Infrared and Raman spectra, ab initio calculations and vibrational assignment for 3-fluoro-1-butyne

The infrared (3500-80 cm⁻¹) and Raman (3500-20 cm⁻¹) spectra of 3-fluoro-1-butyne, CH₃CHFCCH, have been recorded for the gas and solid. Additionally, the Raman spectrum of the liquid has also been recorded to aid in the vibrational assignment. Ab initio electronic structure calculations of energies, geometrical structures, vibrational frequencies, infrared intensities, Raman activities and the potential energy function for the methyl torsion have been calculated to assist in the interpretation of the spectra. The fundamental torsional mode is observed at 251 cm⁻¹ with a series of sequence peaks falling to lower frequency. The three-fold methyl torsional barrier is calculated to be 1441 ± 20 cm⁻¹ (4.12 ± 0.06 kcal mol⁻¹) where the uncertainty is partly due to the uncertainty in values of the V₆ term. A complete vibrational assignment is proposed based on band contours, relative intensities, and ab initio predicted frequencies. Several fundamentals are significantly shifted in the condensed phases compared to values in the vapor state.     

Bromination of 5-Substituted 3H-Furan-2-ones and 3H-Pyrrol-2-ones

The bromination of 5-alkyl(aryl)-3H-furan-2-ones and 5-alkyl(aryl)-3H-pyrrol-2-ones and also their derivatives takes place at the ethylene bond with the formation of 4-monobromo derivatives. N-Phenyl-3H-pyrrol-2-ones are brominated simultaneously at the ethylene bond of the heterocycle and at the phenyl substituent at the nitrogen atom.     

Ab Initio Study of the Dehydrogenation of 3-Pyrroline, 2,5-Dihydrofuran and 2,5-Dihydrothiophene

Abstract

The nature of the transition state structures of the decomposition of 3-pyrroline (1), 2,5-dihydrofuran (2) and 2,5-dihydrothiophene (3) were investigated usingab initio Molecular Orbital (MO) and Density Factional Theory (DFT) methods. The energy barrierof the decomposition of compound 1 is smaller than compound 2 and compound 2 is smaller than compound 3. The energy barriers for the decomposition of compounds 1–3 are 46.20, 50.17, and 61.34 kcal mol^?1, respectively, which is calculated by B3LYP/6-31G^*//HF/6-31G^* level of theory. Which is ingood agreement with reported experimental data. Contrary to the previously reported data, the distance between the cis-2-and-5-hydrogen atoms in compound 1 is greater than compound 2. The transition-state structures of the decomposition of compounds 1–3 are formed by interaction of the cis-2-and-5-hydrogen atoms. Also, the rings of compounds 1–3 in the transition state structures are puckered.     

A Conformational Study on Silacyclohexane. Comparison of ab initio (HF,MP2), DFT,and Molecular Mechanics Calculations. Conformational Energy Surface of Silacyclohexane

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HF, RI‐MP2, RI‐DFT and MM3 methods. All methods predict the chair form to be the dominant conformation and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair‐to‐chair inversion. Each of them consists of two sofa‐like transition states, two twist forms with C₁ symmetry (twist‐C₁), two boat forms with Si in a gunnel position (C₁ symmetry), and one twist form with C₂ symmetry (twist‐C₂). All methods calculate the relative energy to increase in the order chair < twist‐C₂ < twist‐C₁ < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76, 4.80, and 5.47 kcal mol^–1 for the twist‐C₂, twist‐C₁, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol^–1 from MM3 and RI‐DFT calculations, respectively. The boat form with Si at the prow (C_s symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI‐DFT optimization under C_s symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair‐to‐twist‐C₁ transition state, however, and it is clearly not a part of the chair‐to‐chair inversion.     

