# Presentation: 175959

## ACTIVETHERMOCHEMICALTABLES

- BRANKO RUSCIC
- ruscic@anl.gov630.252.4079
- CHEMISTRY DIVISION
- ARGONNE NATIONAL LABORATORY

- Sept. 2001

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## Overview

- Basic Introduction to Thermochemistry ( Chemical Thermodynamics)
- How things are done traditionally (conventional Thermochemical Tables)
- What improvements did we introduce and how are we doing it now (manual Thermochemical Network approach)
- How will CMCS help us improve(automated Active Thermochemical Table, Interacting Active Tables)
- 	(Slides contain more than what will be covered – please do return to this presentation at your convenience and review the finer details)

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## Thermochemistry 101

- Measurable quantities (generally non-transferable):
- Changes relating to chemical reactions, e.g. enthalpies of reaction, kinetic equilibria, etc.OH   +   H2    H2O   +   H	 Hr°T(r) = a ± b; T = xxx(species-interconnecting quantities)
- Changes relating to one species, e.g. spectroscopic measurements of vibronic transitionsOH   X 23/2, v=1, N=5   OH   X 23/2, v=0, N=1;     E = c ± dconveniently condensed as spectroscopic constants (e, B0, Te,...)(species-specific quantities)
- “Distilled” quantities (transferable):
- Enthalpies of formation of various species       OH       +      H2            H2O      +     H      Hr°T = a ± b; T = xxx - Hf°T(OH) - Hf°T(H2) + Hf°T(H2O) + Hf°T(H) = Hr°T
- Entropies, enthalpy increments, heat capacities, etc

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## Thermochemical Tables 101

- Thermochemical Tables list, sorted by chemical species, the “distilled” quantities and their T‐ dependence as a table or polynomial.
- The *raison d’**ê**tre* for the tables is to look up “distilled” quantities at a particular temperature for several species and “reconstruct” the thermo-chemistry of some chemical reaction – whether ever measured or not:       OH       +      H2             H2O       +     H           - Hf°T(OH) - Hf°T(H2) + Hf°T(H2O) + Hf°T(H) Hr°T(r) = a ± b
- Thermochemical Tables can be:
- Un-annotated tabulations: NBS, CRC,...
- Annotated tabulations: Kerr in CRC, WebBook...(single ref. to Hf° source)
- Critical Data Evaluations: Gurvich et al., JANAF,... (provide some insight on the species-specific and species-interconnecting data, and discuss the reasons for preferring one measurement over another)

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## Typical JANAF Page (OH)

- T-dependent functions: Cp°, S°, (H°T – H0°), Hf°, ...

- Selected Hf° at afavorite T (0 or 298 K)

- Discussion & background info

- Molecularinfo

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## How are traditional Thermo-chemical Tables constructed

- Hf°s are developed via an elaborate sequential process.
- During each step a new species is added to the tabulation.
- All Hf°s determined in previous steps are “frozen”.
- The enthalpy of the new species is determined at one T from species-interconnecting measurements that are limited to those that relate the new species solely to species already defined in previous steps.
- T-dependent functions: Cp°, S°, (H°T – H0°), Hf°T, ... are developed by determining the partition function Q, ln Q, T(ln Q)/T and T22(ln Q)/T2  from species-specific quantities and the newly selected single-temperature enthalpy.

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## How are traditional Thermo-chemical Tables constructed (2)

- The sequential process follows a *“standard” order* *of chemical elements*: O  H  halogens  noble gases  chalcogens  pnicogens  carbon period  ...  For every chemical element introduced, the sequence starts at the standard state for that element, for which Hf°=0 by convention.
- Enthalpies of formation have complex hidden dependencies: traceable backward with effort; generally not traceable forward

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## Deficiencies of standard Thermochemical Tables

- Sequential approach results in Hf°s and their error bars that do not properly reflect the global relationships implied by the species-interconnecting data used
-  Values & error bars reflect – at best – only local relationships to nearest neighbors
-  Lack of feedback from species-interconnecting data to already “frozen” enthalpies causes cumulative error
- Hidden relationships in conventional Tables produce Thermodynamic Tables that are really “static”
-  Proper update with new knowledge is nearly impossible since forward relationships are non-transparent
-  Updating one species may improve things locally, but increases the global inconsistencies

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## The Thermochemical Network approach

- Species-interconnecting data form a thermochemical network

- Vertices (nodes – square boxes) are the sought-for Hf°s for every species
- Edges (links – lines with ovals) are the species-interconnecting measurements (reaction enthalpies, equilibria,...)
- There are generally multiple links (competing measurements) between nodes* (**in the example, each number in the oval corresponds to one measurement and its error bar)*

- The best set of Hf°s describing the underlying knowledge is obtained by simultaneous solution of the thermochemical network

