An experimental and theoretical study of molecular structure and vibrational spectra of 2-chloronicotinic acid by density functional theory and ab initio Hartree–Fock calculations

In this work, the Fourier transform Raman and Fourier transform infrared spectra of 2-chloronicotinic acid (2-CNA) are recorded in the solid phase. The molecular geometry, vibrational frequencies, infrared intensities and Raman scattering activities of 2-CNA in ground state have been calculated by using ab initio Hartree–Fock (HF) and density functional (B3LYP and B3PW91) methods with 6-31G(d) and 6-311G(d) basis sets level. On the basis of the comparison between calculated and experimental results and the comparison with related molecule, assignments of fundamental vibrational modes are examined. The optimized geometric parameters (bond lengths and bond angles) obtained by using HF show the best agreement with the experimental values of 2-CNA. Comparison of the observed fundamental vibrational frequencies of 2-CNA and calculated results by density functional (B3LYP and B3PW91) and Hartree–Fock methods indicates that B3LYP is superior to the scaled Hartree–Fock and B3PW91 approach for molecular vibrational problems.     

Theoretical Simulation of Vibrational Sum‐Frequency Generation Spectra from Density Functional Theory: Application to p‐Nitrothiophenol and 2,4‐Dinitroaniline

The molecular orientation of adsorbed molecules forming self‐assembled monolayers can be determined by combining vibrational sum‐frequency generation (SFG) measurements with quantum chemical calculations. Herein, we present a theoretical methodology used to simulate the SFG spectra for different combinations of polarizations. These simulations are based on calculations of the IR vectors and Raman tensors, which are obtained from density functional theory computations. The dependency of the SFG vibrational signature with respect to the molecular orientation is presented for the molecules p‐nitrothiophenol and 2,4‐dinitroaniline. It is found that a suitable choice of basis set as well as of exchange‐correlation (XC) functional is mandatory to correctly simulate the SFG intensities and consequently provide an accurate estimation of the adsorbed molecule orientation. Comparison with experimental data shows that calculations performed at the B3LYP/6‐311++G(d,p) level of approximation provide good agreement with experimental frequencies, and with IR and Raman intensities. In particular, it is demonstrated that polarization and diffuse functions are compulsory for reproducing the IR and Raman spectra, and consequently vibrational SFG spectra, of systems such as p‐nitrothiophenol. Moreover, the investigated XC functionals reveal their influence on the relative intensities, which show rather systematic variations with the amount of Hartree–Fock exchange. Finally, further aspects of the modeling are revealed by considering the frequency dependence of the Raman tensors.     

Vibrational frequencies and structural determination of phosphirane

The normal mode frequencies and corresponding vibrational assignments of phosphirane in are examined theoretically using the GAUSSIAN 98 set of quantum chemistry codes. All normal modes were successfully assigned to one of nine types of motion (C–C stretch, P–C stretch C–H stretch, P–H stretch, CH₂ scissors, CH₂ wag, CH₂ rock, CH₂ twist, and P–H wag) predicted by a group theoretical analysis. Comparing the vibrational frequencies with IR and Raman spectra available in the literature, a set of scaling factors are derived. Predicted infrared and Raman intensities are reported.     

Molecular structure and vibrational raman and infrared spectra of bromochlorofluoromethane and its silicon and germanium analogs: quantum-mechanical DFT and MP2 calculations

DFT(B3LYP) and MP2 calculations with the 6-311G(2d, 2p)-type basis set have been carried out for the prediction of molecular parameters (bond distances, bond angles, rotational constants, and dipole moments) and vibrational Raman and infrared spectra (harmonic wavenumbers, absolute intensities, Raman scattering activities, and depolarization ratios) of bromochlorofluoromethane (HCBrCIF) and its silicon and germanium analogs (HSiBrClF and HGeBrCIF). The predicted geometry and vibrational Raman and infrared spectra of HCBrClF agree well with the available experimental data for this molecule and their deuterated derivatives. This agreement allows one to believe that the predicted molecular parameters and vibrational spectra of HSiBrClF, HGeBrClF, and their deuterated derivatives will guide their future experimental studies.     

