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.     

IR Study of Monomer–Dimer Self-association of 2,2-Dimethyl-3-ethyl-3-pentanol in n-Octane: Determination of the Molar Absorptivities of Monomer and Dimer Bands, and Dimerization Constants Using Novel Equations

The monomer–dimer self-association equilibrium of 2,2-dimethyl-3-ethyl-3-pentanol in n-octane has been studied by IR spectroscopy at four temperatures (288, 298, 308, and 318 K). The solute was chosen to restrict the self-association between solute molecules to dimerization only, owing to steric hindrance of the bulky chains in the vicinity of the OH group. Two linear equations have been derived for the treatment of the experimental data. One of these equations was used to treat the data of the concentration dependent integrated absorbance of the monomer bands for each temperature to obtain the monomer molar absorptivity, ε _m, and dimerization constant, K. The other equation was used to treat the data of concentration-dependent dimer bands to obtain the dimer molar absorptivity, ε _d, and K. Thus, the dimerization constant was determined by two methods. Since the same thermodynamic quantity K is obtained from either the monomer bands or the dimer bands, the difference between them at a given temperature can serve as an assessment of the quality of the experiment. The standard enthalpy and entropy of dimerization were also obtained from a van’t Hoff plot.     

Can the conformational flexibility of cis-decalins be modulated through intramolecular O-H?O hydrogen bonding? Profiling the molecular and supramolecular attributes of designer cis-fused polycyclitols

In order to unravel the modalities of hydrogen bonding in conformationally flexible polycyclitols vis-à-vis their conformationally locked siblings, three diastereomeric perhydro-2,3,4a,6,7,8a-naphthalenehexols, all embodying a 4a,8a-dihydroxy-cis-decalin framework, have been synthesized via sequential stereo-controlled oxyfunctionalization of 1,4,5,8-tetrahydronaphthalene. Variable temperature NMR studies on the cis-fused polycyclitols thus obtained suggest that their inherent conformational flexibility in solution is restrained at ambient temperature owing either to the formation of a stable H-bonded molecular solvate or to the presence of strong intramolecular O-H?O H-bonding. Single crystal X-ray diffraction studies on the three hexols reveal an interesting commonality in their gross molecular packing and a ubiquitous presence of the R²₂(10) dimer motif—a supramolecular synthon rarely encountered in the crystal structures of conformationally locked and axially-rich hexols.     

Stabilization of merocyanine by protonation, charge, and external electric fields and effects on the isomerization of spiropyran: a computational study

Protonation, charging, and field effects on the thermal isomerization of a nitrospiropyran (SP) modified by a thiolated etheroxide chain into merocyanine (MC) are computationally studied at the DFT level. The ring opening leads to cis-MC conformers that then isomerize to the more stable trans forms. While the closed neutral spiropyran is more stable than the conjugated open forms, the merocyanine conformers are significantly stabilized by protonation, electron attachment, and ionization. For protonation on the pyran oxygen atom and electron attachment, the MC conformers are more stable than SP, and unlike for the neutral species, the ring opening is spontaneous at room temperature. Moreover, for the pyran oxygen-protonated form, the ring opening to the cis-merocyanine becomes barrierless. On the other hand, barriers comparable to the neutral remain along the thermal pathway to the cis-merocyanine conformer for ionization or electron attachment, and the barrier for isomerization is significantly higher for the N-protonated SP form. External field effects on the neutral reaction path show that ring opening to the cis-merocyanine is favored when the field reduces the electron density on the pyran part, as also induced by the local field due to O protonation.     

Matrix-isolation infrared and ultraviolet spectroscopic studies of less stable conformers of 1,3,5-hexatriene

The infrared spectra of the 3-trans and 3-cis isomers of 1,3,5-hexatriene in low-temperature Ar matrices deposited from a high-temperature nozzle or deposited under irradiation of the Hg 253.7 nm light show a number of new bands. Correspondingly, the ultraviolet spectra of the 3-trans isomer observed under similar experimental conditions show a new absorption at 276 nm. Most of these new infrared bands and the new ultraviolet absorption are attributable to less stable isomers which have the planar s-cis conformation [or gauche conformation(s) close to the planar s-cis] about either one or both of the two CC bonds.     

NMR study of cis3̄trans isomerism in N-methylated lactams with 11- and 13-membered rings

From ¹H-NMR spectra of 1-methyl-azacyclo-undeca-2-one and 1-methyl-azacyclo-trideca-2-one, the bands corresponding to the cis and trans forms have been assigned and analyzed; based on this analysis, conformational structures about the C-C bond next to nitrogen are proposed. By analysis of the relative areas and shapes of the N-methyl bands measured for the two lactams in 1,1,2,2-tetrachloroethane-d₂, over a broad temperature range, the equilibrium and thermodynamic parameters characterizing the cis—trans isomerism of the amide bond in these lactams have been determined. Peaks corresponding to the cis and trans forms in the ¹³C-NMR spectra of these lactams have also been assigned.     

