Ground State Of Cation Research Articles

The puzzling infrared spectra of the nitric oxide dimer radical cation: a systematic application of Brueckner methods

Since symmetry breaking occurs in the ONNO+ cation with the self-consistent field (SCF) method and inverse symmetry breaking occurs when density functional theory (DFT) methods are used, Brueckner double excitation coupled cluster methods (BD) and BD plus perturbative triple-excitation contributions [BD(T)]have been used to study the geometries and vibrational frequencies for the trans and cis structures of the ONNO+ cation. Double-ζ plus polarization (DZP) basis sets and triple-ζ plus double polarization with f functions (TZ2Pf) basis sets were utilized. The ground state of the trans-ONNO cation is of 2Ag symmetry, which has a slightly (0.36 kcalmol-1) lower energy than the cis conformer (2A1). The controversial vibrational frequency corresponding to the asymmetric N–O stretching mode for both trans and cis structures is predicted as about 1600cm-1. This value is discussed in the context of the many (sometimes variant) experimental assignments.

High-lying Rydberg states and ionization energy of vinyl chloride studied by two-photon resonant ionization spectroscopy

High-lying Rydberg states of jet-cooled vinyl chloride at 9.3–10 eV are investigated using 2+1 resonance-enhanced multiphoton ionization spectroscopy. Four Rydberg series are observed and tentatively assigned as due to the transitions of π→ ns, npσ, npπ, and nf Rydberg orbitals associated with quantum defects of 0.94, 0.89, 0.48, and 0.02, respectively. All of the four series converge to the same ionization energy limit corresponding to the ground state of vinyl chloride cation. The adiabatic ionization energy of vinyl chloride is determined to be 80 720±6 cm −1, in excellent agreement with previously reported values.

Infrared spectroscopy of CH stretching vibrations of jet-cooled alkylbenzene cations by using the “messenger” technique

The CH stretching vibrations of the benzene–Ar, toluene–Ar, and ethylbenzene–Ar clusters prepared in jet expansion were observed in both the neutral and cationic ground states by using infrared–ultraviolet double resonance and infrared photodissociation spectroscopy, respectively. Vibrational frequencies for the in-plane modes of the clusters have been found to be practically the same as those of the corresponding bare molecules. The aromatic CH stretching vibrations showed high frequency shifts upon ionization, and their infrared absorption intensities remarkably decreased. The alkyl CH stretching vibrations were also significantly changed in both frequency and intensity upon ionization. Density functional calculations well reproduced the observed infrared spectra of the neutral and cationic states, and enhancement of hyperconjugation in the cationic state was pointed out.

Acetone n-radical cation internal rotation spectrum: The torsional potential surface

The one color REMPI and two color ZEKE-PFI spectra of acetone-d3 have been recorded. The 3px Rydberg state of acetone-d3 lies at 59 362.3 cm−1 and both of the torsional modes are visible in this spectrum. The antigearing Rydberg (a2) mode, v12*, has a frequency of 62.5 cm−1, while the previously unobserved gearing (b1) mode, v17*, is found at 119.1 cm−1. An ionization potential of 78 299.6 cm−1 for acetone-d3 has been measured. In acetone-d3 n-radical cation ground state, the fundamentals of both of the torsional modes have been observed, v12+ at 51.0 cm−1 and v17+ at 110.4 cm−1, while the first overtone of v12+ has been measured at 122.4 cm−1. Deuterium shifts show that v12+ behaves like a local C3v rotor, but that v17+ is canonical. Combining this data with that for acetone-d0 and aacetone-d6 has allowed us to fit the observed frequencies to a torsional potential energy surface based on an ab initio C2v cation ground state geometry. This potential energy surface allows for prediction of the v17 vibration in acetone-d0 and acetone-d6. The barrier to synchronous rotation is higher in the cation ground state than in the neutral ground state, but significantly lower than in the 3s Rydberg state. The 3px Rydberg and cation ground state potential energy surfaces are found to be very similar to each other, strongly supporting the contention that the 3px Rydberg state has C2v geometry and is a good model for the ion core. The altered 3s Rydberg state potential surface suggests this state has significant valence character.

