Editorial of the PCCP themed issue “Spectroscopy and dynamics of medium-sized molecules and clusters”

The “Spectroscopy and dynamics of medium-sized molecules and clusters” themed issue is aimed to present state-of-the art experimental and theoretical spectroscopy and quantum dynamics. One of the principal topics is the treatment of anharmonicity in medium-sized molecules and clusters either in their ground or excited electronic states. The contributions overview, for instance, accurate descriptions of highly excited states of molecules (including electronic, vibrational and rotational excitation), weakly bound aggregated systems, dynamics of molecules undergoing conformational changes, dynamics of molecules adsorbed on nanostructures, molecules at cold temperatures, molecules in unusual environments (e.g., interstellar space), atmospheric molecules and long range particle transfers.

The articles of this issue cover the microwave, far infra-red, infra-red, UV-Vis and XUV energy domains. The associated spectroscopies, and their specificities and challenges are nicely shown. The theoretical works also reveal the state-of-the-art methodologies used for these purposes, their validity and how to go beyond the common and standard approximations. Specifically, the papers by Csaszar et al. (DOI: 10.1039/C3CP44610G), Tyuterev et al. (DOI: 10.1039/C3CP50275A), Nikitin et al. (DOI: 10.1039/C3CP50799H), Christiansen et al. (DOI: 10.1039/C3CP50283J) and Tennyson et al. (DOI: 10.1039/C3CP50303H) and the perspective article by Herman and Perry (DOI: 10.1039/C3CP50463H) report ab initio and effective Hamiltonian-based approaches for the generation of the rotational–vibrational states and the rovibrational spectra of small (such as H₃⁺, SO₃, CH₄/CD₄) and larger systems (such as pyridine and the complex between pyridine and a silver cation). The close agreement of the theoretical predictions with the available experimental spectra (for both band positions and intensities) is striking, which validates the proposed techniques. These authors point out that such good agreement depends on the accuracy of the multidimensional potential energy surfaces (PESs) incorporated into their nuclear motion treatment calculations. The papers by Hochlaf and co-workers (DOI: 10.1039/C3CP44429E and DOI: 10.1039/C3CP44708A) and by Rauhut et al. (DOI: 10.1039/C3CP50172H) develop new strategies for the generation of full dimensional PESs for atmospherically important charge transfer tetratomic complexes, for scattering calculations and clustering and how to model the high-order terms in the potential energy surface expansions. For electronic computations, the explicitly correlated (R)CCSD(T)-F12 method is considered to correctly describe the repulsive wall, the van der Waals minimum and the large internuclear distances. A diffuse basis set is mandatory to correctly account for long range interactions. Benchmark systems are treated, such as the atmospherically relevant OMgOO⁺ quasi-linear Renner–Teller tetratomic system, and the astrophysically important HCl–He complex. For fitting the PESs, the reference-geometry Harris–Foulkes method is viewed as a powerful tool to model high-order terms within the expansion of multi-dimensional PESs as needed within the calculation of accurate vibrational frequencies beyond the harmonic approximation. The validity of this approach is checked through systematic computations of the fundamental modes of a test suite of 28 molecules.

In a combined theoretical and microwave spectroscopy study, Meijer, van der Avoird and their co-workers (DOI: 10.1039/C3CP51181B) treated the internal dynamics of the benzene dimer. For linalool (an acyclic monoterpene), the groups of Stahl and of Kleiner (DOI: 10.1039/C3CP50271F) recorded for the first time its microwave spectrum in the range from 9 to 16 GHz. The fitted spectroscopic parameters are used to validate the molecular geometry obtained from quantum chemical calculations. From a first principles theoretical point of view, this needs new developments as shown in the case of glycine by Barone, Puzzarini and co-workers (DOI: 10.1039/C3CP50439E), who shed light on the performance of a hybrid CC–DFT model in predicting structure, thermodynamic and spectroscopic constants of such medium-sized molecules. In a related topic, Senent and co-workers (DOI: 10.1039/C3CP50213A) computed the low frequency modes of propane and various monosubstituted isotopologues containing D and ¹³C. Experimentally, Pirali et al. (DOI: 10.1039/C3CP44305A) used a FTIR synchrotron based technique to measure the far infrared spectrum of naphthalene. They derived a set of up-to-date rotational and anharmonic frequencies for the low-frequency modes of naphthalene, which are of great importance for the identification of this PAH-like molecule in interstellar media. The assignments are supported by DFT computations beyond the harmonic approximation. The low frequency modes are very sensitive to anharmonic effects. Again for naphthalene, the theoretical work by Falvo et al. (DOI: 10.1039/C3CP44703K) showed that there are couplings between the electronic and nuclear degrees of freedom during IR emission. Indeed, the simulation of the IR emission spectra of naphthalene and its singly- and doubly-dehydrogenated radicals where various relaxation pathways, including radiative emission and hydrogen loss, are considered, revealed that the fragmentation products significantly contribute to the overall IR emission spectrum, especially to the intensity ratios between bands. Generally, these highly correlated ab initio investigations and these well-resolved experimental studies account correctly for all these effects. They provide some clues that may give insights into the spectral recordings and also explore the far infra-red spectra at low temperatures of medium-sized astrophysically and atmospherically important molecular systems.

