Aa Conformer Research Articles

Precursors of fatty alcohols in the ISM: Discovery of n-propanol

Context. Theories of the origins of life propose that early cell membranes were synthesised from amphiphilic molecules simpler than phospholipids, such as fatty alcohols. The discovery in the interstellar medium (ISM) of ethanolamine, the simplest phospholipid head group, raises the question whether simple amphiphilic molecules are also synthesised in space. Aims. We investigate whether precursors of fatty alcohols are present in the ISM. Methods. To do this, we have carried out a spectral survey at 7, 3, 2 and 1 mm towards the Giant Molecular Cloud G+0.693-0.027 located in the Galactic centre using the IRAM 30 m and Yebes 40 m telescopes. Results. Here, we report the detection in the ISM of the primary alcohol n-propanol (in both conformers Ga-n-C3H7OH and Aa-n-C3H7OH), a precursor of fatty alcohols. The derived column densities of n-propanol are (5.5 ± 0.4) × 1013 cm−2 for the Ga conformer and (3.4 ± 0.3) × 1013 cm−2 for the Aa conformer, which imply molecular abundances of (4.1 ± 0.3) × 10−10 for Ga-n-C3H7OH and of (2.5 ± 0.2) × 10−10 for Aa-n-C3H7OH. We also searched for the AGa conformer of n-butanol [AGa-n-C4H9OH] without success, yielding an upper limit to its abundance of ≤4.1 × 10−11. The inferred CH3OH:C2H5OH:C3H7OH:C4H9OH abundance ratios are 1:0.04:0.006:≤0.0004 towards G+0.693-0.027, that is, they decrease roughly by one order of magnitude for increasing complexity. We also report the detection of both syn and anti conformers of vinyl alcohol, with column densities of (1.11 ± 0.08) × 1014 cm−2 and (1.3 ± 0.4) × 1013 cm−2, and abundances of (8.2 ± 0.6) × 10−10 and (9.6 ± 3.0) × 10−11, respectively. Conclusions. The detection of n-propanol, together with the recent discovery of ethanolamine in the ISM, opens the possibility that precursors of lipids according to theories of the origin of life, could have been brought to Earth from outer space.

X-ray structure of an unusual glycoluril derivative conformation: Structural analysis and influencing factors

The synthesis and X-ray structure of a novel glycoluril derivative are described. The crystal of the glycoluril derivative is obtained by slow evaporation in a refrigerator at about 5 °C. X-ray crystallographic analysis revealed that the compound crystallized in an unusual as conformation instead of the usual clip-shaped aa conformation. Structural analysis shows that the solvation effect, hydrogen bonding, and space-occupying effect influenced the crystallization conformation of the derivative. The X-ray crystallographic analysis in this work provides additional crystallographic evidence of the as conformation of the 3U glycoluril derivative and shows the effect of solvent water on the conformational behavior.

Microwave spectrum of 1-bromobutane

The rotational spectrum of 1-bromobutane has been measured in the range of 8–18GHz using a 480MHz bandwidth chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. 1-bromobutane has five conformers; aa, ag, ga, gg, gg′. Spectra for the 79Br isotopic species and 81Br isotopic species were observed and assigned for the three lowest energy conformers, aa, ga, and gg. Consequentially, the rotational constants, nuclear quadrupole coupling constants, and centrifugal distortion constants were determined and the dipole moment of the aa conformer with 79Br was measured. All the experimental data are in good agreement with the ab initio calculations.

Conformational and spectroscopic study of xanthogen ethyl formates, ROC(S)SC(O)OCH2CH3. Isolation of CH3CH2OC(O)SH

ROC(S)SC(O)OCH2CH3, with R=CH3, (CH3)2CH and CH3(CH2)2, were obtained through the reaction between potassium xanthate salts, ROC(S)SK, and ethyl chloroformate, ClC(O)OCH2CH3. The liquid compounds were identified and characterized by 1H and 13C NMR and mass spectrometry. The conformations adopted by the molecules were studied by DFT methods. 6 conformers were theoretically predicted for R=CH3 and (CH3)2CH, while the conformational flexibility of the n-propyl substituent increases the total number of feasible rotamers to 21. For the three molecules, the conformers can be associated in 3 groups, being the most stable the AS forms – the CS double bond anti (A) with respect to the CS single bond and the SC single bond syn (S) with respect to the CO double bond – followed by AA and SS conformers. The vibrational spectra were interpreted in terms of the predicted conformational equilibrium, presenting the ν(CO) spectral region signals corresponding to the three groups of conformers. A moderated pre-resonance Raman enhancement of the ν(CS) vibrational mode of CH3(CH2)2OC(S)SC(O)OCH2CH3 was detected, when the excitation radiation approaches the energy of a n→π∗ electronic transition associated with the CS chromophore. UV–visible spectra in different solvents were measured and interpreted in terms of TD-DFT calculations. The unknown molecule CH3CH2OC(O)SH was isolated by the UV–visible photolysis of CH3OC(S)SC(O)OCH2CH3 isolated in Ar matrix, and also obtained as a side-product of the reaction between potassium xanthate salts, ROC(S)SK, and ethyl chloroformate, ClC(O)OCH2CH3.

