ORGANIC SYNTHESIS AND INDUSTRIAL ORGANIC CHEMISTRY The Use of Mass Spectra for Estimating the Melting Points of Esters

The possibility of using low-resolution mass spectra as descriptors of molecular structure in estimating the melting points of 79 esters was studied. The melting point T m is one of the most important characteristics of organic compounds. In synthesis of new compounds it is determined first among other physicochemical characteristics, since it is used for identifying the compound and checking its purity. Experimental determination of T m requires fairly large amounts of high-purity sample and in many cases is complicated by low thermal stability of substances. In this connection, approximate estimation of T m is an urgent problem. A recognized leader in estimation of various phys- icochemical characteristics is the QSPR (Quantitative Structure Property Relationships) method based on a search for correlation equations relating the target characteristic and certain molecular characteristics (topological, electronic, etc.) of the compounds from a training sample. The resulting equations are used for further estimation of characteristics of other com- pounds. The topological indices, which are the in- variants of the matrix of the graph reflecting the molecular structure, are mainly used as molecular characteristics. The estimation quality is characterized by the correlation coefficient R between the estimated and experimental values and standard deviations S. Published reviews (1, 2) show that the estimation accuracy in many characteristics already approaches the experimental accuracy. The melting point, whose estimation quality should be considered up to now as insufficient, is among a few exceptions (Table 1) (3 12). A low accuracy of the models is caused in some cases by poor accuracy of experimental values used in training samples. In particular, sometimes the pres- ence of small impurities can significantly decrease T m . The other reason is that, as compared to, e.g., the boiling point, T m is governed by additional factors related to the features of the crystal lattice. Although T m is a function of the structure, up to now there are no fairly general and accurate methods for its estima- tion on the basis of molecular structure. The com- monly used characteristics of molecular structure can- not reflect numerous factors governing T m . Here we studied the possibility of estimating T m of esters from the mass spectra. Previously it was shown that the mass spectra can be successfully used to esti- mate the chromatographic retention indices of a set of substances of various classes, among them esters (13), and standard boiling points of alkanes (14). We used low-resolution mass spectra of 79 esters with the number of carbon atoms from 2 to 15, avail- able from the server of the National Institute of Stan- Table 1. Results of estimating the melting points of vari- ous substances from published data