Vibrational analysis of buta-1,3-diene and its deutero and 13C derivatives and some of their rotational isomers

The wave numbers of trans-2,3-¹³C₂-buta-1,3-diene were calculated using a scaled quantum-chemical force field found at the MP2/6-31G*//MP2/6-31G* level of theory. The obtained results and the theoretical sets of wave numbers for twelve deutero and ¹³C derivatives of the trans form and five deutero and ¹³C derivatives of the gauche form of buta-1,3-diene found previously at the MP2/6-31G*//MP2/6-31G* level are compared with the corresponding experimental vibrational spectra corrected for the Fermi resonance. Combined analysis of the vibrational spectra of the above mentioned isotopomers was performed.     

Spectra and Structure of Silicon-Containing Compounds. XXV. Raman and Infrared Spectra,r 0 Structural Parameters,Vibrational Assignment,and Ab Initio Calculations of Ethyl Chlorosilane-Si-d2

The infrared (3200 to 400 cm^–1) spectra of gaseous and solid and Raman (3200 to 20 cm^–1) spectra of liquid and solid ethyl chlorosilane-Si-d₂, CH₃CH₂SiD₂Cl, have been recorded. Both the gauche and trans conformers have been identified in the fluid phases, but only the gauche conformer remains in the solid phase. Variable temperature (–105 to –150°C) studies of the infrared spectra of CH₃CH₂SiH₂Cl dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 78±11 cm^–1 (0.93±0.13 kJ/mol), with the gauche conformer the more stable form. Utilizing the frequencies of the silicon-hydrogen stretches, from the chlorosilane-Si-d isotopomer, Si—H bond distances of 1.481 and 1.480 Å have been obtained for the gauche conformer and 1.481 Å for the trans conformer. Complete vibrational assignments are proposed for both isotopomers which are consistent with the predicted frequencies utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities and the Raman activities and depolarization values have been obtained from the ab initio calculations. Complete equilibrium geometries have been determined by ab initio calculations employing the 6-31(d), 6-311++G(d,p), and 6-311+G(2d,2p) basis sets with full electron correlation by the Moller–Plesset (MP) perturbation method to second order. Continuing the previously reported rotational constants from five different isotopomers and the ab initio predicted structural parameters, adjusted r ₀ parameters have been calculated, which are compared to the corresponding r _s parameters. The results are discussed and the theoretical values are compared to the experimental values when appropriate.Taken in part from the dissertation of Y. E. Nashed, which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree     

rac‐2,3‐Di­bromo­succinic acid

rac‐2,3‐Di­bromo­succinic acid, C₄H₄Br₂O₄, is the product of the electrophilic addition of bromine to maleic acid. Whereas the carboxyl groups and the bromo ligands are in a gauche arrangement with respect to each other, the tertiary H atoms attached to the central C atoms are in a trans arrangement. The hydroxyl groups form hydrogen bonds with the carbonyl O atoms of neighbouring mol­ecules.     

Vibrational Spectra, Assignment, Conformational Stability and Ab Initio/DFT Calculations for 1-Nitropropane

The infrared spectra (4000–400 cm^{– 1}) of solid and the Raman spectra (3500–30 cm^{– 1}) of liquid and solid 1-nitropropane, CH₃CH₂CH₂NO₂, have been registered. Both the trans and gauche conformers have been identified in the fluid phase, while the trans form remains in the stable solid. Temperature dependence (190–230K) of the liquid 1-nitropropane Raman spectra has been carried out. From these data, the enthalpy difference was determined to be 870 ± 105 J-mol^–1, with the gauche conformer being the more stable rotamer. Ab initio and DFT calculations at different levels of approximation (HF, MP2, B3LYP, B3PW91) gave optimized geometries, harmonic force fields, and vibrational frequencies for the trans and gauche conformers. All the calculations (except the B3PW91/6-31G* level) predicted gauche as the low-energy conformer. Theoretical force constants are analyzed for formulating constraints in the molecular force field model of 1-nitropropane.     

Conformational Equilibria and Large‐Amplitude Motions in Dimers of Carboxylic Acids: Rotational Spectrum of Acetic Acid–Difluoroacetic Acid

We report the rotational spectra of two conformers of the acetic acid–difluoroacetic acid adduct (CH₃COOH–CHF₂COOH) and supply information on its internal dynamics. The two conformers differ from each other, depending on the trans or gauche orientation of the terminal ?CHF₂ group. Both conformers display splittings of the rotational transitions, due to the internal rotation of the methyl group of acetic acid. The corresponding barriers are determined to be V₃(trans)=99.8(3) and V₃(gauche)=90.5(9) cm^?1 (where V₃ is the methyl rotation barrier height). The gauche form displays a further doubling of the rotational transitions, due to the tunneling motion of the ?CHF₂ group between its two equivalent conformations. The corresponding B₂ barrier is estimated to be 108(2) cm^?1. The increase in the distance between the two monomers upon OH→OD deuteration (the Ubbelohde effect) is determined.     

