Proton and Fluorine Spinndash;Lattice Relaxation and Spinndash;Rotational Interaction in Liquid 1,3,5hyphen; and 1,2,4hyphen;Trifluorobenzenes

摘要

The proton and fluorine spinndash;lattice relaxation times,T1, have been measured in degassed liquid 1,3,5hyphen; and 1,2,4hyphen;trifluorobenzenes (135TFB and 124TFB) using pulsed NMR techniques at a frequency of 30 MHz. Within experimental error, the relaxation of each species could always be described with a single exponential. The data are analyzed in terms of dipolar and spinndash;rotational (SR) contributions toT1. The protonndash;fluorine dipolar interaction is theoretically approximated so that a singleT1may be used to analyze the relaxation of each nucleus. The proton relaxation was measured from the mp to 100deg;C and is dominated by dipolar interactions. The fluorine relaxation times, measured from minus;thinsp;20deg;C (supercooled liquid) to approximately 220deg;C, are dominated by dipolar interactions at the lower temperatures and by SR interactions at the higher temperatures. The interhyphen; and intramolecular contributions toT1were separated for F at 0deg;C and for H at 0 and 25deg;C by measuring relaxation as a function of dilution in C6D6. The intramolecularT1values in seconds are: 135TFB:Fthinsp;equals;thinsp;33thinsp;plusmn;thinsp;2, Hthinsp;equals;thinsp;63thinsp;plusmn;thinsp;4 (0deg;) and 87thinsp;plusmn;thinsp;3 (25deg;); 124TFB:Fthinsp;equals;thinsp;35.5thinsp;plusmn;thinsp;2, Hthinsp;equals;thinsp;48thinsp;plusmn;thinsp;3 (0deg;) and 55thinsp;plusmn;thinsp;3 (25deg;). The dipolar correlation times obtained from H relaxation are (4.8thinsp;plusmn;thinsp;0.2)thinsp;times;thinsp;10minus;12sec for 135TFB and (3.1thinsp;plusmn;thinsp;0.2)thinsp;times;thinsp;10minus;12sec for 124TFB at 0deg;C. Rather inaccurate estimates of the SR contribution to fluorineT1and the fluorine SR interaction constantlpar;2Cperp;2thinsp;plus;thinsp;CVerbar;2rpar;1sol;2are made and yield at 0deg;C the approximate values 70 sec and 2.3 kHz, respectively. The data are compared with the theories and experimental results of other investigators.