Quantum Mechanical Force Field Method Research Articles

Intricate spectroscopic profiling, light harvesting studies and other quantum mechanical properties of 3-phenyl-5-isooxazolone using experimental and computational strategies

This manuscript presents the detailed experimental and computational structural, physical property study of 3-phenyl-5-isooxazolone (3P5I). Different experimental spectral studies like IR, UV, Raman and NMR were used to get an insight on the actual structure of the compound. |It was followed by computational simulation of the structure using DFT approach employing B3LYP functional and 6-311++G (d, p) basis set. Scaled quantum mechanical force field method (SQMFF)was used to assign different vibrations in the spectra using normal coordinate analysis (NCA) and found that close agreement with investigational values. Experimental NMR(1H and 13C NMR) spectrum was also compared with the theoretically simulated spectra using GIAO-gauge independent atomic orbital formalism. UV–Vis Spectrum of compound also studied extensively using theoretical and experimental techniques. Most importantly, the compound shows high light harvesting efficiency hence can be used like a very good photosensitiser in DSSC (dye sensitized solar cells). The overall efficiency of the cell is more than 8% in a titanium dioxide based cell, which is high when compared to similar cells. MEP surfaces, HOMO-LUMO, NLO properties, chemical reactivity descriptors, NBO analysis, and Mulliken atomic charge analysis were also executed to predict physical as well as chemical properties. Molecular docking simulations show that the compounds can be used as effective apoptosis agonist and antiasthmatic agent.

Analysis of experimental and simulated vibrational spectra for the antiferroelectric liquid crystal 12OBBB1M6

Vibrational spectra of the S-2-octyl-4-(4′-[4″-dodecyloxyl-benzoyloxy]) benzoyloxybenzoate have been measured using infrared (IR) and Raman techniques in the solid, in the smectic phase, and in CCl4 and CS2 solution. IR absorbance measurements have been carried out in polarized beam for a homogeneously aligned sample to obtain more details about the orientation of vibrational transition dipole moments. In order to investigate the structure and conformation of the molecule in the crystalline state, molecular dynamics simulations have been performed and dihedral angle distributions have been calculated. The molecular structure and vibrational spectra of the molecule mezogenic core for the most stable conformation have been calculated using the density functional theory with the polarized Gaussian basis set 6-31G*. The predicted force field has been scaled using the scaled quantum-mechanical force field method. The calculated spectrum has been compared with the experimental infrared and the Raman spectra, and as a result detailed vibrational assignments have been reported.

Calculation of vibrational spectra for cyanobiphenyl liquid crystals

Density functional theory, B3-LYP with the 6-31G ∗ basis set was applied to study the structures and vibrational infrared (IR) spectra of biphenyl derivatives. The calculated force fields were scaled using the scaled quantum mechanical force field method. The predicted vibrational frequencies were compared with the experimental IR and Raman spectra (500–4000 cm −1). The spectra were interpreted and vibrational assignments were reported. This study shows that the scaled density functional force field approach enables, through the transferability of scale factors, good interpretation of vibrational spectra of large molecules.

Density Functional Theory Studies on Ir Spectra of the Triphenylene Derivatives. A Scaled Quantum Mechanical Force Field Approach

Density functional theory, B3-LYP with the 6-31G* basis set was ap- plied to study the structures and vibrational infrared spectra of triphenylene and the hexasubstituted triphenylene derivatives. The calculated force fields were scaled using the scaled quantum mechanical force field method. The predicted vibrational frequencies were compared with the experimental IR spectra (500-4000 cm -1 ). The spectra were interpreted and vibrational as- signments were reported. This study shows that the scaled density functional force field approach enables, through the transferability of scale factors, good interpretation of vibrational spectra of large molecules.

Vibrational analysis of Uhle's ketone and its oxidized tautomeric derivatives

A case of “disappearance” of the usually strong, characteristic carbonyl band in infrared absorption spectrum of a fused three-ring compound, Uhle's ketone, on oxidation (dehydrogenation) is examined by means of the scaled quantum mechanical force field method using semiempirical AM1, ab initio RHF/3-21G, and density functional B3LYP/6-31G* calculations followed by full normal coordinate analysis. The study confirmed the presence of the keto form in solution characterized by extensive coupling of the carbonyl vibration with skeletal modes leading to lower than usual frequencies and highly reduced band intensities. The results indicate that only the highest level calculations are of real help in explaining this unusual spectral behavior by clarifying the fine details of vibrational assignment.

