## 3 Results and Discussion
### Validation of Vaporisation Enthalpies Using Structure-Property Relationships
#### 3.3.1 Empirical Correlations: Vaporisation Enthalpies versus Kovats Retention Indices  
The Kovats index, \(J_{x}\), represents the retention time of a compound in the gas-chromotographic (GC) column [45]. The retention time reflects the intensity of absorption/desorption of the molecule in the stationary phase. Since the retention times of structurally related molecules are measured under identical experimental conditions (stationary phase, temperature), the \(J_{x}\)-values model the relative differences in the intermolecular interactions of the pure compounds, and the \(J_{x}\)-values are indirectly related to the enthalpies of vaporization. As a rule, the vaporisation enthalpies \(\Delta_{1}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}(298.15\;\mathrm{K})\) and Kovats indices correlate linearly within the series of structurally similar compounds [11]. Kovats retention indices were taken from the original literature [46; 47; 48; 49]. To increase reliability, it is better to correlate the vaporisation enthalpies of alkylbenzenes with the \(J_{x}\)-values measured on a specific GC column. We did not find a comprehensive \(J_{x}\)-set containing all the alkylbenzenes of interest for this work; instead we used a few sets of Kovats indices containing the required molecules. For example, Kovats indices of different iso-propyl- and tert-butyl-substituted benzenes were measured on the non-polar column OV-101 [46]. Results for correlations of experimental enthalpies of vaporisation versus Kovats retention indices on this column are shown in Table 5. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Compounds** & \(\mathbf{T_{\mathrm{fuss}}}\;\mathrm{K}\) & \(\mathbf{\Delta_{\mathrm{cr}}^{1}H_{\mathrm{m}}^{\mathrm{o}}\;\mathrm{at}\;T_{ \mathrm{fuss}}}\) & & & \(\mathbf{\Delta_{\mathrm{cr}}^{1}H_{\mathrm{m}}^{\mathrm{o}}\;\mathrm{c}}\) & \(\mathbf{\Delta_{\mathrm{b}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\;\mathrm{c}}\) & \(\mathbf{\Delta_{\mathrm{cr}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\;\mathrm{e}}\) \\ \cline{3-7}  & & \multicolumn{2}{c}{_WC b_} & & **298.15 K** & \\ \hline
1 & 2 & 3 & 4 & 5 & 6 & 7 \\ \hline
1,2,4,5-tetra-iso-propylbenzene & 393.0 & 19.6 \(\pm\) 0.5 [27] & 49.9 & 18.8 \(\pm\) 0.6 & 68.3 \(\pm\) 1.6 f  & 87.1 \(\pm\) 1.7 \\ \hline
1,4-di-tert-butylbezene & 350.8 & 22.5 \(\pm\) 0.4 [34] & & & & \\  & 350.5 & 21.8 \(\pm\) 0.2 [10] & & & & \\  & 350.5 & 22.6 \(\pm\) 0.1 [36] & & & & \\  & & **22.4 \(\pm\) 0.1 f**  & 63.9 & 21.2 \(\pm\) 0.4 & 60.5 \(\pm\) 0.5 & 81.7 \(\pm\) 0.6 \\ \hline
1-methyl-3,5-di-tert-butylbezene & 307.6 & 18.4 \(\pm\) 0.2 [10] & 59.8 & 18.2 \(\pm\) 0.3 & 62.5 \(\pm\) 0.5 & 80.7 \(\pm\) 0.6 \\ \hline
1,3,5-tri-tert-butylbezene & 343.2 & 11.9 \(\pm\) 0.2 [10] & 34.7 & 10.9 \(\pm\) 0.4 & 69.0 \(\pm\) 0.7 & 79.9 \(\pm\) 0.8 \\ \hline \hline \multicolumn{7}{c}{**52.1 \(\pm\) 6.5 f**} \\ \hline \hline \end{tabular}  
* Uncertainties are presented as expanded uncertainties (0.95 level of confidence with k = 2); b The Walden’s Constant (WC) [43; 44] calculated according to Equation (59) in J\(\cdot\)mol\({}^{-1}\).K\({}^{-1}\) (for details see ESI);c The experimental enthalpies of fusion \(\Delta_{\mathrm{cr}}^{1}H_{\mathrm{m}}^{\mathrm{o}}\) measured at \(T_{\mathrm{fuss}}\) were adjusted to \(T=298.15\;\mathrm{K}\) with the help of Equation (7); d The values of vaporisation enthalpies are calculated in Tables 1 and 2 (given in bold); e Calculated according to the equation: \(\Delta_{\mathrm{cr}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\;(298.15\;\mathrm{K})= \Delta_{\mathrm{r}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\;(298.15\;\mathrm{K})+ \Delta_{\mathrm{cr}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\;(298.15\;\mathrm{K}) +\Delta_{\mathrm{cr}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\;(298.15\; \mathrm{K})\) as the sum of columns 5 and 6 in this table f Average value.  
\end{table} 
Table 4: Phase transition thermodynamics of alkylbenzenes (in kJ\(\cdot\)mol\({}^{-1}\)) a.