Interpretation of the vibrational spectra of methylglyoxal and biacetyl in their first singlet excited electronic states

Laser-induced fluorescence spectra for the first allowed electronic transition (22125 cm^?1) of methylglyoxal (CH₃COCHO) and its perdeutero analog (CD₃COCDO) in a supersonic nozzle beam are quantitatively represented assuming that the potential function governing the CH₃(CD₃) rotation is changed during the transition. In the excited state the potential function is ternary (V₁ = 95 (1 + cos 3θ)cm^?1) as in the fundamental state (V₀ = 134.5 (1 - cos 3θ)cm^?1), but the minima are shifted by an angle of π/3. The spectrum of biacetyl (CH₃COCH₃CO) can be reproduced assuming two uncoupled methyl groups undergoing similar conformational changes during the electronic transition (the estimated potential function is V₁ = 117.5 (1 + cos 3θ) cm^?1 for each methyl group), in perfect agreement with the most recent assignment of the 0-0 transition. These results are consistent with ab initio calculations for the fundamental and first excited singlet states.     

Infrared and Raman spectra and normal coordinate calculations for methylisothiocyanate

The infrared spectra (3200-50 cm^?1) of gaseous and solid CH₃NCS and CD₃NCS and the Raman spectra (3200-10 cm^?1) of the liquids and solids have been recorded. The spectra have been interpreted on the basis of a “pseudo-symmetric top” with C_3v symmetry. An assignment of the fundamental vibrations in both molecules, based on their infrared band contours, depolarization values and group frequencies, is given and discussed. Particularly interesting is the low-frequency region where band maxima were observed at 152 and 80 cm^?1 for CH₃NCS and 139 and 71 cm^?1 for CD₃NCS in the infrared spectra of the gases. A normal coordinate analysis has also been carried out based on C_3v symmetry. Considerable mixing was found between the C_αN stretch and NCS symmetric stretch in both isotopic species. The other normal modes in CH₃NCS are reasonably pure but, for the CD₃NCS molecule, considerable mixing was found between the CD₃ stretches and NCS antisymmetric stretch. The proposed vibrational assignment and the results of the normal coordinate calculations are discussed and compared with the results obtained for similar molecules.     

Infrared spectra of B(OMe)3, ClB(OMe)2 and Cl2BOMe species,isolated CH stretching frequencies and bond strengths

Infrared spectra in the gas phase are reported over the range 3100-500 cm⁻¹ for species of B(OMe)₃, ClB(OMe)₂ and Cl₂BOMe, with CH₃, CD₃ and CHD₂ substitution. A detailed analysis of νCH and νCD data in all three species of Cl₂BOMe yields strong evidence for the presence of three kinds of CH bond, two of them weak and one of them strong. The methyl group is then twisted, probably through 10–20°, out of the eclipsed or staggered conformation. The CHD₂ spectra of the di and trimethoxy compounds are less susceptible to analysis, but suggest also the presence of two weak and strong bonds, the former increasing in weakness as the number of methoxy groups increases. This is as expected from the increased competition likely between the lone pair electrons for the empty boron orbital. The spectra of the CD₃ species permit a clear assignment of νBO, δ_sCH₃, δ_sCD₃ and δ_asCD₃ modes. In Cl(COCH₃)₂, ν_sBO lies at 1278 cm⁻¹.     