The stable form of 1, 1, 1-trifluoro-3-chloropropane molecules in the crystal II modification

The vibrational spectra of CF₃CH₂CH₂Cl in different states of aggregation is investigated and the normal coordinate analysis of trans- and gauche-conformers is carried out. The assignments in vibrational spectra for both the conformers of CF₃CH₂CH₂Cl are given. It is shown that the stable crystalline modification, crystal II, is formed by the trans-conformer of the molecule.     

Analysis of the nitrile and methyl torsional vibrations of n‐propyl cyanide in its S0 electronic state

This work presents a structural and vibrational theoretical study of n‐propyl cyanide as a function of the nitrile and methyl torsional modes. A potential energy hypersurface is built at the MP4(SDQ)/aug‐cc‐pVTZ//MP2/aug‐cc‐pVTZ theory level. The equilibrium structure is found in a gauche conformation. Another minimum is found for the trans form. The maximum appears in a cis conformation. For the first time, the interconversion barriers between the different forms are calculated. A two‐dimensional anharmonic vibrational Hamiltonian is built for the nitrile and methyl torsional modes. We find the vibrational energy levels to organize in two stacks associated to the gauche and trans forms. Fundamental frequencies of 113.12 and 220.54 cm^?1 are predicted for the nitrile and methyl torsions in the equilibrium, gauche, conformer. In addition, we find symmetry allowed transitions between the gauche and trans energy levels stacks. The lowest transition is predicted to appear at 24.49 cm^?1. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Theoretical investigations on structure, electrostatics potentials and vibrational frequencies of Li+ - CH3- O- (CH2- CH2- O)n- CH3 (n=3-7) conformers

Electronic structure, charge distributions and vibrational characteristics of CH₃ O(CH₂ CH₂ O) _n CH₃ (n=3-7) have been derived using the ab initio Hartree Fock and density functional calculations. For tri- to hexaglymes the lowest energy conformers have trans- conformation around the C-C and C-O bonds of the backbone. For heptaglyme (n=7 in the series), however, gauche-conformation around the C-C bonds renders more stability to the conformer and turns out to be 10.1 kJ mol ⁻¹ lower in energy relative to the conformer having trans-orientation around the C-C and C-O bonds. The molecular electrostatic potential topographical investigations reveal deeper minima for the ether oxygen in conformers having the gauche conformation around the C-C bonds over those for the trans- conformers. A change from trans- to gauche-conformation around the C-C bonds of the lowest energy conformer of heptaglyme engenders a triplet of intense bands ∼1,150 cm ⁻¹ in the vibrational spectra. Theoretical calculations predict that Li ⁺ binds strongly to the heptaglyme conformer in the above series. The frequency shifts in the vibrational spectra of CH₃O(CH₂CH₂O) _n CH₃- Li⁺ (n=3-7) conformers have been discussed     

Equilibrium conformation of dimethylaminodichlorophosphine molecule: the use of the results of quantum-chemical calculations and spectroscopic measurements in the analysis of gas-phase electron diffraction data

Geometric parameters and force fields of two stable isomers of dimethylaminodichlorophosphine molecule, a gauche-conformer with C₁ symmetry (A) and anti-conformer with C _s symmetry (D), resulting from internal rotation about the P—N bond, were calculated in the RHF/6-31G* approximation. Using the scaled quantum-chemical force field for the most stable conformer A, the first reliable interpretation of the vibrational spectra of the light and perdeuterated isotopomers of dimethylaminodichlorophosphine was obtained. The root-mean-square vibrational amplitudes, harmonic and anharmonic vibrational corrections, and centrifugal distortion corrections were also calculated. Structural analysis of electron diffraction data was performed with consideration of nonlinear kinematic effects at the first-order level of perturbation theory. The experimental values of the equilibrium geometric parameters were estimated. The results obtained suggest a nonplanar equilibrium configuration of the amino group in the dimethylaminodichlorophosphine molecule.     

Shapes and Internal Dynamics of the 1:1 Adducts of Ammonia with trans and gauche Ethanol: A Rotational Study

Two 1:1 adducts of ammonia with ethanol have been characterized by using pulsed‐jet FT microwave spectroscopy. They are formed with two different (trans and gauche), stable conformers of ethanol. Several internal‐dynamics effects are reflected in the features of the rotational spectra. The trans complex shows the tunneling effects owing to internal rotation of both ammonia and the methyl group. The rotational transitions of the gauche species exhibit a small splitting that is related to tunneling through the potential‐energy barrier between the two equivalent minima.     

