Magnetic resonance tensors in Uracil: Calculation of 13C, 15N, 17O NMR chemical shifts, 17O and 14N electric field gradients and measurement of 13C and 15N chemical shifts

The experimental ¹³C NMR chemical shift components of uracil in the solid state are reported for the first time (to our knowledge), as well as newer data for the ¹⁵N nuclei. These experimental values are supported by extensive calculated data of the ¹³C, ¹⁵N and ¹⁷O chemical shielding and ¹⁷O and ¹⁴N electric field gradient (EFG) tensors. In the crystal, uracil forms a number of strong and weak hydrogen bonds, and the effect of these on the ¹³C and ¹⁵N chemical shift tensors is studied. This powerful combination of the structural methods and theoretical calculations gives a very detailed view of the strong and weak hydrogen bond formation by this molecule. Good calculated results for the optimized cluster in most cases (except for the EFG values of the ¹⁴N3 and ¹⁷O4 nuclei) certify the accuracy of our optimized coordinates for the hydrogen nuclei. Our reported RMSD values for the calculated chemical shielding and EFG tensors are smaller than those reported previously. In the optimized cluster the 6-311+G** basis set is the optimal one in the chemical shielding and EFG calculations, except for the EFG calculations of the oxygen nuclei, in which the 6-31+G** basis set is the optimal one. The optimal method for the chemical shielding and EFG calculations of the oxygen and nitrogen nuclei is the PW91PW91 method, while for the chemical shielding calculations of the ¹³C nuclei the B3LYP method gives the best results.     

Spectral and Computational Studies on Benzil Mono-(2-Pyridyl)Hydrazone

In the present study, structural properties of Mono-(2-Pyridyl) Hydrazone were studied extensively utilizing density functional theory (DFT) employing B3LYP exchange correlation. The Fourier transform infrared (solid phase) was recorded. The vibrational frequencies in the ground state were calculated by using density functional method (B3LYP) with 6-31G* and 6-311G** as basis sets. The spectral studies revealed that the title compound exists in Keto form. Spectral techniques that we employed include ¹H and ¹³C NMR, electronic, thermal techniques. Correlation between experimental chemical shifts and GIAO/B3LYP/6-311G**-calculated isotropic shielding constants, δ_exp = a + bσ_calc, are reported. Good linear regressions between experimental and theoretical results for ¹H and ¹³C were obtained.     

Molecular structure, spectroscopic properties and DFT calculations of 2-(methylthio)nicotinic acid

The analyses of possible conformations, molecular structures, vibrational and electronic properties of 2-(methylthio)nicotinic acid molecule, C₇H₇NO₂S, with the synonym 2-(methylsulfanyl)nicotinic acid have been first presented theoretically. At the same time, FT-IR and micro-Raman spectra of 2-(methylthio)nicotinic acid were recorded in the regions 400–4000 cm^?1 and 100–4000 cm^?1, respectively. In our calculations, the DFTB3LYP method with 6–311G(d, p) basis set was used to have the structural and spectroscopic data about the mentioned molecule in the ground state and the results obtained were compared with experimental values. Furthermore, gauge invariant atomic orbital (GIAO) ¹H and ¹³C NMR chemical shifts in different solvents, UV-vis TD-DFT calculations, the highest occupied molecular orbitals (HOMO-2, HOMO-1, HOMO), lowest unoccupied molecular orbital (LUMO), molecular electrostatic potantial (MEP) surface, atomic charges and thermodynamic properties of molecule have been theoretically verified and simulated at the mentioned level. The energetic behavior of title molecule in different solvent media was investigated by using DFT/B3LYP method with 6–311G(d, p) basis set in terms of integral equation formalism polarizable continuum model (IEFPCM). In addition, the calculated infrared intensities, Raman activities, reduce masses and force constants of the compound under study have been also reported.     

