Zero Electron Kinetic Energy Spectra Research Articles

Spectroscopy of N-methylpyrrole-RG (RG = Ar, Kr) complexes: First excited neutral and ground cationic states

N-methylpyrrole-RG (RG = Ar, Kr) complexes are investigated using resonance-enhanced multiphoton ionization (REMPI) and zero-electron-kinetic-energy (ZEKE) spectroscopy. The RG atom makes the electronic transition to the S1 state allowed, and the REMPI spectrum is blue-shifted compared to the N-methylpyrrole origin, with no bands associated with vibrational excitation of the N-methylpyrrole moiety. The nature of the electronic structure of the S1 state is discussed. Binding energies are obtained for all three electronic states. Adiabatic ionization energies are obtained from the ZEKE spectra, yielding values of 64077 ± 5 cm−1 and 64029 ± 5 cm−1 for RG = Ar and Kr, respectively.

Comment on "Electronic, vibrational and torsional couplings in N-methylpyrrole: Ground, first excited and cation states" [J. Chem. Phys. 154, 224305 (2021)

Two-color (1 + 1') zero-electron-kinetic-energy (ZEKE) and photoionization efficiency (PIE) spectra are reported via different levels in the S1 ← S0 (Ã1A2←X̃1A1) one-photon transition of jet-cooled N-methylpyrrole. The laser radiation is produced using two dye lasers, one with an 1800 l/mm grating and one with 2400 l/mm. We report spectra where the excitation and ionization radiation are produced with both combinations of the dye lasers; these spectra differ markedly. This is attributed to Wood's anomalies with the 2400 l/mm grating: one aspect is a loss in light intensity over a range of wavelengths, attributed to a resonance anomaly. Another is the appearance of a "shadow" ZEKE spectrum and PIE curve at apparently higher ionization wavenumbers; under some conditions, a third ZEKE spectrum was observed-these latter observations arise from higher-order dispersion effects, likely caused by a Rayleigh anomaly. We comment on these observations and report more representative ZEKE and PIE spectra than those presented in a recent paper by our group [A. R. Davies, D. J. Kemp, and T. G. Wright, J. Chem. Phys. 154, 224305 (2021)] for four intermediate S1 levels.

Electronic, vibrational, and torsional couplings in N-methylpyrrole: Ground, first excited, and cation states.

The electronic spectrum associated with the S1 ← S0 (Ã1A2←X̃1A1) one-photon transition of jet-cooled N-methylpyrrole is investigated using laser-induced fluorescence (LIF) and (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy; in addition, the (2 + 2) REMPI spectrum is considered. Assignment of the observed bands is achieved using a combination of dispersed fluorescence (DF), two-dimensional LIF (2D-LIF), zero-electron-kinetic energy (ZEKE) spectroscopy, and quantum chemical calculations. The spectroscopic studies project the levels of the S1 state onto those of either the S0 state, in DF and 2D-LIF spectroscopy, or the ground state cation (D0 +) state, in ZEKE spectroscopy. The assignments of the spectra provide information on the vibrational, vibration-torsion (vibtor), and torsional levels in those states and those of the S1 levels. The spectra are indicative of vibronic (including torsional) interactions between the S1 state and other excited electronic states, deduced both in terms of the vibrational activity observed and shifts from expected vibrational wavenumbers in the S1 state, attributed to the resulting altered shape of the S1 surface. Many of the ZEKE spectra are consistent with the largely Rydberg nature of the S1 state near the Franck-Condon region; however, there is also some activity that is less straightforward to explain. Comments are made regarding the photodynamics of the S1 state.

Torsions of N-methylpyrrole and its cation

Torsional levels in N-methylpyrrole are investigated in the ground (S0) and first excited (S1) neutral states using two-dimensional laser-induced fluorescence (2D-LIF), and in the ground state cation (D0+) using zero-electron-kinetic-energy (ZEKE) spectroscopy. The ZEKE spectra confirm the largely Rydberg nature of the S1 state. The activity seen in both the 2D-LIF and ZEKE spectra are indicative of vibronic (including torsional) interactions and torsional potentials in the three electronic states are deduced, and are consistent with calculated geometries. The adiabatic ionization energy of N-methylpyrrole is derived as 64250 ± 5 cm−1.

