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.     

Structural evidence of anomeric effects in the anesthetic isoflurane

The conformational and structural properties of the inhalational anesthetic isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) have been probed in a supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy. Two conformers of the isolated molecule were identified from the rotational spectrum of the parent and several (37)Cl and (13)C isotopologues detected in natural abundance. The two most stable structures of isoflurane are characterized by an anti carbon skeleton (τ(C(1)-C(2)-O-C(3)) = 137.8(11)° or 167.4(19)°), differing in the trans (AT) or gauche (AG) orientation of the difluoromethyl group. The conformational abundances in the jet were estimated from relative intensity measurements as (AT)/(AG) ≈ 3:1. The structural preferences of the molecule have been rationalized with supporting ab initio calculations and natural-bond-orbital (NBO) analysis, which suggest that the molecule is stabilized by hyperconjugative effects. The NBO analysis of donor-acceptor (LP → σ*) interactions showed that these stereoelectronic effects decrease from the AT to AG conformations, so the conformational preferences can be accounted for in terms of the generalized anomeric effect.     

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     

Raman and infrared spectra,conformational stability,barriers to internal rotation,r 0 structural parameters,ab initio calculations,and vibrational assignment for vinyl chloroformate

The Raman (3200 to 10 cm^–1) and infrared (3500 to 50 cm^–1) spectra of vinyl chloroformate, H₂C=CHOC(O)Cl, have been recorded for both the gas and solid. Additionally, the Raman spectrum of the liquid has been recorded, and depolarization ratios have been obtained. These data have been interpreted on the basis that the only stable conformation present at ambient temperature is thetrans-trans rotamer, where the firsttrans refers to the vinyl moiety relative to the O—CCl bond and the second to the C—Cl bond relative to the=C—O bond. Using harmonic rigid asymmetric top calculations, the infrared vapor phase contours for the C=O and the C=C stretch were predicted for thetrans-trans and for thecis-trans conformer, and were compared with experiment. For both fundamentals thetrans-trans hybrid reproduces the experimental contour, whereas thecis-trans contours fail to do so for both fundamentals. From far-infrared spectrum of the vapor obtained at 0.1 cm^–1 resolution, the C(O)Cl andO-vinyl torsional fundamentals have been observed at 132 and 61 cm^–1, respectively. Ther ₀ structural parameters have been obtained from a combination of ab initio calculated parameters with appropriate offset values and the fit of the microwave rotational constants for the two naturally occurring chlorine isotopes. The structure, barrier to internal rotation, and vibrational frequencies have been determined from ab initio Hartree-Fock gradient calculations, using the 3-21G^* and 6-31G^* basis sets. These results are compared to those obtained experimentally and to similar quantities for some related molecules.     

Ab initio studies of structural features not easily amenable to experiment: Part 20. Structural aspects of ethyl groups

The molecular structures of a number of stable conformations of ethanol, ethylamine, methylethyl ether, methylethylamine and of the ethyl anion have been determined by ab initio geometry optimizations using Pulay's Force method on the 4–21G level. The calculated geometries characterize the extent to which structural groups in a molecule are sensitive to asymmetries in their environment. Characteristic structural trends are consistently found for the CH bond distances and CCH angles in the C₂H₅ groups of trans-ethanol, trans-methylethyl ether and in the ethyl anion. They differ from those previously found for C₂H₅ groups in hydrocarbons. There is qualitative disagreement between the trends calculated for CH bond distances in trans-ethanol and trans-methylethyl ether and those found in the microwave substitution structures of these compounds. Since the substitution parameters are unresolved because of relatively large experimental or model uncertainties, it is presently impossible to decide whether this discrepancy is the result of computational or experimental deficiency. The methyl groups in methylethyl ether and methylethylamine exhibit the characteristic structural distortions which are usually found for CH₃ groups adjacent to electron lone pairs. The CC bond distances in C₂H₅ in the systems studied here are sensitive to the conformational arrangement of ethyl relative to the rest of a system in a way which can be rationalized by orbital interactions involving antibonding orbitals on sp³-hybridized carbon atoms. The calculated conformational stabilities agree qualitatively with experimental trends, except in the case of ethanol where the trans — gauche energy difference is small (about 0.1 kcal mol^?1) and within the uncertainties of the calculations. Our conformational energies for CH₃CH₂NH₂ are in disagreement with a previous ab initio investigation based on a comparison of unoptimized standard geometries. In general, the agreement between calculated structural parameters and corresponding reliable experimental values is very good in all comparable cases.     

