Density functional theory studies on the structures and vibrational spectra of 3,6-dichlorocarbazole and 3,6-dibromocarbazole

Becke 3-Lee-Yang-Parr density functional theory (DFT) calculations using 6-311G** and 6-311G(2df,p) basis sets were carried out to study molecular structures and vibrational spectra of 3,6-dichlorocarbazole and 3,6-dibromocarbazole. The optimized geometries, vibrational frequencies, IR intensities, and Raman activities have been obtained. On the basis of B3LYP calculations, a normal mode analysis was performed to assign the vibrational fundamental frequencies according to the potential energy distributions. The computational frequencies are in good agreement with the observed results.     

An ab initio (RHF) and DFT-B3LYP level spectroscopic studies of BrCCCN and the analysis of atomic polar tensors

Conventional ab initio (RHF) and DFT-B3LYP level calculations in conjunction with a variety of basis sets have been used to investigate the variations in the bond lengths, dipole moment, rotational constant, quadruple coupling constants, infrared frequencies, IR intensities and rotational invariants of BrCCCN. Satisfactory agreements between the B3LYP and experimental values were found for bond lengths, rotational constant, dipole moment and all other parameters. All the calculated bond lengths are in good agreement with each other for a given method having the average standard deviations varying between ±0.005 Å at the B3LYP level. Harmonic vibrational frequencies obtained from the B3LYP calculations show good agreement with the available experimental data. The global atomic polar tensor charges, used for interpreting the IR intensities, of all the atoms of BrCCCN have been calculated at the RHF and B3LYP levels in conjunction with the 6-311g(d) and 6-311++g(d,p) basis sets. Linear regression analysis between calculated and experimental infrared frequencies as well as between IR intensities in a series of 15 similar type of nitrile compounds have been achieved at the B3LYP/6-311++g(d,p) level and gives linear regression coefficients 0.983 and 0.970 respectively. Finally, a number of linear relations were found between r(CN) bond lengths and GAPT charges, and GAPT charges on CHELPG and MK charges at the nitrogen atom for these molecules and proved to be producing satisfactory results at the B3LYP/6-311++g(d,p) level of calculations.     

FT-IR and NMR investigation of 2-(1-cyclohexenyl)ethylamine: a combined experimental and theoretical study

FT-IR and (1)H, (13)C, DEPT, HETCOR, COSY, and NOESY NMR spectra of 2-(1-cyclohexenyl)ethylamine (CyHEA) have been reported for the first time. The vibrational frequencies and (1)H, (13)C NMR chemical shifts of CyHEA (C(8)H(15)N) have been calculated by means of the Hartree-Fock (HF), Becke-Lee-Yang-Parr (BLYP) and Becke-3-Lee-Yang-Parr (B3LYP) density functional methods with 6-31 G(d) and 6-31 G(d,p) basis sets, respectively. The comparison between the experimental and the theoretical results indicates that density functional B3LYP method is superior to the scaled HF and BLYP approach for vibrational frequencies and predicting NMR properties.     

Theoretical prediction of gas-phase infrared spectra of imidazo[1,2-a]pyrazinediones and imidazo[1,2-a]imidazo[1,2-d]pyrazinediones derived from glycine

Imidazo[1,2-a]pyrazine-3,6-diones and imidazo[1,2-a]imidazo[1,2-d]pyrazine-3,8-diones can be produced by pyrolysis of simple amino acids. While such bicyclic and tricyclic amidines were detected and characterized by IR spectroscopy for some alpha-substituted amino acids, the parent systems composed of glycine fragments are unknown up to now. IR spectra for five amidines derived from glycine were calculated by using different semi-empirical (PM3, AM1, MNDO and MINDO/3), HF, and hybrid DFT (B3LYP, B3P86 and B3PW91) methods in conjunction with 6-31G(d) basis set (for HF and DFT). Vibration frequencies in the experimental IR spectra were predicted based upon the B3LYP data, by correcting the calculated wavenumbers by a scaling factor of 0.959. The behavior of most characteristic bands (nu(CX), nu(NH), etc.) and their shifts with respect to such bands in the spectra of alanine and alpha-aminoisobutyric acid derivatives studied before, are discussed. Performance of the semi-empirical methods was tested, bearing in mind possible future needs for IR spectra predictions for larger molecular systems of similar chemical nature; the use of MINDO/3 and MNDO is recommended. A basis set effect on the B3LYP fundamental vibration frequencies for hexahydroimidazo[1,2-a]pyrazine-3,6-dione was studied by varying Pople basis sets from minimal STO-3G to 6-311++G(d, p). No significant improvements were found beyond the 6-31G(d) basis set, which thus can be recommended to predict IR spectra for the amidines and similar molecules.     

