Photoelectron spectroscopic study of the E⊗e Jahn-Teller effect in the presence of a tunable spin-orbit interaction. I. Photoionization dynamics of methyl iodide and rotational fine structure of CH3I+ and CD3I+

The high-resolution single-photon pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X?(+) (2)E(3/2)←X?(1)A(1) transition of CH(3)I and CD(3)I have been recorded. The spectral resolution of better than 0.15 cm(-1) enabled the observation of the rotational structure. CH(3)I(+) and CD(3)I(+) are subject to a weak E?e Jahn-Teller effect and strong spin-orbit coupling. The treatment of the rovibronic structure of the photoelectron spectra in the corresponding spin double group, C(3v)(2)(M), including the effects of the spin-orbit interaction and the vibrational angular momentum, allowed the reproduction of the experimentally observed transitions with spectroscopic accuracy. The relevant spin-orbit and linear Jahn-Teller coupling parameters of the X?(+) ground state were derived from the analysis of the spectra of the two isotopomers, and improved values were obtained for the adiabatic ionization energies [E(I)(CH(3)I)/hc=76931.35(20) cm(-1) and E(I)(CD(3)I)/hc=76957.40(20) cm(-1)] and the rotational constants of the cations. Rovibronic photoionization selection rules were derived for transitions connecting neutral states following Hund's-case-(b)-type angular momentum coupling and ionic states following Hund's-case-(a)-type coupling. The selection rules, expressed in terms of the angular momentum projection quantum number P, account for all observed transitions and provide an explanation for the nonobservation of several rotational sub-bands in the mass-analyzed threshold-ionization spectra of CH(3)I and CD(3)I reported recently by Lee et al. [J. Chem. Phys. 128, 044310 (2008)].     

The Jahn-Teller effect in CH(3)CN(+) (X(2)E) and CD(3)CN(+) (X(2)E) studied by zero kinetic energy photoelectron spectroscopy

The Jahn-Teller effect in CH(3)CN(+) (X(2)E) and CD(3)CN(+) (X(2)E) has been found experimentally by zero kinetic energy (ZEKE) photoelectron spectroscopy using coherent extreme ultraviolet (XUV) radiation. The vibronic bands of CH(3)CN(+) (X(2)E) and CD(3)CN(+) (X(2)E) at about 4500 cm(-1) above the ground states have been recorded. The spectra consist mainly of the Jahn-Teller active C-C[triple bond]N bending (v(8)), the CN stretching (v(2)), the CH(3) (CD(3)) deforming (v(6)), and the C-C stretching (v(4)) vibronic excitations. The Jahn-Teller active vibronic bands (v(8)) have been assigned with a harmonic model including linear and quadratic Jahn-Teller coupling terms, taking into account only the single mode vibronic excitation. The ionization potentials of CH(3)CN and CD(3)CN have also been determined, and their values are 12.2040(+/-0.001) and 12.2286(+/-0.001) eV, respectively.     

Theoretical study of the potential energy surfaces of the Van Der Waals H2O-X2+ (X = Cl or Br) complexes

An ab initio study of the interactions between H2O and Cl2+ and H2O and Br2+ has been performed. We present calculations using both the UMP2 level and the UCCSD(T) level of correlation with the aug-cc-pVTZ basis. The aug-cc-pVQZ basis was tested for selected geometries and was found to yield results similar to the smaller basis. For the H2O-Cl2+ cation, a C2v structure has been identified as the minimum, with De = 6500 cm-1 (78 kJ/mol). A low-lying excited state has De = 6000 cm-1 (72 kJ/mol). The adiabatic and vertical ionization energies of the complex are 10.7 and 11.0 eV, compared to the experimental adiabatic value, 11.5 eV, for free chlorine. For the H2O-Br2+ cation, the calculations are more subtle due to second-order Jahn-Teller effects and result in a Cs structure at the minimum, with De = 6300 cm-1 (75 kJ/mol), yielding an adiabatic ionization energy of 9.9 eV compared to the corresponding experimental value, 10.5 eV, for free bromine. The relatively large binding energies give rise to strong normal mode couplings such that the halogen stretching mode becomes mixed with the water bending and other intermolecular modes, resulting in very large frequency shifts. Vertical ionization energies and ion vibrational frequencies also are reported and used to discuss possible experiments to obtain more precise data for each of the complexes.     

