Showing papers on "Conformational isomerism published in 1968"
TL;DR: In this article, the authors assign conformations and configurations to oximes of several aldehydes and ketones from analysis of their 60-Mc NMR spectra using spin-spin coupling data from the cis isomers (hydroxy cis to hydrogen) of the aldoximes in terms of rotamers I and II.
164 citations
TL;DR: In this article, aldehyde and ketone N,N-dimethyldrazones were assigned configurational and conformations from analyses of their 60-Mc NMR spectra.
102 citations
TL;DR: In this paper, the conformational equilibrium of six 2-alkoxytetrahydropyrans has been studied by means of vicinal coupling constants and dipole moments.
91 citations
TL;DR: In this article, the trans-trans and trans-gauche conformers of diethyl ether have been deduced, together with a valence force field adjusted to give a weighted least-squares fit to the observed frequencies.
80 citations
TL;DR: In this article, the anisotropic effects of the CN double bond, the association between solute and solvent, several aspects of the chemical shifts, and other problems related to configurational isomerism about the double bond are discussed.
50 citations
TL;DR: In this article, the authors assigned conformations and configurations to several hydrazones of aldehydes and ketones from analysis of their 60-Mc NMR spectra.
48 citations
TL;DR: In this paper, the authors investigated the doublet character of IR OH stretching absorptions of many saturated alcohols and showed that molecules with only one minimum energy rotamer have symmetrical OH spectra.
36 citations
TL;DR: In this paper, the vibrational spectra of cyclopropyl carbonyl chloride have been recorded and analyzed with the aid of the infrared spectrum of the corresponding bromide, and the relative stabilities of the two isomers were determined by measuring the intensity-temperature dependence of their carbon-chlorine stretching vibrations in both the vapor and liquid states.
Abstract: The vibrational spectra of cyclopropylcarbonyl chloride have been recorded and analyzed with the aid of the infrared spectrum of the corresponding bromide. The compound exists as a mixture of two discrete rotational isomers in the vapor and liquid states, but not in the solid state. The relative stabilities of the two isomers were determined by measuring the intensity–temperature dependence of their carbon–chlorine stretching vibrations in both the vapor and liquid states. The S‐cis form is shown to be the more stable conformer in both the vapor and liquid states, but the S‐trans form is the more stable in the solid state. Highly oriented polycrystalline masses are obtained on freezing, further indicating that intermolecular forces are unusual in this compound.
34 citations
TL;DR: In this paper, the microwave spectra for six isotopic forms of propargyl alcohol and for the normal form of Propargyl amine have been assigned and the moments of inertia show that the rotamers concerned are those in which the OH and both NH bonds occupy gauche positions, that is, subtend dihedral angles in the region of 60° with respect to the acetylenic group.
Abstract: PROPARGYL alcohol and propargyl amine are of particular structural interest, because they belong to the rare class of substituted derivatives of methanol and methylamine in which the only possibilities of rotational isomerism are those associated with the COH or CNH2 groups. We have assigned microwave spectra for six isotopic forms of propargyl alcohol and for the normal form of propargyl amine. The moments of inertia show that the rotamers concerned are those in which the OH and both NH bonds occupy gauche positions, that is, subtend dihedral angles in the region of 60° with respect to the acetylenic group. In the alcohol, the properties of the potential function for internal rotation are reflected in the occurrence of spectra of molecules in a very low-lying excited state and in the occurrence of series of spectral lines with first-order Stark effects.
25 citations
TL;DR: Piperidine, morpholine, and cis-2,6-dimethylpiperidine in carbon tetrachlorid egives ΔH 0·6 ± 0·2 kcal/mole preference for NH-equatorial as discussed by the authors.
Abstract: Piperidine, morpholine, and ten substituted derivatives show, in the gas-phase, two resolved bands in the first NH-overtone region which are assigned to the NH-equatorial and NH-axial conformers by qualitative and quantitative comparisons of their contours with those predicted by theory. A temperature-dependence study of the intensity ratios indicates that the NH-equatorial conformer predominates for both piperidine and morpholine, with ΔH 0·5 ± 0·1 kcal./mole. Examination of piperidine, morpholine, and cis-2,6-dimethylpiperidine in carbon tetrachlorid egives ΔH 0·6 ± 0·2 kcal./mole preference for NH-equatorial. ΔG Values probably differ little from ΔH values.
25 citations
TL;DR: In this article, a wide range of methods, by which nuclear magnetic resonance (n.m.r) spectroscopy can be used to solve conformational problems are described.
Abstract: Publisher Summary This chapter describes the wide range of methods, by which nuclear magnetic resonance (n.m.r) spectroscopy can be used to solve conformational problems. NMR spectroscopy has proved invaluable in the study of fast exchange-rate processes in both cyclic and acyclic molecules. Four techniques have been used to measure the exchange rates by NMR spectroscopy— namely, (1) the steady-state or high-resolution technique, (2) the spin-echo, (3) double-resonance, and (4) the fast-passage method, of which the first two are by far the most commonly used. The appearance of the high-resolution spectrum of an inverting molecule is governed by the rate of exchange and the internal chemical shift between nuclei in different sites. In the low-temperature spectrum, the high-field peak is due to the conformer with axial hydrogen and the low-field peak to the conformer with equatorial hydrogen. Later, the chapter discusses the validity of the karplus equation.
