Terms Of Conformational Equilibria Research Articles

Vibrational spectra, ab initio calculations and vibrational assignments of 3-butyn-1-ol

The infrared spectra of 3-butyn-1-ol, HC CCH 2CH 2OH, have been recorded as a vapour in the range 3600–50 cm −1 and as a liquid between 3600 and 400 cm −1. Additional spectra of the alcohol isolated in an argon matrix at ca. 5 K were obtained and spectra were recorded after annealing to various temperatures between 10 and 35 K. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 143 K. Spectra of an amorphous solid were recorded at 78 K. In spite of several attempts and many different annealing temperatures, the sample crystallized neither in the IR nor in the Raman cryostats. In the variable temperature Raman spectra, some bands of the liquid changed in relative intensity and were interpreted in terms of conformational equilibria between two of the five possible conformers. Complete assignments were made for all the bands of the most stable conformer gg, in which the OH group is approaching the triple bond, forming an intramolecular hydrogen bond. From various bands assigned to a second conformer aa, in which OH is oriented anti to the C C bond, or a third conformer ag, the conformational enthalpy difference was found to be Δ conf H( ag-gg) = 0.9 kJ mol −1 in the liquid. The two highest energy conformers g’g and ag were not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. The calculations revealed that gg was the low energy conformer and CBS-QB3 calculations suggested the gg conformer was more stable by 5.4 and 4.2 kJ mol −1 relative to ag and aa, respectively, in the vapour. Vibrational wavenumbers and infrared and Raman band intensities for the three low energy conformers are reported from B3LYP/cc-pVTZ calculations.

Vibrational spectra, conformations, ab initio calculations and vibrational assignments of 3-pentyn-2-ol

The infrared spectra of 3-pentyn-2-ol, CH 3C CCH(OH)CH 3, have been recorded as a vapour and liquid at ambient temperature, as a solid at 78 K in the 4000–50 cm −1 range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid. 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, some bands changed in relative 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 in which OH is oriented anti to C 1( a Me). From various bands assigned to a second conformer in which OH is oriented anti to H gem( a H), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ mol −1. The highest energy conformer with OH anti to C 3( a C) was not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis set calculations for which the a H conformer had slightly lower energy, all the calculations revealed that a Me was the low energy conformer. The B3LYP/cc-pVTZ calculations suggested the a Me conformer as more stable by 0.8 and 8.3 kJ mol −1 relative to a H an a C, respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from B3LYP/cc-pVTZ calculations.

Raman spectra, ab initio calculations, phase transitions and conformations of 1,3-disilabutane

Raman spectra of 1,3-disilabutane (SiH3CH2SiH2CH3) were recorded as a liquid at room temperature and at 14 temperatures between 293 and 137 K. Spectra of an amorphous and annealed solid were recorded at 78 K, and very distinct changes occurred on crystallization. In the variable temperature Raman spectra, some bands changed in intensity and were interpreted in terms of conformational equilibria between the two possible conformers. Complete assignments were made for all the bands of the most stable conformer, anti. From three band pairs assigned to the anti and gauche conformers, the conformational enthalpy differences ΔH(gauche-anti) were found to be between 1.3 kJ mol−1 and 1.7 kJ mol−1, using peak heights, and the more stable anti conformer was present in the crystal. A lower value of 0 ± 0.3 kJ mol−1 was obtained employing band areas. Ab initio calculations in the RHF and MP2 approximations and DFT calculations were carried out pointing to the anti conformer as the low energy form.

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

The infrared spectra of ethylmethylfluorosilane (CH 3SiHFCH 2CH 3) 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 Δ H 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.

Photochemical and laser activity of 1,4-bis[ß-(2-quinoxalyl)vinyl]benzene: a new laser dye

The title diolefinic dye (BQxVB) was synthesized and its spectral characteristics were investigated in various solvents as well as microemulsions. A large solvents effect on the fluorescence maxima and quantum yields ( φ f) was observed. The effect of excitation wavelength on both fluorescence and photoisomerization quantum yields have been evaluated in different media and interpreted in terms of conformational equilibrium. The fluorescence quenching of BQxVB by Fe +3 ions was also investigated in aqueous, anionic and cationic micellar solutions. The photochemical behaviour of BQxVB in halomethane solvents (such as CH 2Cl 2, CHCl 3 and CCl 4) has been discussed. The dye solution in N,N-dimethyl formamide (DMF) lases around 460 nm upon pumping with a nitrogen laser ( λ ex = 337.1 nm). Energy transfer from BQxVB to rhodamine B has been studied in ethanol and the critical transfer distance has been determined.

NMR spectra and conformational analysis of substituted 2,3‐dihydro‐[1,5]benzoxazepin‐4(5H)‐one derivatives

AbstractThe 1H and 13C NMR spectra of 26 2,3‐dihydro‐[1,5]benzoxazepin‐4(5H)‐one derivatives and four thiono analogues are interpreted in terms of conformational equilibria. Similar to 1,3,4,5‐tetrahydro‐2H‐1,5‐benzodiazepin‐2‐ones, these compounds favour boat conformations of the seven‐membered ring. In the lactams the conformational energy difference is very low, whereas a difference of 0.3 kcal mol−1 is recorded for the thionolactams, the conformation with the quasi‐axial methyl group also being preferred.