Jahn-Teller effect in CH3D+ and CD3H+: conformational isomerism, tunneling-rotation structure, and the location of conical intersections

High-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectra of CH(3)D and CD(3)H have been recorded at rotational resolution from the adiabatic ionization energy up to 600 cm(-1) of internal energy of the respective cations. The spectra are characterized by the effects of a large-amplitude pseudorotational motion exchanging the equivalent nuclei in each molecule. With increasing internal energy, a transition from the tunneling regime with splittings of the order of 1-10 cm(-1) to the free pseudorotation regime is observed. A theoretical model that treats the simultaneous rotational and pseudorotational motions and incorporates the effects of the geometric phase has been developed. The model provides the appropriate rovibronic symmetries in the C(3v)(M) molecular symmetry group and reaches a near-quantitative agreement with the experimental data. The complete group-theoretical analysis of the rovibronic problem is also given. The analysis of the spectra has revealed the existence of two different isomers for both CH(3)D(+) and CD(3)H(+), which differ in the bond length between the carbon atom and the unique ligand atom. All isomers are subject to a fast pseudorotational motion between three equivalent minima with a period of 3-5 ps in CH(3)D(+) and 18-28 ps in CD(3)H(+). The analysis has also provided the ordering of the tunneling sublevels for each isomer, which enables the location of the twofold conical intersections on the potential energy surface that could not be determined from experiments on CH(4) (+).     

Electronic spectroscopy of the deuterated isotopomers of the NO.methane molecular complex

The molecular complexes formed between a nitric oxide molecule and the various deuterated isotopomers of the methane molecule have been studied in a supersonic jet expansion. The electronic spectrum arising from the transition corresponding to a 3s<--pi* excitation (approximately A (2)Sigma(+)<-- approximately X (2)Pi) located on the NO chromophore has been recorded employing resonance-enhanced multiphoton ionization spectroscopy, with each of CH(4), CH(3)D, CH(2)D(2), CHD(3), and CD(4) as the complexing partner. Rich spectra are obtained, whose appearance changes in a systematic way as the amount of deuteration increases. Unexpectedly, it was possible to record spectra not only in the parent mass channel, but also in various fragment channels; this also led to the identification of some O atom resonances; and their origin is discussed. Discussion is presented of the structure in the spectra, and its possible sources including hindered internal rotation of the methane and NO moieties, overall rotation of the complex, and tunneling. In addition, some guidance has been gleaned from ab initio calculations, and these are discussed in the light of the experimental results.     

Rotational spectra and structure of the Ar2-H2S complex: pulsed nozzle Fourier transform microwave spectroscopic and ab initio studies

This paper reports the rotational spectrum and structure of the Ar2-H2S complex and its HDS and D2S isotopomers. The ground state structure has heavy-atom C2v symmetry with the two Ar atoms indistinguishable and H2S freely rotating as evinced by the fact that asymmetric top energy levels with Kp=odd levels are missing. The rotational constants for the parent isotopomer are: A=1733.115(1) MHz, B=1617.6160(5) MHz and C=830.2951(2) MHz. Unlike the Ar-H2S complex, the Ar2-H2S does not show an anomalous isotopic shift in rotational constants on deuterium substitution. However, the intermolecular potential is still quite floppy, leading to very different centrifugal distortion constants for the three isotopomers. The Ar-Ar and Ar-c.m.(H2S) distances are determined to be 3.820 A and 4.105 A, respectively. The A rotational constants for Ar2-H2S/HDS/D2S isotopomers are very close to each other and to the B constant of free Ar2, indicating that H2S does not contribute to the moment of inertia about the a-axis. Ab initio calculations at MP2 level with aug-cc-pVQZ basis set lead to an equilibrium C2v minimum structure with the Ar-Ar line perpendicular to the H-H line and the S away from Ar2. The centrifugal distortion constants, calculated using the ab initio force field, are in reasonable agreement with the experimental values. However, they do not show the variation observed for different isotopmers. The binding energy of Ar2-H2S has been determined to be 507 cm-1(6.0 kJ mol-1) by CBS extrapolation after correcting for basis set superposition error. Potential energy scans point out that the barrier for internal rotation of H2S about its b axis is only 10 cm-1 and it is below the zero point energy (13.5 cm-1) in this torsional degree of freedom. Internal rotation of H2S about its a- and c-axes also have small barriers of about 50 cm-1 only, suggesting that H2S is extremely floppy within the complex.     

