INTRAMOLECULAR HYDROGEN-BONDING AND MOLECULAR-GEOMETRY OF 2-NITROPHENOL FROM A JOINT GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL INVESTIGATION

The molecular geometry of 2-nitrophenol has been determined by a joint investigation of gas-phase electron diffraction and ab initio molecular orbital calculations. RHF/6-31G*, RHF/6-31G**, and MP2/6-31G* optimizations were used to determine small parameter differences, such as Delta(N=O), Delta(C-C), and Delta(C-N=O), which in turn were utilized as constraints in an electron diffraction structure analysis. The present experimental and calculated geometries are consistent regarding (i) the planarity of the molecule and (ii) all the structural features including strong hydrogen bonding between the nitro group oxygen and hydroxy hydrogen and the structural changes in the rest of the molecule as compared with phenol and nitrobenzene on the one hand and 2-nitroresorcinol on the other. Whereas the structural changes in 2-nitrophenol are less pronounced than those in 2-nitroresorcinol, they are consistent with a resonance form that implies the hydrogen bond and some redistribution of electron density in the benzene ring. The (N=)O...H(-O) and (N=)O...O(-H) nonbonded distances are 1.72 +/- 0.02 and 2.58 +/- 0.01 Angstrom, respectively, and they do not differ, within experimental error, from those of 2-nitroresorcinol. The molecular geometry is characterized by the following bond lengths (r(g)) and angles, obtained in the electron diffraction analysis incorporating the constraints from the molecular orbital calculations: (C-H)(mean), 1.089 +/- 0.007 Angstrom; (C-C)(mean), 1.399 +/- 0.003 Angstrom; C-1-C-2, 1.411 +/- 0.012 Angstrom; C-2-C-3, 1.406 +/- 0.013 Angstrom; C-3-C-4, 1.388 +/- 0.021 Angstrom; C-4-C-5, 1.399 +/- 0.027 Angstrom; C-5-C-6, 1.387 +/- 0.020 Angstrom; C-1-C-6, 1.402 +/- 0.016 Angstrom; C-O, 1.359 +/- 0.009 Angstrom; O-H, 0.969 +/- 0.012 Angstrom; C-N, 1.464 +/- 0.005 Angstrom; (N=O)(mean), 1.233 +/- 0.003 Angstrom; N=O-14, 1.241 +/- 0.009 Angstrom; N=O-15, 1.225 +/- 0.009 Angstrom; angle C-6-C-1-C-2, 121.4 +/- 0.5 degrees; angle C-1-C-2-C-3, 119.4 +/- 0.8 degrees; angle C-2-C-3-C-4, 118.1 +/- 1.6 degrees; angle C-3-C-4-C-5, 122.9 +/- 0.9 degrees; angle C-4-C-5-C-6, 119.3 +/- 0.8 degrees, angle C-1-C-6-C-5, 119.0 +/- 0.8 degrees; angle C-1-C-2-O, 123.9 +/- 0.8 degrees; angle C-O-H, 104.4 +/- 2.2 degrees; angle C-2-C-1-N, 120.8 +/- 0.7 degrees; (angle C-N=O)(mean), 118.4 +/- 0.3 degrees; angle C-N=O-14, 118.2 +/- 1.0 degrees; angle C-N=O-15, 118.6 +/- 1.0 degrees; angle O=N=O, 123.3 +/- 0.4 degrees; nitro group torsion, 7.3 +/- 5.7 degrees.