The microwave spectrum of dichloroacetic acid

The microwave spectrum of dichloroacetic acid consists of many weak absorption lines, of which only a small number has been identified. From the measured frequencies only two rotational constants could be obtained. Determination of the nuclear quadrupole coupling constants of the two chlorine atoms was not possible, since no line splittings were observed.Isotopic substitution of the two chlorine and the two hydrogen atoms shows that the assigned absorption lines belong to the conformation with the hydrogen atom gauche with respect to the hydroxylic group, while the carboxylic group has the common trans geometry. Substitution of the hydrogen atom of the dichloromethyl group by deuterium decreases the inertial moment I_c by 0.2 u Ų, indicating that structure determinations can be hampered by vibrational effects to an almost incredible extent.     

CP/MAS 13C NMR analyses of hydrogen bonding and the chain conformation in the crystalline and noncrystalline regions for poly(vinyl alcohol) films

The chain conformation and hydrogen bonding in the crystalline and noncrystalline regions have been characterized for atactic poly(vinyl alcohol) (PVA) films prepared under different conditions by CP/MAS ¹³C NMR analyses developed recently. The CH resonance lines of the crystalline and noncrystalline components split in different ways, depending significantly on casting solvents and annealing. These lines are found to be successfully resolved into 3–7 constituent lines by the least‐squares curve fitting. In this analysis, nine lines with different chemical shifts are prepared as elementary lines for the curve fitting by assuming the upfield shifts due to the γ‐gauche effect and the downfield shifts due to the formation of the intramolecular hydrogen bonds. The integrated intensities of the constituent lines thus obtained are also interpreted in terms of statistical calculations, assuming the random distribution of the trans and gauche conformations along PVA chains and the statistical distribution of the intramolecular and intermolecular hydrogen bonds between appropriate adjacent OH groups. On the basis of probabilities f_t and f_a for the trans conformation and the intramolecular hydrogen bond obtained through these analyses, the effects of casting solvents and annealing are discussed for both crystalline and noncrystalline components. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1–9, 2000     

Unusual Folding Propensity of an Unsubstituted β,γ‐Hybrid Model Peptide: Importance of the C?H⋅⋅⋅O Intramolecular Hydrogen Bond

The single‐crystal X‐ray diffraction analysis of a β,γ‐hybrid model peptide Boc‐β‐Ala‐γ‐Abu‐NH₂ revealed the existence of four crystallographically independent molecules ( A , B , C and D conformers) in the asymmetric unit. The analysis revealed that unusual β‐turn‐like folded structures predominate, wherein the conformational space of non‐proteinogenic β‐Ala and γ‐Abu residues are restricted to gauche‐gauche‐skew and skew‐gauche‐trans‐skew orientations, respectively. Interestingly, the U‐shaped conformers are seemingly stabilised by an effective unconventional C? H ??? O intramolecular hydrogen bond, encompassing a non‐covalent 14‐membered ring‐motif. Taking into account the signs of torsion angles, these conformers could be grouped into two distinct categories, A / B and C / D , establishing the incidence of non‐superimposable stereogeometrical features across a non‐chiral one‐component peptide model system, that is, “mirror‐image‐like” relationships. The natural occurrence of β‐Ala and γ‐Abu entities in various pharmacologically important molecules, coupled with their biocompatibilities, highlight how the non‐functionalised β,γ‐hybrid segment may offer unique advantages for introducing and/or manipulating a wide spectrum of biologically relevant hydrogen bonded secondary structural mimics in short synthetic peptides.     

Redetermination of sperminium tetrachloride

In the title compound, 1,5,10,14‐tetraazoniatetra­decane tetrachloride, C₁₀H₃₀N₄⁴⁺·4Cl⁻, the sperminium tetracation lies on a centre of symmetry. The two central C—N—C—C torsion angles are gauche and of opposite signs, and all the other torsion angles are trans. All NH groups participate in the three‐dimensional hydrogen‐bond network, which is additionally strengthened by C—H⋯Cl interactions.     