A complete active space self-consistent field (CASSCF) ab initio study of phenol–N2: the properties of a weak hydrogen-bonded system in its S1 excited state

The hydroxy-bound isomer of the phenol–N2 van der Waals complex has been studied, together with the phenol monomer, at the CAS(8,7)/6-31G* and CAS(8,7)/cc-pVDZ levels of theory in the S1 electronically excited state as well as the neutral and cation ground states. Calculated geometries, excitation energies, harmonic vibrational frequencies and rotational constants were in good agreement with experiment, especially when using the Dunning cc-pVDZ basis set, although the greatest deviation from observed values predictably occurred for the S1 excited state. In particular, where rotational constants calculated for phenol were reproduced to within a few megahertz of experiment for the S0 and S1 states, for phenol–N2, rotational fits to the REMPI spectrum showed the CASSCF calculations to have underestimated rotational constants, particularly the A″ and A′ values, by about 7%. Binding energies for the complex compare very well to experimental values for S0 and D0 but were underestimated by about 20% for the S1 state. Vibrational analyses of inter- and intramolecular modes showed good agreement with literature for the neutral and ionic ground states of phenol–N2. However, for the S1 excited state, although good agreement was achieved for the a′ symmetry modes, those of a″ symmetry showed significant errors. The results obtained for the electronically excited state demonstrate that calculations at this level provide a good description of the S1 state and can genuinely be a useful aid to assignment of REMPI/LIF excitation spectra of van der Waals complexes of intermediate bond strength.

Autoionization-detected infrared spectroscopy of intramolecular hydrogen bonds in aromatic cations. II. Unconventional intramolecular hydrogen bonds

A newly developed infrared spectroscopic technique, called autoionization-detected infrared (ADIR) spectroscopy, was applied for a study on hydroxyl–alkyl interactions in cresol and ethylphenol cations. In this technique, vibrational transitions in the ion core of high Rydberg states, which has almost the same vibrational structure as the corresponding bare molecular ion, are measured by detecting the vibrational autoionization signal. The OH stretching vibrations in the rotational isomers of the ortho-, meta-, and para-cresol cations and those of the ethylphenol cations were observed. Remarkable low-frequency shifts of the OH vibration were found only for the cis rotational isomers of the ortho-cresol and ortho-ethylphenol cations, whereas no such shift was found for all the other rotational and structural isomer cations. On the other hand, no remarkable shift of the OH stretch frequency was found for all the isomers in the neutral ground state. These results indicate that an intramolecular hydrogen bond is formed between the hydroxyl and alkyl groups in the cationic ground state of ortho-cresol and ortho-ethylphenol. The remarkable low-frequency shift of the OH vibration also indicates that the alkyl group acts as a proton acceptor in the hydrogen bond. This is a new type of intramolecular hydrogen bond, and the origin of such unconventional hydrogen bond in the cations is discussed.

Assignment of the B̃+ State of the Chlorobenzene Cation Using Photoinduced Rydberg Ionization (PIRI) Spectroscopy

The photoinduced Rydberg ionization (PIRI) spectra of the B̃+ state of the chlorobenzene cation were recorded via the origin, 6b, and 16a16b vibrations of the cation ground state (X̃+). The resonance-enhanced multi photon dissociation spectroscopy (REMPD) spectrum of the B̃+←X̃+ transition of the chlorobenzene cation was also obtained. To date it has been thought that B̃+←X̃+ is an electronically forbidden transition (C2v symmetry), taking place from the 2B1 ground state to a 2B2 excited state. The ability of PIRI to provide spectra from specific lower-state vibrational levels allowed this hypothesis to be tested, because the 16a vibration would be the primary inducing mode in the transition. Assuming a forbidden transition, a comparison between the spectrum from the ground-state origin and that from the 16a16b vibration would necessitate an assignment that gives unlikely vibrational frequencies. It is therefore concluded that the B̃+←X̃+ transition of chlorobenzene is electronically allowed. Configuration interaction of singles (CIS) and complete active space multiconfigurational self-consistent field (CASSCF) calculations with 6-31G** basis sets were performed to ascertain the symmetry assignments of the excited ionic states. These calculations resulted in the possibility that at least one excited state of the cation of 2B1 symmetry lies below any state of 2B2 symmetry. Hence, we propose that the ionic transition observed in the acquired PIRI/REMPD spectra of the cation is an allowed transition to a 2B1 state, thus giving rise to the observation of the origin of the B̃+ state at 18 219 cm-1.

