Physical-Chemical Aspects of Synthetic Auxins 



293 



Table 2. Infrared absorption of some phenoxyacetic acids and other growth sub- 

 stances. 



* S = strong; M = medium; W = weak absorption. 



the absorption spectra for 2,4-D and 2,6-D. A series of such compounds 

 was examined for infrared absorption in this region. The spectra 

 were assigned a code number and were then examined and sorted into 

 two groups, one being those thought to have appreciable biological 

 activity and those having moderate or low biological activity. Follow- 

 ing the sorting, the spectra were decoded and the absorption spectra 

 and biological activity were found to give a positive correlation of 0.80. 



A summary of the absorption characteristics of several of the 

 chlorophenoxyacetic acids, 1 -naphthalene acetic acid, and indole-3- 

 acetic acid is given in Table 2. 



The infrared absorption of substituted aromatic compounds in the 

 800 cm.-i region has been suggested by Bellamy (2) as being due to 

 the in-plane, out-of-plane vibrations of the nuclear substituents. If 

 this is indeed the case, the absorption spectra noted here might sug- 

 gest that the biologically active compounds have a uniformly coupled 

 plus-minus movement of the nuclear substituents, where the com- 

 pounds of lesser biological activity have an alternate vibration pat- 

 tern. In the former case, this behavior would make for a greater pos- 

 sibility of interaction of the ring with a hydrocarbon chain such as 

 forms the skeleton of a protein molecule. 



The apparent success in relating biological activity to infrared ab- 

 sorption spectra of the chlorophenoxyacetic acids raised a question 

 as to whether there may be a similar relation between the electronic 

 configuration of these molecules as indicated by their ultraviolet ab- 



