Ponnadurai Ramasami

Trends in σ-hole strengths and interactions of F3MX molecules (M = C, Si, Ge and X = F, Cl, Br, I)

It is well-established that many covalently-bonded atoms of Groups IV–VII have directionally-specific regions of positive electrostatic potential (σ-holes) through which they can interact with negative sites. In the case of Group VII, this is called “halogen bonding.” We have studied two series of molecules: the F3MX and, for comparison, the H3MX (M = C, Si and Ge; X = F, Cl, Br and I). Our objective was to determine how the interplay between M and X in each molecule affects the σ-holes of both, and consequently their interactions with the nitrogen lone pair of HCN. We find that the relative electronegativities of M and X are not sufficient to explain their effects upon each other’s σ-holes; consideration of charge capacity/polarizability (and perhaps other factors) also appears to be necessary. However the results do support the description of normal σ-hole interactions as being largely electrostatically-driven.

A Combined Experimental and DFT-TDDFT Study of the Excited-State Intramolecular Proton Transfer (ESIPT) of 2-(2′-Hydroxyphenyl) Imidazole Derivatives

We report a combined experimental and computational study of the effect of electron donor and acceptor groups on the excited state intramolecular proton transfer of 2-(2′-hydroxyphenyl) imidazole derivatives in solvents of different polarities. The changes in fluorescence properties, electronic transitions and energy levels are analyzed and discussed. The study was complemented using the Density Functional Theory (DFT)-Time Dependent DFT [B3LYP/6-31G(d)] computations. The calculated absorption and emission spectra of the imidazole derivatives are in good agreement with the experiments, thus allowing an assignment of the UV–vis spectra.

Nickel(II) and copper(II) complexes of allyl 2-(thiophen-2-ylmethylene)hydrazinecarbodithioate: synthesis, X-ray crystal structures, and theoretical study

We report allyl 2-(thiophen-2-ylmethylene)hydrazine-carbodithioate (HL) and its Ni(II) and Cu(II) complexes, [ML2]. The compounds were fully characterized by elemental analysis, IR, 1H-NMR, UV-Vis, and molar conductivity. The crystal structure analysis indicates that the metal is four-coordinate square planar and that a parallel stacking of the molecular planes is present in the crystals, with stacking distances of 3.642 and 3.676 Å for the Ni(II) and Cu(II) complexes, respectively. Gas phase DFT computations indicate that the thione tautomeric form of the free ligand is more stable than the thiol form by 14.52 kJ mol–1. For HL and ML2, comparison between the computed and experimental data shows good agreement.

A DFT study of the [3 + 2] versus [4 + 2] cycloaddition reactions of 1,5,6-trimethylpyrazinium-3-olate with methyl methacrylate.

The reaction between 1,5,6-trimethylpyrazinium-3-olate and methyl methacrylate (MMA) yielding a lactone-lactam has been studied using the DFT method at the B3LYP/6-31G(d) level. It is concluded that formation of the lactone-lactam is a domino process involving three consecutive reactions: (i) a 1,3-dipolar cycloaddition (13DC) reaction between the pyrazinium-3-olate and MMA yielding a [3 + 2] cycloadduct (CA); (ii) a skeletal rearrangement, which converts the [3 + 2] CA into a formal [4 + 2] CA, possessing a diazabicyclo[2.2.2]octane structure; and finally, (iii) an S(N)2 reaction, promoted by halide anion, with concomitant nucleophilic attack of the created carboxylate anion on an iminium carbon with formation of the lactone ring present in the lactone-lactam. Analysis of the four competitive channels associated with the 13DC reaction indicates that this cycloaddition takes place with complete endo stereoselectivity and 6 regioselectivity, yielding [3 + 2] CA. The subsequent skeletal rearrangement also takes place in an elementary step via a non-concerted mechanism. Electron localization function bonding analysis makes it possible to establish that the bicyclo[2.2.2]octane skeleton present in the lactone-lactam complex structure is not attained via a Diels-Alder reaction between pyrazinium-3-olate and MMA.

NIR-Emitting Boradiazaindacene Fluorophores -TD-DFT Studies on Electronic Structure and Photophysical Properties

Density Functional Theory [B3LYP/6-31G(d)] and Time Dependent Density Functional Theory [TD-B3LYP/6-31G(d)] computations have been used to have more understanding of the structural, molecular, electronic and photophysical parameters of recently synthesized near IR-emitting acid switchable di-styryl BODIPY dyes. The structures have been optimized using function B3LYP and basis set used was 6-31G(d) for all the atoms and their geometries which are correlated with corresponding rotational isomers including rotational isomers of diprotonated forms in chloroform solvent. The observed energies of the optimized molecules suggest that there may be rotation about C-C single bond as the observed energy barrier is very low. The results of TD-DFT suggest that there is very good match between the observed and calculated absorptions diprotonated forms of one molecule. There is also good match between experimental and theoretical emission of neutral forms. More deviations are observed in the case emission of the diprotonated forms.

