Density functional theory study of the rotational barriers,conformational preference,and vibrational spectra of 2-formylfuran and 3-formylfuran

The torsional potentials, molecular structures, conformational stability, and vibrational wavenumbers for the rotational isomers of 2-formylfuran and 3-formylfuran are computed using the density functional theory (B3LYP) method with the 6-31+G* basis set. All structures are fully optimized and the optimized geometries, rotational constants, dipole moments, and energies are presented. From the computations, both 2-formylfuran and 3-formylfuran are predicted to exist predominantly in trans conformation with a cis–trans rotational barrier of 11.19 kcal/mol and 8.10 kcal/mol, respectively. The vibrational wavenumbers and the corresponding vibrational assignments of the molecules in the C _s symmetry are examined and the infrared spectra of the molecules are simulated using the wavenumbers and the corresponding intensities obtained from the computations. The effect of solvents on the conformational stability of all the molecules in nine different solvents (heptane, chloroform, tetrahydrofuran, dichloroethane, acetone, ethanol, methanol, dimethylsulfoxide, and water) is investigated. The integral equation formalism in the polarizable continuum model (IEF-PCM) is used for all solution phase computations.     

DFT Study of the Molecular Structure,Conformational Preference,HOMO, LUMO,and Vibrational Analysis of 2-, and 3-Furoyl Chloride

Molecular structure, conformational stability and vibrational wave numbers for the rotational isomers of 2-furoyl chloride and 3-furoyl chloride have been computed using the B3LYP method with the 6-311++G(d,p) basis set. From computations, 2-furoyl chloride was predicated to exist predominantly in cis conformation with cis–trans rotational barrier 40.40 kJ·mol^?1, and 3-furoyl chloride was predicated to exist predominantly in the trans conformation with cis–trans rotational barrier 30.17 kJ·mol^?1. The effects of solvents on the conformational stability of all the molecules in nine different solvents (heptane, chloroform, tetrahydrofuran, dichloroethane, acetone, ethanol, methanol, dimethylsulfoxide and water) were investigated. The integral equation formalism of the polarizable continuum model was used for all solution phase computations. The vibrational wave numbers and the corresponding vibrational assignments of the molecules in C₁ symmetry were examined and the simulated infrared spectra of the molecules are reported. The geometrical parameters, highest occupied and lowest unoccupied molecular orbitals, Infrared intensities, and molecular electrostatic potentials results are reported.     

Characteristics of the hydrogen bond and the structure of Н-complexes of <Emphasis Type="Italic">p</Emphasis>-n-propyloxybenzoic acid and <Emphasis Type="Italic">p</Emphasis>-n-propyloxy-<Emphasis Type="Italic">p</Emphasis>′-cyanobiphenyl

The conformational properties of p-n-propyloxybenzoic acid and p-n-propyloxy-p′-cyanobiphenyl molecules, which can exhibit liquid crystalline properties in the formation of Н-complexes, are studied (DFT/B3LYP)/cc-pVTZ method). It is found that a molecule of p-n-propyloxybenzoic acid has 16 conformers that can be divided into four groups with respect to relative energies (0 kcal/mol, 1.6 kcal/mol, 6.5 kcal/mol, and 8.1 kcal/mol), and a molecule of p-n-propyloxy-p′-cyanobiphenyl has six conformers with relative energies of 0 kcal/mol (two conformers, φ(С_Ph–O–C–C)=180°) and 1.6 kcal/mol (four conformers, φ(С_Ph–O–C–C)=64.4°). In all conformers of the 3-AOCB molecule, phenyl rings are turned at 35° relative to each other. A conformation with the planar arrangement of two rings has a higher energy by 1.5 kcal/mol. Barriers to the internal rotation of different groups are determined and it is established that the structural nonrigidity of the molecules is mainly due to the possible rotation of the–C₂Н₅ moiety about the C–C bond. It is shown that with increasing temperature the vibrational amplitudes of the OC₃H₇ substituent, which enhance the probabilities of transitions between the conformers, become appreciably larger. It is found that p-n-propyloxybenzoic acid and p-n-propyloxy-p′-cyanobiphenyl can form Н-complexes with the medium hydrogen bond. Two types of the structural organization of Н-complexes are considered: linear and angular. The similarity of energies of Н-complexes with different structures (NBO analysis) can be the reason for the occurrence of two liquid crystalline subphases of p-n-propyloxybenzoic acid and p-n-propyloxy-p′-cyanobiphenyl system.     

