Vibrational Energy States Research Articles

Reactivity of vibrationally excited methane on nickel surfaces

Four-dimensional variational calculations have been performed for modeling energy flow between methane (CH4) stretching vibrational energy states as the molecule adiabatically approaches a metallic surface. The model is based on a local mode Hamiltonian for an isolated CH4 molecule and a London-Eyring-Polanyi-Sato potential describing surface–molecule interactions. The results suggest the possibility of mode specific effects on chemical reactivity. Specifically, the symmetric A1 stretch fundamental adiabatically correlates with the localized excitation in the unique CH bond pointing towards the surface. Conversely, the antisymmetric F2 stretch fundamental excitation correlates with A and E vibrations in the CH3 radical, and therefore this degree of freedom is localized away from the reactive CH bond. Landau–Zener semiclassical analysis of nonadiabatic curve crossings predicts a significant velocity dependence to the state specific energy flow dynamics. Since excitation localized in active versus spectator bonds is expected to be more efficient in accelerating CH bond cleavage and adsorption reactivity, these results offer insight into interpreting velocity and vibrationally mediated reaction dynamics of CH4 on catalytic surfaces.

A reduced dimensionality, six-degree-of-freedom, quantum calculation of the H+CH4→H2+CH3 reaction

A reduced dimensionality, time-dependent wave-packet calculation is reported for the H+CH4→H2+CH3 reaction in six degrees of freedom and for zero total angular momentum, employing the Jordan–Gilbert potential energy surface. Reaction probabilities for seven initial vibrational states of nonrotating “CH4,” and for the three lowest energy vibrational states and numerous initial rotational states are presented. Excitation of the C–H stretch, and the bending of H–CH3, enhances the reaction probability more than excitation of the umbrella mode. The six-degree-of-freedom cumulative reaction probability (CRP) for zero total angular momentum is obtained by direct summation over initial state-resolved reaction probabilities. An approximate full-dimensional CRP for zero total angular momentum is obtained using the energy-shift approximation to account for the contribution of degrees of freedom missing in the reduced dimensionality calculations. Then J–K shifting is applied to this CRP to obtain the thermal rate constant which is compared with previous calculations.

The ground state of silylidene (H2C=Si), the silicon analog of vinylidene, from stimulated emission pumping and wavelength-resolved fluorescence spectroscopy

The ground state vibrational energy levels of jet-cooled H2CSi and D2CSi have been studied by a combination of wavelength-resolved fluorescence and stimulated emission pumping (SEP) techniques. By taking advantage of the vibrational selection rules and Franck–Condon factors and selectively pumping upper state single vibronic levels, readily assignable low-resolution emission spectra were obtained. Higher resolution SEP spectra were recorded to give precise measurements of the vibrational band origins of many of the lower-lying vibrational levels. All of the vibrational frequencies, except for the Franck–Condon inactive CH asymmetric stretching mode, ν5, have been determined for both isotopomers. The CH2 rocking mode (ν6) is found to have a very low 263 cm−1 frequency in the ground state. The spectra are complicated by unexpected activity in the out-of-plane bending vibration.

Chemisorption of benzene on metal dimer anions: A study by photoelectron detachment spectroscopy

Photoelectron detachment spectra of M2(C6H6)− (M=Pt, Pd, Pb) have been measured in the gas phase using photon energies of a Nd:YAG laser. The vibrationally resolved ground state transition from the anion to the neutral reveals an adiabatic electron affinity of (2.01±0.05) eV and (0.88±0.05) eV for Pt2(C6H6) and Pd2(C6H6), respectively. A ground state vibrational energy of (24.2±1) meV has been resolved for Pt2(C6H6). The corresponding vibrational energy of Pt2(C6H6)− amounts to (19.0±1.0) meV. The ground state vibrational energies of Pd2(C6H6) and Pd2(C6H6)− are (20.3±1.0) meV and (18.0±2.0) meV, respectively. The small vibrational frequencies suggest a perpendicular coordination (C6v-symmetry) of the benzene-adsorbed transition metal dimers. Pb2, on the other hand, is bound parallel to the benzene plane (C2v-symmetry). A closed shell ground state electron configuration is postulated for Pb2(C6H6) in contrast to the triplet ground state of unreacted Pb2. The vertical electron affinity of Pb2(C6H6) is (1.95±0.05) eV.

