Reduced-size polarized basis sets for calculations of molecular electric properties. II. Simulation of the Raman spectra

The accuracies of the calculated vibrational frequencies and Raman intensities given by two new, highly compact Pol-type basis sets, Z2PolX and Z3PolX, have been determined and compared to the 6-31G(d), PolX, and aug-cc-pVTZ basis sets. Calculation of accurate Raman intensities has previously required large basis sets, but the ZmPolX basis sets are smaller even than PolX, which are the most compact basis sets able to calculate accurate Raman intensities. For the largest compound studied, C5H10O2, Z3PolX required more than an order of magnitude less CPU time than PolX, which has been shown to be 10 times faster than aug-cc-pVTZ. Two sets of test molecules were studied: one was a series of small molecules for which experimental values for absolute Raman activities were available; the second was a series of medium-sized molecules (mainly common organic solvents) where only relative Raman band intensities were available. The accuracies of the Raman intensities given by both of the ZmPolX basis sets were good compared to those of the PolX and aug-cc-pVTZ sets, and much better than the 6-31G(d) values. The errors in even unscaled frequency values <2000 cm(-1) were also acceptable and were slightly lower for Z3PolX than Z2PolX (30 cm(-1) vs. 48 cm(-1)). The combination of good intensity and frequency data meant that for the medium-sized organic molecules there was a close correspondence between the simulated Raman spectra and experimental data, and that the observed bands could easily be assigned on the basis of these calculations. Achieving this level of accuracy in the simulations at modest computational cost should now allow computational methods to be combined with experimental Raman studies much more widely than is currently the case.     

Theoretical IR,Raman and NMR spectra of 1,2- and 1,3-dimethylenecyclobutane

Equilibrium geometries, rotational constants, harmonic vibrational frequencies, infrared intensities, Raman activities, and ¹H and ¹³C NMR spectra were calculated for 1,2-dimethylenecyclobutane and its less stable isomer 1,3-dimethylenecyclobutane by using MP2, DFT (B3PW91), and RHF theoretical methods involving the 6-311++G⁷ basis set.The properties calculated theoretically have been compared with the experimental values. The internal coordinates defined for both isomers were used in the potential energy distribution (PED) analysis. The theoretical vibrational and NMR spectra form the basis to differentiate particular compounds in reaction mixture.     

Density functional theory study of vibrational spectra. 8. Assignment of fundamental vibrational modes of 9,10-anthraquinone and 9,10-anthraquinone-d8

Density functional theory (using Becke's exchange and Lee-Yang-Parr's correlation functionals (BLYP)) and ab initio Hartree-Fock calculations were carried out in order to investigate the molecular structure and vibrational spectra of 9,10-anthraquinone and its perdeuterated analog. The calculated structural and spectral features are in good agreement with the available experimental results. Most of the BLYP/6-31G* non-CH(D) stretching frequencies are slightly lower than reliable experimental assignments; the mean absolute deviation is about 14 cm⁻¹. On the basis of agreement between calculated and experimental results, assignments of the fundamental vibrational modes were examined and some reassignments were proposed. The calculated results can serve as a guide for a future experimental search for the missing fundamentals of the target molecules.     

FT-IR, FT-Raman vibrational spectra and molecular structure investigation of 2-chloro-4-methylaniline: A combined experimental and theoretical study

In this work, the experimental and theoretical vibrational spectra of 2-chloro-4-methylaniline (2Cl4MA, C₇H₈NCl) were studied. FT-IR and FT-Raman spectra of 2Cl4MA in the liquid phase have been recorded in the region 4000–400 cm⁻¹ and 3500–50 cm⁻¹, respectively. The structural and spectroscopic data of the molecule in the ground state have been calculated by using Hartree-Fock (HF) and density functional method (B3LYP) with the 6-31G(d), 6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p) and 6-311G(d), 6-311G(d,p), 6-311+G(d,p), 6-311++G(d,p) basis sets. The vibrational frequencies have been calculated and scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and calculated frequencies are found to be in good agreement. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The DFT-B3LYP/6-311++G(d,p) calculations have been found more reliable than the ab initio HF/6-311++G(d,p) calculations for the vibrational study of 2Cl4MA. The optimized geometric parameters (bond lengths and bond angles) were compared with experimental values of aniline and p-methylaniline molecules.     

