Vibrational frequencies, selected molecule

Solvation of solute vs. basis set -acetic acid and acetate. As a test of molecular orbital theory and the molecular cluster approximation to reproduce experimental vibrational frequencies, the molecules acetic acid and acetate were selected. The gas-phase frequencies of acetic acid were reproduced fairly well with molecular orbital theory. Consequently, discrepancies between observed and calculated frequencies for the aqueous species may be attributed to solvation effects. Acetate represents a bigger challenge because the charged species is likely to have stronger H-bonding associated with its solvation shell. 

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

SFG [4.309, 4.310] uses visible and infrared lasers for generation of their sum frequency. Tuning the infrared laser in a certain spectral range enables monitoring of molecular vibrations of adsorbed molecules with surface selectivity. SFG includes the capabilities of SHG and can, in addition, be used to identify molecules and their structure on the surface by analyzing the vibration modes. It has been used to observe surfactants at liquid surfaces and interfaces and the ordering of interfacial... 

The values of the harmonic vibrational frequencies for the selected set of molecules are presented in Table VII. The expected trends of this property with respect to experiment are well reproduced. The HF results are systematically larger and the correlated vibrational frequencies move closer to... 

Anharmonicity leads to deviations of two kinds. At higher quantum numbers, AE becomes smaller, and the selection rule is not rigorously followed as a result, transitions of A 2 or 3 are observed. Such transformations are responsible for the appearance of overtone lines at frequencies approximately two or three times that of the fundamental line the intensity of overtone absorption is frequently low, and the peaks may not be observed. Vibrational spectra are further comphcated by the fact that two different vibrations in a molecule can interact to give absorption peaks with frequencies that are approximately the sums or differences of their fundamental frequencies. Again, the intensities of combination and difference peaks are generally low. 

At one stage this vibrational photochemistry held the promise of mode selectivity , the possibility of tuning the excitation wavelength to excite one particular vibrational mode in a polyatomic molecule. It would then be possible to select one particular bond for dissociation, simply by changing the wavelength of the irradiation beam according to the vibrational frequency (mode) of that bond. In practice this mode selectivity has not been observed. No matter which specific vibration is excited by multiphoton IR absorption, the energy is very quickly distributed statistically over all the available vibrational modes, so that the weakest bonds dissociate faster. 

Raman intensities were calculated by differentiation of the molecular polarizability with respect to nuclear coordinates. It is often sufficient or desirable to calculate only Raman intensities for selected modes instead of for all 3N - 6 vibrational modes of a large molecule, which can be achieved if the normal modes of the molecule are already known. Therefore, a frequency analysis was performed using numerical differentiation of analytical gradients with respect to Cartesian nuclear coordinates in the first step. This yields vibrational frequencies and normal modes. Then, we used displacements along selected mass-weighted normal coordinates Q/t, for which (static and/or dynamic) polarizabilities are calculated. With a step size SQk,... 

The structural parameters and vibrational frequencies of three selected examples, namely, H2O, O2F2, and B2H6, are summarized in Tables 5.6.1 to 5.6.3, respectively. Experimental results are also included for easy comparison. In each table, the structural parameters are optimized at ten theoretical levels, ranging from the fairly routine HF/6-31G(d) to the relatively sophisticated QCISD(T)/6-31G(d). In passing, it is noted that, in the last six correlation methods employed, CISD(FC), CCSD(FC),..., QCISD(T)(FC), FC denotes the frozen core approximation. In this approximation, only the correlation energy associated with the valence electrons is calculated. In other words, excitations out of the inner shell (core) orbitals of the molecule are not considered. The basis of this approximation is that the most significant chemical changes occur in the valence orbitals and the core orbitals remain essentially intact. On... 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

Vibrational sum-frequency spectroscopy (VSFS) is a second-order non-linear optical technique that can directly measure the vibrational spectrum of molecules at an interface. Under the dipole approximation, this second-order non-linear optical technique is uniquely suited to the study of surfaces because it is forbidden in media possessing inversion symmetry. At the interface between two centrosymmetric media there is no inversion centre and sum-frequency generation is allowed. Thus the asynunetric nature of the interface allows a selectivity for interfacial properties at a molecular level that is not inherent in other, linear, surface vibrational spectroscopies such as infrared or Raman spectroscopy. VSFS is related to the more common but optically simpler second harmonic generation process in which both beams are of the same fixed frequency and is also surface-specific. 

Another technique that shows great promise and is still being developed is laser enrichment, which can be applied to either atoms or molecules (Figure 10.6). The molecular laser process exploits the fact that UFs and UFe molecules have slightly different vibrational frequencies (of the order of 0.5 cm ). The UFe molecules in UFe vapour (supercooled in order to produce sharper absorption bands) are selectively excited with a mneable IR laser, then irradiated with a high-intensity UV laser, whereupon the excited UFe molecules are photodecomposed into UFs (the UFe molecules are unaffected). Under these conditions, the UFs is a solid, and is separated from the UFs (using a sonic impactor). 

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