Vibrational spectra and molecular conformation of diethyl fumarate and especially the maleate

Infrared and Raman spectra of diethyl maleate and diethyl fumarate were recorded in the liquid and solid states. From the spectral changes on phase transition the known conformational equilibrium s-trans/s-cis ? s-trans/s-trans in the case of dialkyl fumarates is confirmed, and the view that dialkyl maleates exist only in a single conformation is corrected. The observation of several bands in the vCC region of IR and Raman spectra of diethyl maleate, and significant intensity shifts in this region with phase transition prove the existence of at least two conformers in liquid dialkyl maleates.     

Electronic spectra of α,β-unsaturated carbonyl compounds—II : An evaluation of increments characteristic of structural features of β-oxy-α,β-unsaturated ketones

By comparing UV spectra of β-alkoxy-α,β-unsaturated ketones of established steric structure, spectral constants characteristic of the cis/trans configuration change and s-cis/s-trans and O-s-cis/O-s-trans conformation changes have been evaluated. These are: Δλ^cis_trans = 0, Δλ^s-cis_s-trans = 8 nm and Δλ^O-s-trans_O-s-cis = 6 nm. A comparison of cis-s-cis enol ethers with the parent enols yielded the increment for the intramolecular (“chelating”) H-bond, Δγ_chel = 24 nm. The methanol-induced bathochromic shift has been found to depend strongly on s-cis/s-trans isomerism. The substituent increments have been shown to be dependent on the degree of substitution in the reference molecule. The results obtained have been summarized in a set of spectral increments complementing the basic system of Woodward and the Fiesers.     

Infrared studies of the less stable cis form of N-methylformmaide and N-methylacetamide in low-temperature nitrogen matrices and vibrational analyses of the trans and cis forms of these molecules

Recently studies of less stable rotational isomers became possible using a combination of low-temperature matrix-isolation infrared spectroscopy and either the high-temperature nozzle technique or irradiation with ultraviolet or near-infrared light. In this paper the infrared spectra of the less stable cis form of N-methylformamide and N-methylacetamide and their variously deuterated species were obtained in low-temperature nitrogen matrices using the high-temperature nozzle technique. The assignments of the bands due to the cis form as well as the trans form were made on the basis of normal-coordinate calculations.     

Infrared spectra and structure of substituted unsaturated carbonyl compounds: XVII. Enamino ketone and aldehyde quaternary ammonium salts

The β-trialkylammonium chloride derivatives of several α,β-unsaturated carbonyl compounds have been shown by infrared spectra to exist in the s-cis and s-trans rotational forms. The equilibrium between these two forms shifts towards the s-cis with the increasing steric requirements of the alkyl substituent linked to carbonyl. The CO and CC stretching frequencies are shifted to higher values as compared with other types of α,β-unsaturated ketones.     

Temperature and Concentration Dependent Circular Dichroism of Mono- and Di-cis Isomers of (3S,3′S)-Astaxanthin Diacetate

The circular dichroism (CD.) spectra of all-trans-(3S, 3′S) astaxanthin diacetate and its 9-cis, 13-cis, 9,9′-di-cis, 9,13′-di-cis, and 9,13-di-cis isomers conform to the rules previously formulated for optically active carotenoids with a 4-oxo-β-end ring containing an asymmetric C-atom [1]. Thus the CD. bands of the all-trans and the di-cis isomers show the same signs whereas those of the mono-cis isomers have opposite signs. The CD. spectra of all the astaxanthin diacetate isomers invert sign upon cooling to ?180°. The CD. spectra of the 9-mono-cis and 9,9′-di-cis isomers and to a lesser extent also those of the 9, 13′-di-cis and 9, 13-di-cis isomers are concentration dependent at ?180°, with the longest wavelength band giving at the higher concentration a bisignate CD. curve under the main absorption characteristic of aggregation. This phenomenon has been observed only in isomers with a 9-cis linkage. It is suggested that steric hindrance prevents such aggregation taking place in the other isomers.     

Mass spectra of some acyclic α,β-unsaturated aldehydes,ketones and esters

The mass spectra of a series of aliphatic acyclic α,β-unsturated aldehydes, ketones and ester have been examined. The spectra do not show evidence for McLafferty rearrangements, alkoxyl migration or for fragmentations which are dependent upon the s-trans or s-cis conformations of the compounds. There is some evidence for cis-trans isomerism about the double bond.     

Structural study on propenylphosphonates

The ¹H and ¹³C NMR, IR and Raman spectra of several cis and trans propenylphosphonic acid derivatives have been measured and discussed. In some compounds NMR chemical shifts and IR data suggest that the PO and CC groups are in a planar s-trans disposition.     