Pulsed field ionization-ZEKE spectroscopy of cresoles and their aqueous complexes: internal rotation of methyl group and intermolecular vibrations

Pulsed field ionization-ZEKE photoelectron spectroscopy and (1 + 1) R2PI spectroscopy have been applied to the cis- and trans-m-cresol.H2O clusters. The internal rotational structure in the S1 state has been re-assigned, and the potential curve has been determined for the cluster. The PFI-ZEKE spectra of the cis- and trans-isomers show low-frequency bands up to 1000 cm-1 above the adiabatic ionization potential IP0. The low-frequency bands are assigned to the internal rotation of the methyl group, the intermolecular stretching and their combination bands in the m-cresol.H2O cluster cation. Level energies and relative transition intensities are reproduced well by a one-dimensional rotor model with a three-fold axis potential. Potential curves for the internal rotation have been determined for both cis- and trans-isomers of m-cresol.H2O cations. The effect of the cluster formation upon the internal methyl rotation, and the interaction between the methyl rotation and the intermolecular vibration are discussed.     

Vibrational structures of methylamine isotopomers in the predissociative ~A states: CH3NHD, CD3NH2, CD3NHD, and CD3ND2

Mass-resolved two-photon (1+1) resonance-enhanced multiphoton ionization spectra of the ~A-X transitions of various methylamine isotopomers (CH(3)NHD, CD(3)NH(2), CD(3)NHD, and CD(3)ND(2)) cooled in the supersonic jet expansion have been measured and analyzed. The band analysis using the Hamiltonian for the internal and overall rotational motions provides the accurate vibrational band positions, allowing for unambiguous assignments for all observed vibrational bands of methylamine isotopomers in the ~A states. Amino wagging (nu(9)) and methyl rocking (nu(7)) modes are found to be Franck-Condon active, and associated anharmonicity constants are precisely determined to give the detailed shape of the potential energy surface in the vicinity of the minimum electronic molecular structure. The barrier height for the nearly free internal rotation about the C-N bond in the ~A state is calculated to be strongly dependent on the excitation of the other higher-frequency vibrational modes, and it is found that the trend is consistent with the experiment. Experimentally measured spectroscopic constants are compared with ab initio calculations, confirming all vibronic assignments. Experimental and theoretical results on all possible HD isotopomers of methylamine in this work, with the earlier report on CH(3)NH(2) and CH(3)ND(2) Baek et al., [J. Chem. Phys. 118, 11026 (2003)], provide the complete spectroscopic characterization of the A state of methylamine.     

B(OCH3)3电子结构的Hel紫外光电子能谱研究

人们知道，Hel紫外光电子能借（PES）提供研究分子轨道能量、能级次序、成键类型以及由光电子峰强度所反映的电离轨道特性等信息是其他手段没有的，因而PES技术已广泛地用于众多化合物分子电子结构的研究中．有机础化合物由于它们高的反应活性作为合成试剂而信受人们重视［‘－     

Ionization from a double bond: rovibronic photoionization dynamics of ethylene, large amplitude torsional motion and vibronic coupling in the ground state of C2H4+

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X-->X+ transition in ethylene and ethylene-d4 have been recorded at a resolution of 0.09 cm(-1). The spectra provide new information on the large amplitude torsional motion in the cationic ground state. An effective one-dimensional torsional potential was determined from the experimental data. Both C2H4+ and C2D4+ exhibit a twisted geometry, and the lowest two levels of the torsional potential form a tunneling pair with a tunneling splitting of 83.7(5) cm(-1) in C2H4+ and of 37.1(5) cm(-1) in C2D4+. A model was developed to quantitatively analyze the rotational structure of the photoelectron spectra by generalizing the model of Buckingham, Orr, and Sichel [Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] to treat asymmetric top molecules. The quantitative analysis of the rotational intensity distributions of allowed as well as forbidden vibrational bands enabled the identification of strong vibronic mixing between the X+ and A+ states mediated by the torsional mode nu(4) and a weaker mixing between the X+ and B+ states mediated by the symmetric CH2 out-of-plane bending mode nu7. The vibrational intensities could be accounted for quantitatively using a Herzberg-Teller-type model for vibronic intensity borrowing. The adiabatic ionization energies of C2H4 and C2D4 were determined to be 84 790.42(23) cm(-1) and 84 913.3(14) cm(-1), respectively.     

Ab initio and electron propagator theory study of boron hydrides

Boron hydrides (BH3, B2H6, B3H7, B4H10, B5H9, and B5H11) and their cations are studied by the coupled cluster CCSD(T) theory, the second-order Mller-Plesset (MP2) perturbation method, and the electron propagator theory in the partial third-order quasi-particle approximation, using the 6-311G(d,p) basis set. The vertical ionization potential energies are calculated, indicating an excellent agreement with the experimental data from photoelectron spectroscopy. Assignments to the experimental spectra are made on the basis of the present computational analyses. A significant Jahn-Teller effect on BH3+ leads to two states, 2A1 and 2B2, with the split energy of 0.14 eV. The triple and double B-H-B bridges are formed in B2H6+ and b-B3H7+, respectively. A new B-H-B bridge is formed while two B-B bonds are broken in B5H11+. The Jahn-Teller effect lowers the symmetry of B5H9 (C4v) to B5H9+ (C2) but slightly influences the structure of ara-B4H10 (C2v). The calculated properties of geometries, vibrational frequencies, and energies are compared with the experimental data available in the literatures.     

