1H, 13C and 19F NMR studies on fluorinated ethers

The enflurane and ethoxyflurane ¹H, ¹³C and ¹⁹F NMR spectra are examined—including sign determination of FF and FH couplings—and considered in the light of previously reported results for methoxyflurane. Conformational differences between methoxyflurane and the former two molecules are indicated by through space FH coupling constants and by the nonequivalence of geminal fluorine nuclei. Populations of conformers about the CC bond are estimated.     

Conformational analysis of 3-(substituted benzo[b]furan-2-yl)-γ-aminobutyric acid,new GABAB ligands,by 1H NMR in D2O

The conformations of 3-(substituted benzo[b]furan-2-yl)-GABA (1–4) in solution (D₂O) are estimated from high-resolution (400 MHz) ¹H NMR coupling data. Conformations and populations of conformers are calculated by means of a modified Karplus-like relationship for the vicinal coupling constants and are compared with X-ray crystallographic investigations. ¹H NMR spectral analysis evidences how 1–4 keep also in solution the preferred g⁻ conformation around the C3C4 bond as found in the solid state while a partial rotation is set up around the C2C3 bond.     

Novel unexpected examples of 5JHH long range couplings in carbohydrates.

⁵J_HH Couplings proceeding through a HCCCCH - fragment rather than a HCOCCH - path in certain L-idofuranose derivatives are described. The probable mechanism is discussed.     

Conformational analysis—XXI : H NMR Conformational Study of Alkyl-Substituted 2-Oxo-1,3,2-Dioxathianes

2-Oxo-1,3,2-dioxathiane and all methyl- and several alkyl-substituted 2-oxo-1,3,2-dioxathianes were prepared for a ¹H NMR conformational study. The conformational energy of the axial SO group in CCl₄, - ΔG^θ_SO = 14.8±0.3kJ mol^?1, was determined by chemical equilibration of the epimeric cis-4,6-dimethyl derivatives and it was found to decrease with the increasing solvent polarity. The conformational equilibria of alkyl-substituted derivatives were solved and the proportions of the conformers estimated using ¹H NMR chemical shifts, vicinal coupling constants and in three cases also dipole moments. The configurational interactions in the C₄C₅C₆ moiety are close to the corresponding values of 1,3-dioxanes.     

An ab initio study of the geometry and energy of the four planar conformers of glyoxylic acid and the glyoxylate ion

The structures and energies of the four planar conformers of glyoxylic acid and the glyoxylate ion have been studied ab initio using the unscaled 4—31G basis set with full geometry optimization. Changes in the CO, OH and CO bond lengths in the conversion of the cc conformer into the ct and tt conformers, and into the tc conformer, are consistent with the formation of four-membered and five-membered hydrogen-bonded ring structures, respectively. Changes in the distances between the nearest non-bonded atoms around each C atom reveal that the internal geometry of the CHO and COOH groups is significantly affected by cis—trans isomerization with respect to the OCCO backbone, and that the geometry of the CHO group is affected by proton dissociation from the COOH group. Furthermore, the movement of the component atoms in each functional group, characterized as clockwise or anticlockwise about the C atom, results in some cases in a rotation of the functional group as a whole. Whereas experiment shows the tc conformer to be more stable than the tt by 1.2 ± 0.5 kcal mol^?1, the calculations find the tt conformer to be the most stable, separated in energy from the ct, tc and cc conformers by 0.4, 1.4 and 10.7 kcal mol^?1, respectively. Augmentation of the 4—31G basis set in several forms, and use of (9,5/4) and (9,5/4,1) basis sets, only served to decrease slightly the tt/tc energy difference, not change the sign. The calculated proton affinity of the glyoxylate ion with respect to the tt conformer is 342.7 kcal mol^?1, compared to 357.7 kcal mol^?1 for the formate ion. The expectation energy differences Δ V_nn, Δ V_ee and Δ V_en for the cis—trans isomerization of the ct and cc conformers are opposite in sign to those for the glyoxal reaction, and in magnitude they all far exceed the ΔE_T values, which shows that hydroxyl group substitution has a much greater influence than a comparison of only the ΔE_T values would suggest.     

Conformation,hydrogen bonding and large amplitude motion investigation on N-methylethylediamine by microwave spectroscopy

The microwave spectrum of N-methylethylenediamine and several deuterated species has been investigated in the frequency range 26.5–40 GHz. The rotational spectra of two different conformers with a NH ⋯ N internal hydrogen bond have been assigned. Both conformers have the methyl group trans to the CC bond. The N atom connected to the methyl group acts as proton donor for a conformer (T1), and as acceptor for the second one (T2g). The former is more stable in energy by 0.65(15) kcal mol⁻¹. Rotational lines of several vibrational satellites have been assigned in order to investigate their large amplitude motions and interactions.     