Reaction of photogenerated fluorine atoms with dopant molecules in solid argon

The products of reactions of dopant CH₄ molecules with F atoms diffusing in solid argon at 20–30 K were identified by ESR and FTIR spectroscopy. The F atoms stabilized in the matrix were generated by UV photolysis of Ar?CH₄(CD₄)?F (1000∶1∶1) samples at 13 K. Subsequent heating above 20 K results in thawing off diffusion of the F atoms and formation of products of their reaction with CH₄: radical-molecular complexes^·CH₃?HF (^·CD₃?DF) and radicals^·CH₃ (^·CD₃). The ESR spectra of the radicala are similar to those observed for matrix-isolated^·CH₃. The^·CH₃?HF complexes are characterized by the IR band of HF stetching vibration at 3764 cm^?1. Two additional splittings on the H (a _H·=2 G) and F(a _F=16G) nuclei of the HF molecule appeal in the ESR spectrum of the complex. The latter splitting is retained in the^·CD₃?DF complex, whereA _D· <0.3G The rate constant of the reaction CH₄+F→^·CH₃+HF is equal to ?10^?25 cm³s^?1 at 20 K. Its activation energy (1.7±0.2 kcal mol^?1) is ?0.5 kcal mol^?1 greater than that in the gas phase. The collinear C_3v-configuration of the^·CH₃?HF complex, which is similar to the configuration of the reagents in the transition state of the reaction considered, was established by the comparison of the exprrimental constants of hyperfine coupling with the results of the quantum-chemical calculation.     

Reactions of photogenerated fluorine atoms with dopant molecules in solid argon

The products of UV photolysis of ternary Ar?CH₄(CD₄)?F₂ mixtures (1:c:c ₀,c, c ₀=0.001–0.01) at 13–16 K were identified by ESR and FTIR spectroscopy. These products are^?CH₃ (^?CD₃) radicals of typesI andII and molecular CH₃F?HF complexes. The latter were characterized by the IR bands of the stretching C?F (1003 cm^?1) and H?F (3774 cm^?1) vibrations. The ESR spectra of radicalsI are asymmetric. The anisotropy of theg-factor (Δg～10^?3) of radicalI indicates that the structure of the radicals is nonplanar. The ESR spectrum of the typeII radical is identical to that of matrix-isolated^?CH₃ (^?CD₃) radicals with the planar structure (Δg<5·10^?5). Under the experimental conditions, the amount of complexes formed in the photolysis is equal to 0.022·c. When the photolysis is ceased, radicalI disappears after ≈10³ s and radicalII is stabilized. The limiting concentrations of the stabilized^?CH₃ and^?CD₃ radicals are equal to 2·10^?2·c and 2·10^?3·c, respectively. A mechanism of the formation of the products is suggested. It is based on the assumption that both matrix-isolated CH₄ and F₂ and their heterodimers CH₄?F₂ are present in the samples and it takes into account the long-range migration of translationally excited flourine atoms. The CH₃F?HF complexes and radicalsI are generated by the photolysis of the CH₄?F₂ heterodimers. The decay of radicalsI is caused by geminate recombination of proximate F...CH₃ pairs. RadicalsII are formed in the reaction of translationally excited fluorine atoms with isolated CH₄ (CD₄) molecules.     

Vibrational analysis of l-alanine and deuterated analogs

Raman spectra of the polycrystalline l-alanine analogs CH₃CH(NH⁺₃)COO^?, CH₃CH(ND⁺₃)-COO^?, CD₃CD(NH⁺₃)COO^?, and CD₃CD(ND⁺₃)COO^? have been obtained. A normal coordinate analysis is carried out based on the experimental frequencies of the four isotopic analogs and a 34 parameter valence-type force field defined in terms of local symmetry coordinates. The final refinement, in which five stretching force constants are constrained to fixed values obtained from bond length data, results in an average error of 7 cm^?1 (0.9%) for the observed frequencies of the four isotopically substituted molecules. Band assignments are given in terms of the potential energy distribution for local symmetry coordinates. For non-deuterated l-alanine, the vibrations above 1420 cm^?1 and below 950 cm^?1 may be described as localized group vibrations. By contrast, the eight modes in the middle frequency range, viz. the three skeletal stretching, the COO^? symmetric stretching, one NH⁺₃ rocking, the symmetric CH₃ deformation, and the two methyne CH deformation vibrations, are very strongly coupled to one another. Some decoupling appears to take place in the perdeutero molecule, and all but five modes can be described as localized group vibrations.     

