Does the Intramolecular Hydrogen Bond Affect the Spectroscopic Properties of Bicyclic Diazole Heterocycles?

The formation of an intramolecular hydrogen bond in pyrrolo[1,2-a]pyrazin-1(2H)-one bicyclic diazoles was analyzed, and the influence of N-substitution on HB formation is discussed in this study. B3LYP/aug-cc-pVDZ calculations were performed for the diazole, and the quantum theory of atoms in molecules (QTAIM) approach as well as the natural bond orbital (NBO) method was applied to analyze the strength of this interaction. It was found that the intramolecular hydrogen bond that closes an extra ring between the C=O proton acceptor group and the CH proton donor, that is, C=O⋯H–C, influences the spectroscopic properties of pyrrolopyrazine bicyclic diazoles, particularly the carbonyl frequencies. The influence of N-substitution on the aromaticity of heterocyclic rings is also discussed in this report.

Theoritical ab initio study of Internal Rotation Barriers, Structures Stabilities and Population of Formamide

The internal rotational barriers for formamide are calculated in gas and solution phases (acetonitrile) at the HF/6-31G* (16.64 and 16.18 kcal/mol, respectively) and MP2/6-31G* (16.86 and 16.71 kcal/ mol, respectively) level of theory. Calculated parameters are compared with experimental data and there is a good agreement between them. Orbital populations are obtained by MPA (mulliken population analysis) and NPA (natural population analysis) methods and bond energies are calculated by the NBO method (natural bond orbitals). The distribution of atomic charges are also given. These calculation indicate that the internal rotational barrier is produced because of change in the distribution of orbital populations of 2p y, 2p z, d yz, d y2 and dz2 orbitals of the nitrogen atom.

THE NUCLEAR BORN–OPPENHEIMER METHOD APPLIED TO NUCLEAR COLLECTIVE MOTION

We deal with the application of the nuclear Born–Oppenheimer (NBO) method to the study of nuclear collective motion. In particular, we look at the description of nuclear rotations and vibrations. The collective operators are specified within the NBO method only to the extent of identifying the type of collective degrees of freedom we intend to describe; the operators are then determined from the dynamics of the system. To separate the collective degrees of freedom into rotational and vibrational terms, we transform the collective tensor operator from the lab fixed frame of reference to the frame defined by the principal axes of the system; this transformation diagonalizes the tensor operator. We derive a general expression for the NBO mean energy and show that it contains internal, collective and coupling terms. Then, we specify the approximations that need to be made in order to establish a connection between Bohr's collective model and the NBO method. We show that Bohr's collective Hamiltonian can be recovered from the NBO Hamiltonian only after adopting some rather crude approximations. In addition, we try to understand, in light of the NBO approach, why Bohr's collective model gives the wrong inertial parameters. We show that this is due to two major reasons: the ad hoc selection of the collective degrees of freedom within the context of Bohr's collective model and the unwarranted neglect of several important terms from the Hamiltonian.

ZERO-POINT FLUCTUATIONS IN THE NUCLEAR BORN-OPPENHEIMER GROUND STATE

The small-amplitude oscillations of rigid nuclei around the equilibrium state are described by means of the nuclear Born-Oppenheimer (NBO) method. In this limit, the method is shown to give back the random phase approximation (RPA) equations of motion. The contribution of the zero-point fluctuations to the ground state are examined, and the NBO ground state energy derived is shown to be identical to the RPA ground state energy.