Conformational Preferences of trans-1,2- and cis-1,3-Cyclohexanedicarboxylic Acids in Water and Dimethyl Sulfoxide as a Function of the Ionization State As Determined from NMR Spectroscopy and Density Functional Theory Quantum Mechanical Calculations

The populations of diaxial (aa) and diequatorial (ee) conformers of trans-1,2- and cis-1,3-cyclohexanedicarboxylic acids (CDCAs; 1 and 2, respectively) and their salts were determined in water and dimethyl sulfoxide (DMSO) solutions from vicinal proton-proton NMR J couplings ((3)J(HH)). Optimized geometries and free energies for these compounds were obtained at the M06-2X/cc-pVTZ(-f)++ level. Although carboxylic acid groups in cyclohexane rings are generally believed to be far more stable (~2 kcal/mol) in equatorial than axial positions, this investigation demonstrated that an aa conformation (normally assumed to be completely insignificant for these compounds) can be favored depending on the medium and ionization state: strong ee preferences (>90%) were observed in water and DMSO for both diacids and their salts, except for the dianion of 1 in DMSO, which was found to be substantially aa (~57%). The possibility of intramolecular hydrogen bonding (H-bonding) was also investigated; the ratios of the ionization constants (K(1)/K(2)) indicated an absence of intramolecular H-bonding because K(1)/K(2) ≪ 10(4) (a standard criterion for non-H-bonding in dicarboxylic acids) for both 1 and 2 in water and also for 2 in DMSO. For 1, K(1)/K(2) increased drastically in DMSO (K(1)/K(2) = 4 × 10(6)), where (3)J(HH) and the ratio K(1)/K(E) = 10, K(E) being the acidity constant of the monomethyl ester of 1, indicated the formation of an intramolecular H-bond for the monoanion in this solvent. An explanation for the observation of compact dianions in solution in terms of the generalized Born equation is also provided.

Vibrational spectra, ab initio calculations, and assignments of the fundamentals of the C2v conformer of n-pentane

Ab initio calculations of the relative energies, optimized geometries and vibrational spectrum of the four conformers of n-pentane have been completed, and a normal coordinate calculation performed to establish the nature of the observed molecular vibrations. From the Raman spectrum of the annealed crystal, in which only aa conformer bands are observable, an assignment of the observed bands to the fundamental vibrations of the aa conformer of C2v symmetry is proposed. The relative energy in the liquid of the ag conformer to the aa conformer is measured in the CCC bending region near 400cm−1 using both n-pentane and n-pentane-d12, and is found to be 164±10cm−1, corresponding to 460±28calmol−1 in excellent agreement with the results of earlier workers obtained by other measurements, but roughly half the value calculated ab initio at the highest level of approximation available in this investigation.

Conformational Changes of trans-1,2-Dichlorocyclohexane Adsorbed in Zeolites Studied by FT-Raman Spectroscopy and Molecular QM/MM Simulations

The conformational behavior of trans-1,2-dichlorocyclohexane (T12D) adsorbed inside zeolites with Faujasite structure (FAU) including sodium Y (Na–Y) and siliceous Y (Si–Y) was investigated by FT-Raman spectroscopy and molecular simulations. The results have clearly shown that the conformational and dynamic properties of T12D strongly depend on the presence of charge-balancing cations as well as Si/Al ratio. Upon loading into Na–Y, the population of the diequatorial (ee) conformer increases compared with pure T12D liquid due to the strong interaction with the extra-framework Na+ ions. Molecular simulations of T12D in Na–Y and Si–Y have also been carried out. The T12D molecule was modeled by quantum and semiempirical quantum chemistry methods and embedded in a zeolite framework that was described by empirical force field. Conformational changes were sampled using quantum mechanics/molecular mechanics molecular dynamics and replica exchange molecular dynamics. Molecular simulations revealed that the ee conformer is preferable versus the diaxial (aa) conformer in both Na–Y and Si–Y frameworks. However, in the Na–Y supercage environment, the Na+ ions polarize the ee conformer stronger than in Si–Y. This leads to a larger shift of the conformational equilibrium in favor of the ee component in Na–Y relative to Si–Y. Computations of the equilibrium population of aa and ee conformers using the dihedral Cl–C–C–Cl angle distributions of aa and ee showed good quantitative agreement with the experimental findings with respect to the dominant ee conformer in both Na–Y and Si–Y.

