Isotope shifts in spectra of molecular liquids

In the IR absorption spectra of low-temperature molecular liquids, we have observed anomalously large isotope shifts of frequencies of vibrational bands that are strong in the dipole absorption. The same effect has also been observed in their Raman spectra. At the same time, in the spectra of cryosolutions, the isotope shifts of the same bands coincide with a high accuracy (±(0.1–0.5) cm^–1) with the shifts that are observed in the spectra of the gas phase. The difference between the spectra of examined low-temperature systems is caused by the occurrence of resonant dipole–dipole interactions between spectrally active identical molecules. The calculation of the band contour in the spectrum of liquid freon that we have performed in this work taking into account the resonant interaction between states of simultaneous transitions in isotopically substituted molecules can explain this effect.     

Vibrational spectra of monoisotopic SiH<Subscript>4</Subscript> and GeH<Subscript>4</Subscript> in low-temperature matrices

IR absorption spectra of monoisotopic ²⁸SiH₄ and ⁷⁶GeH₄ are studied in Ar and N₂ matrices at 10 K. It is shown that the absorption spectra of silane and germane are similar in the regions of the stretching ν₃ and bending ν₄ vibrations. Four groups of bands can be separated out in the spectra of each molecule: (1) narrow bands characteristic of the matrix isolation studies, (2) broad bands, (3) diffuse absorption with a large value of the spectral moment M ₂^* the intensity of which increases upon annealing, and (4) bands of dimers the intensity of which increases quadratically with concentration. The spectra of ²⁸SiH₄ and ⁷⁶GeH₄ in nitrogen matrices contain a triplet in the stretching region and a doublet in the bending region, which is explained by the change in the molecular symmetry from T _d to C _3V on passage from the gas phase to solid nitrogen.     

Structures of vibrational absorption bands of the SiF4 molecule in a low-temperature nitrogen matrix

We have studied the IR absorption spectra of diluted mixtures SiF₄/M = 1/6000–1/10 000 in an N₂ matrix at 11 K (for comparison, the spectra of SiF₄ in Ar and Xe matrices have also been studied). It has been shown that, in solid nitrogen, the appearance of doublets is observed both in the range of the ν₃ band of the SiF₄ (²⁸SiF, ²⁹SiF₄, and ³⁰SiF₄) isotopologues of the SiF₄ molecule and in the range of the ν₁ + ν₄, ν₂ + ν₃, and ν₁ + ν₃ bands of ²⁸SiF₄, whereas, in the range of the 2ν₃ band of ²⁸SiF, a triplet appears. In order to analyze the influence of the matrix on the spectrum of free SiF₄ molecules, we have used a model that makes it possible to successively calculate (i) the spectrum of SiF₄ in terms of the model of local modes, (ii) the structure of a matrix composed by 864 N₂ molecules + a rigid SiF₄ molecule using the Monte Carlo method, and (iii) the interaction of matrix particles with local dipole moments in the approximation of dipole-induced dipole and dipole-quadrupole interactions. The model describes satisfactorily the low-frequency shift of bands in the nitrogen matrix. All obtained experimental and theoretical results are consistent with the assumption that two kinds of stable trapping centers of SiF₄ molecules obeying the T _d symmetry are realized in the nitrogen matrix.     

Spectroscopic parameters of CHFCl2, CHClF2, C2F6, and C3F8 molecules

We investigate the IR spectra of solutions of R-21,-22,-116, and R-218 in liquid argon at 90K. The frequencies, half-widths, and absolute integral intensities of all of the absorption bands recorded are obtained and interpreted. Scientific-Research Institute of Physics, 1, Ul'yanovskaya Str., Petrodvorets, St. Petersburg, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 297–301, May–June, 1997.     

