Vibrational conical intersections in the water dimer

Can classical mechanics sense conical intersection?

Conical intersection (CI) leads to fast electronic energy transfer. However, Hamm and Stock [Phys. Rev. Lett. 109, 173201 (2012)] showed the existence of a vibrational CI and its role in vibrational energy relaxation. In this paper, we further investigate the vibrational energy relaxation using an isolated model Hamiltonian system of four vibrational modes with two distinctively different timescales (two fast modes and two slow modes). We show that the excitation of the slow modes plays a crucial role in the energy relaxation mechanism. We also analyze the system from a mixed quantum-classical (surface hopping method) and a completely classical point of view. Notably, surface hopping and even classical simulations also capture fast energy relaxation, which is a signature of CI's existence.

A Rather Universal Vibrational Resonance in 1:1 Hydrates of Carbonyl Compounds

When the lower frequency OH stretching fundamental of a water molecule is shifted to the 3500 cm^-1 spectral range by the solvation of a carbonyl compound, in this case a ketone, its infrared intensity is shared with a dark state. It is shown by chemical and isotope substitution for more than a dozen systems that the location of this resonance is remarkably substitution-independent. Harmonic and anharmonic model calculations support its assignment to a combination of the water bending overtone and in-plane water libration. This previously unrecognized intramolecular-intermolecular coupling in single solvent water has a strength of 7-10 cm^-1. It may have been sporadically observed before in a few other carbonyl compounds such as amides, without any previous exploration of its potential universality. The resulting generic picosecond energy redistribution channel for aqueous solutions may represent a slow counterpart and doorway model of what happens on a subpicosecond time scale when the hydrogen bonds become stronger, such as in carboxylic acid dimers or protonated water clusters.

Decoding the 2D IR spectrum of the aqueous proton with high-level VSCF/VCI calculations

The aqueous proton is a common and long-studied species in chemistry, yet there is currently intense interest devoted to understanding its hydration structure and transport dynamics. Typically described in terms of two limiting structures observed in gas-phase clusters, the Zundel H₅O₂ ⁺ and Eigen H₉O₄ ⁺ ions, the aqueous structure is less clear due to the heterogeneity of hydrogen bonding environments and room-temperature structural fluctuations in water. The linear infrared (IR) spectrum, which reports on structural configurations, is challenging to interpret because it appears as a continuum of absorption, and the underlying vibrational modes are strongly anharmonically coupled to each other. Recent two-dimensional IR (2D IR) experiments presented strong evidence for asymmetric Zundel-like motifs in solution, but true structure-spectrum correlations are missing and complicated by the anharmonicity of the system. In this study, we employ high-level vibrational self-consistent field/virtual state configuration interaction calculations to demonstrate that the 2D IR spectrum reports on a broad distribution of geometric configurations of the aqueous proton. We find that the diagonal 2D IR spectrum around 1200 cm^-1 is dominated by the proton stretch vibrations of Zundel-like and intermediate geometries, broadened by the heterogeneity of aqueous configurations. There is a wide distribution of multidimensional potential shapes for the proton stretching vibration with varying degrees of potential asymmetry and confinement. Finally, we find specific cross peak patterns due to aqueous Zundel-like species. These studies provide clarity on highly debated spectral assignments and stringent spectroscopic benchmarks for future simulations.

Probing the intermolecular coupled vibrations in a water cluster with inelastic electron tunneling spectroscopy

The hydrogen-bonding networks of water have strong intra- and intermolecular vibrational coupling which influences the energy dissipation and proton transfer in water. Disentangling and quantitative characterization of different coupling effects in water at a single-molecular level still remains a great challenge. Using tip-enhanced inelastic electron tunneling spectroscopy (IETS) based on low-temperature scanning tunneling microscopy, we report the direct quantitative assessment of the intermolecular coupling constants of the OH-stretch vibrational bands of an isolated water tetramer adsorbed on a Au(111)-supported NaCl(001) bilayer film. This is achieved by distinguishing various coupled modes of the H-bonded O-H stretching vibrations through tip-height dependent IET spectra. In contrast, such vibrational coupling is negligible in the half-deuterated water tetramer owing to the large energy mismatch between the OH and OD stretching modes. Not only do these findings advance our understanding on the effects of local environment on the intermolecular vibrational coupling in water, but also open up a new route for vibrational spectroscopic studies of extended H-bonded network at the single-molecular level.