Orbital angular momentum (Renner–Teller) effects in the Πi2 ground state of silicon methylidyne (SiCH)

The ground state vibrational energy levels of jet-cooled SiCH and SiCD have been studied by a combination of laser-induced fluorescence and wavelength-resolved fluorescence techniques. The radicals were produced by a pulsed electric discharge at the exit of a supersonic expansion using tetramethylsilane or methyltrichlorosilane as the precursor. Emission spectra have been obtained by pumping both perpendicular and parallel (vibronically induced) bands, providing complementary information on the Si–C stretching and Si–C–H bending modes. Ground state energy levels up to 4000 cm−1 have been assigned and fitted using a vibrational Hamiltonian that incorporates Renner–Teller, spin–orbit, vibrational anharmonicity, and Fermi resonance interactions. The validity of the derived parameters has been tested using the isotope relations.

High Vibrational State Energy Redistribution in Two Deuterated Cyclopentenes

$^{a}$ S. Rodin-Bercion, D. Cavagnat, L. Lespade and P. Maraval, J. Phys. Chem., 99, 3005 (1995) $^{b}$ L. Lespade, S. Rodin-Bercion and D. Cavagnat, J. Phys. Chem., 101, 2568 (1997)

Potential energy function and vibrational states of N2CO+

A six-dimensional potential energy function (PEF) for the electronic ground state of N2CO+ (X 2A′) has been generated by electronic structure calculations using the restricted open shell coupled cluster RCCSD(T) approach. The ion has a planar trans equilibrium structure with: RNN=1.106 Å, RNC=1.905 Å, RCO=1.127Å, θNNC=175.3°, and θNCO=129.1°. Variational calculations of the vibrational states (J=0) have yielded the following anharmonic wavenumbers for the fundamentals: ν1 (NN stretch) 2287.2, ν2 (CO stretch) 2071.0, ν3 (CN stretch+NCO bend) 546.9, ν4 (CN stretch+NCO bend) 215.2, ν5 (in-plane NNC bend) 123.4, ν6 (out-of-plane NNC bend) 133.8 (all values are in cm−1). For fixed equilibrium coordinates except one, the barriers to linearity have been calculated to be 8 cm−1 for the NNC and 2260 cm−1 for the NCO moieties, the torsional barrier to be 35 cm−1. It has been found that the ν3 and ν4 modes are strongly coupled, the in-plane ν5 and out-of-plane ν6 bending modes possess an inverse anharmonicity and fall into clusters. Using complete active space self-consitent-field CASSCF approach on the collinear cuts of the PEF for low-lying excited states several conical intersections between the Π2 and the Σ+2 states have been located.

Structure Sensitivity in the Kinetics and the Dynamics of CO Oxidation over Stepped Pd(335) Studied by the Molecular Beam Infrared Chemiluminescence Technique: Determination of Working Sites during the Steady-State Reaction

