The Raman spectra of ferrocene-b10 and -d10: librational modes and comments upon previous polarization data

The librational modes of ferrocene-h₁₀ and -d₁₀ have been observed near 62 cm^?1 (-h₁₀) and 40, 52 and 60 cm^?1 (-d₁₀). The polarization measurements at low temperatures can give no useful information concerning the vibrational assignment until the low temperature crystal structure is completely determined.     

A Raman spectroscopic study of the low-frequency vibrations in anisole and anisole-d3

Low-frequency Raman spectra of solid anisole and of solid anisole-d₃ have been recorded at 130 K. The phenyl torsion observed at 148 cm^?1 is shifted to 133 cm^?1 upon deuteration of the methyl group. The twofold torsional barriers calculated from these frequencies are 4033 ± 110 cm^?1 and 4094 ± 123 cm^?1 indicating that coupling to other low-frequency modes in both cases is of the same order of magnitude. The methyl torsional mode was observed at 285 cm^?1 in the spectrum of solid anisole and at 183 cm^?1 in the spectrum of anisole-d₃. The threefold barriers calculated using these frequencies are 1847 ± 20 cm^?1 and 1465 ± 18 cm^?1 respectively. These barrier values indicate that the methyl torsion is coupled to another low-frequency mode. A doublet centered at 230 cm^?1 in anisole is shifted to 245 cm^?1 in anisole-d₃; it is proposed that this is due to a ring mode coupled to the methyl torsion. The splitting is interpreted as an example of Davydov splitting.     

Infrared and Raman spectra,vibrational assignment and normal coordinate calculations for ethylisothiocyanate-d0 and -d5

The i.r. spectra (3500-40 cm⁻¹) of gaseous and solid ethylisothiocyanate, CH₃CH₂NCS, and ethylisothiocyanate-d₅, CD₃CD₂NCS, and the Raman spectra (3200-10 cm⁻¹) of the liquids and solids have been recorded. Additionally, the i.r. spectrum of matrix (N₂) isolated CH₃CH₂NCS was recorded from 3500 to 200 cm⁻¹. The vibrational spectra have been interpreted on the basis of a cis form of C_s symmetry for all three physical states; however, in the spectum of the matrix isolated sample at 10 K, several of the fundamentals are doublets which may be arising from two conformers at this temperature. A vibrational assignment is given based on i.r. band contours, depolarization values, isotopic shifts and group frequencies. The assignment is supported by the Teller—Redlich product ratios. The methyl torsion was observed at 270 cm⁻¹ in the solid phase which gives a periodic barrier of 4.7 kcal/mole. A normal coordinate calculation has been carried out utilizing a modified valence force field consisting of 15 diagonal force constants and seven interaction terms which reproduced the observed frequencies with an error of 1.45% for the -d₀ molecule and 3.40% for the -d₅ compound. Because of the large CNC angle (∼ 145°) and low barrier to internal rotation of the asymmetric rotor, the skeletal bend and torsion couple to give a very broad band with two maxima and apparent P band heads. These results are compared to corresponding studies of similar molecules.     

Isotopic mixed crystal exciton spectra in the far infrared: Naphthalene

The far infrared vibrational exciton spectra of isotopic mixed crystals of naphthalene-h₈ and d₈ were studied. The two observed translational phonon modes were determined to fall into the amalgamated band limit while the lowest energy B_3u, A_u and B_1u vibrational exciton bands were found to be in the separated band limit. The lowest energy B_3u “butterfly” mode with its large (15 cm^?1) exciton splitting was found to agree well with CPA calculations of mixed crystal spectra. A peak at 185 cm^?1 was also assigned as a peak in the vibrational exciton density-of-states of the B_3u mode.     

The methyl and methoxyl torsional modes and the lattice vibration in the low-frequency Raman spectrum of 1,4-dimethoxybenzene

The low-frequency (10–450 cm^?1) Raman spectra of solid (at 300 K and 130 K) and liquid (at 335 K) 1,4-dimethoxybenzene-d₀ and 1,4-dimethoxybenzene-d₅ have been measured. The methyl nad methoxyl torsional transitions have been identified and the corresponding torsional barriers calculated. Upon deuleration the methyl torsional barrier is reduced by 450 cm^?1, implying a coupling between the methyl torsion and a low-frequency ring mode. As far as the torsions are considered, the internal dynamic situation in 1,4-dimethoxybezene resembles that in amisole. A tentative assignment of the observed lattice bands in given. Certain changes in the spectrum when going from the solid to the melt are attributed to the coexistence of both cis and trans conformers in the liquid state.     

