How does electron-spin resonance spectroscopy distinguish triplet carbenes from singlet carbenes?

Electron-spin resonance (ESR) spectroscopy distinguishes between triplet and singlet carbenes by detecting the presence or absence of unpaired electron spins, which generate a magnetic moment.12

In a triplet carbene, the two nonbonding electrons occupy different orbitals with parallel spins, resulting in a net magnetic moment. Because these substances possess unpaired electrons, they are paramagnetic and can be detected by ESR spectroscopy. The technique identifies the triplet state through the interaction of these magnetic moments with an external magnetic field, which can reveal information about the molecule's angular momenta.132

In contrast, singlet carbenes have their two nonbonding electrons paired in the same orbital with opposite spins. Because all electrons are paired, singlet carbenes show no net magnetic moment and are therefore "ESR-silent" (they do not produce an ESR signal). Consequently, researchers use ESR to confirm the triplet state of carbenes, such as the ground state of methylene, while other methods like optical spectroscopy are typically required to study the singlet state.12