Abul Kalam Biswas

First Principles Studies toward the Design of Silylene Superbases: A Density Functional Theory Study

In this paper we have reported for the first time some designed silylene superbases using DFT calculations. These divalent Si(II) compounds can act as powerful neutral organic superbases in the gas phase and in the solvent phase. The DFT calculations performed with the B3LYP/6-311+G**//B3LYP/6-31+G* level of theory showed that one of the designed silylene derivatives :Si(N═PY3)2 [Y = -N═C(NMe2)2] (8) can fall in the range of hyperbase with gas phase proton affinity ∼310 kcal/mol. In THF the calculated proton affinity of 8 was found to be 327.5 kcal/mol. The proton affinities computed at the B3LYP/6-311+G**//B3LYP/6-31+G* level for some simple silylenes have been found to be good agreement with the corresponding experimentally measured values. Phosphazene groups attached to the divalent silicon center of silylenes enhanced the basicity of the Si center significantly and further acted as a second protonation site. The calculated second proton affinity of the silylene derivative, 8 in THF was found to be 285.5 kcal/mol. We have shown that the dimerization and cyclization of such silyene superbases were less likely and the monomeric forms would be more stable than their corresponding dimers. The calculated proton affinities also showed a good correlation with the HOMO-LUMO energy gap and energy difference between the singlet and triplet states (ΔES-T) of the silylene systems. The isodesmic reactions have been employed to examine the stability of the silylene molecules by calculating the silylene stabilization energy (SiSE). The reactivity of silylene molecules has been presented in terms of the nucleophilicity, electronegativity, and hardness of such systems. The Lewis basic properties of these silylene systems have also been explored.

Can fused-pyrrole rings act as better π-spacer units than fused-thiophene in dye-sensitized solar cells? A computational study

Fused-pyrrole spacer groups have been exploited for the first time computationally to improve the efficiency of dye-sensitized solar cells (DSSCs). Known dye 1 (TPA-TTAR-T-A) containing tetrathienoacene as a spacer group has achieved 10% efficiency, which is comparable to that of ruthenium complex dyes. The DFT study reveals that fused-pyrrole as a spacer in dye 3 can augment the efficiency of DSSCs and can perform better than dye 1. The pyrrole ring possesses lower resonance energy (21.0 kcal mol−1) than the commonly used thiophene (29–34 kcal mol−1) spacers. The higher conjugative effect of fused-pyrrole rings influence the short-circuit current density (Jsc), including driving force of electrons (ΔGinjection) and singlet excited state lifetime (τ). Further, open-circuit photovoltage (Voc), which depends on dipole moments (μnormal) and number of electrons transferred from the dye to the TiO2 surface (Δq), are also influenced with such conjugative effects. The calculated λmax (539 nm) and ΔGinjection (−1.82 eV) of dye 3 is much higher than the dye 1 (λmax = 497 nm and ΔGinjection = −1.28 eV). The calculated results show that the singlet excited state lifetime of dye 3 (2.2 ns) is higher than the dye 1 (1.7 ns). Dye 3 should have better Jsc value than 1 as the obtained ΔGinjection, τ and λmax values are higher in the former case. The DFT calculations also suggest that the open-circuit photovoltage (Voc) of 3 would be higher than 1 and hence can augment the efficiency of the designed dye 3. The study was extended with tetrapyrrolobenzothiadiazole spacer groups (dyes 6 and 8) which are also shown to be promising candidates to improve the efficiencies of DSSCs. Importantly, the tendency of aggregation of dyes can also be prevented by introducing bulky groups in the fused-pyrrole ring which can also help to enhance the efficiency of designed dyes relative to reported dyes. The exploration of novel sensitizers with fused-pyrrole spacer groups would provide potential candidates for DSSCs.

Revealing Germylene Compounds to Attain Superbasicity with Sigma Donor Substituents: A Density Functional Theory Study

Compounds of GeII are shown for the first time to function as superbases. Two B(N=PiPr3 )2 groups attached to a germanium(II) center show a gas-phase proton affinity of 296.2 kcal mol-1 , which is close to the range of a hyperbase as revealed by B3LYP-D3/6-31G(2d,p) level of theory. These DFT calculations showed better agreement of geometrical parameters for the reported stable germylene compound 1 than previously reported calculations. A systematic study with different substitutions of GeII revealed that such a system can achieve basicity close to a hyperbase. The stabilities of these superbases were examined with dimerization energy and singlet-triplet state energy difference (ΔES-T ). Furthermore, the calculated gas-phase proton affinity values also show good correlation with the most negative valued point (Vmin ) in electron-rich regions from the molecular electrostatic potential. The high PA values of compounds were also supported by ionization potential, electron affinity, absolute electronegativity, and absolute hardness calculations. The energetics for the reaction with BH3 and AlMe3 further suggest that the lone pair of GeII can act as a Lewis base and display higher donor-acceptor bond strengths.