Microwave and vibrational spectra, ab initio calculations, conformational stabilities and assignments of the fundamentals of the C s conformer of n-butylsilane

The vibrational spectrum of n-butylsilane is described. Complete assignments of fundamentals are made for the anti–anti (aa) conformer of C s symmetry. The relative stabilities of the five different conformers are calculated, and the (aa) conformer emerges as the conformer of lowest relative energy, and thus the most abundant, entirely dominating the infrared and Raman spectra as confirmed by the spectra of the annealed crystal. The dipole moments of all the conformers are calculated to lie between 0.88 and 1 Debye, similar to the dipole moments of the conformers of n-butylgermane. Microwave measurements of this molecule seeded in a molecular jet were made using Fourier-transform microwave spectroscopy and confirmed the presence of the aa conformer, in addition to the anti–gauche (ag) and gauche–anti (ag) conformers. Measurement of rotational spectra for all four 13C-substituted isotopologues as well as the 29Si and 30Si species for the aa conformer allowed a heavy atom structure determination for this conformer.

Microwave and vibrational spectra, ab initio calculations, conformational stabilities and assignments of the fundamentals of the C s conformer of n-butylgermane

The FT-microwave spectrum of n-butylgermane, CH 3CH 2CH 2CH 2GeH 3 has been investigated from 4000 to 18,000 MHz and the microwave spectra have been observed for all of the five naturally occurring germanium isotopologues for the anti–anti (aa) conformer. The dipole moment for the 74Ge containing species has been measured, giving a total dipole moment of 0.881 (26) D. In addition, the vibrational spectrum of n-butylgermane is described. Modestly complete assignments are made for the aa conformer. The relative stabilities of the five conformers are calculated, and the anti–anti (aa) conformer is found to be the most stable in all calculations done. This conclusion is confirmed by the infrared and Raman spectrum of the annealed crystal. The dipole moments of all conformers are calculated to be approximately equal and less than 1 D, ranging from approximately 0.8 to 0.9 D.

Conformational Barrier for Fullerene−Porphyrin−Fullerene Triad

A recent study (J. Phys. Chem. B 2006, 110, 5337) proposed that fullerene-porphyrin-fullerene triad (C60PC60) could be utilized as a photoinduced switch due to the difference in electron transfer directionality for the cis and trans conformer. It is found that the rotational barrier between the SS and AA conformers of C60PC60 is about 3-5 kcal/mol, which can be facilely controllable in experiment. The rotational energy barrier for the anion system is slightly higher than the neutral C60PC60, whereas the total energy of the neutral triad is decreased by about 60 kcal/mol upon electron attachment. The corresponding reorganization energy is very small. These results reveal that the C60PC60 is a potential candidate for electrochemical machinery and energy storage material.

Dependence of the l-Alanyl-l-Alanine Conformation on Molecular Charge Determined from Ab Initio Computations and NMR Spectra

The l-alanyl-l-alanine (AA) molecule behaves differently in acidic, neutral, and basic environments. Because of its molecular flexibility and strong interaction with the aqueous environment, its behavior has to be deduced from the NMR spectra indirectly, using statistical methods and comparison with ab initio predictions of geometric and spectral parameters. In this study, chemical shifts and indirect spin-spin coupling constants of the AA cation, anion, and zwitterion were measured and compared to values obtained by density functional computations for various conformers of the dipeptide. The accuracy and sensitivity of the quantum methods to the molecular charge was also tested on the (mono)-alanine molecule. Probable AA conformers could be identified at two-dimensional potential energy surfaces and verified by the comparison of the computed parameters with measured NMR data. The results indicate that, whereas the main-chain peptide conformations of the cationic (AA+) and zwitterionic (AAZW) forms are similar, the anion (AA-) adopts also another, approximately equally populated conformer in the aqueous solution. Additionally, the NH2 group can rotate in the two main chain conformations of the anionic form AA-. According to a vibrational quantum analysis of the two-dimensional energy surfaces, higher-energy conformers might exist for all three charged AA forms but cannot be detected directly by NMR spectroscopy because of their small populations and short lifetimes. In accord with previous studies, the NMR parameters, particularly the indirect nuclear spin-spin coupling constants, often provided an excellent probe of a local conformation. Generalization to peptides and proteins, however, has to take into account the environment, molecular charge, and flexibility of the peptide chain.