A comparative multi-state multi-dimensional quantum-classical dynamics on compact polycyclic aromatic hydrocarbons (CPAHs) by parallel TDDVR method

The newly implemented parallelized quantum–classical dynamical approach, namely, Time Dependent Discrete Variable Representation (TDDVR) method is applied to the spectroscopically important phenanthrene, acenaphthene and pyrene, where several conical intersections exist in their four lowest singlet excited electronic states (X, A, B and C), to illustrate their various dynamical aspects. This parallel version shows almost linear scalability with increasing number of computing processors. First principles dynamics is performed to the adopted Hamiltonians to obtain photoelectron (PE) spectra, zero electron kinetic energy (ZEKE) spectra, population dynamics and many other dynamical observables. The reduced densities of the wave packet (WP) in coupled electronic surfaces are used to elucidate the intrinsic dynamical features. The TDDVR calculated PE and assigned ZEKE spectra of all CPAHs are found to be good agreement with the previous experimental data and other available theoretical results.

Variations in Duschinsky rotations in m-fluorotoluene and m-chlorotoluene during excitation and ionization.

We investigate Duschinsky rotation/mixing between three vibrations for both m-fluorotoluene (mFT) and m-chlorotoluene (mClT), during electronic excitation and ionization. In the case of mFT, we investigate both the S1 → S0 electronic transition and the D0 + ← S1 ionization, by two-dimensional laser-induced fluorescence (2D-LIF) and zero-electron-kinetic energy (ZEKE) spectroscopy, respectively; for mClT, only the D0 + ← S1 ionization was investigated, by ZEKE spectroscopy. The Duschinsky mixings are different in the two molecules, owing to shifts in vibrational wavenumber and variations in the form of the fundamental vibrations between the different electronic states. There is a very unusual behavior for two of the mFT vibrations, where apparently different conclusions for the identity of two S1 vibrations arise from the 2D-LIF and ZEKE spectra. We compare the experimental observations to the calculated Duschinsky matrices, finding that these successfully pick up the key geometric changes associated with each electronic transition and so are successful in qualitatively explaining the vibrational activity in the spectra. Experimental values for a number of vibrations across the S0, S1, and D0 + states are reported and found to compare well to those calculated. Assignments are made for the observed vibration-torsion ("vibtor") bands, and the effect of vibrational motion on the torsional potential is briefly discussed.

Torsions, low-frequency vibrations, and vibration-torsion ("vibtor") levels in the m-chlorotoluene cation.

Zero-electron-kinetic-energy (ZEKE) spectra are presented for m-chlorotoluene (mClT), employing different low-lying torsional and vibration-torsional ("vibtor") levels of the S1 state as intermediates. The adiabatic ionization energy is determined to be 71 319 cm-1 ± 5 cm-1 (8.8424 ± 0.0006 eV). It is found that the activity in the ZEKE spectra varies greatly for different levels and is consistent with the assignments of the S1 levels of m-fluorotoluene (mFT) deduced in the recent fluorescence study of Stewart et al. [J. Chem. Phys. 150, 174303 (2019)] and the ZEKE study from Kemp et al. [J. Chem. Phys. 151, 084311 (2019)]. As with mFT, the intensities in the ZEKE spectra of mClT are consistent with a phase change in the torsional potential upon ionization, allowing a large number of torsions and vibtor levels to be observed for the cation. Vibration-induced modifications of the torsional potential are discussed. Calculated vibrational wavenumbers for the S0, S1, and D0 + states are also presented.

Observation of the onset of torsion-induced, mode-specific dissipative intramolecular vibrational redistribution (IVR)

Evidence is found showing that coupling with vibration-torsion (“vibtor”) levels of both in-plane and out-of-plane vibrations is instrumental in causing dissipative intramolecular vibrational redistribution (IVR). Both zero-electron-kinetic-energy (ZEKE) spectroscopy and two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy are employed to investigate a series of bands located ∼1200 cm−1 above the S1 ← S0 origin in p-fluorotoluene. Transitions in this wavenumber region have been the focus of a number of studies probing IVR. By recording both ZEKE and 2D-LIF spectra, a prepared S1 population is projected onto both the ground state cation and ground state neutral energy states, respectively, giving added confidence to the assignments. The spectral region under discussion is dominated by a pair of fundamental bands, but for the first time, we present explicit evidence that this is complicated by contributions from a number of overtones and combinations, including vibtor levels. We deduce that very different extents of coupling are present across a 60 cm−1 window of the spectrum, even though the density of states is similar; in particular, one of the fundamentals couples efficiently to the increasing bath of levels, while one does not. We explain this by the influence of serendipitous near-coincidences of same-symmetry levels.

