Anita Das

Renner-Teller intersections along the collinear axes of polyatomic molecules: H2CN as a case study.

The tetra-atomic C(2)H(2)(+) cation is known to form Renner-Teller-type intersections along its collinear axis. Not too long ago, we studied the nonadiabatic coupling terms (NACTs) of this molecule [G. J. Halász et al., J. Chem. Phys. 126, 154309 (2007)] and revealed two kinds of intersections. (i) By employing one of the hydrogens as a test particle, we revealed the fact that indeed the corresponding (angular) NACTs produce topological (Berry) phases that are equal to 2pi, which is a result anticipated in the case of Renner-Teller intersections. (ii) However, to our big surprise, repeating this study when one of the atoms (in this case a hydrogen) is shifted from the collinear arrangement yields for the corresponding topological phase a value that equals pi (and not 2pi). In other words, shifting (even slightly) one of the atoms from the collinear arrangement causes the intersection to change its character and become a Jahn-Teller intersection. This somewhat unexpected novel result was later further analyzed and confirmed by other groups [e.g., T. Vertesi and R. Englman, J. Phys. B 41, 025102 (2008)]. The present article is devoted to another tetra-atomic molecule, namely, the H(2)CN molecule, which just like the C(2)H(2)(+) ion, is characterized by Renner-Teller intersections along its collinear axis. Indeed, we revealed the existence of Renner-Teller intersections along the collinear axis, but in contrast to the C(2)H(2)(+) case a shift of one atom from the collinear arrangement did not form Jahn-Teller intersections. What we found instead is that the noncollinear molecule was not affected by the shift and kept its Renner-Teller character. Another issue treated in this article is the extension of (the two-state) Berry (topological) phase to situations with numerous strongly interacting states. So far the relevance of the Berry phase was tested for systems characterized by two isolated interacting states, although it is defined for any number of interacting states [M. V. Berry, Proc. R. Soc. London, Ser. A 392, 45 (1984)]. We intend to show how to overcome this limitation and get a valid, fully justified definition of a Berry phase for an isolated system of any number of interacting states (as is expected).

Polyradical Character of Triangular Non-Kekulé Structures, Zethrenes, p-Quinodimethane-Linked Bisphenalenyl, and the Clar Goblet in Comparison: An Extended Multireference Study.

In this work, two different classes of polyaromatic hydrocarbon (PAH) systems have been investigated in order to characterize the amount of polyradical character and to localize the specific regions of chemical reactivity: (a) the non-Kekulé triangular structures phenalenyl, triangulene and a π-extended triangulene system with high-spin ground state and (b) PAHs based on zethrenes, p-quinodimethane-linked bisphenalenyl, and the Clar goblet containing varying polyradical character in their singlet ground state. The first class of structures already have open-shell character because of their high-spin ground state, which follows from the bonding pattern, whereas for the second class the open-shell character is generated either because of the competition between the closed-shell quinoid Kekulé and the open-shell singlet biradical resonance structures or the topology of the π-electron arrangement of the non-Kekulé form. High-level ab initio calculations based on multireference theory have been carried out to compute singlet–triplet splitting for the above-listed compounds and to provide insight into their chemical reactivity based on the polyradical character by means of unpaired densities. Unrestricted density functional theory and Hartree–Fock calculations have been performed for comparison also in order to obtain better insight into their applicability to these types of complicated radical systems.

A tri-atomic Renner-Teller system entangled with Jahn-Teller conical intersections

The present study concentrates on a situation where a Renner-Teller (RT) system is entangled with Jahn-Teller (JT) conical intersections. Studies of this type were performed in the past for contours that surround the RT seam located along the collinear axis [see, for instance, G. J. Halász, Á. Vibók, R. Baer, and M. Baer, J. Chem. Phys. 125, 094102 (2006)]. The present study is characterized by planar contours that intersect the collinear axis, thus, forming a unique type of RT-non-adiabatic coupling terms (NACT) expressed in terms of Dirac-δ functions. Consequently, to calculate the required adiabatic-to-diabatic (mixing) angles, a new approach is developed. During this study we revealed the existence of a novel molecular parameter, η, which yields the coupling between the RT and the JT NACTs. This parameter was found to be a pure number η = 22/π (and therefore independent of any particular molecular system) and is designated as Renner-Jahn coupling parameter. The present study also reveals an unexpected result of the following kind: It is well known that each (complete) group of states, responsible for either the JT-effect or the RT-effect, forms a Hilbert space of its own. However, the entanglement between these two effects forms a third effect, namely, the RT/JT effect and the states that take part in it form a different Hilbert space.

