Why several of isomers of octane such as 2,3-dimethylhexane and 2,4-dimethylhexane doesn't have a value for melting point from any databases?
I recommend to include more than one (secondary) reference and to widen the querry -- if there is access -- to general chemistry data bases like Chemical Abstracts (by the American Chemical Society), or Reaxys (by Elsevier), and Springer Materials, too.

In addition, do not stick to one chemical name, and equally include in your querry unique identifiers like the CAS registry number -- applicable regardless how the organic nomenclature is applied (including databases in other languages) -- rather than one chemical name.  By doing so, you may rapidly find addtional data, for example among chemical suppliers like Sigma-Aldrich, that lists 2,3,4-trimethylpentane (CAS-RN 565-75-3) in its catalog with a melting point of −110 °C and a boiling point of 113-114 °C.

Beside electronic ressources, consider additional ones available in print, like volumes in Landolt-Bornstein dedicated to condensed matter (now included in Springer Materials).