• Anjan Chattopadhyay

      Articles written in Journal of Chemical Sciences

    • A comparative study of the spectroscopic features of the low-lying electronic states of H2F+ and H2Cl+ ions

      Anjan Chattopadhyay

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      Configuration interaction studies of H2F+ and H2Cl+ ions, using 6-311G ($3d$, $3p$) basis sets, have revealed several interesting differences in their spectral behaviour. Both of them are having bent ground state (1A1) equilibrium geometries, but there is a huge difference (∼ 1.93 eV) between their energy barrier to linearity. Their first two excited states are found to be linear ${}^3\Pi_u$ and ${}^1\Pi_u$ states, correlating to 3B1/3A1 and 1B1/1A1 Renner-Teller pairs, respectively, in the C2v symmetry. Considering only the allowed singlet-singlet transitions at the ground state equilibrium geometry, the lowest energy transitions found to have transition moment values of 0.65 D and 0.48 D for H2F+and H2Cl+, respectively, appearing in the far UV region. Conical intersections take place during the symmetrical stretching of two H-Cl bonds in the chloronium ion for the first two pairs of excited states (11A2/11B1 and $1^1$A2/$1^1$B1) in the C2v symmetry. This intersection may initiate pre-dissociation from the upper bound adiabatic 1A″ state to the lower repulsive 1A" state in the C$_s$ symmetry. Fluoronium ion is expected to dissociate via a single electronic state due to the absence of such intersection.

    • Comparative study of spectroscopic properties of the low-lying electronic states of 2,4-pentadien-1-iminium cation and its N-substituted analogues

      Anjan Chattopadhyay

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      Semiempirical and ab initio-based CI methods have been employed to study the low-lying electronic states of 2,4-pentadien-1-iminium cation and its N-substituted analogues with electron-donating (methyl, isopropyl) and electron-withdrawing (fluoromethyl) groups on nitrogen. Variations of the dihedral angles ($\Gamma_3$, $\Gamma_4$) of the ground state have given the global minima and global maxima at (180°, 180°) and (90°, 0°) conformations, respectively, with some exceptions in the case of fluoromethyl compound. Increase in the +I effect on nitrogen shifts the TICT conical intersection point away from the 90° ($\Gamma_3$ dihedral angle) value, when the $\Gamma_4$ value is kept fixed at 180°. Transition moment values of the allowed S0(1A$_g$ -like) → S1 (2B$_u$-like) transitions are expectedly higher than the forbidden S0(1A$_g$ -like) → S2(2A$_g$ -like) transitions by almost 5.6 D. Radiative lifetime values of the first excited states are calculated to be around 215 ps for all the four compounds. At (180°, 180°) conformation the vertical excitation energy (VEE) between the S0 and S1 states of the 2,4-pentadieniminium cation is found to be 3.3 eV, which corresponds to the absorption wavelength value of roughly 375 nm. The VEE value increases with substituents having +I effect on nitrogen, while for the fluoromethyl compound it is calculated to be around 2.85 eV. The energy gap between the first two excited singlet states is found to have the least value in the isopropyl-substituted compound, where the S2 state contains a huge contribution from the HOMO2→LUMO2 configuration.

    • A computational investigation of the photochemical oxaziridine and amide conversion process of open-chain conjugated nitrone with electron-withdrawing trifluoromethyl group on nitrogen

      Praveen Saini Anjan Chattopadhyay

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      This computational study investigates the photo-excitation process and subsequent photoproduct formation steps through non-radiative deactivation channels in open-chain conjugated N-substituted nitrone systems (model compounds of corresponding retinylnitrones) having electron-withdrawing groups on nitrogen. Calculations mostly based on CASSCF/6-31G* and CASMP2/6-31G* level of theories on a representative system with N-trifluoromethyl substituent have predicted initial photo-excitation to a planar singlet state. This photochemical path is subsequently followed by a barrierless non-radiative channel towards the lowest-energy conical intersection (CI) geometry having a terminal CNO kink, and situated at 30 kcal/mol below the planar excited state. Following the direction of its gradient difference (GD) vectors, an oxaziridine-type species (RC−O = 1.38 Å, RN−O = 1.53 Å, < CNO= 55.8°) appears at 3–6 kcal mol−1 below the ground state nitrone system through a transition state (along its reverse direction of minimum-energy path), situated on the reaction pathway. This species with an elongated N-O bond seems to be heading towards an amide geometry. On the other hand, in the opposite GD vector direction a proper oxaziridine geometry has been obtained with a much shorter N-O bond distance (RN−O = 1.42 Å).

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