Dynamics of the CH(A2Delta) product from the reaction of C2H with O2 studied by Fourier transform visible spectroscopy

The reaction C(2)H + O(2) --> CH(A(2)Delta) + CO(2) is investigated using Fourier transform visible emission spectroscopy. C(2)H radicals, produced by 193 nm photolysis of C(2)H(2), react with O(2) molecules at low total pressures to produce electronically excited CH(A(2)Delta). Observation of the CH(A(2)Delta-X(2)Pi) electronic emission to infer nascent rotational and vibrational CH(A(2)Delta) distributions provides information about energy partitioning in the CH(A(2)Delta) fragment during the reaction. The rotational and vibrational populations of the CH(A(2)Delta) product are determined by fitting the rotationally resolved experimental spectra with simulated spectra. The CH(A(2)Delta) product is found to be rotationally and vibrationally excited with T(rot) congruent with 1150 K and T(vib) congruent with 1900 K. The mechanism for this reaction proceeds through one of two five-atom intermediates and requires a crossing between electronic potential surfaces. The rotational excitation suggests a bent geometry for the final intermediate of this reaction before dissociation to products, and the vibrational excitation involves an elongation of the C-H bond from the compressed transition state to the final CH(A) state.     

Ab initio configuration interaction study of the B- and C-band photodissociation of methyl iodide

Multireference spin-orbit configuration interaction calculations have been carried out for the valence and low-lying Rydberg states of CH(3)I. Potential energy surfaces along the C-I dissociation coordinate (minimal energy paths with respect to the umbrella angle) have been obtained as well as transition moments for excitation of the Rydberg states. It is shown that the B and C absorption bands of CH(3)I are dominated by the perpendicular (3)R(1),(1)R?(E)←X??A(1) transitions, while the (3)R(2)(E),?(3)R(0(+) )(A(1))←X??A(1) transitions are very weak. It is demonstrated that the bound Rydberg states of the B and C bands are predissociated due to the interaction with the repulsive E and A(2) components of the (3)A(1) state, with the (3)A(1)(E) state being the main decay channel. It is predicted that the only possibility to obtain the I((2)P(3/2)) ground state atoms from the CH(3)I photodissociation in the B band is by interaction of the (3)R(1)(E) state with the repulsive (1)Q(E) valence state at excitation energies above 55,000 cm(-1). The calculated ab initio data are used to analyze the influence of the Rydberg state vibrational excitation on the decay process. It is shown that, in contrast to intuition, excitation of the ν(3) C-I stretching mode supresses the predissociation, whereas the ν(6) rocking vibration enhances the predissociation rate.     

Vibrational spectroscopic study of the electron transfer for the Cl- + CH3Cl S(N)2 reaction in the gas phase

The vibrational spectra study of electron transfer of Cl- + CH3Cl --> ClCH3 + Cl- reaction has been examined by density function theory (DFT) calculations at 6-311 + +G** level in this paper. This reaction includes old bond rupture, new bond formation and electronic transfer in the intermolecular. The vibrational frequencies and vibrational modes of reactant, precursor complex, transition state, successor complex and product are analyzed. The relationship and the change among them can confirm the rupture of bond, the formation of bond and the process of electron transfer.     

Franck-Condon simulation of the single vibronic level emission spectra of HSiF and DSiF including anharmonicity

Potential energy functions (PEFs) of the X (1)A(') and A (1)A(") states of HSiF have been computed using the coupled-cluster single-double plus perturbative triple excitations and complete-active-space self-consistent-field multireference internally contracted configuration interaction methods, respectively, employing augmented correlation-consistent polarized-valence quadruple-zeta basis sets. For both electronic states of HSiF and DSiF, anharmonic vibrational wavefunctions and energies of all three modes have been calculated variationally with the ab initio PEFs and using Watson's Hamiltonian for nonlinear molecules. Franck-Condon factors between the two electronic states, allowing for Duschinsky rotation, were computed using the calculated anharmonic vibrational wavefunctions. These Franck-Condon factors were used to simulate the single vibronic level (SVL) emission spectra recently reported by Hostutler et al. in J. Chem. Phys. 114, 10728 (2001). Excellent agreement between the simulated and observed spectra was obtained for the A (1)A(")(1,0,0)-->X (1)A(') SVL emission of HSiF. Discrepancies between the simulated and observed spectra of the A (1)A(")(0,1,0) and (1,1,0) SVL emissions of HSiF have been found. These are most likely, partly due to experimental deficiencies and, partly to inadequacies in the ab initio levels of theory employed in the calculation of the PEFs. Based on the computed Franck-Condon factors, minor revisions of previous vibrational assignments are suggested. The calculated anharmonic wave functions of higher vibrational levels of the X (1)A(') state show strong mixings between the three vibrational modes of HSi stretching, bending, and SiF stretching.     

