Conformational analysis,barriers to internal rotation,vibrational assignment and ab initio calculations of chloroacetone

The infrared (3500-20 cm⁻¹) and Raman (3200-10 cm⁻¹) spectra have been recorded for gaseous and solid chloroacetone (1-chloro-2-propanone), CH₂ClC(O)CH₃. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the molecule exists predominantly in a gauche conformation having a “near cis” structure of C₁ symmetry (dih ClCCO=142°C) in the vapor but for the liquid a second conformer having a trans structure (chlorine atom oriented trans to the methyl group) with C_s point group symmetry is present. From a study of the Raman spectrum of the liquid at variable temperatures, the trans conformation has been determined to be more stable than the gauche form by 1042±203 cm⁻¹ (2.98±0.6 kcal mol⁻¹ and is the only conformer present in the spectrum of the annealed solid. From ab initio calculations at the 3-21G* and 6-31G* basis set levels optimized geometries for both the gauche and trans conformers have been obtained and the potential surfaces governing internal rotation of the symmetric and asymmetric rotors have been obtained. The observed vibrational frequencies and assignments to the fundamental vibrations for both the gauche and trans conformers are compared to those calculated with the 3-21G* basis set. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.     

Conformational analysis,barriers to internal rotation,ab initio calculations and vibrational assignment of fluoroacetone

The infrared (3500-20 cm⁻¹) and Raman (3200-10 cm⁻¹) spectra have been recorded for gaseous and solid fluoroacetone (1-fluoro-2-propanone), CH₂FC(O)CH₃. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the molecule exists predominantly in the cis (fluorine atom oriented cis to the methyl group) conformation in the vapor but for the liquid a second conformer having a trans orientation (fluorine atom oriented trans to the methyl group) is present. From a study of the Raman spectrum of the liquid at variable temperatures the trans conformation has been determined to be more stable than the cis form by 416 ± 54 cm⁻¹ (1.19 ± 0.15 kcal mol⁻¹) and is the only conformation present in the spectrum of the annealed solid. The asymmetric torsional fundamental for the more stable cis conformer has been observed in the far infrared spectrum of the gas at 69.6 cm⁻¹ with six accompanying hot band transitions proceeding to lower frequency. The corresponding mode for the high energy trans conformer is extensively overlapped but is distinguishable at ∼65 cm⁻¹. From these data the asymmetric torsional potential function governing internal rotation about the CC bond has been determined and the potential coefficients are: V₁ = 675 ± 2, V₂ = 991 ± 5, V₃ = 74 ± 1 and V₄ = 54 ± 2 cm⁻¹. The cis to trans and trans to cis barriers are 1332 ± 5 and 731 ± 5 cm⁻¹, respectively, with an enthalpy difference of 601 ± 8 cm⁻¹ (1.72 ± 0.02 kcal mol⁻¹). From ab initio calculations at the 3-21G and 6-31G* basis set levels optimized geometries for both the cis and trans conformers have been obtained and the potential surface governing internal rotation of the asymmetric top determined. The observed vibrational frequencies with their assignments for both the cis and trans conformers are compared to those from the ab initio calculations. All of these results are compared to the corresponding quantities for some similar molecules.     

Vibrational and conformational analysis of n-propylamine by means of i.r. spectroscopy and ab initio MO calculations

The gas-phase i.r. absorption spectra of normal and amino-deuterated n-propylamine were observed. Most of the observed bands were assigned with the help of ab initio MO calculations for the normal frequencies. The ab initio force constants were scaled to fit the observed spectrum by a least squares method. The existence of five rotational isomers is suggested from an analysis of the NH₂ wagging and torsion bands. The gauche-conformers about the CN axis are found to occupy about 70 % of all n-propylamine molecules, and the gauche-conformers about the CC axis are found to be more abundant than the trans-conformers.     

Raman spectra,conformational stability,structural parameters,vibrational assignment,and ab initio calculations for epifluorohydrin

The Raman spectra (3200–100 cm⁻¹) of epifluorohydrin, OCH₂CH(CH₂F), in variable solvents, as well as that of the gas have been recorded and several of the bands due to the two less stable conformers have been identified. The variable solvent studies were inconclusive on the relative conformer stabilities. The conformational energy differences and optimized geometries for all three conformers have been obtained from ab initio calculations with the 3–21G, 4–31G and 6–31G* basis sets. A normal coordinate analysis has also been performed for each conformer with a force field determined from the 3–21G basis set. Assignment of the vibrational fundamentals observed in the Raman spectra of the fluid phases is proposed based on the normal coordinate calculations. In the liquid phase, one of the conformers with a large dipole moment predominates and it appears to be the gauche-I form which is the only one found in the solid. Utilizing the three rotational constants previously reported for each conformer, along with restricted relative distances for several of the structural parameters among the conformers from ab initio calculations, r₀ structural parameters for the heavy atoms have been determined.     

