The vacuum ultraviolet absorption spectrum of norbornadiene: Vibrational analysis of the singlet and triplet valence states of norbornadiene by configuration interaction and density functional calculations.

Michael H Palmer,Cesare Grazioli,Søren Vrønning Hoffmann,R Alan Aitken,Monica De Simone,Nykola C Jones,Marcello Coreno

doi:10.1063/5.0053962

Abstract

A synchrotron-based vacuum ultraviolet (VUV) absorption spectrum of norbornadiene (NBD) is reported, and the extensive vibrational structure obtained has been analyzed. The previously known 5b13s-Rydberg state has been reinterpreted by comparison with our recent high-resolution photoelectron spectral analysis of the X2B1 ionic state. Additional vibrational details in the region of this Rydberg state are observed in its VUV spectrum when compared with the photoelectron 2B1 ionic state; this is attributed to the underlying valence state structure in the VUV. Valence and Rydberg state energies have been obtained by configuration interaction and time-dependent density functional theoretical methods. Several low-lying singlet valence states, especially those that arise from ππ* excitations, conventionally termed NV1 to NV4, have been examined in detail. Their Franck-Condon (FC) and Herzberg-Teller (HT) profiles have been investigated and fitted to the VUV spectrum. Estimates of the experimental 00 band positions have been made from these fits. The anomaly of the observed UV absorption by the 1A2 state of NBD is attributed to HT effects. Generally, the HT components are less than 10% of the FC terms. The calculated 5b13s lowest Rydberg state also shows a low level of HT components. The observed electron impact spectra of NBD have been analyzed in detail in terms of triplet states.

Highlights

We reported synchrotron-based, high-resolution photoelectron spectra (PES) for both norbornadiene (NBD) and quadricyclane (QC).[1]
Our recent PES study[1] of NBD showed much more vibrational detail than in earlier studies. Overlay of this new PES with its theoretical assignment, as in the supplementary material under SM4, onto our ultraviolet absorption (UV) + VUV absorption spectrum enables us to pinpoint the origins of several Rydberg states more precisely; this is performed below
A direct comparison of the earlier UV absorption, resonant multiphoton ionization (REMPI) spectrum, and our VUV absorption, shown in the supplementary material, SM3, demonstrates that our VUV absorption spectrum shows all the features observed in the REMPI spectra, together with the additional sub-structure

Summary

Introduction

We reported synchrotron-based, high-resolution photoelectron spectra (PES) for both norbornadiene (NBD) and quadricyclane (QC).[1]. The PES for both compounds were previously analyzed in considerable detail by a combination of Møller–Plesset fourth order perturbation theory, which included single, double, and quadruple [MP4(SDQ)] substitutions, configuration interaction (CI), multi-configuration self-consistent field (MCSCF), and density functional theoretical (DFT) methods. Both are isoelectronic with cycloheptatriene (CHT), which we studied recently, but lack conjugated double bonds.[2,3] Our theoretical approaches will be similar to those that were successful with CHT

Methods

Results

Conclusion