Ionization Energy of p-Fluoroaniline and Vibrational Levels of p-Fluoroaniline Cation Determined by Mass-Analyzed Threshold Ionization Spectroscopy

Mass-analyzed threshold ionization (MATI) and two-color resonant two-photon ionization (2C-R2PI) methods were used for detailed studies of the adiabatic ionization energy of p-fluoroaniline (PFA) and the vibrations of this molecule in the cationic ground state. The threshold ion spectra were recorded for PFA via the 0° vibrationless and the 6a1, 11, 121, 6a1l1, and 6a1121 vibrational levels of the S1 state. The adiabatic ionization energy of PFA is found to be 62 543 ± 4 cm-1 by the MATI spectroscopy and 62 550 ± 7 cm-1 by the 2C-R2PI spectroscopy. Results show that the active modes are mostly related to in-plane ring vibrations of the ion. All of these experimental data are presented for the first time. Ab initio and density functional theory calculations were also performed for predicting the ionization energy and vibrational frequencies. Comparative studies show that the measured and calculated results are in very good agreement.

Potential-energy function for intramolecular proton transfer in the malonaldehyde cation

The reaction path and the corresponding energy profile for intramolecular hydrogen transfer in the ground state of the malonaldehyde cation have been characterized employing the UHF/MP2, CASSCF and CASPT2 methods. It is found that the malonaldehyde cation represents the interesting case of a very low-barrier intramolecular hydrogen bond. The relevance of these findings for the infrared spectroscopy of cations of intramolecularly hydrogen-bonded π-systems and for femtosecond time-resolved photoelectron spectroscopy of the exited-state dynamics of these systems are briefly discussed.

The infrared spectrum of the nitric oxide dimer cation: Problems for density functional theory and a muddled relationship to experiment

Ab initio and density functional theory (DFT) methods have been used to study the geometries, vibrational frequencies, and infrared intensities for the trans-, cis-, and gauche-structures of the ONNO+ cation. Five different functionals were employed for comparison. Double-ζ plus polarization (DZP) basis sets and triple-ζ plus double polarization with f functions (TZ2Pf) basis sets were utilized. The ground state of the trans-ONNO cation is of Ag2 symmetry. The prominent infrared absorption is predicted as ∼1900 cm−1 based upon the DFT methods. However, this DFT prediction is suspect since ONNO+ exhibits inverse symmetry breaking, dissociating to the physically absurd limit ON+1/2 plus NO+1/2. This inverse symmetry breaking phenomenon was discussed in an important 1997 paper by Bally and Sastry [J. Phys. Chem. A 101, 7923 (1997)]. Therefore, a higher theoretical level, Brueckner coupled-cluster method was ultimately applied, and the harmonic vibrational frequency of this mode was predicted to be about 1550–1650 cm−1. The important matrix isolation infrared experiments of Jacox et al. [J. Chem. Phys. 93, 7609 (1990)], Lugez et al. [J. Chem. Phys. 110, 10345 (1999)], Hacaloglu et al. [J. Phys. Chem. 94, 1759 (1990)], Andrews et al. [J. Phys. Chem. A 103, 4167 (1999)], and Strobel et al. [J. Phys. Chem. 99, 872 (1995)] are carefully considered.

A comparison of hydrogen-bonded and van der Waals isomers of phenol⋅⋅nitrogen and phenol⋅⋅carbon monoxide: An ab initio study

The hydrogen-bonded and van der Waals isomers of phenol⋅⋅nitrogen and phenol⋅⋅carbon monoxide in their neutral electronic (S0) and cation ground state (D0) were studied using ab initio HF/6-31G*, MP2/6-31G*, and B3LYP/6-31G* methods. The hydrogen-bonded isomers have the ligand bound via the hydroxyl group of the phenol ring, while the van der Waals isomers studied have the ligand located above the aromatic ring. For both complexes, the hydrogen-bonded isomer was found to be the most stable form for both the S0 and the D0 states. For phenol⋅⋅carbon monoxide, twice as many isomers as compared to phenol⋅⋅nitrogen were found. The hydrogen-bonded isomer with the carbon end bonded to the hydroxyl group was the most stable structure for both the S0 and the D0 states.