Infrared and Raman spectra,conformational stability,barriers to internal rotation,ab initio calculations and vibrational assignment of ethyl methyl selenide

The Raman spectra (3200–10 cm⁻¹) of ethyl methyl selenide in the gas, liquid and solid phases and the infrared spectra (3200–30 cm⁻¹) of the gas and solid have been recorded. Qualitative depolarization ratios have been obtained for the lines in the Raman spectrum of the liquid. By a variable temperature Raman study of the liquid, it has been determined that the gauche conformer is more stable than the trans rotamer by 158±16 cm⁻¹ (452±46 cal mol⁻¹), and the gauche conformer is the rotamer present in the solid. A complete vibrational assignment for the gauche conformer is presented. All of these data are compared to the corresponding quantities obtained from ab initio Hartree—Fock gradient calculations employing the STO-3G* and 4–31G*/MIDI-4* basis sets. Complete equilibrium geometries have been calculated for both rotamers and the results are discussed and compared with the corresponding quantities for some similar molecules.     

Pressure Effect on Conformational Equilibria of Analog Peptides in Aqueous Solution by Raman Spectroscopy

We investigated the pressure effect on the conformational equilibria of glycinamide (GA) and 2-chloroacetamide (MCA) in aqueous solution by Raman spectroscopy. Scattering intensities in the CH₂ scissoring mode of GA and the NH₂ rocking mode of MCA in aqueous solution were decomposed into two component bands by ab initio MO calculations at the HF/6-31G(d,p) level. From the pressure dependence of the Raman band intensities, we determined the difference in the partial molar volume (PMV) between the cis and trans conformers of each for GA and MCA. The volume changes for the isomerization of the cis to trans conformer are ?(1.9 ± 0.3) and ?(1.5 ± 0.3) cm³-mol^?1 for GA and MCA, respectively. The volume difference between the cis and trans conformers is due to the hydration effect, which seems to be mainly the result of local effects of solute–solvent interactions in both cases. This contribution is due to the influence of the solute–solvent interaction with water molecules on the PMV of the cis conformer being less than that of the trans conformer.     

Infrared and raman spectra,conformational stability,barriers to internal rotation,ab initio calculations and vibrational assignment of difluoroacetyl chloride

The Raman (3100–10 cm⁻¹) and infrared (3100–30 cm⁻¹) spectra of difluoroacetyl chloride, CHF₂CClO, in the gas and solid phases have been recorded. Additionally, the Raman spectrum of the liquid with qualitative depolarization ratios has been obtained. From these data, a trans/gauche equilibrium is proposed in the gas and liquid phases, with the trans conformer (hydrogen atom eclipsing the oxygen atom and trans to the chlorine atom) the more stable form in the gas, but the gauche rotamer is more stable in the liquid and is the only form present in the annealed solid. From the study of the Raman spectrum of the gas at different temperatures, a value of 272 ± 115 cm⁻¹ (778 ± 329 cal mol⁻¹) was determined for ΔH, with the trans conformer the more stable form. Similar studies were carried out on the liquid and a value of 109 ± 9 cm⁻¹ (312 ± 26 cal mol⁻¹) was obtained for ΔH, but now the gauche conformer is the more stable form. A potential function for the conformational interchange has been determined with the following potential constants: V₁ = 397 ± 23, V₂ = −101 ± 5, V₃ = 474 ± 3, V₄ = −50 ± 3, and V₆ = 10 ± 2 cm⁻¹. This potential has the trans rotamer more stable by 179 ± 31 cm⁻¹ (512 ± 89 cal mol⁻¹) than the gauche conformer. A complete vibrational assignment is proposed for both conformers based on infrared band contours, Raman depolarization data, group frequencies and normal coordinate calculations. The experimental conformational stability, barriers to internal rotation, and fundamental vibrational frequencies are compared with those obtained from ab initio Hartree-Fock gradient calculations employing both the RHF/3-21G^* and RHF/6-31G^* basis sets, and to the corresponding quantities obtained for some similar molecules.     