Vibrational frequencies and structural determination of cyanogen azide

The vibrational frequencies and corresponding normal mode assignments of cyanogen azide are examined theoretically using the Gaussian03 set of quantum chemistry codes. All normal modes were successfully assigned to one of seven types of motion predicted by a group theoretical analysis (NN stretch, NN stretch, N–C stretch, CN stretch, NNN bend, NN–C bend, and N–CN bend). Theoretical infrared and Raman intensities are reported. The molecular orbitals and bonding of cyanogen azide are examined.     

A DFT study of the vibrational spectra of 1-, and 2-nitrotriphenylene

The infrared (IR) and Raman spectra, and intensities of triphenylene, 1-, and 2-nitrotriphenylene were investigated by the density functional theory (DFT, B3LYP method) with 6-311+G** basis set. Normal mode assignments are proposed with particular emphasis on the nitro group vibrations. Compared to 2-nitrotriphenylene (2-NTRP) 1-nitrotriphenylene (1-NTRP) is predicted to show asymmetric nitro stretches at higher frequencies. Through the vibrational study, the structure–spectroscopic relationships of these nitro polycyclic aromatic hydrocarbons (nitro-PAHs) are made, and possible insights into their differential mutagenic potencies correlated. The geometrical distortions of the TRP structure upon nitro group substitution and correlations between structural parameters and vibrational data as well as structure–function relationships related to the mutagenicity of this important class of polycyclic aromatic hydrocarbons are discussed.     

Vibrational investigation on FT-IR and FT-Raman spectra, IR intensity, Raman activity, peak resemblance, ideal estimation, standard deviation of computed frequencies analyses and electronic structure on 3-methyl-1,2-butadiene using HF and DFT (LSDA/B3LYP/B3PW91) calculations

FT-IR and FT-Raman (4000–100 cm⁻¹) spectral measurements of 3-methyl-1,2-butadiene (3M12B) have been attempted in the present work. Ab-initio HF and DFT (LSDA/B3LYP/B3PW91) calculations have been performed giving energies, optimized structures, harmonic vibrational frequencies, IR intensities and Raman activities. Complete vibrational assignments on the observed spectra are made with vibrational frequencies obtained by HF and DFT (LSDA/B3LYP/B3PW91) at 6-31G(d,p) and 6-311G(d,p) basis sets. The results of the calculations have been used to simulate IR and Raman spectra for the molecule that showed good agreement with the observed spectra. The potential energy distribution (PED) corresponding to each of the observed frequencies are calculated which confirms the reliability and precision of the assignment and analysis of the vibrational fundamentals modes. The oscillation of vibrational frequencies of butadiene due to the couple of methyl group is also discussed. A study on the electronic properties such as HOMO and LUMO energies, were performed by time-dependent DFT (TD-DFT) approach. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. The thermodynamic properties of the title compound at different temperatures reveal the correlations between standard heat capacities (C) standard entropies (S), and standard enthalpy changes (H).     

Relationship between the distortion of polyene chains from planarity and the Raman activity of the in-phase CH out-of-plane wag

The relationship between the distortion of polyene chains from planarity and the Raman activity of the in-phase CH out-of-plane wag is examined by performing density functional theory calculations. The vibrational wavenumbers, vibrational modes, Raman activities and infrared absorption intensities of several oligoene (or polyene) molecules are calculated for model structures having various kinds of distortion around the CC double and/or single bond(s). The results of calculations show that the molecular distortion induces the Raman activity of the in-phase CH out-of-plane wag which should be Raman-inactive for the planar structure. The Raman activity of the in-phase CH out-of-plane wag is particularly enhanced when the molecule is uniformly helical. The origin of a weak Raman band at about 1010 cm⁻¹ of trans-polyacetylene is discussed on the basis of the calculated results.     

