Effects in the Infrared Spectra of Liquid Water. ChemRxiv. Preprint.A quantitative characterization of intermolecular and intramolecular couplings that modulate the OH-stretch vibrational band in liquid water has so far remained elusive. Here, we take up this challenge by combining the centroid molecular dynamics (CMD) formalism, which accounts for nuclear quantum effects, with the MB-pol potential energy function, which accurately reproduces the properties of water across all phases, to model the infrared (IR) spectra of various isotopic water solutions with different levels of vibrational couplings, including those that cannot be probed experimentally. Analysis of the different IR OH-stretch lineshapes provides direct evidence for the partially quantum-mechanical nature of hydrogen bonds in liquid water, which is emphasized by synergistic effects associated with intermolecular coupling and many-body electrostatic interactions. Furthermore, we quantitatively demonstrate that intramolecular coupling, which results in Fermi resonances due to the mixing between HOH-bend overtones and OH-stretch fundamentals, are responsible for the shoulder located at ∼3250 cm -1 of the IR OH-stretch band of liquid water. File list (2) download file view on ChemRxiv manuscript.pdf (1.43 MiB) download file view on ChemRxiv supporting_info.pdf (0.93 MiB)

We apply a recently proposed ring polymer surface hopping (RPSH) approach to investigate the real-time nonadiabatic dynamics with explicit nuclear quantum effects. The nonadibatic electronic transitions are described through Tully's fewest-switches surface hopping algorithm and the motion of the nuclei are quantized through the ring polymer Hamiltonian in the extended phase space. Applying the RPSH method to simulate Tully's avoided crossing models, we demonstrate the critical role of the nuclear tunneling effect and zero-point energy for accurately describing the transmission and reflection probabilities with low initial momenta. In addition, in Tully's extended coupling model, we show that the ring polymer quantization effectively captures decoherence, yielding more accurate reflection probabilities. These promising results demonstrate the potential of using RPSH as an accurate and efficient method to incorporate nuclear quantum effects into nonadiabatic dynamics simulations.

We investigate photoinduced proton-coupled electron transfer (PI-PCET) reactions through a recently developed quasi-diabatic (QD) quantum dynamics propagation scheme. This scheme enables interfacing accurate diabatic-based quantum dynamics approaches with adiabatic electronic structure calculations for on-the-fly simulations. Here, we use the QD scheme to directly propagate PI-PCET quantum dynamics with the diabatic partial linearized density matrix path-integral approach with the instantaneous adiabatic electron-proton vibronic states. Our numerical results demonstrate the importance of treating protons quantum mechanically in order to obtain accurate PI-PCET dynamics as well as the role of solvent fluctuation and vibrational relaxation on proton tunneling in various reaction regimes that exhibit different kinetic isotope effects. This work opens the possibility to study the challenging PI-PCET reactions through accurate diabatic quantum dynamics approaches combined with efficient adiabatic electronic structure calculations.

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We apply a recently-developed quasi-diabatic (QD) propagation scheme to simulate proton-coupled electron transfer (PCET) reactions. This scheme enables a direct interface between an accurate diabatic dynamics approach and the adiabatic vibronic states. It explicitly avoids theoretical efforts to pre-construct diabatic states for the transferring electron and proton or reformulate diabatic dynamics methods to the adiabatic representation, both of which are non-trivial tasks. Using partial linearized path-integral approach and symmetrical quasi-classical approach as the diabatic dynamics methods, we demonstrate that the QD propagation scheme provides accurate vibronic dynamics of PCET reactions and reliably predict the correct reaction mechanism without any a priori assumptions. This work demonstrates the possibility to directly simulate challenging PCET reactions by using accurate diabatic dynamics approaches and adiabatic vibronic information.

High-level DFT calculations, coupled with appropriate isodesmic reactions, are employed to investigate the effects of monoheteroatom substitution, cyclization, and unsaturation on the stability, multiplicity, and reactivity of amino-, oxy-, silyl-, phosphino-, and thioalkylcarbenes. The results of these calculations are compared to those of di-tert-butylcarbene, 2,2,5,5-tetramethylcyclopentanylidene, and 2,2,5,5-tetramethylcyclopent-3-enylidene as the reference molecules. The calculated singlet-triplet energy gaps (DeltaE(S-T)) demonstrate the following trend: (amino approximately = oxy) > thio > phosphino > alkyl > silyl. In contrast to the previous reports, isodesmic reactions show that pi-donor/sigma-acceptor amino substituents stabilize not only the singlet but also the triplet states. The stabilization of the triplet states by amino substitution is much less than the singlet states. The DeltaE(S-T) values of all the carbenes are increased through cyclization, while the introduction of unsaturation causes small and rather random changes. These changes are carefully probed by means of isodesmic reactions for the singlet and triplet states, separately. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity, and proton affinity issues showing amino- and phosphinoalkylcarbenes to be more nucleophilic, more basic, and less electrophilic than oxy- and thioalkylcarbenes, respectively. This detailed study offers new insights into the chemistry of these novel carbenes.

The nonadiabatic dynamics of model proton-coupled electron transfer (PCET) reactions is investigated for the first time using a surface-hopping algorithm based on the solution of the mixed quantum-classical Liouville equation (QCLE). This method provides a rigorous treatment of quantum coherence/decoherence effects in the dynamics of mixed quantum-classical systems, which is lacking in the molecular dynamics with quantum transitions surface-hopping approach commonly used for simulating PCET reactions. Within this approach, the protonic and electronic coordinates are treated quantum mechanically and the solvent coordinate evolves classically on both single adiabatic surfaces and on coherently coupled pairs of adiabatic surfaces. Both concerted and sequential PCET reactions are studied in detail under various subsystem-bath coupling conditions and insights into the dynamical principles underlying PCET reactions are gained. Notably, an examination of the trajectories reveals that the system spends the majority of its time on the average of two coherently coupled adiabatic surfaces, during which a phase enters into the calculation of an observable. In general, the results of this paper demonstrate the applicability of QCLE-based surface-hopping dynamics to the study of PCET and emphasize the importance of mean surface evolution and decoherence effects in the calculation of PCET rate constants.

Carbon dioxide (CO 2 )-enhanced oil recovery and sequestration are both processes that are associated with the separation and storage of gas in organic-rich shale formations. The current study investigates the applicability of kerogen, an amorphous and insoluble organic matter abundant in unconventional shale formations, for the separation of the mixture of gases (CO 2 and CH 4 ) in dry and wet (brine) conditions for an effective storage and injection operation. Here, through molecular dynamics, thermodynamics, and kinetics, we investigate the CO 2 transportation and adsorption behavior on three-dimensional kerogen molecular models from the Bakken, which contains nonperiodically arrayed functional groups. The diffusion/separation of CO 2 and CH 4 is probed subject to a varying range of concentrations as well as pressure from atmospheric to high (30 bar) and realistic temperatures (333−393 K) to represent an unconventional reservoir system. It is found that kerogen models from the Bakken would demonstrate an unprecedented CO 2 sorption selectivity over methane in the presence of brine (formation or interstitial water, a mixture of water and salt). Moreover, the concentration of brine shows a positive effect for CO 2 /CH 4 selectivity that supports our goals of sequestration and enhanced production. Based on the quantitative results, the developed kerogen model is suggested as an appropriate framework for CO 2 sequestration and injection to further facilitate hydrocarbon-improved recovery in organic-rich shale reservoirs and promote sequestration in a major shale formation.