Unambiguous Signature of the Berry Phase in Intense Laser Dissociation of Diatomic Molecules

We report strong evidence of Berry phase effects in intense laser dissociation of D₂⁺ molecules, manifested as Aharonov-Bohm-like oscillations in the photofragment angular distribution (PAD). Our calculations show that this interference pattern strongly depends on the parity of the diatom initial rotational state, (-1) ^j. Indeed, the PAD local maxima (minima) observed in one case ( j odd) correspond to local minima (maxima) in the other case ( j even). Using simple topological arguments, we clearly show that such interference conversion is a direct signature of the Berry phase. The sole effect of the latter on the rovibrational wave function is a sign change of the relative phase between two interfering components, which wind in opposite senses around a light-induced conical intersection (LICI). Therefore, encirclement of the LICI leads to constructive ( j odd) or destructive ( j even) self-interference of the initial nuclear wavepacket in the dissociative limit. To corroborate our theoretical findings, we suggest an experiment of strong-field indirect dissociation of D₂⁺ molecules, comparing the PAD of the ortho and para molecular species in directions nearly perpendicular to the laser polarization axis.

Coupling between intra- and intermolecular motions in liquid water revealed by two-dimensional terahertz-infrared-visible spectroscopy

The interaction between intramolecular and intermolecular degrees of freedom in liquid water underlies fundamental chemical and physical phenomena such as energy dissipation and proton transfer. Yet, it has been challenging to elucidate the coupling between these different types of modes. Here, we report on the direct observation and quantification of the coupling between intermolecular and intramolecular coordinates using two-dimensional, ultra-broadband, terahertz-infrared-visible (2D TIRV) spectroscopy and molecular dynamics calculations. Our study reveals strong coupling of the O-H stretch vibration, independent of the degree of delocalization of this high-frequency mode, to low-frequency intermolecular motions over a wide frequency range from 50 to 250 cm-1, corresponding to both the intermolecular hydrogen bond bending (≈ 60 cm-1) and stretching (≈ 180 cm-1) modes. Our results provide mechanistic insights into the coupling of the O-H stretch vibration to collective, delocalized intermolecular modes.

Nonadiabatic effects in electronic and nuclear dynamics

Due to their very nature, ultrafast phenomena are often accompanied by the occurrence of nonadiabatic effects. From a theoretical perspective, the treatment of nonadiabatic processes makes it necessary to go beyond the (quasi) static picture provided by the time-independent Schrödinger equation within the Born-Oppenheimer approximation and to find ways to tackle instead the full time-dependent electronic and nuclear quantum problem. In this review, we give an overview of different nonadiabatic processes that manifest themselves in electronic and nuclear dynamics ranging from the nonadiabatic phenomena taking place during tunnel ionization of atoms in strong laser fields to the radiationless relaxation through conical intersections and the nonadiabatic coupling of vibrational modes and discuss the computational approaches that have been developed to describe such phenomena. These methods range from the full solution of the combined nuclear-electronic quantum problem to a hierarchy of semiclassical approaches and even purely classical frameworks. The power of these simulation tools is illustrated by representative applications and the direct confrontation with experimental measurements performed in the National Centre of Competence for Molecular Ultrafast Science and Technology.

An extended E⊗e Jahn-Teller Hamiltonian for large-amplitude motion: Application to vibrational conical intersections in CH3SH and CH3OH

An extended E⊗e Jahn-Teller Hamiltonian is presented for the case where the (slow) nuclear motion extends far from the symmetry point and may be described approximately as motion on a sphere. Rather than the traditional power series expansion in the displacement from the C_3v symmetry point, an expansion in the spherical harmonics is employed. Application is made to the vibrational Jahn-Teller effect in CH₃XH, with X = S, O, where the equilibrium CXH angles are 83° and 72°, respectively. In addition to the symmetry-required conical intersection (CI) at the C_3v symmetry point, ab initio calculations reveal sets of six symmetry-allowed vibrational CIs in each molecule. The CIs for each molecule are arranged differently in the large-amplitude space, and that difference is reflected in the infrared spectra. The CIs in CH₃SH are found in both eclipsed and staggered geometries, whereas those for CH₃OH are found only in the eclipsed geometry near the torsional saddle point. This difference between the two molecules is reflected in the respective high-resolution spectra in the CH stretch fundamental region.