The Jahn-Teller effect in the triply degenerate electronic state of methane radical cation

A quantum dynamics study is performed to examine the complex nuclear motion underlying the first photoelectron band of methane. The broad and highly overlapping structures of the latter are found to originate from transitions to the ground electronic state, X(2)T(2), of the methane radical cation. Ab initio calculations have also been carried out to establish the potential energy surfaces for the triply degenerate electronic manifold of CH(4)(+). A suitable diabatic vibronic Hamiltonian has been devised and the nonadiabatic effects due to Jahn-Teller conical intersections on the vibronic dynamics investigated in detail. The theoretical results show fair accord with experiment.     

Rovibrational photoionization dynamics of methyl and its isotopomers studied by high-resolution photoionization and photoelectron spectroscopy

High-resolution photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of CH(3), CH(2)D, CHD(2), and CD(3) have been recorded in the vicinity of the first adiabatic ionization threshold following single-photon excitation from the ground neutral state using a narrow-bandwidth vacuum-ultraviolet laser. The radicals were produced from the precursor molecules methyl-bromide, methyl-iodide, dimethyl-thioether, acetone, and nitromethane by 193 nm excimer photolysis in a quartz capillary and were subsequently cooled to a rotational temperature T(rot) approximately equal to 30 K in a supersonic expansion. Nitromethane was identified as a particularly suitable photolytic precursor of methyl for studies by photoionization and threshold photoelectron spectroscopy. Thanks to the cold rotational temperature reached in the supersonic expansion, the rotational structure of the threshold ionization spectra could be resolved, and the photoionization dynamics investigated. Rydberg series converging on excited rotational levels of CH(3) (+) could be observed in the range of principal quantum number n=30-50, and both rotational autoionization and predissociation were identified as decay processes in the threshold region. The observed photoionization transitions can be understood in the realm of an orbital model for direct ionization but the intensity distributions can only be fully accounted for if the rotational channel interactions mediated by the quadrupole of the cation are considered. Improved values of the adiabatic ionization thresholds were derived for all isotopomers [CH(3): 79 356.2(15) cm(-1), CH(2)D: 79 338.8(15) cm(-1), CHD(2): 79 319.1(15) cm(-1), and CD(3): 79 296.4(15) cm(-1)].     

Theoretical study of multiphoton ionization of cyclohexadienes and unimolecular decomposition of their mono- and dications

Quantum chemical calculations of the geometric structure, vertical excitation energies, and ionization potentials for the isomeric pair of 1,3- and 1,4-cyclohexadienes and their mono- and dications have been performed employing a variety of theoretical methods and basis sets. The computed ionization potentials and electronic excitation energies are used to evaluate the range of internal energies available for fragmentation of the cations following multiphoton resonance ionization of the cyclohexadienes in intense laser field. The conditions governing the competition between multiple ionization and decomposition of the ions are also discussed. Calculations of stationary points on the potential energy surfaces for various fragmentation channels and relative product yields at different available internal energies are then utilized to analyze the trends in branching ratios of major dissociation products of the 1,4-cyclohexadiene(2+) dication, which include C(3)H(3)(+) + C(3)H(5)(+), C(2)H(3)(+) + C(4)H(5)(+), and C(4)H(3)(+) + C(2)H(5)(+).     

Electronic structure of H2CS3 and H2CS4: an experimental and theoretical study

The HeI photoelectron spectra of H2CS3 and H2CS4 in the gas phase have been obtained for the first time. A complete theoretical study involving the calculation of the ionization energies using orbital valence Green's functional (OVGF) and population analysis was performed. Calculations of cation-radical forms were carried out in order to interpret the main characters of the six highest occupied molecular orbitals (HOMOs). The first vertical ionization potentials are 8.74 and 8.56eV for H2CS3 and H2CS4, and attributed to {9b2(nS(C=S))}-1 and {8a"(3ppi*(S-S), nS)}-1, respectively. Meanwhile, the energy sequence of three types of sulfur 3p lone-pair have been discussed: 3ppi(S-S)*相似文献   

Excited-state intramolecular proton transfer reaction modulated by low-frequency vibrations: an effect of an electron-donating substituent on the dually fluorescent bis-benzoxazole

