Dissociation channels of the 1-buten-2-yl radical and its photolytic precursor 2-bromo-1-butene

The work presented here is the first in a series of studies that use a molecular beam scattering technique to investigate the unimolecular reaction dynamics of C(4)H(7) radical isomers. Photodissociation of the halogenated precursor 2-bromo-1-butene at 193 nm under collisionless conditions produced 1-buten-2-yl radicals with a range of internal energies spanning the predicted barriers to the unimolecular reaction channels of the radical. Resolving the velocities of the stable C(4)H(7) radicals, as well as those of the products, allows for the identification of the energetic onset of each dissociation channel. The data show that radicals with at least 30.7 +/- 2 kcal/mol of internal energy underwent C-C fission to form allene + methyl, and radicals with at least 36.7 +/- 4 kcal/mol of internal energy underwent C-H fission to form H + 1-butyne and H + 1,2-butadiene; both of these observed barriers agree well with the G3//B3LYP calculations of Miller. HBr elimination from the parent molecule was observed, producing vibrationally excited 1-butyne and 1,2-butadiene. In the subsequent dissociation of these C(4)H(6) isomers, the major channel was C-C fission to form propargyl + methyl, and there is also evidence of at least one of the possible H + C(4)H(5) channels. A minor C-Br fission channel produces 1-buten-2-yl radicals in an excited electronic state and with low kinetic energy; these radicals exhibit markedly different dissociation dynamics than do the radicals produced in their ground electronic state.     

Theoretical and experimental investigations of the electronic Rydberg states of diazomethane: assignments and state interactions

The electronic states of diazomethane in the region 3.00-8.00 eV have been characterized by ab initio calculations, and electronic transitions in the region 6.32-7.30 eV have been examined experimentally using a combination of 2 + 1 REMPI spectroscopy and photoelectron imaging in a molecular beam. In the examined region, three Rydberg states of 3p character contribute to the transitions, 2(1)A2(3p(y) <-- pi), 2(1)B1(3p(z) <-- pi), and 3(1)A1(3p(x) <-- pi). The former two states are of mostly pure Rydberg character and exhibit a resolved K structure, whereas the 3(1)A1(3p(x) <-- pi) state is mixed with the valence 2(1)A1(pi* <-- pi) state, which is unbound and is strongly predissociative. Analyses of photoelectron kinetic energy distributions indicate that the ground vibrational level of the 2(1)B1(3p(z)) state is mixed with the 2(1)A2(3p(y)) nu(9) level, which is of B1 vibronic symmetry. The other 2(1)A2(3p(y)) vibronic states exhibit pure Rydberg character, generating ions in single vibrational levels. The photoelectron spectra of the 3(1)A1(3p(x) <-- pi) state, on the other hand, give rise to many states of the ion as a result of strong mixing with the valence state, as evidenced also in the ab initio calculations. The equilibrium geometries of the electronic states of neutral diazomethane were calculated by CCSD(T), using the cc-pVTZ basis, and by B3LYP, using the 6-311G(2df,p) basis. Geometry and frequencies of the ground state of the cation were calculated by CCSD(T)/cc-pVTZ, using the unrestricted (UHF) reference. Vertical excitation energies were calculated using EOM-CCSD/6-311(3+,+)G* at the B3LYP optimized geometry. The theoretical results show that the 2(1)A2(3p(y) <-- pi) and 2(1)B1(3p(z) <-- pi) states have geometries similar to the ion, which has C(2v) symmetry, with slight differences due to the interactions of the electron in the 3p orbital with the nuclei charge distributions. The geometry of the 3(1)A1(3p(x) <-- pi) state is quite different and has Cs symmetry. The experimental and theoretical results agree very well, both in regard to excitation energies and to vibrational modes of the ion.     

Selective synthesis of di(propen-1-yl)sulfone and di(propen-1-yl)sulfoxide

Convenient selective methods of synthesis of the title compounds, based on oxidation of di(propen-1-yl)sulfide with 30% H₂O₂, have been developed.Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, 664033 Irkutsk. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 675–679, March, 1992.     