Infrared cryogenic studies: Part 14. Chloromethane in various matrices at 10 K

Infrared spectra are reported for chloromethane in argon, krypton, nitrogen, and carbon monoxide matrices at 10 K. No evidence was found for rotation of chloromethane in any of the matrices. In Kr band splitting of the A modes due to intermolecular coupling was observed. In CO, and possibly N₂, minor perturbations of the v₃ mode due to multiple trapping sites were observed. Concentration studies in all matrices show the growth of multimer bands, the predominant species is believed to be an open-chain dimer.     

Stereochemical effects in the chemical ionization mass spectra of cyclic diols

The chemical ionization mass spectra of configurational isomers of many cyclic diols give substantial differences which are characteristic of their stereochemistry. For the cis-isomer of 1,3- and 1,4-cyclohexanediols, formation of a stable intramolecular proton bridge involving the OH groups gives rise to dominant MH⁺ peaks, suppressing the (M-H)⁺ peaks found in the spectra of the trans-isomers and monoalcohols. The stability of the proton bridge in cis-1,3-cyclohexanediol structures is decreased by a sterically interfering cis-5-methyl substituent, but increased by a cis-5-OH group due to additional proton solvation. cis-Stereochemistry also gives increased formation of the dimeric M₂H⁺ ions, but decreased formation of trimers, at higher diol concentrations, for the 1,3- and 1,4-diols. The similarity of the CI spectra of cis- and trans-1,2-cyclohexanediol are explicable in terms of the similarities of the most stable proton-bridged conformers; the reduced ring flexibility in cis- and trans-1,2-cyclopentanediols makes such similar structures unfavorable, as shown by the substantial differences between their CI spectra. The substantial, but expected, variations in behavior with temperature and reagent gas are useful for maximizing stereochemical effects on CI spectra; conditions of lowest energy are usually, but not always, the most useful.     

Infrared investigation of aluminum- and chromium-substituted magnetites and of the lacunar spinels resulting from their oxidation

When Fe³⁺ ions are substituted by aluminum or chromium on magnetite octahedral sites, the ir spectrum shows the conversion of an inverse spinel to a normal spinel. Both broad bands of magnetite are gradually replaced by the four characteristic bands of normal spinels II–III. They are also observed for solid solutions, FeCr₂O₄FeAl₂O₄, with, however, a further band at 780 cm^?1 which may be assigned to Al³⁺ ions on tetrahedral sites. Low-temperature (<400°C) oxidation of these compounds whose sizes are less than 2000 Å results in lacunar spinels III–III. The ir spectrum of these solids is characterized by two absorption bands (as for inverse spinels II–III) except for compounds close to pure γFe₂O₃ in which an order of vacancies could be put in evidence.     

Calorimetric study of thermodynamics of production of polybutene-1, polypentene-1, 4-methylpentene-1 and properties of reagents between 0 and 670 K

The heat capacitiesC ₀ ^p of polybutene-1, polypentene-1, poly-4-methylpentene-1 and 4-methylphentene-1 were studied calorimetrically from 6 to (500–700) K. Temperatures, enthalpies of melting of various crystalline forms and the parameters of the glass transition were determined. The thermodynamic functionsH ⁰(T)?H⁰(0), S⁰(T) andG ⁰(T)?H⁰ (0) were calculated between 0 K and (500–700) K. From the calorimetric values obtained and literature data, the thermodynamic characteristics of the following processes were estimated for the corresponding alkenes-1, andcis-andtrans-alkenes-2: the polymerization of alkenes-1, the monomer-isomerization polymerization ofcis- andtrans-alkenes-2 to polyaklenes-1 and the isomerization ofcis- andtrans-aklenes-2 to alkenes-1 in the same temperature interval at standard pressure.     

Vibrational spectra,conformational equilibria,ab initio calculations and vibrational assignments of ethylmethylfluorosilane