The subtle electronic effects of alkyl groups on the conformational equilibria and intramolecular hydrogen-bond strength in cis-3-alkoxycyclohexanols

1H NMR data for cis-3- n-propoxycyclohexanol ( cis-3-PCH) and cis-3-isopropoxy-cyclohexanol ( cis-3-ICH) show that a concentration increase shifts the conformational equilibrium from the diaxial (aa) conformer, stabilized by an intramolecular hydrogen bond (IAHB), to the diequatorial (ee) conformer [ X ee = 42% and 21% (at 0.01 mol L −1) to 58% and 56% (at 0.40 mol L −1), in CCl 4, respectively] due to intermolecular hydrogen bonds (IEHB), as confirmed by IR data. The Δ ν values, obtained by IR spectra, indicated that increasing the size of the OR group [R = CH 3, CH 2CH 2CH 3 and CH(CH 3) 2], increases the IAHB strength, due to an increase in the inductive effect of R group, which makes the oxygen lone pairs more available for an IAHB with OH group, in opposition to the steric effect. The percentage of ee conformer increases with the solvent basicity for cis-3-PCH and cis-3-ICH, from 48% and 36% in CCl 4 to 97% and 96% in DMSO, respectively. Values of 4.58, 6.06 and 6.33 kcal mol −1 for the IAHB strength in cis-3-PCH, cis-3-ICH and cis-3-TCH ( cis-3- tert-butoxycyclohexanol), respectively, were obtained, from the theoretical data through the CBS-4M method, confirming the experimental results and indicating that the IAHB strength increases with the increasing bulk of OR substituent in this series of compounds.

Synthesis of trans/ cis 4-substituted 3-furyl-2-phenethyltetrahydroisoquinolin-1-ones: conformation of the trans-4-(pyrrolidinylcarbonyl) derivative

The title compounds were synthesized starting from homophthalic anhydride and an imine. The amides obtained showed unexpected values for 3 J 3,4 that cannot be used to deduce their configuration and conformation. This problem was resolved for one representative compound (the 4-(pyrrolidinylcarbonyl) derivative) by means of detailed NMR studies, X-ray diffraction and theoretical calculations. The compound has the trans configuration. In the solid state, its conformation is with dipseudoaxial ( aa) oriented substituents at positions 3 and 4. In different solvents and in the gas-phase, the majority of the data reveal that the observed value of 3 J 3,4 results from an equilibrium of the ee and aa conformers.

1,3-Diaxial steric effects and intramolecular hydrogen bonding in the conformational equilibria of new cis-1,3-disubstituted cyclohexanes using low temperature NMR spectra and theoretical calculations

The conformational equilibria of 3-X-cyclohexanol [X = F ( 1), Cl ( 2), Br ( 3), I ( 4), Me ( 5), NMe 2 ( 6) and MeO ( 7)] and of 3-X-methoxycyclohexane [X = F ( 8), Cl ( 9), Br ( 10), I ( 11), Me ( 12), NMe 2 ( 13) and MeO ( 14)] cis isomers were determined from low temperature NMR spectra and PCMODEL calculated coupling constants. The energy differences between aa and ee conformers were obtained from these data ( Δ G J av and Δ G PC av , respectively) and also by the additivity principle from data for the monosubstituted cyclohexanes (Δ G Ad). H-1 and H-3 hydrogen vicinal coupling constants and Δ G J av values showed that the diequatorial conformer is predominant in the conformational equilibrium of the compounds studied at low temperature. However, Δ G PC av data show that compounds 6 and 7 constitute an exception, since they are almost equally populated by ee and aa at room temperature, due to stabilization of their aa conformer by an intramolecular hydrogen bond. Δ G Ad values, obtained according to the additivity principle, show a better agreement for compounds 2 and 3, since the 1,3-diaxial steric effect is counterbalanced by the formation of an intramolecular hydrogen bond (IAHB). For the remaining compounds, Δ G Ad values underestimate the energy differences, since the 1,3-diaxial steric effect, between X and OH or OCH 3, is absent in the monosubstituted compounds used as references. Moreover, the Δ G PC av , calculated from the coupling constants, obtained through the PCMODEL program, are rather smaller than the Δ G J av values, since the program does not have parameters for the effect, observed in this report, of a substituent at γ position on coupling constants values for the hydrogen under consideration.

