INTRAMOLECULAR MOTION AND MOLECULAR-STRUCTURE OF N-NITROPYRROLIDINE -A GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL STUDY

The molecular structure and intramolecular motion in N-nitropyrrolidin e has been reinvestigated utilizing a joint electron diffraction/ab in itio approach. Both methods give a C-envelope conformation (E-3 or its enantiomeric E-4 form) of the pyrrolidine ring with pyramidal ring ni trogen and the nitro group bending in opposite direction to the out-of -plane carbon in the ring. The barrier heights to inversion at the nit rogen atom in the ring and torsion of the nitro group have been calcul ated. Two transition stales for pseudorotation both having N-envelope conformation (E-1) of the pyrrolidine ring were detected, one with an equatorial and the other with an axial arrangement of the nitro group. The inversion at the ring nitrogen is found to be considerably couple d to pseudorotation of the ring. The C-C and C-N bond length differenc es computed at the MP2/6-311++G* level were used as constraints in th e electron diffraction analysis. The most are (r(g) distances and r(a) angles, with total errors): r(C-N)(mean) 1.485+/-0.018 Angstrom; r(C- C)(mean) 1.535+/-0.018 Angstrom; r(N-N) 1.376+/-0.005 Angstrom; r(N-O) 1.230+/-0.003 Angstrom; r(C-H) 1.111+/-0.005 Angstrom; angle C-N-C 11 2.5+/-2.2 degrees; angle O-N-O 124.6+/-1.2 degrees; angle H-C-H 111.0/-2.8 degrees; puckering amplitude. q(0), 10.8+/-0.6 degrees; pseudoro tation phase angle. f, 76.0+/-1.6 degrees: angle between the C-N-C pla ne and the N-N bond 35.1+/-2.2 degrees: nitro group torsion 0.2+/-3.4 degrees.