The Molecule. Spectra

From electron diffraction studies, distances and angles in the tetrahedral Pb(CH3)4 molecule have been determined [12] and collected below together with values from older measurements [1,7]  

From stretching and bending force constants, a C-Pb-C angle of 109.5 and distance of 3.74 A between the methyl groups was estimated [4], whereas from y(CH) vibrations a C-H distance of 1.094 A was predicted [13] see also [10]. The bond lengths Pb-C = 2.17 A and C-H = 1.10 A, and the angles C-Pb-C = 109.5 , H-C-Pb = 109.3 , and H-C-H = 109.8 have been calculated by the MNDO method [18, 19], whereas a value of 2.247 A has been obtained from Hartree-Fock calculations accounting for relativistic effects [20]. 

The minimum distance of approach of rotating methyl groups was estimated to be 3.046 A [2] and it was inferred that, using a Pb-C distance of 2.23 A, the rotation of the methyl groups is not [2] or probably not hindered [3]. The barrier to methyl rotation is rather low [15]. 

According to X-ray diffraction [17] and neutron scattering measurements [16], a threefold axis exists at the molecular site of solid Pb(CH3)4, the molecule being compressed along this axis space group Pa3, Z = 8 [16, 17]. Pb(CH3)4 is isomorphous with Sn(CH3)4 [16]. Calorimetric studies showed no evidence that Pb(CH3)4 can crystallize in more than one form [3, 5]. The van der Waals volume of lead derived from Pb(CH3)4 data is calculated to be 17.8 cm /mol [11]. 

The partial charge on the methyl groups was estimated to be 0.004 [6]. For measured data of the density of states, see [16]. 

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Electron Spin Resonance Electron spin resonance measures the effect of micro-waves on a molecule with spin (usually a free radical or triplet) in a magnetic field. The detection can be either through the absorption of microwave energy (conventional ESR) or the effect of a microwave frequency on the emission of light (Fluorescence Detected Magnetic Resonance FDMR). Because the transition energy of the electron in a molecule depends on the interaction of that electron with much of the molecule, spectra have many lines and contain substantial information about the structure of the species being studied. 

Franck-Condon principle According to this principle the time required for an electronic transition in a molecule is very much less than the period of vibration of the constituent nuclei of the molecule. Consequently, it may be assumed that during the electronic transition the nuclei do not change their positions or momenta. This principle is of great importance in discussing the energy changes and spectra of molecules. 

These characteristic absorption regions called group frequencies allow the analyst to detect the different elemental patterns and from them to reconstruct the molecule either by dej duct ion or by comparison with library reference spectra. The libraries contaih severaY hundred thousand spectra. 

Resonance Raman reflection spectroscopy of monolayers is possible, as illustrated in Fig. IV-14 for cetyl orange [157]. The polarized spectra obtained with an Ar ion laser allowed estimates of orientational changes in the cetyl orange molecules with a. 

An interesting point is that infrared absorptions that are symmetry-forbidden and hence that do not appear in the spectrum of the gaseous molecule may appear when that molecule is adsorbed. Thus Sheppard and Yates [74] found that normally forbidden bands could be detected in the case of methane and hydrogen adsorbed on glass this meant that there was a decrease in molecular symmetry. In the case of the methane, it appeared from the band shapes that some reduction in rotational degrees of freedom had occurred. Figure XVII-16 shows the IR spectrum for a physisorbed H2 system, and Refs. 69 and 75 give the IR spectra for adsorbed N2 (on Ni) and O2 (in a zeolite), respectively. 

The resonance vector analysis has been used to explore all of the questions raised above on the fate of the polyad numbers in larger molecules, the most thoroughly investigated case so far probably being C2FI2- This molecule has been very extensively probed by absorption as well as stimulated emission pumping and dispersed fluorescence teclmiques [, 53, 70 and 71], the experimental spectra have been analysed in... 

Rosenstock H M, Wallenstein M B, Wahrhaftig A L and Frying H 1952 Absolute rate theory for isolated systems and the mass spectra of polyatomic molecules Proc. Natl Acad. Sci. USA 38 667-78... 

There are two fimdamental types of spectroscopic studies absorption and emission. In absorption spectroscopy an atom or molecule in a low-lying electronic state, usually the ground state, absorbs a photon to go to a higher state. In emission spectroscopy the atom or molecule is produced in a higher electronic state by some excitation process, and emits a photon in going to a lower state. In this section we will consider the traditional instrumentation for studying the resulting spectra. They define the quantities measured and set the standard for experimental data to be considered. 

The SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

In principle, every nucleus in a molecule, with spm quantum number /, splits every other resonance in the molecule into 2/ -t 1 equal peaks, i.e. one for each of its allowed values of m. This could make the NMR spectra of most molecules very complex indeed. Fortunately, many simplifications exist. 

Figure Cl.5.4. Comparison of near-field and far-field fluorescence images, spectra and lifetimes for the same set of isolated single molecules of a carbocyanine dye at a PMMA-air interface. Note the much higher resolution of the near-field image. The spectmm and lifetime of the molecule indicated with the arrow were recorded with near-field excitation and with far-field excitation at two different excitation powers. Reproduced with pennission from Trautman and Macklin [125].

Figure Cl.5.8. Spectral jumping of a single molecule of terrylene in polyethylene at 1.5 K. The upper trace displays fluorescence excitation spectra of tire same single molecule taken over two different 20 s time intervals, showing tire same molecule absorbing at two distinctly different frequencies. The lower panel plots tire peak frequency in tire fluorescence excitation spectmm as a function of time over a 40 min trajectory. The molecule undergoes discrete jumps among four (briefly five) different resonant frequencies during tliis time period. Arrows represent scans during which tire molecule had jumped entirely outside tire 10 GHz scan window. Adapted from...

Schaaff T G efa/1997 Isolation of smaller nanocrystal Au molecules robust quantum effects in the optical spectra J. Phys. Chem. B 101 7885... 

PERMUTATIONAL SYMMETRY AND THE ROLE OF NUCLEAR SPIN IN THE VIBRATIONAL SPECTRA OF MOLECULES IN DOUBLY DEGENERATE ELECTRONIC STATES ... 

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