Sheeraz Ahmad Bhat

Vibrational and electronic spectral analysis of 2,3-pyrazinedicarboxylic acid: A combined experimental and theoretical study

ABSTRACT Fourier transform infrared and Raman spectra of 2,3-pyrazinedicarboxylic acid were recorded and analyzed using density functional theory. The complete assignments of the anharmonic vibrational modes have been performed based on potential energy distribution. The anharmonic frequencies were computed using vibrational second-order perturbation theory as well as vibrational self-consistent field and correlation corrected vibrational self-consistent field methods. Mode–mode coupling strength is also estimated using two-mode representation of quartic force field approximation. The intra- and intermolecular interactions were also studied in the dimer and trimer forms of the title molecule. The ultraviolet–visible absorption spectra in ethanol, methanol, and acetonitrile solvents were recorded and analyzed using time-dependent density functional theory involving the polarization continuum model. The observed and calculated results are well comparable. Molecular electrostatic potential and the highest occupied and the lowest unoccupied molecular orbital analyses are also reported.

Anharmonic vibrational spectra and mode-mode couplings analysis of 2-aminopyridine

Vibrational spectra of 2-aminopyridine (2AP) have been analyzed using the vibrational self-consistence field theory (VSCF), correlated corrected vibrational self-consistence field theory (CC-VSCF) and vibrational perturbation theory (VPT2) at B3LYP/6-311G(d,p) framework. The mode-mode couplings affect the vibrational frequencies and intensities. The coupling integrals between pairs of normal modes have been obtained on the basis of quartic force field (2MR-QFF) approximation. The overtone and combination bands are also assigned in the FTIR spectrum with the help of anharmonic calculation at VPT2 method. A statistical analysis of deviations shows that estimated anharmonic frequencies are closer to the experiment over harmonic approximation. Furthermore, the anharmonic correction has also been carried out for the dimeric structure of 2AP. The fundamental vibration bands have been assigned on the basis of potential energy distribution (PED) and visual look over the animated modes. Other important molecular properties such as frontier molecular orbitals and molecular electrostatics potential mapping have also been analyzed.

Anharmonic vibrational spectroscopy, NBO charges and global chemical reactivity studies on the charge transfer PDCA−.AHMP+ single crystal using DFT calculations

The charge transfer (CT) complex of the 2-amino-4-hydroxy-6-methylpyrimidine and 2,3 pyrazinedicarboxylic acid (PDCA-.AHMP+) was synthesized and its single crystal was grown by solution method. The structure of the crystalline complex has been investigated by single crystal X-ray diffraction (SCXRD). The vibrational features of the complex have been studied with the help of FTIR spectra and DFT computation. The anharmonic corrections in vibrational frequencies are made using the GVPT2 method at B3LYP/6-311++G(d,p) level of theory. The frontier molecular orbitals and global chemical reactivity have been calculated to understand the pharmacological aspect of the synthesized crystal. Furthermore, Hirshfeld electrostatic potential (ESP) surface, void space in the crystal structure and natural as well as Mulliken atomic charges are studied.

Nature and potency interactions of the hydrogen bond through the NBO analysis for charge transfer complex between 2-amino-4-hydroxy-6-methylpyrimidine and 2,3-pyrazinedicarboxylic acid

The intermolecular interactions in complex formation between 2-amino-4-hydroxy-6-methylpyrimidine (AHMP) and 2,3-pyrazinedicarboxylicacid (PDCA) have been explored using density functional theory calculations. The isolated 1:1 molecular geometry of proton transfer (PT) complex between AHMP and PDCA has been optimized on a counterpoise corrected potential energy surface (PES) at DFT-B3LYP/6-31G(d,p) level of theory in the gaseous phase. Further, the formation of hydrogen bonded charge transfer (HBCT) complex between PDCA and AHMP has been also discussed. PT energy barrier between two extremes is calculated using potential energy surface (PES) scan by varying bond length. The intermolecular interactions have been analyzed from theoretical perspective of natural bond orbital (NBO) analysis. In addition, the interaction energy between molecular fragments involved in the complex formation has been also computed by counterpoise procedure at same level of theory.The intermolecular interactions in complex formation between 2-amino-4-hydroxy-6-methylpyrimidine (AHMP) and 2,3-pyrazinedicarboxylicacid (PDCA) have been explored using density functional theory calculations. The isolated 1:1 molecular geometry of proton transfer (PT) complex between AHMP and PDCA has been optimized on a counterpoise corrected potential energy surface (PES) at DFT-B3LYP/6-31G(d,p) level of theory in the gaseous phase. Further, the formation of hydrogen bonded charge transfer (HBCT) complex between PDCA and AHMP has been also discussed. PT energy barrier between two extremes is calculated using potential energy surface (PES) scan by varying bond length. The intermolecular interactions have been analyzed from theoretical perspective of natural bond orbital (NBO) analysis. In addition, the interaction energy between molecular fragments involved in the complex formation has been also computed by counterpoise procedure at same level of theory.

Anharmonic vibrational spectral analysis of L-(-)-xylose molecule

In this work, a detailed vibrational analysis of L-(-)-xylose molecule has been carried out. The geometrical parameters and anharmonic spectrum have been calculated and compared with XRD, FTIR (4000–400cm−1) and FT-Raman (4000–50cm−1) observed data. The simulated data along with IR and Raman intensities were calculated using DFT/B3LYP level of theory in combination with 6-311++G(d,p) basis set. The experimental and theoretical results are found to be in a good agreement with each other. Moreover, thermodynamic properties, molecular electrostatic potential (MEP) and natural bond orbital (NBO) analysis of L-(-)-xylose are also reported. The calculated HOMO and LUMO energies confirm the charge transfer within the molecule.

Quantum chemical and spectroscopic investigations of 4-Hydroxy-7-methyl-1,8-naphthyridine-3-carboxylic acid

The FTIR (4000–400cm−1) and the FT-Raman spectra (4000–50cm−1) of 4-Hydroxy-7-methyl-1,8-naphthyridine-3-carboxylic acid are recorded and investigated. The spectra are interpreted using anharmonic frequency computations by VPT2, VSCF and PT2-VSCF methods within DFT/6-311G(d,p) framework. The root mean square (RMS) values indicate that VSCF computed frequencies are in close agreement with the observed frequencies. The combination and overtone bands are also identified in the FTIR spectrum. The intermolecular O-H⋯O hydrogen bonding interactions are discussed in the dimer structure of the molecule. The magnitudes of the coupling between pair of modes are also computed. The electronic spectra in water and ethanol solvents are analyzed using TD-B3LYP/6-311++G(d,p) level of theory. Molecular electrostatic potential (MEP) and HOMO-LUMO analysis are also performed.