Manifestation of the <Emphasis Type="Italic">gauche</Emphasis> effect in conformers of benzenesulfonic acid hydrazide

Conformational properties of a benzenesulfonic acid hydrazide molecule and its para-nitro and para-methyl derivatives, which have found wide application as porofors and biologically active compounds, are studied. It is found that the benzenesulfonic acid hydrazide molecule has six conformers with relative energies of 0//0 kcal/mol, 0.34//0.98 kcal/mol, 2.51//2.25 kcal/mol, 2.54//2.56 kcal/mol, 2.90//3.28 kcal/mol, 6.64//6.43 kcal/mol (MP2//DFT(B3LYP) with the cc-pVTZ basis set), each conformer has enantiomer. The conformers differ from each other in the relative orientation of the fragments of the–SO2NHNH2 group, the energies of the frontier orbitals, the direction and value of the dipole moments. It is shown that the introduction of a nitro or methyl group into the para-position practically does not affect the conformational properties of the sulfonyl hydrazide group. Change in the structure of benzenesulfonic acid hydrazide during the crystal–gas transition is considered and it is revealed that in the crystal the conformation similar in structure to one of the high-energy conformers of the free molecule is stabilized. The NBO analysis of the electron density distribution is performed and it is shown that the occurrence of the gauche effect in all conformers of the molecules under study can be interpreted by the manifestation of the total action of strong anomeric effects between the lone pairs of nitrogen atoms and antibonding orbitals of S=O, N–H, C–S, and N–S bonds.     

Vibronic spectra,ab initio calculations,and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part V: Oxalyl chloridefluoride (COCl–COF)

The structure and conformational dynamics of the COCl–COF molecule in the ground and lowest excited electronic states were investigated theoretically by the CASPT2/cc-pVTZ method. The equilibrium geometric parameters, harmonic vibrational frequencies, potential functions of internal rotation, and adiabatic transition energies were obtained. According to the results of calculations, the molecule in the ground electronic state exist as the trans and gauche (dOCCO ~30–40°) conformers with a low potential barrier to gauche–gauche transition therefore it is impossible to exclude existence of the cis conformer (instead of gauche) with a very broad and flat potential minimum. For all the investigated excited electronic states of oxalyl chloridefluoride molecule calculations predicted the trans and cis conformers. The strong coupling of internal rotation around the C–C bond and non-planar vibrations of carbonyl fragments was found for the excited electronic states. The results of calculation were utilized for reanalysis of experimental \( \tilde{A}^{1} A^{\prime \prime} \leftarrow \tilde{X}^{1} A^{\prime} \) and \( \tilde{a}^{3} A^{\prime \prime} \leftarrow \tilde{X}^{1} A^{\prime} \) vibronic spectra reported in Kidd and King (J Mol Spectrosc 50:209–219 (1974), and ibid. 48:592–599 (1973)). The vibrational assignment that does not contradict the vibrational spectroscopy data and results of calculations was obtained.     

Relative stability of 5-methyl- and 3,5-dimethyltetrahydro-1,3-oxazine conformers

According to HF and DFT quantum chemical calculations and ¹H NMR data, the chair–chair conformational equilibrium of 5-methyltetrahydro-1,3-oxazine shifts toward the conformer with equatorial orientation of the methyl group on C⁵ and axial orientation of the NH proton. 3,5-Dimethyltetrahydro-1,3-oxazine exists preferentially as chair conformer with equatorial orientation of the 5-methyl group; orientation of the N-methyl group almost does not affect the conformational equilibrium.     