Identification and conformational significance of amide group vibrational modes contributing to the near-infrared spectra of lactams of trans and mixed configuration

After identifying relevant fundamental vibrational bands in the infrared, amide group overtone and combination vibrational modes contributing to the near-i.r. absorption spectra of a series of lactams of trans and of mixed cis and trans conformation have been elucidated. Experimental studies reveal that the spectral behavior of the trans lactams (11- through 13-membered rings) parallels that for trans open-chain secondary amides more closely than that for small-ring, cis lactams. These findings demonstrate the potential utility of near-i.r. spectrometry to serve as a probe for the conformation of the secondary amide grouping. In addition, a (ν_NH + δ_NH) combination band found in the spectra of both conformational classes of lactam may be able to distinguish between cyclic and acyclic secondary amide groupings. These spectral characteristics are sufficiently distinctive that evidence for both cyclic cis and trans components is readily discerned in the near-i.r. spectra of 2-azacyclononanone, a lactam of mixed conformation.     

Structure of polypiperylene chain segments according to high-resolution 13C NMR spectra

The microstructure of stereoregular 1,4-trans-and 1,4-cis-polypiperylenes, as well as polymers prepared from the trans-and cis-piperylene isomers via cationic polymerization in the presence of TiCl₄, was studied by high-resolution ¹³C NMR spectroscopy. Polypiperylene synthesized through the cationic polymerization of the cis isomer had a more diversified morphology of the macromolecular chain, i.e., had higher relative amounts of 1,2-cis-units and combinations of irregular-addition 1,4-trans-units. It was shown that ¹³C NMR spectra give the most comprehensive and independent information on the details of structure of the piperylene macromolecular chain.     

Application of the Seth-Paul-Van Duyse equation—II : Transmission of polar effects by the CHCH group

The use of the improved Seth-Paul-Van Duyse equation (SPVDE) provides a new statistically significant method for determination of transmissive factors of polar effects for various bridge groups. The method was applied to the CO stretching frequencies of s-cis and s-trans conformers of 98 R₁CHCHCOR₂ compounds. The transmissive factors for CHCH group in both conformations s-cis and s-trans have been calculated with an accuracy about of one order higher than found by using the methods described earlier. The applicability of the improved SPVDE to the 137 CO stretching frequencies of various R₁CHCHCOR₂ compounds has been proved.     

PREPARATION AND PROPERTIES OF NEW VISUAL PIGMENT ANALOGUES FROM 5,6-DIHYDRORETINAL AND CATTLE OPSIN*

Abstract— On irradiation of all-trans 5,6-dihydroretinal (I), two opsin-active isomers are formed. Both these isomers (believed to be the 11-cis and 9-cis isomers) individually couple with cattle opsin to form complexes which have maximal absorption at 465 nm. These complexes satisfy all the established criteria characteristic of synthetic visual pigments.     

Processus concurrents de photoisomerisation d'hexatrienes polymethyles: effet de longueur d'one

The ground-state conformer composition is shown to play a determining role in the orientation of the photochemical reactions of two hexatrienes. A pronounced wavelength effect is observed for Z-2,6-dimethyl-1,3,5-heptatriene; photocycloaddition leads to a bicyclo[3.1.0]hexene and [1,5] sigmatropic hydrogen migration occurs from an s-cis-s-trans conformation preferentially excited by longer wavelength light, whereas the Z-E interconversion is predominantly observed as a result of excitation of the s-trans-s-trans conformation by shorter wavelength light.     

An ab initio study of some structural features and the rotational barriers in performic acid,formic acid and hydrogen peroxide

Calculations on performic acid at the 4-31G level, with and without bond functions with complete geometry optimization, and at the (9, 5) level, with and without polarization functions and rigid rotation, all give no sign of a well in the potential energy curve for rotation about the O/O bond axis in the region of 50° – 90° ; and all but the unaugmented 4-31G basis set find the cis-cis planar conformer to be the most stable form. Calculations at the (9,5) level with rigid rotation find the energies of the other planar conformers, relative to the cis-cis conformer, to be 0.94, 1.50 and 14.80 kcal mol^?1 for the trans-trans, cis-trans and trans-cis structures respectively. These energies and also that for the barrier separating the cis-cis and cis-trans conformers, 1–2 kcal mol^?1, are discussed in relation to corresponding data for formic acid, hydrogen peroxide and several planar four heavy-atom molecules. Dipole moment calculations using the same basis sets would seem to favor a skew conformation as the most stable for performic acid, but comparisons between calculated and experimental values for formic acid and for hydrogen peroxide cast doubt on the validity of such results.     

Die Schwingungsspektren der isomeren Komplexe N,N′-Bis(dimethylgallium)-cis- und trans-N,N′-dimethyloxamideine kritische Analyse

Reaction of Ga(CH₃)₃ with N,N′-dimethyloxamide affords two isomeric N,N′-bis(dimethylgallium)-N,N′-dimethyloxamide complexes, characterized by crystal structure analysis as having cis-and trans-configuration with respect to the central oxamide CC bond and belonging to the point groups C_2v (cis) and C_2h (trans), respectively. Both isomers which are formed in varying ratio, depending upon the reaction conditions, have been isolated in pure form; their vibration spectra (IR/RE) are analyzed in detail in points of alternative behaviour, and are assigned in all bands, especially in the G-C and Ga-X frequency region. The assignment receives further confirmation from a comparison with the vibrational data of the homologous N,N′-bis(dimethylgallium)oxamide (cis/trans isomer mixture).     

