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Novel Insights into the Mechanism of the Ortho / Para Spin Conversion of Hydrogen Pairs: Implications for Catalysis and Interstellar Water
Author(s) -
Limbach HansHeinrich,
Buntkowsky Gerd,
Matthes Jochen,
Gründemann Stefan,
Pery Tal,
Walaszek Bernadeta,
Chaudret Bruno
Publication year - 2006
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200500559
Subject(s) - chemical physics , chemistry , spin (aerodynamics) , hydrogen , inductive coupling , molecule , physics , thermodynamics , organic chemistry , quantum mechanics
The phenomenon of exchange coupling is taken into account in the description of the magnetic nuclear spin conversion between bound ortho ‐ and para ‐dihydrogen. This conversion occurs without bond breaking, in contrast to the chemical spin conversion. It is shown that the exchange coupling needs to be reduced so that the corresponding exchange barrier can increase and the given magnetic interaction can effectively induce a spin conversion. The implications for related molecules such as water are discussed. For ice, a dipolar magnetic conversion and for liquid water a chemical conversion are predicted to occur within the millisecond timescale. It follows that a separation of water into its spin isomers, as proposed by Tikhonov and Volkov ( Science 2002 , 296 , 2363), is not feasible. Nuclear spin temperatures of water vapor in comets, which are smaller than the gas‐phase equilibrium temperatures, are proposed to be diagnostic for the temperature of the ice or the dust surface from which the water was released.