Structural,spectroscopic and docking properties of resorcinol,its -OD isotopomer and dianion derivative: a comparative study

Structural and vibrational properties of resorcinol, its -OD isotopomer and dianion salt were compared with the help of quantum-chemical and spectroscopic techniques. The relative stability computed at the MP4/6-311++G(d,p) level predicted that the syn-syn rotameric form of the resorcinol molecule is about 0.7 kcal/mol higher in energy than the more stable anti-syn and anti-anti forms. Vibrational frequencies calculated for the optimized α-resorcinol and its -OD isotopomer unit cells showed that out-of-plane bending vibrations tend to shift to higher wavenumbers for the solid phase compared to the non-condensed phase indicating strong intermolecular packing forces. OH/OD isotopic shifts were identified for stretching and binding modes, which supported by DFT findings. Infrared, Raman and proton NMR data confirmed a notable change in the overall electronic properties of resorcinol upon the abstraction of the hydroxyl protons. In addition, resorcinol, in its three rotameric configurations, exhibited a stable complexation with keratin-7. While the highly electronegative nature of oxygen atoms contributed effectively to the binding affinity of resorcinol towards keratin-7, careful docking analysis showed that the steric factor play the major role in the keratolytic activity of resorcinol.     

Potential energy surface and proton HFI constants of the cyclopentane radical cation

According to the data of UB3LYP/6-31G* and UMP2/cc-pVTZ calculations, the adiabatic potential energy surface of the cyclopentane radical cation is very intricate and combines six types of stationary structures of C _s and C ₂ symmetry. Ten equivalent C _s structures with the totally symmetric electronic state (C _s (² A′)) correspond to global minima. Conformational transitions between the global minima occur along the inversion and pseudorotation coordinates, for each pair of minima the conformational transition occurring in one stage (through the only transition state). The inversion barrier is ～2 kcal/mol; pseudorotation barriers are ～4–8 kcal/mol. The structure of the potential surface provides the interpretation of the EPR data as a result of dynamic averaging over 20 C _s (² A′) and C ₂ (² A) stationary structures.     

3-Cyclopropyl-1,2-dimethyldiaziridine: synthesis and study of molecular structure by gas electron diffraction method

The molecular structure and conformational behavior of 3-cyclopropyl-1,2-dimethyldiaziridine have been for the first time experimentally studied by gas-phase electron diffraction and quantum chemical calculations. The two most stable conformers at 298 K possess anti and gauche mutual ring orientation (with prevalence of the anti conformer) whereas only one anti conformer is observed in solution. The determined structural parameters of gaseous 3-cyclopropyl-1,2-dimethyldiaziridine have been compared with those for 3,3-bidiaziridine structural analogues in the crystal phase. The simple and convenient procedure for the synthesis of 3-cyclopropyl-1,2-dimethyldiaziridine comprising cyclopropane and diaziridine rings in one molecule was developed. The standard enthalpy of formation of 3-cyclopropyl-1,2-dimethyldiaziridine in the gas phase was calculated using Gaussian-4 theory, yielding value of 281.9?±?5.0 kJ/mol.     

Vibrational studies,NMR analysis,modeling of electronic and thermodynamical parameters of 1,3-bis(4-benzamido)triazene

The optimized geometry, vibrational wavenumbers, ¹H and ¹³C chemical shift values of 1,3-bis(4-benzamido)triazene, BBT, in the ground state were computed with the Hartree-Fock (HF) and density functional theory method (PBE1PBE) with 6-311+G(2d,p) basis set. The harmonic vibrational wavenumbers of BBT were calculated and the scaled values were compared with the experimental FT-IR spectra. A detailed interpretation of the NMR spectra of BBT was reported. The calculated data are in reasonably good agreement with experimental measurements. Moreover, the log P value was estimated with ChemBioOffice Ultra 11.0, ACD/LogP, and ALOGPS programs.     

