Spectroscopy, molecular rotational

Radio- and microwaves also have several other fields of application in spectroscopy. Molecular rotational transitions correspond to this wavelength region. Radiometers can be used for passive remote sensing, of e.g. temperature and air humidity, and radar systems can be utilized for active measurements of e.g. oil slicks at sea. Finally, radio astronomy is a fascinating field, yielding information on the most remote parts of the universe. 

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. 

The longest wavelengths of the electromagnetic spectmm are sensitive probes of molecular rotation and hyperfine stmcture. An important appHcation is radio astronomy (23—26), which uses both radio and microwaves for chemical analysis on galactic and extragalactic scales. Herein the terrestrial uses of microwave spectroscopy are emphasized (27—29). 

Bulanin M. O., Orlova N. D. Spectroscopical investigations of molecular rotational motion in condensed media, In Spectroscopy of Interacting Molecules, ed. M. O. Bulanin, Leningrad State University, Leningrad, pp. 55-97 (1970). 

Microwave spectroscopy is generally defined as the high-resolution absorption spectroscopy of molecular rotational transitions in the gas phase. Microwave spectroscopy observes the transitions between the quantised rotational sublevels of a given vibrational state in the electronic ground state of free molecules. Molecular... 

For liquids, the collision rate is close to 1030 collisions s 1. Microwave spectroscopy, which studies molecular rotation, only uses dilute gases to obtain pure rotational states of sufficient lifetime. Rotational transitions are broadened by molecular collisions, because the pressure is close to a few tenths of a bar, as shown in Fig. 1.6. 

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],... 

Patel et al. °"> successfully operated parametric oscillators in the infrared region (2.5 - 25 pm) using the nonlinear characteristics of tellurium and selenium single crystals. This frequency range is important for the molecular spectroscopy of rotational-vibrational... 

The above-mentioned examples demonstrate that the ability to control and detect extreme molecular rotation opens up a very fmitfiil area of investigation in molecular dynamics and spectroscopy. 

A. I. Burshtein and S. 1. Tempkin, Spectroscopy of Molecular Rotation in Gases and Liquids , Cambridge University Press, Cambridge, 1994. 

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. 

NMR analysis of. sm>-D-ring taxane analogues <1999BML3041, 2000JNP726> supports the hypothesis that the oxetane serves to rigidify the overall molecular backbone (see Section 2.06.12.3). NOE and nuclear Overhauser effect spectroscopy (NOESY) experiments have been used to establish stereochemistry in taxanes, their synthetic precursors, and model structures <2005JOC3484, 2001S1013>. Fluorescence spectroscopy and rotational-echo double... 

Like proton transfer, the study of photoisomerization reactions has also benefited from developments in ultrafast spectroscopy, as cis-trans isomerization reactions typically occur on or within a few picoseconds, i.e. of the order of the timescale of a molecular rotation. 

R-type transition in spectroscopy. As a result of light absorption in this transition the difference A — J — J" between the quantum numbers of the angular momentum in excited (J ) and ground (J") state equals +1, and the angular momentum of the molecule increases. The transition with transition dipole moment d l at frequency u>o — fl corresponds to a diminution in the angular momentum of molecular rotation, and we have A = J — J" = — 1. Such a transition is called a P-type transition. 

The field gradient is measured at a fixed point within the molecule, the translational part of the wave-function is thus of no consequence for (qap)-The effect of molecular rotation does, however, modify (qap) but the relationship between the rotating and stationary (qap) s has already been treated in the chapter dealing with microwave spectroscopy. In the present context, we are interested in the field gradients in a vibrating molecule in a fixed coordinate system. The Born-Oppenheimer approximation for molecular wave-functions enables us to separate the nuclear and electronic motions, the electronic wave functions being calculated for the nuclei in various fixed positions. The observed (qap) s will then be average values over the vibrational motion. 

The determination of accurate molecular structure from molecular rotational resonance (MRR) spectra has always been a great challenge to this branch of spectroscopy [/]. There are three basic facts which make this task feasible (1) the free rotation of a rigid body is described in classical as well as in quantum mechanics by only three parameters, the principal inertial moments of the body, Ig, g = x, v, z ... 

This chapter is not intended as a general review. The intention is rather to present the most recent versions or variants of both, the rs- and the r0-method, on a small common background of theory but in sufficient depth and detail to enable the reader to follow, e.g., the flow of programs coded for the purpose. The recent versions of these two methods are the least restrictive with respect to size and composition of the set of isotopomers required and are applicable also to the largest molecules that can be treated by molecular rotational spectroscopy (perhaps ten heavy atoms plus hydrogen). 

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