Identifying complex Fermi resonances in p-difluorobenzene using zero-electron-kinetic-energy (ZEKE) spectroscopy.

The vibrations of the ground state cation ( 2B2g) of para-difluorobenzene (pDFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. A comprehensive set of ZEKE spectra were recorded via different vibrational levels of the S1 state (<00 + 1300 cm-1). The adiabatic ionization energy for pDFB was measured as 73 869 ± 5 cm-1. Use of different intermediate levels allows different cationic vibrational activity to be obtained via the modification of the Franck-Condon factors for the ionization step, allowing the wavenumbers of different vibrational levels in the cation to be established. In addition, assignment of the vibrational structure in the ZEKE spectra allowed interrogation of the assignments of the S1 ← S0 transition put forward by Knight and Kable [J. Chem. Phys. 89, 7139 (1988)]. Assignment of the vibrational structure has been aided by quantum chemical calculations. In this way, it was possible to assign seventeen of the thirty vibrational modes of the ground state pDFB+ cation. Evidence for complex Fermi resonances in the S1 state, i.e., those that involve more than two vibrations, was established. One of these was investigated using picosecond time-resolved photoelectron spectroscopy. In addition, we discuss the appearance of several symmetry-forbidden bands in the ZEKE spectra, attributing their appearance to a Rydberg state variation of an intrachannel vibronic coupling mechanism.

High-resolution electron spectroscopy and molecular structures of Cu–(2,2′-bipyridine) and Cu-(4,4′-bipyridine)

Copper complexes of 2,2′-bipyridine (22BIPY) and 4,4′-bipyridine (44BIPY) were prepared in a laser-vaporization supersonic molecular beam source and identified by laser photoionization time-of-flight mass spectrometry. Electronic spectra and molecular structures were studied with pulsed-field ionization zero electron kinetic energy (ZEKE) electron spectroscopy, density functional theory (DFT) and second-order Møller–Plesset perturbation (MP2) calculations, and spectral simulations. Adiabatic ionization energies and metal–ligand and ligand-based vibrational frequencies of Cu–22BIPY and Cu–44BIPY were measured from the ZEKE spectra. Ground electronic states and molecular structures of the two complexes were determined by comparing the spectroscopic measurements with the theoretical calculations. The ground state of Cu–22BIPY ( 2 B1, C2v) has a planar bidentate structure with Cu binding to two nitrogen atoms and two pyridine molecules in the cis configuration. The ground state of Cu–44BIPY ( 2 A, C2) has a monodentate structure with Cu binding to one nitrogen and two pyridines in a twisted configuration. The ionization energy of Cu–22BIPY is considerably lower and its bond energy is much higher than that of Cu–44BIPY. The different ionization and dissociation energies are attributed to the distinct metal binding modes of the two complexes. It has been found that the DFT calculations yield the correct structures for the Cu–22BIPY complex, whereas the MP2 calculations produce the best structures for the Cu–44BIPY complex.

Binding sites and electronic states of group 3 metal-aniline complexes probed by high-resolution electron spectroscopy

Group 3 metal-aniline complexes, M(aniline) (M = Sc, Y, and La), are produced in a pulsed laser-vaporization molecular beam source, identified by photoionization time-of-flight mass spectrometry, and investigated by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and quantum chemical calculations. Adiabatic ionization energies and several low-frequency vibrational modes are measured for the first time from the ZEKE spectra. Metal binding sites and electronic states are determined by combining the ZEKE measurements with the theoretical calculations. The ionization energies of the complexes decrease down the metal group. An out-of-plane ring deformation mode coupled with an asymmetric metal-carbon stretch is considerably anharmonic. Although aniline has various possible sites for metal coordination, the preferred site is the phenyl ring. The metal binding with the phenyl ring yields syn and anti conformers with the metal atom and amino hydrogens on the same and opposite sides of the ring, respectively. The anti conformer is determined to be the spectral carrier. The ground electronic state of the anti conformer of each neutral complex is a doublet with a metal-based electron configuration of nd(2)(n + 1)s(1), and the ground electronic state of each ion is a singlet with a metal-based electron configuration of nd(2). The formation of the neutral complexes requires the nd(2)(n + 1)s(1) ← nd(1)(n + 1)s(2) electron excitation in the metal atoms.