Infrared and Raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of 1-fluoro-1-methylsilacyclobutane

The infrared (3500–40 cm⁻¹) spectra of gaseous and solid 1-fluoro-1-methylsilacyclobutane, c-C₃H₆SiF(CH₃), have been recorded. Additionally, the Raman spectrum (3500–30 cm⁻¹) of the liquid has been recorded and quantitative depolarization values have been obtained. Both the axial and equatorial (with respect to the methyl group) conformers have been identified in the fluid phases. Variable temperature (−55–−100°C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 267±10 cm⁻¹ (3.19±0.12 kJ mol⁻¹), with the axial conformer being the more stable form and the only conformer remaining in the polycrystalline solid. A complete vibrational assignment is proposed for the axial conformer and many of the fundamentals for the equatorial conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G* and 6-311++G** basis sets at the levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. The results are discussed and compared to those obtained for some similar molecules.     

A comparative study by AM1, PM3 and ab initio HF/3-21G methods on the structure and reactivity of monosubstituted carbanion 1,2,4-triazolium ylides

A full conformational analysis of six 1,2,4-monosubstituted carbanion 1,2,4-triazolium ylides 4 a–f was performed using AM1, PM3 and HF/3-21G methods. The C-type conformers were found as the most stable structures by these different methods. This study also includes a qualitative estimation of the chemical behavior of triazolium ylides 4 a–f as nucleophilic agents on the level of ylide carbon atoms. The ab initio 3-21G method seems to be the most suitable in the characterization of these molecular systems.     

Infrared, and Raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of 1-chlorosilacyclobutane

The infrared spectra (3500–40 cm⁻¹) of gaseous and solid and the Raman spectra (3500–30 cm⁻¹) of liquid and solid 1-chlorosilacyclobutane, c-C₃H₆SiClH, have been obtained. Both the axial and equatorial conformers with respect to the chlorine atom have been identified in the fluid phases. Variable temperature (−105 to −150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 211±17 cm⁻¹ (2.53±0.21 kJ/mol), with the equatorial conformer being the more stable form and the only conformer remaining in the annealed solid. At ambient temperatures, approximately 26% of the axial conformers are present in the vapor phase. A complete vibrational assignment is proposed for the equatorial conformer, and many of the fundamentals of the axial conformers have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been determined for both rotamers by ab initio calculations employing the 6-31G(d) basis set at the levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. Structural parameters have also been obtained using MP2/6-311+G(d,p) ab initio calculations. The r₀ parameters for both conformers are obtained from a combination of the ab initio predicted values and the twelve previously reported microwave rotational constants. The results are discussed and compared to those obtained for some similar molecules.     

Vibrational spectra, conformations, ab initio calculations and vibrational assignments of 3-pentyn-2-ol

The infrared spectra of 3-pentyn-2-ol, CH₃CCCH(OH)CH₃, have been recorded as a vapour and liquid at ambient temperature, as a solid at 78 K in the 4000–50 cm⁻¹ range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid.

Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, some bands changed in relative intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer in which OH is oriented anti to C₁(a_Me). From various bands assigned to a second conformer in which OH is oriented anti to H_gem(a_H), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ mol⁻¹. The highest energy conformer with OH anti to C₃(a_C) was not detected.

Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis set calculations for which the a_H conformer had slightly lower energy, all the calculations revealed that a_Me was the low energy conformer. The B3LYP/cc-pVTZ calculations suggested the a_Me conformer as more stable by 0.8 and 8.3 kJ mol⁻¹ relative to a_H an a_C, respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from B3LYP/cc-pVTZ calculations.     