The infrared spectra of ethylmethylfluorosilane (CH₃SiHFCH₂CH₃) have been recorded as a vapour, liquid and solid at 78 K in the 4000–50 cm⁻¹ range and isolated in an argon matrix at ca. 5 K. Infrared spectra of two different solid phases were obtained after annealing to temperatures of 120 and 130 K, and recooling to 78 K. Although the IR spectra were quite similar in the MIR region, certain differences were noted in the FIR region below 400 cm⁻¹. The most stable conformer MeMe was present after annealing to 130 K, but three bands belonging to MeH were detected after annealing to 120 K. Various infrared bands changed intensity when the argon matrix was annealed to temperatures between 20 and 35 K, and some of these were related to changes in the conformational abundance.Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, various bands changed in intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer MeMe. From various bands assigned to the three conformers, the conformational enthalpy difference ΔH from MeMe to the intermediate energy conformer MeH was found to be 0.5 kJ mol⁻¹ and to the highest conformer MeF was 0.7 kJ mol⁻¹. At ambient temperature this leads to 39% MeMe, 32% MeH and 29% of the MeF conformer in the liquid.Ab initio calculations in the RHF, MP2, DFT approximations and very accurate G2 calculations were carried out. With one exception, the MeMe conformer had the lowest enthalpy in all these calculations, the MeH had the intermediate and the MeF the highest enthalpy, and the calculations were in good agreement with the measurements.     

Study on the spin-crossover transition in [Fe(cis-/trans-stpy)4(X)2] (stpy: styrylpyridine, X: NCS, NCBH3) under high pressure toward ligand-driven light-induced spin change

Toward the realization of a ligand-driven light-induced spin change (LD-LISC) around room temperature, we have investigated the spin-crossover phenomenon in [Fe(stpy)₄(X)₂] (stpy = styrylpyridine, X = NCS, NCBH₃) under high pressure. The spin transition temperature increases from 110 to 220 K with increasing applied pressure up to 0.75 GPa for [Fe(trans-stpy)₄(NCS)₂], while [Fe(cis-stpy)₄(NCS)₂] shows the high-spin state in the temperature region between 2 and 300 K even at 0.75 GPa. In the case of X = NCBH₃, due to the stronger ligand field of NCBH₃, the spin transition temperature increases from 240 to 360 K with increasing applied pressure up to 0.50 GPa for [Fe(trans-stpy)₄(NCBH₃)₂]. In the case of [Fe(cis-stpy)₄(NCBH₃)₂], the spin state is the high-spin state in the temperature region between 2 and 300 K. However, the spin transition appears at 125 K under 0.5 GPa and the transition temperature increases with increasing applied pressure. In this way, we have decided the applied pressure region of 0.65-1.09 GPa where [Fe(stpy)₄(NCBH₃)₂] undergoes LD-LISC at room temperature.     

Dielectric and raman spectroscopic studies on the rotational isomerism of t-butyl formate

Static dielectric constants, refractive indices and densities have been measured for t-butyl formate and acetate in cyclohexane at increasing concentrations and at two temperatures, 20°C and 60°C. The ΔH value for the cis-trans equilibrium of t-butyl formate and the dipole moment of the trans conformation has been calculated from the dielectric measurements taking the ΔS value calculated from the sum-over-states for the cis and trans conformations and assuming the dipole moment of the cis conformation to be equal to 1.94 D as found for t-butyl acetate, which has the cis conformation only.The relative intensities of the Raman bands corresponding to specific vibrations modes of the cis and trans conformations of t-butyl formate are measured in cyclohexane and acetonitrile at various concentrations. The enthalpy difference ΔH is also measured in the liquid state and in acetonitrile by variation of the intensity ratio of the bands with temperature.All thermodynamic quantities obtained from dielectric or Raman intensity measurements are compared with each other and with the theoretical values. The solvation energy difference between cis and trans conformations in cyclohexane and acetonitrile as well as in the liquid state are also compared with theoretical values. The large deviation of solvation energy difference between the experimental value and Onsager's model value are well described by an additional term, which considers dipole-dipole interaction.     

Density functional theory study of ring-chain equilibria for the cross-metathesis of cyclohexene and cis,cis-cycloocta-1,5-diene with functionalized olefins

The computational modeling of ring-chain equilibria for the ring-opening cross-metathesis of cyclohexene (CH) with 1,2-dicarbomethoxy-ethylene (DCE), 1,4-dicarbomethoxy-but-2-ene (DCB) and ethylene at T = 298.15 K using the B3LYP/6-31G(d,p) level of theory revealed that CH and ring-opened products equilibrium is shifted towards the thermodynamically stable six-membered ring. The calculations demonstrated that carbonyl-containing olefins can completely drive the equilibrium in the cross-metathesis with cis,cis-cycloocta-1,5-diene (COD) towards the ring-opened products.     