TL;DR: In this paper, the dipole moments, NMR, IR and Raman spectra of trans-1,2-dihalogenoindans were determined and linear relations were found between the squares of the Dipole moments and the vicinal proton spin coupling constants.
TL;DR: In this article, the CCN out-of-plane bending and angle bending vibrations of normal and branched-chain aliphatic mononitriles have been studied in the range 667−250 cm−1.
TL;DR: In this article, the relative stabilities of the various possible conformations of the ring system, and their rates of interconversion when more than one is observable, depend on repulsion between non-bonded atoms, ringstrain, and (when C-5 is trigonal) conjugation.
Abstract: Oxidation of cyclotriveratrylene (CTV) with chromic acid gives cyclotriveratrylen-5-one (3) accompanied by some cyclotriveratrylen-5,10,15-trione (9), which is formed in higher yield by similar oxidation of the monoketone. The ketone (3) is converted into the corresponding secondary alcohol (4) by hydride reduction and into the tertiary carbinol (6) by methyl-lithium. The latter alcohol easily loses water to form the 5-methylene derivative (7); the isopropyl analogue cannot be isolated, since it spontaneously dehydrates to the 5-isopropylidene derivative (8).The relative stabilities of the various possible conformations of the ring system, and their rates of interconversion when more than one is observable, depend on repulsion between non-bonded atoms, ring-strain, and (when C-5 is trigonal) conjugation. The conformations have been established by 1H n.m.r., i.r., and u.v. spectra. CTV itself exists only in the ‘crown’ conformation (A); the 1H n.m.r. spectrum remains unchanged up to 200°. The ‘crown’ conformation with pseudo-equatorial OH is also the most stable one for the secondary alcohol (4): both the labile secondary alcohol (5), isolated by reduction of the ketone (3), and the methyl carbinol (6) exist in the ‘flexible’ form (C).In the compounds with C-5 trigonal, conjugation, which is not possible in the ‘crown’ conformation, also plays a part, so that the ketone (3) adopts only the ‘flexible’ conformation (B) with C2 symmetry and an enlarged C–CO–C angle. In the less highly conjugated methylene derivative the ‘crown’ conformation (A) also occurs, although the ‘flexible’ one (B) still predominates (K= 3·2). The two conformers equilibrate slowly at room temperature. The isopropylidene derivative (8), however, exists predominantly in the ‘crown’ conformation.In the presence of dilute acid both the ‘crown’ and ‘flexible’ alcohols react with ethanol to give the ethyl ethers; the rate for the latter is at least 6 × 104 times faster. That the ‘crown’⇌‘flexible’ conformational equilibrium for the intermediate carbonium ion is faster than its chemical reaction and lies, like that of the ketone (3), on the side of the flexible form is indicated by the fact that the flexible ether (12) is the primary product of etherification of both the alcohols (4 and 5).
TL;DR: In this article, two isomeric cyclic sulfites were isolated in a 3 to 2 ratio from the ester exchange reaction between 2-methylpentane-2.4-diol and dimethyl sulfite.
Abstract: From the ester exchange reaction between 2-methylpentane-2.4-diol and dimethyl sulfite, two isomeric cyclic sulfites were isolated in a 3 to 2 ratio. On the basis of IR and NMR data it was concluded that the isomers exist in the rigid chair conformation and the isomerism springs from the pyramidal SO orientation. An axial SO is assigned to the liquid isomer (predominant) and an equatorial SO to the solid compound (minor). The 25 ev mass spectra of the two isomers at room temperature is herein described.
TL;DR: The infrared spectrum of methylaminobis(trifluoromethyl)phosphine, MeNH·P(CF3)2, has a well resolved doublet in the N-H stretching region and this is shown to result from the presence of two rotational isomers in thermal equilibrium as mentioned in this paper.
Abstract: The infrared spectrum of methylaminobis(trifluoromethyl)phosphine, MeNH·P(CF3)2, has a well resolved doublet in the N–H stretching region and this is shown to result from the presence of two rotational isomers in thermal equilibrium. The methyl proton n.m.r. spectrum over a range of temperature is consistent with this. The infrared spectrum of the corresponding ethyl compound shows the effect much less, and with the t-butyl and phenyl derivatives only one conformer is sterically stable. Assignments are suggested for the detailed infrared spectra.
TL;DR: The preferred rotational conformation of the acetyl group of (−)-10-nor-8-oxoneomenthol (I), (−)- 10-nor 8 -oxone-menthols (I, II), (−-10-or-8 -oxon-mentshol (II, III, and IV) has been examined by a combination of conformational analysis by a priori calculation and variable-temperature circular dichroism and infrared spectrum measurements as mentioned in this paper.