Kinetics and dynamics of CO oxidation have been studied on a stepped Pd(335) surface at a steady-state condition and compared with those on flat Pd(111). The infrared (IR) chemiluminescence technique was applied to determine where the active catalytic sites are on the Pd(335) surface. Since the vibrational energy state of the product CO2 is sensitive to the structures of the reaction sites on Pd surfaces, information about the working reaction sites during the steady-state CO oxidation can be obtained from the IR emission spectra of the product CO2. The production rate of CO2 was higher on Pd(335) than on Pd(111), indicating that the steps on the surface enhance the catalytic activity for CO oxidation under the steady-state condition. However, the rate data do not necessarily show the real active sites for the CO + O recombination reaction. At a surface temperature of 850 K, the vibrational Boltzmann temperature (TV) of the product CO2 on Pd(335) was quite different from (much lower than) that on Pd(111), although the Pd(335) surface has four-atom wide (111) terraces. The lower TV value on Pd(335) was similar to that on Pd(110)(1 × 1), indicating that a relatively linear activated CO2 complex was formed. Therefore, during the steady-state CO oxidation on Pd(335), the reaction does not take place on the (111) terrace sites, but mostly on the step sites at 850 K. On the contrary, as the CO coverage increased at a lower surface temperature and at a high CO/O2 ratio, the TV values on Pd(335) tend to approach those on Pd(111), indicating that the contribution of the active sites on the steps is decreased and the working reaction sites shift to the (111) terrace sites.

Spectroscopic determination of ground and excited state vibrational potential energy surfaces

Far-infrared spectra, mid-infrared combination band spectra, Raman spectra, and dispersed fluorescence spectra of non-rigid molecules can be used to determine the energies of many of the quantum states of conformationally important vibrations such as out-of-plane ring modes, internal rotations, and molecular inversions in their ground electronic states. Similarly, the fluorescence excitation spectra of jet-cooled molecules, together with electronic absorption spectra, provide the information for determining the vibronic energy levels of electronic excited states. One- or two-dimensional potential energy functions, which govern the conformational changes along the vibrational coordinates, can be determined from these types of data for selected molecules. From these functions the molecular structures, the relative energies between different conformations, the barriers to molecular interconversions, and the forces responsible for the structures can be ascertained. This review describes the experimental and theoretical methodology for carrying out the potential energy determinations and presents a summary of work that has been carried out for both electronic ground and excited states. The results for the out-of-plane ring motions of four-, five-, and six-membered rings will be presented, and results for several molecules with unusual properties will be cited. Potential energy functions for the carbonyl wagging and ring modes for several cyclic ketones in their S1(n,pi*) states will also be discussed. Potential energy surfaces for the three internal rotations, including the one governing the photoisomerization process, will be examined for trans-stilbene in both its S0 and S1(pi,pi*) states. For the bicyclic molecules in the indan family, the two-dimensional potential energy surfaces for the highly interacting ring-puckering and ring-flapping motions in both the S0 and S1(pi,pi*) states have also been determined using all of the spectroscopic methods mentioned above. Here, the effect of the electronic transition on the potential energy surface and hence the molecular structure can be ascertained.

Potential Energy Function and Vibrational States of the Electronic Ground State of N4+

Three different six-dimensional potential energy functions for the electronic ground state X2 of the N4+ have been generated by the RCCSD-T method and the B3LYP and B97-1 density functional approaches. The potentials in their analytic forms have been used in variational calculations of the vibrational states (J = 0 and 1). The RCCSD-T rotational B0 constant of 0.1117 cm-1 is in excellent agreement with the experimental value of 0.11205 cm-1. The anharmonic wavenumbers for the fundamentals have been calculated to be ν1 = 2275.6, ν2 = 390.3, ν3 = 2239.3 (expt: 2234.5084), ν4 = 90.7, and ν5 = 133.8, and the zero-point vibrational energy is 2675.3 (all values in cm-1). All large isotope shifts observed in a cold matrix for ν1 and ν3 have been very well reproduced. Both density functional approaches yielded good agreement for bending fundamentals but failed to desribe accurately the symmetric and antisymmetric stretching vibrations. The dissociation energy, the quadrupole moment and the dipole polarizabilities have been evaluated as well.