Conformational and structural studies of 1-fluorosilacyclobutane from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

The infrared spectra (3500 to 40 cm⁻¹) of gaseous and solid and the Raman spectra (3500 to 30 cm⁻¹) of liquid and solid 1-fluorosilacyclobutane, c-C₃H₆SiFH, have been obtained. Both the axial and equatorial conformers with respect to the fluorine atom have been identified in the fluid phases. Variable temperature (−105 to −150 °C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 282 ± 27 cm⁻¹ (3.37 ± 0.32 kJ/mol), with the equatorial conformer the more stable form and the only conformer remaining in the annealed solid. At ambient temperature there is approximately 21 ± 2% of the axial conformer present in the vapor phase. From isolated Si–H stretching frequencies the Si–H (r₀) distances are calculated to be 1.484 and 1.485 Å for the equatorial and axial conformers, respectively. Structural parameters have been predicted from MP2/6-311 + G(d,p) ab initio calculations and the adjusted r₀ parameters for both conformers were obtained from a combination of the ab initio predicted values and the six previously reported microwave rotational constants. Along with the Si–H bond distance, the Si–C, and C–C distances of 1.865(5), and 1.571(5) Å, respectively, for the equatorial conformer are significantly different from the values for these parameters previously reported from an election diffraction study. Both the SiC and CC distances and the SiF distance are longer by 0.002 and 0.004 Å, respectively, for the axial conformer. Structural parameters have also been obtained for silacyclobutane, c-C₃H₆SiH₂ and ethylsilylfluoride, CH₃CH₂SiH₂F, from combined ab initio predicted values and previously reported rotational constants. Several of these newly determined parameters are significantly different from those previously reported for both molecules. Complete equilibrium geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been determined for both rotamers by ab initio calculations employing the 6-31G(d) basis set at the level of Moller–Plesset (MP) to second order. A complete vibrational assignment supported by normal coordinate calculations is proposed for the equatorial conformer, and several of the fundamentals of the axial conformer have also been identified. The results are discussed and compared to corresponding quantities for some similar molecules.     

Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of 3-methyl-3-butenenitrile

The Raman (3500-30 cm⁻¹) spectra of liquid and solid and the infrared (3500-40 cm⁻¹) spectra of gaseous and solid 3-methyl-3-butenenitrile, CH₂C(CH₃)CH₂CN, have been recorded. Both cis and gauche conformers have been identified in the fluid phases but only the cis form remains in the solid. Variable temperature (−55 to −100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 163±16 cm⁻¹ (1.20±0.19 kJ mol⁻¹), with the cis conformer the more stable rotamer. It is estimated that there is 48±2% of the gauche conformer present at  25°C. A complete vibrational assignment is proposed for the cis conformer based on infrared band contours, relative intensities, depolarization ratios and group frequencies. Several of the fundamentals for the gauche conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries have been obtained for both rotamers by ab initio calculations employing the 6-31G(d), 6-311G(d,p), 6-311+G(d,p) and 6-311+G(2d,2p) basis sets at the levels of restricted Hartree-Fock (HF) and/or Møller-Plesset perturbation theory to the second order (MP2). Only with the 6-311G(2d,2p) and 6-311G(2df,2pd) basis sets with or without diffuse functions is the cis conformer predicted to be more stable than the gauche form. The potential energy terms for the conformational interchange have been obtained at the MP2(full)/6-311+G(2d,2p) level, and compared to those obtained from the experimental data. The results are discussed and compared to the corresponding quantities obtained for some similar molecules.     

Semi-empirical (PM3 and AM1) and ab initio molecular orbital calculations of 1,2,4-oxadiazoles, 4,5-dihydro-1,2,4-oxadiazoles and 4,4-di-n-butyl-2-phenylbenzo-1,3-oxazine

Molecular orbital calculations using semi-empirical (PM3 and AM1) and ab initio (HF/6-31G) types have been carried out on several 3,5-disubstituted 1,2,4-oxadiazoles 1a–d, 5-n-butyl-3,5-diaryl-4,5-dihydro-1,2,4-oxadiazoles 2a–d, and 4,4-di-n-butyl-2-phenylbenzo-1,3-oxazine 3. A comparison of the results by the two computational procedures has been made. Transformation of the oxadiazole ring to 4,5-dihydro-1,2,4-oxadiazole having both aryl and n-butyl groups at C-5 exhibited interesting conformational features. Also, examination of 1,3-oxazine 3 gave an idea about the structure of this compound. The rotational barrier of each phenyl group in 1a and 1d has been calculated using the ab initio method HF/6-31G(d).     