FT-IR and NMR investigation of 1-phenylpiperazine: a combined experimental and theoretical study

FT-IR and (1)H, (13)C, DEPT, COSY, NOESY, HETCOR, INADEQUATE NMR spectra of 1-phenylpiperazine (pp) have been reported for the first time except for its (1)H NMR spectrum. The vibrational frequencies and (1)H, (13)C NMR chemical shifts of pp (C(10)H(14)N(2)) have been calculated by means of the Hartree-Fock (HF) and Becke-Lee-Yang-Parr (BLYP) or Becke-3-Lee-Yang-Parr (B3LYP) density functional methods with 6-31G(d) and 6-31G(d,p) basis sets, respectively. Comparison between the experimental and the theoretical results indicates that density functional B3LYP method is superior to the scaled HF and BLYP approach for predicting vibrational frequencies and NMR properties.     

NMR spectra, GIAO and charge density calculations of five-membered aromatic heterocycles

The B3LYP/6-31+G(d) molecular geometry optimized structures of 17 five-membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311+G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N-methylpyrrole (2) and thiophene (7) using the larger 6-311++G(d,p) basis sets at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,p) and B3LYP/cc-pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the (1)H and 13C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6-31 basis set. The values of nJHH(n=3, 4, 5) and rmnJ(CH)(n=1, 2, 3, 4) were predicted well using the larger 6-31+G(d,p) and 6-311++G(d,p) basis sets and at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz-Kollman (MK); CHELP and CHELPG] for the B3LYP/6-311++G(d,p) geometry optimized structures of 1-17 were used to explore correlations with the experimental proton and carbon chemical shifts.     

Quantum DFT and DF–DFT study of vibrational spectra of sulfuric acid,sulfuric acid monohydrate,formic acid and its cyclic dimer

Rigorous theoretical treatment of vibrational frequencies is critically important for the interpretation of unassigned experimental vibrational spectra and accurate determination of thermodynamic properties of molecular clusters. IR spectra of trans monomers of sulfuric and acetic acids, sulfuric acid monohydrate and cyclic dimer of the formic acid have been studied using DFT and DF–DFT methods using BLYP, B3LYP and PW91 with 12 different Pople and Dunning basis sets. New data for above-mentioned structures have been reported, scaling factors have been calculated and a comprehensive analysis of the performance of BLYP, B3LYP and PW91 methods has been performed. Comparison of the obtained results with experiments shows that results of pure PW91 and BLYP are better than predictions of well-established hybrid B3LYP method. Our analysis shows on the existence of the considerable difference in scaling factors weighted to high and low frequencies. In the case of formic acid dimer, the deviation the predicted low frequencies from the experimental data is considerable that leads to quite large (∼6–7 kcal mol⁻¹) uncertainties in the absolute values of the cluster Gibbs free energy. In order to determine an efficient computational strategy that comprises accuracy and reasonable computational costs, the effect of density fitting (DF) and basis set superposition error (BSSE) on the vibration frequencies has been investigated. We found that application of the DF that substantially (2.5–3.5 times) increases the performance of pure PW91 and BLYP methods gives excellent results, which are very close to those of conventional DFT. This suggests that DF–DFT is a viable low-cost alternative to conventional DFT in predicting vibrational spectra. It has been found that while vibrational spectra obtained using the counterpoise correction (CP) for the BSSE do not deviate much from uncorrected ones, the difference in absorption intensities between corrected and uncorrected spectra obtained using small and medium-sized basis sets is considerable. This suggests that application of DF–DFT uncorrected for the BSSE with large basis sets is a more efficient strategy of predicting vibrational spectra than the application of conventional DFT with small basis sets.     