A study of the unimolecular dissociation of the 2-buten-2-yl radical via the 193 nm photodissociation of 2-chloro-2-butene

This work investigates the unimolecular dissociation of the 2-buten-2-yl radical. This radical has three potentially competing reaction pathways: C-C fission to form CH3 + propyne, C-H fission to form H + 1,2-butadiene, and C-H fission to produce H + 2-butyne. The experiments were designed to probe the branching to the three unimolecular dissociation pathways of the radical and to test theoretical predictions of the relevant dissociation barriers. Our crossed laser-molecular beam studies show that 193 nm photolysis of 2-chloro-2-butene produces 2-buten-2-yl in the initial photolytic step. A minor C-Cl bond fission channel forms electronically excited 2-buten-2-yl radicals and the dominant C-Cl bond fission channel produces ground-state 2-buten-2-yl radicals with a range of internal energies that spans the barriers to dissociation of the radical. Detection of the stable 2-buten-2-yl radicals allows a determination of the translational, and therefore internal, energy that marks the onset of dissociation of the radical. The experimental determination of the lowest-energy dissociation barrier gave 31 +/- 2 kcal/mol, in agreement with the 32.8 +/- 2 kcal/mol barrier to C-C fission at the G3//B3LYP level of theory. Our experiments detected products of all three dissociation channels of unstable 2-buten-2-yl as well as a competing HCl elimination channel in the photolysis of 2-chloro-2-butene. The results allow us to benchmark electronic structure calculations on the unimolecular dissociation reactions of the 2-buten-2-yl radical as well as the CH3 + propyne and H + 1,2-butadiene bimolecular reactions. They also allow us to critique prior experimental work on the H + 1,2-butadiene reaction.     

Experiments and quantum-chemical calculations on Rydberg states of H2CS in the region 5.6-9.5 eV

Absorption spectrum of H(2)CS in the region 5.6-9.5 eV was recorded with a continuously tunable light source of synchrotron radiation. After we subtracted absorption bands of CS(2), our spectrum clearly shows vibrational progressions associated with transitions (1)A(1)(pi,pi*)-X (1)A(1) and (1)B(2)(n,4s)-X (1)A(1) in the region 5.6-6.7 eV. A spectrum from which absorption of C(2)H(4) and CS(2) are subtracted shows several discrete bands in the region 6.9-9.5 eV. A Rydberg state (1)B(2)(n,4p(z)) lying below Rydberg state (1)A(1)(n,4p(y)) is confirmed, and the C-H symmetric stretching (nu(1)) and CH out-of-plane bending (nu(4)) modes for a transition (1)B(2)(n,4s)-X (1)A(1) are identified. New transitions to Rydberg states associated with excitation to 5s-11s, 5p(z)-7p(z), 5p(y)-7p(y), and 3d-6d are identified based on quantum defects and comparison with vertical excitation energies predicted with time-dependent density-functional theory (TD-DFT) and outer-valence Green's-function (OVGF) methods. For lower excited states predictions from these TD-DFT6-31+G calculations agree satisfactorily with experimental values, but for higher Rydberg states the OVGF method using aug-cc-pVTZ basis set augmented with extra diffuse functions yields more accurate predictions of excitation energies.     

Mass spectrometric study of the vacuum pyrolysis of perfluorohexene-2 and determination of the ionization potential of the perfluoro-2-buten-4-yl radical

The ionization potential (IP) of the perfluoro-2-buten-4-yl radical, generated at 950C and 1·10⁵ torr by vacuum pyrolysis of perfluorohexene-2, was found to be 10.3 ± 0.1 eV by the electron impact method. This value is close to the IP of the perfluoro radicals n-C₃F₇ (10.06 ± 0.1 eV) and i-C₃F₇ (10.5 ± 0.1 eV); however, it is 1.8 eV higher than that of the perfluoroallyl radical, possibly because of the weak delocalization of the unpaired electron. At pyrolysis temperatures above 800C, the perfluoro-2-buten-4-yl radical decomposed to the tetrafluoroallene and trifluoromethyl radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1220–1223, May, 1991.     

Carbon 1s excitation spectroscopy of propyne, trifluoropropyne, and propargyl alcohol

The C 1s excitation spectra of propyne (HC2CH3), 3,3,3-trifluoropropyne (HC2CF3), and propargyl alcohol (HC3CH3OH) have been studied using synchrotron radiation and ion time-of-flight mass spectrometry. Discrete peaks below the carbon 1s ionization thresholds are compared and assigned, aided in part by ab-initio calculations incorporating an explicit C 1s hole. Calculated C 1s ionization potentials are in good agreement with previously reported experimental values. Calculated absolute excitation energies consistently underestimate the transition term values, but calculated relative excitation energies and intensities are in good agreement with the experimental results. The spectra are dominated by intense C 1s --> pi transitions. In the case of propyne, C 1s excitations from each of the three chemically inequivalent carbon atoms are observed. The effect of electronegative substitution is found to be different for the C 1s --> Rydberg transitions than for transitions to unoccupied valence levels, with Rydberg transition energies shifting with changes in the C 1s ionization potentials but valence transition energies showing only small changes with electronegative substitution. The C 1s (3a1,4a1) --> pi (6e) transitions of trifluoropropyne are shifted to lower energy relative to propyne even though the electronegative fluorine atoms cause a significant shift to higher energy in the corresponding C 1s IPs.     

