Gauche Structures Research Articles

Positron binding to hydrocarbon molecules: calculation using the positron–electron correlation polarization potential

Positron binding energies of naphthalene and alkanes (CnH2n+2 with n = 3 − 16) are calculated using the correlation polarization potential approach, where the short-range positron–electron correlation potential is modeled by the density-functional expression taken from a homogenous electron gas model. In the case of naphthalene, the calculated positron binding energy is found to reasonably agree with the experimental measurement. In the case of CnH2n+2 with the linear all-trans conformation we found positive positron binding energies for n ≥ 8 while the positron is not bound for n ≤ 7. This result cannot reproduce the previous experimental study, where the positron was bound for all alkanes with 3 ≤n ≤ 16. In addition, our calculated positron binding energies for n ≥ 9 are much larger than the experimental values although the generalized gradient approximation could improve the calculated values. We also investigated the conformer dependence of the positron binding energy for C16H34 and found that the positron binding energies significantly depend on the conformational structure; hairpin-like and crown-like structures generally have large positron binding energies, while single and multiple gauche structures have smaller binding energies.

Balanced design for the feasible super rocket fuels: A first-principle study on gauche CHN7 and CHN3

On the basis of the framework of cubic gauche nitrogen (cg-N), six one-eighth methanetriyl groups (>CH-) substitutes and fifteen one-fourth >CH- substitutes were optimized using the first-principle calculations based on density functional theory (DFT). Both one-eighth and one-fourth substitutes still keep the gauche structures with the simple formula CHN7 and CHN3, respectively. The most thermodynamic stable gauche CHN7 and CHN3 are P21 qtg-C2H2N14 I and P21 qtg-C4H4N12 III, respectively. No probability density of C-C single bonds and high probability densities of C-N-C structures were found in the two substitutes. Although gauche CHN7 and CHN3 lose energy density in contrast to cg-N, they win kinetic stability and combustion temperature (Tc). Thus, they are more feasible than cg-N, and more effective than the traditional rocket fuels.

Assessing the stable conformations of ibuprofen in solution by means of Residual Dipolar Couplings

Detailing the conformational equilibria between global and local minimum energy structures of anti-inflammatory α-arylpropionic acids directly in solution is of the utmost importance for a better understanding of the structure-activity relationships, hence providing valuable clues for rational structure-based drug design studies. Here the conformational preferences of the widely used pharmaceutical ibuprofen were investigated in solution by NMR spectroscopy in weakly ordering phases. A thorough theoretical treatment of the anisotropic interactions that are relevant for NMR spectra led to a conformational model characterized by six pairs of symmetry-related conformers, in particular four couples of gauche structures, with a total probability of 93%, and 2 couples of trans structures, counting for the remaining 7%.

Comparison of the One-Electron Oxidations of CO-Bridged vs Unbridged Bimetallic Complexes: Electron-Transfer Chemistry of Os2Cp2(CO)4 and Os2Cp*2(μ-CO)2(CO)2 (Cp = η5-C5H5, Cp* = η5-C5Me5)

The one-electron oxidations of two dimers of half-sandwich osmium carbonyl complexes have been examined by electrochemistry, spectro-electrochemistry, and computational methods. The all-terminal carbonyl complex Os2Cp2(CO)4 (1, Cp = η5-C5H5) undergoes a reversible one-electron anodic reaction at E1/2 = 0.41 V vs ferrocene in CH2Cl2/0.05 M [NBu4][B(C6F5)4], giving a rare example of a metal–metal bonded radical cation unsupported by bridging ligands. The IR spectrum of 1+ is consistent with an approximately 1:1 mixture of anti and gauche structures for the 33 e– radical cation in which it has retained all-terminal bonding of the CO ligands. Density functional theory (DFT) calculations, including orbital-occupancy-perturbed Mayer bond-order analyses, show that the highest-occupied molecular orbitals (HOMOs) of anti-1 and gauche-1 are metal–ligand delocalized. Removal of an electron from 1 has very little effect on the Os–Os bond order, accounting for the resistance of 1+ to heterolytic cleavage. The Os–Os bo...