<Emphasis Type="Italic">Ab initio</Emphasis> study of scandium fluoride molecules: ScF,ScF<Subscript>2</Subscript>, AND ScF<Subscript>3</Subscript>

Equilibrium geometric parameters, normal mode frequencies and intensities in IR spectra, atomization enthalpy, and relative energies of low-lying electronic states of scandium fluoride molecules (ScF, ScF₂, and ScF₃) are calculated by the coupled-cluster method (CCSD(T)) in triple-, quadruple, and quintuple-zeta basis sets with the subsequent extrapolation of the calculation results to the complete basis set limit. The ScF molecule is also studied by the CCSDT technique. The error in the approximate calculation of triple excitations in the CCSD(T) method does not exceed 0.002 Å for the equilibrium internuclear distance R _e, 4 cm^?1 for the vibrational frequency, and 0.2 kcal/mol for the dissociation energy of the molecule. In the ground electronic state \(\tilde X^2 \) A ₁(C _2ν ) of ScF₂ molecules, R _e(Sc-F) = 1.827 Å and α_e(F-Sc-F) = 124.2°; the energy barrier to bending (linearization) h = E _min(D _g8h ) ? E _min(C_2ν) = 1652 cm^?1. The relative energies of Ã²Δ _g and \(\tilde B^2 \)Π _g electronic states are 3522 cm^?1 and 14633 cm^?1 respectively. The bond distance in the ScF₃ molecule (\(\tilde X^1 \) A′₁, D _3h ) is refined: R _e(Sc-F) = 1.842 Å. The atomization enthalpies Δ_at H ₂₉₈ ⁰ of ScF _k molecules are 139.9 kcal/mol, 289.0 kcal/mol, and 444.8 kcal/mol for k = 1, 2, 3 respectively.     

Theoretical study on coordination of methanol clusters to 3-methyl-4-pyrimidone

Density functional theory (DFT), MP2, and couple cluster ab initio methods were employed to investigate the microsolvation of 3-methyl-4-pyrimidone (3M4P) surrounded by methanol (MeOH) molecules. Structures are analyzed based on hydrogen bonds with a focus on relative energies, interaction energies, hydrogen bond cooperativity, hydrogen bonding geometries, and redshifts in the frequencies of O–H and C=O stretching modes. Our results show that there is no preferential orientation of MeOH attacks on the carbonyle site of 3M4P; both trans and cis 3M4P-MeOH complexes have same chance to be observed. cis 3M4P-MeOH and 3M4P-MeOH complex in which MeOH is located on N lie 0.56 and 3.11 kJ/mol at CCSD(T)/6-31+G(d,p) (0.63 and 1.67 kJ/mol at MP2/6-311++G(d,p)) above trans 3M4P-MeOH. MeOH dimers form more stable 3M4P-(MeOH)₂ complexes compare to 3M4P-(MeOH)₂ complexes in which individual MeOH molecules bind to carbonyl and N. Relative energies of 3M4P-(MeOH)₃ computed using various DFT methods point out the complex formed by linear MeOH trimer along methyl group of 3M4P (cis 3M4P-(MeOH)₃) as lowest. Carbonyl group is predicted as preferential site for hydrogen bond interaction. Besides O–H…O and O–H…N hydrogen bonds, 3M4P-(MeOH)₂ and 3M4P-(MeOH)₃ complexes are also stabilized by H–O…H–C weak interactions.     

Theoretical investigation on the geometries and electronic properties of cesium–silicon CsSi<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2–12) clusters

The structures, relative stabilities, and electronic properties of pure Si _n and Cs-doped silicon clusters (n = 2–12) are systematically investigated using the density functional theory at the B3LYP level. The optimized structures indicated that the lowest-energy structures of CsSi _n are similar to those of pure Si _n clusters and prefer the 3-dimensional configuration for n = 3–12. The relative stabilities of CsSi _n clusters are analyzed based on the averaged binding energy, fragmentation energy, second-order energy difference, and HOMO–LUMO energy gap. It is found that CsSi₆ and CsSi₉ are the magic clusters, and the doping of Cs atom reduces the chemical stabilities of Si _n frame. The Mulliken population analysis pointed out that the charges in the corresponding CsSi _n clusters always transfer from Cs atom to Si _n host in the range of 0.80–0.91 electron. In addition, the partial density of states, infrared, and Raman spectra is discussed.     