Computational investigation of the vibrational and electronic states of S2N2

The structures and vibrational frequencies of the ground and excited states of S(2)N(2) have been calculated using density functional (DF) methods. Time-dependent DF theory (TDDFT) has been used to calculate the excitation energies of the lowest 20 singlet-singlet transitions using a variety of methods. All computational methods predict a small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap. There is some disagreement in the ordering of the b(2g) and b(3g) pi orbitals. This is reflected in the ordering of the B(2u) and B(3u) states from the TDDFT calculations. The excitation energies and oscillator strengths strongly suggest it is the transitions to these states that are responsible for the experimental electronic spectrum. The calculated geometries and vibrational frequencies for these two states show that both have C(2v) equilibrium structures. Modelling of the vibrational progressions and band shapes suggest that the ordering of the states is B(2u)相似文献   

Jet-cooled phosphorescence excitation spectrum of the T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one

The T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one (2CP) was investigated by using phosphorescence excitation (PE) spectroscopy in a free-jet expansion. The origin band, near 385 nm, is the most intense feature in the T1(n,pi*) <-- S0 PE spectrum. A short progression in the ring-bending mode (nu'(30)) is also observed. The effective vibrational temperature in the jet is estimated at 50 K. The spectral simplification arising from jet cooling helps confirm assignments made previously in the room-temperature cavity ringdown (CRD) absorption spectrum, which is congested by vibrational hot bands. In addition to the origin and nu'(30) assignments, the jet-cooled PE spectrum also confirms the 28(0)(1) (C=O out-of-plane wag), 29(0)(1) (C=C twist), and 19(0)(1) (C=O in-plane wag) band assignments that were made in the T1(n,pi*) <-- S0 room-temperature CRD spectrum. The temporal decay of the T1 state of 2CP was investigated as a function of vibronic excitation. Phosphorescence from the v' = 0 level persists the entire time the molecules traverse the emission detection zone. Thus the phosphorescence lifetime of the v' = 0 level is significantly longer than the 2 micros transit time through the viewing zone. Higher vibrational levels in the T1 state have shorter phosphorescence lifetimes, on the order of 2 micros or less. The concomitant reduction in emission quantum yield causes the higher vibronic bands (above 200 cm(-1)) in the PE spectrum to be weak. It is proposed that intersystem crossing to highly vibrationally excited levels of the ground state is responsible for the faster decay and diminished quantum yield. The jet cooling affords partial rotational resolution in the T1(n,pi*) <-- S0 spectrum of 2CP. The rotational structure of the origin band was simulated by using inertial constants available from a previously reported density functional (DFT) calculation of the T1(n,pi*) state, along with spin constants obtained via a fitting procedure. Intensity parameters were also systematically varied. The optimized intensity factors support a model that identifies the S2(pi,pi*) <-- S0 transition in 2CP as the sole source of oscillator strength for the T1(n,pi*) <-- S0 transition.     

Theoretical study of the electronic gas-phase spectrum of glycine, alanine, and related amines and carboxylic acids

A theoretical study on the origin of the common electronic excitations in amino acids is presented, focusing on the excited states of glycine, alanine and the related substructures formic acid, acetic acid, propionic acid, ammonia, methylamine, and ethylamine. Special attention is given to the valence excitation from the nonbonding lone-pair on the carboxylic oxygen atom to the antibonding pi-orbital (n(O) --> pi*(CO)) and the first Rydberg excitation from the nonbonding lone-pair on the nitrogen atom (n(N) --> 3s). From extensive calculations on formic acid and methylamine, different basis sets and electron correlation treatments are benchmarked using a hierarchy of coupled cluster (CC) methods, consisting of CCS, CC2, CCSD, CCSDR(3), and CC3, in combination with augmented correlation consistent basis sets. The dependence of the excitation energies on the size of the backbone structure in the two groups of molecules is investigated, and 0-0 transition energies for the n(O) --> pi*(CO) and n(N) --> 3s transitions are calculated for the smallest molecules. Excellent agreement with experimental values is found where secure experimental assignments are available. A few outstanding problems in the experimental assignments found in the literature are described for both the carboxylic acids and the amines. Final predictions for vertical excitation energies are given for all molecules, including glycine and alanine where no gas-phase experimental results are available. Finally, calculations on protonated amino acids are presented showing an isolation of the n(O) --> pi*(CO) from higher lying states by as much as 1.9 eV for alanine.     

