Experimental and ab initio theoretical vibrational Raman optical activity of alanine

The vibrational Raman optical activity (ROA) spectra of l-alanine in water, 1 N NaOH and 1 N HCl between 720 and 1500 cm⁻¹ measured in backscattering are reported. Unlike the associated vibrational circular dichroism (VCD), the main ROA features are relatively insensitive to pH changes. Ab initio Raman and ROA intensities were evaluated using 6-31G and 6-31G* basis sets and found to agree remarkably well with the experimental parameters in the lower-frequency region.     

Theoretical study on the structures,force field,and vibrational spectra of cyclooctatetraene and cyclooctatetraene-d8

The structures and force field of 1,3,5,7-cyclooctatetraene (COT) have been studied using ab initio theory at the SCF level with the 4-21G basis set. The quadratic force field of the D_2d structure obtained by systematic scaling of the ab initio force constants successfully reproduces the observed frequencies of COT and COT-d₈ with a mean deviation of less than 10 cm⁻¹ for non-CH stretching modes. On the basis of the calculated results, assignments of the fundamental vibrations are examined. The normal mode υ₅ is reassigned to a weak band at 758 cm⁻¹ in the Raman spectrum of COT and to a weak band at 591 cm⁻¹ in the Raman spectrum of COT-d₈. The calculations favor the assignment of υ₂₆ given by Lippincott et al. [J. Am. Chem. Soc. 73, 3370 (1951)] over the revised assignment of Perec [Spectrochim. Acta 47A, 799 (1991)]. The calculations also furnish reliable prediction for the inactive A₂ fundamentals of COT and COT-d₈. The fundamental frequencies and IR and Raman intensities of ¹³CC₇H₈, which constitutes about 9% of COT in natural abundance, are also calculated. Only ν₁₀ (calculated at 908 cm⁻¹) of the formal inactive A₂ modes has appreciable Raman intensity (0.23 Å⁴/amu). A spectral feature due to this fundametal is identified in the liquid Raman spectrum of Tabacik and Blaise [C. R. Acad. Sci. Ser. II 303, 539 (1986)] as a weak peak at 908 cm⁻¹.     

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.     

Structure,vibrational assignment,normal coordinate analysis,and ab initio calculations of methylisocyanate

The infrared (3200-30 cm⁻¹) and Raman (3200-10 cm⁻¹) spectra of gaseous and solid methylisocyanate, CH3NCO, have been recorded. Additionally, the Raman spectrum of the liquid has been obtained and qualitative depolarization ratios have been measured. The CNC bend has been observed in the far infrared and low frequency Raman spectra of the gas at approximately 172 cm⁻¹. An additional far infrared band at ≈50 cm⁻¹ has tentatively been assigned as the methyl torsional mode, although it could be due to the Δν = 1, Δl = ± 1 transitions of the CNC bending mode. A complete assignment of the vibrational fundamentals is proposed. The structural parameters, force constants, and vibrational frequencies have been determined from ab initio Hartree—Fock gradient calculations using the 6-31G* basis set. Additionally, structural parameters have been obtained with the 6-311 + + G** basis set with electron correlation at the MP2 level which are compared to those obtained from the microwave data and electron diffraction study. These results are compared with the corresponding quantities obtained for similar molecules.     

Raman Optical Activity of Glucose and Sorbose in Extended Wavenumber Range

Raman optical activity (ROA) is pursued as a promising method for structural analyses of sugars in aqueous solutions. In the present study, experimental Raman and ROA spectra of glucose and sorbose obtained in an extended range (50–4000 cm⁻¹) are interpreted using molecular dynamics and density functional theory, with the emphasis on CH stretching modes. A reasonable theoretical basis for spectral interpretation was obtained already at the harmonic level. Anharmonic corrections led to minor shifts of band positions (up to 25 cm⁻¹) below 2000 cm⁻¹, while the CH stretching bands shifted more, by ∼180 cm⁻¹, and better reproduced the experiment. However, the anharmonicities could be included on a relatively low approximation level only, and they did not always improve the harmonic band shapes. The dependence on the structure and conformation shows that the CH stretching ROA spectral pattern is a sensitive marker useful in saccharide structure studies.     

Conformational stability and barriers to internal rotation of methacrolein (CHO and CDO) from far infrared spectral data,ab initio calculations and the microwave spectrum of methacrolein-d1

