Abnormal abundance of HNC in interstellar spaces has been the motivation of many experimental as well as theoretical studies of the branching ratio [HNC]/[HCN] in the dissociation of HCNH, right after its formation from electron capture by HCNH+, available in the upper atmosphere. In the present work we were interested in nonadiabatic studies involving the dissociation channel of HCNH leading to the formation of HNC. This study reports for the first time that the conical intersection (CI) between the states 1 2Σ+ and 22Σ+ exists only in some bent geometry (and not in the collinear geometry) where both of these states have A′ symmetry. This finding is important as this CI is crucial in the dissociation of HCNH. We further report that these two states strongly couple with 12Π, the lowest electronic state of collinear HCNH. Hence we construct a three-state Hilbert subspace (HSS), comprising of the states 12Π, 12Σ+, and 2 2Σ+, in a configuration space where these states interact very strongly and the adiabatic-to-diabatic transformation angles (mixing angle) yield meaningful values of topological (Berry) phase. This leads to the construction of the corresponding three-state diabatic potentials. We advocate that these diabatic potentials, considering both the linear as well as bent configurations, nicely elucidate the formation of the HNC molecule by the CH bond dissociation of HCNH molecule. © 2013 American Chemical Society.

DEBASIS MUKHOPADHYAY

Chemistry

A DAS

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

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The University of Calcutta has secured the Fifth position among the Universities of India in the prestigious &quot;Indian University Ranking 2019&quot; list, released by the NIRF of the Ministry of Human Resource Development, Government of India.

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It was established on 24 January 1857, and was one of the first institutions in Asia to be established as a multidisciplinary and Western-style university. Its alumni and faculty include several heads of state, heads of government, social reformers, prominent artists, only academy award winner and Dirac medal winner in India, many Fellows of the Royal Society and five Nobel laureates as of 2019 which is highest in South Asia.

University Of Calcutta

Journal of Physical Chemistry A

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. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.

Molecular Physics

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

We perform ab initio calculation using quantum chemistry package (MOLPRO) on the excited states of Na3 cluster and present the adiabatic PESs for the electronic states 22E′ and 12A1′, and the non-adiabatic coupling (NAC) terms among those states. Since the ab initio calculated NAC elements for the states 22E′ and 12 A1′ demonstrate the numerical validity of so called Curl Condition, such states closely form a sub-Hilbert space. For this subspace, we employ the NAC terms to solve the adiabatic-diabatic transformation (ADT) equations to obtain the functional form of the transformation angles and pave the way to construct the continuous and single valued diabatic potential energy surface matrix by exploiting the existing first principle based theoretical means on beyond Born-Oppenheimer treatment. Nuclear dynamics has been carried out on those diabatic surfaces to reproduce the experimental spectrum for system B of Na3 cluster and thereby, to explore the numerical validity of the theoretical development on beyond Born-Oppenheimer approach for adiabatic to diabatic transformation. © 2011 American Institute of Physics.

Fulltext

Journal of Chemical Physics

Ab initio calculations on the excited states of Na3 cluster to explore beyond Born-Oppenheimer theories: Adiabatic to diabatic potential energy surfaces and nuclear dynamics

The tetra-atomic C2 H2+ 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, 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 2π , 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 π (and not 2π ). 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 H2 CN molecule, which just like the C2 H2+ 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 C2 H2+ 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). © 2010 American Institute of Physics.

Journal	Journal of Physical Chemistry A
Publisher	AMER CHEMICAL SOC
ISSN	1089-5639
Open Access	No