Matrix and gas infrared spectra of nitroethane isotopic species

IR and FIR gas phase and Ar matrix isolation spectra of the isotopic species of nitroethane, CH₃CH₂NO₂, CH₃CD₂NO₂ and CD₃CD₂NO₂ are reported. An assignment will be proposed, based mainly on matrix spectra and normal coordinate analysis. An approximate 25 parameter valence force field is given. The analysis of the gas phase spectra is complicated by the fact that only very few band contours comparable to quasirigid model envelopes occur. The phenomenon and unusual band widths and splittings in the matrix spectra are related to the nearly free internal rotation of the nitro group. The Q branch sequence associated with the methyl torsion shows a remarkable splitting into doublets probably also related to the NO₂ rotation. A value of 1080 ± 20 cm^?1 for the methyl torsional barrier is derived.     

Raman and Infrared Spectra, Ab Initio Calculations, Normal Coordinate Analysis, and Conformational Stability of 2-Methoxypropene

The Raman (3500–40 cm^–1) and infrared (3500–70 cm^–1) spectra of gaseous and solid 2-methoxypropene, CH₃O(CH₃)C=CH₂, and the isotopomers, CD₃O(CH₃)C=CH₂ and CH₃O(CD₃)C=CD₂ have been recorded. In addition, the Raman spectra of the liquids have been recorded with qualitative depolarization measurements. All of these data indicate that only one conformer is present in the fluid phases at ambient temperature and this form is the cis conformer, which remains in the solid. Assignments are provided for the fundamentals of all three isotopomers for the cis conformer with C_s symmetry. The far-infrared spectra of all three isotopic species have been recorded at a resolution of 0.1 cm^–1 in the gas and 1.0 cm^–1 in the solid. The parameters of the potential function governing the asymmetric torsion are determined to be V₃ = 1485 ± 9 cm^–1 and V₆ = –55 ± 4 cm^–1 for the d₀ compound, where only two terms were determined, since a second conformer was not evident. The barrier to internal rotation for the methyl group attached to the oxygen atom is 1370 ± 8 cm^–1 and the C—CH₃ barrier is 772 ± 5 cm^–1. Ab initio calculations with full electron correlation have been carried out by the perturbation method to second order to obtain the equilibrium structural parameters, harmonic force constants, fundamental frequencies, infrared intensities, Raman activities, depolarization values, and conformational stability. The predicted values have been compared to the experimental values where appropriate.     

Spectra and structure of organophosphorus compounds: XVII. Infrared and Raman spectra. vibrational assignment and barriers to internal rotation for t-butylphosphine

The infrared spectra of gaseous and solid tertiary-butylphosphine, [(CH₃)₃CPH₂], have been recorded from 50 cm^?1 to 3500 cm^?1. The Raman spectra of gaseous, liquid and solid (CH₃)₃CPH₂ have been recorded from 10 to 3500 cm^?1. A vibrational assignment of the 42 normal modes has been made. A harmonic approximation of the methyl torsional barrier from observed transitions in the solid state gave a result of 4.22 kcal mol^?1 and 3.81 kcal mol^?1 in the gaseous state. Hot band transitions for the phosphino torsional mode have been observed. The potential function for internal rotation about the C-P bond has been calculated. The two potential constants were determined to be: V₃ = 2.79 ± 0.01 kcal mol^?1 and V₆ = 0.07 ± 0.01 kcal mol^?1.     

Spectres de vibration et conformations du propionate et de l'isobutyrate de méthyle

Infrared and Raman spectra (3600–3620cm^?1) of methyl propionate CH₃CH₂-COOCH₃, CH₃CH₂COOCD₃ and methyl isobutyrate (CH₃)₂CHCOOCH₃, (CH₃)₂CHCOOCD₃, in liquid and crystalline states, have been recorded. Rotational isomerism, by rotation around the C-C bond α to the carbonyl group, is detected and the energy difference between the conformers is 1.1 ±0.3 kcal mol^?1 for methyl propionate and 0.5 ±0.1 kcal mol^?1 for methyl isobutyrate. Vibrational assignments in terms of group frequencies are proposed for each conformer, only the more stable being present in the crystal.     