Internal rotation in propionic acid: near-infrared-induced isomerization in solid argon

The conformational system of propionic acid (CH3CH2COOH) is studied in solid argon. It is predicted by the ab initio calculations that this molecule has four stable conformers. These four structures are denoted Tt, Tg+/-, Ct, and Cg+/-, and they differ by the arrangement around the C-O and Calpha-C bonds. The ground-state Tt conformer is the only form present at 8 K after deposition of an argon matrix containing propionic acid. For the CH3CH2COOH and CH3CH2COOD isotopologues, narrow-band excitation of the first hydroxyl stretching overtone of the conformational ground state promotes the Calpha-C and C-O internal rotations producing the Tg+/- and Ct conformers, respectively. A subsequent vibrational excitation of the produced Tg+/- form induces its conversion to the Cg+/- conformer by rotation around the C-O bond. In the dark, all of the produced conformers decay to the conformational ground state at different rates. The decay kinetics and its temperature dependence allow the identification of the conformers by IR absorption spectroscopy, which is supported by ab initio calculations of their vibrational spectra. For the CH3CH2COOD isotopologue, the excitation of molecules isolated in different matrix sites results in site-dependent photoisomerization rates for the Calpha-C and C-O internal rotations, which also confirm the identification of the photoproducts.     

The Huggins band of ozone: assignment of hot bands

The "hot bands" of the Huggins band of ozone are assigned, in both the 218 K and the 295 K spectrum. The assignment is based on intensities calculated with three-dimensional vibrational wave functions for the electronic ground state (X) and the excited state (B). The hot-band structures in the 218 K spectrum all can be assigned to transitions starting from vibrational states with one quantum of stretching excitation in the ground electronic state. The 295 K spectrum shows new structures, which are due to transitions originating from vibrational states in the X state with two quanta of excitation of the stretching modes--despite very small Boltzmann factors. All structures in the low-energy range of the 295 K spectrum, even the very weak ones, thus can be uniquely interpreted. The significance of hot bands results from the strong increase of Franck-Condon factors with excitation of the stretching modes in both the lower and/or the upper electronic states, whose equilibrium bond lengths differ significantly.     

IR spectroscopy of hydrogen-bonded 2-fluoropyridine-methanol clusters

The electronic and infrared spectra of 2-fluoropyridine-methanol clusters were observed in a supersonic free jet. The structure of hydrogen-bonded clusters of 2-fluoropyridine with methanol was studied on the basis of the molecular orbital calculations. The IR spectra of 2-fluoropyridine-(CH3OH)n(n = 1-3) clusters were observed with a fluorescence-detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The structures of the clusters are similar to those observed for 2-fluoropyridine-(H2O)n (n = 1-3) clusters. The existence of weak hydrogen bond interaction through aromatic hydrogen was observed in the IR spectra. The theoretical calculation also supports the result. The vibrational frequencies of CH bonds in CH3 group are affected by hydrogen bond formation although these bonds do not directly relate to the hydrogen bond interaction. The B3LYP/6-311 ++G(d,p) calculations reproduce well the vibrational frequency of the hydrogen-bonded OH stretching vibrations. However, the calculated frequency of CH stretching vibration could not reproduce the IR spectra because of anharmonic interaction with closely lying overtone or combination bands for nu3 and nu9 vibrations. The vibrational shift of nu2 vibration is reproduced well with molecular orbital calculations. The calculation also shows that the frequency shift of nu2 vibration is closely related to the CH bond length at the trans position against the OH bond in hydrogen-bonded methanol.     

Matrix infrared spectroscopic and computational investigations of the lanthanide-methylene complexes CH2LnF2 with single Ln-C bonds

