Small molecules with polar functional groups, including substituted phenethylamines, are commonly analyzed by liquid chromatographymass spectrometry (

LC-MS) with electrospray ionization (ESI). Analyte molecules are mostly detected in protonated and cation-adducted forms through positive-ion ESI-MS. However, the ESI of substituted phenethylamines commonly provides an intense signal of fragment ions by ESI in-source collision-induced dissociation (IS-CID), which hinders the unambiguous identification of phenethylamines. This phenomenon was approximated as a unimolecular dissociation model, and the dissociation efficiency was evaluated by various quantum chemistry calculations to determine the ESI IS-CID efficiency. The calculated results were consistent with the experimental data, when the dissociation threshold energy of phenethylamines was calculated using the post-Hartree-Fock (post-HF) method, CCSD(t)/cc-pVTZ//MP2(full)/6-311++G(d,p). In contrast to post-HF methods, the utilization of density functional theory calculations with a suitable functional is recognized as an accurate and competitive low-cost approach. In particular, ωB97-XD, M06-2X-D3 and recently developed Minnesota functionals, such as M11, MN12-SX and MN15, provided reliable results, as in the case of the post-HF method. The results obtained by the recently developed double hybrid functionals, DSD-PEBP86-D3(BJ), PBE0-DH and PBE-QIDH, were also reliable. The consideration of ESI IS-CID can facilitate the identification of analyte molecules because most phenethylamines, except for N-methylated analogs, provide an intense signal in the ESI mass spectrum.