Ab initio studies of methylenecarbene and isoelectronic species

Optimum equilibrium geometries, energetics, harmonic vibrational frequencies, and infared intensities within the double harmonic approximation are computed for methylenecarbene, CCH₂, and isoelectronic species involving silicon and germanium at both the SCF level of theory and the level of second-order perturbation theory using a 6-311G(2df, 2p) basis set or its equivalent. Optimum equilibrium geometries and energetics are also computed at both levels of theory using a smaller 6-311G(d, p) basis set or its equivalent. This investigation of these species is the first to include all of the systems with germanium. In addition, this present work is the first study to includef-type polarization functions in a systematic investigation of the molecular structure and properties of all the molecules in the series. Acetylenic structures are also computed for energy comparisons. Of all the linear isomers, only acetylene is found to be a minimum on the potential energy surface. However, all of the C_2v structures are found to be local minima. Both the C_2v and linear structures will serve as a basis for future work involving mapping the entire hyperenergy surfaces of all of the molecular systems in the series.     

Vibrational frequencies and structural determination of 1,1-difluoro-1,2-propadiene

The normal mode frequencies and corresponding vibrational assignments of 1,1-difluoro-1,2-propadiene in C_2v symmetry are examined theoretically using the 98 set of quantum chemistry codes. All normal modes were successfully assigned to one of 11 types of motion (C=C stretch, C–H stretch, C–F stretch, CH₂ scissors, CF₂ scissors, CH₂ wag, CF₂ wag, CH₂ rock, CF₂ rock, CF₂ twist, and C=C=C bend) predicted by a group theoretical analysis. Comparing the vibrational frequencies with IR and Raman spectra available in the literature, a set of scaling factors is derived. Predicted infrared intensities and Raman activities are reported. Predicted normal mode frequencies of 1,1-difluoro-1,2-propadiene-d₂ are reported.     

trans-1,2-Dicyano-cyclopropane and other cyano-cyclopropane derivatives

In this work we present the experimental vibrational absorption (VA), vibrational circular dichroism (VCD) and Raman spectra for (+)-trans-1(S),2(S)-dicyanocyclopropane and its dideuterio derivative, trans-1(S),2(S)-dicyano-1(S),2(S)-dideuteriocyclopropane, along with VA, VCD, Raman and Raman optical activity (ROA) spectral simulations. Here we investigate the applicability of various local and non-local exchange-correlation (XC) functionals, hybrids and meta-hybrids to reproduce the vibrational spectra of this strained ring system, which also bears two cyano groups. At the highest level of theory, B3PW91/ aug-cc-pVTZ, we also investigated the trans-, cis- and gem-dicyanocyclopropane (trans-, cis-, and gem-DCCP), cyanocyclopropane (CCP) and the parent molecule cyclopropane (CP). In doing so we have investigated the electronic effects (coupling) between the cyano groups and the cyclopropane ring. In addition to providing an interpretation of the experimentally observed vibrational spectra for these molecules, this work also provides benchmark calculations for other methods, especially semi-empirical based wave function and density functional theory (DFT) based methods, such as SCC-DFTB and PM6. For the semi-empirical DFT based methods to be used for 3-membered ring systems, one ought to document their reliability for systems which were not used in the parameterization. The small 3- and 4-membered ring systems are good test systems because they contain non-standard bonding, which may be difficult to determine accurately with the approximations used in the SCC-DFTB and other semi-empirical methods. Like molecular mechanics force fields, semi-empirical methods, based on DFT and wave function quantum mechanics (WFQM), must be benchmarked against high level ab initio and DFT calculations and experimental data. In addition to bonding, the changes in the electric dipole moment, magnetic dipole moment, electric dipole-electric dipole polarizability, electric dipole-magnetic dipole polarizability and electric dipole-electric quadrupole polarizability with respect to nuclear displacement and nuclear velocity can be determined by the VA, VCD, Raman and ROA intensities. Hence it is important that the semi-empirical based DFT and wave function methods not only be parameterized to determine energies, gradients and Hessians, but also the electric and magnetic moments and their derivatives that determine the electronic and magnetic properties of these molecules and their interactions with matter and radiation. This will allow biochemists, biophysicists, molecular biologists, and physical biologists to use experimental and theoretical VA, VCD, Raman and ROA spectroscopies to probe biophysical and biochemical function and processes at the molecular level. Festschrift in Honor of Philip J. Stephens’ 65th Birthday.     

