The CM-L and CM+-L complexes for the three coinage metals, CM = Cu, Ag and Au interacting with three key ligands, L = CO, N-2 and H-2, are investigated. Calculations are undertaken using various quantum chemical methods to ascertain the equilibrium geometries and interaction energies; the latter are calculated at the coupled-cluster level of theory, and extrapolated to the basis set limit. Some of the neutral species are found to be particularly challenging, and significant disparity is found in some cases for the geometry and/or harmonic vibrational wavenumbers. In addition, molecular orbital diagrams, atomic charges, and orbital contour plots are presented, in order to gain insight into the interactions occurring in these complexes.

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 Si 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 Si 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.

Accurate interatomic potentials were calculated for the interaction of a singly-charged silicon cation, Si+, with a single rare gas atom, RG (RG = Kr-Rn), as well as a singly-charged germanium cation, Ge+, with a single rare gas atom, RG (RG = He-Rn). The RCCSD(T) method and basis sets of quadruple-ζ and quintuple-ζ quality were employed; each interaction energy is counterpoise corrected and extrapolated to the basis set limit. The lowest electronic term (2P) of each cation was considered, and the interatomic potentials calculated for the diatomic terms that arise from these: 2Π and 2Σ+. Additionally, the interatomic potentials for the respective spin-orbit levels were calculated, and the effect on the spectroscopic parameters was examined. Variations in several spectroscopic parameters with the increasing atomic number of RG were examined. The presence of incipient chemical interaction was also examined via Birge-Sponer-like plots and various population analyses across the series. In the cases of heavier RG, these were consistent with a small amount of electron transfer from the heavier RG atom to the cation, rationalizing the spin-orbit splittings. This was also supported by the observed larger-than-expected spin-orbit splittings for the Si+-RG complexes. Finally, each set of RCCSD(T) potentials including spin-orbit coupling was employed to calculate transport coefficients for the cation moving through a bath of the RG. The calculated ion mobilities showed significant differences for the two atomic spin-orbit states, arising from subtle changes in the interaction potentials.

RCCSD(T) potentials for Si+ and Ge+ interacting with RG atoms (RG = He–Rn) are presented, including extrapolating to the basis set limit and spin–orbit coupling. These potentials are used to derive spectroscopic and ion transport parameters.

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.

Two-dimensional laser-induced fluorescence (2D-LIF) spectra are reported for a set of features in the S1 <-> S0 electronic spectrum of m-fluorotoluene. Two transitions previously assigned to fundamentals are reassigned in the present work. The main reassignments are confirmed with zero-electron-kinetic-energy (ZEKE) spectroscopy. Interactions in the S0, S1 and D0+ electronic states involving vibrational and vibration-torsional ("vibtor") levels are identified via observed activity. The obtained vibrational wavenumbers compare well to the results of quantum chemical calculations.

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.

The vibrations of pyrrole, N-deuteropyrrole, N-fluoropyrrole, N-aminopyrrole and N-methylpyrrole are studied. The evolution of the vibrational wavenumbers of pyrrole is examined, as the mass of the nitrogen-bonded hydrogen atom is artificially increased. It is found that some vibrations are very sensitive to the mass of the substituent bonded to the nitrogen, and this can be viewed as vibrations mixing as that mass increases; however, these mixings stablilize by the time a mass of 14 mu is reached. A consistent numbering scheme for the ring-localized vibrations of N-substituted pyrroles is then put forward. A discussion of the vibrations of the cations of pyrrole and N-substituted pyrroles is then presented. Calculated vibrational wavenumbers are compared to experimental ones for pyrrole, N-deuteropyrrole and N-methylpyrrole, as well as the pyrrole cation.

Torsional levels in N-methylpyrrole are investigated in the ground (S-0) and first excited (S-1) neutral states using two-dimensional laser-induced fluorescence (2D-LIF), and in the ground state cation (D-0(+)) using zero-electronkinetic-energy (ZEKE) spectroscopy. The ZEKE spectra confirm the largely Rydberg nature of the S-1 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).