The papers by Adamo et al. (DOI: 10.1039/C3CP43784A), by Daniel and Lindh et al. (DOI: 10.1039/C3CP51150B) and by Pratt and co-workers (DOI: 10.1039/C3CP51403J) investigate the electronic spectroscopy of a wide class of molecules including seminaphthofluorone molecular species, which are relevant for the design of new compounds for technological (white emitting dyes) and biological (ratiometric probes) applications, metal bearing molecules (BrMCl; M = Cu, Ag, Au) and medium sized chromophores. For BrMCl (M = Cu, Ag, Au), Daniel and Lindh et al. showed that multi configurational methods such as CASSCF/MS-CASPT2 together with consideration of scalar relativistic effects are mandatory for the description of the wavefunctions of these species. Mono determinantal techniques do not, however, allow enough flexibility for these purposes. In addition, they state that spin unrestricted KS-DFT leads to a reasonable description of such features and hence represents a cheap computational alternative for exploring the electronic excited states of such molecules. The work by Adamo et al. is paving the way for these findings since it shows that Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT) successfully describe complex molecular systems in solution for the analysis of their intermingled spectroscopic properties and for design. Through the comparison of the electronic spectra of 2-phenylindole and N-phenylcarbazole, Pratt and co-workers showed that the S1–S0 electronic transition is affected by intramolecular motions such as ring twist between the two chromophores and their attached phenyl groups.

The spectroscopy and dynamics of small clusters are illustrated by Zhou's (DOI: 10.1039/C3CP44588G) and by Delgado-Barrio's groups (DOI: 10.1039/C3CP50282A), by Suhm and Zehnacker-Rentien and co-workers (DOI: 10.1039/C3CP50708D), by Bonnet et al. (DOI: 1039/C3CP50524C), by Carbonnière and Pouchan (DOI: 10.1039/C3CP50424G) and by Miron's group (DOI: 10.1039/C3CP50249J). For instance, Zhou et al. considered the IR photodissociation of mass selected homoleptic nickel carbonyl clusters, where two-center and three-center bridge bonded complexes are characterized. Delgado-Barrio et al. performed quantum chemical computations of doped helium clusters, where Coriolis couplings play a key role in modifying the spectral dopant profile in ³He. Suhm and Zehnacker-Rentien et al. investigated the aggregation behavior of racemic and enantiopure 1-indanol by FTIR spectroscopy, resonant ion dip IR spectroscopy, spontaneous Raman scattering in supersonic jets and a dispersion-corrected DFT approach. They showed evidence for chirality-sensitive aggregation where long range forces play crucial roles. Bonnet and co-workers adapted the semiclassical Wigner treatment to direct photodissociations with the aim of accurately predicting final state distributions at relatively low computational cost, introducing this technique as a powerful interpretative tool. As an application, they treated methyl iodide photodissociation. Their results approach gently those deduced by rigorous quantum dynamics. In the topic of water solvation of biological molecules, the theoretical study by Carbonnière and Pouchan provides physically reasonable structures of microhydrated thymine clusters, from the mono- to the penta-hydrated species through the exploration of the potential energy surfaces with a global search algorithm of minima, where anharmonic effects are taken into account. The XUV photoionization of small Xe clusters by Miron et al. gives new tools for the structural characterization of these clusters, where specific electron spectra are measured for corner/edge/face/bulk Xe atoms of the aggregate.

The evolution of the spectroscopic properties and dynamics from isolated molecules to clusters and bulk is enlightened in the theoretical investigation by Carrington and co-authors (DOI: 10.1039/C3CP00065F). These authors calculated the anharmonic vibrations of the carboxyl group adsorbed on an anatase TiO₂ surface in acetic acid. This is the first time vibrational spectra for different adsorption sites of an organic molecule have been computed and compared without neglecting anharmonicity and coupling of the attaching group. They identify hence the mode of adsorption of the dye on the semiconductor surface. The applications of such studies are in the fields of accurate spectroscopy of adsorbed molecules, catalysis, hydrogen storage and for the development of dye-sensitized solar cells.

In summary, the articles composing this themed issue tackle new and yet unresolved challenges in molecular science. They discuss different approaches to overcome the difficulties associated with the numerical treatment of anharmonicity in complex molecular systems and their probes by new experimental techniques. Finally, this themed issue presents new strategies and the combination of existing approaches to go beyond the harmonic approximation and beyond the Franck–Condon principle. The further developments emphasized here should help in the analysis and interpretation of recent experimental studies of highly excited molecular systems and astrophysical observations.

I would like to thank all contributors to this themed issue and their participation in its success. The two other guest editors, D. Lauvergnat (U. Paris Sud, France) and R. Linguerri (U. Paris-Est Marne-La-Vallée), are thanked for their help and for the valuable time they spent during all steps of preparation. R. Linguerri is thanked also for his help for elaborating the front cover of this issue.

---