Vibrations stretching

Huang Z S, Jucks K W and Miller R E 1986 The vibrational predissociation lifetime of the HF dimer upon exciting the free-H stretching vibration J. Cham. Phys. 85 3338-41... 

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

Owrutsky J C, Li M, Locke B and Hochstrasser R M 1995 Vibrational relaxation of the CO stretch vibration in hemoglobin-CO, myoglobin-CO, and protoheme-CO J. Rhys. Chem. 99 4842-6... 

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

B2.5.11 and B2.5.12. The energy levels of the C-I stretching vibration are not drawn to scale. In reality their separation is much smaller. Adapted from [109]. 

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...

Van der Waals complexes can be observed spectroscopically by a variety of different teclmiques, including microwave, infrared and ultraviolet/visible spectroscopy. Their existence is perhaps the simplest and most direct demonstration that there are attractive forces between stable molecules. Indeed the spectroscopic properties of Van der Waals complexes provide one of the most detailed sources of infonnation available on intennolecular forces, especially in the region around the potential minimum. The measured rotational constants of Van der Waals complexes provide infonnation on intennolecular distances and orientations, and the frequencies of bending and stretching vibrations provide infonnation on how easily the complex can be distorted from its equilibrium confonnation. In favourable cases, the whole of the potential well can be mapped out from spectroscopic data. 

The intennolecular bending and stretching vibrations of Van der Waals complexes typically have wave-numbers... 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

It is tlius of great interest to carry out experiments tliat excite tire intennolecular bending and stretching vibrations directly, witliout exciting tire monomers as well. These transitions he deep in tire far infrared, typically in tire 20-... 

Figure C2.18.7. The integrated absorbance of tire Si-Cl stretching vibration at 625 cm and tire SiO-H stretching vibration at 3740 cm as a function of time during tire (A) SiCl and (B) H2O half-reactions at 600 K and 10 Torr. Reproduced from [95].

Once prepared in S q witli well defined energy E, donor molecules will begin to collide witli batli molecules B at a rate detennined by tire batli-gas pressure. A typical process of tliis type is tire collision between a CgFg molecule witli approximately 5 eV (40 000 cm or 460 kJ mor ) of internal vibrational energy and a CO2 molecule in its ground vibrationless state 00 0 to produce CO2 in tire first asymmetric stretch vibrational level 00 1 [11,12 and 13]. This collision results in tire loss of approximately AE= 2349 cnA of internal energy from tire CgFg,... 

An ultrashort mid-IR pulse excited a C-H stretching vibration (-3000 cm ) of neat acetonitrile at 300 K. The loss of C-H stretching energy occurred in 3 ps. Only 1% of that energy was transferred to the C N stretch (2250 cm ), where it remained for -80 ps. Most of the energy was lost from the C-H stretch by the process,... 

Explicit forms of the coefficients Tt and A depend on the coordinate system employed, the level of approximation applied, and so on. They can be chosen, for example, such that a part of the coupling with other degrees of freedom (typically stretching vibrations) is accounted for. In the space-fixed coordinate system at the infinitesimal bending vibrations, Tt + 7 reduces to the kinetic energy operator of a two-dimensional (2D) isotropic haiinonic oscillator. 

The appearance of the (normally small) linear term in Vis a consequence of the use of reference, instead of equilibrium configuration]. Because the stretching vibrational displacements are of small amplitude, the series in Eqs. (40) should converge quickly. The zeroth-order Hamiltonian is obtained by neglecting all but the leading terms in these expansions, pjjjf and Vo(p) + 1 /2X) rl2r and has the... 

In an atomic level simulation, the bond stretch vibrations are usually the fastest motions in the molecular dynamics of biomolecules, so the evolution of the stretch vibration is taken as the reference propagator with the smallest time step. The nonbonded interactions, including van der Waals and electrostatic forces, are the slowest varying interactions, and a much larger time-step may be used. The bending, torsion and hydrogen-bonding forces are treated as intermediate time-scale interactions. 

Band 4, 5-9411 (1683 cm. ). Aryl carboxylic acid C=0 stretching vibration (Table IV). 

When the above analysis is applied to a diatomic species such as HCl, only k = 0 is present since the only vibration present in such a molecule is the bond stretching vibration, which has a symmetry. Moreover, the rotational functions are spherical harmonics (which can be viewed as D l, m, K (Q,< >,X) functions with K = 0), so the K and K quantum numbers are identically zero. As a result, the product of 3-j symbols... 

The above figure shows part of the infrared absorption spectrum of HCN gas. The molecule has a CH stretching vibration, a bending vibration, and a CN stretching vibration. 

Carbon-hydrogen stretching vibrations with frequencies above 3000 cm are also found m arenes such as tert butylbenzene as shown m Figure 13 33 This spectrum also contains two intense bands at 760 and 700 cm which are characteristic of monosub stituted benzene rings Other substitution patterns some of which are listed m Table 13 4 give different combinations of peaks... 

In addition to sp C—H stretching modes there are other stretching vibrations that appear at frequencies above 3000 cm The most important of these is the O—H stretch of alcohols Figure 13 34 shows the IR spectrum of 2 hexanol It contains a broad peak at 3300 cm ascribable to O—H stretching of hydrogen bonded alcohol groups In... 

Section 15 14 The hydroxyl group of an alcohol has its O—H and C—O stretching vibrations at 3200-3650 and 1025-1200 cm respectively The chemical shift of the proton of an O—H group is variable (8 1-5) and depends on concentration temperature and solvent Oxygen deshields both the proton and the carbon of an H—C—O unit Typical... 

The C=0 stretching vibrations of these compounds may be viewed on Learning By Modeling... 

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

Stretching vibration (Section 13 20) A regular repetitive mo tion of two atoms or groups along the bond that connects them... 

---