n-Propyl cyanide and isocyantde: rotational and structural isomerization

Ab initio calculations with an STO-3G basis and geometry optimization have been performed on n-propyl cyanide and isocyanide in four rotational conformations, trans, cis, and gauche, with, in the latter case, two different dihedral angles, 90° and 120° from the trans position, being employed. The trans and gauche 120° isomers are predicted to be the most stable for both the cyanide and isocyanide, and the cyanide—isocyanide energy difference is calculated to be approximately 22 kcal mole^?1 for each rotational isomer. The results of a population analysis are employed to discuss the electronic structures of the cyanide, isocyanide, and the isomerization process.     

Ab initio molecular orbital calculation on the rotational isomerism of 1,2-dichloroethane and 1-chloropropane

Ab initio MO calculations on the total energies of the rotational isomers of 1,2-dichloroethane and 1-chloropropane were performed to estimate the energy differences and the potential functions. The torsional energy levels were calculated by the harmonic rotator model. The calculation including the configuration interaction with single and double excitations on 1,2-dichloroethane improved the agreement with the experimental energy difference between the trans and gauche forms. Vibrational frequencies of 1,2-dichloroethane were also calculated by the ab initio MO method. The results are in good correlation with experimentally established values by Shimanouchi.     

Comprehensive structural characterization of polyisoprene synthesized via cationic mechanism

The microstructure of polyisoprene synthesized with ^tBuCl/TiCl₄ initiating system is investigated using 1D and 2D (HSQC and HMBC) NMR spectroscopy. It is found that trans‐1,4‐units with regular (head‐to‐tail) and inverse (tail‐to‐tail) and (head‐to‐head) enchainments are predominant structures of unsaturated part of polymer chain, while 1,2‐ and 3,4‐units are presented in minor amounts. The new methodology for the quantitative calculation of the content of different structural units in polyisoprene chain including both types of inverse trans‐1,4‐addition (tail‐to‐tail and head‐to‐head) is proposed. It is shown that head groups consist of tert‐butyl group connected to trans‐1,4‐unit of polyisoprene chain. In addition, two types of chlorine‐containing end groups are found (trans‐4,1‐Cl and 4,3‐Cl), while conjugated double bonds at the chain end are totally absent. The methodology for the calculation of number‐average functionality by tert‐butyl head and chlorine end groups, respectively, is developed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2430–2442     

Daytime Reactions of 1,8‐Cineole in the Troposphere

Relative rate coefficients for the gas‐phase reaction of chlorine atoms (Cl) and hydroxyl radicals (OH) with 1,8‐cineole were determined by Fourier‐transform infrared (FTIR) spectroscopy between 285 and 313 K at atmospheric pressure. The temperature dependence of both reactions shows simple Arrhenius behaviour which can be represented by the following expressions (in units of cm³ molecule^?1s^?1): k(1,8‐cineole+OH)=(6.28±6.53)×10^?8exp[(?2549.3±155.7)/T] and k(1,8‐cineole+Cl)=(1.35±1.07)×10^?10exp[(?151.6±237.7)/T]. Major products of the titled reactions were identified by solid‐phase microextraction (SPME) coupled to a GC‐MS. Additionally, the first step of the reaction was theoretically studied by ab initio calculations and a reaction mechanism is proposed.     