Long-wave Raman spectra of some normal alcohols

Long-wave Raman spectra of some normal alcohols (from n-pentanol to n-decanol) in the liquid phase were registered. The regularities in the dependencies of Raman bands frequencies on the number of carbon atoms in the hydrocarbon chain were deduced. The calculations of Raman spectra of the studied molecules, their equilibrium structures and possible conformers were carried out in the approximation B3LYP/cc-pVDZ. These results in combination with the analysis of literature data allowed to explain the observed regularities in Raman band positions in the spectral range of 200–600 сm⁻¹ and their shifts upon increasing length of the chains. It was found that the plane configurations dominate in the liquid phase for molecules with short- and moderate-chain lengths. The elongation of the chain leads to the decrease of the fraction of plane conformers and in n-decanol the plane structure is completely absent.     

Ethyl acetate: Gas phase infrared spectra,ab initio calculation of structure and vibrational frequencies and assignment

Gas phase IR spectroscopic measurements of ethyl acetate are reported and the vibrational bands are assigned for the trans—trans and trans—gauche conformers. An extensive ab initio calculation has been carried out for both the conformers to obtain molecular structure, barrier heights, vibrational frequencies, force field, IR intensities and multipole moments. Theoretical calculations are of great help for assignment of the free molecule frequencies, which often show deviation from the condensed phase values.     

Assignment and anharmonicity analysis of overtone and combination bands observed in the resonance Raman spectra of carotenoids

The resonance Raman spectra of all-trans carotenoids have been observed in the region of 5000-500 cm⁻¹ for samples in glassy solution at 77 K and in the in vivo state at room temperature. Prominent bands in the wavenumber region higher than 2000 cm⁻¹ are assigned to either overtones or combinations of three modes due to skeletal stretches and the CH₃ in-plane rock. From the wavenumbers of the observed Raman bands, anharmonicity constants for these three modes (including cross-term constants) are obtained. It is found that, for each carotenoid studied, the cross-term anharmonicity constant between the CC and CC stretches is significantly larger than the other anharmonicity constants.     

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.     

Asymmetric torsional potential function and conformational analysis of furfural by far infrared and Raman spectroscopy

The far i.r. (400-50 cm⁻¹) spectra of gaseous and solid furfural (2-furancarboxaldehyde), c-C₄H₃O (CHO), have been recorded. Additionally, the Raman (3500-20 cm⁻¹) spectra of the gas and liquid have been obtained at variable temperatures and the spectrum of the solid at 25 K. These data have been interpreted on the basis that the molecule exists in two different conformations in the fluid states and that the conformation which has the two oxygen atoms oriented in a trans configuration, OO-trans, is most stable (ΔH ⩽ 1 kcal/mol) in the gas; however, the conformation which has the two oxygen atoms oriented cis, OO-cis, is preferred in the liquid (ΔH = 1.07 ± 0.03 kcal/mol) and is the only rotamer present in the spectra of the solid. The asymmetric torsional fundamental for the OO-trans rotamer has been observed at 146.25 cm⁻¹ in the far i.r. spectrum of the vapor and has five accompanying “hot bands”. The corresponding fundamental for the OO-cis rotamer has been observed at 127.86 cm⁻¹ along with a “hot band” which occurs at 127.46 cm⁻¹. From these data a cosine-based potential function governing internal rotation of the CHO top has been determined and the potential coefficients have values of V₁ = 173 ± 2, V₂ = 3112 ± 20, V₃ = 113 ± 2 and V₄ = −198 ± 6 cm⁻¹. This potential is consistent with an enthalpy difference between the more stable OO-trans and high energy OO-cis conformers being 286 ± 24 cm⁻¹ (818 ± 67 cal/mol) and a trans to cis barrier height of 3255 ± 20 cm⁻¹ (9.31 ± 0.06 kcal/mol). These results are compared to the corresponding quantities obtained previously from microwave spectroscopy and theoretical methods.     

Conformational analysis of stereoisomers of 2,4-dimethyl-5-isopropyl-1,3,2-dioxaborinane

The investigation of conformation conversions of cis- and trans-isomers of 2,4-dimethyl-5-isopropyl-1,3,2-dioxaborinane using ab initio quantum-chemical approximation HF/6-31G(d) and the hybrid DFT-method PBE/3z showed that the routes of the transformations presuppose an equilibrium between sofa conformers with the different orientation of substituents at the atoms C⁴ and C⁵ of the ring that convert into each other through transition states corresponding to the equatorial and axial conformations of the 2,5-twist form. Based on experimental data of ¹H NMR spectra and calculated vicinal spin-spin coupling constants the quantitative conformational composition of cis- and trans-isomers was established and also the ΔG^o value of the conformation equilibrium was determined.     

