isotemer

Isotemer is a term used in theoretical chemistry and speculative science to describe a class of molecular systems that are chemically indistinguishable from a reference species yet differ in their vibrational and dynamical properties due to selective isotopic substitution. Unlike conventional isotopologues, which are primarily differentiated by mass, isotemers are defined to retain identical electronic structure and thermodynamic properties under standard conditions while exhibiting altered kinetic behavior and spectroscopic signatures that arise from subtle mass‑dependent dynamics.

Concept and scope: In thought experiments, isotemers illustrate how isotopic arrangement can influence reaction pathways without changing reactivity in a simple thermodynamic sense. The idea relies on the prominence of zero‑point energy differences and isotope effects on transition states, which may affect rates and selectivity in certain environments or at low temperatures.

Production and observation: Realizing isotemers would require precise placement of isotopes at specified molecular sites. In practice, isotopic labeling and advanced synthesis can approximate isotemers for experimental tests, particularly in spectroscopy and kinetic isotope effect studies.

Applications: Potential uses include studying isotope effects in catalysis, validating computational methods for predicting kinetic isotope effects, and designing materials with tailored vibrational properties for thermal management and spectroscopic calibration.

Limitations: The concept remains largely theoretical and constitutes a niche, speculative framework rather than a widely adopted scientific category.

See also: isotopologues, isotopomers, isotopic labeling, isotope effects, zero-point energy.