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     

Disilene R*RSi=SiRR* (R* = SitBu3) mit siliciumgebundenen Me‐ und Ph‐Gruppen R: Bildung,Nachweis, Thermolyse,Struktur: Verbindungen des Siliciums. 154 [1]. Ungesättigte Siliciumverbindungen. 61 [1]

Compounds of Silicon. 154 [1]. Unsaturated Silicon Compounds. 61 [1] Disilenes R*RSi=SiRR* (R* = SitBu₃) with Silicon‐Bound Me and Ph Groups R: Formation, Identification, Thermolysis, Structure Dehalogenations of the 1, 2‐disupersilylsilanes R*MeBrSi—SiBrMeR* (gauche : trans 1.15 : 1.00) and R*PhClSi—SiBrPhR* (gauche : trans = 2.7 : 1.0) in THF with equimolar amounts of NaR* (R* = SitBu₃ = Supersilyl) lead at —78 °C under exchange of bromine for sodium to the disilanides R*MeBrSi—SiNaMeR* and R*PhClSi—SiNaPhR* which are identified by protonation and bromination (formation of R*RXSi—SiX′RR* with R = Me, X/X′ = Br/H, Br/Br: gauche : trans = 1.15 : 1.00, and R = Ph, X/X′ = Cl/H, Cl/Br: gauche : trans = 2.7 : 10, respectively). These eliminate at about —55 °C NaHal with formation of non‐isolable trans‐R*MeSi=SiMeR* and isolable trans‐R*PhSi=SiPhR*. The intermediate existence of the disilene R*MeSi=SiMeR* could be proved by trapping it with PhC≡CPh (formation of a [2+2] cycloadduct; X‐ray structure analysis). In the absence of trapping agents, R*MeSi=SiMeR* decomposes into a mixture of substances, the main product of which is R*MeHSi—SiMeR*—SiHMeR*. The light yellow disilene R*PhSi=SiPhR* has been characterized by spectroscopy (Raman: ν(Si=Si) = 592 cm^—1; UV/VIS: λ_max = 398 nm with ∈ = 1560; ²⁹Si‐NMR: δ(>Si=) = 128 ppm) and by X‐ray structure analysis (planar central framework >Si=Si<; Si=Si distance 2.182Å). R*PhSi=SiPhR* is reduced by lithium in THF with formation of a red radical anion which decomposes at room temperature into hitherto non‐identified products. At about 70 °C, R*PhSi=SiPhR* decomposes with intramolecular insertion of the Si=Si group into a C—H bond of a Ph group and with change of configuration of the R* groups, which at first are trans then cis‐positioned (X‐ray structure analysis of the thermolysis product).     

How CO2 Interacts with Carboxylic Acids: A Rotational Study of Formic Acid–CO2

The rotational spectra of the 1:1 formic acid–carbon dioxide molecular complex and of its monodeuterated isotopologues are analysed in the 6.5–18.5 and 59.6–74.4 GHz frequency ranges using a pulsed jet Fourier transform microwave spectrometer and a free‐jet absorption millimetre wave spectrometer, respectively. Precise values of the rotational and quartic centrifugal distortion constants are obtained from the measured frequencies, and quadrupole coupling constants are determined from the deuterium hyperfine splittings. Structural parameters are estimated from the moments of inertia and their differences among isotopologues: the complex has a planar structure with the two subunits held together by a HC(O)OH???O=C ? O (2.075 Å) and a HC(OH)O???CO₂ (2.877 Å) interactions. The ab initio intermolecular binding energy, obtained at the counterpoise corrected MP2/aug‐cc‐pVTZ level of calculation, is D_e=17 kJ mol^?1.     

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     

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.     

Conformational Panorama and Chirality Controlled Structure–Energy Relationship in a Chiral Carboxylic Acid Dimer

Chirality recognition in dimers of tetrahydro‐2‐furoic acid (THFA) was studied in a conformer‐specific manner using rotational spectroscopy and theoretical approaches. THFA shows a strong preference for the trans‐ over the cis‐COOH configuration. Two drastically different scenarios are possible for the detectable (THFA)₂: a kinetically preferred dimer bound by feeble interactions between two trans THFAs or a thermodynamically favored dimer with a double hydrogen‐bonded ring structure between two cis subunits. To identify the conformers responsible for the extremely dense rotational spectra observed, it was essential not only to locate several hundred homo/heterochiral (THFA)₂ minima in ab initio calculations but also to evaluate the energetic connectivities among the minima. The study further reveals an interesting chirality dependent structure–energy ordering relationship. A method for enantiomeric excess (ee) determination of THFA is presented using a recently proposed chiral self‐tag approach.     

