Proposal of a Standard Unit for Turnover Frequencies : Hz

The authors of this manuscript propose turnover frequencies to be expressed in an International System of Units existing unit (Hz), facilitating the comparison between different catalytic systems.

Since the Greek civilization, humanity has been dealing with measurement as a science.Originally, measures were taken as multiples of a known object like a foot or an inch.After the French revolution, the French government imposed the metric system as an attempt to standardize measurements, although its use is not yet spread all over the world, especially in countries originated from the former British Empire.Nevertheless, throughout the 20 th century, the metric system has established itself as the International System of Units incorporated it and some other basic units to allow the measurement of virtually all magnitudes on nature. 1 Derived units such as joule, watt and hertz (Hz) are used to make simpler the expression of quantities, facilitating their use and comparison.
In catalysis, even in present days, we also face a lack of uniformity when expressing a very useful parameter for the efficiency of a catalytic system: turnover frequencies (TOFs).This parameter is also sometimes called "activity", especially in polymerization and is used to refer to the number of moles of substrate that can be converted per mole of catalyst per unit of time.In the literature, we usually find this presented with the dimension time −1 , but Proposal of a Standard Unit for Turnover Frequencies: Hz the unit of time is not standardized.A search performed on ISI database 2 tagging "turnover frequencies" and retaining the 15 most recent papers listed serves as a sample of this non-uniformity, as shown in Table 1.
In order to show that this is not only a recent discrepancy, we also performed a research considering the 15 most cited papers on ISI database using the same tag, on the same day (Table 2).
It becomes then unnatural for a researcher to compare different catalytic systems without calculations.By using Hz, an existing unit which expresses cycles per second, this comparison becomes immediate, as shown in Table 3, where reactions were grouped and TOFs recalculated having Hz as unit.
The use of Hz to express TOFs is not only a more elegant alternative, but it allows an immediate comparison of different catalytic systems under different conditions.It is also possible to use multiplicative prefixes.For example, for many relevant industrial applications, the turnover frequency varies between 10 −2 -10 2 s −1 , while it is in the range of 10 3 -10 7 s −1 for enzymes, 33 and these two data could be classified into mHz, Hz, kHz or MHz, highlighting distinct activities.
Thus, we propose the use of the standard unit of frequency hertz whenever TOFs and activities are referred, in order to facilitate comparison of scientific work and didactically consolidate even further the image of the catalytic cycle and the International System of Units.

Table 1 .
Data collected from the 15 most recent papers on ISI database (May 5 th , 2014) having "turnover frequency" as a topic a Theoretical value.

Table 2 .
Data collected from the 15 most cited papers (May 5 th , 2014) on ISI database which have "turnover frequency" as a topic.TOF presented is the highest reported

Table 3 .
Compared turnover frequencies expressed in Hz