Electronic spectra and photodissociation of vinyl chloride: a symmetry-adapted cluster configuration interaction study

The vertical absorption spectrum and photodissociation mechanism of vinyl chloride (VC) were studied by using symmetry-adapted cluster configuration interaction theory. The important vertical pi --> pi* excitation was intensively examined with various basis sets up to aug-cc-pVTZ augmented with appropriate Rydberg functions. The excitation energy for pi --> pi* transition obtained in the present study, 6.96 eV, agrees well with the experimental value, 6.7-6.9 eV. Calculated excitation energies along with the oscillator strengths clarify that the main excitation in VC is the pi --> pi* excitation. Contrary to the earlier theoretical reports, the results obtained here support that the C-Cl bond dissociation takes place through the n(Cl-)sigma(C-Cl)* state.     

Synthesis and reactions with <Emphasis Type="Italic">N</Emphasis>-nucleophiles of 1-(thien-2-yl)- and 1-(4-hydroxyphenyl)-3,4,4-trichloro-3-buten-1-ones

Acylation of thiophene and phenol with 3,4,4-trichloro-3-butenoyl chloride afforded the corresponding 1-(thien-2-yl)- and 1-(4-hydroxyphenyl)-3,4,4-trichloro-3-buten-1-ones, whose reaction with amines led to the formation of 3-amino-1-(thien-2-yl, 4-hydroxyphenyl)-4,4-dichloro-2-buten-1-ones The heterocyclization of the initial ketones into pyrazole structure was not observed, and the reaction with hydrazine hydrate provided bispyrazole products, N,N′-bis(5-thien-2-yl)- and N,N′-bis[5-(4-hydroxyphenyl)-1H-pyrazol-3-ylmethylene]hydrazines.     

Ab initio calculations of triplet excited states and potential-energy surfaces of vinyl chloride: insights into spectroscopy and photodissociation dynamics

The equilibrium geometries and harmonic vibrational frequencies of three low-lying triplet excited states of vinyl chloride have been calculated using the state-averaged complete active space self-consistent field (CASSCF) method with the 6-311++G(d,p) basis set and an active space of four electrons distributed in 13 orbitals. Both adiabatic and vertical excitation energies have been obtained using the state-averaged CASSCF and the multireference configuration-interaction methods. The potential-energy surfaces of six low-lying singlet states have also been calculated. While the 3(pi, pi*) state has a nonplanar equilibrium structure, the 3(pi, 3s) and 3(pi, sigma*) states are planar. The calculated vertical excitation energy of the 3(pi, pi*) state is in agreement with the experiment. The singlet excited states are found to be multiconfigurational, in particular, the first excited state is of (pi, 3s) character at the planar equilibrium structure, of (pi, sigma*) as the C-Cl bond elongates, and of (pi, pi*) for highly twisted geometries. Avoided crossings are observed between the potential-energy surfaces of the first three singlet excited states. The absorption spectra of vinyl chloride at 5.5-6.5 eV can be unambiguously assigned to the transitions from the ground state to the first singlet excited state. The dissociation of Cl atoms following 193-nm excitation is concluded to take place via two pathways: one is through (pi, sigma*) at planar or nearly planar structures leading to fast Cl atoms and the other through (pi, pi*) at twisted geometries from which internal conversion to the ground state and subsequent dissociation produces slow Cl atoms.     

Conical for stepwise, glancing for concerted: the role of the excited-state topology in the three-body dissociation of sym-triazine

The highly debated three-body dissociation of sym-triazine to three HCN products has been investigated by translational spectroscopy and high-level ab initio calculations. Dissociation was induced by charge exchange between the sym-triazine radical cation and cesium. Calculated state energies and electronic couplings suggest that sym-triazine is produced in the 3s Rydberg and pi* <-- n manifolds. Analysis of the topology of these manifolds along with momentum correlation in the dissociation products suggest that the 3s Rydberg manifold characterized by a conical intersection of two potential energy surfaces leads to stepwise dissociation, while the pi* <-- n manifold consisting of a four-fold glancing intersection leads to a symmetric concerted reaction.     

