See especially Properties Data for Chemicals and Materials and Spectra and Spectral Data (UB Library Research Guides)

• DOE Data Explorer - Master repository of scientific research data for U.S. Dept. of Energy sponsored research
• Fundamental Physical Constants (NIST)
• Landolt-Bornstein Substance / Property Index - Keyword searchable as well as molecular formula, element system, CAS Registry Numbers, chemical name, and zeolite indicies. UB has most of the older print volumes on the Lockwood Science & Engineering Reference shelves (QC61.L33 & QC61.L332).
• Smithsonian Physical Tables (9th rev. ed.)