Second Harmonic Generation, Electrooptical Pockels Effect, and Static First-Order Hyperpolarizabilities of 2,2′-Bithiophene Conformers: An HF, MP2, and DFT Theoretical Investigation

The static and dynamic electronic (hyper)polarizabilities of the equilibrium conformations of 2,2′-bithiophene (anti-gauche and syn-gauche) were computed in the gas phase. The calculations were carried out using Hartree-Fock (HF), Møller-Plesset second-order perturbation theory (MP2), and density functional theory methods. The properties were evaluated for the second harmonic generation (SHG), and electrooptical Pockels effect (EOPE) nonlinear optical processes at the typical λ=1064 nm of the Nd:YAG laser. The anti-gauche form characterized by the S–C2–C2′–S dihedral angle of 137° (MP2/6-311G**) is the global minimum on the potential energy surface, whereas the syn-gauche rotamer (S–C2–C2′–S = 48°, MP2/6-311G**) lies ca. 0.5 kcal/mol above the anti-gauche form. The structural properties of the gauche structures are rather similar to each other. The MP2 electron correlation effects are dramatic for the first-order hyperpolarizabilities of the 2,2′-bithiophenes, decreasing the HF values by ca. a factor of three. When passing from the anti-gauche to the syn-gauche conformer, the static and frequency-dependent first-order hyperpolarizabilities increase by ca. a factor of two. Differently, the electronic polarizabilities and second-order hyperpolarizabilities of these rotamers are rather close to each other. The syn-gauche structure could be discriminated from the anti-gauche one through its much more intense SHG and EOPE signals.

Structural, torsional, vibrational and response electric properties of 2,2′-bitellurophene rotamers. An ab initio and density functional theory investigation

The molecular structure, conformational behaviour, vibrational spectra and electronic (hyper)polarizabilities of tellurophene and 2,2′-bitellurophene rotamers were determined in gas by correlated ab initio and density functional theory calculations. The torsional potential for the rotation around the C2–C2′ inter-ring bond shows two minima corresponding to anti-gauche and syn-gauche structures and three maxima to planar anti and syn forms and to perpendicular conformation. The potential energy curve is rather flat over the entire 0°–180° twisting range and free rotation cannot be excluded. The IR and Raman spectra of the gauche structures are rather similar to each other, vibrational transitions being scarcely helpful for an unambiguous identification of the rotamers. The dipole moment and the first-order hyperpolarizability increase on passing from the anti-gauche to the syn-gauche conformation by a factor of five and four, respectively. The second harmonic generation nonlinear optical process can be useful to identify the 2,2′-bitellurophene rotamers. On the other hand, the electronic polarizabilities of these structures are much more closer to each other, being predicted to be within 2–13 %.

Towards a first-principles model of Fermi resonance in the alkyl CH stretch region: Application to 1,2-diphenylethane and 2,2,2-paracyclophane

The spectroscopy of two flexible hydrocarbons, 1,2-diphenylethane (DPE) and 2,2,2-paracyclophane (TCP) is presented, and a predictive theoretical model for describing the alkyl CH stretch region of these hydrocarbons is developed. Ultraviolet hole-burning spectroscopy identified two isomers of DPE and a single conformation of TCP present in the supersonic jet expansion. Through the analysis of the ground state low-frequency vibronic spectroscopy obtained by dispersed fluorescence, conformational assignments were made for both DPE and TCP. The two isomers of DPE were found to retain the low energy structures of butane, being present in both the gauche and anti structures. TCP forms a C(2) symmetric structure, differing from the predicted lower energy C(3) conformation by the symmetry of the ethano bridges (-CH(2)CH(2)-) linking the phenyl substituents. Resonant ion-dip infrared spectroscopy is used to record single-conformation IR spectra of the two conformers of DPE and the single conformer of TCP in the alkyl CH stretch region and in the mid-IR that covers the CH bend fundamentals. A local mode Hamiltonian that incorporates cubic stretch-bend coupling is developed. Its parameters are obtained from density functional theory methods. Full dimensional calculations are compared to those that use reduced dimensional Hamiltonians in which anharmonic CH stretches and scissor modes are Fermi coupled. Excellent agreement is found. Scale factors of select terms in the reduced dimensional Hamiltonian are determined by fitting the theoretical Hamiltonian to the anti-DPE spectrum. The scaled Hamiltonian is then used to predict successfully structures for the remaining lower symmetry experimentally determined spectra in the alkyl CH stretch region.

