Heavy-atom tunneling in organic transformations

Substitutional control of non-statistical dynamics in the thermal deazetization of tetracyclic azo compounds

Dynamical control of reactivity for the deazetization of endo,endo-9,10-diazatetracyclo[3.3.2.02,4.06,8]dec-9-ene (3) is studied using on-the-fly quasi-classical trajectory (QCT) calculations at the density functional theory (DFT) level. Two degenerate homotropilidenes, 4 and 5, are formed simultaneously from a single transition state (TS). The ratio of the cyclohexadienyl substituted product, 4, and the dynamical product, i.e. bridgehead substituted product, 5, can be neatly controlled by tuning the topology of the potential energy surface (PES). A steep descent post-TS favors the cyclohexadienyl substituted product while a shallow descent increases the dynamical outcome. Chemical demonstration of the same is achieved by symmetrical and asymmetrical substitution of functional groups along the cleaving (C3-C4) bond. Asymmetric mono-functionalization makes the PES broader, thereby reducing the slope post-TS. This creates a favourable situation for the dynamical products, 5b-5d, to become the major ones. On the contrary, symmetric bi-functionalization makes the cyclohexadienyl substituted product, 4m-4o, overwhelmingly (>85%) predominating. As a corollary to this phenomenon, substitution of the C3-C4 bond by the heavier isotopologues of H/C restricts its motion along the IRC path by the Newtonian kinetic isotope effect. This facilitates bond-opening along the C10-C11 dynamical pathway. Hence, for isotopic substitution, the situation is reversed and the bifunctionalized 3 is more dynamically activated. Simultaneous substitution by the heavier isotopologue of C and H causes deviation from the geometric mean of individual isotopic substitution towards the dynamical product, 5. Therefore, the dynamic control in 3 becomes prominent either via functional group asymmetry or through a Newtonian kinetic isotope effect for symmetric bifunctionalization.

Is Quantum Above-Barrier Reflection Important for Molecular Barrier Crossing Rates?

Understanding quantum tunneling and above-barrier reflection effects on unimolecular and bimolecular reaction rate constants remains challenging to this very day. In many applications, especially when considering moderate-to-high temperatures, the "standard" procedure is to use the parabolic barrier approximation. Recent work has shown though that this may be insufficient, and one cannot ignore anharmonicity. In this work, we study the analytic theory, including anharmonicity obtained when expanding the thermal rate up to order ℏ4. Such theories need high-order derivatives of the potential at the barrier top. We show that such derivatives are computed straightforwardly for six different reactions. We suggest a straightforward methodology for assessing whether the parabolic barrier approximation is valid and show that when the reaction asymmetry is large, this may lead to significant quantum above-barrier reflection and transmission coefficients, which are less than unity.

Quantum Tunneling in Reactions Modulated by External Electric Fields: Reactivity and Selectivity

Quantum tunneling and external electric fields (EEFs) can promote some reactions. However, the synergetic effect of an EEF on a tunneling-involving reaction and its temperature-dependence is not very clear. In this study, we extensively investigated how EEFs affect three reactions that involve hydrogen- or (ground- and excited-state) carbon-tunneling using reliable DFT, DLPNO-CCSD(T1), and variational transition-state theory methods. Our study revealed that oriented EEFs can significantly reduce the barrier and corresponding barrier width (and vice versa) through more electrostatic stabilization in transition states. These EEF effects enhance the nontunneling and tunneling-involving rates. Such EEF effects also decrease the crossover temperatures and quantum tunneling contribution, albeit with lower and thinner barriers. Moreover, EEFs can modulate and switch on/off the tunneling-driven 1,2-H migration of hydroxycarbenes under cryogenic conditions. Furthermore, our study predicts for the first time that EEF/tunneling synergy can control the chemo- or site-selectivity of one molecule bearing two similar/same reactive sites.

Heavy-Atom Tunneling in the Covalent/Dative Bond Complexation of Cyclo[18]carbon-Piperidine

Recent quantum chemical computations demonstrated the electron-acceptance behavior of this highly reactive cyclo[18]carbon (C₁₈) ring with piperidine (pip). The C₁₈–pip complexation exhibited a double-well potential along the N–C reaction coordinate, forming a van der Waals (vdW) adduct and a more stable, strong covalent/dative bond (DB) complex by overcoming a low activation barrier. By means of direct dynamical computations using canonical variational transition state theory (CVT), including the small-curvature tunneling (SCT), we show the conspicuous role of heavy atom quantum mechanical tunneling (QMT) in the transformation of vdW to DB complex in the solvent phase near absolute zero. Below 50 K, the reaction is entirely driven by QMT, while at 30 K, the QMT rate is too rapid (k_T ∼ 0.02 s^–1), corresponding to a half-life time of 38 s, indicating that the vdW adduct will have a fleeting existence. We also explored the QMT rates of other cyclo[n]carbon–pip systems. This study sheds light on the decisive role of QMT in the covalent/DB formation of the C₁₈–pip complex at cryogenic temperatures.