Application of SQMFF Vibrational Calculations to Transition States: DFT and ab Initio Study of the Kinetics of Methyl Azide and Ethyl Azide Thermolysis

DFT including nonlocal corrections and ab initio calculations at MP2 and MP4 levels of theory have been performed in order to provide information concerning the mechanism of the rate limiting step of the thermal decomposition of methyl azide and ethyl azide. The chemically interesting points of the ground-state potential energy surface have been fully optimized, and a detailed normal-mode analysis for the reagents and the transition states is presented. The well-established scaled quantum mechanical force field method has been used to obtain reliable vibrational frequencies for these molecular structures. The force fields of transition states have been modified by using the scale factors computed for the force fields of the azides in their ground state. Finally, the activation energies and the Arrhenius preexponential factors for the rate constant have been computed according to transition state theory. The best values for the activation energies are provided by B3-LYP/6-311+G**. For the preexponential factor, the agreement with experiment seems to be independent of the level of theory used.

An application of the DFT-based scaled quantum mechanical force field method to a weakly bonded system: N 2O 4

The adequacy of the density functional derived scaled quantum mechanical force field method of Rauhut and Pulay has been tested for a weakly bound dimer, N 2O 4. The Becke3–Lee–Yang–Parr/6-31G * harmonic force field has been scaled with the transferable scale factors of Rauhut and Pulay (developed originally for `common' organic compounds without any reference to the N–O bond) resulting in 20.7 and 38 cm −1 for the mean and the maximal individual deviations, respectively, as compared with experimental spectra. Consequently, the force field method of Rauhut and Pulay may give reliable results for non-common compounds as well.

Geometry, force field, and fundamental frequencies of 4a,4b-dihydrophenanthrene, a scaled quantum-mechanical force field study

Geometries of both cis- and trans-4a,4b-dihydrophenanthrene are optimized by restricted Hartree—Fock method with 4–21G basis. At this level of theory, the trans-conformer is predicted 8.0 kcal/mol lower in energy than the cis one. The quadratic force field of the trans-conformer obtained from empirical scaling of the ab initio force constants reproduced the observed frequencies with mean deviation of about 10 cm −1. Based on the calculation an earlier assignment of two observed frequencies is confirmed and assignment of the rest of the observed frequencies is given. The observed frequencies at 1555 and 1534 cm −1 are concluded not to be fundamental frequencies of the subject molecule. This study shows again the practical value of the scaled quantum-mechanical force field method in the assignment of fundamental frequencies and identification of reaction products.

Ab initio calculations of the structure and infrared spectrum of As 2O and As 4O

The structures of linear and cyclic As 2O and bridge and terminally bonded As 4O are calculated using analytic gradient techniques in conjunction with atomic pseudopotentials. The harmonic vibrational frequencies of these species are calculated using the scaled quantum-mechanical force field method with scaling parameters obtained from AsO and As 4. The method used is tested on P 2O and P 4O where a comparison with all-electron ab initio structures and frequencies is made.

Ab initio study of the structure and vibrational frequencies of the 2B2 state of AlC2H4

The structure and harmonic frequencies of the 2B2 state of AlC2H4 have been determined at the SCF level using a triple-ζ plus polarization (TZ2P) basis set. A binding energy of the complex of 9–10 kcal mol–1 has been determined with electron correlation calculated using configuration interaction including all single and double excitations (CISD). The vibrational frequencies have been predicted using the scaled quantum-mechanical force field method and yields values in better agreement with experiment than those from a CISD calculation. A more weakly bound complex at a longer Al⋯C2H4 distance is found, but may be a computational artefact.

Ab initio calculations of the structure and infrared spectrum of PO 2, P 2O and P 4O

The structure of PO 2 and of the different forms of P 2O and P 4O are calculated using analytic gradient techniques. The harmonic vibrational frequencies of these species are calculated using the scaled quantum-mechanical force field method with scaling parameters that we obtain from PO, PO 2 − and P 4. The harmonic frequencies of P 2O are also calculated at the MP2 level. The calculated harmonic frequencies are compared with the measured infrared spectra of these species.

Calculations of the infrared and vibrational circular dichroism spectra of ethanol and its deuterated isotopomers

The scaled quantum mechanical force field method together with the Stephens formalism for the evaluation of rotational strengths has been used to calculate infrared and vibrational circular dichroism (VCD) spectra of ethanol and its deuterated isotopomers. For the IR spectra, agreement between the calculated and experimental spectra is extremely good; the root-mean-square deviation between the calculated and experimental frequencies for all 12 isotopomers is 15 cm{sup {minus}1}. The calculated VCD spectra are also in good agreement with the experimental ones when the evaluation of the rotational strengths is carried out using the distributed origin gauge. Both the IR and VCD results confirm the earlier conjecture that the gauche conformer predominates over the trans, even though the trans has a slightly lower SCF energy.