Molecular structure and conformational preferences of chloro(dimethyldithio)phosphite, ClP(SCH 3) 2, as studied by gas electron diffraction and quantum-chemical calculations

Free ClP(SCH 3) 2 molecule has been studied by gas electron diffraction (GED), B3PW91/6-311+G* (DFT) and MP2/6-31+G* calculations. Each conformer is characterised by two dihedral angles τ(CSP lp) where lp denotes the direction of the electron lone pair on the P atom; assumed to coincide with normal to the plane passing through three points lying at the unit distances along each of three P bonds. DFT calculations indicate that the most stable conformer is a gauche+, gauche− ( g+ g−) conformer of C s symmetry characterised by the angles τ a,b=(CSP lp)±51.5°. It is followed by an anti, gauche+ conformer ( ag+) at ΔE=0.16 kJ mol −1, an anti, gauche− conformer ( ag−) at ΔE=1.3 kJ mol −1, a g− g− conformer at ΔE=5.1 kJ mol −1 and an aa conformer at ΔE=13.0 kJ mol −1. Whereas MP2 calculated results revealed that relative energies to be as follows: 3.6, 0.0, 1.5, 6.6, 13.0 kJ mol −1, respectively. That is ag+ conformer is the most stable one. The calculated standard free energies at 298.15 K indicate that the mole fractions in the gas phase at this temperature are (DFT/MP2): χ( g+ g−)=55/36%, χ( ag+)=24/34%, χ( ag−)=12/23% and χ( g− g−)=8/7%. Experimental GED data agree well with ag+ model alone and do not confirm any presence of g+ g− conformer, the presence of up to 10% ag− conformer cannot be completely excluded. Natural bond orbital (NBO) analysis of the wavefunctions suggest that while bond distances and valence angles are determined by anomeric effects, the relative stabilities of the five conformers are not determined by such effects alone. Structure optimisation of F 2PSMe molecule by both theoretical methods indicates that the most stable conformation is anti, while the energy of a gauche conformer is about 6.1/10.5 kJ mol −1 higher. NBO analysis of the wavefunctions indicates that the relative stabilities of the two conformers as well as the differences between bond distances and valence angles may be determined by anomeric effects.

An FT-Raman Spectroscopic Study of the Conformational Behavior of trans-1,4-Dichlorocyclohexane Adsorbed in Zeolites

We have examined the conformational properties of trans-1,4-dichlorocyclohexane (DCC) adsorbed in several zeolites, including completely siliceous ZSM-5, Na−Y and siliceous Y, by using FT-Raman spectroscopy. The results indicate that the conformational behavior of DCC is dependent on the zeolite framework structure, Si/Al ratio, and the charge-balancing cations. The conformational behavior of DCC in ZSM-5 differs from that in nonpolar solvents such as carbon tetrachloride in that the population of diequatorial (ee) conformer is significantly increased relative to the diaxial (aa) form upon adsorption. The ee and aa conformers are still in dynamic equilibrium as in solution, and lowering the temperature shifts the equilibrium further toward the ee configuration. The situation for DCC incorporated inside Na−Y is markedly different. Upon adsorption, the conformation of DCC is frozen by interactions with the extraframework sodium ions in the supercages of Na−Y. These interactions also lock the guest molecules...

Intramolecular OH/π interaction: the molecular structure and conformations of 3-hexyn-1,6-diol

The molecular structure and conformational composition of 3-hexyn-1,6-diol have been studied by the gas electron diffraction method and by ab initio calculations. Four conformers (AA, G+G−, G+G+ and GA) have been taken into account. The experimental data are in accordance with a conformational mixture consisting of 62.2% GG (equal parts of G+G− and G+G+) and 37.8% GA with a standard deviation of 10.3%. Similar results were, however, obtained when a small contribution (0≤x≥12%) from the high energy AA conformer was introduced and the other forms were properly corrected (GG (62.2+x%); GA (37.8−2x%)). From the experimental data the total amount of gauche contribution in the two parts of the molecule is accordingly estimated to be in the region 76.0–86.3%. The GG and GA conformers are characterized by the formation of two, respectively one, OH⋯π hydrogen bonds.