Oscillator strengths of the bands of the B̃2 A′1—X̃2 A″2 system of CD3 and a spectroscopic measurement of the recombination rate comparison with CH3

Absolute CD₃ concentrations were measured in the flash photolysis of d₆-HgMe₂. An oscillator strength of (0.99 ± 0.10) × 10^?2 was recorded for the 0O band of the B?—X? system, and a recombination rate coefficient of (4.9 ± 0.4) × 10^?11 cm³ molecule^?1 s^?1 was derived. It is suggested that the probability of recombination per collision is virtually the same as for the CH₃ radical.Some new bands of the B?—X? system have been discovered and tentatively assigned: from a study of the temperature dependence of the intensity of the 47300 cm^?1 transition, Herzberg's scheme ‘b’ has been established for the vibrational assignment.     

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.     

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.     

Low frequency raman spectra of isotopic mixed benzophenone crystals

In this paper, we present an experimental study of vibrational lattice modes in isotopic mixed crystals of benzophenone-h₁₀ and -d₁₀. Our results are discussed using theoretical and experimental data concerning other molecular isotopic mixed crystals. The spectra show two regions; in the first (ν < 100 cm^?1) we did not observe an interaction between the various vibrational lattice bands; in the second region (ν > 100 cm^?1), the interaction appears clearly. The 111.5 cm^?1 and 73 cm^?1 torsional modes of benzophenone-h₁₀ have the same behaviour as the external modes throughout the whole concentration range (0–100% of benzophenone-d₁₀).     

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.     

Infrared and Raman matrix isolation studies of methylamine

Infrared (4000-50 cm^?1) and Raman spectra are reported of methylamine, methylamine-d₁ and methylamine-d₂ trapped in argon and nitrogen matrices at 4–20 K. An anomalous intensity variation was found for the NH₂ wagging mode of methylamine isolated in nitrogen matrices, while in argon matrices the NH₂ wagging absorption exhibited a complex structure due to matrix site effects. A normal coordinate analysis was carried out using a new assignment of the NHD twisting frequency. Barriers to internal rotation in argon and nitrogen matrices, calculated from the observed torsional frequencies, are compared with the gas phase value.     

Absorption and emission spectra of Ca2 in solid krypton

The calcium van der Waals molecule, Ca₂, has been formed by codepositing calcium and krypton atoms on a substrate at 12 K. Absorption spectra revealed a structured band at 666 nm with 117 ± 2 cm^? spacings. Calcium-44 isotopic spectra confirmed the assignment and located the band origin at 14 432 ± 4 cm^?1. Emission spectra, pumping the 11 ← 0 absorption, exhibited a very strong 14 000 cm^?1 band with 78 ± 2 cm^?1 spacings to the band origin and unrelaxed emission with 78 ± 2 and 117 ± 2 cm^?1 spacings above the band origin from v′ levels up to 6. The transition ¹∑⁺_u(¹S + ¹P) ← ¹∑⁺_g (¹S + ¹S) and the bonding in ground and excited Ca₂ states fit a simple molecular orbital model.     

Fluorescence spectra of the mixed crystal system anthracene-perdeuteroanthracene. Mixed exciton band

The fluorescence spectra of the mixed crystal system anthracene (A-h₁₀)-perdeuteroanthracene (Ad₁₀) have been studied over the full concentration range at temperatures between 1.6 and 77°K. There exists a collective (mixed) A-h₁₀-A-d₁₀ S₁ exciton band at all concentrations, the lower edge of which represents the emitting level for the intrinsic fluorescence at low temperatures. This edge shifts from 25097 cm^?1 in the neat A-h₁₀ crystal to 25156 cm^?1 in the neat A-d₁₀ crystal. The edge position is a non-linear function of the mixed crystal concentration. From that one gets the “critical” distances U_cr⁺ = 92 cm^?1 and U_cr^? = 131 cm^?1 between the molecular S₁ levels of a guest and the host anthracene (non-deuterated or deuterated), using a formula given by Lifshitz. For the critical distance U_cr⁺ one finds just a bound guest state above the host band, and for U_cr^?1 a bound guest state below the host band.The transition from the S₁ band edge to the S₀ ground state is always forbidden, in the same way as in the neat A-h₁₀ crystal. Only transitions to levels of intramolecular and lattice vibrations of the ground state have been observed. At all concentrations the fluorescence transitions from the lower S₁ band edge of the mixed crystal take place to the vibronic levels of both A-h₁₀ and A-d₁₀. The difference in the frequencies of equivalent intramolecular vibrations of A-h₁₀ and A-d₁₀ results in the appearance of vibronic doublets in the intrinsic fluorescence. The relative intensities of the two doublet components depend strongly on both concentration and temperature. This is due to the influence of quasi-resonance and exciton-superexchange upon the guest states inside the mixed exciton band.For all concentrations one observes two transitions originating from levels inside the S₁ exciton band.     