Molecular structure, vibrational spectral studies of pyrazole and 3,5-dimethyl pyrazole based on density functional calculations

In this work, the experimental and theoretical vibrational spectra of pyrazole (PZ) and 3,5-dimethyl pyrazole (DMP) have been studied. FTIR and FT-Raman spectra of the title compounds in the solid phase are recorded in the region 4000-400 cm(-1) and 4000-50 cm(-1), respectively. The structural and spectroscopic data of the molecules in the ground state are calculated using density functional methods (B3LYP) with 6-311+G** basis set. The vibrational frequencies are calculated and scaled values are compared with experimental FTIR and FT-Raman spectra. The observed and calculated frequencies are found to be in good agreement. The complete vibrational assignments are performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanical (SM) method. 13C and 1H NMR chemical shifts results are compared with the experimental values.     

Spectral analysis of acetylcholine halides by density functional theory calculations

The optimized molecular structures, vibrational frequencies and ¹H and ¹³C NMR chemical shifts of acetylcholine halides (F, Cl, and Br) have been investigated using density functional theory (B3LYP) method with 6-311G(d) basis set. The comparison of their experimental and calculated IR, R and NMR spectra of the compounds has indicated that the spectra of three optimized minimum energy conformers can simultaneously exist in one experimental spectrum. Thus, it was concluded that the compounds simultaneously exist in three conformations in the ground state. The calculated optimized geometric parameters (bond lengths and bond angles), vibrational frequencies and NMR chemical shifts for the minimum energy conformers were seen to be in a good agreement with the corresponding experimental data. All the assignments of the theoretical frequencies were performed by potential energy distributions using VEDA 4 program.     

Determination of structural and vibrational spectroscopic properties of 2-, 3-, 4-nitrobenzenesulfonamide using FT-IR and FT-Raman experimental techniques and DFT quantum chemical calculations

In this work, the molecular conformation and vibrational analysis of 2-, 3-, 4-nitrobenzenesulfonamide (abbreviated as 2-, 3-, 4-NBSA) were presented for the ground state using experimental techniques (FT-IR and FT-Raman) and density functional theory (DFT) employing B3LYP exchange correlation with the 6-311++G(d,p) basis set. The complete assignments of fundamental vibrations were performed on the basis of the experimental results and total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The effects of the nitro group substituent on the characteristic benzene sulfonamides bands in the spectra were discussed. Raman activities calculated by DFT method have been converted to the corresponding Raman intensities using Raman scattering theory. Optimized structure of compounds were interpreted and compared with the earlier reported experimental values for studied molecules. The observed and the calculated geometric parameters and vibrational wavenumbers were compared and found to be in good agreement.     

Vibrational spectrum of the spin crossover complex [Fe(phen)(2)(NCS)(2)] studied by IR and Raman spectroscopy, nuclear inelastic scattering and DFT calculations

The vibrational modes of the low-spin and high-spin isomers of the spin crossover complex [Fe(phen)(2)(NCS)(2)] (phen = 1,10-phenanthroline) have been measured by IR and Raman spectroscopy and by nuclear inelastic scattering. The vibrational frequencies and normal modes and the IR and Raman intensities have been calculated by density functional methods. The vibrational entropy difference between the two isomers, DeltaS(vib), which is--together with the electronic entropy difference DeltaS(el)--the driving force for the spin-transition, has been determined from the measured and from the calculated frequencies. The calculated difference (DeltaS(vib) = 57-70 J mol(-1) K(-1), depending on the method) is in qualitative agreement with experimental values (20-36 J mol(-1) K(-1)). Only the low energy vibrational modes (20% of the 147 modes of the free molecule) contribute to the entropy difference and about three quarters of the vibrational entropy difference are due to the 15 modes of the central FeN(6) octahedron.     