From vibrational spectroscopy and quantum tunnelling to periodic band structures – a self-supervised, all-purpose neural network approach to general quantum problems

A general, feedforward neural network strategy for the treatment of a broad range of quantum problems including rotational and vibrational spectroscopy, tunnelling and band structure calculations is presented in this study.

Stereoelectronic and dynamical effects dictate nitrogen inversion during valence isomerism in benzene imine

Non-classical processes such as heavy-atom tunneling and post transition-state dynamics govern stereoselectivity for benzene imine ⇌ 1H-azepine.

Protonation of Cytosine-Rich Telomeric DNA Fragments by Carboxylated Carbon Nanotubes: Insights from Computational Studies

In this work, we studied, using computational methods, the protonation reactions of telomeric DNA fragments being due to interaction with carboxylated carbon nanotubes. The applied computational methodology is divided into two stages. (i) Using classical molecular dynamics, we generated states in which carboxyl groups are brought to the vicinity of nitrogen atoms within the cytosine rings belonging to the DNA duplex. (ii) From these states, we selected two systems for systematic quantum chemical studies aimed at the analysis of proton-transfer reactions between the carboxyl groups and nitrogen atoms within the cytosine rings. Results of molecular dynamics calculations led to the conclusion that sidewall-functionalized carbon nanotubes deliver carboxyl groups slightly more effectively than the on-tip-functionalized ones. The latter can provide carboxyl groups in various arrangements and more diverse quality of approach of carboxyl groups to the cytosines; however, the differences between various arrangements of carboxyl groups are still not big. It was generally observed that narrow nanotubes can access the cytosine pocket easier than wider ones. Quantum chemical calculations led however to the conclusion that a direct proton transfer from the carboxyl group to the nitrogen atom within the cytosine ring is impossible under normal conditions. Precisely, we detected either very high activation barrier for the proton-transfer reaction or instability of the reaction product, i.e., its spontaneous decomposition toward reaction substrates.

Quantum mechanical tunnelling: the missing term to achieve sub-kJ mol-1 barrier heights

To predict barrier heights at low temperatures, it is not enough to employ highly accurate electronic structure methods. We discuss the influence of quantum tunnelling on the comparison of experimental and theoretical activation parameters (E_a, ΔH^‡, ΔG^‡, or ΔS^‡), since the slope-based experimental techniques to obtain them completely neglect the tunnelling component. The intramolecular degenerate rearrangement of four fluxional molecules (bullvalene, barbaralane, semibullvalene, and norbornadienylidene) were considered, systems that cover the range between fast deep tunneling and small but significant shallow tunnelling correction. The barriers were computed with the composite W3lite-F12 method at the CCSDT(Q)/CBS level, and the tunnelling contribution with small curvature tunnelling. While at room temperature the effect is small (∼1 kJ mol^-1), at low temperatures it can be considerable (in the order of tens of kJ mol^-1 at ∼80 K).

Hopping Conductance in Molecular Wires Exhibits a Large Heavy-Atom Kinetic Isotope Effect

We report a large kinetic isotope effect (KIE) for intramolecular charge transport in π-conjugated oligophenyleneimine (OPI) molecules connected to Au electrodes. ¹³C and ¹⁵N substitution on the imine bonds produces a conductance KIE of ∼2.7 per labeled atom in long OPI wires >4 nm in length, far larger than typical heavy-atom KIEs for chemical reactions. In contrast, isotopic labeling in shorter OPI wires <4 nm does not produce a conductance KIE, consistent with a direct tunneling mechanism. Temperature-dependent measurements reveal that conductance for a long ¹⁵N-substituted OPI wire is activated, and we propose that the exceptionally large conductance KIEs imply a thermally assisted, through-barrier polaron tunneling mechanism. In general, observation of large conductance KIEs opens up considerable opportunities for understanding microscopic conduction mechanisms in π-conjugated molecules.

Switch chemistry at cryogenic conditions: quantum tunnelling under electric fields

While the influence of intramolecular electric fields is a known feature in enzymes, the use of oriented external electric fields (EEF) to enhance or inhibit molecular reactivity is a promising topic still in its infancy. Herein we will explore computationally the effects that EEF can provoke in simple molecules close to the absolute zero, where quantum tunnelling (QT) is the sole mechanistic option. We studied three exemplary systems, each one with different reactivity features and known QT kinetics: π bond-shifting in pentalene, Cope rearrangement in semibullvalene, and cycloreversion of diazabicyclohexadiene. The kinetics of these cases depend both on the field strength and its direction, usually giving subtle but remarkable changes. However, for the cycloreversion, which suffers large changes on the dipole through the reaction, we also observed striking results. Between the effects caused by the EEF on the QT we observed an inversion of the Arrhenius equation, deactivation of the molecular fluxionality, and stabilization or instantaneous decomposition of the system. All these effects may well be achieved, literally, at the flick of a switch.