Solid state and theoretical evaluation of β-fluoroethyl esters indicate a fluorine-ester gauche effect

Single crystal X-ray diffraction studies and a theoretical analysis indicate a preferred conformation for O-β-fluoroethyl esters, where the CF and CO(CO) bonds are gauche rather than anti to each other. The OCCF dihedral angles for three compounds and five independent structures indicate a range of only 63.4-69.6°. Evaluation of a rotational energy profile around this bond in a model system (β-fluoroethyl acetate) predicted a similar dihedral angle and the gauche conformation to be the minimum on the rotational energy profile. High level ab initio calculations measured the gauche conformer to be 0.95 kcal mol⁻¹ lower in energy than the anti conformer and application of a solvation model further increased this differential to 1.6 kcal mol⁻¹, consistent with a previous solution state (NMR) evaluation of this system.     

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.     

Far-Infrared Spectra, Conformational Stability, Barriers to Internal Rotation, Ab Initio Calculations, r0 Structural Parameters, and Vibrational Assignment of Ethyl Methyl Ether

The far-infrared spectra (350–35 cm^–1) of gaseous ethyl methyl ether-d ₀ and ethyl methyl-d ₃-ether have been recorded at a resolution of 0.10 cm^–1. For the d ₀ species, the fundamental asymmetric torsion of the more stable trans conformer (two methyl moieties are trans to one another) has been observed at 115.40 cm^–1 with four upper state transitions falling to lower frequency, whereas, for the gauche form, it has been observed at 93.56 cm^–1 with two excited states falling to lower frequency. the corresponding series for the d ₃ species start from 106.00 and 87.10 cm^–1, respectively. From these data, the asymmetric torsional potential coefficients for the d ₀ species have been determined to be: V ₁ = 572 ± 30; V ₂ = 85 ± 8; V ₃ = 619 ± 30; V ₄ = 175 ± 18, and V ₆ = –28 ± 3 cm^–1. The trans to gauche and gauche to gauche barriers were calculated to be 958 cm^–1 (11.5 kJ/mol) and 631 cm^–1 (7.55 kJ/mol), respectively, with an energy difference of 550 ± 6 cm^–1 (6.58 ± 0.07 kJ/mol). Utilizing three conformer pairs, variable temperature studies (–105 to –150°C) of the infrared spectra of the d ₀ sample dissolved in liquid krypton gave an enthalpy difference of 547 ± 28 cm^–1 (6.54 ± 0.33 kJ/mol) with the trans conformer the more stable rotamer. It is estimated that there is only 4% of the gauche conformer present at ambient temperatures. The structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies, which have been determined experimentally, are compared to those obtained from ab initio gradient predictions from RHF/6-31G* and with full electron correlation at the MP2 level with three different basis sets. The adjusted r ₀ structural parameters have been obtained for the trans conformer from combined ab initio MP2/6-311+G** predictions and previously reported microwave rotational constants. The reported distances should be accurate to 0.003 Å and the angles to 0.5°. These results are compared to the corresponding quantities obtained for some similar molecules.     

Conformational analysis of allylamine. An ab initio molecular orbital study

The conformational characteristics of allylamine were investigated by the ab initio STO -3G basis set. The results indicate that the molecule exists in a number of stable conformations through rotations about the CC? NH and CC? CN bonds. The TE (trans-CCNLP , LP representing lone-pair electrons, and eclipsed-CCCH) is the most stable, while TC (trans-CCNLP and cis-CCCN), GE (gauche-CCNLP and eclipsed-CCCH), G′C(gauche′-CCNLP and cis-CCCN), and G′E (gauche′-CCNLP and eclipsed-CCCH) conformations are less stable, respectively, by 0.41, 0.67, 0.92, and 1.14 kcal mol^?1. These results are in general consistent with previous experimental results. Rationale for the conformational characteristics and order of stabilities are explored.