Electronic transitions in Li4CoCl4.10H2O

The electronic spectrum of Li₄CoCl₆.10H₂O was recorded at liquid nitrogen temperature in the 4,000–25,000 cm^?1 spectral region. The simi larity of this spectrum to that of CoCl₂ permitted us to assume O_h syn metry of the [CoCl₆]^4? cluster in our sample. The band assignment was performed in the crystal field approximation using Tanabe and Sugano's energy matrices for Dq = 730 cm^?1, B = 820 cm^?1 and C/B = 4.4.The large number of bands and high intensity of the maxima in the regio 19,000–21,000 cm^?1 is discussed.     

A precise determination of the first ionization potential of benzene

The first ionization potentials of benzene and benzene-d₆ have been precisely determined by the extrapolation of three-photon resonant Rydberg states in the four-photon ionization spectrum of the jet-cooled molecule. The convergence of resolved transitions in two Rydberg series for principal quantum numbers as high as 14 (-h₆) and 15 (-d₆) establish adiabatic thtesholds of 74573.0 ± 2.0 cm^?1, and 74592.5 = ± 1.2 cm^?1, respectively. These results are crucial for the understanding of the many excited states of benzene in terms of quantum defect theory. Precise quantum defects have been obtained for several Rydberg series and their variation with principal quantum number is reported. The results strongly suggest that the R? series of Wilkinson is derived from aπ(e_1g)→ nf±1 Rydberg excitation.     

Infrared and raman spectra,vibrational assignment and barriers to internal rotation for methyldisilylamine

The Raman spectra of gaseous and liquid (SiH₃)₂NCH₃ and (SiH₃)₂NCD₃ have been recorded to within 10 cm^?1 of the exciting line. The IR spectra of (SiH₃)₂NCH₃ and (SiH₃)₂NCD₃ have been recorded from 80 cm^?1 to 3800 cm^?1 in the gaseous state, and from 80 cm^?1 to 450 cm^?1 in the solid state. A vibrational assignment has been made, and from the low-frequency vibrational data, an upper limit of 3.3 kcal mol^?1 was calculated for the barrier to internal rotation of the silyi groups, whereas a barrier of ~450 cal was calculated for internal rotation of the methyl group. It is concluded that there exists a significantly strong d_π—p_π interaction in methyldisilylamine.     

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.     

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.     

Vibrational spectra of cyanosubstituted cyclopropanes—III. Cyanocyclopropane and cyanocyclopropane-1-d1

The vibrational spectra of cyanocyclopropane have been reinvestigated with the gas phase i.r. spectrum reported for the first time. The i.r. spectra of cyanocyclopropane-1-d₁ have been measured from 4000 to 200 cm⁻¹ in the liquid and the gaseous phases and from 4000 to 400 cm⁻¹ in the polycrystalline state. Raman spectra of the liquid down to 50 cm⁻¹ were obtained. A vibrational assignment of all fundamentals is proposed. With the present data, we had to reassign several fundamentals published in the literature.     

Raman study of vibrational relaxation of cyclohexane in benzene solutions

The Raman profiles of the ν₅ mode (802 cm^?1) of cyclohexane, ν₅ (723 cm^?1) of cyclohexane-_d12 and ν₂ (992 cm^?1) of benzene and its deuterated analogs have been measured as a function of concentration in the benzene—cyclohexane liquid system. The vibrational time correlation functions of cyclohexane in benzene solutions have been calculated by Fourier inversion of isotropic band contours. The concentration dependence of the experimental vibrational correlation times computed from the correlation functions and from the half width at half height have been compared with that predicted theoretically for various mechanisms of band broadening. We have tested the Fischer—Laubereau dephasing model and the Knapp—Fischer concentration-fluctuation model. We have found that the latter model reproduces well experimental data only for the ν₂ mode of benzene in solution.