Potential energy function and vibrational states of HN3 and DN3

Potential energy function and vibrational states of HN3 and DN3

Potential energy function and vibrational states of HN3 and DN3

Eight different six-dimensional potential energy functions for the electronic ground state of the HN3 have been generated by the CCSD(T) method and various density functional approaches. The potentials in their analytic forms have been used in variational calculations of the vibrational states (J = 0). The calculated anharmonic wavenumbers for the fundamentals agree with the known experimental values to within 7cm-1(HN3) and 16cm-1(DN3) for the CCSD(T) potential. The best density functional approach (B3LYP) yields fundamentals which are within 10cm-1(HN3) and 44cm-1(DN3), with the exception of the ν2 which is in error by 43cm-1(HN3) and 95cm-1(DN3). Also the experimental isotope shifts for 15N substituted species are very well reproduced for HN3. The barrier to linearity of the HN2 moiety has been calculated to be 11578cm-1(CCSD(T)). Due to the near-linearity of the NNN group, for which a barrier of only 327cm-1 has been calculated, the overtones and combination levels of the in-plane ν5 and the out-of-plane ν6 bending states fall in clusters in higher excited states. The vibrational energies for all states up to the NH(ND) stretching wavenumbers and their assignments are given.

Imaging spectroscopy of recombination fragments of

Dissociative recombination of vibrationally cold has been studied at the ion storage ring CRYRING, with an accurate measurement of the kinetic energy of the recombination fragments. When the incident electron energy exceeds , fragments are observed with a kinetic energy of only . The energetics suggest that this low-energy channel derives from either recombination of the state or recombination of the metastable state. High-quality ab initio calculations have been used to obtain the state vibrational relaxation time (10 ms), excitation energy (2.25 eV) and radiative lifetime (0.030 s). The dissociative recombination cross section has been obtained in a model calculation. The results imply that the state has too small a recombination cross section and too short a radiative lifetime to explain the experimental results, which were obtained after 15 s of storage in CRYRING. We conclude that the 120 meV channel is due to recombination of into the asymptotic limit. Possible mechanisms are discussed.

Large Amplitude Motions in 2,3-Cyclopentenopyridine

Large amplitude motions in 2,3-cyclopentenopyridine have been investigated by microwave spectroscopy andab initiocalculations. The rotational spectra of the ground and of several low energy vibrational excited states have been assigned. The potential energy functions of the ring puckering and 1,3-ring-twisting motions have been obtained by applying a flexible model to the experimental data.

A potential energy surface for the electronic ground state of N 2O

A potential energy surface for the electronic ground state of N 2O is optimized using a variational procedure with an exact vibrational Hamiltonian. In the optimization, the ab initio force field of Martin, Taylor and Lee is taken as a starting point, and the observed vibrational band origins up to 15000 cm −1 reported by Campargue and co-workers are involved. The RMS error of this fitting to the 60 observed Σ state vibrational energy levels is 0.34 cm −1. The rovibrational energy levels for the Σ and Π vibrational states are calculated to test the refined potential.

Photoionization spectroscopy of ionic metal dimers: LiCu and LiAg

Electronic spectra are reported for the heteronuclear metal dimers LiCu and LiAg, with resonant one-color two-photon ionization (R2PI). The dimers are produced in a pulsed supersonic molecular beam by laser vaporization of either a copper or silver rod coated with a thin film of vacuum deposited lithium metal. A total of twelve excited electronic states for LiCu and seven for LiAg are observed. Analysis of the vibrational progressions yields ground and excited state vibrational frequencies and dissociation energies for both LiCu and LiAg. In addition, selected vibronic bands are rotationally resolved. This data, together with that obtained by Morse and co-workers for LiCu [J. Chem. Phys. (to be published)], gives bond lengths for LiCu and LiAg (r0″=2.26 and 2.41 Å, respectively). The bond lengths for LiCu and LiAg are significantly shorter than expected by comparison to the homonuclear diatomics Li2 and Cu2 or Ag2. Dissociation energies in the heteronuclear dimers are also much greater than the mean of the corresponding homonuclear dimer values. These trends indicate that ionic character plays a leading role in the ground-state bonding.