A study on quantum chemical calculations of 3-, 4-nitrobenzaldehyde oximes

The molecular geometry, vibrational frequencies, 1H and 13C NMR chemical shifts, UV-vis spectra, HOMO-LUMO analyses, molecular electrostatic potentials (MEPs), , thermodynamic properties and atomic charges of 3- and 4-Nitrobenzaldehyde oxime (C7H6N2O3) molecules have been investigated by using Hartree-Fock (HF) and density functional theory (DFT/B3LYP) methods with the 6-311++G(d, p) basis set. The calculated optimized geometric parameters (bond lengths and bond angles), the vibrational frequencies calculated and 13C and 1H NMR chemical shifts values for the mentioned compounds are in a very good agreement with the experimental data. Furthermore, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) have been simulated and the transition states, energy band gaps and molecular electrostatic potential (MEP) maps for each oxime compound have been determined. Additionally, we also report the infrared intensities and Raman activities for the compounds under study.     

Experimental (FT-IR and FT-Raman), electronic structure and DFT studies on 1-methoxynaphthalene

In this work, FT-IR and FT-Raman spectra of 1-methoxynapthalene (C(11)H(10)O) have been reported in the regions 4000-400 cm(-1) and 3500-100 cm(-1), respectively. Density functional method (DFT) has been used to calculate the optimized geometrical parameters, atomic charges, vibrational wavenumbers and intensity of the vibrational bands. The vibrational frequencies have been calculated and scaled values are compared with experimental FT-IR and FT-Raman spectra. The structure optimizations and normal coordinate force field calculations are based on density functional theory (DFT) method with B3LYP/3-21G, B3LYP/6-31G, B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p) basis sets. The complete vibrational assignments of wavenumbers are made on the basis of potential energy distribution (PED). The optimized geometric parameters are compared with experimental values of naphthoic acid. The results of the calculation shows excellent agreement between experimental and calculated frequencies in B3LYP/6-311++G(d,p) basis set. The effects due to the substitutions of methyl group and carbon-oxygen bond are also investigated. A study on the electronic properties, such as excitation energies and wavelengths, were performed by time-dependent DFT (TD-DFT) approach. HOMO and LUMO energies are calculated that these energies show charge transfer occurs within the molecule.     

DFT calculation and assignment of vibrational spectra of aryl and alkyl chlorophosphates

DFT calculations of vibrational spectra of chlorophosphates using wide range of basis sets and hybrid functionals were performed. Good agreement between calculated and experimental vibrational spectra was reached by the combination of non-empirical functional PBE0 with both middle and large basis sets. The frequencies of the stretching vibrations of the phosphate group calculated using semi-empirical functional B3LYP for all basis sets deviate significantly from the experimental values. The number of polarization functions on heavy atoms was shown to be a key factor for the calculation of vibrational frequencies of organophosphates. The importance of consideration of all the stable rotamers for a complete assignment of fundamental modes was shown.      

Spectroscopic, electronic structure and natural bond orbital analysis of o-fluoronitrobenzene and p-fluoronitrobenzene: a comparative study

Experimental FTIR, FT-Raman and FT-NMR spectroscopic studies of o-fluoronitrobenzene and p-fluoronitrobenzene have been carried out. A detailed quantum chemical calculations have been performed using DFT/B3LYP method with 6-311++G** and 6-31G** basis sets. Complete vibrational analyses of the compounds were performed. The temperature dependence of thermodynamic properties has been analysed. The atomic charges, electronic exchange interaction and charge delocalisation of the molecule have been performed by natural bond orbital (NBO) analysis. Molecular electrostatic surface potential (MESP), total electron density distribution and frontier molecular orbitals (FMOs) are constructed at B3LYP/6-311++G** level to understand the electronic properties. The charge density distribution and site of chemical reactivity of the molecules have been obtained by mapping electron density isosurface with electrostatic potential surfaces (ESP). The electronic properties, HOMO and LUMO energies were measured by time-dependent TD-DFT approach. (1)H and (13)C NMR spectra were recorded and (1)H and (13)C nuclear magnetic resonance chemical shifts of the molecule were calculated. The (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of the molecules in chloroform solvent and in gas phase were calculated by using the Gauge-Independent Atomic Orbital (GIAO) method and are found to be in good agreement with experimental values. The theoretical parameters obtained at B3LYP levels have been compared with the experimental values.     