The barriers to methyl group rotation in s-cis- and s-trans-methyl nitrite

Ab initio Hartree-Fock calculations at the 4–21 level reproduce the structures of s-cis- and s-trans-methyl nitrite as well as the surprisingly large difference in the methyl group rotational barriers of the two conformers. Furthermore, the computations reveal the significant structural changes accompanying internal rotation. These structural changes, together with the localized orbitais and population analyses, provide information for an understanding of the unusual barriers. The extraordinarily low barrier of the s-trans conformer results from stabilization of the higher-energy eclipsed form by an attractive interaction involving the in-plane methyl hydrogen atom and the lone pair of the nitrogen atom. The high barrier of the s-cis form is due to additional stabilization of the staggered conformation by a hyperconjugative interaction of the out-of-plane C-H groups with the NO double bond.     

Ultrafast Spectroscopic Study on Non-adiabatic UV Protection Mechanism of Hemicyanines

In this work, we firstly elucidated the ultraviolet light protection dynamics mechanism of the typical hemicyanines, i.e.. Hemicy and DHemicy, by combining the theoretical calculation method and the transient absorption spectra. It is theoretically and experimentally demonstrated that both Hemicy and DHemicy have strong absorption in UVC (200-280 nm), UVB (280-300 nm), and UVA (320-400 nm) regions. Moreover, after absorbing energy, Hemicy and DHemicy can jump into the excited states. Subsequently, Hemicy and DHemicy relax to \begin{document}$\mathrm{S}_0 $\end{document} states from \begin{document}$\mathrm{S}_1 $\end{document} states rapidly by the non-adiabatic transition at the conical intersection point between the potential energy curves of \begin{document}$\mathrm{S}_1 $\end{document} and \begin{document}$\mathrm{S}_0 $\end{document} states, and are accompanied by the trans-cis photoisomerism. The transient absorption spectra show that trans-cis photoisomerization occur within a few picoseconds. Thus, the ultraviolet energy absorbed by Hemicy and DHemicy could be relaxed ultrafastly by the non-adiabatic trans-cis photoisomerization processes.     

The stereochemistry of 2,4-disubstituted γ-butyrolactones and 2,4,6-trisubstituted-5,6-dihydro-4H-1, 3-oxazines

2,4-Disubstituted butyrolactones and 2,4,6-trisubstituted-5,6 dihydro-4H-1,3-oxazines show similar features in their ¹H and ¹³C- NMR spectra. Two geminal ring hydrogens of cis isomers give rise to a complex ABXY spectra when the substituent is alkyl or aryl. In spectra of trans isomers these patterns are degenerated. When R is OMe(in 4) or OCOMe (in 6) the difference in chemical shifts of geminal protons and vicinal coupling constants cannot be used for diagnosis. In ¹³C spectra ring carbons C-2 and C-3 in lactones and C-4 and C-5 in oxazine of trans isomers show a small but consistent shift to higher fields.     

Conformational analysis of substituted polyacetylenes

A conformational analysis of isolated chains of polymethylacetylene (PMA), polypentylacetylene (PPA), and poly(t-butylacetylene) (PTA) was carried out taking into account interactions between nonbonded atoms and torsional potentials. It was found that the trans configuration of all three polymers is more stable than the cis configuration, the difference in potential energy between the trans and the cis isomers however being very small for PTA, leading to the possibility of observing a trans^→ cis isomerization in some solvents. The calculations show that the substituted polyacetylenes are not found in a planar conformation, the larger deviations from planarity being found with the bulkier substituents: PTA > PPA > PMA. A correlation could be established between the UV absorption limit of the samples and the minimum torsional angle of the potential-energy functions. This relation predicts that the absorption limit is shifted to long wavelengths on increasing the planarity of the molecule. Moreover, UV spectra could be calculated from the potential-energy functions, and it is shown that the potential-energy functions of other substituted polyacetylenes can be calculated from their experimental UV spectra.     

Transferts protoniques de sels d'ammonium substitues—XI : Identification des isomères geometriques de l'ion trimethyl 1-cis (2,6) piperidinium par rmn du 1H et du 13C

The spectra of acidic 1-protons and of tertiary 2,6-protons of the 1-cis (2,6) trimethyl- piperidinium ion in various acidic solvents are used to assign structure to cis and trans isomers. ¹³C spectra are described for the ion and the corresponding free amine, with and without an N-methyl substituent. Their chemical shifts are compared with the analogous cyclohexane derivatives.     

Benzo- and indoloquinolizines. XII—carbon-13 n.m.r. study of the conformation of the 1,2,3,4,4a,6,7,8,9,13b-decahydro-9a H-pyrido[1,2-f]phenanthridine isomers

The influence of the configuration and the conformation on the ¹³C n.m.r. spectrum of 1,2,3,4,4a,6,7,8,9,13b-decahydro-9aH-pyrido[1,2-f] phenanthridine was investigated. These observations, coupled with the low temperature spectrum allowed us to confirm the trans-syn-cis as the preferred conformation for one of the four isomers. At ?40°C, 2–3% of another conformer could be detected and identified as the cis-anti-cis-2.