Gas phase conformational behavior of selenomethionine: A computational elucidation

Selenium containing amino acids are known to play numerous key biological roles in various lifesupporting processes. In the current theoretical investigation DFT(B3LYP) and MP2 methods are used to study the gas phase conformers of the selenomethionine molecule in view of their relative stabilities, theoretically predicted harmonic frequencies, HOMO-LUMO energy gaps, rotational constants, and dipole moments. The number and type of intramolecular H-bond interactions existing in the selenomethionine conformers, which play key roles in determining the energy of the conformers, are also analyzed. The predicted geometries as well as the relative stabilities of the conformers suggest that the structural aspects and energies of the conformers may depend on the level of theory and the size of the basis set used. A comparison of the vibrational frequencies furnished in this study with the previous experimental and theoretical results obtained at MP2/6-31++G(d,p) and B3LYP/6-311++G(d,p) levels promotes the interpretation of the vibrational spectroscopy data on biologically relevant molecules.     

Cyclohexane in nanotubes: Direct chair–chair interconversion

DFT PBE/3ζ study of conformational transformations of cyclohexane inside nanotubes with chirality indices of (8,0) and (5,5) showed that the encapsulated molecule is characterized by shortened C–C bonds, some electric charge, and reduced barrier to chair–chair interconversion in comparison to the free molecule. Unlike free cyclohexane, no twist conformer as intermediate minimum on the potential energy surface was localized for the encapsulated molecule.     

Theoretical study on structures and infrared spectroscopy of Cu<Superscript>2+</Superscript>(H<Subscript>2</Subscript>O)Ar<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 1–4)

The binding energy of Cu²⁺(H₂O) is computed to be 98.4 kcal/mol and thus one-photon photodissociation is not possible in the 3400–3800 cm^–1 (9.7–10.9 kcal/mol) region. To study whether the infrared photodissociation processes of Cu²⁺(H₂O) can occur by multiple argon atoms tagging technique, density functional and CCSD(T) methods are used to investigate the geometries, OH stretching frequencies and the argon atom binding energies of Cu²⁺(H₂O)Ar _n (n = 1–4) complexes. Various isomers are found resulting from the different coordination sites of argon atoms. The OH stretches in these complexes are shifted to lower frequencies than those of the free water molecule, and the corresponding vibrational red shifts are progressively smaller as more argon atom is added to Cu²⁺ while binding an argon atom to an OH site should lead to additional sizable red shift to the OH stretching vibrations.     

Gas electron diffraction and quantum chemical studies of the molecular structure of 2-nitrobenzenesulfonic acid

A combined gas electron diffraction and quantum chemical (B3LYP/6-311+G**, B3LYP/cc-pVTZ) study of the molecular structure of 2-nitrobenzenesulfonic acid (2-NBSA) is performed. Quantum chemical calculations show that the 2-NBSA molecule has five conformers, and the Gibbs energy of one of them is lower by more than 4.5 kcal/mol than the energy of the other conformers. It is found experimentally that the saturated vapor of 2-NBSA at T = 394(5) K contains only the low-energy conformer that has an intramolecular hydrogen bond between the H atom of the hydroxyl group and one of the O atoms of the NO₂ group. The C-C-S-O(H) torsion angle determining the position of the S-O(H) bond is ?72(7)°, while the NO₂ group is substantially turned relative to the benzene ring plane (C1-C2-N-O = 40(5)°). The following experimental values of the internuclear distances are obtained for this conformer (Å): r _h1(C-H)_av = 1.07(2), r _h1(C-C)_av = 1.401(4), r _h1(C-S) = 1.767(6), r _h1(S=O)_av = 1.412(4), r _h1(S-O) = 1.560(6), r _h1(N-O)_av = 1.217(5), r _h1(C-N) = 1.461(8), r _h1(O-H) = 0.99(3).     