Infrared spectra of protonated neurotransmitters: dopamine

The infrared (IR) spectrum of the isolated protonated neurotransmitter dopamine was recorded in the fingerprint range (570-1880 cm(-1)) by means of IR multiple photon dissociation (IRMPD) spectroscopy. The spectrum was obtained in a Fourier transform ion cyclotron resonance mass spectrometer equipped with an electrospray ionization source, which was coupled to a free electron laser (FEL). The spectroscopic studies are complemented by quantum chemical calculations at the B3LYP and MP2 levels of theory using the cc-pVDZ basis set. Several low-energy isomers with protonation occurring at the amino group are predicted in the energy range 0-50 kJ mol(-1). Good agreement between the measured IRMPD spectrum and the calculated linear absorption spectra is observed for the two gauche conformers lowest in energy (ΔE) and free energy (ΔG) at both levels of theory, denoted g-1 and g+1. Minor contributions of higher lying gauche isomers cannot be ruled out spectroscopically but their calculated energies suggest only minor population in the sampled ion cloud. In all these gauche structures, one of the three protons of the ammonium group is pointing toward the catechol subunit, thereby maximizing the intramolecular NH-π interaction of the positive charge with the aromatic ring. In total, 16 distinct vibrational bands are observed in the IRMPD spectrum and assigned to individual normal modes of the energetically most stable g-1 conformer, with deviations of less than 24 cm(-1) (average 11 cm(-1)) between measured and calculated frequencies. Comparison with neutral dopamine reveals the effects of protonation on the geometric and electronic structure.

The conformational landscape of 5-methoxytryptamine studied by rotationally resolved fluorescence spectroscopy and resonant ionization spectroscopy

Rotationally resolved electronic spectra of three different conformers of 5-methoxytryptamine were recorded in a molecular beam. 5-Methoxy substitution reduces the number of observed conformers to three compared to seven that have been reported for tryptamine. Quantum chemical calculations indicate that anti-rotamers of the methoxy-group are more stable relative to the syn-forms. Two gauche structures (Gpy(in) and Gph(in)) of the ethylamino group with respect to the indole chromophore were found to be less stable than the other seven. The lowest electronically excited state has been identified as the (1)L(b) one for all observed conformers which was confirmed by quantum-chemical calculations. Based on the comparison of rotational constants obtained from fits using evolutionary algorithms with those from calculations, the three observed conformers were determined to be the Gpy(up), Gph(up), Gpy(out) ethylamino side-chain conformations.

Generalized Anomeric Effect in CH4−nClnS. Energetic and NBO Analyses

Abstract The conformational preference for gauche structures over anti ones in the simple model molecules CH4−nClnS (where n=1 and 2) was examined in detail by means of Natural Bond Orbital (NBO) analysis. We also compared the origin of the anomeric effect in CH2ClSH and CH2FSH. The energetic preference at MP2/6-311+G(d,p) level was slightly greater in CH2ClSH than in CHCl2SH. NBO analysis at the HF/6-311+G(d,p)//MP2/6-311+G(d,p) level indicates that the conformational preference for gauche conformers over anti ones is the result of a wide variety of orbital interactions. However, the interaction between the lp(S) and σ∗(C–Cl) is the most important factor for stabilizing the gauche conformers. In all compounds studied here, the effect of electrostatic and steric contributions involved in the Lewis term on stability of gauche conformers was less important than the charge delocalization.