Conformational analysis of benzoanellated nine-membered rings, Part 1. 1,4,5,7-tetrahydro-3 H-2,6-benzodithionin derivatives

The NMR spectra of compounds 1–5, recorded at different temperatures, are discussed and interpreted in terms of conformational equilibrium. Ground state conformations are found to be chiral, ring inversion barriers are surprisingly high (ca 48 kJ/mol).

Identification of a low spin state associated with conformational equilibria in methemoglobin

Low-temperature Mössbauer and Electron Spin Resonance (ESR) studies on paramagnetic horse methemoglobin demonstrate the existence of a low spin state of hemoglobin which is not identical to the typical Hb+-OH low spin state. The amount of this species was found to increase when samples were gradually cooled instead of being quenched into liquid nitrogen. This state is found in the pH range from 6.0 to 8.9, although an increase of the relative population of this state is found below pH 6.9. This species is formed from Hb+-H2O and Hb+-OH. Temperature studies indicate that this species can be reversibly formed from the usual high spin and low spin species even at 210 K, where water is already completely frozen, ruling out a direct perturbation due to the freezing process. These results are interesting in terms of conformational equilibrium in the region of the heme pocket. The relative intensity of the various species is explained in terms of energy barriers between the different conformations which limit the rate at which the usual high spin and low spin states are converted to this state.

ChemInform Abstract: CONFORMATION AND VIBRATIONAL SPECTRA OF 1,5‐HEXADIYNE (BIPROPARGYL) AND 1,5‐HEXADIYNE‐1,6‐D2

Abstract The infrared spectra of 1,5-hexadiyne (bipropargyl) and 1,5-hexadiyne-1,6- d 2 as vapours, liquids, as solutes in various solvents and as crystalline solids at low temperatures and at high pressures have been recorded. Raman spectra were obtained for the liquids, including semiquantitative polarization measurements, and for the low temperature crystals. The data were interpreted in terms of two conformers, anti and gauche , in the vapour and liquid state and one, the anti, in the crystalline forms. A phase transition for 1,5-hexadiyne was observed at ca. 240 K. Both the high and low temperature crystals had molecules in the anti conformer. Interpretation of the spectra in terms of conformational equilibria was facilitated by a thorough vapour phase band contour analysis. With a few exceptions, all the vibrational fundamentals for both conformers were assigned and found to be in good agreement with results from normal coordinate calculations.

Conformation and vibrational spectra of 1,5-hexadiyne (bipropargyl) and 1,5-hexadiyne-1,6- d2

The infrared spectra of 1,5-hexadiyne (bipropargyl) and 1,5-hexadiyne-1,6- d 2 as vapours, liquids, as solutes in various solvents and as crystalline solids at low temperatures and at high pressures have been recorded. Raman spectra were obtained for the liquids, including semiquantitative polarization measurements, and for the low temperature crystals. The data were interpreted in terms of two conformers, anti and gauche, in the vapour and liquid state and one, the anti, in the crystalline forms. A phase transition for 1,5-hexadiyne was observed at ca. 240 K. Both the high and low temperature crystals had molecules in the anti conformer. Interpretation of the spectra in terms of conformational equilibria was facilitated by a thorough vapour phase band contour analysis. With a few exceptions, all the vibrational fundamentals for both conformers were assigned and found to be in good agreement with results from normal coordinate calculations.

Molecular-mechanics calculations for cyclic acetals of pentofuranoses, related pentitols, and C-glycosides

Molecular-mechanics calculations are reported for the geometries and energies of 2,3- and 3,5- O-isopropylidene-α- and -β- D-ribo- and - L-lyxo-furanoses, and for the 2,3- and 3,5- O-methylene derivatives of 1,4-anhydro- D-ribitol and - L-lyxitol and the related O-isopropylidene derivatives. Calculations were also performed for model compounds of C-glycosides, where the hydroxyl group of the ribo- and lyxo-furanose derivatives was replaced by a methyl group. The results are discussed in terms of conformational equilibria, constitutional equilibria of 2,3- and 3,5- O-alkylidene derivatives, and configurational equilibria (anomeric and C-4-epimeric). The predictions are generally in good agreement with the available experimental data.

Variable temperature i.r. spectra of aliphatic and aromatic 1,2-diketones

The variable temperature i.r. spectra of a series of aliphatic ( 4–16) and aromatic ( 17–25) 1,2-diketones are reported. Changes in the appearance of bands assignable to the CO and OH stretching vibrations, at different temperatures, are interpreted in terms of conformational equilibria between high and low energy forms.

