Calculation Of Rotational Constants Research Articles

Geometry optimizations with spinor-based relativistic coupled-cluster theory.

Development of analytic gradients for relativistic coupled-cluster singles and doubles augmented with a non-iterative triples [CCSD(T)] method using an all-electron exact two-component Hamiltonian with atomic mean-field spin-orbit integrals (X2CAMF) is reported. This enables efficient CC geometry optimizations with spin-orbit coupling included in orbitals. The applicability of the implementation is demonstrated using benchmark X2CAMF-CCSD(T) calculations of equilibrium structures and harmonic vibrational frequencies for methyl halides, CH3X (X = Br, I, and At), as well as calculations of rotational constants and infrared spectrum for RaSH+, a radioactive molecular ion of interest to spectroscopic study.

A DFT study of the dissociation, ionization, and UV/Visible spectra of methyl hypobromite

The energetics and the molecular structures of CH3OBr in the ground electronic, S0, second excited, S2, sextet excited, S6, and cationic ground, D0+ states have been investigated theoretically. The HBr elimination of CH3OBr is found to have two dissociation pathways involving two transition states with an energy difference of 5.28kcal/mol. The B3LYP optimization of the S2 state, which is found to be 1.95eV above the S0 state, shows a tendency of O−Br bond dissociation, resulting in CH3O and Br formation. The optimization of the S6 state, which is found to be 2.35eV above the S0 state, shows a tendency of C−O bond dissociation, forming CH3 and OBr. The ionization energy of the anticipated dissociation products was calculated and found to agree well with the values reported experimentally. For CH3OBr, which has no experimental value for the ionization energy, our ionization energy calculations predicted a value of 9.98eV. Utilizing the excited states calculations and the ionization energy calculations, a design of a pump–probe experiment has been provided in order to explore the ultrafast dissociation dynamics of CH3OBr through S2 and S6 states. The vertical excitation energy calculations were carried out to explore the UV/Visible spectrum of CH3OBr, and the frequency calculations were performed to explore its IR spectrum. The excitation and ionization of CH3OBr result in significant molecular changes including bond lengths, bond angles, dihedral angles, atomic charges, dipole moments, and rotational constants. The excitation to the S2 results in a significant elongation of the O−Br bond, which reflects its tendency for the dissociation through this bond. The excitation to the S6 state results in a remarkable elongation in the C−O bond, reflecting its tendency to dissociate along this bond. The calculations of atomic charges and dipole moment indicate noticeable changes due to excitation and ionization. The electron density contour of HOMO’s indicates the specific site of the HOMO from which the excitation and ionization takes place. The calculations of rotational constants A, B, and C indicated the structural changes through the three mutually perpendicular axes x, y, and z. The effect of ionization to D0+ state on CH3O, CH3, and OBr was investigated following the same thread used for the mother molecule, CH3OBr. CH3O was found to suffer dramatic changes in the molecular structure. It was found to form the CH2OH isomer upon ionization, in which the C−O bond has more double bond character. Moreover, the ionization induces a significant change in the planarity of the CH3O, which changes to the fully planar CH2OH isomer.

Rotational constants and dipole moments of interstellar polyynes: a comparative MP2 and density functional (BP86) study

Abstract By means of theoretical calculations the rotational constants and dipole moments of the first members of the polyynes families (cyanopolyynes, methylcyanopolyynes and isocyanopolyynes) have been calculated at the MP 2 6-31 G ∗ and BP 86 6-31 G ∗ levels. The calculations of rotational constants given in this paper show that the predicted values using density functional theory are in reasonable agreement with the experimental and the MP2 results. A good evaluation of the dipole moments requires the introduction of the electronic correlation, which can be made using density functional calculations. The results are considerably improved using a least squares fit between the experimental and calculated dipole moments including electronic correlation.

Quantum mechanical calculation of rotational constants and Franck-Condon factors of diatomic molecules

In this paper, the question of reliability of the Morse potential as a potential curve for a diatomic molecule is investigated on the basis of calculating the rotational constant. It is shown that the Morse potential describes well potential curves of X1Σq+ and B1Πu electronic states of a Na2 molecule. Calculations of Franck-Condon factors for X1Σq+ ↔ B1Πu band of a Na2 molecule using wave functions of the Morse potential confirm the known correlation between the values of Franck-Condon factors and rotational constants of combined electronic states.

The accuracy of predicted radioastronomical frequencies and the spectrum of hydroxyl

IF radioastronomers are to use predicted frequencies to search for new interstellar species, they require some fairly precise indication of the accuracy of the theoretical calculations. Calculations of rotational constants are not very accurate by the standards of radiofrequency spectroscopy, but computations of Λ-doublet splittings have seemed remarkably accurate1–3. However, these calculations were presented without error limits and as it is not possible to produce these from pure theory we have used the hydroxyl radical to probe the limits of accuracy. The experimental frequencies are known from observations4,5 of the interstellar maser action in OH to a high precision, and it is a particularly suitable case for computation. We show here that the prediction for the lowest rotational level is accurate to 1 MHz; an accuracy higher than conventional terrestrial experimental spectroscopy.