The far i.r. spectra of gaseous methacrolein (2-methylpropenal), CH₂C(CH₃)CHO and methacrolein-d₁ (2-methylpropenal-1-d₁) have been recorded in the region 350-50 cm⁻¹ at a resolution of 0.10 cm⁻¹. The fundamental asymmetric torsions of the d₀ and d₁ compounds for the more stable s-trans conformer have been observed at 169.82 and 158.83 cm⁻¹, respectively, with each band having at least three additional “hot bands” associated with it. The corresponding fundamentals for the s-cis conformers have been observed at 163.74 and 151.26 cm⁻¹ for the d₀ and d₁ compounds, respectively, with one well defined “hot band” in each case. From these data the asymmetric torsional potential coefficients have been determined to be: V₁ = 1148 ± 27; V₂ = 3421 ± 232; V₃ = −89 ± 15; and V₄ = −127 ± 36 cm⁻¹. The s-trans to s-cis barrier was calculated to be 3950 ± 42 cm⁻¹ with the s-trans being more stable than the s-cis conformer by 1057 ± 42 cm⁻¹ (3.02 ± 0.12 kcal/mol). The barrier to internal rotation of the methyl group for the s-trans conformer is 444 ± 3 cm⁻¹ (1.27 ± 0.01 kcal/mol) whereas the corresponding barrier for the s-cis conformer is 441 ± 2 cm⁻¹ (1.26 ± 0.01 kcal/mol). The fact that both the methyl and asymmetric torsion shift with the 1-d₁ substitution indicates that these two tops are kinetically coupled. The presence of the second conformer was confirmed by a study of the i.r. (3500-50 cm⁻¹) and Raman (3200-10 cm⁻¹) spectra of gaseous and solid methacrolein. From these data, a reassignment of some of the fundamentals was necessary. The microwave spectrum of methacrolein-d₁ was recorded from 19.0 to 39.0 GHz and the a-type R-branches assigned. Utilizing the rotational constants for the d₀ and d₁ molecules, some structural information has been obtained for the heavy atom parameters. These data are compared to the corresponding quantities from ab initio calculations at the 6-31G* level. All of these results for methacrolein are compared to the corresponding quantities of acrolein.     

Analysis of torsional spectra of molecules with two internal C3ν, rotors—XXVI. A far infrared study of 1,1-dimethylhydrazine

The infrared spectra of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, and two isotopomers, (CD₃)₂NNH₂ and (CH₃)₂NND₂, have been recorded in the region between 600 and 100 cm⁻¹. Very rich and complex spectra were obtained and analysis of the data has been carried out. The interpretation of the spectra arising from the two methyl torsional modes of the −d₀ compound was carried out using a semi-rigid model, and the resulting potential function obtained is V₃₀ = 1685 ± 12 cm⁻¹ (4.82 ± 0.04 kcal mol⁻¹); V₀₃ = 1827 ± 16 cm⁻¹ (5.22 ± 0.05 kcal mol⁻¹); V₆₀ = −92±5cm⁻¹ (−0.26 ± 0.02 kcal mol⁻¹); V₀₆ = −41 ± 6cm⁻¹ (−0.12 ± 0.02 kcal mol⁻¹) and V^′₃₃ = −51 ± 5 cm⁻¹ (−0.15 ± 0.01 kcal mol⁻¹). Ab initio gradient calculations were carried out employing the 3–21G and 6–31G* basis sets, as well as the 6–31G* basis set with electron correlation at the MP2 level. The structural parameters, conformational stability, and three-fold barriers to internal rotation have been determined and the gauche conformer is calculated to be more stable than the trans form by 783 cm⁻¹ (2.24 kcal mol⁻¹) with the MP2/6–31G* basis set. These calculations were also used to re-evaluate the previously reported assignment of the fundamental modes, and to obtain a potential function for the asymmetric torsion. All of these results are discussed and compared with corresponding quantities for some similar compounds.     

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.     

Vibrational spectra of tetramethyleyclobutane-1-one-3-thione and the fully deuterated derivative

Raman and i.r. spectra of tetramethylcyclobutane-1-one-3-thione (TMCBOT) and the fully deuterated derivative TMCBOT-_d¹² have been recorded. A fairly complete set of vibrational frequencies and assignments are given for the two molecules. The CO stretching mode was observed as a very strong Fermi doublet in the infrared spectrum of TMCBOT at 1811/1782 cm⁻¹. For TMCBOT-d₁₂ a similar doublet was observed at 1808/1775 cm⁻¹. The CS stretching mode was assigned to bands at 1303 cm⁻¹ for TMCBOT and 1307 cm⁻¹ for the deuterated molecule.     

Conformation-sensitive Raman lines of mononucleotides and their use in a structure analysis of polynucleotides: guanine and cytosine nucleotides

Raman spectra are presented for nine crystals containing the guanosine residue and ten crystals containing the cytidine residue whose conformations are known from their X-ray crystallographic analyses. A nearly complete set of assignments of all the observed Raman lines in the 1700—150 cm⁻¹ range is proposed on the basis of a previous normal coordinate treatment of guanine and cytosine with a set of force constants determined by an ab initio MO method, and on the basis of a mutual comparison of the observed spectra. A number of conformation sensitive Raman lines are found here, and several rules on the structure—spectrum correlations are proposed. Raman spectral features in the 1400—1300 cm⁻¹ and 700—600 cm⁻¹ ranges seem to reflect sensitively and regularly the conformation of the guanosine residue, namely its ribose-ring puckering state at the torsion angle around its glycosidic bond. A spectral feature in the 1300—1200 cm⁻¹ range is found to be sensitive to the cytidine conformation. The position of a strong Raman line in the 900—750 cm⁻¹ region, on the other hand, seems to indicate a particular set of torsion angles along the PO5′C5′C4′C3′O3′ backbone. In the light of these proposed rules, the so-called B-form poly [d(GC)].· poly[d(GC)] in solution must have an O4′endo-anti guanosine, a C2′endo-anti cytidine, and an “alternating B” backbone as proposed by Klug [7] while its Z-form should have a C3′ endo-syn guanosine, a form of cytidine in between C2′endoC1′exo-anti cytidine, and a Z_I form backbone, as defined by Wang [41].     