H, C NMR and JCH coupling constants investigation of 4-Phenylpyridine: A combined experimental and theoretical study

Proton coupled and uncoupled ¹³C, ¹H, DEPT, COSY and HETCOR NMR spectra of 4-Phenylpyridine (4-Phpy) have been reported for the first time except for its ¹H NMR spectrum. In order to provide a precise structural elucidation for carbon atoms those have very close chemical shifts to each other, the magnitude of ⁿJ_CH (n=1,2,3) coupling constants of 4-Phpy (C₁₁H₉N) have also been investigated. ¹³C, ¹H NMR chemical shifts and ^1-3J_CH coupling constants of 4-Phpy have been calculated by means of B3LYP density functional method with 6-311++G(d,p) basis set. Moreover, the optimized parameters (bond lengths, bond and torsion angles) of 4-Phpy have been calculated with B3LYP at 6-31G(d) level in methanol (ε=32.63). Comparison between the experimental and the theoretical results indicates that density functional B3LYP method is able to provide satisfactory results for predicting NMR properties.     

Density functional study of the molecular structures,infrared and Raman spectra of carbon suboxide,its sulfur and selenium analogues

Carbon suboxide, and its sulfur and selenium analogues in D _∞h symmetry have been studied in the gas phase by a density functional method using B3LYP as the functional. The basis sets employed are 6-31++G(d,p), 6-311++G(d,p), cc-pVDZ and all calculations have been carried out using Gaussian 03W. Molecular parameters, namely bond lengths, rotational constants, quadrupole moments, and infrared and Raman frequencies are predicted for these molecules. Atomization energies have also been predicted. The calculated molecular parameters and vibrational spectra of the parent molecule, namely carbon suboxide, are in good agreement with literature data. Therefore, data from the present theoretical gas phase study are expected to be valid for the molecular structures and vibrational spectra of carbon subsulfide and carbon subselenide. The results from this study could be used as a reference for these molecules.     

A combined experimental and theoretical study on 8-hydroxy-2-quinolinecarboxylic acid

The mid-IR and Raman spectra of 8-hydroxy-2-quinolinecarboxylic acid (8HQC) were recorded. These spectra were interpreted with the help of B3LYP/6–311 ++G(d,p) calculations and potential energy distribution (PED) analysis. As a result of the calculations, seven tautomers were determined among many stable conformations. The experimental spectra were concordant with the theoretical data of one tautomer. In the functional group region overtone and combination bands were detected and assigned. In addition, because of several peaks in the IR spectrum, it was proposed that the 8HQC exhibits dimerization in condensed phase. Possible dimeric forms of 8HQC were evaluated at the same level of theory, and it has been seen that the calculation results confirm the above proposal. ¹H and ¹³C NMR chemical shifts of 8HQC have been calculated, and compared with the experimental data. The frontier molecular orbital properties and the atomic charges were also theoretically obtained and presented.     

Nuclear spin relaxation and magnetic shielding tensors of atoms constituting cyanogen bromide molecule

The temperature dependence of the correlation time describing reorientation kinetics of cyanogen bromide in CDCl₃ solution has been determined on the basis of the linewidths of the ¹⁴N NMR signal. It has been found that the longitudinal spin relaxation of the ¹⁵N nucleus occurs by shielding anisotropy and spin-rotation mechanisms, whereas for the ¹³C nucleus these mechanisms are of lesser importance. In the latter case the scalar relaxation of the second kind due to carbon-bromine coupling is the predominant relaxation mechanism. The parameter values: ¹ J(¹³C—⁷⁹Br) = 349 ± 10 Hz, T ₁ (⁷⁹Br, 303 K) = 2.31 ± 0.22 × 10^?7 s, Δσ(¹⁵N) = 565 ± 16 ppm and Δσ(¹³C) = 276 ± 120 ppm have been determined from the relaxation data analysis. The shielding anisotropy parameters Δσ(¹⁵N) = 580 ± 50 ppm and Δσ(¹³C) = 274 ± 9 ppm have been independently determined using ¹³C and ¹⁵N NMR in liquid crystalline solvent. The experimentally determined shielding tensors for sp-hybridized atoms in the investigated compound and in a series of bromoacetylenes have been compared with the results of quantum mechanical calculations [GIAO, DFT B3LYP/6-311 + +G(2d,p)]. The ‘heavy atom effect’ shielding bromine-bonded carbons is of the order of — 25 ppm and concerns mainly the σ⊥ component.     