Preparation of New Nitrogen-Bridged Heterocycles. 60. Syntheses and Conformational Analyses of Bis(indolizin-1-yl) Disulfides

Some bis(indolizin-1-yl) disulfides, readily obtainable from the treatment of 1-(benzoylthio)indolizines with piperidine, were prepared and their conformations were investigated. In comparison with those of 1-(benzoylthio)indolizines, the (1)H-NMR spectra of these disulfides showed considerable high field shifts (delta 0.13-0.82 ppm) on each pyridine ring proton and the UV spectra exhibited significant bathochromic and hyperchromic shifts. These results supported strongly the participation of an intramolecular pi-pi interaction between the two indolizine rings in these molecules and, hence, of a particular gauche (cis) conformation. However, the conformational considerations and molecular calculations (Mopac PM3) for some bis(indolizin-1-yl) disulfides showed the presence of four more stable gauche forms in which two are enantiomeric, resulting in three types of gauche structures. These three types of gauche structures were confirmed by X-ray analyses.

NBO and AIM analyses of the anomeric effect in fluoromethanthiol

The origin of generalized anomeric effect in fluoromethanthiol, CH 2FSH, has been investigated by means of ab initio calculations and Natural Bond Orbital (NBO) and Atoms In Molecules (AIM) analyses. The calculated energy data at various computational levels show that the electron correlation increases the relative energy of less stable conformer ( anti), while augmenting the basis set with diffuse and polarization functions acts in the opposite way. The NBO analysis reveals that a very close to pure p-type lone pair orbital and a sp 0.41 hybrid-type lone pair orbital participate in the electron donation to the σ*(C–F) orbital in the gauche and anti structures, respectively. The NBO analysis also shows that the electrostatic and steric contributions included in the Lewis term tend to keep anti conformer, whereas delocalization contribution tends to gauche conformer, as expected for an anomeric effect. The origin of the anomeric effect in CH 2FSH has been also compared with CH 2FOH. The obtained results of thermodynamic calculations and NBO and AIM analyses show that the anomeric effect in CH 2FSH is slightly smaller than CH 2FOH.

Friction induced conformational changes on large‐scale and small‐scale polyester sliding surfaces

AbstractPure and internal PTFE‐lubricated polyesters are slid against steel and stainless steel on large‐scale and small‐scale tribological test rigs under normal loads between 8 and 150 MPa and temperatures of 80–200 °C. The polymer sliding surfaces are analysed by Raman spectra according to a two‐phase system of trans (oriented) and gauche (non‐oriented) structures. Comparing the spectra of pure and internal lubricated polyesters reveals a clear band at 713 cm−1 correlated to PTFE depositions. For small‐scale tests, the crystalline content and overall orientation is larger (40–50%) while no significant increase in the PTFE‐related band is observed. A PTFE/polyester film is transferred onto the steel counterface without back‐transfer to the polymer surface. Large‐scale polymer surfaces have a lower orientation (30–40%) and contradictory to small‐scale samples, PTFE lubricants are observed on the polymer surface. The formation of a PTFE film on the polyester sliding surface is then favourable for low friction and stabilization in wear rates. Orientation and wear effects depend on the movability of wear debris and plastification of the surface, illustrated by Raman spectroscopy. Copyright © 2006 John Wiley & Sons, Ltd.

Intramolecular Hydrogen Bonding in Disubstituted Ethanes. A Comparison of NH···O- and OH···O- Hydrogen Bonding through Conformational Analysis of 4-Amino-4-oxobutanoate (succinamate) and Monohydrogen 1,4-Butanoate (monohydrogen succinate) Anions

