The Quantum Chemical Studies On Spectroscopic Properties Of Interstellar Molecules HC_2nX(X=S, B, Al) And C_2nX(X=P, As)

Using density functional theory (DFT), the equilibrium geometries and the vibrational frequencies of the ground states for linear chains HC_2nB (n = 1-10) have been investigated at the B3LYP/6-31G^** level. Under the optimized ground-state geometries, EOM-CCSD/cc-pvDZ has been used to calculate the vertical excitation energies (â–³E) for the 1¹âˆ‘⁺←X¹âˆ‘⁺ transition in HC_2nB (n = 1-5). On the basis of present calculations, the explicit nonlinear expression for the size dependence of the excitation energy in linear carbon chains HC_2nB has been suggested. CASSCF/6-31G^* method has been used to locate both the ground and excited states for HC_2nB (n =1-4). Different bonding character leads to nonlinear relationship betweenâ–³E and n.The geometries and stabilities in their ground states for linear carbon clusters HC_2nS (n = 1-9) have been reported using the B3LYP method with the 6-31G^* basis set. The predicted rotational constants show excellent agreement with the experimental and previously theoretical values. The calculated vertical excitation energies at CASPT2/cc-pvTZ level for the 2²âˆâ†X²âˆtransition of HC_2nS (n = 1-5) are 3.16, 2.66, 2.05, 1.78 and 1.55 eV, respectively, in good agreement with the corresponding observed values of 3.01, 2.48, 2.10, 1.84 and 1.65 eV,respectively. In addition, the 1²âˆ‘⁺←X²âˆtransition of HC_2nS (n = 1-5) is predicted to have relatively larger oscillator strengths, and they can be observed experimentally. Also, the exponential decay curves for these vertical excitation energies obtained from experiments and theoretical calculations are illuminated.Structures and stabilities of linear carbon chains AlC_2nH (n = 1-5) in their ground states have been investigated by the CCSD and B3LYP approaches. The CASSCF calculations have been used to determine geometries of selected excited states of AlC_2nH (n = 1-5). Optimized geometries indicate single-triple bond alternation structure for the ground states. The vertical excitation energies and emission energies of low-lying excited states have been calculated by the CASPT2 method. The predicted vertical excitation energies for the allowed 1¹âˆâ†X¹âˆ‘⁺ transition in AlC₂H is 3.70 eV, and this is in good agreement with the corresponding observed values of 3.57 eV. The equilibrium geometries of linear carbon clusters C₂X and C₄X (X = P, As) in their ground states have been optimized by density functional (DFT-B3LYP) and coupled cluster with single-double substitution (CCSD). The CASSCF calculations have been used to determine geometries of selected excited states of C₂X and C₄X (X = P, As). The vertical excitation energies of low-lying excited states have been calculated by the CASPT2 method. The predicted vertical excitation energies for the allowed 1²â–³â†X²âˆtransitions in C₂P and C₂AS are 2.03 and 1.84 eV, respectively, which agree very well with the corresponding observed values of 1.98 and 1.81 eV, respectively, and this confirms that assignment in the experiment is reliable.