Low-temperature equilibriums in solutions of isocyanide-phosphine complexes of palladium(II) chloride

cis-[PdCl₂(CNR)(PPh₃)] [R = Cy, t-Bu, C(Me)₂CH₂C(Me)₃] have been synthesized via the interaction of [(PPh₃)ClPd(μ-Cl)₂PdCl(PPh₃)] with isocyanide in CH₂Cl₂ at room temperature with 90–98% yield and characterized by means of mass spectrometry as well as ¹H, ¹³C{¹H, ³¹P}, and ³¹P{¹H} NMR spectroscopy. The complexes structure in the solid phase has been elucidated by means of X-ray diffraction analysis. Dynamic processes in the solutions of the complexes in CDCl₃ and CD₂Cl₂ at temperature of–95 to 60°С have been studied by means of ¹H and ³¹P NMR spectroscopy. It has been found that the studied compounds existed exclusively in the cis-[PdCl₂(CNR)(PPh₃)] form in the solutions. In the case of cis-[PdCl₂(CNCy)(PPh₃)] in CH₂Cl₂, the conformational transitions of the equilibrium forms (the transition of the substituent in the cyclohexyl cycle between the equatorial and axial positions) are slowed down, the equatorial conformer prevailing in the solution (2: 1 at–95°С). Quantum-chemical simulation (DFT) has revealed that the standard Gibbs energy of the conformational transition from the axial form of cis-[PdCl₂(CNCy)(PPh₃)] into the equatorial one in the CH₂Cl₂ solution at 178 K equals–2.5 kJ/mol, being in agreement with the experimental data.     

Quantum-chemical simulation of structure and conformational flexibility of 5,7-di(<Emphasis Type="Italic">tert</Emphasis>-butyl)-2-(8-hydroxyquinolin-2-yl)-1,3-tropolone

Density functional theory [DFT B3LYP/6-311++G(d,p)] simulation has revealed stable tautomers and conformers of polydentate ligand system based on 5,7-di(tert-butyl)-2-(8-hydroxyquinolin-2-yl)-1,3-tropolone with different structures of the coordination nodes, capable of formation of metal chelates. It has been shown that the tautomeric NH- and OH- forms with exo and endo location of the hydroxy group in the quinoline fragments (close in energy, ΔE_ZPE = 0.2–2.4 kcal/mol) are stabilized by intramolecular hydrogen bonds. Energy barriers of the interconversion of these forms via rotation about the C–OH bond of the phenolic fragment are of ΔE_ZPE^≠ = 2.1–4.2 kcal/mol, whereas the barrier of rotation about the bond between the quinoline and tropolone fragments is higher (ΔE_ZPE^≠ = 18.2 and 19.6 kcal/mol).     

Electron diffraction and quantum-chemical studies of the molecular structure of 2-methylbenzenesulfochloride

A combined electron diffraction and quantum-chemical (MP2/6-31G**) study of the molecular structure of 2-methylbenzenesulfochloride at 336(5) K was carried out. It was found that the gas phase contained only one conformer, C ₁. The following structural parameters were obtained: r _h1(C-H)_av = 1.095(8) Å, r _h1(C-C)_Ph = 1.402(4) Å, r _h1(C_Ph-C_meth) = 1.507(13) Å, r _h1(C_Ph-S) = 1.763(6) Å, r _h1(S=O) = 1.418(4) Å, r _h1(S-Cl) = 2.048(5) Å, ∠(H-C-H)_meth/av = 107.3(96)°, ∠(Cl-S-O)_av = 106.4(3)°, ∠C_Ph-S-Cl = 100.8(9), ∠O=S=O = 120.8(10)°. The C_C-C_S-S-Cl torsion angle that defines the position of the S-Cl bond relative to the plane of the benzene ring is 75.6(20)°. The B3LYP/6-311+G** calculated barriers of internal rotation of the methyl and sulfochloride groups are 1.2 kcal/mol and V ₀₁ = 10.2 (V ₀₂ = 4.1) kcal/mol, respectively.