Microwave spectra, vibrational spectroscopy

The dipole and polarization selection rules of microwave and infrared spectroscopy place a restriction on the utility of these techniques in the study of molecular structure. However, there are complementary techniques that can be used to obtain rotational and vibrational spectrum for many other molecules as well. The most useful is Raman spectroscopy. 

Rotational Raman spectroscopy is a powerful tool to determine the structures of molecules. In particular, besides electron diffraction, it is the only method that can probe molecules that exhibit no electric dipole moment for which microwave or infrared data do not exist. Although rotational constants can be extracted from vibrational spectra via combination differences or by known correction factors of deuterated species the method is the only one that yields directly the rotational constant B0. However for cyclopropane, the rotational microwave spectrum, recording the weak AK=3 transitions could be measured by Brupacher [20],... 

Mierowave studies in moleeular beams are usually limited to studying the groimd vibrational state of the complex. For eomplexes made up of two molecules (as opposed to atoms), the intermolecular vibrations are usually of relatively low amplitude (though there are some notable exceptions to this, such as the ammonia dimer). Under these eireumstances, the methods of classical microwave spectroscopy can be used to determine the structme of the complex. The principal quantities obtained from a microwave spectrum are the rotational constants of the complex, which are conventionally designated B and C in decreasing order of magnitude there is one rotational constant B for a linear complex, two constants (A and Bor B and C) for a complex that is a symmetric top and three constants (A, B and C) for an... 

A number of halogenomethanes have been subjected to other forms of molecular spectroscopy. High-resolution Stark spectra of several transitions of the V3 band of CH3F have been studied by means of a CO2 laser measurements of the hyperfine structure on certain rotational transitions in CH2F2 have been made using a molecular beam maser spectrometer the millimetre-wave spectrum of ground-state CDCla and the microwave spectrum of CD3I in excited vibrational states have also been observed. 

For three isotopic species in their vibrational ground states, the following rotational constants (in MHz) were derived from the microwave spectrum [1], see also the tables [2] of molecular constants from microwave spectroscopy (for axis system, see Fig. 1 [1]). 

The possibility of directly measuring molecular stractures is an important advantage of microwave spectroscopy. In vibrational spectroscopy, isotopic substitution helps the interpretation of the spectra but the assigned structmes are only the calculated ones which offer the best match between the calculated vibrational spectrum and the observed one. It is also difficult to determine relative abundances from electronic spectroscopy because the relative intensity of the observed electronic transitions of the chromophore can be affected by the dynamics of the excited state. Both techniques are complementary vibrational spectroscopy can address the conformational preferences of large systems which microwave spectroscopy cannot... 

Excitation spectra arise from transitions between different quantum states of the system, corresponding to different nondegenerate solutions of the Schrodinger equation. In quantum chemistry it is common practice to treat the solution of the Schrodinger equation within the Bom-Oppenheimer approximation [1] and separate the electronic and nuclear degrees of freedom. Consequently the excitation spectra are also separated into an electronic and a roto-vibrational spectrum. The former is studied mainly in optical (UV/vis) spectroscopy experiments and will constitute the main subject of this chapter the latter, which can be investigated by infrared, microwave or Raman spectroscopy measurements, provides fine-structure corrections to the electronic spectrum. 

Figure 6 Spectral pattern and relative intensities of C3S transition lines. Reproduced with permission from Tang J and Saito S (1995) Microwave spectrum of the C3S molecule in the vibrationally excited states of bending modes V4 and Vg. Journal of Molecular Spectroscopy 92.

The Stable carbonyl and thiocarbonyl halide molecules have been studied by IR as well as Raman spectroscopy. Normal coordinate analyses based on force constants transferred from other molecules (Urey-Bradley type), or from ab initio calculations, have aided in the vibrational assignments. Some of the unstable molecules which have been observed in the microwave have been characterized by infrared spectroscopy. The somewhat lower sensitivity of this method means that long path lengths of the gas may be needed. The identification of the various stable and unstable species in the microwave spectrum is simplified by the fact that the absorption lines are usually well resolved from each other. The widths of the bands in the infrared may make the transient species difficult to detect against the stronger absorptions of the stable side products. IR and Raman spectroscopies do have the advantage that they can be used on solid and liquid samples. Since the bands in a low temperature rare gas matrix have a narrower profile, the infrared spectrum is usually simplified over the room temperature gas phase spectrum. Moreover, the vibrational frequencies are only mildly perturbed by solid state effects. For example, CF Se has not been observed in the vapor phase, yet its vibrational dynamics are known from its matrix isolation spectrum. Table 9 gives the vibrational data for the carbonyl, thiocarbonyl, seleno-carbonyl and formyl halides. 

It is characteristic of the technology of microwave spectroscopy that frequencies are measurable to very high precision. Until the introduction of infrared lasers, microwave spectroscopy far outran vibrational spectroscopy in the precision and accuracy of spectral measurements. The primary piece of information obtained from a microwave spectrum is the rotational constant, and given the precision available with this type of experiment, high-precision values of the rotational constant are obtained. This, in turn, implies that very precise values of the bond length of a diatomic molecule can be deduced from a microwave spectrum. In practice, measurement precision corresponding to a few parts in 10,000 is achieved. 

Rotational spectroscopy and microwave astronomy are the most accurate way to identify a molecule in space but there are two atmospheric windows for infrared astronomy in the region 1-5 im between the H2O and CO2 absorptions in the atmosphere and in the region 8-20 xrn. Identification of small molecules is possible by IR but this places some requirements on the resolution of the telescope and the spacing of rotational and vibrational levels within the molecule. The best IR telescopes, such as the UK Infrared Telescope on Mauna Kea in Hawaii (Figure 3.13), are dedicated to the 1-30 xm region of the spectrum and have a spatial resolution very close to the diffraction limit at these wavelengths. 

A significant application of microwave spectroscopy is in the determination of barriers to internal rotation of one part of a molecule relative to another. Internal rotation is a vibrational motion, but has effects observable in the pure-rotation spectrum. If the barrier to internal rotation is very high, then the internal torsion is just like any other vibrational mode, and the rotational constants are affected in the usual way Bv = Be —... 

The molecule is pyramidal, having C3v symmetry with the nitrogen atom at the apex. The molecular dimensions have been determined by electron diffraction (266) and by microwave spectroscopy (161,271). The molecule with this symmetry will have four fundamental vibrations allowed, both in the infrared (IR) and the Raman spectra. The fundamental frequency assignments in the IR spectrum are 1031, vt 642, v2 (A ) 907, v3 (E) and 497 cm-1, v4 (E). The corresponding vibrations in the Raman spectrum appear at 1050, 667, 905, and 515 cm-1, respectively (8, 223, 293). The vacuum ultraviolet spectrum has also been studied (177). The 19F NMR spectrum of NF3 shows a triplet at 145 + 1 ppm relative to CC13F with JNF = 155 Hz (146, 216, 220,249, 280). 

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