Relative strengths of amide NH...O- and carboxyl OH...O- hydrogen bonds were investigated via conformational analysis of succinamate and monohydrogen succinate anions with the aid of vicinal proton-proton NMR couplings and B3LYP DFT quantum mechanical calculations for a variety of solvents. New experimental results for succinamate are compared with those obtained from previous studies of monohydrogen succinate. While some computational results for monohydrogen succinate were published previously, the results contained herein are the product of a more powerful methodology than that used earlier. The experimental results clearly show that intramolecular hydrogen-bond formation is more favored in aprotic solvents than in protic solvents for both molecules. Furthermore, the preference of the succinate monoanion for the gauche conformation is much stronger in aprotic solvents than that of succinamate, indicating that the OH...O- hydrogen bond is substantially stronger than its NH...O- counterpart, despite the approximately 5 kcal cost for formation of the E configuration of the carboxyl group needed to make an intramolecular hydrogen bond. The actual energy differences between formation of internal hydrogen bonds for monohydrogen succinate and succinamate anion were estimated by comparison of the relative values of K1 of the respective acids in water and DMSO by a procedure first developed by Westheimer. Recent theoretical work with succinamate highlights the necessity of considering substituent orientational degrees of freedom to understand the conformational equilibria of the central CH2-CH2 torsions in disubstituted ethanes. Similar methodology is applied here to succinic acid monoanion, by mapping potential-energy surfaces with respect to the CH2-CH2 torsional, carboxyl-substituent rotational, and carboxyl-proton E/Z isomeric degrees of freedom. Boltzmann populations were compared with gauche populations estimated from the experimentally determined coupling constants. The quantum mechanical results for succinamate show a much weaker tendency toward hydrogen bonding than for the succinic acid monoanion. However, the theoretical methods employed appear to substantially overestimate contributions from intramolecularly hydrogen-bonded structures for the succinic acid monoanion when compared with experimental results. Natural bond orbital analysis, applied to the quantum mechanical wave functions of fully optimized gauche and trans structures, showed a strong correlation between the population of amide sigma*(N-H) and carboxyl sigma*(O-H) antibonding orbitals and apparent hydrogen-bonding behavior.

Accurate torsional potentials in conjugated systems: ab initio and density functional calculations on 1,3-butadiene and monohalogenated butadienes

Accurate calculations of the torsional potentials for rotation around the carbon–carbon single bond of all conceivable monohalogenated 1,3-butadienes C4 H 5X, (X ∈ [F, Cl, Br]), are presented. The parent compound, 1,3-butadiene, is also included as a benchmark and reference case. Large-scale ab initio calculations were performed at the second-order Møller–Plesset perturbation theory (MP2) and the coupled cluster, CCSD(T), levels. Additionally, density functional methods were applied. In all compounds considered, the anti- or s-trans-conformation is the most stable. For all three halogens, the 2-halo-1,3-butadiene is the most stable isomer, followed by the cis-1-halo-1,3-butadiene. Depending on the position and the type of halogen, the original 1,3-butadiene torsional potential is modified in a different manner. The modifications are particularly visible in the region of the syn- or s-cis and the gauche structures and in the barrier heights.

Dimers of Ethanol

The energy difference between the gauche and trans structures of ethanol is determined at the CCSD(T)/aug-cc-pVTZ level. Five classes of dimer structures exist due to the two monomer conformations. In a systematic way 600 structures were generated as initial structures for the dimer geometry optimization. On the MP2/6-311++(3df,2p) level the following minima on the potential energy surface were found: 10 conformations with oxygen as the proton donor and 14 conformations with only carbon as the proton donor. Saddle points and barrier heights of five rearrangement reactions were calculated on the MP2/6-31++G(2d,p) level. The consequences of these rearangement reactions are: only six conformations of the ethanol dimers exist with oxygen as the proton donor and at low temperatures some conformations with only carbon atoms as the proton donor may be stable. A qualitative agreement exists between the experimental OH stretching spectrum and the corresponding calculated spectrum by use of the double harmonic ap...

High resolution electronic spectroscopy of three n-alkylbenzenes: ethyl-, propyl-, and butylbenzene

Rotationally resolved S1−S0 fluorescence excitation spectra of ethylbenzene, two conformers of n-propylbenzene, and two conformers of n-butylbenzene have been observed and assigned. The data obtained provide information about the equilibrium properties of each molecule, including their geometries in the S1−S0 states, their electronic distributions, and their dynamical behavior following the absorption of light. Trans structures are found to have S1 states that are Lb1 in character with relatively long fluorescence lifetimes. Gauche structures are found to have S1 states that are mixed (1Lb/1La) in character with relatively short fluorescence lifetimes. Possible reasons for these differences in properties are discussed.