A Raman spectroscopic study of the rotational isomerism of linear permethylated stannylsilanes Me3Sn(SiMe2)nSnMe3 with n=1, 2, 3, 4

Abstract The IR and Raman vibrational spectra of the methyl(trimethylstannyl)silanes Me 3 Sn(SiMe 2 ) n SnMe 3 with n =1 ( 1 ), 2 ( 2 ), 3 ( 3 ) and 4 ( 4 ) were recorded at various temperatures. Compound 2 consists of a mixture of two conformers, gauche (G) and anti (A), in the liquid state. By variable temperature Raman spectroscopy, the enthalpy difference Δ H = H gauche − H anti was determined from the relative intensities of the symmetric SiSn stretching bands as 0.8±0.3 kJ mol −1 . In the Raman spectrum of the crystalline solid, the lines of the G conformer disappear completely, facilitating the assignment of the fundamental vibrations for the A form. No indication of the presence of a third ( ortho ) conformer could be detected. HF-SCF ab initio calculations using electron core potentials for the heavy atoms predict the existence of three conformers on the potential surface. In the Raman spectra of 3 , the symmetric SiSn stretching vibration splits into three components, which can be assigned to the G + G + , GA and AA conformers. The G + G − rotamer could not be observed. As its energy is significantly larger than that for the other conformations due to repulsive four bond interactions between the SnMe 3 groups (pentane effect), the concentration in the equilibrium mixture is negligibly small. For 4 , eight (2 3 ) spectroscopically distinct (classical) conformations are possible. In the room temperature Raman spectrum, the SiSn stretching vibration consists of a line at 330 cm −1 with a broad shoulder at 320 cm −1 . In the solid (−60°C), a single rotamer (probably the all anti form) survives. Upon heating, several conformational states with higher energies are populated which could not be resolved by Raman spectroscopy.

Conformational Studies in the Cyclohexane Series. 3. The Dihalocyclohexanes

The energies of axial and equatorial fluoro-, chloro-, and bromocyclohexanes as well as diaxial, axial−equatorial, and diequatorial 1,2-, 1,3-, and 1,4-dihalocyclohexanes were calculated using the hybrid density functional methods B3LYP and B3P86 as well as MP2 and QCISD and the 6-311G* and 6-311+G(2df,p) basis sets. The best agreement with experimental data was found with QCISD/6-311+G(2df,p). Solvent effects on the relative energies were calculated using the SCIPCM reaction field model. The effect of the halo substituents on the geometry of the cyclohexane ring was examined, and it was found that the effect of an axial substituent was local flattening of the cyclohexane ring, but no effect was found at the remote ring carbons, and no evidence of 1,3-diaxial interactions between the halogen and the axial ring hydrogens was found. In the case of the 1,2-dihalides, the calculations reproduce the preference for the diaxial form with X = Cl, but also predict that the energy difference between diaxial and diequatorial will be quite small when X = F. This appears to be related to the preference for the gauche form of 1,2-difluoroethane. The calculated relative energies of the 1,4-dihalocyclohexanes are in good agreement with electron diffraction data. A calculation of the electrostatic effects in the 1,4-dichlorocyclohexane reproduced the observed preference for the aa conformer.

Conformational Analysis of 1,4-Disilabutane and 1,5-Disilapentane by Combined Application of Gas-Phase Electron Diffraction and ab Initio Calculations and the Crystal Structure of 1,5-Disilapentane at Low Temperatures

The gas-phase structures of the conformers of 1,4-disilabutane and 1,5-disilapentane have been analyzed from electron-diffraction data augmented by flexible restraints derived from ab initio calculations. This allowed the simultaneous refinement of 22 and 29 parameters for 1,4-disilabutane and 1,5-disilapentane, respectively. 1,4-Disilabutane has been found to be present in the vapour predominantly in the anti (A) form (76(2)% from the experiment, 83% predicted by theory). Consistency in the geometries is found between theoretical predictions and experimental findings, except for the torsion angle ∠(SiCCSi) of the gauche (G) conformer [exptl 78.5(21)°, theor 68.0°]. The AA conformer of 1,5-disilapentane was always found to be the lowest energy structure, while some doubt still remains about the ordering of the AG and G(+)G(−) conformers. The AA conformer is found to be the sole form present in the crystal [C2/c, a = 15.585(8), b = 4.704(3), c = 9.895(6) A, β = 95.77(4)°, Z = 4]. Good agreement is found fo...