAC‐CI (symmetry‐adapted cluster configuration interaction) method. The outer‐valence ionization spectra calculated by the SAC‐CI method, including spin–orbit interaction, reproduced the experimental photoelectron spectra well, and quantitative assignments are given. When the extent of effective through‐cage interaction between the bridgehead halogen lone‐pair orbitals was defined in the usual way by orbital‐energy splitting, it was found to be larger than that mediated by other cages such as cubane, and was further enhanced by hexafluorination. The origin of the orbital‐energy splitting is analyzed in terms of cage structure, and it is pointed out that its relation to the degree of interaction between the bridgehead substituents is not as simple as is often assumed.