Chemical dynamics of large amplitude motion

It is one of the marvellous ironies of chemical physics that intellectually and experimentally the most challenging scientific areas to explore and understand always turn out to be the most critically important. For better or for worse, such is the case with chemical dynamics of large amplitude motion. The reason why is maddeningly simple. Physics achieves its most well understood paradigms in the two extreme limits of (i) unconstrained, free particle motion and (ii) small amplitude harmonic motion. Similarly, the canonical image of introductory chemistry involves either (i) ideal gases of essentially non-interacting atoms/molecules, or (ii) molecules with well defined “structures”, about whose local minima we perturbatively allow for small amplitude vibrational oscillations along normal mode coordinates.

The truth is that “real” chemistry, i.e. the making and breaking of bonds, often ventures far from these time honoured and paradigmatic pictures. Indeed, “real” chemistry becomes interesting precisely in that annoyingly nether region where oscillation of a breaking bond is unrecognizably far beyond the harmonic limit, and yet where motion is nowhere yet near that of a free particle. This is the interesting and yet challenging world of large amplitude dynamics, where eigenfunctions do not factor cleanly into products, simple quantum mechanical solutions do not generally exist and the coupling between conventionally well isolated degrees of freedom (e.g., vibration/rotation) can become the dominant terms in the Hamiltonian. It is the deeper exploration of this challenging yet fascinating frontier domain of spectroscopy and dynamics that this issue celebrates.

Particularly intriguing effects are observed when molecular rotation is hindered by solvating helium atoms (c0cp00193g). Hydrogen bonds of increasing strength (c002653k, c002193h, c002056g, c001705a, b925578h) lead to a smooth transition from free rotation to librational oscillation. As little as three atoms are enough to unravel an intricate quantum dynamics (c001124j, c002193h). Four (c002803g, c002067b), five (c002593c, c002774j, b922023b) and six atoms (c003073m, c001944e) still remain a major challenge for any complete treatment. This is particularly true if more than one electronic state is involved (c003593a, c002593c). By separating soft internal rotations and ring puckering modes from their rigid molecular framework, high resolution spectroscopy in the frequency (c002156c, c001705a, c002314k, c000757a, c002803g, c002193h) and time domain (b925388b) can achieve an in depth understanding of important molecular subspaces. Different conformations and vibrational states interact with each other via tunnelling and reveal accurate energy differences (c002156c, c002067b). The associated conformational changes are of essential significance in biomolecular interactions (c002811h). When more degrees of freedom are intimately coupled, one quickly reaches the limits of a full-dimensional quantum theory. Still, one can learn a lot from reduced dimensionality pictures, in particular when there is proton motion in a heavy atom frame (c001253j, c002345k, c002774j, c003140b). In this case, one can reach as far as acidic dissociation on surfaces (c002299n). If the environment cannot induce chemistry in a strong bond, overtone excitation may do the job (c003073m). Not surprisingly, almost all contributions address the issue of quantum tunnelling for some of the atoms involved. Spectral intensities turn out to provide valuable information on the dynamics (c002803g, b925578h, c002345k, c002056g). Furthermore, it becomes clear that algorithmic advances are essential when dealing with large amplitude motion (c001124j, c002811h, c001253j).

Although this issue collects work on a wide scale of interaction strengths and anharmonicities, most contributions have a common goal: to uncover regularity in a world of coupled degrees of freedom. This is where the large amplitude may actually help, rather than just enlarge the relevant phase space. We thank all contributors for their nice work, we hope that there will be unexpected insights from viewing this work together, and we thank the PCCP staff for handling the themed issue in a professional manner.

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