OH/OD isotopic shift factor

The force constant is assumed to be the same for OH and OD bonds. According to Equation (2.26), the OH/OD isotopic shift factor should be 1.3736. Sometimes, the reciprocal value of i is used, which for the harmonic OH oscillator is equal to 0.728. 

Table 2.25 OH/OD isotopic shift factors / for surface hydroxyl groups before and after ...

However, analysis of published data shows that the measured isotopic shift factors for surface hydroxyls substantially deviate from this value. Reported OH and OD frequencies for various materials are summarized in Table 2.24. In most cases, the isotopic shift factor is around 1.356-1.358 and thus smaller than anticipated. For isolated hydroxyls, the factor is almost independent of v(OH) occasionally, a slight decrease with decreasing v(OH) is seen. A few significant deviations from the anticipated value stand out. For convenience, the values of i lower than 1.353 are highlighted by italics in Table 2.24. Most of these low values are associated... 

Table 2.24 Reported stretching frequencies of isolated surface OH and respective OD groups and calculated isotopic shift factors / v(OH), P(OD), OH group Sample cm cm / r Ref. ...

Similar variations of the isotopic shift factor have been reported for bulk hydroxyls (161,643—644). Various models were proposed and the anomalous isotopic shift factor was associated with the anharmonicity of the OH bond (161,644). In a recent review (643), three main causes of the anomalous OH/OD isotopic effects were identified (i) higher anharmonicity of the OH vibrations as compared to OD (ii) contribution of the bending modes via coupHng, and (iii) tunneling effects (taking place with strongly hydrogen-bonded systems). It was also pointed out that weak H-bonds complicate the situation because under these circumstances, the anharmonicity decreases. 

The OH and OD stretching modes of free OH radicals are at 3568.0 and 2632.1 cm respectively (645), which corresponds to an isotopic shift factor of 1.3556. This value is again lower than the theoretical one and is similar to the values reported in Table 2.24 for isolated surface hydroxyls. These measured frequencies of the stretching modes are known to be affected by anharmonicity. The harmonic OH and OD frequencies (calculated on the basis of overtone modes) are at 3735.2 and 2720.9 cm, respectively. Thus, the isotopic shift factor for the harmonic frequencies is 1.3728, which is very close to the theoretical value of 1.3736. The same value for the hydroxyl anion, OH, is 1.3726. Therefore, one can conclude that the deviations of the isotopic shift of free OH radicals and ions are mainly the result of anharmonicity. Similar calculations for isolated surface OH groups (21) show that also in this case, the deviations are mainly caused by anharmonicity (isotopic shift factor of the harmonic frequencies of 1.372-1.373). 

The exchange of with can be used to distinguish between OH and NH groups if their bands appear in the spectral region where both kinds of species can absorb. In principle, the H/D exchange should provide the same information, because the OH/OD and NH/ND isotopic shift factors are different (1.3744 and 1.3693, respectively, for harmonic oscillators). However, the deviations from theory (see above) make the results... 

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