Ethylene frequency shifts

The simple harmonic oscillator picture of a vibrating molecule has important implications. First, knowing the frequency, one can immediately calculate the force constant of the bond. Note from Eq. (11) that k, as coefficient of r, corresponds to the curvature of the interatomic potential and not primarily to its depth, the bond energy. However, as the depth and the curvature of a potential usually change hand in hand, the infrared frequency is often taken as an indicator of the strength of the bond. Second, isotopic substitution can be useful in the assignment of frequencies to bonds in adsorbed species, because frequency shifts due to isotopic substitution (of for example D for H in adsorbed ethylene, or OD for OH in methanol) can be predicted directly. 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

In many cases multiple probe molecules are used in order to deduce the existence of acid strength distributions. For example, the heterogeneity of the acid sites in H-ZSM-5 was studied by monitoring the adsorption of a series of weak electron donor molecules, such as hexane, fluorobenzene, chlorobenzene, benzene, toluene, p-xylene, and ethylene, to the acid sites. The frequency shift of the acidic OH group was taken as a... 

Now we return to Eq. (III.13) which, as we have discussed, gives the rotational energy levels within the experimental uncertainties for many asymmetric top molecules including ethylene oxide. Since, according to this equation, the Zee-man perturbation of the levels is linear with respect to the g-values and susceptibility anisotropies, the same must be true for the Zeeman splittings of the rotational transition frequencies. Thus, from each measured Zeeman satellite with a frequency shift Av H) with respect to the zero field frequency ... 

In their now classic study of the effect of surface forces on adsorbed molecules, Sheppard and Yates (26) found that some of the Raman-active vibrations of methane, ethylene, and hydrogen appeared in the infrared when these materials were adsorbed on silica. The frequency shifts for the molecule on going from the gas phase to the adsorbed phase were all rather small, indicating that no chemical change in the species was brought about by the adsorption. 

Brockmeier has studied this temperature eflFect at 22-313 K for PE and copolymers of ethylene with propylene (48). The results obtained by monitoring the frequencies of the three doublets could not be attributed to a simple contraction of the unit cell. London dispersion forces and changes in the helix of the molecule were possible complicating factors. Shen et al. have observed the shifting of the rocking doublet, ca. 725 cmfrom 10 K to room temperature for solution-crystallized PE (49). The 731 and 720 cm components are polarized along the a and b axes of the unit cell, respectively. Shen found no preferential correlation of either frequency s shift with x-ray measurements of expansion of a crystalline dimension. However, a linear relation was found for the frequency of the 731 cm" band with the specific volume of the unit cell. In both IR studies, frequency shifts were minimal below approximately 50 K which Brockmeier suggested arose from the lower limit of molecular motion in the crystal (48). 

Ebarvia et at reported two piezoelectric quartz sensors for caffeine based on MIPs [443,447]. In one work they coated a piezoelectric quartz sensor with MIPs templated by caffeine. They used the copolymerization reaction of methacryhc acid and ethylene glycol dimethacrylate monomers, using azobis(isobutyronitrile) as the initiator in chloroform. The sensor response was reported to be linear in the range of 1 x 10 mg mL and 1x10 mg mL in a stopped flow measurement mode [447]. In another work they used imprinted polymethacrylic acid to the same end, and reported a linear relationship between the frequency shift and the logarithm of the concentration between 1 X 10 up to 1 mg/mL and a detection limit of 5.9 X 10 mg/mL [443]. 

When halogens are substituted for the hydrogens next to the double bond, the stretching frequency shifts to higher values. For example, one fluorine substituted in ethylene will shift the band for ethylene from 1628 to 1650 cm while if a second fluorine atom is substituted on the same atom, the band will appear at 1730 cm" The presence of the halogen also enhances the intensity of the band. 

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

As presented in the example of ethylene oxide above, it is often beneficial to obtain the IR spectra of isotopomers of the system under study. The isotopomers also were useful in the interpretation of the IR spectra of cyclopropene. In Table 2 the observed and calculated (MP2/6-31G ) isotopic shifts for three of the isotopomers of cyclopropene are given. Comparison of the calculated shifts with those observed indicates that theory reproduces well experimental results. Such calculated shifts can be extremely useful in assigning the origins (symmetries) of the fundamental vibrational frequencies of the parent molecule. 

---