Vibrational dynamics of aqueous hydroxide solutions probed using broadband 2DIR spectroscopy

We employed ultrafast transient absorption and broadband 2DIR spectroscopy to study the vibrational dynamics of aqueous hydroxide solutions by exciting the O-H stretch vibrations of the strongly hydrogen-bonded hydroxide solvation shell water and probing the continuum absorption of the solvated ion between 1500 and 3800 cm(-1). We observe rapid vibrational relaxation processes on 150-250 fs time scales across the entire probed spectral region as well as slower vibrational dynamics on 1-2 ps time scales. Furthermore, the O-H stretch excitation loses its frequency memory in 180 fs, and vibrational energy exchange between bulk-like water vibrations and hydroxide-associated water vibrations occurs in ∼200 fs. The fast dynamics in this system originate in strong nonlinear coupling between intra- and intermolecular vibrations and are explained in terms of non-adiabatic vibrational relaxation. These measurements indicate that the vibrational dynamics of the aqueous hydroxide complex are faster than the time scales reported for long-range transport of protons in aqueous hydroxide solutions.

Diffuse Vibrational Signature of a Single Proton Embedded in the Oxalate Scaffold, HO2CCO2(-)

To understand how the D2d oxalate scaffold (C2O4)(2-) distorts upon capture of a proton, we report the vibrational spectra of the cryogenically cooled HO2CCO2(-) anion and its deuterated isotopologue DO2CCO2(-). The transitions associated with the skeletal vibrations and OH bending modes are sharp and are well described by inclusion of cubic terms in the normal mode expansion of the potential surface through an extended Fermi resonance analysis. The ground state structure features a five-membered ring with an asymmetric intramolecular proton bond. The spectral signatures of the hydrogen stretches, on the contrary, are surprisingly diffuse, and this behavior is not anticipated by the extended Fermi scheme. We trace the diffuse bands to very strong couplings between the high-frequency OH-stretch and the low-frequency COH bends as well as heavy particle skeletal deformations. A simple vibrationally adiabatic model recovers this breadth of oscillator strength as a 0 K analogue of the motional broadening commonly used to explain the diffuse spectra of H-bonded systems at elevated temperatures, but where these displacements arise from the configurations present at the vibrational zero-point level.

Quantum-mechanical approach to predissociation of water dimers in the vibrational adiabatic representation: Importance of channel interactions

The results of application of the quantum-mechanical adiabatic theory to vibrational predissociation (VPD) of water dimers, (H2O)2 and (D2O)2, are presented. We consider the VPD processes including the totally symmetric OH mode of the dimer and the bending mode of the fragment. The VPD in the adiabatic representation is induced by breakdown of the vibrational adiabatic approximation, and two types of nonadiabatic coupling matrix elements are involved: one provides the VPD induced by the low-frequency dissociation mode and the other provides the VPD through channel interactions induced by the low-frequency modes. The VPD rate constants were calculated using the Fermi golden rule expression. A closed form for the nonadiabatic transition matrix element between the discrete and continuum states was derived in the Morse potential model. All of the parameters used were obtained from the potential surfaces of the water dimers, which were calculated by the density functional theory procedures. The VPD rate constants for the two processes were calculated in the non-Condon scheme beyond the so-called Condon approximation. The channel interactions in and between the initial and final states were taken into account, and those are found to increase the VPD rates by 3(1) orders of magnitude for the VPD processes in (H2O)2 ((D2O)2). The fraction of the bending-excited donor fragments is larger than that of the bending-excited acceptor fragments. The results obtained by quantum-mechanical approach are compared with both experimental and quasi-classical trajectory calculation results.

Collective vibrations of water-solvated hydroxide ions investigated with broadband 2DIR spectroscopy

The infrared spectra of aqueous solutions of NaOH and other strong bases exhibit a broad continuum absorption for frequencies between 800 and 3500 cm(-1), which is attributed to the strong interactions of the OH(-) ion with its solvating water molecules. To provide molecular insight into the origin of the broad continuum absorption feature, we have performed ultrafast transient absorption and 2DIR experiments on aqueous NaOH by exciting the O-H stretch vibrations and probing the response from 1350 to 3800 cm(-1) using a newly developed sub-70 fs broadband mid-infrared source. These experiments, in conjunction with harmonic vibrational analysis of OH(-)(H2O)n (n = 17) clusters, reveal that O-H stretch vibrations of aqueous hydroxides arise from coupled vibrations of multiple water molecules solvating the ion. We classify the vibrations of the hydroxide complex by symmetry defined by the relative phase of vibrations of the O-H bonds hydrogen bonded to the ion. Although broad and overlapping spectral features are observed for 3- and 4-coordinate ion complexes, we find a resolvable splitting between asymmetric and symmetric stretch vibrations, and assign the 2850 cm(-1) peak infrared spectra of aqueous hydroxides to asymmetric stretch vibrations.