A. Lee Smith

Infrared spectra-structure correlations for organosilicon compounds

Abstract The concept of group frequencies is particularly useful in the interpretation of the infrared spectra of organosilicon compounds. Because of the insulating effect of the silicon atom, spectra of these compounds can be considered approximately as the sum of independent vibrations of the substituent groups. In this paper, many of the characteristic absorptions of these groups are discussed in terms of their use in identifying and characterizing molecular species. The molecular motions which give rise to some of these absorptions are also discussed. Some of the perturbing influences on the group frequencies are enumerated, and examples given of their effect. Numerous references are made to the results of other workers on individual molecules. A chart is presented which summarizes the correlations given.

The Infrared Spectra of the Methyl Chlorosilanes

The infrared spectra of the series SiCl4 through SiMe4 have been obtained in the gas phase over the range 2–25 microns using CaF2, NaCl, and KBr prisms. The spectra are compared with Raman data for these compounds, and vibrational assignments are made. The methyl band at about 8 microns is shown to be associated with the symmetrical CH3 deformation, the bands around 12 microns with the methyl rocking and Si–C stretching vibrations, and the bands between 16 and 25 microns with the SiCl stretching modes. A comparison of frequencies for the members of the series has made it possible to assign fundamentals for all motions except those involving torsion of the Me group around its axis.

Infrared spectra of organosilicon compounds in the CsBr region

Abstract A number of organosilicon compounds show characteristic bands in the 15–38 μ range. These absorptions arise from SiCl, SiBr and SiI stretching, SiOSi symmetrical stretching SiCHCH2 hydrogen deformation, ring vibrations in Si-aromatic compounds, SiOC bending SiH2 and SiH3 rocking, and other skeletal vibrations. Some of these absorptions fall within narrow wavelength intervals and some are good group frequencies which correlate well with other group parameters. In many cases frequencies can be sorted out and assigned to discrete vibrational modes. Using these concepts it is possible to make assignments with reasonable confidence and correct some misassignments which have appeared in the literature.

Vibrational Spectra of Me2SiCl2, Me3SiCl, Me3SiOSiMe3, (Me2SiO)3, (Me2SiO)4, (Me2SiO)x, and Their Deuterated Analogs

This work had three objectives: to improve the reported assignments of the vibrational frequencies in several organoslllcon monomers and oligomers; to assign the vibrational frequencies of polydimethylsiloxane (PDMS); and to relate the vibrational spectra of PDMS to its other physical properties, especially at sub-ambient temperatures. Infrared and Raman spectra were obtained on the title compounds, and five sets of low-temperature IR spectra were obtained on PDMS. Some low-temperature spectra were also taken on the cyclosiloxanes. Assignment of all frequencies was accomplished, although in some cases, e.g., the methyl rocking frequencies at 650–870 cm−1 and the skeletal deformations below 400 cm−1, our confidence in their correctness Is less than complete. The low-temperature study showed striking changes In the IR spectrum of PDMS below its glass transition temperature T8. Bands became sharper and in some cases showed splitting. This behavior is consistent with the concept of polymer motion being frozen below T8. Except for the appearance of a strong, sharp band at 662 cm−1, not much change was seen in the spectrum at the “cold crystallization” point. Possibly the expected spectral changes were swamped by absorption from the more plentiful amorphous portion of the polymer matrix.

Optimizing the Operating Parameters of Infrared Spectrometers

In order to achieve best results with modern ir spectrometers, users must take considerable care to optimize the setting of the slit program, response time, gain, and scan time. In this paper, explicit recommendations are given for carrying out such adjustments in a logical and self-consistent manner. First, a set of conditions is derived for the recording of general purpose spectra. From this starting point, settings may be modified to record spectra under the following special conditions: low noise level, limited optical energy, high resolution, and rapid scanning. Proper use of the scan speed suppression control is discussed.

Are Group Frequencies Obsolete

In spite of the diminishing number of publications on the subject, group frequencies remain an important research tool for infrared spectroscopists. The capabilities of FT-IR present us with opportunities for discovering useful new correlations. High-quality conventional spectra, vapor spectra, and matrix isolation spectra are easy to obtain. Large libraries of FT-IR spectra are being generated and made available in digital form for fast library searching. Computerized spectrum interpretation is becoming a reality. All these applications, for best utility, require wavelength accuracy of ±l/cm, and this accuracy is easily obtainable from FT-IR spectrometers. Unfortunately, the need for sample integrity and careful sample handling is often unappreciated and may be the fatal flaw that prevents us from realizing the full potential of the infrared method. Careful sample selection, the use of good sample handling techniques, and evaluation of the resulting spectrum by a knowledgeable spectroscopist can minimize problems. Several examples of the uses of group frequencies accurate to ±l/cm in organosilicon chemistry are given.

Use of Nomographs for Routine Calculations in Infrared Spectrophotometry

Various devices are used to reduce the labor of calculating the concentrations of components with mutually interfering bands in quantitative infrared analysis. Computers (7) or IBM equipment (8,10) may be used for complex systems, or successive approximation methods (3,5,11) may be satisfactory. For systems of two or three components, exact solutions can be obtained by solving simultaneous equations (4,6), but such a procedure becomes tedious when large numbers of samples are involved. It is the purpose of this note to point out that nomographs can be easily constructed which give exact solutions to these simultaneous equations with a minimum of effort. Several forms of nomographs have been suggested for this purpose (2,9,12), but the type described here seems to offer some advantages in ease of construction and use.

Happiness is a Class II Spectrum

In 1967, the Coblentz Society and the American Society for Testing Materials jointly initiated a pilot project to generate a limited number of research quality (class II) infrared reference spectra. Six infrared laboratories participated, each producing 25 spectra from samples selected from a group of 57 authentically pure chemical compounds. Three of the compounds were run by all laboratories in order to provide a basis for comparison and evaluation. This paper reports the results of the project. Points covered include a discussion of why class II spectra are important to the future of infrared spectroscopy; some of the difficulties encountered by the participating laboratories and ways of overcoming these difficulties; and a critical comparison of spectra from the same compounds produced on different spectrometers and under different sampling conditions.