Experimental (UV,NMR, IR and Raman) and theoretical spectroscopic properties of 2-chloro-6-methylaniline

In this work, the experimental and theoretical UV, NMR and vibrational spectra of 2-chloro-6-methylaniline (2-Cl-6-MA, C₇H₈NCl) were studied. The ultraviolet absorption spectra of compound that dissolved in ethanol were examined in the range of 200–400 nm. The ¹H, ¹³C and DEPT NMR spectra of the compound were recorded. FT-IR and FT-Raman spectra of 2-Cl-6-MA in the liquid phase were 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 were calculated using density functional theory (DFT) employing B3LYP exchange correlation and the 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 were 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. Isotropic chemical shifts were calculated using the gauge-invariant atomic orbital (GIAO) method. Comparison of the calculated NMR chemical shifts and absorption wavelengths with the experimental values revealed that DFT method produces good results.     

Spectroscopic analysis,AIM, NLO and VCD investigations of acetaldehyde thiosemicarbazone using quantum mechanical simulations

The prepared Acetaldehyde thiosemicarbazone (ATSC) have been investigated by both the experimental and theoretical methods; through this work, the essentiality of elucidation of molecular fragments source linear and non-linear optical properties was explored. The stability of the structure and entire calculations have been performed on HF and B3LYP methods with 6-311++G(d,p) level of basis set. The Mulliken charge profile, electronic, optical and hyper polarizability analyses have been carried out in order to evaluate nonlinear optical (NLO) performance of the present compound. The exact optical location of the ATSC was determined by executing UV–Visible calculations on TDSCF method. The existence of the molecular group for the inducement and tuning of NLO properties were thoroughly investigated by performing fundamental vibrational investigation. The optical energy transformation among frontier molecular levels has been described in UV–Visible region. The Gibbs energy coefficient of thermodynamic functions was monitored in different temperature and it was found constant irrespective of temperatures. The appearance of different chemical environment of H and C was monitored from the ¹H and ¹³C NMR spectra. The vibrational optical polarization characteristics with respect to molecular composition in the compound have been studied by VCD spectrum. The bond critical point, Laplacian of electron density, electron kinetic energy density and total electron energy density have calculated and analysed using AIM study.     

Molecular structure,NMR and vibrational spectral analysis of 2,4‐difluorophenol by ab initio HF and density functional theory

Quantum chemical calculations of energies, geometries and vibrational wavenumbers of 2,4‐difluorophenol (2,4‐DFP) were carried out by using ab initio HF and density functional theory (DFT/B3LYP) methods with 6‐311G(d,p) as basis set. The optimized geometrical parameters obtained by HF and DFT calculations are in good agreement with related molecules. The best level of theory in order to reproduce the experimental wavenumbers is the B3LYP method with the 6‐311G(d,p) basis set. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. A detailed interpretation of the infrared and Raman spectra of 2,4‐DFP is also reported. The entropy of the title compound was also performed at HF/6‐311G(d,p) and B3LYP/6‐311G(d,p) levels of theory. The isotropic chemical shift computed by ¹H, ¹³C NMR analyses also shows good agreement with experimental observations. The theoretical spectrograms for FT‐IR and FT‐Raman spectra of the title molecule have been constructed. Copyright © 2009 John Wiley & Sons, Ltd.     

Density-dependent magnetic shielding in gas phase 13C N.M.R.

Density-dependent ¹³C nuclear magnetic shielding has been found for each of the pure gases CH₄, C₂H₆, C₂H₄, CO and CO₂, and for several binary mixtures of gases. For methane gas the density dependence is greater at higher temperatures in contrast to expectation and the observed temperature dependence of the shielding at zero density is attributable to nuclear motion. ¹³C magnetic shielding is considerably higher in the gas phase than in the liquid phase and the difference varies for chemically non-equivalent ¹³C nuclei by amounts which are well above the level of experimental error.     