Polarized Raman spectra,thermodynamic functions and bidders to internal rotation of 2,3-dimethoxy toluene

Polarized Raman spectra of 2,3-dimethoxy toluene have been recorded in the region 50–4000 cm⁻¹ and IR spectra in the region 200–4000 cm⁻¹. All the 63 (40a′ + 23a″) normal modes of vibration have been assigned assuming a C_s point group. Consistent assignments for the internal modes of vibration of methyl (CH₃) and methoxy (OCH₃) groups have been proposed. In addition thermodynamic functions have been computed over the temperature range 100–1500 K on a MIGHTY II computer and barriers to internal rotations for the three methyl (CH₃) tops and the two methoxy (OCH₃) tops about their respective axes have been determined, using the assigned torsional frequencies and assumed structural parameter for the 2,3-dimethoxy toluene. The barrier heights have been found to be greater than 2.5 kcal mol⁻¹ for all five tops.     

IR-photodissociation of size selected molecular clusters and their structures

Infrared photodissociation spectra of (CH₃NH₂) _n clusters were measured fromn=2 ton=6 near the monomer absorption of the C-N stretching mode at 1044 cm^?1 using a cw-CO₂ laser. The clusters were size-selected by scattering from a helium beam. The spectrum of cold dimers shows a red (1038 cm^?1) and a blue (1048 cm^?1) shifted peak which is attributed to the non-equivalent position of the C-N in the open dimer structure. The larger clusters exhibit only one peak between 1045.4 cm^?1 and 1046.0 cm^?1 caused by the equivalent position of the C-N in the cyclic structures of the larger clusters. Structure calculations confirm these results. Secondly, the mixed complexes C₂H₄-CH₃COCH₃ and C₂H₄-(CH₃COCH₃)₂ were investigated. The dimer spectrum, measured around the monomer frequency of the out-of-plane bending mode of C₂H₄ at 949 cm^?1, shows two peaks at 946.2 cm^?1 and 961.3 cm^?1. This splitting is attributed to two different isomers that are found in configuration calculations. A similar behaviour is found for the trimer.     

Vibrational spectra and normal coordinate calculations for (CH3)3GECN and (CD3)3GECN

Infrared spectra of (CH₃)₃GeCN and (CD₃)₃GeCN have been obtained over the range of 4000–100 cm^?1 from solutions of benzene and dichloromethane, and solid films deposited on windows at liquid nitrogen temperature. Raman spectra have been photographed in dichloromethane solution. Assignments for all fundamentals except the internal torsions have been made, and normal coordinate calculations carried out by a symmetry force field to confirm the proposed assignments.     

Metastable ion study of organosilicon compounds part II—hexamethyldisiloxane

Unimolecular reactions of the metastable silicenium ion (CH₃)₃SiOSi(CH₃)₂ ⁺ generated by dissociative ionization of bexamethyldisiloxane were investigated by mass-analysed ion kinetic energy (PUKE) Spectrometry. The characteristic fragmentations observed were losses of CH₄ and (CH₃)₂Si?O molecules. Complete scrambling of all methyl groups prior to these reactions was found by investigating the MIKE spectra of deuterium labelled analogues (CD₃)₃SiOSi(CH₃)₂⁺ and (CH₃)₃SiOSi(CD₃)₃⁺. The loss of methane was accompanied by a large kinetic energy release (T_0.5 = 482 meV). The MIKE spectra of silicenium ions were compared with those of their carbon analogues. The most predominant reaction of metastable (CH₃)₃COC(CH₃)₃⁺ ion was the loss of CH₂?C(C H₃)₂ leading to protonated acetone. Significant differences between the ion fragmention characteristics of silicon and carbon compounds were found.     

CN stretching and NO2 deformation vibrations and normal coordinate analysis of m-dinitrobenzene and m-dinitrobenzene-d4

Two bands appear for each CN stretching and nitro deformation vibration in the infrared and Raman spectra of m-dinitrobenzene and m-dinitrobenzene-d₄. The 907 cm^?1 bending mode in the vibrational spectra of m-dinitrobenzene undergo 30 cm^?1 upward shift upon d₄ substitution. A normal coordinate analysis pointed out that the 937 cm^?1 bending and 727 cm^?1 CN stretching vibrations as well as 18b CD in-plane deformation are mixed to a great extent. The other nitro bending mode undergo also an inverse isotopic effect (2 cm^?1 upward shift) due to coupling with the 18a CD in-plane deformation vibration.     