Coupling between vibrations, and between vibrations and torsions—a generalization of intramolecular vibrational redistribution (IVR)—provides routes to internal energy delocalization, which can stabilize molecules following photoexcitation. Following earlier work on p-fluorotoluene (pFT), this study focuses on m-fluorotoluene (mFT) as probed via the S1 ↔ S0 electronic transitions and the D0+ ← S1 ionization, using two-dimensional laser-induced fluorescence and zero-electron-kinetic energy spectroscopy, respectively. Wavenumbers are reported for a number of vibrations in the S0, S1, and D0+ states and found to compare well to those calculated. In addition, features are seen in the mFT spectra, not commented on in previous studies, which can be assigned to transitions involving vibration–torsion (“vibtor”) levels. Comparisons to the previous work on both m-difluorobenzene and mFT are also made, and some earlier assignments are revised. At lower wavenumbers, well-defined interactions between vibrational and vibtor levels are deduced—termed “restricted IVR,” while at higher wavenumbers, such interactions evolve into more-complicated interactions, moving toward the “statistical IVR” regime. It is then concluded that a comparison between mFT and pFT is less straightforward than implied in earlier studies.

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.

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.

Zero-kinetic-energy (ZEKE) spectra are presented for m-fluorotoluene, employing different low-lying (<350 cm-1) intermediate torsional and vibration-torsional ("vibtor") levels of the S1 state. The adiabatic ionization energy (AIE) is found to be 71 997 ± 5 cm-1 (8.9265 ± 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 deduced in the recent fluorescence study of Stewart et al. [J. Chem. Phys. 150, 174303 (2019)]. For cation torsional levels, the most intense band corresponds to changes in the torsional quantum number, in line with the known change in the phase of the torsional potential upon ionization. This leads to the observation of an unprecedented number of torsions and vibtor levels, with the pronounced vibtor activity involving out-of-plane vibrations. Interactions between levels involving torsions are discussed, with evidence presented, for the first time it is believed, for modification of a torsional potential induced by a vibration. Also, we discuss the possibility of distortion of the methyl group leading to a change from G6 molecular symmetry to Cs point group symmetry.

Two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy is employed to identify contributions to fluorescence excitation spectra that arise from both overlapping bands and coupling between zero-order states (ZOSs). Evidence is found for the role of torsional motion in facilitating the coupling between vibrations that particularly involves the lowest-wavenumber out-of-plane vibrational modes. The experiments are carried out on jet-cooled p-fluorotoluene, where the molecules are initially in the lowest two torsional levels. Here we concentrate on the 390–420 cm−1 features in the S1 ← S0 excitation spectrum, assigning the features seen in the 2D-LIF spectrum, aided by separate dispersed fluorescence spectra. The 2D-LIF spectra allow the overlapping contributions to be cleanly separated, including some that arise from vibrational-torsional coupling. Various coupling routes open up because of the different symmetries of the lowest two torsional modes; these combine with the vibrational symmetry to provide new symmetry-allowed vibration-torsion (‘vibtor’) interactions, and the role of the excited m = 1 torsional level is found to be significant. GRAPHICAL ABSTRACT

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.

Two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy is a powerful tool allowing overlapped features in an electronic spectrum to be separated, and interactions between vibrations and torsions to be identified. Here the technique is employed to assign the 790-825 cm-1 region above the origin of the S1 ← S0 transition in para-fluorotoluene, which provides insight into the unusual time-resolved results of Davies and Reid [Phys. Rev. Lett. 109, 193004 (2012)]. The region is dominated by a pair of bands that arise from a Fermi resonance; however, the assignment is complicated by contributions from a number of overtones and combinations, including vibration-torsion ("vibtor") levels. The activity in the 2D-LIF spectra is compared to the recently reported zero-electron-kinetic-energy spectra [Tuttle et al., J. Chem. Phys. 146, 244310 (2017)] to arrive at a consistent picture of the energy levels in this region of the spectrum.

The form of molecular vibrations, and changes in these, give valuable insights into geometric and electronic structure upon electronic excitation or ionization, and within families of molecules. Here, we give a description of the phenyl-ring-localized vibrational modes of the ground (S-0) electronic states of a wide range of ortho-disubstituted benzene molecules including both symmetrically- and asymmetrically substituted cases. We conclude that the use of the commonly-used Wilson or Varsanyi mode labels, which are based on the vibrational motions of benzene itself, is misleading and ambiguous. In addition, we also find the use of the 1141 labels for monosubstituted benzenes [A.M. Gardner, T.G. Wright. J. Chem. Phys. 135 (2011) 114305], or the recently-suggested labels for para-disubstituted benzenes [A. Andrejeva, A.M. Gardner, W.D. Tuttle, T.G. Wright, J. Molec. Spectrosc. 321, 28 (2016)] are not appropriate. Instead, we label the modes consistently based upon the Mulliken (Herzberg) method for the modes of ortho-difluorobenzene (pDFB) under C-s symmetry, since we wish the labelling scheme to cover both symmetrically- and asymmetrically-substituted molecules. By studying the vibrational wavenumbers from the same force field while varying the mass of the substituent, we are able to identify the corresponding modes across a wide range of molecules and hence provide consistent assignments. We assign the vibrations of the following sets of molecules: the symmetric o-dihalobenzenes, o-xylene and catechol (o-dihydroxybenzene); and the asymmetric o-dihalobenzenes, o-halotoluenes, o-halophenols and o-cresol. In the symmetrically-substituted species, we find a pair of in-phase and out-of-phase carbon-substituent stretches, and this motion persists in asymmetrically-substituted molecules for heavier sub-stituents. When at least one of the substituents is light, then we find that these evolve into localized carbon-substituent stretches. (C) 2017 Elsevier Inc. All rights reserved.