Benchmarking the performance of spin-component scaled CC2 in ground and electronically excited states

A generalization of the spin-component scaling and scaled opposite-spin modifications of second-order M?ller-Plesset perturbation theory to the approximate coupled-cluster singles-and-doubles model CC2 (termed SCS-CC2 and SOS-CC2) is discussed and a preliminary implementation of ground and excited state energies and analytic gradients is reported. The computational results for bond distances, harmonic frequencies, adiabatic and 0-0 excitation energies are compared with experimental results to benchmark their performance. It is found that both variants of the spin-scaling increase the robustness of CC2 against strong correlation effects and lead for this method even to somewhat larger improvements than those observed for second-order M?ller-Plesset perturbation theory. The spin-component scaling also enhances systematically the accuracy of CC2 for 0-0 excitation energies for pi --> pi* and n --> pi* transitions, if geometries are determined at the same level.     

Ion-pair dissociation dynamics of H2S in the photon energy range 15.26-15.55 eV

The H(+) velocity map images from the ion-pair dissociation of H(2)S + hν → SH(-)(X(1)Σ(+), υ = 0, 1) + H(+) have been measured at the excitation energies 15.259, 15.395, and 15.547 eV, respectively. The experimental results show that most of the available energies are transformed into the translational energies. The angular distributions of the fragments SH(-)(X(1)Σ(+), υ = 0) indicate that the dissociation occurs via pure parallel transition with limiting anisotropy parameter of +2. Because the ion-pair dissociation usually occurs via the predissociation of Rydberg states, this suggests that the ion cores of the excited Rydberg states have linear geometries. The geometries and electronic structures of the linear H(2)S(+) have been calculated employing the quantum chemistry calculation method at the CASPT2/avqz level. The electronic structures for the ion-pair states have been calculated at the CASSCF/avtz level, which indicates that the equilibrium geometries of the ion-pair states have bent geometries.     

Electronic Structure and Photophysical Properties of 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine

Electronic structure and photophysical properties of 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine are studied theoretically with quantum chemical methods as well as 2D site and 3D cube representations. The theoretical results reveal that the first excited state is an intramolecular charge transfer excited state. The change in dipole moment for the first excited state of the excitation is fitted, and the calculated result the change in dipole moment ￠1=6.40 D is consistent with the experimental result ￠1=6.90 D. The polarizability is also fitted. The large changes in dipole moment and the polarizability of the excitation show that S1 is of large nonlinear optical (NLO) effect. The NLO will promote efficient two-photon-absorption cross sections. The excited state properties of dpbt with different external electronic fields are also discussed theoretically.     

Ab initio calculation of the vibrational and electronic spectra of trans- and cis-azobenzene

We report accurate geometries and harmonic force fields for trans- and cis-azobenzene determined by second-order M?ller-Plesset perturbation theory. For the trans isomer, the planar structure with C(2h) symmetry, found in a recent gas electron diffraction experiment, is verified. The calculated vibrational spectra are compared with experimental data and density functional calculations. Important vibrational frequencies are localized and discussed. For both isomers, we report UV spectra calculated using the second-order approximate coupled-cluster singles-and-doubles model CC2 with accurate basis sets. Vertical excitation energies and oscillator strengths have been determined for the lowest singlet n(pi)* and (pi)(pi)* transitions. The results are compared with the available experimental data and second-order polarization propagator (SOPPA) and density functional (DFT) calculations. For both isomers, the CC2 results for the excitation energies into the S(1) and S(2) states agree within 0.1 eV with experimental gas-phase measurements.     

Theoretical investigation of molecular properties of the first excited state of the thiophenoxyl radical

Accurate ab initio study of the lowest excited state (A (2)B(2)) of the thiophenoxyl radical is presented. The calculated equilibrium geometries, excitation energies, and harmonic vibrational frequencies show that the A (2)B(2) <-- X (2)B(1) excitation in C(6)H(5)S has different characteristics than the analogous transition in the phenoxyl radical. Vertical excitation energies for other low-lying (<4.5 eV) excited states of the thiophenoxyl radical are also presented and compared with available experimental data.