Vibrational spectra of ethyl iodedes

The infrared spectra of CH₃CH₂I, CD₃CH₂I, and CH₃CD₂I of the vapors and the solids at 170°C have been recorded from 4000-200 cm^?1. The Raman spectra of the liquids and vapors have also been recorded and depolarization values have been measured. Assignment of the eighteen fundamental vibrations has been based on depolarization values, band contours, group-frequency correlations, and normal coordinate calculations. A critical discussion of the CH stretching assignments in CH₃CH₂X molecules is presented.     

Perturbations of molecular extravalence excitations by rare-gas fluids

In this paper we report the results of an experimental study of the vacuum ultraviolet absorption spectra of molecular impurity states of methyl iodide in Ar (density range ? = 0–1.4 g cm^?3) and in Kr (? = 0–2.3 g cm^?3), of carbon disulphide in Ar (? = 0–1.4 g cm^?3) and of formaldehyde in Ar (? = 0–1.25 g cm^?3). The experimental results provide new information regarding medium perturbations of intravalenc transitions, of the lowest extravalence transitions and of transitions to mixed valence—Rydberg configurations, which serve as a diagnostic tool to distinguish between different types of electronic excitations. All the lowest extravalence molecular excitations exhibit appreciable blue spectral shifts at moderate and at high fluid densities, intravalence transitions are practically insensitive to medium effects, while excitations to mixed valence—Rydberg configurations are characterized by a moderate blue spectral shift. New information has been obtained concerning the energetics of molecular ionization processes in a dense fluid. The high n = 2–5 Rydberg states of CH₃l exhibit a large red shift at moderate (? = 0–0.5 cm^?3) Ar densities. The ionization potential E_g and the effective Rydberg constant G for CH₃I in Ar was found to decrease from G = 13.6 eV and E_g = 9.55 eV at ? = 0 and E_g = 9.08 eV and constant G for CH₃l in Ar was found to decrease from G = 13.6 eV and E_g = 9.55eV at ? = 0 and E_g = 9.08 eV and G ≈ 7.15 eV at ? = 0.5 g cm^?3. Experimental evidence was obtained for the identification of n = 2 molecular Wannier impurity states of CH₃I and of CH₂O in liquid Ar. These spectroscopic data result in E_g ≈ 8.6 eV for CH₃I in liquid Ar and E_g ≈ 10.2 eV for CH₂O in liquid Ar.     

Potential energy distributions and normal coordinates of methyl,methyl-d3 and partially deuterated methyl chloride,bromide and iodide

The potential energy distributions and normal coordinates (L^?1 matrices) for twelve methyl halides, CH₃X, CH₂DX, CD₂HX and CD₃X (X = Cl, Br, I) have been calculated from known structural data. General harmonic force fields for methyl chloride, bromide and iodide previously determined from the most complete available isotopic frequency, Coriolis and centrifugal distortion data were used. The vibrational modes of these molecules are compared and discussed.     

Vibrational spectra and structure of gaseous and solid dimethylboric anhydride

The Raman spectra of gaseous, liquid and solid dimethylboric anhydride (CH₃)₂BOB(CH₃)₂ have been recorded from 10–3500 cm^?1. The IR spectra from 4000–30 cm^?1 have also been recorded. The spectra of the gaseous phase have been interpreted in terms of C₂ symmetry implying a bent B-O-B skeleton with the B(CH₃)₂ groups twisted and consistent with a rather larger barrier to internal rotation about the B-O bonds. The spectra of the crystalline state, however, suggest that the molecular symmetry is altered upon solidification. Isotopic substitution of the oxygen atom by ¹⁸O confirmed that the B-O-B skeleton is linear in the solid state, and the spectra have been interpreted in terms of D_2h molecular symmetry.