Experimental vibrational spectra (Raman,infrared) and DFT calculations on monomeric and dimeric structures of 2‐ and 6‐bromonicotinic acid

In this work, the Fourier transform infrared and Raman spectra of 2‐bromonicotinic acid and 6‐bromonicotinic acid (abbreviated as 2‐BrNA and 6‐BrNA, C₆H₄BrNO₂) have been recorded in the region 4000–400 and 3500–50 cm⁻¹. The optimum molecular geometry, normal mode wavenumbers, infrared intensities and Raman scattering activities, corresponding vibrational assignments and intermolecular hydrogen bonds were investigated with the help of B3LYP density functional theory (DFT) method using 6‐311++G(d,p) basis set. Reliable vibrational assignments were made on the basis of total energy distribution (TED) calculated with scaled quantum mechanical (SQM) method. From the calculations, the molecules are predicted to exist predominantly as the C1 conformer. Copyright © 2009 John Wiley & Sons, Ltd.     

Molecular structure,experimental and theoretical 1H and 13C NMR chemical shift assignment of cyclic and acyclic α,β‐unsaturated esters

The experimental ¹H and ¹³C NMR spectra of 13 phenyl cinnamates and four 4‐methylcoumarins were investigated and their chemical shifts assigned on the basis of the two‐dimensional spectra. For the unsubstituted cinnamic acid phenyl ester, optimized molecular structures were calculated at a B3LYP/6‐311++G(d,p) level of theory. ¹H and ¹³C NMR chemical shifts were also calculated with the GIAO method at the B3LYP/6‐311 + G(2d,p) level of theory. The comparison between experimental and calculated NMR chemical shift suggests that the experimental spectra are formed from the superposition spectra of the two lowest energy conformers of the compound in solution. The most stable s‐cis configuration found in our studies is also the conformation adopted for a related phenyl cinnamate in solid state. The experimental results were analyzed in terms of the substituent effects. Copyright © 2013 John Wiley & Sons, Ltd.     

A combined experimental and computational studies of 3,3,6,6-Tetramethyl-9-(4-Methoxyphenyl)3,4,6,7,9,10 hexahydroacridine-1,8-dione

ABSTRACT

In this work, we have recorded the Fourier Transform Infrared (FTIR) and Ultra-Violet Visible (UV–Vis) spectra of 3,3,6,6-Tetramethyl-9-(4-Methoxyphenyl)3,4,6,7,9,10 hexahydroacridine-1,8-dione (C₂₄H₂₉NO₃) in the spectral range 4000–400?cm^?1 and 190–1400?nm, respectively. The thermo gravimetric (TG) analysis of the compound has been performed to check the thermal stability of the compound. The molecular geometry and complete vibrational spectra in the ground state are calculated by Hartree Fock (HF) and Density Functional Theory (DFT) using6-311G(d,p) basis set. The calculated vibrational harmonic frequencies are scaled using a proper scale factor, yielding a good agreement with the experimental data. Stability of the molecule arising from hyperconjugative interactions, charge delocalisation has been studied using natural bond orbital analysis (NBO). Mulliken charges, MEP mapping and temperature dependence on the thermodynamic properties in the optimised ground state have been calculated. UV–Visible spectrum of the molecule was calculated by using TD-DFT approach and the results were compared with the experimental one. We have calculated the several molecular parameters like ionisation potential, electron affinity, global hardness, electron chemical potential, electronegativity and global electrophilicity based on HOMO and LUMO energy values calculated at B3LYP/6-311G(d,p) level of theory. The calculated optimised structural parameters and vibrational wavenumbers are found to be in good agreement with the experimental results.     

Calculation of torsional and rotational raman spectra of hydrogen peroxide

We present calculated intensity distributions in torsional, rotational, and torsional-rotational Raman lines in spectra of hydrogen peroxide. Ab initio calculations of polarizability tensor components as functions of internal rotation angle were carried out in the HF/6-311G approximation. It is shown that the structure and transformational properties of the polarizability tensor components of hydrogen peroxide in extended molecular symmetry group G₄(EM) permit formation of purely rotational and torsional and rotational-torsional Raman spectra. Common expressions to calculate Raman line intensities governed by torsional and rotational motions of the non-rigid symmetric top molecule are obtained. The torsional components of the line intensities have been calculated by estimating the appropriate matrix elements. The contribution of rotational components has been calculated using the 3j-symbols technique.     

