Acetaldehyde internal rotation

Absolute activity, 12, 13 Absolute intensity, 192 Acetaldehyde barrier height of internal rotation, 378, 382, 383, 388 Acetonitrile, in clathrate, 20... 

Platinum-cobalt alloy, enthalpy of formation, 144 Polarizability, of carbon, 75 of hydrogen molecule, 65, 75 and ionization potential data, 70 Polyamide, 181 Poly butadiene, 170, 181 Polydispersed systems, 183 Polyfunctional polymer, 178 Polymerization, of butadiene, 163 of solid acetaldehyde, 163 of vinyl monomers, 154 Polymers, star-shaped, 183 Polymethyl methacrylate, 180 Polystyrene, 172 Polystyril carbanions, 154 Potential barriers of internal rotation, 368, 374... 

Consider acetaldehyde, CH3CHO. Figure 8.3 shows a for the methyl and CHO protons to differ substantially, so that Vq ax Jax- The low barrier to internal rotation causes condition (1) to be satisfied. Hence the first-order analysis of the preceding paragraphs is applicable. We have an A3X case and the spectrum consists of a doublet (from the methyl protons) whose lines are of equal intensity and a quartet (from the CHO proton) whose lines have the intensity ratios 1 3 3 1 the doublet and quartet are well separated and show the same splitting (Fig. 8.9). 

Abraham and Pople (1960) measured the temperature dependence of the spin coupling constant in acetaldehyde and propionaldehyde. They were able to show that in the most stable forms the carbonyl group eclipses the methyl group in propionaldehyde and a hydrogen atom in acetaldehyde. Powles and Strange (1962) made more extensive measurements of JHH by the spin-echo method and assumed an earlier value of 1-16 kcal mole-1 for the energy barrier to internal rotation. 

Most barriers to internal rotation turn out to be repulsive dominant. Such is the case for methanol, methylamine, propane, propene, hydrazine, and, as has been seen, ethane and ethyl fluoride. Attractive dominant barriers are indicated for acetaldehyde, hydroxylamine, and hydrogen peroxide. 

Very recently lijima and Tsuchiya have developed the necessary theory for correcting B to Bz in the case of molecules with internal rotation of one symmetrical group, e.g. molecules containing a single methyl group." The results have been applied to acetaldehyde," acetyl chloride," and acetyl bromide."... 

The tables may be used to calculate barrier heights in cases where appropriate spectroscopic data are not available, but where experimental values of heat capacity or entropy are known at one or more temperatures. The calorimetrically determined value of the barrier height may then be used in conjunction with the tables to calculate internal rotation contributions to thermodynamic properties over an extended temperature range. Examples of this procedure include calculations for ethane," propene, acetaldehyde, buta-1,2-diene, acetic acid, hexafluoro-ethane, 3-methylthiophen, and 2-methylthiophen. Where spectroscopic values of the barrier height have subsequently been determined, satisfactory agreement has been obtained with the earlier calorimetric values. The agreement between calorimetric (8.16 kJ mol ) and subsequent micro-wave [(8.28 0.07) kJ mol ] values of the barrier height in propene... 

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. 

The barrier to internal rotation of the methyl group of acetaldehyde was initially determined by Kilb et al. from an analysis of the microwave spectrum. Since then, the values of the potential constants have been continually revised. Recently, Crighton and Bell combined the available microwave and infrared data with ab initio theory and refined the torsional parameters. Table 17 collects their internal rotation parameters. 

Table 17. Internal rotation parameters for X A acetaldehyde, thioacet-aldehyde and selenoacetaldehyde.

Figure 2 Internal rotation in dimethyl ether, methanol, acetone, acetaldehyde, and propene in descending order. In- keletal-plane and out-of-skeletal-plane hydrogen atoms are designated dark and light biue, respectively. The depiction is for simultaneous rotation to the SS conformers in the dimethyl compounds...

D Anna et al. (2003) showed that the NO3 reaction with CH2O proceeds by H-atom abstraction so that the sole products of this reaction are HNO3 HCO. Mora-Diez and Boyd (2002) examined the mechanism of the reactions between NO3 and formaldehyde and acetaldehyde theoretically comparisons with experiment are consistent with a direct abstraction mechanism. Alv ez-Idaboy et al. (2001a) reached a similar conclusion for NO3 reaction with formaldehyde, acetaldehyde, propanal, n-butanal, and 2-methylpropanal. Their calculations showed that all reactions proceed via abstraction of the a-carbonyl H-atom the dependence of the rate constant on molecular size was shown to be attributable to the increase in the internal rotational partition function with the size of the aldehyde. 

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