Transferability of anharmonic force fields in simulations of molecular vibrations

Quantum chemical and spectroscopic investigations of 3-methyladenine

FTIR, FT-Raman and UV-Vis spectra of 3-methyladenine have been recorded and investigated using quantum chemical calculations. The molecular geometry and vibrational spectra of 3-methyladenine in the ground state are computed by using HF and DFT methods with 6-311G(d,p) basis set. VSCF, CC-VSCF methods based on 2MR-QFF and PT2 (Barone method) have been utilized for computing anharmonic vibrational frequencies. These methods yield results that are in remarkable agreement with the experimental data. The magnitudes of coupling between pair of modes have been also computed. Vibrational modes are assigned with the help of visual inspection of atomic displacements. The electronic spectra, simulated at TD-B3LYP/6-311++G(d,p) level of theory, are compared to the experiment. The global quantities: electronic chemical potential, electrophilicity index, chemical hardness and softness based on HOMO and LUMO energy eigenvalues are also computed at B3LYP/6-311++G(d,p) level of theory.

CH Stretching Region: Computational Modeling of Vibrational Optical Activity

Most organic compounds provide vibrational spectra within the CH stretching region, yet the signal is difficult to interpret because of multiple difficulties in experiment and modeling. To better understand various factors involved, the ability of several harmonic and anharmonic computational approaches to describe these vibrations was explored for α-pinene, fenchone, and camphor as test compounds. Raman, Raman optical activity (ROA), infrared absorption (IR), and vibrational circular dichroism (VCD) spectra were measured and compared to quantum chemical computations. Surprisingly, the harmonic vibrational approach reasonably well reproduced the measured spectral patterns, including the vibrational optical activity (VOA). The CH stretching, however, appeared to be more sensitive to the basis set and solvent variations than lower-frequency vibrations. For a higher accuracy in frequencies and spectral shapes, anharmonic corrections were necessary. Accurate harmonic and anharmonic force fields were obtained with the mPW2PLYP double-hybrid functional. A limited vibrational configuration interaction (LVCI) where the CH stretching motion was decoupled from other vibrations provided the best simulated spectra. A balanced harmonic oscillator basis set had to be used, containing also states indirectly interacting with fundamental vibrations. A simpler second-order perturbational approach (PT2) appeared less useful. The modeling provided unprecedented agreement with experimental vibrational frequencies; spectral shapes were reproduced less faithfully. The possibility of ab initio interpretation of the CH spectral region for relatively large molecules further broadens the application span of vibrational spectroscopy.

Anharmonic vibrational studies of L-aspartic acid using HF and DFT calculations

The experimental and theoretical studies on the structure, molecular properties and vibrational spectra of L-aspartic acid are presented. The molecular structure, harmonic and anharmonic vibrational frequencies, molecular properties, MEP mapping, NBO analysis and electronic spectra of L-aspartic acid have been reported. Computed geometrical parameters and anharmonic frequencies of fundamental, combination and overtone transitions were found in satisfactory agreement with the experimental data. The UV-Vis spectrum of present molecule has been recorded and the electronic properties such as HOMO and LUMO energies and few low lying excited states were carried out by using time dependent density functional theory (TD-DFT) approach. Natural Bond Orbital (NBO) analysis has been performed for analyzing charge delocalization throughout the molecule. Molecular electrostatic potential map has also been used for quantitative measure of the chemical activities of various sites of the molecule.