Molecular structure and vibrational spectra of phenobaraitone by density functional theory and ab initio hartree-Fock calculations

Quantum chemistry calculations have been performed using Gaussian03 program to compute optimized geometry, harmonic vibrational frequency along with intensities in IR and Raman spectra at RHF/6-31++G** and B3LYP/6-31++G** levels for phenobarbitone (C₁₂H₁₂N₂O₃) in the ground state. The scaled harmonic vibrational frequencies have been compared with experimental FT-IR and FT-Raman spectra. Theoretical vibrational spectra of the title compound were interpreted by means of potential energy distributions (PEDs) using MOLVIB program. A detailed interpretation of the infrared spectra of the title compound is reported. On the basis of the agreement between the calculated and observed results, the assignments of fundamental vibrational modes of phenobarbitone were examined and some assignments were proposed. The theoretical spectrograms for FT-IR and FT-Raman spectra of the title compound have been constructed.     

Ab Initio Calculations on Nitramide and Its Methyl Derivatives( Ⅱ )——The Harmonic Force Fields and Vibrational Spectra of Nitramide and Its Isotopic Derivatives

The harmonic force field and the vibrational spectrum of nitramide were calculated by using the ab initio gradient program TEXAS at the Hartree-Fock level with a 4-21G basis set. The directly computed theoretical harmonic force field was scaled by using empirical scale factors which are transferred from other molecules and provided an a priori prediction of fundamental frequencies and intensities. The average deviations between predicted vibrational frequencies of nitramide and experimental IR spectrum in an argon matrix are 63 cm-1 for symmetric vibrations and 41 cm-1 for antisymmetric modes. A new set of scale factors was optimized in this paper. These scale factors reduced the average deviations to 2. 3 cm-1 for symmetric modes and 0. 8 cm-1 for antisymmetric ones. The vibrational spectra of three isotopic derivatives of nitramide were predicted by using the force field resulted from the optimized set of scale factors, which are in good agreement with their experimental data in an argon matrix.     

Measuring molecular force fields: Terahertz,inelastic neutron scattering,Raman, FTIR,DFT, and BOMD molecular dynamics of solid l-serine

A vibrational analysis of polycrystalline l-serine is provided using experimental terahertz, FTIR, Raman and inelastic neutron scattering (INS) spectra, calculated INS spectra – and Born–Oppenheimer molecular dynamics (BOMD) simulations from which the power spectra for the electronegative elements are compared to the THz spectra. Corrections are made to density functional theory (DFT) calculations for van der Waals interactions. Assignments and potential energy distributions are included for all 3N = 336 normal modes of an eight molecule supercell, including those for 48 non-bonded whole molecule translating and rotating vibrations, of which three are acoustic modes, usually not considered. Calculated and observed frequencies differ by an average 3 cm⁻¹ (s = 4). The INS spectrum of these modes below 100 cm⁻¹, calculated from energy second derivatives, show a remarkable similarity to the experimental 10 K spectra. The calculated low frequency modes are insensitive to small changes in cell parameters and geometry. THz intensities are represented by power spectra and not calculated explicitly. Nevertheless, power spectra of 13 ps BOMD trajectories at classical temperatures of 20 K, 400 K, and 500 K are markedly similar to the experimental terahertz spectra at 77 K and 298 K. Calculations on a serine crystal supercell 2 × 2 × 2 molecules deep appear to include, in a crude but fortuitously accurate way, enough of the principle out of phase dispersion to yield a match with experimental frequencies and intensities.     

Spectroscopic,quantum computational and molecular docking studies on 1-phenylcyclopentane carboxylic acid

The vibrational wavenumbers of optimized molecular structure of 1-phenylcyclopentane carboxylic acid (1PCPCA) molecule have been calculated by quantum chemical theory and compared with experimental results. The density functional theory (DFT) approach is followed using the method B3LYP and 6-311++G(d,p) basis set. Using potential energy distribution, all the assignments of the basic vibrational modes were calculated. Natural bond orbital (NBO) and atoms in molecules (AIM) topological studies applied to get the intermolecular interactions of the compound. ¹H and ¹³C chemical shift of NMR was estimated on the molecule and also compared with the experimental spectra. In order to find the band gap, the time-dependent (TD-DFT) method is used to get the higher order energy levels properties and also compared with experimental data of UV–vis spectrum. From the analysis of various spectroscopic studies, there is a good relationship between the experimental and theoretical values obtained. Quantum characters, bio-active nature and reactive areas of the molecule are revealed by Fukui function, molecular electrostatic potential (MEP) and Hirshfeld surface studies. The human enzyme steroidogenic types and their protein targets were tested with this molecule by molecular docking.     