Laser-ablated lanthanide metal atoms were condensed with CH(2)F(2) in excess argon at 6 K or neon at 4 K. New infrared absorption bands are assigned to the oxidative addition product methylene lanthanide difluorides on the basis of deuterium substitution and vibrational frequency calculations with density functional theory (DFT). Two dominant absorptions in the 500 cm(-1) region are identified as lanthanide-fluoride stretching modes for this very strong infrared absorption. The predominantly lanthanide-carbon stretching modes follow a similar trend of increasing with metal size and have characteristic 30 cm(-1) deuterium and 14 cm(-1) (13)C isotopic shifts. The electronic structure calculations show that these CH(2)LnF(2) complexes are not analogous to the simple transition and actinide metal methylidenes with metal-carbon double bonds that have been investigated previously, because the lanthanide metals (in the +2 or +3 oxidation state) do not appear to form a π-type bond with the CH(2) group. The DFT and ab initio correlated molecular orbital theory calculations predict that these complexes exist as multiradicals, with a Ln-C σ bond and a single electron on C-2p weakly coupled with f(x) (x = 1 (Ce), 2 (Pr), 3(Nd), etc.) electrons in the adjacent Ln-4f orbitals. The Ln-C σ bond is composed of about 15% Ln-5d,6s and 85% C-sp(2) hybrid orbital. The Ln orbital has predominantly 6s and 5d character with more d-character for early lanthanides and increasing amounts of s-character across the row. The Ln-F bonds are almost purely ionic. Accordingly, the argon-neon matrix shifts are large (13-16 cm(-1)) for the ionic Ln-F bond stretching modes and small (～1 cm(-1)) for the more covalent Ln-C bond stretching modes.     

Vibrational spectroscopy of nitroalkane chains using electron autodetachment and ar predissociation

If the binding energy of an excess electron is lower than some of the vibrational levels of its host anion, vibrational excitation can lead to autodetachment. We use excitation of CH stretching modes in nitroalkane anions (2700-3000 cm(-1)), where the excess electron is localized predominantly on the NO2 group. We present data on nitroalkane anions of various chain lengths, showing that this technique is a valid approach to the vibrational spectroscopy of such systems extending to nitroalkane anions at least the size of nitropentane. We compare spectra taken by using vibrational autodetachment with spectra obtained by monitoring Ar evaporation from Ar solvated nitroalkane anions. The spectra of nitromethane and nitroethane are assigned on the basis of ab initio calculations with a detailed analysis of Fermi resonances of CH stretching fundamentals with overtones and combination bands of HCH bending modes.     

Vibrational analysis of excited and ground electronic states of all-trans retinal protonated Schiff-bases

We report on vibrational coherence dynamics in excited and ground electronic states of all-trans retinal protonated Schiff-bases (RPSB), investigated by time-resolved Degenerate Four-Wave-Mixing (DFWM). The results show that wave packet dynamics in the excited state of RPSB consist of only low-frequency (<800 cm(-1)) modes. Such low-frequency wave packet motion is observed over a broad range of detection wavelengths ranging from excited state absorption (～500 nm) to stimulated emission (>600 nm). Our results indicate that low-frequency coherences in the excited state are not activated directly by laser excitation but rather by internal vibrational energy redistribution. This is supported by the observation that similar coherence dynamics are not observed in the electronic ground state. Challenging previous experimental results, we show that the formation of low-frequency coherence dynamics in RPSB does not require significant excess vibrational energy deposition in the excited state vibrational manifolds. Concerning ground state wave packet dynamics, we observe a set of high-frequency (>800 cm(-1)) modes, reflecting mainly single and double bond stretching motion in the retinal polyene-chain. Dephasing of these high-frequency coherences is mode-dependent and partially differs from analogous vibrational dephasing of the all-trans retinal chromophore in a protein environment (bacteriorhodopsin).     

Impact of extended pi conjugation on methyl rotor-induced IVR in aromatic molecules

Laser-induced fluorescence excitation and resolved fluorescence spectra following excitations of the single vibronic levels (SVL) of p-vinyltoluene (p-VT) and p-vinylfluorobenzene (p-VFB) have been measured in a seeded supersonic free-jet expansion. A complete vibronic assignment of the fluorescence spectrum measured following excitation of the 0(0)0-band of p-VT has been presented. Normal vibrational modes in the S0 and S1 states of the molecule have been calculated by CASSCF method, and the correlation between the two set of modes is made by expressing the excited-state normal modes in terms of those of the ground state. The calculations predict that in the excited state methyl and vinyl torsional motions of p-VT are extensively mixed with many of the out-of-plane modes of the aromatic ring. Our resolved fluorescence spectral data measured following SVL excitations essentially agree with such predictions. In the excited state, the molecule exhibits a dramatically low threshold for the rotor-induced IVR in a supersonic jet expansion. Several mechanisms have been discussed to explain the phenomenon.     

