Methylation effect on positron annihilation and binding.

Positron binding and annihilation has been reported for many molecules including large alkane and aromatic compounds [1]. Recently, a good-to-excellent agreement has been obtained between measured binding energies (ε_b) and the theoretical predictions which take into account electron-positron correlations, including virtual-positronium formation [2]. Here, we report positron annihilation and ε_b for both methylated benzene and pyridine molecules. In both cases, a correlation is observed between the increases in annihilation rate and ε_b with increasing number of methyl groups. For nitrogen containing compounds, such as methylpyridines, it is observed that methyl groups located far from the nitrogen exhibited the highest annihilation rate, compared to those closer to it. However, different subsitution sites (m-, o-, p- or 2-, 3-, 4-) for the methylated groups in each series do not show a significant change in ε_b values (i.e., ∆ε_b/ε_b < 5%). Additional results, including the influence of geometry and molecular parameters will be disccussed. [1] E. Arthur-Baidoo et. al., Phys. Rev. A 109, 062801 (2024). [2] J. Hofierka et al., Nature 606, 688-693 (2022)