Using recently developed full-dimensional coupled quasi-diabatic ab initio potential energy surfaces including four electronic (1ππ, 1ππ*, 11πσ*, and 21πσ*) states, the tunneling dynamics of phenol photodissociation via its first excited singlet state (S1 ← S0) is investigated quantum mechanically using a three-dimensional model. The lifetimes of several low-lying vibrational states are examined and compared with experiment. The deuteration of the phenoxyl hydrogen is found to dramatically increase the lifetime, attesting to the tunneling nature of the nonadiabatic dissociation. Importantly, it is shown that owing to the conical intersection topography tunneling in this system cannot be described in the standard adiabatic approximation, which eschews the geometric phase effect since the nonadiabatically computed lifetimes, validated by comparison with experiment, differ significantly from those obtained in that limit.

•Importance of conical intersections in the ground state.•Low-lying axial and equitorial structures including conical intersections.•Spin–orbit coupling similar to that in methoxy.•Conical Intersection topography differs from that of methoxy, ethoxy and isopropoxy.Previous descriptions of the cyclopentoxy radical have considered the ground state as an isolated electronic state; however, the doublet ground and first excited states of the cyclopentoxy radical arise from three electrons in the oxygen 2pπ orbitals, yielding states that may not be adequately described by a single state model. In this work, a prelude to the determination of the photoelectron spectrum of cyclopentoxide, the electronic structure of the ground and first excited state of cyclopentoxy is studied. Both axial and equatorial structures are considered. The ground state is found to have an axial configuration. Conical intersections of the ground and excited state potential energy surfaces in both axial and equatorial nuclear configurations are found, with the minimum energy axial (equatorial) conical intersection lying within ∼515 (260) cm−1 of the axial (equatorial) ground state minimum. The barriers to axial–equatorial interconversion and to ring opening are estimated. The implications for the simulation of the cyclopentoxide photoelectron spectrum are discussed.Graphical abstract

Diabatizations achieved by diagonalization of a property operator or as the extremum of a molecular property are numerous and widely used, although for a particular system a given property method may have limited accuracy or even fail catastrophically. These failures are usually analyzed in terms of limitations of the chosen property or method. Here we introduce an alternative perspective, failure attributable to singularities in the defining equations. The singular subspace is analogous to the conical intersection seam in potential energy surfaces. Using the archetypical NH3 nonadiabatic photodissociation, it is shown that for two states the diabatization condition has singularities on a subspace of dimension N – 2, where N = 3Natom – 6, is the number of internal coordinates. This singular subspace is distinct from the N – 2-dimensional seam of conical intersections of the electronic Hamiltonian and results incorrectly, in singular derivative couplings between diabatic states in unexpected regions of nuclear coordinate space. Simple indicators are developed that provide ways to anticipate and avoid these singularities.

The nonadiabatic photodissociation CH2OH(12A) + hv → CH2OH(2,32A) → CH2O + H or HCOH(cis or trans) + H is addressed using trajectory surface hopping dynamics on a quasi-diabatic representation, Hd, of the 1,2,32A coupled, adiabatic potential energy surfaces. We focus on dynamics originating on the 22A potential energy surface. The Hd is based exclusively on electronic structure data obtained from a multireference configuration interaction single and double excitation expansion, composed of over 67 million configuration state functions, and treats all nine internal degrees of freedom in an even-handed manner. Each simulation is based on bundles of 10000 trajectories randomly selected from harmonic Wigner distributions and propagated for up to 1 ps. The bimodal distribution in the kinetic energy release spectrum is explained in terms of direct versus quasi-statistical dissociation.

Full-dimensional state-to-state quantum dynamics of the photodissociation of NH3(Ã1A2″) is investigated on newly developed coupled diabatic potential energy surfaces. For the first time, the rovibrational distributions of the nonadiabatically produced NH2(X̃2B1) product have been determined quantum mechanically. In agreement with experimental observations, NH2(X̃2B1) produced from the 00 and 21 states of NH3(Ã1A2″) was found to be dominated by its ground vibrational state with an N = Ka propensity, shedding light on the quantum-state-resolved nonadiabatic dynamics facilitated by conical intersections and setting the stage for the elucidation of vibrationally mediated photodissociation.Keywords: conical intersection; nonadiabatic transition; photodissociation; product state distribution; reaction dynamics;

Vibrationally mediated photodissociation of NH3 and ND3 in the A band allows the exploration of the excited-state potential energy surface in regions that are not accessible from the ground vibrational state of these polyatomic systems. Using our recently developed coupled ab initio potential energy surfaces in a quasi-diabatic representation, we report here a full-dimensional quantum characterization of the à ← X̃ absorption spectra for vibrationally excited NH3 and ND3 and the corresponding nonadiabatic dissociation dynamics into the NH2(Ã2A1) + H and NH2(X̃2B1) + H channels. The predissociative resonances in the absorption spectra have been assigned with appropriate quantum numbers. The NH2(Ã2A1)/NH2(X̃2B1) branching ratio was found to be mildly sensitive to the initial vibrational excitation prior to photolysis. Implications for interpreting experimental data are discussed.

Using multireference configuration interaction wave functions composed of 17–52 million configuration state functions, 18 points on the 12A–22A seam and 162 points on the 22A–32A seam of conical intersections relevant to the collisional quenching of OH(A2Σ+) by H2 are determined and analyzed. In the vicinity of planar nuclear configurations, the former seam corresponds to a 12A′–12A″ seam of intersection and the latter corresponds to a 12A′–22A′ seam. For the previously studied 22A–32A seam, two regions not previously examined are reported: (i) an out-of-plane region that connects smoothly to the 12A′–22A′ seam for planar structures and (ii) a Rydberg region that includes D3h/C3v structures where the 22A–32A seam is a 22A′–32A′ seam for D3h structures. Some of the nonplanar points on the 22A–32A seam of conical intersection are found to have OH and H2 distances comparable to those of the reactant molecules and energies below that of the reactant asymptote. These nonplanar entrance channel conical intersections suggest new mechanisms for the quenching reaction. The Rydberg region introduces new connectivity and symmetry issues. For the 12A–22A [12A′–12A″] seam, which unlike the 22A–32A [12A′–22A′] seam cannot continuously deform from planar to nonplanar structures except through confluences, no evidence of nonplanar points on the conical intersection seam was found. The continuous conical parameters, gI,J, hI,J, sxI,J, and syI,J and the associated vectors gI,J, hI,J, and sI,J, are determined and discussed. The conical parameters are made continuous by a prescribed rotation of the degenerate wave functions. The continuity of these conical parameters makes it possible to construct a quasi-diabatic representation of the coupled adiabatic potential energy surfaces.

The low energy photoionization spectrum of propyne (CH3–CCH), which reveals the vibronic structure of the propyne cation, is simulated using vibronic coupling theory. The spin–orbit interaction is included using an intensity borrowing approach, enabling determination of the (X̃2E1/2,3/2, v = 0) splitting and the relative photoionization intensity of these closely spaced levels. The results are compared with recent experimental studies and misstatements are corrected.