•Detailed assignment of S1 ← S0 spectrum of p–chlorofluorobenzene has been achieved.•Comparison of derived vibrational wavenumbers to quantum chemical calculations is made.•Similarities to the spectra of p–difluorobenzene and p–dichlorobenzene are highlighted.The S1 ← S0 (Ã1B2 ← X̃1A1) electronic transition of para-chlorofluorobenzene has been investigated using resonance-enhanced multiphoton ionization (REMPI) spectroscopy. 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 addition, we have also partially reassigned a previously-published spectrum of para-dichlorobenzene.Download high-res image (63KB)Download full-size image

•Vibrations of p-difluorobenzene, pDFB were compared to benzene and fluorobenzene.•This was not useful, owing to vibrations evolving between these molecules.•Instead, normal modes of pDFB are used to assign many para-disubstituted benzenes.•Changing wavenumbers owing to mass and electronic effects are rationalized.We give a description of the phenyl-ring-localized vibrational modes of the ground states of the para-disubstituted benzene molecules including both symmetric and asymmetric cases. In line with others, we quickly conclude that the use of Wilson mode labels is misleading and ambiguous; we conclude the same regarding the related ones of Varsányi. Instead we label the modes consistently based upon the Mulliken (Herzberg) method for the modes of para-difluorobenzene (pDFB). Since we wish the labelling scheme to cover both symmetrically- and asymmetrically-substituted molecules, we apply the Mulliken labelling under C2v symmetry. By studying the variation of the vibrational wavenumbers with mass of the substituent, we are able to identify the corresponding modes across a wide range of molecules and hence provide consistent assignments. Particularly interesting are pairs of vibrations that evolve from in- and out-of-phase motions in pDFB to more localized modes in asymmetric molecules. We consider the para isomers of the following: the symmetric dihalobenzenes, xylene, hydroquinone, the asymmetric dihalobenzenes, halotoluenes, halophenols and cresol.

It is found that a simple electrostatic model involving competition between the attractive dispersive interaction and induced-dipole repulsion between the two RG atoms performs extremely well in rationalizing the M+-RG2 geometries, where M = group 1 metal and RG = rare gas. The Li+-RG2 and Na+-RG2 complexes have previously been found to exhibit quasilinear or linear minimum-energy geometries, with the Na+-RG2 complexes having an additional bent local minimum [A. Andrejeva, A. M. Gardner, J. B. Graneek, R. J. Plowright, W. H. Breckenridge, T. G. Wright, J. Phys. Chem. A, 2013, 117, 13578]. In the present work, the geometries for M = K–Fr are found to be bent. A simple electrostatic model explains these conclusions and is able to account almost quantitatively for the binding energy of the second RG atom, as well as the form of the angular potential, for all 36 titular species. Additionally, results of population analyses are presented together with orbital contour plots; combined with the success of the electrostatic model, the expectation that these complexes are all physically bound is confirmed.

Ab initio calculations were employed to investigate M+–RG2 species, where M+ = Ca, Sr, Ba, and Ra and RG = He–Rn. Geometries have been optimized, and cuts through the potential energy surfaces containing each global minimum have been calculated at the MP2 level of theory, employing triple-ζ quality basis sets. The interaction energies for these complexes were calculated employing the RCCSD(T) level of theory with quadruple-ζ quality basis sets. Trends in binding energies, De, equilibrium bond lengths, Re, and bond angles are discussed and rationalized by analyzing the electronic density. Mulliken, natural population, and atoms-in-molecules (AIM) population analyses are presented. It is found that some of these complexes involving the heavier group 2 metals are bent whereas others are linear, deviating from observations for the corresponding Be and Mg metal-containing complexes, which have all previously been found to be bent. The results are discussed in terms of orbital hybridization and the different types of interaction present in these species.

•nd Rydberg series of Cu using (2 + 1) REMPI spectroscopy.•3d104p1 (2P3/2,1/2) ← 3d104s1 (2S1/2) transitions seen using (1 + 1′) REMPI.•Metastable Cu atoms observed using REMPI, survive many 100s of μs.Resonance-enhanced multiphoton ionization (REMPI) spectra of laser-ablated copper atoms entrained in a supersonic free jet expansion are reported. Depending on the ionization scheme employed, and the conditions under which the copper atoms are produced, very different spectra are produced, which are discussed. In some circumstances, high proportions of metastable atoms survive the ablation and expansion process and are clearly seen in the spectra. The spectroscopic transitions for the observed lines are identified, and it is noted that some caution is merited in the assumption that only ground state copper atoms are present following laser ablation.