The vibrations of the ground state cation (X[combining tilde]2B1) of para-chlorofluorobenzene (pClFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. ZEKE spectra were recorded using different vibrational levels of the S1 state as intermediate levels, for which assignments were put forward in an earlier paper [W. D. Tuttle, A. M. Gardner, and T. G. Wright, Chem. Phys. Lett., 2017, 684, 339]. These different intermediate levels dramatically modify the Franck-Condon factors for the ionization step. The adiabatic ionization energy (AIE) for pClFB was measured as 72 919 ± 5 cm-1, and analysis of the vibrational structure in the ZEKE spectra allowed further interrogation of the assignments of the REMPI spectrum. Assignment of the vibrational structure has been achieved by comparison with corresponding spectra of related molecules, via quantum chemical calculations, and via shifts in bands between the spectra of the 35Cl and 37Cl isotopologues. In this way it was possible to assign twenty out of the thirty vibrational modes of the ground state pClFB+ cation. Additionally, evidence for Fermi resonances between some vibrational levels was found in the S1 state, but no large-scale intramolecular vibrational redistribution (IVR) was seen in the spectra here. Finally, we discuss trends in AIE shifts for benzenes with one or two halogen atoms or methyl substituents.

We consider two key parameters that have been proposed to be important for vibrational energy delocalization, closely related to intramolecular vibrational redistribution (IVR), in molecules. These parameters are the symmetry of the molecule, and the presence of torsional (internal rotor) modes of a methyl group. We consider four para-disubstituted benzene molecules and examine their vibrational character. The molecules selected are para-difluorobenzene, para-chlorofluorobenzene, para-fluorotoluene, and para-xylene. This set of molecules allows the above parameters to be assessed in a systematic way. The probe we use is zero-electron-kinetic-energy (ZEKE) spectroscopy, which is employed in a resonant scheme, where the intermediate levels are selected vibrational levels of the S1 excited electronic state, with wavenumbers up to 1300 cm-1. We conclude that symmetry, and the presence of a methyl groups, do indeed have a profound effect on "restricted" IVR at low energies. This is underpinned by serendipitous coincidences in the energies of the levels, owing to small shifts in vibrational wavenumbers between molecules, so bringing levels into resonance. Additionally, methyl groups play an important role in opening up new routes for coupling between vibrations of different symmetry, and this is critical in the transition to "statistical" IVR at lower energies for molecules that contain them. Further, the presence of two methyl groups in the symmetrically-substituted p-xylene causes more widespread IVR than does the single methyl group in the asymmetrically-substituted p-fluorotoluene.

The protonated water clusters present in the D-region of the ionosphere have been postulated to be formed from cluster ions such as O2+·(H2O)n and NO+·(H2O)n, although the detailed mechanism of the underlying reactions is not understood. Second order Møller-Plesset perturbation theory based Born-Oppenheimer ab initio molecular dynamics (AIMD) simulations of the reactions of the O2+·(H2O)n and NO+·(H2O)n cluster ions to form protonated water clusters reveal different mechanisms for the O2+ and NO+ based ions. AIMD simulations of O2+·(H2O)n=2-5 with initial velocities of the atoms sampled from the Maxwell-Boltzmann distribution at 220 K show that following charge transfer, a reaction to form a protonated water cluster and OH occurs rapidly where the neutral O2 molecule is just a spectator. In contrast, the reaction of NO+·(H2O)n=4,5 has been hypothesised to involve an intracluster reaction, but no reaction is observed in AIMD simulations using thermal initial velocities. However, it is shown that reactions to form protonated water clusters do occur in simulations when a water molecule collides with a NO+·(H2O)4 cluster.