Theoretical studies on vibrational spectra of some halides of group IVB elements

The vibrational spectra of group IVB elements halides MX4 (M=Ti(IV), Zr(IV), Hf(II); X=F, Cl, Br and I), have been investigated by ab initio RHF, MP2 and density functional theory B3LYP method with LanL2DZ basis sets. The optimized geometries, calculated vibrational frequencies and Far-IR intensities of MX4 are evaluated via comparison with experimental data. The vibrational frequencies, calculated by these methods, are compared to each other. The results indicate that B3LYP method is more reliable than RHF and MP2 methods for the frequencies calculations for these compounds. With this method, some vibrational frequencies of M2X6(2+)(M=Ti(IV), Zr(IV) and Hf(II); X=F, Cl, Br and I) are also predicted.     

Theoretical study on infrared vibrational spectra of boric-acid in gas-phase using density functional methods

Density functional theory (DFT) methods with various exchange-correlation functionals such as SVWN, BVWN, BVWN5, BLYP, B1LYP, B3LYP, B3PW91, and BH and H are employed in a theoretical study of molecular boric-acid in gas-phase. In the calculations, the split valence 6-311++G** and 6-31G* basis sets were used. The geometry, zero-point vibrational energies (ZPVEs), and harmonic infrared vibrational (IR) frequencies are predicted. The calculated C_3h-symmetry geometrical parameters are compared with Hartree–Fock (HF) calculation results and experimental data. IR frequencies predicted by the BLYP, B3LYP, and B3PW91 calculations are in good agreement with experimental data. The frequency calculations presented here also suggest that the C_3h-symmetrical structure corresponds to a minimum in the potential energy surface, but neither is D_3h- or C₃-symmetrical structure.     

Quantum‐chemical,IR, NMR,and X‐ray diffraction studies on 2‐(4‐chlorophenyl)‐1‐methyl‐ 1H‐benzo[d]imidazole

The title molecule, 2‐(4‐chlorophenyl)‐1‐methyl‐1H‐benzo[d]imidazole (C₁₄H₁₁ClN₂), was prepared and characterized by ¹H NMR, ¹³C NMR, IR, and single‐crystal X‐ray diffraction. The molecular geometry, vibrational frequencies, and gauge including atomic orbital (GIAO) ¹H and ¹³C NMR chemical shift values of the title compound in the ground state have been calculated by using the Hartree‐Fock (HF) and density functional theory (DFT/B3LYP) method with 6‐31G(d) basis sets, and compared with the experimental data. The calculated results show that the optimized geometries can well reproduce the crystal structural parameters, and the theoretical vibrational frequencies and GIAO ¹H and ¹³C NMR chemical shifts show good agreement with experimental values. The energetic behavior of the title compound in solvent media has been examined using B3LYP method with the 6‐31G(d) basis set by applying the Onsager and the polarizable continuum model (PCM). Besides, molecular electrostatic potential (MEP), frontier molecular orbitals (FMO) analysis, and nonlinear optical (NLO) properties of the title compound were investigated by theoretical calculations. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Investigation of structure, vibrational and NMR spectra of oxycodone and naltrexone: a combined experimental and theoretical study

In this work, two important opioid antagonists, naltrexone and oxycodone, were prepared from thebaine and were characterized by IR, (1)H NMR and (13)C NMR spectroscopy. Moreover, computational NMR and IR parameters were obtained using density functional theory (DFT) at B3LYP/6-311++G** level of theory. Complete NMR and vibrational assignment were carried out using the observed and calculated spectra. The IR frequencies and NMR chemical shifts, determined experimentally, were compared with those obtained theoretically from DFT calculations, showed good agreements. The RMS errors observed between experimental and calculated data for the IR absorptions are 85 and 105 cm(-1), for the (1)H NMR peaks are 0.87 and 0.17 ppm and for those of (13)C NMR are 5.6 and 5.3 ppm, respectively for naltrexone and oxycodone.     