An ab initio study of the CH3I photodissociation. II. Transition moments and vibrational state control of the I* quantum yields

Multireference spin-orbit configuration interaction calculations of transition moments from the X A1 ground state to the 3Q0+, 3Q1, and 1Q excited states responsible for the A absorption band of CH3I are reported and employed for an analysis of the photofragmentation in this system. Contrary to what is usually assumed, the 3Q0+(A1), 3Q1(E), and 1Q(E)<--X A1 transition moments are found to be strongly dependent on the C-I fragmentation coordinate. The sign of this dependence is opposite for the parallel and perpendicular transitions, which opens an opportunity for vibrational state control of the photodissociation product yields. The computed absorption intensity distribution and the I* quantum yield as a function of excitation energy are analyzed in comparison with existing experimental data, and good agreement between theory and experiment is found. It is predicted that significantly higher I* quantum yield values (>0.9) may be achieved when vibrationally hot CH3I molecules are excited in the appropriate spectral range. It is shown that vibrational state control of the I*/I branching ratio in the alkyl (hydrogen) iodide photodissociation has an electronic rather than a dynamic nature: Due to a different electron density distribution at various molecular geometries, one achieves a more efficient excitation of a particular fragmentation channel rather than influences the dynamics of the decay process.     

A femtosecond velocity map imaging study on B-band predissociation in CH3I. II. The 2(0)1 and 3(0)1 vibronic levels

Femtosecond time-resolved velocity map imaging experiments are reported on several vibronic levels of the second absorption band (B-band) of CH(3)I, including vibrational excitation in the ν(2) and ν(3) modes of the bound (3)R(1)(E) Rydberg state. Specific predissociation lifetimes have been determined for the 2(0)(1) and 3(0)(1) vibronic levels from measurements of time-resolved I*((2)P(1/2)) and CH(3) fragment images, parent decay, and photoelectron images obtained through both resonant and non-resonant multiphoton ionization. The results are compared with our previously reported predissociation lifetime measurements for the band origin 0(0) (0) [Gitzinger et al., J. Chem. Phys. 132, 234313 (2010)]. The result, previously reported in the literature, where vibrational excitation to the C-I stretching mode (ν(3)) of the CH(3)I (3)R(1)(E) Rydberg state yields a predissociation lifetime about four times slower than that corresponding to the vibrationless state, whereas predissociation is twice faster if the vibrational excitation is to the umbrella mode (ν(2)), is confirmed in the present experiments. In addition to the specific vibrational state lifetimes, which were found to be 0.85 ± 0.04 ps and 4.34 ± 0.13 ps for the 2(0)(1) and 3(0)(1) vibronic levels, respectively, the time evolution of the fragment anisotropy and the vibrational activity of the CH(3) fragment are presented. Additional striking results found in the present work are the evidence of ground state I((2)P(3/2)) fragment production when excitation is produced specifically to the 3(0)(1) vibronic level, which is attributed to predissociation via the A-band (1)Q(1) potential energy surface, and the indication of a fast adiabatic photodissociation process through the repulsive A-band (3)A(1)(4E) state, after direct absorption to this state, competing with absorption to the 3(0)(1) vibronic level of the (3)R(1)(E) Rydberg state of the B-band.     

Core excitations of naphthalene: vibrational structure versus chemical shifts

High-resolution x-ray photoelectron emission (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectra of naphthalene are analyzed in terms of the initial state chemical shifts and the vibrational fine structure of the excitations. Carbon atoms located at peripheral sites experience only a small chemical shift and exhibit rather similar charge-vibrational coupling, while the atoms in the bridging positions differ substantially. In the XPS spectra, C-H stretching modes provide important contributions to the overall shape of the spectrum. In contrast, the NEXAFS spectrum contains only vibrational progressions from particular C-C stretching modes. The accuracy of ab initio calculations of absolute electronic transition energies is discussed in the context of minute chemical shifts, the vibrational fine structure, and the state multiplicity.     