Dressed adiabatic and diabatic potentials to study conical intersections for F + H2

We follow a suggestion by Lipoff and Herschbach [Mol. Phys. 108, 1133 (2010)] and compare dressed and bare adiabatic potentials to get insight regarding the low-energy dynamics (e.g., cold reaction) taking place in molecular systems. In this particular case, we are interested to study the effect of conical intersections (ci) on the interacting atoms. For this purpose, we consider vibrational dressed adiabatic and vibrational dressed diabatic potentials in the entrance channel of reactive systems. According to our study, the most one should expect, in case of F + H(2), is a mild effect of the (1, 2) ci on its reactive/exchange process--an outcome also supported by experiment. This happens although the corresponding dressed and bare potential barriers (and the corresponding van der Waals potential wells) differ significantly from each other.

Dressed adiabatic and diabatic potentials for the Renner-Teller/Jahn-Teller F + H2 system.

We follow a suggestion by Lipoff and Herschbach (Mol. Phys. 2010, 108, 1133) and compare dressed potentials to get insight regarding the low-energy dynamics (e.g., cold reaction) taking place in molecular systems. In this particular case we are interested in studying the effect of topological effects on the interacting atoms. For this purpose we consider dressed adiabatic and adiabatic-via-dressed diabatic potentials in the entrance channel of reactive systems. In a recent study of this kind for the F + H2 system (J. Chem. Phys. 2012, 136, 054104), we revealed that a single Jahn-Teller conical intersection is expected to have only a mild effect on the dynamics. This fact implies that the Born-Oppenheimer approximation is expected to be valid for this system at least for low enough energies. In the present article this study is extended to include also the Renner-Teller effect as produced by the two lower degenerate Π states. As a result we consider three electronic states which enforce the use of the adiabatic-to-diabatic transformation (ADT) matrix A. The results indicate that the topological effects as produced by the extended Renner/Teller-Jahn/Teller system are strong to the level that, most likely, abolishes the Born-Oppenheimer approximation for this system, all this in contrast to our previous findings (see above publication).

Local electron correlation treatment in extended multireference calculations: Effect of acceptor-donor substituents on the biradical character of the polycyclic aromatic hydrocarbon heptazethrene

The implementation of a local correlation (LC) treatment of multireference (MR) configuration interaction approaches within the COLUMBUS program system is reported. The LC treatment is based on the weak pairs approximation of Sæbø and Pulay (Ann. Rev. Phys. Chem. 1993, 44, 213) and a geometrical analysis of Walter et al. (Chem. Phys. Lett. 2001, 346, 177). The removal of simultaneous single excitations out of the weak pairs is based on the reference doubly occupied space only, leading to a straightforward program implementation and a conceptual simplicity in terms of well-defined localized orbitals. Reductions of up to an order of magnitude in the configuration space expansion and in computer time for the Davidson diagonalization step are found. The selection of the active and the virtual orbital spaces is not affected by this procedure. This treatment is successfully applied to the singlet biradical heptazethrene and its different acceptor-donor substituents: 4,12-dicyanoheptazethrene, 4,12-diaminoheptazethrene, and 4-amino-12-cyanoheptazethrene. Simultaneous insertion of pairs of donor and acceptor groups increases the biradical character; for push-pull substitution, this effect is significantly smaller. In addition, results obtained from spin-corrected unrestricted density functional theory calculations are supported by our MR calculations.