Ab initio calculations were employed to determine the geometry (MP2 level), and dissociation energies [MP2 and RCCSD(T) levels], of the MIIa+–RG2 species, where MIIa is a group 2 metal, Be or Mg, and RG is a rare gas (He–Rn). We compare the results with similar calculations on MIa+–RG2, where MIa is a group 1 metal, Li or Na. It is found that the complexes involving the group 1 metals are linear (or quasilinear), whereas those involving the group 2 metals are bent. We discuss these results in terms of hybridization and the various interactions in these species. Trends in binding energies, De, bond lengths, and bond angles are discussed. We compare the energy required for the removal of a single RG atom from M+–RG2 (De2) with that of the dissociation energy of M+–RG (De1); some complexes have De2 > De1, some have De2 < De1, and some have values that are about the same. We also present relaxed angular cuts through a selection of potential energy surfaces. The trends observed in the geometries and binding energies of these complexes are discussed. Mulliken, natural population, and atoms-in-molecules (AIM) population analyses are performed, and it is concluded that the AIM method is the most reliable, giving results that are in line with molecular orbital diagrams and contour plots; unphysical amounts of charge transfer are suggested by the Mulliken and natural population approaches.

Potential energy curves for the interaction of B+ (1S) with RG (1S), RG = He–Rn, have been calculated at the CCSD(T) level of theory employing quadruple-ζ and quintuple-ζ quality basis sets. The interaction energies from these curves were subsequently point-by-point extrapolated to the basis set limit. Rovibrational energy levels have been calculated for each extrapolated curve, from which spectroscopic parameters are determined. These are compared to previously determined experimental and theoretical values. The potentials have also been employed to calculate the transport coefficients for B+ traveling through a bath of RG atoms. We also investigate the interactions between B+ and the rare gases via contour plots, natural population analysis (NPA), and molecular orbital diagrams. In addition, we consider the atoms-in-molecules (AIM) parameters. The interactions here are compared and contrasted with those for Li+–He and Be+–RG; it is concluded that there is significant and increasing dative covalent bonding for the Be+–RG and B+–RG complexes for RG = Ar–Rn, while the other species are predominantly physically bound.

We propose a new definition of the effective radius of an atomic ion: the bond distance (Re) of the ion/He diatomic complex minus the van der Waals radius of the helium atom. Our rationale is that He is the most chemically inert and least polarizable atom, so that its interaction with the outer portions of the electron cloud causes the smallest perturbation of it. We show that such radii, which we denote RXHe, make good qualitative sense. We also compare our RXHe values to more traditional ionic radii from solid crystal X-ray measurements, as well as estimates of such radii from “ionic” gas-phase MF, MOM, MF+, and MO molecules, where M is a metal atom. Such comparisons lead to interesting conclusions about bonding in ionic crystals and in simple gas-phase oxide and fluoride molecules. The definition is shown to be reasonable for −1, +1, and even for many of the larger +2 atomic ions. Another advantage of the RXHe definition is that it is also consistently valid for ground states and excited states of both neutral atoms and atomic ions, even for open-shell np and nd cases where the electron clouds of the ions are not spherically symmetric and RXHe thus depends on the “approach” direction of the He atom. Finally, we note that when there is a contribution from covalent bonding with the He atom, and/or in cases where the ion is small and has a very high charge, so that there is distortion even of the He 1s electrons, RXHe is not expected to be representative of the size of the ion. We then suggest that in these cases small, and sometimes unphysical, values of RXHe are diagnostic of the fact that simple “physical” interactions have been supplemented by a “chemical” component.

