Measurement of the IR Spectrum

Block diagram of a dispersive infrared spectrometer. The sample beam passes through the sample cell while the reference beam passes through a reference cell that contains only the solvent. A rotating mirror alternately allows light from each of the two beams to enter the monochromator where they are compared. The chart recorder graphs the difference in light transmittance from the two beams. 

Block diagram of an interferometer in an FT-IR spectrometer. The light beams reflected from the fixed and moving mirrors are combined to fomi an interferogram, which passes through the sample to enter the detector. 

Another characteristic in the octane spectrum is the absence of any identifiable C—C stretching absorptions. (Table 12-1 shows that C—C stretching absorptions occur around 1200 cm . ) Although there are seven C—C bonds in octane, their dipole moments are small, and their absorptions are weak and indistinguishable. This result is common for alkanes with no functional groups to polarize the C—C bonds. 

---

A way to ascertain the coordination state of the anchored ions and, hence, their accessibility to adsorbing molecules is represented by the quantification of their capacity to adsorb molecular probes such as CO (and NO) and by the measurement of the IR spectrum of the resulting surface carbonyls (and nitrosyls). Thus, we obtain information about the propensity of the surface ions to insert ligands in their incomplete coordination spheres and, hence, indirect information about the location and structure of sites existing prior to adsorption. Of course, as mentioned previously, the probing with complexing molecules is always associated with a perturbation of the surface structures (this phenomenon is equivalent to surface relaxation). This unavoidable effect must be considered when structures of sites prior to adsorption are proposed. 

The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

One of the important functions of this infrared microscope is the measurement of the IR spectrum from a spatial region smaller than the diffraction limit. This possibility is already illustrated in Figure 29.4e. The TFD-IR spectrum, that corresponds to the IR absorption spectrum, was measured from a fluorescence region smaller than the IR diffraction limit. Infrared spectroscopy in a sub-micron region will be possible by using a high NA objective lens with the confocal optical system. 

Attenuated Total Reflection (ATR).4c A sample brought in contact with the totally reflecting surface of a high-refractive-index material (the ATR crystal), will, on IR irradiation, give an evanescent wave in the less dense medium that extends beyond the reflecting interface. This wave will be attenuated in regions of the IR spectrum where the sample absorbs energy. Observation of such waves constitute ATR measurements. Only the small amounts of beads necessary to cover the area of the ATR crystal are required. 

Vinogradova et al.182) found that the rate of polymerization of butadiene in petroleum ether at 20 °C reaches a maximum value when the ratio of [TMEDA] [Li] is about 4. Measurement of the flow times of a dilute solution of high molecular weight poly(butadienyl)lithium containing an equal amount of TMEDA before and after termination, suggested that the chains are largely in the non-aggregated form. Analysis of the IR spectrum showed that the stoichiometry of the complexed chain end is RLi TMEDA. 

A ceU for combined infrared-ultraviolet spectra devised by Leftin is shown in Fig. 23. The sample is mounted with platinum wire in a rectangular quartz cage which is a close fit in the quartz adsorption cell used to observe the ultraviolet spectrum. The uv cell is sealed directly to a pyrex tube which connects with the IR cell at the other end. Sodium chloride windows are used for measurement of the infrared spectrum. Provision is made for gas admission and evacuation and for heating of the sample by an external furnace. 

Bondarenko et al., fl4) have adopted the momentum and character of the molecular rotational motion as an indication of one or the other state of aggregation. They assumed that all molecules in free internal rotation participate as a gas. This corresponds to the presence of P— and R—branches in the IR spectrum of the bands that are being investigated. From spectroscopic measurements, they concluded that above SOO C at 250 atm, the supercritical water phase was essentially gas—like. In addition, Franck and Roth (IS) have measured both the IR spectrum and Raman spectrum of water in the supercritical region. These measurements indicated limited hydrogen bonding above 400 0 with almost no bonding above 500 0 and 250 atm. 

Perhaps more instructive than studies dealing solely with the OH region of the IR spectrum are those studies in which a probe molecule is used either to quantify the Br0nsted sites, or to qualify the type of site as to Brpnsted or Lewis. To this end numerous studies have been conducted. Pyridine is typically the adsorbent of choice as it is well known to exhibit characteristic IR bands at approximately 1450 and 1540 cm corresponding to Lewis and Brpnsted sites, respectively. Additionally, one can measure the disappearance of the acidic OH stretch bands with pyridine addition to identify the bands corresponding to acidic hydroxyl groups. 

An important consideration in spectroscopic measurements concerns the bandwidth of the laser sources. In order to resolve the vibrational resonances in a conventional approach, one needs, in the conventional scheme, a tunable source that has a narrow bandwidth compared to the resonance being studied. For t5q)ical resolutions, this requirement implies, by uncertainty principle, that IR pulses of picosecond or longer duration must be used longer. On the other hand, ultrafast pulsed IR sources with broad bandwidths are quite attractive from the experimental standpoint. In order to make use of these sources, two t5q)es of new experimental techniques have been introduced. One technique involves mixing the broadband IR source ( 300 cm ) with a narrowband visible input ( 5 cm ). By spectrally resolving the SF output, we may then obtain resolution of the IR spectrum limited only by the linewidth of the visible source [M, M]- This result follows from the fact that SF vis satisfied for the SFG process. The second new approach involves the... 

As seen in Figure 2b (Curves 2 and 3), the adsorption of CO2 on Ce02 produces extremely complex spectra, both in presence of CO2 and after evacuation of the gas phase. Furthermore, the band at 3618 cm"l is very close to the position of the hydrogenocarbonate species in the case of alumina. These two reasons are sufficient to eliminate the possibility to obtain selective measurement of surface properties of alumina in this region of the IR spectrum. 

The IR spectrum of diazomethane is characterized by the strong NN stretching band (2102 cm in the gas phase, 2075 cm in the solid state, 2096 cm Mn Ar or N2 matrices, Moore and Pimentel, 1964 a, 1964 b and references cited therein). Moore and Pimentel s two papers are still, in spite of their age, an excellent account of the IR spectrum of diazomethane. They also include measurements and discussions of the three isotopically-labeled diazomethanes CHDN2, CD2N2, and CH2 N " N. Fadini et al. (1978) measured the CN and NN stretching bands of the three isotopic diazomethanes CH2 " N2, CH2 " N N, and CH2 N " N. They were found at the following wavenumbers CN 1136, 1112, and 1170 cm NN 2097, 2075, and 2073 cm respectively. 

Reaction of K2[OsCl6] and KSCN with precipitation of the products by (Bu"4N)+ yields (Bu"4N)3[Os(NCS)j] IR, electronic absorption and Mossbauer spectra were measured, and the magnetic susceptibility was measured from 90 to 295 K (ji,g = 1.78 BM at 295 K). On the basis of the IR spectrum in particular no clear conclusion could be drawn as to whether the thiocyanate ligand was N or S bonded. Subsequent investigation showed that a mixture of isomers (Bu"4N)j[Os(NCS) (SCN)6 ] is formed in the reaction, " and these may be separated by high-voltage electrophoresis or ion exchange. ... 

Sample form represents the initial physical form of a sample that is the state in which the Raman spectrum was recorded (since it was not changed), The sample-form for registration of the IR spectrum is presented after the signs / IR , More detailed information on sample preparation for IR measurements is presented in the Experimental part. 

---