A soleil-babinet compensator based Raman optical activity spectrometer: interpretation of roa couplets in (+)-(3R)-methylcyclohexanone

The Raman optical activity (ROA) spectrum of (+)-(3R)-methylcyclohexanone from 150 to 1400 cm⁻¹ measured using a novel ROA spectrometer is reported. The spectrometer features a Soleil-Babinet compensator to produce high-quality circularly polarized laser radiation. The principal ROA couplets are assigned and interpreted in terms of pairs of coupled vibrational motions.     

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.     

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.     

Vibrational circular dichroism,absolute configuration and predominant conformations of volatile anesthetics: 1,2,2,2-tetrafluoroethyl methyl ether

Vibrational absorption and circular dichroism spectra of (−)-1,2,2,2-tetrafluoroethyl methyl ether have been measured in CCl₄ solution in the 2000–900 cm⁻¹ region. These spectra are compared with the ab initio predictions of absorption and VCD spectra obtained with density functional theory using B3LYP/6-31G* and B3PW91/6-311G(2d) basis sets for different conformers of (R)-1,2,2,2-tetrafluoroethyl methyl ether. The results suggest that the trans-conformer of 1,2,2,2-tetrafluoroethyl methyl ether is predominant in the solution phase and that the (−)-enantiomer has the R-configuration.     

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.     

A full vibrational assignment (4000-50 cm−1) of 1, 10-phenanthroline and its perdeuterated analogue

Full infrared and Raman vibrational assignments (4000-50 cm⁻¹) of 1,10-phenanthroline are presented and full infrared assignments (4000-50 cm⁻¹) of 1,10-phenanthroline-d₈ are given. Four fundamentals are newly reported for 1,10-phenanthroline and several changes are recommended to earlier assignments for this compound. The assignments of the fully deuterated analogue are newly reported.     

Spectra and structure of silicon compounds. XVII Raman and infrared spectra and vibrational assignment for hexamethyldisilane and ab initio calculations for disilane and hexamethyldisilane

The IR spectra of gaseous and solid hexamethyldisilane between 4000 and 25 cm⁻¹ and the far-IR spectrum of the liquids from 450 to 25 cm⁻¹ have been recorded. The Raman spectra have been recorded from 3500 to 10 cm⁻¹ for all three physical phases. Assisted by ab initio calculations, the vibrational spectrum of hexamethyldisilane has been assigned under D_3d symmetry and the results of a normal coordinate analysis are discussed. No spectral features indicative of free internal rotation have been observed. Gradient ab initio calculations have been carried out for the disilane and hexamethyldisilane molecules using different types of basis sets. The structural parameters, rotational constants, unscaled and scaled frequencies and harmonic force constants have been reported for both disilane and hexamethyldisilane.     

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.     

Density functional theory and Ab initio studies of vibrational spectroscopic (FT-IR,FT-Raman and UV) first order hyperpolarizabilities,NBO, HOMO–LUMO and TD-DFT analysis of the 1,2-Dihydropyrazolo (4,3-E) Pyrimidin-4-one

Fourier transform Raman and Fourier transform infrared spectra of 1,2-Dihydropyrazolo (4,3-E) Pyrimidin-4-one were recorded in the regions 3500–100 cm⁻¹ and 4000–400 cm⁻¹ respectively in the solid phase. 1,2-Dihydropyrazolo (4, 3-E) Pyrimidin-4-one is used to treat hyperuricemia and its complication including chronic gout. The equilibrium geometry harmonic vibrational frequencies, infrared intensities and Raman intensities were calculated by Hartee Fock and density functional B3LYP methods with 6-31G (d, p) basis set, using Gaussian 03W program package on a Pentium IV/1.6 GHz personal computer. The thermodynamic functions of the title compound were also performed at the above methods and basis set. A detailed interpretation of the infrared and Raman spectra of 1,2-Dihydropyrazolo (4,3-E) Pyrimidin-4-one is reported. Stability of the molecule arising from hyper conjugative interactions, charge delocalization has been analyzed using natural bond orbital (NBO) analysis. UV–vis of the compound was recorded. The calculated HOMO and LUMO energies show that chemical activity of the molecule. The first order hyperpolarizability (β) of this novel molecular system and related properties of 1,2-Dihydropyrazolo (4,3-E) Pyrimidin-4-one are calculated using HF/6-31G (d, p) method on the finite field approach. The experimental spectra also coincide satisfactorily with those of theoretically constructed spectra.