Probing the Properties of Polynuclear Superhalogens without Halogen Ligand via ab Initio Calculations: A Case Study on Double‐Bridged [Mg2(CN)5]−1 Anions

An ab initio study of the superhalogen properties of eighteen binuclear double‐bridged [Mg₂(CN)₅]^?1 clusters is reported herein by using various theoretical methods. High‐level CCSD(T) results indicate that all the clusters possess strong superhalogen properties owing to their high vertical electron detachment energies (VDEs), which exceed 6.8 eV (highest: 8.15 eV). The outer valence Green's function method provides inaccurate relative VDE values; hence, this method is not suitable for this kind of polynuclear superhalogens. Both the HF and MP2 results are generally consistent with the CCSD(T) level regarding the relative VDE values and—especially interesting—the average values of the HF and MP2 VDEs are extremely close to the CCSD(T) results. The distributions of the extra electrons of the anions are mainly aggregated into the terminal CN units. These distributions are apparently different from those of previously reported triple‐bridged isomers and may be the reason for the decreased VDE values of the clusters. In addition, comparisons of the VDEs of binuclear and mononuclear superhalogens as well as studies of the thermodynamic stabilities with respect to the detachment of various CN^?1 ligands are also performed. These results confirm that polynuclear structures with pseudohalogen ligands can be considered as probable new superhalogens with enhanced properties.     

An ab initio and matrix isolation infrared study of the 1:1 C2H2–CHCl3 adduct

The details of weak C–Hπ interactions that control several inter and intramolecular structures have been studied experimentally and theoretically for the 1:1 C₂H₂–CHCl₃ adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C₂H₂ and CHCl₃ species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels. Both the computational and experimental data indicate that the C₂H₂–CHCl₃ complex has a weak hydrogen bond involving a C–Hπ interaction, where the C₂H₂ acts as a proton acceptor and the CHCl₃ as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl₃ and a hydrogen in C₂H₂. The combination of the C–Hπ interaction and the secondary ClH interaction determines the structure and the energetics of the C₂H₂–CHCl₃ complex. In addition to the vibrational assignments for the C₂H₂–CHCl₃ complex we have also observed and assigned features owing to the proton accepting C₂H₂ submolecule in the acetylene dimer.     

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.     

IR Low‐Temperature Matrix and ab Initio Study on β‐Alanine Conformers

For the first time the argon‐matrix low‐temperature FTIR spectra of β‐alanine are recorded. They reveal a quite complicated spectral pattern which suggests the presence of several β‐alanine conformers in the matrix. To interpret the spectra, the eighteen β‐alanine conformers, stable in the gas phase, are estimated at the B3LYP and MP2 levels combined with the aug‐cc‐pVDZ. Ten low‐energy structures are reoptimized at the QCISD/aug‐cc‐pVDZ and B3LYP and MP2 levels by using the aug‐cc‐pVTZ basis sets. Assignment of the experimental spectra is undertaken on the basis of the calculated B3LYP/aug‐cc‐pVDZ anharmonic IR frequencies as well as careful estimation of the conformer population. The presence of at least three β‐alanine conformers is demonstrated. The detailed analysis of IR spectra points to the possible presence of five additional β‐alanine conformers.     

微波作用下1-苯基-5-[5-(芳胺羰基甲硫基)-4-苯基-1,2,4-三唑-3-甲硫基]四唑的合成及生物活性

采用微波和相转移催化法通过1-苯基-5-(4-苯基-1,2,4-三唑-5-巯基-3-甲硫基)四唑(2)与2-氯乙酰芳胺(3)反应高效、快速地合成了10种尚未见文献报道的1-苯基-5-[5-(芳胺羰基甲硫基)-4-苯基-1,2,4-三唑-3-甲硫基]四唑. 其结构经 IR, ¹H NMR, ¹³C NMR 和元素分析表征. 生物活性实验结果表明, 该类化合物在较低浓度下部分化合物对小麦芽有很好的促进作用.     

