Vibrational spectroscopic (FTIR and FTRaman) investigation using ab initio (HF) and DFT (LSDA and B3LYP) analysis on the structure of Toluic acid

Rotational spectroscopy of the atmospheric photo-oxidation product o-toluic acid and its monohydrate

o-Toluic acid, a photo-oxidation product in the atmosphere, and its monohydrate were characterized in the gas phase by pure rotational spectroscopy. High-resolution spectra were measured in the range of 5-14 Hz using a cavity-based molecular beam Fourier-transform microwave spectrometer. Possible conformers were identified computationally, at the MP2/6-311++G(2df,2pd) level of theory. For both species, one conformer was identified experimentally, and no methyl internal rotation splittings were observed, indicative of relatively high barriers to rotation. In the monomer, rocking of the carboxylic acid group is a large amplitude motion, characterized by a symmetrical double-well potential. This and other low-lying out-of-plane vibrations contribute to a significant (methyl top-corrected) inertial defect (-1.09 amu Å(2)). In the monohydrate, wagging of the free hydrogen atom of water is a second large amplitude motion, so the average structure is planar. As a result, no c-type transitions were observed. Water tunneling splittings were not observed, because the water rotation coordinate is characterized by an asymmetrical double-well potential. Since the minima are not degenerate, tunneling is precluded. Furthermore, a concerted tunneling path involving simultaneous rotation of the water moiety and rocking of the carboxylic acid group is precluded, because the hilltop along this coordinate is a virtual, rather than a real, saddle-point. Inter- and intramolecular non-covalent bonding is discussed in terms of the quantum theory of atoms in molecules. The percentage of o-toluic acid hydrated in the atmosphere is estimated to be about 0.1% using statistical thermodynamics.

Spectroscopic and quantum chemical electronic structure investigations of 3,4-dihydrocoumarin and 3-methylcoumarin

A complete vibrational analysis of 3,4-dihydrocoumarin and 3-methylcoumarin have been performed according to SQM force field method based on ab initio and DFT calculation 6-311++G(d,p) basis set and their frequencies are compared. The influences of carbon-oxygen bond and methyl group to the vibrational frequencies of the title compounds have been discussed. The pronounced decrease of the lone pair orbital occupancy and the molecular stabilization energy show the hyperconjugation interaction from the NBO analysis. Calculations of molecular orbital geometry show that the visible absorption maxima of DHC and 3MC correspond to the electron transition between frontier orbitals such as translation from HOMO to LUMO. Gauge-including atomic orbital (GIAO) 1H and 13C chemical shift calculations have been calculated. Area of high, neutral and low electrostatic potential is determined for DHC and 3MC.

Conformational analysis, UV-VIS, MESP, NLO and NMR studies of 6-methoxy-1,2,3,4-tetrahydronaphthalene

The detailed HF and B3LYP/6-311++G(d,p) comparative studies on the complete FT-IR and FT-Raman spectra of 6-methoxy-1,2,3,4-tetrahydronaphthalene [MTHN] have been studied. In view of the special properties and uses, the present investigation has been undertaken to provide a satisfactorily vibrational analysis of 6-methoxy-1,2,3,4-tetrahydronaphthalene. Therefore, a thorough Raman, IR, molecular electrostatic potential (MESP), non-linear optical (NLO) properties, UV-VIS, HOMO-LUMO and NMR spectroscopic investigation are reported complemented by B3LYP theoretical predictions with basis set 6-311++G(d,p) to provide novel insight on vibrational assignments and conformational stability of MTHN. Potential energy surface scans (PES) of the CH3 group are undertaken to shed light on the rather complicated conformational interchanges in the compound under investigation.

Quantum chemical determination of molecular geometries, interpretation of FT-IR, FT-Raman spectra and charge transfer properties for N-(2-cyanoethyl)-N-methylaniline

FT-Raman and FT-IR spectra were recorded for N-(2-cyanoethyl)-N-methylaniline sample in solid state. The equilibrium geometries, harmonic vibrational frequencies, infrared and Raman scattering activities were computed using DFT method. Results obtained at this level of theory were used for a detailed interpretation of the infrared and Raman spectra, based on the potential energy distribution (PED) of the normal modes. Molecular parameters such as bond length, and bond angle were calculated with the same method. The intra-molecular charge transfer was calculated by means of natural bond orbital analysis (NBO). Hyperconjugative interaction energy was more during the π-π(*) transition. Energy gap of the molecule was found using HOMO and LUMO calculation, hence the less band gap, which seems to be more stable. Atomic charges of various atoms of title molecule and other thermo-dynamical parameters were calculated using same levels of calculation. The correlation equations between heat capacity, entropy, Gibb's free energies changes with temperatures were fitted by quadratic formula. UV-VIS spectral analyses of title molecule have been researched by theoretical calculations. In order to understand electronic transitions of the compound, TD-DFT calculations on electronic absorption spectra in gas phase and solvent were performed. The calculated frontier orbital energies, absorption wavelengths (λ), oscillator strengths (f) and excitation energies (E) for gas phase in different solvent are also illustrated.

Vibrational spectroscopic studies, molecular orbital calculations and chemical reactivity of 6-nitro-m-toluic acid

The potential energy surface scan for the selected dihedral angle of 6-nitro-m-toluic acid (NTA) has been performed to identify stable conformer. The optimized structure parameters and vibrational wavenumbers of stable conformer have been predicted by density functional B3LYP method with 6-311++G(d,p) basis set. The formation of dimer species through carboxylic acid group of the title molecule has also been discussed. The theoretical dimer geometries have been compared with that of monomer and the variations of bond lengths and bond angles upon dimerization were also discussed. Natural bond orbital (NBO) analysis has been performed on both monomer and dimer geometries. The significant changes in occupancies and the energies of bonding and anti-bonding orbitals upon dimerization have been explained in detail. The predicted frontier molecular orbital energies at B3LYP/6-311++G(d,p) method set show that charge transfer occurs within the molecule. The nucleophilic and electrophilic sites obtained from the molecular electrostatic potential (MEP) surface were compared with their derived fitting point charges. The vibrational wavenumbers of NTA affected profusely by the nitro group substitution in comparison to the toluic acid have been interpreted in this work.