Contrasting Dynamics in Isoelectronic Anions Formed by Electron Attachment

Cyanogen NCCN and cyanoacetylene HCCCN are isoelectronic molecules, and as such, they have many similar properties. We focus on the bond cleavage in these induced by the dissociative electron attachment. In both molecules, resonant electron attachment produces CN- with very similar energy dependence. We investigate the very different dissociation dynamics, in each of the two molecules, revealed by velocity map imaging of this common fragment. Different dynamics are manifested both in the excess energy partitioning and in the angular distributions of fragments. Based on the comparison with electron energy loss spectra, which provide information about possible parent states of the resonances (both optically allowed and forbidden excited states of the neutral target), we ascribe the observed effect to the distortion of the nuclear frame during the formation of core-excited resonance in cyanoacetylene. The proposed mechanism also explains a puzzling difference in the magnitude of the CN- cross section in the two molecules which has been so far unexplained.

Prebiotic chemical origin of biomolecular complementarity

The early Earth, devoid of the protective stratospheric ozone layer, must have sustained an ambient prebiotic physicochemical medium intensified by the co-existence of shortwave UV photons and very low energy electrons (vLEEs). Consequently, only intrinsically stable molecules against these two co-existing molecular destructors must have proliferated and thereby chemically evolved into the advanced molecules of life. Based on this view, we examined the stability inherent in nucleobases and their complementary pairs as resistance to the molecular damaging effects of shortwave UV photons and vLEEs. This leads to the conclusion that nucleobases could only proliferated as their complementary pairs under the unfavorable prebiotic conditions on early Earth. The complementary base pairing not only enhances but consolidates the intrinsic stability of nucleobases against short-range UV photons, vLEEs, and possibly many as-yet-unknown deleterious agents co-existed in the prebiotic conditions of the early Earth. In short, complementary base pairing is a manifestation of chemical evolution in the unfavorable prebiotic medium created by the absence of the stratospheric ozone layer.

Investigating Possible Dipole-Bound States of Cyanopolyynes: the Case for the C5 N- Anion Detected in Interstellar Space

We present results of quantum structure calculations aimed at demonstrating the possible existence of dipole-bound states (DBS) for the anionC 5 N - ${{\rm{C}}_5 {\rm{N}}^ - }$ , a species already detected in the Interstellar medium (ISM). The positive demonstration of DBS existence using ab initio studies is an important step toward elucidating possible pathways for the formation of the more tightly bound valence bound states (VBS) in environments where free electrons from starlight ionization processes are known to be available to interact with the radical partner of the title molecule. Our current calculations show that such excited DBS states can exist inC 5 N - ${{\rm{C}}_5 {\rm{N}}^ - }$ , in agreement with what we had previously found for the smallercyanopolyyne in the series: theC 3 N - ${{\rm{C}}_3 {\rm{N}}^ - }$ anion. This system has a very weakly bound anion with binding energies of about 3 and 9 cm-1 for the1 Σ + ${^1 \Sigma ^ + }$ and3 Σ + ${^3 \Sigma ^ + }$ DBS, respectively.

Role of the Electron-Dipole Interaction in Photodetachment Angular Distributions

The importance of including long-range electron-molecule interactions in treatments of photodetachment and/or photoionization is demonstrated. A combined experimental and computational study of CN^- detachment is presented in which near threshold anisotropy parameters (β) are measured via photoelectron imaging. Calculated β values, based on an EOM-IP-CCSD/aug-cc-pVTZ Dyson orbital, are obtained using free-particle and point dipole models. The results demonstrate the influence of the molecular dipole moment in the detachment process and provide an explanation of the recently reported near threshold behavior of the overall photodetachment cross section in CN^- detachment.

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.

Density Functional Prediction of Quasiparticle, Excitation, and Resonance Energies of Molecules With a Global Scaling Correction Approach

Molecular quasiparticle and excitation energies determine essentially the spectral characteristics measured in various spectroscopic experiments. Accurate prediction of these energies has been rather challenging for ground-state density functional methods, because the commonly adopted density function approximations suffer from delocalization error. In this work, by presuming a quantitative correspondence between the quasiparticle energies and the generalized Kohn–Sham orbital energies, and employing a previously developed global scaling correction approach, we achieve substantially improved prediction of molecular quasiparticle and excitation energies. In addition, we also extend our previous study on temporary anions in resonant states, which are associated with negative molecular electron affinities. The proposed approach does not require any explicit self-consistent field calculation on the excited-state species, and is thus highly efficient and convenient for practical purposes.

Structural properties of possible interstellar valence anions of the series HCnN- (n = 3, 5, 7, 9)

In this contribution we investigate the structural properties of stable anions of small carbon clusters, with one nitrogen and one hydrogen atoms attached to the C-cluster, to surmise their possible existence in the Interstellar Medium (ISM). Many possible configurational (geometrical) isomers with positive vertical electron detachment values are presented, and arranged according to their relative energy. Specific attention is paid to the structures of the lowest-energy valence isomers, the chain-structures of HC₇N^- and HC₉N^- anions with quasilinear and linear geometry, respectively. They exhibit relatively large permanent dipole moments (2.697 and 5.034 Debye) and adiabatic electron affinities (AEAs) of 1.13 and 1.35 eV. Isomers of the HNC_n^- family and many branched structures are possible stable species viable for detection in the ISM.

The quest to uncover the nature of benzonitrile anion

Anionic states of benzonitrile are investigated by high-level electronic structure methods.

EOM-CC guide to Fock-space travel: the C2 edition

Electronic structure calculations for C₂, C₂⁻, and C₂²⁻ using the CC/EOM-CC family of methods. Results illustrate that EOM-CCSD provides an attractive alternative to MR approaches.

Real and Imaginary Excitons: Making Sense of Resonance Wave Functions by Using Reduced State and Transition Density Matrices

Within non-Hermitian quantum mechanics, metastable electronic states can be represented by isolated L²-integrable complex-valued wave functions with complex energies. An analysis scheme of the real and imaginary parts of resonance wave functions by using reduced transition density matrices and natural transition orbitals is presented. While the real parts of excitons describe changes in the electron density corresponding to the bound part of the resonance, the imaginary excitons can be interpreted as virtual states facilitating one-electron decay into the continuum. The different nature of real and imaginary excitons is revealed by exciton descriptors, in particular hole-particle separation and their correlation. Singular values and respective participation ratios quantify the extent of collectivity of the excitation and a number of distinct decay channels. The utility of the new tool is illustrated by the analysis of bound and metastable excited states of cyanopolyyne anions.