High-spin electronic states of lanthanide-arene complexes: Nd(benzene) and Nd(naphthalene)

Neodymium (Nd) complexes of benzene and naphthalene were synthesized in a laser-ablation supersonic molecular beam source. High-resolution electron spectra of these complexes were obtained using pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy. Second-order Møller-Plesset perturbation calculations were employed to aid spectral and electronic-state assignments. The adiabatic ionization energies were measured to be 38 081 (5) cm(-1) for Nd(benzene) and 37 815 (5) cm(-1) for Nd(naphthalene). For the Nd(benzene) complex, the observed frequencies of 831 and 286 cm(-1) were assigned to C-H out-of-plane bending and Nd(+)-C(6)H(6) stretching modes in the (6)A(1) ion state and 256 cm(-1) to the Nd-C(6)H(6) stretching mode in the (7)A(1) neutral state. To confirm these assignments, the ZEKE spectrum of the deuterated species was recorded, and the corresponding vibrational frequencies were measured to be 710 and 277 cm(-1) in the ion state and 236 cm(-1) in the neutral state. For the Nd(naphthalene) complex, the observed vibrational modes were C(10)H(8) bending (394 cm(-1)), Nd(+)-C(10)H(8) stretching (286 and 271 cm(-1)), Nd(+)-C(10)H(8) bending (80 cm(-1)), and C(10)H(8) twisting (105 cm(-1)) in the (6)A(') ion state and metal-ligand bending (60 cm(-1)) and ligand twisting (55 cm(-1)) in the (7)A(') neutral state. The formation of the ground state of the Nd(benzene) complex requires 4f → 5d and 6s → 5d electron excitation of the Nd atom, whereas the formation of the ground state of Nd(naphthalene) involves the 6s → 5d electron promotion.

Electronic states and pseudo Jahn-Teller distortion of heavy metal-monobenzene complexes: M(C6H6) (M = Y, La, and Lu)

Monobenzene complexes of yttrium (Y), lanthanum (La), and lutetium (Lu), M(C(6)H(6)) (M = Y, La, and Lu), were prepared in a laser-vaporization supersonic molecular beam source and studied by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and ab initio calculations. The calculations included the second-order perturbation, the coupled cluster with single, double, and perturbative triple excitation, and the complete active space self-consistent field methods. Adiabatic ionization energies and metal-benzene stretching frequencies of these complexes were measured for the first time from the ZEKE spectra. Electronic states of the neutral and ion complexes and benzene ring deformation were determined by combining the spectroscopic measurements with the theoretical calculations. The ionization energies of M(C(6)H(6)) are 5.0908 (6), 4.5651 (6), and 5.5106 (6) eV, and the metal-ligand stretching frequencies of [M(C(6)H(6))](+) are 328, 295, and 270 cm(-1) for M = Y, La, and Lu, respectively. The ground states of M(C(6)H(6)) and [M(C(6)H(6))](+) are (2)A(1) and (1)A(1), respectively, and their molecular structures are in C(2v) point group with a bent benzene ring. The deformation of the benzene ring upon metal coordination is caused by the pseudo Jahn-Teller interaction of (1(2)E(2)+1(2)A(1)+2(2)E(2)) e(2) at C(6v) symmetry. In addition, the study shows that spectroscopic behaviors of Y(C(6)H(6)) and La(C(6)H(6)) are similar to each other, but different from that of Lu(C(6)H(6)).

Electronic States and Metal–Ligand Bonding of Gadolinium Complexes of Benzene and Cyclooctatetraene

Gadolinium (Gd) complexes of benzene (C(6)H(6)) and (1,3,5,7-cyclooctatetraene) (C(8)H(8)) were produced in a laser-vaporization supersonic molecular beam source and studied by single-photon pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy. Adiabatic ionization energies and metal-ligand stretching frequencies were measured for the first time from the ZEKE spectra. Metal-ligand bonding and electronic states of the neutral and cationic complexes were analyzed by combining the spectroscopic measurements with ab initio calculations. The ground states of Gd(C(6)H(6)) and [Gd(C(6)H(6))](+) were determined as (11)A(2) and (10)A(2), respectively, with C(6v) molecular symmetry. The ground states of Gd(C(8)H(8)) and [Gd(C(8)H(8))](+) were identified as (9)A(2) and (8)A(2), respectively, with C(8v) molecular symmetry. Although the metal-ligand bonding in Gd(C(6)H(6)) is dominated by the covalent interaction, the bonding in Gd(C(8)H(8)) is largely electrostatic. The bonding in the benzene complex is much weaker than that in the cyclooctatetraene species. The strong bonding in Gd(C(8)H(8)) arises from two-electron transfer from Gd to C(8)H(8), which creates a strong charge-charge interaction and converts the tub-shaped ligand into a planar form. In both systems, Gd 4f orbitals are localized and play little role in the bonding, but they contribute to the high electron spin multiplicities.