Molecular structure, IR spectra of 2-mercaptobenzothiazole and 2-mercaptobenzoxazole by density functional theory and ab initio Hartree–Fock calculations

Quantum chemistry calculations have been performed using Gaussian03 program to compute optimized geometry, harmonic vibrational frequency along with intensities in IR and Raman spectra and atomic charges at RHF/6-31+G*, B3LYP/6-31+G* and B3LYP/6-31++G* levels for 2-mercaptobenzothiazole (MBT, C₇H₅NS₂) and 2-mercaptobenzoxazole (MBO, C₇H₅NOS) in the ground state. The scaled harmonic vibrational frequencies have been compared with experimental FT-IR and FT-Raman spectra. The results show that the scaled theoretical vibrational frequencies is very good agreement with the experimental values. A detailed interpretation of the infrared and Raman spectra of 2-mercaptobenzothiazole and 2-mercaptobenzoxazole was reported. Comparison of calculated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes.     

Excited state ab initio and Franck-Condon simulation of S1 → S0 fluorescence excitation spectra of p-, m-, and o-difluorobenzenes

Although difluorobenzenes (DFBs) are well-known organic molecules to understand the electronic structure and spectroscopy of benzene and its derivatives, few theoretical investigations have been performed to simulate their fine spectra and assign their vibrational bands. In this work, the fluorescence excitation (FEX) spectra of the first excited singlet states for three DFBs molecules (para-, meta- and ortho-difluorobenzene) were simulated by the Franck-Condon calculations with the displaced harmonic oscillator approximation plus the distorted correction. The calculated results indicated that the spectral profiles of three DFBs are primarily described by the Franck-Condon progression of their totally symmetric vibrational modes. Specifically, it is found that modes v(3) and v(5) of para-DFB, v(8) and v(9) of meta-DFB, and ortho-DFB play the most important roles in the fluorescence spectra. By taking into account the contributions of the distorted effect, we could assign most of the dominant overtones from the nontotally symmetric vibrational modes, and the results agree well with the experimental assignments. Some inferred and unassigned vibrational transitions in experiment were confirmed according to the present calculated results. In addition, in the simulated fluorescence spectra, we tentatively assigned several combination bands with relative moderate intensity and weak vibrational lines which appeared in the experimental observations but the corresponding assignments were not given. The present work reproduced satisfactorily the experimental FEX spectra of p-, m-, and o-DFBs derivatives and provided a useful method to simulate the FEX spectra of dihalogenated benzene molecules.     

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.     

Synthesis,molecular conformation,vibrational, electronic transition,and chemical shift assignments of 4-(thiophene-3-ylmethoxy)phthalonitrile: a combined experimental and theoretical analysis

This work presents the synthesis and characterization of a novel compound, 4-(thiophene-3-ylmethoxy)phthalonitrile (TMP). The spectroscopic properties of the compound were examined by FT-IR, FT-Raman, NMR, and UV techniques. FT-IR and FT-Raman spectra in solid state were observed in the region 4000–400 cm⁻¹ and 3500–50 cm⁻¹, respectively. The ¹H and ¹³C NMR spectra were recorded in CDCl₃ solution. The UV absorption spectrum of the compound that dissolved in THF was recorded in the range of 200–800 nm. The structural and spectroscopic data of the molecule in the ground state were calculated using density functional theory (DFT) employing B3LYP exchange correlation and the 6-311++G(d,p) basis set. The vibrational wavenumbers were calculated and scaled values were compared with experimental FT-IR and FT-Raman spectra. The complete assignments were performed on the basis of the experimental results and total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. Isotropic chemical shifts (¹³C NMR and ¹H NMR) were calculated using the gauge-invariant atomic orbital (GIAO) method. A study on the electronic properties, such as HOMO and LUMO energies, were performed by time-dependent DFT (TD-DFT) approach. The HOMO and LUMO analyses have been used to elucidate information regarding charge transfer within the molecule. Comparison of the calculated frequencies, NMR chemical shifts, absorption wavelengths with the experimental values revealed that DFT method produces good results.     