A DFT and MP2 Study on the Molecular Structure and Vibrational Spectra of Halogenosubstituted Phosphoryl and Thiophosphoryl Compounds

The molecular geometrical parameters, rotational constants, dipole moments and vibrational infrared properties of a series of phosphoryl compounds (OPX _i Y _j Z _k , X, Y, Z = F, Cl, Br; i+ j + k = 3) and their thio analogs are predicted by density functional and MP2 calculations using the 6-311G(2d,2p) basis set. Both methods yielded similar results. The predicted molecular parameters and the vibrational Raman and infrared spectra agree well with the available experimental data. The Raman Scattering Activities (RSA) and depolarization ratios (Dep) of the molecules are obtained by DFT calculations. Considering the different substitution modes of various halogen atoms, the resultant changes in the geometrical and vibrational properties are discussed. Such studies permit detailed information to be obtained concerning unknown molecules and can define the guidelines for synthesizing molecules of particular characteristics.     

Vibrational frequencies and structural determinations of tetraisocyanatogermane

The normal mode frequencies and corresponding vibrational assignments of Ge(NCO)₄ are examined theoretically using the 98 set of quantum chemistry codes. Each of the vibrational modes was assigned to one of six types of motion predicted by a group theoretical analysis (Ge–N stretch, N–C–O symmetric stretch, N–C–O asymmetric stretch, N–C–O bend, Ge–N–C bend, and N–Ge–N bend) utilizing the T_d symmetry of the molecule. Uniform scaling factors were derived for each type of motion. Predicted infrared and Raman intensities are reported.     

Vibrational frequencies and structural determination of 1,3-difluoro-1,2-propadiene

The normal mode frequencies and corresponding vibrational assignments of 1,3-difluoro-1,2-propadiene in are examined theoretically using the GAUSSIAN 98 set of quantum chemistry codes. All normal modes were successfully assigned to one of eight types of motion (CC stretch, C–H stretch, C–F stretch, H–C–F scissors, H–C–F wag, H–C–F rock, H–C–F twist, and CCC bend) predicted by a group theoretical analysis. Comparing the vibrational frequencies with IR and Raman spectra available in the literature, a set of scaling factors are derived. Predicted infrared and Raman intensities are reported.     

Ab initio and DFT studies of the molecular structures and vibrational spectra of succinonitrile

The Molecular structure, conformational stability and vibrational frequencies of succinonitrile NCCH2CH2CN have been investigated with ab initio and density functional theory (DFT) methods implementing the standard 6-311++G* basis set. The potential energy surfaces (PES) have been explored at DFT-B3LYP, HF and MP2 levels of theory. In agreements with previous experimental results, the molecule was predicted to exist in equilibrium mixture of trans and gauche conforms with the trans form being slightly lower in energy. The vibrational frequencies and the corresponding vibrational assignments of succinonitrile in both C2h and C2 symmetry were examined theoretically and the calculated Infrared and Raman spectra of the molecule were plotted. Observed frequencies for normal modes were compared with those calculated from normal mode coordinate analysis carried out on the basis of ab initio and DFT force fields using the standard 6-311++G* basis set of the theoretical optimized geometry. Theoretical IR intensities and Raman activities are reported.     

Quantum chemical determination of molecular geometries and interpretation of FTIR and Raman spectra for 2,3,4-and 2,3,6-tri-fluoro-benzonitriles

Raman and FTIR spectra for 2,3,4- and 2,3,6-tri-fluoro-benzonitriles have been recorded in the regions 50–4000 cm⁻¹ and 400–4000 cm⁻¹, respectively. Measurement of depolarization ratios for the Raman lines has also been made. Optimized geometrical parameters, charge distributions and vibrational wavenumbers were calculated using ab initio quantum chemical method. Normal coordinate analysis has also been carried out to help assign the fundamentals of these molecules. Each vibration has been assigned using observed wavenumbers in the IR and Raman spectra and their relative intensities, depolarization ratios of the Raman lines, the calculated frequencies, vector displacements and potential energy distributions (PEDs).     