Conformations of some saturated carbonyl compounds. Part III. Infrared spectra of isomeric propyl and butyl formates and acetates

I.r. spectra of n-propyl, isopropyl, n-butyl, isobutyl, s-butyl, and t-butyl formates and acetates are reported. The effects on these spectra of cooling liquid films and of solidification are interpreted in terms of conformational equilibria. Evidence for the presence of rotational equilibria has been obtained in all the esters except for isopropyl formate, isopropyl, and t-butyl acetate. Conformers are considered to arise from rotation about the HCO–O bond in t-butyl formate (for which ΔH° has been estimated as 150 ± 20 cal mol–1) and from rotation about the O–alkyl bond in isobutyl formate (for which ΔH° has been estimated as 745 ± 40 cal mol–1).

Conformations of some αβ-unsaturated carbonyl compounds. Part III. Infrared solution spectra of methyl, [2H3]methyl, ethyl, and [2H5]ethyl acrylates and trans-crotonates

Infrared spectra of methyl, [2H3]methyl, ethyl, and [2H5]ethyl acrylates and trans-crotonates in solution in carbon disulphide are reported. Band intensity measurements at temperatures between –110 and 35 °C together with changes in the appearance of these bands on solidification of the solution are interpreted in terms of conformational equilibria between high- and low-energy forms. The additional enthalpies (ΔH°) of the high-energy forms are remeasured for methyl acrylate as 326 ± 18 cal mol–1 and for methyl trans-crotonate as 306 ± 30 cal mol–1. The additional entropy (ΔS°) of the high-energy form of methyl trans-crotonate is estimated as 1·5 ± 1·0 cal mol–1 K–1.

Conformations of some saturated carbonyl compounds. Part I. Infrared spectra of methyl formate, acetate, propionate, and n-butyrate; [2H3]methyl propionate and [2H3]methyl n-butyrate

I.r. spectra of methyl formate, acetate, propionate, n-butyrate; and [2H3]methyl propionate and n-butyrate are reported. The effects of cooling solutions in carbon disulphide from 35 to –110°, and of solidification, on these spectra are interpreted in terms of conformational equilibria. One pair of conformers is suggested for methyl propionate for which the additional enthalpy (ΔH°) for the high energy form is 87 ± 19 cal mol–1. Two pairs of conformers are suggested for methyl n-butyrate for which the additional enthalpies of the high-energy forms are 92 ± 27 and 440 ± 20 cal mol–1. It was not possible to determine ΔS° from the data.

Conformations of some saturated carbonyl compounds. Part II. Infrared spectra of ethyl formate, acetate, propionate, and n-butyrate, and [2H5]ethyl n-butyrate

Infrared spectra of ethyl formate, acetate, propionate, and n-butyrate, and [2H5]ethyl n-butyrate are reported. The effects on these spectra of cooling solutions in carbon disulphide to –100 °C and of solidification are reported and are interpreted in terms of conformational equilibria. Evidence for the presence of one pair of conformers, derived probably from rotation about the O–Et bond, has been found in all the esters examined; an enthalpy difference (ΔH°) of 315 ± 25 cal mol–1 between high- and low-energy conformers has been obtained for ethyl acetate. Evidence has also been obtained for the presence of an additional equilibrium in ethyl and [2H5]ethyl n-butyrates associated with rotations within the n-butyl chain.

Conformations of some αβ-unsaturated carbonyl compounds. Part II. Infrared and Raman spectra of methyl and ethyl acrylates and trans-crotonates

I.r. spectra of methyl and ethyl acrylates and trans-crotonates in solution and vapour states and Raman spectra in the liquid state are reported and vibrational assignments are suggested. Intensity measurements, at temperatures between –110° and 200°, of i.r. bands are interpreted in terms of conformational equilibria between high- and low-energy forms. The additional enthalpy (ΔH) of the high-energy forms are, for methyl acrylate 315 ± 45 cal. mole–1 in solution in CS2 and 400 ± 100 cal. mole–1 in the vapour, and for methyl trans-crotonate 520 ± 25 cal. mole–1 in the vapour state. Assignments of bands which show temperature-dependence in the regions 900–1000 cm.–1 and 1150–1350 cm.–1 are discussed, and the reasons for the lack of observed temperature variation of bands in the 1600–1800 cm.–1 region are explained in terms of comparisons with related compounds. It is concluded that the low-energy form is probably the s-trans conformation and the high-energy form is possibly the s-cis-conformation or a non-planar conformation.

Conformations of some αβ-unsaturated carbonyl compounds. Part I. Infrared spectra of acraldehyde, crotonaldehyde, methyl vinyl ketone, and ethylideneacetone

Infrared spectra of acraldehyde, crotonaldehyde, methyl vinyl ketone, and ethylideneacetone in solution and vapour states are reported. Band intensity measurements at temperatures between –110° and 200° are interpreted in terms of conformational equilibrium between the s-trans and non-trans forms of the ketones. The additional enthalpy (ΔH) of the non-trans forms are: [graphic omitted] No evidence is found for more than one form of the aldehydes and the rotational structure of vibrational bands of both acraldehyde and crotonaldehyde suggest these exist in the s-trans conformation. Vibrational assignments are provided for the various conformations detected in these compounds.