Etude comparée des spectres de vibration du tiapride. Base et chlorhydrate

Raman and infrared spectra of tiapride were recorded from 4000 to 200 cm⁻¹. Assignments of the fundamental vibrations involved in intra or intermolecular H-bonds as NH⋯O, N⁺H⋯O and N⁺H⋯Cl⁻ are discussed. Analysis of some characteristic vibrations leads to the conclusion that the conformations are quite independent of the solvent effects. The change from tiapride base to chlorhydrate that occurs during the crystallization involves a new order of the aliphatic chain which changes the planarity of the chelated form.     

Derivative analysis of Raman spectra of liquid water in the OH (D) stretching region

Second derivative analysis of Raman spectra of H₂O, D₂O and HOD in liquid phase at room temperature for parallel and perpendicular polarized modes in the OH and OD stretching regions is reported. Five components obtained at approximately 3215, 3375, 3455, 3535 and 3640 cm⁻¹ for the second derivative plots of Raman spectra of liquid water are explained as due to the presence of three types of associated water species with (i) both OH bonds involved in moderately strong hydrogen bonds (SS), (ii) both OH bonds involved in weak hydrogen bonds (WW), and (iii) one OH bond involved in strong and one in weak hydrogen bonds (SW) respectively. The derivative plots obtained for Raman spectra of D₂O and HOD also contain features expected to be present on the basis of this model.     

A comparative study of H+ and Li+ interactions with oxygen bases

Ab initio SCF calculations have been performed to investigate the structural and electronic features of the interactions of H⁺ and Li⁺ with the oxygen bases H₂CO and OH^?. The data indicate that the OH interactions are primarily covalent while the LiO interactions are primarily electrostatic, although the LiO interaction in LiOH has considerable covalent character.     

Thermodynamics of vinyl ethers—XVII : Thermodynamic stability of 1,2-dialkoxyethylenes

The relative thermodynamic stability of the monoalkoxy- and 1,2-dialkoxyethylene systems [OCCH(C) and OCCO, respectively] has been studied by chemical equilibration of suitable isomeric compounds. Although a single alkoxy substituent stabilizes the CC bond by about 25 kJ mol^?1, the 1,2-dialkoxyethylene system is no more stable than the monoalkoxyethylene (“ordinary” vinyl ether) system. On the contrary, the MeOCCOMe system was found to be about 4 kJ mol^?1 (on an enthalpy basis) less stable than the system MeOCCH.     

Conformational studies of 5-acetyl-10-cyano-10,11-dihydro-5H-dibenz[b,f]azepine

5 - Acetyl - 10 - cyano - 10,11 - dihydro - 5H - dibenz[b,f]azepine, 3, has been synthesized and its conformations in solution have been studied by variable temperature NMR. At ?55° in CDCl₃ solution, 3 shows the signals due to four different conformational isomers. At 55°, signals due to only two conformers can be seen. The ABX pattern of each of the four conformers was analyzed using double resonance experiments and the LAOCN3 computer program. The relative abundances of the isomers in the mixtures were estimated by computer additions of different proportions of the spectra calculated for the separate isomers. These spectral observations are discussed in detail and interpreted in terms of slow inversion of the seven-membered ring by torsion of the 4a55a bonds and restricted twisting of the C10C11 ethylene bridge.     

The influence of small monovalent cations on neighbouring N…HO hydrogen bonds

The influence of small monovalent metal ions on hydrogen bonds of the OH…N type is studied with the example Li⁺/H₂ONH₃. The net stabilization effect is discussed and compared with the OH…O system. The applicability of the semi-empirical CNDO/2 method and ab initio calculations with extended and minimal basis sets is investigated.     

σ,π-conjugation and 199Hg shielding constants in benzyl derivatives of mercury

The ¹⁹⁹Hg chemical shifts in a number of benzylmercuric chlorides containing methyl substituents in various positions of the benzene nucleus are studied by the heteronuclear ¹H-{¹⁹⁹Hg} double resonance technique. Meta- and para-methyl substituents are shown to have no pronounced action on the shielding whereas ortho-methyl groups each shift the signal by 30 ppm downfield. The observed effect is due to an increase in the population of conformers with the CHg bond lying out of the aromatic cycle plane. The departure from planarity favours conjugation between the electrons of the CHg bond and the π-electronic system of the ring. The J(¹H¹⁹⁹Hg) and J(¹³C¹⁹⁹Hg) coupling constant values obtained in this work confirm the latter conclusion.     

Application of the Seth-Paul-Van Duyse equation—III : Transmission of polar effects by the furan ring

The improved Seth-Paul-Van Duyse equation (SPVDE) has been used to determine transmissive factors of polar effects for the furan ring. The SPVDE was applied to the CO stretching frequencies of OO-trans and OO-cis conformers of a series of substituted 5-phenyl-2-furancarbonyl compounds measured in carbon tetrachloride. The transmissive factors for furan ring in both conformation OO-trans and OO-cis have been calculated with a higher accuracy than found by using the methods described earlier. The applicability of the improved SPVDE to the 96 CO stretching frequencies of various 5-phenyl-2-furancarbonyl compounds has been proved.     