Vibrational analysis of 1,4-diiodobutane

Infrared and Raman spectra were obtained for 1,4-diiodobutane, and normal-coordinate calculations were made using a transferred 48-parameter modified v force field. This compound sometimes crystallizes in the GG' conformation with C₂ symmetry and sometimes in the TG conformation. No evidence was obtained for the presence of the TT (C_2h symmetry) or GG (C_i) conformers, but one or two additional conformers are present that must have a nonplanar chain of carbon atoms.     

Spectra and structure of small ring compounds—LX. Raman and infrared spectra,conformational stability,normal coordinate analysis,and ab initio calculations of cyclobutyl acetylene

The Raman spectra (3400 to 10 cm⁻¹ of gaseous, liquid (with qualitative depolarization values) and solid cyclobutyl acetylene, c-C₄H₇CCH, have been recorded. Additionally, the infrared spectra (3500 to 90 cm⁻¹ of the gas and solid have been obtained. The spectra of the fluid phases are consistent with two stable conformers existing at ambient temperature. These data have been interpreted on the basis that the equatorial conformer is more stable than the high energy axial form in both the gas- and liquid-phases, and is the only conformer present in the solid. Two Q-branches are observed in the low frequency vibrational spectra of the gas at 133 and 118 cm⁻¹ and are assigned to the fundamental ring puckering vibration and an associated upper state transition of the low energy equatorial conformer. These data have been used to approximate the form of the potential function governing ring inversion. Experimental values for the enthalpy difference between the two conformers have been determined for both the gas, 282 ± 49 cm⁻¹, and the liquid, 181 ± 15 cm⁻¹, from relative intensities of a pair of Raman lines over 71 and 100°C temperature ranges, respectively. The structure, conformational stability, inversion barrier and vibrational frequencies have been determined by ab initio calculations using the 3-21G and/or 6-31G* basis sets. These calculated results are discussed in comparison to those determined from experiment and to corresponding quantities for some similar molecules.     

Infrared,Raman and NMR spectra,conformational stability,and ab initio calculations of divinylmethoxyborane

The infrared spectra (4000–50 cm⁻¹) of gaseous and solid divinylmethoxyborane, (CH₂=CH)₂BOCH₃, as well as the Raman spectra (3500–20 cm⁻¹) of the liquid and solid have been recorded. Qualitative depolarization values have been obtained from the Raman spectrum of the liquid. All normal modes, except the torsions, have been assigned based on infrared band contours, depolarization values, group frequencies, and normal coordinate calculations. From a comparison of the spectra in the fluid and solid states, it is concluded that the molecule exists predominantly in a single conformation in all physical states. Frequencies and potential energy distributions for the normal modes have been calculated with the 3–21G basis set. A comparison of these calculated frequencies to the observed spectra is consistent with the predominant form having a “planar” heavy atom skeleton with C_s, symmetry. From the variable low temperature ¹³C NMR data, a barrier to rotation about the B-O bond of 10.1 ± 0.1 kcal mol⁻¹ has been determined, which is in excellent agreement with a barrier of 8.5 kcal mol"1 obtained from ab initio calculations. Structural parameters, conformational stability, and barriers to internal rotation have been obtained from ab initio Hartree-Fock gradient calculations employing both the 3–21G and 6–31G^* basis sets. The results are compared to the corresponding data for some similar organoboranes.     

Spectra and structure of organophosphorus compounds—XXX. Vibrational and conformational study of methoxy difluorophosphinoxide-d0 and -d3