Accurate 13-C and 15-N molecular crystal chemical shielding tensors from fragment-based electronic structure theory

Standard nuclear magnetic resonance (NMR) spectroscopy experiments measure isotropic chemical shifts, but measuring the chemical shielding anisotropy (CSA) tensor can provide additional insights into solid state chemical structures. Interpreting the principal components of these tensors is facilitated by first-principles chemical shielding tensor predictions. Here, the ability to predict molecular crystal CSA tensor components for ¹³C and ¹⁵N nuclei with fragment-based electronic structure techniques is explored. Similar to what has been found previously for isotropic chemical shifts, the benchmarking demonstrates that fragment-based techniques can accurately reproduce CSA tensor components. The use of hybrid density functionals like PBE0 or B3LYP provide higher accuracy than generalized gradient approximation functionals like PBE. Unlike for planewave density functional techniques, hybrid density functionals can be employed routinely with modest computational cost in fragment approaches. Finally, good consistency between the regression parameters used to map either isotropic shieldings or CSA tensor components is demonstrated, providing further evidence for the quality of the models and highlighting that models trained for isotropic shifts can also be applied to CSA tensor components.     

Experimental and theoretical FT‐IR and FT‐Raman spectroscopic analysis of N1‐methyl‐2‐chloroaniline

In this work, the experimental and theoretical vibrational spectra of N1‐methyl‐2‐chloroaniline (C₇H₈NCl) were studied. FT‐IR and FT‐Raman spectra of the title molecule in the liquid phase were 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 were calculated by using density functional method (B3LYP) with the 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 were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. ¹³C and ¹H NMR chemical shifts results were compared with the experimental values. The optimized geometric parameters (bond lengths and bond angles) were given and are in agreement with the corresponding experimental values of aniline and p‐methyl aniline. Copyright © 2008 John Wiley & Sons, Ltd.     

Molecular structure,spectroscopic (FTIR,FTIR gas phase,FT‐Raman) first‐order hyperpolarizability and HOMO–LUMO analysis of 4‐methoxy‐2‐methyl benzoic acid

Vibrational spectral analysis was carried out for 4‐methoxy‐2‐methyl benzoic acid (4M2MBA) by using Fourier transform infrared (FT‐IR) (solid, gas phase) and FT‐Raman spectroscopy in the range of 400–4000 and 10–3500 cm⁻¹ respectively. The effects of molecular association through O H···O hydrogen bonding have been described by the single dimer structure. The theoretical computational density functional theory (DFT) and Hatree‐Fock (HF) method were performed at 6–311++G(d,p) levels to derive the equilibrium geometry, vibrational wavenumbers, infrared intensities and Raman scattering activities. The scaled theoretical wavenumbers were also shown to be in good agreement with experimental data. The first‐order hyperpolarizability (β₀) of this novel molecular system and related properties (β, α₀ and Δα) of 4M2MBA are calculated using the B3LYP/cc‐pvdz basis set, based on the finite‐field approach. A detailed interpretation of the infrared and Raman spectra of 4M2MBA is reported. The theoretical spectrograms for FT‐IR and FT‐Raman spectra of the title molecule were also constructed and compared with the experimental one. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of the frequency of intramolecular motions on the NMR relaxation in liquid state temperature regime

Equations for the spectral densities of complex motion of a spin pair undergoing internal motion and isotropic/anisotropic overall rotation have been considered. The fluctuations of the interproton distances, caused by internal motion, have been taken into account in the theoretical equations. A method allowing a distinction between the isotropic and the anisotropic overall rotation of molecules has been proposed. The effect of the activation parameters of internal motions (known from the solid state study) on the measured T ₁ relaxation of the ¹³C and ¹H–¹H cross-relaxation rates has been analysed for methyl-β-D-galactopyranoside in DMSO-d6 solution. The conformational trans-gauche jumps of the methylene group are not fast enough to affect the T ₁ value of carbon C6 in the liquid state temperatures regime. Only the methyl group rotation is a very fast internal motion. This motion influences the carbon C7 relaxation and methyl protons–anomeric proton cross-relaxation. The values of interatomic distances between anomeric H(C1) and H(C5) as well as the three methyl protons H(C7) have been calculated from the cross-relaxation rates. The distance H(C1)–H(C7) fluctuates due to the rotation of methyl group. The application of the ‘model-free approach’ to study molecular dynamics in solutions is discussed.