Infrared spectroscopy of the phenol-N2 cluster in S and D: Direct evidence of the in-plane structure of the cluster

The OH stretching vibration of jet-cooled phenol-N2 in the neural and cationic ground states was observed by using infrared–ultraviolet double resonance spectroscopy and infrared photodissociation spectroscopy, respectively. The OH vibration showed a small but significant low-frequency shift of 5 cm−1 upon the cluster formation in the neutral, while the shift drastically increased up to 159 cm−1 in the cation. These results represent the direct evidence of the in-plane cluster structure, in which phenolic OH is hydrogen bonded to N2, as was proposed in the zero kinetic energy photoelectron study [S. R. Haines et al., J. Chem. Phys. 109, 9244 (1998)].

Autoionization-detected infrared spectroscopy of jet-cooled aromatic cations in the gas phase: CH stretching vibrations of isolated p-ethylphenol cations

The first observation of CH stretching vibrations of isolated aromatic molecular cations in the gas phase was performed by using autoionization-detected infrared (ADIR) spectroscopy. Aromatic and alkyl CH stretching vibrations of p-ethylphenol ( p-EP) in the cationic ground state were observed and compared with those of the neutral ground state. Distinct high-frequency shifts of these vibrations upon ionization were found. CH stretching vibrations of the p-EP–Ar cluster cation were also measured by infrared photodissociation (IRPD) spectroscopy, providing essentially the same vibrational spectrum as that of the bare cation.

Theoretical studies of internal methyl rotations in m-xylene: comparison of Franck–Condon factors with the experimental spectra

The potential energy surfaces of internal rotations of two methyl groups were calculated with the ab initio MO method for m-xylene (1,3-dimethylbenzene) in the ground (S 0), first excited (S 1), and cation ground (D 0) states. The internal rotation levels and their wavefunctions on the theoretically calculated potential energy surfaces were obtained by solving Schrödinger's equation for double methyl rotors. The Franck–Condon factors for D 0←S 1 and S 1←S 0 transitions were calculated and compared with the experimental spectra. A good agreement was obtained not only in level spacings but also in relative intensities in the spectra. The interaction between two methyl rotors was also found to be small, and the rotational levels can be labeled by the irreducible representation of the direct product G 6×G 6 group.

Conformational energy and dynamics of 9-ethylfluorene

The S1 excited state and cation ground state of jet cooled 9-ethylfluorene have been studied experimentally using resonant enhanced multiphoton ionization and zero electron kinetic energy (ZEKE) photoelectron spectroscopy. The spectroscopy has identified two conformations of the ethyl chain which are labeled symmetric and unsymmetric both of which exist in the supersonic expansion. Density functional quantum chemical calculations are used to calculate the ground state and cation energies of each conformer as well as the barrier to conformer interconversion via a bond rotation. Dynamics on the S1 surface are measured using picosecond and nanosecond ZEKE photoelectron spectroscopy. Fast irreversible vibrational redistribution is measured at energies ⩾990 cm−1 and the ZEKE spectra are shown to have a unique signature for each of the two isomers. Picosecond and nanosecond ZEKE spectroscopy are used to search for conformer interconversion but even at the highest energy probed (2648 cm−1) no evidence is seen for a dynamic barrier crossing. Statistical density of states calculations are used to predict the relative populations of each conformer expected as a function of excess energy as well as related Rice–Ramsperger–Kassel–Marcus calculations to predict the expected isomerization rates.

ZEKE electron spectroscopy of azulene and azulene–argon

Mass-selected ion-current spectra and zero-kinetic-energy (ZEKE) electron spectra were obtained for azulene and its van der Waals (vdW) complex with Ar in supersonic jets by two-photon (1+1′) resonant ionization through the second singlet electronic excited states (S2). Ab initio calculations were also carried out to study the optimized geometries and vibrational modes for azulene in the neutral and cation ground states (S2 and D0). Lennard-Jones (LJ) potential energy calculations including `charge–charge-induced-dipole interactions' were also made for azulene–Ar. The main results may be summarized as follows. (1) The adiabatic ionization energies have been determined as: Ia(azulene)=59781±5 cm−1 and Ia(azulene–Ar)=59708±5 cm−1. The difference in Ia is 73 cm−1. (2) Several vibrational frequencies of (azulene)+ have been observed and identified on the basis of ab initio theoretical calculations. (3) A vibrational progression with a spacing of 9–10 cm−1 in the ZEKE spectra of azulene–Ar, has been assigned experimentally and theoretically to the vdW bending vibration bx+1 along the long axis of azulene. (4) From the calculated LJ potential energy minima, it has been found that Ar is shifted by 0.10Å along the long axis of azulene from the position in the neutral electronic ground state. (5) The observed vdW vibrational progressions have been reproduced by Franck–Condon calculations, suggesting that Ar is shifted by 2° for (azulene–Ar)+ with respect to their neutral S2 state.

