  The model-potential approach previously developed by the authors to study
positron interactions with molecules is used to calculate the positron binding
energy for $n$-alkanes (C$_n$H$_{2n+2}$) and the corresponding cycloalkanes
(C$_n$H$_{2n}$). For $n$-alkanes, the dependence of the binding energy on the
conformation of the molecule is investigated, with more compact structures
showing greater binding energies. As a result, thermally averaged binding
energies for larger alkanes ($n\gtrsim 9$) show a strong temperature dependence
in the range of 100-600 K. This suggests that positron resonant annihilation
can be used as a probe of rotational (trans-gauche) isomerization of
$n$-alkanes. In particular, the presence of different conformers leads to
shifts and broadening of vibrational Feshbach resonances in the annihilation
rate, as observed with a trap-based low-energy positron beam.
