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Simon Freed 



required to specify the system will be less at lower temperatures. The system 

 will redistribute itself from higher to lower energy levels so that only the more 

 basic ones remain appreciably occupied. Fewer chemical species are now 

 present and also active. There has been, in a sense, a reduction in chemical 

 noise differing in its frequency spectrum from the continuum characteristic 

 of an electrical conductor. Chemical noise reflects the structural properties 

 of molecules and may consist of dominant discrete frequencies associated 

 with virtual continua of modulations. Usually these represent couphng of the 

 electronic system of the molecule in a given atomic configuration with its 



Fig. 1 . The variation of absorption spectrum of praseodymium chloride with 

 temperature. Line drawings of visible absorption spectra of crystals of anhydrous 

 praseodymium chloride (PrClg) at room temperature, at that of liquid nitrogen, 

 and that of liquid helium. Sharper spectra, improved resolution, and fewer hnes 

 are evident at lower temperatures. The fewer hnes correspond to fewer energy 

 states which are occupied by the praseodymium ions. At room temperature the 

 blocks of diffuse spectra are actually not uniform in intensity but are more 

 intense as a rule in those regions where the spectrum of the crystal at 77°K 

 possessed its most intense line spectrum. The greater diffuseness of the lines and 

 their increased numbers at the higher temperature may be regarded as chemical 

 noise associated with the spectroscopic signals from the more stable states at the 



lowest temperature. 



own vibrations, restricted rotations, etc. If the molecules are complex, fluctua- 

 tions between difl'erent atomic configurations may contribute to the noise. 

 In addition, coupling of the molecule in each of its states with the molecules 

 of its environment in different configurations leads to more and more densely 

 spaced energy levels which I referred to as the continua. 



A reduction in temperature removes thennal energy required to activate 

 some motions and effect changes in configurations, and reduces the number 

 of perturbations of a given configuration. Not only are fewer species present 

 but each species is more sharply defined; thus, less infonnation is required 

 for specifying the system than at higher temperature. Clearly, the system is 

 now more specific in its reactions than at higher temperature and its specificity 

 can be related to more sharply defined geometric configurations. The chemical 

 system has become a more precise probe. 



The following illustrations have been selected for the simplicity of their 

 phenomena rather than for their direct relevance to biology. 



The sharp absorption spectrum of a crystal of a rare earth salt (Fig. 1) 

 shows very clearly that at the lower temperature fewer lines are present; they 

 are sharper and more clearly resolved and the general diffuse background 

 prominent at the higher temperature (not shown in the line drawing of the 

 figure) becomes decidedly weaker. There are then fewer kinds of absorption 

 centers at the lower temperature and, because the stable states are exposed to 