Abstract: The preferred rotational conformation of the acetyl group of (+)-10-nor-8-oxoneomenthol (I), (−)-10-nor-8-oxomenthol (II), (−)-10-nor-8-oxocarvomenthol (III), and (+)-10-nor-8-oxoneocarvomenthol (IV) has been examined by a combination of conformational analysis by a priori calculation and variable-temperature circular dichroism and infrared spectrum measurements. Compound I exhibited an inversion in the sign of the Cotton effect upon a change in the solvent. This phenomenon was interpreted as indicating the conformational change Ia\ightleftarrowsIb. The cause of the change was ascribed to the formation and subsequent rupture of the intramolecular hydrogen bond. Compound II exhibited a solvent-depending circular dichroism curve similar to the curve of I; this was explained as indicating the existence of a conformational equilibrium between the free conformer, IIa, and the internally-hydrogen-bonded conformers, IIb, IIc, and IId. Furthermore, compounds III and IV were found to take preferentially the IIIa–I...
TL;DR: The electron spin resonance (e.s.r.) spectrum of 4,4′-dimethoxybiphenyl in aluminium chloride-nitromethane is remarkably temperature dependent as mentioned in this paper.
Abstract: The electron spin resonance (e.s.r.) spectrum of 4,4′-dimethoxybiphenyl in aluminium chloride–nitromethane is remarkably temperature dependent. The changes are explained by postulating the rapid interconversion of conformational isomers, leading to line-width alternation.
TL;DR: In this paper, the sound absorption data have been analyzed to yield barrier heights between rotamers for tri-n-alkylamines and substituted triethylamines in the liquid phase.
TL;DR: In this article, the e.s.r. spectra obtained by reduction of m-dinitrobenzene and 3,5dinitronzoic acid with aqueous alkaline sodium dithionite have been re-investigated.
Abstract: The e.s.r. spectra obtained by reduction of m-dinitrobenzene and 3,5-dinitrobenzoic acid with aqueous alkaline sodium dithionite have been re-investigated. In addition to the dominant interaction with one nitrogen nucleus which has been previously reported, a further splitting of 0·24 gauss due to the other nitrogen nucleus has been resolved in both spectra. The spectra exhibit line-width effects characteristic of the slow interconversion of two asymmetric conformers of the nitro-radical anion in which the environments of the nitro-groups are exchanged. A mean life-time of 1·1 × 10–6 sec. has been estimated for the conformers of the 3,5-dinitrobenzoate anion radical. The asymmetry of these conformers is attributed to strong interaction between the solvent and one nitro-group.
TL;DR: In this article, the relative importance of the following standard interactions is elucidated: gauche butane (known), propane, gauche propylamine, and ethylamines.
Abstract: The relative importance of the following ‘standard interactions’ is elucidated: gauche butane (known), propane, gauche propylamine, and ethylamine. Axial–equatorial equilibria are thence predicted for aminocyclohexane, its N-methyl derivatives, and other compounds. Results are compared with experimental data (some new) and the conclusion that N-substitution in aminocyclohexane increases the proportion of axial conformer is discussed.
TL;DR: In this article, the effects of splitting and temperature sensitivity are explained by conformational isomerism, arising from the rotation of the isopropenyl group, and the reasons for both splitting and non-splitting are discussed in terms of the i.r.-spectra of a number of substances for different conditions of substitution.
TL;DR: In this article, an attempt has been made to assign most of the fundamental vibrational frequencies to normal modes of vibration and the extent of hydrogen bonding (inter-and intra-molecular association) has also been investigated in the monocarboxylic acid, finding evidence for hydrogen bonding involving C-H bonds and for the presence of conformational isomerism in dilute solutions of the mono-acid.
Abstract: Infrared studies of cyclobutanemonocarboxylic acid, cyclobutane-1,1-dicarboxylic acid, and cyclobutane-cis-1,2-dicarboxylic acid are reported. An attempt has been made to assign most of the fundamental vibrational frequencies to normal modes of vibration and the extent of ‘hydrogen bonding’(inter- and intra-molecular association) has also been investigated in the monocarboxylic acid.Evidence is found for hydrogen bonding involving C–H bonds and for the presence of conformational isomerism in dilute solutions of the mono-acid.
TL;DR: In this paper, the e.s.r. spectra of the nitroxide radicals produced by oxidation of 4-methyl, 4-n-propyl-, and 4-t-butyl-piperidine, piperazine, and 4N-methylpiperazine have been measured in aqueous solution.
Abstract: The e.s.r. spectra of the nitroxide radicals produced by oxidation of 4-methyl-, 4-n-propyl-, and 4-t-butyl-piperidine, piperazine, and 4-N-methylpiperazine have been measured in aqueous solution. The spectra have been analysed in terms of a fast exchange between two unequally populated chair forms and estimates have been made of the free-energy differences between conformational isomers.