Knowledge Bank

From infrared and Raman spectra a complete assignment of vibrational fundamentals has been obtained for cis- and $trans-HCIC=CFH,DCIC=CFH,HCIC=CFD$, and $DCIC=CFD$. For $cis-HCIC=CFH$, the $a\prime$ fundamentals, in $cm-1$, are: $3114,\sim 3100,1661,1335$, $1231,1062,812,656$, and 205; and the $a\prime \prime$ fundamentals are: 857, 735, and 442. For trans-HCIC = CFH, the a' fundamentals, in $cm-1$, are: $3103,\sim 3095,1651,1296,1218,1127,875,447$, and $\sim 269$; and the $a\prime \prime$ fundamentals are: 888, 784, and $\sim 269$. The assignment of isomeric configurations, based on the vibrational spectra, is also supported by the nuclear magnetic resonance coupling constants (in Hz). For the cis isomer: $JHH(cis)=3.3$ and $JHF(trans)=27.2$; for the trans isomer: $JHH(trans)=8$ and $JHF(cis)=10$. From the molecular parameters and the temperature dependence of the equilibrium constant for cis-trans isomerization, $\Delta E\circ \circ$(electronic) was found to be $780\pm 22$ cal/mole, the cis isomer having the lower energy.