Novel insights into the mechanism of the ortho/para spin conversion of hydrogen pairs: implications for catalysis and interstellar water.
Novel insights into the mechanism of the ortho/para spin conversion of hydrogen pairs: implications for catalysis and interstellar water.
复制标题
对氢对的正/对自旋转换机制的新见解：对催化和星际水的影响。
作者：
H. Limbach;G. Buntkowsky;Jochen Matthes;Stefan Gründemann;T. Pery;Bernadeta Walaszek;B. Chaudret
在描述束缚的正对二氢和对二氢之间的磁性核自旋转换时，考虑了交换耦合现象。与化学自旋转化不同，这种转化发生时不会破坏键。结果表明，交换耦合需要降低，相应的交换势垒才能增加，给定的磁相互作用才能有效地诱导自旋转换。讨论了对水等相关分子的影响。据预测，对于冰，偶极磁转换和液态水的化学转换将在毫秒时间尺度内发生。由此可见，Tikhonov和Volkov （Science 2002, 296, 2363）提出的将水分离成自旋异构体是不可行的。彗星中水蒸气的核自旋温度比气相平衡温度小，被认为可以用来诊断水从冰或尘埃表面释放出来的温度。
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.