Modeling of band shapes in the low-temperature molecular liquid spectra affected by resonance dipole–dipole interaction

The band shape theory of the vibrational transitions accompanied by an excitation of a strongly IR-active fundamental is elaborated for low-temperature molecular liquids with an account for strong intermolecular resonance dipole–dipole interactions. The developed model of a liquid uses the regular (fcc structure) crystal whose structure is then destroyed by introducing random, normally distributed displacements of particle positions, by assigning arbitrary molecular orientations as well as a varied number of randomly distributed vacancies. This model was applied to several molecular liquids , those having triply degenerate modes were studied more in detail.     

Integral intensities of absorption bands of silicon tetrafluoride in the gas phase and cryogenic solutions: Experiment and calculation

The spectral characteristics of the SiF₄ molecule in the range 3100–700 cm^?1, including the absorption range of the band ν₃, are studied in the gas phase at P = 0.4–7 bar and in solutions in liquefied Ar and Kr. In the cryogenic solutions, the relative intensities of the vibrational bands, including the bands of the isotopically substituted molecules, are determined. The absorption coefficients of the combination bands 2ν₃, ν₃ + ν₁, ν₃ + ν₄, and 3ν₄ are measured in the solution in Kr. In the gas phase of the one-component system at an elevated pressure of SiF₄, the integrated absorption coefficient of the absorption band ν₃ of the ²⁸SiF₄ molecule was measured to be A(ν₃) = 700 ± 30 km/mol. Within the limits of experimental error, this absorption coefficient is consistent with estimates obtained from independent measurements and virtually coincides with the coefficient A(ν₃) = 691 km/mol calculated in this study by the quantum-chemical method MP2(full) with the basis set cc-pVQZ.     

Observation of simultaneous ν1(SF6) + ν3(NF3) and ν2(SF6) + ν3(NF3) transitions enhanced by the resonance dipole-dipole interaction with the ν1 + ν3 and ν2 + ν3 states of the SF6 molecule

IR spectra of the solution of SF₆ molecules in liquid NF₃ at 84 K have been recorded. In a solvent transmission window of 1500–1750 cm⁻¹, two wide absorption bands with pronounced peaks in the high-frequency part are observed. The profile of these bands is explained by the influence of the resonance dipole-dipole (RDD) interaction of the states of the simultaneous transition ν₁(SF₆) + ν₃(NF₃) and ν₂(SF₆) + ν₃(NF₃) with the states (ν₁ + ν₃) and (ν₂ + ν₃) of the SF₆ molecules, respectively. The use of three isotopic modifications ³²SF₆, ³³SF₆, and ³⁴SF₆ has allowed us to vary the resonance detuning and thus to change the strength of the RDD interaction. With the liquid near the melting point being represented as a close-packed cubic crystal, the profile was calculated and its spectral characteristics were determined. The frequencies of the main peaks coincide with the experimental values accurate to the error.     

An IR spectroscopic study of liquid ozone and ozone dissolved in liquid argon

We have measured and interpreted the IR spectra of liquid ozone films at 78–85 K and ozone dissolved in liquid argon at 91–95 K. A less hindered rotation of ozone molecules in argon manifests itself as an intensity redistribution, caused by the Coriolis interaction, from the states ν₃(B ₁) and ν₁ + ν₃(B ₁) to the states ν₁(A ₁) and 2ν₁(A ₁), respectively. The occurrence of wings in the contours of the bands ν₁(A ₁), 2ν₁(A ₁), and 2ν₃(A ₁) in liquid Ar and their absence in the spectrum of O₃ also confirms the conclusion that the rotational motion of ozone molecules in an inert solvent at low temperatures is relatively less hindered. Maxima of ozone bands in Ar solution are shifted toward lower frequencies compared to those in the gas phase by 1–30 cm^?1, which corresponds to the following shifts of harmonic frequencies of the molecule: Δω₁ = ?1.85(5) cm^?1, Δω₂ = ?0.67(7) cm^?1, Δω₃=?7.20(5) cm^?1. It was found that the absorption band of the ν₃ mode in the spectrum of O₃ in the liquid phase has a complicated asymmetric contour because of the resonance dipole-dipole interaction. The first and second spectral moments of this band have been determined to be M ₁ = 1030.6 cm^?1 and M ² = 240.0 cm^?2.