Understanding the formation of [3+2] and [2+4] cycloadducts in the Lewis acid catalysed reaction between methyl glyoxylate oxime and cyclopentadiene: a theoretical study

The formation of the formal [3+2] and [2+4] cycloadducts in the Lewis acid (LA) catalysed reaction of cyclopentadiene (Cp, 1) with methyl glyoxylate oxime (MGO, 2a) has been theoretically studied using DFT methods. Coordination of BF3 LA to the oxygen atom of MGO 2a not only increases the electrophilicity of the oxime, but also makes the corresponding tautomeric BF3:nitrone complex 8b the reactive species. The reaction is characterised by the nucleophilic attack of Cp 1 on the carbon atom of the corresponding BF3:nitrone complex 8b. The subsequent ring closure at the end of the reaction allows the formation of the [3+2] or [2+4] cycloadducts. ELF bonding analysis of selected points on the intrinsic reaction coordinate of the two competitive endo paths enables us to establish that the formation of the formal [3+2] or [2+4] cycloadducts takes place through stereoisomeric transition state structures with a similar electronic structure.

Synthesis, molecular structure, FT-IR, Raman, XRD and theoretical investigations of (2E)-1-(5-chlorothiophen-2-yl)-3-(naphthalen-2-yl)prop-2-en-1-one

A novel (2E)-1-(5-chlorothiophen-2-yl)-3-(naphthalen-2-yl)prop-2-en-1-one [C17H11ClOS] compound has been synthesized and its structure has been characterized by FT-IR, Raman and single-crystal X-ray diffraction techniques. The isomers, optimized geometrical parameters, normal mode frequencies and corresponding vibrational assignments of the compound have been examined by means of the density functional theory method, employing, the Becke-3-Lee-Yang-Parr functional and the 6-311+G(3df,p) basis set. Reliable vibrational assignments and molecular orbitals have been investigated by the potential energy distribution and natural bonding orbital analyses, respectively. The compound crystallizes in the monoclinic space group P2₁/c with the unit cell parameters a=5.7827(8)Å, b=14.590(2)Å, c=16.138(2)Å and β=89.987 (°). The CC bond of the central enone group adopts an E configuration. There is a good agreement between the theoretically predicted structural parameters and vibrational frequencies and those obtained experimentally.

Rotational barrier and thermodynamical parameters of furfural, thiofurfural, and selenofurfural in the gas and solution phases: theoretical study based on density functional theory method

This work aims to study the trans and cis conformers of furfural, thiofurfural and selenofurfural in the gas and solution phases. Assuming that there is equilibrium between these conformers, the transition state has also been investigated. All computations have been done using density functional theory method with B3LYP as the functional and 6-311++G(d,p) as the basis sets. The optimized molecular structures and related parameters of these conformers are reported. The infrared wavenumbers and Raman activities of these conformers are also reported with appropriate assignments. The energy differences between the trans and cis conformers, associated barriers and thermodynamical parameters have been derived from the computations. It is found that the structural parameters are not much different in the gas and solution phases. However, in the gas phase, the trans conformer is always more stable, but increasing the polarity of the solvent leads to the cis conformer becoming more stable. The rotational barrier is always larger than the energy difference and both of them increase when the solvent becomes more polar. Some of the results for furfural compare satisfactorily with literature and therefore the data from this work will be useful for thiofurfuraldehyde and selenofurfuraldehyde, as their literature is limited.

Structural, energetic and vibrational properties of some van der Waals complexes of CO2, OCS and OCSe

As part of a study of the properties of some chalcogen-bonded complexes with NH3, H2O, PH3 and H2S, we have investigated the oxygen-bound species containing CO2, OCS and OCSe by means of molecular orbital calculations at the ab initio level. The structures of the NH3, H2O and PH3 complexes are all similar, with a primary C…X interaction (X = N, O, P) and one of the hydrogen atoms approaching an oxygen atom in a weak secondary attraction. The H2S complexes show a greater variety of alternative structures. The changes in the monomer geometrical parameters, the interaction energies and the harmonic vibrational spectra vary in general in a systematic way as the acid and the base are changed. Deviations from this systematic behaviour have been rationalised.

Ion‐Pair SN2 Substitution: Activation Strain Analyses of Counter‐Ion and Solvent Effects

The ion-pair SN 2 reactions of model systems MnF(n-1) +CH3Cl(M(+) =Li(+), Na(+), K(+), and MgCl(+); n=0, 1) have been quantum chemically explored by using DFT at the OLYP/6-31++G(d,p) level. The purpose of this study is threefold: 1) to elucidate how the counterion M(+) modifies ion-pair SN 2 reactivity relative to the parent reaction F(-) +CH3Cl; 2) to determine how this influences stereochemical competition between the backside and frontside attacks; and 3) to examine the effect of solvation on these ion-pair SN2 pathways. Trends in reactivity are analyzed and explained by using the activation strain model (ASM) of chemical reactivity. The ASM has been extended to treat reactivity in solution. These findings contribute to a more rational design of tailor-made substitution reactions.