We present high level ab initio potential energy curves for the Mn+−RG complexes, where n = 1, 2, RG = rare gas, and M = Be and Mg. Spectroscopic constants have been derived from these potentials, and they generally show very good agreement with the available experimental data. The potentials have also been employed in calculating transport coefficients for M+ moving through a bath of RG atoms, and the isotopic scaling relationship is examined for Mg+ in Ne. Trends in binding energies, De, and bond lengths, Re, are discussed and compared to similar ab initio results involving the corresponding complexes of the heavier alkaline earth metal ions. We identify some very unusual behavior, particularly for Be+−Ne, and offer possible explanations.

We report electronic spectra of the Au–Xe complex for the first time. The transitions are recorded in the vicinity of the Au atomic 6p← 6s transitions. Structured spectra are found close to both the 62P1/2 and 62P3/2 states. The former is assigned as a 2Π1/2 state in line with previous work on Au–Ar and Au–Kr; the possible assignment of the second spectrum is discussed. In addition, a large basis set extrapolated RCCSD(T) potential energy curve for the ground state, X2Σ+, is presented and derived spectroscopic parameters reported. More qualitative calculations are presented for electronically-excited states which arise from the Au(52D) + Xe and Au(62P) + Xe asymptotes, as well as some higher-lying states. The ab initio results are employed in the assignment of the reported spectra.

The potential energy curves of Br cations interacting with rare gas (RG = He−Ar) atoms are calculated employing the RCCSD(T) technique for the non-spin−orbit states, with spin−orbit coupling being included analytically, using an atoms-in-molecule approach. The curves are calculated employing large basis sets extrapolated to the complete basis set limit, and the energies are corrected for basis set superposition error. Comparison is made to the limited potentials available previously. Gaseous ion mobilities are obtained using the potential energy curves of the states of the complex that correlate to the Br+(3PJ°) + RG asymptotes. Excellent agreement is obtained with the experimental data in He; however, the experimental error bars are such that we are unable to determine which spin−orbit state(s) are present in the mobility measurements. Finally, a high-resolution photoelectron spectrum is simulated for the ionization of Br−Ar.

We report the results of a (2 + 1) resonance-enhanced multiphoton ionization (REMPI) study of the Ẽ2Σ+(4sσ) Rydberg state of NO–Kr. We present an assignment of the two-photon spectrum based on a simulation, and discuss it in the context of previously-reported spectra of NO–Ne and NO–Ar. In addition, we report on spectra in the region of the vNO = 1 level of the Ẽ, and ′ 4s and 3d Rydberg states of NO–Rg (Rg = Ne–Kr). Since the NO vibrational frequency is affected by electron donation from the rare-gas (Rg) atom to the NO+ core, as well as by the penetration of the Rydberg electron, the fundamental NO-stretch frequency reflects the interactions in the complex. The results indicate that the 4s Rydberg state has a strong interaction between the NO+ core and the Kr atom, as was the case for NO–Ar and NO–Ne. For the 3d Rydberg states, although penetration is not as significant as for the 4s Rydberg states, it does play an important role, with subtle angular effects being notable.

We present the results of ab initio calculations, which include spin-orbit coupling, to support our recently-proposed interpretation of (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy of the Au–Ar complex. Mixing between the 2Σ1/2+ and 2Π1/2 (···6p1) states leads to a dramatic decrease of the well-depth of the latter, as well as a perturbation in the outer wall.Spin-orbit coupling is important in the Au(6p)–Ar interaction.

Optimized geometries and vibrational frequencies are calculated for Ca+−X and Y−Ca+−X complexes (X, Y = H2O, N2, CO2, O2, and O), required for understanding the chemistry of calcium in the upper atmosphere. Both MP2 and B3LYP optimizations were performed employing 6-311+G(2d,p) basis sets. In some cases a number of different orientations had to be investigated in order to determine the one of lowest energy, and in cases involving O and O2, different spin states also had to be considered. In order to establish accurate energetics, RCCSD(T) single-point energy calculations were also employed, using aug-cc-pVQZ basis sets. Accurate dissociation energies for the Ca+−X and X−Ca+−Y species are derived and discussed. Comparison with available experimental results is made where possible.

High-quality ab initio potential energy curves are presented for the Zn+–Rg series (Rg = He–Rn). Calculations are performed at the RCCSD(T) level of theory, employing aug-cc-pV5Z quality basis sets, with ‘small core’ relativistic effective core potentials being used for Kr–Rn. We present spectroscopic information for the titular species, derived from our potential energy curves, and compare to previous results. We also present calculated ion transport data and show that mobility minima occur for a number of these systems, albeit at low gas temperatures for some of them.Mobilities of Zn+ in rare gases at 100 K showing mobility minima.