Excited-state intramolecular proton transfer (ESIPT) reaction has been studied in a molecule showing dual fluorescence, the 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (BBMP), and its isotopomers, where the methoxy, and alternatively, the OH group has been deuterated. Attention is focused on the influence of electron donating OCH(3) substituent on fast excited state reaction. Comparison between the resonance-enhanced multiphoton ionization spectrum and the laser-induced excitation of the primary and phototautomeric emissions has been done. The geometry, electron density distribution, vibrational structure as well as the potential energy profiles in the S(0) and S(1) states of four possible rotameric forms of BBMP were calculated with application of the density functional theory (DFT). It allowed identifying the most probable conformer and assessing the role of low-frequency motions for the ESIPT efficiency.     

Microwave investigation of the CO-CH4 van der Waals complex

Rotational spectra of eight isotopomers of the weakly bound van der Waals complex CO-CH4 were recorded in the frequency range from 4 to 19 GHz using a pulsed molecular beam Fourier transform microwave spectrometer. For the isotopomers containing methane monomers of Td symmetry, namely, 12C16O-12CH4, 12C16O-13CH4, 12C16O-12CD4, 13C16O-12CH4, and 13C18O-12CH4, three rotational progressions were observed that correlate to the jm=0, 1, and 2 rotational levels of free methane. For those containing partially deuterated methane monomers with C3V symmetry, namely, 12C16O-12CH3D and 12C16O-12CHD3, only two progressions were recorded, correlating to the jk=0(0) and 1(1) rotational levels of free CH3D and CHD3, respectively. The van der Waals bond distance R, intermolecular stretching frequency nus, and the corresponding stretching force constant ks were derived from the obtained spectroscopic results. The results obtained for the jm=0 ground state are compared to the previous infrared and millimeter wave data. A 17O nuclear quadrupole coupling constant was determined from the resolved hyperfine structure of 13C17O-12CH4 and was used to obtain angular information about the carbon monoxide subunit. A Coriolis interaction was deduced from the irregular spectral pattern involving levels with jm=1. Qualitative information about the extent of the perturbation was obtained from a comparison of spectroscopic constants of different isotopomers.     

Study of the Electronic Structure of Ni(eta(5)-C(5)H(5))(NO) by Variable-Photon-Energy Photoelectron Spectroscopy and Density Functional Calculations

Photoelectron spectra, with photon energies varying from 18 to 120 eV, have been measured for Ni(eta(5)-C(5)H(5))(NO). Relative partial photoelectron cross sections and branching ratios have been evaluated for the first three valence ionization bands. He I and He II photoelectron spectra have been remeasured for Ni(eta(5)-C(5)H(5))(NO) and Ni(eta(5)-C(5)H(4)CH(3))(NO). In the latter case, the fine structure on the first band differs from that in the previously published spectrum. Density functional calculations have been carried out to determine the ionization potentials of the lowest lying states of Ni(eta(5)-C(5)H(5))(NO) as well as the corresponding photoionization cross sections and the resulting branching ratios using the LCGTO-DF and LDKL-DF methods, respectively. Both experimental and theoretical investigations lead to an ion state ordering (2)E(1) < (2)E(2) approximately (2)A(1)< (2)E(1) and an assignment of (2)E(1) states to the first and third bands with the (2)A(1) and (2)E(2) states comprising the second band. This differs from the original assignment in the literature, where the (2)A(1) ionization was assigned to a high-energy shoulder on the first band. The separation of this shoulder from the main band maximum of 0.23 eV (1850 +/- 81 cm(-)(1)) suggests that it may be caused by excitation of the NO stretching vibration in the ion. The neutral molecule has a NO stretch of 1832 cm(-)(1); the calculated energies for the neutral molecule and the cation are 1845 and 1911 cm(-)(1), respectively. Agreement between calculated and experimental ionization energies and good matching of the theoretical and measured branching ratios support the new assignment of the photoelectron spectrum.     

氯代苯阳离子的密度泛函理论研究

采用B3LYP方法及6-311G(d,p)和6-311+G(d,p)基组,对12种氯代苯阳离子进行了理论研究,优化其电子基态的结构,计算了对应分子的垂直电离势(VIP)和绝热电离势(AIP).依据Jahn-Teller理论,确定了1,3,5-C6H3Cl3+和C6Cl6+离子分别具有C2v(2B1)和D2h(2B2g)结构(对应分子分别为D3h和D6h结构).其余10个离子的构型的对称点群与对应分子相同,但构型参数值有明显差别.用B3LYP方法计算的各氯代苯分子的垂直电离势和绝热电离势与实验值符合得很好.