Mobility of solid tert-butyl alcohol studied by deuterium NMR

The molecular mobility of solid deuterated tert-butyl alcohol (TBA) has been studied over a broad temperature range (103–283 K) by means of solid-state 2H NMR spectroscopy, including both line shape and anisotropy of spin–lattice relaxation analyses. It has been found that, while the hydroxyl group of the TBA molecule is immobile on the 2H NMR time scale (τC > 10(–5) s), its butyl group is highly mobile. The mobility is represented by the rotation of the methyl [CD3] groups about their 3-fold axes (C3 rotational axis) and the rotation of the entire butyl [(CD3)3-C] fragment about its 3-fold axis (C3′ rotational axis). Numerical simulations of spectra line shapes reveal that the methyl groups and the butyl fragment exhibit three-site jump rotations about their symmetry axes C3 and C3′ in the temperature range of 103–133 K, with the activation energies and preexponential factors E1 = 21 ± 2 kJ/mol, k(01) = (2.6 ± 0.5) × 10(12) s(–1) and E2 = 16 ± 2 kJ/mol, k(02) = (1 ± 0.2) × 10(12) s(–1), respectively. Analysis of the anisotropy of spin–lattice relaxation has demonstrated that the reorientation mechanism of the butyl fragment changes to a free diffusion rotational mechanism above 173 K, while the rotational mechanism of the methyl groups remains the same. The values of the activation barriers for both rotations at T > 173 K have the values, which are similar to those at 103–133 K. This indicates that the interaction potential defining these motions remains unchanged. The obtained data demonstrate that the detailed analysis of both line shape and anisotropy of spin–lattice relaxation represents a powerful tool to follow the evolution of the molecular reorientation mechanisms in organic solids.     

Chemistry of the phenoxathiins and isosterically related heterocycles. XX. Assignment of the 13C-NMR spectra of phenarsazine-10-chloride and several substituted analogs. The effect of substitution on the anisotropic reorientation of the phenarsazine system

The total assignment of the ¹³C-nmr spectra of phenarsazine-10-chloride and several substituted analogs is reported. Spin-lattice (T₁) relaxation measurements have shown these systems to reorient anisotropically. In the case of the parent system and the 3-chloro substituted system, the axis of anisotropic reorientation has been shown to pass approximately through the center of the molecule. In the case of benzo[c]phenarsazine-7-chloride, the axis of anisotropic reorientation, which has been accurately defined, is shifted 23° from that in the previous cases, the shift occurring in the direction of the benzo-moiety.     

Conformational mobility of the six-membered ring in 5-oxo-1,3-cyclohexadiene and its derivatives

Molecular mechanics and MNDO calculations showed that the six-membered ring in the molecule of 5-oxo-1,3-cyclohexadiene possesses high conformational mobility. The transition from a planar equilibrium conformation to a distorted sofa conformation in which the C(sp²)-C(=O)-C(sp³)-C(sp²) torsion angle is equal to ±30° increases the energy of the molecule by less than 1 kcal mol^–1. The influence of steric (R = Me, Et, Prⁱ, Bu^t) and electronic (R = NH₂, NO₂) effects of substituents R on the equilibrium conformation and mobility of the carbocycle has been analyzed. Both types of substituents at unsaturated C atoms do not change the equlibrium conformation or flexibility of the six-membered ring. Substituents at saturated C atoms cause the transition of the carbocycle to the distorted sofa conformation and significantly restrict its mobility. The electronic structures of 5-oxo-1,3-cyclohexadiene and its amino and nitro derivatives have been analyzed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 849–854, May, 1995.     

Structure and Catalytic Mechanism of 3-Ketosteroid Dehydrogenases

3-Ketosteroid dehydrogenases (KSTDs) are FAD-dependent enzymes that introduce a double bond in the A ring of 3-ketosteroid substrates to initiate degradation of the steroid nucleus. Δ¹-KSTD desaturates the C1-C2 bond of the steroid, while Δ⁴-KSTD targets the C4-C5 bond. Crystal structures with bound products showed that Δ¹- and Δ⁴-KSTD use different amino acid residues to catalyze an otherwise mechanistically very similar reaction (Δ¹-KSTD: Tyr318, Tyr119, and Tyr487; Δ⁴-KSTD: Ser468, Tyr319, and Tyr466). However, the substrates are rotated by ∼40° about an axis perpendicular to their plane to bring the target bond (C1-C2 or C4-C5) in the right position.     

Nitration of dithieno[3,2-b:3′,2′-d]pyridine and dithieno[3,2-b:3′,4′-d]pyridine

Nitration of dithieno[3,2-b:3′,2′-d]pyridine ( 4 ) and dithieno[3,2-b:3′,4′-d]pyridine ( 5 ) has been studied. Nitration of 4 occurred in both positions of the C ring, while 5 was predominantly substituted on the 3,4-fused ring. The structures of the nitro derivatives were proven by extensive use of ¹H and ¹³C nmr spectroscopy.     

