Sometimes you have no choice but to go back to the original journal literature to find the data you need. SciFinder often provides both experimental and calculated data for individual compounds. Search for chemical data using a molecular formula, chemical name, chemical structure, reaction components, author name, or topic of interest.
Reaxys is a unique web-based chemistry database consisting of deeply excerpted compounds and related factual properties, reaction and synthesis information as well as bibliographic data, navigated and displayed via an actionable interface.
(Mallet Chemistry Library, University of Texas at Austin)
Searching for Data, in General
The resources listed here are useful in identifying additional data collections and for discussions on the critical evaluation of data:
Handbooks and Tables in Science and Technology
3rd ed. Powell, R.H. Oryx Press: Phoenix, AZ, 1994. (BSEL Reference Z7405 .T3 P68 1994)
Descriptions of over 3,600 handbooks and tables in science, technology and medicine. Indexed by subject, author/editor and title.
The Search for Data in the Physical and Chemical Sciences
Arny, L. R. New York: Special Libraries Association, 1984. (BSEL QC5.3 .A76 1984)
Very useful for identifying sources for data not included in the standard tools, providing information on many smaller, important government data sets. Also, good overview of data as an entity (locating it, critical evaluation of it) and the key players.