FT-IR, FT-Raman spectra, density functional computations of the vibrational spectra and molecular conformational analysis of 2,5-di-tert-butyl-hydroquinone

The purpose of finding conformer among six different possible conformers of 2,5-di-tert-butyl-hydroquinone (DTBHQ), its equilibrium geometry and harmonic wavenumbers were calculated by the B3LYP/6-31G(d,p) method. The infrared and Raman spectra of DTBHQ were recorded in the region 400-4000 cm(-1) and 50-3500 cm(-1), respectively. In addition, the IR spectra in CCl(4) at various concentrations of DTBHQ are also recorded. The computed vibrational wavenumbers were compared with the IR and Raman experimental data. Computational calculations at B3LYP level with two different basis sets 6-31G(d,p) and 6-311++G(d,p) are also employed in the study of the possible conformer of DTBHQ. The complete assignments were performed on the basis of the potential energy distribution (PED) of the vibrational modes, calculated using VEDA 4 program. The general agreement between the observed and calculated frequencies was established.     

Physicochemical Characterization of the Loganic Acid–IR,Raman, UV-Vis and Luminescence Spectra Analyzed in Terms of Quantum Chemical DFT Approach

The molecular structure and vibrational spectra of loganic acid (LA) were calculated using B3LYP density functional theory, the 6–311G(2d,2p) basis set, and the GAUSSIAN 03W program. The solid-phase FTIR and FT-Raman spectra of LA were recorded in the 100–4000 cm⁻¹ range. The assignment of the observed bands to the respective normal modes was proposed on the basis of the PED approach. The stability of the LA molecule was considered using NBO analysis. The electron absorption and luminescence spectra were measured and discussed in terms of the calculated singlet, triplet, HOMO, and LUMO electron energies. The Stokes shift derived from the optical spectra was 20,915 cm⁻¹.     

FT-IR,FT-Raman,NMR spectra and DFT calculations on 4-chloro-N-methylaniline

In this work, the vibrational spectral analysis was carried out by using FT-IR and FT-Raman spectroscopy in the range 400–4000 and 50–3500 cm^?1 respectively, for the title molecule. The structural and spectroscopic data of the molecule in the ground state were calculated by using density functional method using 6-311++G(d,p) basis set. The vibrational frequencies were calculated and scaled values were compared with experimental FT-IR and FT-Raman spectra. The observed and calculated frequencies are found to be in good agreement. The complete assignments of all the vibrational mode were performed on the basis of the total energy distributions (TED). ¹³C and ¹H NMR chemical shifts results were given and are in agreement with the corresponding experimental values. The theoretically constructed FT-IR and FT-Raman spectra exactly coincides with experimental one.     

Theoretical and experimental study of montmorillonite intercalated with tetramethylammonium cation

Structural and vibrational features of Na-montmorillonite and montmorillonite intercalated with tetramethylammonium cation (TMA⁺) were characterized theoretically and experimentally. Theoretical study was performed using density functional theory with inclusion of dispersion corrections. The analysis of the hydrogen bonds in the calculated models has shown that the Na⁺ cations coordinated by six water molecules (Na-M model) are bound to montmorillonite layers by moderate hydrogen bonds between water molecules and basal oxygen atoms of the tetrahedral sheets. Hydrated Na⁺ cations are stabilized by relatively strong hydrogen bonds among water molecules. In the intercalate model, the TMA⁺ cation is fixed in the interlayer space by weak hydrogen bonds between the methyl groups and basal oxygen atoms of montmorillonite layers. The calculated vibrational spectra are in a good agreement with the measured infrared spectra. The detailed analysis of the simulated vibrational spectra allowed unambiguous identification of corresponding bands in the measured spectra and their assignment to the particular vibrational modes. For example, calculations clearly distinguished between AlMgOH and AlAlOH stretching vibrations and also between the coupled vibrations of the methyl groups of the TMA⁺ cations.