Microwave spectroscopy, with highly

Microwave spectroscopy can determine the magnitude of V6 in S0 but not the sign, since the potential well is too small to localize even the m = 0 wavefunction. S, <— S0 absorption spectra of cold molecules with 1 cm"1 resolution can reveal the magnitude of V6 in S, a technique pioneered by Ito and coworkers.4 Pratt and coworkers7 and Miller and coworkers8 have made major contributions to the high-resolution optical spectroscopy of rotor-containing molecules. 

The techniques available to achieve molecular structure determinations are limited. They include structural analysis with diffraction techniques—such as electron, neutron, and x-ray diffraction—and various absorption and emission techniques of electromagnetic radiation—such as microwave spectroscopy and nuclear magnetic resonance (NMR). For molecules with unpaired spins a companion technique of electron spin resonance spectroscopy (ESR) is highly informative. 

Microwave spectroscopy is probably the ultimate tool to study small alcohol clusters in vacuum isolation. With the help of isotope substitution and auxiliary quantum chemical calculations, it provides structural insights and quantitative bond parameters for alcohol clusters [117, 143], The methyl rotors that are omnipresent in organic alcohols complicate the analysis, so that not many alcohol clusters have been studied with this technique and its higher-frequency variants. The studied systems include methanol dimer [143], ethanol dimer [91], butan-2-ol dimer [117], and mixed dimers such as propylene oxide with ethanol [144]. The study of alcohol monomers with intramolecular hydrogen-bond-like interactions [102, 110, 129, 145 147] must be mentioned in this context. In a broader sense, this also applies to isolated ra-alkanols, where a weak Cy H O hydrogen bond stabilizes certain conformations [69,102]. Microwave techniques can also be used to unravel the information contained in the IR spectrum of clusters with high sensitivity [148], Furthermore, high-resolution UV spectroscopy can provide accurate structural information in suitable systems [149, 150] and thus complement microwave spectroscopy. 

Closely related to conformational energy differences are barriers to single-bond rotation and to pyramidal inversion. Here the experimental data are restricted to very small systems and derive primarily from microwave spectroscopy, from vibrational spectroscopy in the far infrared and from NMR, but are generally of high quality. Comparisons with calculated quantities are provided in Table 8-3 for single-bond rotation barriers and Table 8-4 for inversion barriers. The same models considered for conformational energy differences have been surveyed here. 

This kind of microwave spectroscopy is the best technique available for determining the structure of small molecules in the gas phase. Microwave frequencies can be measured with extremely high accuracy, permitting bond length measurements literally... 

Within the frequency range up to 200 GHz the klystron can be replaced (at a price) by synthesisers, coupled with solid state microwave amplifiers and passive or active frequency multipliers. These devices have very high frequency stability, are easily modulated either in frequency or power, and are readily compatible with computer control of all their main fbnctions. The klystron is gradually becoming redundant, but has an honoured place in the development of microwave spectroscopy. 

High-accuracy molecular dimensions for the parent monocyclic heterocycles have been determined by micro-wave spectroscopy and these can be found in Section 2.3.3.2 (Table 7). The ring dimensions obtained by X-ray diffraction for the 2-carboxylic acid derivatives of furan, thiophene, selenophene, and tellurophene (Table 3) are generally in good agreement with those obtained for the parent heterocycles using microwave spectroscopy (Table 7). 

Highly accurate molecular geometries for thiophene, deuteriothiophene, and C-labeled thiophene have been obtained by microwave spectroscopy. The molecular structure has also been determined by ED. Results from ED compare reasonably well with microwave spectral analysis, except for the C-H bond lengths, which are somewhat smaller than those determined from microwave spectroscopy. 

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