Molecules rotational-vibrational spectroscopy

This is the classic work on molecular rotational, vibrational and electronic spectroscopy. It provides a comprehensive coverage of all aspects of infrared and optical spectroscopy of molecules from the traditional viewpoint and, both for perspective and scope, is an invaluable supplement to this section. 

I. N. Levine (1975) Molecular Spectroscopy (John Wiley Sons, New York). A survey of the theory of rotational, vibrational, and electronic spectroscopy of diatomic and polyatomic molecules and of nuclear magnetic resonance spectroscopy. 

The upper state can also be formed in energetically excited ro-vibrational states. Most photoelectron experiments do not have enough resolution to observe rotational levels, except in rare cases, but vibrational resolution is commonly achieved. Therefore, it is possible to carry out limited vibrational spectroscopy of cations and reactive neutral molecules using this approach. 

The vibrational and rotational motions of the chemically bound constituents of matter have frequencies in the IR region. Industrial IR spectroscopy is concerned primarily with molecular vibrations, as transitions between individual rotational states can be measured only in IR spectra of small molecules in the gas phase. Rotational - vibrational transitions are analysed by quantum mechanics. To a first approximation, the vibrational frequency of a bond in the mid-IR can be treated as a simple harmonic oscillator by the following equation ... 

Vibrational spectroscopy measures and evaluates the characteristic energy transitions between vibrational or vibrational-rotational states of molecules and crystals. The measurements provide information about nature, amount and interactions of the molecules present in the probed substances. Different methods and measurement principles have been developed to record this vibrational information, amongst which IR and Raman spectroscopy are the most prominent. The following focuses on these two techniques, the corresponding instrumentation and selected applications. 

A technique which is not a laser method but which is most useful when combined with laser spectroscopy (LA/LIF) is that of supersonic molecular beams (27). If a molecule can be coaxed into the gas phase, it can be expanded through a supersonic nozzle at fairly high flux into a supersonic beam. The apparatus for this is fairly simple, in molecular beam terms. The result of the supersonic expansion is to cool the molecules rotationally to a few degrees Kelvin and vibrationally to a few tens of degrees, eliminating almost all thermal population of vibrational and rotational states and enormously simplifying the LA/LIF spectra that are observed. It is then possible, even for complex molecules, to make reliable vibronic assignments and infer structural parameters of the unperturbed molecule therefrom. Molecules as complex as metal phthalocyanines have been examined by this technique. 

The next most useful is vibrational spectroscopy but identification of large molecules is still uncertain. In the laboratory, vibrational spectroscopy in the infrared (IR) is used routinely to identify the functional groups in organic molecules but although this is important information it is not sufficient to identify the molecule. Even in the fingerprint region where the low wavenumber floppy vibrational modes of big molecules are observed, this is hardly diagnostic of structure. On occasion, however, when the vibrational transition can be resolved rotationally then the analysis of the spectrum becomes more certain. 

Electronic spectroscopy The study of rotational, vibrational and electronic motion in molecules following absorption of radiation in the visible region of the spectrum... 

An electric dipole operator, of importance in electronic (visible and uv) and in vibrational spectroscopy (infrared) has the same symmetry properties as Ta. Magnetic dipoles, of importance in rotational (microwave), nmr (radio frequency) and epr (microwave) spectroscopies, have an operator with symmetry properties of Ra. Raman (visible) spectra relate to polarizability and the operator has the same symmetry properties as terms such as x2, xy, etc. In the study of optically active species, that cause helical movement of charge density, the important symmetry property of a helix to note, is that it corresponds to simultaneous translation and rotation. Optically active molecules must therefore have a symmetry such that Ta and Ra (a = x, y, z) transform as the same i.r. It only occurs for molecules with an alternating or improper rotation axis, Sn. 

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

The concept of atropisomerism developed to a considerable extent following other developments in chemistry, especially those in spectroscopy. Early work by Kohlrausch (4) and Mizushima (3), based on Raman spectra and dipole moment studies, established that rotational isomers—rotamers—must exist in 1,2-dichloroethane. Pitzer established that there are three energy minima when ethane is rotated about its C—C axis (6). Rotamers about single bonds have been found in a wide variety of organic compounds since then, mainly as a result of the application of vibrational spectroscopy to organic molecules (7). 

Barriers to single-bond rotation and pyramidal inversion derive principally from microwave spectroscopy, from vibrational spectroscopy in the far infrared and (for the larger barriers) from NMR. Although the number of systems for which data are available is limited (and the systems themselves primarily limited to very small molecules), in some cases barriers are known to high accuracy (to within 0.1 kcal/mol). 

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],... 

Microwave spectroscopy often gives more definite and precise information on the structure of polyatomic molecules than vibration-rotation and electronic spectra. For example, consider the simplest oxime formald-oxime, CH2=NOH. There are two likely structural configurations for this... 

In extending the studies of vibrational coherence to rotational coherence in isolated molecules, we formulated the concept of rotational recurrences (echoes ), which led to rotational coherence spectroscopy. A polarized picosecond (and later femtosecond) pulse was used to orient a molecular ensemble (Fig. 7). The molecules then rotate freely with different speeds... 

Absorption of microwave radiation to excite molecular rotation is allowed only if the molecule has a permanent dipole moment. This restriction is less severe than it may sound, however, because centrifugal distortion can disturb the molecular symmetry enough to allow weak absorption, especially in transitions between the higher rotational states which may appear in the far IR (c. 100cm-1). Microwave spectroscopy can provide a wealth of other molecular data, mostly of interest to physical chemists rather than inorganic chemists. Because of the ways in which molecular rotation is affected by vibration, it is possible to obtain vibrational frequencies from pure rotational spectra, often more accurately than is possible by direct vibrational spectroscopy. 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

VIBRATION-ROTATION TUNNELING SPECTROSCOPY OF MOLECULES AND DIMERS... 

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