The i.r. (4000-40 cm⁻¹) and Raman (4000-10 cm⁻¹) spectra of gaseous, liquid and solid methoxy difluorophosphinoxide, CH₃OP(O)F₂, and the deuterated analog have been recorded. Results obtained from variable solvent and matrix isolation studies are consistent with the existence of both trans (CO bond trans to the PO bond) and gauche (dihedral angle approximately 120° from the trans form) conformers in the fluid phases. From simulations of observed gas phase i.r. band profiles, it was possible for assignments to be made to the individual conformers for a number of the fundamentals. Variable temperature studies carried out for the gaseous and liquid phases give energy differences between the gauche and trans conformers of 451 ± 100 cm⁻¹ (1.29 ± 0.3 kcal/mol) and 69 ± 20 cm⁻¹ (197 ± 57 cal/mol), respectively. Furthermore, these data are consistent with the gauche form being the thermodynamically preferred conformer for the gas phase whereas the trans conformer is preferred in the liquid phase and the only conformer present in the annealed solid. The methoxy torsional mode of the gauche conformer has been assigned to a very strong band observed in the far i.r. spectrum of the gas phase at 42 cm⁻¹. The matrix isolation spectra of the normal compound in Ar, CO and N₂ matrices indicated no changes in the conformational equilibrium among these different matrices and this equilibrium remains unchanged upon annealing the matrices.     

Spectra and structure of phosphorus—boron compounds—XXVI. Infrared and Raman spectra,conformational stability and normal coordinate analysis of ethyldichlorophosphine-borane

The Raman (3500-10 cm⁻¹) and infrared (3500-50 cm⁻¹) spectra of solid ethyldichlorophosphine-borane, CH₃CH₂P(BH₃)Cl₂ and its deuterated analog, CH₃CH₂P(BD₃)Cl₂ have been recorded. Additionally, the infrared spectra of the gases and the Raman spectra of the liquids have been recorded and qualitative depolarization ratios have been obtained. Based on the fact that several distinct Raman lines disappear on going from the liquid to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers, with the trans conformer being more stable in the liquid phase, and the only one present in the solid phase. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the gauche and trans conformers was determined to be nearly zero. Based on Raman depolarization data, group frequencies, isotopic shift factors and infrared band contours, a complete vibrational assignment has been proposed for the trans conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The BH₃ torsion has been observed at 188 cm⁻¹, while the BD₃ torsion was not observed. The methyl torsions in the spectra of the solids have been observed at 209 and 202 cm⁻¹ for the “light” and deuterated species, respectively. From the torsional data, barriers to internal rotation have been calculated. The asymmetric torsional mode has been observed for the trans conformer in the infrared spectra of the gas phase at 108 and 104 cm⁻¹ for the BH₃ and BD₃ species, respectively. These results are compared with similar quantities for some corresponding organophosphine—borane compounds.     

Conformational and vibrational analysis of butyronitrile

IR and Raman spectra have been obtained for butyronitrile and are interpreted with the aid of normal coordinate calculations. This compound exists as trans and gauche conformers in the liquid and amorphous solid states, but only the gauche conformer is present in the crystalline solid. Simultaneous calculations for propionitrile and the two conformers of butyronitrile result in an average error of 4.7 cm⁻¹, or 0.42%. Vibrational assignments are made for all three molecules in terms of ordinary symmetry coordinates.     

Spectra and structure of organophosphorus compounds—XXXVII. Infrared and Raman spectra,conformational stability,vibrational assignment and normal coordinate analysis of ethyldichlorophosphine-d0 and -d5

The infrared (3500 to 40 cm⁻¹) and Raman (3500 to 10 cm⁻¹) spectra have been recorded for the gaseous and solid phases of ethyldichlorophosphine, CH₃CH₂PCl₂, and CD₃CD₂PCl₂. Additionally, the Raman spectra of the liquids were recorded and qualitative depolarization values were obtained. In the spectrum of the gas the gauche conformer is predominant with about 65% abundance whereas in the spectrum of the liquid at ambient temperature the amount of gauche conformer is reduced compared to the gas phase and at −100°C the trans conformer predominates. The trans conformer is the more stable species in the solid. A variable temperature study was carried out on the Raman spectrum of the liquid and ΔH and ΔS values of 190 ± 30 cm⁻¹ (543 ± 87 cal/mol) and 2.86 ± 0.3 eu were determined, respectively, with the trans conformer being more stable. Similar variable temperature studies have been carried out on a number of conformer peaks in the infrared spectrum of the gas and a ΔH value of 53 ± 38 cm⁻¹ (152 ± 110 cal/mol) was obtained, again with the trans conformer being more stable. All the fundamental modes of both conformers have been assigned utilizing band contours, depolarization values, isotopic shift factors and group frequencies. A normal coordinate calculation has been carried out utilizing a modified valence force field to calculate the frequencies and potential energy distribution for both conformers. The barriers to methyl rotation of the trans and gauche conformers are 2.2 ± 0.1 and 2.3 ± 0.1 kcal/mol, respectively. These results are compared to similar quantities for some corresponding molecules.