Molecularly Tuning the Radicaloid N-H···O═C Hydrogen Bond.

Substituent effects on the open shell N–H···O═C hydrogen-bond has never been reported. This study examines how 12 functional groups composed of electron donating groups (EDG), halogen atoms and electron withdrawing groups (EWG) affect the N–H···O═C hydrogen-bond properties in a six-membered cyclic model system of O═C(Y)–CH═C(X)N–H. It is found that group effects on this open shell H-bonding system are significant and have predictive trends when X = H and Y is varied. When Y is an EDG, the N–H···O═C hydrogen-bond is strengthened; and when Y is an EWG, the bond is weakened; whereas the variation in electronic properties of X group do not exhibit a significant impact upon the hydrogen bond strength. The structural impact of the stronger N–H···O═C hydrogen-bond are (1) shorter H and O distance, r(H···O) and (2) a longer N–H bond length, r(NH). The stronger N–H···O═C hydrogen-bond also acts to pull the H and O in toward one another which has an effect on the bond angles. Our findings show that there is a linea...