Potential barriers hindering internal

Barrirrrs (see Potential barriers hindering internal rotation)... 

The Si PES, calculated by Nonella and Huber (1986), has a shallow minimum above the ground-state equilibrium, or expressed differently, a small potential barrier hinders the immediate dissociation of the excited S complex. Although the height of the barrier is less than a tenth of an eV, it drastically affects the dissociation dynamics, even at energies which significantly exceed the barrier. The excited complex lives for about 5-10 internal NO vibrational periods before it breaks apart. The photodissociation of CH3ONO through the Si state exemplifies indirect photodissociation or vibrational predissociation (Chapter 7). 

Hindered Rotation (kT to) With hindered rotation, the potential energy of the internal rotation is restricted by a potential barrier, Vq, whose magnitude varies as the two parts of the molecules rotate past each other in a cyclic fashion. For example, in the molecule H3C-CCI3, the potential varies as the hydrogen atoms on one carbon move past the chlorine atoms on the other. 

In indirect photofragmentation, on the other hand, a potential barrier or some other dynamical force hinders direct fragmentation of the excited complex and the lifetime amounts to at least several internal vibrational periods. The photodissociation of CH3ONO via the 51 state is a representative example. The middle part of Figure 1.11 shows the corresponding PES. Before CH30N0(5i) breaks apart it first performs several vibrations within the shallow well before a sufficient amount of energy is transferred from the N-0 vibrational bond to the O-N dissociation mode, which is necessary to surpass the small barrier. 

Although eqs. (7) and (8) are devised for free internal rotation with no hindering potential barriers, modification of the computational procedure (Appendix I) permits adequate treatment of cases involving small potential barriers. The assumptions made in this treatment of rotations and internal rotations have been tested17 and the equations seem adequate for kinetics purposes. 

A vibrational degree of freedom may be replaced by internal rotation (torsion) around a a bond. In this case the microwave spectrum of the molecule is modified by torsion-rotation interaction. By studying this effect on the rotational spectrum, the internal rotation potential barrier can be determined. The hindering potential of CH3N3 was found to be V3 = 695 20 cal/mole (the subscript 3 stands for the 3-fold axis of the hindering potential). The potential is rather small but is not smaller than the value expected from a hyperconjugation effect . 

Other estimated values that have been reported include 200 cm ( ) and 37 cm (5 ). The inactive torsional frequency is treated as a hindered Internal rotation. We use an estimated potential barrier of 8.0 kcal mol ( ) to calculate heat capacity contributions for hindered rotation from the table of Pitzer and Brewer (9). Contributions below 201 K could not be obtained by... 

In the next step it will be shown how the information on the hindering potential may be inferred from the fine structure of rotational spectra. We start with the pure internal-rotation Hamiltonian Hj [Eq. (5)], which contains one mass-geometry-dependent constant, F, and the potential parameters. First, assume V3 alone is important. Both parameters may be incorporated in a reduced potential barrier s, defined as... 

The most accurate information on the magnitudes of potential barriers (b) is derived from microwave spectra. Review papers on the determination of banier heists hindering internal rotation have been publi ed by Wilson and Lowe Extensive data on barriers are given in these summaries see also Refs. ... 

The effects seen above with sugars are to be expected with other complex molecules which possess internal motions that can be described by a potential function possessing more than one minimum and which are hindered by a relatively small potential barrier in the solid state. 

The correction to be made to to obtain is a function of three properties the temperature T, the potential energy barrier V of the hindered internal rotation and the partition function of the free internal rotation. 

Most internal rotations are hindered and it is therefore necessary to take this potential barrier to rotation into account, which can be different for R and RH. The subscript h will be given to a hindered internal rotation. Applying equation (17) to R and RH ... 

This permits a reconstruction of full 2-D potential energy surface for Ar + HF(v l), which is shown in Fig. 3, and exhibits the double minimum behavior (ArHF and ArFH configurations) predicted from multiproperty fits on rare gas-HCi complexes by Hutson and Howard.The barrier between the two minima is considerably above the j l HF rotor energy, and thus the Ar-HF complex is an example of a strongly hindered internal rotor. 

Many applications of Kilpatrick and Pitzer s procedure for calculating thermodynamic properties of molecules with compound rotation have been reported. In all cases possible potential energy cross-terms between rotating tops have been neglected. Contributions from internal rotation of symmetric tops have been calculated using the appropriate tables." These tables have also been used in calculations for the internal rotation of asymmetric tops hindered by a simple -fold cosine potential. 3-Fold potential barriers have been assumed in calculations for the —OH rotations in propanol and 1-methylpropanol, the —SH rotations in propane-1-thiol, butane-2-thiol, 2-methylpropane-l-thiol, and 2-methylbutane-2-thiol, the C—S skeletal rotations in ethyl methyl sulphide, diethyl sulphide, isopropyl methyl sulphide, and t-butyl methyl sulphide, and the C—C skeletal rotations in 2,3-dimethylbutane, and 2-methylpropane-l-thiol. 2-Fold cosine potential barriers have been assumed in calculations in the S—S skeletal rotations in dimethyl disulphide and diethyl disulphide. ... 

The internal dynamics of the methyl group immensely complicates the spectroscopy of these molecules. Of course, this aspect of the problem also provides much of the spectroscopic interest. When the methyl hydrogens of acetaldehyde oscillate around the CC axis, they experience forces arising from the CHO frame of the molecule which vary sinusoidally. As a result, the potential function for internal rotation can be represented by a cosine function in which the crest to trough distance measures the height of the potential barrier. Since the energy barrier to methyl rotation is low in acetaldehyde, the internal motion is one of hindered internal rotation, rather than torsional oscillation. 

---