A critical analysis of electronic density functionals for structural,energetic, dynamic,and magnetic properties of hydrogen fluoride clusters

We present extensive computational results on density functional calculations for hydrogen fluoride species (HF)_n (with 1≤n≤6) and compare them to results from other approaches and experiments, where available. Among the calculated properties we discuss equilibrium structural parameters, vibrational frequencies, electric dipole moments, IR intensities, dissociation energies, barriers for rearrangement by proton tunneling, NMR chemical shifts and spin couplings for ¹H and ¹⁹F, and magnetic susceptibilities. It is found that density functional (particularly BLYP) and even more so hybrid approaches (particularly B3LYP) provide useful results. However, we show that due to some characteristic deficiencies, these are in general not competitive with more quantitative results from large basis set MP2 calculations. The calculated magnetic properties do not indicate any “aromaticity” connected to a hypothetical electronic ring current. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1695–1719, 1997     

[M(Im)2X2]型配合物的Far-IR和Raman光谱的理论研究

本文用从头计算RHF和密度泛函B3LYP方法以及LanL2DZ，SDD和6-31G(d)基组计算了配合物M(Im)₂X₂ (Im=imidazole；M=Zn(Ⅱ)，Pd(Ⅱ)，Pt(Ⅱ)；X=F，Cl，Br，I)的几何构型以及Far-IR和Raman振动频率。计算结果表明，对Zn(Ⅱ)配合物而言,B3LYP/6-31G(d)方法得到的几何参数与实验值吻合得最好，B3LYP/SDD次之。在计算Far-IR和Raman振动频率时，发现采用6-31G(d)基组，两种方法计算的结果差别不大。对LanL2DZ和SDD基组而言，对计算结果影响较大的是理论方法，基组影响甚微，个别的振动频率基组影响较大，相比较而言，SDD基组得到的结果更好一些。本文所使用的两种计算方法都能得到与实验值比较吻合的结果，而用从头计算RHF方法计算的结果与实验值更接近一些。在此基础上，预测了Pd(Ⅱ)和Pt(Ⅱ)配合物的Far-IR和Raman振动频率。     

Comparison between optimized geometries and vibrational frequencies calculated by the DFT methods for the Anderson-type heteropolyanion: hexamolybdoaluminate(III), [AlIII(OH)6Mo6O18]3-

Optimized geometries and vibrational frequencies were calculated for the hexamolybdoaluminate(III), [AlIII(OH)6Mo6O18]3-, Anderson-type heteropolyanion with the HF, B3LYP, B3PW91, B3P86 and B1LYP methods of theory using the LanL2DZ, SDD and combination of LanL2DZ with 6-31G (d, p) basis sets. The agreement between the optimized and experimental geometries was in the decreasing order: HF, B3P86, B3PW91, B1LYP and B3LYP. The calculated frequencies by the B3LYP have the smallest mean root mean square (RMS) error. The effect of the basis set on the calculated bond lengths and frequencies by the density functional calculations (DFT) methods was minor. The agreement between the previously reported IR and Raman spectra and the calculated values is, in general, good.     

Structure, stability and vibrational spectrum of the hydrogen-bonded complex between HNO3 and H2O. Ab initio and DFT studies

The structure, stability and vibrational spectrum of the binary complex between HONO2 and H2O have been investigated using ab initio calculations at SCF and MP2 levels with different basis sets and B3LYP/6-31G(d,p) calculations. Full geometry optimization was made for the complex studied. It was established that the hydrogen-bonded H2O...HONO2 complex has a planar structure. The corrected values of the dissociation energy at the SCF and MP2 levels and B3LYP calculations are indicative of relatively strong OH...O hydrogen-bonded interaction. The changes in the vibrational characteristics (vibrational frequencies and infrared intensities) arising from the hydrogen bonding between HONO2 and H2O have been estimated by using the ab initio calculations at SCF and MP2 levels and B3LYP/6-31G(d,p) calculations. It was established that the most sensitive to the complexation is the stretching O-H vibration from HONO2. In agreement with the experiment, its vibrational frequency in the complex is shifted to lower wavenumbers. The predicted frequency shift with the B3LYP/6-31G(d,p) calculations (-439 cm(-1)) is in the best agreement with the experimentally measured (-498 cm(-1)). The intensity of this vibration increases dramatically upon hydrogen bonding. The ab initio calculations at the SCF level predict an increase up to five times; at the MP2 level up to 10 times and the B3LYP/6-31G(d,p) predicted increase is up to 17 times. The good agreement between the predicted values of the frequency shifts and those experimentally observed show that the structure of the hydrogen-bonded complex H2O...HONO2 is reliable.