We investigate the A∼2Σ+ state of NO–Ne by high-level ab initio methods, and by (1 + 1) resonance-enhanced multiphoton ionization spectroscopy (REMPI). Despite being able to obtain high-quality spectra of NO–Ar, NO–Kr and NO–Xe, no spectrum of NO–Ne was observed. It is shown that this state is very weakly bound; and that the overlap between the zero-point vibrational energy level in the X∼2Π state, and the bound levels of the A∼2Σ+ state is very small: hence the Franck–Condon factors are close to zero. The location of the Ne atom outside the 3s Rydberg orbital is the cause of the observations.The A∼ state of NO–Ne is very weakly bound, supporting only two stretch vibrations.

We have collected (2 + 1) Resonance-Enhanced Multiphoton Ionization (REMPI) spectra of van der Waals complexes in which a NO molecule is attached to either CO, N2, or both N2 and Ar. The energy region probed corresponds to electronic transitions of uncomplexed NO(X2Π) to the 4s and 3d Rydberg states, and we discuss the observed spectra in light of the expected perturbations to these electronic levels induced by complexation. We employ a model in which the van der Waals partners are assumed to reside within the Rydberg orbital, and discuss the importance of core penetration in the description of the electronic structure. By performing calculations on NO+ interacting with both N2 and Ar, we identify the global minimum as being a non-planar structure. Further, the N2 and Ar are found to interact with the NO+ largely independently, and we find some evidence for this from the REMPI spectrum of NO–{N2, Ar}.

We study different conformers of the toluene dimer using unconstrained geometry optimizations at the MP2 level of theory. We reoptimize these employing counterpoise-corrected MP2 gradients, and subsequently perform single-point counterpoise-corrected CCSD(T) interaction energy calculations. An antiparallel-stacked structure is found to be the most stable of the three isomers and has an interaction energy that is narrowly below that of a cross structure; a parallel-stacked structure is the least stable of the three isomers. We find no evidence for a stable T-shaped isomer, that is, no minimum on the potential energy surface corresponding to this structure.Non-counterpoise-corrected MP2/6-31++G∗∗ optimized geometries of the three toluene dimers. The CCSD(T) energies suggest the energy ordering is unchanged, with the lowest-energy isomer on the left.

We report electronic spectra of the Au–Xe complex for the first time. The transitions are recorded in the vicinity of the Au atomic 6p← 6s transitions. Structured spectra are found close to both the 62P1/2 and 62P3/2 states. The former is assigned as a 2Π1/2 state in line with previous work on Au–Ar and Au–Kr; the possible assignment of the second spectrum is discussed. In addition, a large basis set extrapolated RCCSD(T) potential energy curve for the ground state, X2Σ+, is presented and derived spectroscopic parameters reported. More qualitative calculations are presented for electronically-excited states which arise from the Au(52D) + Xe and Au(62P) + Xe asymptotes, as well as some higher-lying states. The ab initio results are employed in the assignment of the reported spectra.

We report the results of a (2 + 1) resonance-enhanced multiphoton ionization (REMPI) study of the Ẽ2Σ+(4sσ) Rydberg state of NO–Kr. We present an assignment of the two-photon spectrum based on a simulation, and discuss it in the context of previously-reported spectra of NO–Ne and NO–Ar. In addition, we report on spectra in the region of the vNO = 1 level of the Ẽ, and ′ 4s and 3d Rydberg states of NO–Rg (Rg = Ne–Kr). Since the NO vibrational frequency is affected by electron donation from the rare-gas (Rg) atom to the NO+ core, as well as by the penetration of the Rydberg electron, the fundamental NO-stretch frequency reflects the interactions in the complex. The results indicate that the 4s Rydberg state has a strong interaction between the NO+ core and the Kr atom, as was the case for NO–Ar and NO–Ne. For the 3d Rydberg states, although penetration is not as significant as for the 4s Rydberg states, it does play an important role, with subtle angular effects being notable.