The performance of different density functional methods in the calculation of molecular structures and vibrational spectra of platinum(II) antitumor drugs: cisplatin and carboplatin

A comparison of eight density functional models for predicting the molecular structures, vibrational frequencies, infrared intensities, and Raman scattering activities of platinum(II) antitumor drugs, cisplatin and carboplatin, is reported. Methods examined include the pure density functional protocols (G96LYP, G96PW91, modified mPWPW and original PW91PW91), one‐parameter hybrid approaches (mPW1PW and mPW1LYP), and three‐parameter hybrid models (B3LYP and B3PW91), as well as the HF and MP2 levels of theory. Different effective core potentials (ECPs) and several basis sets are considered. The theoretical results are discussed and compared with the experimental data. It is remarkable that the mPW1PW protocol introduced by Adamo and Barone [J Chem Phys 1998, 108, 664], is clearly superior to all the remaining density functional methods (including B3LYP). The geometry and vibrational frequencies of cisplatin and carboplatin calculated with the mPW1PW method, and the ECP of Hay and Wadt (LanL2DZ basis set) are in better agreement with experiment than those obtained with the MP2 method. The use of more elaborated ECP and the enlargements of basis sets do not significantly improve the results. A clear‐cut assignments of the platinum‐ligand vibrations in cisplatin and carboplatin are presented. It is concluded that mPW1PW is the new reliable method, which can be used in predicting molecular structures and vibrational spectra of large coordination compounds containing platinum(II). © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 901–912, 2001     

Ab initio and DFT predictions of infrared intensities and Raman activities

Relative infrared (IR) intensities and relative Raman activities have been computed for vibrations of test molecules, including from two to nine heavy atoms, using second-order Moller-Plesset perturbation theory (MP2), and three hybrid density functionals (B3LYP, M05, and M05-2X). The basis set convergence of vibrational properties is discussed. Our results demonstrate that B3LYP offers the most cost-effective choice for the prediction of molecular vibrational properties, but the predictions of another two tested hybrid functionals are very similar and in very good agreement with experimental data. MP2 shows good performance for the IR intensities, whereas the quality of prediction of the relative Raman activities should be characterized as only moderate. B3LYP calculations of the relative IR intensities using highly compact Sadlej's Z3PolX basis set retain the high accuracy of the more CPU expensive Sadlej's pVTZ and much more expensive aug-cc-pVTZ calculations. Relative Raman activities are more sensitive to basis set effects and require at least Sadlej's pVTZ to obtain quantitative results.     

Vibrational spectroscopic study of poly(dimethylsiloxane)-ZnO nanocomposites

A series of poly(dimethylsiloxane)-zinc oxide (PDMS-ZnO) nanocomposites having different concentrations of ZnO nanoparticles (0, 1, 5, 10 and 20 wt%) have been prepared. Raman and FTIR-ATR spectroscopic analysis was performed in order to determine the interaction between the ZnO nanoparticles and PDMS polymer matrix. Density functional theory (DFT) using the (B3-LYP)/6-311++G(2df,2p) method was used to investigate the vibrational spectra of PDMS. A complete vibrational assignment is supported by the normal coordinate analysis, calculated Raman activities as well as IR intensities.The presence of ZnO nanoparticles in PDMS gives rise to significant differences in relative intensities of the characteristic vibrational bands with respect to the cross-linked polymer. The changes in relative intensities of Raman bands, as well as swelling measurements, were used to explain the effect of ZnO nanoparticles on the cross-linked structure of PDMS nanocomposites. It is established that ZnO nanoparticles influence the cross-linking density of the polymer matrix.     

Vibrational frequencies and structural determination of tetraethynylstannane

The normal mode frequencies and corresponding vibrational assignments of Sn(CCH)₄ are examined theoretically using the 98 set of quantum chemistry codes. Each of the vibrational modes was assigned to one of six types of motion predicted by a group theoretical analysis (Sn–C stretch, CC stretch, C–H stretch, CC–H bend, Sn–CC bend, and C–Sn–C bend) utilizing the T_d symmetry of the molecule. A set of uniform scaling factors were derived for each type of motion. Predicted infrared and Raman intensities are reported.