Classical and approximate quantum investigations of vibrational energy transfer in S1 p-difluorobenzene

A simple model potential energy surface is constructed and used in both quasiclassical trajectory calculations and quantum vibrational close-coupling, infinite order sudden approximation calculations of collision-induced vibrational energy transfer from four vibrational states of S1 p-difluorobenzene. Classical and quantum state-to-state cross sections are compared for excitation of the two lowest energy vibrational states and collision with He or Ar. Qualitatively, the same trends are seen in both sets of results. Classical cross sections, however, are significantly larger at very low collision energies as a consequence of the binning procedures used to determine classical final states and, in the case of the Ar collider, as a result of the possible breakdown of the sudden approximation. Rotational excitation of the p-difluorobenzene molecule is also investigated and found to have only small effects on the dominant energy transfer channels. The theoretical results are compared with recent experimental results of Mudjijono and Lawrance [J. Chem. Phys. 104, 7444 (1996)]. The classical results, for the He, Ne, Ar, and Kr collision partners, show good agreement with experiment, reproducing the major energy transfer channels and the experimental collision partner dependence. Quantum results agree well with experiment for the He collider and are also used to assign experimentally ambiguous product states and to investigate vibrational energy transfer channels that are not experimentally observable. The propensity toward the transfer of multiple quanta of vibrational energy is analyzed and, in general, found to increase with the intermolecular well depth and with the mass of the collision partner. The He collision partner, however, behaves anomalously.

Infrared chemiluminescence study of vibrationally excited CO 2 formed by catalytic oxidation of CO over single crystal and polycrystalline Pt surfaces

The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on Pt(110), Pt(111) and polycrystalline Pt surfaces. The CO 2 molecules produced by the oxidation of CO on Pt(110)-(1 × 2) and Pt(111) were vibrationally excited but rotationally rather cool. However, the vibrational temperature ( T v) is higher on Pt(110)-(1 × 2) than on Pt(111), while T v is much lower on Pd(110) than on Pd(111). On polycrystalline Pt surfaces, the vibrational and rotational energy states of product CO 2 were altered by the pretreatment (annealing in vacuo or in air), which strongly affects the surface structure. These results suggest that the vibrational and rotational states of the CO 2 product are sensitive to the surface structure, i.e. the transition state of CO oxidation is strongly affected by the structure of the reaction sites.

Structure-sensitivity of the Dynamics of CO Oxidation on Pd(111), Pd(110) and Polycrystalline Pd Surfaces: Infrared Chemiluminescence Study of the Product CO2

Abstract The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on Pd(111), Pd(110) and polycrystalline Pd surfaces. The product CO2 molecules were vibrationally and rotationally excited in all cases. However, the vibrational energy states of CO2 were different among these three surfaces. This result means that the structure of the activated CO2 complex (i.e., the dynamics of CO oxidation) depends on the surface structures. In addition, the steady-state reaction rates of CO oxidation as a function of surface temperature were also different among the Pd surfaces. These results show that the CO oxidation on Pd is structure-sensitive not only in the kinetics but also in the dynamics.

Optothermal detection of nonradiative relaxation channels in electronically excited molecules

Optothermal detection has been used to observe nonradiative relaxation channels in aniline, p-bromoaniline, and trans-stilbene. p-Bromoaniline has no detectable fluorescence due to a heavy atom effect which increases the rate of intersystem crossing to the triplet state. An optothermal spectrum of p-bromoaniline was observed with the origin at 32 625 cm−1. For trans-stilbene, the differences between the laser excitation spectrum and the optothermal spectrum of the S1 state clearly show the onset of isomerization at ∼1250 cm−1 above the origin. Absolute quantum yields of fluorescence, Franck–Condon factors, nonradiative rates, and radiative rates have been obtained for a series of vibronic transitions. For low energy vibrational states, there is good agreement between the current study and previous work. For vibrational energies above the barrier of isomerization, predicted quantum yields do not agree with our experimental results.