Chemistry of the phenoxathiins and isosterically related heterocycles. XIV. Total assignment of the 13C-Nmr spectrum of pyrrolo[3,2,1-kl]phenothiazine

Total assignment of the ¹³C-nmr spectrum of the novel tetracyclic phenothiazine, pyrrolo[3,2,1-kl]phenothiazine is described. Assignments were determined from model systems, ¹H-¹³C spin-coupling behavior in conjunction with selective excitation techniques and spin-lattice (T₁) relaxation measurements. The anisotropic reorientation of this ring system via a principal axis passing through the center of the molecule, which provides a qualitative basis for signal discrimination, is also discussed.     

1,7‐Dideaza‐2′‐deoxy‐6‐nitronebularine: a pyrrolo[2,3‐b]pyridine nucleoside with an intramolecular hydrogen bond stabilizing the syn conformation

The title compound [systematic name: 1‐(2‐deoxy‐β‐D‐erythro‐pentofuranosyl)‐4‐nitro‐1H‐pyrrolo[2,3‐b]pyridine], C₁₂H₁₃N₃O₅, forms an intramolecular hydrogen bond between the pyridine N atom as acceptor and the 5′‐hydroxy group of the sugar residue as donor. Consequently, the N‐glycosylic bond exhibits a syn conformation, with a χ torsion angle of 61.6 (2)°, and the pentofuranosyl residue adopts a C2′‐endo envelope conformation (²E, S‐type), with P = 162.1 (1)° and τ_m = 36.2 (1)°. The orientation of the exocyclic C4′—C5′ bond is +sc (gauche, gauche), with a torsion angle γ = 49.1 (2)°. The title nucleoside forms an ordered and stacked three‐dimensional network. The pyrrole ring of one layer faces the pyridine ring of an adjacent layer. Additionally, intermolecular O—H...O and C—H...O hydrogen bonds stabilize the crystal structure.     

Polar aspects of intramolecular interactions in 2‐amino‐5‐nitro‐4‐methylpyridines and 2‐amino‐3‐nitro‐4‐methylpyridines

The dipole moments of twelve 2‐N‐substituted amino‐5‐nitro‐4‐methylpyridines ( I‐XII ) and three 2‐N‐substituted amino‐3‐nitro‐4‐methylpyridines ( XIII‐XV ) were determined in benzene. The polar aspects of intramolecular charge‐transfer and intramolecular hydrogen bonding were discussed. The interaction dipole moments, μ_int, were calculated for 2‐N‐alkyl(or aryl)amino‐5‐nitro‐4‐methylpyridines. Increased alkylation of amino nitrogen brought about an intensified push‐pull interaction between the amino and nitro groups. The solvent effects on the dipole moments of 2‐N‐methylamino‐5‐nitro‐4‐methyl‐( I ), 2‐N,N‐dimethylamino‐5‐nitro‐4‐methyl‐ ( II ) and 2‐N‐methylamino‐3‐nitro‐4‐methylpyridines ( XIII ) were different. Specific hydrogen bond solute‐solvent interactions increased the charge‐transfer effect in I , but it did not disrupt the intramolecular hydrogen bond in XIII.     

Solid triphenylmethanol: A molecular material that undergoes multiple internal reorientational processes on different timescales

In solid triphenylmethanol, the molecules are arranged in hydrogen-bonded tetramers, and it is already well established that the hydrogen bonding in this material undergoes a dynamic switching process between different hydrogen bonding arrangements. In addition to this motion, we show here, from solid-state ²H NMR studies of the deuterated material (C₆D₅)₃COH, that each phenyl ring in this material undergoes a 180°-jump reorientation about the C₆D₅-C(OH) bond, with an activation energy of ca. 50 kJ mol⁻¹. The timescale for the phenyl ring dynamics is several orders of magnitude longer than the timescale for the hydrogen bond dynamics in this material, and is uncorrelated with the dynamics of the hydrogen bonding arrangement.     

The Structure of 3-Phenyl-4-(N-Carbamyl-1,4,2-Oxathiazolimin-3-yl)Sydnone,C11H7N5O4S·0.8 CH2Cl2

The title compound (M_r = 373) crystalizes in the othorhombic space group P bna with a = 10.410(2), b = 11.658(4), c = 23.108(3) Å, V = 2804.4 Å Z = 8. The single crystal intensity data were collected using MoK∞ radiation (λ = 0.7093Å) at room temperature. The crystal and molecular structure was solved with the final agreement index R = 0.039 for 1046 observed reflections. The bond lengths N(1)- C(7) and C(7)-C(8) of the title compound are slightly longer than those of 3-substituted sydnone derivatives. This may be attributed to the steric effect arising from the interaction of the phenyl ring and the 4-substituent with the neighboring atoms of sydnone ring. Both the title compound and 4-acetyl-3-(p-tolyl)sydnone have smaller dihedral angles between the sydnone ring and the plane of the sp² orbital of the double bond of the 4-substituent and both have shorter C(7)-C(9) bond lengths than those of other similar sydnone derivatives.     