Insertion of activated acetylenes into the metal-hydride bond of [(η5-C5H5)2M(CO)H] (M = Nb,Ta)

Reactions of [Cp₂M(CO)H] (M = Nb, Ta; Cp = η⁵-C₅H₅) with various acetylenes RCCR having electron-withdrawing groups were investigated. They give the σ-al- kenyl complexes [Cp₂M(CO)(CRCHR)] via insertion of the alkyne into the MH bond. On the basis of ¹H and ¹⁹F NMR data the reactions were shown to be: (i) regioselective, monosubstituted alkynes giving only the α-R metallated complex; (ii) stereoselective, exclusive formation of the Z-isomer being observed with hexafluorobut-2-yne. The Z isomer has been shown to exist as two conformers, the steric requirements of the ligands creating a barrier to rotation of the alkenyl group around the MC σ bond.     

Lineare oligophosphaalkane: XVIII. Addition Ph-funktioneller methylenbisphosphane an die MoMo-Dreifachbindun in (η5-C5H5)2Mo2(CO)4

By means of the addition of the PH-functional methylenebisphosphanes R¹R²-PCH₂PR³H (PCP) to the MoMo triple bond in (η⁵-C₅H₅)₂Mo₂(CO)₄(MoMo) the complexes (η⁵-C₅H₅)₂Mo₂(CO)₄(PCP) containing a five-membered ring system Mo₂P₂C are obtained. Starting with unsymmetrically substituted methylenebisphosphanes R′₂PCH₂PRH only one isomer is formed, while the disecondary derivatives RHPCH₂PHR (as the diastereomeric mixture) gave two isomers of (η⁵-C₅H₅)₂Mo₂(CO)₄(PCP) (A₂ and AB) as indicated by the ³¹P{¹H} and ¹³C{¹H} NMR spectra.X-ray structural analysis of the derivative of the racemate of t-BuHPCH₂PH(t-Bu) space group C2/c, monoclinic, a 18.034(2), b 14.909(1), c 11.106(1) Å, α 90, β 99.788(8), γ 90°) reveals a puckered Mo₂P₂C five-membered ring system (dihedral angle PMoMo′P′ 54.4(2)°) with square-pyramidal coordination geometry at the Mo atoms. Two of the CO ligands (C(6)O(1) and C(6′)O(1′)) are almost coplanar with the molybdenum atoms, while the terminal CO groups (C(7)O(2) and C(7′)O(2′)) are about orthogonal (dihedral angle C(7)MoMo′C(7′) 88.4(3), MoMo′ 3.2109(4), MoP 2.4567(8), PC(8) 1.834(3), PH(P) 1.37(3) Å).     

Conformer assignment of OH stretches in ethanol and isopropanol

Rigid harmonic asymetric top contour simulation is applied to assign the OH stretch transitions observed in the vapour phase infrared spectra of ethanol and isopropanol to gauche and trans conformers. For hetanol, the high frequency ν(OH) absorption must be assigned to the trans, for isopropanol the high frequency absorption is found to be due to the gauche conformer.     

The sign of the four-bond FH coupling in methoxyflurane

The methoxyflurane NMR spectrum was reexamined looking for a through-space transmitted four-bond FH coupling. This coupling was measured and its sign determined relative to the corresponding vicinal FH coupling. This sign determination required a triple irradiation technique, which is also described. A Karplus-like equation for the vicinal FH couplings yields the relative populations of both preferential conformers, and determines the positive sign of both three- and four-bond FH couplings.     

Conformational interactions between a phenyl ring and a side-chain halide substituent: A 1H NMR and molecular mechanics study of some 2-aryl-1-halopropanes

¹H NMR data are reported for a series of 2-aryl-1-halopropanes. Vicinal coupling constants in the CH₂CH—fragment show that the rotamer populations about the CC bond are sensitive to para substituents. The ratio of anti:gauche aryl/halide conformers is greatest when the para substituent is the electron-donating ethyl group and least when it is the strongly electron-withdrawing nitro group. This points to a non-steric conformational interaction involving the ring and the sidechain heteroatom. Comparison of the empirical results with conformational preferences predicted from molecular mechanics calculations using the COSMIC force field suggests that the interaction serves to enhance the population of the anti arrangement.     

The 1H NMR spectra of methylmethoxyaluminium chloride and methylmethoxyaluminium iodide

It has been shown by means of ¹H NMR, IR and cryometrical measurements that, with oxygen bridges, Me(OMe)AlCl and Me(OMe)AII are trimers in solutions.Two isomers, cis and trans, are present, which are responsible for the multiple ¹H NMR spectra. The proton signals of MeAl and OMe groups are assigned in each isomer.