The electronic states of 1,2,5-thiadiazole studied by VUV absorption spectroscopy and ab initio configuration interaction methods

Abstract

The 1,2,5-thiadiazole VUV absorption spectrum over the range 5-12 eV has been obtained for the first time. It shows broad bands centred near 5. 0, 7.2, 7.7, 8.7, 9.6 and 10.6 eV; several of these show well resolved vibrational structure. A number of valence states and Rydberg states relating to the first and second ionisation energies have been identified. The doubling of some bands in the 4.7 eV region (previously reported) is confirmed; a 3s Rydberg state shows a similar phenomenon, which we also attribute to a non-planar upper state; a study of the (planar) equilibrium geometry of the corresponding triplet Rydberg state shows it is a saddle point. Electronic excitation energies for valence (singlet and triplet) and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple-zeta + double polarisation basis set, augmented by diffuse (Rydberg) orbitals. The theoretical study shows the nature of the more intense Rydberg state types, and positions of the main valence and Rydberg bands. There is generally a good correlation between the theoretical intensities and the experimental envelope. Reconsideration of the order of cationic states in the UV-photoelectron spectrum, crucial to the Rydberg state interpretation, has led to the sequence: 2 B 1 < 2 B 2 < 2 A 2 < 2 A 1 . Study of the excitation energies to specific upper states, also supports this order. The value of IE 1 is refined to 10.111 eV. Equilibrium structures show the p-and r-cations are planar, in contrast to the lowest triplet state.

Excitation spectra of aromatic molecules within a real-spaceGW-BSE formalism: Role of self-consistency and vertex corrections

“…[80][81][82] The mean absolute difference across these three molecules is 0.78 eV for G 0 W 0 -BSE, 0.68 eV for G 0 W 0 Γ LDA , 0.14 for mixed GW -BSE, 0.25 for QSGW -BSE, 0.24 for QSGW Γ LDA -BSE, and 0.25 for TDLDA. For 1,2,5-thiadiazole, the cited calculation only include singles and doubles excitations, 83 and the mean absolute difference is 1.26 eV for G 0 W 0 -BSE, 1.14 eV for G 0 W 0 Γ LDA , 0.58 eV for mixed GW -BSE, 0.33 eV for QSGW -BSE, 0.29 for GW Γ LDA -BSE, and 0.61 for TDLDA. BSE calculations that follow perturbative GW calculations are all smaller than the excitation energies from the best previous theoretical calculations.…”

Assessment of cytotoxic and cytogenetic effects of a 1,2,5-thiadiazole derivative on CHO-K1 cells. Its application as corrosion inhibitor

“…Triaromatic rings are the support of some of the most efficient and selective ligands for asymmetric catalysis [6]. Polyaromatics also have characteristic physical properties which could lead to their application as organic conductors or semiconductors [7,8]. Moreover, heterocyclic structural moieties are also found in some biomaterials, colorants and pigments [9].…”

Dissociation pathways for the molecular cation of 3,4‐dichloro‐1,2,5‐thiadiazole: A time‐of‐flight mass spectrometry and computational study

“…Various experimental techniques such as microwave spectroscopy and gas‐phase electron diffraction confirmed the planar structure of these thiadiazoles . Also, various theoretical studies pertaining to the structure and stabilities including its radical cation are also available in the literature . For thiadiazole, there are mainly four isomeric forms such as 1,2,3‐thiadiazole; 1,2,4‐thiadiazole; 1,2,5‐thiadiazole and 1,3,4‐thiadiazole which occur in nature.…”

“…The photochemistry and spectroscopy of 1,2,5‐thiadiazoles have been the subject of investigations, both experimentally and theoretically, especially in the UV region. The recent work by Palmer includes the experimental studies related to the UV‐VUV absorption spectrum in the gas phase, and the theoretical assignments of various observed band systems . These spectra show vibronic structure in the first band which originate from a π‐π* transition.…”