Potential energy surfaces and vibrational spectra for isotopomers of N2O

Potential energy surfaces and vibrational spectra for the four isotopomers (^l5N¹⁴N¹⁶O,^l4N^I5N¹⁶O,¹⁵N₂ ¹⁶O and¹⁵N₂ ¹⁸O) of N₂O have been investigated with the vibrational self-consistent field-configuration interaction method. It is shown that the isotopomers with the same end atom have similar values of the potential parameters, and that substitution with different end atoms can affect the potential obviously. The calculated vibrational levels are in good agreement with the observed values by the optimization of several potential parameters (f ₁ ⁽¹⁾,f ₁₃ ⁽⁰⁾,f ₃ ⁽¹⁾ which are sensitive to isotopic substitutions. Project supported by the National Natural Science Foundation of China (Grant No. 29673029).     

2‐Amino‐8‐(2‐deoxy‐2‐fluoro‐β‐d‐arabinofuranosyl)imidazo[1,2‐a][1,3,5]triazin‐4(8H)‐one monohydrate,a 2′‐deoxyguanosine analogue with an altered Watson–Crick recognition site

The title compound, C₁₀H₁₂FN₅O₄·H₂O, shows an anti glycosyl orientation [χ = −123.1 (2)°]. The 2‐deoxy‐2‐fluoroarabinofuranosyl moiety exhibits a major C2′‐endo sugar puckering (S‐type, C2′‐endo–C1′‐exo, ²T₁), with P = 156.9 (2)° and τ_m = 36.8 (1)°, while in solution a predominantly N conformation of the sugar moiety is observed. The conformation around the exocyclic C4′—C5′ bond is −sc (trans, gauche), with γ = −78.3 (2)°. Both nucleoside and solvent molecules participate in the formation of a three‐dimensional hydrogen‐bonding pattern via intermolecular N—H...O and O—H...O hydrogen bonds; the N atoms of the heterocyclic moiety and the F substituent do not take part in hydrogen bonding.     

Ab initio studies on the mechanism of the fluoroketene-imine cycloaddition reaction: Via a gauche or trans intermediate

The cycloaddition reaction of FCH(double bond)C(double bond)O and NH₂CH(double bond)NH leading to 2-azetidinone was studied theoretically at the level of RHF /6–31G and RHF /6–31G *. Two possible mechanisms via a gauche or trans intermediate were compared. The obtained results show that the reaction proceeds in a gauche manner much easier than in a trans one. © 1996 John Wiley & Sons, Inc.     

Conformation et Spectrometrie de RMN. III. Spectres 13C de Diméthylamino Cyclanols et Dérivés

The ¹³C NMR spectra of 2-dimethylaminocyclohexanols and the four trans-3-dimethylamino-2-decahydronaphtols are described. The gauche interactions allow precise estimation of chemical shifts for each carbon atom; thus, band attribution can be resolved without mistake. In the case of the cis and the trans diequatorial compounds, the conformational perturbations which we have suggested before are verified by our present measurements on the substituted carbon atoms. We also show effects on adjacent atoms, which were unobservable with the usual IR and ¹H NMR techniques.     

Conformational studies on 2-fluoro-1,2-disubstituted ethanes by NMR spectroscopy. Influence of electronegativity on vicinal proton–proton and fluorine–proton coupling constants

The analysis of the ABKX spectra of thirteen compounds of the series RC(H-K)(F-X)C(H-A)(H-B)X gave the four vicinal proton-proton and fluorine-proton coupling constants. These coupling constants of conformationally mobile structures were used (i) to calculate the populations of the rotational states of the ? CHF? CH₂? bond, (ii) to calculate the vicinal trans proton-proton J(HH)^t and gauche and trans fluorine-proton coupling constants J(FH)^g and J(FH)^t and (iii) to give the unambiguous assignment of protons H-A and H-B. The dependence of the gauche and trans coupling constants with substituent electronegativity is explored. The results extend known correlations towards smaller electronegativity values. More quantitatively, the results and those in the literature, excluding those where deformations of torsional or bond angles occur, give a good fit of the data: a linear fit for J(HH)^t = 15.0-0.77 Σ(ΔE), an exponential fit for J(FH)^g = 15.35 exp [-0.266 Σ (ΔE)] and a linear fit for J(FH)^t = 65.75 - 7.52 Σ (ΔE), where Σ (ΔE) is the sum of the electronegativity difference between hydrogen and the six atoms or groups on the CH? CF fragment.     

1,21‐Henicosanediol

In the molecular structure of the title compound, C₂₁H₄₄O₂, the hydro­carbon skeleton has an all‐trans conformation. One of the terminal hydroxyl groups is also trans with respect to the skeleton, while the other is gauche. In the crystal structure, the mol­ecules lie parallel to the b axis, forming layers with a thickness of b/2. The packing is similar to that in the smectic A phase of liquid crystals. These features are similar to those of the homologues with an odd number of C atoms, but different from those with an even number.