Pulsed-field ionization zero electron kinetic energy spectrum of the ground electronic state of BeOBe+

The ground electronic state of BeOBe(+) was probed using the pulsed-field ionization zero electron kinetic energy photoelectron technique. Spectra were rotationally resolved and transitions to the zero-point level, the symmetric stretch fundamental and first two bending vibrational levels were observed. The rotational state symmetry selection rules confirm that the ground electronic state of the cation is (2)Σ(g)(+). Detachment of an electron from the HOMO of neutral BeOBe results in little change in the vibrational or rotational constants, indicating that this orbital is nonbonding in nature. The ionization energy of BeOBe [65480(4) cm(-1)] was refined over previous measurements. Results from recent theoretical calculations for BeOBe(+) (multireference configuration interaction) were found to be in good agreement with the experimental data.

High-resolution electron spectroscopy, preferential metal-binding sites, and thermochemistry of lithium complexes of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons are model systems for studying the mechanisms of lithium storage in carbonaceous materials. In this work, Li complexes of naphthalene, pyrene, perylene, and coronene were synthesized in a supersonic metal-cluster beam source and studied by zero-electron-kinetic-energy (ZEKE) electron spectroscopy and density functional theory calculations. The adiabatic ionization energies of the neutral complexes and frequencies of up to nine vibrational modes in the singly charged cations were determined from the ZEKE spectra. The metal-ligand bond energies of the neutral complexes were obtained from a thermodynamic cycle. Preferred Li∕Li(+) binding sites with the aromatic molecules were determined by comparing the measured spectra with theoretical calculations. Li and Li(+) prefer the ring-over binding to the benzene ring with a higher π-electron content and aromaticity. Although the ionization energies of the Li complexes show no clear correlation with the size of the aromatic molecules, the metal-ligand bond energies increase with the extension of the π-electron network up to perylene, then decrease from perylene to coronene. The trends in the ionization and metal-ligand bond dissociation energies of the complexes are discussed in terms of the orbital energies, local quadrupole moments, and polarizabilities of the free ligands and the charge transfer between the metal atom and aromatic molecules.

Metal coordination converts the tub-shaped cyclo-octatetraene into an aromatic molecule: Electronic states and half-sandwich structures of group III metal-cyclo-octatetraene complexes

Group III (Sc, Y, and La) metal-(1,3,5,7-cyclo-octatetraene) (COT) complexes were produced in a laser-vaporization molecular beam source and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy. Adiabatic ionization energies and metal-ligand stretching frequencies were measured from the ZEKE spectra. Metal-ligand bonding and low-lying electronic states of the neutral and ionized complexes were analyzed by combining the spectroscopic measurements with the molecular orbital treatment and density functional theory calculations. The ionization energies and metal-ligand stretching frequencies of these complexes are in the order of Sc&amp;gt;Y&amp;gt;La. The ground electronic state of the neutral complexes is A21, whereas the ground state of the ions is A11. The molecular symmetry is C8v in both neutral and ionic ground states. Although free COT is a nonaromatic molecule with a tublike structure, coordination of the group III metal atoms converts the tub-shaped molecule into a planar, aromatic structure. This conversion is induced by a two-electron transfer from the metal atoms to the ligand upon the formation of the complexes.

Comment on “A New Proposal for the Ground State of the FeO− Cluster in the Gas Phase and for the Assignment of Its Photoelectron Spectra”