Spectroscopic and structural elucidation of L-tyrosine-containing dipeptides valyl-tyrosine and tyrosyl-alanine: solid-state IR-LD spectroscopy, quantum chemical calculations and vibrational analysis

L-Tyrosine-containing dipeptides valyl-tyrosine (H-Val-Tyr-OH) and tyrosyl-alanine (H-Tyr-Ala-OH) are characterized structurally by means of quantum chemical ab initio calculations and solid-state linear-dichroic infrared (IR-LD) spectroscopy. The IR-characteristic bands are assigned by application of reducing-difference procedure for polarized IR-spectra interpretation. Infrared data obtained are supported as well by the made vibrational analysis. The structures of both peptides are predicted on the basis of conformational analysis and structural information, obtained by the shown IR-spectroscopic tool.     

卤取代对三氮系1-3H原子转移互变异构影响的理论研究

用量子化学从头计算方法研究了卤取代对三氮系1-3H原子转移互变异构的影响,探讨卤取代对降低反应活化能和稳定N＝N双键的电子效应.对反应机理作出解释.     

C D : a computer program to generate 1D, 2D and 3D grids of functions dependent on the molecular ab initio electron density

A program to compute many functions dependent on the electron density ρ(r) from the results of ab initio molecular calculations is presented. The program allows the generation of different one-, two-, and three-dimensional grids for further graphical representation or numerical analysis. Other options like extracting separate atom contributions to the function computed or locating maximum and minimum values are also implemented. A number of illustrative applications regarding different ρ(r)-dependent functions are presented and the performance and portability of the program is discussed.     

The Detection of tert-Butylthiosulfoxylic Acid,t-BuSSOH,and 1-Oxatrisulfane,HSSOH, in the Gas Phase – Experimental Results and Ab Initio Calculations

Flash vacuum pyrolysis (FVP) of tert-butylthiosulfinic acid S-tert-butylester, t-BuS(O)St-Bu, at a temperature of 500 °C and a pressure of 0.07 hPa leads to the formation of tert-butylthiosulfoxylic acid, t-BuSSOH ( 1 ), and 2-methylpropene as byproduct. 1 has been identified in the gas phase by its IR absorption bands at ν(OH) = 3598 cm^–1, δ(SOH) = 1149 cm^–1 and ν(SO) = 718 cm^–1. At higher temperatures (700 °C) the elimination of a second mole of 2-methylpropene and the shift of ν(SO) to higher wavenumbers (750 cm^–1) indicate the formation of 1-oxatrisulfane, HSSOH. Different sulfenic acids RSOH (R = Me, i-Pr, t-Bu) were synthesized by FVP in order to study the influence of the substituent R on the vibrational wavenumbers ν(OH), ν(SO) and δ(SOH) observed in the gas phase. The strongest effect results for δ(SOH) leading to a decrease by 6 wavenumbers if the methyl group is substituted by a tert-butyl group. The assignment of the experimental wavenumbers has been supported by theoretical values obtained from ab initio calculations at the MP2(fc)/6-311G** level. Furthermore, the theoretical studies show that of all compounds RS₂OR′ (R = R′ = H, Me; R = Me (H), R′ = H (Me)) the unbranched chain isomers RSSOR′ are energetically favored over the branched chain isomers. Relaxed potential energy surface scans at the MP2(fc)/6-311G** level have been carried out to study the rotational isomers of the branched molecules RS(Y)XR′ (R = R′ = H, Me; R = Me (H), R′ = H (Me); X = O (S), Y = S (O)). Of the three conformations (+)syn-clinal, (–)syn-clinal, and anti-periplanar resulting from molecular model considerations only the two latter ones correspond to local minima on the calculated potential curve. The (–)syn-clinal conformation is slightly favored for all other constitutional isomers except for HS(O)SH and MeS(O)SH, which prefer the anti-periplanar conformation.