An analysis of the conformers of 1,5-hexadiene

High level theoretical methods have been used to predict the energetic ordering of the conformations of 1,5-hexadiene. Presented are optimized geometries and relative energies for ten energetically distinct conformations of the molecule and two enantiomeric conformations. Previous work had predicted that the gauche (or skew) conformation about the central carbon—carbon bond is the lowest in energy. However, we have found that the conformational energies of 1,5-hexadiene, unlike n-butane, are remarkably similar. We postulate this to be the reason for poor performance (for example, in comparisons with variable temperature NMR studies) of some molecular mechanics simulations on molecules of this type in the past. With the DZP CCSD(T) method, the gauche(120)/anti/gauche(120) conformer is predicted to be the global minimum, followed 0.06kcalmol−1 higher in energy by the gauche(120)/anti/gauche(−120) structure, followed another 0.03 kcal mol−1 higher in energy by the gauche(120)/gauche(60)/gauche(−120) arrangement. The three earlier reported molecular mechanics conformational energy surfaces all rise much more steeply from the gauche(120)/anti/gauche(120) and gauche(120)/gauche(60)/gauche(−120) structures.

High-level theoretical study of the conformational equilibrium of n-pentane

An accurate calculation of the energy differences between stationary points on the torsional potential energy surface of n-pentane is performed using ab initio Hartree–Fock theory, advanced many-body methods such as MP2, MP3, CCSD, and CCSD(T), as well as density functional theory, together with basis sets of increasing size. This study focuses on the four conformers of this compound, namely, the all staggered trans–trans (TT), trans–gauche (TG), gauche–gauche (G+G+), and gauche–gauche (G+G−) structures, belonging to the C2v, C1, C2, and C1 symmetry point groups, respectively. A focal point analysis up to 635 basis functions is carried out to determine when the series of relative energies of the four conformers approach convergence. It is found that relative to the minimum energy TT conformer, the energy differences of the TG, G+G+, and G+G− conformers obtained from ab initio methods are 0.621, 1.065, and 2.917 kcal mol−1, respectively. Converged energy differences obtained with three density functionals, B3PW91, B3LYP, and MPW1K, are found to be considerably higher than those computed ab initio. Mole fractions of the various conformers are evaluated at different temperatures from thermostatistical data accounting for vibrational and rotational entropies, as well as zero-point vibrational energies in the rigid rotor-harmonic oscillator approximation.

Stereochemistry and Stereoelectronics of Azines. 13. Conformational Effects on the Quadrupolarity of Azines. An Ab Initio Quantum-Mechanical Study of a Lateral Synthon

The quadrupole moment of formaldazine, H2C=N-N=CH2, has been studied for the trans structure (Ð(C-N-N-C) = τ = 180≡) and a series of gauche structures (τ > 120≡). Restricted Hartree-Fock theory, second-order Moller-Plesset theory, and quadratic CI theory have been used in conjunction with the basis sets 6-31G*, 6-31G**, 6-311G** and 6-311++G**. Formaldazine is a quadrupolar molecule with primitive quadrupole moment tensor components of Qxx = -22.4, Qyy = -20.4 and Qzz = -25.6 DA at the theoretical level QCISD/6-311++G**. The examination of the theoretical level dependency shows that the reliable computation of a quadrupole moment requires the use of a flexible basis set. A large part of the component Qzz = -25.6 DA is due to the π-system and compares, on a per electron basis, with the Qzz value of benzene. Conformational changes of the azines in the range 120° < τ < 180≡ have but a minute effect on the energy and are associated with only minor electronic relaxation. These conformational changes alter the quadrupole moment tensor components less than ΔQxx = +0.4, ΔQyy = +1.6 and ΔQzz = -1.0 DA at QCISD/6-311++G**//QCISD/6-31G*. The direction of these changes is explained by consideration of the rotation of the CN-π-systems and a small reduction of the CN bond polarity in the gauche structures. The Qzz component of formaldazine is representative of the quadrupole moment tensor component along the direction of the C2 axis of the azine bridge as such. Hence, the results of this study suggest that azines can engage in strong quadrupole-quadrupole interactions and can be employed as lateral synthons in crystal engineering.