In “A New Proposal for the Ground State of the FeOCluster ...” by Hendrickx and Anam,1 the authors attempt to interpret the photoelectron spectrum of FeOby proposing that the ground state of FeOis not a 4∆ state, the currently accepted assignment, but instead is a 6Σ+ state. The photoelectron spectrum2,3 of FeOis complex, given that the ground electronic state of neutral FeO is a 5∆ state.4 However, the proposed reassignment of the anion ground state is clearly incorrect and appears to be based on an incomplete and highly selective reading of the experimental data on this system. The key issues are summarized below. First, this proposed assignment cannot be reconciled with the very high resolution (∼0.001 cm-1) autodetachment spectrum of FeOreported by Andersen et al.5 This paper, which was inexplicably not referenced by Hendrickx, presents fully rotationally resolved electronic spectra of FeOthat originate from a ground state with the spin-orbit fine structure characteristic of a 4∆ state; specifically, bands originating from the Ω ) 7/2, 5/2, and 3/2 spin-orbit components with an average spacing of 233 cm-1 were identified. None of this fine structure would have been seen from a 6Σ+ state, which should be a Hund’s case (b) molecule with no spin-orbit splitting. Moreover, the experimental rotational constant of 0.4972 cm-1 for the Ω ) 7/2 multiplet yields a bond length of 1.641 A, in good agreement with the bond length of 1.635 A calculated by Hendrickx for the 4∆ state of FeO-, but in poorer agreement with the bond length of 1.683 A calculated for the 6Σ+ state. Second, the zero-electron-kinetic-energy (ZEKE) spectrum of FeOmeasured by Drechsler et al.,6 which was referenced by Hendrickx, exhibits partial rotational resolution on the origin band that was indeed fit very well using the rotational constants from Andersen et al., with an overall band contour consistent with detachment from a 4∆ anion state. In addition, the ZEKE spectrum exhibited spin-orbit fine structure that was shown to be fully consistent with the 4∆ spin-orbit splittings of Andersen et al. Neither the rotational structure nor fine structure in the ZEKE spectrum is consistent with a 6Σ+ ground state for the anion. Finally, we concur with Hendrickx and Anam that the photoelectron spectrum of FeOis not fully understood. At low electron binding energy, it is dominated by transitions to the 5∆ ground state of FeO but also exhibits minor transitions that have been tentatively assigned6 to an excited 7Σ+ excited state of FeO. This state is not accessible by one-electron detachment from the anion 4∆ state. However, while the 7Σ+ state is accessible by one-electron detachment from the proposed anion 6Σ+ ground state, the 5∆ state, which is responsible for most of the transitions in this region, is not (as pointed out by Hendrickx). Thus, even leaving out the issues considered in the preceding paragraphs, the proposed reassignment of the FeOground state is perplexing, as it appears to cause more problems than it solves.

Binding sites, rotational conformers, and electronic states of Sc–C6H5X (X=F, CH3, OH, and CN) probed by pulsed-field-ionization electron spectroscopy

Scandium (Sc) complexes of fluorobenzene (C(6)H(5)F), toluene (C(6)H(5)CH(3)), phenol (C(6)H(5)OH), and benzonitrile (C(6)H(5)CN) are produced in a laser-vaporization molecular beam source. These complexes are studied with pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy and density functional theory calculations. Adiabatic ionization energies and low-frequency metal-ligand and ligand-based vibrational modes are measured from the ZEKE spectra. Metal binding sites and strengths and electronic states are obtained by comparing the ZEKE spectra with the theoretical calculations. The ionization energies of Sc-C(6)H(5)X (X = F, CH(3), OH, and CN) follow the trend of CN > F > OH > CH(3), whereas the bond energies are in the order of CN > CH(3) approximately OH > F. The metal-ligand stretching frequency of Sc(+)-C(6)H(5)CN is nearly twice as those of the other three complexes. All neutral complexes are in low-spin doublet ground states and singly-charged cations are in singlet states. The preferred Sc binding site in these complexes are the phenyl ring with X = F, CH(3), and OH and the nitrile group with CN. For the phenol complex, two rotational conformers are identified in different OH orientations.

High-resolution electron spectroscopy and σ/π structures of M(pyridine) and M+(pyridine) (M=Li, Ca, and Sc) complexes

Metal-pyridine (metal = Li, Ca, and Sc) complexes are produced in laser-vaporization molecular beams and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy and theoretical calculations. Both sigma and pi structures are considered for the three complexes by theory, and preferred structures are determined by the combination of the ZEKE spectra and calculations. The Li and Ca complexes prefer a sigma bonding mode, whereas the Sc complex favors a pi mode. Adiabatic ionization energies and metal-ligand vibrational frequencies are determined from the ZEKE spectra. Metal-ligand bond dissociation energies of the neutral complexes are obtained from a thermodynamic cycle. The ionization energies follow the trend of Li-pyridine (32,460 cm(-1)) < Ca-pyridine (39,043 cm(-1)) < Sc-pyridine (42,816 cm(-1)), whereas the bond energies are in the order of Ca-pyridine (27.0 kJ mol(-1)) < Li-pyridine (49.1 kJ mol(-1)) < Sc-pyridine (110.6 kJ mol(-1)). The different bonding modes between the main group metals and transition element are discussed in terms of Sc 3d orbital involvement. The bond energy differences between the Li and Ca metals are explained by the number of valence s electrons and the size of the metal atoms.