Electronic spectroscopy and dynamics of the monomer and Arn clusters of 9-phenylfluorene

The spectrum of the S1 electronic state of jet-cooled 9-phenylfluorene–Arn, n=0–4, has been measured by two color resonant enhanced multiphoton ionization spectroscopy. The cation ground states of these complexes have also been studied by mass analyzed threshold ionization (MATI) spectroscopy in a 1+1 excitation process with various intermediate states in S1. Ab initio calculations in conjunction with the spectroscopy have determined that the phenyl ring at the 9 position is perpendicular to the plane of the fluorene moiety yielding an overall symmetry of Cs. The Ar complexes for n=1–3 exhibit multiple isomers which are identified in the S1 spectrum and confirmed by MATI spectroscopy. The structure of these isomers is determined by spectral analysis and additivity rules as well as atom–atom calculations using a Lennard-Jones potential. Vibrational dynamics from selected S1 vibronic levels are observed by the appearance of the picosecond or nanosecond time delayed MATI spectra. Vibrational redistribution and dissociation of the clusters are measured with nanosecond and picosecond time resolution. It is found that different isomers of the n=1 cluster show dramatically different rates of redistribution for several vibronic bands.

Van Der Waals versus Hydrogen-Bonding in Complexes of Indole with Argon, Water, and Benzene by Mass-Analyzed Pulsed Field Threshold Ionization

By using the technique of mass-analyzed threshold ionization spectroscopy, we were able to measure ionization energies of indole−Ar (62505 cm-1), indole−H2O (59433 cm-1), and the indole−benzene (59833 cm-1) complexes, as well as their dissociation energies in the cationic ground state. The dissociation energies in the neutral ground state are calculated from the experimental data. The ionic E0 = 537 ± 10 cm-1 and neutral dissociation energy D0 = 451 ± 15 cm-1 of the indole−Ar complex are much smaller than those of the indole−H2O complex; E0 = 4790 ± 10 cm-1, D0 = 1632 ± 15 cm-1 and of the indole−C6H6 complex: E0 = 4581 ± 10 cm-1, D0 = 1823 ± 15 cm-1. This demonstrates the van der Waals character of the indole−Ar complex and the hydrogen bonding type in indole−water. Furthermore, we conclude that the indole−benzene complex is hydrogen-bonded with the benzene π-cloud serving as electron donator and indole serving as hydrogen donator.

Radical Cation of N,N-Dimethylpiperazine: Dramatic Structural Effects of Orbital Interactions through Bonds

The radical cation of N,N-dimethylpiperazine (DMP) has been studied using time-resolved optical absorption and resonance Raman spectroscopy. Different quantum-chemical methods were used to calculate the molecular structures and vibrational force fields in the ground state of the radical cation and in the resonant excited state. An excellent agreement between theoretical and experimental vibrational frequencies as well as resonance Raman intensities could be achieved. It is concluded that through--bond interaction between the formal lone pair on one amino nitrogen and the odd electron on the other is strong enough to lead to a symmetric charge-delocalized molecular structure of the DMP radical cation, with a chair-type geometry.

Observation of van der Waals vibrations in zero kinetic energy (ZEKE) photoelectron spectra of toluene-Ar van der Waals complex

Zero kinetic energy (ZEKE) photoelectron spectra due to van der Waals (vdW) vibrations have been observed for the toluene-Ar vdW complex in a supersonic free jet. ZEKE photoelectron measurements were carried out by using two-color (1 + 1′) resonant excitation followed by pulsed-field ionization (PFI). The first laser was tuned to the S 1 origin and its several specific vdW vibrational levels, while the second laser was scanned to observed a very low frequency vdW vibrational structure in the cation ground state ( D 0). From ZEKE photoelectron spectra thus obtained via several S 1 vdW vibrational levels, we have been able to identify the non-totally symmetric vdW bending mode ( b x +: 17 cm −1) and the vdW stretching mode ( s z +: 49 cm −1) for the toluene-Ar cation for the first time.