Does the position of the electron-donating nitrogen atom in the ring system influence the efficiency of a dye-sensitized solar cell? A computational study

AbstractWe have reported a number of new metal-free organic dyes (2–6) that have cyclic asymmetric benzotripyrrole derivatives as donor groups with peripheral nitrogen atoms in the ring, fluorine and thiophene groups as π-spacers, and a cyanoacrylic acid acceptor group. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were employed to examine the influence of the position of the donor nitrogen atom and π-conjugation on solar cell performance. The calculated electron-injection driving force (ΔGinject), electron-regeneration driving force (ΔGregen), light-harvesting efficiency (LHE), dipole moment (μnormal), and number of electrons transferred (∆q) indicate that dyes 3, 4, and 6 have significantly higher efficiencies than reference dye 1, which exhibits high efficiency. We also extended our comparison to some other reported dyes, 7–9, which have a donor nitrogen atom in the middle of the ring system. The computed results suggest that dye 6 possesses a higher incident photon to current conversion efficiency (IPCE) than reported dyes 7–9. Thus, the use of donor groups with peripheral nitrogen atoms appears to lead to more efficient dyes than those in which the nitrogen atom is present in the middle of the donor ring system. Graphical AbstractThe locations of the nitrogen atoms in the donor groups in the designed dye molecules have an important influence on DSSC efficiency

Visible-Light-Induced Efficient Selective Oxidation of Nonactivated Alcohols over {001}-Faceted TiO2 with Molecular Oxygen.

In the presence of molecular oxygen, a {001}-faceted nanocrystalline anatase TiO2 catalyst enabled the selective oxidation of nonactivated aliphatic alcohols to the corresponding aldehydes or ketones under visible light. The reaction shows excellent conversion and selectivity towards the formation of the carbonyl products without over-oxidation to the corresponding carboxylic acids. The exceptional reactivity of the catalyst is possibly due to the absorption of visible light originating from a stronger interaction of alcohol with the {001} facet, which facilitates the modification of the band structure of TiO2 , thus facilitating the photogenerated hole transfer and subsequent oxidation processes. The experimental results have also been corroborated by first-principles quantum chemical DFT calculations.

The effect of σ/π, σ and π donors on the basicity of silylene superbases: a density functional theory study

In this study, we examined computationally that the combination of π- and σ-donor substituents results in higher basicity of silylene compounds compared to the usage of π-donor substituents alone. The designed silylenes possess three types of ligands: sigma (σ) donors (4–6), a combination of π-and σ-donors (7–9) and π-donors (10–12). We examined the effect of phosphazene and substituted phosphazene substituents on σ-donor boron atoms to enhance the basicity of the Si(II) centre. Gas phase proton affinity (PA) of 337.1 kcal mol−1 was predicted for compound 6 with only σ-donor ligand, which is in the range of hyperbases. The pairing of π- and σ-donor substituents leads to the proton affinity value ∼341.4 kcal mol−1 for compound 9, which is higher than that of the corresponding π-donor ligand 12 (310.1 kcal mol−1). PA values predicted for these silylenes are comparable to the strongest inorganic superbases such as alkali-metal hydroxides, hydrides, and oxides. The optimized geometry of 9 suggests that the smaller distance between the nitrogen atom of the phosphazene substituent and Si (1.97 A) facilitates donation of the lone pair electron of N to a vacant orbital of Si and subsequently enhances basicity. A combination of π- and σ-donor (7–9) and only σ-donor (4–6) ligands are the preferred ligands for enhancing the basicity of silylene compounds over π-donor ligands alone (10–12). Stability analysis by dimerization energy elucidated that the studied compounds should exist as monomers. The PA values further corroborated well with the ionization potential (IP), electron affinity (EA), absolute electronegativity (χ) and absolute hardness (η).

Conformational preference of glycinamide in solution: An answer derived from combined experimental and computational studies

Conformational problems are often subtle but very important in controlling many intricate features in chemistry and biochemistry. We have performed the conformational analysis of glycinamide using NMR experiments and computational studies. (1)H NMR experiments suggest the prevalence of intramolecular hydrogen bonded conformation of glycinamide (2B) in acetonitrile, whereas, non-intramolecular hydrogen bonded conformation 2A is favoured in dimethylsulfoxide. The NOESY experiments carried out for glycinamide in DMSO-d6, showed stronger NOE interaction of the NHa-atom of amide group with CH2 than that of NHb-atom confirming the presence of conformer 2A. DFT calculations performed with explicit DMSO molecules also suggested a clear preference for the conformer 2A. The molecular dynamics simulations performed with the explicit DMSO molecules also showed that the intermolecular hydrogen bonding exists between the solvent and solute molecules to stabilize the conformer 2A. The present study sheds light on the debate of conformational preference of neutral glycinamide in the present literature.