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## How is the Network constructed

- The Thermochemical Network is defined by the relationships implied by the species-interconnecting data.
- Ideal network:
- Encompasses the complete universe of thermochemically relevant measurements
- The network is terminated only by references to elements in standard states for which Hf°T = 0.
- Practical network (amenable to manual treatment):
- Localized by judiciously selecting a related subset of chemical species
- The excised sections are substituted with references to Hf°s that are not likely to be affected by the solution of the network and can be treated as constants [e.g. measurements on aromatic compounds are not likely to affect Hf°(H), Hf°(H2O), or Hf°(CO2)]
- Species-specific data is exploited to bring the whole Network to a common denominator (e.g. all species-interconnecting measurements are reduced to Hr°s at the same T).

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## How is the Network solved

- Statistical analysis of the Network is an iterative process that proceeds both through linear and error-weighted space and examines the Network for internal consistency.
- The error bar of each and every species-interconnecting measurement is examined within the context of the network
- During this process, “wrong” measurements (i.e. measurements with optimistic error bars) are identified. Adjustments to error bars of “offending” measurements are proposed and retested.
- Once the Network has reached self-consistency, a simultaneous solution to all Hf°s is found by minimizing in error-weighted space.
- Species-specific data, together with solved Hf°s, is used to obtain the full complement of thermodynamic functions and their T-dependance.

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## Active Thermochemical Table

- An Active Thermochemical Table (ATcT) is an automated machine designed to produce a standard Thermochemical Table using the Network methodology.
- ATcT consists of:
- Engine: carries out the statistical analysis and simultaneous solution
- Input data: species specific and species-interconnecting
- Core input data (e.g. anointed by the Table Keeper or otherwise agreed on)
- Amendments to core data (new measurements, “what if” data, etc...)
- and produces
- Output data: an internally-consistent Thermochemical Table,i.e. a full complement of thermodynamic functions for every species considered;the output can be queried just as any normal Thermochemical Table
- Standard version (e.g. anointed) when based on core data alone
- Tentative/private/test version(s) when input includes various amendments

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## Advantages of an Active Thermochemical Table

- 	ATcT provides a new paradigm to thermochemistry
- The output Thermochemical Table is superior to conventional tables:
- Values are globally consistent with with all input data
- Error bars properly reflect all relationships presented as input
- Allows painless propagation of new data and its consequences
- Opens a new venue of rapid availability of latest information (including tentative information)
- Allows “what if” tests of:
- New data for consistency (or lack thereof) with the existing knowledge
- Educational explorations, including hypothetical data
- By finding the weakest links it can suggest which new experiments/ calculations will have the highest impact on current knowledge
- Supporting documents relating to input data are easily incorporated for examination

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## Interacting Active Tables

- By expanding the Active Table idea, one can envision adding “intelligence” to other identifiable databases at key levels of scale
- For example, an Active Rate Table placed above the ATcT would make use of the relationships implied in a tabulation of elementary kinetic rate measurements (e.g. rate determinations that are relative to other rates or rely on incorporating other rates, forward-reverse rate relationships implied by thermochemistry, ...)
- Or, down at the bottom there is, at least in principle, an Active Table of Natural Constants, reproducing the periodic re-evaluations of fundamental physical constants by Cohen & Taylor/Mohr & Taylor (which also involve solving a network of measurements).
- A series of stacked Interacting Active Tables, placed at strategic scale levels can exchange information up and down the scales in an automated fashion, until global consistency across all involved scales is reached

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## Benefits of theInteracting Active Tables approach

- The setup allows an accelerated convergence to solutions by:
- Producing data that is globally consistency across all scales (adding a *new dimension* to the flow of information between scales).
- Rapid propagation of new knowledge through all scales.
- Convenient seamless access to data across scales.
- Exploratory “what if” tests and explicit rapid* *check of new data for global consistency (a *new paradigm* to how science is conducted)
- Statistical analysis of the available databases (whether including new data or not) aimed to provide information on the "weakest links" in the databases and hence suggesting new high-impact experiments/calculations (another *new paradigm* to how science is conducted)

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## More Information

- For more information
- login to the _DocuShare_, drill down to the folder_Use Cases/Collected Use Cases/Active Table Use Cases _	and examine:
- _Introduction to Active Thermochemical Tables_
- _Scheme for Intro to Active Thermochemical Tables_
- _A Note on Interacting Active Tables_
- _Scheme of Interacting Active Tables_
- 	Visit also the various ATcT-nn Use Cases in the same folder
- e-mail to _Branko Ruscic_ or call 630.252.4079
- see J. Phys. Chem. A **102**, 10889 (1998)and J. Phys. Chem. A **103**, 8625 (1999) for examples of local thermochemical networks

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