<Emphasis Type="Italic">Ab initio</Emphasis> calculation of nitrogen oxide dimer structure and its anion-radical

A comparative quantum chemical analysis has been made for the most stable dimer of nitrogen oxide with the structure cis-ONNO in a singlet state ¹A₁ by ab initio method of SCF MO LCAO, allowing for electron correlation according to Meller-Plesset perturbation theory of the second order (MP2), and density functional technique (DFT). The computations by MP2 method show anion-radical (ONNO)^? to have a strong bond between nitrogen atoms (N-N 1.44 Å) in contrast to molecular weakly bound cis-dimer with equilibrium distance N-N 2.23 Å. Molecular orbital structure of the dimer and its anions was examined that made it possible to suggest a reason of preferable stabilization of nitrogen oxide dimer in the cis-form. Calculated high affinity to electron (E_a = ?1.55-?1.69 eV) for the molecular dimer ONNO (¹A₁) explains an intense strengthening of N-N bond in anion-radical and confirms the experimental data on a possibility of surface anion-π-radical formation on electron donor centers. The DFT computations indicate that this technique poorly reproduces the experimental geometry and electron structure of the cis-dimer ONNO having predicted a triplet ground state with the equilibrium distance N-N ≈2 Å instead of a singlet one with N-N 2.26 Å. The comparison between MP2 and DFT calculations for complex dimer ONNO with copper cation reveals the energy state of the complex (Cu-O₂N₂)⁺ corresponding to stabilization of anion-π-radical (N₂O₂)^? {term-³A₂, Cu(d)⁹-(ONNO)^?1} to be highly overestimated by DFT.     

Organometallic compound with Lewis base—IV : Dimerization of methyl crotonate by aluminum alkyl-tertiary amine complex

Aluminum alkyl-tertiary amine complex was found to induce the catalytic dimerization of methyl crotonate (MCr) to dimethyl 2-methylpent-4-ene-1,3-dicarboxylate (1) and dimethyl 2-methylpent-cis-3-ene-1,3-dicarboxylate (2). The of the γ-hydrogen of the MCr molecule. Dimer 2 is formed through the isomerization of dimer 1. The complex of AlR₃ with a bidentate ligand, sparteine, produces dimer 1, selectively. The complex of AlR₃ with monodentate ligand NEt₃, on the other hand, induces the isomerization of dimer 1 to the cis-form of dimer 2. The coordination number of aluminum alkyl-tertiary amine complex seems to control the dimerization mechanism of MCr.     

Preparation,spectroscopic assignments and relative stabilities of cis and trans species of o,o′-azodioxytoluene in the solid state and in solution

Crystalline samples of cis- and of trans-o,o′-azodioxytoluene have been isolated. The cis crystals isomerize spontaneously to trans. IR spectra were recorded for the pure dimeric species as well as for o-nitrosotoluene and assignments of specific peaks were made. In solution the cis form is the energetically more stable species as revealed from NMR assignments on cis and trans samples.     

UV-spektroskopische Untersuchungen an cis-und trans-Bis(phenyl)bis(phosphan)platin(II)-Verbindungen

A long-wavelength absorption band in the UV spectra of compounds of the typecis-and trans-bis(phenyl)bis(tri-n-butylphosphane) platinum(II) and cis-bis-(phenyl)bis(tris[4-(dimethylamino)phenyl]phosphane)platinum(II) is probably due to the influence of substituents in the phenyl rings in the form of a phenyl → platinum charge-transfer-transition.     

Protonation et hydrolyse d'enamine. Epimerisation des immoniums intermediaires

At pH 5 (AcOHAcONa) protonation of the enamine of 3,6,6-trimethylnorpinane 2-one (3-methylnopinone) yields the cis immonium ion 6 (X  OAc), which isomerizes to the thermodynamically more stable trans immonium ion 6′ (X  OAc). Under more strongly acid conditions (aqueous hydrochloric or perchloric acids), the enamonium salts 7 (X  Cl or ClO₄) are formed; these isomerize, with a rate increasing with decreasing acidity to the cis immonium salts 6, stable under these conditions. Epimerization of the cis salt 6 (X  Cl), occurs in ethereal hydrogen chloride, the rate increasing also with decreasing acidity. At pH 5, hydrolysis of the enamine yields 3-methylnopinone, the proportion of the less thermodynamically stable trans isomer increasing with a rise in reaction temperature.     

Molecular modeling of ring-chain equilibria for the ring-opening cross-metathesis of cis,cis-1,5-dimethyl-cycloocta-1,5-diene with ethylene at T=298.15 K

The molecular modeling of ring-opening cross-metathesis of cis,cis-1,5-dimethyl-cycloocta-1,5-diene (1,5-DM-COD), cis,cis-1,6-dimethyl-cycloocta-1,5-diene (1,6-DM-COD) and cis,cis-cycloocta-1,5-diene (COD) with ethylene (ethenolysis) at T=298.15 K using the B3LYP/6-31G(d,p) level of theory reveals that ring-chain equilibrium constants are dependent on the nature of cyclic diene. The ring-chain equilibria for the ethenolysis of 1,5-DM-COD is completely shifted to the formation of monomeric 2-methyl-hexa-1,5-diene.