Study of internal ring rotation and molecular reorientation in the liquid crystal 5O.7 by deuterium N.M.R. spectroscopy

Abstract

The spectral densities of motion were determined by deuterium N.M.R. relaxation measurements in the nematic, smectic A and smectic C phases of 4-n-pentyloxybenzylidene-d ₁-4′-heptylaniline and 4-n-pentyloxybenzylidene-4′-heptylaniline-2,3,5,6-d ₄. By examining two atomic sites on a 5O.7 molecule, we were able to gain information on the reorientation motion and internal rotation of the aniline ring. It was also found that director fluctuations make some contribution to the spectral density J ₁ (ω). We use the superimposed rotations model to account for the internal ring motion and the small step rotational diffusion model for the molecular reorientation. The derived rotational diffusion constants for the spinning and tumbling motions appear to give physically plausible activation energies in the mesophases of 5O.7.     

5,10:13,14-Disecosteroids: novel modified steroids containing 10- and 9-membered rings

In this paper a synthetic pathway to the modified 5,10:13,14-bisfragmentation cholestane derivatives 8-14 is described. The synthesis involves introduction of the 5α- and 14α-hydroxyl groups in the cholestane molecule and subsequent cleavage of the C(5)-C(10) bond in 5α,14α-dihydroxycholestan-3β-yl acetate (4) with the HgO/I₂ reagent and the C(13)-C(14) bond in the stereoisomeric 14α-hydroxy-5,10-secosteroids 5 and 6 with the Pb(OAc)₄/I₂ reagent. Complete and unambiguous ¹H and ¹³C NMR resonance assignments of the obtained secosteroids, as well as the solution conformations of their 10- and 9-membered rings were determined by extensive analysis of 1D and 2D NMR spectral data. The structures and the solid-state conformations of 5,10-secosteroids 5-7 were confirmed by X-ray analysis. All diseco-compounds have a novel 5,10:13,14-disecocholestane skeleton.     

Polarity and structure of 2-(1-methylbenzimidazol-2-yl)-1-phenyl- and -1,2-diphenyl-1-nitroethenes

Polarity of 2-(1-methylbenzimidazol-2-yl)-1-phenyl- and -1,2-diphenyl-1-nitroethenes was determined and their structure was studied using electronic and ¹H, ¹³C NMR spectroscopy, dipole moments measuring, XRD analysis, and quantum-chemical calculations. It was shown that the 2-(1-methylbenzimidazol-2-yl)-1-nitro-1-phenylethene has Z-configuration both in crystal and solution. The nitro group and benzimidazole substituent in its molecule are removed from the plane of the double bond. For 1,2-diphenyl-1-nitroethene E-structure is typical.     

Synthesis of spiro[1,4-benzothiazine-2,2′-pyrans] starting from methyl β-D-arabino-2-hexulopyranosonate

Dedicated to Professor Klaus Burger on the occasion of his 60th birthday Methyl β-D -arabono-2-hexulopyranosonate 1 has via the novel glycosyl donor 3 been transformed into the thiophenyl glycosides 4 and 5. Catalytic hydrogenation of the nitro compound 4 in alkaline solution led to spontaneous cyclization and deprotection to form the cyclic hydroxamic acid 7. The related lactams 8 and 9 were obtained from amine 5. The spiro[1,4-benzothiazine-2,2′-pyrans] 7–9 are the first representatives of a novel class of heterocycles structurally related to bioactive natural products. As shown by the values for J_3′,4′ and J_4′,5′ the glycosides 4, 5 and 6 adopt a ⁵C₂ conformation of the pyranoid ring whereas the 1,4-benzothiazine system in 7–9 forces a conformational change into the ²C₅ conformation.     

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.     

Nitration of dithieno[3,4-b:3′,2′-d]pyridine and dithieno[2,3-b:3′,2′-d]pyridine

The nitration of dithieno[3,4-b:3′,2′-d]pyridine ( 2 ) and dithieno[2,3-b:3′,2′-d]pyridine ( 3 ) has been studied. Nitration of 2 occurred in both positions of the c-fused thiophene ring, while 3 was predominantly substituted in the 2-position. The structures of the nitro derivatives were proven by extensive use of ¹H and ¹³C nmr spectroscopy.     

Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin

Broadband microwave spectra were recorded over the 2-18 GHz frequency range for a series of four model aromatic components of lignin; namely, guaiacol (ortho-methoxy phenol, G ), syringol (2,6-dimethoxy phenol, S ), 4-methyl guaiacol ( MG ), and 4-vinyl guaiacol ( VG ), under jet-cooled conditions in the gas phase. Using a combination of ¹³C isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the r_C(1)-C(6) bond between the two substituted C-atoms lengthens, leading to clear bond alternation that reflects an increase in the phenyl ring resonance structure with double bonds at r_C(1)-C(2), r_C(3)-C(4) and r_C(5)-C(6). Syringol, with its symmetric methoxy substituents, possesses a microwave spectrum with tunneling doublets in the a-type transitions associated with H-atom tunneling. These splittings were fit to determine a barrier to hindered rotation of the OH group of 1975 cm⁻¹, a value nearly 50 % greater than that in phenol, due to the presence of the intramolecular OH⋅⋅⋅OCH₃ H-bonds at the two equivalent planar geometries. In 4-methyl guaiacol, methyl rotor splittings are observed and used to confirm and refine an earlier measurement of the three-fold barrier V₃=67 cm⁻¹. Finally, 4-vinyl guaiacol shows transitions due to two conformers differing in the relative orientations of the vinyl and OH groups.     

堆积的腺嘌呤体系新失活通道的动力学模拟

采用半经典动力学方法模拟了激光诱导下π堆积的腺嘌呤体系最低激发态的失活过程. 模拟激光脉冲仅作用于一个腺嘌呤分子. 发现随着激发态腺嘌呤分子(A)与基态腺嘌呤分子(A′)之间距离的缩短, 它们的相互作用显著增强. 分子间的相互作用导致了一条新的失活通道, 即C₂原子与C₂′原子靠拢成键, 形成“成键的激基复合体”的中间体. 中间体的寿命约为390 fs. C₂原子的畸变和H₂′原子的环面外振动导致中间体失活. 失活后C₂－C₂′断裂, 释放的键能转化为分子动能, 腺嘌呤分子恢复基态的平面结构.     

Heteronuclear NMR spectroscopic investigations of hydrogen bonding in 2‐(benzo[d]thiazole‐2′‐yl)‐N‐alkylanilines

The 2‐(benzo[d]thiazole‐2′‐yl)‐N‐alkylanilines have previously revealed the presence of a strong intramolecular hydrogen bond. This in turn gives rise to a more complicated multiplet for the protons attached to the carbon adjacent to the amino group. This intramolecular hydrogen bond was investigated by a deuterium exchange experiment using heteronuclear NMR spectroscopy (¹H, ¹³C, ¹⁵ N and ²H). We observed changes in the multiplet structure and chemical shifts providing further evidence that the deuterium replaces the hydrogen in the intramolecular hydrogen bond. A time course study of the D₂O exchange confirmed the presence of a strong hydrogen bond. The comparison of the structures obtained by X‐ray crystallography showed a very small difference in planarity between the two‐substituted and four‐substituted amino compounds. In both the cases, the phenyl ring is not absolutely coplanar with the thiazole unit. The existence of this intramolecular hydrogen bond in 2‐(benzo[d]thiazole‐2′‐yl)‐N‐alkylanilines was further confirmed by single crystal X‐ray crystallography. Copyright © 2015 John Wiley & Sons, Ltd.     

Rotational Dynamics of Adamantanecarboxylic Acid in Complex with β-cyclodextrin

The reorientation of 1-adamantanecarboxylic acid (AdCA) within the β-cyclodextrin (β-CD) cavity is investigated by means of multiple-field ¹³C NMR relaxation. The dissociation constant describing the complexation equilibrium is determined using translational diffusion measurements for the guest during a titration by the host in D₂O/DMSO solvent mixture. The changes in apparent diffusion properties of AdCA during the titration are at 25 °C well described assuming the formation of a 1:1 complex, whereas at 0 °C the data indicate the presence of a 2:1 (guest:host) complex. The ¹³C NMR relaxation parameters for the AdCA molecule bound inside the β-CD cavity are extracted. Despite the high association constant, indicating a strong interaction between the two molecules, the guest molecule is quite mobile. The reorientation of the bound AdCA at 25 °C can be described by either the Lipari–Szabo or the axially symmetric rotational diffusion model. The motion is extremely anisotropic: the adamantyl group rotates fast around the β-CD symmetry axis, inside its cylindrical cavity. At lower temperature, the relaxation properties are no longer possible to explain using these models. Instead, the data are analyzed using extended, three-step spectral density of Clore et al. [J. Am. Chem. Soc. 112, 4989 (1990)].