Kinetics of the Strain-Promoted Oxidation-Controlled Cycloalkyne-1,2-quinone Cycloaddition: Experimental and Theoretical Studies

Stimulated by its success in both bioconjugation and surface modification, we studied the strain-promoted oxidation-controlled cycloalkyne–1,2-quinone cycloaddition (SPOCQ) in three ways. First, the second-order rate constants and activation parameters (ΔH⧧) were determined of various cyclooctynes reacting with 4-tert-butyl-1,2-quinone in a SPOCQ reaction, yielding values for ΔH⧧ of 4.5, 7.3, and 12.1 kcal/mol, for bicyclo[6.1.0]non-4-yne (BCN), cyclooctyne (OCT), and dibenzoazacyclooctyne (DIBAC), respectively. Second, their reaction paths were investigated in detail by a range of quantum mechanical calculations. Single-configuration theoretical methods, like various DFT and a range of MP2-based methods, typically overestimate this barrier by 3–8 kcal/mol (after inclusion of zero-point energy, thermal, and solvation corrections), whereas MP2 itself underestimates the barrier significantly. Only dispersion-corrected DFT methods like B97D (yielding 4.9, 6.4, and 12.1 kcal/mol for these three reactions) and high-level CCSD(T) and multireference multiconfiguration AQCC ab initio approaches (both yielding 8.2 kcal/mol for BCN) give good approximations of experimental data. Finally, the multireference methods show that the radical character in the TS is rather small, thus rationalizing the use of single-reference methods like B97D and SCS-MP2 as intrinsically valid approaches.

Topological effects in low-lying electronic states of linear N2H2+ and HBNH+ associated with onset of bending

ABSTRACT The current decade has seen illuminating and extensive study of interplay of topological effects such as Renner–Teller and Jahn–Teller (JT) effects in small molecules. Study of such effects in HCNH molecule showed an interesting feature that for slightly bent system, a pair of JT conical interactions (CIs) appear only for certain nonplanar configuration, in contrast to its appearance in CS configuration (molecular plane) for C2H2 +. Moreover, since the feature of appearance of JT-CIs in two 2Σ+ states of HCNH in some bent configuration may be critically associated to the abundance of HNC in interstellar spaces, as advocated recently by Das and Mukhopadhyay (J. Phys. Chem. 117, 8680 (2013)), the interest in such topological studies have remained relevant. In this article we have investigated such topological effects for slightly bent N2H2 + and HBNH+. Interestingly, the appearance of JT-CIs are, for HBNH+, in molecular plane while for N2H2 +, in certain nonplanar configuration, in sharp contrast to the effect in the otherwise similar pair of molecular systems, C2H2 + and HCNH. GRAPHICAL ABSTRACT

Investigation of plausible mechanistic pathways in hydrogenation of η5-(C5H5)2Ta(H)=CH2: an analysis using DFT and AIM techniques

AbstractIn this manuscript, we investigate two plausible pathways for addition of H2 across the bond Ta=C in η5-(C5H5)2Ta(H)=CH2. One of the investigated reaction pathways involves a single concerted step with a four-membered transition state keeping the oxidation state of tantalum unaltered, where as the other pathway deals with a two step reaction with α-insertion of H2 to produce a 16e− Ta(III)–methyl species and a subsequent oxidative addition. We must emphasize that an experimental study by Bregel et al. [J Am Chem Soc 2002, 124:13827–13832] on a derivative of the investigated chemical system in the present study showed that the two step strategy of α-insertion followed by subsequent oxidative addition is the preferred one. Our numerical investigations using DFT and AIM calculations lead to a similar conclusion. To establish our conclusion, we employ various basis sets to obtain the free energy of activation of the reaction. The AIM technique especially helps us to characterize the bond critical points at the optimized geometries of the reactants, products, transition states, and intermediates for the two step mechanism. FigurePathways in hydrogenation of η5-(C5H5)2Ta(H)=CH2

Dependence of the knock-on electron cross-section on muon energy in high, medium and low Z elements

Using the recent experimental results of various authors the dependence of the knock-on electron cross-section on primary muon energy has been studied in detail. It is found that the knock-on production cross-section increases with primary muon energy upto at least 32 GeV in accordance with Bhabhas theory for energy transfers greater than 15 MeV.