We present a computational approach of general applicability to simulate the vibronic line shapes of absorption and electronic circular dichroism (ECD) spectra in rigid exciton-coupled dimers based ...on a time-dependent expression of the spectra and quantum dynamical calculations. We adopt a diabatic model of interacting states localized on the monomers whose electronic potential energy surfaces are described within harmonic approximation, including the effect of displacements, frequency changes, and normal-mode mixings. Spectra that fully account for the effect of all nuclear degrees of freedom of the system are obtained through a hierarchical representation of the Hamiltonian in blocks, defined so that few blocks accurately describe the short-time dynamics of the system. With this approach, on the ground of time-dependent density functional theory calculations, we simulate the absorption and ECD spectra of a covalent compound representing a “dimer” of anthracene, in the spectral region of the 1La monomer transition, obtaining results in good agreement with the experiment.
The excited-state behavior in the Franck-Condon (FC) region of the d(ApG) and d(GpA) dinucleoside monophosphate (A = adenine, G = guanine) is investigated in aqueous solution by means of ...time-dependent Density Functional Calculations, including solvent effects by the Polarizable Continuum Model and using three different functionals, i.e. PBE0, M052X, and CAM-B3LYP. Our analysis, focussed mainly on the two stacked dimers formed by 9-methyl-adenine and 9-methyl-guanine in the same arrangement as in the B structure of DNA (AG and GA), provides absorption and ECD spectra in very good agreement with their experimental counterparts. The sequence dependence of the dinucleoside excited-state properties is fully reproduced and it is explained on the grounds of the different interactions among the nucleobases' frontier orbitals existing in AG and GA. Our calculations predict that the 9Me-G --> 9Me-A charge transfer (CT) excited state is, at least, as stable as the lowest energy bright state of the dimers and that it is more stable than the intra-strand CT state in the (9Me-A)(2) stacked dimer. On the other hand, in AG and GA the 9Me-G --> 9Me-A CT state is significantly coupled to the dimer bright excited states and deviations from the decay rate predicted on the ground of the Marcus Theory are possible.
By using calculations rooted in the time dependent density functional theory (TD-DFT) we have investigated how the lowest energy excited states of a face-to-face π-stacked cytosine dimer vary with ...the intermonomer distance (R). The perfomances of different density functionals have been compared, focussing mainly on the lowest energy single excited state of the dimer (S₁)₂. TD-PBE0, TD-LC-ωPBE, and TD-M05-2X provide a picture very similar to that obtained at the CASPT2 level by Merchan et al. (J Chem Phys 2006, 125, 231102), predicting that (S₁)₂ has a minimum for R ~ 3 Å, with a binding energy of ~0.5 eV, whereas TD-B3LYP, TD-CAM-B3LYP, and TD-PBE understimate the binding energy. However, independently of the functional employed, no low-energy spurious charge transfer transitions are predicted by TD-DFT calculations, also when a nonsymmetric dimer is investigated, providing encouraging indications for the use of TD-DFT for studying the excited state of π-stacked nucleobases.
Abstract
Tax revenue in many low- and middle-income countries is inadequate for funding investments in public goods and human capital. With high levels of informality and limited state capacity, many ...tax authorities have difficulty determining the true tax base and collecting taxes efficiently and equitably. Tax authorities are increasingly adopting new technologies to improve administrative processes, reduce taxpayer compliance costs, and enhance their overall effectiveness. This paper reviews the recent literature on the use of technology for tax administration. It highlights the potential of technology to improve tax collection by helping to identify the tax base, monitor compliance, and facilitate compliance. It also identifies possible limitations to the use of technology arising from inadequate infrastructure and connectivity, lack of adoption or resistance by taxpayers and tax collectors, lack of institutional mainstreaming, and an unsupportive regulatory environment.
Vertical models for the simulation of spectroscopic line shapes expand the potential energy surface (PES) of the final state around the equilibrium geometry of the initial state. These models ...provide, in principle, a better approximation of the region of the band maximum. At variance, adiabatic models expand each PES around its own minimum. In the harmonic approximation, when the minimum energy structures of the two electronic states are connected by large structural displacements, adiabatic models can breakdown and are outperformed by vertical models. However, the practical application of vertical models faces the issues related to the necessity to perform a frequency analysis at a nonstationary point. In this contribution we revisit vertical models in harmonic approximation adopting both Cartesian (x) and valence internal curvilinear coordinates (s). We show that when x coordinates are used, the vibrational analysis at nonstationary points leads to a deficient description of low-frequency modes, for which spurious imaginary frequencies may even appear. This issue is solved when s coordinates are adopted. It is however necessary to account for the second derivative of s with respect to x, which here we compute analytically. We compare the performance of the vertical model in the s-frame with respect to adiabatic models and previously proposed vertical models in x- or Q 1-frame, where Q 1 are the normal coordinates of the initial state computed as combination of Cartesian coordinates. We show that for rigid molecules the vertical approach in the s-frame provides a description of the final state very close to the adiabatic picture. For sizable displacements it is a solid alternative to adiabatic models, and it is not affected by the issues of vertical models in x- and Q 1-frames, which mainly arise when temperature effects are included. In principle the G matrix depends on s, and this creates nonorthogonality problems of the Duschinsky matrix connecting the normal modes of initial and final states in adiabatic approaches. We highlight that such a dependence of G on s is also an issue in vertical models, due to the necessity to approximate the kinetic term in the Hamiltonian when setting up the so-called GF problem. When large structural differences exist between the initial and the final-state minima, the changes in the G matrix can become too large to be disregarded.
While much knowledge is being generated on the impact of the pandemic, we still know very little on its implications on taxation in lower-income countries. Yet, tax is crucial to fund crisis response ...and recovery, in addition to broader development plans and expanded government expenditure. This paper starts addressing this gap using an unique dataset of survey data from Rwanda. We document two significant shifts in taxpayers' views during the pandemic: perceptions about the fairness of the tax system improve by 40 per cent, and their attitudes to compliance become more conditional on the provision of public services of sufficiently good quality. We put these results in the broader context of crisis response. We show that they are not simply linked to individual experiences of the crisis or access to relief, but they are more likely linked to generalised improvements in solidarity and patriotism.
Enantiomerically pure alleno‐acetylenic oligomers of defined lengths were synthesized by the palladium‐mediated oxidative homocoupling of optically pure 1,3‐diethynylallenes. The large amplification ...of their chiroptical properties strongly suggests the formation of helical secondary structures. This assignment is supported by time‐dependent quantum chemical calculations.
Abstract
Many African countries struggle to collect an adequate amount of tax revenue to support needed investments in public services. This paper examines how African countries may take advantage of ...recent advances in technology to improve tax administration. It provides an overview of the potential and challenges of different tax categories in Africa: consumption taxes, real estate taxes, trade taxes and income taxes. It then describes the ways in which technology solutions may be deployed to address these challenges by helping to identify the tax base, monitor compliance and facilitate compliance. Lastly, it provides insights from interviews with senior tax administrators across the continent on their practical experiences in adopting technology for taxation.
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