3-二甲氨基-2-甲基丙烯醛S2激发态结构动力学

采用共振拉曼光谱和完全活性自洽场理论计算研究了3-二甲氨基-2-甲基丙烯醛（DMAMP）光激发到S2（ππ^＊）态后的光物理性能,在B3LYP／6—311＋＋G（d,p）水平计算确定了DMAMP与其三种异构体之间的基态异构化能垒,指认了振动光谱．采用涵盖紫外强吸收带的激光波长,获得了DMAMP在环己烷、乙腈和甲醇溶剂中的A-带共振拉曼光谱,含时密度泛函方法计算确定了该光谱中基频的相对强度,发现振动-电子耦合发生在S2（）态的Franck—Condon区域．CASSCF计算方法确定低单重和三重激发态、势能面锥形交叉点和系间窜跃点的激发能．共振拉曼光谱强度模式分析和CASSCF计算获得了DMAMP的A一带短时结构动力学和其后的衰变动力学表明,C1=06和C2=C3之间的瞬时去共轭效应发生在S2（ππ^＊）态的Franck—Condon区域,激发态电荷重分布机制表明,C3和二甲氨基之间以及C1和C2之间的共轭增强效应发生在波包离开Franck-Condon区域后．C1=06和C2=C3之间的去共轭效应使得-C3=N（CH3）2沿着C2-C3键旋转更加容易,C1-C2之间以及C3和N（CH3）的共轭增强效应使得绕C1-C2和C3-N5旋转变得比较困难．这些表明DMAMP初始结构动力学沿着CI-1（S2／S0）交叉点展开,而沿CI-2（S2／S0）和[CI-3（S2／S0）交叉点展开的几率可以忽略,提出了DMAMP分子受光激发从S2,FC（ππ^＊）经由各锥形交叉点和各系间窜跃点回到S0或T1,min的两个衰变通道。     

Reaction dynamics and vibrational spectroscopy of CH3D molecules with both C-H and C-D stretches excited

State-resolved reactions of CH3D molecules containing both C-H and C-D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C-H symmetric stretch and the C-D stretch (nu1 + nu2), the combination of the C-D stretch and the C-H antisymmetric stretch (nu2 + nu4), and the combination of the C-D stretch and the first overtone of the CH3 bend (nu2 + 2nu5). The simulation for the previously unanalyzed (nu2 + nu4) state yields a band center of nu0 = 5215.3 cm(-1), rotational constants of A = 5.223 cm(-1) and B = 3.803 cm(-1), and a Coriolis coupling constant of zeta = 0.084. The reaction dynamics largely follow a spectator picture in which the surviving bond retains its initial vibrational excitation. In at least 80% of the reactive encounters of vibrationally excited CH3D with Cl, cleavage of the C-H bond produces CH2D radicals with an excited C-D stretch, and cleavage of the C-D bond produces CH3 radicals with an excited C-H stretch. Deviations from the spectator picture seem to reflect mixing in the initially prepared eigenstates and, possibly, collisional coupling during the reaction.     

Laser-induced fluorescence and pure rotational spectroscopy of the CH2CHS (vinylthio) radical

Laser-induced fluorescence (LIF) excitation spectra of the B-X (2)A(") electronic transition of the CH(2)CHS radical, which is the sulfur analog of the vinoxy (CH(2)CHO) radical, were observed under room temperature and jet-cooled conditions. The LIF excitation spectra show very poor vibronic structures, since the fluorescence quantum yields of the upper vibronic levels are too small to detect fluorescence, except for the vibrationless level in the B state. A dispersed fluorescence spectrum of jet-cooled CH(2)CHS from the vibrationless level of the B state was also observed, and vibrational frequencies in the X state were determined. Precise rotational and spin-rotation constants in the ground vibronic level of the radical were determined from pure rotational spectroscopy using a Fourier-transform microwave (FTMW) spectrometer and a FTMW-millimeter wave double-resonance technique [Y. Sumiyoshi et al., J. Chem. Phys. 123, 054324 (2005)]. The rotationally resolved LIF excitation spectrum for the vibronic origin band of the jet-cooled CH(2)CHS radical was analyzed using the ground state molecular constants determined from pure rotational spectroscopy. Determined molecular constants for the upper and lower electronic states agree well with results of ab initio calculations.     

Laser-induced fluorescence spectroscopy of NC3S

In a discharged supersonic jet of acetonitrile and carbon disulfide, we have for the first time observed an electronic transition of the NC(3)S radical using laser-induced fluorescence (LIF) spectroscopy. A progression originating from the C-S stretching mode of the upper electronic state appears in the excitation spectrum. Each band of the progression has a polyad structure due to anharmonic resonances with even overtones of bending modes. Rotationally resolved spectra have been observed by high-resolution laser scans, and the electronic transition is assigned to A 2Pii-X 2Pii. For the vibronic origin band, the position and the effective rotational constant of the upper level have been determined to be 21 553.874(1) and 0.046 689(4) cm(-1), respectively. The dispersed fluorescence spectrum from the zero vibrational level of A 2Pi3/2 has also been observed; its vibrational structure is similar to that of the LIF excitation spectrum, showing a prominent C-S stretching progression with polyad structures. The vibrational frequencies of the C-S stretching mode in the ground and excited electronic states are determined to be 550 and 520 cm(-1), respectively. Fluorescence decay profiles have been measured for several vibronic levels of the A state.